Exploration of plastic-like phases in alkylurea systems. A thermal and X-ray study

Many of alkylureas (H2NCONHCnH2n+1, Cn-U) exhibited phase transitions and those found for C4,8-U had fairly large entropy of transition, indicating formation of quasi-plastic phases. X-ray structure determinations of C2,5-14-U revealed a characteristic common hydrogen bond network to result in the formation of two-dimensional, plate-like supramolecules having crossed arrangement of alkyl groups. The crystal structures of C2,5-14-U were found to be built by stacking the corresponding supramolecules along the a axis. The phase transitions of C10,13,14-U and those of C8,12-U were found to accompany shifts of supramolecules along the crystal c axis and along both the b and c axes, respectively. The transitions found for C4,8-U were proved to be of order-disorder type associated with the disordering of the alkyl groups. The transition of C4-U was found to be associated with a drastic twist of the plane of the NCON moiety. The transition temperatures of C4-U and C8-U were depressed significantly by doping C3-U and C7-U, respectively, to form corresponding mixed crystals.