Sadhan Jana

Tailoring Mechanical Properties of Aerogels for Aerospace Applications

Silica aerogels are highly porous solid materials consisting of three-dimensional networks of silica particles and are typically obtained by removing the liquid in silica gels under supercritical conditions. Several unique attributes such as extremely low thermal conductivity and low density make silica aerogels excellent candidates in the quest for thermal insulation materials used in space missions. However, native silica aerogels are fragile at relatively low stresses. More durable aerogels with higher strength and stiffness are obtained by proper selection of silane precursors and by reinforcement with polymers. This paper first presents a brief review of the literature on methods of silica aerogel reinforcement and then discusses our recent activities in improving not only the strength but also the elastic response of polymer-reinforced silica aerogels. Several alkyl-linked bis-silanes were used in promoting flexibility of the silica networks in conjunction with polymer reinforcement by epoxy.

Dispersion of nanofillers in high performance polymers using reactive solvents as processing aids

Melt mixing of nanoparticles with high performance polymers is not feasible due to severe shear heating and formation of particle aggregates. An alternative scheme was investigated in the present study involving the use of low molecular weight reactive solvents as processing aid and dispersing agent. Dispersion of nanosize fumed silica particles in polyethersulphone (PES) matrix was studied with the aid of small amounts of low molecular weight epoxy. Viscosity and processing temperatures of PES were significantly reduced and fumed silica particles were successfully dispersed to nanoscales. Epoxy component was polymerized after dispersion of fumed silica to recover the mechanical properties. Significant improvement in barrier resistance and heat deflection temperature over neat PES was observed.

Chaos, Symmetry, and Self-Similarity: Exploiting Order and Disorder in Mixing Processes

Fluid mixing is a successful application of chaos. Theory anticipates the coexistence of order and disorder—symmetry and chaos—as well as self-similarity and multifractality arising from repeated stretching and folding. Experiments and computations, in turn, provide a point of confluence and a visual analog for chaotic behavior, multiplicative processes, and scaling behavior. All these concepts have conceptual engineering counterparts: examples arise in the context of flow classification, design of mixing devices, enhancement of transport processes, and controlled structure formation in two-phase systems.

Shape memory polymers and their nanocomposites: a review of science and technology of new multifunctional materials.

This paper aims to present a review of recent progress made on shape memory polymers (SMPs) and their nanocomposites. Developments in allied fields are also presented in an effort to identify the current and future trends in this area. A new classification of SMP--rubberlike and mesomorphic systems--is suggested. The underlying physical mechanisms of shape memory actions, polymer-nanofiller interactions, and the resultant properties of SMP nanocomposites are discussed. Examples are presented to highlight the influence of processing conditions, filler geometry and filler surface characteristics, and the nature of matrix polymers on shape memory properties. A short description of current and potential applications is also presented.

Experimental and computational studies of mixing in complex Stokes flows: the vortex mixing flow and multicellular cavity flows

A complex Stokes flow has several cells, is subject to bifurcation, and its velocity field is, with rare exceptions, only available from numcrical computations. We present experimental and computational studies of two new complex Stokes flows: a vortex mixing flow and multicell flows in slender cavities. We develop topological relations between the geometry of the flow domain and the family of physically realizable flows; we study bifurcations and symmetries, in particular to reveal how the forcing protocol’s phase hides or reveals symmetries. Using a variety of dynamical tools, comparisons of boundary integral equation numerical computations to dye advection experiments are made throughout. Several findings challenge commonly accepted wisdom. For example, we show that higher-order periodic points can be more important than period-one points in establishing the advection template and extended regions of large stretching. We demonstrate also that a broad class of forcing functions produces the same qualitative mixing patterns. We experimentally verify the existence of potential mixing zones for adiabatic forcing and investigate the crossover from adiabatic to non-adiabatic behaviour. Finally, we use the entire array of tools to address an optimization problem for a complex flow. We conclude that none of the dynamical tools alone can successfully fulfil the role of a merit function; however, the collection of tools can be applied successively as a dynamical sieve to uncover a global optimum.

Effect of Plasticization of Epoxy Networks by Organic Modifier on Exfoliation of Nanoclay

Plasticization of cross-linked epoxy networks by hydrocarbon chains of quaternary ammonium ions and its effect on exfoliation behavior of nanoclay particles in mixtures of aromatic and aliphatic epoxies were investigated. It was found that quaternary ammonium ions, apart from catalyzing epoxy curing reactions, are capable of plasticizing cross-linked epoxy chains, the effect of which was observed in terms of large reduction in glass transition temperature and lowering of the values of storage modulus of cured epoxy networks. The effect of plasticization on storage modulus was found to be small for aromatic epoxy and large for aliphatic epoxy. As a consequence, the aromatic epoxy−clay system produced complete exfoliation of clay galleries, while the systems with mixtures of aliphatic and aromatic epoxy resulted in intercalated systems, even though the extent of curing of epoxy was the same in all cases.

Reinforcement of Silica Aerogels Using Silane-End-Capped Polyurethanes

Proper selection of silane precursors and polymer reinforcements yields more durable and stronger silica aerogels. This paper focuses on the use of silane-end-capped urethane prepolymer and chain-extended polyurethane for reinforcement of silica aerogels. The silane end groups were expected to participate in silica network formation and uniquely determine the amounts of urethanes incorporated into the aerogel network as reinforcement. The aerogels were prepared by one-step sol-gel process from mixed silane precursors tetraethoxysilane, aminopropyltriethoxysilane (APTES), and APTES-end-capped polyurethanes. The morphology and mechanical and surface properties of the resultant aerogels were investigated in addition to elucidation of chemical structures by solid-state (13)C and (29)Si nuclear magnetic resonance. Modification by 10 wt % APTES-end-capped chain-extended polyurethane yielded a 5-fold increase in compressive modulus and 60% increase in density. APTES-end-capped chain-extended polyurethane was found to be more effective in enhancement of mechanical properties and reduction of polarity.

Surface Modification and Reinforcement of Silica Aerogels Using Polyhedral Oligomeric Silsesquioxanes

This study evaluated polyhedral oligomeric silsesquioxane (POSS) molecules as useful, multifunctional reinforcing agents of silica aerogels. Silica aerogels have low-density and high surface area, although their durability is often compromised by the inherent fragility and strong moisture absorption behavior of the silica networks. POSS molecules carrying phenyl, iso-butyl, and cyclohexyl organic side groups, and several Si-OH functionalities were incorporated into silica networks via reactions between Si-OH functionalities in POSS molecules and silanes. Solid state (13)C and (29)Si NMR spectra established that greater than 90% of POSS molecules grafted onto silica networks and led to an increase in fractal dimensions. An almost 6-fold increase in compressive modulus was achieved with less than 5 wt % trisilanol phenyl POSS, and a 50-fold decrease in polarity with negligible changes in density were seen in aerogels modified with less than 5 wt % trisilanol isobutyl POSS.

From Reynolds's stretching and folding to mixing studies using horseshoe maps

Osborne Reynolds’s seminal idea of stretching and folding being the basis of fluid mixing has a direct bearing on the interpretation of mixing processes involving dynamical systems tools, in particular, horseshoe maps. Horseshoes offer the only direct, mathematically rigorous, experimental verification of chaos in a flow. In this work these ideas are formalized and developed, with the goal of exploiting the concepts in experimental mixing studies, particularly in the case of alternating doubly symmetric flows. Methods to represent and to identify horseshoes are developed. Application examples to three different flows—focusing primarily on errors arising from imperfect placement and reconstruction—are presented.

Compatibilization of PBT–PPE blends using low molecular weight epoxy

Abstract Compatibilization of blends of an immiscible polymeric system consisting of poly(butylene terephthalate) (PBT) and poly(phenylene ether) (PPE) by a set of low molecular weight epoxy, varying in chain flexibility and functionality, was studied in terms of morphology and mechanical properties. Evidences, such as reduction of epoxide peak in FT-IR spectra and viscosity and glass transition temperatures higher than PBT of melt-blended materials of PBT–epoxy indicated formation of PBT–epoxy copolymers, which in turn compatibilized and stabilized the morphology in PPE–PBT–epoxy blends. Significant improvements in tensile and impact strengths resulted.