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Dissertation/Thesis Abstract

Phase Behavior in Blends of Asymmetrical Polyolefins
by Nedoma, Alisyn Jenise, Ph.D., University of California, Berkeley, 2010, 129; 3444440
Abstract (Summary)

This dissertation presents the most comprehensive study of χ to date for a single pair of homopolymers. Polyisobutylene (component B) and deuterated polybutadiene with 63 % 1,2 addition (component C) were selected for this study because they exhibit a large window of miscibility and may be tailored to cross the spinodal at experimentally accessible temperatures. Binary blends were designed across a range of values for NB/ NC and the composition of the blend, &phis;B, to study the effect of these parameters on the measured value, χ sc. In addition to the strict temperature dependence presumed for χ, this study documented a composition and molecular weight dependence. The empirical expression for χsc, measured using small angle neutron scattering, was three times more dependent on composition then the expression for χ used to predict thermodynamic behavior. Despite this three-fold diminished dependence on &phis;B, the composition-dependent χ profoundly affected the phase behavior of binary blends.

A range of values was studied for NB/ NC ≤ 1, and in all cases &phis;B,cirt was found to be < 0.5, in stark contrast to the expectation of Flory-Huggins Theory that &phis;B,crit ≥ 0.5. This effect was shown to result from the combined effects of a composition-dependent χ and N B/NC removed from values of 1. Remarkable agreement was obtained between the predicted phase diagrams and measured phase transitions, over a range of values for NB/ NC and &phis;B, by accounting for the composition and molecular weight dependence of χ.

The miscibility of binary B/C blends was used as the basis for designing a diblock copolymer (component A-C) to order immiscible binary blends of polyisobutylene and deuterated polybutadiene with 89 % 1,2 addition (component A). The copolymer comprised one block chemically identical to component C (miscible in component B) and one block chemically identical to component A. This is in contrast to the majority of ternary blend studies which comprise A/B/A-B polymer systems with neutral interactions between each homopolymer and the corresponding block of the diblock copolymer. Ternary A/B/A-C blends exhibit a favorable interaction between the B homopolymer and C block, demonstrated by the miscibility of B/C blends. The A-C diblock copolymer surfactant can produce microstructures when added to A/B blends at much lower concentrations of copolymer than for an analagous A-B copolymer.

This dissertation introduces the use of lamellar structure factor that fits scattering profiles unsuitable for the microemulsion fit. In addition, the lamellar fits include as adjustable parameters the size of each microdomain and corresponding interfacial width. These fit values agree quantitatively with independently generated predictions using self-consistent field theory, indicating a broad understanding of the physical parameters that affect thermodynamic behavior in the A/B/A-C system studied.

This dissertation presents a study for which the concentration of diblock copolymer was fixed and the composition of the A and B homopolymers was systematically varied across a range of compositions including &phis;A,crit. The experiment corresponded to tracing the copolymer isopleth on a ternary phase prism. Theoretical groups have predicted a rich phase behavior along the isopleth for similar ternary systems, however, the observed phase behavior was quantitatively identical for all blends studied. Self-consistent field theory predictions agreed with fit values of the domain spacing and microdomain widths. There was no discernible correlation between &phis;A and phase behavior. This finding, and that of the study with critical A/B/A-C blends, together suggest that NA/NB correlates strongly with the phase behavior of a blend, while &phis; A does not. This relationship, captured by mean-field theory, provides a simple method for tuning the phase behavior of polymer nanocomposites without using additional surfactant. (Abstract shortened by UMI.)

Indexing (document details)
Advisor: Balsara, Nitash P.
School: University of California, Berkeley
Department: Chemical Engineering
School Location: United States -- California
Source: DAI-B 72/05, Dissertation Abstracts International
Subjects: Polymer chemistry, Chemical engineering, Nanotechnology
Keywords: Nanocomposite, Phase behavior, Polyolefins
Publication Number: 3444440
ISBN: 978-1-124-52244-9
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