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Application is made of Pao's theory for viscoelastic flow together with measurements of the relaxation spectrum to analyze the flow behavior of polymer melts. One result for linear polyethylene is that over a limited range of shear rate, the shear stress becomes a many-valued function of shear rate. This behavior arises as a natural consequence of the viscoelastic nature of polymers and appears to unify many previous experimental observations of the behavior of polymer melts at high shear rates. The theory predicts an instability in the flow curve of linear polyethylene. It predicts that this discontinuity occurs at different values depending on whether the shear stress is increasing or decreasing. The recoverable strain is predicted to pass through a maximum and the velocity of a material particle in the unstable region can have a number of values at different instants in time. One interpretation of this would be that the flow can occur as a “stick-slip” process. The effect of hydrostatic pressure on the flow curve is also discussed. All of the results of this analysis have been observed experimentally by others.

Solution temperatures in xylene and n-octadecane of polyethylene single-crystal aggregates were measured by differential scanning calorimetry. Crystals were irradiated to suppress morphological changes during the heating process. The measurements, as a function of reciprocal lamellar thickness, yield values for the equilibrium dissolution temperatures Td0 and end surface free energy σe of Td0=113.7±1.2°C and σe=93±8 erg/cm2 for dissolution in xylene, and Td0=129.4±1.6°C and σe=92±10 erg/cm2 for dissolution in octadecane.

SynopsisThe discrete relaxation spectrum of an ABS (acrylonitrile-butadiene-styrene) polymer a t 190°C. was calculated by using results from tensile relaxation moduli and the principle of reduced variables. The shift factor was found to conform well to the WLF equation, and the free volume fraction at T, was calculated to be 0.026 in good agreement with the universal value. The values of the thermal expansion coefficient of free volume were calculated to be 9.8 X 10-4 deg.-l and 7.0 X lo-' deg.-l, respectively, from the WLF coefficients and from dilatometric results. The width of the entanglement plateau of the relaxation spectrum was observed to be a factor of approximately 2 larger than that calculated from molecular weights between entanglement couplings determined either from rubber elasticity theory or from an assumed molecular model which di+ counts the presence of the butadiene in the ABS system. By using Pao's theory, flow curves a t 190°C. were calculated both from the discrete relaxation spectrum and from the dynamic modulus.However, the stress values of these curves were found to be about a decade higher than those experimentally determined from capillary flow measurements. Nevertheless, the shapes of the curves are in good agreement, and an explanation is suggested for existing discrepancies. Flow instability, processing variables, and residual strains are discussed in light of the flow curves and the calculated recoverable shear strains.These curves were essentially identical.

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