Abstract
AbstractTo examine the effect of spatial confinement on the crystallization behavior of polymers and the correlation between overall crystallization kinetics and microphase-separated morphology in the melt, the crystallization kinetics of a diblock copolymer, poly(ethylene oxide)-block-polybutadiene (PEO-b-PB), blended with PB homopolymers (h-PB) with a broad range of molecular weight was studied. A nearly symmetric PEO-b-PB with Mb,PEO = 7500 and Mb,PB = 5500 was blended with h-PBs to yield the blends exhibiting “completely wet brush”, “partially wet brush”, and “completely dry brush” phase behavior in the melt state. In the completely wet-brush system, PEO microdomains in the melt state systematically transformed from lamellae to cylinder to sphere with increasing h-PB composition. In the case of completely dry-brush, the lamellar identity of PEO domain retained throughout the blend composition but the lamellar units became increasingly isolated as characterized by the formations of cylindrical and spherical vesicles at high h-PB compositions. Partially wet-brush represents an intermediate case, where the transformation in domain morphology was still accessible with increasing h-PB composition but the transition points were “delayed” due to partial solubilization of h-PB in the PB microdomains. Crystallization kinetics studies of the three blend systems indicated that the crystallization rate was highly sensitive to the extent of solubilization of h-PB in the PB domain (or the degree of wetting) at intermediate to high h-PB composition. Since the degree of wetting determined the domain/vesicle morphology and domain connectivity, our results asserted that crystallization kinetics may serve as a sensitive probe for microdomain morphology and connectivity of block copolymers in the melt state.To establish a preliminary model for the crystal orientation in block copolymers consisting of a soft amorphous phase, the crystal orientation in the lamellae-forming PEO-b-PB crystallized at different Tc was assessed from the melting point and the corresponding thickness of PEO lamellae in the second part of the study. The c-axis of the PEO chains was postulated to align perpendicularly to the lamellar surface at Tc ＝ 23 and 25 ℃. The crystalline stems however started to tilt away from the lamellar interface at high Tcs, because the tilt could increase the cross sectional area of the PB blocks, which would then allow the stretched PB chains to relax. The result suggested that the conformational entropy of the amorphous blocks may be an important factor controlling the crystal orientation in the crystalline domain.