Polylactide Stereocomplex Crystallites as Nucleating Agents for Isotactic Polylactide

摘要

A nucleation efficiency scale for isotactic poly(L-lactide) (PLLA) was ob- tained with self-nucleation and nonisothermal differential scanning calorimetry exper- iments. The maximum nucleation efficiency occurred at the highest concentration of self-nucleating Bites, and the minimum efficiency occurred in the absence of these sites (pure PLLA polymer melt). Blends of PLLA and isotactic poly(D-lactide) (PDLA) led to the formation of a 1/1 stereocomplex. In comparison with the homopolymer (PLLA), the stereocomplex had a higher melting temperature and crystallized at higher tempera- tures from the melt. Small stereocomplex crystallites were formed in PLLA/PDLA blends containing low concentrations of PDLA. These crystallites acted as heteroge- ~ neous nucleation sites for subsequent PLLA crystallization. Using the PLLA nucleation Iefficiency scale, we evaluated a series of PLLA/PDLA blends (0.25-15 wt % PDLA). A maximum nucleation efficiency of 66% was observed at 15 wt % PDLA. The nucleation I efficiency was largely dependent on the thermal treatment of the sample. The nucle- ~ ating ability of the stereocomplex was most efficient when it was formed well before , PLLA crystallization. According to the efficiency scale, the stereocomplex was far superior to talc, a common nucleating agent for PLLA, in its ability to enhance the rate of PLLA crystallization. In comparison with the PLLA homopolymer, the addition of , PDLA led to reduced spherulite sizes and a reduction in the overall extent of PLLA , crystallization. The decreased extent of crystallization was attributed to the hindered mobility of the PLLA chains due to tethering by the stereocomplex.