(45g) Investigating Polymeric Thin Film Vapour Uptake and Their Properties Using the Quartz Crystal Microbalance
AIChE Annual Meeting
Sunday, October 28, 2018 - 5:15pm to 5:30pm
Our research offers reports on the use of Quartz Crystal Microbalance (QCM), to determine vapour uptake in such thin polymeric films in real time. We hypothesize that the vapour uptake of such thin films will deviate significantly due to film thickness. These include their diffusion rates, critical relative humidity, and density changes, from bulk properties 11-12. Ultimately, we set about explicitly outline an experimental protocol for investigating these properties in thin polymeric films systematically.
The set of polymers studied include PMMA, poly-vinyl acetate, PVP, and the pharmaceutically relevant copolymer copovidone (Kollidon VA-64); all of which were dissolved in either methanol or toluene and spun cast onto gold coated QCM sensors. These spun casted films are confirmed to be homogeneous using atomic force microscopy (AFM). The uptake of these film can be described using the Sauerbrey approximation at all humidityâs. Variations in the study of these films include; annealed samples (typically >12 hours, under vacuum and +50K over their Tg at RTP), or samples âas spin-coatedâ. This is relevant to understanding the impact of residual solvent, and internal stresses in the film.
The data obtained from the QCM was corroborated with ellipsometry to obtain a thickness. Using the Sauerbrey model we could then obtain density of the films at varying thicknesses. The obtained densities as a function of thickness was found to agree well with literature values for all three polymers studied, decreasing in densities for small thicknesses. At increased thicknesses, the film densities matched that of the bulk.
Using a humidity chamber set-up, the uptake of moisture of these films as a function of relative humidity was investigated between 0-94% RH at 25 oC. The sorption data could be obtained from interval and integral sorption experiments; ie a single step from initial to final relative humidity or multiple steps, respectively. Such experiments provides an approach to determine the various possible diffusion mechanisms for polymers. This is an advancement on seminal work produced over a decade ago by Vogt et al 12. Additionally, it could also offer insights into the critical relative humidity at 25 oC for the polymers of choice by observing the dissipation shifts. This also offered interesting insight into the hysteresis observed and at times not observed for various polymers; especially when compared to bulk experiments performed using dynamic vapour sorption.
All these works combined, showcase the strength of the QCM as a surface specific tool at understanding and isolating interfacial phenomena in polymers. Using the QCM, it is possible to determine the densities of films as a function of thickness and also determine the kinetics of uptakes for the different mechanisms at the solid-substrate, bulk, and solid-interface.
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