Identification of finite shear-elasticity in the liquid state of molecular and polymeric glass-formers

Abstract

Finite shear elasticity has been identified in the liquid state. The study is expanded to a van der Waal's molecular glass-former, o-terphenyl, and to an ordinary polymer melt, polybutylacrylate, as a function of their molecular weight. These fluids exhibit shear elasticity at the sub-millimetre scale and far above any phase transition. This macroscopic property challenges the conventional terminal relaxation modes (-process or terminal viscoelastic times (reptation)) in terms of individual molecular process. IntroductionTo define three states of matter, such as solid, liquid or glass and the transitions between them, the shear and bulk moduli are certainly the most useful and relevant characteristic features [1]. The shear modulus presents a substantial advantage over bulk modulus because specific instrumentation has been elaborated to routinely measure the dynamic properties of viscoelastic materials (materials displaying dynamic behaviours intermediate between liquid and solid). These apparatuses are dynamic mechanical analysers, rheometers, shear resonators, surface force apparatus, among others. Based on the same principle, these devices measure the stress or motion transmitted by simple contact between the sample and the surface submitted to small mechanical oscillatory solicitations. This mechanical technique is currently the only one able to probe long time scales.The boundary conditions between the surface and the sample play a major role in this measurement since the efficiency of transmission of the motion and, thus, the quality of the measurement are completely dependent on the interaction between the surface and the material. To date, progress in instrumentation sensitivity and techniques has allowed the measurement of shear modulus with a high precision and over six decades of magnitude [2]. These improvements have considerably widened the frequency window and the modulus range, enabling the detection of subtle properties that could not been considered before. Recent developments have shown that it is also

Identification of Shear Elasticity at Low Frequency in Liquid n-Heptadecane, Liquid Water and RT-Ionic Liquids [emim][Tf2N]

“…We reveal in all cases, a weak but non-ambiguous solid-like low frequency response at weak shear strain solicitation. The present work thus extends, details and generalizes the identification of solid-like correlations previously reported above the melting point, of glycerol [4], of o-terphenyl [5], of polypropylene glycol [3] and more generally in polymer melts [1,3].…”

“…However, several recent results have revived the assumption of long-range elastic correlations and therefore of long time scales, hitherto neglected in the liquid state even away from any phase transition. The measurement of macroscopic finite shear elasticity at the sub-millimeter scale has been essentially enabled by minoring the disturbance of the liquid during the mechanical solicitation and by improving the boundary conditions of the liquid to the substrate [1][2][3][4][5]. This method was successfully applied to glass formers (polymer melts, polypropylene glycol, o-terphenyl, glycerol) revealing in the low frequency range (typically 10 − 2 -10 2 rad/ s), a finite shear modulus and a finite viscous modulus.…”

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

“…In 2005, L. Noirez and co-authors reported on elastic behaviors in the isotropic phase of SCLCPs using a conventional rheometer and at the millimeter scale [14]. Then, solid-like behaviors (G′ and G″ constant with G′ > G″) were identified using the total wetting protocol [28] in various glass formers (glycerol, o-terphenyl) and ordinary polymers [29][30][31][32]. In 2007, S-Q Wang [55] reproduced the results of [15] using the same protocol.…”

“…The understanding of the shear-induced phase has thus to be related to a cohesive property hitherto neglected in the isotropic phase. 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 finally examine the case of a short alkyl chain assimilated to a spacer: The heptadecane.…”

Importance of Interfacial Interactions to Access Shear Elasticity of Liquids and Understand Flow Induced Birefringence from Liquid Crystals to Worm-Like Micellar Solutions

“…Recent experimental developments have shown that reinforcing the surface contacts using high energy surface improves the measurement of the dynamic response [14][15][16][17][18][19][20][21]. When the liquid molecules are strongly in interaction with the surface, the noise observable at low frequency grows and forms a well-defined signal, stronger than the viscous response (Fig.…”