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Title:Liquid state theory of the local and long wavelength structure and thermodynamic properties of block copolymer liquids
Author(s):David, Edwin Frank
Doctoral Committee Chair(s):Schweizer, Kenneth S.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Physical
Plastics Technology
Abstract:This work presents a theoretical study of the structure, thermodynamics, and phase behavior of block copolymer melts. First, the formal extension of the liquid state "polymer reference interaction site model" (PRISM) theory to AB copolymers is presented. Within a simple "symmetric" model, analytical and numerical predictions are made for the fluctuation stabilization, peak scattering wavevector, and effective chi-parameter. The conditions for contact with Leibler's mean field theory are also established. Beyond this, a novel mechanism for the disordered phase stabilization is suggested wherein a depletion of energetically unfavorable contacts, and clustering of like species, reduces the driving force toward further segregation. This is manifest in a fluctuation renormalized effective chi-parameter, and a destruction of all k $\not=$ 0 spinodal instabilities for finite chains. The chain length scaling of these fluctuation corrections is identical to the Brazovskii-Leibler-Fredrickson-Helfand phenomenological field theory. Additionally, finite thickness chain models exhibit stronger stabilization effects brought on by local interchain packing correlations.
The roles of density and concentration fluctuations in the "medium induced" interactions experienced by a tagged chain in the liquid are also examined. Under certain assumptions the liquid state theory copolymer solvation potential becomes similar in form to previous field theoretic results, but where the fluctuation renormalized scattering function and chi-parameter replace the corresponding mean-field quantities. An application to the symmetric diblock yields good agreement with existing simulation data for the chain stretching phenomenon.
Structure-property relations in homopolymer melts are studied as well. Consistent with the experimental trend observed for olefin-based polymer melts, the bulk cohesive energy density is lower for more branched, or lower aspect ratio, species due to less efficient chain packing. The consequences of these correlation effects on blend miscibility are also discussed.
Finally, conformationally asymmetric diblock melts are investigated. Our conclusion for flexible polymers is that purely entropic packing interactions do not drive the system close to a microphase boundary. Predictions for diblocks interacting through attractive van der Waals forces, but where the Flory chi-parameter is identically zero, reveal rich dependencies of the apparent microphase spinodal boundaries on the degree of structural mismatch between blocks, consistent with experiment. This demonstrates the importance of coupled thermal and packing interactions, and the fundamental inseparability of enthalpic and entropic contributions to the effective chi-parameter governing miscibility. Length-scale-dependent "effective compositions" above and near the apparent microphase spinodal temperature are also calculated. Generally, large enrichments of the local composition are predicted in the highly fluctuating regime, even when the monomers are dilute in the bulk sense.
Issue Date:1995
Rights Information:Copyright 1995 David, Edwin Frank
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9624333
OCLC Identifier:(UMI)AAI9624333

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