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

Many of alkylureas (H2NCONHCnH2nC1, 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. q 2004 Elsevier B.V. All rights reserved