Session Y02: Polymers in Extreme Environments

Live

Improving the ability of materials to absorb large amounts of kinetic energy from ballistic impact is important for a broad range of applications including body armor and protection of satellites. Carbon nanotubes (CNTs) have exceptional mechanical properties, especially very high axial stiffness and tensile strength. We investigate the energy absorption characteristics and associated deformation behavior of an initially low modulus, low strength, porous, unoriented nonwoven multi-walled carbon nanotubes (MWCNT) mats. The mats are comprised of unaligned, individual tubes and tube bundles bonded together into networks by both covalent and secondary bonds. Their ultra-high aspect ratios and large surface area enhance physical interactions between tubes. Employing a laser-induced micro-projectile impact test (LIPIT), μ-size silica projectiles are impacted at 200-1000 m/s resulting in extreme strain rates (10⁷ s^-1). The morphology of the MWCNT mat evolves dramatically during the deformation event resulting in strong strain hardening as the tubes and bundles are able to reconfigure and align. The mats exhibit record high specific energy absorption (E_p*), up to 12 MJ/kg, far beyond the of ~1 MJ/kg performance of common protection materials such as Kevlar, Dyneema, steel and aluminum.   

Live

Recent studies on amorphous homopolymers have shown that the high-rate microballistic fracture behavior of the polymer is directly related to its entanglement density. Based on this insight, we ask the question, “What is the relationship between chain entanglements and fracture behavior of matrix-free polymer grafted nanoparticle films?” In this work, we address this question by using laser-induced projectile testing (LIPIT) to study the fracture behavior of poly(methyl acrylate)-grafted silica nanoparticle thin films. By systematically varying the molecular mass of the poly(methyl acrylate), we find that kinetic energy loss, as well as the ballistic limit, is strongly dependent on the molecular mass of the polymer grafts that displays a non-monotonic trend. We show that the maximum energy absorption occurs when increase in molecular mass causes a transition from a close-packed concentrated polymer brush with no entanglements to a semi-dilute polymer brush with chain interdigitation. Thus the failure mechanism here is attributed to be pullout of short chain segments. Finally, we show that chain scission is the dominant failure mechanism above this transition point.   

Live

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.   

Live

The performance of materials under very high strain rate deformation and to very large strains is important for applications such as body armor and protection of spacecraft. The high strain rate performance of polymeric materials is both interesting and challenging because they exhibit very significant strain rate, pressure, and temperature dependent deformation behaviors. In addition, they feature many diverse types of nanoscale morphological features and when confined to thin films or other small scale geometries their characteristics like chain mobility, entanglements, microdomain shape and orientation can strongly influence their mechanical behavior compared to the bulk state. Over the past decade, the laser-induced projectile impact test (LIPIT) has been developed and applied to explore the deformation, failure, and recovery of very small test scale specimens of a variety of materials subjected to extreme deformation rates. LIPIT involves micro-projectiles accelerated at targets by rapid expansion of a elastomer launch membrane by a gas bubble produced by a laser ablation, achieving projectile velocities from 100 – 4000 m/s and strain rates on the order of 10⁶ -10⁸ s^-1. The nanoscale high rate deformation behavior of glassy homopolymers, block copolymers, grafted polymer nanoparticles and nonwoven nanofiber and MWCNT mats will be discussed with reference as to how these miniaturized tests on small samples can be translated to provide insight into macro scale performance.   

Live

Understanding the failure behavior of materials subjected to extreme impact events can lead to the development of protective systems that can protect human life and critical infrastructure from those events. Here, we report the failure behavior of monolithic polycarbonate plates subjected to ultra high strain rates due to the impact of a 4 mm projectile traveling in the velocity range of 3-6 km/s achieved using a two-stage light gas gun (2SLGG). Commercial grade polycarbonates with two different molecular weights have been considered. The glass transition temperature (Tg) determined by dynamic mechanical analysis (DMA) and dielectric thermal analysis (DETA) shifts to a higher temperature with increasing strain-rate. High-speed videos taken during the impact event captures the fluid-like flow of the ejecta material in the back-face debris clouds (BFDC). This behavior is different from PMMA, another glassy polymer, where glass-like failure with finely particulated BFDCs has been observed. Although Tg increases with increasing strain rate, the adiabatic heating process during the perforation event leads to a liquid-like flow behavior. The change in failure behavior as a function of molecular weight and plate thickness will be presented.   

Live

Interfacial polymer flows play a significant role in many domains. To understand these complex flows, it is crucial to simultaneously characterize the mechanical behavior of the fluid as well as the boundary conditions at the interfaces. Here, near-surface flow of neutral and charged polyacrylamides were investigated using total internal reflection fluorescence microscopy (TIRFM). Near-surface polymer solutions display non-linear stress/strain-rate relation, consistent with bulk shear-thinning mechanical behavior. This near-surface rheology is accompanied with non-trivial hydrodynamic boundary conditions. Neutral polymers display a chain-sized adsorbed layer on glass that is weekly dependent of the polymer concentration and shear rate. Conversely anionic polymers show apparent slip lengths ranging from a few nanometers to several microns. The viscosity dependence of the slip length is in good agreement with a simple two-layer depletion model, which can be suppressed by the addition of salt. These results show that TIRFM is an efficient tool to study the dynamics of complex fluids at interfaces, and that effective boundary conditions can be easily tuned over several orders of magnitude in microfluidic contexts.   

Live

In traditional dynamic impact experiments such as projectile impact testing, a common metric for accessing a material’s ability to withstand such impact is the ballistic limit, which is defined as the maximum projectile velocity a material can sustain without puncture. In this contribution, we quantify the ballistic limit of thin polymer films using a recently developed micro-projectile impact test called laser-induced projectile impact testing (LIPIT). Specifically, we use LIPIT to study the puncture behavior of free-standing polycarbonate thin films with thickness ranging from 60 nm to 500 nm at microprojectile impact velocities ranging from 50 m/s to 200 m/s. We show that the critical film thickness that defines the ballistic limit changes with impact velocity. For a given impact velocity, find that the ballistic resistance is related to the increase in mass of the system, which can be controlled by increasing the thickness of the polymer film. These results have important implications in understanding the length scale-dependent puncture resistance of polymer thin films relevant to dynamic impact applications.   

Not Participating

Nematic liquid-crystal elastomers combine heterogeneous molecular orientation fields with mechanically anisotropic cross-linking. One example is the cross-linked double-twist structure found in collagen fibrils and keratin macrofibrils, both important structural components of many animal tissues. Theoretical treatments of elastomers under strain typically impose a deformation field throughout the structure. However, in many experimental setups, the deformation is only imposed at the boundaries — for example at the ends of long fibrils. Since in real systems the molecular director and cross-link density fields can be heterogeneous, the strain field within the elastomer is generally heterogeneous as well. We present a general thermodynamic framework for modelling the deformation of a nematic liquid crystal elastomer. We derive coupled partial differential equations which fully characterize a spatially heterogeneous deformation gradient field for a given set of boundary conditions. To demonstrate the utility of this method we solve for the full deformation gradient field within an anisotropically cross-linked double-twist cylinder subjected to experimentally relevant boundary conditions.   

Live

Metamorphosis in living systems has inspired the formation of complex 3D geometries from compact 2D domains, which can be replicated in soft materials through origami. Self-oscillating active polymer gels that exhibit chemo-mechanical transduction, can be enabling materials for designing self-folding structures. Here, we develop a generic theoretical framework, based on finite element method, to simulate mechanical response of both active and inactive polymer gels. Our approach combines the Flory Huggins theory to include polymer-solvent interactions and non-Gaussian statistical mechanics with limited polymer compressibility, to account for large elastic deformations in polymer gels. Through our 3D simulations, we capture chemo-mechanical oscillations in active polymer gels, which transform to neutrally swollen gel in the limiting case. Also, we investigate the role of friction between polymer and solvent on the dynamic behavior of polymer gels. Moreover, as a special case, we can simulate gels with incompressible polymer matrix and Gaussian chain statistics. Our findings can be useful for designing self-folding polymeric shapes to be used as smart miniaturized devices.   

Live

The 2D phase behavior of assemblies of silica-coated gold nanorods (NRs), silica-coated hematite nano-ellipsoids (NEs), and their binary mixtures with silica nanospheres at a liquid surface was studied by in situ scanning electron microscopy (SEM). All the nanoparticles (NPs) were functionalized with polyethylene glycol (PEG) and then dispersed in ionic liquid (IL) 1-ethyl 3-methylimidazolium ethyl sulfate ([EMIM][EtSO4]). The NPs gradually segregated to the IL surface until the self-assembled monolayer reached a jammed state. The evolution of the assembly was directly visualized by SEM. The dense assemblies were characterized by the nematic and smectic order parameters (global and local) and orientational correlation functions. Partially adsorbed NPs with orientations out of the interfacial plane reduced the in-plane orientational order in the jammed monolayers. NP-NP interactions and the fraction of out of plane NPs were shape-dependent; NEs preferentially associated side-by-side to form flexible stacks while NRs did not. In the dense assemblies of binary mixtures of NRs and NEs with nanospheres, uniformly mixed and phase-separated morphologies were obtained controlled by the NP-NP interactions and NP adsorption kinetics.   

Live

An advanced laser-induced projectile impact test (LIPIT) apparatus was used to investigate high rate deformation of lamellar block copolymer (BCP) particles. By employing polyvinyl alcohol (PVA) as surfactant, chloroform as solvent and homogenization processing of a 42 kg/mol-32 kg/mol PS-PDMS BCP, (45 vol % PDMS) nearly defect-free, concentric microspheres were produced. These particles (diameters from 2.8 to 3.8 microns), were then individually launched at supersonic velocities (300~700 m/s) to impact a gold coated silicon substrate at normal incidence. Extreme microstructural deformations were observed after impact such as layer kinking to form axisymmetric chevron boundaries and layer compression as well as different types of overall particle shapes, depending on the magnitude of the KE at impact. Interestingly, spreading/wetting of the particle over the substrate occurred at higher speeds (480 m/s < v_i < 700 m/s), due to the shock compression adiabatic heating enabling rapid lateral diffusion of polymers along inter-material dividing surface (IMDS). The effects of different substrate surface energies and thermal conductivities on the deformation-spreading were also explored.   

On Demand

We employ Monte Carlo simulations to study the packing efficiency and phase behavior of polymers under extreme plate-like confinement [1]. The latter is realized through the presence of flat, parallel walls in one dimension with the inter-wall distance being equal to the diameter of the spherical monomers. The hard sphere and square well models are employed to describe monomer interactions [2]. We study in detail how intensity and range of attraction, packing density and chain flexibility and length affect the ability of chains to pack and crystallize. The characteristic crystallographic element (CCE) descriptor [3] is used to gauge local structure and its similarity to reference 2-D crystals. A simple model is proposed to explain the observed simulation trends and the established ordered morphologies.

[1] P. Ramos, N. C. Karayiannis and M. Laso, J. Comput. Phys. 375, 918 (2018).
[2] M. Herranz et al., Polymers 12, 1111 (2020).
[3] P. Ramos et al., Crystals (2020, to appear); DOI: 10.20944/preprints202010.0294.v1   

On Demand

Hydrogels mimicking the mechanical responses of biomaterials can lead to new applications of these materials. An example of such biomaterials is resilin protein, which is primarily responsible for power-amplified activities in many species like locomotion, feeding, and defense. To mimic the power-amplified activities in biological species, a synthetic material needs to be highly stretchable, resilient, and retractable. Here, we present a hydrogel system synthesized through a simple chemical reaction scheme using hydrophilic acrylic acid and methacrylamide, and hydrophobic poly(propylene glycol) diacrylate [PPGDA]. These gels show tunable elastic modulus of 15-100 kPa, stretchability up to 8.6 times, and resilience of 98%. These gels achieve a retraction velocity of 16 m/s and an acceleration of 4×10³ m/s² when released from a stretched state. These values are comparable to those observed in biological species during power amplification. Because of their high stretchability and resilience, these gels have been utilized to launch projectiles over a long distance. The stability of these gels in a saline environment opens up their application in developing undersea soft-robotics, prosthetics, and engineered devices.