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

Abstract

Enantiomeric substituted polylactides, poly(l-2-hydroxybutyrate) [P(l-2HB)] and poly(d-2-hydroxybutyrate) [P(d-2HB)] with different weight-average molecular weights (Mw = 1.8 × 103–4.1 × 104 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 Mw. SC crystallites were only crystalline species formed during melt-crystallization of the blends, in the crystallization temperature (Tc) range of 55–160 °C (except at 70 °C). At Tc = 70 °C, in addition to SC crystallites, a significant amount of homo-crystallites were formed for average Mw exceeding 2.7 × 104 g mol−1, and equal amounts of SC crystallites and homo-crystallites were formed for average Mw exceeding 3.2 × 104 g mol−1. However, a critical average Mw 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 Mw below 1 × 104 g mol−1 and the normal type of homo-crystallites were formed for Mw above 1 × 104 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 Mw = 9 × 103–1 × 104 g mol−1 and Mw above 9 × 103 g mol−1, respectively.