Enhancing the Crystallization Performance of Poly(L-lactide) by Intramolecular Hybridizing with Tunable Self-assembly-type Oxalamide Segments

Abstract

In this work, hydroxyl-terminated oxalamide compounds N1,N2-bis(2-hydroxyethyl)oxalamide (OXA1) and N1, N1′-(ethane-1,2-diyl)bis(N2-(2-hydroxyethyl)oxalamide (OXA2) were synthesized to initiate the ring-opening polymerization of L-lactide for preparation of oxalamide-hybridized poly(L-lactide) (PLAOXA), i.e., PLAOXA1 and PLAOXA2. The crystallization properties of PLA were improved by the self-assembly of the oxalamide segments in PLAOXA which served as the initial heterogeneous nuclei. The crystal growth kinetics was studied by Hoffman-Lauritzen theory and it revealed that the nucleation energy barrier of PLAOXA1 and PLAOXA2 was lower than that of PLA. Consequently, PLAOXA could crystallize much faster than PLA, accompanied with a decrease in spherulite size and half-life crystallization time by 74.8% and 86.5% (T = 125 °C), respectively. In addition, the final crystallinity of PLAOXA1 and PLAOXA2 was 6 and 8 times higher, respectively, in comparison with that of neat PLA under a controlled cooling rate of 10 °C/min. The results demonstrate that the hybridization of oxalamide segments in PLA backbone will serve as the self-heteronucleation for promoting the crystallization rate. The higher the content of oxalamide segments (PLAOXA2 compared with PLAOXA1) is, the stronger the promotion effect will be. Therefore, this study may provide a universal approach by hybridizing macromolecular structure to facilitate the crystallization of semi-crystalline polymer materials.