Dynamics of Hairy-Rod Polymer Solutions in Simple Shear Flow:  Aging Effects

Abstract

We investigated the dynamics of solutions of a model hairy-rod polyester using optical rheometry and find that in the concentrated regime significant time effects dominate its response in simple shear flow. They are characterized by an enhanced viscosity and shear thinning behavior, accompanied by a corresponding increase of the absolute value of the induced birefringence and the relevant relaxation time upon cessation of shear. However, the most dramatic effect is the change of sign and large increase of the absolute value of the birefringence, signifying the dominance of form effects, associated with the formation of large anisotropic aggregates at longer times, which are oriented in the vorticity direction. This observation of anomalous birefringence is in qualitative agreement with the theoretical approach of Cates (J. Phys. II (Fr.) 1992, 2, 1109) who considered a presmectic local (anisotropic) ordering resulting from the interplay of particle interactions and the external field.

Dynamics of Nondilute Hairy-Rod Polymer Solutions in Simple Shear Flow

“…For this polymer, the number-average countour length is L n = 64.4 nm, and its persistence length is typically l = 20 nm, typical of wormlike chain conformation . The overlap concentration ( c * = 0.09 wt %) was determined from light scattering measurements at different concentrations . The onset of the concentrated regime is marked by the concentration c ** ∼ b -1 L e -2 , with b ≈ 1.2 nm the average diameter of the molecule and L e = 42 nm the effective contour length defined as 1 c ** was estimated to be around 2.5 wt %.…”

“…The various solutions were prepared in tetrachloroethane, a good solvent with relatively high boiling point (140 °C) to reduce evaporation problems, and density ρ = 1.6 g/mL, by adding the polymer to the solvent under strong stirring at 80 °C for 12 h. All solutions were used fresh; i.e., immediately after this treatment, they were cooled to 25 °C and loaded into the rheometer. In this way, the aging problem and its consequences were not a concern . The concentrations examined covered the range 0.36−7.24 wt %.…”

“…HPR's already investigated with rheo-optics include poly( n -hexyl isocyanate) and poly( p -phenylenes). , All are quite polydisperse in size and exhibit a tendency to form aggregates in solution, which essentially coexist with wormlike chains. , The latter systems represent a continuation of our above-mentioned longstanding program on the dynamics of well-characterized HRP's. The main findings from these studies suggest the presence of a two-step decay characterizing the relaxation of birefringence relating to different cluster sizes; the orientation of the formed aggregates in a direction perpendicular to the flow field with their constituents oriented along the flow. ,, An explanation of this remarkable effect calls for a kind of pretransitional smecticlike ordering resulting from the coupling of the external field and the steric interactions of the polymers …”

Rheo-optical Analysis of Functionalized Graphene Suspensions

“…However, turbidity alone does not allow one to measure both the dichroism and orientation angle of the distribution; moreover, turbidity requires rather concentrated systems. The drawbacks of turbidity measurements can be overcome by using linear flow dichroism. , Measuring the time- or rate-dependent evolution of the linear flow dichroism has the ability to elucidate the nature of particle motion and the average degree of alignment simultaneously and can be used even for nanoparticles. − Flow dichroism has been used in the past to study both rodlike and disklike nanoparticles in suspension. ,− In all cases where disklike nanoparticles are studied, the particles were made of stiff materials such as clay or gold. ,, Using flow dichroism and static light scattering, we study the behavior of functionalized graphene sheets (FGSs) in dispersion to investigate their 2D nature in suspension. Graphene oxide sheets have been suggested to exist in a crumpled state, , but more precise experiments found only relatively flat configurations .…”

Orientation dynamics of dilute functionalized graphene suspensions in oscillatory flow

“…These changes for anisotropic particles are directly related to change in the microstructural state [17][18][19][20][21][22]. Successful applications of this optical technique to determine flow-induced alignment of rodlike and disklike materials have been reported [15,21,[23][24][25][26][27][28][29] but they have been mainly focusing on steady-state shear flow or simple transient shear flow. In the present work we probe the orientation evolution in transient oscillatory flow for molecularly thin functionalized graphene sheet suspension.…”