Fabrication of Thermo-responsive Cotton Fabrics Using Poly(vinyl caprolactam-co-hydroxyethyl acrylamide) Copolymer

1 Fabrication of Thermo-responsive Cotton Fabrics Using 1 Poly(vinyl caprolactam-co-hydroxyethyl acrylamide) 2 Copolymer3 4 Min Xiao1, Edurne González 1, Alexis Martell Monterroza2, Margaret Frey1,* 5 6 1Department of Fiber Science & Apparel Design, Cornell University, Ithaca NY 14850, USA 7 2Department of Chemistry & Chemical Biology, Cornell University, Ithaca NY 14850, USA 8 9 Abstract10 A thermo-responsive polymer with hydrophilic to hydrophobic transition behavior, 11 poly(vinyl caprolactam-co-hydroxyethyl acrylamide) (P(VCL-co-HEAA)), was prepared 12 by copolymerization of vinyl caprolactam and N-hydroxyethyl acrylamide via free radical 13 solution polymerization. The resulting copolymer was characterized by Fourier transform 14 infrared spectroscopy (FTIR), 1H nuclear magnetic resonance (NMR), gel permeation 15 chromatography (GPC), differential scanning calorimetry (DSC) and thermogravimetric 16 analysis (TGA). The lower critical solution temperature (LCST) of P(VCL-co-HEAA) 17 was determined at 34.5°C. This thermo-responsive polymer was then grafted onto cotton 18 fabrics using 1,2,3,4-butanetetracarboxylic acid (BTCA) as crosslinker and sodium 19 hypophosphite (SHP) as catalyst. FTIR and energy dispersive X-ray spectroscopy (EDS) 20 studies confirmed the successful grafting reaction. The modified cotton fabric exhibited 21 thermo-responsive behavior as evidenced by water vapor permeability measurement 22 confirming decreased permeability at elevated temperature. This is the first demonstration 23 that a PVCL based copolymer is grafted to cotton fabrics. This study provides a new 24 thermo-responsive polymer for fabrication of smart cotton fabrics with thermally 25 switchable hydrophilicity. 26 27 Keywords: thermo-responsive polymer, poly(vinyl caprolactam) (PVCL), LCST, water 28 vapor permeability, smart cotton fabric 29 30 * Corresponding author. Tel.:+1-607-255-193731 32 Email address: [email protected] 33 34 This is the accepted manuscript of the article that appeared in final form in Carbohydrate Polymers 174 : 626-632 (2017), which has been published in final form at https://doi.org/10.1016/j.carbpol.2017.06.092. © 2017 Elsevier under CC BYNC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/) 2 1. Introduction 35 Cotton fabrics have many desirable properties, which include high absorbency, comfort, 36 dyeability and low cost. Cotton consists of nearly 99% cellulose and the hydroxyl groups 37 (-OH) available on the cellulose backbone have been used in variety of modification 38 strategies to impart a new functionality to cotton fabric, such as wrinkle resistance and 39 antimicrobial properties. In recent years, an emerging modification strategy has been 40 centered on grafting stimuli-responsive polymer materials onto cellulose-based natural 41 fibers to create materials that can respond to changes (temperature, pH or light) in the 42 environment (Stuart et al., 2010; Yang, Esteves, Zhu, Wang, & Xin, 2012). A particular 43 research interest is creation of thermo-responsive smart textiles with potential 44 applications in skin care products, wound dressing products, smart permeability, 45 deodorant fabrics, reversible wettability and physiological parameter monitoring (Hu, 46 Meng, Li, & Ibekwe, 2012). 47 Thermo-responsive or temperature-sensitive polymers are a kind of smart materials that 48 respond to changes in temperature and undergo a phase transition at the lower critical 49 solution temperature (LCST) (De Las Heras Alarcón, Pennadam, & Alexander, 2005). At 50 temperatures below LCST, these macromolecules are hydrophilic and soluble in water 51 due to the dominant hydrogen bonding between hydrophilic segments of the polymer 52 chain and water molecules. Alternatively, at temperatures above LCST, these 53 macromolecules become hydrophobic and phase separate in water due to the strengthened 54 hydrophobic interactions among hydrophobic segments (Ivan M Okhapkin, Irina R 55 Nasimova, Elena E Makhaeva, & Alexei R Khokhlov, 2003; Qiu & Park, 2001). While 56 most polymers increase their water solubility as the temperature increases, polymers with 57 an LCST decrease their water solubility as the temperature increases. 58 Poly (N-isopropylacrylamide) (PNIPAm) and poly(vinyl caprolactam) (PVCL) are 59 attractive thermoresponsive polymers with LCST in the range of physiological 60 temperature, a property which makes them especially attractive for potential use in drug 61 delivery, biochemistry, bioengineering or sensors (Bae, Okano, Hsu, & Kim, 1987; 62 Schmaljohann, 2006). Although PNIPAm has been more widely studied, the use of PVCL 63 3 is a better alternative for bioapplications due to its higher biocompatibility (Cortez-Lemus 64 & Licea-Claverie, 2016). 65 Grafting of PNIPAAm to cotton fabrics has already been reported by a few authors (T. 66 Chen, Fang, Zhong, Chen, & Wang, 2015; Wang et al., 2016). However, to our 67 knowledge, this is the first demonstration that a PVCL based copolymer is used to 68 manufacture smart fabrics. In this study, we synthesize and immobilize thermo-69 responsive copolymer poly(vinyl caprolactam-co-hydroxyethyl acrylamide), P(VCL-co-70 HEAA), onto cotton to obtain thermally switchable hydrophilicity. Hydroxyethyl 71 acrylamide (HEAA) is used as co-monomer in order to add –OH functional groups to the 72 copolymer. These –OH groups are able to graft the copolymer to cotton fabrics via a 73 carboxylic acid-based crosslinker (BTCA). Furthermore, it should be noted that PHEAA 74 is also biocompatible and has been widely used for biomedical applications (Zhang, Chu, 75 Zheng, Kissel, & Agarwal, 2012). The properties of the copolymer are characterized by 76 FTIR, 1H NMR, GPC, DSC and TGA. The lower critical solution temperature (LCST) is 77 also determined. FTIR and EDS studies are conducted to confirm the grafting reaction, 78 and scanning electron microscopy (SEM) is used to study the surface morphology of 79 modified cotton. Additionally, the thermo-responsive behavior of the resulting cotton 80 fabric is investigated through water vapor permeability measurement. 81 82 2. Experimental 83 2.1 Materials 84 Standard desized, scoured, and bleached plain woven cotton fabrics (density 110 g/m2) 85 were purchased from Testfabrics, Inc.. Vinyl caprolactam (VCL), 2,2’-azobis(2-methyl-86 propionitrile) (AIBN), dimethyl formamide (DMF), diethyl ether, 1,2,3,4-87 Butanetetracarboxylic acid (BTCA), and sodium hypophosphite monohydrate (SHP) 88 were all purchased from Sigma Aldrich. N-(2-Hydroxyethyl) acrylamide (HEAA) was 89 supplied by Santa Cruz Biotechnology. Deuterium oxide (D2O) was purchased from 90 Cambridge Isotope Laboratories, Inc.. All chemicals were used as received without 91 further purification. 92 4 2.2 Synthesis of P(VCL-co-HEAA) copolymer 93 P(VCL-co-HEAA) copolymer was synthesized by copolymerizing vinyl caprolactam 94 (VCL) and hydroxyethyl acrylamide (HEAA) via free radical polymerization using AIBN 95 as initiator and DMF as solvent. Reactions were performed in a three neck round bottom 96 flask equipped with a reflux condenser and a N2 inlet. The initial feed molar ratio of VCL 97 to HEAA was 80:20. First, HEAA (16.10g, 140 mmol), VCL (77.84 g, 560 mmol) and 98 DMF (345 mL) were added to the flask. The temperature was increased to 60 °C and the 99 reaction mixture was stirred for 15 minutes under N2 flow until all components were 100 completely dissolved. Then, AIBN initiator (0.57 g, 3.5 mmol) was thoroughly dissolved 101 in 5 mL of DMF and then injected into the flask to start the polymerization. The reactions 102 were performed at 60°C for 16 hours under continuous N2 flow. Polymerization was 103 stopped by cooling down the reaction to room temperature. Afterwards, the polymer was 104 precipitated in diethyl ether, filtered and dried in a vacuum oven at 50 °C overnight. 105 2.3 Characterization of P(VCL-co-HEAA) copolymer 106 2.3.1 1H Nuclear magnetic resonance (NMR) 107 The 1H-NMR experiment of the copolymer was recorded at room temperature with an 108 INOVA 400 spectrometer operating at 400 MHz and using D2O as solvent. 109 2.3.2 Gel permeation chromatography (GPC) 110 The molecular weight of the P(VCL-co-HEAA) copolymer was measured by a Waters 111 ambient-temperature GPC equipped with a Waters 1515 isocratic HPLC pump and a 112 Waters 2414 refractive index detector at 50°C. Dimethyl formamide (DMF) with 0.1% 113 lithium bromide was used as mobile phase at a flow rate of 0.5 mL/min. The obtained 114 molecular weight value was referred to polystyrene standards. 115 2.3.3 Thermogravimetric analysis (TGA) 116 Thermogravimetric analysis (TGA) of the P(VCL-co-HEAA) copolymer was performed 117 from 30 to 800°C at a heating rate of 10°C /min using a nitrogen purge on TGA Q500, 118 TA Instruments. 119 2.3.4 Differential scanning calorimetry (DSC) 120 Thermogram of the copolymer was acquired with a TA instruments DSC Q2000. The 121 5 procedure included a heat/cool/heat sequence at a rate of 10°C/min in the temperature 122 between 0°C and 300°C to remove any effect of thermal history. 123 2.3.5 Lowest critical solution temperature (LCST) 124 The lower critical solution temperature (LCST) of P(VCL-co-HEAA) in aqueous solution 125 was measured on a Spectramax 384 spectrophotometer. Optical transmittance of 1 wt % 126 polymer solution in water was measured at 500 nm as a function of temperature. The 127 LCST value of the polymer was determined at the temperature showing an optical 128 transmittance of 50%. 129 2.4 Grafting of P(VCL-co-HEAA) to cotton fabrics 130 Grafting of P(VCL-co-HEAA) to cotton fabrics was performed using BTCA as 131 crosslinker and SHP as catalyst. A solution was prepared with 250g/L, 20g/L, and 30g/L 132 concentrations of P(VCL-co-HEAA), BTCA, and SHP, respectively. Each cotton sample 133 was soaked in the solution overnight at room temperature and then padded in a laboratory 134 padder with two dips and two nips to reach a wet pickup of (120±5)%. The sample was 135 dried at 85°C for 10 min and then cured in an oven at 160°C for 20 min. Finally, the 136 sample was rinsed with deionized water and air-dried in a conditioning room 137 (21.0±2.0°C, 65.0±4.0% relative humidity) for 24 h. 138 2.5 Characterization of thermo-responsive cotton fabrics 139 2.5.1 Add-on 140 The weight of the conditioned cotton fabrics was recorded before and after the grafting 141 process. The add-on of the thermo-responsive cotton fabrics was calculated as the relative 142 weight increase of the fabric as shown in the following equation. 143 Add-on (%) = (𝑚𝑚𝑓𝑓−𝑚𝑚0 𝑚𝑚0) × 100 (1) 144 where 𝑚𝑚0 is the initial weight of the fabric and 𝑚𝑚𝑓𝑓 is the final weight of the fabric grafted 145 with P(VCL-co-HEAA). 146 2.5.2 FTIR 147 The Fourier transform infrared (FTIR) spectra of cotton fabrics were collected on a FTIR 148 spectrometer (Magna 560, Nicolet Instrument Technologies, Fitchburg, WI, USA) using 149 6 a diamond attenuated total reflectance (ATR) accessory. The data were averaged over 64 150 scans with a resolution of 4 cm-1 in the range of 4000 to 600 cm-1 for each sample. 151 2.5.3 SEM-EDS studies 152 The surface morphology of cotton fabrics was examined on a field emission scanning 153 electron microscope (LEO 1550 FESEM). The samples were mounted on aluminum stubs 154 and sputter-coated with gold and scanned at 5 kV for SEM imaging. Energy dispersive 155 X-ray spectroscopy (EDS) study was conducted to analyze the elemental compositions of 156 cotton fabrics after grafting with P(VCL-co-HEAA) copolymer. 157 2.5.4 Water vapor permeability (WVP) measurements 158 Water vapor permeability of the modified cotton fabrics was measured in accordance with 159 BS 7209:1990 Test Method. Briefly, the test cotton fabric was sealed over the open mouth 160 of a test dish which contains water, and the assembly placed in a controlled atmosphere. 161 Over a period of time, successive weightings of the assembled dish were made and the 162 rate of water vapor permeation through the test fabric was determined. Six test fabrics 163 (three for treated samples and three for untreated samples) were tested in a similar manner 164 and concurrently to determine WVP at room temperature (21°C) and also at 50°C. 165 3. Results and discussion 166 3.1 Synthesis and characterization of P(VCL-co-HEAA) copolymer 167 7 168 Figure 1. 1H-NMR of P(VCL-co-HEAA) copolymer 169 0100 200 300 400 500 600 700 800 0 20 40 60 80 100 P(VCL-co-HEAA) PVCL Temperature ( 0 C) Weight (%) 170 (a) 171 8 050 100 150 200 250 300 -1.0 -0.5 0.0 0.5 P(VCL-co-HEAA) PVCL Temperature ( 0 C) Heat Flow (W/g) 172 (b) 173 174 (c) 175 Figure 2. (a) TGA thermograms of PVCL and P(VCL-co-HEAA) copolymer; (b) DSC 176 thermograms of PVCL and P(VCL-co-HEAA) copolymer. The second heating cycle of a 177 heat/cool/heat sequence is shown; (c) Transmittance of PVCL and P(VCL-co-HEAA) copolymer in 178 an aqueous solution as a function of temperature. 179 9 Initial concentration of monomers and their reactivity ratios affect compositions of a 180 copolymer. Figure 1 is 1H-NMR spectrum of P(VCL-co-HEAA) copolymer using D2O 181 as the solvent. The molar composition of the copolymer was determined by integrating 182 the peaks corresponding to VCL protons at 4.2 ppm (a) and HEAA protons at 3.5 ppm 183 (b). A feed monomer ratio of 80/20 mol % VCL/HEAA yielded a 51/49 mol % 184 VCL/HEAA in the final copolymer. This is attributed to the low reactivity ratio of PVCL 185 as shown by other authors (Ivan M. Okhapkin, Irina R. Nasimova, Elena E. Makhaeva, 186 & Alexei R. Khokhlov, 2003; Shah, Pal, Gude, & Devi, 2010). The more reactive HEAA 187 monomer was preferentially incorporated into the copolymer even at a lower feeding ratio 188 to VCL. The weight average molecular weight (Mw) of P(VCL-co-HEAA) copolymer 189 was determined by GPC to be 10,0551 g/mol with a polydispersity value of 1.9. 190 The thermal properties of PVCL and P(VCL-co-HEAA) copolymer were analyzed by 191 TGA studies, as shown in Figure 2(a). The initial weight loss was due to the liberation of 192 absorbed moisture. The sharp weight decrease was associated with the thermal 193 degradation of the polymer. The TGA thermograms showed that the synthesized 194 copolymer had a decomposition temperature of up to 420°C. In contrast, PVCL 195 decomposed at 428°C. This suggested that the addition of the hydrophilic comonomer 196 HEAA did not affect the thermal stability of the copolymer remarkably. P(VCL-co-197 HEAA) is thermally stable for the subsequent pad-dry-cure finishing process where the 198 grafting reaction with cellulose was achieved at 160°C. 199 The glass-transition temperatures (Tg) of PVCL and P(VCL-co-HEAA) copolymer were 200 shown in Figure 2(b) and determined to 190°C and 160°C respectively. Introducing the 201 hydrophilic comonomer HEAA into the copolymer led to a decrease in the Tg. This can 202 enhance the crosslinking reaction between cellulose, the crosslinker and the copolymer 203 in that the molecular chains of the copolymer starts to move at temperatures above Tg, 204 leading to an increased contact with cellulose and the crosslinker. 205 Figure 2(c) shows the thermo-responsive behavior of PVCL and the copolymer. The 206 LCST of the polymer was correlated with the temperature dependent phase separation of 207 the polymer and determined by monitoring the optical transmittance change as a function 208 16 Figure 6 shows the weight loss of modified and unmodified cotton fabrics as a function 301 of time at room temperature (21°C) and also at 50°C. The weight loss value was the 302 average of three test fabrics with a standard deviation of less than 1mg. The slope of the 303 curve is the rate of water vapor permeation through the test fabric. Based on equations 304 (2) and (3), the WVP values were calculated and shown in Table 2. The correlation 305 coefficient (R2) are all greater than 0.999, indicating a good linear regression fitting. The 306 WVP studies demonstrated that at room temperature, P(VCL-co-HEAA) copolymer 307 exhibited hydrophilicity and the modified cotton fabrics behaved similarly to the 308 untreated cotton fabric with similar rate of water vapor permeation through the fabric and 309 therefore similar WVP. At 50°C, the copolymer underwent phase transition resulting from 310 aggregation of the chain molecules due to the intermolecular interactions between the 311 hydrophobic groups at elevated temperature. The grafted cotton fabrics switched from 312 hydrophilic to hydrophobic. Therefore, the treated cotton fabrics exhibited lower rate of 313 water vapor permeation and also WVP than the untreated cotton. It is also worthy to point 314 out that 6.7 wt% add-on of the fabric treated with 150g/L of the copolymer was sufficient 315 enough to achieve the same water vapor permeability with the fabric treated with 250g/L 316 of the copolymer. This thermally switchable hydrophilicity of cotton fabric makes it 317 applicable to protective clothing and other smart textiles. 318 319 Table 2. Water vapor permeability of cotton fabrics treated with P(VCL-co-HEAA) copolymer 320 Concentration Water vapor permeability (g/m2/day) Room Temperature (50°C) Slope (mg/h) R2 WVP Slope (mg/h) R2 WVP 0 (control) 150 g/L 0.157 0.151 0.9991 0.9996 695 669 0.417 0.306 0.9984 0.9983 1849 1362 200 g/L 0.157 0.9998 697 0.306 0.9982 1359 250 g/L 0.146 0.9993 649 0.308 0.9915 1367 321 4. Conclusions 322 A thermo-responsive copolymer P(VCL-co-HEAA) was synthesized by free radical 323 polymerization at 60°C. A feed monomer ratio of 80/20 mol %VCL/HEAA yielded a 324 17 51/49 mol % of ratio of VCL/HEAA in the copolymer due to a higher reactivity of HEAA. 325 The copolymer had a glass transition temperature of 160°C and a decomposition 326 temperature of up to 420°C as evidenced by DSC and TGA studies. The LCST value of 327 the copolymer was 34.5°C. Thermo-responsive cotton fabrics were successfully 328 fabricated by a grafting reaction using BTAC as crosslinker and SHP as catalyst. FTIR 329 and EDS studies confirmed the deposition of P(VCL-co-HEAA) copolymer onto cotton. 330 The modified fabrics exhibited temperature-responsive behavior in the water vapor 331 permeability measurements. Compared to the unmodified cotton, 6.7 wt% add-on of the 332 fabric grafted with the copolymer was sufficient enough to exhibit decreased water 333 permeability at elevated temperature due to hydrophilic to hydrophobic transition. This 334 study provides another alternative for fabrication of smart textiles and application in 335 functional clothing. 336 337 Acknowledgements 338 This work was supported by the USDA National Institute of Food and Agriculture, Hatch 339 project NYS-329402 and Hatch multistate project NC-1194 NYC-329816. Any opinions, 340 findings, conclusions, or recommendations expressed in this publication are those of the 341 author(s) and do not necessarily reflect the view of the National Institute of Food and 342 Agriculture (NIFA) or the United States Department of Agriculture (USDA). This work 343 made use of the Cornell Center for Materials Research Shared Facilities which are 344 supported through the NSF MRSEC program (DMR-1120296) and the Cornell NMR 345 facility supported in part by the NSF MRI program (CHE-1531632). This work also made 346 use of the Nanobiotechnology Center shared research facilities at Cornell. 347 348 References 349 Bae, Y. H., Okano, T., Hsu, R., & Kim, S. W. (1987). Thermo‐sensitive polymers as on‐off 350 switches for drug release. Die Makromolekulare Chemie, Rapid Communications, 8(10), 351 481-485. 352 Chen, S.-C., Wu, Y.-C., Mi, F.-L., Lin, Y.-H., Yu, L.-C., & Sung, H.-W. (2004). 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