Session W45: Emerging Trends in Polymer Composites for Applications in Energy and National Security

The Effect of Morphology on Ion Transport and Electrochemical Performance in Polymer-Based Composite Electrolytes

Solid polymer electrolytes are promising next generation energy storage materials and they are compatible with traditional roll-to-roll manufacturing methods and stack pressure requirements. Inorganic solid electrolytes such as oxides, sulfides, and halides, while exhibiting much higher room temperature ionic conductivity than common polymer electrolytes, all require high stack pressure to maintain contact at the electrode interfaces. Thus, composite electrolytes combining the high ionic conductivity of inorganic electrolytes and the processibility and ability to form conformal contact at the interfaces are promising to fulfill all the requirements for advanced battery technologies. In this presentation, I discuss the development of composite electrolytes with different morphologies and their effect on ion transport and performance.

In the first part, we develop a composite electrolyte with a three-dimensionally (3D) interconnected lithium aluminum titanium phosphate (LATP) ceramic electrolyte framework, and a backfilled polymer electrolyte serving as a processibility phase to enhance mechanical flexibility and ensure conformal contact at the electrode-electrolyte interfaces. We demonstrate that the composite exhibits improved transference number compared to neat polymer and enhanced dendrite resistance compared to dense ceramics. We also discuss the interface resistance within the 3D composite. By replacing the dual-ion-conducting polymer phase with a single-ion-conducting polymer phase, we elucidate factors that control the limiting current density of the composite electrolyte.

In the second part, we discuss the effect of particle size and spatial distribution on the ion transport barrier in a model composite electrolyte consisting of Li_0.34La_0.56TiO₃ (LLTO) ceramic and a single-ion-conducting polymer electrolyte. Broadband dielectric spectroscopy (BDS) reveals that the total ionic conductivity in these polymer-ceramic composites increases with the addition of LLTO nanorods, compared with neat polymer electrolyte. For comparison, composites made from a commercial source of LLTO with micrometer size particles do not show enhancement in conductivity. The spatial distribution of LLTO particles within the polymer matrix also plays a role in ion transport of the composite. These investigations shed light on how to design composite electrolytes to maximize favorable ion transport paths and minimize barriers.   

Morphology of Polymers and Ions on the Atomic-Scale Revealed by Cryogenic Transmission Electron Microscopy

The atomic-scale structures of proteins have been revealed by cryogenic transmission electron microscopy (cryo-TEM).  We leverage this development to image the molecular arrangement and crystal motifs in synthetic polymers.  We compare images of self-assembled nanosheets and nanofibers with simulation results, thereby validating the atomic-scale interactions assumed in the simulation with experimental data on the same scale.  We use atomically-defined amphiphilic polypeptoid block copolymers synthesized by solid-phase synthesis, and focus on halogen atoms that are either covalently or ionically bonded to the monomers of polypeptoids.  The covalently bonded case enables a study of the role of halogen-bonding in lattice structures in polymer self-assembly. The ionically bonded case, the polypeptoid which is positively charged, contains chloride counterions.   Cryo-TEM enables imaging of the counterions, providing direct images that we used to test theories of Stern layers and counterion condensation.    

Molecular Simulations of Polymer Nanocomposites

In this talk I will describe molecular simulations of two different polymer nanocomposites. A common strategy to control the nanoparticle (NP) distribution in composites is to graft polymer chains onto the NP surfaces. I will describe the use of a new method, theoretically-informed Langevin dynamics (TILD) simulations, to calculate equilibrium phase diagrams for grafted NPs in polymer melts. The phase diagram for NPs densely grafted with short A chains and blended with long B homopolymer chains is significantly shifted compared to that for an equivalent AB homopolymer blend. Adding additional A homopolymer leads to an increase in miscibility of the gNPs on the gNP-rich side of the phase diagram. The extra A homopolymer helps to compatibilize the interface between the gNPs and the matrix B chains. Our results are consistent with both experiments and modeling of poly(methyl methacrylate) (PMMA)-grafted silica NPs in poly(styrene-ran-acrylonititrile) (SAN) and PMMA-NP/SAN/PMMA composites.

A second polymer nanocomposite system of current interest is that of rubber filled with silica nanoparticles. It is important to understand the effects of high-pressure hydrogen gas on rubber materials used in hydrogen infrastructure. I will describe atomistic molecular dynamics simulations of the interface between a silica nanoparticle and a hydrogen gas-saturated, crosslinked rubber. The hydrogen gas preferentially adsorbs to the silica-rubber interface and has higher mobility at the interface than in the bulk. Hypothesizing that the excess gas may impact the strength of the weak, non-covalent interaction across the rubber-filler interface, we calculated the work of adhesion (WOA) in the presence of H2 gas.  A linear decrease in the WOA was found with increasing gas concentration.   

Understanding the character of photoresponse in glassy polymeric materials with molecular modeling

Highly cross-linked thermoset polymers have several excellent properties for use in structural applications, such as high glass transition temperature, high elastic modulus and yield stresses, and good environmental stability. However, these rigid materials also tend to experience brittle failure and poor ductility. Here, we consider azobenzene-loaded polymer glasses as a route to developing rigid resins and adhesives that rapidly change mechanical properties on irradiation. Although experiments and simulations have shown that photoactive groups can trigger strong property changes in polymeric materials, the vast majority of work in responsive polymers has been carried out on soft, compliant elastomers and solvent-swollen gels. Further, the response time of photo-activated soft materials is often on the order of minutes or hours. We explore the isomerization of photo-activated azobenzene incorporated in deeply glassy epoxy-amine networks. Using large-scale atomistic molecular simulations, we show that the character of the local environment near each responsive group strongly alters the "waiting time" between the photo-stimulus and individual molecular photo-isomerizations. Importantly, the median wait time for isomerization can be varied by orders of magnitude by tuning the strength of interactions between the dispersed azo-compounds and the surrounding matrix. Interestingly, the molecular simulations predict a wait time distribution having a power-law decay with exponents near unity, where the extent of the power-law distribution grows with decreasing temperature or increasing density, up to a point where the median time diverges with density. Overall, we find that the density and energies act as key predictors of isomerization events in the glass. These results suggest that future efforts to correlate isomerization with descriptions of the local packing and covalent environments will be central for a rational chemical design of responsive resins.   

Polymers, Composites, and Aerospace in 2030: Inventing the Stuff That Makes the Future

Over a hundred years ago, the pioneers of aviation took flight in no small part due to polymeric innovations. Unquestionably, the future of aerospace will look as different as the Wright Flyer and Curtiss June Bug does from the F35 and today's unmanned aerial vehicles (UAVs). The fundamental role of polymers and composites will remain unchanged though – they will be a crucial ingredient that enables these future machines to push the performance envelope. However, the demands will far surpass those required from today's structures, canopies, coatings, and optoelectronic subsystems. Crucial to this success will be asking "how do we optimize multifunctionality and manufacturability while forecasting the service life of an adaptive and responsive system of materials that includes polymers". Current research at the Air Force Research Laboratory will be used to discuss these challenges and highlight how digital technologies are being used to accelerate discovery, development, and deployment.