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Licensed Unlicensed Requires Authentication Published by De Gruyter 2016 (Print 1978)

Molecular Orientation of the Polystyrene Matrix

From: the Effect of Molecular Orientation on the Mechanical Properties of Rubber-Modified Polystyrene

  • W. Retting

Abstract

Unoriented and oriented specimens of two rubber-modified polystyrenes (designated HIPS I and II) were examined by 8 members of the IUPAC working party “Structure and Properties of Commercial Polymers”.

The raw materials were characterized by several methods. It was found that the two materials, apart from their different glass transition temperatures Tg (unlike HIPS II, HIPS I contains no paraffin oil as a lubricant), differ mainly with regard to their effective rubber phase volume: although both materials were prepared with the same rubber content, the rubber particles of HIPS II contain more polystyrene and therefore have a larger volume.

The molecular orientation of the stretched specimens, distributed to all participants, as well as of the specimens stretched individually by some participants was determined by several methods (birefringence, linear thermal expansion, frozen-in stress, shrinkage). Because of its higher Tg, HIPS I becomes more highly oriented when both materials are streched at the same temperature.

The dependence of the matrix orientation of the materials, as well as of the corresponding deformation of the embedded rubber particles, on the stretching conditions was examined. Owing to the molecular relaxation mechanisms, the orientation of the matrix relaxes during the stretching process. If the specimens are stretched with constant crosshead speed at sufficiently high temperatures, the generation and relaxation of orientation obeys the superposition principle, and in addition the generation of orientation follows a linear relationship with strain. Furthermore, if the materials are stretched within a suitable temperature range above Tg, some of their properties can be explained very well with the aid of the theory of rubber elasticity. The deformation of the rubber particles depends on the draw ratio of the matrix and on the stretching conditions. Only at low temperatures and short streching times do the rubber particles deform in conformity to the matrix. Otherwise their deformation falls short of the matrix deformation.

The influence of orientation on the mechanical properties of the materials was also investigated. Strength and extensibility, both measured parallel to the direction of orientation, are improved by orientation. The unusual increase in extensibility is due to a transition from deformation by craze formation to deformation by shear yielding. – HIPS I is somewhat stronger, especially if it is oriented, and in the oriented state is able to deform more easily by shear yielding. On the other hand HIPS II, which tends more to crazing, shows, if not oriented, an extensibility that is considerably higher than that of the unoriented HIPS I. The mechanical properties of the oriented materials are definite functions of the molecular orientation of the matrix material, independent of the stretching conditions and of the deformation of the rubber particles.

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