Macromolecules zyxwvu 1993,26, 5125-5128 5125 Application of the Onsager Model to the LC-Isotropic Phase Transition of Binary Mixtures of a Nematic Liquid with Diluents Susanne Birkel and Claus D. Eisenbach' Makromolekulare Chemie 11 and Bayreuther Institut fiir Makromolekiilforschung (BIMF), Universitiit Bayreuth, 0-95440 Bayreuth, Germany Jaan Noolandi Xerox Research Centre zyxwvu of Canada, 2660 Speakman Drive, Mississauga, Ontario, Canada L5K 2Ll Received February 23, 1993; Revised Manuscript Received June 3, 1993 ABSTRACT A model nematic compound, zyxwvu bis(4-(4'-benzyloxycarbonyloxybenzoyloxy)phenyl)adipate zyxw (l), has been diesolved in two different solvents (adipic acid dinitrile and hydroquinone dimethyl ether) which represent parts of the building blocks of the pure liquid crystalline compound 1 with two mesogen unit dipoles pointing in opposite directions and is obtained by a stepwise reaction using protective groups. The behavior of this LC model compound at the isotropic-nematic phase transition as a function of temperature at a given solvent volume fraction is in good agreement with the classical Onsager model for the entropic ordering of hard rods with increasing number density. The corresponding endotherms are also very sharp, indicating that intermolecular interactions (aside from excluded volume) are not dominant and that the system can respond effectively to rapid changes in number density resulting from changes in solvent density with temperature. Introduction Rigid, rodlike molecules interacting with each other mainly through steric, excluded volume interactions form an interesting model system which can be used to study the nematic-isotropic phase transition, and to test the validity of theoretical models. A well-known theory of the ordering of hard rods with a large aspect ratio zyxwvu Lld at sufficientlyhigh density is due to 0nsager.l In this picture the free energy of the system includes the entropy of long rigid rods, as well as the excluded volume interaction for a large aspect ratio (neglecting the effect of the shapes of the rod ends) based on a second-virial-coefficient approx- imation. The theoretical description can be used to numerically calculate the bulk isotropic-nematic phase transition, as well as properties of the equilibrium iso- tropic-nematic interface.2 The question remains, however, as to how good this simple model is for a real system. Although the Onsager model is widely regarded as ac- counting for the formation of nematic ordering in liquid crystals: it is useful to have an independent assessment of the theory based on observed properties at the phase transition. Other treatments applying the Flory-Huggins theory to nematic-isotropic phase equilibria have also been In this paper we present the results of experiments using a model nematic compound bis(4-(4'-benzyloxycarbonyl- oxybenzoy1oxy)phenyl)adipate (1) dissolved in turn in two different solvents (adipic acid dinitrile [AI and hydro- quinone dimethyl ether [Bl) which themselves represent parts of the building blocks of the pure LC compound. The structural characteristics of these model compounds are that the dipoles of the oxyphenylcarbon- yloxyphenyloxy mesogen are in opposite directions because the adipic ester linkage is exclusively formed through the hydroxy hydroquinone group. This intramolecular com- pensation of the two mesogen unit dipoles is advantageous for the experiments described below, since the mesogenic 0024-929719312226-5125$04.00/0 properties of the model compound predominately result only from its isotropic shape. Experimental Section The LC model compound 1 was synthesized by reacting adipic acid dichloride with 4-(benzyloxycarbonyloxy)(4'-hydroxyphen- y1)benzoate (1:2 mole ratio) in dichloromethane with pyridine as HCl scavenger; the 4-hydroxybenzoic acid hydroquinone ester derivative has been obtained by first reacting 4-(benzyloxycar- bonyloxy) benzoyl chloride with 4-(tert-butyldimethylsilyloxy)- phenol and subsequent removal of the silyl protecting group. Experimental details including the synthesis of the monofunc- tional mesogen starting material will be given elsewhere+ T, zy = ppm): 1.89 (m, 4H), 2.65 (m, 4H), 5.29 (8, 2H), 7.17 (d, 4H, J = 9.2 Hz), 7.24 (d, 4H, J = 9.2 Hz), 7.35 (d, 4H, J = 8.7 Hz), 7.43 (m, lOH), 8.23 (d, 4H, J = 8.7 Hz). I3C NMR (CDCb-dl, 62.5 MHz, TMS, 6, ppm): 24.21,33.88,70.67,121.21,122.49,122.52, 127.04, 128.58, 128.74, 128.91, 131.87, 134.45, 148.15, 148.17, 152.82, 155.09, 164.07, 171.54. The diluents adipic acid dinitrile (A) and hydroquinone dimethyl ether (B) were obtained from Fluka. The binary mixture of 1 with diluents A or B was prepared by accurately weighing the diluent (0.1 up to 20 mg) in a small test tube; then the LC compound 1 was weighed until the total weight of the binary mixture was exactly 100mg. For complete mixing of the two components, the test tube was sealed, heated about 10 K above the melting temperature T, (about 165 OC), agitated at this temperature, and then cooled down to room temperature. The composition of the binary mixtures and the phase transition temperatures together with the heats of fusion are given in Tables I and 11. Differential scanning calorimetry (DSC)was performed with a Perkin-Elmer DSC 11. The sample weight was 2-5 mg. The melting temperature T, and the nematic-isotropic phase tran- sition temperature T, were taken as the correspondingendotherm peak maximum temperature. Gallium, indium, tin, azobenzene, and acetanilide were used as calibration standards. 1H NMR and 13C NMR spectra of the model compound were recorded at 250 or 62.5 MHz on a Bruker AC 250 MHz NMR in CDCls-dl referenced to a standard of Me& (TMS) at 25 OC (- 5% or - 40% w/v solutions; s = singlet, d = doublet, m = multiplet). The optical micrographswere obtained zyx with a Leitz microscope Laborlux R pol, which was equipped with a Linkam Scientific 155 "C; zyxwvu Tc = 200 OC. 'H NMR (CDCb-dl, 250 MHz, TMS, 6, zyxwvutsrqpo 0 1993 American Chemical Society