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
Jan 15, 2016
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)