@article{talwar_scanu_raghavan_khan_2008, title={Influence of binary surfactant mixtures on the rheology of associative polymer solutions}, volume={24}, ISSN={["0743-7463"]}, DOI={10.1021/la801030n}, abstractNote={Hydrophobically modified alkali-soluble emulsion polymers (HASE) are a class of comblike associative polymers that can impart high viscosities to aqueous solutions. The rheology of HASE solutions can be tuned by the addition of surfactants, such as nonylphenol ethoxylates (NP e), where e is the length of the hydrophilic (ethoxylate) chain. While previous studies have considered individual surfactants, our focus here is on binary surfactant mixtures. We find that equimolar NP4-NP12 mixtures significantly enhance the zero-shear viscosities of HASE solutions as compared to equivalent amounts of NP8, especially at high overall surfactant concentrations. Dynamic rheological measurements suggest that the higher viscosities are due to increases in the lifetime of hydrophobic junctions in the polymer-surfactant network. In contrast to the above results, equimolar NP4-NP8 mixtures are rheologically identical to equivalent solutions of NP6. The differences between the two sets of mixtures are further correlated with cloud point measurements and thereby with the overall hydrophilic-lipophilic balance (HLB) of the surfactant system.}, number={15}, journal={LANGMUIR}, author={Talwar, Sachin and Scanu, Lauriane and Raghavan, Srinivasa R. and Khan, Saad A.}, year={2008}, month={Aug}, pages={7797–7802} }
 @article{chiou_raghavan_khan_2001, title={Effect of colloidal fillers on the cross-linking of a UV-curable polymer: Gel point rheology and the Winter-Chambon criterion}, volume={34}, ISSN={["1520-5835"]}, DOI={10.1021/ma010281a}, abstractNote={The effect of colloidal silica fillers on the cross-linking behavior of a model UV-curable polymer system (thiol-ene) is studied using in situ rheology and real-time FTIR spectroscopy. The validity of the Winter-Chambon criterion (convergence of the loss tangents at the gel point) is examined for the cross-linking of these filled polymers, some of which are strongly flocculated dispersions (physical gels) prior to chemical cross-linking. Two different types of colloidal silica particles are studied: one with octyl chains tethered to the surface and the other with methyl surface groups. The Winter-Chambon criterion is satisfied for all samples containing the methyl-terminated silica. However, the criterion breaks down for samples containing the octyl-modified silica, with the loss tangents not converging at any single point. This suggests the absence of a self-similar critical gel at the gel point in the latter case. Neither type of silica particles alters the mechanism of the cross-linking reaction, as revealed by FTIR spectroscopy, but they do retard the cross-linking kinetics. An alternate method is suggested for determining the chemical gel point of filled systems that fail to obey the Winter-Chambon criterion. This method involves monitoring the critical strain (limit of the linear viscoelastic region) at various UV exposure times. A dramatic increase is observed in the critical strain at the gel point, indicating a transition from weak, physical bonds to strong, covalent cross-links.}, number={13}, journal={MACROMOLECULES}, author={Chiou, BS and Raghavan, SR and Khan, SK}, year={2001}, month={Jun}, pages={4526–4533} }
 @article{shay_raghavan_khan_2001, title={Thermoreversible gelation in aqueous dispersions of colloidal particles bearing grafted poly(ethylene oxide) chains}, volume={45}, number={4}, journal={Journal of Rheology (New York, N.Y.)}, author={Shay, J. S. and Raghavan, S. R. and Khan, S. A.}, year={2001}, month={Jul}, pages={913–927} }
 @article{raghavan_hou_baker_khan_2000, title={Colloidal interactions between particles with tethered nonpolar chains dispersed in polar media: Direct correlation between dynamic rheology and interaction parameters}, volume={16}, ISSN={["0743-7463"]}, DOI={10.1021/la9815953}, abstractNote={Colloidal interactions between particles dispersed in a liquid can be suitably tailored by modifying the surface chemistry of the particles. In the case of fumed silica particles, the surface can be systematically altered from hydrophilic to hydrophobic by replacing a portion of the original silanol (Si−OH) groups by nonpolar alkyl chains. In this study, we probe the effect of surface modification of fumed silica on their rheology and microstructure in polar media. Variables of interest include the length of the tethered alkyl chain and the extent of surface coverage. For the continuous phase, we examine a range of polyether liquids comprising different architectures and molecular weights. We find that when the alkyl chains are C8 or longer, and are attached at saturation levels, a dense nonpolar surface layer is formed on each silica unit. Such particles experience strong interactions in polar media, leading to the formation of a volume-filling network (gel). We show that these interactions arise as a re...}, number={3}, journal={LANGMUIR}, author={Raghavan, SR and Hou, J and Baker, GL and Khan, SA}, year={2000}, month={Feb}, pages={1066–1077} }
 @article{raghavan_walls_khan_2000, title={Rheology of silica dispersions in organic liquids: New evidence for solvation forces dictated by hydrogen bonding}, volume={16}, ISSN={["0743-7463"]}, DOI={10.1021/la991548q}, abstractNote={Dispersions of hydrophilic fumed silica are investigated in a range of polar organic media. The silica forms stable, low-viscosity sols exhibiting shear thickening behavior in a host of liquids, including ethylene glycol and its oligomers and short-chain alcohols, such as n-propanol. In contrast, the silica flocculates into colloidal gels in other liquids, such as glycols with methyl end-caps and longer-chain alcohols. We suggest that there is a causal relationship between the hydrogen-bonding ability of the liquid and the colloidal microstructure observed. In strongly hydrogen-bonding liquids, a solvation layer is envisioned to form on the silica surface through hydrogen bonding between liquid molecules and surface silanol groups (Si−OH). This gives rise to short-range, non-DLVO repulsions (“solvation forces”) which stabilize the silica particles. In contrast, in the case of liquids with limited hydrogen-bonding ability, silanols on adjacent silica particles are envisioned to interact directly by hydroge...}, number={21}, journal={LANGMUIR}, author={Raghavan, SR and Walls, HJ and Khan, SA}, year={2000}, month={Oct}, pages={7920–7930} }
 @article{english_raghavan_jenkins_khan_1999, title={Associative polymers bearing n-alkyl hydrophobes: rheological evidence for microgel-like behavior}, volume={43}, number={5}, journal={Journal of Rheology (New York, N.Y.)}, author={English, R. J. and Raghavan, S. R. and Jenkins, R. D. and Khan, S. A.}, year={1999}, month={Sep}, pages={1175–1194} }
 @misc{khan_fedkiw_baker_fan_raghavan_hou_1999, title={Composite electrolyte containing surface modified fumed silica}, volume={5,965,299}, number={1999 Oct. 12}, publisher={Washington, DC: U.S. Patent and Trademark Office}, author={Khan, S. A. and Fedkiw, P. S. and Baker, G. L. and Fan, J. and Raghavan, S. R. and Hou, J.}, year={1999}, month={Oct} }
 @article{walker_raghavan_royer_smith_wignall_melnichenko_khan_spontak_1999, title={Enhanced miscibility of low-molecular-weight polystyrene polyisoprene blends in supercritical CO2}, volume={103}, ISSN={["1089-5647"]}, DOI={10.1021/jp990551f}, abstractNote={While ongoing efforts continue to explore the high-pressure phase equilibria of polymer blends, few studies have attempted to address the impact of a supercritical (sc) fluid on such equilibria. In this work, we report on the phase behavior of an upper critical solution temperature (UCST) polymer blend in the presence of supercritical carbon dioxide (scCO2), a nonselective plasticizing agent. Blends composed of low-molecular-weight polystyrene and polyisoprene have been examined as a function of temperature in scCO2 by visual inspection, small-angle neutron scattering, and spectrophotometry. In the presence of scCO2, the cloud point temperature is depressed by as much as 28 °C, depending on both blend composition and CO2 pressure. Complementary studies performed with nitrogen decouple the plasticization efficacy of CO2 from free-volume compression due to hydrostatic pressure. Existence of a pressure yielding a maximum in CO2-induced cloud point depression is established. These results provide evidence for...}, number={26}, journal={JOURNAL OF PHYSICAL CHEMISTRY B}, author={Walker, TA and Raghavan, SR and Royer, JR and Smith, SD and Wignall, GD and Melnichenko, Y and Khan, SA and Spontak, RJ}, year={1999}, month={Jul}, pages={5472–5476} }
 @article{raghavan_riley_fedkiw_khan_1998, title={Composite polymer electrolytes based on poly(ethylene glycol) and hydrophobic fumed silica: Dynamic rheology and microstructure}, volume={10}, ISSN={["0897-4756"]}, DOI={10.1021/cm970406j}, abstractNote={Dynamic rheological techniques are used to probe the microstructures present in fumed silica-based composite polymer electrolytes. These electrolytes are obtained by dispersing hydrophobic fumed silica in a poly(ethylene glycol)−lithium salt solution and display high conductivities (σ298K > 10-3 S/cm), mechanical stability, and easy processability. The materials behave as soft solids (gels) under ambient conditions due to the presence of a three-dimensional network of silica entities. Network formation occurs as a result of van der Waals (dispersion) forces between the nonpolar surface layers on silica units. Factors which affect the van der Waals interaction, and hence the gel network density, include the nature of the PEG end group, the presence of ionic species, and the size of the hydrophobic surface group on the silica. The composites also exhibit shear-thinning behavior due to the shear-induced disruption of network bonds, and this behavior can be advantageously utilized in electrolyte processing.}, number={1}, journal={CHEMISTRY OF MATERIALS}, author={Raghavan, SR and Riley, MW and Fedkiw, PS and Khan, SA}, year={1998}, month={Jan}, pages={244–251} }
 @article{fan_raghavan_yu_khan_fedkiw_hou_baker_1998, title={Composite polymer electrolytes using surface-modified fumed silicas: conductivity and rheology}, volume={111}, ISSN={["0167-2738"]}, DOI={10.1016/S0167-2738(98)00151-9}, abstractNote={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 (C1 or C8) to polar polyethylene oxide (PEO) 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.}, number={1-2}, journal={SOLID STATE IONICS}, author={Fan, J and Raghavan, SR and Yu, XY and Khan, SA and Fedkiw, PS and Hou, J and Baker, GL}, year={1998}, month={Aug}, pages={117–123} }
 @article{raghavan_khan_1997, title={Shear-thickening response of fumed silica suspensions under steady and oscillatory shear}, volume={185}, ISSN={["0021-9797"]}, DOI={10.1006/jcis.1996.4581}, abstractNote={Suspensions of fumed silica in polypropylene glycol exhibit shear-thickening under steady shear and "strain-thickening" under oscillatory shear. Strain-thickening refers to a sharp increase in the complex viscosity eta* observed at critical combinations of strain-amplitude and frequency. Two regimes of strain-thickening behavior have been found: The first occurs at high critical strains and low frequencies, whereas the second occurs at high critical frequencies and a constant lower strain. Strain-thickening in the first regime can be explicitly correlated with steady shear-thickening, using a modified version of the Cox-Merz rule. Accordingly, strain-thickening data for the complex viscosity eta* as a function of the maximum dynamic shear-rate gamma0omega can be superposed against shear-thickening data for the steady viscosity as a function of the steady shear rate. Such a correlation between the two kinds of thickening phenomena has not been reported previously. The combination of shear- and strain-thickening behavior can be qualitatively explained using a clustering mechanism, which attributes the various phenomena to the formation of temporary, flow-induced clusters. The two regimes of strain-thickening are a result of differences in the relative ease of cluster formation.}, number={1}, journal={JOURNAL OF COLLOID AND INTERFACE SCIENCE}, author={Raghavan, SR and Khan, SA}, year={1997}, month={Jan}, pages={57–67} }