Richness of Side-Chain Liquid-Crystal Polymers: From Isotropic Phase towards the Identification of Neglected Solid-Like Properties in Liquids

Abstract

Very few studies concern the isotropic phase of Side-Chain Liquid-Crystalline Polymers (SCLCPs). However, the interest for the isotropic phase appears particularly obvious in flow experiments. Unforeseen shear-induced nematic phases are revealed away from the N-I transition temperature. The non-equilibrium nematic phase in the isotropic phase of SCLCP melts challenges the conventional timescales described in theoretical approaches and reveal very long timescales, neglected until now. This spectacular behavior is the starter of the present survey that reveals long range solid-like interactions up to the sub-millimetre scale. We address the question of the origin of this solid-like property by probing more particularly the non-equilibrium behavior of a polyacrylate substituted by a nitrobiphenyl group (PANO2). The comparison with a polybutylacrylate chain of the same degree of polymerization evidences that the solid-like response is exacerbated in SCLCPs. We conclude that the liquid crystal moieties interplay as efficient elastic connectors. Finally, we show that the "solid" character can be evidenced away from the glass transition temperature in glass formers and for the first time, in purely alkane chains above their crystallization temperature. We thus have probed collective elastic effects contained not OPEN ACCESSPolymers 2012, 4 1110 only in the isotropic phase of SCLCPs, but also more generically in the liquid state of ordinary melts and of ordinary liquids.

Harmonic strain-optical response revealed in the isotropic (liquid) phase of liquid crystals

“…19 This synchronized birefringence is coherent with the stress results observed at low strain amplitudes on various liquids indicating an elastic pre-regime existing at the sub-millimeter scale prior to the conventional flow or viscous behavior. [4][5][6][7][8][9] This low frequency elastic plateau is also identified in the isotropic phase of LCP95 as illustrated in the stress measurements carried out at 0.200 mm on wetting substrate (Fig. 1(b)) and in agreement with previous results.…”

“…1(b)) and in agreement with previous results. [4][5][6][7][8][9] Finally, the birefringence induced in the isotropic phase cannot be interpreted as a surface induced property. Wall or capillary effects rule out since the probed gap distances far exceed coherence lengths of the order of several nanometers.…”

“…These experiments suppose that a dynamic coupling exists between the pretransitional fluctuations (clusters of molecules with identical directions coexisting within the isotropic phase) and elastic properties uncovered at low frequency (typically below the Hz) in various liquids. [4][5][6][7][8][9] This strategy is efficient since a strong and reversible optical response is highlighted in the isotropic phase upon a low frequency low strain mechanical solicitation. This strain-induced optical birefringence is visible up to 15 C above the isotropic transition and has the advantage of producing a true black phase between crossed polarizers in its inactivated state.…”

“…Comparison of dynamic relaxation data of the LCP95 in the isotropic phase (T SAI þ 2 C, e ¼ 120 lm, 1% strain amplitude, ARES2 rheometer): On conventional substrate (glass): The shear modulus is not measurable and loss modulus G 00 scales with x 2 depicting a Newtonian behavior (blue squares (ٗ): conventional glass substrate) and on wetting substrate (red circles (᭺): alumina substrate): the shear modulus (elastic response) G 0 dominates at low frequency. 7 The storage modulus G 0 dominates indicating a low frequency elastic behavior. (c) Optical setup: The isotropic phase fills the gap between two fixtures and is observed between crossed polarizers with a microscope (magnification lens: Â100).…”

Synthesis of Isothianaphthene (ITN) and 3,4-Ethylenedioxy-Thiophene (EDOT)-Based Low-Bandgap Liquid Crystalline Conjugated Polymers

“…In general, LC materials consist of a rigid mesogen core and flexible side chains. In the case of polymer LCs, main-chain-type [1] and side-chain-type polymer LC [2] have been studied. Aromatic groups of a monomer repeat unit play the role of mesogenic core in the case of main-chain type polymer LCs.…”