Dynamic Infrared Spectroscopy in Tension and Shear

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Abstract
The utility of polarized infrared spectroscopy for the study of polymeric systems undergoing dynamic deformation has been established by several groups following the pioneering work of Noda and Marcott. All previous studies have utilized tensile deformation, and tensile experiments are complicated by sample thinning. One must also rely on the assumption of transverse isotropy to determine the three elements of the dynamic absorption tensor. An alternative approach involves the use of shear rather than tensile deformation. Experimentally, two different shear geometries are readily achievable. In one, the IR beam is passed normal to the shear plane and the polarizers must be set at ±45° to the flow direction. Since no sample thinning is involved with no net flow of material in or out of the sampling volume, the trace of the dynamic absorption matrix must be zero, and all three elements of the dynamic absorption tensor can be obtained in a single experiment. In the second geometry, the IR beam is passed parallel to the shear plane. In this configuration, the optical signals are modulated at twice the deformation frequency, and the magnitude of the dynamic dichroism scales with the square of the shear strain. Since the shear strains are usually quite small, severe sensitivity problems can arise with this second configuration.