Mixed Isodimorphic/Isomorphic Crystallization in Aliphatic Random Copolycarbonates

In this work, we investigate 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). Both copolymers display apparent isodimorphic behavior as they crystallize over the entire composition range, exhibiting pseudoeutectic points at 45 and 76 mol % of PC8 content for PC7/PC8 and PC12/PC8, respectively. However, the evolution in melting enthalpies (ΔH_m) and crystallinities (X_c) deviate from the expected pseudoeutectic behavior, indicating mixed isodimorphic/isomorphic crystallization, a behavior reported for the first time recently by us in poly(hexamethylene-ran-octamethyelene carbonate) (PC6/PC8). Further understanding of this behavior was obtained through structural and conformational characterization employing in situ synchrotron radiation wide- and small-angle X-ray scattering (WAXS/SAXS) and Fourier-transform infrared spectroscopy (FT-IR). For PC7/PC8 copolymers, a new third crystalline phase, from now on named the γ phase, different from PC7- and PC8-type crystalline phases (or any of their polymorphs: δ, α, and β), emerged for intermediate compositions, i.e., 34–45 mol % PC8, in line with the atypical variations of ΔH_m and X_c. For PC12/PC8 copolymers, a coexistence of the γ/PC8 type phase was found at PC8-rich contents. According to FT-IR results, the γ phase adopted a polyethylene-like conformation in all cases despite the presence of C═O groups. However, compared with those compositions where homopolymer-like phases dominate the crystallization, there is a stronger dipole–dipole interaction in the γ phase, evidenced by a shift of absorption band associated with C═O stretching. This evidence aligns with our previous work in PC6/PC8 copolymers, demonstrating that the mixed isodimorphic/isomorphic crystallization found in PC8-based random copolycarbonates is likely a general case. Additionally, 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. At the same time, the position of the pseudoeutectic point shifts toward compositions rich in PC8, demonstrating how the chemical structure affects the exact location of the pseudoeutectic point in these random copolycarbonates.