Interfacial Effects in Montmorillonite Filled Polyester Thin Films

Abstract

The behavior of thin supported and non-supported films has been shown to be distinct from the bulk polymer. Separately, high surface area fillers such as montomorillonite layered silicates (MLS) result in improved properties. In this study, we investigate thin films of polyethylene terephthalate (PET) with MLS. The thermal expansivity and glass transi- tion temperature (T g ) were determined using ellipsometry. The results show that with increasing MLS concentration, the glass transition drops for the thin and thick film plateaus. For intermediate thicknesses, a modest influence of the MLS polymer interface on the characteristic length is found. The liquid layer thickness decreases with increased MLS presence. approximately 60 nm. From the experimental results they observed the significant reduction in deviation in T g and CTE of thin film from bulk film as the alkyl chain length was increased from methyl to n-octyl. They attributed to increase in the cooperative segmental length with the addi- tion of solvent. Utilizing a combination of polymer physics (molecular mobility) and polymer miscibility (interactions), Pham and Green (10) explain the contradictory T g -thickness results via a three layer model. Next to a substrate, the poly- mer chains bend and fold over leading to higher density, decreased mobility and higher glass transition temperatures. At the free surface, lower density, higher mobility and lower glass transitions are to be expected. The higher mobility re- sults from an increase in relaxation time due to entropic ef- fects such as disentanglements, confinement effects or by chain-end segregation. Between the free surface and the re- gion close to the substrate, polymer chains have intermediate mobility. When the substrate-polymer interaction is non in- teracting, the thickness of the substrate layer is lower than when there is a high degree of substrate-polymer interaction. The consequence is that the intermediate region in a non- interacting coating is thicker than in an interacting system. In the former the glass transition increases with increasing film thickness. The free surface mobility dominates leading to decreased T g 's with decreasing film thickness. In a film where positive substrate-polymer interactions occur, the glass transition decreases with increasing film thickness as the lower mobility layer dominates.