Two-dimensional correlation infared analysis (2D-IR) based on dynamic infrared spectroscopy used as a probe of the viscoelastic behavior of side chain liquid crystalline polyurethanes

Dynamic infrared dichroism techniques were employed in the study of a complex side chain liquid crystalline segmented polyurethane. The dynamic spectra obtained were further analyzed using two-dimensional correlation infrared spectroscopy (2D-IR) that facilitates an easier interperation of the dynamic response. The behavior of the side chain mesogens is monitored by the cyano tag at the end of the molecule while the hard segments response is found in the carbonyl spectral region. This study of the different parts of the macromolecule leads to the understanding of the elastic and the viscous orientation behavior of the polymer under conditions of dynamic strain. In particular, we find that the elastic component of the strain aligns smectic layers parallel and hard domains perpendicular to the direction of strain. The viscous strain component, on the other hand, induces a perpendicular smectic layer and parallel hard domain orientation behavior. The utilization of two-dimensional infrared spectroscopy made posssible the discovery of the presence of paracrystalline hard segment "domains" within this material that have slightly different viscoelastic responses. All of these observations are consistent with a model proposed in earlier work that the hard segments and the smectic layers change orientation as the applied strain is increased from low to high strains. Finally, we show conclusive evidence for the coupling of the mechanical deformation behavior of the smectic layers and the hard domains and identified the two primary relaxation times in this system.