Heterostereocomplex‐ and Homocrystallization and Thermal Properties and Degradation of Substituted Poly(lactic acid)s, Poly( l ‐2‐hydroxybutanoic acid) and Poly( d ‐2‐hydroxy‐3‐methylbutanoic acid)

Heterostereocomplex‐ and homocrystallization behavior, thermal properties and degradation of neat poly( l ‐2‐hydroxybutanoic acid) [P( l ‐2HB)], poly( d ‐2‐hydroxy‐3‐methylbutanoic acid) [P( d ‐2H3MB)], and their equimolar blend are first investigated. Regime I and II kinetics are observed for neat P( l ‐2HB), whereas regime II and III kinetics are seen for the blend. The growth geometry of the neat P( l ‐2HB) is linear and circular while that of the blend is spherical, whereas that of the neat P( d ‐2H3MB) changes from linear to spherical, depending on crystallization temperature ( T c ). The main crystalline species is heterostereocomplex (HTSC) in the blend for a wide T c range of 0–180 °C and a very small amount of P( d ‐2H3MB) homocrystallites form for melt‐crystallization at T c below 70 °C and solution‐crystallization. The equilibrium melting temperature of P( l ‐2HB)/P( d ‐2H3MB) HTSC crystallites (234.5 °C) is higher than those of P( l ‐2HB) and P( d ‐2H3MB) homocrystallites (114.9 and 208.6 °C, respectively). The activation energy values for thermal degradation of the P( l ‐2HB)/P( d ‐2H3MB) blend (190–219 kJ mol −1 ) are between those of neat P( l ‐2HB) and P( d ‐2H3MB) (164–180 and 210–380 kJ mol −1 , respectively), reflecting that the interaction between the polymers with opposite configurations is similar to or lower than that between the polymers with the same configurations at a high temperature in the melt.