Liquid crystal elastomers

1
Liquid crystal elastomers
Liquid Crystal elastomer
Normal isotropic elastomer
2
Monodomain and polydomain samples
Aligned
few microns
Unaligned Polydomain
3
Mechanical anisotropy
Anisotropy in monodomain

(All samples synthesised by Dr Ali Tajbakhsh)
(soft elasticity)
D
tan d
35º
V
50º
(like isotropic rubber)
70º
80º
Frequency (s-1)
4
Mechanical anisotropy
Master curve constructed using time-Temperature
superposition. (Scaled to 35º)
1.8
1.6
1.4
1.2
tan d
1.0
0.8
0.6
0.4
0.2
-1
0
1
2
10
10
10
10
Frequency (s-1)
5
Mechanical anisotropy
Polydomain compared with monodomain
tan d
Frequency (s-1)
6
Stretched Polydomain

• Stretching a polydomain material and clamping it
  during dynamic mechanical analysis shows same
  behaviour as monodomain.

7
Mechanical anisotropy
Stretched Polydomain
tan d
8
Time-resolved experimentsWAXS
2-D intensified CCD detector
X-rays
Stretch
Oscillatory shear
..and then shear
COMPUTER
Optical chopper
9
Azimuthal integration
Fit to I a b exp(-c (cos(f-d))2). d
shows azimuthal tilt
10
Variation in tilt angle
We can successfully obtain WAXS data at 1s
time-resolution without loss of image quality by
binning over many cycles.
0.5 mm
Strain movement of arm
0
84
83.5
83
-0.5 mm
82.5
82
81.5
Tilt angle / degrees
81
80s
80.5
10s
40s
80
79.5
79
0
50
100
150
200
250
300
350
Time (degrees of shear cycle)
11
Time-resolved Optical experiments
Amplified photodiode
Red diode laser
Oscillatory stretch
COMPUTER
Optical chopper
12
Amplitude and phase shift
Amplitude / Arbitrary units
0
70º
60º
55º
50º
40º
Phase shift (cycles)
1
0.1
0.01
1
0.1
0.01
0
1
0.1
0.01
0.1
0.01
Frequency / s-1

• High temperatures amplitude independent of
  frequency phase shift increases
• Medium temperatures amplitude decreases with
  frequency phase shift shows hump
• Low temperatures amplitude independent of
  frequency phase shift decreases

13
Model (assume linear)

• Two processes causing changes in transparency on
  stretching.
• One fast (affine deformation? Thinning?)
• One slow (disappearance of domain boundaries?)
• Both equilibrium transparency linear with strain
  (for small amplitude)

14
Derivation of model

• (small) sinusoidal imposed strain
• gives sinusoidal light transmission

15
Amplitude data qualitative fit
BUCKLING
Amplitude / Arbitrary units
0
f / s-1
1
0.1
0.01
70º
60º
55º
50º
40º

• High temperatures amplitude independent of
  frequency
• Medium temperatures amplitude decreases with
  frequency
• Low temperatures amplitude independent of
  frequency

k 10
k 1
k 0.1
k 0.01
k 1e-3
1e-4
10
16
Phase shift data quantitative fit
c1 2.16
70º k 6.55 s-1
Data give good fit to model, with
temperature-dependent rate constant
60º k 2.62 s-1
55º k 0.75 s-1
Phase shift (cycles) d / 2 p
50º k 0.26 s-1
0.03
0.02
40º k 0.022 s-1
0.01
Data consistent with Activation energy EA 200
kJ mol-1 (assume Arrhenius equation)
0.00
w / s-1
0.01
0.1
10
17
Phase shift t-T superposition?
Scaled to 50º
w / s-1
18
Fitting our data

• Assume t-T superposition, scaled for 50 degrees

offset
At 50º C k 0.26 s-1
c1 17.4
w / s-1
19
Step-strain
Fits first-order mono-exponential I I0 - A
exp(-k t) k increases with temperature
50º C k 0.17 s-1
40º C k 0.034 s-1
60º C k 4.9 s-1
10s
5s
1s
3s
100s
200s
2s
20
Comparison of first-order rate constants

• The sinusoidal and step data agree (within error)
• Activation energy 200 kJ mol-1 . (What does this
  mean?)

1 / T (K-1)
ln (k / s-1)