Mixed Isodimorphic/Isomorphic\nCrystallization in Aliphatic\nRandom Copolycarbonates

In this work, we investigate the crystallization behavior\nof two\nseries of novel aliphatic random copolycarbonates: poly(heptamethylene-<i>ran</i>-octamethylene carbonate) (PC7/PC8) and poly(octamethylene-<i>ran</i>-dodecamethylene carbonate) (PC12/PC8). Both copolymers\ndisplay apparent isodimorphic behavior as they crystallize over the\nentire composition range, exhibiting pseudoeutectic points at 45 and\n76 mol % of PC8 content for PC7/PC8 and PC12/PC8, respectively. However,\nthe evolution in melting enthalpies (Δ<i>H</i><sub>m</sub>) and crystallinities (<i>X</i><sub><i>c</i></sub>) deviate from the expected pseudoeutectic behavior, indicating\nmixed isodimorphic/isomorphic crystallization, a behavior reported\nfor the first time recently by us in poly(hexamethylene-<i>ran</i>-octamethyelene carbonate) (PC6/PC8). Further understanding of this\nbehavior was obtained through structural and conformational characterization\nemploying in situ synchrotron radiation wide- and small-angle X-ray\nscattering (WAXS/SAXS) and Fourier-transform infrared spectroscopy\n(FT-IR). For PC7/PC8 copolymers, a new third crystalline phase, from\nnow on named the γ phase, different from PC7- and PC8-type crystalline\nphases (or any of their polymorphs: δ, α, and β),\nemerged for intermediate compositions, i.e., 34–45 mol % PC8,\nin line with the atypical variations of Δ<i>H</i><sub>m</sub> and <i>X</i><sub>c</sub>. For PC12/PC8 copolymers,\na coexistence of the γ/PC8 type phase was found at PC8-rich\ncontents. According to FT-IR results, the γ phase adopted a\npolyethylene-like conformation in all cases despite the presence of\nCO groups. However, compared with those compositions where\nhomopolymer-like phases dominate the crystallization, there is a stronger\ndipole–dipole interaction in the γ phase, evidenced by\na shift of absorption band associated with CO stretching.\nThis evidence aligns with our previous work in PC6/PC8 copolymers,\ndemonstrating that the mixed isodimorphic/isomorphic crystallization\nfound in PC8-based random copolycarbonates is likely a general case.\nAdditionally, as the number of methylene groups in the repeating unit\nof the second comonomer in the PC8-based copolymers increases, the\ncomposition range where the γ phase is observed narrows. At\nthe same time, the position of the pseudoeutectic point shifts toward\ncompositions rich in PC8, demonstrating how the chemical structure\naffects the exact location of the pseudoeutectic point in these random\ncopolycarbonates.