(45g) Investigating Polymeric Thin Film Vapour Uptake and Their Properties Using the Quartz Crystal Microbalance

Thin polymeric films are crucial in numerous areas, most notably electronics, adhesives, food processing, and pharmaceuticals ^1-4. Sub 500 nm films and the deviation of their properties from bulk behaviour is a well-established field. Numerous studies have been reported suggesting changes in the arrangement of their molecular chains, their glass transition temperatures (Tgs’s), densities; all of which are interrelated with one another ^{3, 6-10}. Such studies can be done for free standing or substrate supported films, the latter of which researchers have more explicitly explored which includes the effect of nanoconfinement and is the subject of our focus.

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 humidity’s. Variations in the study of these films include; annealed samples (typically >12 hours, under vacuum and +50K over their Tg at RTP), or samples “as spin-coated”. 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.

References:

1. Frisch, H. L. (1980). Sorption and transport in glassy polymers–a review. Polymer Engineering & Science, 20(1), 2-13.
2. Forrest, J. A., & Dalnoki-Veress, K. (2001). The glass transition in thin polymer films. Advances in Colloid and Interface Science, 94(1-3), 167-195.
3. Forrest, J. A., Dalnoki-Veress, K., Stevens, J. R., & Dutcher, J. R. (1996). Effect of free surfaces on the glass transition temperature of thin polymer films. Physical review letters, 77(10), 2002.
4. Lachman, L., & Drubulis, A. (1964). Factors influencing the properties of films used for tablet coating I.
5. Effects of plasticizers on the water vapor transmission of cellulose acetate phthalate films. Journal of pharmaceutical sciences, 53(6), 639-643.
6. Van der Lee, A., Hamon, L., Holl, Y., & Grohens, Y. (2001). Density profiles in thin PMMA supported films investigated by X-ray reflectometry. Langmuir, 17(24), 7664-7669.
7. Nguyen, H. K., Labardi, M., Capaccioli, S., Lucchesi, M., Rolla, P., & Prevosto, D. (2012). Interfacial and annealing effects on primary α-relaxation of ultrathin polymer films investigated at nanoscale. Macromolecules, 45(4), 2138-2144.
8. Reiter, G., & Napolitano, S. (2010). Possible origin of thickness‐dependent deviations from bulk properties of thin polymer films. Journal of Polymer Science Part B: Polymer Physics, 48(24), 2544-2547.
9. Mortazavian, H., Fennell, C. J., & Blum, F. D. (2015). Structure of the Interfacial Region in Adsorbed Poly (vinyl acetate) on Silica. Macromolecules, 49(1), 298-307.
10. Ellison, C. J., & Torkelson, J. M. (2003). The distribution of glass-transition temperatures in nanoscopically confined glass formers. Nature materials, 2(10), 695.
11. Kim, S., Mundra, M. K., Roth, C. B., & Torkelson, J. M. (2010). Suppression of the T g-nanoconfinement effect in thin poly (vinyl acetate) films by sorbed water. Macromolecules, 43(11), 5158-5161.
12. Vogt, B. D., Soles, C. L., Lee, H. J., Lin, E. K., & Wu, W. L. (2004). Moisture absorption and absorption kinetics in polyelectrolyte films: influence of film thickness. Langmuir, 20(4), 1453-1458.