Crystallization and Solid-State Structure of Poly(l‑2-hydroxy-3-methylbutanoic acid)

The side-chain-substituted poly­(lactic acid)­s (substituted PLAs) have attracted much attention as novel bio-based polymers, but their possibility as crystalline polymeric materials still remains unknown. In this study, the crystallization behavior and solid-state structure are comprehensively investigated for poly­(l-2-hydroxy-3-methylbutanoic acid) [P­(L-2H3MB)] with M n = 6.2 × 104, i.e., stereoregular substituted PLA having isopropyl side chains. The equilibrium melting temperature of P­(L-2H3MB) is 240 °C, which is significantly higher than that of poly­(l-lactic acid) (PLLA) (≈200 °C). In addition, the isothermal and nonisothermal crystallization behaviors show that the crystallization of P­(L-2H3MB) is extremely faster than that of PLLA. X-ray diffraction results suggest that P­(L-2H3MB) has two or more crystal modifications, but only one modification appears with a high crystallinity (≈60%) when melt-crystallized at 155–180 °C.