DSpace at VNU: Synthesis and optical investigation of amphiphilic diblock copolymers containing regioregular poly(3-hexylthiophene) via post-polymerization modification

Synthetic Metals 217 (2016) 172–184 Contents lists available at ScienceDirect Synthetic Metals journal homepage: www.elsevier.com/locate/synmet Synthesis and optical investigation of amphiphilic diblock copolymers containing regioregular poly(3-hexylthiophene) via post-polymerization modification Thu Anh Nguyenb , Trung Thanh Nguyena , Le-Thu T Nguyena , Thang Van Lea,c , Ha Tran Nguyena,c,* a Faculty of Materials Technology, Ho Chi Minh City University of Technology (HCMUT), Vietnam National University, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Viet Nam b National Key Lab of Polymer and Composite Materials, Viet Nam National University, Ho Chi Minh, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Viet Nam c Materials Technology Key Laboratory (Mtlab), Vietnam National University, Ho Chi Minh City, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Viet Nam A R T I C L E I N F O Article history: Received 24 January 2016 Received in revised form 13 March 2016 Accepted 26 March 2016 Available online April 2016 Keywords: Poly(3-hexylthiophene) Poly(N,N-dimethylamino-2-ethyl methacrylate) Poly(2-hydroxyethyl methacrylate) Atom transfer radical polymerization (ATRP) Amphiphilic diblock copolymer A B S T R A C T This paper reports on the synthesis and properties of amphiphilic diblock copolymers composed of a regioregular poly(3-hexylthiophene) (P3HT) block and a block of poly(N,N-dimethylamino-2-ethyl methacrylate-random-2-hydroxyethyl methacrylate) (P(DMAEMA-r-HEMA)) Well-defined rod-coil P3HT-b-P(DMAEMA-r-HEMA)) diblock copolymers were synthesized via the combination of quasiliving Grignard metathesis (GRIM) polymerization, end group modification, atom transfer radical polymerization (ATRP), and post-polymerization modification of diblock copolymer precursors and exhibited an average molecular weight of around 11,000 g/mol with low polydispersities below 1.5 The P3HT-b-P(DMAEMA-r-HEMA)) diblock copolymers were easily converted to amphiphilic diblock copolymers due to esterification of HEMA hydroxyl groups and amine quaternization of DMAEMA units to yield anionic or cationic amphiphilic diblock copolymers, respectively The structure and properties of the resulting diblock copolymers were characterized by proton nuclear magnetic resonance (1H NMR), gel permeation chromatography, Fourier transform infrared, UV–vis spectroscopy, and differential scanning calorimetry ã 2016 Elsevier B.V All rights reserved Introduction The regioregular poly(3-hexylthiophene) (P3HT) polymer has attracted significant interest owing to its potential in a variety of applications, including light-emitting diodes (OLED’s), field-effect transistors (OFET’s), optical sensors, smart windows and polymeric solar cells [1–9] The great importance of conjugated rod-coil block copolymers as a powerful tool towards supramolecular architectures with novel functions and physical properties has been well recognized In addition, the ability of diblock copolymers to selfassemble creates a new route for tuning the molecular organization and the resulting electronic and optoelectronic properties [10,11] The p-p interaction between the conjugated rods adds * Corresponding author at: Faculty of Materials Technology, Ho Chi Minh City University of Technology (HCMUT), Vietnam National University, 268 Ly Thuong Kiet, District 10, Ho Chi Minh City, Viet Nam E-mail address: nguyentranha@hcmut.edu.vn (H.T Nguyen) http://dx.doi.org/10.1016/j.synthmet.2016.03.035 0379-6779/ã 2016 Elsevier B.V All rights reserved value in providing controlled structures and functionality Furthermore, the microphase separation of conjugated rod-coil block copolymers may lead to nanoscale morphologies, such as lamellar, spherical, cylindrical and microporous structures These nanostructures may not only give rise to interfacial effects, but also open a new way for electronic processes Also, the combination of a stimulus (such as light, pH or temperature)-responsive coil segment with tunable photo-physical properties of the conjugated rod segment could enable the discovery of novel multifunctional sensory materials [12,13] Several classes of p-conjugated rod-coil block copolymers have been reported in the literature, including polyfluorene (PFO), polycarbazole, polyphenylene and polythiophene as rod segments, and polymethylmethacrylate (PMMA), poly(N,N-dimethylamino-2-ethyl methacrylate) (PDMAEMA), polystyrene (PS) and poly(2-vinylpiridine) (P2VP) as coil segments [14–20] Roil-coil diblock copolymers containing regioregular P3HT have already been reported by a number of research groups, such as the synthesis of P3HT-b-polystyrene, P3HT-b-PMMA, T.A Nguyen et al / Synthetic Metals 217 (2016) 172–184 P3HT-b-poly(isobornyl methacrylate) by atom transfer radical polymerization (ATRP) [21–23] Recently, amphiphilic diblock copolymers including P3HT as the hydrophobic rod segment and other hydrophilic coil segments have emerged as new unique materials due to their versatile microphase separation behavior and possibilities for fine-tuning of the supramolecular architecture of the polymers [24–26] For example, Craley et al [27] have reported the synthesis of poly (3-hexylthiophene)-block-poly(acrylic acid) (P3HT-b-PAA), which exhibited solvatochromic behavior in several solvents of varying polarity P3HT-b-PAA was obtained by hydrolysis reaction of the P3HT-b-poly(tert-butyl acrylate) precursor, which was synthesized via ATRP of tert-butyl acrylate using a bromoester terminated P3HT macroinitiator An allyl-terminated P3HT was converted to a hydroxypropyl terminated P3HT, which was subsequently endgroup modified to give the bromoester terminated P3HT However, in this procedure, the hydroboration-oxidation of the allyl group to form hydroxyl group with a maximum conversion efficiency below 80–85% could result in a mixture of bromoester terminated P3HT chains and those having no functional end groups that lead to the formation of the mixture including a homopolymers and the resulted diblock copolymers after controlled radical polymerization process Lohwasser and Thelakkat [28] reported the synthesis of poly(3-hexylthiophene)-block-poly(4-vinylpyridine) (P3HT-bP4VP) via preparing an alkoxyamine-terminated P3HT as a macroinitiator for nitroxide-mediated radical polymerization (NMRP) of 4VP P3HT-b-P4VP diblock copolymers with 55 and 77 wt% of P4VP were obtained, exhibiting microphase separation and colloidal structures in solution Mohamed et al [29] reported the synthesis of amphiphilic poly(3-hexylthiophene)-graft-poly (ethylene oxide) (P3HT-g-PEO) rod–coil conjugated random copolymers via non-controlled oxidative polymerization of 3HT with FeCl3 and click chemistry These P3HT-g-PEO diblock copolymers exhibited micellar morphologies in aqueous solutions with spherical particle diameters of approximately 60–75 nm More recently, Kumari et al [30] has reported the synthesis of poly (3-hexyl thiophene)-block-poly(N-isopropylacrylamide) (P3HT-bPNIPAM) diblock copolymers via the azideÀalkyne “click” reaction between an alkyne-terminated P3HT and an azide end-functionalized PNIPAM However, the use of excess P3HT-alkyne to react with PNIPAM-azide required purification of unreacted P3HT, leading to a relatively low yield (60%) Generally in the synthesis of other P3HT-containing diblock copolymers via alkyne-azide “click” reaction strategy, greater than an equivalent amount of either end-functionalized homopolymers was normally used to ensure high coupling conversion, which necessitates isolation of excess homopolymers [31] On the other hand, in the most case, especially in the synthesis of amphiphilic diblock copolymers consisting of a permanently hydrophobic block and a strongly hydrophilic block, the direct copolymerization method is difficult to carry out because of the solubility discrepancy between amphiphilic components in a common solvent In such case, the use of protecting group chemistries or a polymer post-modification approach is required For that reason, in this contribution, we present the synthesis of a new type of diblock poly(3-hexylthiophene)-block-poly (N,N-dimethylamino-2-ethyl methacrylate-random-2-hydroxyethyl methacrylate) (P3HT-b- P(DMAEMA-r-HEMA)) copolymers via the combination of Grignard metathesis (GRIM) method and ATRP of N,N-dimethylamino-2-ethyl methacrylate and 2-hydroxyethyl methacrylate co-monomers, that provides control of the number average molecular weight and narrow polydispersity index of each segment, resulting in well-defined diblock copolymers Moreover, the coil block of these copolymers contains both types of pendant side groups, the hydroxyl moieties of HEMA units and the dimethylamino groups of DMAEMA units, which could be 173 easily converted via esterification and quaternization to give P3HTb-polyanion and P3HT-b-polycation copolymers, respectively (Fig 1) The thermal and optical properties of these copolymers were investigated for insights into their microphase separation and crystallization behavior These amphiphilic ionic copolymers are envisaged to be useful for forming a variety of self-assembly structures in solutions such as micelles and vesicles, or for preparing conducting polymer nanostructures [32–34] Experiment 2.1 Materials 3-Hexylthiophene (!99%), N-bromosuccinimide (99%), iodine (!99.8%), iodobenzene diacetate (98%), N,N-dimethylformamide (DMF, 99.8%), sodium borohydride (NaBH4, 99%), phosphorus(V) oxychloride (POCl3, 99%), copper(I) bromide (CuBr, 98%), N,N,N0 ,N00 , N00 -pentamethyldiethylenetriamine (PMDETA, 99%) were purchased from Aldrich Ni(dppp)Cl2 and i-PrMgCl in tetrahydrofuran (THF) (2 mol/L) were purchased from Acros and stored in glove box at room temperature N,N-dimethylamino-2-ethyl methacrylate (99.8%) and 2-hydroxyethyl methacrylate (99.8%) were purchased from Acros and were distilled and stored in freezer Potassium acetate (KOAc, 99%), sodium carbonate (99%), and magnesium sulphate (98%) were purchased from Acros and used as received Chloroform (CHCl3, 99.5%), toluene (99.5%) and tetrahydrofuran (THF, 99%) were purchased from Fisher/Acros and dried using molecular sieves under N2 Dichloromethane (99.8%), n-hexane (99%), n-heptane (99%), methanol (99.8%), ethyl acetate (99%) and diethyl ether (99%) were purchased from Fisher/Acros and used as received 2.2 Characterization H NMR spectra were recorded in deuterated chloroform (CDCl3) with TMS as an internal reference, on a Bruker Avance 500 MHz spectrometer FT-IR spectra, collected as the average of 64 scans with a resolution of cmÀ1, were recorded from KBr disk on a FT-IR Bruker Tensor 27 Elemental analyses were recorded on a Carlo Elba Model 1106 analyzer MALDI TOF analysis was performed using a Voyager Elite apparatus in linear mode using trans-2-[3-(4-tertbutylphenyl)-2-methylprop-2-enylidene]-malonitrile (DCTB) as matrix Nitrogen laser desorption at a wavelength equal to 337 nm was applied Size exclusion chromatography (SEC) measurements were performed on a Polymer PL-GPC 50 gel permeation chromatograph system equipped with an RI detector, with THF 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 UV–vis absorption spectra of polymers in solution and polymer thin films were recorded on a Shimadzu UV-2450 spectrometer over a wavelength range of 300– 700 nm Differential scanning calorimetry (DSC) measurements were carried on a DSC Q20 V24.4 Build 116 calorimeter under nitrogen flow, at a heating rate of 10  C/min Contact angle measurements were performed on an OCA 20 contact angle system (Dataphysics, Germany) The AFM images were obtained using an agilent spm 5500 atomic force microscopy (AFM) The obtained diblock copolymers were casted from chroloform solution (5% concentration) to form thin film thickness of 15–20 mm on the glass substrate for four-probe electrical measurement 2.3 Synthesis of 2-bromo-3-hexylthiophene To a solution of 3-hexylthiophene (5 g, 29.7 mmol) in anhydrous THF (50 mL) in a 200 mL flask, a solution of N-bromosuccinimide (5.29 g, 29.7 mmol) was added slowly at  C under nitrogen The 174 T.A Nguyen et al / Synthetic Metals 217 (2016) 172–184 mixture was stirred at  C for h After that, 50 mL of distilled water was added to the reaction mixture, and the mixture was extracted with diethyl ether The organic layer was washed with a solution of Na2S2O3 (10%), a solution of KOH (10%), and dried over anhydrous MgSO4 The organic layer was distilled to give a colorless oil (6.7 g, 92%) 1H NMR (500 MHz, CDCl3), d (ppm): 7.19 (d, J = 5.6 Hz, 1H), 6.82 (d, J = 5.6 Hz, 1H), 2.59 (t, J = 7.3 Hz, 2H), 1.59 (s, br, 2H), 1.33 (m, none, 6H), 0.91 (t, J = 6.2 Hz, 3H) 13C NMR (75.5 MHz, CDCl3), d (ppm): 141.0, 128.2, 125.1, 108.8, 31.6, 29.7, 29.4, 28.0, 22.6, 14.1 2.4 Synthesis of 2-bromo-3-hexyl-5-iodothiophene Iodine (1.42 g, 11.18 mmol) and iodobenzene diacetate (1.965 g, 6.1 mmol) were added to a solution of 2-bromo-3-hexylthiophene (2.5 g, 11.1 mmol) in dichloromethane (25 mL) at  C The mixture Scheme Synthetic routes for the synthesis of ionic sulfonated P3HT-b-P(DMAEMA-r-HEMA) and quaternized P3HT-b-P(DMAEMA-r-HEMA) diblock copolymers T.A Nguyen et al / Synthetic Metals 217 (2016) 172–184 was stirred at room temperature for h Then, aqueous Na2S2O3 (10%) was added, and the mixture was extracted with diethyl ether and dried over anhydrous MgSO4 The solvent was evaporated to obtain crude product, which was purified by silica column chromatography (eluent: n-heptane) to give pure 2-bromo-3hexyl-5-iodothiophene as a pale yellow oil (3 g, 86%) 1H NMR (500 MHz, CDCl3), d (ppm): 6.97 (s, 1H), 2.52 (t, J = 7.54 Hz, 2H), 1.56 (quint, 2H), 1.32 (m, 6H), 0.89 (t, J = 6.4 Hz, 3H) 13C NMR (75.5 MHz, CDCl3), d (ppm): 144.3, 137.0, 111.7, 71.0, 31.5, 29.6, 29.2, 28.8, 22.5, 14.1 2.5 Synthesis of regioregular head-to-tail poly(3-hexylthiophene) with H/Br end groups (polymer 4) A dry, 500 mL three-neck flask was flushed with nitrogen and was charged with 2-bromo-3-hexyl-5-iodothiophene (24.37 g, 65 mmol) After three azeotropic distillations by toluene, anhydrous THF (220 mL) was added via a syringe, and the mixture was stirred at  C for h i-PrMgCl (2 M solution in THF, 30.87 mL, 61.75 mmol) was added via a syringe and the mixture was continuously stirred at  C for h The reaction mixture was allowed to cool down to  C The mixture was transferred to a flask containing a suspension of Ni(dppp)Cl2 (800 mg, 1.475 mmol) in THF (25 mL) The polymerization was carried out for 24 h at  C, followed by addition of M HCl After termination, the reaction was stirred for 15 and extracted with CHCl3 The polymer was precipitated in cold methanol and washed several times with n-hexane The polymer was characterized by 1H NMR and GPC GPC: Mn = 7100 g/mol, Ð = 1.18 Yield: 70% FT-IR (cmÀ1): 721, 819, 1376, 1454, 1510, 2853, 2922, 2953 H NMR (500 MHz, CDCl3), d (ppm): 6.96 (s, 1H), 2.90 (t, J = 7.5 Hz, 2H), 1.79 (sex, 2H), 1.52 (q, 6H), 0.94 (t, J = 6.4 Hz, 3H) Maldi-ToF 175 (m/z): 1410, 1576, 1742, 1908, 2074, 2240, 2406, 2572, 2738, 2904, 3070, 3236 GPC: Mn = 7000 g/mol Polydispersity index (Ð) = 1.18 UV–vis (CHCl3): lmax = 450 nm 2.6 Synthesis of regioregular head-to-tail poly(3-hexylthiophene) with CHO/Br end groups (polymer 5) Polymer (1 g) was dissolved in 260 mL of anhydrous toluene under nitrogen DMF (5.12 mL, 66.3 mmol) and phosphorus(V) oxychloride (POCl3) (5.30 mL, 58 mmol) were then added to the solution The reaction was performed at 75  C for 24 h The solution was cooled down to room temperature, followed by the addition of a saturated aqueous solution of sodium acetate The solution was stirred for h Then, the polymer was extracted with CHCl3 The polymer was precipitated in cold methanol and washed with cold n-hexane After drying under vacuum, 96 mg of polymer was obtained The yield was 93% FT-IR (cmÀ1): 721, 819, 1376, 1453, 1509, 1649, 2854, 2923, 2953 1H NMR (500 MHz, CDCl3), d (ppm): 9.99 (s, 1H), 6.96 (s, 1H), 2.78 (t, 2H), 1.69 (sex, 2H), 1.49 (q, 6H), 0.89 (t, 3H) Maldi-ToF (m/z): 1602, 1768, 1934, 2100, 2266, 2432, 2598, 2764, 2930, 3096, 3262 2.7 Synthesis of regioregular head-to-tail poly(3-hexylthiophene) with CH2OH/Br end group (polymer 6) Polymer (500 mg) was dissolved in 30 mL of anhydrous THF under nitrogen NaBH4 (41.8 mg) was then added The mixture was kept stirring at room temperature for h Then, the solvent was evaporated under vacuum The polymer was precipitated in cold methanol After drying under vacuum, 480 mg of the polymer was obtained The yield was 96% FT-IR (cmÀ1): 724, 817, 1376, 1453, 1509, 1561, 2853, 2922, 2953 1H NMR (500 MHz, CDCl3), d (ppm): Fig Schematic illustration of P3HT-b-polycation and P3HT-b-polyanion diblock copolymers generated from P3HT-b-P(DMAEMA-r-HEMA) 176 T.A Nguyen et al / Synthetic Metals 217 (2016) 172–184 Fig FT-IR spectra of P3HT-macroinitiator and P3HT-b-P(DMAEMA-r-HEMA) 6.96 (s, 1H), 2.78 (t, 2H), 3.7 (t, 2H), 1.69 (sex, 2H), 1.49 (q, 6H), 0.89 (t, 3H) Maldi-ToF (m/z): 1440, 1606, 1772, 1938, 2104, 2270, 2436, 2602, 2768, 2934, 3100 2.8 Synthesis of bromoester-terminated poly(3-hexylthiophene) (P3HT-macroinitiator) Polymer (500 mg) was dissolved in 20 mL of anhydrous THF under nitrogen To this solution, triethylamine (1 mmol) and 2bromoisobutyryl bromide (0.83 mmol) were added Then the reaction was carried out at 50  C overnight The polymer was extracted by CHCl3 The solution was washed two times with distilled water The polymer was precipitated in cold methanol After drying under vacuum, 475 mg of the polymer was obtained The yield was 95% FT-IR (cmÀ1): 724, 818, 1376, 1451, 1509, 1561, 1735, 2853, 2922, 2953 1H NMR (500 MHz, CDCl3), d (ppm): 6.96 (s, 1H), 5.29 (t, 2H), 2.78 (t, 2H), 1.93 (t, 6H), 1.69 (sex, 2H), 1.49 (q, 6H), 0.89 (t, 3H) Maldi-ToF (m/z): 1420, 1586, 1752, 1918, 2084, 2250, 2416, 2582, 2748, 2914, 3080 GPC: Mn = 7200 g/mol, Ð = 1.28 Mn estimated by 1H NMR = 7000 g/mol it became homogeneous, and then placed in a 60  C oil bath When the macroinitiator solution was added by cannula into the monomer solution, the mixture solution became homogeneous with a dark orange color After the solution was allowed to react for 16 h at 60  C, the resultant polymer solution was diluted with 20 mL of THF The solution was then passed through a column of Al2O3 to remove copper The polymer solution was concentrated and then precipitated into n-heptane The precipitated polymer was collected by vacuum filtration and subsequently washed with n-heptane, followed by drying under vacuum to give 165 mg of the desired product corresponding to a conversion of 83% 2.9 Synthesis of poly(3-hexylthiophene)-block-poly(N,Ndimethylamino-2-ethyl methacrylate-random-2-hydroxyethyl methacrylate) (P3HT-b-P(DMAEMA-r-HEMA)) P3HT-b-P(DMAEMA-r-HEMA) was synthesized by ATRP using the P3HT-macroinitiator 0.1 g of P3HT-macroinitiator (MnNMR = 7000) was placed in a 25 mL flask, to which mL of degassed THF was added by syringe The P3HT-macroinitiator solution was stirred until it became homogeneous A solution containing N,Ndimethylamino-2-ethyl methacrylate (DMAEMA) (0.28 mmol, 44.0 mg), 2-hydroxyethyl methacrylate (HEMA) (0.168 mmol, 21.9 mg), PMDETA (0.028 mmol, 4.9 mg) and CuBr (2.0 mg, 0.014 mmol) was prepared separately, and was degassed by three freeze-pump-thaw cycles The monomer solution was stirred until Fig 1H NMR spectrum of P3HT-b-P(DMAEMA-r-HEMA) T.A Nguyen et al / Synthetic Metals 217 (2016) 172–184 177 Fig GPC traces of P3HT-macroinitiator (dash line) and P3HT-b-P(DMAEMA-r-HEMA) (solid line) FT-IR (cmÀ1): 724, 818, 1376, 1451, 1509, 1561, 1735, 2853, 2922, 2953, 3550 1H NMR (500 MHz, CDCl3); d (ppm): 6.97 (s, 1H), 4.08 (s, 4H), 3.95 (s, 2H), 2.77 (s, 2H), 2.58 (s, 2H), 2.43 (s, 6H), 1.98 (m, 6H), 1.69 (sex, 2H), 1.48 (q, 6H), 0.89 (t, 3H) GPC: Mn = 12500 g/mol Ð = 1.42 FT-IR (cmÀ1): 724, 818, 1090 1376, 1451, 1509, 1561, 1735, 2853, 2922, 2953 1H NMR (500 MHz, CDCl3); d (ppm): 7.5–8.5 (m, 3H), 6.97 (s, 1H), 4.08 (s, 4H), 3.95 (s, 2H), 2.77 (s, 2H), 2.58 (s, 2H), 2.43 (s, 6H), 1.98 (m, 6H), 1.69 (sex, 2H), 1.48 (q, 6H), 0.89 (t, 3H) 2.11 Quaternization of P3HT-b-P(DMAEMA-r-HEMA) 2.10 Sulfonation of P3HT-b-P(DMAEMA-r-HEMA) Sulfonation reactions were carried out in THF at room temperature For a typical reaction, 60 mg of P3HT-b-P (DMAEMA-r-HEMA) was dissolved in 20 mL of THF and purged by N2 Once the diblock copolymer was dissolved in the solvent, triethylamine (2 equivalents to HEMA) was added The solution of 2-sulfobenzoic acid cyclic anhydride (SBA) (2 equivalents to HEMA) in about 10 mL of THF was then slowly added into the reaction The solution turned turbid immediately, and the reaction was stopped after 24 h The resulting reaction mixture was precipitated in n-heptane to obtain the sulfonated P3HT-b-P (DMAEMA-r-HEMA) diblock copolymer The yield was 95% Quaternization reactions were carried out in THF at 60  C for 18 h For a typical reaction, 100 g of P3HT-b-P(DMAEMA-r-HEMA) was dissolved in 20 mL of THF and purged by N2 To this solution CH3I (1.19 mg) was added The solution turned turbid gradually, and the reaction was stopped after 24 h The obtained cationic diblock copolymer was isolated by concentration of the reaction solution under reduced pressure and precipitation in cold n-heptane The obtained cationic diblock copolymer was dried to constant weight under vacuum The degree of quaternization of P3HT-b-P(DMAEMA-r-HEMA) was estimated by 1H NMR spectroscopy FT-IR (cmÀ1): 724, 818, 1090 1376, 1451, 1509, 1561, 1735, 2853, 2922, 2953 1H NMR (500 MHz, CDCl3); d (ppm): 6.97 (s, 1H), 4.08 Table Macromolecular characteristics of P3HT-b-P(DMAEMA-r-HEMA) synthesized by ATRP using P3HT-Br (nexp = 7000 g/mol) as the macroinitiator and CuBr/PMDETA ([CuBr]/ [PMDETA] = 1/2) as the catalytic complex Entry Temperature ( C) 60 60 Conversiona (%) 83 79 DMAEMA HEMA P3HT-b- P(DMAEMA-r-HEMA) d Mntheb Mnexpc Mntheb Mnexpc f 2610 2484 2490 2260 1296 1260 1640 1500 1 MnMMR Ðe 11130 10760 1.42 1.44 a Conversion as determined after precipitation in cold n-heptane: Conv = (m À mI À mCu À mL)/mM where m denotes the weight of product, and mI, mCu, mL, mM the weights of the initiator, copper catalyst, ligand (PMDETA) and monomers, respectively b DMAEMA and HEMA theoretical number-average-molar mass as calculated by [DMAEMA] or [HEMA]0/[P3HT-Br]0  Conv(%)  MwDMAEMA(orHEMA) assuming a living process c DMAEMA (or HEMA) experimental number-average molar mass as determined by 1H NMR spectroscopy (see Fig 3): Mnexp = DPexp  MwMMA(orMSp) where DPexp is the experimental degree of polymerization, as calculated from the relative intensities of a-amino methylene protons of DMAEMA (d = 2.32 ppm), a-methylene proton of HEMA (d = 4.00 ppm) and the methine (ring) protons of P3HT (d = 6.98 ppm) d Initiation efficiency as calculated from MntheofP(DMAEMA-r-HEMA)/MnexpofP(DMAEMA-r-HEMA) e Dispersity index as determined by GPC in THF at 35  C 178 T.A Nguyen et al / Synthetic Metals 217 (2016) 172–184 Fig 1H NMR spectrum of sulfonated P3HT-b-P(DMAEMA-r-HEMA) (s, 4H), 3.95 (s, 2H), 3.62 (s, 3H), 2.77 (s, 2H), 2.58 (s, 2H), 2.43 (s, 6H), 1.98 (m, 6H), 1.69 (sex, 2H), 1.48 (q, 6H), 0.89 (t, 3H) Results and discussion well-defined poly(3-hexylthiophene)-block-poly The (N,N-dimethylamino-2-ethyl methacrylate-random-2-hydroxyethyl methacrylate) (P3HT-b-P(DMAEMA-r-HEMA)) diblock copolymers were prepared via the combination of ‘quasi-living’ GRIM polymerization of the P3HT block and subsequent atom transfer radical polymerization (ATRP) of DMAEMA and HEMA comonomers according to Scheme In the first state, the P3HT-macroinitiator was synthesized via steps, including a controlled ‘quasi-living’ GRIM polymerization of the 2-bromo-3-hexyl-5-iodothiophene monomer The obtained P3HT with H/Br end groups had a GPC recorded number average molecular weight (Mn) value of 7100 g/mol, which is close to the theoretical one, and moderate polydispersity index of 1.18 Then, a quantitative conversion of Br-P3HT-H into a-bromo-v-bromoisobutyrate poly(3-hexylthiophene) (7) was achieved by a 3-step procedure Based on the integral ratio between the methine (ring) protons of P3HT at 6.96 ppm and methyl protons of the v-bromoisobutyrate end group at 1.95 ppm, an Mn value of 7000 g/mol was estimated Finally, the bromoester terminated P3HT was used as the macroinitiator for the ATRP of DMAEMA and HEMA co-monomers, in presence of CuBr and PMDETA as catalytic system The feed ratio of DMAEMA/HEMA co-monomers of about 1.7 was established for obtaining a good control over the ATRP ([DMAEMA]/[HEMA]/[P3HTMacroinitiator]/[CuBr]/[PDMAEMA] = 20/12/1/1/2) The unreacted monomers were eliminated and the resulting P3HT-b-P(DMAEMAr-HEMA) diblock copolymers were collected via precipitation in cold n-heptane Fig compares the FT-IR spectra of the obtained P3HT-b-P (DMAEMA-r-HEMA) and the P3HT macroinitiator A strong Fig 1H NMR spectrum of quaternized P3HT-b-P(DMAEMA-r-HEMA) T.A Nguyen et al / Synthetic Metals 217 (2016) 172–184 179 Fig UV–vis spectra of sulfonated P3HT-b-P(DMAEMA-r-HEMA) in different solvents and in a solid state film absorption signal at 1725 cmÀ1 appears in the spectrum of the diblock copolymer and is attributed the stretching vibration of the carbonyl groups of the P(DMAEMA-r-HEMA) block The polymerization degree of the P(DMAEMA-r-HEMA) block was calculated from the the 1H NMR spectrum (Fig 3) by comparing the relative signal intensities of the dimethylamino group (peak p) of DMAEMA moieties and methylene hydroxyl group of HEMA moieties (peak k) with the methine (ring) protons of P3HT at d = 6.96 ppm As a result, the NMR recorded molecular weight of the P(DMAEMA-r-HEMA) block was around 4000 g/mol, and the molar fraction of HEMA in this block was about 0.45 Accordingly, the numbers of DMAEMA and HEMA units were calculated via 1H NMR to be 16 and 13, respectively The GPC curves of the P3HT-b-P(DMAEMA-r-HEMA) diblock copolymer and P3HT-macroinitiator are shown in Fig 4, revealing single peaks and relatively narrow molecular weight distributions, indicating well-controlled chain growth during the ATRP process Fig UV–vis spectra of quaternized P3HT-b-P(DMAEMA-r-HEMA) in different solvents and in a solid state film 180 T.A Nguyen et al / Synthetic Metals 217 (2016) 172–184 Table Solution UV–vis spectral peaks for regioregular P3HT, sulfonated P3HT-b-P(DMAEMA-r-HEMA) and quaternized P3HT-b-P(DMAEMA-r-HEMA) Solvent sulfonated P3HT-b-P(DMAEMA-r-HEMA) (l = nm) quaternized P3HT-b-P(DMAEMA-r-HEMA) (l = nm) P3HT (l = nm) Chloroform Tetrahydrofuran Toluene Dichloromethane Ethyl acetate Water (pH < and pH > 10) Solid-state film 445 452 452 449 520, 550, 610 513, 555, 605 515, 550, 600 450 452 452 451 505, 559, 610 520, 555, 610 520, 550, 607 452 447 451 455 Insoluble Insoluble 527, 558, 610 The GPC recorded Mn and Ð of P3HT-b-P(DMAEMA-r-HEMA) were around 12500 g/mol and 1.42, respectively The molecular weight characteristics of P3HT-b-P(DMAEMA-rHEMA) are summarized in Table Similar results of comparable experiments with the same conditions are shown, indicating the repeatability of the experimental method As seen in Table 1, the P (DMAEMA-r-HEMA) block was obtained with a relatively good approximation between theoretical and experimental molar masses, attesting for an initiation efficiency close to The coil block of P3HT-b-P(DMAEMA-r-HEMA) was sulfonated via esterification reaction between the hydroxyl group of HEMA units and 2-sulfobenzoic acid cyclic anhydride (SBA), following the method previously described [35] In this reaction, triethylamine was used as the catalyst and a 2–1 molar ratio of SBA to ÀÀOH groups was employed to ensure complete sulfonation It was noted that upon the addition of SBA, the reaction solution turned turbid immediately, indicating a change in the solubility of the diblock copolymer as a result of the formation of anionic moieties The 1H NMR spectrum of the sulfonated P3HT-b-P(DMAEMA-r-HEMA) showed the appearance of aromatic proton peaks (peaks q) at 7.4–8.1 ppm assigned to the phenyl group of sulfobenzoic moieties, suggesting that 2-sulfobenzoic acid cyclic anhydride was covalently linked to the OH groups (Fig 5) The resulting relative intensity of the phenyl peak regions (peaks q), with respect to either the signal of the methylene group in the side chain of HEMA units (peak k) or the signal of thiophene moieties (peak a or b), indicated that all the hydroxyl side groups were sulfonated Fig Absorption spectra of sulfonated P3HT-b-P(DMAEMA-r-HEMA) in THF/methanol mixtures with varied volume ratios (A), and an image showing (from left to right) color changes of the copolymer solutions with increasing methanol content (B) T.A Nguyen et al / Synthetic Metals 217 (2016) 172–184 181 Fig 10 Absorption spectra of quaternized P3HT-b-P(DMAEMA-r-HEMA) in THF/methanol mixtures with varied volume ratios (A), and an image showing (from left to right) color changes of the copolymer solutions with increasing methanol content (B) On the other hand, the dimethylamino pedant groups of P3HT-b-P(DMAEMA-r-HEMA) were quaternized using iodomethane (CH3I), with an equimolar ratio of dimethylamino groups to CH3I A change in the solubility of the diblock copolymer in THF was realized as the solution became turbid after h, with the turbidity increasing gradually, indicating the formation of quaternized P3HT-b-P(DMAEMA-r-HEMA) Fig shows the 1H NMR spectrum of the obtained quaternized diblock copolymer The occurrence of quaternization reaction was confirmed by the appearance of the methyl peak (peaks q) at 3.60 ppm By integration of corresponding 1H NMR signals, a degree of quaternization was estimated to be approximately 80% The optical behavior of sulfonated and quaternized P3HT-b-P (DMAEMA-r-HEMA) diblock copolymers in solvents of different Fig 11 Water contact angles on quaternized P3HT-b-P(DMAEMA-r-HEMA) (A) and sulfonated P3HT-b-P(DMAEMA-r-HEMA) (B) films The inset photographs show profiles of water droplets on the copolymer surfaces taken at t = s and t = 35 s after deposition of the droplets 182 T.A Nguyen et al / Synthetic Metals 217 (2016) 172–184 Fig 12 DSC thermograms of P3HT-b-P(DMAEMA-r-HEMA) (a) and corresponding sulfonated and quaternized copolymers (b and c, respectively) polarities was studied, using UV–vis to monitor changes in the p-p* transition of P3HT chains It should be noted that the solubility of P3HT chains depends on the polarity of the solvent used Fig and Fig show the UV–vis absorbance spectra of sulfonated and quaternized P3HT-b-P(DMAEMA-r-HEMA)s in different solvents, respectively, and the position of absorption peaks are summarized in Table In less polar aprotic solvents, such as THF, chloroform, dichloromethane and toluene, the absorption maxima of both sulfonated and quaternized P3HT-bP(DMAEMA-r-HEMA)s were around 445–450 nm, indicating that the P3HT block adopted a coil conformation in these solvents In polar solvents such as ethyl acetate and water (basic water for the sulfonated copolymer and acidic water for the quaternized copolymer), the UV–vis spectra of both ionic P3HT-b-P(DMAEMAr-HEMA) copolymers showed a main absorption peak at around 513–520 nm and and two vibronic peaks at 550–559 nm and 605–610 nm, similar to the spectra of these copolymers and of P3HT in the solid state (Table 2) This indicates aggregation of the P3HT block As such, polar solvents caused long-range order in solution of P3HT segments, while the other block remained as random coil Since THF is a good solvent for the regioregular P3HT block and a bad solvent for the ionic sulfonated/quaternized P(DMAEMA-rHEMA) block and vice versa for methanol, in these homogeneous solvents these diblock copolymers coexist in two conformations Thus, to study this phenomenon, their optical absorption behavior in mixtures of THF/methanol with varied volume ratios from 0/100 to 100/0 (%/%) was investigated As shown in Figs A and 10 A for both ionic diblock copolymers, in pure THF, the p-p* absorption band of the regioregular P3HT chains was located at 452 nm indicative of the coil conformation The ionic diblock copolymers adopted predominantly coil-like conformations at low methanol contents (below or equal to 20% and 50% for sulfonated and quaternized P3HT-b-P(DMAEMA-r-HEMA)s, respectively) Further increasing the methanol content led to bathochromic shifts of this band, accompanied by gradual appearance of two vibronic peaks at higher wavelengths This phenomenon corresponds to conformational changes toward the formation of aggregates of P3HT chains due to their poor solubility in methanol In pure methanol, the aggregation of P3HT chains was evidenced by the presence of a main peak at 515 nm and two vibronic peaks at 560 nm and 610 nm The conformational changes of the ionic diblock copolymers were additionally indicated by characteristic color changes from yellow to purple, as shown in Figs B and 10 B It should be noted that the presence of an ionic block, such as the sulfonated and quaternized P(DMAEMA-r-HEMA) blocks, can impact the hydrophilicity of P3HT-based materials Thus, the surface wettability of the regioregular P3HT and the ionic diblock copolymer films was determined by carrying out contact angle measurements Fig 11 shows the water contact angle as a function of time after deposition of a water droplet on surfaces of the homopolymeric P3HT and ionic copolymers As a reference, the homopolymeric P3HT surface was expectedly hydrophobic and the water contact angle remained constant at 92 (Ỉ1 ) throughout the duration of the experiment The water contact angles on copolymer surfaces were high, at first But after a couple of seconds, the water droplets collapsed and the surfaces suddenly changed from hydrophobic to hydrophilic as evidenced by the abrupt drop of contact angle from 92 to 59  (Ỉ1 ) for quaternized P3HT-b-P(DMAEMA-r-HEMA) (Fig 11A) and to 18 (Ỉ1 ) for sulfonated P3HT-b-P(DMAEMA-r-HEMA) (Fig 11B), and remained unchanged afterwards The thermal properties of the diblock copolymers were studied via DSC Fig 12 shows that the thermal profile of the non-ionized P3HT-b-P(DMAEMA-r-HEMA) copolymer exhibits a glass transition of the P(DMAEMA-r-HEMA) block at 32.8  C and a melting transition of the P3HT block at 204.7  C (DH = 11.27 J/g) Note that the melting peak temperature (Tm) and and melting enthalpy of a previously reported linear P3HT homopolymer (Mn = 4500 g/mol) were 239  C and 17.14 J/g, respectively [36] This illustrates that the coil P(DMAEMA-r-HEMA) block can hinder the crystallization of P3HT Upon either sulfonation or quaternization of the P (DMAEMA-r-HEMA) block, the glass transition temperature (Tg) slightly increased to 45.1 and 43.5  C, respectively, as a result of intermolecular ionic associations of ionic block chains [37,38] Accordingly, these ionic copolymers featured melting transitions of crystallized P3HT block chains at around 190–200  C but composed of overlapping multiple endotherms The multiple melting behavior has previously been reported for P3HT and is attributed to a “meltingÀrecrystallizationÀmelting” process [39,40] Thus, the final melting enthalpy did not reflect the degree of crystallinity of the copolymer samples At room temperature, the DC conductivity of P3HT-b-P (DMAEMA-r-HEMA) diblock copolymers film was measured about  10À11 S/cm On the contrary, the DC conductivity of sulfonated P3HT-b-P(DMAEMA-r-HEMA) and quaternized P3HT-b-P (DMAEMA-r-HEMA) diblock copolymers were measured about  10À10 S/cm and  10À10 S/cm, respectively which are higher than the DC conductivity of precursor P3HT-b-P(DMAEMA-rHEMA) This phenomenon can be explained that the ionic sulfonated/quaternized P3HT-b-P(DMAEMA-r-HEMA) diblock copolymers could be self-doped by ionic pendant groups such as SO3À or IÀ The micro- and nanoscopic morphologies of thin deposits of the diblock copolymer P3HT-b-P(DMAEMA-r-HEMA), quaternized P3HT-b-P(DMAEMA-r-HEMA) and sulfonated P3HT-bP(DMAEMA-r-HEMA) were investigated by AFM in intermittentcontact mode These films have been prepared by drop-casting onto glass substrates from a good solvent (CHCl3) for both P3HT and P(DMAEMA-r-HEMA) segments followed by annealing at 150  C for 24 h Thin films of the studied copolymers show a fibrillar (nanowire-like) morphology (Fig 13) This observation is attributed to the microphase separation between flexible P (DMAEMA-r-HEMA) segments and P3HT rod-segments Indeed, this fibrillar morphology is typical from the crystalline assembly of P3HT into p-stacked structures, as observed and described for highly regioregular poly(thiophene)s and other conjugated polymers T.A Nguyen et al / Synthetic Metals 217 (2016) 172–184 183 Fig 13 AFM images of P3HT-b-P(DMAEMA-r-HEMA) (A) and corresponding sulfonated and quaternized copolymers (B and C, respectively) Conclusion We have successfully prepared new rod-coil P3HT-b-P (DMAEMA-r-HEMA) diblock copolymers via the combination of the GRIM method, end group modifications and ATRP of N,N-dimethylamino-2-ethyl methacrylate-and 2-hydroxyethyl methacrylate co-monomers These diblock copolymers were characterized by 1H NMR, GPC and FT-IR methods P3HT-b-P (DMAEMA-r-HEMA) could be facilely converted to either cationic or anionic diblock copolymers upon quaternization of DMAEMA units or sulfonation of HEMA moieties, respectively The resulting ionic diblock copolymers exhibited the typical behavior of amphiphilic diblock copolymers and were found to be readily soluble in a variety of organic solvents as well as in acidic/basic water It is worth pointing out that the introduction of either quaternized or sulfonated side groups in the coil block allowed for 184 T.A Nguyen et al / Synthetic Metals 217 (2016) 172–184 significant improvement of the surface 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structural transition around 54  C in regioregular poly(3-hexylthiophene) with high crystallinity as revealed by infrared spectroscopy, Macromolecules 44 (23) (2011) 9341–9350  ... appearance of the methyl peak (peaks q) at 3.60 ppm By integration of corresponding 1H NMR signals, a degree of quaternization was estimated to be approximately 80% The optical behavior of sulfonated and. .. methacrylate-random-2-hydroxyethyl methacrylate) (P3HT-b- P(DMAEMA-r-HEMA)) copolymers via the combination of Grignard metathesis (GRIM) method and ATRP of N,N-dimethylamino-2-ethyl methacrylate and 2-hydroxyethyl... via esterification and quaternization to give P3HTb-polyanion and P3HT-b-polycation copolymers, respectively (Fig 1) The thermal and optical properties of these copolymers were investigated for insights