INVESTIGATION OF A MONOTROPIC LIQUID-CRYSTAL POLYURETHANE BASED ON BIPHENOL, 2,6-TOLYLENE DIISOCYANATE, AND A 6 METHYLENE CONTAINING FLEXIBLE SPACER .2. IR SPECTROSCOPIC PHASE CHARACTERIZATION

The mesophase to crystal phase transition observed upon heating the monotropic liquid crystal polyurethane (2,6-LCPU-6), based on the mesogenic biphenol 4,4'-bis(6-hydroxyhexoxy)biphenyl (BHHBP) and 2,6-tolylene diisocyanate (2,6-TDI), has been investigated by differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), and infrared (IR) spectroscopy. Hexafluoroisopropyl alcohol (HFIP) fast solvent-evaporation casting resulted in 2,6-LCPU-6 thin films with a glassy mesophase morphology. The mesophase to crystal exothermic transition has been observed by DSC between 130 and 140-degrees-C, depending on sample preparation. It is accompanied by a substantial increase of H-bonding between urethane C=O and NH, as observed by IR. Curve-fitting analysis of the conformationally sensitive amide I region revealed three bands: ordered H-bonded carbonyl groups, disordered H-bonded carbonyl groups, and "free" carbonyl groups. The prime feature of the 130-degrees-C transition is the substantial increase of ordered H-bonded carbonyl groups at the expense of disordered H-bonded carbonyl groups. Crystal melting occurs between 180 and 210-degrees-C and is accompanied by the complete disappearance of the ordered H-bonded peak along with substantial changes in the frequency and width at half-height of the disordered H-bonded peak.