Defect-mediated creep of structured materials

Abstract

Low-stress creep measurements on a nematic liquid crystal polymer indicate that it is a viscoelastic solid, with a modulus of 100 dynes/cm 2 and a yield stress of 50 dynes/cm 2 . Both smectics and nematics are viscoelastic solids at very low stress levels, with a modulus that is related to their defect texture. At stress levels somewhat above the yield stress, there is a yielding regime where the deformation rate and defect spacing are power laws in the applied stress. We understand these power laws using the ideas developed long ago by Orowan for the motion of line defects in crystalline solids. The exponents of these power laws are different for nematics and smectics, but the nematic universality class also appears to apply to superplastic metals and ceramics.

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

“…To our knowledge, the only prediction of long‐range elastic correlations in the (generic) liquid state is due to Volino 64. The equilibrium dynamic properties are predicted to be dimension‐dependent, which is in agreement with the dimensional character observed for the low‐frequency elasticity 5–16, 47…”

“…This high‐frequency zone corresponds precisely to the conventional viscoelastic curve of the shear modulus scaling as ω 2 (an experimental illustration is shown in Fig. 3(b) and elsewhere9–14). The viscoelastic behaviour is the product of the nonlinear regime of the elastic component.…”

“…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.…”

“…The low‐frequency elasticity is observable by improving the wettability of the substrate 9–14. Is it lost by damaging the boundary conditions?…”

Dynamic interfacial properties between a flexible‐chain polymer and a thermotropic liquid crystalline polymer investigated by an ellipsoidal drop retraction method

“…From the weighted relaxation spectrum of LC‐5000, we can find a peak with a relaxation time of about 50 s and an increase of λ* H (λ) with λ at λ > 10 3 s. The latter trend suggests a long relaxation time of LC‐5000 at λ > 10 3 s. An accurate determination of such a long relaxation time needs at least 10 4 s for the creep test, which is well beyond the time of our creep tests, 2400 s. Moreover, the storage modulus exhibits a plateau at low frequency, which is a characteristic solidlike behavior. Colby et al28 observed the same trend, arguing that a network of defect lines essentially makes the material a viscoelastic solid with a yield stress. Thus, we may conclude that there are two characteristic relaxation times for LC‐5000, λ 1 ≈ 50 s and λ 2 > 10 3 s. The long relaxation time λ 2 is related to the relaxation of TLCP textures and defects.…”