Abstract
Self-assembly is a process that autonomously organizes the building blocks into ordered patterns or structures from a disordered state. These ordered structures usually possess a hierarchy of length scales and represent to a state of lowest free energy. Self-assembly is reversible and can be manipulated by proper controls of the external conditions (e.g. temperature, concentration). Recently, self-assembly has been exploited as a practical strategy for constructing ordered nanostructures and has thus become an essential part in nanotechnology. Block copolymers are capable of self-assembling into a series of long-range ordered morphology governed by interblock segregation strength and the volume fraction of the constituting blocks. Therefore, they have been considered as one of the most important self-assembled polymer materials. Three topics concerning diblock copolymer blends have been studied in this dissertation, including (1) phase transition, (2) crystallization-induced perturbations of microdomain structure and (3) cocrystallization behavior. The diblock system studied in here was poly(ethylene oxide)-block-polybutadiene (PEO-b-PB). A lamellae-forming PEO-b-PB was blended with a low molecular weight homopolymer PB (h-PB) to yield PEO spheres dispersed in the PB matrix. The first experimental evidence on the existence of face-centered cubic (FCC) phase in diblock copolymer/homopolymer blends was disclosed here. The thermal reversibility experiment verified that the FCC packing was a thermodynamically stable phase at elevated temperature. The driving force of the order-order transition (OOT) between BCC and FCC phase was postulated to stem from the balance between packing frustration and interfacial energy. More importantly, the BCC-FCC OOT was found to occur if the system contained the corresponding precursor prior to transition. The lack of such a precursor may be responsible for the prevalent lack of BCC-FCC OOT among diblock copolymer systems in the bulk. Crystallization in the microdomains of crystalline-amorphous diblock copolymers may induce domain coalescence when the crystallization temperature lies above the Tg of the amorphous matrix. We have systematically studied the effect of crystallization on the microdomain morphology of PEO-b-PB/h-PB blends exhibiting spherical morphology in the melt. Isothermal crystallization was found to deform the originally spherical domain into ellipsoidal object with aspect ration of ca. 1.3. Drastic perturbation of the domain structure was induced by annealing the crystalline samples near the onset of melting, where two to three microdomains coalescence into a rod object. We have also examined the role of h-PB in the domain coalescence. It was observed that the blend containing higher h-PB content exhibited a stronger resistance against microdomain coalescence. The resistance was presumed to stem from the diffusion barrier associated with the rejection of a portion of h-PB originally dissolved in the coronal regions of the micelles during the coalescence process. The binary blends of a short symmetric PEO-b-PB and a long asymmetric PEO-b-PB have been studied to examine the cocrystallization behavior of the longer and the shorter PEO blocks confined in the lamellar microdomains. Cocrystallization was found to occur over a wide range of undercooling whereas the corresponding blends of PEO homopolymers with similar molecular weights and compositions displayed phase-segregated crystallization. In contrast to the kinetically trapped solid solutions formed in homopolymer blends, the cocrystallization behavior observed here may be driven thermodynamically to attain a lower interfacial energy in the system while allowing the long PB blocks in the asymmetric diblock to relax conformationally.