The tensile strength of decamethylene dismethylazodicarboxylate vulcanizates of GR‐S, 50°C. polybutadiene, 5°C. polybutadiene, and natural rubber has been determined as a function of temperature and/or the degree of crosslinking. A theory of tensile strength of amorphous polymers which adequately describes the effect of the degree of crosslinking on strength is proposed. The theoretical expression for the strength of amorphous materials is then modified to include the effect of crystallization. The complete expression for the tensile strength of crystallizable elastomers, which includes amorphous elastomers as a special case, is: where ω0 = 1 − (1 + δ2λ3)−1/2, fc = volume fraction crystallinity, M = primary molecular weight, Mc = molecular weight per crosslink in the uncrystallized polymer, λ = extension ratio at break, and δ, K = parameters. The theory is shown to be good for GR‐S and semiquantitatively correct for a crystallizable polymer, 5°C. polybutadiene.

In general, extension of an elastomer results in a degree of preferred orientation of the molecular chains composing the amorphous phase. Therefore the amorphous fraction of a partially crystalline elastomer must be related to the integrated intensity of the amorphous diffraction halo rather than to the intensity at any one azimuth. A noteworthy exception is natural rubber, for which simple meridional measurements suffice. A Geiger-counter apparatus with beam monitor and temperature-controlling accessories is described for making accurate measurements of the x-ray intensities scattered at any azimuth and at small or moderate Bragg angles. Measurements of crystallinity in natural rubber are in essential agreement with the findings of previous workers. When polybutadiene is extended at room temperature, molecular orientation occurs, but little if any crystallization. Measurements at lowered temperatures show that the crystalline fraction becomes appreciable at about 0°C and that it increases with further reduction in temperature and with increasing extension ratio. Preferred orientation of the crystalline regions in extended polybutadiene has been measured quantitatively with the object of providing jointly with birefringence measurements a value of the birefringence of a single crystal of polybutadiene.

The semitheoretical expression, ft.no-cs .-Ft.nco-cs' wheref.=volume fractio~ crystalline, t.no=observed birefringence, c=the stress-optical coefficient, s=the str~ss on actual cross sectIon, F= a known function of crystallite orientation, and t.n.o= the birefringence of a smgle. polymer crystal, is proposed for relating birefringence, stress, and degree of crystallinity in polycrystalline elastomers under tensile stress. This expression is shown to yield results in substantial agreement wIth. x-ray results for the degree of crystallinity of 5°C polybutadiene when t.nco=O.157. Birefringence-densI~y da~a o?, natural rubber (due to L. R. G. Treloar) are shown to be a special case of the foregoing ex-preSSIOn, yreldmg a value t.n.o=O.218 for natural rubber which is in good agreement with the theoretical value for this quantity.

In general, extension of an elastomer results in a degree of preferred orientation of the molecular chains composing the amorphous phase. Therefore the amorphous fraction of a partially crystalline elastomer must be related to the integrated intensity of the amorphous diffraction halo rather than to the intensity at any one azimuth. A noteworthy exception is natural rubber, for which simple meridional measurements suffice. A Geiger-counter apparatus, with beam monitor and temperature-controlling accessories, is described for making accurate measurements of the x-ray intensities scattered at any azimuth and at small or moderate Bragg angles. Measurements of crystallinity in natural rubber are in essential agreement with the findings of previous workers. When polybutadiene is extended at room temperature, molecular orientation occurs, but little if any crystallization. Measurements at lowered temperatures show that the crystalline fraction becomes appreciable at about 0° C and that it increases with further reduction of temperature and with increasing extension ratio. Preferred orientation of the crystalline regions in extended polybutadiene has been measured quantitatively with the object of providing jointly with birefringence measurements a value of the birefringence of a single crystal of polybutadiene.

The theory of strength proposed above appears to describe satisfactorily the effect of crosslinking on the strength of amorphous elastomers, and to apply semiquantitatively to crystallizable elastomers. Further work, involving the effect of primary molecular weight, constitution, swelling, and additives is required to substantiate the general applicability of the theory. The mechanism of rupture, in particular, is poorly understood.