Confined crystallization behaviors and phase morphologies of PVCH-PE-PVCH/PE homopolymer blends

Abstract

To investigate confined crystallization of polymer chains with/without fixed chain ends in nanoscale spaces, a series of blends of poly(vinylcyclohexane)-b-poly(ethylene)-b-poly(vinylcyclohexane) (PVCH-PE-PVCH) and ethylene polymers with different chain lengths, i.e. (i) paraffin (M¯n = 364 g/mol) and (ii) polyethylene (homo-PE, M¯w = 15 000 g/mol), were prepared by spin-coating. The phase morphologies of these triblock copolymer/homopolymer (Hp) blends with various volume fractions of Hp were observed by Atomic Force Microscope (AFM), and were theoretically simulated by employing the dissipative particle dynamics (DPD) technique. The crystallization behaviors of PE chains, which located in block copolymer and physically blended with block copolymer could be regards as with/without fixed chain ends respectively, in these well-designed microphase-separated spaces were also explored by differential scanning calorimetry (DSC). AFM results show that short chain paraffins can be well dissolved in PVCH-PE-PVCH, which causes a transformation from cylindrical PE microdomains to lamellar ones with the increasing of the weight fraction of paraffin (ϕp). However, homo-PE (relative long chain) is dissolved poorly in PVCH-PE-PVCH and cannot induce this transformation, while the macrophase separation takes place at a lower weight fraction of the homo-PE (ϕh). Our simulations for the phase structures of these systems suitably reproduce the experimental observations, and reveal that the entropic interactions consequently determine the mesoscale phase morphologies. DSC results indicate that the melting temperature (Tm) of PE blocks (fixed at both chain ends) gradually rises with increasing ϕp, and reaches the peak value when the microphase structures transform from cylinders to lamellae. This implies that the confinement degree of PE block in cylinders is stronger than that in lamella due to the less conformational entropy caused by fixed chain ends. In PVCH-PE-PVCH/homo-PE system, the Tm of PE block rises slightly with increasing ϕh, while the Tm of homo-PE (both chain ends free) within the nanoscale PE microdomains is not changed. It demonstrates that the restricted effect, which affects the Tm in PVCH-PE-PVCH, could be ascribed to the fixed PE chain ends linked to the glassy PVCH. However, the supercooling degree (▵T) of homo-PE crystallization dissolved in PE block microdomains decreased sharply compared to that of pure homo-PE crystallization. It shows that the driving force for homo-PE crystallization is increased due to the spatial confinement which results in the crystallization to be difficult.