Competing polymer-substrate interactions mitigate random copolymer adsorption

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.