Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session A54: Smart and Responsive Polymers and Soft Materials I: Micro-Length Scale PhenomenaFocus
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Sponsoring Units: DPOLY GSOFT DBIO Chair: Chelsea Davis, Purdue University Room: BCEC 254A |
Monday, March 4, 2019 8:00AM - 8:12AM |
A54.00001: Mechanochromic Polycarbonate: Seeing Plasticity with Color Steven Yang, Yuval Vidavsky, Meredith Silberstein We use force-driven chromism to detect plasticity in polycarbonate. We create this functionality by embedding spiropyran, a mechanochromic compound, in the polymer backbone. While there are many examples of force-driven functionality in elastomers, there are few for glassy polymers. In prior experiments, the activation of spiropyran in glassy polymers was limited to tests near Tg, or with an added plasticizer. With glassy polycarbonate, we demonstrate tensile activation at room temperature. Spiropyran activation begins after yield and increases with hardening during plastic flow. For monotonic tensile tests, the activation is inversely related to the strain rate, while the stress is comparable. These results suggest that for glassy polymers, the activation of mechano-responsive compounds is coupled with stress, plastic flow, and time. Understanding how these factors contribute to activation will enable using mechano-responsive polymers for quantitative sensing. |
Monday, March 4, 2019 8:12AM - 8:24AM |
A54.00002: A statistical mechanical model for the electrostriction of polymers Matthew Grasinger, Kaushik Dayal Dielectric elastomers (DEs) are soft materials that polarize in the presence of an electric field. Using statistical mechanics, the thermodynamics of a DE chain subject to electrical loading and kinematic constraints is investigated. The equations of the monomer orientation density and the approximate chain free energy are derived. Solutions are presented in the small chain stretch limit and near the fully stretched limit. A closed-form approximation for the free energy of a DE chain is developed using asymptotic matching and shown to agree well with the numerical solutions. Next, averaging over chain orientations, the chain-scale response is built up to an electromechanically coupled DE constitutive model. Some preliminary results from the constitutive model are presented, which includes a contribution, at the macromolecular scale, to the electrostriction of DEs. The contribution to electrostriction is shown to be dependent on the lengths and orientations of chains in the DE network. |
Monday, March 4, 2019 8:24AM - 8:36AM |
A54.00003: Simulating Polymeric Microstructures With Encoded Pre-Determined Deformability James Waters, Joanna Aizenberg, Anna Christina Balazs Biological systems are able to translate stimuli into meaningful actions, reshaping themselves in response to changes in their environments. Liquid crystalline elastomers (LCE’s) represent a synthetic means of achieving this type of behavior, undergoing large deformations when nematic-isotropic phase transitions occur within the material. We use a finite element simulation method to predict the thermal response of LCE microplates, in a particular instance of this phenomenon. The shape changes are dictated by the director orientation in the nematic state, programmed in by a magnetic field present during cross-linking. After calibrating our simulation parameters against experimental results, we are able to accurately predict the observed experimental response of LCE microplates with a range of initial nematic orientations. |
Monday, March 4, 2019 8:36AM - 8:48AM |
A54.00004: Rational Design of Strain-Adaptive Elastomers through Polymer Architectures Heyi Liang, Mohammad Vatankhah-Varnosfaderani, Sergei Sheiko, Andrey Dobrynin Designing materials capable of mimicking the mechanical properties of soft biological tissues is important for tissue engineering, soft robotics, and wearable electronics. The biological tissues show a unique combination of the mechanical softness and strong strain-stiffening, which make it difficult to replicate in synthetic elastomers composed of linear polymers. Using a combination of theoretical calculations and molecular dynamics simulations, we have developed a universal materials design strategy which encodes the stress-strain curve of soft materials into the molecular architecture of graft polymer networks. Such networks can be made either by chemical crosslinking of graft polymer strands or by self-assembly of linear-bottlebrush-linear triblock copolymers. The mechanical response of such networks is controlled by the architectural parameters (i.e. network strand length, side chain length, grafting density, and composition of triblock copolymers). Our approach is verified by synthesizing PDMS networks of combs and bottlebrushes with mechanical properties of jellyfish, lung, and arterial tissue. This technique lays the foundation for computationally driven design of soft materials. |
Monday, March 4, 2019 8:48AM - 9:00AM |
A54.00005: Detecting Bond Breakage and Fracture in Tough Hydrogels Using Mechanoluminescence Gabriel Sanoja, Rint Sijbesma, Costantino Creton Synthetic hydrogels are soft materials composed of 3-D polymer networks swollen with water. They are promising synthetic analogues of tissues but their excessive brittleness remains an important limitation. Though there have been advances in the design of tough hydrogels, it is not yet possible to quantitatively predict toughness from molecular design. A promising strategy towards this endeavor is based on the incorporation of mechanoluminescent probes in these materials. Upon force-induced bond breakage, emission of visible light allows for spatio-temporal mapping of molecular bond scission during failure. |
Monday, March 4, 2019 9:00AM - 9:12AM |
A54.00006: Light-Actuated Liquid Crystal Elastomer Waveguides Alexa Kuenstler, Ryan Hayward The ease with which light can be spatially and temporally modulated makes it an attractive stimulus for the actuation of soft materials. However, many devices require direct line-of-sight access for deployment, severely compromising their utility in confined geometries where remote control is required. One way to circumvent these limitations is to employ actuators that act as waveguides to deliver light over distances that make flood illumination impractical. To this end, we present a method to fabricate liquid crystal elastomer fibers that can adopt defined 3D conformations in response to waveguided visible light due to localized photothermal deformation. By exploiting the photo-mediated reduction of gold salt, spatially-defined regions of nanoparticles are patterned in aligned fibers to define the location and direction of bending. Fibers are demonstrated to bend simultaneously in orthogonal directions on a characteristic time scale of seconds. Furthermore, these fibers are shown to waveguide light, thereby removing the need for line-of-sight access to achieve complex shape change. |
Monday, March 4, 2019 9:12AM - 9:48AM |
A54.00007: Detection of Molecular Fracture in Elastomers with Mechanophores Invited Speaker: Costantino Creton Fracture of soft materials involves large deformations before eventual macroscopic failure occurs and the damage to the material is much less localized at the crack tip than in stiff and brittle materials such as inorganic glasses. Although fracture of simple elastic networks made of flexible polymers connected by covalent bonds, obeys relatively well the classical Lake and Thomas theory, fracture of complex soft materials is much harder to predict from knowledge of the molecular structure and architecture. Organic chemists have recently developed new molecules that can be incorporated in networks and give an optical signal (fluorescence, luminescence or change in absorption) in response to the application of a force. These optical signals can then be used to obtain quantitative information about bond scission and energy dissipation during a macroscopic fracture event. Previous work1 has shown that it is possible to toughen and stiffen soft materials by incorporating a minority percolating filler into a stretchable matrix. Using model interpenetrated networks we demonstrate here, by incorporating mechanophore molecules in the filler or in the matrix network, how such materials break in two stages, by first softening the stiff filler network, losing mechanical percolation, and then breaking the stretchable matrix. Such mechanisms active at the crack tip are responsible for the much higher toughness of these stiffened materials and should be widely generalizable to elastomers and hydrogels reinforced by particles forming a percolating network. |
Monday, March 4, 2019 9:48AM - 10:00AM |
A54.00008: Electric field induced bending of Ionic polymer electrolyte membrane Chathuranga Prageeth Rajapaksha, Chenrun Feng, Camilo Piedrahita, Jinwei Cao, Thein Kyu, Antal Istvan Jakli Electroactive polymers (EAPs) are promising candidates for future soft robotics due to their light weight, low cost and ease of fabrication. Among different kinds of EAPs Ionic electroactive polymers (IEAPs) are more attractive, since they are low voltage driven. Here we report about electric field induced studies of solid-state ionic polymer electrolyte membranes (IPEMs). Three different kinds of electrodes (gold, carbon and PEDOT: PSS based) were tested to optimize performance. Highly ionic conductive IPEMs were prepared by using Poly (ethylene glycol) diacrylate (PEGDA), thiosiloxane and ionic liquid(1-Hexyl-3-methylimidazolium hexafluorophosphate) as described by Piedrahita et al [1]. Different ionic liquid concentrations were tested to achieve large bending displacements. In addition to the reversible short-term low voltage (<3V) responses, we also studied responses up to 20V ac and dc voltages in wide-range of time scales. Not only non-linearity and hysteresis, but also a so far not understood inversion of displacement was also observed at large voltages and large time scales. These studies will provide important information on electrode effects and applicability limits of IEAPs. |
Monday, March 4, 2019 10:00AM - 10:12AM |
A54.00009: Programmable Assembly of Responsive Capillary Multipoles Jinhye Bae, Nakul P Bende, Arthur A Evans, Junhee Na, Christian Santangelo, Ryan Hayward The ability to program inter-particle interactions with well-defined selectivity, directionality, strength, and range remains a central challenge in efforts to realize complex materials by self-assembly of simple building blocks. Capillary attraction or repulsion owing to the geometrical distortion of the three-phase contact line by non-spherical particles has emerged as a powerful means to program two-dimensional assembly. To date, however, the focus has been almost exclusively on assembling rigid particles having shapes of limited complexity, restricting both the types of assemblies and their reconfigurability. We take advantage of the stimuli-responsiveness and the low-energy bending deformation of hydrogel particles, to show stimulus-controlled modulation of shape-induced capillary assembly. We anticipate that such surface tension-driven modification of the capillary interaction will suggest a rich area for fundamental studies and opportunities to further tailor the inter-particle interactions. |
Monday, March 4, 2019 10:12AM - 10:24AM |
A54.00010: Structure Dependent Ice Inhibition in Physically Crosslinked Hydrogels by Crystallization of Hydrophobic Crosslinks Pablo Sepulveda-Medina, Chao Wang, Bryan Vogt Confinement of water at the nanoscale can enable significant supercooling that is not possible in bulk water, but common strategies to confine water lead to low water content. We have recently demonstrated near complete ice inhibition in physically crosslinked hydrogels that use the hydrophobic association of perfluorinated moieties to crosslink and confine the water. However, the confining structure and water content in these hydrogels are coupled, which leads to some questions about the underlying physics. Here we demonstrate a similar route to modulate the nanostructure of a physically crosslinked hydrogel based on 2-hydroxyethyl acrylate (HEA) and n-octadecyl acrylate (ODA) without changing the copolymer composition and water content by crystalline crosslinks. Crystallization in the dry state leads to sheets of crystalline ODA, while melting and re-crystallizing the ODA in a hydrated copolymer leads to spherical nanodomains. The temperature-dependent structure was probed by SAXS, WAXS and SANS. DSC revealed that crystallization in the state leads to significantly more unfrozen water inside the hydrogel (10 to 15 wt%). The hydrogel morphology appears to control the unfrozen water content in these hydrogels under cryogenic temperatures. |
Monday, March 4, 2019 10:24AM - 10:36AM |
A54.00011: Rheological Signature of a Thermally-Gelling Nanoemulsion Meysam Hashemnejad, Abu Zayed Md Badruddoza, Brady Zarket, Patrick Doyle We report the rheological behavior of a new thermoresponsive oil-in-water nanoemulsion system. The nanoemulsion undergo a transition to a gel at elevated temperature. The gelation mechanism is entirely different from previous reports and nanoemulsion droplets play a major role in thermoresponsive behavior. The formulation contains FDA approved amphiphilic triblock copolymer as gelling agents. Nanoemulsions were also prepared using a low-energy process. The thermogelling formulation exhibits unique rheological behaviors. The power-law shear thinning behavior at sol and gel states were observed. Yielding behavior of the nanoemulsion gels was characterized using large amplitude oscillatory shear (LAOS) experiments. These gels display strain-softening behavior under application of large shear-deformation prior to failure of the material. The material turn back to the initial state simultaneously after cessation of applied high shear stress/strain. |
Monday, March 4, 2019 10:36AM - 10:48AM |
A54.00012: Oscillating chemo-mechanical Belousov-Zhabotinsky (BZ) hydrogels Baptiste Blanc, Ning Zhou, Eric Liu, S.Ali Aghvami, Bing Xu, Hyunmin Yi, Seth Fraden
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Monday, March 4, 2019 10:48AM - 11:00AM |
A54.00013: The Effects of Mesogen Spacer and Linker on the Actuation of Liquid Crystal Elastomers Jan-Michael Carrillo, Bobby G Sumpter, Suk-kyun Ahn Experiments have shown that the actuation temperature of thermo-responsive main-chain liquid crystal elastomers (LCE) can be controlled by adjusting the spacer length (NL) of reactive mesogen (i.e., LC monomer). Actuation occurs when the order parameter of LC mesogens changes at the nematic-isotropic transition temperature (TNI). The precursor LC oligomers, which are connected LC units via linking thru an alkyl diamine chain extender, exhibits an increasing TNI as NL is increased. This behavior is opposite to what is observed in LC monomers. To elucidate this behavior, we performed isothermal-isobaric (NPT) ensemble coarse-grained molecular dynamics simulation of stiff-chains that are connected to flexible spacers at different temperatures, where TNI is determined when the smoothly decaying orientational correlation function, g2(r), transitions to an oscillating decay function. Simulations show that increasing NL decreases TNI for both monomers and oligomers. However, the dangling beads (ND), representing the alkyl length of the diamine linkers in the experiments, amplifies the decrease of TNI when the linker length (NL) is short. We infer that the combination of NL and ND changes the shape anisotropy of the LC mesogen, affecting its ability to transition to the nematic phase. |
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