A Study of the molecular orientation of isotactic polypropylene as a function of draw ratio and the effect of annealing time on the same.
Ogada, Tom Ochieng
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The birefringence (BF) due to molecular orientation induced in isotactic polypropylene (IPP) films by hot-stretching at 115oC centi-grade has been studied, as a function of anneling time and draw (extension) ratio. The specimens were stretched between 20% and 600% (draw ratios of 1.2 to 7). The general variation of BF versue extension and ''orientation function'' (OF) versus extension curves appeared to be described by an inverse tangential function, suggesting that there is a tendency of BF and OF values towards equilibrium values. The films that had been pre-stretch annealed for varying times showed a lowering of BF with pre-stretch annealing times at low extensions (0-200%) but tended to a common magnitude irrespective of the annealing time, at higher extensions. The observed lowering of BF in the low extensions ceased after a certain period of annealing. These observations have been explained by invoking the crystalline and amorphous nature of the IPP, and the microstructural changes, that result from the applied tensile stresses under the given thermal conditions.