Showing papers by "Josep Salud published in 2002"

Binary mixtures of nCB and nOCB liquid crystals. Two experimental evidences for a smectic A nematic tricritical point

Abstract: Modulated differential scanning calorimetry (MDSC) has been used to obtain specific heat measurements on octyloxycyanobiphenyl (8OCB), octylcyanobiphenyl (8CB) and nonyloxycyanobiphenyl (9OCB) liquid crystals and on several binary mixtures of 8OCB + 9OCB and 8CB + 9OCB. The order of the mesophase transitions, smectic A(SmA) to nematic(N) and N to isotropic (I) on pure components and on binary mixtures has been studied and, in addition, both binary mesophase diagrams have been built. For each set of mixtures, there exists a tricritical point (TCP) composition at the SmA–N transition. The TCP compositions have been found to be X9OCB = 0.63 for the system 8OCB + 9OCB and X9OCB = 0.42 for the system 8CB + 9OCB. In addition, the specific heat critical exponents (α) through second order SmA to N transition have been obtained between 8OCB(or 8CB) and the TCP composition for the two sets of binary mixtures. When these α-values were plotted against the normalised nematic range, a common and uniform crossover trend was found. Indeed, this fact suggests a uniform behaviour in terms of the McMillan ratio for the nCB and nOCB series of liquid crystals. Moreover, for these compounds there exists a common value of (TAN/TNI)TCP of 0.99 and also a value of (TAN/TNI)3D-XY of about 0.96.

Polymorphism of 2,2-Dichloropropane: Crystallographic Characterization of the Ordered and Disordered Phases

Abstract: The polymorphism of 2,2-dichloropropane ((CH3)2CCl2) has been further investigated by both thermal and X-ray powder diffraction experiments. From the former the phase transitions between the different phases have been characterized at normal pressure as well as at high pressures (up to 200 MPa). From the p−T slopes of the two-phase coexistence lines, the volume variations at the transition points have been calculated and compared with those obtained by means of X-ray powder diffraction characterization. The existence of two low-temperature stable ordered phases (III and II), one high-temperature orientationally disordered phase (rhombohedral, Ib) and one additional monotropic orientationally disordered phase Ia, has been confirmed. The structure of the low-temperature ordered phase II has been determined by X-ray powder diffraction and Rietveld profile refinement as monoclinic C2/c, with lattice parameters a =10.6402(3) A, b = 5.4074(2) A, c = 10.7295(3) A, and β = 116.274(3)° at 175.2 K. The strength of ...