Bulletin of the American Physical Society
APS March Meeting 2019
Volume 64, Number 2
Monday–Friday, March 4–8, 2019; Boston, Massachusetts
Session F30: Extreme Deformation II: Rate and Size Effects in Glasses, Networks, and FibersFocus
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Sponsoring Units: DPOLY GSOFT DFD DBIO Chair: Alfred Crosby, University of Massachusetts Amherst Room: BCEC 162B |
Tuesday, March 5, 2019 11:15AM - 11:27AM |
F30.00001: Deformation of Matrix-Free, Glassy Single Component Polymer NanoComposite at Extreme High Strain Rates Jinho Hyon, Edwin Thomas, Jason Streit, Richard Vaia Very thin, freestanding glassy polystyrene (PS) films show unexpectedly large energy absorption under rapid axisymmetric tensile loading at ballistic strain rates (~ 107/s). For supersonic microprojectile (3.7 μm diameter) impact (350~800 m/s), the more mobile and less entangled near-surface regions of the PS facilitate crazing and dramatically increase craze multiplication and subsequent growth with accompanying large adiabatic temperature rise of the highly deforming film. Here, we investigate the influence on the high rate deformation of grafting of the PS chains to nanoparticle (NP) surfaces. The covalent anchoring of several hundred polymer chains to individual silica NPs and the well-entangled coronal regions between NPs improve the stress transfer through the composite, yielding polymer nanocomposites with excellent energy absorption. The single component nanocomposite PS grafted nanoparticle (PSgNP) films (~ 1% v/v, 16nm diameter SiO2 NPs) show 25% enhanced high kinetic energy absorption per unit mass of the target film over the previous record specific energy absorption of the thin, freestanding homopolymer PS films. |
Tuesday, March 5, 2019 11:27AM - 11:39AM |
F30.00002: High Rate Fracture Behavior of Polycarbonate Films via Supersonic Microprojectile Impact Testing Edwin Chan, Wanting Xie, Christopher Soles, Jae-Hwang Lee The fracture behavior of amorphous polymer glasses is strongly linked to the entanglement density. Traditionally, it is established that polymers exhibiting strain hardening, such as polycarbonate, tend to undergo ductile rather than brittle fracture due to its high entanglement density. In this work, we use Laser-induced Projectile Impact Testing (LIPIT) to study the supersonic fracture behavior of polycarbonate films as a function of entanglement density. We show that the kinetic energy for microparticle penetration through the polymer film is affected by the molecular mass of the polymer or the plasticizer content. We show that this variation in the penetration energy to a change in the mechanism of fracture. Specifically, the fracture mechanism transitions from crazing to shear yielding due to changes in the entanglement density of polycarbonate film. |
Tuesday, March 5, 2019 11:39AM - 11:51AM |
F30.00003: Strain Rate Effects During Ultra-High Strain Rate Penetration of Polymeric Materials M. Hunter Bowering, W. F. Heard, Thomas E. Lacy, Jr, Charles U. Pittman, Jr., Santanu Kundu Energy dissipation during penetration of a material is an important consideration in designing lightweight armor to protect structures, equipment, and personnel from impact damage. A series of impact tests, with projectile velocities in the range 2-7 km/s, was performed on monolithic plates of ultra-high molecular weight polyethylene (UHMWPE), high density polyethylene (HDPE), and poly(methyl methacrylate) (PMMA). A relationship between back face debris cloud (BFDC) velocity and impact velocity was developed for each material. Damage zone sizes were compared, offering insights into the effects of molecular architecture on stress delocalization and energy dissipation during perforation. Monolithic plate thicknesses were varied in the UHMWPE and HDPE target populations to assess thickness effects on damage zone size and BFDC. Comparison of the apparent failure mechanisms and damage metrics, in conjunction with thermal analysis, were used to explain the relative performance of each material. PMMA demonstrated glass-like failure with finely particulated BFDCs, while perforation of HDPE resulted in fluid-like BFDCs. UHMWPE damage morphology possessed qualities of both PMMA and HDPE. |
Tuesday, March 5, 2019 11:51AM - 12:03PM |
F30.00004: The Role of Fast Polymer Dynamics on the Mechanical Toughness of Polymeric Materials Christopher Soles, Kanae Ito, Adam B Burns, Kevin A. Masser, Joseph L. Lenhart, Madhu Sudan Tyagi It is understood that there is a link between molecular relaxations in glassy polymers and mechanical toughness. The notion is that these relaxations dissipate the energy of impact and thereby enhance toughness. Decades of research have focused on correlating the mechanical toughness of a polymer with the relaxation processes quantified by relatively slow characterization techniques such as dynamic mechanical analysis, dielectric spectroscopy, or solid-state nuclear magnetic resonance. However, there is a disconnect because the time and length scale of the molecular mechanisms are typically several orders of magnitude faster and more localized than the experimental techniques used to characterize them. We revisit this by using quasielastic neutron scattering (QENS) to quantify both the collective excitation and molecular relaxations that occur on the time scale of ps to ns, and see how these motions correlate with mechanical toughness. We demonstrate a strong correlation between these fast polymer relaxations and toughness. We show that these fast polymer relaxations play a critical role in predicting the ability of a material mitigate impact under ballistic conditions where the strain rates can approach an inverse microsecond. |
Tuesday, March 5, 2019 12:03PM - 12:15PM |
F30.00005: A Quasi-mimetics Approach for Uncovering Starch-based Hybrid Materials Yin Fang, Yuanwen Jiang, Endao Han, Yiliang Lin, Xianghui Xiao, Jin Wang, Heinrich M Jaeger, Bozhi Tian Materials design through biomimetics aims to achieve functions similar to those found in nature. However, this requires a thorough understanding of the system used for mimicking (SuM), which would become a challenge when SuM is complex. Here, we propose a quasi-mimetics approach, which focuses on partial components in the SuM. Different from biomimetics, quasi-mimetics aims to probe the elusive behaviors in the complex SuM, as well as to uncover new functions not found in SuM. We illustrate this approach with a starch particle-embedded hydrogel composite where noodle dough serves as a SuM. The starch hybrid composite displays unprecedentedly high stretchability both in air and water, self-healing behavior, and strain-dependent mechanical training effect. Our mechanistic study showed that the unique mechanical features of the starch hybrid gel are related to the formation of covalent bonds between the interface of the starch particles and polymer network, as well as the dynamic hydrogen bonds in the hydrogel matrix. The new quasi-mimetics approach can be broadly applied to other material exploration and device applications. |
Tuesday, March 5, 2019 12:15PM - 12:27PM |
F30.00006: Starch embedded hydrogels: Linking macroscopic mechanical properties with microscopic particle configurations Endao Han, Yin Fang, Xianghui Xiao, Jin Wang, Bozhi Tian, Heinrich M Jaeger When noodles are made, a flour dough can be stretched to an extremely long length without breaking. What can we learn from it in order to synthesize innovative hydrogels? Mimicking the microstructure of flour dough, we synthesized a hybrid hydrogel by embedding wheat starch granules in an alginate-PAA gel network. This hybrid hydrogel shows many amazing mechanical properties, such as substantially improved stiffness and toughness, extremely high stretchability (up to 8000% strain), and persistent memory that is rewritable with training. With the help of x-ray microtomography, we found direct links between the macroscopic stress-strain curves and the microscopic particle structures when such hybrid hydrogels are deformed under uniaxial extension. This quasi-mimetic process allows us to not only produce materials with novel properties, but also explore new physics with simple, highly controlled systems. |
Tuesday, March 5, 2019 12:27PM - 1:03PM |
F30.00007: The Thermoviscoelastic Behavior of a Main-Chain Liquid Crystal Elastomer Invited Speaker: Thao Nguyen Liquid crystal elastomers (LCEs) combine the anisotropic ordering of liquid crystals and viscoelastic behavior of an elastomeric network. This leads to remarkable mechanical properties, including reversible shape change in response to temperature or light, deformation induced soft elasticity and anisotropy, and extreme dissipation behavior. In this presentation, I will describe our efforts to characterize the rate-dependent and temperature-dependent large deformation behavior of an acrylate main-chain LCE networks to investigate the effects of mesogen order and chain alignment on the stress-strain response, soft elasticity behavior, and hysteresis. The experimental results showed that time-temperature superposition can be applied to the stress response of the LCEs in the nematic state throughout the soft elasticity plateau. All LCE networks, polydomains and monodomains stretched parallel and perpendicular to the director, exhibited the same rate-dependence for the modulus and hysteresis. However, the onset strain and duration of the soft elasticity plateau were relatively insensitive to the strain rate. These findings suggest that the rate-dependent stress response of the acrylate LCEs in the nematic state is insensitive to mesogen reorientation. |
Tuesday, March 5, 2019 1:03PM - 1:15PM |
F30.00008: Puncture of Polymer Gels at Small Size Scales Shruti Rattan, Alfred Crosby Understanding failure processes of polymer gels is critical for numerous applications, ranging from adhesives to protective materials. We discuss the failure processes associated with the deep indentation and puncture of soft gels with an axisymmetric probe. We commonly observe that the first critical transition, associated with puncture, occurs at nominal stresses that can be as much as 100 to 1000 times the elastic modulus. To understand how soft gels can sustain such stress levels prior to initial failure, we have studied puncture as a function of size scale, velocity, and material network structure. Spherically-tipped indenters of radii, R=0.4-66 um were used to characterize puncture at length scales well above the network mesh size (nm), on the same order of magnitude as the elasto-capillary length (um), and significantly below the elasto-fracture length (mm). Critical energy release rate was found to be in agreement with the predicted scaling from the classical Lake-Thomas model modified for gel fracture via the failure mechanism of chain pull-out and plastic yielding of micelles. These experiments and proposed relationships provide new insight into how gels fail and how design paradigms may be shifted to more effectively engineer with soft gels. |
Tuesday, March 5, 2019 1:15PM - 1:27PM |
F30.00009: Size-dependent viscoelasticity of electrospun polymer nanofibers Shengqiang Cai Electrospun polymer nanofibers have garnered significant interest due to their strong size-dependent material properties, such as tensile moduli, strength, toughness, and glass transition temperatures. These properties are closely correlated with polymer chain dynamics. In most applications, polymers usually exhibit viscoelastic behaviors such as stress relaxation and creep, which are also determined by the motion of polymer chains. However, the size-dependent viscoelasticity has not been studied previously in polymer nanofibers. Here, we report the first experimental evidence of significant size-dependent stress relaxation in electrospun Nylon-11 nanofibers as well as size-dependent viscosity of the confined amorphous regions. In conjunction with the dramatically increasing stiffness of nano-scaled fibers, this strong relaxation enables size-tunable properties which break the traditional damping-stiffness tradeoff, qualifying electrospun nanofibers as a promising set of size-tunable materials with an unusual and highly desirable combination of simultaneously high stiffness and large mechanical energy dissipation. |
Tuesday, March 5, 2019 1:27PM - 1:39PM |
F30.00010: The Complex Role of Crystalline Structure in the Mechanical Properties of UHMWPE Fibers Christopher Henry, Giuseppe R Palmese, Nicolas J Alvarez The mechanical properties of semi-crystalline polymeric materials are largely determined by the architecture and amount of the crystalline domains. It is known for UHMWPE fibers that the toughest and stiffest fibers exhibit a structure consisting of extended chain crystals and lamella. This structure is produced via a post-drawing process where the fiber is deformed causing the re-organization of the polymer chains. There is a lack of understanding regarding how the crystalline structure develops during drawing and what role the various structural aspects play in the mechanical properties. We show, using SAXS/WAXS, that various crystalline structures can be produced during the manufacturing of the PE fibers and that these architectures develop along unique pathways when drawn. Using a VADER1000 to perform post-drawing and tensile testing, understanding of the fibers mechanical response is found. We show that unique structures exhibit distinct relaxations and a critical stress at failure of PE fibers. This study provides a much deeper understanding of crystalline behavior in extension and allows for improved design of the post-drawing process. |
Tuesday, March 5, 2019 1:39PM - 1:51PM |
F30.00011: Energy Storage and Release in Twisted, Buckled, and Helical Fibers Adam Fortais, Kari Dalnoki-Veress The buckling and twisting of slender, elastic fibers is a deep and well-studied field. Recently, there has been great interest in applying this knowledge to the development of miniature linear actuators (artificial muscle), but also to investigate linkages with the formation of helices in nature (e.g. tendrils) as well as DNA supercoiling. A slender rod that is twisted with respect to a fixed end will spontaneously form a hockle, or loop, to relieve the torsional stress that builds. Further twisting results in the formation of plectonemes — a helical excursion in the fiber that extends with additional twisting. Here we investigate the energy stored and subsequently released by hockles and plectonemes as they are pulled apart, in analogy with force spectroscopy studies of DNA and protein folding. Hysteresis loops in the snapping and unsnapping inform the stored energy in the twisted fiber structures. |
Tuesday, March 5, 2019 1:51PM - 2:03PM |
F30.00012: Fiber networks with inter-fiber adhesion: role of adhesion in extreme network mechanics Vineet Negi, Ahmed Sengab, Catalin Picu Many soft materials of biological and industrial interest are composed from nanofibers. In such cases, inter-fiber adhesion may produce fiber bundling and organization on scales larger than that of individual components. We study both non-cross-linked [1] and cross-linked [2] networks with adhesive interactions between filaments. We determine the parametric range in which adhesion reorganizes the network and study the mechanical behavior of the resulting structures. We observe a broad range of tunable properties, including softening in tension, large strain range in which the structure responds linearly to applied strains, and adhesion-dependent elastic moduli. The results provide guidelines for material design and demonstrate that controlling inter-fiber adhesion may lead to fibrous materials with exceptional properties and behavior. |
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