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Phase transitions and reentrant phenomena in liquid crystals having both rigid and flexible intramolecular
joints
W. Pyżuk, E. Górecka, J. Mieczkowski, J. Przedmojski
To cite this version:
W. Pyżuk, E. Górecka, J. Mieczkowski, J. Przedmojski. Phase transitions and reentrant phenomena in liquid crystals having both rigid and flexible intramolecular joints. Journal de Physique II, EDP Sciences, 1992, 2 (7), pp.1465-1477. �10.1051/jp2:1992213�. �jpa-00247743�
Classification
Physics Abstracts
61.30 64.79M
Phase transitions and reentrant phenomena in liquid crystals having both rigid and flexible intramolecular joints
W. Pyiuk (I), E. G6recka (1), J. Mieczkowski (2) and J. Przedmojski (3j
(1) Laboratory of Dielectrics and Magnetics, Department of Chemistry. University of Warsaw, Al. 2wirki I Wigury 101, 02-089 Warsaw, Poland
(2) Laboratory of Natural Compounds, University of Warsaw, 02-093 Warsaw, Poland (3) Laboratory of Crystallography, Warsaw Technical University, 02-663 Warsaw, Poland
(Received 10 December I99I, accepted in final form 24 March 1992)
R6sumk. Plusieurs cristaux liquides compos6s de trois groupements phdnyl s6par6s par un
groupement -CH(CH~)CH~-COO-, ainsi que par des groupements azo et azoxy, ont 6td
examin6s par AED, m6thodes optiques et par rayons X. Pour des esters de l'acide di-3-(4'- nitro)phdnylbutyrique et de 4'-alkoxy-phdnylazo-phdnol-4 ayant comme terminaison une chaine
dodecyloxy ou bien plus longue, ainsi que pour des composds azoxy relatif, on observe (mEme au- dessous de 5 K) une dtroite phase ndmatique rdentrante ou inverse entre )es phases smectiques :
monocouche et partiellement bicouche. Pour des homologues plus longs dans la s6rie des
composds azoxy, on a constat6 l'existence d'autres phases smectiques ce qui implique l'apparition,
sur [es diagrammes des phases, de nouveau points multicritiques, par exemple le point A~ C~ -N". Sur chaque ligne s6parant )es phases smectiques A de la phase n6matique [es transitions sont faiblement du premier ordre ou du deuxibme or&e ce qui mbne dans certain cas h
plus qu'un point tricritique. Sur la ligne A
j N/Aj N'~ on observe h Tm/T~~ = 0, 834 un simple point tricritique N Aj l'apparition des autres ddpend du choix des constituants du systbme binaire. Dans le cas de quake composds azoxy on a constatd une transition du deuxibme ordre
entre [es phases smectiques partiellement bicouches, A~ et Cd- La transition est accompagnde
d'un brttsque changement de la chaleur sp6cifique qui vane lindairement avec la longueur de la queue de la moldcule. Pour des homologues suivants de la s6rie des compos6s azoxy on observe
diffdrentes ddpendances en tempdrature de la distance entre les couches de la phase A~.
Abstract. Two series of liquid-crystalline compounds having three phenyl rings separated by
flexible spacer -CH(CH~~CH~-COO- and by rigid azo and azoxy group, were studied by DSC, optical and X-ray methods. For esters of dl-3-(4'-nitro)-phenylbutyric acid with 4'-alkoxy-
phenylazo-phenol-4 having dodecyloxy or longer terminal chains as well as for related azoxy
compounds, a narrow (even below 5 K) reentrant or inverted nematic phase appearing between
partly bilayer and monolayer smectics A was observed. For higher homologues of the azoxy series additional smectic phases appear, leading to the occurrence of new multicritical points, e-g- the critical end point A~ C~ N". On each of the lines, which separate nematic from smectic A phases, transitions are of weakly first or second-order and more than one tricritical point can
occur. On the Aj-N/Aj-N'~ line, a simple N-Aj tricritical point is observed at
Tm/T~~
=
0.834. The presence of further critical points depends on the components of the binary system involved. Four of the azoxy compounds studied undergo a second order phase transition
between partly bilayer smectics, A~ and C~. Such a transition is accompanied by a jump in the
specific heat, varying linearly with the length of the molecular tails. Various temperature dependences of the layer spacing in the A~ phase are observed for subsequent homologues from the azoxy series.
1. Introduction.
Phase transitions in liquid crystals are extremely sensitive to details of molecular structures.
For this reason reentrant phenomena [1-3] are observed within a rather limited class of
calamitic dipolar molecules. The relation between molecular structure and reentrance
phenomena of mesogens has been examined in details in the early 1980's [2, 4], but only compounds consisting of rigid rings joined by rigid links, like -CH
= CH-, -N(O )N-,
etc., were reported. Flexible joints, like -O(CH~)~-COO-, have been introduced into
liquid crystalline molecules only occasionally [5]. As a result, there is a lack of physicochem-
ical and structural studies of relevant systems. In the present work the influence of molecular
rigidity on the reentrance phenomena is studied. For this purpose we synthesised two series of compounds having a novel chiral flexible joint -CH (CH~)CH~-COO-. This joint has been
specially designed for prospective studies of some rare optically active phases (N~~*, Cl), so far known only in a few pure compounds [6]. The new compounds are based on the classical
formula :
P-Ri-S12-R2-523-R3-T (I)
where P and T are strongly polar (nitro) and long-chain terminal (alkoxy) groups,
Rj, R2 and R3 are phenyl rings connected by joints Sj~ (3-substituted butyric group) and S~~ (for two synthesised series : azo and azoxy group, respectively). For short, the m-th
homologues of both series :
O~N-C~H~-CH (CH~ )CH~COO-C ~H~-N
= N-C~H~-OC~H~
~ ~ j (II)
O~N-~jH~-CH (CH~ )CH~COO-C~H~-N(O )N-C4II~-OC~H~
~ ~ j (III)
are hereafter abbreviated as mAZO and mAZY.
In the present study we examine the phase properties and thermal effects related to phase transitions, especially those related to the reentrant nematic and partly bilayer smectic A.
Also some structural properties of the SmA~ phase are presented. Particular attention is paid
to the DSC studies, rarely performed for reentrant mesogens, and to the phase diagram analysis. Both methods reveal some novel triple as well as tricritical points in the systems investigated. It has been shown that the presence of the latter points depends on the choice of
components of a binary system.
2. Experimental.
2, I MATERIAL PREPARATION. Esters of dl-3-(4'-nitro)phenylbutyric acid with 4'-alkoxy- phenylazo-phenol-4 were synthesised according a many-step reaction scheme based on
crotonic acid, 4-nitrophenol and n-alkanols as starting materials. The reaction scheme will not be given here because it is rather obvious and the experimental procedure is a matter of
routine. The only modifications were : (I) alkylation of 4-nitrophenol, in DMF, with alkyl
chlorides rather than bromides or iodides, (it) diazotionization of long chain amines in THF rather than in water solutions. Final azo compounds were oxidized with m-chloroperbenzoic
acid to give azoxy derivatives. Crude products were carefully purified by recrystallization and
column chromatography. Materials obtained in this way were usually mixtures of a few
crystalline forms. When heated low-temperature forms usually undergo direct transitions to
more stable polymorphic forms. In some cases melting to liquid-crystalline phases followed by crystallization and subsequent melting, a phenomenon of double melting well known from
organic chemistry, was observed. The details, however, are not the subject of this paper. As is
seen in tables I and II, the lowest melting temperatures of stable polymorphic forms for mAZO and mAZY compounds are ca. 90 °C and 70 °C, respectively.
Table I. Melting points (*), phase transition temperatures (in °C) and in parentheses, enthalpies (in Jg~~) for the homologous AZO series :
O~N-C~H~-CH (CH~ )CH~COO-C~H~-N = N-C6H4-OC~H2
m + I
m Cry SmA
i N~~ SmA~ N iso
101,1 l14.5 (2.5)
2 l17,1 126.2 (3.2)
6 92.4 55.3 107,1 (2.8)
8 90.2 65.0 (0.J) 105.2 (2.9)
10 90.5 63.5 103.9 (3.2)
II 93.0 60.0 101.9 (3.5)
12 92.6 55.5 77.0 (0.00) 95.0 (0.00) 102.0 (4.4)
13 91.8 56.5 64.5 102.6 (0.J) (**) 103.6 (5.0)
14 96.0 61.0 105.7 (6.J)
15 94.7 107.6 (7.00)
16 103.2 109.0 (7.70)
17 100.2 l10.7 (8.04)
18 100.2 ill.7 (8.75)
(*) Melting of the most stable crystalline form. (**) Subsequent peaks not fully resolved.
2.2 DSC METHOD. Calorimetric measurements were performed with a DSC-7 Perkin-
Elmer setup. Thermograms were recorded for 1.5 mg samples at a heating rate of 5 K/mn.
Special care was taken to fix the base line as flat as possible and to stabilize the flow of an inert gas. Under these conditions the signal-to-noise ratio was optimum and thermal effects -as small as 20 mJg~ could be measured easily. Typical thermograms corresponding to different
phase transitions are presented in figure1.
2.3 OPTICAL AND X-RAY METHODS. X-ray measurements were performed on a Guinier
camera for samples having temperatures stabilized within I K. For ortho- and conoscopy
observations a Jenapol U polarizing microscope, equipped with a Mettler FPB2HT hot stage
was used. To increase the precision of the conoscopy we used a separated elaborated optical
system with a tunable He-Ne laser, performing the measurements on 100~Lm thick
homeotropically oriented materials. The sample temperature was kept within 0,I K in all
optical measurements.
Table II. Melting points (*), phase transition temperatures (in °C) and in parentheses, enthalpies (in Jg~~) or specific heat changes (**) (in Jg~ K~ ~)for the homologous AZY series,
O~N-C~H~-CH (CH )CH ~COO-C~H~-N(O )N-C~H~-OC~H~
~ ~ j.
m Cry Sm5 Sm4 SmAj N'~ Smcd SmA~ N iso
6 83.6 125.5 (1.7)
8 87.4 73.8 (0.09) 122.0 (2 8)
10 97.8 90.1 (0.20) 120.7 (2.6)
11 88.0 92.0 (0.I3J 121.5 (2.6)
12 73.5 92.0 (0.01) 102.0 (0.00) l12.5 (0.00) 121.9 (3.3)
13 71.6 47 82.0 (0.12) 86.1 (0.15) 121.7 (0.20)(***) 122.1 (4.3)
14 70.1 57.8 (0.19) 73.1 (0.14) 77.4 (0.II) 124.8 (5.58)
15 (****) 74.9 56 (0.I8j 62.9 (0.06J 66.2 (0.07) 68.2 (0.09) 126.0 (6.52)
16 71.9 66.1 (0.47) 70.5 (0.14) 128.7 (6.94)
17 72.7 61 65.7 (0.57) 69.7 (0.22) 129.9 (7.14)
18 76.2 65.1 66.1 70.5 (0.28) 131.5 (7.70)
(*) Melting of the most stable crystalline form. (**) For the A~ C~ transition. (***) Subsequent peaks not fully resolved. (****) Phase transition temperatures lower ca. 2 °C than those expected for
pure compound.
3. Results.
3,I PHASE DIAGRAMS. The data collected in table I of phase transitions between liquid
crystalline phases for the AZO and AZY series were established from microscopic
observations and DSC studies. Smectic phases were identified from textures, miscibility tests and conoscopic as well as X-ray studies. The relevant phase diagrams are presented in
figures 2. Both diagrams reveal areas of monolayer and partly bilayer smectics A, separated by a nematic gap. In contrast to the rigid-core compounds [4, 7] : (I) smectic tongues are
located rather close together and close to the nematic-isotropic transition line (it) a reentrant
nematic appears for unusually long terminal substituents (m =12,13) and becomes an
inverted phase while the upper nematic vanishes (m
= 14, 15) (iii) a reentrant nematic is very narrow, e-g- only 3 K for ISAZY.
Another feature of the phase diagrams is a relatively narrow width of the smectic tongues,
ca, one-third of that found for the reentrant homologous series [4, 7, 8]. A quantitative comparison of the width would require the use of a normalised equation of temperature
(T)-chain length (m) phase diagrams. Resulting parameters could be correlated with molecular constitution. Assuming phase boundaries to be skewed parabolas with apices at
(mo, To), we write :
y=Ax±Bxfl, p=1/2,
where x
= m/mo I, y
= T/To I and x
= T/To I, y = m/mo I, for the A~ and
Ai phase, respectively. (Note that hyperbolic boundaries seem also to be reasonable for the upper smectic). From the above equations we found the dimensionless width, B (A~
= 0.148, for the AZY series. The value of 0.242 found for the series having CN, CH
= CH and COO
as P, Si~ and S~~ groups [8], indicates an about twice broader phase for classical three-ring compounds.
(al
f 5.o
~
d
#
91 es
(bi (cl
I j ~~
d>
~ 25
57 59 65 67 Se
<d>
I
~ 60
#
65 lo
temperature °C)
Fig, I. Rough thermograms showing selected phase transitions as well as the ability of the DSC method to detect small transition enthalpies and specific heat changes. (al SmAj
- N'~ in IIAZY- l3AZY (0.433 mol.fr.) mixture, AH
=
48 mJg~ (b) left : Sm4
- SmAj in I4AZY, AH
= 0,19 Jg~ '
right Sm4
- Smcd in I6AZY, AH
= 0.47 Jg~ ' (c) Sm4
- SmC~ in I7AZY, AH
=
0.57 Jg~~,
followed by Smcd
- SmAd> AC~ 0.22 JK~ ' g~' Note different vertical scales. Curves normalized to I mg samples.
140 140
iso
iso
6 N SmAd
~ l
)©
N SmAd
(
80
Smcd
f 80
~,
~
~'~
60 So
SmAl .
Sm5 Sm4
40 40
7 9 11 13 15 11 7 9 11 13 15 ii
carbon chain length carbon chain length
Fig. 2. Phase diagrams for two homologous series, mAZO (left) and mAZY (right).
As our results suggest the shape of the SmA~ area is universal within a homologous series,
whereas the shape of the SmAi boundary is not. We obtained mo =11.87± 0.05 and
practically the same width, B(A~), of the upper smectic phase for pure compounds of the
AZY series as well as for 6AZY-I3AZY and IIAZY-I3AZY binary systems (see Sect. 3.2.
for the relevant phase diagrams of mixtures). A different behaviour of these smectic phases
can be explained by taking into account their local and long range molecular ordering. Within the partly bilayer smectic phase, which is composed of highly polar asymmetric molecules, clusters of monomers and dimers exist [9]. The presence of dimers results in a loosy structure of the phase with recesses which can be easily filled by monomers of different molecular lengths. In this way the SmA~ phase becomes less sensitive to the molecular composition of
binary systems than the SmAj phase, as in the latter a strong local ordering is present.
The phase diagram for the AZO series cannot be determined in the vicinity of the expected A~ N~~ Aj triple point, because of a strong tendency to crystallization. The AZY series
has lowered melting temperatures, thus the reentrant nematic phase becomes monotropic
(e.g, for m
= 13). Moreover, the compounds can be easily supercooled to reveal additional smectic phases promoted by stabilization effects due to the presence of a transverse dipole
moment of the azoxy group. From two smectics, which limit the low temperature area of the
reentrant phase, the upper phase was identified as a partly bilayer SmC~ by X-ray
measurements and texture observations. The characteristic N C transition bars appeared at
the SmC~ N~~ transition on both cooling and heating. The only texture observed for the
lower Sm4 phase is a fan texture with sharp fan boundaries, which also appears when cooling
an ideally homeotropic smectic Aj. Small colorful plates are a typical texture of the Sm5
phase.
In the presence of the smectics C~ and Sm4 some new triple points should be observed on
phase diagrams of binary mixtures. Actually, the A~ C~ N~~ point is to be seen in figure 3 for the I4AZY-I6AZY system. From a variety of possible NAC systems diagrams [10], a diagram with a critical end point is realized, because in this case the second-order
~ SmAd
SmAd 6
~
yo Smcd .
f . .
SmAl Smcd
$f
~
Sm4
Sm5
so so
o~o o.5 1-o o-o o.5 1-o
composition composition
Fig. 3. Phase diagrams for binary systems I4AZY-I6AZY (left) and I6AZY-IBAZY (right).
Biphasic regions are too narrow to be observed or presented on these diagrams.
C~ A~ transition line meets two lines of the very weak first-order transitions N* A~ and N* C~ (see Sect. 2.2, for details). Previously found phase diagrams of similar topology II
are related to other N~~ -A-C points, e-g- to the Lifshitz point, as the second-order N C transition line has been there observed.
There are some predictions in the literature on the biaxiality of the nematic phase in the
vicinity of NAC points [12]. Our conoscopic measurements exclude a biaxial birefringence greater than 0.003 for ISAZY.
3.2 DSC STUDIES. Systematic DSC studies were performed for both series. The effective
enthalpies determined are collected in tables I and II. As the DSC data they also include, beside enthalpies of phase transitions, contributions from pretransitional anomalies of the
specific heat. Not all transitions shown on the phase diagrams were accessible for calorimetric
measurements because of the tendency to crystallization of some compounds.
The results for the N Ai transition correspond to the well-known fact that the broader the
nematic phase, the lower the transition enthalpy. From an extrapolation procedure we
expected the transition for 6AZY to be second order. Unfortunately, this compound does not
supercool sufficiently to reveal the smectic phase. However, from an analysis of the transition
enthalpies in the 6AZY-I3AZY system, the tricritical point was located at 0.79 mol fraction of 6AZY at TN~/TN~ = 0.834. This value corresponds, in terms of molecular statistical
theories [13-15], to the ratio of the isotropic attractive pair potential to the anisotropic one amounting to 3
=
0.38. Much higher values of 3 were reported recently for reentrant
compounds having only rigid spacers [16].
DSC studies reveal well-measurable enthalpies of the transitions on both sides of the
SmA~ phase. However, in I2AZO and I2AZY having the narrowest A~ phase, the thermal effects did not appear at any scanning rate. The observed second-order transitions may be
