Enhanced crystallization kinetics of poly(lactide) with oxalamide compounds as nucleators: effect of spacer length between the oxalamide moieties

Abstract

Poly(lactide), PLA, suffers from low crystallization rate. To speed up the crystallization rate, oxalamide compounds with the formula of C₆H₅NHCOCONH(CH₂)_nNHCOCONHC₆H₅ (n = 2, 4, 6, 8, 12) are synthesized as nucleating agents (NA_(n)). Their thermal properties and nucleation efficiency can be tailored by tuning the aliphatic spacer length, i.e., –(CH₂)_n–. The melting temperatures and crystallization temperatures of the NA_(n) decrease monotonically with increasing n value while the thermal decomposition temperature remains constant. The aliphatic spacer length (n) of NA_(n) influences obviously the crystallization kinetics of PLA, resulting in nucleation efficiency in the sequence of NA₍₂₎ > NA₍₄₎ > NA₍₁₂₎ ≈ NA₍₈₎ > NA₍₆₎. Polarized optical microscopy results show that NA_(n) could self-assemble into needle-like superstructures which subsequently promote the crystallization of PLA macromolecules. The differences in superstructural geometry and the nucleation activity of NA_(n) are responsible for the variation in the nucleation efficiency.