Crystallization and morphology of the trigonal form in random propene/1-pentene copolymers

The melting and crystallization behaviour of a series of isotactic propene/1-pentene random copolymers, with 1-pentene contents up to 50 mol%, was investigated by DSC and temperature resolved WAXD/SAXS. The role of the 1-pentene comonomer in the development of the trigonal modification (δ-form) of i-PP was studied and the results were compared with those reported in the literature for PP copolymers with 1-hexene. The crystallizing capability of the δ-form, which develops in the composition range between ca. 10 and 50 mol% of 1-pentene content, only slightly decreases with concentration of 1-pentene. This result is correlated with the limits imposed to cell expansion by the crystal density. The crystallinity degree calculated from the deconvolution of the WAXD patterns is in fair agreement with the results of the DSC analysis, from which the value of the melting enthalpy of the perfect i-PP δ-form has been estimated to be around 140 J/g. The crystallization kinetics of the trigonal modification is characterized by a composition-dependent induction time followed by a relatively fast development of structural order. The sharp WAXD reflections combined with the SAXS data suggest that, notwithstanding the intrinsic intrachain structural disorder, thin and wide lamellae characterize the morphology of the δ-form crystallites.