Abstract
Adsorption of surfactants onto amorphous polymers at the solid-solution interface is of direct relevance to many industrial sectors ranging from food, pharmaceuticals, paints, paper and photographic colour films. Although it is widely accepted that surfactants play the underpinning role in these applications, little is currently understood about the interactions between surfactants and polymeric materials at the molecular level. This lack of progress is mainly due to the inability of most existing techniques in probing this type of structural information at the wet interface. Specular neutron reflection (SNR) is a recently developed technique capable of detecting structural information with resolution down to a few angstroms (A). When combined with deuterium labelling, it is possible to distinguish the surfactant from the polymeric species at the interface. The aim of this work is to explore the appropriate experimental approach that utilizes the potential of neutron reflection to unravel molecular information about the actions of surfactants. A major progress that was made in the project was the development of experimental protocols for the formation of smooth polymeric thin films onto neutron transparent substrates. This experimental process was substantially supported by spectroscopic ellipsometry (SE), a home-based laboratory optical system that was also highly sensitive to film thickness and composition. This exploratory work has mainly used model polymeric samples that are of broader implications to various technological applications. A nonionic alkyl ethoxylate surfactant, such as C12E5 was chosen because its interfacial behaviour has been widely examined. Measurements were made over a wide concentration range around the critical micellar concentration (cmc), using specially designed cells. In the case of PMMA (poly(methylmethacrylate)), adsorption of C12E5 was found to be completely reversible with no observable penetration of the surfactant into the polymer. The area per surfactant at the cmc (Acmc) was found to be around 50A2. Good agreement was obtained between SE and SNR, but the latter revealed additional information about the in-situ conformational structure of the surfactant. It was found that at the cmc, the dodecyl chain layer was only 4-SA thick, indicating that the alkyl chains lay virtually flat on PMMA surface. The ethoxylate headgroup layer was however some 16A thick, indicating the projection of the headgroups into the aqueous solution. In contrast, when anionic SDS (sodium dodecyl sulphate) and cationic C12TAB (dodecyl trimethyl ammonium bromide) were used, a much reduced adsorption was observed. While these results highlight the role of the type of surfactant headgroups, it shows the power of SNR and the effectiveness of the experimental approach adopted in unravelling molecular information at the interface. This experimental approach was subsequently extended to PBMA and PS for the assessment of different polymeric substrates. The varying extent of surfactant adsorption and the ingress of the surfactant associated with film swelling have clearly shown that with careful control of experimental conditions, detailed structural information can be reliably revealed from the experimental procedures that I have developed.