No abstract

No abstract

Measurements of thermal diffusivity were carried out with semicrystalline and amorphous polymers stretched in the solid and in the molten state to different elongations. For semi‐crystalline materials two regions in the curves of the diffusivity versus the elongation can be detected. The change occurs at an elongation of 1.4 for the polymers stretched in the solid state and at an elongation of 3 for the polymers stretched in the molten state. For amorphous polystyrene the shape of the curves of thermal diffusivity versus elongation also depends on the mode of stretching. Along with thermal diffusivity measurements, wide angle X‐ray scattering investigations were also performed and give explanations for the differences in thermal behavior.

Melts of a commercial poly(methy1 methacrylate) product and a commercial polystyrene were elongated homogeneously up to Hencky strains of E > 4 by means of the rotary clamp technique. For this purpose, certain modifications to the rheometer were necessary which are described in detail. Both polymer melts show strain hardening, i. e. the elongational viscosities are higher than the threefold of the linear viscoelastic limiting viscosity measured in shear. The degree of hardening is very different: For the poly(methy1 methacrylate) product the hardening is only small, and so are the recoverable portions of the total strains, whereas for the polystyrene the opposite is true. This different behaviour is connected with a difference of the linear viscoelastic relaxation spectra at long relaxation times. The test results can be described efficiently by the Lodge-Wagner rubberlike liquid constitutive equation.