1. Photo-Actuation in Liquid Crystal Elastomers Mark Warner
Cavendish Laboratory, Cambridge.
Nematic elastomers.
changes in liquid crystal ordering gives polymer shape change.
heat or light-induced distortions of rubber.
Actuation
thermo-actuation: 400+ % strains, lift weights (artificial muscle).
photo-actuation, photo-stationarity, dynamics.
future problems – polydomains, light polarisation, curling.

2. Strips of Nematic Elastomer
Monodomains 20cm x 1.5cm x 1mm
perfect order
transparent
birefringent (bifocal contact lenses)
Monodomain first made by H Finkelmann.
Allows tests of L(T).

3. Thermal Contractions L(T), with load, Finkelmann and Wermter.
Background for liquid crystals, polymers, elastomers.
Click to play film

4. n Nematic Liquid Crystals. Polymerise rods to make long chains
Fluid of rod-like molecules, e.g. O
CH3 N
p-azoxyanisole (PAA) n
Cool:
Transition Tni
heat or illuminate
Polymerise rods to make long chains

5. Nematic Polymers have shape anisotropy.
(CH2)10 O Si
O-C6H13 N
Nematic Polymers have shape anisotropy.
Pendant mesogens
(side chain nematic polymers)
[Mesogens in back-bone, main chain nematic polymers]
Rods order nematically, induce shape change in backbones.

6. ] Crosslink nematic polymers to form nematic elastomers
Nematic polymers - anisotropic random walks:
Gaussian distribution (prolate spheroid)
Elongated mean square shape with effective step lengths: ]
parallel to director
perpendicular to director
Anisotropic Gaussian distribution of spans
Mean shape is anisotropic
Crosslink nematic polymers to form nematic elastomers

7. Free energy density (aside) initial shape current shape
isotropic shear modulus (energy scale, 105J/m3)
# density of strands
Hyper-elastic – large deformation at const. volume,
Deformation spontaneous (because of order parameter change).
[Imposed – director rotation, soft elasticity, . . .]
(aside)

8. } Stress perpendicular to director: soft or semi-soft stress-strain
Küpfer and Finkelmann
Stress perpendicular to director:
soft or semi-soft stress-strain
Plateau slope & threshold depend on extent of semi-softness C
usual rubber modulus after plateau B A
and
(N) }
(Softness,
threshold)
differ only in crosslinking history
(I)
(also and )

9. Put in Extend softly until director rotation complete. Shape is
(then hard)
length of plateau
Put in

10. 1995 – I. Kundler and H. Finkelmann
Macromol. Chem. Rap. Commun. 16, 679
hard
soft
universal collapse of all data.
director rotation
singular edges or
plateau strain

11. Quasi-convexification – put stripes in where needed for lowest energy.
Solved completely by De Simone, Conti, Dolzmann et al
(soft)
De Simone et al (1/4 of strip)
(Depends on strip aspect ratio.)
Zubarev et al (1998)
(crossed polars)

12. Isotropic Nematic - Change order parameter
Chains & solid change shape
Cool [or photo-decay]
Chain shape distribution
initial sphere l
final prolate spheroid
(i.e. cooling or photo-decay)
Heat [or light - photoisomerisation]
Return to the free energy density

13. Minimise over with given and
Distortion
Minimise over with given and
Spontaneous distortion
(cooling or photo recovery)
(MW, KPG & TAV, 1988)
Anisotropy (measure by neutron scattering) Strains 3% - 400%.

14. Experiment: Master curve really Tni
Tajbakhsh and Terentjev Cavendish Laboratory
3.5 3
Strain L/L0
2.5
Cross-section
~2mm2
Load=15g
Load=10g
Load=5g
No Load 2
1.5 1
Master curve 20 40 60 80
100
120
Temperature (C)
really
Tni

15. Nematic elastomer strips2
Tajbakhsh and Terentjev
Cavendish Laboratory
300% length change 6
Click to play film

16. Birubber strip, H Finkelmann, Freiburg.
Click to play film
Photo-elasticity

17. Photoisomerisation Ideas
Absorb photon into dye molecule, change molecular conformation.
Azo benzene
trans isomer
cis isomer
(straight)
(bent)
Ideas
Azo-nematic rods - photochromes.
Bent rods disrupt nematic order. Reduce order parameter with light rather than temperature
Photo-elastic reaction analogous to T-induced shape changes.

19. Optical and thermal strains.
Illuminate or raise temperature
order parameter Q decreases
sample contracts
(Finkelmann et al, Phys. Rev. Letts. 2001)
Optical
Thermal
Q important

20. Photo-mechanical Recovery
contraction
recovery (dark)
T = 25C.

21. Photo-stress & nematic order at fixed length
Order and mechanical response go together.
(EM Terentjev et al)

22. Effective shifts along curve.
cis (“impurity”) conc.
(bent form)
Shift in effective temperature:
Effective shifts along curve.
Irradiation I depletes rod (trans) conc. from fo to f:
(total conc. photochromes)
Photo-dynamics initially:
The cis conc. grows:
Initially the effective temperature increases as

23. Initial photo-strain rate
Re-examine Initial Slopes
flatter
steeper
Optical
Thermal

24. Assume rubber dynamics slave to changes in
Combine trans loss and gain rates:
back reaction cis to trans
loss of trans
Full strain dynamics
(gives shift in effective temperature)
Dynamics of “impurity” conc:
Effective rate:
Photostationary
Assume rubber dynamics slave to changes in

25. Measure separately or calc.
Know
Theory uses
photostationary state from
(describes data for differing To)
data for To = 25oC

26. Palffy-Muhoray.
Idea: nematic elastomer loaded with green dye, exposed to laser pulse.
Absorbs laser energy – heats up, differential thermal contraction?
[Dye photoisomerises and lowers nematic order – photo-contraction???]
Green laser pulse
Click to play film
Test if:
dependent on light polarisation?
polydomain elastomer responds?
there is a stationary bend?

27. Mahadevan et al (Phys. Rev. Letts., 2004)
Light intensity I(x) falls with x (absorption length d )
Contraction decreases with x
Bending (curling) of beam or sheet
thickness
rad. curvature

28. Balance torques – get 2 neutral planes at depths xn
Curvature (1/R) non-monotonic in d/w
(absorption length/thickness)
Optimal d ~ w/3

29. Ikeda (Nature, 2003) – photo-bending of elastomer sheets
Click to play film
Most peculiar dynamics – why does it continue curling after eclipsing itself?!
What should the photo-stationary shape be?

30. Ikeda polydomain photo-elastomers
Light incident
Ikeda polydomain photo-elastomers
Curl direction depends on light polarisation
i.e. not a heating effect

31. Ikeda
Click to play film
Uncurling in the absence of UV.

32. The Future. Summary. Actuation.
Thermal and Photo-control of nematic order give mechanical strain (up to 400+%)
Times tuneable: minutes to seconds – vary chemistry and stimulate back-reactions.
Subsecond reaction for small strains (1%).
Actuation.
Remote actuation (light controlled, e.g. specialist environments).
Micro-chemical syntheses, pumps, valves on micron scale?
Optical control of variable holographs & of optical components.
The Future.
Non-linear dynamics. cis life time t (Q) and isomerisation rate h (Q) – depend on Q(f) and internal stresses.
Light polarisation effects, response of polydomain rubber.
Photo director rotation. Photo-induced soft shape change?
Photostationary curled shapes; dynamics of complex shapes?