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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...

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The time-of-flight (TOF) method for the determination of electron-diffusion coefficients D and drift velocities w described earlier could not be used to investigate a number of gases because a Gieger—Müller counter was used for an electron detector. A new experimental apparatus using differential pumping and an electron-multiplier detector was developed to remove this restriction. Using this new apparatus, D and w have been measured for He, Ar, H2, N2, CO, CO2, CH4, and C2H4. Considerable divergence of D values from published values by the Townsend method have been obtained, indicating a fundamental difference in the methods.

Articles you may be interested inDipolebound excited states of the I−CH3CN and I−(CH3CN)2 ion-molecule complexes: Evidence for asymmetric solvation Laser induced fluorescence measurements of the X 2Σ+ state of CN produced by electron impact on CH3CN J. Chem. Phys. 95, 754 (1991);Thermal electron attachment to nitromethane, methylcyanide, methyliodide, and methylbromide is compared with capture of electrons by these molecules from highly excited Rydberg states of atoms. Data on thermal electron attachment to CH 3 NO, are consistent with a three-body attachment process, with the nature of the third body being important. The thermal energy electron attachment rate constant for CH 3 CN is :s; 1.24X 10-" cm 3 sec-I. Some results on dissociative electron attachment and io~ pairing processes in CH 3 N0 2 , CH 3 CN, CH 3 I, and CH3Br are also presented.

Liquid-crystalline elastomers are produced by the introduction of crosslinking into liquid-crystalline polymer systems. This crosslinking results in materials with a number of unusual properties. Of particular note are the effects of an applied stress coupled with such a system. Theory predicts that these should include stress-induced phase transitions and spontaneous elongation of samples in the liquid-crystalline phase; such properties have now been observed experimentally. The materials exhibit a number of other properties, also induced by the coupling of network elasticity to the liquid-crystalline phase. These include a temperature-induced memory effect; materials with a chiral mesophase exhibit piezoelectric behaviour.

Free-standing monodomain liquid crystal elastomer samples are shown to have a complete memory of the orientational configuration at the time of cross-linking. This memory is demonstrated through samples in which the parent polymer system is first aligned in a magnetic field prior to cross-linking. These films show reversible nematic-isotropic phase transitions and x-ray scattering patterns characteristic of nematic phases. The liquid crystal elastomer films exhibit a remarkable memory effect, in that the sample may be held at temperatures well above the nematic-isotropic transition for extended periods (> 2 weeks), but on cooling into the liquid crystal phase region, both the original director alignment and the degree of preferred orientation are recovered. It is demonstrated that these novel memory effects are equilibrium in nature. The origins of this phenomena in terms of coupling between the mesogenic side-chains and the polymer network are discussed

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