Formation and Glassy Properties of Irreversibly Adsorbed Polymer Nanolayers

Abstract

The growth of nanotechnology and increasing employment of polymer films in the energy, pharmaceutical, and sustainability sectors merit accurate characterization of polymers reduced to the nanometer size scale. However, an increased influence of interfaces due to the large surface-to-volume ratio inherent to these geometrically-confined systems can result in substantial deviations from bulk behavior.

Annealing a supported polymer film in the melt state, a common practice to relieve residual stresses and erase thermal history, can result in the growth of an irreversibly adsorbed layer. This layer of polymer chains physically adsorbed to the substrate interface has been shown to influence thin film properties such as diffusion, viscosity, and glass transition temperature (Tg). Understanding the implications of annealing on the properties of polymer films is essential in their proper selection and processing for applications. This dissertation examined the formation and glassy properties of irreversibly adsorbed nanolayers with regard to interfacial influences, composition, and confinement in an effort to better predict how annealing can impact the properties of films.

We used a fluorescence bilayer method to selectively label and measure the Tg of polystyrene adsorbed layers and investigate the role of interfacial competition in determining their Tg. Comparing exposed and buried adsorbed layers revealed that the free surface strongly influenced Tg at short annealing times (tads), while chain adsorption dictated Tg at long tads.

We then synthesized a series of homopolymers and random copolymers of styrene and methyl methacrylate and evaluated the growth and structure of their adsorbed layers. Composition-based trends in adsorbed layer plateau thickness, growth kinetics, and sensitivity to leaching solvent were used to propose a mechanism for copolymer adsorption, explaining how different segment-substrate interaction strengths in a single chain mitigate adsorption in random copolymers.

Finally, we compared the adsorption kinetics of thick and thin films of poly(tert butylstyrene) (PtBS) to reveal an elevated effective annealing temperature (Teff) in thin films that was used to determine Tg at the substrate. The relationship between adsorption kinetics and Teff enabled development of a general method for predicting adsorbed layer thickness in confined films for any annealing time and temperature.