A Study of the Viscoelastic Behavior of Novel Side Chain Liquid Crystalline Polyurethanes Using Dynamic Infrared Spectroscopy

Side chain liquid crystal polyurethanes are a new class of materials designed to exhibit mechano-optic properties. This paper reports phase modulated dynamic Fourier transform infrared spectroscopy (FT-IR) experiments performed to study the response of these materials to sinusoidal mechanical perturbations. In dynamic FT-IR, the viscoelastic reorientation of various segments of the macromolecule can be monitored as a function of the applied strain. For the polyurethane under study, the cyano band is used to follow mesogen movements, and the urethane carbonyl is used to track the hard segment. We present evidence for two types of hard segments: those involved in hydrogen bonding within hard domains, and those found in “lone” hard segments in the soft matrix. We also present evidence for two types of mesogens: those found in smectic layers, and those not involved in smectic ordering at the hard domain interface. The hard domains and the smectic layers have strong viscous components to their mechanical response. The “free” mesogens and the “lone” hard segments, on the other hand, exhibit a more elastic response.