Thermal Stability of Shear-Induced Crystal Nuclei of Poly(l -lactic acid)

Knowledge of the conditions required to obtain an isotropic melt of the polymer of interest, which before was subjected to shear and crystallization, is important for understanding and prediction of crystallization in polymer processing. In this work, the thermal stability of crystallization precursors of qualitatively different structures, namely, classical crystal remnants and oriented shish-like nuclei, is analyzed for poly(l-lactic acid) (PLLA). Shearing the melt at different rates leads to generation of shish-like and point-like nuclei which grow at identical temperature to crystals of similar stability/melting temperature. Analysis of the crystallization kinetics and of the semicrystalline morphology by differential scanning calorimetry and polarized-light optical microscopy, respectively, after subjecting the melt to different maximum temperatures, showed that disordering/isotropization of remnants of crystals, which melt close to 179 °C, occurs in a temperature window about 5-10 K higher than the melting temperature. Shish-like nuclei, generated at 135 °C during shearing the melt 10 s at 2 s-1, survive heating to a higher temperature than in the case of self-nuclei, however, with their number/nucleation efficacy exponentially decreasing with time over a period of several 103 s at 190 °C. In contrast to many other polymers, oriented shish-like structures disappear well below the equilibrium melting temperature, allowing mild melting conditions to obtain an isotropic structure. The stability of the remaining structures in the melt seems to be caused/controlled by molecular interactions rather than by slow diffusion of the chain segments toward a random-coiled structure as the longest relaxation times (reptation) at about 180 °C are several orders of magnitude shorter than the observed lifetime of the ordered structures.