Dynamic infrared spectroscopy in tension and shear

The utility of polarized infrared spectroscopy for the study of polymeric s ystems undergoing dynamic deformation has been established by several group s following the pioneering work of Noda and Marcott. All previous studies h ave utilized tensile deformation, and tensile experiments are complicated b y sample thinning. One must also rely on the assumption of transverse isotr opy to determine the three elements of the dynamic absorption tensor. An al ternative approach involves the use of shear rather than tensile deformatio n. Experimentally, two different shear geometries are readily achievable. I n one, the IR beam is passed normal to the shear plane and the polarizers m ust be set at +/-45 degrees to the flow direction. Since no sample thinning is involved with no net how of material in or out of the sampling volume, the trace of the dynamic absorption matrix must he zero, and all three elem ents of the dynamic absorption tensor can be obtained in a single experimen t. In the second geometry, the IR beam is passed parallel to the shear plan e. In this configuration, the optical signals are modulated at twice the de formation 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 .