SUMMARY The principal polarizabilities of poly(p‐phenylene terephthalamide)—PPT—fibres were calculated using the molecular model of Northolt, together with the values of bond polarizabilities given by Denbigh and also by Bunn & Daubeny. Following generally accepted practice, the Lorentz‐Lorenz expression was utilized to derive the principal refractive indices (and the birefringence) of the PPT fibres. The experimental values of the refractive indices, for four different PPT samples, were obtained with the aid of the Pluta polarizing interference microscope; some comments on the relevant procedures being included. Finally, discussion on the discrepancy between the calculated and observed values of refractive indices is attempted.
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Hamza et al. (1978) studied this question.
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