Plasma polymer chemical gradients for evaluation of surface reactivity: epoxide reaction with carboxylic acid surface groups

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Abstract
Organic surfaces of specific chemistry can be fabricated using plasma polymerisation. A recently developed deposition approach allows gradients in surface carboxyl group concentration to be fabricated over a distance of 13 mm from a mixed acrylic acid/octadiene plasma. The utility of these surfaces in the investigation of the chemisorption of volatile specie using small area XPS analysis has been demonstrated using halogen-substituted epoxide molecules. The combination of automated small area XPS analysis and chemical gradients has been used to reduce the number of samples required by a factor of 20 compared with the traditionally employed one data point-one sample approach. We believe that chemical gradients are a flexible and widely applicable route to reducing the sample preparation load in such a study of adsorbate–surface reactivity. The reactivity of epoxy functionalities with surfaces is of great importance in the field of adhesion science; most high performance paints and adhesives are based on epoxy resins. Here, the inter-relationship between the amount of epoxide chemisorption and the carboxylic acid concentration is determined. It is apparent that the level of chemisorption is strongly dependent on the carboxyl concentration, exhibiting a threshold surface composition below which reaction was not observed. The influence of the halogen tag atom is made clear; it is shown that a fluorine tagged epoxide has enhanced reactivity compared to chlorine or bromine substituted epoxides. It is also observed that degradation of the epifluorohydrin molecule occurs to form an inorganic fluorine species at the plasma polymer surface.