2810
E. AI. BARIZALL, 11, R. S.PORTER, AND J. F. JOHNSON
Heats of Transition for Nematic Mesophases’
by Edward M. Barrall, 11, Roger S. Porter, and Julian F. Johnson California Research Corporation, Richmond, California (Receiued February 17, 1964)
Heats of transition have been measured for three pure compounds which exhibit a mesophase of the nematic type. The compounds are p-azoxyanisole, anisaldazine, and N-prnethoxybenzylidene-p-phenylazoaniline. Their liquid crystal or mesophase range is separated by first-order transitions from both a solid crystalline phase and an isotropic or true liquid state. Heats and temperatures for the two transitions of each purified compound were measured with an extensively calibrated custom-built differential thermograph. The nature of nematic mesophase transitions is discussed along with limited thermal data previously available on these compounds.
Nematic mesophases or liquid crystals represent a phase which is distinguishable from bot,h a solid crystalline phase a t lower temperatures and an isotropic or normal liquid phase a t higher temperatures by firstorder transitions.2 Thermal data are rare for liquid crystals.3 Definitive data are required for an insight into the field of order and flow of liquid crystals. I n this study, heats of transition for solid-nematic and nematic-isotropic states have been measured for three pure compounds which exhibit liquid crystal phases of the nematic type using differential thermal analysis (d.t.a.). The three conipounds studied are pazoxyanisole, anisaldazine, and N-p-methoxybenzylidene-p-phenylazoaniline. These compounds are hereafter referred to as PAA, AAD, and IIBPA. The conipound AIBPA has been referred to as p-anisal-paminoazobenecne. The source, structurc, methods of purification, and compositional analyses for these three compounds have been previously described. Published heat of transition data for PAA are evaluated in the light of these results. New heats of transition are reported herein for the other two compounds. The only previous iriforniation available is that the heat for thc nematic-isotropic transition for LlBPA is small, as estimated from double refraction near the transition.5 Calorimetric mcasurmients for transition heats and temperatures of transition were made with an advancedtype differential thermograph. The design and calibration of this custom thermograph have been described recently.fia,b To achieve greatest precision Thc Joirvnal of I’hpical Chemistrl/
and accuracy, different d.t.a. block designs and sample preparation procedures were used to memure transition temperature and heat absorption, respectively. Experimental Sample Preparation. Samples for the measurement of transition temperatures were prepared by diluting PAA, AAD, and MBPA with 500-mesh carborundum and grinding gently with a few drops of benzene. The carborundum had been purified as described p r e v i ~ u s l y . ~ The concentration of the liquid crystal compound was determined by extracting weighed aliquots of the dry carborundum mixture with chloroform and benzene and reweighing. Concentrations are shown in Table I. Samples for the determination of heats of transformation were prepared by precisely weighing about 10 mg. of the pure compound onto a 1-cm. square of aluminum foil. Foils were folded carefully to form small leakproof packets. Ammonium chloride, ammonium bromide, and NBS benzoic acid, prepared in the same way, were used to calibrate the apparatus (1) Part I V of a series on order and flow of liquid crystals. (2) 1’. L. Jain, J. C. Lee, and It. D. Spence, J. Chem.Phys., 23,878
(1955). (3) G.13. Brown and W . G . Shaw, Chem. Rev., 57, 1049 (1957). (4) It. S. Porter and J. F. Johnson, J. A p p l . Phys., 34, 51 (1963). (5) V. Pi. Tsvetkov, Acta Physicochim. (IJSSR), 19, 86 (1944). (6) (a) E.X I . Barmll, 11. J. F. Gernert, R. S. Porter, and J. F. Johnson, Anal. Chem., 35, 1837 (1963); (b) E.M . Barrall, 11. R. s. Porter, and J. F . Johnson. ibid., 36, 2172 (1964). (7) E. >I. Rarrall, 11, and L. B. Rogers, ibid., 34, 1101 (1962).
I i E A T S OF
2811
TRANSITION FOR NEMATIC ATESOPHASES
Table I : Thermographic Data on Three Liquid Crystal Compounds -. Cornpound
PAA AAII MBPA
----Phase
7 -
Tba
113.50 161.96 142.07
transition----
Solid-nematic-----
I
T,
T,
117.60 168,90 147.20
125.02 175.71 153.30
Nematic-isotropic--T,
--
-
Tb
133.30 177.28 177.48
133.85 180.46 179.07
T,
136.72 185.53 181.80
Compound concn. i n d.t.a. tests Compound wt. '70 wt., g.
10.25 8.30 8.49
0,0146 0 0143 0.0188
a Temperatures indicate the beginning of the d.t.a. endotherm, TI,, the endothermal minimum, T,, and the temperature, T,, at which the recorder returned to the previously established base line for 4.7"/min. heating rate. E w h temperature is the average of nine separate thermographic runs.
for calorimetry.fib The ammonium salts were Baker Analyzed reagent grade chemicals. Procedure. All thermograms were recorded with the differential temperature signal as a function of sample temperature on an 2-y recorder as described previously.6~7 The sample temperature and the sample half of the differential temperature were measured with the same thermocouple. Transition temperatures were measured with the thermocouples located in contact with, and in the center of, the 0.1-g. carborundum-diluted samples. The samples were contained in glass tubes. The block (block A) design has been described previously.6b Duplicate runs at four heating rates (4.7, 5.8, 11.4, and 12,7'/min.) showed no shift in the vertex of the endothermal minimum with heating rate. Calibration. The temperature axis was calibrated with the melting points of NBS benzoic acid (m.p. 121.8'), salicylic acid (m.p. 158.3'), and potassium thiocyanate (m.p. 177.0') diluted in the same manner as the samples in carborundum. The temperature at the minimum of the endotherms corresponded closely to the reported melting points. The absolute temperature errors on the various portions of the thermographic endotherms were: beginning (Tb), +0.09', minimum (Tm), +O.O1io, and end ( T p ) , = k O . O 9 O . The reason for this variation is the judgment which must be exercised in determining the beginning and end of the endotherm from peak shape. The location of the vertex is less dependent upon such judgments. Runs at different heating rates were made on the same saniples of PAA and AAD, since relocation of the samples during the melting process was not a problem with these materials. Sublimation of the R/IBPA melt necessitated the use of an identical, freshly weighed sample for each run in block A. Calorimetry using block A suffers from large errors when samples of differing thermal conductivities and
physical states are studied.Gb Calorimetric measurements were thercfore carried out in the block of different design (block B). Sample and calibration runs were carried out at the same heating rates as in block A. Block B produces peaks which are too broad for precise transition temperature measurements. The peak areas were determined by the automatic integration methodss Integration errors were limited to less than +1.5%. Ammonium bromide and chloride and benzoic acid were used in the calorimetric calibration since the transition temperatures bracket the temperature range of interest for the compounds studied. The temperatures and heats of solid-solid transition of ammonium chloride (183.1O, 1073 cal./mole) and ammonium bromide (137.2', 882 cal./mole) have been determined by ArelLg The thermal data for the fusion of benzoic acid (121.8', 33.9 cal./g.) have been tabulated by Rossini, et aZ.I0 The data obtained for these solidliquid and solid-solid phase changes are found to fit the same calibration curve. This is consistent with the successful removal of extraneous sample variables by using block B. The precision of the calorimetric studies was determined by running each sample and calibration threcb times and determining the standard deviation of the set.
Results Transition Temperatures. Figure 1 shows reprcsentative differential thermograms for p-azoxyanisole (PAA), anisaldazine (.4AD), and r\;-p-mcthoxybenayli(8) K. W. Gardiner, 11. F. Klaver, l p . Bnumann, and J. V. .lohiiron, "Gas Chromatography," Arademic Press, N e w Y o r k , N. Y.,1962, p. 349.
(9) A. Arell, Ann. Acad. Sci. Fennicne. Ser. A V I , 57, 42 (1980). (10) 1'. D. Itossini, D. D. Wagman, W. €1. ICvans, S. Leviue, axid I.
Jaffe, "Selected Values of Chemical Thermodynarnir Properties," National Bureau of Standards Circular 500, I!. S. Government Printing Office, Washington, D. C.. 1%52.
Volzrmc 68,Number IO
Octolwr, 1.96'4
2812
E. M. BARRALL, 11, R. S. PORTER, ANI) J. F. J O I ~ N Y O N
T
A 4.7'C/MIN.
/B
17W7 ia0.48
14207
1WW
177.48
1ei.w
147.20
I3330 133.85
-c
117,bO
6.7
12.2
TEMPERATURE, 'C
Figure 1. Thermographic traces of three liquid crystal compounds, block A : anisaldnzine, AAD ; N-p-methoxybenzylidene-p-phenylazoaniline,MBPA; p-azoxyanisole, PAA.
dene-pphenylazoaniline (MBPA) a t a heating rate of 4.7"/min. Table I gives the temperatures of the beginning, minimum, and end of the phase transition endotherms. These three temperatures define the thermographic characteristics for each mesophase transition. Figure 2 shows thermograms for PAA a t four heating rates. The location of the peaks on the temperature axis is not affected by heating rate. This is because of the choice of geometry for the d.t.a. system. I n Fig. 2 the heat absorbed by the solid-nematic transition was 0.420 cal., and 0.0102 cal. for the nematic-isotropic transition of PAA. These results indicate that each transition may be considered a single event without a distinct pretransition caloric effect as has been reported." There is generally good agreement between the solidnematic transition temperatures obtained from visual and d.t.a. meas~renients.~The visually observed nematic-isotropic transition is somewhat higher, 1-5', than the vertex value but generally lower than the endotherm conclusion temperature as seen by d.t.a. This difference was seen by Martin and Muller.11 The difference increases with decreasing heat of transition and, therefore, may be due in part to pretransition effects which are discussed below. Transition Heats. Table I1 lists the results of the calorimetric studies carried out by d.t.a. on three liquid crystal compounds. These new calorimetric values for PAA agree excellently with the combined data obtained by Rfartin and Muller.11 These values differ considerably, however, from an earlier extensive study of the nematic-isotropic transition for PAA.12*13Existing calorimetric values for PAA may be compared in Table
I
133.84
117
TEMPERATURE,%
Figure 2. Thermographic trace of p-azoxyanisole at four heating rates, block A. A 0.0150-g. sample of PAA was thermographed a t the indicated heating rates. The sensitivity on the AT axis for the 117.6" endotherm was: A, 0.09"/in.; B, O.lSO/in.; C, 0.36O/in.; D, 0.72O/in. The sensitivities for the 133.84' endotherm were: A, 0.05'/in.; €3-D, 0.09O/in.
111. There have been no previous reports for heats of transitions for the other two compounds. Table I1 : Heats of Transition for Three Liquid Crystal Compounds
Liquid crystal compound
PAA AAD MBPA
Heat absorbed phase transition--SolidNematicnematic. isotopic, cal./g. oal./g.
28.1 f 0.9 26.5 f 0.5 25.9 f 0 . 6
0.68 f 0.02 0.59 f 0.02 0 . 4 1 f 0.02
High resolution nuclear magnetic resonance (n.m.r.) spectra obtained on these three compounds provide insight into nematic mesophase transitions." I n the high temperature isotropic state, spectra indicate normal liquid behavior. I n the nematic state, (11) H.Martin and F. H. Mllller, Kolloid-Z., 187, 107 (1963). (12) K. Kreutzer, Ann. P h y s i k , (5) 33, 192 (1938). (13) K. Kreutzer and W. Kast, Naturwiss., 25, 233 (1937). (14) R. S. Porter, unpublished results.
Table 111: Thermal Dat,a for Transitions of p-Axoxyanisole
--Solid-nrinatic---
r
Method
Worker
This work Pchenck and Srhneider and Buhncro Llrtrtin and Miillerb
Ilekocko Hulett“
Kreutxer” e
n.t.a.
cal./g.
OC.
28.8 29.8
1?J2
I17
Depression nematic4sotropic temperature Clausius-Clapeyron pressure dependence Ice calorimet,er
See It. Schenck, “Kristalline Flussigkeiten,” IApzig, 1905, pp. 84-89.
Discussion Changes in physical properties have been observed a t temperatures near first- as well as second-order transitions. Pretransition effects may be expected to be abnormally large in systems capable of forming liquid crystals.16 By the theory of Frenkel, the largest prctransitions, viz., heterophase fluctuations, are to be expccted in cascs where two phases differ but slightly from one another and where the heat of transition is small.17 These fcaturcts facilitate the formation of nuclci of one phase in another. Indeed, a t a few degrees below nematic-isotropic transitions, a number of physical properties have been found to change rapidly with temperature, indicative of pretransition. These include dcnsity, specific heat, viscosity, dielectric constant, optical transparencies, and flow and magnetic birefringence.l6 There is also commonly a real discontinuity in these physical properties a t nematic-isotropic transitions which is characteristic of first-ordcr transition^.^^^^ Precise density and niagnctic and flow birrfringence rneasuremcnts also indicate pretransitions on the liquid side of the nematic-isotropic transformations.4 Small aggregates have been found in the isotropic state of the
OC.
...
1). t .a.
n.m.r. spectra are much more complex and suggest that all aromatic protons are unique. This means that commonly flcxible groups, such as methyl ethers, are free to rotate in the isotropic or true liquid state. I n contrast, bonds in the nematic state must be fixed in nonrotating positions. These rcsults are in accord with conclusions that molecules in nematic mesophascs are packed such that they have freedom of rotation about one axis only, which is ordinarily the long axis.’5 The heat for nematic-isotropic transition thus involved rnergy for both molecular separation and internal rotation.
Host,
117.6
Ice calorimeter
-Nematic-isotropicTemp.,
Temp..
0.68 0.68
...
...
(128) 182 132
...
...
132
0.71
..
...
131
1 .79
See ref. 11.
28.2
133.9
Heat, cal./g.
0.69 0.68
See ref. 12 and 13.
nematic-forming compounds studied h e x 6 The aggregates or molecular swarms contain tens to several hundred molecules and cxist up to 5’ above the nematic-isotropic transition. The fact that they are of such small dimensions is in good agreement with the apparent abscnce of aggregates in light scattering rneasurcmcnts on the isotropic ~ t a t e . ~ By thc theory of Frenkel, the extent of pretransition phenomena should be rclatcd to the heat of transition. Data on the three compounds studied here agree with this conccpt. The tcrnperature range for pretransitions for the scries increases with decreasing heat for the nematic-isotropic t]ransition. This is revealed by viscosity data,4’19density data,4 and by magnetic and flow birefringence measurenients.5 Experimental cvidencc of several types clearly indicates pretransitions in liquid crystals. These effects appear adequatcly interpreted by Frenkel’s heterophase fluctuation theory. Therefore, it seems unnecessary to separate the caloric effects due to socalled first-order and prctransition effects as has bcen done previously.I1
Acknowledgment. The authors express appreciation to Messrs. D. Trujillo and A. R. Rruzzone for help with the expcrimrntal work.
Discussion A. A. ASTOXIOI.( N:itional 1lescarc.h Council, Ot,t,uwa). I wish t o point out t,tiat’ in the system porous glass-.watrr,, also, (1.5) 11. Williams, J . C‘hetn. Phyn., 39, 381 (1963).
(IS) W.A. H o y e r and A. W.Nolle. ibid., 24, 803 (1956). (17) J. Frenkel, Z h . K k u p r r i m . i Tcor. Fiz., 9, 952 (l939), a n d “Kinet,ic T h e o r y of L i q u i d s , ” C1:rrendon Press, Oxford, 1946. (18) E. R a u e r and %J.Rernrrmont, .I. Phiis. R a d i u m , 7, 19 (19%). (19) R. 8. I’ort,er and J. F. .Johnson, .I. Phys. Chpm., 66,1826 (1962).
