As one in a series of studies relating the rheological properties of mechanical mixtures of two polymer components to the degree of mixing, the temperature dependence of tensile stress relaxation behavior of two types of mixed systems, i.e., poly(methyl methacrylate)–poly(vinyl acetate) system and lightly crosslinked poly(methyl methacrylate–poly(vinyl acetate)) system, was investigated over a temperature range covering the glass transition temperatures of both polymer components. The time–temperature superposition procedure was carried out for comparison of several parameters, such as fractional free volume and its thermal expansion coefficient, which were determined on the basis of the free volume concept from the viscosity in relation to the William‐Landel‐Ferry equation, with those of the individual polymer components. Although the fractional free volume and its thermal expansion coefficient thus determined for the mixed systems were apparent values, the results may, at least qualitatively, deny the simple additivity of the free volumes of the two‐polymer mixed phases and suggest the existence of a sort of physical interaction between the phases, i.e., the internal pressure induced by one phase or the other due to a difference in thermal expansion coefficient between the phases.
The viscoelastic properties of some commercial textile fibers were measured by means of several longitudinal-vibration methods over a range of frequency from 2 X 10-1 to 2 X 105 cps at 20°C, 65% R.H., under a static tension of 0.4 g/den and a dynamic strain of less than 0.1%. In the above experiments, the nonlinear characteristics of vibrational properties were negligibly small, and the experimental results were described by the complex dy namic modulus function E*(ω) on the assumption that the Boltzmann superposition principle was valid. Generally speaking, for all the textile fibers tested the real part of the complex dy namic modulus was found to be flat over this frequency range except there was some increase in the supersonic range. The imaginary part of the complex dynamic modulus, on the other hand, seemed to increase at both ends of the frequency range covered. It was concluded from our experimental results that for any textile fiber there is not so anomalous a dispersion as that of various rubberlike materials found in the same frequency range. Furthermore, according to the method of representation by a continuous distribution of relaxation times, the shape of the so-called "relaxation spectrum" of any textile fiber was found to be fairly flat over the relaxation-time range from 1 X 10-6 to 1 X 10° sec. With the exception of slight increases at both ends of this relaxation-time range the spectrum can be represented by the so-called "box distribution." This is very differ ent from the spectra of rubberlike materials over the same relaxation-time range.
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