G. Harikrishnan

Probing nanodispersions of clays for reactive foaming

Nanodispersions of clays in polyurethane components have been prepared. Nanoclays (both natural and organically modified) of various aspect ratios are used. The fillers are dispersed separately in polyurethane components, viz., polyol and polyisocyanate. The nanodispersions are characterized by the combined use of solution rheology, X-ray scattering, cryo-electron microscopy, and IR spectroscopy. Reactive foaming of these nanodispersions is carried out to make polyurethane nanocomposite foams. The status of the dispersion of fillers in components and in foams has been compared to investigate the effect of the foaming process in exfoliation. Interpretation of the results from different characterization techniques describes the state of the dispersion of fillers in components and in foam. The rheological and physicochemical behaviors of nanodispersions are shown to have a significant influence on the properties of nanocomposite foams.

Thermally Tailored Gradient Topography Surface on Elastomeric Thin Films

We report a simple method for creating a nanopatterned surface with continuous variation in feature height on an elastomeric thin film. The technique is based on imprinting the surface of a film of thermo-curable elastomer (Sylgard 184), which has continuous variation in cross-linking density introduced by means of differential heating. This results in variation of viscoelasticity across the length of the surface and the film exhibits differential partial relaxation after imprinting with a flexible stamp and subjecting it to an externally applied stress for a transient duration. An intrinsic perfect negative replica of the stamp pattern is initially created over the entire film surface as long as the external force remains active. After the external force is withdrawn, there is partial relaxation of the applied stresses, which is manifested as reduction in amplitude of the imprinted features. Due to the spatial viscoelasticity gradient, the extent of stress relaxation induced feature height reduction varies across the length of the film (L), resulting in a surface with a gradient topography with progressively varying feature heights (hF). The steepness of the gradient can be controlled by varying the temperature gradient as well as the duration of precuring of the film prior to imprinting. The method has also been utilized for fabricating wettability gradient surfaces using a high aspect ratio biomimetic stamp. The use of a flexible stamp allows the technique to be extended for creating a gradient topography on nonplanar surfaces as well. We also show that the gradient surfaces with regular structures can be used in combinatorial studies related to pattern directed dewetting.

Clay nanoplatelet induced morphological evolutions during polymeric foaming

Remarkable evolutionary changes in cell morphology during reactive polymer nanocomposite foaming are observed by controlled foaming of suspensions of montmorillonite clay in the oligomeric polyurethane component. Delaminated nanoplatelets, when present as a networked cluster in suspensions, are shown to have very high efficiency in generating gas embryos for bubble nucleation. In the post-nucleation foaming period, clay nanoplatelets show an additional de-wetting behavior. The packing fraction of clay platelets in suspension and the consequent suspension rheology affect the final foam morphology.

A simple transient method for measurement of thermal conductivity of rigid polyurethane foams

Rigid polyurethane foams (PU) are widely used as thermal insulators in various applications. The thermal conductivity of the foam is the key parameter that governs the efficiency of thermal insulation provided by the foam. The usual technique employed to measure thermal conductivity is based on the rate of steady state heat transfer across a known thickness, induced by two different known temperatures at two opposite surfaces of the foam. We introduce a technique based on the transient measurement of heat transfer measured by an embedded needle probe. This technique is not only rapid but the instrumentation required for such a measurement is simple and the cost is only a fraction of the steady state counterpart. The values of thermal conductivity obtained by both methods are compared and found to agree within 4% over the range of 0.02—0.03 W/mK, which is the usual range of thermal conductivity for commercial rigid PU foams. The sensitivity of the needle probe technique is demonstrated by measuring the thermal conductivity values of foams made with various concentrations of chemical blowing agent (water). The present technique is also shown to be effective for measuring the thermal conductivity of small samples, especially, free rise cup foams for which the steady state technique can not be used.

An aqueous pathway to polymeric foaming with nanoclay

The property of osmotic swelling of a hectorite clay suspension in water is exploited for dispersing it in a polymeric foam matrix. This green route eludes the surface modification of clay as well as the mediation by organic solvents for clay dispersion during foaming. The route follows the initial swelling and partial delamination of clay galleries by water molecules, followed by further dispersion in an oligomeric isocyanate foaming component, before conducting reactive foaming. A nominal mass fraction of clay provides an effective barrier, restricting the gas phase mass transport in the foam. Electron microscopic investigations show that clay particles are preferentially located on the lamellar part of the microscopic foam cell, rather than on plateau borders.

Modeling Diffusion in Foamed Polymer Nanocomposites

Two-way multicomponent diffusion processes in polymeric nanocomposite foams, where the condensed phase is nanoscopically reinforced with impermeable fillers, are investigated. The diffusion process involves simultaneous outward permeation of the components of the dispersed gas phase and inward diffusion of atmospheric air. The transient variation in thermal conductivity of foam is used as the macroscopic property to track the compositional variations of the dispersed gases due to the diffusion process. In the continuum approach adopted, the unsteady-state diffusion process is combined with tortuosity theory. The simulations conducted at ambient temperature reveal distinct regimes of diffusion processes in the nanocomposite foams owing to the reduction in the gas-transport rate induced by nanofillers. Simulations at a higher temperature are also conducted and the predictions are compared with experimentally determined thermal conductivities under accelerated diffusion conditions for polyurethane foams reinforced with clay nanoplatelets of varying individual lamellar dimensions. Intermittent measurements of foam thermal conductivity are performed while the accelerated diffusion proceeded. The predictions under accelerated diffusion conditions show good agreement with experimentally measured thermal conductivities for nanocomposite foams reinforced with low and medium aspect-ratios fillers. The model shows higher deviations for foams with fillers that have a high aspect ratio.

Polymer concentration regulated aging in aqueous Laponite suspensions

By rheologically examining poly(vinyl alcohol)-water-Laponite multicomponent suspensions, we report evidence of concentration of adsorbing polymer strictly regulating the aging dynamics in multicomponent Laponite suspensions. The study is performed in the dilute polymer concentration regime. We analyze the observed aging pattern by fixing the non-monotonic transitions observed between temporally achieved suspension elastic moduli and polymer concentration, as the border of a sub-regime. This divides the aging pattern into three sub-regimes, across which the suspension elastic modulus shows an oscillatory relationship with polymer concentration. We also observe that suspensions belonging to first two sub-regimes remained colloidally stable during the timescale of measurement. In the third sub-regime, macrophase separation occurred, while suspensions were aging. For an explanation to this observation, we conducted three complementary measurements on select suspensions from each sub-regime, after they attained structurally arrested state. They are (a) small-angle neutron scattering, (b) structure recovery upon shear melting, and (c) zeta potential estimation. The collective evidences are analyzed to reach a possible explanation. The observation and analysis highlight the effect of concentration of adsorbing polymer on aging clay suspensions and have direct applications in stabilization, rheology modification, as well as polymer nanocomposite preparation.

Deducing Multiple Interfacial Dynamics during Polymeric Foaming

Several interfacial phenomena are active during polymeric foaming, the dynamics of which significantly influence terminal stability, cell structure, and in turn the thermomechanical properties of temporally evolved foam. Understanding these dynamics is important in achieving desired foam properties. Here, we introduce a method to simultaneously portray the time evolution of bubble growth, lamella thinning, and plateau border drainage, occurring during reactive polymeric foaming. In this method, we initially conduct bulk and surface shear rheology under polymerizing and nonfoaming conditions. In a subsequent step, foaming experiments were conducted in a rheometer. The microscopic structural dimensions pertaining to the terminal values of the dynamics of each interfacial phenomena are then measured using a combination of scanning electron microscopy, optical microscopy, and imaging ellipsometry, after the foaming is over. The measured surface and bulk rheological parameters are incorporated in time evolution equations that are derived from mass and momentum transport occurring when a model viscoelastic fluid is foamed by gas dispersion. Analytical and numerical solutions to these equations portray the dynamics. We demonstrate this method for a series of reactive polyurethane foams generated from different chemical sources. The effectiveness of our method is in simultaneously obtaining these dynamics that are difficult to directly monitor because of short active durations over multiple length scales.

Polymeric foaming with nanoscale nucleants: a surface nanobubble mechanism.

The dimensionally restricted, diffusion-driven volumetric change of almost flat nucleated surface nanobubbles hosted on dispersed nanoscale surfaces is proposed as the probable mechanism of heterogeneous bubble generation during polymer-nanoscale-nucleant suspension foaming. By conducting numerical simulations, this hypothesis is used to predict the final bubble sizes upon polymeric foaming with nanoscale nucleants and to compare them with reported experimentally determined values. The volumetric change in the bubble hosted on the miniscule surface is envisaged to occur due to two parallel diffusion processes: 1) through the contact line of the bubble cap with the surface, and 2) through the curved gas-polymer interface. The foaming conditions determine the direction and molar rate of both these diffusions. The mechanism explains the relative nucleating efficiency of nanoscale surfaces experimentally observed during reactive and nonreactive polymeric foaming by predicting the growth or dissolution of the bubble. In the case of nonreactive thermoplastic foaming, the size of the bubbles released to the bulk from the nanoscale surface varies in a near linear fashion with respect to the size of the nucleants, limited to a maximum nucleant size. Beyond this maximum, the size of bubble generated is independent of the nucleant size. However, increase in the initial nanoscopic contact angle does not significantly affect the bubble size upon detachment from the surface.