Metal-ion-mediated healing of gels

Several researchers have demonstrated the biomimicking attributes of stimuli responsive synthetic hydrogels such as sensitivity, selectivity, mobility, and memory. In this work we demonstrate yet another attribute, namely healing in hydrogels. We show that certain hydrogels having a flexible aliphatic side chain with a terminal carboxyl group can show healing in the presence of transition metal ions. On bringing two initially dried gel pieces into contact with each other in a dilute copper chloride solution the pieces were found to weld and the strength of the weld-line was found to increase gradually with time. The welded gel pieces could be separated by leaching out the metal ions and the separated gel pieces can be welded again by subsequent treatment with the metal ions

Deformation induced hydrophobicity: implications in spider silk formation

A theoretical framework, which considers the effect of strong inter-polymer associations on phase separation of a deforming polyacrylamide solution is presented. It is shown that deformation induces effective hydrophobicity in the stretched polymer chains resulting in the formation of strong cooperative hydrogen bonding between the polymer chains. This finding has implications in providing insights into the mechanisms of the way spiders spin silk by rapidly deforming a freshly secreted protein solution. It has been argued that in a manner analogous to the model system showed here, it is likely that a rapid deformation induces phase separation of the solution into a polymer rich phase, which eventually forms the fiber. This work also naturally provides strategic hints for making silk like strong fibers by synthetic means at mild conditions

A unified wall slip model

A unified slip model is developed, which predicts wall slip by either a disentanglement mechanism or by debonding mechanism, depending upon the adhesive energy of the wall-polymer pair. The model is based on the transient network theory, in which the activation processes of adsorption and desorption are considered to occur at the wall in parallel to the stretching of the adsorbed chains. It is shown that the stick-slip transition occurs due to the local non-monotonic flow behavior near the wall irrespective of the mechanism of slip. The model predictions of the critical wall shear stress are in good agreement with experimentally observed values of the critical stress for various adhesive energies of wall polymer pair. Another important prediction of the model is that the temperature dependence of the critical wall shear stress for debonding is different than that of disentanglement mechanism under certain experimental conditions. This may be useful for discerning the correct mechanism of slip. The unified model encompasses different systems (viz. entangled solutions and melts) and diverse mechanisms (viz. disentanglement and debonding) in a common mathematical framework

Slipping fluids: a unified transient network model

Wall slip in polymer solutions and melts play an important role in fluid flow, heat transfer and mass transfer near solid boundaries. Several different physical mechanisms have been suggested for wall slip in entangled systems. We look at the wall slip phenomenon from the point of view of a transient network model, which is suitable for describing both, entangled solutions and melts. We propose a model, which brings about unification of different mechanisms for slip. We assume that the surface is of very high energy and the dynamics of chain entanglement and disentanglement at the wall is different from those in the bulk. We show that severe disentanglement in the annular wall region of one radius of gyration thickness can give rise to non-monotonic flow curve locally in that region. By proposing suitable functions for the chain dynamics so as to capture the right physics, we show that the model can predict all features of wall slip, such as flow enhancement, diameter-dependent flow curves, discontinuous increase in flow rate at a critical stress, hysteresis in flow curves, the possibility of pressure oscillations in extrusion and a second critical wall shear stress at which another jump in flow rate can occur

Strain-Rate Frequency Superposition in Large-Amplitude Oscillatory Shear

In a recent work, Wyss, {\it et.al.} [Phys. Rev. Lett., {\bf 98}, 238303 (2007)] have noted a property of `soft solids' under oscillatory shear, the so-called strain-rate frequency superposition (SRFS). We extend this study to the case of soft solids under large-amplitude oscillatory shear (LAOS). We show results from LAOS studies in a monodisperse hydrogel suspension, an aqueous gel, and a biopolymer suspension, and show that constant strain-rate frequency sweep measurements with soft solids can be superimposed onto master curves for higher harmonic moduli, with the {\it same} shift factors as for the linear viscoelastic moduli. We show that the behavior of higher harmonic moduli at low frequencies in constant strain-rate frequency sweep measurements is similar to that at large strain amplitudes in strain-amplitude sweep tests. We show surface plots of the harmonic moduli and the energy dissipation rate per unit volume in LAOS for soft solids, and show experimentally that the energy dissipated per unit volume depends on the first harmonic loss modulus alone, in both the linear and the nonlinear viscoelastic regime.Comment: 10 pages, 25 figures, accepted for publication in Physical Review E. Incorporates referee comment

Exploring the utility of an axisymmetric semi-hyperbolic die for determining the transient uniaxial elongation viscosity of polymer melts

The estimation of elongation viscosity of polymeric fluids from pressure drop measurements across converging dies remains an attractive method due to its cost effectiveness. However, the elongation viscosity so measured can be contaminated by large shear effects at the walls unless specific stress boundary conditions can be imposed. Such experiments are in general difficult to perform. In this work we examine an alternative simpler method that relies on a combination of experimental pressure drop measurements in semi-hyperbolic dies and matching viscoelastic CFD simulations to obtain the transient elongation viscosity data. The calculations of the transient uniaxial elongation viscosity from this methodology for an LDPE melt are compared with data obtained using extensional rheometers in order to explore the utility of the strategy

Morphology of polymers precipitated from a supercritical solvent

The precipitation of polymers via the rapid expansion of a supercritical chlorodifluoromethane solution to ambient conditions across a fine diameter capillary has been studied experimentally. The morphology of the polymers precipitated-polycaprolactone, poly(methyl methacrylate) and a styrene/methyl methacrylate block copolymer-is influenced strongly by conditions of the expansion process. Conditions of high temperature, high polymer concentration, low pressure or low capillary L/D ratio enhance the formation of high aspect ratio fibers, while opposite conditions favor the formation of spherical particles of micron size. Each of the conditions favoring fiber formation favors precipitation farther upstream in the expansion process. Based on one-dimensional compressible flow calculations using a virial equation of state for pure solvent, it is proposed that fiber formation occurs when a polymer-rich phase is rejected from solution in the entry region to the capillary. The location of precipitation is shown to be crucial in determining the characteristic time scale for the density reduction process, which may be as small as 10-7 s

Dynamics of end-tethered chains at high surface coverage

A molecular model for wall slip based on recent tube theories is extended to account for the effects of entanglements between tethered chains that occur at higher surface coverage. Three regimes of surface coverage are identified. Regime I is a low surface coverage regime (the mushroom regime), which has been discussed earlier by us. In regime II the tethered chains undergo a cooperative constraint release process due to which the slip velocity increases with surface coverage while the slip length becomes independent of the surface coverage. In regime III the tethered chains start to become entangled with each other thereby causing the interfacial modulus, the critical wall shear stress and the critical shear rate to decrease with surface coverage. Our model is different from previous scaling models in that it provides a constitutive equation for tethered chains. As a result, it offers scope for quantitative prediction of microscopic and macroscopic experimental slip data based solely on molecular information. Our model also predicts scaling laws for the various slip parameters in the three regimes of surface coverage. These laws are in general agreement with previously reported scaling models and experimental data