Confined crystallization and morphology of melt segregated PLLA-b-PE and PLDA-b-PE diblock copolymers

The crystallization behavior of strongly segregated diblock copolymers composed of polyethylene (PE) and poly(L-lactide) or racemic poly(lactide) (PLA) blocks has been investigated by differential scanning calorimetry (DSC), small-angle X-ray scattering (SAXS), wide-angle X-ray scattering (WAXS), and transmission electron microscopy (TEM). In both systems the crystallization of PE block was confined within the preexisting lamellar domain. In the double-crystalline PLLA-b-PE, coincident crystallization of PLLA and PE blocks was observed during cooling process because the crystallization rate of the PLLA block was retarded by the covalent linkage with the PE block. When the PLLA block was self-nucleated, a complete separation of the crystallization process of both blocks was achieved. Polarized optical microscopy confirmed that neither PLLA nor PE blocks could form spherulites in view of the large segregation strength that effectively confined the crystallization within the lamellar microdomains. High-speed DSC was applied to reduce reorganization during the scan so that values closer to the equilibrium melting points (T-m degrees), employing the Hoffman-Weeks treatment, could be obtained for PLLA. The T-m degrees for the PLLA block was depressed as compared to homo-PLLA by confinement effects. The crystallization of the PE block within the amorphous-crystalline PLDA-b-PE was strictly confined, and the microdomain morphology established in the melt state was essentially unperturbed, regardless of whether the crystallization occurred when the PLDA block was glassy or rubbery (hard or soft confinement). In the case of the double-crystalline PLLA-b-PE system, the crystallizations of both PE and PLLA blocks were also effectively confined within the respective lamellar microdomains, irrespective of which is the leading crystallizing component prescribed by the crystallization history. The confinement effect was a consequence of the large segregation strength coupled with the solidification of the microdomains of the leading crystallizing component, which subsequently imposed a hard confinement effect on the crystallization of the second block. TEM revealed the ordered lamellar morphology, and in one case the crystalline lamellae of the PE block were visualized within the microphase-separated lamellae due to an isothermal crystallization pretreatment.