Liquid crystal elastomers Normal isotropic elastomer Liquid Crystal elastomer

Monodomain and polydomain samples Unaligned Polydomain Aligned few microns

Mechanical anisotropy 35º 50º 70º 80º tan  D V (like isotropic rubber) (soft elasticity) Frequency (s -1 ) Anisotropy in monodomain (All samples synthesised by Dr Ali Tajbakhsh)

Mechanical anisotropy tan  Master curve constructed using time-Temperature superposition. (Scaled to 35º) Frequency (s -1 )

Mechanical anisotropy Frequency (s -1 ) tan  v monodomain d monodomain polydomain Polydomain compared with monodomain

l Stretching a polydomain material and clamping it during dynamic mechanical analysis – shows same behaviour as monodomain. Stretched Polydomain

Mechanical anisotropy degrees 0 50 degrees 0 v monodomain d monodomain d stretched polydomain v stretched polydomain polydomain Stretched Polydomain tan 

Time-resolved experiments:WAXS X-rays 2-D intensified CCD detector Oscillatory shear Stretch…..and then shear COMPUTER Optical chopper

Fit to: I = a + b * exp(-c * (cos(  -d))^2). “d” shows azimuthal tilt  Azimuthal integration

Variation in tilt angle Time (degrees of shear cycle) 80s 10s 40s Strain: movement of arm +0.5 mm mm We can successfully obtain WAXS data at 1s time-resolution without loss of image quality by binning over many cycles. Tilt angle / degrees

Time-resolved Optical experiments Red diode laser Amplified photodiode Oscillatory stretch COMPUTER Optical chopper Phase shift amplitude Sinusoidal strain Strain light

Amplitude and phase shift 70º60º55º50º Phase shift (cycles) Amplitude / Arbitrary units Frequency / s High temperatures: amplitude independent of frequency; phase shift increases Medium temperatures: amplitude decreases with frequency; phase shift shows “hump”Medium temperatures: amplitude decreases with frequency; phase shift shows “hump” Low temperatures: amplitude independent of frequency; phase shift decreases BUCKLING 40º BUCKLING

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

strain Transmission of light First-order rate constant Equilibrium transmission for slow process (small) sinusoidal imposed strain… …gives sinusoidal light transmission: amplitude phase shift Derivation of model

Amplitude data: qualitative fit BUCKLING Amplitude / Arbitrary units º60º High temperatures: amplitude independent of frequency; Medium temperatures: amplitude decreases with frequency;Medium temperatures: amplitude decreases with frequency; Low temperatures: amplitude independent of frequency; 55º50º40º f / s -1 1e-4 10 k = 1e-3 k = 0.1 k = 0.01 k = 10 k = 1

 / s Phase shift data: quantitative fit Phase shift (cycles) =  70º k = 6.55 s -1 60º k = 2.62 s -1 55º k = 0.75 s -1 50º k = 0.26 s -1 40º k = s -1 c 1 = 2.16 ln (k / s -1 ) 1 / T (K -1 ) Data give good fit to model, with temperature- dependent rate constant Data consistent with Activation energy E A = 200 kJ mol-1 (assume Arrhenius equation)

Phase shift: t-T superposition?  / s -1 Scaled to 50º

Fitting our data l Assume t-T superposition, scaled for 50 degrees:  / s -1 + offset At 50º C k = 0.26 s -1 c 1 = 17.4

Step-strain 100s200s 10s 5s 1s 2s 3s 40º C k = s -1 50º C k = 0.17 s -1 60º C k = 4.9 s -1 Fits first-order mono-exponential I = I 0 - A exp(-k t) k increases with temperature

The sinusoidal and step data agree (within error) Activation energy 200 kJ mol -1. (What does this mean?) ln (k / s -1 ) 1 / T (K -1 ) Comparison of first-order rate constants