Vapor uptake in polymer materials: Simulations and Theory
Guido Ritsema van Eck is a PhD student in the Department of Materials Science and Technology of Polymers. Promotors are prof.dr. J.H. Snoeijer and dr.ir. S.J.A. de Beer from the faculty of Science & Technology.
Polymer brushes are a class of coatings consisting of end-anchored polymer chains, which possess a range of technologically interesting properties. Proposed applications for brushes include fouling-resistant layers, surfaces with low friction and adhesion, and coatings that would increase the selectivity and absorption capacity of sensors and separation processes. Additionally, polymer molecules are generally responsive to changes in their environment, and retain this property in brush form. Polymer brushes can therefore also be employed as "smart" materials, whose properties can be switched on demand or made dependent on environmental conditions. While early polymer brush research focused on brushes in liquid environments, the use of polymer brushes in air or solvent vapors has also become a topic of interest in the last decades. Experimental studies have shown that many of the interesting properties of polymer brushes in liquid also extend to brushes in solvent vapor. However, fundamental research into vapor swelling of brushes is relatively limited. In this thesis, the validity of several simple but previously untested assumptions is examined using coarse-grained molecular dynamics simulations as the primary tool.
First, an extensive literature review on fundamental and experimental brush-in-air research is provided. Secondly, we examine whether the vapor swelling of polymer brushes is appropriately described by Flory-Huggins-like theories. We use a modified Flory-Huggins model by Birshtein and Lyatskaya, which was originally developed for brushes in mixed solvents. The predictions of this model are compared to the results of coarse-grained molecular dynamics simulations of the swelling process. We find that the modified Flory-Huggins model provides an appropriate qualitative description of vapor swelling, and identify general conditions for vapor absorption and for the formation of an adsorption layer. Next, we compare the swelling behavior of brushes in vapor to that of non-anchored polymer films. Since the anchored chains in a brush need to extend to absorb solvent, theories such as the one described above predict a polymer brush to swell less than a non-anchored coating of the same chains under identical circumstances. This was tested experimentally by allowing polymer brushes with a hydrolyzable anchoring group to de-graft in a humid environment, producing non-anchored films with the exact same chain length distribution as the polymer brushes. The swelling of these brushes and films was then measured by ellipsometry in a humidity-controlled environment. Additionally, coarse-grained molecular dynamics simulations of brushes and free films were performed. Both the experiments and the simulations support the prediction that brushes display reduced swelling compared to free polymer films.
After this, we investigate the vapor swelling of polymer brushes that contain two poorly miscible polymer species in various chain architectures. As expected, we find that swelling increases with the number of unfavorable polymer-polymer contacts. Interestingly, the swelling enhancement in the mixed brush is largest at low vapor concentrations, making mixed polymer brushes potentially interesting for sensing applications. The final chapter of this thesis presents exploratory research into Schroeder's paradox, a discrepancy between the swelling of gels by a saturated vapor and the corresponding liquid that has not been conclusively explained. Experimentally, we find that macroscopic samples of chemically crosslinked poly(hydroxyethyl methacrylate) produced by free radical polymerization do indeed display this discrepancy. Additionally, these samples change their degree of swelling acccordingly when moved from the liquid to the saturated vapor or vice versa. We also reproduce the effect in a (microscopic) simulated system, in the absence of chemical details. These results suggest that Schroeder's paradox may be a more common phenomenon than previously thought, and allow us to exclude a hypothesis based on a Van der Waals loop in the swelling isotherm.
