@article{vatankhah-varnosfaderani_daniel_zhushma_li_morgan_matyjaszewski_armstrong_dobrynin_sheyko_spontak_et al._2017, title={Bottlebrush elastomers: a promising molecular engineering route to tunable, prestrain-free dielectric elastomers (Conference Presentation)}, volume={10163}, ISBN={["978-1-5106-0812-2"]}, ISSN={["0277-786X"]}, DOI={10.1117/12.2261913}, abstractNote={Electroactive polymers (EAPs) refer to a broad range of relatively soft materials that change size and/or shape upon application of an electrical stimulus. Of these, dielectric elastomers (DEs) generated from either chemically- or physically-crosslinked polymer networks afford the highest levels of electroactuation strain, thereby making this class of EAPs the leading technology for artificial-muscle applications. While mechanically prestraining elastic networks remarkably enhances DEs electroactuation, external prestrain protocols severely limit both actuator performance and device implementation due to gradual DE stress relaxation and the presence of a cumbersome load frame. These drawbacks have persisted with surprisingly minimal advances in the actuation of single-component elastomers since the dawn of the “pre-strain era” introduced by Pelrine et al. (Science, 2000). In this work, we present a bottom-up, molecular-based strategy for the design of prestrain-free (freestanding) DEs derived from covalently-crosslinked bottlebrush polymers. This architecture, wherein design factors such as crosslink density, graft density and graft length can all be independently controlled, yields inherently strained polymer networks that can be readily adapted to a variety of chemistries. To validate the use of these molecularly-tunable materials as DEs, we have synthesized a series of bottlebrush silicone elastomers in as-cast shapes. Examination of these materials reveals that they undergo giant electroactuation strains (>300%) at relatively low fields (<10 V/m), thereby outperforming all commercial DEs to date and opening new opportunities in responsive soft-material technologies (e.g., robotics). The molecular design approach to controlling (electro)mechanical developed here is independent of chemistry and permits access to an unprecedented range of actuation properties from elastomeric materials with traditionally modest electroactuation performance (e.g., polydimethylsiloxane, PDMS). Experimental results obtained here compare favorably with theoretical predictions and demonstrate that the unique behavior of these materials is a direct consequence of the molecular architecture.}, journal={ELECTROACTIVE POLYMER ACTUATORS AND DEVICES (EAPAD) 2017}, author={Vatankhah-Varnosfaderani, M. and Daniel, W. F. M. and Zhushma, A. P. and Li, Q. X. and Morgan, B. J. and Matyjaszewski, K. and Armstrong, D. P. and Dobrynin, A. V. and Sheyko, S. S. and Spontak, Richard and et al.}, year={2017} }
 @article{armstrong_spontak_2017, title={DESIGNING DIELECTRIC ELASTOMERS OVER MULTIPLE LENGTH SCALES FOR 21ST CENTURY SOFT MATERIALS TECHNOLOGIES}, volume={90}, ISSN={["1943-4804"]}, DOI={10.5254/rct.17.82660}, abstractNote={ABSTRACTDielectric elastomers (DEs) constitute an increasingly important category of electroactive polymers. They are in a class of generally soft materials that, upon exposure to an electric stimulus, respond by changing size, shape, or both. Derived from network-forming macromolecules, DEs are lightweight, robust and scalable, and they are capable of exhibiting giant electroactuation strains, high electromechanical efficiencies, and relatively low strain-cycling hysteresis over a broad range of electric fields. Due primarily to their attractive electromechanical attributes, DEs are of growing interest in diverse biomedical, (micro)robotic, and analytical technologies. Since the seminal studies of these electroresponsive materials (initially fabricated mainly from chemically cross-linked acrylic and silicone elastomers), advances in materials design over multiple length scales have resulted in not only improved electromechanical performance but also better mechanistic understanding. We first review the fundamental operating principles of DEs developed from conventional elastomers that undergo isotropic electroactuation and then consider more recent advances at different length scales. At the macroscale, incorporation of oriented fibers within elastomeric matrices is found to have a profound impact on electroactuation by promoting an anisotropic response. At the mesoscale, physically cross-linked thermoplastic elastomer gel networks formed by midblock-swollen triblock copolymers provide a highly tunable alternative to chemically cross-linked elastomers. At the nanoscale, the chemical synthesis of binetwork and bottlebrush elastomers permits extraordinarily enhanced electromechanical performance through targeted integration of inherently prestrained macromolecular networks.}, number={2}, journal={RUBBER CHEMISTRY AND TECHNOLOGY}, author={Armstrong, Daniel P. and Spontak, Richard J.}, year={2017}, pages={207–224} }
 @article{cooper_arutselvan_liu_armstrong_lin_khan_genzer_dickey_2017, title={Stretchable Capacitive Sensors of Torsion, Strain, and Touch Using Double Helix Liquid Metal Fibers}, volume={27}, ISSN={["1616-3028"]}, DOI={10.1002/adfm.201605630}, abstractNote={Soft and stretchable sensors have the potential to be incorporated into soft robotics and conformal electronics. Liquid metals represent a promising class of materials for creating these sensors because they can undergo large deformations while retaining electrical continuity. Incorporating liquid metal into hollow elastomeric capillaries results in fibers that can integrate with textiles, comply with complex surfaces, and be mass produced at high speeds. Liquid metal is injected into the core of hollow and extremely stretchable elastomeric fibers and the resulting fibers are intertwined into a helix to fabricate capacitive sensors of torsion, strain, and touch. Twisting or elongating the fibers changes the geometry and, thus, the capacitance between the fibers in a predictable way. These sensors offer a simple mechanism to measure torsion up to 800 rad m−1—two orders of magnitude higher than current torsion sensors. These intertwined fibers can also sense strain capacitively. In a complementary embodiment, the fibers are injected with different lengths of liquid metal to create sensors capable of distinguishing touch along the length of a small bundle of fibers via self‐capacitance. The three capacitive‐based modes of sensing described here may enable new sensing applications that employ the unique attributes of stretchable fibers.}, number={20}, journal={ADVANCED FUNCTIONAL MATERIALS}, publisher={Wiley}, author={Cooper, Christopher B. and Arutselvan, Kuralamudhan and Liu, Ying and Armstrong, Daniel and Lin, Yiliang and Khan, Mohammad Rashed and Genzer, Jan and Dickey, Michael D.}, year={2017}, month={May} }
 @article{vatankhah-varnoosfaderani_daniel_zhushma_li_morgan_matyjaszewski_armstrong_spontak_dobrynin_sheiko_et al._2017, title={Bottlebrush Elastomers: A New Platform for Freestanding Electroactuation}, volume={29}, ISSN={["1521-4095"]}, DOI={10.1002/adma.201604209}, abstractNote={Freestanding, single-component dielectric actuators are designed based on bottlebrush elastomers that enable giant reversible strokes at relatively low electric fields and altogether avoid preactuation mechanical manipulation. This materials design platform allows for independent tuning of actuator rigidity and elasticity over broad ranges without changing chemical composition, which opens new opportunities in soft-matter robotics.}, number={2}, journal={ADVANCED MATERIALS}, author={Vatankhah-Varnoosfaderani, M. and Daniel, W. F. M. and Zhushma, A. P. and Li, Q. X. and Morgan, B. J. and Matyjaszewski, K. and Armstrong, D. P. and Spontak, Richard and Dobrynin, A. V. and Sheiko, S. S. and et al.}, year={2017}, month={Jan} }
 @article{armstrong_mineart_lee_spontak_2016, title={Olefinic Thermoplastic Elastomer Gels: Combining Polymer Crystallization and Microphase Separation in a Selective Solvent}, volume={5}, ISSN={["2161-1653"]}, DOI={10.1021/acsmacrolett.6b00677}, abstractNote={Since selectively swollen thermoplastic elastomer gels (TPEGs) afford a wide range of beneficial properties that open new doors to developing elastomer-based technologies, we examine the unique structure-property behavior of TPEGs composed of olefinic block copolymers (OBCs) in this study. Unlike their styrenic counterparts typically possessing two chemically different blocks, this class of multiblock copolymers consists of linear polyethylene hard blocks and poly(ethylene-co-α-octene) soft blocks, in which case, microphase separation between the hard and the soft blocks is accompanied by crystallization of the hard blocks. Here, we prepare olefinic TPEGs (OTPEGs) through the incorporation of a primarily aliphatic oil that selectively swells the soft block and investigate the resultant morphological features through the use of polarized light microscopy and small-/wide-angle X-ray scattering. These features are correlated with thermal and mechanical property measurements from calorimetry, rheology, and extensiometry to elucidate the roles of crystallization and self-assembly on gel characteristics and establish useful structure-property relationships.}, number={11}, journal={ACS MACRO LETTERS}, author={Armstrong, Daniel P. and Mineart, Kenneth P. and Lee, Byeongdu and Spontak, Richard J.}, year={2016}, month={Nov}, pages={1273–1277} }
 @article{al-mohsin_mineart_armstrong_spontak_2017, title={Tuning the performance of aqueous photovoltaic elastomer gels by solvent polarity and nanostructure development}, volume={55}, ISSN={["1099-0488"]}, DOI={10.1002/polb.24242}, abstractNote={In this study, a sulfonated pentablock ionomer is considered for use as an aqueous gel electrolyte in photovoltaic elastomer gels (PVEGs) containing photosensitive dyes. Depending on the casting solvent employed, these materials order into different nanoscale morphologies, some of which inherently consist of a continuous pathway through which ions and other polar species are able to diffuse, while others transform into continuous channels upon exposure to water. Here, we examine the effect of solvent polarity during film casting, vapor annealing, and liquid immersion on block ionomer morphology and PVEG photovoltaic performance. Casting the block ionomers from a mixed nonpolar/polar solvent promotes the formation of dispersed ion-rich spherical microdomains. Alternatively, the use of a single polar solvent produces coexisting nonpolar cylinders and lamellae. Exposure of either morphology to polar solvent vapor causes the block ionomers to restructure into a lamellar morphology, whereas exposure of dispersed ion-rich microdomains to water induces a transformation to an irregular morphology composed of continuous ionic channels, which provide an effective pathway for ion diffusion and, consequently, the highest photovoltaic efficiency. In addition to their photovoltaic efficacy, these aqueous gels possess improved mechanical properties (in terms of tensile strength and elastic modulus) in the presence of photosensitive dyes. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016}, number={1}, journal={JOURNAL OF POLYMER SCIENCE PART B-POLYMER PHYSICS}, author={Al-Mohsin, Heba A. and Mineart, Kenneth P. and Armstrong, Daniel P. and Spontak, Richard J.}, year={2017}, month={Jan}, pages={85–95} }