Neutron Reflectometry Studies of the Hydrated Structure of Polymer Thin Films

Abstract: Polymer coatings are inexpensive surface modifications providing a wide variety of functions. There is an ever-present motivation to improve the films’ performance and to decrease the cost and the environmental footprint of their production. This thesis includes the study of the structural and functional parameters of polymer coatings that are aimed at preventing biological fouling, the unwanted attachment of organic molecules and organisms on surfaces. The focus was on thin films prepared by the self-initiated photografting and photopolymerization method. This is a UV-initiated polymerization reaction that does not require additional chemicals beyond the monomers and the solvent. Since biofouling is a prominent problem in wet environments, the emphasis was placed on the hydrated structure of the films. Neutron reflectometry was selected as a primary method for these studies, since it is a powerful method for investigating the structure of polymer thin films, especially in the hydrated state due to the labelling offered by isotope substitution. This allows the determination of the solvent volume fraction depth profile, which reveals the chain segment density profile in the hydrated film. To resolve fast changes in the film structure and to study the chemical composition, spectroscopic ellipsometry and infrared absorption spectroscopy was implemented in a setup for in-situ measurements in parallel with neutron reflectometry.  This thesis contains an introduction and five research articles, and it can be divided into two main parts: the first focusing on the polymerization reaction and the resulting polymer films and the second on the reflectivity method and instrumentation development. Firstly, uncharged hydrophilic polymer layers were prepared by self-initiated photografting and photopolymerization and the hydrated structure of these was investigated. It was found that the films follow a stretched profile indicating negligible crosslinking, and that the growth dynamics is determined by the balance of grafting and removal through radiation damage. Studying sequential grafting of oppositely charged polyelectrolytes confirmed the results on growth dynamics and showed the effects of electrostatic interactions between the monomers. This also demonstrated that the polymerization method is ill suited for preparing block co-polymers due to the removal of material from the previous layer. However, these studies also show that the growth of the second layer tends to proceed from the substrate, forming a system where the two kinds of chains co-exist and interact in the same layer. The grafting of random co-polymers was also investigated by comparing the anti-fouling performance of layers made from a mixture of oppositely charged monomers to layers made using zwitterionic polymers, resulting in no significant difference. This was attributed to the pairwise deposition of oppositely charged monomers, further emphasizing the importance of the Coulomb force in defining the structure of the charged films. The second part of this work focuses on instrument development. Here the building and testing of an angle-dispersive reflectometer is presented, and the design and first applications of an in-situ setup for measuring spectroscopic ellipsometry and infrared spectroscopy along with neutron reflectometry is described. By investigating the structure of the polymer films prepared by self-initiated photografting and photopolymerization, this work improved the understanding of this method, facilitating the development of new applications in the future. By combining additional methods with neutron reflectometry, both fast changes in the structure and the chemical evolution of the samples can be investigated. However, the differences in the sensitivities and the structural models required by the probes present new challenges in modelling.