Crystallization of Strictly Linear Poly(ethylene-ran-acrylic acid) Copolymer: Impacts of Pendant Group Insertion and Hydrogen Bonding

Functionalized polyolefins hold great promise as a material group for a future society. The hierarchical structure formation process during crystallization and melting of such polymers is greatly influenced by the inter- and intrachain interactions induced by functional groups. Herein, we report the comprehensive structural analysis of strictly linear poly(ethylene-ran-acrylic acid), i.e., polyethylene bearing pendant carboxyl (−COOH) groups randomly along the chain (PE-COOH), during the crystallization and melting processes. Additional use of the corresponding methyl ester (PE-COOMe) and polyethylene (PE) enabled us to separate the effects of hydrogen bonds (H-bonds) among the −COOH groups from those caused by the random insertion of pendant groups. The crystallizable methylene sequences were divided by the randomly inserted pendant groups, giving rise to the sequence-length selective crystallization. That is, longer sequences crystallized at higher temperatures and vice versa. This significantly hindered the crystallization rate, crystal thickness, and crystallinity and led to a strong melt memory effect. The H-bonding between −COOH groups acted as physical cross-linking for the PE chains and hindered the chain motion. This further slowed the crystallization and reinforced the melt memory effect. On the other hand, the H-bonds contributed to increased crystallinity and melting point of PE-COOH crystallized at high temperatures, compared to those of PE-COOMe. Thus, it was found that the H-bonding of −COOH groups stabilized the crystals formed at high temperatures. This work presents a unified view of the crystallization and melting behavior of strictly linear PE with interacting pendant groups, providing the possibility to precisely control the crystalline morphology in scales of both lamella and spherulite by altering the thermal history.