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Article Preparation, Structure, and Properties of Silk Fabric Grafted with 2-Hydroxypropyl Methacrylate Using the HRP Biocatalyzed ATRP Method

Jinqiu Yang, Shenzhou Lu ID , Tieling Xing * ID and Guoqiang Chen

National Laboratory for Modern Silk, Soochow University, 215123, ; [email protected] (J.Y.); [email protected] (S.L.); [email protected] (G.C.) * Correspondence: [email protected]; Tel.: +86-512-6706-1175

 Received: 13 April 2018; Accepted: 19 May 2018; Published: 21 May 2018 

Abstract: transfer radical polymerization (ATRP) is a “living”/controlled radical polymerization, which is also used for surface grafting of various materials including . However, the commonly used metal complex catalyst, CuBr, is mildly toxic and results in unwanted color for textiles. In order to replace the transition metal catalyst of surface-initiated ATRP, the possibility of HRP biocatalyst was investigated in this work. 2-hydroxypropyl methacrylate (HPMA) was grafted onto the surface of silk fabric using the horseradish peroxidase (HRP) biocatalyzed ATRP method, which is used to improve the crease resistance of silk fabric. The structure of grafted silk fabric was characterized by Fourier transform infrared spectrum, X-ray photoelectron spectroscopy, thermogravimetic analysis, and scanning electron microscopy. The results showed that HPMA was successfully grafted onto silk fabric. Compared with the control silk sample, the wrinkle recovery property of grafted silk fabric was greatly improved, especially the wet crease recovery property. However, the whiteness, breaking strength, and moisture regain of grafted silk fabric decreased somewhat. The present work provides a novel, biocatalyzed, environmentally friendly ATRP method to obtain functional silk fabric, which is favorable for application and has potential for medical materials.

Keywords: horseradish peroxidase; atom transfer radical polymerization; bio-catalysis; silk

1. Introduction Silk is a natural . It is widely used in the field because of its outstanding mechanical strength, wear comfortability, and elegant luster, which has been praised as the queen of [1–3]. However, silk has some shortcomings, such as bad crease recovery property and photo-yellowing stability [4–6]. Silk is very easy to wrinkle during wearing, especially after sweating or washing. This shortage seriously affects the utilization of silk and limits its application scope. In recently years, high-quality and functional silk has be prepared to improve the performance of the end products and satisfy the specific requirements with vinyl monomers using the grafting technique [7,8]. Silk fabric can be grafted with vinyl monomer by conventional radical polymerization through various chemical initiators or by irradiation, and the controlled/living radical polymerization process (CRP). Atom transfer free radical polymerization (ATRP) is the most widely used CRP method [9].The ATRP method is used in mild reaction conditions, which provide well-defined polymers and show high tolerance for monomer structures with a variety of functional groups [10–13]. Surface-initiated ATRP has been applied in materials to provide their wettability, wrinkle resistance, anti-bacterial property, and flame retardancy, etc. Teramoto [14] prepared diacetate-graft-poly(lactic acid)s (CDA-g-PLAs) through ATRP, and the thermal characteristics of cellulose diacetatewaereimproved. Kang [15] synthesized ethyl cellulose-graft-poly(2-hydroxyethyl methacrylate) (EC-graft-PHEMA) copolymers using the ATRP in

Polymers 2018, 10, 557; doi:10.3390/polym10050557 www.mdpi.com/journal/polymers Polymers 2018, 10, x FOR PEER REVIEW 2 of 9 characteristicsPolymers 2018, 10, 557of cellulose diacetatewaereimproved. Kang [15] synthesized ethyl2 of 9 cellulose-graft-poly(2-hydroxyethyl methacrylate) (EC-graft-PHEMA) copolymers using the ATRP in methanol to improve the hydrophilic property of ethyl cellulose. Xing [16] prepared methanol to improve the hydrophilic property of ethyl cellulose. Xing [16] prepared multi-functional silk multi-functional silk with flame retardancy and antibacterial properties using flame retardant with flame retardancy and antibacterial properties using flame retardant dimethyl methacryloyloxyethyl dimethyl methacryloyloxyethyl phosphate (DMMEP) as the first monomer and dimethylaminoethyl phosphate (DMMEP) as the first monomer and dimethylaminoethyl methacrylate (DMAEMA) as the methacrylate (DMAEMA) as the second monomer using the ATRP method. second monomer using the ATRP method. However, a major disadvantage of the ATRP method is the usage of the transition metal However, a major disadvantage of the ATRP method is the usage of the transition metal complex complex catalyst, which is usually used inrelatively large amounts. The commonly used metal catalyst, which is usually used inrelatively large amounts. The commonly used metal catalyst, CuBr, catalyst, CuBr, is mildly toxic and renders ATRP environmentally unfavorable. The copper ion is mildly toxic and renders ATRP environmentally unfavorable. The copper ion residuals are difficult to residuals are difficult to remove from the materials and hinder the application of the remove from the polymer materials and hinder the application of the resulting polymers in biomedical resulting polymers in biomedical and food fields [17,18]. Meanwhile, the colored catalyst is easy to and food fields [17,18]. Meanwhile, the colored catalyst is easy to stain the grafted materials and stain the grafted materials and results in the unwanted color for textiles. results in the unwanted color for textiles. can be an alternative to the transition metal catalyst because of their non-toxicity and Enzymes can be an alternative to the transition metal catalyst because of their non-toxicity eco-friendly nature [8,19]. A lot of enzymes have catalytic active sites comprising metal ions like and eco-friendly nature [8,19]. A lot of enzymes have catalytic active sites comprising metal ions peroxidase. Horseradish peroxidase (HRP; EC1.11.1.7) is a kind of heme protein, containing active like peroxidase. Horseradish peroxidase (HRP; EC1.11.1.7) is a kind of heme protein, containing sites on the ferric protoporphyrin ring. Recently, HRP-catalyzed ATRP has been used for the active sites on the ferric protoporphyrin ring. Recently, HRP-catalyzed ATRP has been used for the synthesis of polymers. Sigget al. [20] catalyzed N-isopropyl-acrylamide using activators generated synthesis of polymers. Sigget al. [20] catalyzed N-isopropyl-acrylamide using activators generated by electron transfer for the ATRP (ARGET ATRP) method using HRP. This method allows an ATRP by electron transfer for the ATRP (ARGET ATRP) method using HRP. This method allows an ATRP process to be conducted with a tiny amount of transition metal catalyst in thepresence of excess process to be conducted with a tiny amount of transition metal catalyst in thepresence of excess reducing agent such as ascorbic acid, which could effectively scavenge and remove dissolved reducing agent such as ascorbic acid, which could effectively scavenge and remove dissolved oxygen oxygen from the polymerization system. Renggli [21] obtained a protein cage nano reactor using from the polymerization system. Renggli [21] obtained a protein cage nano reactor using HRP as HRP as biocatalyst, which was further polymerized with an acrylate. These works provided the biocatalyst, which was further polymerized with an acrylate. These works provided the foundation foundation for HRPcatalyzed grafting using the surface-initiated ATRP method. for HRPcatalyzed grafting using the surface-initiated ATRP method. The active group of HRP contains a ferric protoporphyrin structure to catalyze ATRP, which is The active group of HRP contains a ferric protoporphyrin structure to catalyze ATRP, which is equivalent to the CuBr/ligand metal complex catalyst. The schematic of silk fabric grafted with equivalent to the CuBr/ligand metal complex catalyst. The schematic of silk fabric grafted with monomers using the HRP-catalyzed ATRP method is shown in Figure 1. monomers using the HRP-catalyzed ATRP method is shown in Figure1.

FigureFigure 1. SchematicSchematic ofof silk silk grafted grafted with with monomers monomers using using HRP HRP biocatalytic biocatalytic ATRP (MATRP = vinyl (M monomer,= vinyl monomer,Mn = polymer M withn = npolymer repeat units, with NaAsc n =repeat sodium units ascorbate,, NaAsc DHA = = dehydroascorbicsodium ascorbate, acid). DHA = dehydroascorbic acid). In this work, HRP was used as biocatalyst for the ATRP grafting of 2-hydroxypropyl methacrylate (HPMA)In this on silkwork, surface HRP towas improve used theas wrinklebiocatalyst resistance for the of silkATRP fabric. grafting Sodiumascorbate of 2-hydroxypropyl (NaAsc) methacrylatewith higher water(HPMA) solubility on silk than surface ascorbic to acid impr wasove used the as thewrinkle reducing resistance agent inof ARGET silk fabric. ATRP. SodiumascorbateThe structure and (NaAsc) properties with of thehigher grafted water silk solubilit fabric werey than investigated. ascorbic acid was used as the reducing agent in ARGET ATRP. The structure and properties of the grafted silk fabric were investigated. 2. Materials and Methods 2. Materials and Methods 2.1. Materials and Reagents 2.1. MaterialsDegummed and Reagents silk fabric, with a 36 g/m2 density, was purchased from Suzhou Huajia Silk Group. 2-HydroxypropylDegummed silk methacrylate fabric, with (HPMA)a 36 g/m2 wasdensity, purchased was purchased from Macklin. from Suzhou Triethylamine Huajia Silk (TEA) Group. and 2-Hydroxypropyltetrahydrofuran (THF) methacrylate were distilled (HPMA) under was reduced purchased pressure from beforeMacklin. use. Triethylamine Horseradish (TEA) peroxidase and tetrahydrofuran (THF) were distilled under reduced pressure before use. Horseradish peroxidase

Polymers 2018, 10, 557 3 of 9

(HRP; EC1.11.1.7) was supplied by Aladdin Reagent and stored at −20 ◦. All other reagents in this study were used without further purification.

2.2. Preparation of the Silk Macroinitiator The silk fabric (1 g) reacted with 2-bromoisobutyryl bromide (BriB-Br) (2.22 mL) in the presence ofTEA (1.23 mL) and 4-(dimethylamino) pyridine (DMAP, 0.5 g). The mixture was stirred at 10 ◦C for 1 h, then warmed up to 50 ◦C for 24 h. The silk sample was thoroughly washed with water and finally dried at 60 ◦C in vacuum oven [10,15]. Thus, the silk-Br macroinitiator was prepared.

2.3. Surface-Initiated ATRP

2.3.1. HRP-Mediated Grafting of HPMA on Silk Fabric’s Surfaces The silk-Br macroinitiator (1 g) was incubated with 50 mL of phosphate buffer (1/15 M, pH 8.0), containing a certain volume of monomer (HPMA), L-sodiumascorbate (NaAsc), and HRP in a 100 mL round-bottom flask ([HPMA] = 0.16 mol/L, [HRP] = 0.027 mmol/L, n(HRP):n(NaAsc) = 1:150). Then, the flask was evacuated and filled with nitrogen for three times. The mixture was vibrated in the water bath at 60 ◦C for certain time (Sample b: 8 h, Sample c: 16 h, Sample d: 24 h). Then, HRP catalyzed silk-grafted-poly(HPMA) (HC-silk-g-PHPMA) sample was obtained and washed with methyl alcohol and water, and finally dried at room temperature under vacuum to a constant weight.

2.3.2. CuBr-Mediated Grafting of HPMA on Silk Fabric’s Surfaces The silk-Br macroinitiator (1 g) was immersed into a reaction mixture containing certain HPMA, CuBr/PMDETA(N,N,N0,N00,N00-pentamethyldiethylenetriamine), and 50 mL of deionized water in a 100 mL round-bottom flask ([HPMA] = 0.24 mol/L, [CuBr] = 0.16 mmol/L, n(PMDETA):n(CuBr) = 2:1). After sealing it with a polytetrafluoroethylene three-way stopcock, the flask was evacuated and flushed with nitrogen, which was repeated three times. The mixture was placed in water bath and polymerized under oscillation at 80 ◦C for 6 h. The sample was rinsed with dilutedhydrochloric acid to remove the blue color caused by CuBr, and then washed with acetylacetone/ethanol (volume ration: 1/5) and water, and dried under a vacuum oven. Thus, CuBr/PMDETA catalyzed silk-grafted-poly(HPMA) (CC-silk-g-PHPMA) sample was obtained.

2.3.3. Grafting Yield Calculation Grafting yield was calculated as follows:

w − w Grafting yield (%) = 2 1 × 100 (1) w1

in which w1 and w2 denote the weight of the control silk and the PHPMA grafted silk fabric, respectively.

2.4. Characterization and Measurements

2.4.1. Fourier Transform Infrared (FT-IR) Analysis The FTIR spectra were recorded using a Nicolet5700 FTIR (Nicolet Co., Madison, WI, USA). The scan range was from 4000 to 400 cm−1.

2.4.2. X-ray Diffraction (XRD) Analysis XRD patterns were obtained at a scanning rate of 1◦/min using an X’Pert PRO MPD diffractometer (Holland Panalytical, Almelo, Holland). The voltage and current of the X-ray source were 40 kV and 30 mA, respectively. Polymers 2018, 10, 557 4 of 9

2.4.3. X-ray Photoelectron Spectroscopy (XPS) Analysis X-ray photoelectron spectroscopy (XPS) was carried out on a Thermo ESCALAB 250 X-ray photoelectron spectroscopy(Thermo Fisher Scientific, Waltham, MA, USA) using Al Ka (1486.6 eV) excitatior with pass energy of 20 eV at a reduced power of 150 W. The samples were attached to the spectrometer probe with double-sided tape, and the X-ray beam was 500 µm.

2.4.4. Thermal Properties Thermogravimetic analysis (TGA) measurements were performed on a Diamond 5700 thermal analyzer at a heating rate of 10 ◦C/min with a temperature range from 40 to 600 ◦C. The open aluminum cell was swept with N2 during the analysis.

2.4.5. Scanning Electron Microscopy (SEM) Analysis Morphology of the silk samples was observed at 3.00 k magnification by a Hitachi TM3030 Desktop SEM (Hitachi TM3030, Hitachi Ltd., Tokyo, ) at an acceleration voltage of 3 kV under vacuum condition. The samples were mounted on a conductive adhesive tape and coated with before testing.

2.4.6. Crease-Resistant Recovery and Physical Properties Measurement The wrinkle recovery angle of the fabric was measured according to AATCC66-2003 (Wrinkle Recovery of : Recovery Angle). Each result was the average of six measurements. The tensile strength of the fabric was tested by an Instron 3365 Universal Testing Machine (IllinoisTool Works Inc., High Wycombe, Buckinghamshire, UK) according to ISO 13934-1-2013. The sample was cut into the size of 30 cm × 5 cm and the average value was obtained after 5 times tests. The whiteness of silk fabric was measured by WSD III whiteness instrument (Wenzhou Darong Textile Instrument Co., Ltd., Wenzhou, China), and the result was the average of eight measurements. Moisture regain (MR) was evaluated in the standard conditions at 20 ◦C and 65% relative humidity (RH) (ISO 2060: Determination of moisture content and moisture regain of textile-oven-drying method, 1994).MR was calculated according to the following equation:

m − m MR (%) = 1 0 × 100 (2) m0 in which m0 is weight of dried fabric, and m1 is weight of moist fabric.

3. Results and Discussion

3.1. Fourier Transform Infrared (FTIR)Spectra and X-ray Diffraction (XRD) Curves Figure2 shows the FTIR spectra and XRD curves of the silk samples before and after grafting. The FTIR spectra (Figure2A) of the silk all show the characteristic absorption peaks of silk fibroin −1 −1 −1 including amide IυC=O 1726 cm , amide IIδN-H 1623 cm , and amide IIIυC-N1260 cm . Compared with the control silk fabric, additional peak appeared at 1726 cm−1 for HC-silk-g-PHPMA, which is characteristic absorption peak of carbonyl stretching vibration of ester, indicating that the HPMA monomers were successfully grafted onto silk fabric.XRD patterns are analyzed as shown in Figure2B. It could be seen that control silk fabric and the grafted silk fabric all exhibited a major X-ray diffraction peak at 20.5◦, which is characteristic peak of silk with highly ordered β-structure [4]. The position and intensity of the major X-ray diffraction peak did not change regardless of the grafting. That is to say, the grafting has no effect on the crystalline region of silk fibers, and it is also reasonable to assume that the grafting is not harsh and causes no damage to the crystalline region of silk fibers. Polymers 2018, 10, 557 5 of 9 Polymers 2018, 10, x FOR PEER REVIEW 5 of 9

esterνc=o amideⅡδN-H 20.5° Polymers 1726 2018 , 10 , x 1520 FOR PEER REVIEW 5 of 9 d

esterνc=o amideⅡδN-H 20.5° 1726 1520 dc

b d

c Transmittance Intensity c ba d b Transmittance Intensity ca a 1623 1260 b amideⅠνc=o amide ⅢνC-N 1800 1600 1400 1200 1000 800 a -1 10 20 30 40 1623 Wavenumbers 1260 /cm 2θ/° amideⅠνc=o amide( AⅢ) νC-N (B) 1800 1600 1400 1200 1000 800 -1 10 20 30 40 Figure 2.FTIR Wavenumbersspectra (A) /cmand XRD curves (B) of (a) the control silkθ/° fabric, (b) 15.62% of Figure 2. FTIR spectra (A) and XRD curves (B) of (a) the control2 silk fabric, (b) 15.62% of HC-silk-g-PHPMA, (c)(A 29.01%) of HC-silk-g-PHPMA, and (d) 38.87% of HC-silk-g-PHPMA.(B) HC-silk-g-PHPMA, (c) 29.01% of HC-silk-g-PHPMA, and (d) 38.87% of HC-silk-g-PHPMA. Figure 2.FTIR spectra (A) and XRD curves (B) of (a) the control silk fabric, (b) 15.62% of 3.2. X-ray Photoelectron Spectroscopy (XPS) 3.2. X-rayHC-silk-g-PHPMA, Photoelectron Spectroscopy (c) 29.01% of (XPS) HC-silk-g-PHPMA, and (d) 38.87% of HC-silk-g-PHPMA. The XPS C1s spectra of silk fabric are shown in Figure 3. The C1s spectrum of the control silk The3.2.fabric X-ray XPS contains Photoelectron C1sspectra three distinct Spectroscopy of silk peaks fabric (XPS) at are284.5 shown (C–C), in 286.0 Figure (C–OH/C–N),3. The C1s and spectrum 288.1 eV of (N–C=O) the control [22], silk while the C1s spectrum of HC-silk-g-PHPMA contains three distinct peaks at 284.5 (C–C), 286.3 fabric containsThe XPS three C1s distinctspectra of peaks silk fabric at 284.5 are shown (C–C), in 286.0 Figure (C–OH/C–N), 3. The C1s spectrum and 288.1 of the eV control (N–C=O) silk [22], (C–OH), and 288.6 eV (O–C=O). The C–OH (286.3 eV) and O–C=O (288.6 eV) mainly originated from whilefabric the C1s contains spectrum three of distinct HC-silk-g-PHPMA peaks at 284.5 contains(C–C), 286.0 three (C–OH/C–N), distinct peaks and at 288.1 284.5 eV (C–C), (N–C=O) 286.3 [22], (C–OH), the hydroxyl and ether of HPMA. Table 1 liststhe -to-nitrogen (C/N) ratio of silk fabric, which and 288.6while the eV C1s (O–C=O). spectrum The of HC-silk-g-PHPMA C–OH (286.3 eV) contai andns O–C=O three distinct (288.6 peaks eV) mainlyat 284.5 originated(C–C), 286.3 from increased after grafting with HPMA. Less N element content was detected for the grafted silk the hydroxyl(C–OH), and and 288.6 ether eV (O–C=O). of HPMA. The Table C–OH1 (286.3liststhe eV) carbon-to-nitrogen and O–C=O (288.6 eV) (C/N) mainly ratio originated of silk from fabric, sample compared with the control silk fabric. The reason was that the surface of the grafted silk the hydroxyl and ether of HPMA. Table 1 liststhe carbon-to-nitrogen (C/N) ratio of silk fabric, which whichfabric increased was covered after graftingby PHPMA with poly HPMA.mer, which Less mainly N element contains content C, O, and was H detected elements. forN element the grafted increased after grafting with HPMA. Less N element content was detected for the grafted silk silk samplewas weakened compared after with grafting, the control further silkindicating fabric. th Theat monomers reason was were that successfully the surface grafted of the onto grafted the silk sample compared with the control silk fabric. The reason was that the surface of the grafted silk fabricsilk was fabric covered using byHRP PHPMA biocatalytic polymer, ATRP method. which mainly contains C, O, and H elements. N element fabric was covered by PHPMA polymer, which mainly contains C, O, and H elements. N element was weakened after grafting, further indicating that monomers were successfully grafted onto the silk was weakened8000 after grafting, further indicating that monomers8000 were successfully grafted onto the b fabric using HRP biocatalytic BE/ eV FWHM/eV ATRP % method. a BE/eV FWHM/eV % silk fabric using HRP biocatalytic C-CATRP method. C-C 284.5 1.21 69.3 C-C 284.5 1.21 71.2 C-OH/C-N 286.0 1.20 20.0 C-OH 286.3 1.28 23.5 C-C O-C=O 288.6 1.02 5.3 80006000 N-C=O 288.1 1.17 10.7 80006000 a b BE/ eV FWHM/eV % C-C BE/eV FWHM/eV % C-C 284.5 1.21 69.3 C-C 284.5 1.21 71.2 C-OH/C-N 286.0 1.20 20.0 C-OH 286.3 1.28 23.5 C-C O-C=O 288.6 1.02 5.3 60004000 N-C=O 288.1 1.17 10.7 60004000

Counts / s C-OH C-OH Counts / / s Counts N-C=O 40002000 40002000 O-C=O

Counts / s C-OH C-OH Counts / / s Counts N-C=O 20000 20000 O-C=O 290 288 286 284 282 290 288 286 284 282 Binding energy / eV Binding energy / eV 0 0 Figure 2903.The curve 288 fitting 286 analysis 284 of C1s 282 spectra of (a) the290 control 288 silk fabric 286 and 284 (b) 38.87% 282 ofHC-silk-g-PHPMA.Binding energy / eV Binding energy / eV Figure 3.The curve fitting analysis of C1s spectra of (a) the control silk fabric and (b) 38.87% FigureTable 3. The1. curveThe element fitting analysisweight ofpercent C1s spectraof (a) ofth (ea ) thecontrol control silk silkfabric fabric and and (b) ( b)38.87% 38.87% of ofHC-silk-g-PHPMA. HC-silk-g-PHPMA.ofHC-silk-g-PHPMA. Table 1. The element weight percent Elementof (a) contentthe control/% silk fabric and (b) 38.87% Silk sample C/N TableofHC-silk-g-PHPMA. 1. The element weight percent of (a)C the controlO silk fabric andN (b) 38.87% ofHC-silk-g-PHPMA. Control 69.98 24.08 5.94 11.78 Element content/% silk-g-PHPMASilk sample 71.20Element 26.31 Content/% 2.49 28.59C/N Silk Sample C O N C/N Control CON69.98 24.08 5.94 11.78

Controlsilk-g-PHPMA 69.9871.20 26.31 24.08 2.49 5.94 28.59 11.78 silk-g-PHPMA 71.20 26.31 2.49 28.59

Polymers 2018, 10, 557 6 of 9

3.3. ThermalPolymers Properties 2018, 10, x FOR PEER REVIEW 6 of 9

Figure3.3. Thermal4 shows Properties the TG (A) and DTG (B) curves of the silk fabric. It can be seen from Figure4A that the evaporationPolymers 2018 of, 10 the, x FOR absorbed PEER REVIEW moisture caused slight weight loss of silk fabric when temperature6 of 9 was Figure 4 shows the TG (A) and DTG (B) curves of the silk fabric. It can be seen from Figure 4A less than 200 ◦C. The weight loss ratio of the control silk fabric was 32.5% when the weight loss rate 3.3.that Thermal the evaporation Properties of the absorbed moisture caused slight weight loss of silk fabric when ◦ reachedtemperature its highest was point less at than 320 200C, °C. which The weight could loss be ratio attributed of the control to the silk decomposition fabric was 32.5% of when silk fabricthe into Figure 4 shows the TG (A) and DTG (B) curves of the silk fabric. It can be seen from Figure 4A small moleculesweight loss including rate reached CO itsand highest H O. point The at decomposition 320 °C, which could process be attributed of the grafted to the decomposition silk fabric contained that the evaporation of 2the absorbed2 moisture caused slight weight loss of silk fabric when of silk fabric into small molecules including CO2 and H2O. The decomposition process◦ of the grafted two stages,temperature and the was weight less than loss 200 rate °C. reached The weight its highestloss ratio pointof the atcontrol the firstsilk fabric stage was 331 32.5%C and when second the stage ◦ silk fabric contained two stages, and the weight loss rate reached◦ its highest point at the first stage 410 Cweight (for 29.01% loss rate of reached HC-silk-g-PHPMA), its highest point at and 320 335°C, which and 416couldC be (forattributed 38.87% to the of HC-silk-g-PHPMA),decomposition 331 °C and second stage 410 °C (for 29.01% of HC-silk-g-PHPMA),◦ and 335 and 416 °C(for 38.87% of respectively.of silk Fromfabric into Figure small4B, molecules the peaks including at 331 CO (b,2 DTG)and H2 andO. The 335 decompositionC (c, DTG) process of HC-silk-g-PHPMA of the grafted in HC-silk-g-PHPMA), respectively. From Figure 4B, the peaks at 331 (b, DTG) and 335 °C (c, DTG) of silk fabric contained two stages, and the weight loss rate reached its highest point at the first stage the firstHC-silk-g-PHPMA stage were caused in bythe thefirst decomposition stage were caused of silkby the fibroin, decomposition which corresponded of silk , towhich the peak at ◦ 331 °C and second stage 410 °C (for 29.01% of HC-silk-g-PHPMA), and 335 and 416 °C(for 38.87% of 320 C (a,corresponded DTG) of control to the peak silk at fabric. 320 °C At (a, the DTG) second of cont stage,rol silk the fabric. grafted At the silk second fabric stage, had the additional grafted peaks HC-silk-g-PHPMA), respectively. From Figure 4B, the peaks at 331 (b, DTG) and 335 °C (c, DTG) of at 410 andsilk 416fabric◦C, had which additional can bepeaks explained at 410 and by 416 the °C, decomposition which can be explained of poly(HPMA) by the decomposition [23]. When of heated HC-silk-g-PHPMA in the first stage were caused by the decomposition of silk fibroin, which poly(HPMA)◦ [23]. When heated up to 600 °C, the final residual of the control silk fabric (28.5%) was up to 600correspondedC, the final to the residual peak at of 320 the °C control (a, DTG) silk of fabriccontrol (28.5%)silk fabric. was At morethe second than stage, that ofthe the grafted grafted silk more than that of the grafted silk fabric (15.4%), which may be due to the decomposition of the fabric (15.4%),silk fabric which had additional may be due peaks to at the 410 decomposition and 416 °C, which of can the be poly-(HPMA) explained by the into decomposition small molecules of such poly-(HPMA) into small molecules such as CO2 and H2O. This phenomenon further indicated the poly(HPMA) [23]. When heated up to 600 °C, the final residual of the control silk fabric (28.5%) was as CO2 andmonomers H2O. were This successfully phenomenon grafted further onto the indicated silk fabric the using monomers HRP-catalyzed were atom successfully transfer radical grafted onto more than that of the grafted silk fabric (15.4%), which may be due to the decomposition of the the silkpolymerization fabric using HRP-catalyzed method. atom transfer radical polymerization method. poly-(HPMA) into small molecules such as CO2 and H2O. This phenomenon further indicated the monomers were successfully grafted onto the silk fabric using HRP-catalyzed atom transfer radical 100 polymerization method. 90 80 a 100 %

/ 70 90 60 80 ab 50 320 dW/ dt dW/ %

/ 70 40 410 60 c

weight retention retention weight 30 a b 50 320335 20 b dt dW/ 416 40 c 410 10 c 331 weight retention retention weight 30 a 100 200 300 400 500 600 100 200 300335 400 500 600 20 Temperature / ℃ b Temperature /℃416 c 10 (A) (B331) 100 200 300 400 500 600 100 200 300 400 500 600 Figure 4. TG curvesTemperature (A) and / ℃ DTG curves ( B) of (a) the control Temperaturesilk fabric, /℃ (b) 29.01% of Figure 4. TG curves (A) and DTG curves (B) of (a) the control silk fabric, (b) 29.01% of HC-silk-g-PHPMA, HC-silk-g-PHPMA, and(A )(c) 38.87% of HC-silk-g-PHPMA. (B) and (c) 38.87% of HC-silk-g-PHPMA. 3.4. ScanningFigure 4. Electron TG curves Microscopy (A) and (SEM) DTG curves (B) of (a) the control silk fabric, (b) 29.01% of HC-silk-g-PHPMA, and (c) 38.87% of HC-silk-g-PHPMA. 3.4. ScanningFigure Electron 5 shows Microscopy the morphology (SEM) of silk fabrics. The control silk fabric had a smooth and uniform appearance in portrait orientation. The surfaces of the grafted silk fabric were covered with PHPMA Figure3.4. Scanning5 shows Electron the morphology Microscopy (SEM) of silk fabrics. The control silk fabric had a smooth and uniform and became tough. Moreover, the surfaces of the silk fabric were rougher as the grafting yield of the Figure 5 shows the morphology of silk fabrics. The control silk fabric had a smooth and uniform appearancesilk fabric in portrait became orientation.higher. The surfaces of the grafted silk fabric were covered with PHPMA and becameappearance tough. in Moreover,portrait orientation. the surfaces The surfaces of the of silk the fabricgrafted weresilk fabric rougher were covered as the grafting with PHPMA yield of the and became tough. Moreover, the surfaces of the silk fabric were rougher as the grafting yield of the silk fabric became higher. silk fabric became higher.

Figure 5. Cont. Polymers 2018, 10, 557 7 of 9

Polymers 2018, 10, x FOR PEER REVIEW 7 of 9

Figure 5.SEM images of (a) the control silk fabric, (b) 29.01% the HC-silk-g-PHPMA, and (c) 38.87% the FigureHC-silk-g-PHPMA. 5. SEM images of (a) the control silk fabric, (b) 29.01% the HC-silk-g-PHPMA, and (c) 38.87% the HC-silk-g-PHPMA. 3.5. Crease-Resistant Recovery 3.5. Crease-ResistantCompared Recovery with the control silk fabric, the crease-resistant recovery properties of the silk fabric grafted with HPMA using HRP biocatalys is and metal complex catalysis method were both Compared with the control silk fabric, the crease-resistant recovery properties of the silk fabric improved (Table 2). With the increase of grafting yield, the dry crease recovery angle (DCRA) and graftedwet with crease HPMA recovery using HRP angle biocatalys (WCRA) isof and HC-silk-g-PHPMA metal complex catalysisincreased method up to were18.41% both and improved (Table2).51.56%,respectively.This With the increase of can grafting be attributed yield, theto tw dryo reasons: crease recovery(1) The copoly anglemerization (DCRA) reaction and wet of crease recoverymonomer angle (WCRA) and silk offabric HC-silk-g-PHPMA occurred in amorphous increased area up of tosilk 18.41% fiber, andwhich 51.56%, weaken respectively. the hydrogen This can be attributedbonding to twoin amorphous reasons: (1)area The and copolymerization decreased the creep reaction deformation of monomer and permanent and silk deformation fabric occurred in amorphouscaused area by breaking of silk fiber, up of which hydrogen weaken bonding; the (2) hydrogen The grafting bonding copolymerization in amorphous in amorphous area and area decreased limited relative slippage of silk macromolecules. Additionally, the WCRA of grafted silk fabric the creepincreased deformation higher than and that permanent of the DRCA, deformation which is favorable caused for by silk breaking fabric easy up to of wrinkle hydrogen at wet bonding; (2) The graftingstate. This copolymerization result can be attributed in amorphous to the occurrence area limited of grafting relative under slippage wet state. of The silk DCRA macromolecules. and Additionally,WCRA of the CC-silk-g-PHPMA WCRA of grafted also have silk the fabric same increased increase trend higher with HC-silk-g-PHPMA. than that of the DRCA, which is favorable for silk fabric easy to wrinkle at wet state. This result can be attributed to the occurrence of grafting under wetTable state. 2.The CRA The of DCRA silk fabric and with WCRA different ofgrafting CC-silk-g-PHPMA yield under dry and alsowet conditions. have the same increase trend with HC-silk-g-PHPMA.Grafting samples Grafting yield/% DCRA/° WCRA/° 0 201 128 15.62 220 168 Table 2. The CRA of silk fabric with different grafting yield under dry and wet conditions. HC-silk-g-PHPMA 29.01 233 180 38.87 238 194 ◦ ◦ CC-silk-g-PHPMAGrafting Samples Grafting37.82 Yield/% DCRA/ 229 WCRA/ 195 0 201 128 3.6.Physical Properties 15.62 220 168 ComparedHC-silk-g-PHPMA with control silk fabric, HC-silk-g-PHPMA29.01 and 233 CC-silk-g-PHPMA 180 were somewhat damaged in the whiteness index and breaking38.87 strength (Table 238 3). For HC-silk-g-PHPMA, 194 the inactive enzymes adhered to the surface of silk fabrics reduced the whiteness of silk fabrics. For CC-silk-g-PHPMA 37.82 229 195 CC-silk-g-PHPMA, the colored CuBr stained the silk fabric and also caused the decrease of whiteness. The mechanical properties of fibers partly depend on the orientation of macromolecule. 3.6. PhysicalAfter Propertiesgrafting with HPMA, the orientation of macromolecule of silk fiber was changed, and the breaking strength decreased. The balance moisture regain of grafted silk fabrics reduced because the Comparedhydrophilic with groups control of silk silk fabric fabric, surface HC-silk-g-PHPMA were covered by the andpolymers. CC-silk-g-PHPMA The physical properties were somewhat of damagedHC-silk-g-PHPMA in the whiteness and index CC-silk-g-PHPMA and breaking strengthwere basically (Table 3the). Forsame, HC-silk-g-PHPMA, but the advantage the of inactive enzymesHC-silk-g-PHPMA adhered to the surface is the usage of silk of fabricsHRP biocatalyst. reduced the whiteness of silk fabrics. For CC-silk-g-PHPMA, the colored CuBr stained the silk fabric and also caused the decrease of whiteness. The mechanical properties of fibers partly depend on the orientation of macromolecule. After grafting with HPMA, the orientation of macromolecule of silk fiber was changed, and the breaking strength decreased. The balance moisture regain of grafted silk fabrics reduced because the hydrophilic groups of silk fabric surface were covered by the polymers. The physical properties of HC-silk-g-PHPMA and CC-silk-g-PHPMA were basically the same, but the advantage of HC-silk-g-PHPMA is the usage of HRP biocatalyst. Polymers 2018, 10, 557 8 of 9

Table 3. Whiteness, breaking strength, and moisture regain of silk fabrics.

Grafting Samples Grafting Yield/% Whiteness/% Breaking Strength/N Moisture Regain/% 0 79.02 479.74 8.45 15.62 75.68 428.25 8.03 HC-silk-g-PHPMA 29.01 74.26 415.36 7.96 38.87 70.39 391.59 7.78 CC-silk-g-PHPMA 37.82 70.23 396.86 7.98

4. Conclusions In conclusion, silk fabric was successfully grafted with 2-hydroxypropyl methacrylate (HPMA) using the HRP-mediated ATRP method. The structure of control silk and grafted silk fabric was characterized by Fourier transform infrared, XRD, XPS, TG, and SEM. The results indicated HPMA was grafted onto the surface of silk fabric, and the copolymerization occurred in the amorphous region of silk fabric. Compared with the control sample, the grafted silk fabric showed greatimprovement in the crease-resistant recovery property, especially in the wet crease recovery angle. However, the whiteness, breaking strength, and moisture regain of grafted silk fabric decreased.In comparison with HC-silk-g-PHPMA and CC-silk-g-PHPMA, its properties were nearly the same, and the biocatalyst HRP was applied in the preparation of HC-silk-g-PHPMA. Consequently, this work provides a biocatalyzed ATRP method with which to obtain functionalsilk fabric, which might realize the potential application of the ATRP grafting method in textile and medical materialsmodification.

Author Contributions: Tieling Xing and Guoqiang Chen conceived and designed the experiments; Jinqiu Yang and Shenzhou Lu performed the experiments and analyzed the data; Jinqiu Yang and Tieling Xing wrote the paper. All authors discussed the results and improved the final text of the paper. Acknowledgments: The work is supported by NaturalScience Foundation of Province (BK20151242, BK20171239), National Nature Science Foundationof China (51741301), the Six Talent Peaks Project of Jiangsu Province (JNHB-066), and Priority Academic Program Development of Jiangsu Higher Institutions (PAPD). Conflicts of Interest: The authors declare no conflict of interest.

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