Competing polymer–substrate interactions mitigate random copolymer adsorption

Abstract

Annealing a supported polymer film in the melt state, a common practice to relieve residual stresses and erase thermal history, can result in the development of an irreversibly adsorbed nanolayer. This layer of polymer chains physically adsorbed to the substrate interface has been shown to influence thin film properties such as viscosity and glass transition temperature. Its growth is attributed to many simultaneous interactions between individual monomer units and the substrate stabilizing chains against desorption. A better understanding of how these specific polymer-substrate interactions influence the growth of the adsorbed layer is needed, particularly given how strongly the properties of geometrically-confined polymeric systems are impacted by interfaces. Here, we use homopolymers and random copolymers of styrene and methyl methacrylate to form adsorbed layers and examine the influence of chemical composition and the resulting polymer-substrate interactions on adsorbed layer growth and structure. Ellipsometric measurements reveal a non-monotonic trend between composition and thickness of the adsorbed layers that is inconsistent with the behavior normally exhibited by random copolymers, being intermediate to their respective homopolymers. We examine this trend in terms of plateau thickness and growth kinetics at two different annealing temperatures and propose a mechanism for how different polymer-substrate interactions combine to influence adsorption when copolymer films are annealed. By introducing compositional heterogeneity, this mechanism extends the study of irreversible adsorption to complex chemistries and provides for a more general understanding of how annealing should be accounted for in the proper selection and processing of polymer thin films for applications in nanotechnology.

Revealing Interfacial Interactions in Random Copolymer Adsorbed Layers by Solvent Leaching

“…Figure depicts the growth of these adsorbed layers from silicon‐supported films annealed for different times at T g + 45 K, which have been isolated by either toluene (closed symbols) or THF (open symbols). Toluene‐washed PS (circles) and PMMA (squares) adsorbed layers exhibited similar thicknesses and growth rates, in agreement with our previous report 24. Washing instead with THF elicited little change to the adsorbed layer thicknesses ( h ads ) of PS.…”

“…The solvent‐leaching experiments presented in this letter extend our investigation of how different polymer‐substrate interactions combine to influence irreversible adsorption in systems with compositional heterogeneity 24. Our findings reveal a distinct sensitivity of MMA‐containing polymers to hydrogen‐bonding solvents.…”

“…In this study, adsorbed layers were isolated by soaking and rinsing in either toluene, tetrahydrofuran (THF), or chloroform, and were subsequently dried overnight prior to characterization. Adsorbed layer thickness was determined via ellipsometry using the protocol published previously 24…”

“…Adsorbed nanolayers of homopolymers, typically isolated via solvent leaching, have been experimentally characterized in terms of thickness,11,16,20–22 density,12,13 surface topography,12,16 and T g 11,16,23. However, limited consideration has been given to the irreversible adsorption of polymers with heterogeneous compositions 24,25. Extending the current framework for understanding irreversible adsorption to include copolymers is an important step in determining its generality and applicability in more complex systems.…”

“…In a recent study, we used a model system of polystyrene (PS), poly(methyl methacrylate) (PMMA), and poly(styrene‐random‐methyl methacrylate) [P(S ‐r‐ MMA)] to examine how competition among specific polymer‐substrate interactions affects the growth of irreversibly adsorbed layers of random copolymers 24. Although PS interacts weakly with a silica substrate through van der Waals forces and PMMA interacts with silica more strongly via hydrogen bonding,26 we found that PS and PMMA homopolymer adsorbed layers achieved similar plateau thicknesses due to a tradeoff between adsorbed layer structure and adsorbed amount.…”