X-Ray Diffraction Studies of Crystallization in Elastomers

Abstract

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.

Thermodynamics of Deformation

“…The maximum internal energy change of 11.5 J/g obtained in large strain deformation cycles between extension ratios of 1-8.9 corresponds to about 18% crystal Unity using 64 J/g for the heat of fusion of natural rubber. This is in good agreement with x-ray scattering estimates of stressinduced crystallinity by other workers (70)(71)(72) of the viork of unloading.…”

Drawing and extrusion of semi-crystalline polymers

“…The same mechanism has to be active in the case of permanently by chemical crosslinks or temporarily by entanglements formed network [251] in natural [252] or polychloroprene [253] rubber that crystallizes upon stretching. Such crystallization may be followed by EM [253], X-ray scattering [254], birefringence [255], decrease of retractive force [256], or calorimetry [257]. According to Schultz [258] the crystallization of such amorphous microfibrillar structures may be explained by spinodal decomposition [104,105] of an amorphous and crystalline component with c~ = 0.5.…”

Strain-Induced Crystallization of Natural Rubber: A Review of X-Ray Diffraction Investigations

“…60 When the stretch ratio reaches a given value, the crystallinity increases almost linearly. It can be seen in this figure that the onset of crystallization for unfilled NR occurs for a stretch ratio of 4 approximately.…”

“…By measuring the crystallinity at different stretch ratios, the degree of crystallinity versus stretch ratio was investigated in a sequential manner, [57][58][59][60] which gave the typical results shown in Figure 6. 60 When the stretch ratio reaches a given value, the crystallinity increases almost linearly.…”