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Alignment of diblock copolymers with electric fields
The use of electric fields to control the orientation of copolymer microstructures in a thin film geometry is demonstrated. Copolymers of polystyrene-block-poly(methyl methacrylate), P(S-b-MMA), and polystyrene-block polyisoprene, P(S-b-I), were prepared by living anionic techniques for use in this study. An emphasis was given on scattering and reflectivity techniques to elucidate information regarding the mechanism of alignment. Alignment is understood, for all copolymer systems, in terms of a simple dielectric body argument whose energy is dependent on a dielectric mismatch parameter. ^ The observed pathway along which the alignment is realized depends on the initial state of the sample. Vastly different behavior is observed for thick films, where the effect of the substrate can be effectively ignored, and thin films, where the surface interactions dominate. In thick films, the alignment mechanism was followed via in-situ small angle scattering (SAXS) experiments and proceeds via disruption of the grains of the copolymer microdomains with a subsequent rotation of the grains along the field direction. Observed similarities in alignment of diblock and triblock copolymers, further strengthen the grain rotation argument. If alignment occurred by an enhancement of interfacial fluctuations or a mechanism requiring a molecular response, distinct differences in the alignment mechanism between the di- and triblock copolymers would be evident. Additionally, it was observed that if a copolymer system is allowed to relax between applied fields, the resulting final alignment was found to be better than the alignment achieved under a constant applied field. This indicates that, under a constant applied field, the copolymer is trapped in a non-equilibrium state that can be overcome by allowing a partial relaxation of the system. ^ In thin films, a direct competition between the interfacial interactions and the electric field force is observed. Complete reorientation occurs only when the applied field overcomes the interfacial interactions. This occurred at a well-defined threshold electric field strength, Et, and was observed to be independent of the film thickness over a range of thicknesses. In these thin films, the mechanism of alignment must proceed through a fluctuation pathway where the applied electric field amplifies the fluctuations until alignment is achieved. Preliminary neutron reflectivity data shows a large disruption of the surface-induced parallel orientation by the electric field that is directly related to the electric field strength, the interfacial interactions, as well as the copolymer initial state. This is consistent with an electric-field induced fluctuation disrupting the surface-induced lamellar stacking. ^
Polymer chemistry|Condensed matter physics
DeRouchey, Jason Edward, "Alignment of diblock copolymers with electric fields" (2002). Doctoral Dissertations Available from Proquest. AAI3039349.