Crystal orientation of poly(ɛ-caprolactone) blocks confined in crystallized polyethylene lamellar morphology of poly(ɛ-caprolactone)-block-polyethylene copolymers

The crystal orientation of poly(ɛ-caprolactone) (PCL) blocks in PCL-block-polyethylene (PE) copolymers has been investigated using two-dimensional small-angle X-ray scattering (2D-SAXS) and 2D wide-angle X-ray diffraction (2D-WAXD) as a function of crystallization temperature Tc and thickness of PCL layers dPCL. The PCL blocks were spatially confined in the solid lamellar morphology formed by the crystallization of PE blocks (PE lamellar morphology), an alternating structure of crystallized PE lamellae and amorphous PCL layers. This confinement is expected to be intermediate between hard confinement by glassy lamellar microdomains and soft confinement by rubbery ones, because the crystallized PE lamellae consist of hard PE crystals covered with amorphous (or soft) PE blocks. The 2D-SAXS results showed uniaxial orientation of the PE lamellar morphology after applying the rotational shear to the sample. Therefore, it was possible to investigate crystal orientation of PCL blocks within the oriented PE lamellar morphology. The 2D-WAXD results revealed that the c axis of PCL crystals (i.e., stem direction of PCL chains) was parallel to the lamellar surface normal irrespective of Tc when 16.5 nm ≥ dPCL ≥ 10.7 nm. However, it changed significantly with changing Tc when dPCL = 8.8 nm; the c axis was perpendicular to the lamellar surface normal at 45 °C ≥ Tc ≥ 25 °C while it was almost random at 20 °C ≥ Tc ≥ 0 °C. These results suggest that the PE lamellar morphology plays a similar role to glassy lamellar microdomains regarding spatial confinement against subsequent PCL crystallization.