Chloroform transport in stress‐whitened polypropylene

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Abstract
The transport property of chloroform in undeformed and stress‐whitened polypropylenes was studied. The rate of penetration of liquid chloroform in whitened samples prepared by uniaxial stretching was slower than that of the undeformed material. In the whitened region, the process appeared to be Fickian diffusion, while in the undeformed material, it was a combination of Fickian and Case‐II. The diffusivity (at 50°C) in the whitened region increased about 8 times as the strain rate of stretching decreased from 0.67 to 0.033/min. The temperature dependence of diffusivity (ε = 0.67/min) showed an activation energy of 10.16 Kcal/mole. The whitening effect was greatly reduced as chloroform was absorbed; the final residual whitening after desorption increased with the rate of stretching as determined by transmission densitometry. For the undeformed polypropylene, Case‐II type of matrix relaxation contributed significantly to the total flux. The transport process was relaxation controlled at the surface: it gradually changed to Fickian diffusion behavior toward the middle plane. The flux contribution of relaxation across the thickness increased with the sorption time, and that of diffusion decreased. These observations seem to indicate that cold drawing impeded the deformation by swelling stress from occurring at the penetration front.

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