polymers 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 Engineering Laboratory for Modern Silk, Soochow University, Suzhou 215123, China;
[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: Atom transfer radical polymerization (ATRP) is a “living”/controlled radical polymerization, which is also used for surface grafting of various materials including textiles. 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 clothing application and has potential for medical materials. Keywords: horseradish peroxidase; atom transfer radical polymerization; bio-catalysis; silk
1. Introduction Silk is a natural protein fiber. It is widely used in the textile field because of its outstanding mechanical strength, wear comfortability, and elegant luster, which has been praised as the queen of fibers [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 cellulose 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
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Polymers 2018, 10, 557of 2 of 9 characteristics cellulose diacetatewaereimproved. Kang [15] synthesized ethyl 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 polymer 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. Enzymes 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 Figure 1.
Figure 1. Schematic grafted with monomers usingusing HRP HRP biocatalytic ATRP (M = vinyl Figure Schematicofofsilk silk grafted with monomers biocatalytic ATRP (M monomer, = vinyl Mn = polymer repeat units, NaAsc sodiumunits ascorbate, DHA== dehydroascorbic acid). monomer, Mwith n = npolymer with n =repeat , NaAsc sodium ascorbate, DHA =
dehydroascorbic acid).
In this work, HRP was used as biocatalyst for the ATRP grafting of 2-hydroxypropyl methacrylate In work, HRPtowas used the as wrinkle biocatalyst for the ATRP grafting of 2-hydroxypropyl (HPMA)this on silk surface improve resistance of silk fabric. Sodiumascorbate (NaAsc) methacrylate (HPMA) on silk surface to improve the wrinkle resistance silk fabric. with higher water solubility than ascorbic acid was used as the reducing agent inofARGET ATRP. Sodiumascorbate with higher water solubility thaninvestigated. ascorbic acid was used as the reducing The structure and(NaAsc) properties of the grafted silk fabric were 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. Materials and Reagents Degummed silk fabric, with a 36 g/m2 density, was purchased from Suzhou Huajia Silk Group. 2-Hydroxypropyl methacrylate purchased from Macklin. Triethylamine (TEA) and Degummed silk fabric, with (HPMA) a 36 g/m2was density, was purchased from Suzhou Huajia Silk Group. tetrahydrofuran (THF) were distilled under pressure use.Triethylamine Horseradish (TEA) peroxidase 2-Hydroxypropyl methacrylate (HPMA) wasreduced purchased from before Macklin. and tetrahydrofuran (THF) were distilled under reduced pressure before use. Horseradish peroxidase
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(HRP; EC1.11.1.7) was supplied by Aladdin Reagent and stored at −20 ◦ C. 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: Grafting yield (%) =
w2 − w1 × 100 w1
(1)
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.
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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 adhesive 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, Japan) at an acceleration voltage of 3 kV under vacuum condition. The samples were mounted on a conductive adhesive tape and coated with gold 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 Woven Fabric: 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: MR (%) =
m1 − m0 × 100 m0
(2)
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 Figure 2 shows the FTIR spectra and XRD curves of the silk samples before and after grafting. The FTIR spectra (Figure 2A) of the silk all show the characteristic absorption peaks of silk fibroin including amide IυC=O 1726 cm−1 , amide IIδN-H 1623 cm−1 , and amide IIIυC-N 1260 cm−1 . 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 Figure 2B. 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.
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esterνc=o amideⅡδN-H
20.5°
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d 20.5°
dc
Intensity Intensity
Transmittance Transmittance
esterνc=o amideⅡδN-H 1520 1726
cb ba
a
1260 amide ⅢνC-N
1623 amideⅠνc=o
1800
1600 1623 amideⅠνc=o
1800
d c d b ca b
1400 1200 1000 -1 Wavenumbers /cm 1260
a
800
10
20
amide(A) ⅢνC-N
1600
2θ/°
30
40
(B)
1400 1200 1000 800 10 20 30 40 -1 Figure 2.FTIR Wavenumbers spectra (A) /cm and XRD curves (B) of (a) the control silk fabric, (b) 15.62% of 2θ/°
Figure 2. FTIR spectra (A) and XRD curves (B) of (a) the control 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 HC-silk-g-PHPMA, (c) 29.01% of HC-silk-g-PHPMA, and (d) 38.87% of HC-silk-g-PHPMA.
3.2. X-ray Photoelectron Spectroscopy (XPS)
3.2. X-ray Photoelectron Spectroscopy (XPS)
The XPS C1s spectra of silk fabric are shown in Figure 3. The C1s spectrum of the control silk
3.2. X-ray Photoelectron Spectroscopy (XPS) fabric contains three distinct at are 284.5 (C–C),in286.0 (C–OH/C–N), and 288.1 eV of (N–C=O) [22], silk The XPS C1s spectra of silkpeaks fabric shown Figure 3. The C1s spectrum the control while the C1s spectrum of HC-silk-g-PHPMA contains three distinct peaks at 284.5 (C–C), 286.3 fabric contains three peaks at 284.5 (C–C), in 286.0 (C–OH/C–N), and 288.1 eVcontrol (N–C=O) The XPS C1s distinct spectra of silk fabric are shown Figure 3. The C1s spectrum of the silk [22], (C–OH), and 288.6 eV (O–C=O). The C–OH contains (286.3 eV)three and O–C=O (288.6 eV) mainly originated from whilefabric the C1s spectrum of HC-silk-g-PHPMA distinct peaks at 284.5 (C–C), 286.3 (C–OH), contains three distinct peaks at 284.5 (C–C), 286.0 (C–OH/C–N), and 288.1 eV (N–C=O) [22], the hydroxyl and ether of HPMA. Table 1 liststhe carbon-to-nitrogen (C/N) ratio of silk fabric, which while the spectrum of HC-silk-g-PHPMA three distinct at 284.5 originated (C–C), 286.3 from and 288.6 eV C1s (O–C=O). The C–OH (286.3 eV)contains and O–C=O (288.6 peaks eV) mainly increased after grafting with HPMA. Less N element content was detected for the grafted silk (C–OH), and 288.6 eV (O–C=O). TheTable C–OH1(286.3 eV) carbon-to-nitrogen and O–C=O (288.6 eV)(C/N) mainly originated from the hydroxyl and ether of HPMA. liststhe ratio of silk fabric, sample compared with the control silk fabric. The reason was that the surface of the grafted silk theincreased hydroxyl and ether of HPMA. Table 1 liststhe carbon-to-nitrogen (C/N) ratio of silk fabric, which which after grafting with HPMA. Less N element content was detected for the grafted fabric was covered by PHPMA polymer, which mainly contains C, O, and H elements. N element increased after grafting with HPMA. Less N element content was detected for the silk silk silk sample compared the control fabric. Themonomers reason was that the surface ofgrafted theonto grafted was weakened afterwith grafting, further silk indicating that were successfully grafted the sample compared with the control silk fabric. The reason was that the surface of the grafted silk fabricsilk was covered PHPMA polymer, which mainly contains C, O, and H elements. N element fabric using by HRP biocatalytic ATRP method. 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 weakened after grafting, further indicating that monomers were successfully grafted onto the 8000 8000 b a fabricsilk using HRP biocatalytic ATRP method. BE/eV FWHM/eV % BE/ eV FWHM/eV % fabric using HRP biocatalytic ATRP method. C-C
4000 6000
1.21 1.20 1.17
69.3 20.0 10.7
BE/ eV FWHM/eV % C-C 284.5 1.21 69.3 C-OH/C-N 286.0 1.20 20.0 N-C=O 288.1 1.17 10.7
a
C-C
C-OH
N-C=O
2000 4000
C-OH
20000
N-C=O
290
6000 8000
Counts / sCounts / s
Counts / s Counts / s
6000 8000
C-C 284.5 C-OH/C-N 286.0 N-C=O 288.1
288 286 284 Binding energy / eV
282
4000 6000
C-C 284.5 C-OH 286.3 O-C=O 288.6
1.21 1.28 1.02
BE/eV FWHM/eV C-C 284.5 1.21 C-OH 286.3 1.28 O-C=O 288.6 1.02
71.2 23.5 5.3
C-C
% 71.2 23.5 5.3
C-C
b
C-OH
2000 4000
O-C=O C-OH
20000
290
O-C=O
288
286
284
282
Binding energy / eV
0 0 Figure 3.The curve fitting analysis of C1s spectra of (a) the control silk fabric and284 (b) 38.87% 290 288 286 282 290 288 286 284 282 Binding energy / eV Binding energy / eV ofHC-silk-g-PHPMA.
Figure 3.The curve fitting analysis of C1s spectra of (a) the control silk fabric and (b) 38.87%
Table 1. curve The element weight of percent of (a) ofthe control silk silk fabric and Figure 3. The fitting analysis C1s spectra (a) the control fabric and(b)(b)38.87% 38.87% of ofHC-silk-g-PHPMA. ofHC-silk-g-PHPMA. HC-silk-g-PHPMA. Table 1. The element weight percent Element of (a) content/% the control silk fabric and (b) 38.87%
sample C/N TableofHC-silk-g-PHPMA. 1. The element Silk weight percent of (a)Cthe controlOsilk fabric and N (b) 38.87% ofHC-silk-g-PHPMA. Control silk-g-PHPMA Silk sample
Silk Sample
Control silk-g-PHPMA Control
silk-g-PHPMA
69.98 24.08 5.94 Element content/% 71.20Element 26.31 Content/%2.49 C O N C69.98 O 24.08 5.94 N 71.20 26.31 2.49 5.94 69.98 24.08
71.20
26.31
2.49
11.78 28.59 C/N
C/N 11.78 28.59 11.78
28.59
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Figure 4 shows the TG (A) and DTG (B) curves of the silk fabric. It can be seen from Figure 4A that 3.3. Thermal Properties Polymers 2018, x FOR PEER REVIEW 6 of 9 the evaporation of10, the absorbed moisture caused slight weight loss of silk fabric when temperature 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 The weight loss ratio of the control silk fabric was 32.5% when the weight loss rate that theC.evaporation 3.3. Thermal Properties of the absorbed moisture caused slight weight loss of silk fabric when ◦ C, which could be attributed to the decomposition of silk fabric into reachedtemperature its highestwas point 320200 lessatthan °C. The weight loss ratio of the control silk fabric was 32.5% when the Figure 4 shows the TG (A) and DTG (B) curves of the silk fabric. It can be seen from Figure 4A weight loss rate reached highest point 320 °C, which could be attributed to the decomposition small molecules including COits and H2 O. Theatdecomposition process of the grafted silk fabric contained that the evaporation of 2the absorbed moisture caused slight weight loss of silk◦ fabric when of silk fabric into small molecules including CO 2 and H 2 O. The decomposition process of and the grafted two stages, and thewas weight loss200 rate its highest the first stagewas 33132.5% C second temperature less than °C.reached The weight loss ratiopoint of the at control silk fabric when the stage silk fabric contained two stages, and the weight loss rate reached its highest point at the first stage ◦ ◦ 410 C weight (for 29.01% of HC-silk-g-PHPMA), and 335 and 416 C (for 38.87% of HC-silk-g-PHPMA), loss rate reached its highest point at 320 °C, which could be attributed to the 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 of silk From fabric into small molecules including H2O. The335 decomposition process of the grafted respectively. Figure 4B, the peaks at 331CO (b,2 and DTG) and C (c, DTG) of HC-silk-g-PHPMA 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 of silk fibroin, which corresponded the peak at in by the the firstdecomposition stage were caused by the decomposition of silk fibroin,towhich 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, DTG) of control silk at fabric. second stage, the grafted silk fabricstage, had the additional corresponded to the peak 320 °CAt(a,the DTG) of control silk fabric. At the second grafted peaks HC-silk-g-PHPMA), respectively. From Figure 4B, the peaks at 331 (b, DTG) and 335 °C (c, DTG) of ◦ C, silk 416 fabric had additional peaks at 410 and °C,decomposition which can be explained by the decomposition of heated at 410 and which can be explained by416 the of poly(HPMA) [23]. When 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 600 C, the final residual the°Ccontrol silk was than stage, that of grafted silk corresponded to the peak atof320 (a, DTG) of fabric control(28.5%) silk fabric. At more the second thethe grafted more than that of the grafted silk fabric (15.4%), which may be due to the decomposition of the fabric (15.4%), which may be due to the decomposition of the poly-(HPMA) into small molecules such silk fabric had additional peaks at 410 and 416 °C, which can be explained by the decomposition of 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 and H2 O.were Thissuccessfully phenomenon further indicated the monomers were successfully grafted onto monomers grafted onto the silk fabric using HRP-catalyzed atom transfer radical than thatHRP-catalyzed of the grafted silk fabric (15.4%),radical which polymerization may be due to themethod. decomposition of the the silk more fabric using atom transfer 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
a
20
a b c
100
200
10
300 400 Temperature / ℃
500
200
Figure 4. TG
300 400 curves (A) and Temperature /℃
ab
320 410
c b
335 320
c
a 600
500
600
416 410
331
100
200
100
200
b c
(A) 100
dW/ dt
dW/ dt
weight retention weight/ % retention / %
80 100 70 90 60 80 50 70 40 60 30 50 20 40 10 30
300335 400 416 Temperature /℃
500
600
500
600
331 (B)
300
400
DTG curves (B) of (a) the control Temperature silk 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. Figure 4.Electron TG curves (A) and DTG curves (B) of (a) the control silk fabric, (b) 29.01% of 3.4. Scanning Microscopy (SEM) HC-silk-g-PHPMA, and (c) 38.87% of HC-silk-g-PHPMA.
3.4. ScanningFigure Electron Microscopy (SEM) of silk fabrics. The control silk fabric had a smooth and uniform 5 shows the morphology appearance portraitMicroscopy orientation. The surfaces of the grafted silk fabric were covered with PHPMA 3.4. ScanninginElectron (SEM)
Figure 5 shows the morphology 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 silk fabrics. Thegrafted control silk hadwere a smooth and uniform appearance in portrait The of surfaces of the silkfabric fabric covered with PHPMA silk fabric becameorientation. higher. appearance in portrait orientation. The surfaces of the grafted silk fabric were covered with PHPMA and became tough. Moreover, the surfaces of the silk fabric were rougher as the grafting 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.
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Figure 5.SEM images of (a) the control silk fabric, (b) 29.01% the HC-silk-g-PHPMA, and (c) 38.87% the
Figure 5. SEM images of (a) the control silk fabric, (b) 29.01% the HC-silk-g-PHPMA, and (c) 38.87% HC-silk-g-PHPMA. the HC-silk-g-PHPMA. 3.5. Crease-Resistant Recovery
3.5. Crease-Resistant ComparedRecovery 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 grafted wet with crease HPMArecovery using HRP biocatalys andHC-silk-g-PHPMA metal complex catalysis bothand improved angle (WCRA)is of increasedmethod up towere 18.41% (Table 2). With the increase ofcan grafting yield, to thetwo dryreasons: crease(1) recovery angle (DCRA) and wet 51.56%,respectively.This be attributed The copolymerization reaction of crease and silkof fabric occurred in amorphous areaup of to silk fiber, and which weakenrespectively. the hydrogenThis can recoverymonomer angle (WCRA) HC-silk-g-PHPMA increased 18.41% 51.56%, bonding in amorphous areaThe andcopolymerization decreased the creep deformation and permanent be attributed to two reasons: (1) reaction of monomer and silkdeformation fabric occurred in caused by breaking up of hydrogen bonding; (2) The grafting copolymerization in amorphous area amorphous area of silk fiber, which weaken the hydrogen bonding in amorphous area and decreased limited relative slippage of silk macromolecules. Additionally, the WCRA of grafted silk fabric the creep deformation and permanent deformation caused by breaking up of hydrogen bonding; increased higher than that of the DRCA, which is favorable for silk fabric easy to wrinkle at wet (2) The grafting in amorphous area limited relative silkDCRA macromolecules. state. Thiscopolymerization result can be attributed to the occurrence of grafting underslippage wet state.ofThe and Additionally, theCC-silk-g-PHPMA WCRA of grafted silk the fabric higher than that of the DRCA, which is WCRA of also have sameincreased increase trend with HC-silk-g-PHPMA. favorable for silk fabric easy to wrinkle at wet state. This result can be attributed to the occurrence of Table 2.The CRA of silk fabric with different grafting yield under dry and wet conditions. grafting under wet state. The DCRA and WCRA of CC-silk-g-PHPMA also have the same increase Grafting samples Grafting yield/% DCRA/° WCRA/° trend with HC-silk-g-PHPMA. 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 ◦ Grafting Samples Grafting Yield/% DCRA/ WCRA/◦ 195 CC-silk-g-PHPMA 37.82 229
3.6.Physical Properties
0
201
128
15.62
220
168
Compared with control silk fabric, HC-silk-g-PHPMA and233 CC-silk-g-PHPMA HC-silk-g-PHPMA 29.01 180were somewhat damaged in the whiteness index and breaking strength (Table 3). For HC-silk-g-PHPMA, the 38.87 238 194 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. After Properties grafting with HPMA, the orientation of macromolecule of silk fiber was changed, and the 3.6. Physical breaking strength decreased. The balance moisture regain of grafted silk fabrics reduced because the Compared with control fabric, HC-silk-g-PHPMA CC-silk-g-PHPMA were somewhat hydrophilic groups of silksilk fabric surface were covered by the and polymers. The physical properties of damaged in the whitenessand index and breaking strength (Table 3). HC-silk-g-PHPMA, HC-silk-g-PHPMA CC-silk-g-PHPMA were basically the For same, but the advantagetheofinactive is the usage of fabrics HRP biocatalyst. enzymesHC-silk-g-PHPMA adhered to the surface of silk 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.
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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
HC-silk-g-PHPMA
15.62 29.01 38.87
75.68 74.26 70.39
428.25 415.36 391.59
8.03 7.96 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 Jiangsu 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 Education Institutions (PAPD). Conflicts of Interest: The authors declare no conflict of interest.
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