@article{krishnan_spontak_2012, title={Factors affecting time-composition equivalence in ternary block copolymer/cosolvent systems}, volume={8}, ISSN={["1744-6848"]}, DOI={10.1039/c1sm06672b}, abstractNote={Time–temperature rheological equivalence is one of the most important and broadly used concepts developed with regard to the viscoelastic behavior of polymers. In this study, we explore the generality of an analogous relationship, time–composition equivalence, in several series of ternary block copolymer/cosolvent systems at ambient temperature. Of particular interest are triblock copolymers solvated with a miscible mixture of midblock-selective solvents to yield physical gels. Such gels, consisting of a midblock-rich network stabilized by glassy endblock-rich microdomains, exhibit remarkable elasticity. The copolymers employed here are styrenic thermoplastic elastomers, whereas the solvents include an aliphatic/alicyclic mineral oil and several different tackifying resins varying in molecular weight and, hence, viscosity. Despite changes in solvent properties, time–composition superpositioning (tCS) yields master curves wherein the composition shift factors consistently scale with cosolvent zero-shear viscosity. Corresponding scaling exponents vary linearly with copolymer concentration and change slope at a morphological transition. Failure of tCS at low frequencies can be largely avoided by implementing copolymers with high-molecular-weight endblocks.}, number={5}, journal={SOFT MATTER}, author={Krishnan, Arjun S. and Spontak, Richard J.}, year={2012}, pages={1334–1343} }
 @article{krishnan_smith_spontak_2012, title={Ternary Phase Behavior of a Triblock Copolymer in the Presence of an Endblock-Selective Homopolymer and a Midblock-Selective Oil}, volume={45}, ISSN={["1520-5835"]}, DOI={10.1021/ma300417u}, abstractNote={Bicomponent block copolymers are known to exhibit rich phase behavior in systems containing one or two block-selective homopolymers or solvents. In this study, we combine these efforts by investigating ternary blends composed of an ABA triblock copolymer, an A-selective homopolymer and a B-selective oil. A styrenic thermoplastic elastomer is selected here because of its ability to form a physical network upon microphase separation and thus impart significant elasticity and toughness to such blends. Synchrotron small-angle X-ray scattering is employed to classify the nanostructures of blends varying in composition, homopolymer molecular weight, and oil type, and the results are used to construct ternary morphology diagrams that reveal the phases present at the glass transition temperature of the styrenic endblocks. Of all the classical and complex morphologies commonly observed in binary copolymer blends and solutions, only the bicontinuous gyroid consisting of styrenic channels in a mixed midblock/oil mat...}, number={15}, journal={MACROMOLECULES}, author={Krishnan, Arjun S. and Smith, Steven D. and Spontak, Richard J.}, year={2012}, month={Aug}, pages={6056–6067} }
 @article{vargantwar_brelander_krishnan_ghosh_spontak_2011, title={(Electro)mechanical behavior of selectively solvated diblock/triblock copolymer blends}, volume={99}, ISSN={["1077-3118"]}, DOI={10.1063/1.3666783}, abstractNote={Thermoplastic elastomeric triblock copolymers swollen with a midblock-selective solvent form a highly elastic physical network that can exhibit remarkable electromechanical properties (high actuation strains and electromechanical efficiency with low hysteresis upon cycling) as dielectric elastomers. One unexplored means of controllably altering the midblock network and the corresponding (electro)mechanical properties at constant copolymer concentration is to substitute non-network-forming diblock for triblock copolymer molecules. In this study, we demonstrate that the incorporation of composition-matched diblock molecules into selectively solvated triblock systems results in softer materials that are less physically crosslinked and thus capable of undergoing electroactuation at reduced electric fields.}, number={24}, journal={APPLIED PHYSICS LETTERS}, author={Vargantwar, Pruthesh H. and Brelander, Sarah M. and Krishnan, Arjun S. and Ghosh, Tushar K. and Spontak, Richard J.}, year={2011}, month={Dec} }
 @article{krishnan_vargantwar_ghosh_spontak_2011, title={Electroactuation of Solvated Triblock Copolymer Dielectric Elastomers: Decoupling the Roles of Mechanical Prestrain and Specimen Thickness}, volume={49}, ISSN={["1099-0488"]}, DOI={10.1002/polb.22331}, abstractNote={Abstract}, number={22}, journal={JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS}, author={Krishnan, Arjun S. and Vargantwar, Pruthesh H. and Ghosh, Tushar K. and Spontak, Richard J.}, year={2011}, month={Nov}, pages={1569–1582} }
 @article{vargantwar_shankar_krishnan_ghosh_spontak_2011, title={Exceptional versatility of solvated block copolymer/ionomer networks as electroactive polymers}, volume={7}, ISSN={["1744-6848"]}, DOI={10.1039/c0sm01210f}, abstractNote={Responsive materials possess properties that change abruptly when exposed to an external stimulus, and electroactive polymers constitute examples of robust, lightweight materials that change shape upon electrical actuation. We demonstrate that solvated block copolymer networks afford tremendous versatility in designing electronic and ionic electroactive polymers. As dielectric elastomers, styrenic block copolymer systems attain extraordinary actuation strains approaching 300%, along with high electromechanical coupling efficiencies. Changing the solvent improves the blocking stress and yields remarkably high energy densities, while providing a unique opportunity for mechanical impedance matching and control of shape recovery kinetics, as well as mode of deformation. Dielectric elastomers composed of acrylic copolymers actuate beyond 100% in-plane strain without any prestrain, whereas block ionomer networks swollen with ionic solutions yield ionic polymer–metal composites, which actuate by bending. Selective solvation of block copolymer networks represents an effective and largely unexplored means by which to tune the function and properties of electroactive polymers through systematic manipulation of copolymer and solvent attributes.}, number={5}, journal={SOFT MATTER}, author={Vargantwar, Pruthesh H. and Shankar, Ravi and Krishnan, Arjun S. and Ghosh, Tushar K. and Spontak, Richard J.}, year={2011}, pages={1651–1655} }
 @article{krishnan_zanten_seifert_lee_spontak_2011, title={Selectively solvated triblock copolymer networks under biaxial strain}, volume={99}, ISSN={["0003-6951"]}, DOI={10.1063/1.3635780}, abstractNote={Triblock copolymers swollen with a midblock-selective solvent provide a test platform by which to interrogate the properties of highly elastic physical gel networks. Here, such networks are biaxially strained and studied by synchrotron small-angle x-ray scattering. Analysis of the form factor reveals that initially spherical micellar cores deform to ellipsoids when strained. The Percus-Yevick hard-sphere model describes the structure factor of micelles exhibiting liquid-like order prior to deformation but requires an attractive potential to match the structure factor under strain. The magnitude of this potential increases with increasing strain, indicating a change in coronal overlap as the network is stretched.}, number={10}, journal={APPLIED PHYSICS LETTERS}, author={Krishnan, Arjun S. and Zanten, John H. and Seifert, Soenke and Lee, Byeongdu and Spontak, Richard J.}, year={2011}, month={Sep} }
 @article{krishnan_seifert_lee_khan_spontak_2010, title={Cosolvent-regulated time-composition rheological equivalence in block copolymer solutions}, volume={6}, ISSN={["1744-6848"]}, DOI={10.1039/c0sm00573h}, abstractNote={The morphological and mechanical attributes of triblock copolymer solutions composed of miscible, midblock-selective solvents are investigated by small-angle scattering and dynamic rheology. Variation in cosolvent composition at constant copolymer concentration has little effect on copolymer morphology, but promotes large differences in matrix relaxation, as evinced by changes in the shape of isothermal frequency spectra. Shifting these spectra in the frequency domain reveals the existence of time–composition equivalence, wherein shift factors scale with the viscosity of the cosolvent mixture.}, number={18}, journal={SOFT MATTER}, author={Krishnan, Arjun S. and Seifert, Soenke and Lee, Byeongdu and Khan, Saad A. and Spontak, Richard J.}, year={2010}, pages={4331–4334} }
 @article{talwar_krishnan_hinestroza_pourdeyhimi_khan_2010, title={Electrospun Nanofibers with Associative Polymer-Surfactant Systems}, volume={43}, ISSN={["1520-5835"]}, DOI={10.1021/ma1013447}, abstractNote={Associative polymers are unique in their structure with pendant hydrophobes attached to their hydrophilic backbone, enabling associations between the hydrophobes and forming junctions in aqueous solutions. In this study, we examine efforts to produce electrospun nanofibers of associative polymers in conjunction with a readily spinnable polymer. Scanning electron micrograph (SEM) images reveal that the solution rheology sets an upper limit to the concentration of associative polymer that can be successfully electrospun. However, addition of nonionic surfactants to the precursor solution results in significant improvement in nanofiber morphology as evinced from reduced beading. Through judicious use of nonionic surfactants to modulate solution viscoelastic properties, we are able to obtain defect-free nanofiber morphology and gain new insights into the fundamentals of the electrospinning process. In particular, we find that solution viscoelasticity as measured in terms of the relaxation time, rather than viscosity as typically hypothesized, controls the nanofiber formation process.}, number={18}, journal={MACROMOLECULES}, author={Talwar, Sachin and Krishnan, Arjun S. and Hinestroza, Juan P. and Pourdeyhimi, Behnam and Khan, Saad A.}, year={2010}, month={Sep}, pages={7650–7656} }
 @article{shankar_krishnan_ghosh_spontak_2008, title={Triblock copolymer organogels as high-performance dielectric elastomers}, volume={41}, ISSN={["1520-5835"]}, DOI={10.1021/ma071903g}, abstractNote={Block copolymers and nanostructured materials derived therefrom are becoming increasingly ubiquitous in a wide variety of (nano)technologies. Recently, we have demonstrated that triblock copolymer organogels composed of physically cross-linked copolymer networks swollen with a midblock-selective solvent exhibit excellent electromechanical behavior as dielectric elastomers. In-plane actuation of such organogels, collectively referred to as electroactive nanostructured polymers (ENPs) to reflect the existence of a self-organized copolymer morphology, is attributed to the development of an electric-field-induced surface-normal Maxwell stress. In this study, we examine the composition and molecular weight dependence of the electromechanical properties afforded by organogels prepared from poly[styrene-b-(ethylene-co-butylene)-b-styrene] (SEBS) triblock copolymers selectively swollen with EB-compatible aliphatic oligomers. These materials undergo ultrahigh actuation displacement at significantly reduced electri...}, number={16}, journal={MACROMOLECULES}, author={Shankar, Ravi and Krishnan, Arjun K. and Ghosh, Tushar K. and Spontak, Richard J.}, year={2008}, month={Aug}, pages={6100–6109} }
 @article{spontak_shankar_bowman_krishnan_hamersky_samseth_bockstaller_rasmussen_2006, title={Selectivity- and size-induced segregation of molecular and nanoscale species in microphase-ordered triblock copolymers}, volume={6}, ISSN={["1530-6992"]}, DOI={10.1021/nl061205u}, abstractNote={Microphase-ordered block copolymers serve as model systems to elucidate the potential of molecular self-assembly and organic templates to fabricate functionalized polymeric materials. Both aspects are related to the incorporation of secondary species such as low-molar-mass compounds or nanoparticles within the copolymer matrices. Since the resulting properties of such functionalized copolymers critically depend on the morphology of the blend or composite, the nonrandom distribution of such inclusions within the copolymer matrix must be understood. Using a self-consistent field theoretical approach, we quantitatively evaluate the segregation and interfacial excess of low-molar-mass and nanoscale species in ordered triblock copolymers as functions of block selectivity and inclusion size. The predictions are found to agree with the morphology observed in a model triblock copolymer/nanoparticle composite, thereby demonstrating the generality of this approach. Our results suggest a wide correspondence in the structure-forming effect of molecular and nanoscale inclusions that will have implications in the design and processing of functional nanostructured polymers.}, number={9}, journal={NANO LETTERS}, author={Spontak, Richard J. and Shankar, Ravi and Bowman, Michelle K. and Krishnan, Arjun S. and Hamersky, Mark W. and Samseth, Jon and Bockstaller, Michael R. and Rasmussen, Kim O.}, year={2006}, month={Sep}, pages={2115–2120} }