Random copolymers crystallize in three modes according to the comonomer inclusion/exclusion balance during crystallization: isodimorphism, isomorphism, and comonomer exclusion.1,2 Considering a PAxBy copolymer, the B co-units are excluded from PA crystals in the comonomer exclusion case, limiting their crystallization to a narrow composition range and vice-versa, leaving a wide intermediate composition range without crystallization. In contrast, in isomorphic copolymers, total comonomer inclusion leads to the formation of a new phase, PA-B, in all composition ranges that differ from that of the homopolymers. As a result, the properties, such as the melting temperature (Tm), vary linearly with composition. Finally, isodimorphism can be regarded as an intermediate case (with a comonomer inclusion/exclusion balance) in which partial comonomer inclusion leads to crystallization for all compositions. But, due to the excluded co-units, there is a Tm depression as a function of the composition, resulting in a pseudo-eutectic behaviour, also extrapolated to other properties. Recent studies of apparent isodimorphic copolymers have revealed that their pseudo-eutectic behaviour arises from a combination of different crystallization modes, specifically mixed crystallization modes.
In this lecture, we will present the results our group has gathered regarding the mixed crystallization modes, focusing on our research on (a) copolycarbonates3,4, in which we identified key characteristics of isodimorphism alongside unusually high crystallinity and the formation of a new crystalline phase in a limited composition range, leading to a new mixed isodimorphic/isomorphic crystallization mode; and (b) in copolyesters,5 where we noted the absence of comonomer inclusion on one side of the pseudo-eutectic, defining another new mixed crystallization mode of comonomer exclusion and isodimorphism. These findings introduce two novel mixed crystallization modes for random copolymers but also open new questions about why these modes arise.