Summarizing, the following important conclusions may be drawn from these experiments on a typical unaccelerated soft gum compound. 1. The existence of the inversion point in the stress-temperature curves is shown to be due solely to ordinary volume thermal expansion, and may be eliminated by correcting for this thermal expansion. 2. The curves given in Figure 8a show that, for compounds of this type, the change of entropy with elongation accounts for more than 90 per cent of the total stress at room temperature, while the internal-energy contribution is less than 10 per cent and, to a first approximation, may be neglected. In other words, the retractive force is due almost entirely to the tendency of the extended rubber molecules to return to a less ordered curled-up state. This is in direct contrast to the elasticity exhibited by ordinary bodies, in which case elasticity is due to intermolecular forces. 3. The contribution of the entropy force to the total force is well represented by the theoretical expression of James and Guth. This agreement constitutes our main reason for interpreting the entropy force as being due to the kinetic motion of the rubber molecules.
The physical significance of stress-strain curves and of isometrics obtained by the relaxation method is discussed and clarified. Stress-strain curves taken at various temperatures give the correct dependence of stress upon temperature if they are taken sufficiently fast so that stress relaxation does not mask the temperature dependence. Isometrics obtained after previous relaxation of the sample are shown to depend upon duration and temperature of the relaxation by a numerical factor only. The basis for this behavior is the factorization of the stress into a factor depending upon extension and temperature only which corresponds to the equation of state and another factor depending upon the temperature T* and the duration of the relaxation process. For simple stress relaxation, the same factorization holds with T* equal to T. A general theory is formulated for time dependent elastic phenomena by generalizing Boltzmann's theory. The theory explains why factorization does not hold for creep, in agreement with experiment.
Throughout the discussion, it has been stressed that the superior properties of USE rubber may be correlated with the production of an unspoiled,and unpolymerized hydrocarbon different from that found in ordinary market grades. Greater softness, greater solubility, improved flex cracking, faster rate of vulcanization, elimination of smoking, reduction of creping, heavy compression for shipment, the presence of formaldehyde as enzyme poison, antioxidant, and possible polymerization retarder-all these factors support the suggestion that USE may contain a more unsaturated rubber molecule than is present in usual market grades.
appearance of a peak in the curve. Again, the other samples show a similar behavior, except that the temperature at which the bending of the curve occurs, in the frequency range here studied, is higher for higher styrene content. For the 70/30 sample it does not yet occur above -26°C, while for 50/50 the bending is already pronounced at 16°C. This appearance of a peak can be interpreted as the result of' a relaxation phenomenon, though the curves do not correspond exactly to those calculated with a single relaxation time.Finally it is of interest to note that while at a constant frequency the losses increase first slowly and then more and more rapidly with decreasing temperature, this trend is reversed at very low temperatures (approx. -40°C for 60/40), and the losses decrease again. This fact stands out particularly well in looking at the change of the resonance curve. In lowering the temperature the resonance becomes wider, then is no longer observable, reappears again and becomes narrow. This characteristic temperature is higher for higher styrene content; for 70/30 it could not be observed above -50°C, the lower limit of our temperature range. This characteristic temperature is approximately the second-order transformation temperature of these substances. 12 As it is assumed that below the transformation point the segments of the chain-molecules "freeze in," and are no longer able to orient as freely as above this temperature, we may conclude that the relaxation phenomena observed are linked to this segment mobility. In agreement with this is the observation that the loss-frequency curve below this temperature has a different character from that above, and does not show the characteristic peak, at least not in our frequency range.Measurements have been made of the direct current conductivity of rubbers loaded with carbon black. Shawinigan and Continental R-40 blacks compounded in natural rubber and GR-S were studied, and the resistivities were determined as functions of time, temperature, concentration and elongation. Resistance decreased with time, at first very rapidly, then more slowly, approaching an equilibrium value. This behavior seems to be independent of the type of black used. Temperature coefficients of resistance (at 50°C) were positive for Shawinigan stocks, negative for samples containing R-40, and tended to
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