Abstract
We studied the crystallization behavior in a block copolymer/homopolymer blend system exhibiting “dry-brush” phase behavior in the melt. A nearly symmetric poly(ethylene oxide)-block-polybutadiene (PEO-b-PB) was blend 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 thickness expansion of the PB domain without introducing transformation in microdomain morphology. Even though the lamellar identity of PEO domain retained throughout the blend composition, the global structure of the lamellar unit transformed from lamellar to cylindrical to spherical vesicles with increasing h-PB composition. The fixed cooling rate experiment in the differential scanning calorimetry (DSC) revealed that the crystallization of PEO in the lamellar phase proceeded at the undercooling comparable to that of PEO homopolymer over the major composition range (wh-PB £ 0.7). The corresponding isothermal crystallization kinetics was properly described by the classical Avrami equation, indicating that the crystallization started from heterogeneous nucleation followed by long-range crystal growth. In neat PEO-b-PB, the long-range growth was driven by breaking out the melt morphology (which led to percolated lamellar structure and spherulitic), while in the blends it was assisted by the connected PEO lamellae in the melt (which led to preservation of melt structure). At very high h-PB composition (wh-PB 3 0.8) where the PEO lamellae formed spherical vesicles in the melt, a crystallization exotherm located at very large undercooling (ca. 90K) emerged upon cooling. The isothermal crystallization experiment revealed that the corresponding crystallization followed a simple first-order kinetics prescribed by a homogeneous nucleation-controlled crystallization wherein the crystallization started from homogeneous nuclei followed by essentially instantaneous crystal growth to fill domain space. Crystallization modes in the present dry-brush system may thus be breakout, templated, or confined depending upon the composition of h-PB. In general, the confinement effect exerted by dry-brush blending was far less effective than the corresponding wet-brush blending in which the confinement started to operate at wh-PB ? 0.48 (i.e. the composition where the PEO domains transformed into cylinders).