Composition dependence of crystallization behavior observed in crystalline-crystalline diblock copolymers

Abstract

The crystallization behavior of poly(ε-caprolactone) (PCL) blocks starting from a solid morphology formed in advance by the crystallization of polyethylene (PE) blocks (PE-crystallized morphology) in PCL-WPE diblock copolymers has been investigated by a time-resolved synchrotron small-angle X-ray scattering (SR-SAXS) method as a function of composition (or volume fraction of PE blocks Φpe in the system). The PE-crystallized morphology and the crystallized state of PCL blocks (i.e., melting temperature and crystallinity of PCL blocks) were examined by static SAXS and differential scanning calorimetry methods, respectively. When PCL-B-PE with ΦPE ≤ 0.58 was quenched from a micropbase-separated melt into low temperatures Tc (30°C ≤ Tc ≤ 45°C), the PE block crystallized first to yield rae PE lamellar morphology, an alternating structure consisting of thin PE crystals and amorphous PE + PCL layers, followed by the crystallization of PCL blocks starting from this PE lamellar morphology, where two different crystallization behaviors of PCL blocks were observed depending on Tc; at higher Tc (≥ 40°C) the Avranri index was ca. 3, indicating a heterogeneous crystallization in 3D space. However, it was smaller (∼2) at lower Tc (≤38°C), suggesting a confined crystallization within the PE lamellar morphology. The temperature at which the crystallization mechanism changed was almost independent of ΦPE in these copolymers. The late stage of PCL crystallization (or post-Avrami process) was not significantly dependent on ΦPE and similar to that of crystalline homopolymers. For PCL-b-PE with ΦPE ≥ 0.73, the PE block crystallized on the basis of molten microdomains to yield the PE-crystallized microdomain, and eventually PCL crystallization was confined within this microdomain for every Tc at early and late stages. These results were consistent with our previous conclusions derived from the investigation of resulting morphology. ©2008 The Society of Polymer Science.