Surface and interfacial segregation in end functionalized polymers

Date of Completion

January 1993

Keywords

Chemistry, Polymer

Degree

Ph.D.

Abstract

Polymer surfaces and interfaces are key to a number of technologically important areas: adhesion, lubrication, composites, etc. Surface and interfacial segregation have been shown to occur in polymer blends, polymer solutions, and block copolymers. In this work, the effect of polymer end groups on the surface and interface are examined.^ Surface tension as a function of end group composition and molecular weight are studied in end functionalized poly(dimethyl siloxane), and found to depend on Mn$\sp{-1}$ and ($\gamma\sb{\rm e}$-$\gamma\sb{\rm r}$) where $\gamma\sb{\rm e}$ and $\gamma\sb{\rm r}$ are the surface energies of the end group and repeat unit. The interfacial tension between these materials and methanol or water also has an Mn$\sp{-1}$ dependence, but the end groups reorient to present the most energetically favorable species at the interface. Mean field lattice calculations support the experimentally observed trends and quantify the relative enthalpic and entropic contributions.^ The surface of end fluorinated polystyrene is found to be highly enriched in the low energy end group. The surface induces an oscillating composition profile that propagates into the bulk in agreement with Fredrickson's mean field theory. The surface excess is predicted to go as Mn$\sp{-1/2}$, and this trend is observed experimentally. Surface rearrangement to expose the more energetically favored segments occurs in contact with saturated water vapor even below the glass transition temperature. The process has a (time)$\sp{1/2}$ dependence suggesting a diffusion controlled process. Diffusion coefficients and activation energies are calculated. The activation energies suggest that the process is related to the $\alpha$ and $\beta$ relaxations of the polystyrene.^ A fluorescence dequenching technique to monitor the interfacial reaction of two immiscible polymers bearing end groups capable of a specific interaction is investigated. While the experimental system did not have sufficient sensitivity to draw conclusions about the interfacial interaction, the technique shows promise for studying other systems that mimic reactive processing conditions.^ End groups can significantly affect surface and interfacial properties. The effects must be understood as all polymers (except cyclic polymers) inherently have end groups that can change the surface or interfacial properties whether by default or design. These effects can potentially be exploited to accomplish desired goals. ^

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