Stereocomplex crystallization and homo-crystallization of enantiomeric poly(2-hydroxybutyrate)s: Effects of molecular weight and crystallization conditions

摘要

Enantiomeric substituted polylactides, poly(l-2-hydroxybutyrate) [P(l-2HB)] and poly(d-2-hydroxybutyrate) [P(d-2HB)] with different weight-average molecular weights (M _w = 1.8 × 10 ~3-4.1 × 10 ~4 g mol ~(-1)) were synthesized by acid-catalyzed polycondensation. The effects of molecular weights of P(l-2HB) and P(d-2HB) and crystallization conditions on stereocomplex (SC) crystallization and homo-crystallization of neat P(l-2HB), neat P(d-2HB), and their equimolar blends were investigated by means of differential scanning calorimetry and wide-angle X-ray scattering. Only SC crystallites were formed during solution- crystallization of the blends without the formation of homo-crystallites, irrespective of average M _w. SC crystallites were only crystalline species formed during melt-crystallization of the blends, in the crystallization temperature (T _c) range of 55-160 °C (except at 70 °C). At T _c = 70 °C, in addition to SC crystallites, a significant amount of homo-crystallites were formed for average M _w exceeding 2.7 × 10 ~4 g mol ~(-1), and equal amounts of SC crystallites and homo-crystallites were formed for average M _w exceeding 3.2 × 10 ~4 g mol ~(-1). However, a critical average M _w above which predominant homo-crystallization and no SC crystallization occur was not observed. The equilibrium melting temperature of SC crystallites obtained by the Hoffman-Weeks procedure was 239.1 °C, which was lower than the highest value (279 °C) reported for polylactide SC crystallites. In solution-crystallization of the neat P(l-2HB) and P(d-2HB), a new type of homo-crystallites were formed for M _w below 1 × 10 ~4 g mol ~(-1) and the normal type of homo-crystallites were formed for M _w above 1 × 10 ~4 g mol ~(-1). In contrast, melt-crystallization of the neat P(l-2HB) and P(d-2HB) generated the new and the normal type homo-crystallites for M _w = 9 × 10 ~3-1 × 10 ~4 g mol ~(-1) and M _w above 9 × 10 ~3 g mol ~(-1), respectively.