Novel aspects in the microphase separation of block copolymers

Citation

Zheng, Wei (1997) Novel aspects in the microphase separation of block copolymers. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/N3Z2-AV91. https://resolver.caltech.edu/CaltechETD:etd-04222005-131237

Abstract

Several novel aspects of the microphase separation transition of block copolymers have been studied. These aspects are exemplified by the following three systems: (1) ABC triblock copolymers; (2) Weakly charged diblock copolymers; (3) Diblock copolymers in confined geometries. For ABC triblock copolymers, results from theoretical calculations of the morphological phase diagrams in the strong segregation limit are presented. The chain conformation free energy is approximated following an approach proposed by Ohta and Kawasaki. Our study focuses on two unique features of the ABC triblock copolymers, namely, the dependence of the morphology on the sequence of the triblock chain and the relative strength of the various interaction parameters. Our results compare favorably with experimental observations. In addition, we predict the existence of some new structures that have yet to be observed experimentally. For weakly charged diblock copolymers, a theoretical framework is developed. This framework combines the Random Phase Approximation and the Poisson-Boltzmann equation in order to consistently treat the electrostatic interactions between all charged species and the free energy contributions from the connectivity of the diblock copolymers. A Landau-Ginzburg effective free energy is derived and is used to study the microphase separation of charged-neutral block copolymers with an arbitrary amount of added salt in the weak segregation limit. Study of the spinodal limit of the system shows not only greatly enhanced compatibility between A and B blocks in comparison with the corresponding neutral system, but also inhibition of microphase separation under certain conditions. A criterion for microphase separation is derived and phase diagrams under various conditions are presented. Study of concentration fluctuation near the order-disorder transition demonstrates that the breaking of the interchange symmetry leads to new scaling of fluctuation corrections at a fixed value of monomer charge density [alpha]. The morphology of diblock copolymer melt confined between two solid walls in the strong segregation limit is studied by extending the method developed by Ohta and Kawasaki to include surface effects. We focus on two new features which are absent in simple diblock copolymers: the competition between the surface interactions and the confinement effects, and the breaking of the rotational and translational symmetry. The first new feature is demonstrated by studying the equilibrium properties of symmetric/nearly symmetric diblock copolymers confined between two identical walls with a small preferential surface affinity. The second feature is illustrated by studying the phase behavior of diblock copolymers with arbitrary value of volume fraction. For phase transition involves morphologies with three dimensional structure, the broken rotational and translational symmetry leads to the dependence of the transition volume fraction on the distance between the two plates. The influence of surface effects on diblock copolymers confined between two distinct plates is also studied by presenting phase diagrams for two special cases.

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