This is the accepted manuscript made available via CHORUS. The article has been
published as:
Phase behavior and characterization of
heptamethyltrisiloxane-based de Vries smectic liquid
crystal by electro-optics, x rays, and dielectric
spectroscopy
S. P. Sreenilayam, D. M. Agra-Kooijman, V. P. Panov, V. Swaminathan, J. K. Vij, Yu. P.
Panarin, A. Kocot, A. Panov, D. Rodriguez-Lojo, P. J. Stevenson, Michael R. Fisch, and
Satyendra Kumar
Phys. Rev. E 95, 032701 — Published 10 March 2017
DOI:
10.1103/PhysRevE.95.032701
1
The Phase behaviour and the characterization of heptamethyltrisiloxane 1
based de-Vries SmA* liquid crystal by electro-optics, x-rays and 2
dielectric spectroscopy 3
4
S. P. Sreenilayam
a
, D. M. Agra-Kooijman
b†
, V. P. Panov
a
, V. Swaminathan
a
, J. K. Vij
a*
, Yu. P. 5
Panarin
a, c
, , A. Kocot
d
, A. Panov
e
, D. Rodriguez-Lojo
e
, P. J. Stevenson
e
, M. Fisch
b§
and 6
Satyendra Kumar
b,f
7
8
a
Department of Electronic and Electrical Engineering, Trinity College,-The University of Dublin, 9
Dublin 2, Ireland 10
11
b†
Department of Physics and
§
College of Applied Engineering Sustainability and Technology 12
Kent State University, Kent, OH 44242 13
14
c
School of Electrical and Electronic Engineering, Dublin Institute of Technology, Dublin 8, 15
Ireland 16
d
Institute of Physics, Silesian University, Katowice, Poland 17
e
School of Chemistry and Chemical Engineering, Queens University, Belfast, BT7 1NN, U. K. 18
19
20
f
Division of Research and Department of Physics, University at Albany, Albany, NY 12222 21
22
A heptamethyltrisiloxane liquid crystal (LC) exhibiting Iso-SmA*- SmC* 23
phases has been characterized by calorimetry, polarizing microscopy, x-ray 24
diffraction, electro-optics and dielectric spectroscopy. Observations of a large 25
electro-clinic effect, a large increase in the birefringence (Δn) with electric field, a 26
low shrinkage in the layer thickness(~1.75%) at 20
0
C below the SmA* to SmC* 27
transition, and low values of the reduction factor (~0.40) suggest that SmA* phase 28
in this material is of the de-Vries type. The reduction factor is a measure of the 29
layer shrinkage in SmC* phase and it should be zero for an ideal de-Vries. 30
Moreover, a decrease in the magnitude of Δn with decreasing temperature 31
indicates the presence of the temperature-dependent tilt angle in the SmA* phase. 32
The electro-optic behavior is explained by the generalized Langevin-Debye model 33
as given by Shen et al. [Phys. Rev. E 88, 062504 (2013)]. The soft mode dielectric 34
relaxation strength shows a critical behavior when the system goes from SmA* to 35
SmC* phase. 36
*
Corresponding Author:
jvij@.tcd.ie 37
2
1. INTRODUCTION 38
In liquid crystalline (LC) compounds, the phase transition from the orthogonal (SmA) to 39
tilted (SmC) smectic phases is associated with an appearance of tilt (θ) between the molecular 40
long axis n and layer normal z (Fig. 1a) Due to this tilt, the layer spacing in the SmC phase (d
C
) 41
is smaller than in SmA (d
A
). In the realm of the rigid-rod molecular model being valid (Fig. 1a), 42
the smectic layer thickness d
C
is reduced from d
A
by cosθ [1-3]. In conventional SmC LCs, θ 43
varies from zero to ~30º depending on temperature. The large layer contraction in ferroelectric 44
SmC* induces chevron structures which in turn results into zigzag defects [4]. These defects 45
present a roadblock to a successful commercialization of the ferroelectric LC (FLC) devices. The 46
FLC devices intrinsically have faster switching modes [5] than their nematic counterparts that 47
are currently predominantly used in the industry. The objective is therefore to eliminate these 48
zigzag defects by making the smectic layer thickness almost independent of temperature so as to 49
have the most desirable features of FLCs in the next generation of displays. 50
In 1972, Diele et al. reported a non-chiral LC with the same layer spacing in the SmC and 51
SmA [6]. To explain it, de-Vries proposed a new type of SmA phase where the molecules are 52
tilted as in SmC with two possible structures. In one case [7,8], SmC-like layers are stacked in a 53
random fashion. In other words tilt directions with the same tilt angle in different layers are 54
randomly oriented. This implies that the azimuthal angle (φ) varies randomly from on layer to 55
next: no long range correlations in the azimuthal angle of the smectic layers was proposed to 56
exist in this case. In the second model of de-Vries [9], the molecules are tilted and the 57
correlation in the tilt direction exists within a single layer too, i.e. φ has a finite-correlation 58
length. If the correlation length is much smaller than wavelength of the visible light, then the 59
phase in optical experiments should behave as ‘a uniaxial SmA’. The results of both de-Vries 60
models should be that the directors in SmA phase would be distributed on to a cone as shown in 61
Fig. 1b. 62
The chiral de-Vries materials show electro-optic behavior due to the field-induced 63
azimuthal reorientation of the molecules on the cone and the apparent tilt angle measured by an 64
optical experiment in SmA thus increases with the field. They exhibit a significantly large 65
electroclinic effect due to the azimuthal reorientation and the induced tilt becomes saturated at 66
“high” electric fields once the degeneracy in the azimuthal angle is lost (the azimuthal angle is 67
condensed to values within narrow limits). For the zero external field, the maximum of the 68
molecular orientational distribution function is at the cone angle (volcano distribution) rather 69
than at the layer normal. The de-Vries behavior can be described by the reduction factor defined 70
3
as,
cos
/
/
; where δ(T) is the tilt angle for the layer shrinkage 71
relative to layer thickness d
AC
at the smectic A-C transition and θ
opt
is the optical tilt angle 72
determined by the polarizing optical microscopy [10, 11]. An ideal de-Vries material producing 73
defect-free bookshelf geometry in SmC* phase will have the reduction factor R=0. 74
75
FIG. 1: Schematic representation of (a) conventional SmA-SmC (rigid rod model) and 76
(b) de-Vries SmA-SmC (diffuse cone model) phase transition. Here, z is the layer 77
normal, n is the molecular long axis orientation, θ is the angle between n and z, d
C
and 78
d
A
are the layer spacings in SmC and SmA phases, respectively. 79
Several research groups reported de-Vries type behavior in smectic LCs composed of 80
non-chiral [12-14] and chiral [15-19] molecules. LC materials that behave as ‘good de-Vries 81
like’ so far are siloxane-terminated TSiKN65 compound [20], its carbosilane-terminated 82
analogue W599 [21] and the 2-phenylpyrimidine derivative 8422[2F3] [22]. For these materials, 83
the layer contraction at the smectic A*–C* transition lies in the range of 0.65 to 1%. In this 84
paper, we present experimental results on the calorimetric, optical, polarization measurements, 85
dielectric spectroscopy and x-ray diffraction on heptamethyl- trisiloxane derivative MSi
3
MR11, 86
which exhibits a strong electroclinic effect with birefringence strongly increasing at SmA* - 87
SmC* phase transition. Experimental results suggest that the SmA* phase in this material is of 88
de-Vries type. The reduction factor for this material is found to be ~0.40. The electro-optic 89
response and the induced polarization are found to be in agreement with the generalized 90
Langevin-Debye model. The soft mode relaxation strength of de-Vries type SmA* phase as a 91
function of temperature exhibits critical nature when the system undergoes a transition to SmC* 92
phase. 93
Conventional
SmA
n
z
n
z
θ
d
A
d
C
Shrinkage
(a)
de-Vries SmA
d
A
d
C
d
A
≈d
C
n
z
θ
(b)
SmC
SmC
4
2. EXPERIMENTAL 94
The molecular structure and the transition temperatures of the MSi
3
MR11 are shown in 95
Fig. 2a. This compound was resynthesized and it has two chiral centres. The synthetic procedure 96
is given in the appendix A. One of the objectives here is to see whether two chiral centres give 97
rise to a similar phenomenon as compounds with one chiral centre. The mesogenic core of MR11 98
consists of a biphenyl 2-chloro-3-methylpentanoate unit. Here ‘M (mono-substituted)’stands for 99
the number of siloxane end groups attached to the mesogen MR11. The mesogen MR11 [23] 100
with 11 methylene units is attached to a trisiloxane backbone. The purity of the sample was 101
found to much higher through its analysis by NMR than for the previously synthesized sample 102
[23]. An analysis for the purity of the sample is given in the appendix A. The transition 103
temperatures (Fig. 2a) are obtained on cooling under quasi-equilibrium condition with a cooling 104
rate of ~1°C min
-1
using polarizing microscopy. 105
106
(a) 107
Cr 16ºC [-5.12 J/g] SmC* 47ºC [-1.06 J/g] SmA* 59ºC [-5.56 J/g] Iso 108
109
(b) 110
10 20 30 40 50 60 70
SmA*
SmA*
Iso
Iso
Cr
Cr
SmC*
SmC*
ΔH=6.09 J/g
ΔH=0.89 J/g
ΔH=5.4 J/g
ΔH=-5.12 J/g
ΔH=-5.56 J/g
Heat Flow (mW)
T(
o
C)
Cooling10
o
C/ min
Heating10
o
C/ min
ΔH=-1.06 J/g
111
(c) 112