Influence of asymmetric ratio of polystyrene-block-poly(methyl methacrylate) block copolymer on the crystallization rate of PLA

To investigate the influence of the asymmetric ratio of polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) on the crystallization rate of polylactide (PLA), we synthesized PS-b-PMMA samples featuring distinct asymmetric ratios by using mercaptan/ε-caprolactam living polymerization and subsequently melt-blended the polymers with PLA. Optical micrographs showed that the PMMA block facilitated the dispersion of PS-b-PMMA in the PLA matrix because of the favorable miscibility between PLA and PMMA. The results of nonisothermal and isothermal experiments indicated that adding 0.5 wt% PS-b-PMMA increased the crystallization rate of PLA when the value of the asymmetric ratio was ≤4. However, when the asymmetric ratio of PS-b-PMMA approached 6, the long-chain PMMA block retarded the chain mobility of PLA and lowered the crystallization rate. The results of polarization optical microscopy revealed that incorporation of PS-b-PMMA enhanced the crystallization rate of PLA through heterogeneous nucleation. The PS block in the PLA matrix provided the nucleation sites, whereas the PMMA block interacted with the PLA polymer chain and facilitated crystal growth. Lastly, wide-angle X-ray diffraction patterns suggested that incorporating PS-b-PMMA in the PLA matrix did not change the crystalline structure of PLA when the block-length ratio between PS and PMMA was varied. The asymmetric ratio of PS-b-PMMA strongly influenced the crystallization, and we optimized the asymmetric ratio of PS-b-PMMA in order to accelerate PLA crystallization.