SynopsisA device for measuring the elasticity of polymer melts: has been designed by one of us (B. Maxwell). The device was used to obtain the relaxation modulns in shear of a linear polyethylene melt. From these data a discrete relaxation spectrum was derived. The range of the obtained spect,rum was confirmed to correspond to the terminal zone of the "entanglement plateau" of the spectrum. The limiting dynamic viscosity (as frequency approaches zero) was obtained by integrating the relaxation modulus with respect to time. The viscosity and its activation energy were found to agree closely with the flow viscosity and the flow activation energy, respectively, involved in capillary flow.Even though it is well known that polymer melts are generally viscoelastic, in their flow studies they are often treated as if they are purely viscous liquids. This oversimplification is particularly serious in some materials where the elastic response to an external force is of the same magnitude as the viscous response. In processing polymeric materials, for example, there is a strong indication that the elastic energy plays an important role in melt the extent to which strains are frozen into the final product ifi also related to the elastic contribution and affects the mechanical properties of the solid.While there is a very large amount of data available on the flow viscosity of polyethylene melt^,^'-^ only a few investigations have been carried out on the viscoelastic properties within the linear range. ' We have constructed a device suitable for making measurements of stress relaxation on a polymer melt at elevated temperatures. In stress relaxation a strain is applied and held constant while the decaying stress is measured as a function of time. Hence, the possibility of errors arising from friction, as in a creep experiment, is eliminated. If the strain is kept small enough, the mathematics developed for linear viscoelasticity is applicable, thus simplifying the analysis and practical application of the data.The device for performing stress relaxation experinients in shear is shown in Figure 1. The sample is premolded in the form of two hollow cylinders. I t is then loaded in the apparatus and melted under nitrogen atmosphere. The shear strain is applied by rotating the inverted cup.
Articles you may be interested inDynamical aspects of mixing schemes in ethanol-water mixtures in terms of the excess partial molar activation free energy, enthalpy, and entropy of the dielectric relaxation process The mechanical relaxation time in a glassy polymer depends on the magnitude of strain. The stress relaxation modulus of a styrene acrylonitrile and polybutadiene composite system (ABS) was measured at strains ranging from 0.005 to 0.10. The relaxation time was observed to shorten by up to four orders of magnitude. In addition. there was observed a decrease in the elastic contribution to the mod ulus. These two aspects of nonlinear viscoelasticity are interpreted in terms of the excess enthalpy associated with dilatation under strain, a crucial factor for ductile behavior and the formation of crazes. Up to 0.9 cal/g of the excess enthalpy associated with the stress-induced dilatation is obtained from the differential scanning calorimetry study.
Sometimes the polymer coating on an optical fiber is observed to have separated from the fiber over a small portion of the interface. Irregularities on the capstans and sheaves of draw, rewind, coloring, and cabling machines can initiate such delaminations. Subsequent growth would not be anticipated under the condition of radial compressive stress that might be expected for a coating shrinking over a relatively rigid fiber as the composite cools during manufacture. Compressive stress is indeed found at the interface when a single-layer coating is used. However, for a two-layer system, having a high-modulus secondary over a low-modulus primary (for improved protection against microbending), the different rates of thermal expansion can lead to radial tension at the silica/primary interface, and this tension can “drive” the growth of delaminations. A principal result of this study is that the analysis predicts the primary coating, although rubbery, to be approximately in a state of uniform hydrostatic tension. This tensile stress is of substantial magnitude because of constraints imposed by the relatively stiff secondary coating and by the fiber. The existence of significant radial tension at the fiber surface is consistent with experimental observations of induced delaminations, which are seen to grow long after cessation of external disturbances.
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