Research Article pubs.acs.org/journal/ascecg
Modification of Rice Straw for Good Thermoplasticity via Graft Copolymerization of ε‑Caprolactone onto Acetylated Rice Straw Using Ultrasonic-Microwave Coassisted Technology Lili Zhen,†,‡ Guangzhi Zhang,† Kai Huang,† Xuehong Ren,†,‡ Rong Li,†,‡ and Dan Huang*,†,‡ †
Key Laboratory of Eco-Textile (Jiangnan University), Ministry of Education, 1800 Lihu Road, Wuxi 214122, China Jiangsu Engineering Technology Research Center for Functional Textiles, School of Textiles & Clothing, Jiangnan University, 1800 Lihu Road, Wuxi 214122, China
‡
ABSTRACT: Developing renewable resources for industrial applications, rather than using severely depleted natural fossil, is the biggest need of recent times. In this regard, rice straw (RS) has already been considered as an alternative material. However, some structural modifications need to be performed to make RS suitable for different applications. The objective of this study was to improve thermoplasticity of RS, required for industrial purposes. Acetylated rice straw-graf t-poly(ε-caprolactone) (ARS-g-PCL) copolymers were synthesized via ring-opening polymerization (ROP) of ε-CL onto ARS in the presence of stannous octoate (Sn(Oct)2), N,N-dimethylacetamide (DMAC), using a rapid and efficient sonochemistry-assisted microwave process. The effects of microwave power, ultrasonic power, reaction time, temperature, and amount of ε-caprolactone on grafting percentage (PG) were examined. The sonochemistry-assisted microwave process significantly increased graft efficiency. The optimized conditions for obtaining the maximum PG of 39.6% are as follows: (i) 4:1 weight ratio of ε-caprolactone to rice straw, (ii) microwave/ultrasonic power of 200 W/270 W for 40 min, (iii) reaction temperature 140 °C. The structure and properties of ARS-g-PCL were analyzed using FT-IR, NMR, XRD, and SEM, whereas thermal performances were tested using TGA and DSC. A single glass transition at ∼100 °C and a broad endothermic peak near 185 °C can be seen in the thermogram of ARS-g-PCL, indicating good thermoplasticity compared with ARS. In addition, thin films of ARS-g-PCL can be prepared without any additives. Our findings together indicate that ARS-g-PCL has potential to substitute materials derived from natural resources for different industrial applications. KEYWORDS: ε-Caprolactone, Graft copolymer, Rice straw, Thermoplastic property, Ultrasonic-microwave coassisted technology
1. INTRODUCTION
Modifying structure of plant-derived materials via graft polymerization may be useful to make them suitable for several industrial applications. In this context, many reports have been published on both starch5−8 and cellulose9 grafting. Graft polymerizations onto lignocellulosic biomass derived from a number of different plant sources have also been reported.10−12 However, graft polymerization onto whole plant material is a particularly less studied area, and major contributions in this category have been in the area of wood−plastic composites.13 Although there have been reports on graft polymerizations onto cereal straw, this subject has not received adequate attention commensurate with the abundance of straw as a source of biomass. In addition, existing literature does not provide any information regarding the percentages of total grafted polymer bound to major straw components.
Over the last decades, there has been an increasing interest in developing novel biological materials based on biodegradable and renewable materials. Rice straw (RS) is one of the most abundant renewable biological materials (agricultural residues) in the world. However, there are some disadvantages, such as water sensitivity, insolubility in common organic solvents and high softening temperature, associated with rice straw. These drawbacks make RS unsuitable for some industrial uses that are presently served by synthetic polymers derived from petrochemicals. It is well-known that RS decomposes before its melting temperature as there is no adequate gap between the temperature that is sufficient to break intermolecular bonds and the degradation temperature.1 Therefore, it is difficult to mold RS in sheet or any other shape without using an adhesive like urea resin or isocyanate resin.2 In the present scenario, there is a growing urgency to develop novel biobased products as well as some innovative technologies for minimizing widespread dependence on fossil fuel.3,4 © 2016 American Chemical Society
Received: September 9, 2015 Revised: January 13, 2016 Published: February 4, 2016 957
DOI: 10.1021/acssuschemeng.5b01039 ACS Sustainable Chem. Eng. 2016, 4, 957−964
Research Article
ACS Sustainable Chemistry & Engineering
cellulose backbone and C−H in cellulose and hemicelluloses, respectively. 13 C NMR (400 MHz, solid state); δ (ppm): 170.7 (−CO, C7); 104.4 (carbons in the cellulose backbone, C1); 72.7 (carbons in the cellulose backbone, C2, C3, C5); 64.4 (carbons in the cellulose backbone, C6); 20.6 (−CH3, C8). 2.2.2. Ring-Opening Polymerization (ROP) of ε-CL onto Acetylated Rice Straw (ARS). In a special three-necked flat-bottomed flask fitted with a reflux condenser, 5 g of absolutely dry ARS, 20 g of ε-CL, Sn(Oct) 2 (1 wt % to ARS), and 20 mL of N,Ndimethylacetamide (DMAC) were taken under argon atmosphere. The reaction mixture was processed at 140 °C under ultrasound (270 W) and microwave (200 W) for 40 min. Subsequently, the reaction mixture was diluted with methylene chloride, and the product was precipitated in anhydrous methanol. The solid−liquid separation was performed through filtration. The solid obtained was washed with ethyl alcohol and distilled water, respectively, and dried in a vacuum oven at 70 °C. The crude product was finally purified with toluene extraction for 12 h using a Soxhlet extractor to remove unreacted caprolactone and polycaprolactone homopolymer (possibly formed during polymerization). The extracted pure sample (ARS-g-PCL) was dried at 80 °C for 3 days in a vacuum drying oven. IR (iTR) (cm−1): 2944 (CH symmetric vibration of CH3); 2865 (CH symmetric vibration of −CH3); 1743 (CO stretch of ester carbonyl); 1370 (CH bending in acetyl group); 1230 (CO stretch of COCO); 1630, 899 (β-glucosidic linkages between the sugar units in hemicelluloses and celluloses). 1 H NMR (400 MHz, CDCl3); δ (ppm): 1.38 (m, 2 H −CH2− in C4′); 1.64 (m, 4 H, −CH2− in C3′and C5′); 1.94−2.11 (m, 9 H, methyl protons of acetyl); 2.30 (m, 2 H, methylene proton in CO CH2− in C2′); 3.71 (m, 2 H, methylene proton in CH2OH, C6′), 4.05 (m, 2 H, −OCH2−); 3.54 (m, 1 H), 4.42 (m, 2 H), 4.81 (m, 1 H) and 5.06 (m, 1 H) belong to the methylene proton in cellulose backbone and CH in cellulose and hemicelluloses. 13 C NMR (400 MHz, solid state); δ (ppm): 173.3 (−CO, C1′); 170.7 (−CO, C7); 104.4 (carbons in the cellulose backbone, C1); 72.7 (carbons in the cellulose backbone, C2, C3, C5). 64.4 (carbons in the cellulose backbone, C6, C6′); 34.2 (−CH2 in PCL, C2′); 28.8 (−CH2 in PCL, C5′); 25.3(−CH2 in PCL, C3′, C4′); 20.6 (−CH3, C8). 2.3. WPG (Weight Percent Gain) and Percent Grafting (PG). To determine efficiency of graft copolymerization, weight percent gain (WPG) is often used. Here WPG describes the percent increase in the weight of ARS-g-PCL copolymers with respect to the weight of ARS used for the reaction. We calculated WPG according eq 1:
In a previous study, we have shown that the thermoplasticity of RS can be improved by acetylation of RS,14 however, we could not achieve good thermoplasticity because of poor flexibility of short branched acetyl chain. Acetylated RS film was very brittle without plasticizer, with tensile strength of 1.3 ± 0.26 MPa and breaking elongation of 0.85 ± 0.02%, which is the same as cellulose acetate. To enhance thermoplasticity of modified RS and cast RS film with relatively higher tensile strength and breaking elongation whithout the present of plasticizer, we later attempted to introduce a flexible long branched chain via graft copolymerization of ε-caprolactone (εCL) directly onto RS (unpublished original); however, we did not achieve any obvious improvement in thermoplasticity due to lower degree of grafting. Nishio15 found that in contrast to unmodified cellulose, acetylated cellulose derivatives, because of their relatively good solubility in some organic solvents, make the grafting reaction possible; however, the solubility varies according to the degree of substitution (DS). The objective of our current work was to develop an efficient technique for enhancing thermoplasticity of RS. Guided by the facts stated above, we attempted ring-opening polymerization (ROP) of ε-CL onto acetylated rice straw (ARS) to prepare acetylated rice straw-grafted-polycaprolactone (ARS-g-PCL) and employed sonochemistry-assisted microwave process in ring-opening polymerization. We investigated the effects of graft copolymerization reaction conditions on weight percent gain (WPG) and grafting percentage (PG) of ARS-g-PCL. The structure of the final grafted polymer was established using ATR-FTIR, 1H NMR and solid-state 13C NMR, whereas the SEM study was performed to examine morphology of ARS-gPCL thin films. Furthermore, thermal stability and thermoplasticity were tested.
2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. The rice straw (RS) was obtained from a local countryside (Wuxi, China). ε-Caprolactone (CL) of >99% purity, purchased from Sigma-Aldrich, was distilled over calcium hydride and then collected over 4 Å molecular sieves. Toluene was distilled from sodium benzophenone ketyl prior to use. N, Ndimethylacetamide (DMAC) was dried over molecular sieves. Commercial grade stannous octoate (Sn(Oct)2) of 95% purity, 95% ethanol, benzene, acetic anhydride, glacial acetic acid, and 98% sulfuric acid were purchased from Sinopharm Group Inc., China. 2.2. Synthesis of RS-based Thermoplastic Copolymers. 2.2.1. Acetylation of RS. Acetylation of RS was carried out following a previous method reported elsewhere.14 After acetylation, 20 mL of deionized water was added to the mixture and allowed to hydrolyze at 40 °C for 6 h. The product was then cooled to room temperature and slowly poured over a large volume of water in a beaker with constant stirring; solid−liquid phases were separated through filtration. The solids obtained were then washed with water until the pH became neutral. Next, the acetylated rice straw (ARS) powders were ovendried at 80 °C until it reached a constant weight. The weight percent gain (WPG) and percent acetyl content (PAC) of the acetylated rice straw were found as 20% and 1.4, respectively. IR (iTR) (cm−1): 2937 cm−1 (−CH in −CH2− and −CH− stretching); 2915 (CH symmetric vibration of −CH3); 1750 (C O stretch of ester carbonyl); 1381 (CH in acetyl group, bending); 1230 (CO stretch of COCO); 1634, 899 (β-glucosidic linkages between the sugar units in hemicelluloses and celluloses, respectively). 1 H NMR (400 MHz, CDCl3); δ (ppm): 1.27 (m, 1 H, methyl protons in lignin); 2.04−2.14 (m,12 H, methyl protons of acetyl); 3.55 (m, 1 H); 3.74 (m, 2 H); 4.08 (m, 2 H); 4.42 (m, 3 H); 4.81 (m, 2 H); 5.09 (m, 2 H) and 5.33 (m, 1 H) belong to the methylene proton in
WPG% = (Wr − W0)/W0 × 100
(1)
where W0 is the initial oven-dried weight of ARS, and Wr is the ovendried weight of ARS-g-PCL. Percent grafting (PG) is defined as the percentage of PCL chain segments modified onto ARS. We obtained PG from eq 2:
PG% = (Wg − W0)/W0 × 100
(2)
where W0 is the initial oven-dried weight of ARS, and Wg is the ovendried weight of ARS-g-PCL. 2.4. Preparation of ARS-g-PCL Film. ARS-g-PCL films were prepared following a method reported elsewhere.16 ARS-g-PCL powder (20 g) was dissolved in 400 mL of methylene chloride and stirred for 3 h at room temperature. The solution was then centrifuged at 4000 rpm for 5 min to exclude a small amount of insoluble material. Thereafter, dibutyl phthalate (DBP) was added to the methylene chloride solution of ARS-g-PCL, and the resulting mixture was stirred for 1 h. DBP (0%, 20 wt % to soluble ARS-g-PCL) was used as a plasticizer to improve thermoplasticity of the modified RS. The ARS-gPCL films were prepared by casting the solution onto an aclinic polytetrafluoroethylene (PTFE) molds at 20 °C (humidity 60%). 2.5. Characterization. 2.5.1. Fourier Transform Infrared (FT-IR) Spectroscopy. FT-IR spectra of RS, ARS and ARS-g-PCL were recorded on an attenuated total reflectance (ATR) spectrophotometer 958
DOI: 10.1021/acssuschemeng.5b01039 ACS Sustainable Chem. Eng. 2016, 4, 957−964
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ACS Sustainable Chemistry & Engineering
Scheme 1. Graft Copolymerization of ε-CL onto ARS
(Nicolet iS10; Thermo-Fisher Scientific, USA). The samples were placed on a germanium plate, and each sample was scanned for 16 times at a resolution of 4 cm−1. 2.5.2. Nuclear Magnetic Resonance (NMR) Spectroscopy. The 1H NMR spectra of ARS and ARS-g-PCL were recorded on an AVANCE III 400 MHz NMR spectrometer (Bruker Co., Switzerland) using CDCl3 as solvent. Chemical shifts are reported in parts per million (ppm, δ) downfield from the internal standard TMS. In order to exclude the insoluble, ARS and ARS-g-PCL were dissolved in dichloromethane and then centrifuged for 5 min (4000 r/min). The supernatant was concentrated by rotary evaporation and dried in a vacuum oven to obtain samples suitable for recording 1H NMR. Solid state 13C NMR spectra of the original RS, ARS and ARS-gPCL were recorded on a 400 MHz NMR spectrometer (AVANCE III, Bruker, Switzerland) at a 13C frequency of 101 MHz, using the combined techniques of proton dipolar decoupling (DD), magic angle spinning, and cross-polarization (CP/MAS 13C SSNMR). 13C RF (radio frequency) field strengths of 101 kHz corresponding to 90° pulses of 4 ms were used for the matched spin-lock cross-polarization transfer. The spinning speed was set at 3 kHz. The contact time was 1 ms, whereas the acquisition time was 10 ms. A typical number of 10 000 scans were used for each spectrum. Chemical shifts were referenced with respect to TMS (tetramethyl silane) signal. 2.5.3. Wide-Angle X-ray Diffraction (XRD). XRD patterns of RS, ARS, and ARS-g-PCL were obtained at room temperature using a D8 Advance XRD instrument (WAXD, Bruker AXS Company, Germany) at a scanning speed of 4°/min for 2θ ranging from 3° to 60° (voltage, 40 kV; current, 40 mA). 2.5.4. Scanning Electron Microscopy (SEM). SU-1510 (Hitachi, Japan) was used to obtain SEM images of the studied samples. The samples were coated with a thin layer of gold by sputtering before SEM imaging. An accelerating voltage of 5 kV with accounting time of 100 s was employed. 2.5.5. Thermal Analysis. Thermogravimetric analysis (TGA) was performed for RS, ARS, and ARS-g-PCL using SDT Q600 thermogravimetric analyzer (TA Instruments, USA). The samples were heated at a rate of 10 °C/min from 30 to 600 °C under nitrogen atmosphere. The DSC (differential scanning calorimetry) thermograms were recorded using Q-200 thermal analyzer (TA Instruments, USA). The samples were completely dried at 105 °C for 4 h before DSC measurements, and the dried samples were first heated from an ambient temperature (25 °C) to 200 °C at a heating rate of 20 °C/ min under nitrogen atmosphere, and subsequently quenched to −50 °C. Then the second scans were run from −50 to +300 °C to record stable thermograms at a heating rate of 10 °C/min under nitrogen atmosphere. 2.5.6. Tensile Testing of the Films. Tensile strength of the films was measured using a universal strength tester (Zwicki-Line Testing Machine, BZ 2.5, Zwick Roll). The samples were cut into a dimension of 150 mm × 10 mm; a sharp knife was used to avoid jagged edges. The length of effective part was 100 mm. These specimens were elongated at the rate of 10 mm/min at room temperature. Seven films, prepared under identical conditions, were tested; the data are reported as average ± one standard deviation.
caprolactone to ARS (1.0, 2.0, 3.0, 4.0, 5.0, 6.0, and 7.0) to study their effects on WPG and PG. Table 1 summarizes the results. Table 1. Effects of Weight Ratio of ε-Caprolactone to ARS on WPG and PGa entry
CL:ARS (%w/w)
1 2 3 4 5 6 7
1:1 2:1 3:1 4:1 5:1 6:1 7:1
WPG (%) 10.5 15.2 22.8 30.3 36.4 42.9 45.1
± ± ± ± ± ± ±
1.0 1.5 1.2 1.5 2.0 1.8 1.9
PG (%) 6.5 11.6 16.5 23.9 23.2 23.0 23.4
± ± ± ± ± ± ±
0.8 1.0 1.5 1.6 1.5 1.8 1.7
a The reaction was carried out at 130−140 °C employing microwaveultrasonic wave (100 W/270 W) for 40 min with different εcaprolactone to ARS ratios and 1% catalyst concentration.
Table 1 shows that when the ratio of ε-caprolactone to ARS increases from 1:1 to 4:1, PG improves gradually from (6.5 ± 0.8)% to (23.9 ± 1.6)%, whereas WPG increases from (10.5 ± 1.0)% to (30.3 ± 1.5)%. When the ratio reaches above 4:1, PG tends to be the same, with a slight fall in certain cases; however, WPG continues increasing with increase in the amount of εcaprolactone. This is probably due to the insufficient amount of ε-caprolactone being available for reacting with hydroxyl groups in ARS at lower ratios of ε-caprolactone to ARS, resulting in lower grafting ratio and less weight gain. On the other hand, with increase in ε-caprolactone concentration, there would be a number of collisions between ε-caprolactone and the active center in ARS, resulting in increasing PG and WPG. At 4:1 εcaprolactone to ARS ratio, the number of accessible hydroxyl groups in ARS probably reaches equilibrium, thus PG does not increase with the further increase in ε-caprolactone to ARS ratio higher than 4:1. It is important to note that higher concentration of ε-caprolactone is particularly very beneficial for homopolymerization, generating caprolactone oligomer; larger the ε-caprolactone to ARS ratio, greater is the proportion of homopolymerization. Therefore, we chose weight ratio of εcaprolactone to ARS as 4:1 while optimizing other conditions of the grafting polymerization. 3.1.2. Effects of Microwave and Ultrasonic Power. We investigated the influences of ultrasonic/microwave irradiation power, keeping weight ratio of ε-caprolactone to ARS at 4:1, as optimized. Table 2 summarizes the effects of microwave power and ultrasonic power on WPG and PG of ARS. Entries 1, 3−14 confirm that the reaction temperature is affected by microwave power and ultrasonic power. Under microwave power (0 W)/ ultrasonic power (270 W), the reaction mixture reaches a temperature of 30 °C (entry 3), whereas under microwave power (50 W)/ultrasonic power (270 W), the reaction mixture reaches a temperature of 90°−100 °C (entry 4). In these
3. RESULTS AND DISCUSSION 3.1. Optimization of Conditions for Graft Copolymerization. To synthesize graft copolymer with high grafting efficiency, we optimized the conditions for graft polymerization of ARS. However, it was difficult to measure the grafting yield of the final polymer because of multicomponent chemical structure and heterogeneity of ARS; the degree of grafting was also difficult to measure. Instead, we evaluated the extent of graft copolymerization by determining the weight percent gain and the percent grafting of the final polymeric product (Scheme 1). 3.1.1. Effects of ε-Caprolactone to ARS ratio. We carried out series of graft reactions at different weight ratio of ε959
DOI: 10.1021/acssuschemeng.5b01039 ACS Sustainable Chem. Eng. 2016, 4, 957−964
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ACS Sustainable Chemistry & Engineering Table 2. Effects of Microwave and Ultrasonic Powera entry
T (°C)
MW (W)
US (W)
time (min)
WPG (%)
1b 2c 3 4 5 6 7 8 9 10 11 12 13 14
20 150 ∼30 90−100 130−140 130−140 130−140 130−140 130−140 130−140 130−140 130−140 140−150 140−150
0 0 0 50 100 150 200 200 200 200 200 200 250 300
0 0 270 270 270 270 0 90 180 360 450 270 270 270
∞ 600 40 40 40 40 40 40 40 40 40 40 40 40
0 40.2 ± 1.8 1.5 ± 0.8 5.2 ± 0.6 22.3 ± 1.6 35.3 ± 1.5 29.7 ± 1.5 38.9 ± 1.8 44.1 ± 1.9 46.5 ± 1.7 47.8 ± 1.6 46.4 ± 1.8 49.2 ± 1.5 50.5 ± 1.7
Table 3. Effect of Reaction Time on WPG and PGa
PG (%) 0 34.5 0 ∼0 16.9 28.1 25.6 34.1 37.8 39.0 37.7 39.6 37.1 36.4
±2
± ± ± ± ± ± ± ± ± ±
1.4 1.7 1.6 1.7 1.9 1.7 1.6 1.8 1.9 1.6
entry
time (min)
1 2 3 4 5 6
20 30 40 50 60 70
WPG (%) 29.7 37.2 46.4 50.0 49.3 50.4
± ± ± ± ± ±
1.5 1.7 1.8 1.7 1.9 1.7
PG (%) 24.9 30.4 39.6 41.7 41.2 40.3
± ± ± ± ± ±
1.4 1.6 1.8 1.8 1.7 1.9
a All reactions were carried out at 140 °C under fixed microwave power/ultrasonic wave power (200 W/270 W) with ε-caprolactone to ARS ratio of 4:1 and catalyst concentration of 1%.
weight ratio of ε-caprolactone to ARS (4:1) and microwave power/ultrasonic wave power (200 W/270 W). As evident from Table 3, WPG and PG increase significantly with increasing reaction time; PG reaches up to 39.6% (±1.8%) when the reaction time is less than 40 min. When reaction goes over 40 min, grafting rate tends to be stable. Therefore, the suitable reaction time is 40 min. 3.2. Characterization. 3.2.1. FT-IR Analysis. Figure 1 illustrates the FT-IR spectra of RS, ARS, and ARS-g-PCL.
a
Reactions were carried out using CL to ARS ratio of 4:1 in N,Ndimethylacetamide (DMAC) with stannous octoate as catalyst (1% to ARS). bThe reaction was carried out at room temperature. cThe reaction was carried out at 150 °C in oil bath.
experiments, temperature was controlled by automatically adjusting microwave and ultrasonic power. Furthermore, under fixed ultrasonic power of 270 W but with increasing microwave power (0−200 W), PG increases from 0% to 39.6% (±1.8%), while temperature increases from room temperature (20 °C) to 140 °C (entries 3−6, 12). As evident from entries 13 and 14, further increase in microwave power does not help in increasing PG, instead, it decreases PG significantly. On the other hand, WPG increases continuously to 50.5% (±1.7%) with continuous increase in microwave power up to 300 W. Higher microwave power can provide more energy to drive ROP of ε-caprolactone, resulting in decreased PG; however, too high microwave power is particularly advantageous for homopolymerization of εcaprolactone. Compared to ε-caprolactones, caprolactone oligomers are difficult to graft on ARS because of the steric hindrance, thus a large amount of polycaprolactone (PCL) is generated with increasing degree of homopolymerization. Moreover, part of acetylated straw graft copolymer is degraded under too severe reaction condition such as too high microwave power. High microwave power can also initiate ester exchange reaction. Results presented in Table 2 (entries 7−12) indicate that the effect of ultrasonic power on WPG and PG is weaker than that of microwave power. As evident from Table 2, in the absence of ultrasonic power, the graft rate can reach a value of more than 25% at 300 W of microwave power; therefore, grafting can be achieved without ultrasonic power. However, the graft polymerization of ε-caprolactone to ARS can further be promoted to a certain extent by employing ultrasound due to ultrasonic cavitation. We obtained the highest PG under ultrasonic power of 270 W. However, small portion of acetylated straw graft copolymer could be degraded under severe ultrasonic cavitation. On the basis of the above results, we optimized microwave power and ultrasonic power at 200 W and 270 W, respectively. At this condition, the reaction temperature is about 140 °C. 3.1.3. Effect of Reaction Time. Table 3 presents effects of reaction time on WPG and PG. All the reactions were carried out varying the reaction time but under the same optimal
Figure 1. FT-IR spectra of RS, ARS (WPG 20%), and ARS-g-PCL (PG 39.6%).
Comparing the FT-IR spectrum of ARS with that of RS, a few new absorption peaks and increased intensity of certain peak are evident. Unlike FT-IR spectrum of RS, spectrum of ARS not only shows absorption peaks at 1743 cm−1 (−CO, stretching) and 1500−1200 cm−1 (−CO, stretching and −CH3, bending), it also shows increase in the intensity of the absorption peak at 2937 cm−1 (−CH in −CH2− and −CH−, stretching), indicating obvious changes in the chemical structure of RS after acetylation. In the FT-IR spectrum of ARS-g-PCL sample, a reduction in the intensity of −OH groups at 3448 cm−1(stretching) is evident (Figure 1), indicating the conversion of −OH groups. The increased intensities of the peaks corresponding to C−H groups in −CH2− and −CH− at 2800−2950 cm−1 (stretching) and −CO at 1743 cm−1 (stretching) confirm the ringopening copolymerization of ε-caprolactone with ARS. In addition, the reduction in the intensity of −CH3 at 1370 cm−1 (bending) indicates ester exchange reaction during grafting reaction; ε-caprolactone segment with more −CH2− replaced part of acetyl groups, decreasing the proportion of −CH3 groups.17 Therefore, FT-IR spectra confirm reduction in −OH groups as well as increase in caprolactone groups. 3.2.2. 1H NMR and Solid-State 13C NMR Analysis. Figure 2a illustrates 1H NMR spectra of the studied compounds. In the 1 H NMR spectrum of ARS, the methyl protons of acetyl appear at δ = 2.04−2.14 ppm, whereas the peaks at δ = 3.55−5.33 ppm 960
DOI: 10.1021/acssuschemeng.5b01039 ACS Sustainable Chem. Eng. 2016, 4, 957−964
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cellulose backbone appear at 104.4 ppm (C1) and 64.4−72.7 ppm (C4, C2, C3, C5, and C6), respectively. The introduction of PCL chain segments on the molecular chains of acetyl rice straw is confirmed by the peaks at 173.3 ppm, which is ascribed to the carbonyl carbons (C1′) in caprolactone. The changes in the peaks corresponding to methylene carbons (C2′, C3′, C4′, and C5′) further confirm successful grafting of PCL on ARS. The double peak at 64.4 ppm is characteristics to the carbons in the cellulose backbone (C6) and methylene carbons (C6′) of PCL.20−23 Therefore, we can infer that the PCL chain segments are successfully grafted on residual hydroxyl of ARS. 3.2.3. Analysis of SEM Micrographs. We investigated the surface morphology of the samples using SEM. Figure 3 illustrates the SEM micrographs of the surfaces of RS, ARS, and ARS-g-PCL. Alternate smooth and irregular surfaces can be seen on the epidermis of RS before acetylation, as shown in Figure 3a. It has been suggested that smooth surfaces correspond to small sieve tube bundles embedded in subepidermal sclerenchyma, whereas irregular surfaces had two types of excrescences, and presented trichomes.24 As evident from Figure 3b that all the surface cells of RS are completely destroyed during acetylation, the fibrillar structures disappear, and the surface is covered with a layer of coagulum. Although ARS-g-PCL shows similar surface morphology as ARS, the surface is more uniform in the former (Figure 3c). The uniformity in ARS-g-PCL surface might be due to covering of particles on straw by flexible PCL segments grafted on ARS, thus forming a smoother surface. This result is consistent with the XRD findings, indicating successful grafting of PCL chains on ARS backbone. 3.2.4. XRD Analysis. We investigated the morphological changes in the samples using XRD (Figure 4). The XRD curves
Figure 2. 1H NMR (a) and 13C NMR (b) specta of ARS (WPG 20%) and ARS-g-PCL (PG 39.6%).
correspond to the methylene proton in cellulose backbone and C−H in cellulose and hemicelluloses. In the 1H NMR spectrum of ARS-g-PCL, the above proton peaks are also present. The typical methylene proton peaks of PCL chain segments can be seen at δ = 2.30 ppm (CO−CH2−, C2′); δ = 4.05 ppm (−O− CH2−); δ = 3.71 (CH2−OH, C6′); δ = 1.38 ppm (−CH2−, C4′); δ = 1.64 ppm (−CH2−, C3′, C5′).18 On the basis of these assignments, we can predict that the molecular structure of ARS-g-PCL includes PCL side chain, and ε-caprolactone is attached with ARS successfully by ring opening copolymerization reaction. Additionally, certain lignin and their derivatives can easily get dissolved in a solvent, but they are gradually removed during acetylation, grafting reaction, and posttreatment process; none of the spectrum shows any characteristic absorption peaks for the protons of the aromatic ring of syringyl lignin and guaiacyl lignin (6.20−6.40 ppm and 7.00− 7.26 ppm).19 In addition, reduced intensity of the methyl protons of acetyl group (δ = 1.94−2.11 ppm) is evident in the 1 H NMR spectrum of ARS-g-PCL, suggesting replacement of some acetyl group by caprolactone through ester exchange reaction during grafting polymerization. Figure 2b illustrates 13C NMR spectra of ARS (WPG 20%) and ARS-g-PCL (PG 39%); all the carbons are assigned with numbers in the chemical structures of the compounds. The peaks at 20.6 ppm (−CH3, C8), 170.7 ppm (−CO, C7) assigned to the carbons of acetyl group. The carbons in the
Figure 4. XRD patterns of RS, ARS (WPG 20%), ARS-g-PCL (PG 39.6%), and PCL.
for RS and ARS show that diffraction peak localized at 2θ of 16.5°, assigned to crystal planes (101), (101) of cellulose (cellulose I) in raw RS, disappears after acetylation in ARS,
Figure 3. SEM images of RS (a), ARS (b, WPG 20%), and ARS-g-PCL(c, PG 39.6%). 961
DOI: 10.1021/acssuschemeng.5b01039 ACS Sustainable Chem. Eng. 2016, 4, 957−964
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ACS Sustainable Chemistry & Engineering
(22%) and RS (24%), suggesting that ARS-g-PCL gets lost with volatile products and does not contribute to char formation. From DTG curves presented in Figure 5a, two degradation temperatures can be seen for RS (297 and 354 °C), ARS (WPG 20%) (384 and 440 °C), and ARS-g-PCL (PG 39%) (310 and 408 °C). This is consistent with the finding that ARS and ARSg-PCL are more thermally stable than RS. Both degradation temperatures are higher in ARS and ARS-g-PCL than those of RS. ARS-g-PCL loses most of its weight at the first decomposing temperature and decomposes faster compared to raw RS and ARS. This is undoubtedly due to the disintegration of intramolecular interactions such as hydrogen bonds. The high stability and close-packed assembly of rice straw structure are interrupted and loosened by the PCL chain in ARS-g-PCL, thus strong intramolecular interactions such as hydrogen bonds are replaced by weaker van der Waals force. Besides, part of macromolecular structure in ARS is slightly destroyed in ring opening copolymerization reaction and post treatment processes. In addition, the first degradation temperature (Tmax = 310 °C) of ARS-g-PCL is higher than that of RS, confirming the presence of PCL in PCL-g-ARS.25 As evident from Figure 5b, ARS-g-PCL exhibits an endothermic peak at 50 °C, attributed to the melting of crystalline segment of ARS-g-PCL. The presence of the melting peak demonstrates that the thermoplasticity of RS is further improved due to the grafting reaction. The melting temperature of PCL-g-ARS (50 °C) is lower than that of PCL (∼60 °C), because the degree of polymerization of PCL branched chain grafted on ARS is low, and only a few PCL segments form crystalline region ; furthermore, crystalline region contains several crystalline defects. ARS-g-PCL presents low melting point and melting enthalpy, thus making graft copolymer chain segments to act as plasticizer.26 In addition, the thermogram of ARS-g-PCL showed a glass transition at ∼100 °C and a broad endothermic peak near 185 °C emerges (Figure 5b), suggesting an increase of amorphous region and improved the plasticity of ARS-g-PCL compared to ARS. Therefore, ARS-g-PCL can be considered for exploring different thermoplastic application. 3.3. Thermoplastic Application of ARS-g-PCL. 3.3.1. Digital Photo and SEM of ARS-g-PCL Film. Figure 6a
while a diffraction peak at 2θ = 8.0° appears in ARS due to the formation of cellulose acetate crystal. The crystallinity fell to 33% from 36% of the straw. Acetylation of RS increases accessibility and reactivity of RS cellulose. In the XRD curve of ARS-g-PCL, the characteristic diffraction peaks located at 2θ = 8.0° and 2θ = 16.5° are absent, and wide scattering peak buries the sharp diffraction peak at 2θ = 21.6°, formed by PCL segments crystal. The diffraction peak at 2θ = 21.6° of ARS-gPCL is not as obvious as that of PCL. This can be explained in the following way. The PCL segments grafted on ARS have a low degree of polymerization degree and they disperse on the surface of ARS. On the other hand, cellulose acetate in ARS has a strong diffraction peak at 2θ = 22.6° and this peak covers the sharp peak characteristic to PCL at 21.6°. The above findings suggest low degree of crystallinity of PCL segments grafted on ARS backbone. 3.2.5. Thermogravimetric Analysis. We examined glass transition temperature, thermal stability, and partial melting of the polymers using TG and DSC instruments. Figure 5 depicts
Figure 5. TG, DTG (a), and DSC (b) of RS, ARS (WPG 20%), and ARS-g-PCL (PG 39.6%).
the thermograms of the studied samples; the measurement was carried out under nitrogen atmosphere. An obvious decrease of weight by 5% due to loss of moisture between 50 and 100 °C can be seen in the thermogram of RS (Figure 5a), whereas there are no significant changes in ARS and ARS-g-PCL because of their low content of hydroxyl groups. This indicates retention of less number of unsubstituted hydroxyl groups in ARS and ARS-g-PCL, thus making them hydrophobic. TG curves in Figure 5a show higher degradation temperature for ARS and ARS-g-PCL compared to that of RS, indicating higher thermal stability of ARS and ARS-g-PCL. ARS is thermally the most stable polymer, even more than ARS-g-PCL. The lower thermal stability of the ARS-g-RS is probably due to the fact that part of easily degradable macromolecular structure in ARS gets slightly destroyed in the course of grafting carried out at high temperature.20 In addition, we found that the residue left at 500 °C (17%) for ARS-g-PCL, was less than that of ARS
Figure 6. Digital photo (a) and SEM image (b) of ARS-g-PCL (PG 39.6%) film with 20% DBP.
illustrates a digital photograph of ARS-g-PCL thin film. The color of ARS-g-RS is transparent brown. The SEM image (Figure 6b) reveals smooth and uniform texture of the film. Therefore, ARS-g-PCL has good film forming ability. 3.3.2. Mechanical Property of ARS-g-PCL Film. Table 4 depicts tensile properties of ARS14 and ARS-g-PCL films in absence or presence of plasticizer dibutyl phthalate (DBP, 20%). In absence of plasticizer (entry 3), ARS-g-PCL film exhibits good mechanical performance with an average maximum breaking strength of 15.87 (±1.58) MPa and average 962
DOI: 10.1021/acssuschemeng.5b01039 ACS Sustainable Chem. Eng. 2016, 4, 957−964
Research Article
ACS Sustainable Chemistry & Engineering
(3) Mistri, E.; Bandyopadhyay, N. R.; Ghosh, S. N.; Ray, D. Development of green composites from furfuryl palmitate. Ind. Eng. Chem. Res. 2010, 49 (22), 11357−11362. (4) Esteban, L. S.; Carrasco, J. E. Biomass resources and costs: Assessment in different EU countries. Biomass Bioenergy 2011, 35, S21−S30. (5) Chen, L.; Qiu, X.; Deng, M.; Hong, Z.; Luo, R.; Chen, X.; Jing, X. The starch grafted poly(l-lactide) and the physical properties of its blending composites. Polymer 2005, 46 (15), 5723−5729. (6) Sugih, A. K.; Picchioni, F.; Janssen, L. P. B. M.; Heeres, H. J. Synthesis of poly-(ε)-caprolactone grafted starch co-polymers by ringopening polymerisation using silylated starch precursors. Carbohydr. Polym. 2009, 77 (2), 267−275. (7) Witono, J. R.; Noordergraaf, I. W.; Heeres, H. J.; Janssen, L. P. Graft copolymerization of acrylic acid to cassava starch–evaluation of the influences of process parameters by an experimental design method. Carbohydr. Polym. 2012, 90 (4), 1522−1529. (8) Sheikh, N.; Akhavan, A.; Ataeivarjovi, E. Radiation grafting of styrene on starch with high efficiency. Radiat. Phys. Chem. 2013, 85, 189−192. (9) Guo, Y.; Wang, X.; Shen, Z.; Shu, X.; Sun, R. Preparation of cellulose-graft-poly(ε-caprolactone) nanomicelles by homogeneous ROP in ionic liquid. Carbohydr. Polym. 2013, 92 (1), 77−83. (10) Teli, M. D.; Sheikh, J. Graft copolymerization of acrylamide onto bamboo rayon and fibre dyeing with acid dyes. Iran. Polym. J. 2012, 21 (1), 43−49. (11) Zahedi, M.; Tabarsa, T.; Ashori, A.; Madhoushi, M.; Shakeri, A. A Comparative Study on Some Properties of Wood Plastic Composites Using Canola Stalk, Paulownia, and Nanoclay. J. Appl. Polym. Sci. 2013, 129 (3), 1491−1498. (12) Wan, Z.; Xiong, Z.; Ren, H.; Huang, Y.; Liu, H.; Xiong, H.; Wu, Y.; Han, J. Graft copolymerization of methyl methacrylate onto bamboo cellulose under microwave irradiation. Carbohydr. Polym. 2011, 83 (1), 264−269. (13) Khalil, H. A.; Tehrani, M.; Davoudpour, Y.; Bhat, A.; Jawaid, M.; Hassan, A. Natural fiber reinforced poly(vinyl chloride) composites: A review. J. Reinf. Plast. Compos. 2013, 32 (5), 330−356. (14) Zhang, G.; Huang, K.; Jiang, X.; Huang, D.; Yang, Y. Acetylation of rice straw for thermoplastic applications. Carbohydr. Polym. 2013, 96 (1), 218−226. (15) Nishio, Y. Material functionalization of cellulose and related polysaccharides via diverse microcompositions. Adv. Polym. Sci. 2006, 205 (1), 97−151. (16) Hyppölä, R.; Husson, I.; Sundholm, F. Evaluation of physical properties of plasticized ethyl cellulose films cast from ethanol solution Part I. Int. J. Pharm. 1996, 133, 161−170. (17) Chen, G.; Dufresne, A.; Huang, J.; Chang, P. R. A Novel Thermoformable Bionanocomposite Based on Cellulose Nanocrystalgraft-Poly(ε-caprolactone). Macromol. Mater. Eng. 2009, 294 (1), 59− 67. (18) Klébert, S. Modification of Cellulose Acetate by Reactive Processing - Chemistry, Structure and Properties. Ph.D. Thesis, Budapest University of Technology and Economics, Budapest, 2007. (19) Számel, G.; Klébert, S.; Sajó, I.; Pukánszky, B. Thermal Analysis of Cellulose Acetate Modified with Caprolaction. J. Therm. Anal. Calorim. 2008, 91 (3), 715−722. (20) Mayumi, A.; Kitaoka, T.; Wariishi, H. Partial substitution of cellulose by ring-opening esterification of cyclic esters in a homogeneous system. J. Appl. Polym. Sci. 2006, 102 (5), 4358−4364. (21) Li, J.; Xie, W.; Cheng, H. N.; Nickol, R. G.; Wang, P. G. Polycaprolactone-Modified Hydroxyethylcellulose Films Prepared by Lipase-Catalyzed Ring-Opening Polymerization. Macromolecules 1999, 32 (8), 2789−2792. (22) Kusumi, R.; Lee, S.-H.; Teramoto, Y.; Nishio, Y. Cellulose Estergraft-poly(ε-caprolactone): Effects of Copolymer Composition and Intercomponent Miscibility on the Enzymatic Hydrolysis Behavior. Biomacromolecules 2009, 10 (10), 2830−2838. (23) Kusumi, R.; Teramoto, Y.; Nishio, Y. Structural characterization of poly(ε-caprolactone)-grafted cellulose acetate and butyrate by solid-
Table 4. Tensile Properties of ARS and ARS-g-PCL Films entry
ARS-g-PCL film
DBPa (%)
δb (%)
σc (MPa)
1 2 3 4
14
0 20 0 20
0 1.40 ± 0.03 3.35 ± 0.70 4.72 ± 0.55
0 1.11 ± 0.32 15.87 ± 1.58 11.20 ± 0.84
ARS
ARS-g-PCL
a
DBP: Dibutyl phthalate. elongation.
b
σ: Tensile strength.
c
ε: Breaking
elongation at break of 3.35% (±0.70%). Entry 4 shows that the strength of the ARS-g-PCL film reduces slightly after adding plasticizer DBP, but the elongation at break improves obviously up to 4.72% (±0.55%). Because a plasticizer can reduce stress between macromolecules, the brittleness and modulus of ARSg-PCL film are also reduced. Therefore, depending on the performance required for a certain application, ARS-g-PCL film can be prepared by varying the amount of plasticizer. As evident from the entries 1−4, ARS-g-PCL exhibits good plasticity because of internal plasticization of PCL chain segments. Moreover, upon grafting PCL chain segments to ARS, the elongation at break and the breaking strength increase significantly. In addition, we did not study the effect of PCL homopolymers on tensile properties of ARS-g-PCL films. According to grafting experiments, ungrafted PCL homopolymers share at least 50% of total amount of reacted monomers after reaction. Reuse of high amount of homopolymers from the grafting reaction is important in industry. Therefore, in future work we will study the potential applications of ungrafted PCL homopolymers.
4. CONCLUSIONS We developed and optimized an efficient method to modify RS structurally for improving thermal stability, solvent resistance and enhancing thermoplasticity. The grafted copolymer (ARSg-PCL) prepared via ROP could be casted to form transparent films in absence or presence of plasticizer dibutyl phthalate (DBP). Compared to acetylated rice straw films, ARS-g-PCL grafted copolymer films exhibit improved elongation rate as well as breaking strength. Moreover, adequate flexibility and sufficient mechanical strength make ARS-g-PCL graft copolymer an inexpensive and biodegradable thermoplastic material that may find its application in different industrial purposes.
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AUTHOR INFORMATION
Corresponding Author
*Prof. Dan Huang. E-mail address:
[email protected]. Tel./ fax: +86510 85912009. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was funded by Industrialization of Research Findings Projects of Colleges and Universities of Jiangsu Province (Grant No. JHBZ 2011-27).
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REFERENCES
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DOI: 10.1021/acssuschemeng.5b01039 ACS Sustainable Chem. Eng. 2016, 4, 957−964
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DOI: 10.1021/acssuschemeng.5b01039 ACS Sustainable Chem. Eng. 2016, 4, 957−964
