PHASE-BEHAVIOR AND STRUCTURE IN MESOGEN CONTAINING POLYURETHANES

The mesophase to crystal phase transition observed upon heating the mo notropic liquid crystal polyurethane (2,6-LCPU-6), based on the.mesoge nic biphenol 4,4'-bis(6-hydroxyhexoxy)biphenyl (BHHBP) and 2,6-toluene diisocyanate (2,6-TDI), has been investigated by differential scanning calorimetry (DSC), wide angle x-ray scattering (WAXS) and infrared (I R) spectroscopy. Hexafluoroisopropanol (HFIP) fast solvent-evaporation casting resulted in 2,6-LCPU-6 thin films with a glassy mesophase mor phology. The mesophase to crystal exothermic transition has been obser ved by DSC between 130 and 140-degrees-C, depending on sample preparat ion. It is accompanied by a substantial increase of H-bonding between urethane C=O and N-H, as observed by IR. Curve fitting analysis of the conformationally sensitive amide I region revealed three bands; order ed H-bonded carbonyl groups, disordered H-bonded carbonyl groups, and ''free' carbonyl groups. The prime feature of the 130-degrees-C transi tion is the substantial increase of ordered H-bonded carbonyl groups a t the expense of disordered H-bonded carbonyl groups. Crystal melting occurs between 180 and 210-degrees-C and is accompanied by-the complet e disappearance of the ordered H-bonded peak along with substantial ch anges in the frequency and width at half-height of the disordered H-bo nded peak