Challenging Isodimorphism Concepts: Formation of Three Crystalline Phases in Poly(hexamethylene- ran-octamethylene carbonate) Copolymers

Yilong Liao; Ricardo A Pérez-Camargo; Haritz Sardon; Antxon Martínez de Ilarduya; Wenxian Hu; Guoming Liu; Dujin Wang; Alejandro J Müller

doi:10.1021/acs.macromol.3c01265

Challenging Isodimorphism Concepts: Formation of Three Crystalline Phases in Poly(hexamethylene- ran-octamethylene carbonate) Copolymers

Macromolecules. 2023 Oct 11;56(20):8199-8213. doi: 10.1021/acs.macromol.3c01265. eCollection 2023 Oct 24.

Authors

Yilong Liao¹, Ricardo A Pérez-Camargo¹, Haritz Sardon¹, Antxon Martínez de Ilarduya², Wenxian Hu^{3

4}, Guoming Liu^{3

4}, Dujin Wang^{3

4}, Alejandro J Müller^{1

5}

Affiliations

¹ POLYMAT and Department of Polymers and Advanced Materials: Physics, Chemistry, and Technology, Faculty of Chemistry, University of the Basque Country UPV/EHU, Paseo Manuel de Lardizábal, 3, Donostia-San Sebastián 20018, Spain.
² Department of Chemical Engineering, Polytechnic University of Catalonia ETSEIB-UPC, Diagonal 647, Barcelona 08028, Spain.
³ Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Engineering Plastics, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China.
⁴ University of Chinese Academy of Sciences, Beijing 100049, P. R. China.
⁵ Ikerbasque, Basque Foundation for Science, Plaza Euskadi 5, Bilbao 48009, Spain.

Abstract

In this work, poly(hexamethylene-ran-octamethylene carbonate) copolycarbonates were synthesized by melt polycondensation in a wide range of compositions. The copolymers displayed some of the characteristic isodimorphic thermal behavior, such as crystallization for all the compositions and a pseudoeutectic behavior of the melting temperature (T_m) versus composition. The pseudoeutectic point was located at 33 mol % poly(octamethylene carbonate) (POC) content (i.e., corresponding to the PH₆₇O₃₃C copolymer). Surprisingly, the crystallinities (X_c) for a wide range of copolymer compositions were higher than those of the parent components, a phenomenon that has not been observed before in isodimorphic random copolymers. The structural characterization, performed by wide-angle X-ray scattering (WAXS) and small-angle X-ray scattering experiments, revealed unexpected results depending on composition. On the one hand, the poly(hexamethylene carbonate) (PHC)- and POC-rich copolymers crystallize in PHC- and POC-type crystals, as expected. Moreover, upon cooling and heating, in situ WAXS experiments evidenced that these materials undergo reversible solid-solid transitions [δ-α (PHC) and δ-α-β (POC)] present in the parent components but at lower temperatures. On the other hand, a novel behavior was found for copolymers with 33-73 mol % POC (including the pseudoeutectic point), which are those with higher crystallinities than the parent components. For these copolymers, a new crystalline phase that is different from that of both homopolymers was observed. The in situ WAXS results for these copolymers confirmed that this novel phase is stable upon cooling and heating and does not show any crystallographic feature of the parent components or their solid-solid transitions. FTIR experiments confirmed this behavior, revealing that the new phase adopts a polyethylene-like chain conformation that differs from the trans-dominant ones exhibited by the parent components. This finding challenges the established concepts of isodimorphism and questions whether a combination of crystallization modes (isodimorphism and isomorphism) is possible in the same family of random copolymers just by changing the composition.