Microphase and macrophase separation in binary and ternary block copolymer blends

Banaszak, Michal (1991) Microphase and macrophase separation in binary and ternary block copolymer blends. Doctoral (PhD) thesis, Memorial University of Newfoundland.

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Abstract

In this thesis we investigate some properties of microphase separated copolymer blends by means of the theory of polymer mixtures developed by K. M. Hong and J. Noolandi, carrying out numerical self-consistent calculations for copolymer solvent blends and modifying a fourth order expansion of the free energy for copolymer/homopolymer blends. In all cases we restrict attention to the lamellar structure. -- Using the numerical self-consistent calculations we carry out systematic studies for copolymer/selective solvent blends in both the weak and strong segregation regimes. Comparison with earlier results of Whitmore and Noolandi for copolymer/neutral solvent blends is provided. -- We also study the lamellar structure of binary A-b-B/A and ternary A-b-B/A/B copolymer/homopolymer blends near the microphase separation transition. The approach we have developed in this case combines perturbative solutions to the modified diffusion equation with a model for the total A and B polymer density profiles. As test of this procedure we have compared numerical self-consistent calculations for binary copolymer/selective solvent blends with the modification of fourth order expansion introduced in this thesis. We have used the procedure to calculate the domain and subdomain thicknesses, the interfacial width, swelling of the copolymers by the homopolymers, and individual polymer density profiles, and their dependence on the copolymer and homopolymer degrees of polymerization, overall composition, and Flory interaction parameter. The results are compared with three sets of experiments on copolymer/homopolymer blends. They are consistent with the picture that added homopolymers tend to penetrate within the copolymers and swell them laterally, and that the degree to which this occurs depends on the relative molecular weights of the copolymers and homopolymers, as indicated in experiments of Hashimoto and coworkers and others. The tendency of added homopolymers to cause an increase or decrease in the domain thickness correlates with their tendency to stabilize or destabilize the microphase. -- Finally we construct phase diagrams of ternary, A-b-B/A/B, copolymer/homopolymer blends. This work is an extension of earlier research by Whitmore and Noolandi for binary and ternary blends. The approach, again, uses a perturbative solution to the modified diffusion equation to calculate the polymer distribution functions, but it employs only one wavenumber in the fourth order expansion of the free energy, significantly simplifying the numerical calculations. The main results of these calculations are phase diagrams for a variety of model systems containing symmetric and asymmetric copolymers mixed with homopolymers of varying molecular weights, and for a PS-b-PI/PS/PI mixture. We also compare induced microphase formation in ternary and binary blends

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