Article pubs.acs.org/Macromolecules
Mapping Substrate Surface Field Propagation in Block Polymer Thin Films Cameron K. Shelton† and Thomas H. Epps, III*,†,‡ †
Department of Chemical and Biomolecular Engineering and ‡Department of Materials Science and Engineering, University of Delaware, Newark, Delaware 19716, United States S Supporting Information *
ABSTRACT: We isolated the key substrate−polymer interactions responsible for the propagation of substrate surface field effects in block polymer (BP) thin films through a modified approach to the Owens and Wendt interfacial energy formalism. This modification captured the influence of longrange surface energy components on through-film nanostructure orientation in BP thin films, and it provides a framework for manipulating BP thin film behavior without the need for extensive parameter space exploration. Optical microscopy (OM) of gradient thickness films on chlorosilane-modified substrates provided a high-throughput approach for identifying the critical propagation depth of substrate−polymer interfacial energy effects. Atomic force microscopy (AFM) was combined with OM to verify changes in free surface nanostructure as a function of film thickness. Using a model poly(methyl methacrylate-b-n-butyl acrylate) BP thin films system, we mapped the critical propagation depth as a function of interfacial energy difference and found a nearly linear increase in propagation depth at low interfacial energy differences followed by the onset of a plateau at high interfacial energy differences. Our results connect seemingly disparate trends found in the substrate surface field propagation literature and demonstrate a more translatable approach for improving BP thin film through-film orientation via appropriate chemical tailoring of substrate surfaces.
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interactions and enhance film uniformity in an ever-expanding list of BP systems.30 BP molecular weight (Mw),31−33 annealing temperature,23,29 and substrate−surface interactions18,22,23,26,27,34−36 are known to be key parameters that influence the propagation of substrate surface effects. The Mw effect is related mainly to chain mobility and segregation strength. Shorter polymer chains (lower Mw) have fewer entanglements, smaller entropic penalties for reorganization, and greater freedom of movement in comparison to longer polymer chains (higher Mw).32,37,38 Additionally, higher Mw polymers typically have a larger segregation strength and highly entangled conformations that impede microdomain breakup/reorganization and reduce propagation effects.4 As a result, substrate−polymer interaction effects have a shorter propagation depth in higher Mw systems.31 The annealing temperature also affects the mobility of polymer chains and segregation strength. Higher annealing temperatures assist chain diffusion and reduce segregation strength, thereby allowing an increase in the propagation of substrate surface effects. Zhang et al. explored the structural reorganization of polymer chains at different annealing temperatures in a poly(styrene-b-methyl methacrylate) (PS− PMMA) BP. They mapped the propagation depth of substrate
INTRODUCTION Block polymers (BPs), with their ability to self-assemble into highly ordered, periodic assemblies with nanoscale dimensions over large length scales, have received significant attention for potential thin film applications such as nanolithography, nanotemplating, and nanoporous membranes.1−9 However, control over thin film (
