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Pressure induced change of order of cholesteric-smectic-A phase transition
15 April 1977
CHEMICAL PHYSICS LETTERS
Vohrme 47, number 2
PRESSURE INDUCED CHANGE OF ORDER OF C~DL~TE~~-SME~IC-A P. PULPAL
PHASE T~~S~IO~
and G. SCHERFR
&prtment 0fPizysieal Chemistry. ~niye~ity of ~a~e~~or~, D-4 79 Paderborn. Germany Received 27 December
1976
The pressure dependence of the cholesteric-smectic-A phase transition of cholesteryt oIey1 carbonate was determined by optical measurements up to 4.5 kbar and 95°C. The phase transition was indicated by the divergence of the wavelength of maximum light reflection. The corresponding phase diagram shows two straight lines which intersect at 1.43 kbar and 52,0°C thus pointing to a change in transition order. Taking into account the results of Keyes et al. a relatively strong correlation between change of positional and orientational long range order can be stated.
The cholesteric-smectic-A phase transitions of pure substances at atmospheric pressure known up to now are generally first order, i.e. the transitions are accompanied by a discontinuity in enthalpy and volume. Approaching this transition by temperature or pressure variatioq a divergence of the wavelength of maximum Jight reflection hR of the cholesteric phase can be observed. That means according to the relation [ 1 f XR =8.Ziz,
(1)
where Fi denotes the average refractive index, an untwisting of the cholesteric helix with the pitch z. The main reason for the discontinuity in volume is the change of the positional Iong range order by the ordering of the molecules into the planar arrangement of the smectic phase. This effect is connected to a large extent with the change of the long range orientational order, the vanishing of the twist. Despite of this connection the behaviour of the pitch seems to be continuous in many cases even if that of the volume is discontinuous at the phase transition point. To get more information about the correlation between the change of positional and orientational order by the phase transition cholesteric-smectic-A we make use of the results of Keyes and co-workers [2]_ They found by fight transm~sion measurements at a constant laser wavelength that the choiesteric-smectic-AA phase transition of cholesteryl oleyl carbonate (CCC) changes from frst to higher order at 2.66 kbar and 60.3°C 286
(“tricritical point”). Whereas Keyes et al. located the phase transition by the sudden change in the intensity of the transmitted light - an effect caused essentially by the change of the positional order - we take that point as the phase transition point where h, continuously diverges, that is the cholesteric twist disappears. The aim of this ~vestigation is to answer the question if the derivative of the temperature dependence of the phase transition pressure determined in the fatter manner exhibits a discontinuity at higher pressures thus revealing a correlation between the change of positional and orientational order. The measurements were performed with a high pressure optical cell described previously [3]. The pressure dependence of hR was determined at constant temperature. One of the measured fifteen isotherms is shown in fig. 1. On the basis of the measured quantities (hR,i/pi) near the transition the phase transition pressure @,) was obtained by means of a minimizaticin [4J of the
following function:
F-C
i
[hR,i-
A(1 --p-/pI u )“I2
+min
,
where A, B and pu denote the parameters of the function. The plot of the pu values versus phase transition temperature tYu in fig. 2 yields two straight lines which intersect at 1.43 kbar ol,r) and 52.O”C (i?UI). Thus
VooIume47,number2
$5 April L977
CHEMICAL PHYSICS LETTERS
Tabfe I Phase tEGIsitiOn temperatures in “C (SA = sac&c-A, CR = cholesteric, I = isotropic) of cholesteryl oleyl carbonate Authors
3001 1.00
I
t
1.25
1.50
1.75
2.00
Pikbail Fig. 1. Pressure dependence of the wavelength of
reflection XR for choiesteryl oleyl carbonate
maximum at temperature
Smecttc A
Cholesterlc
3
Pi. 2. The pu - i& phase diagram for
r*c1-
choiesteryl oleyl carbon-
ate. the change of order of the cholesteric-smectic-A phase transition from first to higher order found by Keyes et al. at higher pressures can &o be seen in a discontinuity in dp,/d9, of a phase diagram based upon the change of orientational order. This result points to a rela~vely strong correlation between the
CR
SA
E
Keyes etal., Eastmaa product, data taken from fig_ I in ref. 121
17
28
* Pohmann et af., product purification by c~mato~phy
24
40.0
Poltmann et at., Eastman product, no purification
16
30.0
Adamski and DyIik-Gtomiec [ 71
2s
35
change of the positional and the orientationa.I order. Applying the Clausius-Clapeyron equation to the function pu (8,) in the iow pressure range of fig_ 2 one would obtain by means of the AVvalue offI& t cm3 moPt [Sj (transition volume at atmospheric pressure) 150* lOcalmol- i for the transition enthalpy. The agreement of this value with that of 160 2 10 caf mof-i directly measured by differential scanning cabrimetry [4] within the measuring accuracy favours also the correlation between the change of orientaticnaf and positional order in the case of the cbofesteric-smecticA phase transition. The discrepancy in the pLc--rYuvalue of the “tricritical point” found by Keyes et al. and us is probabIy expIained by the fact that they used the CCC of Eastman Organic Chemicals without further purification- The Eastman product available to us was not uniform. The transition temperatures of our purified COC are compared with data of other authors in table I _ We would like to thank H. Stegemeyer for heQfu1 discussions, This work was supported by the Deutsche Forschun~gemeins~haft.
References [1] H. de Vries, ActaCryst. 4 (1951) 229. [2] P-H. Keyes, HT. Weston and W.5, Dar&Is, Phys Rev. L&ten 31(1973) 628. 131 P. Polhnarm, 3. Phys. E 7 (1974) 490. [4 J 3-A. Nelder and R. Mead, Computer J. 7 (E965) 308. IS] R. Lotenz and Ii_ Stegemeyer, to be pubLished_ [6] K. Bergmann and H, Stegemeyer, to be pubEsfred_ f7f P. Adamski and A. Dylik-Gromiec, &%ol.Cryn. Liquid Cryst. 25 (1974) 281.
CHEMICAL PHYSICS LETTERS
Vohrme 47, number 2
PRESSURE INDUCED CHANGE OF ORDER OF C~DL~TE~~-SME~IC-A P. PULPAL
PHASE T~~S~IO~
and G. SCHERFR
&prtment 0fPizysieal Chemistry. ~niye~ity of ~a~e~~or~, D-4 79 Paderborn. Germany Received 27 December
1976
The pressure dependence of the cholesteric-smectic-A phase transition of cholesteryt oIey1 carbonate was determined by optical measurements up to 4.5 kbar and 95°C. The phase transition was indicated by the divergence of the wavelength of maximum light reflection. The corresponding phase diagram shows two straight lines which intersect at 1.43 kbar and 52,0°C thus pointing to a change in transition order. Taking into account the results of Keyes et al. a relatively strong correlation between change of positional and orientational long range order can be stated.
The cholesteric-smectic-A phase transitions of pure substances at atmospheric pressure known up to now are generally first order, i.e. the transitions are accompanied by a discontinuity in enthalpy and volume. Approaching this transition by temperature or pressure variatioq a divergence of the wavelength of maximum Jight reflection hR of the cholesteric phase can be observed. That means according to the relation [ 1 f XR =8.Ziz,
(1)
where Fi denotes the average refractive index, an untwisting of the cholesteric helix with the pitch z. The main reason for the discontinuity in volume is the change of the positional Iong range order by the ordering of the molecules into the planar arrangement of the smectic phase. This effect is connected to a large extent with the change of the long range orientational order, the vanishing of the twist. Despite of this connection the behaviour of the pitch seems to be continuous in many cases even if that of the volume is discontinuous at the phase transition point. To get more information about the correlation between the change of positional and orientational order by the phase transition cholesteric-smectic-A we make use of the results of Keyes and co-workers [2]_ They found by fight transm~sion measurements at a constant laser wavelength that the choiesteric-smectic-AA phase transition of cholesteryl oleyl carbonate (CCC) changes from frst to higher order at 2.66 kbar and 60.3°C 286
(“tricritical point”). Whereas Keyes et al. located the phase transition by the sudden change in the intensity of the transmitted light - an effect caused essentially by the change of the positional order - we take that point as the phase transition point where h, continuously diverges, that is the cholesteric twist disappears. The aim of this ~vestigation is to answer the question if the derivative of the temperature dependence of the phase transition pressure determined in the fatter manner exhibits a discontinuity at higher pressures thus revealing a correlation between the change of positional and orientational order. The measurements were performed with a high pressure optical cell described previously [3]. The pressure dependence of hR was determined at constant temperature. One of the measured fifteen isotherms is shown in fig. 1. On the basis of the measured quantities (hR,i/pi) near the transition the phase transition pressure @,) was obtained by means of a minimizaticin [4J of the
following function:
F-C
i
[hR,i-
A(1 --p-/pI u )“I2
+min
,
where A, B and pu denote the parameters of the function. The plot of the pu values versus phase transition temperature tYu in fig. 2 yields two straight lines which intersect at 1.43 kbar ol,r) and 52.O”C (i?UI). Thus
VooIume47,number2
$5 April L977
CHEMICAL PHYSICS LETTERS
Tabfe I Phase tEGIsitiOn temperatures in “C (SA = sac&c-A, CR = cholesteric, I = isotropic) of cholesteryl oleyl carbonate Authors
3001 1.00
I
t
1.25
1.50
1.75
2.00
Pikbail Fig. 1. Pressure dependence of the wavelength of
reflection XR for choiesteryl oleyl carbonate
maximum at temperature
Smecttc A
Cholesterlc
3
Pi. 2. The pu - i& phase diagram for
r*c1-
choiesteryl oleyl carbon-
ate. the change of order of the cholesteric-smectic-A phase transition from first to higher order found by Keyes et al. at higher pressures can &o be seen in a discontinuity in dp,/d9, of a phase diagram based upon the change of orientational order. This result points to a rela~vely strong correlation between the
CR
SA
E
Keyes etal., Eastmaa product, data taken from fig_ I in ref. 121
17
28
* Pohmann et af., product purification by c~mato~phy
24
40.0
Poltmann et at., Eastman product, no purification
16
30.0
Adamski and DyIik-Gtomiec [ 71
2s
35
change of the positional and the orientationa.I order. Applying the Clausius-Clapeyron equation to the function pu (8,) in the iow pressure range of fig_ 2 one would obtain by means of the AVvalue offI& t cm3 moPt [Sj (transition volume at atmospheric pressure) 150* lOcalmol- i for the transition enthalpy. The agreement of this value with that of 160 2 10 caf mof-i directly measured by differential scanning cabrimetry [4] within the measuring accuracy favours also the correlation between the change of orientaticnaf and positional order in the case of the cbofesteric-smecticA phase transition. The discrepancy in the pLc--rYuvalue of the “tricritical point” found by Keyes et al. and us is probabIy expIained by the fact that they used the CCC of Eastman Organic Chemicals without further purification- The Eastman product available to us was not uniform. The transition temperatures of our purified COC are compared with data of other authors in table I _ We would like to thank H. Stegemeyer for heQfu1 discussions, This work was supported by the Deutsche Forschun~gemeins~haft.
References [1] H. de Vries, ActaCryst. 4 (1951) 229. [2] P-H. Keyes, HT. Weston and W.5, Dar&Is, Phys Rev. L&ten 31(1973) 628. 131 P. Polhnarm, 3. Phys. E 7 (1974) 490. [4 J 3-A. Nelder and R. Mead, Computer J. 7 (E965) 308. IS] R. Lotenz and Ii_ Stegemeyer, to be pubLished_ [6] K. Bergmann and H, Stegemeyer, to be pubEsfred_ f7f P. Adamski and A. Dylik-Gromiec, &%ol.Cryn. Liquid Cryst. 25 (1974) 281.