The use of model cellulose gel beads to clarify flame-retardant characteristics of layer-by-layer nanocoatings

Layer-by-Layer (LbL) assembled nanocoatings are exploited to impart flame-retardant properties to cellulosic substrates. A model cellulose material can make it possible to investigate an optimal bilayer (BL) range for the deposition of coating while elucidating the main flame-retardant action thus allowing for an efficient design of optimized LbL formulations.

Carbohydrate Polymers 255 (2021) 117468 Contents lists available at ScienceDirect Carbohydrate Polymers journal homepage: www.elsevier.com/locate/carbpol The use of model cellulose gel beads to clarify flame-retardant characteristics of layer-by-layer nanocoatings ăklỹkaya a, *, Rose-Marie Pernilla Karlsson a, c, Federico Carosio b, Lars Wồgberg a, c, * Oruỗ Ko a Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 56, SE-100 44 Stockholm, Sweden Dipartimento di Scienza Applicata e Tecnologia, Politecnico di Torino, Alessandria Site Viale Teresa Michel 5, 15121, Alessandria, Italy c Wallenberg Wood Science Center, KTH Royal Institute of Technology, SE-100 44 Stockholm, Sweden b A R T I C L E I N F O A B S T R A C T Keywords: Layer-by-Layer assembly Flame-retardant Thermal stability Cellulose gel beads Layer-by-Layer (LbL) assembled nanocoatings are exploited to impart flame-retardant properties to cellulosic substrates A model cellulose material can make it possible to investigate an optimal bilayer (BL) range for the deposition of coating while elucidating the main flame-retardant action thus allowing for an efficient design of optimized LbL formulations Model cellulose gel beads were prepared by dissolving cellulose-rich fibers followed by precipitation The beads were LbL-treated with chitosan (CH) and sodium hexametaphosphate (SHMP) The char forming properties were studied using thermal gravimetric analysis The coating increased the char yield in nitrogen to up to 29 % and showed a distinct pattern of micro intumescent behavior upon heating An optimal range of 10–20 BL is observed The well-defined model cellulose gel beads hence introduce a new scientific route both to clarify the fundamental effects of different film components and to optimize the composition of the films Introduction Cellulose is the most abundant biopolymer on earth (Klemm et al., 2005), the most common sources of cellulose being wood and cotton Cotton fibers have been one of the major constituents in textiles, and wood fibers have a broad application in the pulp and paper industry Cellulose-based materials are inexpensive, biodegradable and recy­ clable, but the inherent flammable character of cellulose limits its application or requires flame-retardant treatment for specific applica­ tions Recent developments have shown that treatment of the fiber surfaces with thin layers of polymers and nanoparticles, through the LbL technique, can impart excellent flame protection both for textiles (Li, Schulz, & Grunlan, 2009) and for wood fibers (Koklukaya et al., 2015) The surfaces of fibers from both cotton and wood are however rough and chemically heterogeneous and they are not suitable for fundamental investigations of the assembly of multilayers and the effects of LbL coatings Different model cellulose surfaces with different degrees of crystallinity have therefore been developed (Aulin et al., 2009) The most commonly used films have been prepared by spin coating of dis­ solved cellulose onto smooth silica surfaces (Aulin et al., 2009; ăklỹkaya et al., 2018) It is not possible to dissolve cellulose in con­ Ko ventional solvents due to its relatively high molecular mass and close packing of the glucan macromolecule in a crystalline structure How­ ever, regenerated cellulose materials can be prepared using solvents such as N-Methylmorpholine-N-Oxide (NMMO) (Johnson, 1969), cupriethylenediamine (CED) (Schweizer, 1857) or lithium chloride in N, N-dimethylacetamide (LiCl-DMAc) (McCormick, 1981) Through the regeneration of cellulose in suitable liquids, materials with different shapes can be prepared such as films (Wendler et al., 2012), fibers (Woodings, 2003), hydrogels (S Wang et al., 2016), spheres (Oliveira & Glasser, 1996) etc., and the degree of crystallinity of these materials is dependent on the choice of solvent Carrick et al (Carrick et al., 2014) and Karlsson et al (Karlsson et al., 2018) demonstrated the use of LiCl-DMAc to prepare nm smooth cellulose spheres with a crystallinity below 1% Regenerated cellulose fibers widely used in textiles (i.e., lyocell, viscose, and rayon) are prepared using different regeneration processes (Wendler et al., 2012), but the use of native and regenerated cellulose can be limited by the thermal instability and flammability of the cellulose Flame-retardancy can be imparted to cellulosic materials via chemical additives in the wet state (Hall et al., 1999), pad-cure coating (Horrocks, 2011), spray coating (Helmstetter, 1998) and recently, the LbL technique (Holder et al., 2017) The LbL assembly technique has been employed to apply an efficient flame-retardant coating on substrates such as textile (Li, Schulz, & Grunlan, 2009; * Corresponding authors at: Department of Fiber and Polymer Technology, KTH Royal Institute of Technology, Teknikringen 56, SE-100 44 Stockholm, Sweden E-mail addresses: oruc@kth.se (O Kă oklỹkaya), wagberg@kth.se (L Wồgberg) https://doi.org/10.1016/j.carbpol.2020.117468 Received September 2020; Received in revised form 26 November 2020; Accepted 27 November 2020 Available online December 2020 0144-8617/© 2021 The Authors Published by Elsevier Ltd This is an open access (http://creativecommons.org/licenses/by-nc-nd/4.0/) article under the CC BY-NC-ND license O Kă oklükaya et al Carbohydrate Polymers 255 (2021) 117468 Srikulkit et al., 2006), polyurethane foam (Kim et al., 2011), wood fibers ¨klükaya et al., 2020; Ko ¨klükaya et al., 2018), (Koklukaya et al., 2015; Ko and cellulose nanofibril based aerogels (Koklukaya et al., 2017) LbL treatment is based on the consecutive adsorption of layer constituents at the solid-liquid interface (Decher, 1997) Different systems have been investigated to impart micro-intumescent flame-retardant nanocoatings on cotton fabrics using CH as a carbon source/blowing agent and SHMP as an acid source (Guin et al., 2014; Leistner et al., 2015; Mateos et al., 2014) However, there is limited fundamental understanding of the flame-retardant mechanism and the effects of the LbL-assembled mul­ tilayers prior to their deposition as coatings on wood fiber/cellulose surfaces The conventional approach is to perform the depositions on the selected substrate with variable parameters, perform the complete characterization and then trace back to optimal layer number and coating mode of action In this way, the mechanisms behind the improved flame-retardant characteristics of LbL films composed of poly (allylamine hydrochloride) (PAH) and montmorillonite clay (MMT) on polyamide as well as for LbL systems comprising of CH and ammonium polyphosphates (APP) applied to a cotton substrate could be indirectly identified (Apaydin et al., 2014; Jimenez et al., 2016) It was shown that the flame-retardant mode of action of (PAH/MMT) coating occurred in the condensed phase The 40 BLs of (PAH/MMT) coating protected the underlying polyamide from an external heat flux of 25 kW/m2 (Apaydin et al., 2014) It was also shown for a (CH/APP) multilayer coating that the flame-retardant behavior was due to a combination of a condensed phase forming an aromatic char layer and a gas phase releasing non-flammable volatiles that promote the micro-intumescence phenomenon (Jimenez et al., 2016) Based on earlier investigations it is apparent that the common approach is to investigate the optimal deposition conditions and flame-retardant mechanism after a complete characterization of the treated substrates (Apaydin et al., 2014; Guin et al., 2014; Jimenez et al., 2016; Mateos et al., 2014) Within this context, model substrates such as silicon oxide have also been employed in order to investigate the compositional and morphological changes occurring within the coating after the exposure to a flame or to a heat flux (Koklukaya et al., 2017; Maddalena et al., 2018) This approach has the limitation of focusing only on the coating disregarding the effects of the substrate that is replaced by silicon oxide Thus, although the earlier studies helped to identify the overall flame-retardant effect of LbL multilayers, the use of a small scale and a controlled preliminary screening approach involving the substrate of interest would have allowed for the optimal design of the coating architecture and compo­ sition while providing a meaningful insight on the crucial interactions occurring during combustion between the deposited LbL coating and the substrate In order to address such questions, we propose a simple and yet effective strategy for the study and design of a flame-retardant LbL assembly of nanocoatings directly on model cellulose beads To this aim, we have used dissolved carboxymethylated fibers to prepare cellulose-based hydrogel beads to be used as model cellulose substrates in combination with the LbL technique to deposit intumescent coatings of CH and SHMP This system is a good candidate for studying the fundamental processes behind the development of intumescence coating as it has already been used to confer flame-retardant properties to cotton (Guin et al., 2014) These authors reported an improved cellulose char formation combined with the formation of a barrier consisting of sub-micron-sized bubbles as the main mechanism for optimal flame-retardancy (Guin et al., 2014; Jimenez et al., 2016) In the present work we are using our model system to investigate a much deeper un­ derstanding of the fundamentals behind the flame-retardant action of the CH/SHMP system Model experiments were also performed using silicon oxide model surfaces (Carosio et al., 2018; Koklukaya et al., ăklỹkaya et al., 2018) to 2017) and flat model cellulose surfaces (Ko investigate the molecular details of the morphology, thickness, and roughness of the formed films A correlation between optimal char forming ability and deposited BL range is identified The smooth surface texture of cellulose gel beads provided a clear view on the structural changes of the nanocoating during degradation pointing out a micro-intumescent behavior The results also demonstrate the excellent applicability of the cellulose beads as model substrates for cellulose rich materials in a variety of fundamental studies Experimental 2.1 Materials The cellulose fibers employed in this study were obtained from a ă dissolving grade pulp supplied by Domsjă o Fabriker AB, Ornskă oldsvik, Sweden The cellulose content of the pulp was 93 % and the degree of polymerization was about 780 (provided by the manufacturer) N,Ndimethylacetamide (DMAc) (>99.5 %, GC grade), lithium chloride (LiCl), and acetic acid (Sigma-Aldrich) were used as received CH (Mw = 60 000, 95 % deacetylation) was supplied by GTC Union Corp., Qingdao, China, and SHMP (crystalline, +200 mesh, 96 %) was obtained from Sigma-Aldrich, Stockholm, Sweden Poly(vinyl amine) (PVAm), com­ mercial name Xelorex 6300, was supplied by BASF PVAm was dialyzed and freeze-dried prior to use Monochloroacetic acid, methanol, iso­ propanol, ethanol, HCl, NaOH, and NaCl were all analytical grade pur­ chased from Merck, Stockholm Sweden 2.2 Cellulose gel bead preparation The cellulose fibers were first carboxymethylated following to the method previously described by Wågberg et al (Wågberg et al., 2008) and the degree of substitution (D.S) was calculated by conductometric titration (Katz & Beatson, 1984) to be 0.13 which corresponds to a charge density of 795 μeqv/g g of the carboxymethylated pulp was then dissolved in 100 mL solution of wt% LiCl/DMAc following the steps previously described by Karlsson et al (Karlsson et al., 2018) and Carrick et al (Carrick et al., 2014) The water in the pulp was first sol­ vent exchanged by displacement with ethanol and the ethanol was subsequently exchanged with DMAc using a filtration procedure Each solvent was displaced over a period of two days during which the solvent was changed at least twice per day After this first step, the DMAc in which the pulp was to be dissolved was dehydrated by heating and keeping it for 30 at a temperature of 105 ◦ C The LiCl was also dehydrated during this 30 in an oven at 105 ◦ C After the dehy­ dration, the DMAc was allowed to cool and the LiCl was added and dissolved The pulp was added to the DMAc/LiCl solution at a temper­ ature of ca 40 ◦ C and then instantly placed in a ◦ C fridge and stirred with a magnetic stirrer overnight After about 24 h, the solution was clear The solution was then filtered using a 0.45 μm PTFE syringe filter and the filtrate was employed to form gel beads by drop-wise precipi­ tation through a needle of 0.64 mm into about 95 mL of a non-solvent consisting of 80 mL 0.03 M HCl (aq) with 15 mL ethanol The gel beads formed were allowed to rest in the bottom of the beaker at ◦ C for 24 h The non-solvent was then replaced with deionized water by decanting about 80 mL of the non-solvent four times during two days and stepwise decreasing the concentration of HCl The gel beads were then washed with deionized water for one week in order to remove any residual DMAc/LiCl The wet cellulose gel beads have an average diameter of 2.7 mm and after drying the average diameter of beads was 0.6 mm Prior to LbL treatment, the gel beads were dried at 23 ◦ C and 50 % RH 2.3 Model LiCl/DMAc cellulose films Cellulose dissolved in a solution of DMAc/LiCl was used to prepare non-crystalline cellulose films according to a method similar to that presented by Eriksson et al (Eriksson et al., 2005) P-type, boron doped, thickness 625 μm, single side polished flat silicon wafers (Addison En­ gineering, Inc San Jose, CA) were used as a substrate for the model cellulose surfaces The silicon wafers were cut (10 × 60 mm) into strips O Kă oklỹkaya et al Carbohydrate Polymers 255 (2021) 117468 The silicon wafers and QCM-D crystals were cleaned by rinsing them in a sequence of Milli-Q water, ethanol and Milli-Q water and then drying them with nitrogen gas The surfaces were plasma treated for in a plasma oven (PDC-002, Harrick Scientific Inc.) at 30 W under reduced air pressure Prior to spin coating of the cellulose solution, silicon wafers were treated with 0.1 g/L PVAm solution at pH 7.5 for 15 to form an anchoring layer for the cellulose and then rinsed with Milli-Q water and dried with a stream of nitrogen Spin-coating was performed using a spin coater (KW-4A-2, Chemat Technology, Northridge, CA, USA) and the cellulose solution was placed onto a PVAm treated silicon surface on the spin-coater disk Spin coating was performed at 000 rpm for 30 s These model cellulose surfaces were precipitated by immersion in Milli-Q water The substrates were then placed in Milli-Q water to remove excess solvent and LiCl Finally, the substrates were dried with nitrogen and stored in a desiccator until further use The dry thickness of the non-crystalline cellulose film was measured by AFM to be 38 ± nm represents one bilayer (BL) This process was repeated with adsorption times until the desired number of bilayers had been depos­ ited Containers and filters were replaced after deposition of every 20 BL to avoid any complex formation Coated beads were placed on a Teflon surface after LbL-treatment and dried at 23 ◦ C and 50 % RH Photograph of cellulose gel bead before and after the LbL-treatment is shown in supporting information Figure S1 2.5 Thin film characterization 2.5.1 Quartz crystal microbalance with dissipation A Quartz Crystal Microbalance with Dissipation (QCM-D, Q-Sense ăteborg, Sweden) was used to estimate both the amount of AB, Go adsorbed polyelectrolyte with associated water and the viscoelastic properties of the adsorbed film The normalized frequency change can be related to the adsorbed mass of polyelectrolyte and water and the energy dissipation can be related to the viscoelastic properties of the film (Rodahl et al., 1995) The adsorption of polyelectrolytes and the rinsing steps were monitored until saturation was reached 2.4 Layer-by-Layer deposition CH solution (1 g/L) was prepared in v/v% acetic acid, and a SHMP solution (5 g/L) was prepared in Milli-Q water (18.2 MΩ cm Milli-Q grade water Synergy 185, Millipore Bellerica, USA) Both solutions were stirred with a magnetic stirrer for 24 h to ensure complete disso­ lution and the pH of the solutions was then adjusted to pH using M NaOH for CH and M HCl for SHMP and the electrolyte concentration was adjusted to 10 mM NaCl The silicon wafers were cleaned according to the method previously described with Milli-Q water, ethanol and Milli-Q water and dried with nitrogen gas (Aulin et al., 2008) The sil­ icon wafers were then placed in an air plasma cleaner (PDS 002, Harrick Scientific Corp.) for in order to clean and activate the surface prior to LbL deposition The silicon wafers or model cellulose surfaces were alternately dipped into polyelectrolyte solutions in the order CH and SHMP using an automatic dipping robot (StratoSequence VI, nanoStrata Inc., Tallahassee, Florida, USA) The adsorption time for the first bilayer was in each solution in order to achieve a uniform deposition, while the time for the rest of the depositions was The substrates were rinsed with Milli-Q water (pH 5) three times between each depo­ sition for in each rinsing step without intermediate drying For LbL deposition on cellulose gel beads, a home-made filtration set up was used (Fig 1) Prior to LbL deposition, the cellulose beads were washed according to the previously described procedure, the carboxyl groups ărklund, 1993), and were converted to their sodium form (Wågberg & Bjo the beads were dried at 23 ◦ C and 50 % RH Dried cellulose beads were immersed in cationic CH solution for min, after which the solution was filtered by suction and the beads were rinsed twice with Milli-Q water (pH 5) to ensure removal of loosely adhered and excess polymer The beads were then exposed to anionic SHMP solution by filling the container with a solution and allowing adsorption for The solu­ tion was then filtered by applying vacuum pressure and the beads were rinsed twice with Milli-Q water (pH 5) One such sequence of deposition 2.5.2 Atomic force microscopy An Atomic force microscope (AFM), Nanoscope IIIa (Bruker AXS, Santa Barbara, CA) was used to investigate the surface topography, roughness, and thickness of the multilayer films deposited on model cellulose surfaces prepared on silicon wafers The films were scratched with a sharp blade in the dry state The thickness was measured before and after LbL treatment to determine the thickness of the films formed E and J-type piezoelectric scanners and Scanasyst cantilevers with a nominal resonance frequency of 70 kHz and a 0.4 N/m spring constant were used to scan the surfaces in air The surface roughness value was calculated from acquired images with an area of × μ m2 2.5.3 Nitrogen analysis The ANTEK 7000 nitrogen analyzer (Antek Instruments, Houston, TX, USA) was used to measure the nitrogen content of chitosan adsorbed on the cellulose beads The method is based on combustion of the sample (c.a mg) at 1050 ◦ C in an oxygen-poor atmosphere where nitrogen is oxidized to NO before being further oxidized to excited NO2 in ozone The light emitted when the excited NO2 is converted to its standard state is detected by a photomultiplier tube The system is calibrated with a known amount of CH (Supporting information Figure S2) 2.5.4 Fourier transform infrared spectrometry Spectra of cellulose beads were obtained using a Perkin-Elmer Spectrum 2000 FTIR with an attenuated total reflectance crystal acces­ sory (Golden Gate) ATR-FTIR spectra were recorded in the 4000− 600 cm− region at a resolution of 4.0 cm− and using 16 scans 2.5.5 Thermogravimetric analysis The thermal degradation of the untreated and LbL-treated cellulose Fig a) Photograph of cellulose gel bead in wet swollen state (on the right) and once dried then again swollen in water (on the left), b) Schematic description of the LbL assembly on cellulose gel beads Beads were treated with cationic chitosan (CH) and anionic sodium hexametaphosphate (SHMP) The process was repeated in order to deposit 100 BL The polyelectrolyte concentration was g/L for CH and g/L for SHMP, both in a 10 mM NaCl aqueous solution at pH The rinsing solution was Milli-Q water at pH and c) Schematic of cellulose bead before and after LbL assembly O Kă oklỹkaya et al Carbohydrate Polymers 255 (2021) 117468 beads was investigated by thermogravimetric analysis (TGA) (Mettler Toledo TGA/DSC, Stockholm, Sweden) The samples (5 ± mg) were placed in 70 μL aluminium oxide crucibles and heated at a rate of 10 ◦ C/ from 40 to 800 ◦ C in nitrogen at a flow rate of 50 mL/min sputter coated with Pt/Pd Results and discussion 3.1 Monitoring the build-up of multilayer films on flat model surfaces and on cellulose beads 2.5.6 Heating element The thermal degradation behavior of untreated and LbL treated cellulose beads were monitored using a high-speed camera (IDT N4MS3) with a 2× magnification microscope lens The samples were placed in a small ceramic crucible and covered with a microscope slide cover slip A light emitting diode light source (IDT LED) was used to illuminate the samples The ceramic crucible was placed on a flat heating element (d 10.8 × mm/24 V/50 W/750 ◦ C/Button heater, Rauschert Steinbach GmbH, Germany) with heating rate of 300 ◦ C/min when the temperature was set to ~370 ◦ C The changes in the structure of the samples were recorded using high-speed camera at a frame rate of 100 frames per second A schematic of the experimental setup is shown in Figure S3 The LbL formation of multilayer films consisting of CH and SHMP on model cellulose surface was investigated using QCM-D Fig shows the results from the QCM-D measurements, where the normalized frequency shift for the third overtone and the change in the dissipation are shown as functions of the number of adsorbed layers The frequency shift during the adsorption of CH showed an increase followed by a decrease during SHMP adsorption, as shown in Fig 2a The energy dissipation data (ΔD) showed a significant decrease during adsorption of the CH layer followed by an increase during the adsorption of the SHMP layer Benselfelt et al reported a similar behavior for the adsorption of a multilayer film of PDADMAC/PSS on a model cellulose surface (Benselfelt et al., 2017) Earlier studies have also clearly shown a deswelling of cellulose film due to polyelectrolyte adsorption (Benselfelt et al., 2017; Enarsson & Wågberg, 2008; Notley, 2008; Wang et al., 2011; Vuoriluoto et al., 2015) It can therefore be suggested that the detected changes in the QCM-D measurements following the adsorption of CH are due to a deswelling of the highly charged cellulose film on the QCM crystal due to a neutralization of the charges of the cellulose by the adsorbed CH similar to earlier results (Benselfelt et al., 2017) The decrease in the pH accompanying the addition of CH will naturally add to this effect but, as noted earlier (Xie & Granick, 2002), the charges of the CH will also increase the degree of dissociation of the carboxyl groups in the cellulose, which means that the effect of the pH will probably not be the dominating cause of the deswelling When the SHMP 2.5.7 Scanning Electron microscopy and energy dispersive X-ray analysis A field emission scanning electron microscope (FE-SEM, Hitachi S4800) was used to investigate the surface morphology of cellulose beads before and after the LbL treatment The residues from the heating element test were also investigated with FE-SEM to study the change in morphology Test pieces were coated with a nm thick platinum/ palladium layer using a Cressington 208 HR high-resolution sputter coater The presence of phosphorus in the LbL-treated cellulose beads before and after the heat element test and TGA was performed using an Inca (Oxford Instruments, X-MAX N80) energy dispersive X-ray spec­ trometer (EDX) Dried cellulose beads were mounted on the specimen holder and then cut in half using a razor blade Samples for EDX were not Fig The LbL build-up of five bilayers of (CH/SHMP) thin film on model cellulose surfaces (i.e., 795 μeq/g) using QCM-D measurements a) Change in normalized frequency, b) change in energy dissipation, c) total adsorbed mass calculated using the Sauerbrey equation, and d) chemical structures of polyelectrolytes used The polyelectrolyte concentrations were g/L for CH and g/L for SHMP, both in a 10 mM NaCl aqueous solution at pH The rinsing solution was Milli-Q (MQ) water at pH The adsorption sequence and rinsing were continued until a steady state signal was reached O Kă oklỹkaya et al Carbohydrate Polymers 255 (2021) 117468 was added, there was a significant decrease in frequency and a concomitant increase in the dissipation, which indicate that when the SHMP is adsorbed, the charge of the chitosan is efficiently compensated and probably over-compensated, which means that the cellulose film is again able to swell, and this swelling is much more significant compared to earlier results (Benselfelt et al., 2017) The trends for the subsequent layers are the same, showing both a steady build-up of the LbL film and a reversible swelling and deswelling of the cellulose film which is a clear demonstration of the dynamics of the LbL assembly during the build of the films which indeed is important for the development of the prop­ erties of the films To add further information to the QCM-D measurements and to quantify the amount of polymers in the adsorbed layers the cellulose beads were also used as a substrate for LbL deposition and the amount of CH adsorbed was determined using nitrogen analysis The results showed a steady increase in adsorbed amount during the build-up of 100 BL supporting the results of the QCM-D measurements regarding the steady build-up of LbLs on the cellulose surface (Fig 3) The initial value for the reference sample is probably due to residual nitrogen containing cellulose solvent in the prepared beads and this value could be sub­ tracted from the other layers since the amount of adsorbed polymer was determined from a calibration curve using the CH (Figure S2) The chemical composition of the LbL coatings deposited on cellulose beads has been assessed qualitatively using FTIR spectroscopy in an attenuated total reflection (ATR) configuration Fig shows the spectra of cellulose beads, CH, SHMP and LbL-treated cellulose beads The characteristic peaks of cellulose are described in supporting informa­ tion The peak observed at 1734 cm− was ascribed to protonated car­ boxylic acids The LbL-treated cellulose beads showed peaks at 1655 cm− ascribed to the carbonyl (C=O), at 1592 cm− ascribed to NH2, and at 1153 cm− 1, 1063 cm− 1, and 1028 cm− ascribed to stretching vi­ brations of C–O–C in glucosidic bonds of CH (Osman & Arof, 2003) The presence of SHMP in the coating was shown by two strong signals at 1250 cm− and 865 cm− corresponding to stretching of P=O and P–O–P groups (Drevelle et al., 2005) The absorbance intensity of two peaks increased as the number of bilayers increased indicating the build-up of the multilayer Further evidence of the thin film growth was provided by the fact that the weak signal at 1734 cm− 1, related to C=O stretching vibrations in the carboxylic group present on the beads slowly dis­ appeared during the LbL deposition The absorbance peaks of LbL thin film starts to dominate over the absorbance peaks of cellulose already at 10 BL of deposition 3.2 Thickness and roughness of the films on model surfaces Model cellulose surfaces prepared on silicon wafers were used as substrates for multilayer film formation and the LbL-assembled films were imaged using AFM to characterize the morphology, roughness, and thickness of the dry films The height images of the CH/SHMP films are shown in Fig The thickness of the films was measured using AFM by scanning the film scratched with a scalpel (Figure S4) The thickness and roughness are shown in Fig as functions of the number of deposited bilayers The films deposited on the model cellulose surfaces were somewhat thicker than those deposited on the silicon wafers and the LbL film buildup showed a linear increase in the thickness up to 20 BL, after which the increase in thickness was non-linear with additional BL deposition It has been suggested that the change in the LbL growth with the number of BLs can be due to an in and out diffusion of polyelectrolytes in the LbLs (Guin et al., 2014; Picart et al., 2001) or a type of island growth with increasing number of BLs (Haynie et al., 2011) where an initial un­ evenness propagates and small islands grow into larger islands as the number of BL increases After passing 20 BL, there is a super-linear growth (Abdelkebir et al., 2011) and the roughness was larger for 10 and 20 BL of films deposited on silicon wafers than those deposited on model cellulose surfaces, but the roughness was similar for 50 and 100 BL of film on both surfaces and the formed layers were indeed very smooth A more detailed analysis of the surfaces also shows that there is a granular morphology of 10 and 20 BL of CH/SHMP film on the silicon oxide surfaces, as shown in Figure S5, and also on the cellulose model surfaces as shown in Fig It can also be seen that the smaller granular shape of the surfaces changes into a larger scale unevenness at around 50 BL, which results in a lower roughness value but also fits with the island growth model (Haynie et al., 2011) This behavior has already been observed for non-linear LbL systems and has been ascribed to the in and out diffusion of polyelectrolytes in the LbL structure (Picart et al., 2002) It is not possible to establish the molecular reason for the change in growth detected when passing 20 BL for the present system, but it is clear that there is a steady growth of the LbLs with the number of deposition steps and that the granular structure of the surfaces changes to a more even surface leaving a rather flat and flaw-free surface which is probably essential for good flame-retardancy of the treated surfaces 3.3 Thermogravimetric analysis of cellulose beads Thermogravimetric analysis in nitrogen was used in order to eval­ uate the effect of the LbL coating on the char forming ability of the cellulose gel beads in an oxygen depleted environment This approach can provide preliminary hints on the pyrolysis occurring in the condensed phase of a burning material where the presence of a protec­ tive coating results in an essentially anaerobic atmosphere and reduced heating rates Fig shows the weight loss (TG) and the derivative of the weight loss (dTG) as a function of temperature and Table presents the degradation temperatures and residual amounts of reference and LbLtreated beads The untreated and LbL-treated cellulose beads show similar thermal degradation processes The initial weight loss observed at 100 ◦ C is attributed to the dehydration of water adsorbed by the coating The first significant loss of mass observed at 246 ◦ C is attributed to dehydration due to water entrapped within the cellulose gel beads and depolymer­ ization of non-crystalline cellulose leading to an aliphatic char The rate of mass loss was lower in the LbL-treated than in the reference beads It is suggested that this is due to the charring properties of the LbL coating since the rate at which thermal energy reaches the surface of cellulose beads is reduced by the LbL film (Hribernik et al., 2007) A second degradation step occurs at 308 ◦ C The LbL-treated cellulose beads exhibit no early degradation, which is generally reported to be due to the presence of phosphorus (Guin et al., 2014) A possible explanation is that the temperature at which the phosphorus compound catalyzes the Fig Total amount of CH adsorbed on cellulose beads as a function of number of bilayers deposited, determined by nitrogen analysis using a calibration curve for the CH The polyelectrolyte concentrations were g/L for CH and g/L for SHMP, both in a 10 mM NaCl aqueous solution at pH The rinsing solution was Milli-Q water at pH 5 O Kă oklỹkaya et al Carbohydrate Polymers 255 (2021) 117468 Fig FTIR spectra of a) cellulose gel beads, b) CH, c) SHMP and d) CH/SHMP treated cellulose beads (10, 20, 50 and 100 BL) The polyelectrolyte concentrations were g/L for CH and g/L for SHMP, both in a 10 mM NaCl aqueous solution at pH The rinsing solution was Milli-Q water at pH Fig AFM Scanasyst height images of the reference model cellulose surface and LbL-treated surfaces with different numbers of BLs The images are × μ m2 and the z-range is indicated in the scale bar to the right of the images The polyelectrolyte concentrations were g/L for CH and g/L for SHMP, both in a 10 mM NaCl aqueous solution at pH The rinsing solution was Milli-Q water at pH O Kă oklỹkaya et al Carbohydrate Polymers 255 (2021) 117468 Fig The average a) thickness and b) roughness values of CH/SHMP films deposited on silicon oxide and model cellulose surfaces as functions of the number of bilayers deposited The polyelectrolyte concentrations were g/L for CH and g/L for SHMP, both in a 10 mM NaCl aqueous solution at pH The rinsing solution was Milli-Q water at pH Fig a) Weight loss (TG) and b) derivative of weight loss (dTG) for untreated and CH/SHMP treated cellulose beads in a nitrogen atmosphere (~300 ◦ C/min) by using a specially designed heating element in which the untreated and LbL-treated cellulose beads were heated to a high temperature (~370 ◦ C) and monitored with a high speed camera in order to assess the steep degradation curve observed in TGA evaluations (Figure S6) 370 ◦ C was selected in order to ensure completion of all the main degradation steps observed by TGA All the samples immediately began to exhibit thermal degradation resulting in the formation of a char layer During the degradation, sudden movements (e.g., jumping of the sample) were observed which were attributed to the release of entrap­ ped water vapor within the cellulose beads and in the LbL structures The residues from this test were then investigated using SEM in order to assess the coating morphology Fig shows SEM images of untreated and LbL-treated beads before and after heat application The untreated beads had a relatively smooth morphology After deposition of 10 BLs of CH/SHMP, the beads appeared to have a wrin­ kled morphology A possible explanation of this structural change is the difference in modulus of cellulose and of the coating (Nolte et al., 2005; Stafford et al., 2004) More specifically, the cellulose beads were dry prior to the LbL treatment but during the treatment process, they became completely swollen due to the presence of water Having different moduli, the beads and the thin LbL films create a stress which forms the wrinkled morphology (Chan & Crosby, 2011) upon drying as shown in Fig A further increase in BL number results in an increased coating thickness as shown by AFM, which consequently forms larger wrinkles in the coating After the application of heat, both the untreated and LbL-treated beads maintain their shapes The charring layer of the Table TGA data for untreated and CH/SHMP treated cellulose beads in a nitrogen atmosphere Sample Tmax1 [◦ C] Tmax2 [◦ C] Residue [%] Cellulose bead 10 BL 20 BL 50 BL 100 BL 245 247 246 246 246 308 308 308 308 308 21 25 26 29 dehydration of cellulose overlaps the degradation temperature of non-crystalline cellulose The amount of residue found at 800 ◦ C increased as the number of bilayers deposited increased This behavior can be attributed to the presence of phosphate groups in the SHMP which favor the dehydration of CH towards the formation of an aromatic char which acts as a thermal barrier to limit mass and heat transfer to the cellulose beads (Carosio et al., 2015) In addition, an optimum in char forming efficiency is clearly observable in the 10–20 BL range as the increase of the deposited BL to 50 and 100 yields diminish returns in terms of final residues (Table 1) This suggests that a flame-retardant application of this system should target a BL number in between 10 and 20 BL and is in good agreement with a previous study where 17 BL deposited on cotton yielded optimal flame-retardant properties (Guin et al., 2014) In order to mimic the exposure to a heating rate relevant to a fire scenario, the beads were further characterized at high heating rates O Kă oklỹkaya et al Carbohydrate Polymers 255 (2021) 117468 Fig SEM images of untreated and CH/SHMP treated cellulose beads before (the left-hand column) and after (the right-hand column) heat application a) Un­ treated cellulose bead, b) 10 BL, c) 20 BL, d) 50 BL, and e) 100 BL The higher magnification SEM images of indicated area by square frames are shown adjacent to corresponding SEM image of cellulose bead The polyelectrolyte concentrations were g/L for CH and g/L for SHMP, both in a 10 mM NaCl aqueous solution at pH The rinsing solution was Milli-Q water at pH untreated cellulose bead exhibits obvious cracks and voids due to cel­ lulose pyrolysis and the release of volatile compounds On the contrary, LbL-treated beads show a unique structure which is generally defined as micro-intumescent bubbling (Carosio et al., 2015; Li et al., 2011) The formed sub-micronic bubbles become larger and more distinguishable as the number of BLs deposited is increased The reason for this change is not exactly known but it can be suggested that the bubbles are formed as a consequence of the release of volatile gases inside the beads, and that this in turn creates a stress on the coating layer which yields by creating bubbles This behavior can be related to the thickness and barrier properties of the films formed More extensive model experiments are needed to clarify these mechanisms, but the results show the potential of using the beads to establish the molecular mechanism for different LbL-treatments of cellulose surfaces In addition, it is worth highlighting that a nearly identical post combustion morphology has been observed on cotton fabrics treated by the same CH/SHMP assembly (Guin et al., 2014) This further shows the ability of the approach developed in this work in predicting the coating behavior in real scale testing conditions Fig shows cross-sections of untreated and 100 BL treated cellulose beads together with 50 BL treated cellulose beads after heat treatment and their corresponding EDX spectra The elemental composition of the samples shows that phosphorus was detected only in the LbL-treated beads, indicating the presence of SHMP in the thin films on the beads Phosphorus is homogeneously distributed along the surface of the bead, and it is still present as a protective layer around the bead even after the heat application This is further shown by the elemental analysis line spectrum of the crosssection of a 20 BL treated cellulose bead after thermal gravimetric analysis (Figure S7) Interestingly, no phosphorus was detected in the core of the cellulose bead, indicating that the phosphate action upon heating took place only on the surface of the cellulose bead where the coating was located This suggests that, upon heating, no migration of the phosphate occurs inside the beads and that the main action of SHMP is to mainly favor the coating char formation with limited effects on cellulose The formation of a protective barrier can then limit heat transfer and promote cellulose charring as it is well known that the char forming ability of cellulose is inversely proportional to the heating rate (Alongi et al., 2013) This further explains the observed diminishing returns in performances upon increasing the number of BL as observed by TGA In the 10–20 BL range the assembly produces a continuous and thick enough coating capable of providing good thermal shielding per­ formances; since there is no phosphate migration, increasing the amount of SHMP by adding more layers only slightly improves the coating performances Conclusions Based on the previously reported literature dealing with the use of CH/SHMP LbL coating for cotton flame-retardancy (Guin et al., 2014; Mateos et al., 2014), this work reported a novel approach employing O Kă oklỹkaya et al Carbohydrate Polymers 255 (2021) 117468 Fig Cross-section SEM images and corresponding EDX elemental mapping of untreated, 100 BL treated and, after heat application, 50 BL treated cellulose beads well-defined, non-crystalline cellulose gel beads as a model substrate to examine the molecular mechanism behind the effect of the multilayer coating on the thermal degradation of cellulose FTIR measurements showed that absorbance peaks of pristine cellulose were dominated by the absorbance peaks of coating after deposition of 10 BL Thermogra­ vimetric analysis revealed that cellulose beads coated with CH/SHMP films exhibit a degradation behavior different from that of the uncoated reference beads The multilayer coating of CH/SHMP due to synergetic effect enhanced the char formation by favoring the dehydration of cel­ lulose and the char formed subsequently protected the underlying cel­ lulose resulting in a residue as high as 29 % at 800 ◦ C for 100 BL coated beads under a nitrogen atmosphere In addition, the amount of residue significantly increased by a factor of 3.5 after only 10 BLs had been deposited but a further increase in the BL number did not show a similar increase SEM images and EDX spectra show the formation of a micro-intumescent swollen char layer located on the surface of the LbL-treated beads A correlation of the observed results with previously reported literature (Apaydin et al., 2014; Holder et al., 2017; Jimenez et al., 2016) dealing with the use of LbL assembled coatings for flame-retardancy clearly demonstrates the effectiveness of the proposed approach in providing meaningful insights on optimal BL range, coating mechanism and microstructure changes upon heating The proposed colloidal approach has never been investigated before since common practice of previously reported literature was to investigate the optimal deposition conditions and flame-retardant mechanism after a complete characterization of the treated substrates (Apaydin et al., 2014; Guin et al., 2014; Jimenez et al., 2016; Mateos et al., 2014) Conversely, this model material provides an excellent experimental platform for in­ vestigations aimed at a clear understanding of the effect of different surface treatments on the thermal degradation of cellulose Further developments of the proposed approach might involve the design of cellulose beads characterized by tunable degree of crystallinity (H Li et al., 2020) as well as the study of different LbL assembly encompassing nanoparticles and the implementation of advanced characterization techniques aiming at a deeper investigation of the molecular scale mechanisms of the assembly authors All the authors have given their approval to the final version of the manuscript CRediT authorship contribution statement ă klỹkaya: Investigation, Writing - original draft Rose-Marie Oruỗ Ko Pernilla Karlsson: Investigation, Writing - review & editing Federico Carosio: Investigation, Validation, Writing - review & editing Lars Wågberg: Supervision, Validation, Writing - review & editing Declaration of Competing Interest The authors declare no competing financial interest Acknowledgment ăklỹkaya, and Federico Carosio acknowledge Lars Wồgberg, Oruỗ Ko financial support from SSF (The Swedish Foundation for Strategic Research) and Lars Wågberg and Rose-Marie Pernilla Karlsson also acknowledge The Wallenberg Wood Science Centre for financial support Appendix A Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.carbpol.2020.117468 References Abdelkebir, K., Gaudi`ere, F., Morin-Grognet, S., Coquerel, G., Labat, B., Atmani, H., & Ladam, G (2011) Evidence 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