Shear-induced Structural Changes Research Articles

Variation of dynamic viscoelasticity during yielding and post-yield relaxation in a glassy polymer

A key factor causing the non-linear viscoelastic behaviors in largely deformed glassy polymers has been considered to be shear-induced structural change. To obtain further information on the molecular kinetics of the structural change, the dynamic viscoelastic properties of glassy polymethyl methacrylate were measured during its uniaxial elongation and subsequent relaxation processes. The dynamic data obtained in the yielding process showed that the structure of the quasi-equilibrium glass was successively changed into another stable one exhibiting much faster stress relaxation. in the post-yield relaxation process, on the other hand, the dynamic properties gradually reverted with time, presumably indicating structural recovery in the glass. Furthermore, some hardening mechanism was directly observed in a fully stress-relaxed state of the largely deformed glass.

Shear-Induced Structural Changes Observed in Concentrated Suspensions of Binary Blends of Fluorescent Particles of Differing Size and Color

AbstractShear-induced structural changes were observed in suspensions of binary blends of fluorescent particles of differing size and color. Median particle size ranged from 8.3 to 13.7 microns for the nearly spherical fluorescent particles. Concentrated aqueous suspensions were studied under shear flow in a Couette fixture modified to accommodate a fused silica cup. The color of the fluorescent particle blends was observed through the cup wall using photometric methods. Suspension color near the cup wall depended upon shear rate. A color shift occurred near the shear thinning to shear thickening transition. With increasing shear rate, the color contribution from the larger particles increased as the thickening transition occurred. Draining films of these suspensions on a flat fused silica plate were also studied using photometric readings from both the free surface and through the silica plate. Color measurements indicated highly non-uniform particle size distributions within the draining films, which varied with drain time.

Shearing apparatus for neutron scattering studies on fluids: Preliminary results for colloidal suspensions

A Couette-type concentric cylinder apparatus to investigate liquids at equilibrium and under shear has been constructed and tested. The apparatus is designed for a neutron facility and is optimized as a general purpose adjunct to the small-angle neutron scattering (SANS) equipment. It is versatile and rugged; a wide range of shear rates and operating temperatures can be covered; and controls are fully automated. Test results with sheared colloidal suspensions of 91-nm polystyrene spheres are presented. Evidence of shear-induced structure changes is clear.

Mechanism of Shear-Induced Structural Changes in Liquid Crystals-Cholesteric Mixtures

The reflection spectrum of the cholesteric Grandjean texture has been obtained as a function of mechanical shear. Data are explained in terms of a layered structure consisting of tilted and untilted helical Grandjean texture and a dynamic focal-conic texture for shear rates less than 103 sec−1. The correlated helical structure is shown to tilt under applied shear with little distortion. At shear rates greater than 103 sec−1, homeotropic texture is generated. A theoretical model is proposed which explains observed spectral intensity and wavelength data.