Arjun S. Krishnan

Exceptional versatility of solvated block copolymer/ionomer networks as electroactive polymers

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.

Ultrastretchable, cyclable and recyclable 1- and 2-dimensional conductors based on physically cross-linked thermoplastic elastomer gels

Stretchable conductors maintain electrical conductivity at large strains relative to their rigid counterparts that fail at much lower strains. Here, we demonstrate ultrastretchable conductors that are conductive to at least 600% strain and may be strain-cycled without significant degradation to the mechanical or electrical properties. The conductors consist of a liquid metal alloy injected into microchannels composed of triblock copolymer gels. Rheological measurements identify the temperature window over which these gels may be molded and laminated to form microchannels without collapsing the microscale features. Mechanical measurements identify the gel composition that represents a compromise between minimizing modulus (to allow the polymer to be stretched with ease) and maximizing interfacial adhesion strength at the laminated polymer–polymer interface. The resulting 2D stretchable conductors are notable for their ability to maintain electrical conductivity up to large strains, their mechanical durability, and their ability to be recycled easily with full recovery of the component species.

Factors affecting time–composition equivalence in ternary block copolymer/cosolvent systems

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.

Thermorheological behavior of coexisting physical networks: combining SAFIN and SAMIN organogels

Organogels, like their hydrogel analogs in aqueous media, consist of chemically or physically cross-linked networks that endow simple (or even viscoelastic) organic liquids with solid-like characteristics. Of particular interest here are physical networks composed of either low-molar-mass organic gelators, which tend to form self-assembled fibrillar networks (SAFINs) by site-specific interactions such as hydrogen bonding and/or π–π stacking, or microphase-separated triblock copolymers, which form self-assembled micellar networks (SAMINs) due primarily to bridged midblocks that connect neighboring micelles. In this study, we combine these two physical networks by mixing the gelator 1,3:2,4-dibenzylidene-D-sorbitol with a series of midblock-swollen triblock copolymers differing in molecular weight to produce coexisting SAFIN/SAMIN networks in two midblock-selective solvents differing in polarity. The thermorheological properties of the resultant mixed network systems are investigated for three cases: (i) the order–disorder transition temperature (TODT) of the SAMIN is much lower than the dissolution temperature (Td) of the SAFIN, (ii) TODT < Td and (iii) TODT ≈ Td. Employing dynamic rheology, we show how these transitions are affected by the order in which self-organization occurs.

Cosolvent-regulated time–composition rheological equivalence in block copolymer solutions

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.

(Electro)mechanical behavior of selectively solvated diblock/triblock copolymer blends

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.

Selectively solvated triblock copolymer networks under biaxial strain

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.

Deviation from time-composition equivalence in polymer solutions with selective cosolvents

Time-composition superpositioning (tCS) permits determination of the mechanical properties of polymeric materials over a widely extended time (or frequency) domain by systematically varying composition under isothermal conditions. We have recently reported (Soft Matter, 6, 4331, 2010) the existence of such equivalence in technologically relevant triblock copolymers swollen with miscible, midblock-selective cosolvents differing in chemical constitution and viscosity. In this study, chemically homologous homopolymer and copolymer systems exhibiting rheological properties that deviate from tCS are investigated. With regard to the particular case of selectively solvated triblock copolymers, the source of deviation is explained in the context of endblock hopping.