Physical properties of ultrathin polymer films and quantifying interfacial interactions using a quartz crystal resonator

Interfacial interactions govern every aspect of our daily lives from biological processes to computer technology. Our research has developed along two distinct paths: the effects of substrate surface chemistry and nanoparticles on the physical properties of ultrathin (<50 nm) polymer films and the acoustic response of a quartz crystal resonator in contact with both a fluid medium and spherical particles.; In our ultrathin film study, we investigated several photoresist base polymers but used polystyrene (PS) as a model system. Using ellipsometry, we determined the glass transition temperature (Tg) of our films. For all polymer film samples, Tg decreased relative to bulk values with decreasing film thickness on silicon. Upon substrate surface modification (gold and a carboxylic acid-terminated self-assembled monolayer (SAM)), film Tg's depressed even further relative to those on silicon. We analyzed the data with a three-layer model that yielded a decrease in polymer density at the substrate interface. Similarly, the effect of nanoparticles on a polymer film's T g were dependent on the chemical affinity between the two materials. In our dewetting studies, films on SAM and glass surfaces completely and partially dewetted, respectively, whereas a gold substrate completely suppressed it.; In the quartz resonator studies, we first investigated viscoelastic behavior of glycerol-water mixtures (Newtonian liquids). Since the dielectric relaxation times of the high viscosity mixtures were comparable to the frequency of crystal oscillation, we calculated a frequency dependent complex modulus based on a Maxwell model. We then probed the contact mechanics of spherical glass particles with the quartz resonator surface as a function of humidity. We observed a significant positive increase in resonant frequency and a wet-dry hysteresis---the subsequent dry state bond stiffness was greater than the saturated humid state. This hysteresis was essentially reversible due to elastic deformation of tip asperities as a consequence of finite surface roughness and capillary bridging. We found a logarithmic capillary aging that agreed with the theory that capillary bridging is an energy activated process. From these results, we concluded that the quartz resonator is an accurate, non-destructive probe of dry-state interparticle forces of cohesion in granular media.