Bulletin of the American Physical Society
APS March Meeting 2018
Volume 63, Number 1
Monday–Friday, March 5–9, 2018; Los Angeles, California
Session A52: Mechano-Responsive Polymers and Soft MaterialsFocus
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Sponsoring Units: DPOLY GSOFT Chair: Matthew Green, Arizona State Univ Room: LACC 512 |
Monday, March 5, 2018 8:00AM - 8:12AM |
A52.00001: Mechanophore activation in a crosslinked polymer matrix via instrumented indentation Chelsea Davis, Jeremiah Woodcock, Ketan Khare, Muzhou Wang, Ryan Beams, Stephan Stranick, Aaron Forster, Jeffrey Gilman Recent advances in mechanoresponsive fluorophores will enable a host of unique scientific challenges and opportunities to be addressed. Several mechanophores (MPs) in polymers have been reported, yet the specific deformation required to activate these molecules in a bulk polymer network has not been sufficiently specified. To understand the mechano-activation/deformation relationship of a spirolactam-based MP, scratches were applied to a MP-functionalized glassy network at varying normal loads and lateral displacement rates. This experimental design allowed strain and strain rate effects to be decoupled. Areas of elastic and plastic deformation as well as brittle fracture were observed within each scratch as the normal loading of the indenter increased. The fluorescence intensity increased while the fluorescence lifetime decreased with increasing strain. Contact mechanics models are employed to demonstrate that relatively high degrees of strain are required to initiate the ring-opening activation transition within the spirolactam-based MP. These self-reporting damage sensors can be incorporated within polymeric coatings to allow real time structural health monitoring for a myriad of applications. |
Monday, March 5, 2018 8:12AM - 8:24AM |
A52.00002: Mechanically activated fluorescence in polymer matrix composites Meng Wang, Matthew Green Polymer matrix composites (PMCs) offer design solutions to produce composites for numbers of critical applications. Nanoparticle additives like carbon nanotubes and quantum dots have been investigated for their ability to improve the conductivity, thermal stability, and mechanical strength of traditional composites. Current mechanoresponsive polymers rely on the use of the breakage of covalent bonds to induce self-healing reactions or changes in optical properties. Their active response is mediated by a functional polymer backbone, which limits the polymeric materials available. Thus, we propose to tune the interactions between fluorescent quantum dots, fluorescently labeled carbon nanotubes, and novel matrix chemistries to probe composite failure mechanisms. We find that pronounced changes in fluorescence emerge following plastic deformation, indicating that in these polymeric materials the transduction of mechanical force into the fluorescence occurs in response to mechanical activation. We anticipate that force activation of fluorescence for quenching pairs can serve as a general strategy for the development of new PMCs building blocks that impart polymeric materials with desirable functionalities ranging from damage sensing to enhancements in mechanical strength |
Monday, March 5, 2018 8:24AM - 8:36AM |
A52.00003: Shifting the twist of organic microribbons by external stimulus Yifan Zhang, Yanke Che, jincai zhao Soft materials with morphological changes in response to external stimuli attract intensive interest because of their applications in actuators and sensors. Speed of response, one of the most important parameters, is related to the active components and the elasticity. Here, using predictive design principles, we report a class of layered microribbons exhibited fast electro-induced response. By increasing the length of the side chains of perylene diimides to tune the elastic modulus, we find that twisting rate is inversely proportional to elasticity modulus. Twisting arises from screw dislocations of the monomers by diagonal π-stacking distortion relative to the microribbon axis. Our work offers principles applicable to designing other layered molecular systems when the goal is rapid response to external stimuli. |
Monday, March 5, 2018 8:36AM - 9:12AM |
A52.00004: Challenges and Opportunities of Polymer Mechanochemistry Invited Speaker: Roman Boulatov Polymer mechanochemistry is an emerging field at the interface of chemistry, materials science, physics and engineering. It aims at understanding and exploiting unique reactivities of stretched polymer chains. Macromolecules or their segments become stretched in bulk polymers under mechanical loads or when polymer solutions are sonicated or flow rapidly through abrupt contractions. Increasing amount of empirical data suggest that mechanochemical phenomena are widespread wherever polymers are used. In the past decade, empirical mechanochemistry has progressed enormously, from studying fragmentations of commodity polymers by simple backbone homolysis to demonstrations of self-strengthening and stress-reporting materials and mechanochemical cascades using purposefully designed monomers. Progress in developing the conceptual foundation of mechanochemistry has been more limited. I’ll discuss this progress using specific examples to illustrate the opportunities both for fundamental studies of polymer behavior and for potential applications. I’ll also describe the outstanding challenges facing polymer mechanochemistry, particularly in developing conceptual frameworks within which to rationalize, systematize and generalize empirical mechanochemical observations. I’ll conclude by suggesting 5 important scientific problems that can be advanced by exploiting polymer mechanochemistry. |
Monday, March 5, 2018 9:12AM - 9:24AM |
A52.00005: Stress-Sensing Thermoset Polymer Networks via Grafted Cinnamoyl Mechanophores in Epoxy Ryan Gunckel, Elizabeth Nofen The employment of mechanophores and mechanochemistry in materials has enabled the development of novel force-responsive materials. Studies exploring the force sensing capabilities of the UV-dimerized cinnamoyl moiety have shown that after severing its cyclobutane bond under application of an external force, the moiety will revert to its initial fluorescent state. This has been experimentally demonstrated in literature through embedding the cinnamoyl group into an epoxy thermoset matrix. However, the covalent grafting of these mechanophores into the backbone of a thermoset is noticeably missing from mechanophore literature. Two specific approaches for bonding the cinnamoyl group have been explored. These include bonding the cinnamoyl group to the epoxide within the resin component, as well as a bonding to an amine-based hardener molecule. The goal of this work is to improve upon the fluorescent response of the cinnamoyl moiety as well as mitigate property loss from mechanophore incorporation over previous particulate approaches. Additionally, this work aims to provide a fundamental understanding of mechanochemistry as a whole. |
Monday, March 5, 2018 9:24AM - 9:36AM |
A52.00006: Magnetic switching of the ferroelectric polarization in a patterned medium mediated by magnetostriction and flexoelectricity Shashi Poddar, Pedro de Sa, Ronggang Cai, Laurent Delannay, Bernard Nysten, Luc Piraux, Alain Jonas Multi-component metal/organic multilayers and hybrid nanostructures of 1-3 connected architecture exhibit room temperature tuning of the electrical polarization by a magnetic field but permanent domain reversal by a magnetic field is difficult. In our prior study with 1-3 connected multiferroic hybrid layer consisting of ferromagnetic nickel nanopillars regularly embedded in a ferroelectric co-polymer(PVDF:TrFE) matrix, polarization could be switched using magnetic field up to 0.5 T aided by a weak electric field. In this work, we fabricate the reverse geometry of ferroelectric polymer nanopillars embedded in a Ni matrix. On application of a vertical magnetic field, the Ni matrix with negative magnetostriction expands in the plane and compress out of the plane of the device which being physically clamped to the substrate results in a strain gradient along the ferroelectric nanopillars.The strain gradient in turn produces an equivalent electric field by the flexoelectric effect, which is of the same magnitude as the field required to switch a negatively poled ferroelectric domain of the copolymer. This was confirmed by piezoelectric force microscopy and finite element simulations, where switching reversal of a negatively poled domain was induced by the application of 0.7 T alone. |
Monday, March 5, 2018 9:36AM - 9:48AM |
A52.00007: Strain-induced electrical charge transport in PEDOT:PSS thin films: role of wrinkles and cracks Biporjoy Sarkar, Dillip Satapathy, Manu Jaiswal In this work, we demonstrate that the modulations of electrical charge transport in strained PEDOT:PSS thin films on flexible substrates can be understood in terms of subtle changes in the film morphology. The application of uniaxial strain results in the formation of wrinkle arrays which are oriented in the direction of applied strain. At higher strain magnitude (ε > 4 %), the formation of quasi-periodic parallel cracks oriented in a direction perpendicular to the applied strain is observed. While the wrinkle arrays lead to weakly anisotropic charge transport, the cracks result in strong anisotropy (ΔR/R =60 at 16% strain). We quantify the contributions of wrinkles and cracks toward the resistance changes observed in strained PEDOT: PSS films. Interestingly, the resistance of PEDOT:PSS thin film exhibits a ‘strain memory effect’ when it is subjected to a controlled cyclic process. We demonstrate that this peculiar ‘strain memory effect’ can be understood in terms of the history of wrinkles array formation when the film subject to cyclic strain. The strain-dependent electrical response is remarkably different when PEDOT: PSS films are cast on pre-strained substrates and this response can be reconciled based on diminished polymer: substrate interactions. |
Monday, March 5, 2018 9:48AM - 10:24AM |
A52.00008: Quantitative Studies of Polymer Mechanochemistry Invited Speaker: Stephen Craig In recent years, covalent polymer mechanochemistry has undergone something of a renaissance in which it has been extensively explored for a variety of purposes including (but not limited to) biasing reaction pathways, trapping transition states and intermediates, catalysis, release of small molecules and protons, stress reporting and stress strengthening materials, and soft devices. The extent to which such responses might be useful in bulk materials involves physics on two different length scales: the coupling of force to chemical reactions within an individual polymer strand, and the number of polymer strands within a network that experience various magnitudes of force. This talk will describe quantitative studies on both of these length scales, including the ability of studies on one to inform the other. |
Monday, March 5, 2018 10:24AM - 10:36AM |
A52.00009: Optomechanical Coupling for the Study of the Dynamic Mechanical Properties of Metal-coordinate Gels Irina Mahmad Rasid, Bradley Olsen, Niels Holten-Andersen The dynamic nature of the bonds in associating polymer networks has led to its use in the design of tough and self-healing hydrogels. While the ability of the materials to regain its original stiffness after a recovery period has been documented, such data provides no information on the molecular level processes occurring as the network is damaged, and subsequently as it heals. A luminescent hydrogel, cross-linked with dynamic metal-ligand coordinate complexes has been developed. Through this system, there is an opportunity to visualize and hence analyze these molecular level processes, as the optical and mechanical properties of the gel are inherently coupled. The system was designed such that the relative intensity of the emission in the 400-500nm range, as the hydrogel is stressed, is a measure of the degree of dissociation and re-association of the complexes. In this work, the opto-mechanical properties of the hydrogel was measured through a technique called rheo-fluorescence, where the bond states could be quantified as the material was stressed under steady shear over a range of shear rates. |
Monday, March 5, 2018 10:36AM - 10:48AM |
A52.00010: Nonlinear Mechano-Optical Behavior and Strain-Induced Structural Changes of L-Valine-Based Poly(ester urea)s Keke Chen, Nathan Dreger, Fang Peng, Bryan Vogt, Matthew Becker, Miko Cakmak The uniaxial mechano-optical behavior of a series of L-valine-based poly(ester urea) (PEU) with varying diol lengths was studied. A custom, real-time measurement system was used to capture the true stress, true strain, and birefringence during deformation. PEUs exhibited similar temperature dependent behaviors at temperatures above the glass transition temperature (Tg). A characteristic temperature, defined as the liquid−liquid (Tll) (rubbery−viscous) transition, was found at ~1.05 Tg (K) (Tg +15°C) under the strain rate of 0.017 s−1. Above Tll, the stress optical curve becomes linear and temperature independent. A stretched exponential form was applied to evaluate the isothermal relaxation of PEUs after the deformation. The mean relaxation time was found to exhibit a maximum at Tg +15°C. Real-Time infrared spectroscopy and in situ wide angle x-ray scattering revealed a strain-induced intersegmental structural change during stretching. At the onset of strain hardening, the intermolecular hydrogen bonding was found to adopt different bonding modes and the interchain distance displayed an increase. The hydrogen bonding in the structure strengthens the supramolecular packing. Once disrupted, the reconfigured bonding structure results in an incomplete strain recovery upon heating. |
Monday, March 5, 2018 10:48AM - 11:00AM |
A52.00011: Bio-inspired design of Highly Sensitive and Reversible Mechanochromisms via surface engineering Songshan Zeng, Dianyun Zhang, Luyi Sun In nature, some marine organisms, such as Vogtia and Cephalopods, have evolved to possess camouflage traits by dynamically and reversibly altering their transparency, fluorescence, and coloration via muscle controlled surface structures and morphologies. To mimic this display tactics, we designed similar deformation controlled surface engineering via strain-dependent cracks and folds to realize four types of novel mechanochromic devices: (1) transparency change mechanochromism (TCM), (2) luminescent mechanochromism (LM), (3) color alteration mechanochromism (CAM), and (4) encryption mechanochromism (EM), based on a simple bilayer system containing a rigid thin film and a soft substrate. These devices exhibit a wide scope of mechanochromic response with excellent sensitivity and reversibility. These novel devices are promising for applications in smart windows, dynamic optical switches, strain sensors, encryption, etc. |
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