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 only in the isotropic phase of SCLCPs, but also more generically in the liquid state of ordinary melts and of ordinary liquids.
Highlights
Shear flow is used in solid mechanics as well as in fluid dynamics
A splendid and unexpected birefringence emerges from the isotropic phase of side-chain liquid crystal polymer melts when they are sheared above a critical shear rate (
The confrontation of the timescales involved in the shear-induced phase has revealed that the scheme proposed by the ad hoc theoretical models based on a flow coupling to the lifetime of the orientational pre-transitional fluctuations is not valid
Summary
Shear flow is used in solid mechanics as well as in fluid dynamics. The flow in liquids can be induced by the mechanical displacement of a surface in contact with the fluid (Figure 1). Since 2005, we have been working on, and demonstrating, that improved dynamic measurements enable access to this cohesive property via the identification of a low frequency shear elasticity (solid-like character) in the liquid state in various glass formers away from any phase transition [28,29,30,31,32] We apply this improved protocol first to the isotropic phase of the SCLCP, PANO2, compare it to an ordinary polymer chain (polybutylacrylate melt (PBuA)) of the same degree of polymerization, and examine the case of a short alkyl chain assimilated to a spacer: The heptadecane.
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