Formation and characterization of polyethylene oxide thiourea inclusion compounds (PEO-TU-IC)

This research focuses on formation of polymer-thiourea inclusion compounds and determining its structure, conformation and motions, using polyethylene oxide as guest polymer. Several experimental factors, such as temperature, solvents, polymer-thiourea ratio, and crystallization methods were examined in order to find the most reliable and exact conditions to get the inclusion compounds. Cyclohexyl bromide - thiourea inclusion compound (CB-TU-IC) was prepared by melt in the blend method and used as a reference in order to determine the structure of the PEO-TU-IC prepared in our laboratory. Crystal structure of CB-TU-IC was studied extensively and its crystal structure was found to be rhombohedral while the crystal structure of thiourea was determined as orthorhombic. PEO-TU-ICs were formed either by melt in the blend or cocrystallization method. The X-ray diffraction pattern observed for PEO-U-IC appears to be different from the X-ray diffraction pattern observed for CB-TU-IC and thiourea. This suggests PEO-TU-IC adopts a different crystal structure from both thiourea and CB-TU-IC crystal structures. PEO-TU-IC was characterized by DSC, and FTIR and 13C NMR spectroscopies. It was also demonstrated by FTIR spectroscopy that PEO-TU-IC adopts different structure from either rhombohedral or classic orthorhombic. Infrared bands associated with rhombohedral or classic orthorhombic crystal structure were assigned. The PEO-TU-IC spectrum is different from both and closer to thiourea, suggesting again PEO-TU-IC has a different crystal structure. Since the X-ray diffraction pattern is close to orthorhombic crystal structure, the crystal structure of PEO-TU-IC was attributed to an expanded orthorhombic crystal structure. Comparison of CB-TU-IC and PEO-TU-IC obtained by melt in the blend and co-crystallization in solution show the same tendency in melting temperatures compared with thiourea thermal behavior. Two endotherms are present for CB-TU-IC and PEO-TU-IC, suggesting that a structural reorganization occurs during heating. IR bands associated with thiourea in PEO-TU-IC and CB-TU-IC are shifted to low wavenumbers with respect to thiourea bands but the CB-TU-IC shift is larger. It can be attributed to thiourea rhombohedral host structure in CB-TU-IC. PEO chains in the channels adopt a conformation which is dissimilar to the bulk conformation (TGT), which has trans C-O bonds and gauche C-C bonds because only the PEO bands in PEO-TU-IC that correspond to C-O-C axial stretching and CH2 rocking are weakly observed. In addition, TIP measurements of 1H spin diffusion reveal structural aspects of PEO and its PEO-TU-ICs and the isolation of PEO included chains in the thiourea matrix.