DSC studies on p-(n-alkyl)-p 0 -cyanobiphenyl (RCB's) and p-(n-alkoxy)-p 0 -cyanobiphenyl (ROCB's) liquid crystals G.A. Oweimreen * , M.A. Morsy Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia Received 13 September 1999; received in revised form 24 September 1999; accepted 25 November 1999 Abstract Solidi®cation (supercooled nematic-to-solid or supercooled smectic-A-to-solid), melting (solid-to-nematic or solid-to- smectic-A), smectic-A-to-nematic, and clearing (nematic-to-isotropic or smectic-A-to-isotropic) transition temperatures W s , W m , W AN , and W c , respectively, and enthalpies DH s , DH m , DH AN , and DH c , respectively, have been determined by differential scanning calorimetry (DSC) for members of the p-(n-alkyl)-p 0 -cyanobiphenyl (RCB) and the p-(n-alkoxy)-p 0 -cyanobiphenyl (ROCB) homologous series of liquid crystals, where `R' is the number of carbon atoms in the n-alkyl or n-alkyloxy tails. The study shows that the transition of the less ordered nematic phase to the more ordered smectic-A phase, in liquid crystals that exhibit both phases (those with R8 or 9), involves a transient phase of intermediate order. Broad or overlapping solidi®cation exotherms indicate that two mesogenic aggregations supercool simultaneously. Finally, there is evidence for a transition between two solid forms that occurs at temperatures preceding W m or concurrent with it depending on `R' and the heating rate. This refutes an earlier view that during melting the solid form with the higher melting point solidi®es. # 2000 Elsevier Science B.V. All rights reserved. Keywords: Liquid crystals; Phase transitions; Transition temperatures; Transition enthalpies 1. Introduction The low melting points, chemical and photochemi- cal stability, and electro-optical properties [1,2] of the nematogenic homologous series p-(n-alkyl)-p 0 -cyano- biphenyls (RCB, Scheme 1, A) and p-(n-alkoxy)-p 0 - cyanobiphenyls (ROCB, Scheme 1, B); where R is the number of carbons in the n-alkyl or n-alkyloxy tails, make them suited for use as components for devices based on the twisted nematic or phase change effects and mixtures of them have been used in liquid crystal displays (LCD) [3]. From a thermodynamic point of view these liquid crystals are enantiotropic [4,5] with mesophases that are thermodynamically stable with respect to both the isotropic and crystalline phases, within the tempera- ture range from the melting point, W m , to the clearing point temperature, W c . Since the synthesis and characterization [1,6] of the RCB and ROCB series of liquid crystals several studies were carried out. BDH Chemicals [7] reported the electro-optical properties of these liquid crystals, their melting and clearing temperatures (obtained using a polarizing microscope), and their enthalpies of melting (using differential scanning calorimetry, Thermochimica Acta 346 (2000) 37±47 * Corresponding author. Tel.: 966-3-860-2617; fax: 966-3-860- 4277. E-mail address: ghassan@kfupm.edu.sa (G.A. Oweimreen). 0040-6031/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved. PII:S0040-6031(99)00411-6