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Preparation and characterization of biodegradable polymer blends from poly(3-hydroxybutyrate)/poly(vinyl acetate)-modified corn starch.

INTRODUCTION

Biodegradable polymers have received much attention over several decades due to their sustainable solution to alleviate a growing concern on the environmental impact. Several biodegradable polymers are now available in the commercial market, such as poly(lactic acid) (PLA), poly(hydroxyalkanoate) (PHA), polycaprolactone (PCL), poly(butylene succinate) (PBSU), etc. Among those biodegradable polymers, PHAs are produced by a variety of bacteria, such as Ralstonia eutropha, Alcaligenes latus, Azolohacter vinelandii, and pseudomonads, under a condition of nutrient deficiencies and excess carbon source [1-3]. At present, at least 100 different monomeric units as constituents of PHAs have been identified, depending on the bacterial strain, carbon source and fermentation conditions. Among PHAs, poly(3-hydroxybutyrate) (PHB) is the most common polyester available in the market. However, the price of PHB was relatively high in comparison with general plastics, despite of its high performance and biodegradability.

Besides the aforementioned biodegradable polymers, biopolymers derived from renewable resources, such as starch, cellulose, chitosan, are also an important class of biodegradable materials. Among them, starch offers a competitive price and is considered to be an excellent option in the industry. Starch is one of most abundant natural food sources and primarily composed of amylose and amylopectin. Amylose is a linear polymer of [alpha]-1,4-linked glucose units, whereas amylopectin is a branched polymer of [alpha]-1,4-linked glucose segments connected by 1,6-linkages [4]. Recently, numerous works have been investigated to incorporate starch into the biodegradable polymers, including PHB [5-9], PLA [10-12], PCL [13, 14], and PBSU [15, 16]. The combination of those biodegradable polymers and starch would exploit their respective merits to widely expand their applications. However, the low compatibility of starch with some biodegradable polymers was often encountered and thus it requires some strategies to better improve their compatibility and properties.

Some efforts have been made to investigate the properties of PHB/starch blends. Zhang and Thomas [5] blended PHB with two types of maize starch at a ratio of 70/30 and the results showed that thermal and mechanical properties were greater for blends with high amylose-content starch than those with low amylose-content starch. The authors attributed these improvements to the enhanced hydrogen bonding between the PHB and high-amylose content starch. Innocentini-Mei et al. [17] studied thermal and mechanical properties of PHB blends with natural starch as well as with starch derivatives. Compared with neat PHB, a decrease of Young's modulus in all the blends under investigation was observed. Godbole et al. [18] studied thermal and mechanical properties of the PHB blends with a soluble potato starch and with a thermoplastic starch prepared by mixing starch, water and glycerol. They found that there was no interaction between the PHB and starch. Zhang et al. [19] studied thermal behavior and phase morphology of the PHB blends with starch acetate. The glass transition temperatures of the PHB component in the prepared blends were all close to the value of neat PHB, indicating that PHB and starch acetate were immiscible. To improve the compatibility, Imam et al. [20] indicated that polyethylene oxide) (PEO) could serve an efficient compatibilizer in the poly(hydroxybutyrate-co-hydroxyvalerate) (PHBV)/starch blends. In a recent work of our group, a starch copolymer has been prepared by grafting poly(vinyl acetate) (PVAc) to the soluble potato starch aiming to improve compatibility between starch and PHB [8], This work was undertaken to further discuss the modification of a different starch type, i.e., com starch, by grafting with PVAc. The prepared PVAc-modified com starch (CSV) was then blended with PHB where the grafted PVAc was expected to increase their compatibility. The structural and thermal characterizations on the blends were investigated, in addition to the enzymatic hydrolysis behavior, for better understanding the detailed role of the modified starch and pave the way for the development of new bio-based polymer blends.

EXPERIMENTAL

Materials

The major materials used were poly(3-hydroxybutyrate) (PHB), vinyl acetate (VAc), and com starch. PHB was supplied from Aldrich Chemical Co., Milwaukee. It had a glass transition temperature ([T.sub.g]) at 2[degrees]C and a melting point ([T.sub.m]) at 176[degrees]C. Its number-average molecular weight ([M.sub.n]) was 259 kg/mol with a polydispersity index (PDI) of 2.05 based on polystyrene standard. Vinyl acetate was received from Acros, NJ. It was distilled under reduced pressure and only the distillate in the middle stage was used for polymerization. Com starch was purchased from Nacalai Tesque Inc., Tokyo, Japan. Ceric ammonium nitrate (CAN), Ce[(N[H.sub.4]).sub.2][(N[O.sub.3]).sub.6], obtained from Riedel-de Haen, Germany, was used to initiate the graft-polymerization of vinyl acetate. [alpha]-Amylase was purchased from Sigma, USA, to hydrolyze starch blends.

Synthesis of PVAc-Modified Corn Starch (CSV)

The synthesis of PV Ac-modified com starch (CSV) was based on the procedure as described in our previous study [8] with a slight modification. First, 30 g of com starch was added to a four-necked reactor containing 870 g of deionized water, which was then stirred at 300 rpm and 90[degrees]C for 1 h. After gelatinization, it was cooled down to 50[degrees]C for further reaction with the ceric ion ([Ce.sup.4+]) solution prepared by dissolving 0.018 mol of CAN salt in a 30 mL nitric acid solution (0.50 N). After 5 min, 60 g of VAc monomer was added to the above solution to start the grafting reaction for 4 h. After reaction, the dispersion solution was poured into the ice-cooled ethanol, followed by filtration and thorough wash with water/ethanol (1/1 in volume). The precipitated product (CSV) containing both PVAc homopolymer and starch-g-PVAc copolymer was dried in the air-circulating oven at 50[degrees]C to a constant weight and then in vacuo for another 2 h at 60[degrees]C. The monomer conversion (X%) was calculated by the following equation.

X = [[W.sub.2] - [W.sub.1] x S%]/[[W.sub.1] x M%]] x 100 (1)

where [W.sub.1] and [W.sub.2] represent the weight of the sample drawn out from the dispersion solution before and after drying, respectively. M% and S% are the weight percentages of feeding monomer and com starch in the beginning of grafting process. To calculate the grafting efficiency (GE%, weight percentage of grafted PVAc based on total synthesized PVAc) and grafting ratio (GR, weight ratio of grafted PVAc to com starch), PVAc homopolymer was extracted by acetone using a Soxhlet extractor for 72 h. The residual starch-g-PVAc copolymer was dried in the air-circulating oven at 50[degrees]C to a constant weight and then in vacuo for another 2 h at 60[degrees]C, denoted as [W.sub.3]. Both GE and GR were then calculated according to the following equations.

GE = [[W.sub.3] - [W.sub.1] x S%]/[[W.sub.2] - [W.sub.1] x S%]] x 100 (2)

GR = [[W.sub.3] - [W.sub.1] x S%]/[[W.sub.1] x S%]] x 100 (3)

Preparation of PHB/CSV Blends

Materials were pre-dried in an oven at 60[degrees]C for 2 h. PHB and CSV (PVAc-modified com starch) were dissolved in DMSO at 120[degrees]C and 100[degrees]C, respectively, and then mixed together at a total concentration of 5% (w/v) in a Teflon mold (PHB/CSV: 10/0, 9/1, 7/3, 5/5, 3/7, 1/9, and 0/10 in weight ratio). The homogenized solution was dried in an air-circulating oven at 80[degrees]C for 8 h and later in a vacuum oven at 100[degrees]C for another 5 h to remove DMSO. The prepared blends were then ground into powder and hot pressed to form the sample films at I75[degrees]C.

Structure and Thermal Characterizations of PHB/CSV Blends

The infrared spectra for the PHB, CSV, and their blends were recorded on a Fourier transform infrared spectrophotometer (FT-IR, Magna-IR spectrometer 550, Nicolet) at a resolution of 4 [cm.sup.-1] for 32 scans from 4000 to 400 [cm.sup.-1]. A differential scanning calorimeter (DSC 2920, TA, Instruments) was used to record the thermal behavior of the samples in the nitrogen environment. For the CSV, PVAc, and starch-g-PVAc, samples were first heated up to 150[degrees]C and held for 1 min, followed by cooling down to -30[degrees]C at a cooling rate of 20[degrees]C/min and then reheating up to 150[degrees]C at a heating rate of 10[degrees]C/min to record their glass transition temperatures ([T.sub.g]). For the PHB and PHB/CSV blends, samples were first melted in a hot stage (Linkam THMS 600) at 200[degrees]C for 2 min and then quenched in liquid [N.sub.2] to inhibit crystallization. Immediately, sample was transferred into a DSC cell maintained at -30[degrees]C and then heated to 200[degrees]C at a heating rate of 10[degrees]C/min to record the [T.sub.g], crystallization temperature ([T.sub.c]), and melting temperature ([T.sub.m]). On the other hand, thermogravimetric analysis (TGA 2950, TA Instruments) was used to evaluate the thermal stability of the blends in the nitrogen environment with a heating rate of 20[degrees]C/min from 100[degrees]C to 600[degrees]C after a 10 min period held at 100[degrees]C to remove moisture.

Morphological Observations

Low voltage (3-5 kV) scanning electron microscope (FESEM, Leo 1530, Germany) was used to observe morphologies of samples. To observe starch particles and PVAc latex particles in the dispersion solution after grafting reaction, a drop of dispersion was placed on a copper grid coated with a collodion. It was then air-dried and observed by SEM. In addition, the morphologies of the CSV film formed by directly drying the dispersion as well as the PHB/CSV blends were also observed by SEM. To observe cross-sectional area of the blends, samples were fractured in liquid nitrogen. All specimens were coated with approximately 3 nm thickness of sputtered Pt-Pd to increase conductivity.

Enzymatic Hydrolysis of PHB/CSV Blends. First, 10 mL buffer solution at pH = 7.2 containing 10 mg of [alpha]-amylase (0.1%) was prepared. The dried sample film (1.5 X 1.5 [cm.sup.2]) was weighed first and then immersed into the above solution. It was incubated under 150 rpm at 37[degrees]C. After the [alpha]-amylase treatment for 3 days, the hydrolyzed sample was washed and dried at 80[degrees]C for 24 h till a constant weight. The weight loss percentage was calculated, in addition to the evaluation of structural variation and morphology observation.

RESULTS AND DISCUSSION

When it was tried to blend corn starch with PHB, poor film formability with unsatisfactory properties was observed owing to their incompatibility and poor interfacial adhesion. For solving the problem of interfacial adhesion between the hydrophobic PHB and hydrophilic starch, corn starch was then grafted with hydrophobic PVAc chains. PVAc has been proved to be miscible with PHB [21-23]. It was thus expected that the grafted PVAc chains would improve the compatibility between starch and PHB. Another potential advantage of incorporating PVAc component is that the material's toughness could be increased, since PVAc is known as a leathery polymer. Graft copolymerization of synthetic polymers to polysaccharide chains has been studied for many years, especially in cellulose. Among all the methods used [24-27], it has been shown that ceric ion is the most effective agent in grafting vinyl monomers to a number of polysaccharides [27-30]. Therefore, cerium ammonium nitrate (CAN) was used in this study to initiate the graft polymerization of VAc monomer from com starch. The reaction product, PVAc-modified corn starch denoted as CSV, was then blended with PHB.

Synthesis of PVAc-Modified Corn Starch (CSV)

Basically, ceric ion, a strong oxidizing agent, could oxidize pyranose ring of starch to produce free starch macro-radical ([Ce.sup.4+] + Starch [right arrow][Ce.sup.3+] + Starch* [H.sup.+]), followed by the graft polymerization of VAc monomer via chain polymerization to form starch-g-PVAc graft copolymer. Besides the copolymer formation, VAc monomer tended to polymerize itself to form PVAc homopolymer [29, 31], and the amount of which could be extracted with acetone. Through the gravimetric analysis, the monomer conversion, grafting efficiency, and grafting ratio were determined to be 84.6%, 16%, and 0.27, respectively, which were close to what we determined for PVAc-modified soluble potato starch for a similar preparation condition earlier. This indicates a similar grafting mechanism for both systems. According to these results, the reaction product CSV thus consisted of 47.2% of starch-g-PVAc and 52.8% of PVAc homopolymer.

Figure 1 shows FT-1R spectra of corn starch, extracted PVAc homopolymer, and starch-g-PVAc copolymer. Characteristic absorption bands of starch (Fig. la) include absorption peaks of O--H (3700-3100 [cm.sup.-1]), C-H (2925 and 2887 [cm.sup.-1]), O-H bending of absorbed water (1649 [cm.sup.-1]), C--O-- stretching (1155-1000 [cm.sup.-1]) and the skeletal vibration of pyranose ring (935, 851, 761, 579, and 536 [cm.sup.-1]) [32, 33], As shown in Fig. lb, the characteristic absorption peaks of the extracted PVAc homopolymer are assigned to CH stretching (2975 and 2925 [cm.sup.-1]), C=O stretching (1738 [cm.sup.-1]), and C--O--C stretching (1240 [cm.sup.-1]). For the starch-g-PVAc copolymer (Fig. 1c), the observed absorption peaks at 3700-3100, 1649, 1155, 935, 851, 762, and 575 [cm.sup.-1] are originated from starch, and both peaks at 1742 and 1248 [cm.sup.-1] are contributed from PVAc, respectively, which confirms the successful grafting of PVAc chains from starch.

Morphology and Thermal Characterizations of PV Ac-Modified Corn Starch (CSV)

The corn starch was first gelatinized at 90[degrees]C for 1 h before reaction. The SEM picture as shown in Fig. 2a revealed the gelatinized starch particles had an oblong shape with the length of about 10 [micro]m. Their surfaces were rough and they even had some cracks under a high magnification. During the subsequent graft reaction, it was observed that the translucent solution became a milky dispersion. To closely monitor the reaction process, a drop of dispersion was dispensed on a copper grid coated with a collodion. It was then dried and observed by SEM. A representative starch particle is shown in Fig. 2b, which still has a size of about 10 [micro]m, yet with a smoother surface. More interestingly, it can be seen that there are many small particles attached to the starch particle and also around the starch particle. It has to be emphasized again that the reaction product CSV consisted of PVAc homopolymer and starch-g-PVAc copolymer. As noticed in the previous work [8], some graft copolymer was suggested to stabilize the PVAc latex particles during the reaction. Under a high magnification, PVAc latex particles with an average size of about. 140 nm are observed in Fig. 2c. These PVAc latex particles are not as rigid and spherical as those produced in the conventional emulsion polymerization such as polystyrene latex. This is because the reaction temperature is higher than the glass transition of the PVAc and probably also due to the presence of gelatinized starch. To better understand the structure, the reaction product was cast to form a thin film of PVAc-modified starch, and followed by vacuum drying at 50[degrees]C. It can be clearly seen in Fig. 2d that starch panicles, with the size ranging from 5 to 30 [micro]m, reside on the surface of film. When the cast film was further immersed in the acetone for 24 h, the SEM picture in Fig. 2e shows many voids present in the film due to the extraction of the PVAc latex particles. Slightly larger void sizes are observed due to the softening and merging of the PVAc particles during the drying state above its [T.sub.g] and the subsequent swell and dissolution in the extraction process.

The thermal behavior of the modified starch CSV, extracted PVAc homopolymer, and starch-g-PVAc copolymer was further investigated using DSC and TGA. First, there was no transition temperature such as [T.sub.g] or [T.sub.m] observed for the neat corn starch. This is because of its rigid saccharide structure and strong interand intra-hydrogen bonding. The [T.sub.g] values observed in DSC for CSV and starch-g-PVAc are mainly from the PVAc component but their magnitudes are affected by com starch. Figure 3 shows that the [T.sub.g] values of PVAc chains in the CSV and starch-g-PVAc copolymer are about 37 and 44[degrees]C, respectively; where the [T.sub.g] value for the neat PVAc obtained by extraction from CSV is 35[degrees]C. The higher value for the starch-g-PVAc is attributed to the hindered molecular mobility imposed from starch on the grafted PVAc. In addition, the similar glass transition temperatures observed in the CSV and PVAc suggested a high amount of PVAc homopolymer present in the CSV, in agreement with the calculation of grafting efficiency. A further investigation using TGA also supports this finding. Figure 4 shows the thermal degradation behaviors of neat starch, CSV, starch-g-PVAc, and PVAc. One-stage degradation behavior is observed for the neat starch due to the selective dehydration and transglucosidation [34], where the maximum-rate degradation temperature ([T.sub.max]) is close to 336[degrees]C. The char yield of starch is essentially high, up to 12.2%, attributing to the pyranose ring structure in the common carbohydrate compound. On the other hand, PVAc possesses a two-stage degradation behavior, including the first stage at 365[degrees]C on the degradation of acetate side group, and the second stage at 486[degrees]C corresponding to the degradation of the C--C main chain. The two-stage degradation behavior is still observed for both CSV and starch-g-PVAc, manifesting the combining effect from the neat starch and PVAc. And, their onset degradation temperature (Tonsct) and first-stage maximum-rate degradation temperatures ([T.sub.max1]) are between those respective temperatures of the neat starch and PVAc. In particular, the second-stage degradation behavior closely resembles to that of PVAc homopolymer, indicating the dominant composition of PVAc in the modified starch in agreement with the DSC analysis. The results of thermal properties are summarized in Tables 1-3.

Structure and Thermal Characterizations of PHBICSV Blends

The prepared modified com starch (CSV) was then blended with PHB. The structure and thermal properties of the blends were then investigated. Major regions of the FT-IR spectra of PHB/CSV blends at various compositions are depicted in Fig. 5 for a comparison. Characteristic absorption bands of PHB are attributed to C[H.sub.3] (2973, 1454, 1376 [cm.sup.-1]), C[H.sub.2] (2930 [cm.sup.-1]), C=O stretching (1727 [cm.sup.-1]), and C--O--C (1182 [cm.sup.-1]) [35]. The characteristic absorption ranges of the modified starch CSV including the starch and PVAc components are still observed as those shown in Fig. 1. In general, as the CSV is incorporated into the PHB, the starch absorption peaks become more obvious with increasing the CSV content, especially the -OH absorption peaks and the skeletal pyranose vibration peaks. Furthermore, the C=O stretching peak becomes broader due to the overlapping from the PHB component at 1727 cm 1 and the PVAc component at 1738 [cm.sup.-1], and it gradually shifts to higher wavenumber as the content of CSV is increased. A shoulder is also found around 1661 to 1653 [cm.sup.-1], which is ascribed to the O--H group of absorbed water. From the analysis of FT-IR spectra, it is difficult to ascertain the existence of strong interactions such as hydrogen bonding between the PHB and CSV.

To evaluate the compatibility between the PHB and CSV, the DSC test of the blends was performed. Samples were heated above their melting temperatures and quenched with liquid nitrogen, followed by heating again to record glass transition temperature ([T.sub.g]), crystallization temperature ([T.sub.c]), and melting temperature ([T.sub.m]). Figure 6 shows DSC thermograms of the PHB/CSV blends at various compositions. The neat PHB exhibits [T.sub.g], [T.sub.c], and [T.sub.m] at 2, 48, and 176[degrees]C, respectively. Therefore, in the amorphous state, the PHB is more flexible than the PVAc due to its lower [T.sub.g]. After blending, all the PHB/CSV blends show a single [T.sub.g] value which approaches to the [T.sub.g] of the CSV component as the CSV content is increased. It is known that if the two components with glass transition temperatures of [T.sub.g,1] and [T.sub.g,2] are completely miscible, their blends would exhibit a single [T.sub.g] lying between [T.sub.g,1] and [T.sub.g,2]. On the contrary, if they are immiscible, two [T.sub.g] values corresponding to those of individual components would be observed. However, for the miscibility between the neat polymer and the graft copolymer, the situation becomes more complex. As PHB is mixed well with the PVAc component on the CSV copolymer but not on the starch component, then the system is still considered as compatible, rather than miscible. To better reveal the [T.sub.g]S of the PHB/CSV blends at various compositions, Fig. 7 shows the [T.sub.g]-composition dependence of the PHB/CSV blends which is then fitted by the following Gordon-Taylor equation.

[T.sub.g,Blend] = [[[w.sub.1][T.sub.g,1] + [kw.sub.2][T.sub.g,2]]/[[w.sub.1] + [kw.sub.2]]] (4)

where k is an adjustable parameter, [T.sub.g,Blend], [T.sub.g,1] and [T.sub.g,2] , are [T.sub.g]S of the PHB/CSV blend, PHB, and CSV, respectively, while [w.sub.1] and [w.sub.2] are the weight fractions of PHB and CSV. The experimental data are well-fitted with the Gordon-Taylor equation using the adjusted parameter (k) of 0.11. It was mentioned previously that the CSV consists of the amorphous PVAc and starch-g-PVAc. Therefore, the PHB is expected to be mixed well with the PVAc portion in the CSV. Considering both PHB and PVAc have ester groups, they might have dipole-dipole interactions that result in their well-mixed situation. However, Greco and Martuscelli [21] estimated the interaction parameter of the PHB/PVAc blend from the values of melting-point depression, and found a value of -0.0059. Such a small value suggests that only slight or no dipole-dipole interaction between PHB and PVAc. On the other hand, it was found both Fox equation and Gordon-Taylor equation could well describe the [T.sub.g]-composition dependence. It is thus suggested that there is only London dispersion force between PHB and PVAc [8], Nevertheless, with the help of the amorphous PVAc grafted on the starch, the starch thus can be compatible with the PHB and well dispersed in the matrix. Therefore, the blends of PHB/CSV can have good film formability. In addition, the crystallization temperature of the PHB component during the heating stage increases with increasing the CSV content due to the hindered molecular chain mobility from the CSV. On the other hand, the melting temperature of the PHB component decreased in light of the reduced chain packing due to the disruption effect of the existing CSV. A melting-point depression behavior is generally observed for the crystalline component when it is blended with an amorphous, miscible component [36], As for the crystallinity, two competing effects including the increased hindrance of molecular chain and nucleation effect of modified starch were interplayed. At the low composition of CSV, the former factor dominated, which led to the decreased crystallinity. On the other hand, at the high composition of CSV, the nucleation effect dominated, as a result of increased crystallinity for PHB/CSV (5/5, 3/7). For the highest CSV composition, the melting peak in the DSC trace was not clear due to limited amount of highly restricted PHB, therefore the estimated crystallinity was only for a reference.

The thermal stability of PHB/CSV blends at various compositions is illustrated in Fig. 8. The neat PHB shows one-stage degradation behavior, in contrast to the two-stage degradation behavior for the CSV as seen in the previous discussion. The onset degradation temperature ([T.sub.onset]) of the neat PHB is about 259[degrees]C and the maximum-rate degradation temperature ([T.sub.max]) is at 274[degrees]C. For the PHB/CSV blends, a three-stage degradation behavior is observed. The first degradation stage is associated to the degradation of PHB component with the [T.sub.max] ranging from 297[degrees]C to 305[degrees]C and increasing with an increase in the CSV content. The addition of CSV thus raises the thermal stability of the PHB component. The second and third degradation processes correspond to the respective degradation stages of the CSV. The char yield increases with increasing the CSV content, reaching about 7.4% up to 600[degrees]C. This is associated with the increased CSV content with the chemical structure prone to form a thermal resistance layer to give a high degree of residual carbon as discussed earlier.

Enzymatic Hydrolysis of PHB/CSV Blends

[alpha]-Amylase is known to hydrolyze the [alpha]-1,4-glucosidic bonds of starch, resulting in the water soluble dextrin and glucose. The PHB/CSV blends were thus subjected to the [alpha]-amylase treatment and the structure and morphology of the residual products were then analyzed. A preliminary experiment was carried out on the enzymatic degradation of the PHB/CSV (3/7) blend with time and found that the weight loss already reached a constant value after 3 days. Therefore, all the blend samples were hydrolyzed in the 0.1% [alpha]-amylase buffer solution for three days. Figure 9 shows the weight loss of PHB/CSV blends after [alpha]-amylase treatment. The values correspond well with the respective theoretical starch contents in the blends, indicating that the presence of PHB and PVAc would not have negative effects on the enzymatic degradation of the starch component. On the other hand, it also proves that there is a limit effect of [alpha]-amylase on the degradation of PHB and PVAc. FT-IR spectra of samples at various compositions were recorded to assess the structural variation of hydrolyzed blends after [alpha]-amylase treatment. Figure 10 shows that the characteristic absorption peaks of the starch such as the OH absorption peaks (3100-3700 [cm.sup.-1] and 1649 [cm.sup.-1]) and the skeletal pyranose vibration peaks (579, 761, and 855 [cm.sup.-1]) are nearly discernible. Yet, the characteristic absorption peaks of the PHB and PVAc such as the C[H.sub.3], C=O, and C--O--C stretching peaks are still observed. This further confirms the specific reaction of [alpha]-amylase on the starch component. In order to degrade the PHB polyester, further studies on using combined enzymes such as esterase and amylase that could degrade both starch and PHB will be undertaken. Figure 11 shows the SEM pictures of the PHB, CSV and PHB/CSV blend (5/5) after amylase treatment. The smooth surface of the PHB sample confirms the limited effect of [alpha]-amylase. With increasing the CSV content, the dimension of etched cavities and surface roughness increase progressively due to the attack of [alpha]-amylase on the starch component in the blends.

CONCLUSIONS

In order to improve the compatibility between the com starch and PHB, the com starch was modified by grafting PVAc chains to prepare the PVAc-grafted com starch (CSV). Yet, during the reaction, PVAc homopolymer was also produced due to chain transfer reaction in addition to the graft copolymer. By gravimetric analysis, the monomer conversion, grafting efficiency and grafting ratio were 84.6%, 16%, and 0.27, respectively. The CSV thus consisted of 47.2% of starch-g-PVAc and 52.8% of PVAc homopolymer. The structure of the CSV was verified by FT-IR analysis. After blending PHB with the CSV, structure and thermal properties of the blends were investigated. Only a single [T.sub.g] was found for all the PHB/CSV blends and increased with increasing the CSV content. This indicated that the PHB was well-mixed with the PVAc component in CSV; and with the help of the grafted PVAc, the starch could be compatible with the PHB and well dispersed in the matrix. In addition, the presence of the CSV component could increase the thermal stability of the PHB which had a higher thermal degradation temperature in the blend than the neat PHB. [alpha]-amylase was used to evaluate the hydrolytic degradation of corn starch in the blends. The results showed that the presence of the PHB and PVAc components would not hinder the enzymatic degradation of the corn starch. However, in order to degrade PHB polyester. further studies on using combined enzymes that could degrade both starch and PHB will be undertaken. In addition, mechanical properties of the PHB/CSV would also be studied in the near future.

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Sun-Mou Lai, (1) Wei-Wei Sun, (2) Trong-Ming Don (2)

(1) Department of Chemical and Materials Engineering, National I-Lan

University, I-Lan 260, Taiwan

(2) Department of Chemical and Materials Engineering, Tamkang University, New Taipei City 25137, Taiwan

Correspondence to: Trong-Ming Don; e-mail: tmdon@mail.tku.edu.tw

Contract grant sponsor: Ministry of Science and Technology (MOST) in Taiwan; contract grant number: NSC I0I-2221-E-032-002-MY3.

DOI 10.1002/pen.24071

Published online in Wiley Online Library (wileyonlinelibrary.com).

TABLE 1. Thermal properties including glass transition
temperature ([T.sub.g]), onset degradation temperature
maximum-rate degradation temperature  ([T.sub.max]) (b), and
char yield at 600[degrees]C of the corn starch,  PVAc-modified
corn starch (CSV) (c), CS-g-PVAc copolymer, and
PVAc homopolymer.

              [T.sub.g]     [T.sub.onset]
Sample       ([degrees]C)   ([degrees]C)

Com starch        --             317
CSV               37             328
CS-g-PVAc         44             322
PVAc              35             344

             [T.sub.max]        Char
Sample       ([degrees]C)     yield (%)

Com starch       336            12.2
CSV            358, 485         OO OO
CS-g-PVAc      351, 481          9.5
PVAc           365, 486          4.2

(a) [T.sub.onset] is the temperature at 5 wt% loss.

(b) [T.sub.max] is the differential weight-loss peak temperature.

(c) The CSV consists of 47.2 wt% CS-g-PVAc and 52.8 wt% PVAc.

TABLE 2. Glass transition temperature ([T.sub.g]), crystallization
temperature ([T.sub.c]), melting temperature ([T.sub.w]), melting
heat ([DELTA][H.sub.m]), and crystallinity ([X.sub.c,PHB]) of the
PHB/CSV (a) blends at various compositions.

PHB/CSV    [T.sub.g]      [T.sub.c]      [T.sub.m]
          ([degrees]C)   ([degrees]C)   ([degrees]C)

10/0           2              48            176
9/1            3              44            171
7/3            5              52            170
5/5            6              62            168
3/7            6              84            167
1/9            25            104            167
0/10           37             --             --

PHB/CSV    [DELTA]    ([X.sub.c,PHB])
          [H.sub.m]       (%) (b)
            (J/g)

10/0        68.8           47.2
9/1         43.3           32.9
7/3         39.7           38.9
5/5         48.5           66.4
3/7         23.3           53.2
1/9          0.8            5.5
0/10         --             --

(a) The CSV consists of 47.2 wt% CS-g-PVAc and 52.8 wt% PVAc.

(b) The melting heat of PHB with 100% crystallinity
([DELTA][H.sub.m.sup.[omicron]]) is 146 J/g and the crystallinity
of the PHB component in the blends is calculated by ([X.sub.c,PHB])
= ([DELTA][H.sub.m]/[DELTA][H.sub.m.sup.[omicron]]) X
(1/[W.sub.PHB]) x 100 where [W.sub.PHB] is the weight fraction of
the PHB component in the blend.

TABLE 3. Thermal degradation properties of PHB/CSVa blends at different
compositions including onset degradation temperature (Tonsel) (b),
maximum-rate degradation temperature (Tmax)c and char yield
at 600[degrees]C.

          [T.sub.onset]    [T.sub.max]       Char
PHB/CSV    ([degrees]C)    ([degrees]C)    yield (%)

10/0           259              274            0
9/1            268         297, 359, 476      0.9
7/3            272         298, 360, 475      2.5
5/5            278         299, 356, 477      4.3
3/7            291         304, 360, 474      5.5
1/9            295         305, 355, 484      7.4
0/10           328           358, 485         8.8

(a) The CSV consists of 47.2 wt% CS-g-PVAc and 52.8% wt% PVAc.

(b) [T.sub.onset] is the temperature at 5 wt% loss.

(c) [T.sub.max] is the differential weight-loss peak temperature.
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Author:Lai, Sun-Mou; Sun, Wei-Wei; Don, Trong-Ming
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Date:Jun 1, 2015
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