Crystallizability of substituted poly(lactic acid)s: Effects of alkyl side-chain structure

Abstract

The side-chain-substituted poly(lactic acid)s (SPLAs) having C3–C4 alkyl side chains (Pr, iPr, n-Bu, iBu, and s-Bu for propyl, isopropyl, n-butyl, isobutyl, and sec-butyl groups, respectively) were synthesized from l-amino acids, and their crystallizability was systematically investigated (unknown for Pr, n-Bu, and s-Bu but already known for iPr and iBu). s-Bu was found to be a crystalline polymer like iPr, whereas Pr and n-Bu had no or very little crystallizability like iBu. The crystal structure, melting behavior, and crystallization rate of s-Bu were examined in detail. The crystal structure and crystallinity (ca. 60%) of s-Bu were not affected at all by the crystallization temperature and casting solvent. The melting temperature and spherulite growth rate of s-Bu were compared with those of iPr and referenced data of SPLA with methyl side chains (Me) [i.e., poly(l-lactic acid)] and that with ethyl side chains (Et) [i.e., poly(l-2-hydroxybutanoic acid)]. The magnitude relation among melting temperatures of SPLAs would be iPr > s-Bu > Me > Et, and that among spherulite growth rates iPr > Me ≈ s-Bu > Et.