Understanding the mechanical behaviour of novel liquid crystal elastomers

Liquid crystal elastomers (LCEs) are a class of materials which combine the orientational order and anisotropic properties of liquid crystals with the rubber-like elasticity of conventional elastomers. This thesis investigates the behaviour of an all acrylate LCE which displays a molecular auxetic response when deformed in the direction perpendicular to its nematic director. Auxetic materials have negative Poisson's ratios meaning they expand in one or both of their transverse axes when a longitudinal elongational strain is applied. In addition to a molecular auxetic response, the LCE also deforms via the mechanical Fréedericksz transition (MFT) which is characterised by a sharp rotation of the nematic director once a critical strain is reached. This behaviour appears to be quite different from the `semi-soft elastic' (SSE) response observed in other LCEs. The SSE is characterised by a continuous rotation of the nematic director, and a low energetic cost of deformation.

The molecular auxetic response and MFT in the LCE is investigated using polarised Raman spectroscopy (PRS), broadband dielectric spectroscopy (BDS), and the rheological techniques `dynamic mechanical analysis' (DMA) and `small amplitude shear rheology' (SAOS). PRS provides insight into the state of the nematic order. The PRS measurements reveal that when the LCE is strain perpendicular to the nematic director there is a decrease in the uniaxial order and the emergence of biaxial order. Additionally, at strains near the onset of the molecular auxetic response, the order parameter data suggests that the mesogenic units rotate into the axis that also displays the negative Poisson's ratio. BDS, DMA and SAOS provide insight into glass-formation and the molecular dynamics of the LCE. This LCE is particularly interesting in this regard as chemically identical nematic or isotropic samples can be synthesised. The glass transition temperatures are similar in both the nematic and isotropic phase. For both phases of the LCE, a cross-over in the dynamic behaviour of τ(α) is observed at T*≈330K. However, above T*, τ(α) is Arrhenius for the nematic LCE whereas τ(α) is non-Arrhenius for the isotropic sample. The difference in τ(α) is argued to be related to the presence of pretransitional nematic domains in the isotropic LCE.

The effect of strain on the relaxation dynamics and mechanical response of the nematic LCE is investigated to better understand the molecular auxetic response and the MFT. It is found that the complex Young’s modulus, E*, and the characteristic time-scale of the α relaxation, τ(α), remain constant during small deformations. However, for strains close to the onset of the molecular auxetic response an increase in both E* and τ(α) is observed. Based on these findings it is suggested that the observed molecular auxetic response, which is related to out-of-plane rotations of the mesogenic units, is in turn driven by an effect of constraints on polymer configurations, and the finite extensibility of the network, at large imposed strains.