Critical analysis of the crystal orientation behavior in polyethylene-based crystalline-amorphous diblock copolymer

Orientation of polyethylene (PE) crystals formed over a broad range of undercooling in a polyethylene-block-poly(d,l-lactide) (PE-b-PDLLA) diblock copolymer has been critically examined. Due to the large segregation strength and approximate 50/50 composition in this system, the crystallization took place within one-dimensionally confined lamellar microdomains without forming spherulites. A homogeneous crystal orientation with the PE crystalline stems orienting parallel to the lamellar interface was observed at crystallization temperatures (T_c) between 45 and 102 °C. Once the sample was crystallized at T_c ≥ 40 °C or directly quenched into liquid nitrogen from the melt, the isotropic WAXD pattern indicated that the PE crystals became randomly oriented. Considering that homeotropic orientation was not identified for the present system and other PE-based crystalline-amorphous diblocks, we concluded that PE crystals in the lamellar microdomains always show homogeneous orientation when there is a preferred orientational order. We further organized the thermodynamic and kinetic factors that may govern the preferred crystal orientation in diblock copolymers and concluded that the orientational order should be controlled by the competition between nucleation and crystal growth kinetics. The persistence of homogeneous orientation of PE was attributed to its excellent nucleating power.