Modification of Polymer/Polymer Interfaces Using Block Copolymers and Microgel Particles

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• Richard J Spontak, Departments of Chemical & Biomolecular Engineering and Materials Science & Engineering, North Carolina State University
• Monday 03 July 2006, 11:30-12:30
• T001 [Tower Seminar Room], Materials Science and Metallurgy, Department of.

If you have a question about this talk, please contact Edmund Ward.

The stability of polymer thin films is critical to the development of advanced protective coatings and defect-free multilayer assemblies, in which case the molecular-scale processes responsible for film destabilization must be fully understood. When a thin liquid film contacts a solid substrate, it can remain stable or, depending on the nature and magnitude of intermolecular forces within the film, rupture and dewet from the substrate. Since film stability governs the function of synthetic coatings and numerous biological liquids, independent efforts have sought to elucidate the dewetting behavior of homopolymer or block copolymer thin films on a solid substrate. Dewetting in thin films generally occurs by nucleation and growth (NG) wherein film rupture proceeds by the formation and growth of circular holes that ultimately impinge to produce sessile droplets. Thin film destabilization may alternatively proceed by spinodal dewetting (SD) wherein film thickness fluctuates at the liquid-air interface. The spatiotemporal evolution of surface modulations promotes the formation of a bicontinuous morphology similar to that observed in liquid mixtures that phase-separate by spinodal decomposition. Instability considerations confirm that the dominant mode of surface fluctuations amplifies with increasing time and occurs at a fast-growing wave vector. The dynamic topology of an unstable thin film on a homogeneous substrate normally occurs when a thin film destabilizes and flows from thin to thick regions. If a substrate is chemically heterogeneous, a spinodal-like instability can likewise develop as a consequence of flow from less to more wettable interfacial regions. While most efforts have explored thin film stability by varying film thickness, few studies have reported on the effect of block copolymers as interfacial modifiers in thin films even though copolymers are conventionally used to compatibilize homopolymers. The dewetting rate of polystyrene (PS) from poly(methyl methacrylate) (PMMA) at 180°C, for instance, is greatly reduced by adding a PS-b-PMMA (SM) diblock copolymer to the PS. In this work, we first demonstrate that the dewetting rate and mechanism of PS/SM thin films of constant thickness are both tunable via copolymer concentration. Atomic force microscopy and transmission electron microtomography provide complementary information of these dynamically evolving systems and confirm the existence of interfacial copolymer structures. We also examine the viability of core-shell microgel particles, which can be envisaged as permanent micelles, as thin-film stabilizing agents and provide evidence that surface patterning can be achieved using such particles when they undergo autophobically-driven surface segregation.

This talk is part of the Electron Microscopy Group Seminars series.

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