Advances in the flexoelectrooptic effect using polymerstabilized bimesogenic mixtures

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Advances in the flexoelectro-optic effect using
polymer-stabilized bimesogenic mixtures
Steve Morris
Centre for Molecular Materials for Photonics and
Electronics, Electrical Engineering
Division, Cambridge University Engineering
Department, 9 JJ Thomson Avenue, Cambridge, CB3
0FA, UK
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Outline

• Background
• The flexoelectro-optic effect
• Structure-property relations
• Materials
• Importance of shape
• High-tilt mixtures
• Materials
• Short-pitch mixture
• Long-pitch mixture
• Low driving fields
• Polymer stabilisation
• Effect on tilt angle
• Effect on response times
• Frequency Dependence
• DC cleaning

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Background
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The Flexoelectro-optic effect
In 1987 Meyer and Patel showed that
flexoelectric coupling between an applied
electric field and a chiral nematic liquid
crystal (NLC) results in a fast-switching
electro-optic effect J. S. Patel and R. B.
Meyer, Phys. Rev. Lett. 58, 1538 (1987).
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Structure-property relations
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Materials (odd and even spacers)
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Importance of shape (tilt angle)
For an applied field of E 4 Vmm-1, the tilt
angle of FFO5OCB is f 16o compared with f 7o
for FFO6OCB
podd 305 nm
peven 440 nm
(e/k)odd 1.45 CN-1m-1
(e/k)even 0.4 CN-1m-1
Reduced temperature of Tr 0.84 (Tr T/TIN)
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Importance of shape (response time)
FFO6OCB has the longer pitch yet exhibits a much
faster response time. Using the values for the
pitch and the response time, we find that the
effective visco-elastic ratio for FFO6OCB is g/k
4 x 109 kgN-1m-1s-1 compared with g/k 76 x109
kgN-1m-1s-1 for FFO5OCB.
Reduced temperature of Tr 0.84 (Tr T/TIN)
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Importance of shape (Flexoelastic ratio)
To a first approximation, the ratio of e/K
depends only upon the parity of the spacer and
not on its length, and is approximately 0.5
CN-1m-1 for the even-spaced compounds compared
with 1.5 CN-1m-1 for the opposite spacer
parity.
These values are all at the same reduced
temperature of Tr 0.82
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Importance of shape (Flexoelectric coefficient)
Flexoelastic ratio (e/K) as a function of
flexible spacer length, n, for the non-symmetric
homologous series FFOnOCB.
Values of the flexoelectric coefficient for the
odd- and even-spaced bimesogens are of the order
of 10 0.5 pCm-1 and 7 0.5 pCm-1,
respectively.
These values are all at the same reduced
temperature of Tr 0.82
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Importance of shape
The flexoelectric coefficient is increased for
the odd-spaced bimesogens - can be understood
in that the bent-configuration of the odd-spaced
bimesogens result in a larger bend flexoelectric
coefficient, e33. - Helfrichs relationship,
e33 ? mtq(b/a)2/3, where a is the molecular
length and b is the breadth . - Even though
the dipole moment is along the longitudinal
direction of the mesogenic units, for the bent
conformation this would result in an increase in
the component of the dipole moment in the
transverse direction. - This is supported by
results obtained from shape models which have
shown that the dipole moments are much larger
for cis (bent) isomers than trans (linear)
isomers.
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High-tilt mixtures
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Materials
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Short-pitch mixture
The flexoelectro-optic response for the short
pitch bimesogen mixture at different shifted
temperatures, Ts. The legend for the graphs is as
follows Ts 5oC (?), Ts 10oC (?), Ts 20oC
(?), Ts 30oC (?), Ts 40oC (?), and Ts 50oC
(?).
The phase sequence for this mixture is
I-(96oC)-BP-(92oC)-N-(38oC)-SmX
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Long-pitch mixture
The flexoelectro-optic response for the long
pitch bimesogen mixture at different shifted
temperatures, Ts. The legend for the graphs is as
follows Ts 5oC (?), Ts 10oC (?), Ts 20oC
(?), Ts 30oC (?), Ts 40oC (?), and Ts 50oC
(?).
The phase sequence of this material is
I(85oC)-N-(33oC)-SmX.
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Low driving fields
The electric field strength required for a 22.5o
tilt angle of the optic axis (full intensity
modulation) as a function of the shifted
temperature. The short pitch bimesogen mixture
() and the long pitch bimesogen mixture (?).
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Polymer-stabilisation
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Concentration of polymer
The flexoelectro-optic tilt angles measured for
the 0 (?), 3 (?) and 6 (?) RM257 mixtures as a
function of the r.m.s. applied electric field
value, at a shifted temperature of Ts -20ºC.
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Effect on tilt angle
Polymerised temperature Ts -10oC (circles) Ts
-30oC (squares), Operating temperature Ts
-5oC (closed symbol, solid line) Ts -25oC
(open symbol, dashed line)
Polymerised at low temperature results in a
longer pitch and therefore larger tilt angles.
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Effect on response time
Polymerised temperature Ts -10oC (circles) Ts
-30oC (squares), Operating temperature Ts
-5oC (closed symbol, solid line) Ts -25oC
(open symbol, dashed line)
Polymerised at low temperature and operate at
high temperature enables high tilt angles to be
combined with short response times.
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Frequency dependence
Operating Temperature 50oC (Tc 57oC) (NP)
No polymer. (P) Polymerised samples (cured at
50oC)
After polymerisation tan(f) depends on frequency
as well as field strength.
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DC cleaning
DC cleaning at T 80oC (isotropic), Frequency
1 Hz, timescale 30 mins, Voltage 30
Vpeak-to-peak
DC cleaning improves response and increases
tilt angle for a given field strength. Resitivity
increases due to removal of ions but decreases
with time following DC cleaning.
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Summary

• Odd-shaped (bent) bimesogens exhibit a larger
  flexoelastic ratio due to a larger bend angle and
  transverse dipole moment.
• Even-spaced (elongated) bimesogens exhibit a
  faster response time.
• Large tilt-angles (80o) can be obtained using
  mixtures of bimesogenic compounds.
• Polymer-stabilisation enables high-tilt angles
  to be combined with fast response times.
• DC cleaning can be used to reduce the negative
  effect of the polymer on the tilt angles.

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Acknowledgements

• Physicists/Engineers Matt Clarke (now at
  Rutherford Appleton Laboratory), Ben Broughton
  (now at Sharp Labs Europe), Colin Evans
  (Cambridge University).
• Chemists Andrew Blatch, Steve Perkins and Martin
  Peacock.
• And Prof. Harry Coles

Financial support
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References

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