Surface Modification of Hydrophobic Polymer Films with Amphiphilic Molecules

Abstract

The surface segregation of additive molecules within hydrophobic polymers is of great importance to a range of industrial systems. This thesis specifically addresses the segregation of surfactants within hydrophobic polymer films similar to those used in the babycare and feminine hygiene industry. The aims are to identify
the key drivers of surface segregation, and be able to predict this behaviour.
Pentaethylene glycol monododecylether (C12E5) exhibits surface segregation in both crystalline and amorphous polymers, with the degree of matrix crystallinity shown to significantly influence surfactant segregation. Greater crystallinity has been shown to reduce the surface enrichment of these films which was attributed to the inhibition of movement of surfactant by the highly ordered crystalline domains. The surfactant layers also display different temperature and water resistance depending on the matrix, with the surface layer on amorphous polymers appearing to be more resistant to loss. However, both sample types appear to show two
types of surfactant structures on the surface, with some surfactant appearing to remain anchored to the surface while the remaining surfactant is lost.
Nanometre scale strands were found on C12E5-containing films which were resistant to temperature elevation and water exposure. These were not seen in dodecyldimethylamine oxide (DDAO) / LDPE films. Instead small islands were seen but these islands show little surface activity.
The effect of surfactant hydrophilicity on surface segregation has also been studied by comparing C12E3 and C12E5 in polymer matrices. A larger head group size has been shown to produce a larger surface excess on the surface and this has been related to the greater compatibility of surfactants with a smaller hydrophilic head group.
Initial exploration of the influence of shear stress on interfacial segregation has shown that C12E5 segregates to the interface under shear and quiescent conditions. The lack of any strong influence of shear rate or temperature suggests that additive chemistry and matrix crystallinity are the most important factors in promoting surface modification with surfactant additives.