Structural changes during polystyrene orientation: A study of optical birefringence and wide angle X-ray scattering

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Abstract

Results of coupled optical birefringence and wide-angle X-ray scattering (WAXS) measurements on uniaxially oriented, atactic, glassy polystyrene (PS) samples are reported. Orientations were achieved in tensile test experiments performed slightly above the glass transition temperature. The elongated samples were quenched after orientation. The applied force was kept constant during cooling. The second order moments of the chain orientation distribution function (ODF) were calculated from the azimuthal profile of the scattered X-ray intensity from measurements made on five PS samples with different degrees of orientation. For ODF calculations the first, equatorial halo was used. The influence of the second, meridional halo was removed by resolving the WAXS patterns into three peaks. The intrinsic azimuthal linewidth of the WAXS profile was estimated and taken into account in the ODF calculations. The magnitude of the intrinsic birefringence Δnint was determined using the linear relationship between the birefringence and the second order moment of the ODF. A value for Δnint of −0.05 was obtained. This result was used to calculate the angle between the normal plane of the benzene ring and the chain axis. A comparison with previously published results indicates that the average relative orientation of the benzene ring with respect to the chain axis was changing in our samples during stretching. This structural difference is attributed to strong deformation of the network and retarded relaxation during sample preparation.

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