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Preparation and properties of biodegradable poly(propylene carbonate)/starch composites.

INTRODUCTION

The utilization of carbon dioxide (C[O.sub.2]) has attracted scientific and practical interest in recent years because of environmental pollution and energy shortages. Global warming, known as the greenhouse effect, is caused mostly by the massive release of carbon dioxide into the atmosphere. The contribution of carbon dioxide to global warming is estimated to be about 66% (1-3). Thus, using carbon dioxide is of both scientific and practical interest. On the other hand, plastic waste is mainly derived from packaging materials, such as rubbish bags, agricultural mulch films, industrial product packages and food wrappers, for which recycling is neither practical nor economical. These synthetic polymers, like polyolefins, are nonbiodegradable, and therefore, many approaches to render synthetic polymers degradable have been considered. Among various applications, the copolymerization of carbon dioxide with propylene oxide (PO) using different catalysts, such as organometallic compounds and their complexes, metallic complexes, as well as polymer supporting bimetallic catalysts, has been extensively reported (4-8).

Recently, we have successfully synthesized high-molecular-weight alternating poly(propylene carbonate) (PPC) in very high yields (126 g polymer per gram catalyst) from carbon dioxide and propylene oxide using a supported catalyst. The catalyst was synthesized from zinc oxide and glutaric acid under magnetic stirring followed by supporting them on a perfluorinated compound (9, 10). The structure of PPC is illustrated in Fig. 1a. Such a completely alternating PPC exhibits high transparency, superior mechanical strength and good degradability in surroundings of both soil and buffer solutions (9-11).

Starch is the primary energy storage polymer generated by many plants; it mainly consists of amylose, a linear polymer, and amylopectin, a highly branched polymer. The corresponding structures are shown in Fig. 1b. Starch exists in the form of approximately spherical particles with a size range of 3 to 30 [micro]m or more. Cornstarch has an average granule size of ~10 [micro]m and contains about 25% amylose (12). Because starch is abundant, renewable, inexpensive and degradable, it has been widely used as an additive for synthetic resins to yield biodegradable composites or it has been transformed into a completely biodegradable thermoplastic starch subjected to certain modifications. Many industrial products such as packaging bags, mulch films and fast-food containers have been manufactured from starch-based biodegradable materials to reduce pollution caused by traditional plastics.

[FIGURE 1 OMITTED]

Much effort has been devoted to producing starch-based composites using various aliphatic polyesters like polycaprolactone (PCL) (13-16), polyhydroxybutyrate-co-valerate (PHBV) (17, 18), polylactic acid (PLA) (19), poly(butylene succinate adipate) (PBSA) (20), and so on. Willett reported that starch/PHEE (poly(hydroxyester ether)) composite with 40-wt% unmodified starch exhibited good interface adhesion (21). For the composites from biodegradable polybutylene succinate adipate and granular cornstarch, increasing the starch content led to an increase in modulus and decreases in tensile strength and toughness (20).

Because of its light weight (specific gravity 1.4), starch offers a number of advantages over currently used inorganic fillers in terms of weight. Moreover, since starch shows hydrophilic behavior, it is anticipated that the incorporation of starch can greatly improve the water absorption of the PPC composite, thereby enhancing the biodegradation properties. In this regard, the main aim of this work is to develop a kind of completely biodegradable composite with applicable mechanical properties.

EXPERIMENTAL

Materials

The PPC used in this work was synthesized in our laboratory. The basic information of the PPC is tabulated in Table 1. The starch used was unmodified cornstarch (CS) purchased from Shandong Junen Electric Power Group Golden Corn Co., Ltd., China. The physical and chemical properties of the starch were determined according to the standard of GB12309. Starch was vacuum-dried to a moisture content of <1% prior to blending and processing.

PPC pellets and starch were initially dried in a vacuum oven at 80[degrees]C for 24 hr. Composites with PPC/starch weight ratios of 65/35, 50/50, 40/60, 30/70 were prepared in a Brabender Plasticorder batch mixer at 150[degrees]C and a rotary speed of 30 rev [min.sup.-1] for 6 min. The chamber volume was 50 [cm.sup.3]. For each sample, 50 g of material was fed into the batch. Melt torque of the composites were recorded during the mixing period. For purposes of comparison, the neat PPC was also melt-blended under similar processing conditions in the mixer. The composites subject to blending were stored in a tightly sealed vial to prevent any moisture absorption. The mixture was melt-pressed into sheets 1 mm thick and then cut into standard dogbone tensile bars (ASTM D638) with dimensions of 25 X 4 X 1 [mm.sup.3].

Material Characterization

Molecular weights and polydispersities of the PPC and PPC/starch composites were determined using gel permeation chromatography (GPC) consisting of a Waters Model 515 pump and a Waters Model 410 refractive index detector. Chloroform was used as solvent and the mobile phase was tetrahydrofuran (THF). Calibration was performed with polystyrene standards having molecular weights in the range of 2000-1,950,000 g/mol. Number average ([M.sub.n]), weight average ([M.sub.w]), and peak molecular weights ([M.sub.p]) were calculated using the Waters Empower software. Because of the insolubility of starch in chloroform, only the PPC component of the composites was determined in the chromatograms.

The static tensile properties were measured at 23[degrees]C and relative humidity of 50% [+ or -] 5% using an Instron Model 5566 tensile tester. The crosshead speed was set at 10 mm [min.sup.-1]. Five specimens of each sample were tested, and the average results were reported. Prior to measurements, the samples were conditioned at 23[degrees]C and 50% [+ or -] 5% humidity for 24 hr by placing them in a closed chamber containing a saturated Ca(N[O.sub.3])[.sub.2] * 4[H.sub.2]O solution in distilled water (ASTM E-104). Dynamic mechanical analysis (DMA) was carried out with a DuPont DMA (model 983) at a fixed frequency of 1 Hz and an oscillation amplitude of 0.2 mm. The dimensions of the specimens were 30 X 10 X 1 [mm.sup.3]. The temperature of the specimens ranged from -30[degrees]C to 80[degrees]C at a heating rate of 2[degrees]C/min.

Thermogravimetric analysis (TGA) measurements of the samples were performed in a PerkinElmer TGA-6 under a nitrogen protective atmosphere. The temperature used ranged from 30[degrees]C to 600[degrees]C with a heating rate of 20[degrees]C/min. Prior to the analysis, the samples were dried in a vacuum oven at 80[degrees]C for 24 hr.

FTIR spectra were recorded in a Bruker Vector 22 FT-IR spectrometer. The frequency range of FTIR was 4000-500 [cm.sup.-1] with a resolution of 4.0 [cm.sup.-1] and scan rate of 8 scans/s. Samples were measured in the form of thin films ~60 [micro]m thick, which were prepared by hot-press molding.

The tensile bars were fractured in liquid nitrogen and used for morphology observation with a scanning electron microscope (JEOL JSM-6330F). Prior to the examination, the fractured surfaces were coated with a thin layer of gold.

RESULTS AND DISCUSSION

Composite Preparation

According to previous work (22), high-molecular-weight poly(propylene carbonate) (PPC) may suffer chain-unzipping decomposition or chain-scission decomposition at certain temperatures. Thus, the melt-blending temperature should be controlled to be as low as possible. The changes in molecular weight and polydispersity of various PPCs were determined using GPC. As shown in Table 1, there were no significant changes in molecular weights between the original PPC and the PPC subjected to mixing at 150[degrees]C for 6 min. To maintain the mechanical properties, the PPC/starch composites must be melt-blended at <150[degrees]C. It can also be seen that the addition of starch resulted in obvious decomposition of PPC. This is due to the presence of trace water absorbed in starch. Water can in turn cause the hydrolysis of PPC at high temperature.

The relationship between the torque value and mixing time for PPC/starch composites is depicted in Fig. 2. It is apparent that the torque values became constant after ~3 min of mixing. This means that the melt-blending of 6 min was enough for complete mixing. It can also be seen from the constant torque value that the melt viscosity increased initially with increasing starch content up to 35 wt%. With further increase in starch content, the melt viscosity decreased, obviously demonstrating the improvement of the processability of the composites.

[FIGURE 2 OMITTED]

Mechanical Properties

Natural starch fillers were commonly added to polyolefins to produce composites with certain degradability (23-27). However, the nominal adhesion between starch and polyolefins is poor, owing to the incompatibility between the hydrophilic starch and hydrophobic polyolefins. Consequently, the introduction of unmodified starch to the thermoplastics generally results in the decrease of tensile strength and toughness.

Experimental tensile strengths of the PPC/starch composites and the theoretical values predicted from Eq 1 are shown in Fig. 3. The equation is commonly used in the case of poor adhesion between the filler and the matrix (28)

[sigma] = [[sigma].sub.0] (1 - [[PHI].sub.F.sup.2/3])S (1)

where [sigma] and [[sigma].sub.0] are the tensile strengths of the filled and unfilled blends, respectively: [[PHI].sub.F] is the volume fraction of filler, and S is a stress concentration function. The densities of cornstarch and PPC used in the work were 1.40 g/[cm.sup.3] and 1.30 g/[cm.sup.3], respectively. For simplicity, no stress concentration (S = 1) is assumed. Thus, the theoretical values of PPC/starch composites with varying starch content can be obtained from Eq 1. It is evident from Fig. 3 that the experimental tensile strengths of the composites are much higher than those predicted from Eq 1, indicating good bonding between the PPC matrix and starch. Presumably, the enhanced reinforcement of the composites with starch was produced from the hydrogen bonds between the hydroxyl groups of starch and the carbonyl groups of PPC. Moreover, the experimental tensile strength of the composites increases with increasing starch content up to 60 wt%, with a maximum increment of 23.0% at 35-wt% starch. With further increasing the starch content up to 70 wt%, the tensile strength begin to decrease, obviously because of the agglomeration of cornstarch.

[FIGURE 3 OMITTED]

Figure 4 shows the variations of the modulus and elongation at break of composites with starch content. It is clear that the modulus of composites increases dramatically with increasing starch content. The fact resulted from the stiffening effect of starch particulates within the PPC matrix because the modulus of starch is much higher than that of PPC. However, elongation at break drops sharply from 641% to 1.87% when 35-wt% starch was added. Similarly, the tensile energy at break decreases sharply with increasing starch content. These results indicate that the composites become very brittle.

The curves of storage moduli and loss moduli vs. temperature for pure PPC and PPC/starch composites are shown in Figs. 5a and b, respectively. From Fig. 5a, only one transition peak that corresponds to glass transition temperature ([T.sub.g]) of PPC matrix was observed. No obvious transition was observed for starch owing to its relatively higher [T.sub.g] (20). The storage modulus for PPC/starch composites is significantly higher than that for pure PPC, indicating the increased rigidity of PPC/starch composites.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Thermal Behavior

Figures 6a and b show the typical TG and DTG curves for neat PPC and PPC/50% starch composites. The 5% weight-loss temperatures ([T.sub.-5%]) and the maximum weight loss temperatures ([T.sub.max]) are summarized in Table 2. For PPC/starch composites, two [T.sub.max]s corresponding, respectively, to PPC and starch were observed, as appears in Fig. 6b. The glass transition temperatures ([T.sub.g]s) determined using DMA are also listed in Table 2. The [T.sub.g]s of PPC/starch composites increase greatly with an increase of starch content. The [T.sub.g] of PPC/70% starch composite is over 50[degrees]C, which is 8[degrees]C higher than that of pure PPC. This increase in [T.sub.g] is believed to be associated with the interaction between PPC and starch via hydrogen bonding. Moreover, the addition of starch to PPC led to a marked increase in [T.sub.-5%], for instance, from 241.4[degrees]C for pure PPC to 284.6[degrees]C for PPC/70% starch composite. It is evident from Table 2 that the incorporation of starch into PPC can improve its thermal properties.

[FIGURE 6 OMITTED]

Morphology Observation

SEM micrographs of PPC/35% starch and PPC/70% starch composites are shown in Figs. 7a, b and Figs. 8a, b, respectively. It can be seen that the starch remains in the shape of approximately spherical particles in the composite. Figure 7a shows that the starch fillers are dispersed well in the PPC matrix. There is no obvious agglomeration of starch particles. For PPC/35% starch composite, the micrographs (Figs. 7a, b) show homogeneous dispersion of starch in the PPC matrix without obvious voids between PPC and starch, implying good interfacial adhesion between the two components (24). This should explain why the tensile strength of PPC/35% starch composite is 23% higher than that of pure PPC. For PPC/70% starch composite, however, larger agglomerates and obvious gaps or voids between starch particles and the PPC matrix are observed in Figs. 8a and b. This in turn leads to the decrease of tensile strength for the PPC/70% starch composite. The agglomerates and voids are thought to result from the larger amount of starch added, especially when the amount of starch exceeds the amount of PPC matrix.

[FIGURE 7 OMITTED]

[FIGURE 8 OMITTED]

Figure 9 shows the FTIR spectra of pure PPC, PPC/50% starch composite, and neat starch. The absorption of carbonyl groups at 1754 [cm.sup.-1] for PPC obviously shifted to 1741 [cm.sup.-1] upon blending with starch. Mean-while, the absorption peak at 3397 [cm.sup.-1] corresponding to the -OH group for cornstarch shifted to a smaller wavenumber compared with that for PPC/50% starch (3391 [cm.sup.-1]). Moreover, the intensity of the 1741 [cm.sup.-1] adsorption is significantly larger than the shoulder at 1850 [cm.sup.-1]. These are direct proofs of the interaction between carbonyl groups of PPC and hydroxyl groups of starch via hydrogen bonding.

[FIGURE 9 OMITTED]

CONCLUSIONS

Unmodified cornstarch can be simply melt-blended with biodegradable poly(propylene carbonate) (PPC) to produce completely biodegradable and cost-competitive composites. The tensile strengths of the composites increased with increasing starch content up to 60 wt%, and thereafter decreased with further increase in starch. The stiffness and heat resistance of the composites were greatly improved by starch addition. Thermal analysis tests indicated that the starch addition led to a significant improvement in the thermal stability of the composites. The reinforcement in mechanical properties and the improvement in thermal properties resulted from the interaction between the carbonyl groups of PPC and the hydroxyl groups of starch via hydrogen bonding as evidenced by FTIR investigation. Scanning electron microscopic examination revealed the existence of good interfacial adhesion between the filler and PPC matrix, due to the aforementioned interaction within the composites.
Table 1. GPC Results of the PPCs.

Specimen [M.sub.n] [M.sub.w]

Original PPC 82,100 [+ or -] 800 250,600 [+ or -] 4300
PPC subject to mixing 81,900 [+ or -] 900 236,700 [+ or -] 3900
PPC/50% starch 79,300 [+ or -] 1200 218,300 [+ or -] 5600

Specimen [M.sub.p] Polydispersity

Original PPC 170,500 [+ or -] 4100 3.05 [+ or -] 0.09
PPC subject to mixing 158,100 [+ or -] 4600 2.89 [+ or -] 0.08
PPC/50% starch 169,300 [+ or -] 5900 2.75 [+ or -] 0.12

Table 2. Thermal Properties of the Pure PPC and PPC/Starch Composites.

Specimen [T.sub.g], [T.sub.-5%], [T.sub.max.sup.1],
 [degrees]C [degrees]C [degrees]C

Pure PPC 42.5 241.4 255.9
PPC/35% starch 43.6 261.0 300.4
PPC/50% starch 46.2 278.9 305.9
PPC/60% starch 48.6 281.6 307.4
PPC/70% starch 50.7 284.6 313.4
Cornstarch -- 286.5 --

Specimen [T.sub.max.sup.2],
 [degrees]C

Pure PPC --
PPC/35% starch 331.8
PPC/50% starch 330.6
PPC/60% starch 328.3
PPC/70% starch 332.6
Cornstarch 303.9


ACKNOWLEDGMENTS

We thank the Ministry of Science and Technology of China (Grant No. 2002BA653C), Key Strategic Project of Chinese Academy of Sciences (Grant No. KJCX2206B), and the Guangdong Natural Science Foundation of China (Team Project Grant No. 015007) for financial support of this work.

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X. C. GE (3), X. H. LI (3), Q. ZHU (1), L. LI (2), and Y. Z. MENG (1,2*)

(1) The State Key Laboratory for Optoelectronic Materials and Technology

Zhongshan (Sun Yat-Sen) University

Guangzhou 510275, P. R. China

(2) School of Mechanical & Production Engineering

Nanyang Technology University

North Sprine (N3) 639798, Singapore

(3) Guangzhou Institute of Chemistry

Chinese Academy of Sciences

P.O. Box 1122, Guangzhou 510650, P. R. China

*To whom correspondence should be addressed.
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Author:Ge, X.C.; Li, X.H.; Zhu, Q.; Li, L.; Meng, Y.Z.
Publication:Polymer Engineering and Science
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Date:Nov 1, 2004
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