LITERATURE REVIEW
Introduction to self – healing material
A self-healing polymer is a form of functional polymer material that can repair scratches, cracks, and other mechanical damage, and whose unique self-healing potential is essential for the material lifetime [32] Today, these materials are no longer an "illusion" since artificial materials can fully repair their qualities and structure after being damaged Cracks in buildings, for example, may close on their own, and scratches in the bodywork can be restored to their previous luster
Almost all materials are prone to deterioration and degradation over time; the long-term aging process causes cracks, leading to the material's utility being compromised As a result, the repair is required to extend the life of the material [33] Self-healing materials made from thermoplastic polymers have exceptional features They have significant benefits over standard polymers with monotonous chain architectures in that they fix the damage by themselves (without human intervention) caused by thermal and mechanical means with their intrinsic character of self-healing from injury as inspired by nature [34, 35] Today several forms and mechanisms of healing have been proven and published in science and it is predicted that more self- healing mechanisms will be found with technological advancement Self-healing materials are widely employed in military equipment [36], electrical devices, automobiles [37], airplanes, biological materials [38], and [39] With each passing year, the proportion of applications increases (Figure 1-1) [32]
Figure 1-1: The proportion of the output value of various self-healing fields in 2018(a), 2019 (b), and 2020 (c), respectively (d) The self-healing capability of various repair system [32].
Classification of self-healing polymers
Based on the process used to repair the damage, self-healing materials can be classed as extrinsic or intrinsic Intrinsically self-healing polymers are materials that can repair themselves at the molecular and macroscopic levels of damage caused by a driving force such as heat or light and then the repairing of bond strength (either physical or chemical) even after removing the stimulus from the system [35] Intrinsic-type materials, the healing process is due to the material's inherent properties and functions whereas in extrinsic materials, the damage is recovered by the release of a healing agent inside the cracks from isolated micro-containers embedded in the matrix [40]
In the natural world of biology, the unit of self-healing is the cell, in which different cells perform specific functions Inspired by this design, small artificial capsules that are capable of filling gaps when cracks occur have been developed The
10 concept of self-healing by capsules was first proposed by professor Scott White [32,
40, 41] Self-healing is achieved by introducing microcapsules into the polymer Microencapsulation is a process that encapsulates solid (microscopic) and liquid particles in a shell, thereby isolating and protecting them from the external environment [41] This process has been demonstrated with dicyclopentadiene (DCPD) and a Grubbs catalyst (benzylidene -bis(tricyclohexylphosphine) dichlororuthenium) Both the capsule containing the DCPD and the Grubbs catalyst were mixed into the epoxy resin The monomer itself is relatively inert and polymerization will not take place, when a small crack appears to rupture the DCPD- containing capsules, the reagents will be released and exposed to the catalyst, and the polymerization occurs at room temperature As a result, the cracked material recovers about 75% of its original mechanical properties (Figure 1-2) [42]
Figure 1-2: Describe the healing process with microcapsules proposed by
Each time, the amount of reagent in the microcapsules used is very little, sometimes insufficient to cover the crevices To overcome the difficulty of
11 microencapsulation of a healing agent, an alternative approach similar to the biological vascular system was explored by White and coworkers [43, 44]] This method employs a network of microchips or microfibers to continuously feed healing chemicals to injured locations Bond and colleagues introduced fibers containing a curing agent to an epoxy substrate They have proven that these composite panels can restore up to 97% of their original flexural strength (Figure 1-3) [45]
Figure 1-3: Describe the healing process with microfiber proposed by Bond [45]
The major drawback of extrinsic self-healing materials is that there are many difficulties in designing this material First, the reaction of the catalyst must be maintained even after it is encapsulated in the capsule or microfibers In addition, require broken capsules/ microfibers as well as material damage to release contents that can repair the damage and the monomers must flow at a sufficient rate (with low enough viscosity) to cover the cracks before it polymerizes or full recovery cannot be achieved Finally, the catalysts must rapidly dissolve into the monomer for the reaction to take place effectively and to prevent further propagation of cracks 1.2.2 Intrinsic self – healing polymer
The mechanisms of intrinsic healing processes of micro- and macroscale damages can be based various molecular principles, such as e reversible covalent connections, supramolecular interactions, shape memory effects, or via the use of
12 polymer blends based on different physical and chemical techniques [46, 47] When a reversible polymeric network is disrupted, two local processes, chain cleavage and chain slippage, occur at the rupture site [48] Although chain cleavage is commonly thought of as structural damage, it may also produce a number of functional groups at the surface These functional groups can influence the ensuing interactions in the polymer substance The first step in the intrinsic healing process is the softening and movement of the healing substance toward the damage, and the second stage is discovered to be a hardening process that recovers its original qualities The intrinsic self-healing method offers viable solutions to a variety of repair concepts
Intrinsic self-healing mechanisms can make use of both dynamic covalent interactions (Diels-Alder, trans-esterification, disulfide or radical exchange reactions) and non-covalent bonds (hydrogen bonding, π−π stacking, host-guest interaction stacking or ligand-metal bonding) [35]
The cross-linking of polymeric materials is usually used to provide better mechanical qualities such as high modulus, solvent resistance and high fracture strength Yet, it has a negative impact on polymer refabrication Moreover, strongly cross-linked materials have the drawback of brittleness and cracking Among the various approaches to solve this problem, the introduction of reversible cross-links into polymer chains has been one of the most powerful development approaches [49] The fracture damage of the polymer is often accompanied by chain separation (bonding break) at the cracked surfaces In dynamic bonding polymers, these mobile reversible bonds tend to dissociate first, while other covalent bonds are conserved However, broken dynamic bonds can be re-bonded, connecting the fault surfaces and thereby leading to a complete restoration of the material properties at a macroscopic scale In principle, any polymer with a sufficient number of dynamic bonds may be capable of self-healing However, the poor mobility of the polymer chains often prevents the dynamic bonds from re-linking Furthermore, even without being constrained by chain mobility, the re-formation of these bonds can only occur with a proper closure of two crack surfaces [50] To overcome the crack closure problem,
13 solutions such as manual injection of a solvent or introduction of the shape-memory effect have been applied
In 2002, X Chen et al introduced a self-healing polymer formed via the Diels- Alder reaction between tris-maleimide and tetra-furan After that, researchers have studied many self-healing polymers with different dynamic linkages, including DA [51] reversible bonds, disulfide swapping bonds [52], urea binding [53], siloxane bonding [54], hydrogen bonding [55], ionic bonding [56], metal-ligand interaction [57], host-guest interaction [58, 59] or π−π stacking [60] Some widely used intrinsically self-healing concepts are summarised below:
The Disulfide exchange reaction is one of the earliest recognized covalent reactions, partly because this reaction is found in biological systems, for example, in the process of folding protein or the process of controlling the redox state of the cell [61] A disulfide exchange process depends on the nucleophilic attack of a thiolate anion to a disulfide bond, resulting in the release of a new thiolate from the original disulfide and at the same time the formation of a new disulfide group (Figure 1-4) Because a thiolate anion is needed, this exchange reaction is sensitive to pH (low) – which is a beneficial condition for the exchange reaction The exchange reaction can be started from the thiol mixture through the oxidation process with a basalt In addition, the thiol can be added to the existing disulfide, and the condition is the presence of a base Finally, adding a reducing agent (typically Dithiolthretiol –DTT) to a disulfide mixture will also make the exchange reaction to occur
Figure 1-4: Schematic diagram of a network with the healing through disulfide exchange reactions [61]
Tsarevesky and Matyjaszewski reported an early example of a self-healing polymer based on disulfide exchange [62] Polymers with chemically unstable disulfide bonds in the main chain was synthesized by atom transfer radical polymerization (ATRP) of styrene using an initiator containing disulfide bonds in the chain and pentamethyldiethylenetriamine as a catalyst at 90 °C The disulfide bond in the chain is cleaved by reduction with DDT to give the corresponding thiol- terminated polystyrene These groups recombined with the original disulfide by oxidation with FeCl3 (Figure 1-5)
Figure 1-5: The polymerization and reducing reaction of PS has the end of SH group [62]
Disulfide bonds have received attraction in self-healing polymer systems because these bridges are weaker than the carbon-carbon bonds [63] Thus, with a mechanical impact on the material, this linkage will be broken first The disulfide bond is relatively flexible and can be applied to many types of polymers [46]
Taking advantage of bond interchangeability, several self-healing polymers based on disulfide compounds have been developed In 2012, Lafont and colleagues studied the repairability of epoxy thermosets with disulfide bonds In order to repair these materials, damaged surfaces were kept connected for one minute and exposed to visible light for 12 hours [64] In 2013, Michal et al published self-healing networked polysulfide materials based on disulfide metabolism under UV or heat stimulation (180 o C) in combination with the shape memory properties of the material [65] In 2014, ZhouƯ's research group reported that using tri-n-butylphosphine as a catalyst for room-temperature disulfide exchange, confers self-healing and viability recycling for materials [66] Odriozola and co-workers investigated the exchange bond of aromatic disulfide as a self-healing material the same year [67] Based on previous studies, Bis(4-aminophenyl) disulfide has been used as a bridge in PU These materials show the self-repairing ability without catalysts at room temperature Therein, the cut halves were kept in contact for 2 hours, and the healed sample could be prolonged by hand without being broken (Figure 1-6) [68]
Figure 1-6: Polymer materials based on disulfide bonds can self-heal after damaging without external stimulus [68]
Despite the widespread use of disulfide exchange bonds in materials, several disadvantages of this bond also need to be considered In material systems where excessive thiols are required, oxidation of thiols in the air should be avoided
Self-healing polyurethane
Thermoplastic polyurethanes (TPUs) are polymers that include urethane repeating units and are mostly generated through addition polymerization of polyisocyanates and macropolyols with chain extenders (Figure 1-11) [84] Polyurethane materials generally have high tensile strength, low temperature resistance, wear resistance and corrosion resistance PUs have been widely utilized in automotive, electronics, medical supplies, coatings, and sports equipment industries The addition of self-healing capability has significantly increased the service life and recyclability of PUs [85]
Figure 1-11: A basic polyurethane synthesis method is depicted schematically [85]
The diisocyanate, dihydroxy telechelic prepolymer and chain extender in polyurethane synthesis determine the final characteristics of polyurethanes and their capacity or inability to self-heal Additional factors, such as side reactions during PU production and the presence of catalysts may also influence the ability of the final material to self-repair [48] Han et al have evaluated the effects of different dynamic chain extenders and water molecules on the self-healing of polyurethanes [86] They created two kinds of polyurethanes in this study, using 2,4-pentanedione dioxime (DMG) and bis(4-hydroxyphenyl) disulfide (HPS) as chain extenders, respectively
In addition, PDMS was employed as the soft segment and isophorone diisocyanate (IPDI) as the hard segment The water absorption experiment clearly demonstrated that they absorbed water until reaching saturation The self-healing trials were conducted in both air and water, demonstrating that the underwater self-healing speed of DMG polyurethanes was greatly boosted Water was thought to plasticize these hydrophobic polymers, accelerating their self-healing speed underwater The design of TPUs combining various self-healing processes has grown rapidly in recent years, which is a beneficial technique to combine extraordinary mechanical strength and excellent self-healing efficiency Thus far, self-healing polyurethanes have relied
23 mostly on reversible covalent bonds and non-covalent bonds to achieve many healing cycles
Recently, in 2022, Liu et al used 1,8-menthane diamine (MD) and bis(2- hydroxyethyl) disulfide (HEDS) as chain extenders to prepare polyurethane elastomers MD amino groups and isocyanate react to generate H-bonded urea groups, which increase mechanical characteristics, while MD's tight ring structure facilitates disulfide bond breakdown The tensile strength of the MD-PU-SS elastomer was 24.8 MPa MD-PU-SS toughness and fracture energy were determined to be 274.6 MJ m 3 and 114.43 kJ m 2 , respectively The use of MD enabled MD-PU-
SS to have not only high mechanical qualities but also up to 94% healing efficiency and good recyclability [86]
According to another report, a self-healing polyurethane material was created by creating hydrogen connections between urethane bonds, urea bonds, and carbonyl groups in propylene carbonate The self-healing poly(urea-urethane) (PUU) elastomer's mechanical characteristics and healing effects were investigated [87] Besides urea and urethane groups, a self-repairing polyurethane elastomer has been designed based on the hydrogen bonds of the ureidopyrimidone (Upy) unit capable of forming quadruple H-bonds Sun et al used a UPy functionalized monomer as a chain extender to create a waterborne polyurethane (WPU) with quadruple hydrogen bonds in the main chain The number of quadruple hydrogen bonds was proved to be inversely proportional to the healing time of a scratch (the lowest healing time at 80 °C is 2.5 h), although the mechanical properties of the repaired film recovered to more than 90% of their original value [88].
Self-healing Diels-Alder containing polyurethanes
The Diels-Alder reaction of furan and maleimide moieties has been widely explored in the creation of self-healing polyurethane Despite the fact that most products contain covalently cross-linked structures, the thermally reversible cycloaddition process that results in the breakage of DA bonds takes place at a high
24 temperature When the DA bond is placed at the polymer's backbone or end group, a self-healing polyurethane with a linear structure can be formed Cheng et al developed a novel diol containing DA bonds as well as a thermally induced self- healing and recyclable WPU film (WPU-DA-x) on the basis of the DA/retro-DA reaction They divided the repairing process into three parts based on the development of the self-healing: 1) greater temperature retro-DA reactions; 2) diffusion of degraded pieces; 3) re-association of DA bonds [89]
Most research has been grounded on synthesising functional polyols (including diols) with DA linkages between furan and maleimide groups for polyaddition with polyisocyanates [90]
In method I, functional polyols containing a DA cycloadduct are employed to synthesize PUs with DA linkages on the main chain Furfuryl alcohol first reacted with N-(2-hydroxyethyl)-maleimide to form DA-labeled diol Triethanolamine and hexamethylene diisocyanate (HDI) were polyadditioned to the resulting diol The resultant PU crosslinked networks with dynamic DA cycloadduct connections have remarkable shape memory characteristics [91] Another study reports PUs based on self-made soft segments, isocyanate terminated polydimethylsioxane (NCO-PDMS- NCO) or a mixture of NCO-PDMS-NCO and polycaprolactone diol to develop thermally healable poly(siloxane-urethane) elastomers (PDMS-DA-PU and PDMS/PCL-DA-PU) Via DA and retro-DA reactions, the elastomers demonstrated good self-healing and remoldability The PCL segment was inserted into the polymer chain to improve mechanical characteristics (siloxane-urethane) The polymers' tensile strength, elasticity, and shape recovery ability were enhanced by the microphase separation structure and covalently crosslinked network [92] The Tung oil and 4-maleimidophenol have recently employed to synthesize a polyphenol containing phenolic hydroxyl groups The resulting adduct was polyadditioned with an NCO-terminated PU prepolymer to produce self-healing PU networks derived from the tung oil-based polyphenol The tensile test experiment has demonstrated that
25 the broken and healed thermosets preserved 46-64% and 81-88% of their original tensile strengths and elongations at the break, respectively (Figure 1-12) [93]
Figure 1-12: The self-healing polyurethane network based on phenol-type urethane is depicted schematically [93]
The second approach involves synthesizing linear polyurethanes by incorporating a diol bearing pendant furan groups along the polymer chain Subsequently, the generated reactive polyurethanes are employed in successive Diels-Alder reactions utilizing polymaleimides as crosslinking agents, ultimately yielding thermally-reversible polyurethane networks interconnected via Diels-Alder linkages The incorporation of pendant furan groups within the linear polyurethane chains effectively enabled cross-linking through tri-maleimide end-capped cyclotriphosphazene, forming covalent Diels-Alder bonds This led to a remarkable increase of 293% in tensile stress at break and an impressive 732% enhancement in Young’s modulus The exceptional self-healing capacity of the resulting cross-linked polyurethane films was both qualitatively and quantitatively investigated, leveraging the reversible nature of Diels-Alder reactions Moreover, the introduction of
26 cyclotriphosphazene conferred outstanding flame retardant properties to the synthetic polyurethane elastomers Additionally, the PU-DA-x films exhibited commendable recyclability, capitalizing on the retro-DA/DA process [93]
Figure 1-13: Formation of Reversible Cross-Linked PU-DA-x Films via Diels- Alder Covalent Bonds (a) Synthesis of the PUF Solution (b) Preparation of Flame-
Approach III entails post-modifying thermoplastic PUs with functional furans or maleimides to produce reactive PUs ended with furan or maleimide groups By the creation of dynamic DA adducts, the generated PUs were crosslinked with either polymaleimides or polyfurans via the DA reaction Several techniques have been investigated, such as the furfuryl group modification of a tri-isocyanate toward a shape memory thiourethane thermoset [94] and the interpenetration of both PU and polyDA for hybrid networks With shape memory properties, the two systems were formed from bismaleimidic and bisfuranic semi-crystallized polycaprolactone chains (Ttrans = Tm), whereas the other two networks were made from bis-/trismaleimidic and trisfuranic monomers with glass transitions, which initiated shape recovery behaviors (Ttrans = Tg) The shape recovery capacity appears to be a key to the healing effects of scratches since fracture closure and the healing reaction are impossible without it After having automatically healed at a low temperature of 60 °C for 1-3 days, two materials showed excellent shape memory and scratch-healing capabilities, with the
27 level of mechanical recovery, 70 - 80% PEG reacted with IPDI before being modified with N-hydroxyethyl maleimide in order to produce a reactive polymaleimide from PU prepolymer The PU prepolymer generated was coupled with furfuryl-functionalized polydopamine nanoparticles, yielding thermoreversible PU crosslinked networks with polydopamine nanoparticles covalently integrated via the formation of DA adducts The resultant DA-crosslinked PU composites display the excellent self-healing ability, being exposed to near-infrared (NIR) light [95]
Figure 1-14: Chemical structures of the utilized (thio)urethane multi-maleimide and -furan monomers, and shape-memory networks, cross-linked reversibly by the
Shape memory-induced crack closure in self-healing polymer
Figure 1-15 depicts the basic approaches to repairable polymers that use either physical or chemical processes at the molecular level, or the two strategies may be combined Several self-healing mechanisms involve a mix of physical and chemical events, such as shape memory effects that support the self-healing process [95]
Closing the crack so that mending can occur is a challenging problem for self- healing materials In recent years, scientists have taken the advantage of the "shape memory" characteristic to create the effect of closing cracks, thereby supporting the
"self-healing" response more effectively The shape-recovery effect, also known as
"shape memory", allows the crack surfaces to come into contact so that the bonding reaction between the two crack surfaces can occur and the crack heals, thus eliminating the need for outside intervention to close the crack [96] Many shape- memory polymers as polyurethane, polystyrene, or multi-component polymers (epoxy-polycaprolactone, etc.) have been investigated [97]
Figure 1-15: The basic approaches to self-healing polymers that use either physical or chemical processes [97]
For thermo-stimulated shape-memory polymers, the glass transition and melting transition temperatures are the main temperature points of shape change [75] There are two basic requirements for the memory effect:
• The net-point defines the primary shape (permanent shape of the material) – responsible for shape fixation and shape memory polymer recovery
• Transition chains are responsible for the secondary shape of the material
Figure 1-16 describes the shape changing and recovering process of a shape memory material If the material is above its glass/melting transition point it becomes pliable and can change shape by an external force The temporary shape can be fixed by cooling down below the transition point
Figure 1-16: Shape changing and recovering process of a shape-memory polymer
From 2012 up to now, the incorporation of the shape memory feature into internal self-healing materials based on reversible bonds, including Diels-Alder bonds, disulfide bonds, and hydrogen bonds, etc has been studied in various polymer systems The urethane-thiourethane system that combines shape memory and low- temperature self-healing was developed using DA chemistry Bismaleimide, bisfuranic, bis-/trismaleimidic, or trisfuranic monomers were used in semi-crystalline polycaprolactone (PCL) circuits, with PCL acting as the conformational memory switch segment The effectiveness of wound healing depends on the ability to restore shape, without which the crack cannot close and therefore a healing reaction cannot occur For such network, 70-80% recoveries of the mechanical strengths and almost
30 complete disappearance of scratches were observed after healing at about 60 °C for
1 to 3 days [75] The shape memory effect associated with Diels-Alder reversible chemistry between furan and maleimide was explored in several cases, focusing on a poly(tetramethylene oxide)-co-poly(p-dioxanone)s [99] and polyurethanes [3, 14, 31] Michal et al [65] reported a shape-memory lattice polymer containing reversible disulfide bonds This material was able to recover scratches by shape memory process under thermal stimulation and then by the exchange reaction of disulfide bonds under
UV stimulation Self-healing shape memory gels based on reversible hydrogen bonding were also published in 2014 [100, 101] In May 2016, Yurun Xu et al [102] created self-healing polyurethanes based on a combination of disulfide bonding and shape memory effect With the help of the shape memory effect, scratches almost disappeared after 4 hours at 80 o C, with the mechanical properties almost recovered completely.
SYNTHESIS OF POLYURETHANE WITH DIELS-ALDER
Synthesis strategy of System 1 (DA-PUn)
To synthesize DA-PUn, a four-step procedure was implemented (Scheme 4):
(i) Synthesis of Allyl-PCL diol: In the initial step, allyl groups were introduced into the PCL chain
(ii) Synthesis of Furan-PCL diol: Subsequently, in the second step, furan groups were incorporated into the PCL chain through a thiol-ene reaction
(iii) Synthesis of DA-PCL diol: The following step involved the creation of a polyol bearing DA moieties at both polymer chain ends via a maleimide-furan DA reaction
(iv) Synthesis of DA-PUs: The DA-PCL-diol compound readily reacts with polyisocyanate in the final step to form a crosslinked DA-PU network
Scheme 4: Synthetic pathway to System 1
Experimental
Poly(-caprolactone) diol (CAPA 2403D, manufactured with butanediol as initiator, Mn = 4000 g mol – 1 , Perstorp UK Ltd) were azeotropically dried with toluene before use Allyl glycidyl ether (AGE, 99+%, TCI Chemicals–Japan) was dried over molecular sieves and distilled under vacuum before use Dichloromethane (99.9%, Fisher Chemicals) was dried over CaH2 and distilled Tetrafluoroboric acid diethyl ether complex (HBF4∙Et2O, 90%, Sigma–Aldrich), furfuryl mercaptan (FM, 97+%, Sigma–Aldrich), 2-mercaptoethanol (99%,Sigma–Aldrich), 2,2-dimethoxy-2- phenylacetophenone (DMPA, 99%, Sigma–Aldrich), calcium oxide (96%, Fisher Chemicals), triphenylphosphine (TPP, 99%, Sigma–Aldrich), and 1,1-(Methylenedi- 4,1-phenylene)bismaleimide (bismaleimide, 95%, Sigma–Aldrich) were used as received Hexamethylene diisocyanate trimer (HDI-trimer, tradename Desmodur® N 3600) was kindly provided by Bayer (Vietnam Ltd.) 2,5-Bis-(hydroxymethyl)furan (99%+) was purchased Polyscience Inc Zirconium(IV) acetylacetonate was purchased from Merck
3-Maleimido-1-propanol was synthesized according to the previously reported procedure [94] (can be found in the appendix of this thesis, Appendix 5)
1H NMR spectra were recorded in deuterated chloroform (CDCl3) with TMS as an internal reference, on a Bruker Avance 300 at 300 MHz
Transmission Fourier transform infrared (FT-IR) spectra, collected as the average of 128 scans with a resolution of 4 cm −1 , were recorded from KBr disk on the FT-IR Bruker Tensor 27
Attenuated total reflectance (ATR) FT-IR spectra were collected as the average of 128 scans with a resolution of 4 cm −1 on a FT-IR Tensor 27 spectrometer equipped with a Pike MIRacle ATR accessory with a diamond/ZnSe element
Differential scanning calorimetry (DSC) measurements were carried out with a DSC Q20 V24.4 Build 116 calorimeter under nitrogen flow, at a heating rate of 10 °C/min, from 0 to 170 o C
Mechanical properties were measured with a Tensilon RTC-1210A tensile testing machine, making use of a 1000 N load cell The dog bone shaped samples had an effective gauge length of 11 mm, a width of 2 mm, and a thickness of ± 1.5 mm (meeting the requirements of ASTM D638-type IV but the dimensions were scaled down) At least four specimens were tested for each composition
Scanning electron microscope (SEM) images were obtained on a HITACHI S−4800 microscope, operating at 5 kV
Optical microscopic images were recorded on an Olympus GX51F microscope
Wide-angle powder X-ray diffraction (XRD) patterns were recorded at room temperature on a Bruker AXS D8 Avance diffractometer using Cu Kα radiation (K 0.15406 nm), at a scanning rate of 0.05 o s −1 The data were analyzed using DIFRAC plus Evaluation Package (EVA) software The d-spacings were calculated from peak positions using Cu Kα radiation and Bragg’s law
Field emission scanning electron microscopy (FE-SEM) images were obtained on a FeSem, Su8010, Hitachi, Japan Scanning electron microscope (SEM) images were obtained on a Field Emission Scanning Electron Microscope, Jeol JMS- 6480LV
2.2.3 Assessment methods of healing efficiency by tensile measurements
The tested sample was subjected to damage, followed by healing via heating at
65 o C for either 30 min or 24 hours The healing efficiency was determined as the ratio of the tensile property (i.e stress of break, Young’s modulus, or strain of break) of the healed sample to the reference sample The reference sample (sample of the same composition and not damaged) was also heated and stabilized under the sample conditions with the healed sample
Test healing by tensile measurements: The dog bone-shaped samples were cut through the middle using a razor blade, and the halves were then put back together and placed between two glass slides in an oven at 65 o C under ambient pressure After heating, the samples were kept at room temperature for 8 h to stabilize before tensile measurements
2.2.4 Synthesis of Allyl-PCL diol
According to 1 H NMR, the exact Mn values for PCL-diol (CAPA 2403D) were discovered to be 4090 gmol -1 (found in the appendix of this thesis, Figure Eppendix 2) Commercial PCL diol (CAPA 2403D) with Mn = 4090 gmol -1 (3.45 g, 1.69 mmol of –OH groups) was dissolved in 17.5 mL of dichloromethane in a round–bottom flask To this solution, 0.017 mL (0.12 mmol) of HBF4∙Et2O was added Then, a nitrogen flow was passed over the mixture and the flask was closed with a rubber septum 0.31 mL (4.22 mmol) of AGE was slowly introduced with a syringe during
6 h The reaction mixture was kept at room temperature for 24 hours, and, after that, the acid catalyst was neutralized with solid CaO After filtration of CaO, the solution was concentrated and the product was isolated by precipitation three times in methanol (dichloromethane/ methanol = 1/5, v/v), washed three times with methanol, filtered and dried on vacuum line
Scheme 5: Synthesis of Allyl-PCL polyol: Cationic activated monomer oligomerization of allyl glycidyl ether (AGE) initiated by a commercial PCL-diol
(Mn = 4090 g mol -1 ) to give allyl PCL diol
Diagram 2-1: Schematic procedure of the synthesis of Allyl-PCL-diol
2.2.5 Synthesis of Furan-PCL diol
Following Scheme 4, the second step of the 4-step preparation of DA-PU-n was the synthesis of furan end-block functionalized PCLs (furan-PCL diol) This synthesis was making the furan functionality needed for polyurethane synthesis The furan end-block functionalized PCLs (furan-PCL diol) and ref-PCL polyol were synthesized according to a previously reported two-step procedure [103]
Scheme 6: Synthesis of furan-PCL diol: UV-initiated radical thiol-ene reaction of allyl - diol with either furfuryl mercaptan or ethanol mercaptan in the presence of 2,2-dimethoxy-2-phenylacetophenone as photoinitiator and triphenylphosphine as disulfide reducing agent
In a flask containing a stirring bar and closed with a rubber septum, allyl-PCL- diol was melted at 55 o C under stirring After stopping heating, it remained as a clear liquid and a minimum amount of tetrahydrofuran was added to maintain the polymer in the liquid state at room temperature Then, DMPA (10 mol% with respect to allyl groups), triphenylphosphine (TPP) and the thiol compound (either furfuryl mercaptan or 2-mercaptoethanol) were added in succession The allyl : thiol: TPP molar ratio used was 1: 2 : 20 The reaction mixture was degassed and was purged with nitrogen through a needle using vacuum/nitrogen line After an overnight exposure to UV light (wavelength of 365 nm, with twelve lamps of 9 W circularly oriented), the product was collected by precipitation three times in diethyl ether and was further dried under vacuum (10 –3 torr) at 60 o C to remove any left unreacted thiol
Diagram 2-2: Schematic procedure of the synthesis of furan-PCL diol
2.2.6 Synthesis of DA-PCL polyol
The next step of the four-step synthesis of DA-PU-n was synthesized DA-PCL polyol to make DA links at the interface of hard and soft PU's phases This compound was synthesized by the DA reaction between a furan end-functionalized PCL diol and 3-maleimido-1-propanol (in tetrahydrofuran, at a maleimide to furan molar ratio of 1), which was effective at 60 o C in 24 hours
Scheme 7: Synthesis of DA-PCL polyol by the DA reaction between a furan end- functionalized PCL diol
Diagram 2-3: Schematic procedure of the synthesis of DA-PCL polyol The thesis synthesized a PCL polyol with the chain termini attached to the hydroxyl-bearing DA groups using the DA reaction between a furan end- functionalized PCL diol and 3-maleimido-1-propanol with furan-to-maleimide equimolar ratio
2.2.7 Synthesis of DA-PUs and ref-PU
The last step of the four-step synthesis of DA-PU-n is the crosslink of compound DA-PCL polyol by reacting to polyisocyanate and PCL diol
Scheme 8: Synthetic scheme of functionalized PCL polyols, PUs containing DA bonds at the hard−soft interface 6a, and reference material
40 Diagram 2-4: Schematic procedure of the synthesis of DA-PUn
All reactants and glasswork were previously dried PCL (either PCL-diol, DA- PCL polyol or ref PCL polyol, vacuum dried before use) was dissolved in toluene at
50 °C The catalyst (zirconiumacteylacetonate, 1 mol% with respect to the NCO groups) and the HDI-trimer were incorporated toinitiate the polyurethane formation The ratio of −NCO/−OH functional groups was fixed to 1.05 in allcases Vacuum and stirring were maintained to avoid undesirable side reactions with water and toensure a homogeneous system, until the liquid mixture was injected in molds Samples were cured at 60 °C for 24 hours After opening the molds, the polyurethanes were washed by a Soxhlet extraction in acetone at 60 °C The sample was dried and annealed at 70 °C for 24 hours The density of DA cross-links at the hard−soft interface and the crystallinity of the soft domains, tunable by changing the weight ratio between the DA-PCL polyol and PCL diol, could in principle influence the mechanical properties and the ability to reversibly exchange chemical bonds Thus, a set of networks DA-PU-with variation in the weight fraction (n) of DA-PCL in the soft phase was prepared (n = 100−85−70−50 wt% of DA-PCL polyol).
Results and discussion
The synthetic scheme, reaction conditions and yield/conversions of 4 steps of DA-PUs are summarized in Scheme 9:
Scheme 9: The synthetic scheme, reaction conditions and yield/conversions of 4 steps of DA-PUs
2.3.1 Characterization of Allyl-PCL diol
The signal denoted as a’ corresponds to the methine protons of HO–CH(R)– groups of terminal HO–AGE units The signal denoted as i’ corresponds to the methylene protons of –(AGE)y–CH2(R)– groups of CL units next to AGE As a spectrum shown in Figure 2-1, although signal a’ partially overlaps with others (i.e signals i, q, and d), the total integral value of peaks i, d, and a’ in the range of 4.10– 3.80 ppm were obtained separately Thus, by taking into consideration the integrals of peak i (calculated based on the intensity of signal m and the intensity ratio of peak i and m previously obtained from the spectrum of the starting PCL–diol), peak q (calculated based on the intensity of signal m and the intensity ratio of peak q and m previously obtained from the spectrum of the starting PCL–diol) and peak d (equal to
43 the integral of peak f), an estimation of the integral value of signal a’ was possible The total number of attached AGE units per polymer chain was determined to be 3
Figure 2-1: 1 H NMR spectrum of allyl-PCL diol
2.3.2 Characterization of Furan-PCL diol
Figure 2-2: 1 H NMR spectrum of furan-PCL diol (obtained via the thiol-ene reaction of allyl-PCL diol with furfuryl mercaptan)
Figure 2-3: 1 H NMR spectrum of ref-PCL polyol (obtained via the thiol-ene reaction of allyl-PCL diol with 2-mercaptoethanol)
The signal denoted as a’ corresponds to the protons of HO–CH(R)– groups of terminal HO–AGE units The signal denoted as i’ corresponds to the methylene protons of –(AGE)y– CH2(R)– groups of CL units next to AGE units
1H NMR: The signal denoted as a’ corresponds to the protons of HO–CH(R)– groups of terminal HO–AGE units The signal denoted as i’ corresponds to the methylene protons of –(AGE)y– CH2(R)– groups of CL units next to AGE units From the 1 H NMR analysis of the products (Figure 2-1 for the reaction with furfuryl mercaptan and Figure 2-2 for the reaction with 2-mercaptoethanol), by comparing the signal intensities before and after coupling reactions, using a separate signal corresponding to the polymer backbone as the reference, both the conversion of allyl groups and the number of attached thiol molecules per allyl group could be determined Full conversions of the thiol–ene coupling of the allyl-PCL diol with furfuryl mercaptan and 2-mercaptoethanol were obtained
2.3.3 Characterization of DA-PCL polyol
1H NMR: Comparison of the 1 H NMR spectra of DA-PCL polyol, which was synthesized by the DA reaction between furan-PCL diol and 3- maleimido-1- propanol, with those of furan-PCL diol and 3-maleimido-1-propanol The signal denoted as a′ corresponds to the protons of HO− CH(R)− groups of terminal glycidyl ether units The signal denoted as i′ corresponds to the methylene protons of the caprolactone units next to glycidyl ether units As shown in the spectrum of DA-PCL polyol (Figure 2-4), the maleimide peak at 6.73 ppm and the furan peaks at 7.35, 6.30, and 6.18 ppm almost disappeared, whereas the peaks attributed to the DA adduct bond signals appeared at 6.59−6.33, 5.23, and 3.27−2.87 ppm By comparing the integral ratio between the signal g1 (DA double-bond group, at 6.59−6.33 ppm) and the signal g (maleimide group, at 6.73 ppm) or the signal t (furan group, at 6.18 ppm), the DA reaction conversion was determined to be around 97% The result revealed that on average, each DA-PCL chain contains three pendant DA moieties and hence five hydroxyl groups
Figure 2-4: 1 H NMR analysis of DA-PCL polyol
FT-IR: Figure 2-5 shows a comparison of the FT-IR spectra of DA-PCL polyol, furan-PCLdiol and 3-maleimido-1-propanol The disappearance of the typical maleimide absorption bands at 696 and 829 cm −1 and furan “ring breathing” signal at
1010 cm −1 , as well as the appearance of the band at 862 cm −1 ascribed to the furan– maleimide cycloadduct, confirms the successful occurrence of the DA reaction
Figure 2-5: A comparison of the FT-IR spectra of DA-PCL polyol, furan-PCL diol and 3- maleimido-1-propanol: a) full spectra; b) in the range of 1050-650 cm -1
2.3.4 Characterization of DA-PUs and ref-PU a ATR FT-IR spectra of DA-Pus
Figure 2-6: ATR FT-IR spectrum of DA-PU-100, in comparison with that of the corresponding precursor DA-PCL polyol: a) full spectra; b) in the range of 1050-
Attenuated total reflection (ATR) FT-IR spectra of all PUs featured no absorption bands at 2270 cm −1 , corresponding to the NCO stretching band, and the appearance of urethane vibrational bands at 3340 cm −1 (amide A), 1619 cm −1 (amide I), and 1560 cm −1 (amide II) (Figure 2-6 a) A comparison of the ATR FT-IR spectra of DA-PU-100 and DA-PCL polyol showed the preservation of the band at 862 cm −1
48 ascribed to the DA bond and negligible typical maleimide absorption bands at 829 and 696 cm −1 and furan “ring breathing” signal at 1010 cm −1 , indicating that all the
DA bonds were intact (Figure 2-6 b)
Figure 2-7: ATR FT-IR spectra of DA-PU-100 in three thermal cycles (120 °C
(rDA) for 1 h and subsequently 65 °C (DA) for 24 h
The DA reversible cross-linking in the solid DA-PUs was monitored using ATR FT-IR spectroscopy, and the representative results are shown for DA-PU-100 in Figure 2-7 The spectrum of the sample after heating to 120 °C for 1 h, without subsequent quenching, clearly shows the appearance of maleimide bands at 696 and
829 cm −1 and simultaneously decreased in intensity of the DA band at 862 cm −1 This clearly suggests the partial cleavage of the DA linkages, although some cross-linking could occur during sample handling and measurement at room temperature Subsequently, mild heating at 65 °C reconstructed the disconnected linkages through
DA reaction, as evidenced by the decrease in the maleimide band intensity and the
49 regaining of DA band intensity The repeatability of the DA and rDA reactions was demonstrated for three cycles b DSC data of DA-PU-n
Figure 2-8: DSC thermograms (endo up) of the prepared PU materials
The differential scanning calorimetry (DSC) thermograms of all PUs displayed an endothermic transition of the crystallized PCL soft phase in the range of 34−64 °C, with maxima at 47−56 °C, corresponding to the transition temperature (Ttrans) to trigger the shape-recovery effect (Figure 2-8) The melting maximum and melting endotherm of PCL segments in DA-PUs slightly decreased with the increasing weight fraction of DA-PCL polyol, which declined the crystallinity of the soft phase due to hindered polymer chain alignment The relatively lower Ttrans of DA-PUs with higher
DA moiety contents can be beneficial to enable partial shape recovery at room temperature Additionally, the DSC thermograms of DA-PUs showed an endothermic peak starting at 100 °C and ending at 170 °C, attributed to the rDA reaction and
50 indicating the cleavage of reversibly cross-linked bonds, whereas it was not observed for ref-PU Based on the thermal analysis, 65 °C was chosen as the healing temperature to trigger both shape recovery and the DA reaction, allowing for a mild, single-temperature healing process c XRD analysis of DA-PUs
Figure 2-9: XRD patterns of PU1, DA-PU-70 and DA-PU-100 (a, b, and c, respectively)
Wide-angle powder X-ray diffraction (XRD) measurements were conducted for a further morphological investigation of the DA-PUs For a comparison purpose, a
PU (named as PU1) consisting of a semicrystalline PCL soft phase and DA bonds in the hard phase was prepared by repeating the previously reported structural design and synthetic procedure (Appendix 2) PU1 was composed of PCL diol, HDI isocyanurate trimer, and a DA adduct tetra-ol (formed by reacting 2,5- bis(hydroxymethyl)furan with bismaleimide), and had a hard segment content of 22
51 wt % analogous to that of DA-PU-70 Figure 2-9 compares the XRD spectra of PU1, DA-PU-70, and DA-PU-100 The XRD patterns of both PU1 and DA-PU-70 revealed strong and sharp peaks at 2θ = 21.5, 22.1, and 23.8° and weak reflections at 15.7 and 29.9° attributed to the crystalline PCL soft domains For DA-PU-100, these peaks were significantly weakened, suggesting a low degree of crystallinity in the soft phase due to high cross-link content Two relatively diffused reflections at around 13 and 20° (beneath the diffraction peaks of PCL crystals) were observed for PU1 and DA- PU-70 These peaks could be assigned to irregular packing of the hard segments, which is over shadowed by the presence of the crystallized PCL phase Such typical reflections ascribed to low-order arrangement of hard segments have been previously reported for some HDI isocyanurate trimer-based and other PUs [104-106] The difference of the XRD pattern of DA-PU-100 indicates that increasing the number of chemical cross-links suppressed both crystallization of soft segments and segregation of hard segments Moreover, only PU1 displayed a small diffraction peak at 4.57 Å (19.4°), suggesting that it could be a result of a small fraction of hard segment stacking Despite the modest microphase separation of the hydrogen bonding, hard domains from the soft PCL domains in DA-PUs, as the DA bonds are attached to both soft chains and urethane moieties, must be located in the vicinity of both domains Such a chemical structure would increase the “solubilization” of hard segments in the soft segment phase, which, to some extent, could be advantageous for the molecular motion of DA bonds d Demonstration of PU shape-memory
Shape memory testing (with the sample heated at 75°C, undergoing torsion or elongation, then cooled while maintaining deformation at room temperature for 10 minutes, followed by removal of the deformation force) and thermal activation (heating at 65°C for 5 minutes) to restore the original shape demonstrated that all five
PU samples were capable of retaining secondary shape and fully recovering their initial shape This illustrates the shape memory capability of the DA-PU-70
Figure 2-10: Demonstration of shape-memory behavior of the DA-PU-70 Temporary spiral (A) and stretched strip (B) shapes were programmed by twisted and stretched the samples at 75 o C, followed by cooling down to room temperature to fix the temporary shapes e Healing assessment
Conclusions of chapter 2 (System 1)
PU materials of System 1 with high stiffness, strength, and toughness (Young’s modulus ∼80−225 MPa, ultimate tensile strength ∼16−30 MPa, and toughness
∼26−96 MJ m −3 ) and highly efficient healability at mild temperatures (∼60−70 °C) were successfully obtained The recovery of mechanical properties varied significantly for different compositions The healing efficiencies of the DA-PUs
64 increased in the following order: DA-PU-100 < DA-PU-50 < DA-PU-85 < DA-PU-
70 DA-PU-70 exhibited the best healing ability After only 30 min at 65 °C, it showed recoveries of Young’s modulus of 87% and maximum stress of 70%, whereas maximum healing efficiencies (89% for Young’s modulus and 91% for strength) were obtained after healing for 24 hours
Apparently, both the PCL and reversible DA linkages participated in the healing The PCL-induced shape recovery effect favored intimate contact of scratch surfaces, assisting the recovery of broken DA bonds Therefore, for a DA-PU structure with a proper balance between PCL segments and DA bonds, the movement and partial diffusion of PCL segments and the reconnection reaction of DA groups seemed to occur effectively across the scratch interface, resulting in high healing efficiency
In summary, these materials feature reversible furan−maleimide DA bonds at the interface between the hard and soft domains, along with a crystalline PCL soft segment assisted toughness and shape-memory effect The key is the residence of the
DA cross-links at the hard−soft interface that do not interfere with the soft segment crystallinity and can also provide sufficient mobility for the re-association of disconnected furan and maleimide entities at temperature above the melting temperature of the crystalline PCL phase Therefore, effective reformation of DA groups could be enabled at mild temperatures, without the need for prior DA adduct decomposition at high temperatures to avoid potential degradation (polymerization) of maleimide moieties The materials showed good mechanical properties and were also able to repair complete cuts and macro-damages by a one-step heating at mild temperature
However, it's worth noting that the synthesis process does come with a drawback It requires air-sensitive small-scale syntheses via cationic ring-opening oligomerization and thiol-ene reactions This aspect may pose some challenges in large-scale production or in environments where air sensitivity is a concern.
SYNTHESIS OF DIELS–ALDER CROSSLINKED TELECHELIC POLY(CAPROLACTONETHIOURETHANE)S – SYSTEM 2 (CrosslinkPCLTUs)
Synthetic strategy and reaction scheme of System 2 (CrosslinkPCLTUs) 65 3.2 Experimental
a precursor (FurPCL) and not demanding stringent reaction conditions The method involves initially extending furfuryl thiourethane-telechelic linear polycaprolactones (PCLs) using an excess of bismaleimide (BMI), followed by terminal crosslinking through the Diels-Alder (DA) reaction (Scheme 10) The following synthesis experiments were performed:
(i) Synthesis of the furfuryl thiourethane-telechelic polycaprolactone (FurPCL) as a precursor material ((1) and (2), Scheme 10)
(ii) Synthesis of CrosslinkPCLTPU via a one-pot two-step process: including first the synthesis of the intermediate product MalPCLTU ((3)- step 1, Scheme 10), followed by crosslinking with a trisfuran crosslinker ((4)-step 2, Scheme 10)
Scheme 10: Synthetic pathway to System 2 3.2 Experimental
Poly(-caprolactone) diol (CAPA 2403D, Mn = 4000 g mol –1 , Perstorp UK Ltd) and poly(-caprolactone) diol (CAPA 2201, Mn = 2000 g mol –1 , Perstorp UK Ltd) were azeotropically dried with toluene before use Trisfuran was synthesized according to the previously reported procedure [107] Zirconium (IV) acetylacetonate was purchased from Merck Hexamethylene diisocyanate (99 %), 2-furfurylthiol (97
%), 1,1'-(methylenedi-4,1-phenylene) bismaleimide (bismaleimide, BMI, 95%) and triethylamine (99%) were purchased from Sigma-Aldrich n-Heptane (99%), chloroform (99%), N,N-dimethylformamide (DMF, 99.8%), tetrahydrofuran (99.5%) and toluene (99%) were purchased from Fisher Chemicals
Details of the synthesis of trisfuran can be found in the appendix of this thesis
Analytical methods such as 1 H NMR, FT-IR, (ATR) FT-IR, DSC, Mechanical properties, SEM, Optical microscopic images, XRD, and FE-SEM have been described in section 2.2.2 (which can be found in section 2.2.2, page 32)
Solid state 13 C cross polarisation magic angle spinning (CP MAS) NMR spectra of the networks were recorded with a Bruker AVANCE-III 400 MHz spectrometer operating at 100.6 MHz for 13 C, at a spinning speed of 8000 Hz
Gel permeation chromatography (GPC) measurements were performed on a Polymer PL-GPC 50 gel permeation chromatograph system equipped with an RI detector, with N,N-dimethylformamide as the eluent at a flow rate of 1.0 mL/min Molecular weight and molecular weight distribution were calculated with reference to polyethylene glycol standards
Dynamic mechanical analyses (DMA) were carried out by using the TA instrument (Q800) from -40 - 110 o C at a heating rate of 5 o C /min and frequency of
3.2.3 Synthesis of furfuryl thiourethane-telechelic polycaprolactone (FurPCL)
Furfuryl thiourethane-telechelic polycaprolactone (FurPCL) was synthesized according to a one-pot two-step procedure as described in Scheme 11 In detail: Hexamethylene diisocyanate (2 equiv.), freshly azeotropically dried polycaprolactone diol (1 equiv.) and zirconium(IV) acetylacetonate (1.5 mol% per NCO groups) were dissolved in dry chloroform under nitrogen atmosphere The reaction was refluxed at 65 °C under nitrogen for 4 h After the mixture was cooled down, 2-furfurylthiol (2.4 equiv.) and triethylamine (0.2 v% relative to 2- furfurylthiol) were added and the mixture was stirred at room temperature under nitrogen atmosphere overnight After the reaction, the solution was concentrated and the product was precipitated from chloroform to n-heptane for multiple times to remove the excess 2-furfurylthiol The precipitate was dried under vacuum
68 Scheme 11: One-pot synthesis procedure of FurPCL
Diagram 3-1:Schematic procedure of the synthesis of FurPCL
3.2.4 Synthesis of the DA crosslinked poly(caprolactone-thiourethane) (CrosslinkPCLTU)
Five networks (CrosslinkPCLTU1‒5) with different molar ratios of BMI/FurPCL (Table 3-1) were synthesized In a one-pot procedure, either FurPCL2000 or FurPCL4000 was first coupled with excess BMI forming a maleimide-telechelic precursor (MalPCLTU) via the DA reaction between furan and maleimide groups at 50 o C for 12 h (step 1), followed by addition of the tris-furan crosslinker (step 2) The reaction mixture was then immediately injected in molds or casted on teflon plates, and was cured at 50 °C for 24 hours, followed by Soxhlet extraction to remove the solvent Finally, a series of self-healing crosslinkPCLTUs with various compositional ratios were obtained (Table 3-1)
Scheme 12: Synthesis procedure of CrosslinkPCLTU Table 3-1: Feed compositions of Crosslink PCLTU networks
Entry Networks FurPCL used BMI/FurPCL/tris- furan molar ratio
Content of DA adducts (mmol g -1 ) c
5 CrosslinkPCLTU5 FurPCL4000 b 1.50/1.00/0.34 0.49 a Mn = 2843 g mol -1 (determined by 1 H NMR analysis, see 3.3.1) b Mn = 5316 g mol -1 (determined by 1 H NMR analysis, see 3.3.1) c calculated from the feeding content of bismaleimide, assuming full conversion of DA reaction
Diagram 3-2: Schematic procedure of the synthesis of CrosslinkPCLTU
At the end of the first synthetic step, MalPCLTU, the maleimide-telechelic precursor of CrosslinkPCLTU, was formed by the DA reaction between the furan and maleimide groups of FurPCL and BMI For 1 H NMR, FT-IR and DSC analyses of the intermediate product of step 1 (MalPCLTU), DMF was removed at reduced pressure at 50 °C to give MalPCLTU as a solid.
Results and discussion
The synthetic scheme, reaction conditions and yield/conversions of CrosslinkPCLTPUs are summarized in Scheme 13:
Scheme 13: The synthetic scheme, reaction conditions and yield/conversions of
3.3.1 Characterization of furfuryl thiourethane-telechelic polycaprolactone (FurPCL) a 1 H NMR analysis of furfuryl thiourethane-telechelic polycaprolactone (FurPCLs)
Figure 3-1: 1 H NMR spectrum of FurPCL2000 derived from the PCL-diol with Mn
The integral value of peak H1 (HDI-derived methylene protons in the repeating unit that contains the urethane group) equals 2/3 times of that of peak o The integral of value of peak H1’ (HDI methylene protons in the terminal unit that is next to the thiourethane group) equals 2 times of peak a (furan proton) Thus, from the sum of integrals of peaks in the range of 3.4−2.8 ppm and the calculated integrals of peak H1 and H1’, the integral value of peak H1’’ could be determined Accordingly, the z and y (degrees of repeating units) values were estimated by comparing the integral values of peak H1’’ and peak H1 with peak H1’ (z = H1’’/(2H1’) and y = H1/H1’), to be 0.035 and 1.09, respectively The content of urea groups with respect the total content of allurethane, thiourethane and urea groups was estimated from the integral ratio H1’’/ (H1 + H1’ + H1’’) to be 0.3%, which is insignificant Taking into account the Mn of PCL-diol of 2075.6 g mol -1 (see Appendix 1), the Mn of FurPCL2000 was calculated as follow:
Figure 3-2: 1 H NMR spectrum of FurPCL4000 derived from the PCL-diol with Mn
The integral value of peak H1 (HDI-derived methylene protons in the repeating unit that contains the urethane group) equals that of peak q, which could be estimated based on the integral value of peak m and the unchanged integral ratio q/m (as pre- determined in the 1 H NMR spectrum of PCL4000-diol in Figure 3-2) The integral of value of peak H1’ (HDI methylene protons in the terminal unit that is next to the thiourethane group) equals 2 times of peak a (furan proton) Thus, from the sum of integrals of peaks in the range of 3.4−2.8 ppm and the calculated integrals of peak H1 and H1’, the integral value of peak H1’’ could be determined Accordingly, the z and y (degrees of repeating units) values were estimated by comparing the integral values of peak H1’’ and peak H1 with peak H1’(z = H1’’/(2H1’) and y = H1/H1’), to be 0.079 and 1.15, respectively The content of urea groups with respect the total content of allurethane, thiourethane and urea groups was estimated from the integral ratio
H1’’/(H1+H1’+ H1’’) to be 6.8%, which is insignificant Taking into account the Mn of PCL-diol of 4090 g mol -1 (see Appendix 1), the Mn of FurPCL4000 was calculated as follow:
75 b FT-IR analysis of furfuryl thiourethane-telechelic polycaprolactone (FurPCL)
As shown in Figure 3-3 and 3-4, the FT-IR spectra of FurPCL2000 and FurPCL4000 show no absorption bands at 2270, 2570 and 2500 cm -1 corresponding to the stretching vibrational modes of NCO, SH and OH groups, respectively This indicates that all these groups were completely reacted Besides, compared to the spectra of the PCL-diols, those of FurPCLs revealed new bands of furan groups at
3146, 3117 and 1011 cm -1 [108, 109], amide II (of urethane/thiourethane groups) at
1527 cm -1 , and amide A (of urethane/thiourethane groups) at 3352 cm -1
Figure 3-3: A comparison of the FT-IR spectra of PCL-diols, FurPCL2000 and
Figure 3-4: A comparison of the FT-IR spectra in the range of 3400-2500 cm -1 (left) and 1145-865 cm -1 (right) of PCL-diols, FurPCL2000 and FurPCL4000
3.3.2 Characterization of furfuryl thiourethane-telechelic polycaprolactone (FurPCL) a 1 H NMR analysis of intermediate products of step 1 – MalPCLTU
MalPCLTU is the product at the end of the first step (Scheme 12) For all ratios
1H NMR and FT-IR analysis of MalPCLTUs confirmed that FurPCL reacted completely with the excess BMI to form DA groups, resulting in intermediate products with preserved maleimide end groups The representative 1 H NMR result is shown for MAlPCLTU 1 (the maleimide-telechelic precursor of CrosslinkPCLTU1, Entry 1, Table 3-1) in Figure 3-5, with all the expected signals corresponding to its structure The furan peaks at 7.22, 6.20, 6.12 ppm disappeared, indicating complete reaction of furan groups Simultaneously, the intensity of maleimide peak at 6.76 ppm relative to that of the aromatic signal at 7.25-7.10 ppm was reduced by half when compared with the spectrum of BMI Taking into account the BMI/FurPCL feeding molar ratio of 2, this strongly suggests that half of the maleimide moieties reacted with furan groups and the resulting MAlPCLTU 1 was telechelically end-capped with maleimide groups In addition, the spectrum also shows new peaks attributed to the
DA adduct bond signals at 6.5, 6.45, 5.27, 3.32 and 3.04 ppm, (assigned as peaks b1, c1, a1, s and g in Figure 3-29, respectively)
Figure 3-5: 1 H NMR spectrum of the intermediate products maleimide-telechelic precursor MalPCLTU of CrosslinkPCLTU1 (Entry 1, Table 2-1) b FT-IR analysis of intermediate products – MalPCLTU
The result was further confirmed by FT-IR analysis of MAlPCLTU 1, indicated by the complete disappearance of furan absorption bands at 1011, 3146 and 3117 cm -
1, whereas the signals at 831 and 694 cm -1 corresponding to maleimide end-groups were preserved (Figure 3-6) Similar FT-IR results were also observed for the maleimidetelechelic precursors of CrosslinkPCLTU2–5 (MAlPCLTU 2–5, Figure 3-7)
Figure 3-6: A comparison of the FT-IR spectra in the range of 3400-2500 cm -1 (left) and 1300-650 cm -1 (right) of BMI, FurPCL2000 and MalPCLTU1
Figure 3-7: A comparison of the FT-IR spectra in the range of 1300-650 cm -1 of
79 c Molecular weight characteristics of MalPCLTUs
Figure 3-8: GPC traces of MalPCLTUs and FurPCLs (using DMF as eluent and
Table 3-2: Molecular weight characteristics of MalPCLTUs and FurPCLs
Samples GPC_DMF (PEG) GPC_Chloroform
Mn (g mol -1 ) Đ* Mn (g mol -1 ) Đ* Mn (g mol -1 ) PCL-diol (Mn ~
The GPC traces of MAlPCLTU 1–5 are compared with those of FurPCLs in Figure 3-8, and their molecular weight characteristics obtained from GPC and 1 H NMR analyses are presented in Table 3-2 The results showed that the molecular weight of MalPCLTU precursor increased with decrease in BMI/ FurPCL ratio d DSC data of MalPCLTUs
Figure 3-9: DSC curves of MalPCLTUs
Figure 3-9 shows the DSC curve of MalPCLTUs With the use of longer FurPCL as well as increasing the chain length of MalPCLTU 1–5, their DSC curves showed increases in the melting temperature (from 42 to 60 o C) and melting enthalpy of the crystalline phase of PCL segments The thermograms of all MalPCLTUs also displayed a broad endothermic transition at around 125 o C assigned to the retro-DA reaction where the DA bonds in the backbone of MalPCLTUs were dissociated
3.3.3 Characterization of the DA crosslinked poly(caprolactone- thiourethane) (CrosslinkPCLTU)
The formation of crosslinkPCLTU networks via crosslinking the chain ends of MalPCLTUs using the tris-furan crosslinker was proved by solid-state 13 C cross- polarization magic angle spinning (CP MAS) NMR and ATR FT-IR spectroscopy a Solid state 13 C NMR spectra of CrosslinkPCLTUs
Figure 3-10: Solid state 13 C NMR spectrum of CrosslinkPCLTU1
The solid-state 13 C NMR spectra of CrosslinkPCLTU1–5 are shown in Figure 3-10 and 3-11 The presence of DA linkages is indicated by the peaks at 92 and 82 ppm, whereas the peak at 110 ppm attributed to the furan group is absent [51, 110]
Figure 3-11: Solid state 13C NMR spectra of CrosslinkPCLTU 2‒5 b ATR FT-IR spectra of CrosslinkPCLTU 1‒5
The result was further ascertained by the ATR FT-IR spectra of CrosslinkPCLTU1–5 As shown in Figure 3-12, a comparison of the ATR FT-IR spectra of CrosslinkPCLTU1 after curing for 3 h with those of corresponding precursor (MalPCLTU 1) and tris-furan crosslinker shows decrease in the intensities of maleimide absorption bands at 694 and 832 cm -1 and furan band at 1011 cm -1 After 24 hours being cured, the disappearance of these absorption bands and simultaneous growth of the DA absorption band at 862 cm -1 indicate full curing of the network
Figure 3-12: ATR FT-IR spectra of CrosslinkPCLTU1 after curing for 3 and 24 h, in comparison with those of corresponding precursor (MalPCLTU 1) and tris-furan crosslinker
Figure 3-13: ATR FT-IR spectra of CrosslinkPCLTU 1‒5
Figure 3-14:DSC first heating curves for CrosslinkPCLTUs at heating rate of 10 o C min -1 Differential scanning calorimetry (DSC) first heating scan curves of the resultant CrosslinkPCLTU networks revealed a melting transition (Tm PCL) of the PCL crystalline phase in a range of 20 – 60 o C for CrosslinkPCLTU1–3 and 40 – 70 o C for CrosslinkPCLTU4–5 and a broad endotherm corresponding to the retro-DA process at around 124 o C (starting at 82 o C and ending at 170 o C) (Figure 3-14)
The cooling and second heating scans are shown in Figure 3-15 Detailed data, including Tm PCL, melting enthalpy (∆Hmelt PCL), retro-DAtemperature (Tretro-DA), and enthalpy of the retro-DA reaction (∆Hretro-DA) are summarized in Table 3-3
CrosslinkPCLTU 1 CrosslinkPCLTU 2 CrosslinkPCLTU 3 CrosslinkPCLTU 4 CrosslinkPCLTU 5
Table 3-3: Thermal properties of CrosslinkPCLTUs measured by DSC
(1st) (1st) (1st) (1st) (2nd) (2nd) (2nd) (2nd)
CrosslinkPCLTU4–5 derived from FurPCL4000 exhibited sharp and high PCL melting peaks in the range of ~ 42 - 73 o C For CrosslinkPCLTU1–3 networks derived from FurPCL2000, the relatively high content of DA bonds seems to hamper crystallization of PCL segments, giving rise to low as well as less ordered crystallinity of PCL, evidenced by multiple and low PCL melting peaks in the range of 22 - 55 o C
Also, due to the lower crystalline tendency of FurPCL2000, in an insufficient time scale of a DSC experiment, CrosslinkPCLTU1–3 did not show any crystallization during the subsequent cooling and heating scans (Figure 3-15) The enthalpy of the retro-DA process for CrosslinkPCLTU1–5 was in the range of 21–43 Jg -1 and decreased from CrosslinkPCLTU1 to 5, in agreement with the decreasing amounts of
DA adducts in the networks The subsequent cooling scans of CrosslinkPCLTU1–5 showed very broad exothermic peaks from 170 to below 50 o C attributed to the DA reaction of recovered furan and maleimide moieties (Figure 3-15 a) Because of the short duration of the DSC experiment, the DA adduct formation was incomplete, indicated by the decrease in the retro-DA reaction enthalpy in the second heating scans (Figure 3-15 b) The retro-DA endotherm peaks also shifted to higher temperature ~131 o C in the second heating scan with respect to the first run, indicating the formation of the more exo-stereoisomer DA adduct as a result of higher DA reaction temperature in the DSC experiment [111]
Figure 3-15: DSC data of CrosslinkPCLTU 1‒5 at a scan rate of 10 °C min −1 : a) cooling scans; b) second heating scans; insets: zoom-in of the transitions Cooling and heating rates: 10 o C min -1
87 d Dynamic mechanical analysis (DMA) of CrosslinkPCLTU 1‒5
Table 3-4: Thermal mechanical properties of CrosslinkPCLTUs measured by DMA
Storange modulus G’ at Troom (30 oC) (MPa)
Storange modulus G’ at the onset of the rubbery plateau (MPa) CrosslinkPCLTU1 -8.3 16.8‒58.1 338.7 222.0
Dynamic mechanical analysis (DMA) analysis was further performed in tensile mode in order to evaluate the properties of CrosslinkPCLTUs (Figure 3-16 and Table 3-4) Values of the storage modulus (G′) of CrosslinkPCLTU1‒3 networks with low PCL crystallinity clearly show that lower is the DA crosslinking density of the network formed from longer MalPCLTU precursor chains, lower is the value at room temperature (30 o C) Nevertheless, for CrosslinkPCLTU4‒5, the effect of PCL crystallinity was predominant CrosslinkPCLTU5 exhibited higher G’ value at 30 o C despite its lower content of DA groups as compared with CrosslinkPCLTU4 After the Tg (in the range of -24 to -8 °C) is the fusion of PCL crystalline domains, which occurs at approximately 16 ‒ 66 °C for CrosslinkPCLTU1‒3 and 67 ‒ 84 °C for CrosslinkPCLTU4‒5 A short rubbery plateau is observed after the melting of PCL derivative chains, directly after which is a loss of the elastic properties of the network starting at 90 − 100 °C due to the retro-DA reaction The G’ values at the onset of the rubbery plateau of CrosslinkPCLTU1‒5 was compared, decreasing with decrease in the content of DA adducts in the networks It is worth noted that these networks exhibited rubbery storage moduli from above 5 to more than 200 MPa, which is
88 advantageous as the materials still remain certain performance when the shape memory effect is activated
Figure 3-16: Storage modulus (a) and tan δ (b) obtained by DMA analysis of the
CrosslinkPCLTU 1 CrosslinkPCLTU 2 CrosslinkPCLTU 3 CrosslinkPCLTU 4 CrosslinkPCLTU 5
CrosslinkPCLTU 1 CrosslinkPCLTU 2 CrosslinkPCLTU 3 CrosslinkPCLTU 4 CrosslinkPCLTU 5 ta n
Conclusions of chapter 3 (System 2)
In summary, endowed by the good crystallinity of the bismaleimide-PCL derivative (MalPCLTU) chains, CrosslinkPCLTU5 showed both the best tensile performance and healing behavior Below Tm PCL, the crystalline domains play as physical netpoints resulting in high overall mechanical properties Meanwhile, the relatively lower density of DA bonds enables higher chain mobility when activated in the near viscoelastic state, resulting in efficient gap closure between the cut surfaces and deformation recovery as well as effective DA reaction
In essence, the CrosslinkPCLTUs system has simplified the material synthesis process compared to the creation of DA-PU-n (System 1), while maintaining exceptional properties including full recovery from severe cracks The tensile strength shows an impressive 87% recovery at 70°C after 24 hours of heating
Extended Healing Time: Both System 1 and System 2 require a lengthy 24-hour healing period at a relatively high temperature range of 65 - 70°C This extended duration might be considered impractical in certain applications
A BLEND OF DIELS-ALDER CROSSLINKED POLY(CAPROLACTONETHIOURETHANE)S AND A LINEAR POLYMER
Synthesis strategy of BLENDn
Copolymer P(4VP-r-SMA) interacts with the polymer through the hydrogen interaction of the pyridine ring (Scheme 3) In addition, the SMA component in the copolymer with high mobility will diffuse through the crack to support the polymer's self-healing process.
Experimental
Copolymer poly(4-vinylpyridine-random-stearyl methacrylate) P(4VP-r-SMA) was prepared according to reference [118] The synthesized P(4VP-r-SMA) exhibited a number average molecular weight Mn of 18600 gmol -1 and a molecular weight dispersity (Đ) of 1.59, as determined by gel permeation chromatography (GPC) method The 4VP/SMA molar ratio 1.37/1 as determined by 1 H NMR analysis
Transmission Fourier transform infrared (FT-IR) spectra, collected as the average of 128 scans with a resolution of 4 cm −1 , were recorded from KBr disk on the FT-IR Bruker Tensor 27
Mechanical properties were measured with a Tensilon RTC-1210A tensile testing machine, making use of a 1000 N load cell The dog bone shaped samples had an effective gauge length of 11 mm, a width of 2 mm, and a thickness of ± 1.5 mm (meeting the requirements of ASTM D638-type IV but the dimensions were scaled down) At least four specimens were tested for each composition
Optical microscopic images were recorded on an Olympus GX51F microscope 4.2.3 Synthesis of CrosslinkPCLTU/P(4VP-r-SMA) blend
The preparation procedure of CrosslinkPCLTU/ P(4VP-r-SMA) blend is similar to that of CrosslinkPCLTU5, but with the presence of an amount of P(4VP-r-SMA) in the reaction mixture In detail, after performing step 2 of the procedure described in Part 2.4, the mixture was not immediately injected into the mold but instead P(4VP- r-SMA) was added according to the ratio as shown in Table 3-1 The mixture was stirred at 50 °C for 1 h, then injected into a mold and cured at 50 °C for 24 hours Blend10 and Blend20 samples were prepared using 10 wt% and 20 wt% of P(4VP-r-SMA) with respective to CrosslinkPCLTU5
Diagram 4-1: Schematic procedure of the synthesis of Blend.
Results and discussion
4.3.1 Characterization of P(4VP-r-SMA) and Blend
Blend10 and Blend20 samples were prepared using 10 wt% and 20 wt% of P(4VP-r-SMA) with respective to CrosslinkPCLTU5
Figure 4-1: FT-IR spectras of P(4VP-r-SMA), CrosslinkPCLTU5, Blend20
The FT-IR spectrum of P(4VP-r-SMA), exhibiting bands at 2952 and 2854 cm -
1 (corresponding to the SMA block C-H stretching vibrations) and pyridine vibrational bands at 1594 cm -1 (pyridine C=C stretching mode), 992 and 813 cm -1 (pyridine C-H bending) (Figure Eppendix 8) [119]
A comparison of the ATR FT-IR spectra of CrosslinkPCLTU5 and Blend20 (Figure 4-1) showed the preservation of the band at 1594 cm -1 and 818 cm -1 ascribed to the P4VP ring signal, indicating the appearance of the P4VP moiety
Infrared spectroscopy is a powerful instrument for studying particular interactions in the hydrogen-bonded system, and the frequency shift is widely recognized as a measure of hydrogen bonding strength The peak shift of the pyridine functional group on the IR spectra when engaging in hydrogen bonding was used to evaluate hydrogen bond formation The shift of the pyridine deformation vibrational bands to slightly higher wavenumbers is ascribed to the formation of hydrogen bonding interaction [120-122]
The pyridine ring's distinctive signal moves from 813 cm -1 to 818 cm -1 The creation of hydrogen bonds in the blend polymer caused these changes [120-122]; nevertheless, the interactions were weak, and hence the displacement was relatively
104 minor [119] Additionally, the very slight shift of the (C=O)N-H stretching vibration from 3338 cm -1 to 3342 cm -1 indicating a somewhat lower hydrogen bonds strength of the (C=O)N-H groups suggests that the presence of the linear P(4VP-r-SMA) might moderately interfere with the hydrogen bonding between urethane groups [123, 124]
4.3.2 Self-Healing Studies of Blend sample
Healing tests were applied to the CrosslinkPCLTU/ P(4VP-r-SMA) samples similarly to those applied for the CrosslinkPCLTUs
To assess the healability, surface cut recovery tests using optical microscopy was first performed Macro-scratches were generated using a scalpel blade
Figure 4-2: Optical microscopy (OM) images of Blend10 (A) and Blend20 (B) samples before and after the healing process
As demonstrated in Figure 4-2 A, Blend10 has a reasonably good self-healing capacity, mending the scratches entirely after 10 hours at 70 o C The results showed that the incision was completely healed after 10h at 70 o C, compared with 24 hours of the previously investigated CrosslinkPCLTU5 system For Blend10, it seemed that the presence of a small quantity of P(4VP-r-SMA) resulted in reducing the healing time In this case the diffusion of P(4VP-r-SMA) chains across the crack, along with
105 the PC-derived shape memory effect, assisted effectively the crack closure process Nevertheless, at a higher content of the copolymer of 20 wt%, it was likely that the decreased contents of PCL segments as well as DA groups were insufficient for efficient shape recovery and healing reaction to occur Apparently, a proper balance between the content of the DA linkages, PCL segments and P(4VP-r-SMA) chains as a diffusion agent is necessary
To further assess the ability to heal deformaton damages of Blend10, this sample was subjected to large tensile deformations at strains of 50% of the corresponding strains of break (similarly to the test previously applied for CrosslinkPCLTU5)
Figure 4-3: Stress−strain measurements of the original and healed samples of
After an instantaneous elastic recovery when releasing the load, the elongated sample 70 °C for 10 h to enable full shape recovery and reformation of the broken
DA bonds A comparison of the stress−strain curves of the healed and original samples is shown in Figure 4-3 The result showed that Blend10 exhibited a recovery of strength of 82% after 10 h of healing at 70 o C, which is quite close to that for CrosslinkPCLTU5 (87% after healing at 70 o C for 24 hours)
Apprarently, blending the thiourethane network with a small amount of P(4VP-r-SMA) considerably reduced the healing time, though the tensile properties of the blend material were lower
Table 4-1: Summary of the mechanical properties of CrosslinkPCLTU5 and Blend10 measured by tensile test
Conclusion of chapter 4 (System 3)
The synthesis of System 3 follows the same simple process as for System 2 While there's a slight decrease in mechanical strength recovery for the blend compared to System 2, it's not significantly substantial However, for System 3 the healing time was reduced from 24 hours to 10 hours These preliminary results in this chapter indicate the potential for developing further blend systems to enhance the material's healing efficiency The Blend system can be widely applied in the materials industry to create self-healing and reusable products This is especially useful in applications requiring material flexibility and strong load-bearing capacity.
CONCLUSION AND RECOMMENDATIONS
Summary of current work
The thesis has achieved the aim of proposing and investigating a synthetic pathway to create materials that can heal under moderate conditions without sacrificing mechanical properties by incorporating reversible interactions easily and practically Thus, networks containing dynamic Diels-Alder and thiourethane bonds and shape recovery ability have been obtained Such materials were capable of mild- temperature-triggered healing of scratches, complete cuts, and deformation damages
First, materials incorporate reversible furan-maleimide Diels-Alder (DA) bonds at the interface of rigid and flexible regions, in addition to a crystalline polycaprolactone (PCL) soft segment that enhances toughness and imparts a shape- memory effect The crucial factor lies in the presence of DA cross-links at the junction between the hard and soft domains, which neither disrupts the crystalline structure of the soft segment nor hinders the mobility required for the reconnection of disconnected furan and maleimide entities at temperatures surpassing the melting point of the crystalline PCL phase Consequently, effective regeneration of DA groups becomes feasible at mild temperatures, eliminating the necessity for prior decomposition of DA adducts at elevated temperatures to prevent potential degradation (polymerization) of maleimide components The highest achievement was recorded with DA-PU-70, showcasing strength recoveries exceeding 95% and Young's modulus restoration of 100% after 24h at 65 o C In particular, this PU system has the ability to completely heal large puncture wounds
Second, the materials were prepared via chain-end crosslinking of maleimide- telechelic poly(caprolactone-thiourethane) precursors that contain multiple rigid bismaleimide segments Five polymer systems were prepared by varying the FurPCL precursor chain length and BMI/FurPCL ratio The results have shown that monitoring the chain length of the precursors led to changes in the network mechanical and healing performance The best obtained material
(CrosslinkPCLTU5) showed high tensile strength (36 MPa) and Young’s modulus (330 MPa) and good healing efficiency at mild temperature (complete healing of scratches and cuts and tensile strength recovery of 87% at 70 o C) The best-obtained material demonstrated high tensile strength (~36 MPa) and Young's modulus (~330 MPa), as well as good healing efficiency at low temperatures (full healing of scratches and cuts, and tensile strength recovery of 87% at 70 o C)
A preliminary extension of these results has been made in which the best CrosslinkPCLTU network was blended with the P(4VP-r-SMA) linear copolymer as a H-bond forming and diffusion agent Despite the resulting lower mechanical properties, blending CrosslinkPCLTU5 with 10 wt% of P(4VP-r-SMA) has reduced the healing time while maintaining the healing efficiency This shows enormous potential in technology and application.
Contributions of the thesis’s study
This thesis addresses the critical need for the development of Polyurethane (PU) polymer systems with the capability of self-repair and regeneration at moderate temperatures (~60-70 °C) These polymer systems are engineered to combine high rigidity, durability, and resilience with robust regenerative capabilities
The urgency lies in the significant advancement in polymer system design compared to conventional self-healing polymers The utilization of Dynamic Diels- Alder (DA) linkages at the interface between rigid and soft phases, along with the incorporation of crystalline PCL segments, enhances both the material's rigidity and shape-memory properties This opens up vast potential applications across industries, including manufacturing, healthcare, and materials technology
Reducing the time required for repair and regeneration in these polymer systems is also crucial In today's context, swift resolution of incidents and the reutilization of material resources are paramount Thanks to this progress, the thesis contributes to
Outlook
For the studied self-healing systems, their abilities of multiple healing have not been extensively tested yet and will be further examined in the future
As the preliminary results of the blended system (System 3) gave promising results, more extensive survey and further assessment of the procedure parameters will be made in order to enhance the self-healability and mechanical properties of the materials These include changing the structure and compositions of the blended copolymer and further exploration of other supramolecular polymer networks that can be applied as an interpenetrating component for the DA-crosslinked thiourethane system
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This appendix summarizes the synthesis and characterization of the following materials that were used for the preparation of self-healing PUs in this thesis.
Polycaprolactone diol (PCL-diol)
The degree of polymerization x was determined by comparing the integral values between peak m (corresponding to 2 protons of –OC(O)–CH2(R)– of CL units) and peak o (corresponding to 6 protons of the initiator used in the synthesis of the commercial PCL–diol), to be 17.4 Thus, the number-average molecular weight (Mn) value of 2075.6 g mol –1 was calculated
Figure Eppendix 1: 1 H NMR spectrum of commercial PCL-diol (Mn ~ 2000 g mol -
1) The signal denoted as t’ corresponds to the methylene protons of the terminal
Figure Eppendix 2: 1 H NMR spectrum of commercial PCL-diol (Mn ~ 4000 g mol -
1) The signal denoted as i’ corresponds to the methylene protons HO–CH2(R)– of the terminal units
As peak q partially overlaps with peak i and i’, only a sum of integrals of the three peaks can be identified In addition, the total integral value of peak i and i’ equal to that of peak m Thus, the integral value of peak q (corresponding to four protons of the initiator used in the synthesis of the commercial PCL–diol) could beestimated The degree of polymerization x was determined by comparing the integral values between peak m (corresponding to two protons –OC(O)–CH2(R)– of CL units) and peak i’, to be 35.0 Thus, an Mn value of 4090 g mol –1 was calculated.
Synthesis of PU1
PU1 was synthesized following a previously reported procedure [31] (Scheme Eppendix 2) 1,1-(Methylenedi-4,1-phenylene)bismaleimide was first reacted with 2,5-bis-(hydroxymethyl)furan at a maleimide to furan molar ratio of 1 in tetrahydrofuran at 50 o C The reaction conversion was ~ 96% after 24 hours as indicated by 1 H NMR[11] The obtained DA adduct-tetraol was combined with PCL- diol to react with HDI-trimer to give PU1
Scheme Eppendix 1: Synthesis of PU1.
Photographs of a puncture by a paper pin on a cured natural rubber material
Figure Eppendix 3: Photographs of a puncture by a paper pin on a cured natural rubber material before (left) and after (right) healing at 65 °C for 24 hours After thermal treatment, the sample was bent showing that the puncture was not healed
Figure Eppendix 4: 1 H NMR spectrum of BMI
Synthesis of 3-Maleimido-1-propanol
3-Maleimido-1-propanol was prepared according the previously reported procedure.1 3-Amino-1-propanol (2.04 g, 27.1 mmol) in 20 mL of methanol was added dropwise to a suspension of exo-3,6-epoxy-1,2,3,6-tetrahydrophthalic anhydride (8) (4.5 g, 27.1 mmol) in methanol (180 mL) at 0 o C The mixture was stirred at 0 o C for another 5 min and at room temperature for 30 min The mixture was then refluxed at 62 °C for 3 days and concentrated to give a clear oil This oil was purified by column chromatography using methanol/dichloromethane (1: 15) as eluent to give an intermediate product (DA adduct of furan and 3-maleimido-1- propanol) This intermediate product was refluxed in toluene at 110 o C for 8 h to yield 3-maleimido-1-propanol as a yellowish oil Yield: 63%
Figure Eppendix 5: Synthetic scheme and the 1 H NMR spectrum of 3-maleimido-1- propanol
Figure Eppendix 6:Transmission FT-IR spectrum of 3-maleimido-1-propanol.
Synthesis of trisfuran
At ambient circumstances, a triethylamine-catalyzed thiol-isocyanate reaction occurred between HDI-trimer and 2-furfurylthiol in tetrahydrofuran (THF) In 110
132 mL of THF, 42.513 g (155.6 mmol of NCO groups) of Desmodur N3390 BA was combined with 20 mL (198.3 mmol) of 2-furfurylthiol The reaction was then agitated at ambient temperature overnight with triethylamine (277 àL) The product was precipitated in n-heptane, filtered, and vacuum dried to yield a dark solid Yield: 92% [94]
Figure Eppendix 7: 1 H NMR spectrum of Trisfuran
The peak of the (thio)urethane group appears at ~5.5 ppm, compared with the two original substances The proton of the methylene group attached to the NCO group at 3.2 ppm (symbol 1) completely shifts to the 3.3 ppm position (symbol 16) corresponding to the proton of the methylene group of thiourethane group
The proton of the methylene group attached to the SH group at 3.7 ppm (symbol 10) completely disappeared and the protonation of the methylene group attached to the thiourethane group at 4.2 ppm (symbol 18) appeared In the product spectrum, the proton of the methylene group attached to the SH group completely disappeared at 1.9 ppm (symbol 9) The complete disappearance of the 4 and 8 peaks of butylacetate indicates that this solvent was completely removed during purification
Compared with the spectra of the two original substances, the product spectrum also shows characteristic peaks corresponding to the hydrocarbon structure from HDI
- trimer and furfury thiol The furan group is represented by the characteristic peaks at 6.2, 6.4, and 7.2 ppm In addition, the relative integral value of the peaks is consistent with the chemical structure of tris-furan Thus, trisfuran was successfully synthesized and purified According to the structural formula, the average molecular weight is 847.1 g/mol, so the number of furan groups is equal to 3 x 1000/847.1 3.54 mmol/g
Figure Eppendix 8: FT-IR spectrum of Trisfuran The FT-IR spectrum shows that the starting substance Desmodur N3390 at 2280 cm -1 has a peak of the -NCO group; after the reaction, this peak disappeared
Also, on the spectrum appeared the N-H stretch of the thiourethane group (3339 cm -1 ) (Figure Eppendix 8) and the furan group (1013 cm -1 ), which are the characteristic groups for the structure of trisfuran.
Synthesis of copolymer P(4VP-r-SMA)
Scheme Eppendix 2: Synthesis of copolymer P(4VP-r-SMA)
The chemicals were weighed and put into the flask in turn: 4VP, SMA, solvent THF, catalyst (C) and initiator (I) at the ratio [4VP]/[SMA]/[C]/ [I] = 90/60/1/1, concentration [monomer] = 1.5M The reaction solution was stirred well and performed a "freeze-pump-thaw" cycle with liquid nitrogen 3 times to remove oxygen from the solution
The polymerization reaction was activated and developed in UV medium for 24 hours, after which the product was precipitated in methanol and centrifuged to obtain the solid The product was vacuum vacuumed at room temperature to constant mass to obtain a final product of P(4VP-r-SMA) [118]
Figure Eppendix 9: 1 H NMR spectrum of P(4VP-r-SMA)
The chemical structure of these copolymers were characterized by 1 H NMR measurement (Figure Eppendix 9) In 1 H NMR spectra, two broad proton signals at δ 8.15 – 8.6 and δ 6.27 – 7.07 ppm which contributed to the meta- and orthoprotons of pyridine rings, respectively In addition, signals in the range of δ 2.87 – 4.08 ppm that assigned to methylene protons (-O-CH2-) from stearyl groups The resonance signals observed in the regions between 1.00 – 2.61 ppm indicated the protons in copolymer backbones and -CH2-[CH2]15- in stearyl groups Methyl protons (CH3) of stearyl and methacrylate groups in P(4VP-r-SMA) contributed to the peaks below 1.00 ppm in the spectra Moreover, by measuring the relative intensities of meta- protons from pyridine ring (d, 2H) and of methylene protons (e, 2H) from stearyl group, molar ratio of two units in copolymers could be calculated, particularly, the fraction of 4VP/SA in P(4VP–r-SMA) was 1.37, whereas the feeding ratio of 4VP over SMA) was 1.5/1
Figure Eppendix 10: FT-IR ATR spectrum of P(4VP-r-SMA)
Figure Eppendix 11: GPC traces of P(4VP–r-SMA) (using DMF as eluent and PS standards)
Gel permeation chromatography (GPC) measurements were performed on a Polymer PL-GPC 50 gel permeation chromatograph system equipped with an RI detector; DMF containing 0.1 mol/L of lithium bromide as the eluent at a flow rate of 1.0 mL/min, molecular weights and molecular weight distributions were calculated with reference to polystyrene standards The obtained copolymer product has an average molecular weight of Mw = 18,556 g/mol with a polydispersity Đ = 1.59.