Nucleation and crystallization in double crystalline poly(p-dioxanone)-b-poly(ε-caprolactone) diblock copolymers

The crystallization behavior of three double crystalline diblock copolymers containing poly(epsilon-caprolactone) and poly(p-dioxanone) has been studied via differential scanning calorimetry (DSC), polarized optical microscopy (POM), and wide-angle X-ray scattering (WAXS). Crystallization and melting temperatures and enthalpies are compared among copolymers and to those of the corresponding homopolymers. Only one crystallization exotherm was observed for the diblocks. DSC and WAXS indicated that during isothermal crystallization PPDX crystallized first, followed by PCL. POM revealed a transformation of crystalline morphologies at around 50 degreesC, from granular aggregates at high temperature (where only PPDX is crystalline) to banded spherulites at lower temperature, where both blocks were crystalline. The kinetics of crystallization were studied in detail via spherulite growth rates obtained from POM, and it was found that PPDX crystallization in the diblocks occurred much more slowly than in the homopolymers, this being responsible for the observed coincident crystallization of the two blocks during DSC cooling scans. On the other hand, the crystallized PPDX acts to nucleate the PCL block, so that heterogeneous crystallization was always observed even in copolymers containing a minority of this component (23%), for which "confined" crystallization might be expected. The crystallization kinetics of the PCL in the copolymers is accelerated by the presence of the crystalline PPDX block.