PVT Studies on Dimer Liquid Crystals and Estimation of Transition Entropies at Constant Volume

Abstract

Pressure-volume-temperature (PVT) measurements were performed on <x,io-bis[(4,4'cyanobiphenylyl)oxy]alkane (CBA-re) dimer liquid crystals having -0( 2)"0flexible spacers (re = 9, 10) between the 4,4'-cyanobiphenylyl ends. Both samples exhibit crystalline " nematic (CN) and nematic isotropic (NI) transitions. The contribution in entropy Sv caused by the volume change has been estimated by using the standard thermodynamic relation ASy = -(a!f)AVtT, where AVtr represents the transition volume, a the thermal expansion coefficient, and ß the isothermal compressibility. The transition entropy (AStA at constant volume was elucidated by subtracting ASv from the entropy change (AStA observed under constant pressure. By using the (ASr"fc values derived from the Clapeyron equation, the constant-volume entropies (AS"A at the NI transition were found to be 7.9 and 13.3 J mol"1 K"1, respectively, for CBA-9 and -10. The values of (ASCA for the CN transition were estimated on the basis of DSC data: 53.9 (CBA-9) and 62.4 J mol"1 K"1 (CBA-10). For both transitions, the correction for the volume change was found to be significant, ranging from 40 to 60% of the total transition entropy observed under constant pressure. Nevertheless, the odd-even character of the NI transition remains unaltered. As shown by comparison of (ASCA and (ASnA, the major change (85%) takes place at the CN transition, the liberation of entropy at the NI interphase being much smaller (15%).

Liquid Crystal Dimers

“…Non-symmetric dimers have been prepared in which one mesogenic unit is linked via a terminal position while the other is attached in a lateral position [125,127]; for example, Weiss¯og et al [125] synthesised 20 and this exhibits smectic A and nematic phases. There are insuf®cient numbers of examples of this molecular architecture, however, to comment upon structure-property correlations.…”

Nematic Conformation of Chain Molecules Predominating in the Ordered Mesophase

“…Essentially the same procedure can be applicable for the estimation of the entropy change at constant volume (ΔS CN ) V for the CN transition. The values obtained in this manner are as follows: for CBA-9 (ΔS NI ) V /R ¼ 0.7 À 1.0 and (ΔS CN ) V /R ¼ 7.3 À 9.7, and for CBA-10 (ΔS NI ) V /R ¼ 0.8 À 1.8 and (ΔS CN ) V /R ¼ 7.8 À 9.2 [36][37][38]. In view of the uncertainties involved in the multistage deduction of these values, the correspondence with the conformational entropy changes, S NI conf /R and S CN conf /R, estimated by the RIS/ 2 H NMR technique is reasonable.…”

Spatial Configuration and Thermodynamic Characteristics of Main‐Chain Liquid Crystals

“…Efforts were made to elucidate the thermodynamics involved in formation of the LC state of mainchain compounds [Abe et al, 2005a]. In these studies, the conformational entropies calculated from the spectroscopic observations were shown to compare favorably with the constant-volume transition entropies estimated from PVT analysis [Abe and Nam, 1995;Abe et al, 1995aAbe et al, , 1999Abe et al, , 2003Maeda et al, 1996]. Accumulation of various evidence suggests that the spatial configuration of the flexible spacer and its thermodynamic role remain nearly identical, independent of DP from dimer to polymers [Abe et al, 2005a].…”

“…The change in the specific volume as a function of temperature and pressure has been determined for several dimers (CBA-9 and -10, MBBE-6), trimers (CBA-T9 and -T10) and PBA-copolymer (M w = 43,000 by gel permeation chromatography: chloroform, polystyrene standard at 25 • C [Kato, 2002]) using PVT apparatus manufactured by Gnomix Co. [Abe and Nam, 1995;Abe et al, 1999Abe et al, , 2007Kato, 2002]. The PVT data of PAM-10 ([η] = 0.40 dL/g in methanesulfonic acid at 30 • C [Wojtkowski, 1987]) are taken from the Zoller and Walsh book of PVT data for polymers [1995].…”