Crystallization of a Weakly Segregated Polyolefin Diblock Copolymer

Microphase separation and subsequent crystallization are studied in an ethylene/head-to-head propylene (E/hhP) diblock copolymer using time-resolved small- and wide-angle X-ray scattering (SAXS and WAXS). When the sample is cooled from the melt, the SAXS data show a broad maximum due to correlation-hole scattering, which is replaced by a single sharp Bragg reflection due to the melt microstructure. During subsequent crystallization, two higher-order reflections grow concurrently. Thus, for similar quench depths below the microphase separation transition (T_MST) and the melting temperature (T_m) respectively, microstructures representative of weak and strong segregation are observed. The second- and third-order reflections are superimposed on an intense broad maximum arising from the heterogeneity within the semicrystalline E domains. The second- and third-order SAXS peaks and the broad maximum on which they are superimposed, none of which are visible prior to crystallization, grow in parallel with the development of crystallinity at the unit cell level observed by WAXS. The overall domain periodicity d increases during crystallization but its value is path dependent and varies with the cooling rate from the melt.