(744b) Searching for Glass Transitions in Layer-by-Layer Thin Films
Layer-by-layer (LbL) assemblies are promising for global energy and health applications, but their materials properties are not well understood. LbL assemblies are created from the alternate adsorption of oppositely charged species from solution to a substrate. Particularly, little is known about the thermal properties of LbL assemblies because the supporting substrate impedes characterization. It is not initially clear if electrostatic LbL assemblies possess a glass transition temperature, if they are rubbery or glassy, or if their heat capacity is comparable to their homopolymer constituents. Here, we isolate large areas of LbL assemblies from a low-energy substrate, which facilitates thermal characterization via modulated differential scanning calorimetry (MDSC) and thermal gravimetric analysis (TGA). LbL assemblies of poly(acrylic acid) (PAA) and poly(allylamine hydrochloride) (PAH) were deposited onto hydrophobic substrates, and subsequently isolated. Upon heating, no glass transition was observed. Instead, cross-linking events corresponding with anhydride formation and amidation were observed. Results highlight that PAH/PAA LbL films are glassy, and have low mobility because of the high density of ion pair crosslinks. The techniques presented here are general, and can be applied to any LbL film. This portion of work is in press at Soft Matter.
In some cases, especially for ultra-thin LbL assemblies less than 200 nm thick, direct thermal characterization is challenging. In a separate study, we report an ellipsometric technique that tracks changes in film thickness with temperature for ultra-thin LbL assemblies. This approach allows for the determination of volumetric expansion coefficients in addition to determination of a glass transition temperature. Both electrostatic and hydrogen-bonding LbL assemblies were investigated. Results suggest that thermal events are somewhat dependent upon film thickness, and the origin of this ?confinement effect? is not yet clear.
The work presented here describes general approaches to thermal characterization of both thick and thin LbL assemblies. These techniques allow determination of the film's glass transition, cross-linking reactions, and thermodynamic properties. Our future goal is to create ultra-thin coatings that are thermo-response in ambient conditions.