Strain-induced transitions in liquid-crystal elastomers

Abstract

A novel strain-induced transition of the director orientation in a monodomain cross-linked sidechain liquid-crystal polymer system is reported which is analogous to the Freedericksz transition in simple nematics. This new type of transition has a characteristic threshold strain which arises from an internal elastic barrier to rotation and is related to the level of anisotropy of the polymer chain trajectory.PACS numbers: 64.70.Md, 61.41.+e Side-chain liquid-crystal polymers have the potential to exhibit rich phase behavior [1]. Such materials are formed by linking mesogenic or liquid crystal forming units to a flexible polymer backbone. The inherent chemical connectivity of the system inhibits the natural phase separation of these disparate components and leads to the possibility of interesting effects. At the heart of this behavior is the interaction between the naturally disordered polymer backbone and the long range orientational order of the mesogenic side groups. Small-angle neutron scattering (SANS) experiments on deuterium labeled mixtures have revealed anisotropic chain configurations in liquid-crystal monodomain samples of side-chain polymers, in that the polymer chain is preferentially extended either parallel or perpendicular to the liquid-crystal director [2,3]. In the isotropic phase, such materials exhibit the conventional isotropic random coil configuration of the polymer chain. If we introduce chemical crosslinking between the polymer chains to form a network there exists a mechanism to translate the microscopic coupling in the liquid-crystal phase to macroscopic effects [4]. The consequences of the interactions between a polymer network and the nematic order in a liquidcrystal elastomer were first identified by de Gennes [5] and subsequently a number of new phenomena, including piezoelectricity [6], memory effects [7], spontaneous macroscopic shape changes accompanying phase transitions [8], stress-induced molecular switching [9], and shifts in phase transitions [10], have been experimentally observed. By introducing the cross-links into a liquidcrystal polymer held in magnetic field, an equilibrium monodomain structure can be formed, in that upon removal of the magnetic field the system maintains the monodomain structure while held in the liquid-crystal state [7] shown schematically in Fig. 1(a). Such solid liquid crystals exhibit a reversible phase transition into the isotropic phase. However, quite remarkably on cooling in the absence of the magnetic field, both the director alignment n° and the state of order Q° at the time of cross-linking are recovered [7]. These phenomena arise from an internal field generated by the anisotropic network present at the time of cross-linking. In this Letter we observe for the first time how such an internal field can result in a mechanically induced transition of the director alignment in a side-chain liquid-crystal elastomer. This is essentially a mechanical analog of the well-known Preedericksz transition induced by electric or magnetic fields [11], obs...

Soft elasticity is not necessary for striping in nematic elastomers

“…Were this the case, striping would not be observed. This might explain the observations of Mitchell et al [9] Figures (5) and (6) show the angle θ and shear δ of the third solution as the elastic modulus ratio µ 1 /µ 2 varies. They clearly illustrate the change in the size of the transition region as a function of the ratio µ 1 /µ 2 of the elastic moduli.…”

“…This range decreases as the magnitude of the relative strain term in the free energy decreases, finally reaching the point where the transition can experimentally appear to be discontinuous. This may explain the apparent discontinuous transition observed by Mitchell et al [9] in their stretching experiments.…”

“…For example, the recent rheological experiments of Martinoty et al [8] appear to contradict the assumptions of soft elasticity. Furthermore, the experiments of Mitchell et al [9] on nematic elastomers did not exhibit the striping instability, but rather an apparent jump discontinuity in the director orientation.…”

A Two-Dimensional X-ray Scattering System for In-Situ Time-Resolving Studies of Polymer Structures Subjected to Controlled Deformations

“…It is operable in the temperature range between 293 and 573 K, with a stability of _+2 K over the entire sample, with minimum and maximum deformation rates of 1 and l l0mmmin -I. This device has been employed in the study of macroscopic deformation of liquid-crystalline elastomers and in determining the mechanisms by which a monodomain sample oriented in one direction can be caused to adopt an orthogonal alignment under uniaxial extension (Beattie, Lacey, Pople & Mitchell, 1997;Roberts, Mitchell, Davis & Pople, 1996;Mitchell, Davis & Guo, 1993).…”

Orientational order and finite strain in nematic elastomers

“…For example, nematic elastomers exhibit phase transitions, spontaneous shape changes, response to external fields, and microstructures such as striped domains [1][2][3][4][5][6] .…”