Time dependence effects in the shear of thin polymeric films

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Abstract
Two experimental methods are described which explore the contact time as a shear strength variable. The available data suggest that polymethyl methacrylate, and to a lesser extent polyvinyl alcohol, are unable to respond to the rapid transient pressure pulses generated in simple sliding experiments. One experiment simply involves changing the contact diameter whilst maintaining the constancy of other variables. The second more versatile experiment involves the application of a sinusoidal load to the contact in addition to the constant load. The magnitude of the oscillatory component of the frictional force may be detected as a function of sine wave frequency. The application of high frequencies reveals that high density polyethylene shows a response attenuation. The results from the two experiments are similar and are rationalised in terms of the presence of relaxations which cause a 'slowing down' of the rate of mechanical response.