Phenomena and Problems inLiquid Crystal Elastomers

Mark Warner, Cavendish Cambridge.

Classical RubberLocally a polymeric liquid – mobileMake more complex, keep locally fluid

More complex solids

Nematic fluid

cool

n

Nematic polymers have shape anisotropy

Crosslink:elastomers respond to molecular shape change

monodomain

1

λ/1

0R R

crosslink

block of rubber

Nematic Rubber

anisotropic chains

n

1T

021 Tr nemF

initial shape current shape

35 J/m10~Rμ Tns

Change shape with T

Tajbakhsh and TerentjevCavendish Laboratory

Roughly 300% strains.Temperature changed by hot air blower.Monodomain elastomer.Close to real-time movement.

2

6

3.5

1

1.5

2

2.5

3

20 40 60 80 100 120

Temperature ( C)

Str

ain

L/L 0

Cross-section~2mm2

Load=15gLoad=10gLoad=5gNo Load

Smectic ASmectic A

cool

Smectic liquidsNematic fluid with layered positional order.Layer modulus 107 N/m2.

(DJ Cleaver et al, Sheffield)n k

Smectic CSmectic C

2-D elastomer – layers so strong

(b) 90ºC (heating)

E

(a) 25ºC (heating)

LE

(c) 130ºC

LE

E

(Hiraoka and Finkelmann, 2005)

layers

kn

P

Spontaneous shears of smectic sheet(also possible with slab)

Reduce order by bending some rods- Photo alternative to thermal disruption of order.

Absorb photon into dye molecule

trans isomer cis isomer

Azo benzene

(straight) (bent)

Recoverythermal or stimulated

Optical strains.

ThermalOptical

Can be very fast.Bend.Polydomain response.

Birubber strip, H Finkelmann, Freiburg.

Non-uniform response

Nematic elastomer + green dye guest; laser pulse.

Dye photoisomerisestop has lower nematic order – differential photo-contraction???

Green laser pulse

Palffy-Muhoray

* Curvature of photo-beams very rich (2 neutral planes)* Optically write structures in films

Most peculiar dynamics – why does it continue curling after eclipsing itself?! What should the photo-stationary shape be?

Photo-bending of sheets (Ikeda, Nature, 2003)

E

Uncurling in the absence of UV.(in light – stimulated decay)

Responsive surfaces and thin films

uzuzr

urz

H

u Hzz

a

zr

Substrate

l

light beam localised strains

photo-rubber

Elongation on illumination

Rotate order rather than change magnitude

Stretch transverse to director

• Body accommodates rotating chain distribution.• Need shear & stretch.• Entropy, energy constant.

)(

thereafter hard.

inscribed 2/1)(

231

)(T

021 Tr nemF

Minimised by (Olmsted):2/1

0

2/1)(soft

2/1)(

Stretch transverse to director

stretch

forc

e/ar

ea

hard

21

211

)/(

1)/(

1sin

r

r

Response by rotation pervades all LC elastomer mechanics

E

45o

Photo-bend also for polydomains – depends on light polarisation

k

E

Light incident

Curl direction ↔ light polarisation

(heat a minor effect?)

Polydomain photo-elastomer (thin)

Incident light

1

1

1

Local molecular mobility

Domains suffer director rotation away from E large change in natural shape

(MW & DC, PRL 06)

E

0.8

0 2 4 6 8 10 12

0.9

1.0

0.7

l

I~

1+S

Photo contraction along E non-monotonic with intensity I

recovered , all domains isotropic

director rotation gives strain

back rotation startsorder parameter collapses(“bleaching”) in back-rotated domains

back rotation complete

NMR? Mechanics?Unpolarised light?

SmC* ferro electric

Spontaneous shear ~ 0.4Actuation based on shear.

Ferro-electric films respond to:• stress/strain• electric field• light• heat

k nc

p

k0 k0n0n

z

x

c0 -c0

p0

p

-2^ ^^

^

^

^

^ ^

^^

Slab geometry for filmApply shear -2Reverse polarisationFilm bistable??

Cholesterics – helically twisted nematics:

Elastomers:Separate left from right handed molecules.Change colour on stretching.Lase when pumped – lasing colour changes with stretch . . .

(tuneable laser from an elastic photonic band solid)

Deformations in practice (Quasi-convexification)

Stripes

Macroscopic extension

Kundler & Finkelmann

(crossed polars)

Replace gross deformations by microstructure of (soft) strains with lower energy which satisfies constraints in gross sense.

Practical geometry – put stripes in where needed for lowest energy:

Conti et al (1/4 of strip)

(soft)

Zubarev, Finkelmann et al

Terentjev et al

(Depends on strip aspect ratio.)

Q0

0 initially (and finally)

z0

0Jump away from ; global order S < 0

z

n~Q0

Collapse of local order Q; global order less negative

0Jump back toward

z

n

Q~0

0

Detect by NMR?

0.8

0 2 4 6 8 10 12

0.9

1.0

0.7

l

I~

1+S

Local order Q0 rotated away from E; global S<0

Local and global order = 0

Mahadevan et al (Phys. Rev. Letts., 2004)

Light intensity I(x) falls with x (absorption length d )

Contraction decreases with xBending (curling) of beam or sheet

thickness rad. curvature

w>d: thickw<d: thin film

Balance torques – get 2 neutral planes at depths xnCurvature (1/R) non-monotonic in d/w

(absorption length/thickness)

Optimal d ~ w/3

“thick” “thin”

(more examples)