Isodimorphic crystallization and thermally-induced crystal transitions in poly(octamethylene-ran-decamethylene carbonate): Critical role of comonomer defects

Minor comonomer units can enter the crystalline phase of isodimorphic copolymers as defects, and affect their chain conformation and packing. However, the effects of minor comonomer inclusion on the crystalline structure and crystal transitions of aliphatic copolycarbonates are still unclear. Herein, we synthesized the poly(octamethylene- ran -decamethylene carbonate) with various compositions and systematically studied their crystallization behavior and crystal transitions upon cooling and heating. The random copolycarbonates exhibit isodimorphic crystallization behavior, showing the pseudo-eutectic region at octamethylene carbonate fraction ( f C8 ) of 0.64–0.79. The octamethylene carbonate (C8)-rich copolycarbonates ( f C8  ≥ 0.79) show reversible solid-solid crystal transitions as poly(octamethylene carbonate) (PC8) homopolymer upon cooling and heating, which are temperature- and composition-dependent. With f C8 decreasing, the δ-α transition is observed at a high temperature with more inclusion of comonomer defects, and the crystals are conformational disordered with development of more gauche conformation in methylene segments at a low temperature when compared to PC8 homopolymer. At a high temperature, the chain conformation in methylene segments of C8-rich copolycarbonates are analogous as that of PC8 homopolymer, thus the α-β transition temperature remains unchanged.