SynopsisSt.ress-temperature coefficients have been measured for a high-molecular-weight ethylene-propylene copolymer, containing 45 mole-% of ethylene, chlorinated and vulcanized with a cure system baaed on sulphur and accelerators. From the relationships:the thermodynamic functions [dE/bL] T and -T [ d S / b L ] T have been determined in arange of the extension ratio up to (Y = 6.9. A good agreement has been found between the experimental data and the stress-strain theoretical curve obtained for a non-Gaussian type network.
Since no previous theoretical calculation of the unperturbed dimensions of cis-1,4 polybutadiene took into account the particular nature of the energy barriers to the rotation around single (C-C) bonds adjacent to double bonds, a new matrix calculation has been carried out, the results being given as a function of two energy parameters, AEl and AE2. They show that the best agreement between evaluated and experimental data occurs for values of the above parameters near to those obtained for simpler molecules showing structural analogies with the polymer under consideration; in particultu, the unusual positive vahie of the variation of
Crosslink density of EPDM vulcanizates, both gum and filled, has been evaluated from stress-strain measurements on both dry and swollen samples, by plotting the data according to the Mooney-Rivlin diagram. Values of the C1 constant of swollen specimens were generally found to be much higher than those of the dry ones, and the difference was ascribed to the effect of entanglements, which probably act as stable crosslinks in swollen networks. The Flory-Huggins equation has also been studied and its applicability to EPDM vulcanizates examined ; a large degree of uncertainty was found for the polymer-solvent interaction parameter, μ, that suggests the use of this equation with reservation in the case of EPDM elastomers. Many data of the Mooney-Rivlin C2 constant being available, its dependence on various parameters has been investigated and the hypothesis has been confirmed that it is associated with entanglements and intermolecular structure. The stress-strain curves of samples filled with both mineral and black fillers were analysed using the Mullins and Tobin “strain amplification factor.” This technique, which allows one to describe the behavior of the rubber phase in filled vulcanizates, has given good results that endorse the use of the Mooney-Rivlin plot for the characterization of rubbery networks. The Mooney-Rivlin constants also gave very interesting results for the investigation of the networks obtained from curing elastomer blends.
The study of the stress-temperature curves of ethylene-propylene copolymers having different compositions revealed that ethylene-rich samples crystallize under stretching at room temperature. The results obtained suggest that crystallization is probably the outcome of two phenomena: orientation of the microcrystallites, undetectable by x-ray analysis, and true crystallization under stretching. These phenomena are accompanied by strong negative variations of internal energy; these variations, when occurring at low stretching ratios, cannot be distinguished from the variations of conformational internal energy. Therefore, when calculating the ratio ƒE/ƒ in samples containing equimolecular amounts of the two monomers, and even more in the ethylene-rich ones, at room temperature, the values may be affected by uncertainty due to the occurrence of crystallization. The calculation of ƒE/ƒ at high temperature, where the rubbery network follows Equation (2) perfectly, even in ethylene-rich samples, shows that this ratio is always negative and depends on the stretching ratio α, for any composition.
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