Tuning Melt/Brush Interactions for Functional Polymer Films



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Polymer films are employed in a variety of industries that include electronic materials, anti-fouling coatings, barrier coatings, and adhesives. In many applications, the functionality of these films is controlled by surface and interfacial properties. Polymer brushes can modify the chemistry, energetics and function at a polymer/substrate or polymer/air interface. This thesis describes two systems where the structure and function of a polymer coating are controlled by interactions at a brush/melt interface. First, we investigated the role of substrate neutrality on domain ordering in thin films of poly(styrene-b-methyl methacrylate) (PS-PMMA) lamellar diblock copolymers con ned between a neutral free surface on the top and nearly neutral brushed poly(styrene-r -methyl methacrylate) silicon substrate at bottom. The PS-PMMA lm thickness (t) and brush grafting density (Σ) were systematically varied to examine their impacts on in-plane and out-of-plane ordering. A combination of high resolution microscopy and grazing-incidence small angle X-ray scattering characterizes lateral order and out-of-plane ordering respectively. Lateral correlation lengths at the top of the lm scaled as tn, where the exponent n increased from approximately 0.75 to 1 as Σ decreased from 0.6 to 0.2 nm−2. Out-of-plane defects such bent or tilted domains were detected in all films. Our results demonstrate that preferential interactions at the substrate can contribute to both in-plane and out-of-plane disorder.

Second, we studied the phase behavior of bottlebrush polystyrene (PS) and linear deuterated polystyrene (dPS) in thin films. These nearly athermal systems exhibit wetting and dewetting transitions that drive bottlebrush dispersion or aggregation, respectively. These effects depend on the relative degrees of polymerization of matrix chains Nmto those of bottlebrush side-chains Nsc. When Nm/Nsc is low (≤1.6), the bottlebrushes are dispersed throughout the lm thickness with a slight excess at the free surface and substrate interfaces. When Nm/Nsc is high (≥8), the bottlebrushes are depleted from the interior of the lm and strongly segregated at the interfaces. The interfacial excess is driven by an entropic depletion attraction effect. These studies demonstrate that low concentrations of certain bottlebrush polymer architectures can generate brushlike surfaces and interfaces in any thermoplastic material through a spontaneous, entropy-driven segregation process.



Block copolymers, Nanopatterns, Surface Science


Portions of this document appear in: Stein, Gila E., Nikhila Mahadevapuram, and Indranil Mitra. "Controlling interfacial interactions for directed self assembly of block copolymers." Journal of Polymer Science Part B: Polymer Physics 53, no. 2 (2015): 96-102. And in: Mitra, Indranil, Xianyu Li, Stacy L. Pesek, Boris Makarenko, Brad S. Lokitz, David Uhrig, John F. Ankner, Rafael Verduzco, and Gila E. Stein. "Thin film phase behavior of bottlebrush/linear polymer blends." Macromolecules 47, no. 15 (2014): 5269-5276.