The surface layer and friction of elastomers
Polimery 2001, No 10, 684
SummaryAtomic force microscopy (AFM), microindentation, X-ray photoelectron spectroscopy (XPS), chemical microanalysis and microtribology were used to study the chemical composition of the surface layer (to within 5 nm deep) and the mechanical properties of the technical surface layer (2-6 µm deep) of poly(cis-1,4-isoprene) vulcanizates (IR) of fairly related crosslink densities (Table 1) and differing degrees of crosslink sulfidity (Table 2). The low-M constituents of the curing system and some vulcanization by-products were found to migrate onto the IR surface. Constituents like zinc stearate were found to produce an external layer that exhibited lubricating properties. In (EtOH-Ac2O)-extracted vulcanizates, the friction force was several times higher than that in non-extracted vulcanizates. Hardness exhibited a surface gradient unrelated to the excess of the curing system constituents that migrated onto the surface. The gradient is believed to be due to "maturation" of the spatial crosslink network involving rearrangement of polysulfide bridges and formation of more monosulfide crosslinks. IR's slow heat transfer, responsible for the gradient of vulcanization temperature, cannot be ruled out as a contributing factor. The rising polarity of polysulfide crosslinks resulted in increased free surface energy of the vulcanizates. The mechanical loss of the technical surface layer (augmented hysteresis of surface layer), measured as the hardness-to-Young's modulus ratio (H/E), allows to interpret the microtribological data (Fig. 4). Friction of the vulcanizates was found to rise as the degree of sulfidity of crosslinks was raised (Fig. 5).