Exploring Surfactant and Plasticiser Segregation in Thin PVA Films

Abstract

Surfactant segregation in solid polymer films is a little studied area of scientific research. This investigation launches an initial exploration into the behaviour of surfactant and plasticiser (glycerol) molecules in a poly(vinyl alcohol) (PVA) matrix. By understanding the nature of segregation within these systems, information regarding the compatibility of components can be gained. A greater understanding of what dictates compatibility and small molecule segregation in polymers is of importance to many industrial products and processes.

Using a series of ion beam analysis and neutron reflectometry experiments the segregation behaviour of three non-ionic, and two ionic surfactants is studied in great detail, both in binary non-plasticised and ternary plasticised PVA films. In order to use these techniques effectively, samples were mostly spin-cast, silicon substrate bound polymer films, of thickness <200 nm.
The non-ionic surfactants studied were a series of poly(oxyethylene glycol) alkyl ethers with a 12-carbon lipophilic tail-group (C12Ex). Each of these surfactants exhibited spontaneous segregation to the air and/or substrate interface of the polymer film in binary films with PVA. Upon the introduction of plasticiser under ambient conditions, the amount of surface segregating surfactant is reduced, causing a greater concentration to be present in the bulk. However, when studied under relative humidity control the opposite effect is realised, and the presence of water in the films causes increased surfactant segregation.

The ionic surfactants studied are cetlytrimethylammonium bromide (CTAB) and sodium dodecyl sulphate (SDS). In binary films with PVA, CTAB shows no segregating nature, until glycerol is introduced, at which point substrate and surface segregation is induced. SDS shows development of significant surfactant-plasticiser multi-lamella structures on the surface of the thin films upon the introduction of the glycerol. SDS-glycerol structures like this, with interstitial regions of plasticiser stabilising the surfactant lamellae are unlike anything seen on the surface of a solid polymer film before. Both of these features infer a complex array of interactions with the plasticiser.

Surface energy and compatibility arguments are provided throughout to explain the diverse range of surfactant behaviours discovered in this work. Where surface energy is believed to be the driving force of surface segregation, with lower energy components segregating more, and compatibility to be the force which dictates how segregation behaves upon the introduction of plasticiser. Compatibility arguments are mostly comprised of the hydrogen bonding potential of the compounds within the films, where strong interactions between the polymer, plasticiser and surfactant give rise to increased compatibility, and therefore reduced segregation. Through this work, new, exciting information on the nature of small molecules in polymer systems is realised.