Miscibility And Crystallization Behavior Of Biodegradable Poly(Lactic Acid)/Poly(3-hydroxybuty Rate-co-4-hydroxybutyrate) Blends

Both poly(lactic acid)(PLA) and poly(3-hydroxybutyrate-co-4-hydroxybutyrate)[P(3HB-co-4HB)] are commercially available biodegradable polymers derived from renewable resources like starch. It is expected to obtain a new kind of fully biodegradable polymers by blending PLA and P(3HB-co-4HB). In this work, four groups of PLA/P(3HB-co-4HB) blends, that is, LMW-PLA/P(3HB-co-11%4HB), LMW-PLA/P(3HB-co-20%4HB), HMW-PLA/P(3HB-co-11%4HB), and HMW-PLA/P(3HB-co-20%4HB), were prepared through solvent-casting method by blending PLAs with two different molecular weights (HMW-PLA:Mn=1.11x105, Mw=1.71x105; LMW-PLA:Mn=1.27x104, Mw=1.47×104) and P(3HB-co-4HB)s with two different4HB contents [P(3HB-co-11%4HB):containing11%4HB unit; P(3HB-co-20%4HB):containing20%4HB unit). The miscibility and crystallization behavior of the blends were studied systematically by differential scanning calorimetry (DSC) and polarized optical microscopy. The mechanical properties of the blends were also investigated preliminarily. The main results are as follows:(1) The misciblity of PLA/P(3HB-co-4HB) blends. The thermal behavior of the amorphous samples, obtained by quenched from the melt, were recorded during heating process by DSC. Two glass transition temperatures (Tg) were found in PLA/P(3HB-co-4HB) blends, indicating that the blends were basically immiscible. However, it was also found that the Tg from the PLA side decreased with increasing the content of P(3HB-co-4HB), indicating that miscibility to some extent was still existed between PLA and P(3HB-co-4HB). With increasing the molecular weight of PLA or the content of4HB unit in P(3HB-co-4HB), the miscibility of PLA/P(3HB-co-4HB) blends weakened further.(2) The non-isothermal crystallization behavior of the PLA/P(3HB-co-4HB) blends from the melt. The non-isothermal crystallization behavior of the PLA/P(3HB-co-4HB) blends was investigated by DSC. It was found that neat LMW-PLA presented a strong crystallization peak, while it was inhibited after blending with P(3HB-co-11%4HB) or P(3HB-co-20%4HB). Whereas for HMW-PLA/P(3HB-co-11%4HB) and HMW-PLA/P(3HB-co-20%4HB) systems, the crystallization peaks of the blends were stronger than that of each component, presenting strong synergistic effect.(3) The morphology and spherulitic growth rate of PLA/P(3HB-co-4HB) blends. The isothermal crystallization behavior at120℃for HMW-PLA/P(3HB-co-11%4HB)(60/40) blends, neat HMW-PLA and neat PLA/P(3HB-co-11%4HB) was investigated by polarized optical microscopy. No Apparent spherulites were found in both neat HMW-PLA and the blends but not in neat P(3HB-co-11%4HB). Compared with neat HMW-PLA, the density of the spherulites was lower and the diameter of spherulites was larger in the blends. Moreover, the spherulitic growth rate of the blends was three times as that of neat HMW-PLA.(4) The mechanical properties of the PLA/P(3HB-co-4HB) blends. The mechanical properties of the HMW-PLA/P(3HB-co-11%4HB)(both60/40and40/60) blends, neat HMW-PLA and neat P(3HB-co-11%4HB) were investigated by tensile testing machine. It was found that the strength and the modulus of the blends were between that of the two components, while the elongation at break of the blends was worse than that of the neat components.