Preparation and properties of biodegradable blend containing poly (propylene carbonate) and starch acetate with different degrees of substitution
Highlights
• Starch acetate (SA) acts as reinforcement fillers for poly (propylene carbonate) (PPC). • Strong interfacial adhesion existed between SA and PPC. • Incremental enhancement in mechanical properties was dramatically obtained for PPC/SA (DS < 0.88) blend. • The introduction of SA led to the improvement in the thermal stability of PPC matrix.
Introduction
The green house effect has resulted in dramatic climate change and is a serious environmental issue (Meehl & Washington, 1996), which can be attributed to the massive production of carbon dioxide (CO2) by human activities (Broecker, 1997, Kacholia and Reck, 1997). Much action has been taken to reduce the excessive emission of CO2 (Tollefson, 2010). An alternative approach is to make use of CO2 as feedstock for chemical transformations. The materials for packaging are often made from synthetic polymers, and they are non-biodegradable and unrecyclable, and inappropriate disposal of these plastics resulted in pollution and posed severe environmental problem. Apparently, synthesis of biodegradable polymers useful as packing materials is highly desirable, and has been intensively studied. The copolymerization of carbon dioxide with propylene oxide (PO) could be catalyzed by various catalysts, such as metallic complexes, organometallic compounds and their complexes, as well as polymer supporting bimetallic catalysts (Chen et al., 1991, Gorecki and Koran, 1985, Nishimura et al., 1978, Rokicki and Kuran, 1981, Tsuchida and Kasai, 1980).
Recently, poly (propylene carbonate) (PPC) with a very high molecular weight has been successfully synthesized from CO2 and PO by using supported catalysts in our laboratory (Wang et al., 2002, Zhu et al., 2002). The resulting PPC exhibited an alternating molecular structure as illustrated in Fig. 1, and it also showed excellent mechanical properties and considerable degradability in both soil and buffer (Song et al., 2008, Wu et al., 2009). Further, more than 40 wt% of CO2 could be fixed into PPC (Meng, Du, Tiong, Zhu, & Hay, 2002).
As an inexpensive, biodegradable and renewable resource, starch has attracted much interest since its blending with biodegradable polymers can reduce production costs and improve the mechanical properties of the blend. As a filler, starch tends to reduce the toughness of the blend and to enhance the modulus (Averous, 2004). In our previous work, we reported natural cornstarch (NS) could be melt blended with biodegradable PPC to produce biodegradable and economical blends (Ge, Li, Zhu, Li, & Meng, 2004). The modulus of the resulting blends increased when the content of starch increased. However, the SEM image indicated that the compatibility between hydrophobic PPC and hydrophilic starch was weak. Hence, these blends were brittle and had low impact strength. Moreover, the PPC could get hydrolyzed when the moisture absorbed by the hydrophilic starch fillers reached a certain level (Gaspar et al., 2005, Shogren, 1992).
To successfully blend these two components without significant alteration of the mechanical properties, physical or chemical modification of natural starch may be necessary to improve its compatibility with PPC. Modifications of starch via esterification and grafting have been extensively studied (Fanta & Shogren, 1997). It was reported that poly (methyl acrylate) had been grafted to starch, and the resulting starch-g-poly (methyl acrylate) (S-g-PMA) was then utilized for blending with PPC. The PPC/S-g-PMA blends showed slightly improved thermal stability and mechanical properties, compared with the PPC/NS blends. Nevertheless, the preparation was rather complicated, and the grafting ratio of S-g-PMA was very low, making the potential industrial applications less feasible (Ge, Xu, Meng, & Li, 2005). By converting part or all of the hydroxyl groups into ester groups, acetylation is a simple and economic way to modify natural starch. High degrees of substitution (DS) around 0.2–3 with starch acetate have been extensively investigated in recent years, and the hydrophobicity, melting processability and mechanical properties of the modified starch were shown to be very different from the native starch (Gonzalez & Perez, 2002). It is believed that the hydrophobic nature of starch acetate should improve its compatibility with PPC. In this work, a series of PPC/SA blended with different DS and varied SA contents were prepared, and the correlation between the morphology of the blends and their properties was studied.
Section snippets
Materials
The PPC used in this work with a number-average molecular weight of more than 250,000, and polydispersity of 1.91 was provided by Henan Tianguan Enterprise Group Co. Ltd., China. Natural cornstarch (NS, content of amylose: 25%) was purchased from Shenyang Jilong Corn Co. Ltd., China. The physical and chemical properties of NS were determined by the standard of GB/T8885-2008. Starch was dried at 80 °C in the convection oven for 24 h prior to acetylation and blending. Acetic acid anhydride (AR) was
DS levels of SA
The DS levels of a series of SA determined by titration turned out to be 0.25, 0.51, 0.88, 1.44, and 2.52. Theoretically, the highest DS is 3.0, corresponding to the complete conversion of the 3 hydroxyl groups in one glucose unit to the acetate groups. The SA prepared in this work has a DS level varied from 0.25 to 2.52. Therefore, correlation of the properties of the blend and interfacial chemistry with the DS levels could be comprehensively studied. The objective of this work is to find the
Conclusions
Starch acetate (SA) with different DS (degrees of substitution) levels can be simply melt-blended with biodegradable poly (propylene carbonate) (PPC) to obtain biodegradable and cost-competitive blends. An improved thermal stability was obtained in PPC/SA, compared with PPC/NS and neat PPC. The stronger intermolecular interaction between PPC matrix and SA was confirmed by FTIR technique. SEM results demonstrated the stronger interfacial adhesion and compatibility between SA and PPC matrix than
Acknowledgments
The authors would like to thank the China High-Tech Development 863 Program (2009AA034900 and 2009AA03Z340), Guangdong Province Universities and Colleges Pearl River Scholar Funded Scheme (2010), Guangdong Province Sci & Tech Bureau (Key Strategic Project Grant No. 2008A080800024 and 10151027501000096), and Guangdong Education Bureau (Key Project. cxzd1004); Chinese Universities Basic Research Funding for financial support of this work.
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