Phenomena and Problems in Liquid Crystal Elastomers Mark Warner, Cavendish Cambridge. Classical Rubber Locally a polymeric liquid – mobile Make more complex, keep locally fluid More complex solids

Nematic fluid cool Nematic polymers have shape anisotropy Crosslink: elastomers respond to molecular shape change monodomain

1 l crosslink block of rubber Nematic Rubber anisotropic chains initial shape current shape Change shape with dT

Tajbakhsh and Terentjev Cavendish Laboratory Roughly 300% strains. Temperature changed by hot air blower. Monodomain elastomer. Close to real-time movement. 2 6

Smectic A cool Smectic liquids Nematic fluid with layered positional order. Layer modulus 10 7 N/m 2. (DJ Cleaver et al, Sheffield) nk Smectic C 2-D elastomer – layers so strong

(b) 90ºC (heating) qEqE (a) 25ºC (heating) LELE (c) 130ºC LELE qEqE (Hiraoka and Finkelmann, 2005) layers k n 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 isomercis isomer Azo benzene (straight)(bent) Recovery thermal 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 photoisomerises top 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 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

Minimised by (Olmsted): Stretch transverse to director

stretch force/area hard Response by rotation pervades all LC elastomer mechanics

E 45 o 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 Local molecular mobility Domains suffer director rotation away from E Þ large change in natural shape (MW & DC, PRL 06) E

Photo contraction l along E non-monotonic with intensity I recovered l, all domains isotropic director rotation gives strain back rotation starts order parameter collapses (“bleaching”) in back-rotated domains back rotation complete NMR? Mechanics? Unpolarised light?

SmC* ferro electric Spontaneous shear L ~ 0.4 Actuation based on shear. Ferro-electric films respond to: stress/strain electric field light heat k n c p q L

Slab geometry for film Apply shear -2L Reverse polarisation Film 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.)

Q0Q0 initially (and finally) z q0q0 Jump away from ; global order S < 0 z n ~Q 0 q Collapse of local order Q ; global order less negative Jump back toward z n Q~0 q~q 0 Detect by NMR?

Local order Q 0 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 x Bending (curling) of beam or sheet thickness rad. curvature w>d : thick w<d : thin film

Balance torques – get 2 neutral planes at depths x n Curvature ( 1/R ) non-monotonic in d/w (absorption length/thickness) Optimal d ~ w/3 “thick”“thin” (more examples)