@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{shankar_klossner_weaver_koga_zanten_krause_colina_tanaka_spontak_2009, title={Competitive hydrogen-bonding in polymer solutions with mixed solvents}, volume={5}, ISSN={["1744-683X"]}, DOI={10.1039/b808479c}, abstractNote={Poly(ethylene oxide) (PEO) coil size is investigated in a binary mixture of solvent molecules capable of cooperatively hydrogen-bonding with each other, as well as with the PEO chains. Viscometry reveals a minimum in zero shear rate solution viscosity at a molar ratio of ∼2:1 water:methanol. This viscosity coincides with a minimum in PEO gyration radius and occurs near the conditions where water/methanol mixtures deviate most markedly from ideal solution behavior. A minimum in polymer mean square end-to-end distance is predicted for polymer solutions composed of two hydrogen-bonding solvents.}, number={2}, journal={SOFT MATTER}, author={Shankar, Ravi and Klossner, Rebecca R. and Weaver, Juan T. and Koga, Tsuyoshi and Zanten, John H. and Krause, Wendy E. and Colina, Coray M. and Tanaka, Fumihiko and Spontak, Richard J.}, year={2009}, pages={304–307} }
 @article{shankar_ghosh_spontak_2009, title={Mechanical and actuation behavior of electroactive nanostructured polymers}, volume={151}, ISSN={["0924-4247"]}, DOI={10.1016/j.sna.2009.01.002}, abstractNote={Electroactive polymers (EAPs) can exhibit relatively large actuation strain responses upon electrical stimulation. For this reason, in conjunction with their light weight, robust properties, low cost and facile processability, EAPs are of considerable interest in the development of next-generation organic actuators. Within this class of materials, dielectric electroactive polymers (D-EAPs) have repeatedly exhibited the most promising and versatile properties. A new family of D-EAPs derived from swollen poly[styrene-b-(ethylene-co-butylene)-b-styrene] triblock copolymers has been recently found to undergo ultrahigh displacement at relatively low electric fields compared to previously reported D-EAPs. The present work examines the mechanical and actuation response of these electroactive nanostructured polymer (ENP) systems under quasi-static, and electromechanical loading conditions. Careful measurement of the quasi-static properties under tensile and compressive loading yield similar results that are significantly influenced by the introduction of in-plane strain, as well as by copolymer concentration or molecular weight. Blocking stress measurements reveal that the actuation effectiveness achieved by some of the ENPs is comparable to that of the VHB 4910 acrylic D-EAP, thus providing a novel and efficient avenue to designer D-EAPs for advanced engineering, biomimetic and biomedical applications.}, number={1}, journal={SENSORS AND ACTUATORS A-PHYSICAL}, author={Shankar, Ravi and Ghosh, Tushar K. and Spontak, Richard J.}, year={2009}, month={Apr}, pages={46–52} }
 @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} }
 @misc{shankar_ghosh_spontak_2007, title={Dielectric elastomers as next-generation polymeric actuators}, volume={3}, ISSN={["1744-6848"]}, DOI={10.1039/b705737g}, abstractNote={Due to their versatile properties, robust behavior, facile processability and low cost, organic polymers have become the material of choice for an increasing number of mature and cutting-edge technologies. In the last decade or so, a new class of polymers capable of responding to external electrical stimulation by displaying significant size or shape change has emerged. These responsive materials, collectively referred to as electroactive polymers (EAPs), are broadly classified as electronic or ionic according to their operational mechanism. Electronic EAPs generally exhibit superior performance relative to ionic EAPs in terms of actuation strain, reliability, durability and response time. Among electronic EAPs, dielectric elastomers exhibit the most promising properties that mimic natural muscle for use in advanced robotics and smart prosthetics, as well as in haptic and microfluidic devices. Elastomers derived from homopolymers such as acrylics and silicones have received considerable attention as dielectric EAPs, whereas novel dielectric EAPs based on selectively swollen nanostructured block copolymers with composition-tailorable properties have only recently been reported. Here, we provide an overview of various EAPs in terms of their operational mechanisms, uses and shortcomings, as well as a detailed account of dielectric elastomers as next-generation actuators.}, number={9}, journal={SOFT MATTER}, author={Shankar, Ravi and Ghosh, Tushar K. and Spontak, Richard J.}, year={2007}, pages={1116–1129} }
 @article{shankar_ghosh_spontak_2007, title={Electroactive nanostructured polymers as tunable actuators}, volume={19}, ISSN={["1521-4095"]}, DOI={10.1002/adma.200602644}, abstractNote={Lightweight and conformable electroactive actuators stimulated by acceptably low electric fields are required for emerging technologies such as microrobotics, flat-panel speakers, micro air vehicles, and responsive prosthetics. High actuation areal strains (> 50 %) are currently afforded by dielectric elas-tomers at relatively high electric fields (> 50 V l m –1 ). In this work, we demonstrate that incorporation of a low-volatility, aliphatic-rich solvent into a nanostructured poly[styrene-b - (ethylene-co -butylene)- b -styrene] triblock copolymer yields physically crosslinked micellar networks that exhibit excellent displacement under an external electric field. Such property development reflects solvent-induced reductions in matrix viscosity and nanostructural order, as well as field-enhanced polarization of the styrenic units, which together result in ultra-high areal actuation strains (>200 %) at significantly reduced electric fields (< 40 V l m –1 ) with remarkably low cyclic hysteresis. Use of nanostructured polymers whose properties can be broadly tailored by varying copolymer characteristics or blend composition represents an innovative and tunable avenue to reduced-field actuation for advanced engineering, biomimetic, and biomedical applications.}, number={17}, journal={ADVANCED MATERIALS}, author={Shankar, Ravi and Ghosh, Tushar K. and Spontak, Richard J.}, year={2007}, month={Sep}, pages={2218-+} }
 @article{shankar_ghosh_spontak_2007, title={Electromechanical response of nanostructured polymer systems with no mechanical pre-strain}, volume={28}, ISSN={["1521-3927"]}, DOI={10.1002/marc.200700033}, abstractNote={Abstract}, number={10}, journal={MACROMOLECULAR RAPID COMMUNICATIONS}, author={Shankar, Ravi and Ghosh, Tushar K. and Spontak, Richard J.}, year={2007}, month={May}, pages={1142–1147} }
 @article{sun_shankar_boerner_ghosh_spontak_2007, title={Field-driven biofunctionalization of polymer fiber surfaces during electrospinning}, volume={19}, ISSN={["1521-4095"]}, DOI={10.1002/adma.200601345}, abstractNote={Surface-biofunctionalized synthetic polymer fibers composed of a fiber-forming host polymer and an oligopeptide conjugate are prepared by electrospinning. The conjugate consists of a polypeptide segment and a polymer block that is compatible with the host polymer. Because the more polarizable peptide segment migrates to the surface during electrospinning, peptide surface enrichment (see figure and inside cover) is achieved in a single step without further treatment.}, number={1}, journal={ADVANCED MATERIALS}, author={Sun, Xiao-Yu and Shankar, Ravi and Boerner, Hans G. and Ghosh, Tushar K. and Spontak, Richard J.}, year={2007}, month={Jan}, pages={87-+} }
 @article{kishore_shankar_sankaran_2006, title={Effects of microballoons' size and content in epoxy on compressive strength and modulus}, volume={41}, ISSN={["1573-4803"]}, DOI={10.1007/s10853-006-0801-5}, number={22}, journal={JOURNAL OF MATERIALS SCIENCE}, author={Kishore and Shankar, Ravi and Sankaran, S.}, year={2006}, month={Nov}, pages={7459–7465} }
 @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} }
 @article{kishore_sankaran_2005, title={Gradient syntactic foams: Tensile strength, modulus and fractographic features}, volume={412}, number={02-Jan}, journal={Materials Science & Engineering. A, Structural Materials: Properties, Microstructure and Processing}, author={Kishore, Shankar R. and Sankaran, S.}, year={2005}, pages={153–158} }
 @article{kishore_sankaran_2005, title={Short beam three point bend tests in syntactic foams. Part 1: Microscopic characterization of the failure zones}, volume={98}, number={2}, journal={Journal of Applied Polymer Science}, author={Kishore, Shankar R. and Sankaran, S.}, year={2005}, pages={673–679} }
 @article{kishore_sankaran_2005, title={Short-beam three-point bend test study in syntactic foam. Part III: Effects of interface modification on strength and fractographic features}, volume={98}, number={2}, journal={Journal of Applied Polymer Science}, author={Kishore, Shankar R. and Sankaran, S.}, year={2005}, pages={687–693} }
 @article{kishore_sankaran_2005, title={Short-beam three-point bend tests in syntactic foams. Part II: Effect of microballoons content on shear strength}, volume={98}, number={2}, journal={Journal of Applied Polymer Science}, author={Kishore, Shankar R. and Sankaran, S.}, year={2005}, pages={680–686} }