Loss Modulus Data Research Articles

Viscoelastic features of molten blends of poly(vinyl chloride)/poly(ethylene‐vinyl acetate)

AbstractDynamic viscoelastic measurements and extrusion rheometry results of immiscible and partially miscible PVC/EVA blends from 50 to 90 wt% in PVC are presented and compared with those of plasticized PVCs. From loss modulus data the plateau modulus, GNo, associated with entanglements, is calculated. The variation of GNo with composition is analyzed using the theory of Tsenoglou. A simple model, which accounts for the effect of a structural parameter and the effect of free volume, is used to adjust viscosity data, taken at several frequencies and shear rates. For both types of blends, best fits are obtained using a model that establishes that the free volume of each polymer is not measurably altered by the other.

Generating line spectra from experimental responses. Part II: Storage and loss functions

A computer algorithm is described which allows the determination of a discrete distribution of relaxation times from simulated or smoothed storage or loss modulus data, or of retardation times from simulated or smoothed storage or loss compliance data. The distributions faithfully reproduce the input data and are suitable for data storage as well as for generating any other response curves.

Temperature dependence of polyolefin melt rheology

AbstractThe rheology of polymer melts depends strongly on temperature. Quantifying this temperature dependence is very important for fundamental, as well as practical, reasons. The purpose of this paper is to present a unified framework for handling the temperature dependence of rheological data. We considered the case (by far the most common in polymer melts) where all relaxation times (in the context of linear viscoelasticity) have the same temperature dependence (characterized by a “horizontal shift activation energy”) and all relaxation moduli have the same temperature dependence (characterized by a “vertical shift activation energy”). The horizontal and vertical activation energies were extracted from loss tangent vs. frequency and loss tangent vs. complex modulus data, respectively. This is the recommended method of calculation, as it allows independent estimation of the two activation energies (statistically uncorrelated). It was shown theoretically, and demonstrated experimentally, that neglect of the vertical shift leads to a stress (or modulus) dependent activation energy and necessitates different activation energies for the superposition of loss and storage modulus data. The long standing problem of a stress‐dependent activation energy in long chain branched LDPE was identified as originating from the neglect of the vertical shift. The theory was applied successfully to many polyolefin melts, including HDPE, LLDPE, PP, EVOH, LDPE, and EVA. Linear polymers (HDPE, LLDPE, PP) and EVOH do not require a vertical shift, but long chain branched polymers do (LDPE, EVA). Steady‐shear viscosity data can be superimposed using activation energies extracted from dynamic data.

The determination of the loss tangent of PVAc derived from measurements with a creep torsiometer

Damping characteristics or energy dissipative characteristics of materials can be determined in a variety of ways. If a proven or acceptable constitutive description for a material at hand exists, virtually any physical measurement can be converted into a description of energy loss, provided the measurements are sufficiently accurate and are made over a suitable range of parametric interest. In many applications, particularly acoustics, deformations are so small that linearly viscoelastic behavior represents an appropriate constitutive description. Starting from a description of the apparatus shear creep measurements are converted exactly and approximately to loss tangent (storage and loss modulus) data. A comparison of the approximate and exact representation is made. The description of the test procedure and computations allows the comparison to other methods with respect to ease and accuracy of property determination.

Dynamic Mechanical Properties of Polyethylene Melts: Calculation of Relaxation Spectrum from Loss Modulus

Dynamic mechanical properties of several polyethylene melts were investigated using the Weissenberg rheogoniometer. The use of the rheogoniometer for oscillatory shear testing has been discussed. The methods of determining relaxation spectrum from loss modulus based on second-order approximation of Schwarzl and Staverman, and first- and second-order approximations of Ninomiya and Ferry were found relatively inaccurate in reproducing the original loss modulus and storage modulus data. As a result, the iterative procedure based on Roesler and Twyman's derivation and that based on Tanner's algorithm were employed and found to regenerate accurately the original experimental data. Of all the approximation methods, the one based on the second-order approximation method of Williams and Ferry was subject to least error in its ability to regenerate the original melt data.