Saad Khan - Academia.edu (original) (raw)
Papers by Saad Khan
Solid State Ionics, 1998
We report results from our studies on composite polymer electrolytes based on novel surface-modif... more We report results from our studies on composite polymer electrolytes based on novel surface-modified fumed silicas. The electrolytes were prepared by dispersing fumed silica in a matrix formed by methyl-capped polyethylene glycol and lithium salt. Silicas with widely different surface chemistries were synthesized in order to study the effects of surface modification, with the attached surface groups ranging from non-polar alkyl moieties (C or C) to polar polyethylene oxide (PEO) 1 8 oligomers (MW | 200). We find, rather surprisingly, that the conductivity is independent of the type of surface group present on the silica. Moreover, the conductivity decreases only slightly on addition of fumed silica, even at high weight fraction of solids. In contrast, the rheological properties of the composites are strongly affected by both the silica surface chemistry and weight fraction. Dynamic rheology measurements reveal that fumed silicas with silanol and octyl coverage both flocculate into gels (networks). The resulting materials are mechanically stable, with the elastic modulus of the gel being strongly dependent upon weight fraction of solids. The PEO-modified silica, in contrast, gives rise to a low-viscosity suspension where the silica units exist as distinct, non-interacting species. The findings of this study have significant implications for future work on composite electrolytes, in that we can tailor the mechanical properties of the system without affecting the electrochemical behavior.
Rheologica Acta, 1987
The experimental properties of different polymer melts, polystyrene, high density polyethylene an... more The experimental properties of different polymer melts, polystyrene, high density polyethylene and low density polyethylene are compared for the first time in three different deformations: step shear, step biaxial extension and steady uniaxial extension. Properties of three other melts are also studied in step biaxial and shear experiments. For our comparative purposes some data of Laun and Winter from the literature are used, as well as new data reported here. In all the step strain experiments, the stresses can be factored into a time dependent relaxation modulus and a strain dependent damping function. The data are interpreted using a differential constitutive equation of Larson which satisfies this time-strain separability and has a single parameter that describes the strain softening character of the material. Results show that differences in the properties of the melts are most pronounced in uniaxial extension and least in biaxial extension. All melts follow the Doi-Edwards prediction relatively closely in biaxial extension. In uniaxial extension, the branched material shows a strong strain hardening effect although its shear and biaxial properties are similar to the other melts. The constitutive model gives a reasonably good fit to the data in all three deformations for unbranched materials for the same value of the adjustable parameter; the model, however, fails for the branched low density polyethylene.
Nanoscale Research Letters, 2010
Incorporation of nanoparticles composed of surface-functionalized fumed silica (FS) or native col... more Incorporation of nanoparticles composed of surface-functionalized fumed silica (FS) or native colloidal silica (CS) into a nanostructured block copolymer yields hybrid nanocomposites whose mechanical properties can be tuned by nanoparticle concentration and surface chemistry. In this work, dynamic rheology is used to probe the frequency and thermal responses of nanocomposites composed of a symmetric poly(styrene-b-methyl methacrylate) (SM) diblock copolymer and varying in nanoparticle concentration and surface functionality. At sufficiently high loading levels, FS nanoparticle aggregates establish a load-bearing colloidal network within the copolymer matrix. Transmission electron microscopy images reveal the morphological characteristics of the nanocomposites under these conditions. Keywords Block copolymer Á Colloidal network Á Nanostructured polymer Á Nanocomposite Á Silica Á Nanoparticles Á Fumed silica Á Colloidal silica
Macromolecular Chemistry and Physics, 2004
Journal of Rheology, 2003
Evidence of wall slip and magnitude of yield stress are examined for colloidal gels consisting of... more Evidence of wall slip and magnitude of yield stress are examined for colloidal gels consisting of hydrophobic silica, polyether, and lithium salts using geometries with serrated, smooth, hydrophilic and hydrophobic surfaces. Serrated plates, which provide minimal wall slip, are used to compare different methods of measuring yield stress: conventional extrapolation of shear stress in steady shear experiments and dynamic experiments at large strain amplitudes. In the latter, the yield stress is denoted by the maximum in the elastic stress, the product of the elastic modulus and strain (G Ј ␥), when plotted as a function of strain amplitude. Although excellent agreement is observed in the yield stress values using both these techniques, the dynamic method seems preferable considering its experimental ease, accuracy, and lack of extrapolation. In the presence of smooth geometries, the silica gels show evidence of wall slip with a concomitant decrease in yield stress. Using underestimation of yield stress as a measure of wall slip, we find slip to be unaffected by changes in the gel modulus obtained through incorporation of additional silica or salts. The use of smooth surfaces compared to serrated surfaces leads to approximately a 60% reduction in yield stress for all such samples. Finally, control of wall slip is attempted using plates modified to have different surface energies. Hydrophobic plates reduce slip significantly and produce data comparable to those with the serrated plates. In contrast, hydrophilic plates have minimal effect on slip and produce data analogous to those obtained using smooth plates. These results can be explained based on the fact that the particle-lean layer, responsible for slip, remains so with hydrophilic plates as it repels the hydrophobic silica particles in favor of the polar solvent. In contrast, the hydrophobic silica interacts with the hydrophobic plates, thus reducing slip.
Journal of Rheology, 1997
Rheological and photophysical data are presented for a hydrophobically modified alkali-soluble co... more Rheological and photophysical data are presented for a hydrophobically modified alkali-soluble copolymer, of a constitution similar to materials currently employed as rheology modifiers in water-borne coatings. The copolymer comprises a polyelectrolyte backbone bearing ethoxylate side chains capped with complex alkylaryl groups of a high molar volume. In aqueous alkaline media, the hydrophobes associate dynamically, the topology of the network so formed being dependent on the polymer concentration. Photophysical studies, employing pyrene as a hydrophobic fluorescent probe, indicate the presence of hydrophobic associations. At concentrations below the coil overlap concentration, c*, these associations are predominantly intramolecular. At higher polymer concentrations, intermolecular interactions become more probable. This change in network topology is in qualitative agreement with previous theoretical considerations of associative polymer systems and is reflected in an unusually high...
Journal of Rheology, 1999
Rheological techniques are used to probe the behavior of hydrophobic alkali-swellable emulsion (H... more Rheological techniques are used to probe the behavior of hydrophobic alkali-swellable emulsion (HASE) polymers, bearing n-alkyl hydrophobes, in aqueous alkaline media. The polymers possess a comb-like architecture with a polyelectrolyte backbone (ethyl acrylate-co-methacrylic acid) and hydrophobes (∼16 per polymer chain) tethered to the backbone via polyether side chains. The size of the hydrophobes is varied from n-C8 to n-C20 in this study. It is shown that, at such a level of hydrophobic modification, and at relatively high polymer concentrations, the microstructure in these polymer systems is akin to that existing in concentrated microgels. Thus, the original polymer latex particles swell extensively in alkaline media and disintegrate to form a system of close-packed, compressible (“soft”) aggregates. This is reflected in the rheological response of the system where we observe a high steady shear viscosity with no zero-shear plateau at low shear rates followed by considerable sh...
Journal of Rheology, 2001
Colloidal interactions between particles in a dispersion can be tuned by grafting polymeric chain... more Colloidal interactions between particles in a dispersion can be tuned by grafting polymeric chains onto the surface of the particles. The affinity between the polymeric chains and the continuous-phase liquid controls the strength of these interactions. In our system the polymerliquid affinity is strongly influenced by temperature, and as a result, dramatic changes occur in the dispersion microstructure on heating. The system is an aqueous dispersion of polystyrene ͑PS͒ particles bearing grafted poly͑ethylene oxide͒ ͑PEO͒ chains of low molecular weight ͑ ϳ 2000͒. At room temperature, water is a good solvent for PEO chains, and the dispersion is a stable, low-viscosity sol. As temperature is increased, water becomes a progressively worse solvent for PEO. Beyond a temperature T c there is a sharp transition in microstructure from a stable sol to a volume-filling gel. The sol-gel transition is reversible and the transition temperature T c can be pinpointed using tan ␦ versus temperature plots. Remarkably, T c is more than 100°C lower than the temperature for PEO ͑2000͒ in water, i.e., the gelation occurs under significantly better-thanconditions. T c is independent of particle concentration, but is strongly influenced by the graft density of PEO chains on the particles. The higher the graft density, the higher the T c for gelation; conversely, at very low graft density, the samples are gels even at room temperature. Above T c , the elastic modulus (G Ј) of the gels reveals a power law dependence with particle volume fraction ͑͒, i.e., G Ј ϰ n. The power law exponent n is independent of the PEO graft density, implying that the various gels have a similar microstructure. We suggest that gelation is the result of a weak secondary minimum in the interparticle potential that can develop in the case of short stabilizing moieties and moderate solvent conditions.
Journal of Rheology, 1988
Stress relaxation behavior of concentrated solutions of high molecular weight polystyrene followi... more Stress relaxation behavior of concentrated solutions of high molecular weight polystyrene following a step shear strain is studied using both mechanical and optical birefringence techniques. Using the stress-optic law, which we find to be valid for our solutions, we obtain time-dependent shear stress and first normal stress difference values from birefringence measurement that are free of transducer compliance effects. Similar to the previously reported experimental observations of Fukuda, Osaki and Kurata, we obtain unusually low values for nonlinear shear moduli, much lower than the predictions of Do&Edwards model, for the sample with more than about 60 entanglements per molecule. Moreover, the shear stress and first normal stress difference data measured on this sample do not conform to the Lodge-Meissner relationship, especially at long times, suggesting the formation of regions in which the imposed strain is not homogeneously distributed.
Journal of Rheology, 1995
Suspensions of fumed silica exhibit a wide range of rheological properties, depending on the type... more Suspensions of fumed silica exhibit a wide range of rheological properties, depending on the type of microstructure present. At high silica concentrations, the rheological behavior is ‘‘gel-like,’’ due to the formation of a network consisting of interconnected silica flocs. When large amplitude oscillatory preshear is applied on these systems, the network linkages are disrupted, resulting in the formation of isolated flocs. In this study, we focus on the extent to which the network is restored on cessation of preshear. By applying small amplitude oscillations we can study the development of the elastic modulus (G′) with time, following disruptive shear. We find that the restoration of the network after preshear is instantaneous; however, G′ recovers to different levels depending on the amplitude of the imposed preshear strain. Contrary to expectations, larger preshear strains (which cause a greater degree of microstructural disruption) do not always lead to lower levels of recovered...
Journal of The Electrochemical Society, 2003
The ion-transport properties of composite electrolytes composed of oligomers of poly͑ethylene gly... more The ion-transport properties of composite electrolytes composed of oligomers of poly͑ethylene glycol͒ dimethyl ether, hydrophobic fumed silica, and Li͑CF 3 SO 2) 2 N ͑LiTFSI͒ are investigated using nuclear magnetic resonance ͑NMR͒, electrophoretic NMR ͑ENMR͒, ac impedance spectroscopy, and rheology. The effects of fumed silica and salt concentration on ionic conductivity, diffusivity of ions and oligomers, and lithium transference number (T Li) are examined at 30°C. The fumed silica forms a selfsupporting network with large pores such that the network, regardless of silica concentration, has little effect on ion-transport characteristics. Examination of the effect of salt on ion transport reveals a maximum ionic conductivity at around 1.06 M, which is attributed to a tradeoff of adding more charge carriers balanced against increased ion-ion interactions and reduced mobilities. T Li with respect to salt concentration surprisingly passes through a minimum around 0.35 M. The increase in T Li at higher concentrations is attributed to the mobilities of cations, anions, and solvating oligomer becoming constrained to the same value due to ''loss of free volume.'' The values of T Li at low salt concentrations ͑Ͻ0.35 M͒ are attributed to the ions existing in either a fully dissociated state or primarily as charged complexes. Results of T Li from ENMR and from estimation via pulse field gradient NMR ͑pfg-NMR͒ are compared showing that pfg-NMR consistently overestimates T Li. Finally, a comparison is presented of measured conductivity with that calculated from the Nernst-Einstein equation and diffusivities found from pfg-NMR measurement; we discuss possible reasons why it is inappropriate to estimate ion-pair formation by this comparison.
Journal of The Electrochemical Society, 2004
Lithium dendrite formation is investigated via in situ microscopy in a liquid electrolyte contain... more Lithium dendrite formation is investigated via in situ microscopy in a liquid electrolyte containing polyethylene glycol dimethyl ether ϩ lithium bis͑trifluoromethylsulfonyl͒imide and composite gel-like electrolytes formed by dispersing nanometer-size fumed silica into the liquid. Fumed silicas with either hydrophilic silanol surface groups or hydrophobic octyl surface groups were employed. Dendrites with current density-dependent morphology are formed in liquid electrolyte but addition of fumed silica inhibits their formation, with hydrophilic fumed silica having a more pronounced effect than hydrophobic silica. The dendrite inhibition effect of fumed silica is attributed to its abilities to form a continuous network with elastic-like properties and scavenge impurities from the electrolyte.
Journal of The Electrochemical Society, 2002
Composite electrolytes consisting of methyl-capped poly͑ethylene glycol͒ oligomer ͑Mw Ϸ 250͒, lit... more Composite electrolytes consisting of methyl-capped poly͑ethylene glycol͒ oligomer ͑Mw Ϸ 250͒, lithium bis͑trifluoromethylsul-fonyl͒imide ͑Li:O ϭ 1:20͒, and fumed silica were investigated. In particular, the effects of fumed silica-surface chemistry and weight percentage in the composite on cycling behavior of Li/electrolyte/Li, Li͑Ni͒/electrolyte/Li, and Li/electrolyte/metal oxide cells were studied. Four types of fumed silicas with various surface groups were employed, A200 ͑native hydroxyl groups͒, R805 ͑octyl-modified͒, R974 ͑methyl-modified͒, and FS-EG3 ͑ethylene oxide-modified͒. The presence of fumed silica enhances lithium cyclability by reducing the interfacial resistance and cell-capacity fading, regardless of surface chemistry. However, the extent of the enhancing effect of fumed silica strongly depends on its surface chemistry, with the largest effect seen with A200 and the least effect seen with FS-EG3. Increasing fumed silica weight fraction intensifies the stabilizing effect.
Journal of The Electrochemical Society, 2002
Conductivity and lithium-ion transference numbers are reported for physically gelled composite el... more Conductivity and lithium-ion transference numbers are reported for physically gelled composite electrolytes using lithium hectorite clay as the charge carrier and carbonate solvents ͑ethylene carbonate, propylene carbonate, and dimethyl carbonate͒. Results are compared with those of typical lithium-ion battery electrolytes based on lithium hexafluorophosphate ͑LiPF 6 ͒ and carbonate solvents. Room-temperature conductivities of the composite electrolytes as high as 2 ϫ 10 Ϫ4 S/cm were measured. Because of the nature of the anionic clay particulates creating the gel structure, near-unity lithium-ion transference numbers are expected and were observed as high as 0.98, as measured by the dc polarization method using lithium-metal electrodes. Since the carbonates react with lithium and create mobile ionic species that significantly reduce the observed lithium-ion transference number, care must be taken to minimize or eliminate the presence of the reaction-formed ionic species. These hectorite-based composite systems are possible electrolytes for rechargeable lithium-ion batteries requiring high discharge rates.
Solid State Ionics, 1998
We report results from our studies on composite polymer electrolytes based on novel surface-modif... more We report results from our studies on composite polymer electrolytes based on novel surface-modified fumed silicas. The electrolytes were prepared by dispersing fumed silica in a matrix formed by methyl-capped polyethylene glycol and lithium salt. Silicas with widely different surface chemistries were synthesized in order to study the effects of surface modification, with the attached surface groups ranging from non-polar alkyl moieties (C or C) to polar polyethylene oxide (PEO) 1 8 oligomers (MW | 200). We find, rather surprisingly, that the conductivity is independent of the type of surface group present on the silica. Moreover, the conductivity decreases only slightly on addition of fumed silica, even at high weight fraction of solids. In contrast, the rheological properties of the composites are strongly affected by both the silica surface chemistry and weight fraction. Dynamic rheology measurements reveal that fumed silicas with silanol and octyl coverage both flocculate into gels (networks). The resulting materials are mechanically stable, with the elastic modulus of the gel being strongly dependent upon weight fraction of solids. The PEO-modified silica, in contrast, gives rise to a low-viscosity suspension where the silica units exist as distinct, non-interacting species. The findings of this study have significant implications for future work on composite electrolytes, in that we can tailor the mechanical properties of the system without affecting the electrochemical behavior.
Rheologica Acta, 1987
The experimental properties of different polymer melts, polystyrene, high density polyethylene an... more The experimental properties of different polymer melts, polystyrene, high density polyethylene and low density polyethylene are compared for the first time in three different deformations: step shear, step biaxial extension and steady uniaxial extension. Properties of three other melts are also studied in step biaxial and shear experiments. For our comparative purposes some data of Laun and Winter from the literature are used, as well as new data reported here. In all the step strain experiments, the stresses can be factored into a time dependent relaxation modulus and a strain dependent damping function. The data are interpreted using a differential constitutive equation of Larson which satisfies this time-strain separability and has a single parameter that describes the strain softening character of the material. Results show that differences in the properties of the melts are most pronounced in uniaxial extension and least in biaxial extension. All melts follow the Doi-Edwards prediction relatively closely in biaxial extension. In uniaxial extension, the branched material shows a strong strain hardening effect although its shear and biaxial properties are similar to the other melts. The constitutive model gives a reasonably good fit to the data in all three deformations for unbranched materials for the same value of the adjustable parameter; the model, however, fails for the branched low density polyethylene.
Nanoscale Research Letters, 2010
Incorporation of nanoparticles composed of surface-functionalized fumed silica (FS) or native col... more Incorporation of nanoparticles composed of surface-functionalized fumed silica (FS) or native colloidal silica (CS) into a nanostructured block copolymer yields hybrid nanocomposites whose mechanical properties can be tuned by nanoparticle concentration and surface chemistry. In this work, dynamic rheology is used to probe the frequency and thermal responses of nanocomposites composed of a symmetric poly(styrene-b-methyl methacrylate) (SM) diblock copolymer and varying in nanoparticle concentration and surface functionality. At sufficiently high loading levels, FS nanoparticle aggregates establish a load-bearing colloidal network within the copolymer matrix. Transmission electron microscopy images reveal the morphological characteristics of the nanocomposites under these conditions. Keywords Block copolymer Á Colloidal network Á Nanostructured polymer Á Nanocomposite Á Silica Á Nanoparticles Á Fumed silica Á Colloidal silica
Macromolecular Chemistry and Physics, 2004
Journal of Rheology, 2003
Evidence of wall slip and magnitude of yield stress are examined for colloidal gels consisting of... more Evidence of wall slip and magnitude of yield stress are examined for colloidal gels consisting of hydrophobic silica, polyether, and lithium salts using geometries with serrated, smooth, hydrophilic and hydrophobic surfaces. Serrated plates, which provide minimal wall slip, are used to compare different methods of measuring yield stress: conventional extrapolation of shear stress in steady shear experiments and dynamic experiments at large strain amplitudes. In the latter, the yield stress is denoted by the maximum in the elastic stress, the product of the elastic modulus and strain (G Ј ␥), when plotted as a function of strain amplitude. Although excellent agreement is observed in the yield stress values using both these techniques, the dynamic method seems preferable considering its experimental ease, accuracy, and lack of extrapolation. In the presence of smooth geometries, the silica gels show evidence of wall slip with a concomitant decrease in yield stress. Using underestimation of yield stress as a measure of wall slip, we find slip to be unaffected by changes in the gel modulus obtained through incorporation of additional silica or salts. The use of smooth surfaces compared to serrated surfaces leads to approximately a 60% reduction in yield stress for all such samples. Finally, control of wall slip is attempted using plates modified to have different surface energies. Hydrophobic plates reduce slip significantly and produce data comparable to those with the serrated plates. In contrast, hydrophilic plates have minimal effect on slip and produce data analogous to those obtained using smooth plates. These results can be explained based on the fact that the particle-lean layer, responsible for slip, remains so with hydrophilic plates as it repels the hydrophobic silica particles in favor of the polar solvent. In contrast, the hydrophobic silica interacts with the hydrophobic plates, thus reducing slip.
Journal of Rheology, 1997
Rheological and photophysical data are presented for a hydrophobically modified alkali-soluble co... more Rheological and photophysical data are presented for a hydrophobically modified alkali-soluble copolymer, of a constitution similar to materials currently employed as rheology modifiers in water-borne coatings. The copolymer comprises a polyelectrolyte backbone bearing ethoxylate side chains capped with complex alkylaryl groups of a high molar volume. In aqueous alkaline media, the hydrophobes associate dynamically, the topology of the network so formed being dependent on the polymer concentration. Photophysical studies, employing pyrene as a hydrophobic fluorescent probe, indicate the presence of hydrophobic associations. At concentrations below the coil overlap concentration, c*, these associations are predominantly intramolecular. At higher polymer concentrations, intermolecular interactions become more probable. This change in network topology is in qualitative agreement with previous theoretical considerations of associative polymer systems and is reflected in an unusually high...
Journal of Rheology, 1999
Rheological techniques are used to probe the behavior of hydrophobic alkali-swellable emulsion (H... more Rheological techniques are used to probe the behavior of hydrophobic alkali-swellable emulsion (HASE) polymers, bearing n-alkyl hydrophobes, in aqueous alkaline media. The polymers possess a comb-like architecture with a polyelectrolyte backbone (ethyl acrylate-co-methacrylic acid) and hydrophobes (∼16 per polymer chain) tethered to the backbone via polyether side chains. The size of the hydrophobes is varied from n-C8 to n-C20 in this study. It is shown that, at such a level of hydrophobic modification, and at relatively high polymer concentrations, the microstructure in these polymer systems is akin to that existing in concentrated microgels. Thus, the original polymer latex particles swell extensively in alkaline media and disintegrate to form a system of close-packed, compressible (“soft”) aggregates. This is reflected in the rheological response of the system where we observe a high steady shear viscosity with no zero-shear plateau at low shear rates followed by considerable sh...
Journal of Rheology, 2001
Colloidal interactions between particles in a dispersion can be tuned by grafting polymeric chain... more Colloidal interactions between particles in a dispersion can be tuned by grafting polymeric chains onto the surface of the particles. The affinity between the polymeric chains and the continuous-phase liquid controls the strength of these interactions. In our system the polymerliquid affinity is strongly influenced by temperature, and as a result, dramatic changes occur in the dispersion microstructure on heating. The system is an aqueous dispersion of polystyrene ͑PS͒ particles bearing grafted poly͑ethylene oxide͒ ͑PEO͒ chains of low molecular weight ͑ ϳ 2000͒. At room temperature, water is a good solvent for PEO chains, and the dispersion is a stable, low-viscosity sol. As temperature is increased, water becomes a progressively worse solvent for PEO. Beyond a temperature T c there is a sharp transition in microstructure from a stable sol to a volume-filling gel. The sol-gel transition is reversible and the transition temperature T c can be pinpointed using tan ␦ versus temperature plots. Remarkably, T c is more than 100°C lower than the temperature for PEO ͑2000͒ in water, i.e., the gelation occurs under significantly better-thanconditions. T c is independent of particle concentration, but is strongly influenced by the graft density of PEO chains on the particles. The higher the graft density, the higher the T c for gelation; conversely, at very low graft density, the samples are gels even at room temperature. Above T c , the elastic modulus (G Ј) of the gels reveals a power law dependence with particle volume fraction ͑͒, i.e., G Ј ϰ n. The power law exponent n is independent of the PEO graft density, implying that the various gels have a similar microstructure. We suggest that gelation is the result of a weak secondary minimum in the interparticle potential that can develop in the case of short stabilizing moieties and moderate solvent conditions.
Journal of Rheology, 1988
Stress relaxation behavior of concentrated solutions of high molecular weight polystyrene followi... more Stress relaxation behavior of concentrated solutions of high molecular weight polystyrene following a step shear strain is studied using both mechanical and optical birefringence techniques. Using the stress-optic law, which we find to be valid for our solutions, we obtain time-dependent shear stress and first normal stress difference values from birefringence measurement that are free of transducer compliance effects. Similar to the previously reported experimental observations of Fukuda, Osaki and Kurata, we obtain unusually low values for nonlinear shear moduli, much lower than the predictions of Do&Edwards model, for the sample with more than about 60 entanglements per molecule. Moreover, the shear stress and first normal stress difference data measured on this sample do not conform to the Lodge-Meissner relationship, especially at long times, suggesting the formation of regions in which the imposed strain is not homogeneously distributed.
Journal of Rheology, 1995
Suspensions of fumed silica exhibit a wide range of rheological properties, depending on the type... more Suspensions of fumed silica exhibit a wide range of rheological properties, depending on the type of microstructure present. At high silica concentrations, the rheological behavior is ‘‘gel-like,’’ due to the formation of a network consisting of interconnected silica flocs. When large amplitude oscillatory preshear is applied on these systems, the network linkages are disrupted, resulting in the formation of isolated flocs. In this study, we focus on the extent to which the network is restored on cessation of preshear. By applying small amplitude oscillations we can study the development of the elastic modulus (G′) with time, following disruptive shear. We find that the restoration of the network after preshear is instantaneous; however, G′ recovers to different levels depending on the amplitude of the imposed preshear strain. Contrary to expectations, larger preshear strains (which cause a greater degree of microstructural disruption) do not always lead to lower levels of recovered...
Journal of The Electrochemical Society, 2003
The ion-transport properties of composite electrolytes composed of oligomers of poly͑ethylene gly... more The ion-transport properties of composite electrolytes composed of oligomers of poly͑ethylene glycol͒ dimethyl ether, hydrophobic fumed silica, and Li͑CF 3 SO 2) 2 N ͑LiTFSI͒ are investigated using nuclear magnetic resonance ͑NMR͒, electrophoretic NMR ͑ENMR͒, ac impedance spectroscopy, and rheology. The effects of fumed silica and salt concentration on ionic conductivity, diffusivity of ions and oligomers, and lithium transference number (T Li) are examined at 30°C. The fumed silica forms a selfsupporting network with large pores such that the network, regardless of silica concentration, has little effect on ion-transport characteristics. Examination of the effect of salt on ion transport reveals a maximum ionic conductivity at around 1.06 M, which is attributed to a tradeoff of adding more charge carriers balanced against increased ion-ion interactions and reduced mobilities. T Li with respect to salt concentration surprisingly passes through a minimum around 0.35 M. The increase in T Li at higher concentrations is attributed to the mobilities of cations, anions, and solvating oligomer becoming constrained to the same value due to ''loss of free volume.'' The values of T Li at low salt concentrations ͑Ͻ0.35 M͒ are attributed to the ions existing in either a fully dissociated state or primarily as charged complexes. Results of T Li from ENMR and from estimation via pulse field gradient NMR ͑pfg-NMR͒ are compared showing that pfg-NMR consistently overestimates T Li. Finally, a comparison is presented of measured conductivity with that calculated from the Nernst-Einstein equation and diffusivities found from pfg-NMR measurement; we discuss possible reasons why it is inappropriate to estimate ion-pair formation by this comparison.
Journal of The Electrochemical Society, 2004
Lithium dendrite formation is investigated via in situ microscopy in a liquid electrolyte contain... more Lithium dendrite formation is investigated via in situ microscopy in a liquid electrolyte containing polyethylene glycol dimethyl ether ϩ lithium bis͑trifluoromethylsulfonyl͒imide and composite gel-like electrolytes formed by dispersing nanometer-size fumed silica into the liquid. Fumed silicas with either hydrophilic silanol surface groups or hydrophobic octyl surface groups were employed. Dendrites with current density-dependent morphology are formed in liquid electrolyte but addition of fumed silica inhibits their formation, with hydrophilic fumed silica having a more pronounced effect than hydrophobic silica. The dendrite inhibition effect of fumed silica is attributed to its abilities to form a continuous network with elastic-like properties and scavenge impurities from the electrolyte.
Journal of The Electrochemical Society, 2002
Composite electrolytes consisting of methyl-capped poly͑ethylene glycol͒ oligomer ͑Mw Ϸ 250͒, lit... more Composite electrolytes consisting of methyl-capped poly͑ethylene glycol͒ oligomer ͑Mw Ϸ 250͒, lithium bis͑trifluoromethylsul-fonyl͒imide ͑Li:O ϭ 1:20͒, and fumed silica were investigated. In particular, the effects of fumed silica-surface chemistry and weight percentage in the composite on cycling behavior of Li/electrolyte/Li, Li͑Ni͒/electrolyte/Li, and Li/electrolyte/metal oxide cells were studied. Four types of fumed silicas with various surface groups were employed, A200 ͑native hydroxyl groups͒, R805 ͑octyl-modified͒, R974 ͑methyl-modified͒, and FS-EG3 ͑ethylene oxide-modified͒. The presence of fumed silica enhances lithium cyclability by reducing the interfacial resistance and cell-capacity fading, regardless of surface chemistry. However, the extent of the enhancing effect of fumed silica strongly depends on its surface chemistry, with the largest effect seen with A200 and the least effect seen with FS-EG3. Increasing fumed silica weight fraction intensifies the stabilizing effect.
Journal of The Electrochemical Society, 2002
Conductivity and lithium-ion transference numbers are reported for physically gelled composite el... more Conductivity and lithium-ion transference numbers are reported for physically gelled composite electrolytes using lithium hectorite clay as the charge carrier and carbonate solvents ͑ethylene carbonate, propylene carbonate, and dimethyl carbonate͒. Results are compared with those of typical lithium-ion battery electrolytes based on lithium hexafluorophosphate ͑LiPF 6 ͒ and carbonate solvents. Room-temperature conductivities of the composite electrolytes as high as 2 ϫ 10 Ϫ4 S/cm were measured. Because of the nature of the anionic clay particulates creating the gel structure, near-unity lithium-ion transference numbers are expected and were observed as high as 0.98, as measured by the dc polarization method using lithium-metal electrodes. Since the carbonates react with lithium and create mobile ionic species that significantly reduce the observed lithium-ion transference number, care must be taken to minimize or eliminate the presence of the reaction-formed ionic species. These hectorite-based composite systems are possible electrolytes for rechargeable lithium-ion batteries requiring high discharge rates.