Mixed Isodimorphic/Isomorphic Crystallization in Aliphatic Random Copolycarbonates

The crystallization behavior of two series of novel aliphatic random copolycarbonates: poly(heptamethylene-ran-octamethylene carbonate) (PC7/PC8) and poly(octamethylene-ran-dodecamethylene carbonate) (PC12/PC8) has been studied. Both copolymers exhibit apparent isodimorphic behavior, crystallizing in the entire composition range and exhibiting pseudo-eutectic points. However, melting enthalpies (ΔH_m) and crystallinities (X_c) deviate from the pseudo-eutectic behavior, indicating that a mixed isodimorphic/isomorphic crystallization has occurred, a behavior reported recently by us in poly(hexamethylene-ran-octamethyelene carbonate) (PC6/PC8). Further understanding of this behavior was obtained through structural and conformational characterization by in-situ synchrotron radiation wide-angle X-ray scattering (WAXS) and Fourier-transform infrared spectroscopy (FT-IR). For PC7/PC8 copolymers, a new third crystalline phase, the γ phase, different from PC7- and PC8-type crystalline phases (or any of their polymorphs: δ, α, and β) was detected at intermediate compositions. As the number of methylene groups in the repeating unit of the second comonomer in the PC8-based copolymers increases, the composition range where the γ phase is observed narrows. Additionally, the position of the pseudo-eutectic point shifts towards compositions rich in PC8, demonstrating how the chemical structure affects the exact location of the pseudo-eutectic point in these copolycarbonates.