Photo-oxidative degradation in polyisoprene: surface characterization and analysis by atomic force microscopy

Surface adhesion and surface, mechanical properties of natural rubber cast films have been investigated by atomic force microscopy (AFM) before and after UV irradiation. Analysis in the force versus distance (F-d) mode probed changes in tip-to-surface adhesion. Adhesion was observed to increase as a result of exposure, in accord with the prediction that generation of polar groups in the surface should promote a hydrophilic transition. The hydrophilicity also gives rise to a stronger adhesive contribution from a meniscus force, as demonstrated by comparison of results obtained in air for irradiated, and as-received samples. Calculated values of work of adhesion, based on the Johnson-Kendall-Roberts model, reflected the changes in surface chemistry and the effects of the fluid environment. The outcomes of F-d analyses revealed both tip indentation and polymer extension; the. former was fitted to expressions derived from the Snedden formalism, followed by calculation of an effective Young's modulus. Good agreement was obtained for as-received surfaces with bulk measurements. The photochemistry predicts chain scission events which are likely to account for the observed softening of the polymer. (C) 2001 Society of Chemical Industry.