Abstract
(a) Shape of the isotherm.—The appearance of a “first” upturn in the conventional isotherm is associated with non-gaussian behavior. Crystallization takes place, under usual conditions, in a range of elongation where non-gaussian behavior prevails and tends, initially, to reduce the stress below the value expected for the amorphous non-gaussian network. On further stretching a “second”, steeper up turn may be observed which is associated with a substantial reduction of amorphous content and, therefore, with crystallization. This interpretation, which is coherent with most of the experimental observations (particularly the lack of correspondence between the first upturn and the downturn of the energy component, the swelling behavior and the independence of the first upturn upon the mode of obtaining the isotherm and upon temperature) is satisfactorly fitted in the present picture of behavior of the network under stress. (b) Energy component.—Extensive measurements and theoretical analysis indicate that the ratio fe/f is essentially unaffected by swelling with n-hexadecane and by elongation, even in the elongation range following the gaussian region. The best value, i.e., 0.18, which is affected bya large error (± 0.05) confirms that the unperturbed dimensions of natural rubber molecules increase with temperature, an effect for which molecular description is lacking. Values of fe/f≦0.10, as they may occur for stretched natural rubber networks, are indicative of crystallization. (c) Crystallization under stress.—The following differences between experiments and Flory's approximate theory were noticed: (1) the value of ΔH is about one half the value to be expected on the basis of independent measurements. This indicates that the experimental variation of Tm with α is steeper than predicted; 2) Tm at α=1 appears to be some 30° below the actual isotropic melting temperature; 3) most serious differences between theory and experiments are found in the smooth change of slope of τ vs T curves at constant length in the region immediately following Tm. The part played in these effects by failure to obtain true equilibrium may not be essential judging from the fact that the best attempt made to reach equilibrium conditions did not reveal significant effects. Improved attempts to obtain equilibrium for the present system and using the present experimental technique are precluded by the occurrence of chemical degradation. Further experiments, on different polymer systems focusing attention on the region of incipient crystallization (or, preferably, “final melting”), are suggested.
