Side-chain crystallinity,heat of melting,and thermal transitions in poly[N-(10-n-alkyloxycarbonyl-n-decyl)maleimides] (PEMI)

The heat of melting, the melting temperature T_m, and the sub-T_g transition temperature have been studied from –120°C to above T_m in a series of 11 polyN-(10-n-alkyloxycarbonyl-n-decyl)]-maleimides (PEMI). Side-chains from ethyl to n-docosyl with n even have been included. The contribution to the heat of melting per methylene group shows that the hexagonal paraffin crystal modification is present in these poly(N-maleimides), in agreement with x-ray data for the same compounds. The enthalpy data show that only a part of the outer methylene groups are present in the crystalline aggregates. Furthermore, DSC traces exhibit a typical distribution of crystallite sizes, which become narrower as the side-chains become longer. The critical chain length needed to form a stable nucleus includes nine methylene groups in the outer part of the n-alkyl side-chain. The influence of the side-chain length and crystallinity on the γ-transition temperature of these polymers was also investigated. In the range where these polymers are essentially amorphous the sub-T_g transition temperature decreases continuously as the number of methylene groups in the side-chain increases. This transition is attributed to internal motion within the external side-group without any interaction with the main chain. This is presumably made possibly by the partial rotation of the oxycarbonyl group. We suggest that this transition is similar to the well known γ transition which has been attributed to various segmental motions in all ethylene copolymers and in all homopolymers containing a determined number of? CH₂? units in the main-chain or in the side-chain. Estimates based on the chemical structure, yield a value for the γ transition of ? CH₂? similar to that measured by other methods in polyethylene and related materials.