Contribution from internal viscoelastic

Since dynamic friction of polymers has a large contribution from internal viscoelastic dissipation, plots of the friction force, determined, e.g., by AFM, vs./D are qualitatively similar to plots of tan<5 vs. fa. Hence, by measuring dynamic friction forces under well-controlled conditions, the dynamics of a given polymer can be directly probed at the free surface of a sample specimen. Thereby surface vs. bulk properties can be probed as well as confinement effects. 

When Zotefoam HDPE materials of density 98 kg m" were subjected to a single major compressive impact (419), after recovery at 50 °C for 1 hour, the performance, defined as the energy density absorbed before the compressive stress reached 2.5 MPa was back to 75% of the initial value. Further severe impacts caused a further deterioration of the performance of the recovered foam. Peak compressive strains of 80 to 90% caused some permanent buckling of the cell walls of HDPE foams. The recovery is much slower than the 0.1 second impact time, so is not a conventional linear viscoelastic response. It must be driven by the compressed air in internal cells in the gas, with some contribution from viscoelasticity of the polymer. Recovery of dimensions had slowed to a very low rate after 10 seconds at 20 °C or after 10 seconds at 50 °C. 

Intcrmolecular Contributions. Increasing concentration reduces the effects of excluded volume and intramolecular, hydrodynamic on viscoelastic properties (Section 5). Internal viscosity and finite extensibilty have already been eliminated as primary causes of shear rate dependence in the viscosity. Thus, none of the intramolecular mechanisms, even abetted by an increased effective viscosity in the molecular environment, can account for the increase in shear rate dependence with concentration, e.g., the dependence of power-law exponent on coil overlap c[r/] (Fig. 8.9). Changes in intermolecular interaction with increased shear rate seems to be the only reasonable source of enhanced shear rate dependence, at least with respect to the early deviations from Newtonian behavior and through a substantial portion of the power law regime. 

Here, the internal viscosity is defined as the contribution of the glassy-relaxation process to the zero-shear viscosity. This definition is different from the common understanding of the term used in literature/ although both have similar notions as to the existence of an effect of fast sub-Rouse-segmental motions on polymer viscoelasticity. In the literature the term internal viscosity generally refers to the effect that would lead to a plateau value of the intrinsic viscosity at high frequencies. 

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