Date of Award

2-2012

Document Type

Open Access Dissertation

Degree Name

Doctor of Philosophy (PhD)

Degree Program

Polymer Science and Engineering

First Advisor

Thomas P. Russell

Second Advisor

Ryan C. Hayward

Third Advisor

Todd Emrick

Subject Categories

Engineering | Polymer Science

Abstract

Co-continuous structures generated from the phase separation of polymer blends present many opportunities for practical application. Due to the large interfacial area in such structures and the incompatibility between the components, such non-equilibrium structures tend to coarsen spontaneously into larger sizes and eventually form dispersed morphologies. Here, we utilize various strategies to kinetically stabilize the co-continuous structures in polymer blend systems at nano- to micro- size scales.

In the partially miscible blend of polystyrene and poly(vinyl methyl ether), we took advantage of the spinodal decomposition (SD) process upon thermal quenching, and arrested the co-continuous micro-structures by the addition of nanoparticles. In this approach, the critical factor for structural stabilization is that the nanoparticles are preferentially segregated into one phase of a polymer mixture undergoing SD and form a percolated network (colloidal gel) beyond a critical loading of nanoparticles. Once formed, this network prevents further structural coarsening and thus arrests the co-continuous structure with a characteristic length scale of several microns. Our findings indicate that a key to arresting the co-continuous blend morphology at modest volume fractions of preferentially-wetted particles is to have attractive, rather than repulsive, interactions between particles.

For the immiscible blend of polystyrene and poly(2-vinyl pyridine) (PS/P2VP), we presented a strategy to compatibilize the blend by using random copolymers of styrene and 2-vinylpyridine, controlling the degree of immiscibility between PS and P2VP. Based on such compatibilization, co-continuous structured membranes, having characteristic size down to tens of nanometers, were fabricated in a facile way, via the solvent-induced macrophase separation of polymer blend thin films. The feature size was controlled by controlling the film thickness and varying the molecular weight of the PS homopolymer and the random copolymers. As the processing method (solution casting) is simple and the structures are insensitive to the solvent or substrate choices, this approach shows great potential in the large scale fabrication of co-continuous nanoscopic templates on flexible substrates via roll-to-roll processes. Moreover, we proposed a quasi-binary blend system based on the PS/P2VP pair with the addition of a common solvent. An experimentally accessible phase mixing temperature was achieved, and the co-continuous morphologies were generated via thermally induced spinodal decomposition. The addition of solid particles significantly slowed down the coarsening kinetics and, in some cases, arrested the co-continuous structures at ~6 &mum for a short period of time. This study suggests an alternative means to achieve co-continuous structures in polymer solutions and also provides better understanding of the thermodynamics and kinetics of polymer blend phase separation.

Our research demonstrates several means of kinetically trapping the non-equilibrium interconnected structures at sub-micron to tens-of-nanometer size scales that are germane to several functions including active layers of photovoltaic cells and polymer-based membranes.

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