Crystallization in the vesicle walls templated by dry-brush block copolymer/homopolymer blend

We studied the crystallization behavior in a diblock copolymer/homopolymer blend system exhibiting "dry-brush" phase behavior in the melt. A nearly symmetric poly(ethylene oxide)-block-polybutadiene (PEO-b-PB) was blended with a PB homopolymer (h-PB) having approximately the same molecular weight as that of the PB block, thereby yielding the dry-brush blends wherein h-PB was localized in the PB microdomain, causing expansion of PB domain thickness without introducing transformation in microdomain morphology. Even though the lamellar identity of PEO domain retained throughout the blend composition, the lamellar units became increasingly isolated as characterized by the formations of cylindrical and spherical vesicles at high h-PB compositions. Over the major composition range (ω_h-PB ≤ 0.7), crystallization of PEO blocks in the vesicle wall was able to take place at the undercooling comparable to that of PEO homopolymer, implying that the crystallization mechanism was analogous to the homopolymer crystallization initiated predominantly through heterogeneous nucleation followed by long-range crystal growth. At the compositions (ω_h-PB ≥ 0.8) where most PEO lamellae formed shells of spherical vesicles in the melt, the crystallization was effectively confined within the individual vesicle, and it only occurred at very deep undercooling (ΔT > 70 K). The corresponding isothermal crystallization followed the first-order kinetics prescribed by a nucleation-controlled crystallization wherein the crystallization started from homogeneous nuclei followed by essentially instantaneous crystal growth to fill the vesicle wall. In general, the confinement effect exerted by dry-brush blending was far less effective than the corresponding wet-brush blending.