Article pubs.acs.org/JAFC
Effects of Monomers and Homopolymer Contents on the Dry and Wet Tensile Properties of Starch Films Grafted with Various Methacrylates Zhen Shi,†,‡ Narendra Reddy,‡ Li Shen,§ Xiuliang Hou,† and Yiqi Yang*,†,‡,§,∥,⊥ †
Key Laboratory of Eco-textiles, Ministry of Education, College of Textiles and Garments, Jiangnan University, Lihu Road #1800, Wuxi 214122, People’s Republic of China ‡ Department of Textiles, Merchandising, and Fashion Design, 234 HECO Building, East Campus, University of NebraskaLincoln, Lincoln, Nebraska 68583-0802, United States § College of Chemistry, Chemical Engineering, and Biotechnology, Donghua University, No. 1882 West Yan'an Road, Shanghai 200051, People’s Republic of China ∥ Department of Biological Systems Engineering and ⊥Nebraska Center for Materials and Nanoscience 234 HECO Building, East Campus, University of NebraskaLincoln, Lincoln, Nebraska 68583-0802, United States ABSTRACT: Starch grafted with four different methacrylates was compression molded to form thermoplastic films with good strength and water stability. Starch is an inexpensive and biodegradable polymer but is nonthermoplastic and needs to be chemically modified to make starch suitable for various applications. In this research, starch was grafted with four methacrylates (methyl, ethyl, butyl, and hexyl), and the effect of the length of the alkyl ester group on grafting parameters, thermoplasticity, and properties of thermoplastic films developed have been studied. Influence of grafting conditions on % grafting efficiency, % homopolymers, and % monomer conversion were studied, and the grafted starch was characterized using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and nuclear magnetic resonance (1H NMR). At similar grafting ratios, butyl methacrylate (BMA) provided better strength and elongation to the starch films than the other three methacrylates. Grafting of methacrylates appears to be an economical approach to develop thermoplastic products from starch. KEYWORDS: starch, thermoplastics, methacrylates, grafting
■
extruded or processed into products.3,4 TPS could be used to manufacture films for food packing, extrudates for secondary packing, and micro and nanofibers for textile and medical applications. Similarly, TPS could be used as a resin, and natural fibers could be used as reinforcement to obtain completely degradable composites. However, thermoplastic starch has several limitations. For instance, TPS is mostly water-soluble and provides products with poor strength.3,5 Blending of synthetic polymers, inorganic materials, and additives such as lignin and nanoclay have been used to improve the properties of products developed from thermoplastic starch.6 Cross-linking is another approach that has been commonly used to improve the properties of starch products.7 It was previously shown that solution cast starch films, after being cross-linked with citric acid, had much higher tensile strength and water stability.7 Chemical modifications, such as acetylation, hydroxypropylation, and grafting, have also been used to modify starch and develop thermoplastic products.8,9 Starch acetates have been produced on a commercial scale and used in pharmaceutical, food, and other industries. Although acetylation makes starch thermoplastic and provides products with good tensile
INTRODUCTION Starch is one of the most widely used biopolymers for food and nonfood applications. Large availability, low cost, and easy biodegradability are some of the major features that make starch attractive for various applications. Starch is found in several sources, including cereal grains, vegetables, and fruits, but the basic structure and properties of starch are the same. Nonfood applications of starch include paper, pharmaceutical, biofuel, textile, and packaging, in addition to many others. Although ethanol production, paper, and other applications use starch in its native form, many types of physical and/or chemical modifications are done to make starch suitable for processing and to develop products. Modifications are also necessary to improve the properties, such as increasing its tensile strength and hydrophobicity, and to obtain starch-based products with desired features.1 Modified or unmodified starches are made into products mainly by the wet or dry approach. Wet processing of starch includes dissolving or dispersing starch and casting the solution, mainly to form films. Dry processing of starch includes extrusion, injection, and compression molding to form foams, sheets, films, and other products. Although dry processing of starch is preferred over wet processing, native starch is nonthermoplastic, and it is necessary to modify starch to produce thermoplastic products.2 Thermoplastic starch, commonly referred to as TPS, is made by mixing starch with water and plasticizers under heat and can be © 2014 American Chemical Society
Received: Revised: Accepted: Published: 4668
March 20, 2014 April 29, 2014 May 1, 2014 May 12, 2014 dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
Journal of Agricultural and Food Chemistry
Article
acrylate, methyl methacrylate, and a mixture of methyl and butyl acrylate and extruded them into ribbons.20 Plastics obtained from starch-g-methyl methacrylate were described as being brittle, but those grafted with methyl acrylate and butyl acrylate were flexible and leathery.20 As seen from the literature discussed above, limited study has been done on developing thermoplastics products from grafted starch, and only a few monomers have been studied. The influence of different acrylic monomers, their structures, and chain lengths, and the amount of homopolymers on the tensile properties of compressionmolded films is not known. More importantly, the resistance of the films to water and high humidity and the suitability of the starch films for various applications have not been investigated. In this research, a series of vinyl monomers with increasing alkyl chain lengths were grafted onto starch. Grafting conditions were optimized, and the grafted starch was characterized for thermal behavior and structural changes. Grafted starch was compression molded into thermoplastic films, and the influences of % homopolymers on the dry and wet tensile properties were studied. The ability of the different monomers to improve the strength and water stability were also investigated.
properties and water stability, acetylation is expensive and not affordable to develop commodity products such as films. Recently, acylation of starch using enzymes has been done to avoid the harsh conditions necessary during other chemical modifications.1 Grafting of polymers is another approach that has been commonly used to develop products from starch. Both natural and synthetic polymers have been grafted onto starch to impart desired properties to starch.9 A multitude of synthetic monomers such as acrylamide, polyurethane, polystyrene, acrylonitrile, and vinyl monomers have been grafted onto starch, and the thermal behavior of the grafted materials have been compared to the unmodified starch.9−11 Among the various types of monomers grafted onto starch, acrylic monomers such as methyl, ethyl, and butyl acrylates, and the corresponding methacrylates, are preferred due to their lower cost and ability to provide good performance properties.12 Grafting of acrylic monomers onto starch and other biopolymers has been extensively studied, and the influence of catalysts, initiators, and monomers on the grafting parameters and properties of the products obtained has also been investigated. For example, methyl methacrylate (MMA) was grafted onto sago starch in aqueous media using different initiators of ceric ammonium nitrate and potassium persulfate under nitrogen atmosphere.12 On the basis of FTIR studies, it was suggested that MMA was successfully grafted onto starch and that the grafted starch was suitable for developing thermoplastic products.12 In another study, cassava starch was grafted with poly(methyl methacrylate) using benzoyl peroxide as initiator.13 Starch with 37.2% grafting ratio, 95.5% yield, and 54.3% homopolymer was obtained after the reaction. Ethyl acrylate and acrylamide were grafted onto starch by copolymerization using potassium permanganate-citric acid as initiator, and the reaction conditions were optimized. Grafting of the copolymers onto starch was proven using FTIR.9 In another study, corn starch was first alkenylsuccinylated to different degrees of substitutions and was later graftcopolymerized with acrylic acid.14 Alkenylsuccinylation resulted in higher grafting efficiency and stronger films. The influence of the alkyl chain length on the grafting efficiency was studied using three vinyl monomers [methyl methacrylate (MMA), ethyl methacrylate (EMA), and butyl methacrylate (BMA)] using ceric ammonium nitrate as the initiator. Optimum grafting conditions were developed, and it was found that the reactivity of the methacrylates increased with increasing alkyl group length.15 It was also suggested that the monomers were grafted onto the amorphous and crystalline regions. Crystallinity and thermal stability of the grafted starch were found to decrease after grafting. Starch was graft copolymerized with methacrylonitrile and MMA, EMA, BMA, and HMA. Among the four methacrylates studied, HMA was found to have considerably lower % grafting, which was inferred to result from the lesser accessibility of HMA to the active sites of the carbohydrate.16 Efforts have also been made to develop thermoplastics from grafted starch.17 Starch grafted with methyl acrylates and methacrylates has been injection molded into ribbons, and the properties of the extruded ribbons have been investigated.18,19 Films with tensile values ranging from 8 to 17 MN/mm2 and with elongations from 2% to 150% were obtained, depending on the level of grafting and the aging time.18 In other research, starch-g-PMA with PMA contents between 59% and 67% provided films with tensile strengths ranging from 9 to 24 MN/ m2 and elongations of 30−265%.19 Bagley et al. grafted methyl
■
MATERIALS AND METHODS
Materials. Corn starch (regular) was purchased from the National Starch Company and used as received. Four vinyl monomers (MMA, EMA, BMA, and HMA) were selected for grafting onto starch based on our previous studies on grafting vinyl monomers onto chicken feathers and camelina meal.21,22 Monomers, potassium persulfate used as initiator, sodium bisulfite used as reductant, and other chemicals required for the study were reagent grade. Methods. Grafting. Starch was first dispersed in water, and the required ratios of monomers were added into a four-necked flask along with the initiator and reductant. Oxygen in the flask was removed by displacing it with nitrogen, and the entire reaction was performed under nitrogen atmosphere. Grafting conditions such as monomer concentration to starch (30−140%), reaction time (0.5 to 4 h), and reaction temperature were varied to obtain the optimum grafting conditions. After the required grafting time, the reaction was stopped by adding 2% paradioxybenzene. Grafted starch was thoroughly washed in water to remove unreacted monomers and inorganic salts and later dried and collected. Grafting parameters, such as grafting efficiency, molar grafting ratio (mol/g), monomer conversion (%), and % homopolymers were calculated using equations reported in our previous researches.21 Thermal Analysis. Grafted starch was analyzed to determine the thermal behavior using differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA). DSC studies were done on a Mettler Toledo calorimeter (Model D822e; Mettler Toledo, Columbus, OH) under nitrogen atmosphere with temperatures ranging from 25 °C to 250 °C. Three scans were done for each sample, and the average data were used to draw the melting curves. TGA studies were done on a Sigma T300 analyzer with 10 mg of sample, and three samples were analyzed under nitrogen for each condition. An average of the three samples were used to plot the thermal degradation curves. Fourier Transform Infrared (FTIR). Grafted and ungrafted starch samples were analyzed on a Fourier Transform Infrared Spectrophotometer (Thermo Scientific Model: iS10; Thermo Scientific, Waltham, MA) on an attenuated total reflectance mode using a diamond cell. Samples were scanned 32 times from 500 to 4000 cm−1 at a resolution of 8 cm−1. Three spectras were collected for each sample, and the average values were used for comparison. 1 H NMR. Nuclear magnetic resonance studies were conducted to confirm grafting of the monomers onto starch. For the NMR analysis, samples were dissolved in acetone (1 wt %) and scanned on a Avance 600 MHz Digital NMR spectrometer (Bruker Co. Ltd. Switzerland). A 4669
dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
Journal of Agricultural and Food Chemistry
Article
Figure 1. Influence of grafting temperature on % grafting efficiency (a), % homopolymers (b), % monomer conversion (c), and molar grafting ratio (d). Grafting was performed using 100% of monomers based on weight of starch for 180 min at various temperatures. For each monomer, data points with the different letters indicate statistically significant difference. Dry and Wet Tensile Properties. Compression-molded films were first conditioned at 21 °C and 65% relative humidity for at least 48 h before being tested. To determine the stability under wet conditions, samples were conditioned at 90% humidity for 24 h in a Caron environmental chamber and then immediately tested. Tensile tests to determine the strength, elongation, and modulus were done on an MTS tensile tester (Model Qtest 10, MTS Corporation, Duluth, MN) according to ASTM standard D 882. The gauge length was 5 cm, and the crosshead speed was 10 mm/min. At least 20 samples from three different films were tested, and the average and standard deviations of the strength, elongation, and modulus were reported.
basic proton pulse sequence (zg30) was used with a relaxation delay of 1 s and acquisition time of 3 s and operated at 600 MHz. Spectra were phase corrected interactively using TOPSPIN. Baseline correction was carried out manually using the appropriate factors. Influence of Homopolymers. Homopolymers formed by the reaction between the monomers will affect the thermal behavior and properties of the thermoplastic films developed. To understand the influence of homopolymers, starch was treated (Soxhlet extracted for 24 h) using acetone to remove the homopolymers. The amount of homopolymers in the grafted starch was determined based on the weight differences before and after extraction. A known amount of homopolymers (25−75%) was first dissolved in acetone, and the grafted samples were later added into the solution. Acetone was evaporated, and the samples containing homopolymers were dried and later powdered in a Wiley mill (20 mesh) before being compression molded into films. Developing Thermoplastics. Grafted starch with and without a known amount of homopolymers was compression molded to form films. Samples (about 10−12 g) were placed between two aluminum foils and compression molded on a carver press at different temperatures for 5 min at a pressure of 20 000−40 000 PSI. The different compression conditions were because of the varying thermoplasticities of the grafted samples. After compression, the press was cooled by running cold water, and samples were collected for further analysis.
■
STATISTICS Data obtained was analyzed using SAS by the Tukey’s pairwise comparison method. The significance level was set at 95% with α = 0.05, and a P value of less than 5% were considered insignificant.
■
RESULTS AND DISCUSSION Effect of Grafting Conditions on Grafting Parameters. Effect of Grafting Temperature. Figure 1a,d shows the influence of temperature on grafting parameters. Increasing the temperature decreased the % grafting efficiency for all four monomers studied, as seen from Figure 1a. There was no 4670
dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
Journal of Agricultural and Food Chemistry
Article
Figure 2. Influence of monomer concentration during grafting on % grafting efficiency (a), % homopolymers (b), % monomer conversion (c), and molar grafting ratio (d). Grafting was performed at 60 °C for 60 min using different concentrations of monomers. For each monomer, data points with the different letters indicate statistically significant difference.
significant change in the grafting efficiency when the temperature was between 40 °C and 60 °C, but the grafting efficiency decreased at 70 °C and 80 °C. Increasing the length of alkyl chain from 2 to 6 (MMA to HMA) decreased the % grafting efficiency at all temperatures studied. Previous studies on the grafting of starch with methacrylates have also shown that increasing temperature increased the grafting efficiency up to a certain temperature, and the % grafting efficiency (GE%) decreased at high temperature.15 Therefore, a decrease in % GE at high temperatures was attributed to the increase in termination rate of the starch grafted polymeric chain or due to chain transfer to monomer molecules.15 A decrease in the % GE leads to an increase in % homopolymers when the grafting temperature was 70 °C and 80 °C as seen from Figure 1b. At high temperatures, the monomers move faster and more readily react with each other and form the homopolymers. The % monomer conversion was considerably lower when the temperature was 40 °C, but increased to above 70% for all the monomers when the temperature was 80 °C or higher. Increasing the length of the alkyl chain decreased the % monomer conversion at all temperatures studied, as seen from Figure 1c due to the steric hindrance as the length of the carbon chain increases.16 This is more evident when the molar grafting ratio is considered for the different monomers. Increasing the temperature increased the molar grafting ratio for all the monomers, as seen from Figure 1d. More interestingly, the grafting ratios decreased as the length of the
alkyl chain increased, although the grafting conditions were similar. At a temperature of 60 °C, MMA had a grafting ratio of 5.9 mmol/g, compared to 2.9 mmol/g for HMA. MMA, being a much smaller molecule than HMA, was more easily accessible to starch and was easily grafted and provided higher % grafting. A temperature of 60 °C was found to provide the most optimum grafting for all four monomers studied. Effect of Monomer Concentration. Increasing monomer concentration continually decreased the % grafting efficiency and increased the % homopolymers, as seen from Figure 2a,b. The % homopolymers were less than 5% for all the monomers when the monomer concentration was 20%. Increasing the monomer concentration leads to a substantial increase in % homopolymers up to about 105−110% when the monomer concentration is 180%. When the monomer concentration was increased above 20%, a higher number of monomer molecules are available for reaction, and therefore, higher numbers of monomers were grafted onto starch. When the monomer concentration increased above an optimum level, the monomers collide with each other and form homopolymers because the growing poly(methacrylate) chain has greater compatibility with the monomer molecules.15,16 At low monomer concentrations (20%), the % monomer conversion is significantly lower (Figure 2c) because there are fewer molecules in solution that can react and form grafted chains or homopolymers. The molar grafting ratio increased with increasing monomer concentration for all the monomers, as 4671
dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
Journal of Agricultural and Food Chemistry
Article
Figure 3. Influence of reaction time on % grafting efficiency (a), % homopolymers (b), % monomer conversion (c), and molar grafting ratio (d). Grafting was performed at 60 °C with 1:1 ratio of monomer to starch. For each monomer, data points with the different letters indicate statistically significant difference.
seen from Figure 2d. However, the grafting ratio decreased substantially with increasing length of the alkyl chain due to steric hindrance of the longer chain monomers and subsequent difficulty in being grafted onto starch. Effect of Grafting Time. Grafting time did not show a major effect on % grafting efficiency, as seen from Figure 3a. Similarly, the % homopolymers (Figure 3b) also did not show any significant change with increasing reaction time. However, monomer conversion marginally increased with increasing reaction time, as seen from Figure 3c. Previous studies had shown that increasing reaction time from 30 to 60 min significantly increased % GE for MMA, EMA, and BMA. In our reaction, we added the monomers gradually for almost 30 min, and we therefore did not study a reaction time lower than 1 h. Most of the monomers have either been grafted or converted into homopolymers within 1 h, and we therefore do not observe any major change in grafting parameters with increasing reaction time. As observed before, higher alkyl chain length decreased the grafting ratio, and reaction time did not affect the grafting ratio, as seen from Figure 3d. Characterizing the Grafted Starch. Fourier Transform Infrared Spectroscopy. The characteristic carbonyl group in the methacrylates that appears as CO stretching at about 1730 cm−1 is seen in Figure 4 for all four spectra for the grafted starch. The intensity of this peak is highest for HMA, similar for EMA and MMA, and considerably lower for BMA. Presence of the additional peak in the grafted samples that belongs to the
Figure 4. FTIR spectrum of the unmodified and grafted starch depicting the characteristic peaks belonging to the carbonyl group in the monomers. The spectrum was recorded after extracting the homopolymers from the grafted samples.
methacrylates confirms grafting of the monomers onto starch.9,13 Broad peaks seen between 3500 and 3100 cm−1 are due to the hydroxyl groups in starch and the sharp peaks at about 2900 cm−1 are due to the C−H stretching from the grafted polymers, also suggesting successful grafting.12,13 4672
dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
Journal of Agricultural and Food Chemistry
Article
the grafted starch could be compression molded into films at 165 °C under pressure, thereby preventing excessive thermal damage to starch at high temperatures. 1 H NMR. Grafting of the poly(acrylates) onto starch was evident from the NMR spectra in Figure 7a−d below. As shown in Figure 7a, the H NMR resonances at 3.2−3.8 (protons 2,3,4,5,6), 4.3 (proton 7) and 5.0−5.3 (proton 1) were the characteristic signals from starch. The resonances at 3 and 2.5 ppm were from water and DMSO, respectively. Figure 7b−e illustrated that the H NMR spectrum for the MMA-, EMA-, BMA-, and HMA-grafted starch, respectively. It could be seen that polyacrylates and starch moieties were present in the spectrum. After grafting with acrylates, resonances at 0.9−2.0 ppm appeared, which were the signals of CH3, CH2, and CH groups in the polyacrylate. The resonance of 4.3 ppm (proton 7) decreased or disappeared in the spectra, which clearly confirmed the grafting of polyacrylates onto the hydroxyl group of starch. The signal of COOCH3 group of polyacrylate overlapped with protons 2−6 of the starch, which made the resonance at 3.2−3.8 ppm more complicated. Tensile Properties of Compression Molded Starch Films. Table 1 provides a comparison of the tensile strength of the starch films grafted with the four different monomers and containing different levels of homopolymers. On the basis of the tensile strength in Table 1, it can be observed that increasing the length of alkyl chains from 2 to 4 increased the strength of the films when there was no homopolymer but HMA grafted starch films had considerably lower strength than BMA-grafted films. BMA had strength of 1 MPa compared to 0.2 and 0.3 MPa for MMA and EMA, respectively, whereas HMA-grafted films have a strength of 0.5 MPa. Addition of homopolymers continually increased the strength for all the monomers. With 50% homopolymers, the strength of the MMA- and EMA-grafted films were nearly 10 times higher, 3 times higher for BMA films, and twice higher for HMA films. Films made from the 100% homopolymers showed that increasing the length of the alkyl chain considerably decreased the tensile strength. For instance, MMA-homopolymer films had a strength of 12.8 MPa, compared to 3 MPa for HMAhomopolymer films. MMA- and EMA-grafted films showed considerably lower elongation than BMA and HMA films, especially with the addition of homopolymers. With 50% homopolymers, BMA films had the highest strength, although HMA-grafted films showed higher elongation than the films containing the other monomers. Although MMA and EMA provided films with good dry tensile properties, the films containing up to 75% homopolymers were unstable in water, became too weak, and could not be tested even after conditioning at 90% humidity for 24 h at 21 °C. BMA- and HMA-grafted films showed good stability to moisture, especially in the presence of homopolymers, as seen from Table 1. In fact, the wet strength of the films grafted with HMA and containing 50% and 75% homopolymers is similar to or higher than the dry strength of the films. Tensile properties of the grafted starch films will depend on the extent of grafting, amount of homopolymers, and the inherent properties of the monomers and homopolymers. BMA provides better thermoplasticity, melts the starch and therefore provides the films with good tensile strength. In addition, increasing the alkyl chain length decreased the hydrophilicity, and therefore, BMA- and HMA-grafted films had better water stability than MMA and EMA films.
Thermal Behavior. Thermal degradations of the grafted starch samples were similar as seen from Figures 5 and 6. A
Figure 5. Thermal degradation (% weight loss) of the grafted starch samples when heated to 580 °C.
Figure 6. DSC thermograms illustrating that the starches grafted with higher lengths of alkyl chains had better melting when heated to 250 °C.
sharp decrease in the curve is seen around 220 °C, suggesting the start of degradation. MMA showed a slightly lower weight loss than the other samples up to about 300 °C, whereas HMA and EMA showed lower weight loss between 300 °C and 400 °C. An overall weight loss of about 80% was obtained after heating to 600 °C. DSC thermograms clearly show that starch grafted with longer lengths of alkyl esters (BMA and HMA) melted at higher temperatures and had sharper peaks compared to the starch grafted with MMA and EMA. Among the four monomers studied, EMA-grafted starch had a relatively lower melting enthalpy of about 12.6 J/g compared to 50−53 J/g for the other three monomers. As seen from Figure 6, HMA had a sharp and large melting peak at about 210 °C, whereas MMA and EMA produced broad melting peaks ranging from 160 °C to 200 °C. BMA and HMA are more thermally stable than MMA and EMA and therefore melt at higher temperatures. The sharp melting peaks for HMA and BMA suggest that these two polymers may provide better thermoplastic products but need higher processing temperatures. Although the melting peaks for the grafted starch were between 160 °C and 200 °C, 4673
dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
Journal of Agricultural and Food Chemistry
Article
Figure 7. 1H NMR spectra of the ungrafted starch (a), MMA-grafted starch (b), EMA-grafted starch (c), BMA-grafted starch (d), and HMA-grafted starch (e) with the position of the various hydrogen atoms.
Elongation of the films also showed that increasing the alkyl chain length and amount of monomers increases elongation. Without any homopolymers, MMA had an elongation of 2.8%, compared to 3.2% for HMA. Starch films grafted with HMA were considerably softer and therefore had lower strength but higher elongation (than BMA containing films). Increasing the length of alkyl chains increases the flexibility of the side chains, and therefore, the elongation of the films with BMA and HMA was higher than those of MMA- and EMA-containing homopolymers. Addition of homopolymers did not show an increase in the elongation of MMA- or EMA-grafted films, but
the elongation of the BMA- and HMA-grafted films increased up to 10 times. BMA-grafted films without homopolymers had an elongation of 3.1%, compared to 8.5% when 50% homopolymers were added. HMA-grafted films without any homopolymers had an elongation of 3.2%, compared to 10.4% when the homopolymer content was 50%. Increase in elongation for the BMA and HMA films should be due to the high elongation of the BMA (71%) and HMA (111%) homopolymers. Wet elongation of the BMA- and HMA-grafted films also showed a similar trend with elongations of 8.6 and 4674
dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
Journal of Agricultural and Food Chemistry
Article
Table 1. Dry and Wet Tensile Strength of Starch Films Grafted (2.7 mmol/(g) with Four Different Monomers and Containing Different Ratios of Homopolymersa breaking stress, MPa MMA % homo polymers 0 25 50 75 100
dry 0.23 0.45 2.9 4.1 12.8
± ± ± ± ±
a
0.04 0.01a 0.6d 0.5f 1.0h
EMA wet
dry
0 0 0 0 5.0 ± 1.2H
± ± ± ± ±
0.36 2.4 3.0 3.9 8.5
0.09 0.7c 0.4d 0.5f 0.9g
a
BMA wet
dry
0 0 0 0 5.5 ± 0.7
± ± ± ± ±
1.0 2.5 3.4 4.3 5.8
HMA wet
b
0.1 0.4c 0.2e 0.4f 0.6c
0.4 1.9 1.6 2.1 2.5
± ± ± ± ±
dry E
0.03 0.6D 0.7D 0.6C 0.6C
0.5 0.8 1.0 1.5 3.0
± ± ± ± ±
wet a
0.1 0.1b 0.1b 0.4b 0.8e
0.4 1.4 1.6 2.1 2.5
± ± ± ± ±
0.09E 0.5D 0.7D 0.6C 0.6C
a Films were compression molded at 165 °C for 5 min. For each monomer, data points with the different letters indicate statistically significant difference.
aging. BMA-grafted films showed good dry and wet tensile properties even when the grafting was 30%. Therefore, BMA grafted starch films could be useful for food and other applications. Starch grafted with BMA provided thermoplastic films with good strength and water stability. At similar grafting ratios, although increasing the length of the alkyl esters provided higher thermoplasticity and increased water stability, HMA homopolymers were considerably softer and decreased the strength of the starch films. Increasing the chain length also decreased the grafting ratio due to stearic hindrance from the monomers with longer side chains. Homopolymers considerably affected the tensile properties and water stability. Increasing the % of homopolymers generally increased the tensile strength and elongation. Films grafted with EMA were considerably brittle with elongation of 1−3%, whereas films grafted with BMA and HMA had elongations between 5% and 35%, depending on the amount of homopolymers. Overall, BMA was found to be more suitable for developing thermoplastic starch products with good tensile properties and water stability.
16.4% for the BMA and HMA films containing 50% homopolymers. On the basis of the strength and elongation of starch films grafted with various monomers, BMA was found to be more suitable to obtain films with good strength and elongation. A digital picture of a film made by compression molding starch grafted with BMA is shown in Figure 8. The film is transparent,
■
Figure 8. A digital picture of a film made by compression molding starch grafted with BMA.
AUTHOR INFORMATION
Corresponding Author
*Fax: 402-472-0640; e-mail: [email protected].
indicating that the grafted starch could be melted and made into thermoplastic products. Further studies were done to develop films from BMA with different grafting ratios and study the dry and wet tensile properties, since films grafted with MMA and EMA were weak and could not be tested after conditioning at 90% humidity for 24 h. Increasing the % grafting from 2.1 to 4.2 mmol/g (30% to 60% grafting) substantially increased the strength and elongation of the BMA grafted starch films from 1 to 3.2 MPa. There was also a considerable increase in the wet strength of the films (from 0.4 to 1.8 MPa) because of the increase in the amount of the hydrophobic polymer with increasing % grafting. When the grafting was 50%, the wet strength of the films was about 30% of the dry strength. Elongation of the films also increased from 3% to 12% when the grafting ratio was increased from 2.1 to 4.2 mmol/g. The corresponding increase in the wet elongation was from 6.7% to 25%. Strength and elongation of the films increased with increasing % grafting, since the grafted starch became more thermoplastic and formed better films. In addition, the grafted polymer had better strength and elongation than starch, and increasing the % of the grafted polymer therefore increased the tensile properties. Stability of the films under high humidities and in water is important for various applications, especially for food pack-
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors thank the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1135) at Jiangnan University, Scientific Support Program of Jiangsu Province (BE2011404), the Graduate student innovation plan of Jiangsu Province (CX10B_222Z), the Doctor Candidate Foundation of Jiangnan University (JUDCF10004), agricultural research division, and the USDA-HATCH act and Multistate project S1054(NEB 37-037) at the University of Nebraska Lincoln for their financial support. The China scholarship council is also acknowledged for providing the living expenses for the visiting scholar (Z.S.).
■
REFERENCES
(1) Alissandratos, A.; Halling, P. J. Enzymatic acylation of starch. Biores. Technol. 2012, 115, 41−47. (2) Jimenez, A.; Jose Fabra, M.; Talens, P.; Chiralt, A. Edible and biodegradable starch films: A Review. Food Bioproc. Technol. 2012, 5 (6), 2058−2076.
4675
dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
Journal of Agricultural and Food Chemistry
Article
(3) Cyras, V. P.; Manfredi, L. B.; Ton-That, M.-T.; Vazquez, A. Physical and mechanical properties of thermoplastic starch/montmorillonite nanocomposite films. Carbohyd. Polymr. 2008, 73 (1), 55−63. (4) Arvanitoyannis, I. S.; Vlachos, A. Maize Authentication: Quality Control Methods and Multivariate Analysis (Chemometrics). Crit. Rev. Food Sci. Nutr. 2009, 49 (6), 501−537. (5) He, G. J.; Liu, Q.; Thompson, M. R. Characterization of structure and properties of thermoplastic potato starch film surface cross-linked by UV irradiation. Starch-Starke 2013, 65 (3−4), 304−311. (6) Soares, F. C.; Yamashita, F.; Mueller, C. M. O.; Pires, A. T. N. Thermoplastic starch/poly(lactic acid) sheets coated with cross-linked chitosan. Polym. Test. 2013, 32 (1), 94−98. (7) Reddy, N.; Yang, Y. Citric acid cross-linking of starch films. Food Chem. 2010, 118 (3), 702−711. (8) Singh, A. V.; Nath, L. K.; Guha, M. Synthesis and characterization of highly acetylated sago starch. Starch-Starke 2011, 63 (9), 523−527. (9) Zhang, B.; Zhou, Y. Synthesis and characterization of graft copolymers of ethyl acrylate/acrylamide mixtures onto starch. Polym. Comp. 2008, 29 (5), 506−510. (10) Jyothi, A. N. Starch graft copolymers: Novel applications in industry. Comp. Interface 2010, 17 (2−3), 165−174. (11) Kaewtatip, K.; Tanrattanakul, V.; Szecsenyi, K. M.; Pavlicevic, J.; Budinski-Simendic, J. Thermal properties and morphology of cassava starch grafted with different content of polystyrene. J. Thermal Anal. Calorim. 2010, 102 (3), 1035−1041. (12) Fakhru’l-Razi, A.; Qudsieh, I. Y. M.; Yunus, W.; Ahmad, M. B.; Rahman, M. Z. A. Graft copolymerization of methyl methacrylate onto sago starch using ceric ammonium nitrate and potassium persulfate as redox initiator systems. J. Appl. Polym. Sci. 2001, 82 (6), 1375−1381. (13) Pimpan, V.; Thothong, P. Synthesis of cassava starch-gpoly(methyl methacrylate) copolymers with benzoyl peroxide as an initiator. J. Appl. Polym. Sci. 2006, 101 (6), 4083−4089. (14) Zhu, Z.; Zhang, L.; Li, M.; Zhou, Y. Effects of starch alkenylsuccinylation on the grafting efficiency, paste viscosity, and film properties of alkenylsuccinylated starch-g-poly(acrylic acid). StarchStarke 2012, 64 (9), 704−712. (15) Athawale, V. D.; Rathi, S. C. Effect of chain length of the alkyl group of alkyl methacrylates on graft polymerization onto starch using ceric ammonium nitrate as initiator. Eur. Polym. J. 1997, 33 (7), 1067− 1071. (16) Vazquez, B.; Goni, I.; Gurruchaga, M.; Valero, M.; Guzman, G. M. Synthesis and characterization of graft-copolymers of methacrylonitrile methacrylate mixtures onto amylomaize by the ceric ion method. J. Polym. Sci., Part A: Polym. Chem. 1992, 30 (8), 1541−1548. (17) Willet (Willett, J. L. Starch in Polymer Compositions. In Starch: Chemistry and Technology, 3rd ed.; BeMiller, J., Whistler, R., Eds.; Academic Press: New York, 2009; pp 715−737. (18) Trimnell, D.; Swanson, C. L.; Shogren, R. L.; Fanta, G. F. Extrusion processing of granular starch-g-poly(methylacrylate)-effect of extrusion conditions on morphology and properties. J. Appl. Polym. Sci. 1993, 48 (9), 1665−1675. (19) Dennenberg, R. J.; Bothast, R. J.; Abbott, T. P. A new biodegradable plastic made from starch graft poly(methyl acrylate) copolymers. J. Appl. Polym. Sci. 1978, 22, 459−465. (20) Bagley, E. B.; Fants, G. F.; Burr, R. C.; Doane, W. M.; Russell, C. R. Graft copolymers of polysaccharides with thermopalastic polymers, a new type of filled plastic. Polym. Engg. Sci. 1977, 17 (5), 311−315. (21) Jin, E.; Reddy, N.; Zhu, Z.; Yang, Y. Graft polymerization of native chicken feathers for thermoplastic applications. J. Agric. Food Chem. 2011, 59 (5), 1729−1738. (22) Reddy, N.; Jin, E.; Chen, L.; Jiang, X.; Yang, Y. Extraction, characterization of components, and potential thermoplastic applications of camelina meal grafted with vinyl monomers. J. Agric. Food Chem. 2012, 60 (19), 4872−4879.
4676
dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
Effects of Monomers and Homopolymer Contents on the Dry and Wet Tensile Properties of Starch Films Grafted with Various Methacrylates Zhen Shi,†,‡ Narendra Reddy,‡ Li Shen,§ Xiuliang Hou,† and Yiqi Yang*,†,‡,§,∥,⊥ †
Key Laboratory of Eco-textiles, Ministry of Education, College of Textiles and Garments, Jiangnan University, Lihu Road #1800, Wuxi 214122, People’s Republic of China ‡ Department of Textiles, Merchandising, and Fashion Design, 234 HECO Building, East Campus, University of NebraskaLincoln, Lincoln, Nebraska 68583-0802, United States § College of Chemistry, Chemical Engineering, and Biotechnology, Donghua University, No. 1882 West Yan'an Road, Shanghai 200051, People’s Republic of China ∥ Department of Biological Systems Engineering and ⊥Nebraska Center for Materials and Nanoscience 234 HECO Building, East Campus, University of NebraskaLincoln, Lincoln, Nebraska 68583-0802, United States ABSTRACT: Starch grafted with four different methacrylates was compression molded to form thermoplastic films with good strength and water stability. Starch is an inexpensive and biodegradable polymer but is nonthermoplastic and needs to be chemically modified to make starch suitable for various applications. In this research, starch was grafted with four methacrylates (methyl, ethyl, butyl, and hexyl), and the effect of the length of the alkyl ester group on grafting parameters, thermoplasticity, and properties of thermoplastic films developed have been studied. Influence of grafting conditions on % grafting efficiency, % homopolymers, and % monomer conversion were studied, and the grafted starch was characterized using thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), and nuclear magnetic resonance (1H NMR). At similar grafting ratios, butyl methacrylate (BMA) provided better strength and elongation to the starch films than the other three methacrylates. Grafting of methacrylates appears to be an economical approach to develop thermoplastic products from starch. KEYWORDS: starch, thermoplastics, methacrylates, grafting
■
extruded or processed into products.3,4 TPS could be used to manufacture films for food packing, extrudates for secondary packing, and micro and nanofibers for textile and medical applications. Similarly, TPS could be used as a resin, and natural fibers could be used as reinforcement to obtain completely degradable composites. However, thermoplastic starch has several limitations. For instance, TPS is mostly water-soluble and provides products with poor strength.3,5 Blending of synthetic polymers, inorganic materials, and additives such as lignin and nanoclay have been used to improve the properties of products developed from thermoplastic starch.6 Cross-linking is another approach that has been commonly used to improve the properties of starch products.7 It was previously shown that solution cast starch films, after being cross-linked with citric acid, had much higher tensile strength and water stability.7 Chemical modifications, such as acetylation, hydroxypropylation, and grafting, have also been used to modify starch and develop thermoplastic products.8,9 Starch acetates have been produced on a commercial scale and used in pharmaceutical, food, and other industries. Although acetylation makes starch thermoplastic and provides products with good tensile
INTRODUCTION Starch is one of the most widely used biopolymers for food and nonfood applications. Large availability, low cost, and easy biodegradability are some of the major features that make starch attractive for various applications. Starch is found in several sources, including cereal grains, vegetables, and fruits, but the basic structure and properties of starch are the same. Nonfood applications of starch include paper, pharmaceutical, biofuel, textile, and packaging, in addition to many others. Although ethanol production, paper, and other applications use starch in its native form, many types of physical and/or chemical modifications are done to make starch suitable for processing and to develop products. Modifications are also necessary to improve the properties, such as increasing its tensile strength and hydrophobicity, and to obtain starch-based products with desired features.1 Modified or unmodified starches are made into products mainly by the wet or dry approach. Wet processing of starch includes dissolving or dispersing starch and casting the solution, mainly to form films. Dry processing of starch includes extrusion, injection, and compression molding to form foams, sheets, films, and other products. Although dry processing of starch is preferred over wet processing, native starch is nonthermoplastic, and it is necessary to modify starch to produce thermoplastic products.2 Thermoplastic starch, commonly referred to as TPS, is made by mixing starch with water and plasticizers under heat and can be © 2014 American Chemical Society
Received: Revised: Accepted: Published: 4668
March 20, 2014 April 29, 2014 May 1, 2014 May 12, 2014 dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
Journal of Agricultural and Food Chemistry
Article
acrylate, methyl methacrylate, and a mixture of methyl and butyl acrylate and extruded them into ribbons.20 Plastics obtained from starch-g-methyl methacrylate were described as being brittle, but those grafted with methyl acrylate and butyl acrylate were flexible and leathery.20 As seen from the literature discussed above, limited study has been done on developing thermoplastics products from grafted starch, and only a few monomers have been studied. The influence of different acrylic monomers, their structures, and chain lengths, and the amount of homopolymers on the tensile properties of compressionmolded films is not known. More importantly, the resistance of the films to water and high humidity and the suitability of the starch films for various applications have not been investigated. In this research, a series of vinyl monomers with increasing alkyl chain lengths were grafted onto starch. Grafting conditions were optimized, and the grafted starch was characterized for thermal behavior and structural changes. Grafted starch was compression molded into thermoplastic films, and the influences of % homopolymers on the dry and wet tensile properties were studied. The ability of the different monomers to improve the strength and water stability were also investigated.
properties and water stability, acetylation is expensive and not affordable to develop commodity products such as films. Recently, acylation of starch using enzymes has been done to avoid the harsh conditions necessary during other chemical modifications.1 Grafting of polymers is another approach that has been commonly used to develop products from starch. Both natural and synthetic polymers have been grafted onto starch to impart desired properties to starch.9 A multitude of synthetic monomers such as acrylamide, polyurethane, polystyrene, acrylonitrile, and vinyl monomers have been grafted onto starch, and the thermal behavior of the grafted materials have been compared to the unmodified starch.9−11 Among the various types of monomers grafted onto starch, acrylic monomers such as methyl, ethyl, and butyl acrylates, and the corresponding methacrylates, are preferred due to their lower cost and ability to provide good performance properties.12 Grafting of acrylic monomers onto starch and other biopolymers has been extensively studied, and the influence of catalysts, initiators, and monomers on the grafting parameters and properties of the products obtained has also been investigated. For example, methyl methacrylate (MMA) was grafted onto sago starch in aqueous media using different initiators of ceric ammonium nitrate and potassium persulfate under nitrogen atmosphere.12 On the basis of FTIR studies, it was suggested that MMA was successfully grafted onto starch and that the grafted starch was suitable for developing thermoplastic products.12 In another study, cassava starch was grafted with poly(methyl methacrylate) using benzoyl peroxide as initiator.13 Starch with 37.2% grafting ratio, 95.5% yield, and 54.3% homopolymer was obtained after the reaction. Ethyl acrylate and acrylamide were grafted onto starch by copolymerization using potassium permanganate-citric acid as initiator, and the reaction conditions were optimized. Grafting of the copolymers onto starch was proven using FTIR.9 In another study, corn starch was first alkenylsuccinylated to different degrees of substitutions and was later graftcopolymerized with acrylic acid.14 Alkenylsuccinylation resulted in higher grafting efficiency and stronger films. The influence of the alkyl chain length on the grafting efficiency was studied using three vinyl monomers [methyl methacrylate (MMA), ethyl methacrylate (EMA), and butyl methacrylate (BMA)] using ceric ammonium nitrate as the initiator. Optimum grafting conditions were developed, and it was found that the reactivity of the methacrylates increased with increasing alkyl group length.15 It was also suggested that the monomers were grafted onto the amorphous and crystalline regions. Crystallinity and thermal stability of the grafted starch were found to decrease after grafting. Starch was graft copolymerized with methacrylonitrile and MMA, EMA, BMA, and HMA. Among the four methacrylates studied, HMA was found to have considerably lower % grafting, which was inferred to result from the lesser accessibility of HMA to the active sites of the carbohydrate.16 Efforts have also been made to develop thermoplastics from grafted starch.17 Starch grafted with methyl acrylates and methacrylates has been injection molded into ribbons, and the properties of the extruded ribbons have been investigated.18,19 Films with tensile values ranging from 8 to 17 MN/mm2 and with elongations from 2% to 150% were obtained, depending on the level of grafting and the aging time.18 In other research, starch-g-PMA with PMA contents between 59% and 67% provided films with tensile strengths ranging from 9 to 24 MN/ m2 and elongations of 30−265%.19 Bagley et al. grafted methyl
■
MATERIALS AND METHODS
Materials. Corn starch (regular) was purchased from the National Starch Company and used as received. Four vinyl monomers (MMA, EMA, BMA, and HMA) were selected for grafting onto starch based on our previous studies on grafting vinyl monomers onto chicken feathers and camelina meal.21,22 Monomers, potassium persulfate used as initiator, sodium bisulfite used as reductant, and other chemicals required for the study were reagent grade. Methods. Grafting. Starch was first dispersed in water, and the required ratios of monomers were added into a four-necked flask along with the initiator and reductant. Oxygen in the flask was removed by displacing it with nitrogen, and the entire reaction was performed under nitrogen atmosphere. Grafting conditions such as monomer concentration to starch (30−140%), reaction time (0.5 to 4 h), and reaction temperature were varied to obtain the optimum grafting conditions. After the required grafting time, the reaction was stopped by adding 2% paradioxybenzene. Grafted starch was thoroughly washed in water to remove unreacted monomers and inorganic salts and later dried and collected. Grafting parameters, such as grafting efficiency, molar grafting ratio (mol/g), monomer conversion (%), and % homopolymers were calculated using equations reported in our previous researches.21 Thermal Analysis. Grafted starch was analyzed to determine the thermal behavior using differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA). DSC studies were done on a Mettler Toledo calorimeter (Model D822e; Mettler Toledo, Columbus, OH) under nitrogen atmosphere with temperatures ranging from 25 °C to 250 °C. Three scans were done for each sample, and the average data were used to draw the melting curves. TGA studies were done on a Sigma T300 analyzer with 10 mg of sample, and three samples were analyzed under nitrogen for each condition. An average of the three samples were used to plot the thermal degradation curves. Fourier Transform Infrared (FTIR). Grafted and ungrafted starch samples were analyzed on a Fourier Transform Infrared Spectrophotometer (Thermo Scientific Model: iS10; Thermo Scientific, Waltham, MA) on an attenuated total reflectance mode using a diamond cell. Samples were scanned 32 times from 500 to 4000 cm−1 at a resolution of 8 cm−1. Three spectras were collected for each sample, and the average values were used for comparison. 1 H NMR. Nuclear magnetic resonance studies were conducted to confirm grafting of the monomers onto starch. For the NMR analysis, samples were dissolved in acetone (1 wt %) and scanned on a Avance 600 MHz Digital NMR spectrometer (Bruker Co. Ltd. Switzerland). A 4669
dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
Journal of Agricultural and Food Chemistry
Article
Figure 1. Influence of grafting temperature on % grafting efficiency (a), % homopolymers (b), % monomer conversion (c), and molar grafting ratio (d). Grafting was performed using 100% of monomers based on weight of starch for 180 min at various temperatures. For each monomer, data points with the different letters indicate statistically significant difference. Dry and Wet Tensile Properties. Compression-molded films were first conditioned at 21 °C and 65% relative humidity for at least 48 h before being tested. To determine the stability under wet conditions, samples were conditioned at 90% humidity for 24 h in a Caron environmental chamber and then immediately tested. Tensile tests to determine the strength, elongation, and modulus were done on an MTS tensile tester (Model Qtest 10, MTS Corporation, Duluth, MN) according to ASTM standard D 882. The gauge length was 5 cm, and the crosshead speed was 10 mm/min. At least 20 samples from three different films were tested, and the average and standard deviations of the strength, elongation, and modulus were reported.
basic proton pulse sequence (zg30) was used with a relaxation delay of 1 s and acquisition time of 3 s and operated at 600 MHz. Spectra were phase corrected interactively using TOPSPIN. Baseline correction was carried out manually using the appropriate factors. Influence of Homopolymers. Homopolymers formed by the reaction between the monomers will affect the thermal behavior and properties of the thermoplastic films developed. To understand the influence of homopolymers, starch was treated (Soxhlet extracted for 24 h) using acetone to remove the homopolymers. The amount of homopolymers in the grafted starch was determined based on the weight differences before and after extraction. A known amount of homopolymers (25−75%) was first dissolved in acetone, and the grafted samples were later added into the solution. Acetone was evaporated, and the samples containing homopolymers were dried and later powdered in a Wiley mill (20 mesh) before being compression molded into films. Developing Thermoplastics. Grafted starch with and without a known amount of homopolymers was compression molded to form films. Samples (about 10−12 g) were placed between two aluminum foils and compression molded on a carver press at different temperatures for 5 min at a pressure of 20 000−40 000 PSI. The different compression conditions were because of the varying thermoplasticities of the grafted samples. After compression, the press was cooled by running cold water, and samples were collected for further analysis.
■
STATISTICS Data obtained was analyzed using SAS by the Tukey’s pairwise comparison method. The significance level was set at 95% with α = 0.05, and a P value of less than 5% were considered insignificant.
■
RESULTS AND DISCUSSION Effect of Grafting Conditions on Grafting Parameters. Effect of Grafting Temperature. Figure 1a,d shows the influence of temperature on grafting parameters. Increasing the temperature decreased the % grafting efficiency for all four monomers studied, as seen from Figure 1a. There was no 4670
dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
Journal of Agricultural and Food Chemistry
Article
Figure 2. Influence of monomer concentration during grafting on % grafting efficiency (a), % homopolymers (b), % monomer conversion (c), and molar grafting ratio (d). Grafting was performed at 60 °C for 60 min using different concentrations of monomers. For each monomer, data points with the different letters indicate statistically significant difference.
significant change in the grafting efficiency when the temperature was between 40 °C and 60 °C, but the grafting efficiency decreased at 70 °C and 80 °C. Increasing the length of alkyl chain from 2 to 6 (MMA to HMA) decreased the % grafting efficiency at all temperatures studied. Previous studies on the grafting of starch with methacrylates have also shown that increasing temperature increased the grafting efficiency up to a certain temperature, and the % grafting efficiency (GE%) decreased at high temperature.15 Therefore, a decrease in % GE at high temperatures was attributed to the increase in termination rate of the starch grafted polymeric chain or due to chain transfer to monomer molecules.15 A decrease in the % GE leads to an increase in % homopolymers when the grafting temperature was 70 °C and 80 °C as seen from Figure 1b. At high temperatures, the monomers move faster and more readily react with each other and form the homopolymers. The % monomer conversion was considerably lower when the temperature was 40 °C, but increased to above 70% for all the monomers when the temperature was 80 °C or higher. Increasing the length of the alkyl chain decreased the % monomer conversion at all temperatures studied, as seen from Figure 1c due to the steric hindrance as the length of the carbon chain increases.16 This is more evident when the molar grafting ratio is considered for the different monomers. Increasing the temperature increased the molar grafting ratio for all the monomers, as seen from Figure 1d. More interestingly, the grafting ratios decreased as the length of the
alkyl chain increased, although the grafting conditions were similar. At a temperature of 60 °C, MMA had a grafting ratio of 5.9 mmol/g, compared to 2.9 mmol/g for HMA. MMA, being a much smaller molecule than HMA, was more easily accessible to starch and was easily grafted and provided higher % grafting. A temperature of 60 °C was found to provide the most optimum grafting for all four monomers studied. Effect of Monomer Concentration. Increasing monomer concentration continually decreased the % grafting efficiency and increased the % homopolymers, as seen from Figure 2a,b. The % homopolymers were less than 5% for all the monomers when the monomer concentration was 20%. Increasing the monomer concentration leads to a substantial increase in % homopolymers up to about 105−110% when the monomer concentration is 180%. When the monomer concentration was increased above 20%, a higher number of monomer molecules are available for reaction, and therefore, higher numbers of monomers were grafted onto starch. When the monomer concentration increased above an optimum level, the monomers collide with each other and form homopolymers because the growing poly(methacrylate) chain has greater compatibility with the monomer molecules.15,16 At low monomer concentrations (20%), the % monomer conversion is significantly lower (Figure 2c) because there are fewer molecules in solution that can react and form grafted chains or homopolymers. The molar grafting ratio increased with increasing monomer concentration for all the monomers, as 4671
dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
Journal of Agricultural and Food Chemistry
Article
Figure 3. Influence of reaction time on % grafting efficiency (a), % homopolymers (b), % monomer conversion (c), and molar grafting ratio (d). Grafting was performed at 60 °C with 1:1 ratio of monomer to starch. For each monomer, data points with the different letters indicate statistically significant difference.
seen from Figure 2d. However, the grafting ratio decreased substantially with increasing length of the alkyl chain due to steric hindrance of the longer chain monomers and subsequent difficulty in being grafted onto starch. Effect of Grafting Time. Grafting time did not show a major effect on % grafting efficiency, as seen from Figure 3a. Similarly, the % homopolymers (Figure 3b) also did not show any significant change with increasing reaction time. However, monomer conversion marginally increased with increasing reaction time, as seen from Figure 3c. Previous studies had shown that increasing reaction time from 30 to 60 min significantly increased % GE for MMA, EMA, and BMA. In our reaction, we added the monomers gradually for almost 30 min, and we therefore did not study a reaction time lower than 1 h. Most of the monomers have either been grafted or converted into homopolymers within 1 h, and we therefore do not observe any major change in grafting parameters with increasing reaction time. As observed before, higher alkyl chain length decreased the grafting ratio, and reaction time did not affect the grafting ratio, as seen from Figure 3d. Characterizing the Grafted Starch. Fourier Transform Infrared Spectroscopy. The characteristic carbonyl group in the methacrylates that appears as CO stretching at about 1730 cm−1 is seen in Figure 4 for all four spectra for the grafted starch. The intensity of this peak is highest for HMA, similar for EMA and MMA, and considerably lower for BMA. Presence of the additional peak in the grafted samples that belongs to the
Figure 4. FTIR spectrum of the unmodified and grafted starch depicting the characteristic peaks belonging to the carbonyl group in the monomers. The spectrum was recorded after extracting the homopolymers from the grafted samples.
methacrylates confirms grafting of the monomers onto starch.9,13 Broad peaks seen between 3500 and 3100 cm−1 are due to the hydroxyl groups in starch and the sharp peaks at about 2900 cm−1 are due to the C−H stretching from the grafted polymers, also suggesting successful grafting.12,13 4672
dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
Journal of Agricultural and Food Chemistry
Article
the grafted starch could be compression molded into films at 165 °C under pressure, thereby preventing excessive thermal damage to starch at high temperatures. 1 H NMR. Grafting of the poly(acrylates) onto starch was evident from the NMR spectra in Figure 7a−d below. As shown in Figure 7a, the H NMR resonances at 3.2−3.8 (protons 2,3,4,5,6), 4.3 (proton 7) and 5.0−5.3 (proton 1) were the characteristic signals from starch. The resonances at 3 and 2.5 ppm were from water and DMSO, respectively. Figure 7b−e illustrated that the H NMR spectrum for the MMA-, EMA-, BMA-, and HMA-grafted starch, respectively. It could be seen that polyacrylates and starch moieties were present in the spectrum. After grafting with acrylates, resonances at 0.9−2.0 ppm appeared, which were the signals of CH3, CH2, and CH groups in the polyacrylate. The resonance of 4.3 ppm (proton 7) decreased or disappeared in the spectra, which clearly confirmed the grafting of polyacrylates onto the hydroxyl group of starch. The signal of COOCH3 group of polyacrylate overlapped with protons 2−6 of the starch, which made the resonance at 3.2−3.8 ppm more complicated. Tensile Properties of Compression Molded Starch Films. Table 1 provides a comparison of the tensile strength of the starch films grafted with the four different monomers and containing different levels of homopolymers. On the basis of the tensile strength in Table 1, it can be observed that increasing the length of alkyl chains from 2 to 4 increased the strength of the films when there was no homopolymer but HMA grafted starch films had considerably lower strength than BMA-grafted films. BMA had strength of 1 MPa compared to 0.2 and 0.3 MPa for MMA and EMA, respectively, whereas HMA-grafted films have a strength of 0.5 MPa. Addition of homopolymers continually increased the strength for all the monomers. With 50% homopolymers, the strength of the MMA- and EMA-grafted films were nearly 10 times higher, 3 times higher for BMA films, and twice higher for HMA films. Films made from the 100% homopolymers showed that increasing the length of the alkyl chain considerably decreased the tensile strength. For instance, MMA-homopolymer films had a strength of 12.8 MPa, compared to 3 MPa for HMAhomopolymer films. MMA- and EMA-grafted films showed considerably lower elongation than BMA and HMA films, especially with the addition of homopolymers. With 50% homopolymers, BMA films had the highest strength, although HMA-grafted films showed higher elongation than the films containing the other monomers. Although MMA and EMA provided films with good dry tensile properties, the films containing up to 75% homopolymers were unstable in water, became too weak, and could not be tested even after conditioning at 90% humidity for 24 h at 21 °C. BMA- and HMA-grafted films showed good stability to moisture, especially in the presence of homopolymers, as seen from Table 1. In fact, the wet strength of the films grafted with HMA and containing 50% and 75% homopolymers is similar to or higher than the dry strength of the films. Tensile properties of the grafted starch films will depend on the extent of grafting, amount of homopolymers, and the inherent properties of the monomers and homopolymers. BMA provides better thermoplasticity, melts the starch and therefore provides the films with good tensile strength. In addition, increasing the alkyl chain length decreased the hydrophilicity, and therefore, BMA- and HMA-grafted films had better water stability than MMA and EMA films.
Thermal Behavior. Thermal degradations of the grafted starch samples were similar as seen from Figures 5 and 6. A
Figure 5. Thermal degradation (% weight loss) of the grafted starch samples when heated to 580 °C.
Figure 6. DSC thermograms illustrating that the starches grafted with higher lengths of alkyl chains had better melting when heated to 250 °C.
sharp decrease in the curve is seen around 220 °C, suggesting the start of degradation. MMA showed a slightly lower weight loss than the other samples up to about 300 °C, whereas HMA and EMA showed lower weight loss between 300 °C and 400 °C. An overall weight loss of about 80% was obtained after heating to 600 °C. DSC thermograms clearly show that starch grafted with longer lengths of alkyl esters (BMA and HMA) melted at higher temperatures and had sharper peaks compared to the starch grafted with MMA and EMA. Among the four monomers studied, EMA-grafted starch had a relatively lower melting enthalpy of about 12.6 J/g compared to 50−53 J/g for the other three monomers. As seen from Figure 6, HMA had a sharp and large melting peak at about 210 °C, whereas MMA and EMA produced broad melting peaks ranging from 160 °C to 200 °C. BMA and HMA are more thermally stable than MMA and EMA and therefore melt at higher temperatures. The sharp melting peaks for HMA and BMA suggest that these two polymers may provide better thermoplastic products but need higher processing temperatures. Although the melting peaks for the grafted starch were between 160 °C and 200 °C, 4673
dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
Journal of Agricultural and Food Chemistry
Article
Figure 7. 1H NMR spectra of the ungrafted starch (a), MMA-grafted starch (b), EMA-grafted starch (c), BMA-grafted starch (d), and HMA-grafted starch (e) with the position of the various hydrogen atoms.
Elongation of the films also showed that increasing the alkyl chain length and amount of monomers increases elongation. Without any homopolymers, MMA had an elongation of 2.8%, compared to 3.2% for HMA. Starch films grafted with HMA were considerably softer and therefore had lower strength but higher elongation (than BMA containing films). Increasing the length of alkyl chains increases the flexibility of the side chains, and therefore, the elongation of the films with BMA and HMA was higher than those of MMA- and EMA-containing homopolymers. Addition of homopolymers did not show an increase in the elongation of MMA- or EMA-grafted films, but
the elongation of the BMA- and HMA-grafted films increased up to 10 times. BMA-grafted films without homopolymers had an elongation of 3.1%, compared to 8.5% when 50% homopolymers were added. HMA-grafted films without any homopolymers had an elongation of 3.2%, compared to 10.4% when the homopolymer content was 50%. Increase in elongation for the BMA and HMA films should be due to the high elongation of the BMA (71%) and HMA (111%) homopolymers. Wet elongation of the BMA- and HMA-grafted films also showed a similar trend with elongations of 8.6 and 4674
dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
Journal of Agricultural and Food Chemistry
Article
Table 1. Dry and Wet Tensile Strength of Starch Films Grafted (2.7 mmol/(g) with Four Different Monomers and Containing Different Ratios of Homopolymersa breaking stress, MPa MMA % homo polymers 0 25 50 75 100
dry 0.23 0.45 2.9 4.1 12.8
± ± ± ± ±
a
0.04 0.01a 0.6d 0.5f 1.0h
EMA wet
dry
0 0 0 0 5.0 ± 1.2H
± ± ± ± ±
0.36 2.4 3.0 3.9 8.5
0.09 0.7c 0.4d 0.5f 0.9g
a
BMA wet
dry
0 0 0 0 5.5 ± 0.7
± ± ± ± ±
1.0 2.5 3.4 4.3 5.8
HMA wet
b
0.1 0.4c 0.2e 0.4f 0.6c
0.4 1.9 1.6 2.1 2.5
± ± ± ± ±
dry E
0.03 0.6D 0.7D 0.6C 0.6C
0.5 0.8 1.0 1.5 3.0
± ± ± ± ±
wet a
0.1 0.1b 0.1b 0.4b 0.8e
0.4 1.4 1.6 2.1 2.5
± ± ± ± ±
0.09E 0.5D 0.7D 0.6C 0.6C
a Films were compression molded at 165 °C for 5 min. For each monomer, data points with the different letters indicate statistically significant difference.
aging. BMA-grafted films showed good dry and wet tensile properties even when the grafting was 30%. Therefore, BMA grafted starch films could be useful for food and other applications. Starch grafted with BMA provided thermoplastic films with good strength and water stability. At similar grafting ratios, although increasing the length of the alkyl esters provided higher thermoplasticity and increased water stability, HMA homopolymers were considerably softer and decreased the strength of the starch films. Increasing the chain length also decreased the grafting ratio due to stearic hindrance from the monomers with longer side chains. Homopolymers considerably affected the tensile properties and water stability. Increasing the % of homopolymers generally increased the tensile strength and elongation. Films grafted with EMA were considerably brittle with elongation of 1−3%, whereas films grafted with BMA and HMA had elongations between 5% and 35%, depending on the amount of homopolymers. Overall, BMA was found to be more suitable for developing thermoplastic starch products with good tensile properties and water stability.
16.4% for the BMA and HMA films containing 50% homopolymers. On the basis of the strength and elongation of starch films grafted with various monomers, BMA was found to be more suitable to obtain films with good strength and elongation. A digital picture of a film made by compression molding starch grafted with BMA is shown in Figure 8. The film is transparent,
■
Figure 8. A digital picture of a film made by compression molding starch grafted with BMA.
AUTHOR INFORMATION
Corresponding Author
*Fax: 402-472-0640; e-mail: [email protected].
indicating that the grafted starch could be melted and made into thermoplastic products. Further studies were done to develop films from BMA with different grafting ratios and study the dry and wet tensile properties, since films grafted with MMA and EMA were weak and could not be tested after conditioning at 90% humidity for 24 h. Increasing the % grafting from 2.1 to 4.2 mmol/g (30% to 60% grafting) substantially increased the strength and elongation of the BMA grafted starch films from 1 to 3.2 MPa. There was also a considerable increase in the wet strength of the films (from 0.4 to 1.8 MPa) because of the increase in the amount of the hydrophobic polymer with increasing % grafting. When the grafting was 50%, the wet strength of the films was about 30% of the dry strength. Elongation of the films also increased from 3% to 12% when the grafting ratio was increased from 2.1 to 4.2 mmol/g. The corresponding increase in the wet elongation was from 6.7% to 25%. Strength and elongation of the films increased with increasing % grafting, since the grafted starch became more thermoplastic and formed better films. In addition, the grafted polymer had better strength and elongation than starch, and increasing the % of the grafted polymer therefore increased the tensile properties. Stability of the films under high humidities and in water is important for various applications, especially for food pack-
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS The authors thank the Program for Changjiang Scholars and Innovative Research Team in University (No. IRT1135) at Jiangnan University, Scientific Support Program of Jiangsu Province (BE2011404), the Graduate student innovation plan of Jiangsu Province (CX10B_222Z), the Doctor Candidate Foundation of Jiangnan University (JUDCF10004), agricultural research division, and the USDA-HATCH act and Multistate project S1054(NEB 37-037) at the University of Nebraska Lincoln for their financial support. The China scholarship council is also acknowledged for providing the living expenses for the visiting scholar (Z.S.).
■
REFERENCES
(1) Alissandratos, A.; Halling, P. J. Enzymatic acylation of starch. Biores. Technol. 2012, 115, 41−47. (2) Jimenez, A.; Jose Fabra, M.; Talens, P.; Chiralt, A. Edible and biodegradable starch films: A Review. Food Bioproc. Technol. 2012, 5 (6), 2058−2076.
4675
dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
Journal of Agricultural and Food Chemistry
Article
(3) Cyras, V. P.; Manfredi, L. B.; Ton-That, M.-T.; Vazquez, A. Physical and mechanical properties of thermoplastic starch/montmorillonite nanocomposite films. Carbohyd. Polymr. 2008, 73 (1), 55−63. (4) Arvanitoyannis, I. S.; Vlachos, A. Maize Authentication: Quality Control Methods and Multivariate Analysis (Chemometrics). Crit. Rev. Food Sci. Nutr. 2009, 49 (6), 501−537. (5) He, G. J.; Liu, Q.; Thompson, M. R. Characterization of structure and properties of thermoplastic potato starch film surface cross-linked by UV irradiation. Starch-Starke 2013, 65 (3−4), 304−311. (6) Soares, F. C.; Yamashita, F.; Mueller, C. M. O.; Pires, A. T. N. Thermoplastic starch/poly(lactic acid) sheets coated with cross-linked chitosan. Polym. Test. 2013, 32 (1), 94−98. (7) Reddy, N.; Yang, Y. Citric acid cross-linking of starch films. Food Chem. 2010, 118 (3), 702−711. (8) Singh, A. V.; Nath, L. K.; Guha, M. Synthesis and characterization of highly acetylated sago starch. Starch-Starke 2011, 63 (9), 523−527. (9) Zhang, B.; Zhou, Y. Synthesis and characterization of graft copolymers of ethyl acrylate/acrylamide mixtures onto starch. Polym. Comp. 2008, 29 (5), 506−510. (10) Jyothi, A. N. Starch graft copolymers: Novel applications in industry. Comp. Interface 2010, 17 (2−3), 165−174. (11) Kaewtatip, K.; Tanrattanakul, V.; Szecsenyi, K. M.; Pavlicevic, J.; Budinski-Simendic, J. Thermal properties and morphology of cassava starch grafted with different content of polystyrene. J. Thermal Anal. Calorim. 2010, 102 (3), 1035−1041. (12) Fakhru’l-Razi, A.; Qudsieh, I. Y. M.; Yunus, W.; Ahmad, M. B.; Rahman, M. Z. A. Graft copolymerization of methyl methacrylate onto sago starch using ceric ammonium nitrate and potassium persulfate as redox initiator systems. J. Appl. Polym. Sci. 2001, 82 (6), 1375−1381. (13) Pimpan, V.; Thothong, P. Synthesis of cassava starch-gpoly(methyl methacrylate) copolymers with benzoyl peroxide as an initiator. J. Appl. Polym. Sci. 2006, 101 (6), 4083−4089. (14) Zhu, Z.; Zhang, L.; Li, M.; Zhou, Y. Effects of starch alkenylsuccinylation on the grafting efficiency, paste viscosity, and film properties of alkenylsuccinylated starch-g-poly(acrylic acid). StarchStarke 2012, 64 (9), 704−712. (15) Athawale, V. D.; Rathi, S. C. Effect of chain length of the alkyl group of alkyl methacrylates on graft polymerization onto starch using ceric ammonium nitrate as initiator. Eur. Polym. J. 1997, 33 (7), 1067− 1071. (16) Vazquez, B.; Goni, I.; Gurruchaga, M.; Valero, M.; Guzman, G. M. Synthesis and characterization of graft-copolymers of methacrylonitrile methacrylate mixtures onto amylomaize by the ceric ion method. J. Polym. Sci., Part A: Polym. Chem. 1992, 30 (8), 1541−1548. (17) Willet (Willett, J. L. Starch in Polymer Compositions. In Starch: Chemistry and Technology, 3rd ed.; BeMiller, J., Whistler, R., Eds.; Academic Press: New York, 2009; pp 715−737. (18) Trimnell, D.; Swanson, C. L.; Shogren, R. L.; Fanta, G. F. Extrusion processing of granular starch-g-poly(methylacrylate)-effect of extrusion conditions on morphology and properties. J. Appl. Polym. Sci. 1993, 48 (9), 1665−1675. (19) Dennenberg, R. J.; Bothast, R. J.; Abbott, T. P. A new biodegradable plastic made from starch graft poly(methyl acrylate) copolymers. J. Appl. Polym. Sci. 1978, 22, 459−465. (20) Bagley, E. B.; Fants, G. F.; Burr, R. C.; Doane, W. M.; Russell, C. R. Graft copolymers of polysaccharides with thermopalastic polymers, a new type of filled plastic. Polym. Engg. Sci. 1977, 17 (5), 311−315. (21) Jin, E.; Reddy, N.; Zhu, Z.; Yang, Y. Graft polymerization of native chicken feathers for thermoplastic applications. J. Agric. Food Chem. 2011, 59 (5), 1729−1738. (22) Reddy, N.; Jin, E.; Chen, L.; Jiang, X.; Yang, Y. Extraction, characterization of components, and potential thermoplastic applications of camelina meal grafted with vinyl monomers. J. Agric. Food Chem. 2012, 60 (19), 4872−4879.
4676
dx.doi.org/10.1021/jf5013709 | J. Agric. Food Chem. 2014, 62, 4668−4676
