Effects of strong anchoring on the dynamic moduli of heterogeneous nematic polymers

Choate, Eric P.; Cui, Zhenlu; Forest, M. Gregory

doi:10.1007/s00397-007-0235-2

Cited by 10 publications

(17 citation statements)

References 17 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…From a theoretical point of view, the dynamic properties of simple liquids are considered to be governed by short‐range interparticle interactions,1 whereas in high molecular weight systems (typically polymers in solution or in the bulk) these properties are supposed to be dominated by the intramolecular forces due to chain connectivity 2–4. However, recent converging experimental observations5–14 prove that this distinction is not relevant; in agreement with the pioneering works of Derjaguin et al ,15, 16 much larger solid‐like length‐scale correlations govern liquid dynamics. Conventional approaches do not take into account this macroscopic elasticity since it is postulated that the shear elasticity always vanishes at low frequency.…”

Section: Introductionmentioning

confidence: 84%

The missing parameter in rheology: hidden solid‐like correlations in viscous liquids, polymer melts and glass formers

Noirez

Baroni

Mendil‐Jakani

2009

Polymer International

View full text Add to dashboard Cite

BACKGROUND: Determination of dynamic relaxation consists of measuring the viscous and the elastic components of a material by generally applying a small (oscillatory) deformation. The shear stress is transmitted to the material via contact with a substrate. Dating at least back to Stokes, the no‐slip boundary condition between the fluid and the substrate is supposed to be fulfilled during this measurement. We show that the viscoelastic parameters of fluids are usually not determined under no‐slip boundary conditions and do not originate from the first linear regime. Viscous and viscoelastic fluids (entangled and unentangled polymers, glass formers) measured under no‐slip conditions exhibit a fundamentally different response with a dominant terminal solid‐like response. RESULTS: We show that the terminal behaviour of fluids such as liquid polymers or glass formers measured at the sub‐millimetre scale and far above the glass transition is not viscous but solid‐like. Instead of a viscoelastic behaviour scaling as ω and ω2 (ω is the frequency) for the viscous and the elastic moduli, respectively, the dynamic response is simplified; for low gap thickness, both viscous and elastic moduli are invariant with respect to the frequency (with the elastic modulus being larger than the viscous modulus) and enhanced by two to four orders of magnitude compared to the conventional viscoelastic response. Over a critical strain amplitude, the solid‐like response decreases and is progressively replaced by the conventional viscoelastic behaviour. We discuss the implications of this observation and reconsider the assumptions inherent to a rheology measurement. CONCLUSION: The identification of so far neglected macroscopic elasticity in the fluidic state far above the glass transition temperature in entangled and unentangled polymers and glass formers shows that the liquid state is dominated by long range intermolecular interactions. This information is fundamental to understand and to foresee dynamic behaviour; it sheds further light on nonlinear phenomena such as large time scale relaxations, rheo‐thinning, violation of the no‐slip boundary condition and spectacular shear‐induced instabilities (spurt effect, ‘shark‐skin’ instabilities, gross melt fracture, etc.) that are unpredictable in the frame of the conventional viscoelastic approach. It also implies that the viscoelastic times (reptation, Rouse) in polymers are not the longest relaxation times of these materials. Copyright © 2009 Society of Chemical Industry

show abstract

Section: Introductionmentioning

confidence: 84%

The missing parameter in rheology: hidden solid‐like correlations in viscous liquids, polymer melts and glass formers

Noirez

Baroni

Mendil‐Jakani

2009

Polymer International

View full text Add to dashboard Cite

show abstract

“…Experimental and theoretical literature illustrate the effect of the liquid crystal anchoring and its orientation on the dynamic response [43][44][45][46]. In the present study, the reinforcement of the boundary conditions is achieved by improving the wetting of the liquid to the substrate (alumina substrate) and by probing sub-millimetre gaps.…”

Section: Identification Of a Finite Low Frequency Solid-like Responsementioning

confidence: 94%

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

et al. 2012

View full text Add to dashboard Cite

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.

show abstract

“…2). Similar work on the large variation in director orientation for different (planar and homeotropic) anchoring conditions has also been modeled [16,17] in some depth.…”

mentioning

confidence: 93%

Small Surface Pretilt Strikingly Affects the Director Profile during Poiseuille Flow of a Nematic Liquid Crystal

2010

View full text Add to dashboard Cite

Conventional nematic liquid crystal cells are fabricated with small surface pretilt of the director induced by rubbed polymer alignment. Depending on the orientation of the bounding surfaces, this may lead to two slightly different untwisted director configurations, splay and parallel. This small difference leads to remarkably different director profiles during pressure-driven flow, observed here using optical conoscopy. Data show excellent agreement with numerical modeling from Leslie-Ericksen-Parodi theory.

show abstract

Effects of strong anchoring on the dynamic moduli of heterogeneous nematic polymers

Cited by 10 publications

References 17 publications

The missing parameter in rheology: hidden solid‐like correlations in viscous liquids, polymer melts and glass formers

The missing parameter in rheology: hidden solid‐like correlations in viscous liquids, polymer melts and glass formers

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

Small Surface Pretilt Strikingly Affects the Director Profile during Poiseuille Flow of a Nematic Liquid Crystal

Contact Info

Product

Resources

About