Synthesis, properties, and crystallization of the alternating stereocopolymer poly(L-lactic acid-alt-D-lactic acid) [syndiotactic poly(lactic acid)] and its blend with isotactic poly(lactic acid)

The alternating stereocopolymer poly(L-lactic acid-alt-D-lactic acid), i.e., syndiotactic poly(lactic acid) (s-PLA) as well as isotactic poly(L-lactic acid) (i-PLA) as a reference polymer, was successfully synthesized by the procedure reported by Stayshich and Meyer. The strategy proposed in the present study is applicable to synthesize other alternating or syndiotactic stereocopolyesters from chiral hydroxyalkanoic acids. The physical properties, crystalline modification and crystallization behavior of neat i-PLA and s-PLA, and their blend were investigated by Fourier transform infrared spectrometry, wide-angle X-ray diffractometry, polarized optical microscopy, differential scanning calorimetry, and thermogravimetry. The crystal modification of s-PLA was different from that (alpha- or delta-form) of i-PLA. In marked contrast with isotactic and syndiotactic poly(methyl methacrylate) blends, in i-PLA/s-PLA blends, i-PLA and s-PLA crystallized separately or only i-PLA crystallized, without stereocomplex formation or their co-crystallization. The radial growth rate of spherulites was much lower for neat s-PLA than that for neat i-PLA, whereas the nucleus number per unit area of spherulites (spherulite density) was much higher and the induction period for spherulite growth was shorter for neat s-PLA than that for neat i-PLA. The latter effects dominated the former effects, resulting in rapid crystallization of neat s-PLA compared to that of neat i-PLA during slow cooling from the melt. The thermal stability of neat i-PLA and s-PLA, and their blend was similar to each other, indicating that the effects of tacticity or blending on thermal stability were very low.