Ashish Lele

Rapid self-healing hydrogels

Synthetic materials that are capable of autonomous healing upon damage are being developed at a rapid pace because of their many potential applications. Despite these advancements, achieving self-healing in permanently cross-linked hydrogels has remained elusive because of the presence of water and irreversible cross-links. Here, we demonstrate that permanently cross-linked hydrogels can be engineered to exhibit self-healing in an aqueous environment. We achieve this feature by arming the hydrogel network with flexible-pendant side chains carrying an optimal balance of hydrophilic and hydrophobic moieties that allows the side chains to mediate hydrogen bonds across the hydrogel interfaces with minimal steric hindrance and hydrophobic collapse. The self-healing reported here is rapid, occurring within seconds of the insertion of a crack into the hydrogel or juxtaposition of two separate hydrogel pieces. The healing is reversible and can be switched on and off via changes in pH, allowing external control over the healing process. Moreover, the hydrogels can sustain multiple cycles of healing and separation without compromising their mechanical properties and healing kinetics. Beyond revealing how secondary interactions could be harnessed to introduce new functions to chemically cross-linked polymeric systems, we also demonstrate various potential applications of such easy-to-synthesize, smart, self-healing hydrogels.

In situ rheo-x-ray investigation of flow-induced orientation in layered silicate–syndiotactic polypropylene nanocomposite melt

This article describes experimental results for both the rheology and flow-induced orientation of a series of intercalated syndiotactic polypropylene nanocomposites which were prepared by melt intercalation in the presence or absence of an i-PP/maleic anhydride copolymer. The nanocomposites showed typical rheological signatures of well-dispersed interacalated nanocomposites such as a low frequency plateau in dynamic moduli and an apparent yield transition from very high viscosity at low shear stresses to low viscosity above a yield stress. In situ x-ray diffraction (XRD) measurements during shear provided direct evidence of rheology-microstructure links in these materials. It was found that the clay tactoids could be easily oriented by shear and that a high degree of orientation can be achieved after the yield transition. Further, the rheo-XRD apparatus allowed measurements of the relaxation of orientation upon the cessation of flow. The orientation relaxation time matched the characteristic relaxation times estimated from independent rheological measurements well.

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.

Thermodynamics of hydrogen-bonded polymer gel-solvent systems

A statistical thermodynamic theory, which accounts for hydrogen-bonding interactions between polymeric gels and solvents, is developed. The theory is shown to provide quantitative predictions of swelling behavior of poly(ethylene oxide) gels in chloroform and water and qualitative predictions of thermoreversible volume transitions of poly(N-isopropyl acrylamide) (PNIPA) gel in water. At the LCST of PNIPA gel, the theory predicts a sharp increase in the number of hydrogen-bonds formed between polymer molecules of the gel and a sharp decrease in the hydrogen-bonds formed between polymer molecules and water molecules. The predictions of this theory can have significant implications in designing smart gels based on hydrogen-bonding interactions. Such gels have applications in separations and in biomedical technology.

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.

Designing new thermoreversible gels by molecular tailoring of hydrophilic-hydrophobic interactions

We have shown that the lattice fluid hydrogen bond (LFHB) model can successfully quantify the first-order volume transition in hydrogels. The model predicts that a critical balance of hydrophilic and hydrophobic interactions is required for a gel to exhibit a discontinuous volume transition. In this work we will report the swelling behavior of a new thermoreversible copolymer hydrogel, which has been synthesized from two monomers, whose homopolymers do not show any volume transition in water in the observable range of temperatures. The discontinuous volume transition phenomenon in the copolymer gel was observed only at a critical balance of hydrophilic-hydrophobic interactions. The discontinuous nature of the volume transition is lost with a subtle change in the hydrophilic-hydrophobic balance. The copolymer gel was synthesized from 2-acrylamido 2-methyl propane sulfonic acid (AMPS), which is a hydrophilic monomer, and N-tertiary butylacrylamide (N-t-BAm), which is a hydrophobic monomer. The hydrophilic-h...

Molecular tailoring of thermoreversible copolymer gels: Some new mechanistic insights

We earlier reported the role of hydrophobic and hydrogen bonding interactions on the transition temperatures of thermoreversible copolymer gels. We show here that the chemical structure of the hydrophobe and its concentration determine the transition temperatures [lower critical solution temperature (LCST)] and the heat of transition of new hydrophobically modified poly(N-isopropyl acrylamide) [PNIPAm] copolymer gels. The gels, prepared by copolymerizing NIPAm monomer with hydrophobic comonomers containing increasing lengths of alkyl side groups and a terminal carboxyl acid group, showed lower LCST and lower heat of transition when compared to pure PNIPAm gel. The experimental results were also compared with theoretical calculations based on a lattice-fluid-hydrogen-bond [LFHB] model. We show experimentally and theoretically that a linear correlation exists between the transition temperature and length of the hydrophobic alkyl side group. Also, in apparent contradiction to previous work, we found a reduct...

Thermoreversible hydrogel based on radiation induced copolymerisation of poly(N-isopropyl acrylamide) and poly(ethylene oxide)

Thermoreversible copolymer hydrogel based on poly(ethylene oxide) and poly ( N -isopropyl acrylamide) has been prepared by γ-radiation technique. The utility of 13 C n.m.r. spectroscopy in elucidating the structure and copolymer composition has been demonstrated. The volume transition as a function of temperature in these copolymers has been studied by swelling ratio measurements. Unlike poly(ethylene oxide) homopolymer gel, the copolymer gels show first order volume transition in the temperature range of 35–40°C. These gels are easy to synthesise in any shape and size and are found to be having good mechanical strength even in the fully swollen state. They can have potential applications in controlled drug delivery, bioseparations and biomedical fields.

PREDICTION OF RE-ENTRANT SWELLING BEHAVIOR OF POLY(N-ISOPROPYL ACRYLAMIDE)GEL IN A MIXTURE OF ETHANOL-WATER USING LATTICE FLUID HYDROGEN BOND THEORY

The re-entrant volume phase transition of poly(N-isopropyl acrylamide) gel in ethanol–water mixtures has been predicted by using the extended lattice-fluid-hydrogen bond (LFHB) theory. In our calculations we do not make any arbitrary assumptions for the polymer–solvent interaction parameters. Instead, we determine the interaction parameters by fitting the LFHB theory to the swelling data of the gel in each of the solvents. In addition to predicting the re-entrant transition, the theory predicts selective absorption of ethanol over water, particularly by the collapsed gel. Simultaneously, the hydrogen bonding between water and ethanol is predicted to be enhanced in the presence of the gel. The interpolymer hydrogen bonds increase during the gel collapse region. The polymer–ethanol hydrogen bonds increase and the polymer–water hydrogen bonds decrease continuously with increasing ethanol composition in the outside phase. These predictions are in qualitative agreement with experimental observations and overcome the empiricism in previous theoretical work. A variety of qualitatively different swelling behaviors of gels in mixed solvents is also predicted for varying hydrophilic–hydrophobic balance in the chemical structure of the gels.

Necking in extrusion film casting: The role of macromolecular architecture

Extrusion film casting (EFC) is used on an industrial scale to produce several thousand tons of polymer films and coatings. While significant research has been carried out on necking of films of viscoelastic melts in EFC, the influence of macromolecular chain architecture on the necking behavior is not yet fully understood. In the present research, we have explored experimentally and theoretically the effects of long chain branching and molecular weight distribution on the extent of necking during EFC. Polyethylenes of essentially linear architecture but having narrow and broad molecular weight distributions, and polyethylenes having long chain branching were used for experimental studies. The EFC process was analyzed using the one-dimensional flow model of Silagy et al. [Polym. Eng. Sci. 36(21), 2614–2625 (1996)] in which multimode molecular constitutive equations namely the “extended pom-pom” equation (for long chain branched polymer melts) and the “Rolie–Poly (Rouse linear entangled polymers)” equation...