Crystal Polymorphism and Multistage Phase Transition of Alternating Poly(ether ester)s Synthesized from Long-Spaced Dicarboxylic Acid and Diethylene Glycol

Poly(ether ester)s (PEEs) are an important class of thermoplastics that combine the good physical performance of both polyether and polyester blocks. However, their heterogeneous block structures complicate both the synthesis and the crystallization of PEEs. In this study, we synthesized a series of long-spaced PEEs with an alternative distribution of ether and ester segments from diethylene glycol (DEG) and n-methylene diacids (n = 6–18) and investigated their polymorphic crystallization and phase transition. The melting temperature and crystallinity of PEEs enhance with increasing the CH2 segment length of used diacid monomer. Interestingly, PEEs show temperature-dependent crystal polymorphism and multistage phase transition, depending on the CH2 segment length and crystallization temperature (Tc). Dual and triple crystal polymorphisms are found in the PEEs with medium CH2 segments (growth of hexagonal form II and form I at low and high Tc, respectively) and in those with long CH2 segments (growth of orthorhombic form III, hexagonal form II, and form I at low, medium, and high Tc, respectively). Phase transitions of form III to II and II to I are detected in PEEs upon heating, accompanied by the increase of structural stability. This study provides fundamental guidance for developing PEEs with tunable crystalline structures and properties.