Observation of Surface Rearrangement of Polymers Using ESCA

Abstract

Electron spectroscopy for chemical analysis (ESCA) at cryogenic temperatures is investigated as a means of characterizing the "hydrated" surface composition of polymers. Artifacts and contamination rates are considered. The use of variable take-off-angle, high-resolution ESCA allows rearrangements to be observed, if they result in a change in average surface composition. Rearrangement of the hydrated surface structure back to a dehydrated surface structure occurs at widely varying rates in different polymer systems. Hydroxyethyl methacrylate (HEMA) grafted onto silicone elastomer exhibited, in the hydrated state, a HEMA-enriched surface which rapidly rearranged upon dehydration to a silicone-enriched surface so that it could only be detected with cryogenic sample handling and analysis. A pHEMA-polystyrene (PS) block copolymer also exhibited pHEMA surface enrichment as a result of hydration. This enrichment could be detected at room temperature, but not to the same degree as at low temperature. Biomer and Pellethane did not exhibit measurable rearrangement. A poly(ether urethane) (PEU) with a poly(ethylene glycol) soft segment showed evidence of a subtle rearrangement. A PEU containing a poly(dimethyl siloxane) (PDMS) soft segment and a commercial PEU, Biomer, contaminated with PDMS both exhibited a decrease in PDMS at the surface as a result of hydration. This decrease was more pronounced in samples analyzed at low temperature than in samples analyzed at room temperature.