Session D68: Highly Loaded and Morphologically Enhanced Polymer Nanocomposites

Infiltration of polymers into nanoparticle packings to produce highly loaded nanocomposites

Conventional methods of nanocomposite fabrication involve mixing and dispersing nanoparticles into a polymer matrix, making it challenging to produce composites with extremely high volume fractions (> 50 vol%) of nanoparticles. Recently our group has shown that such nanocomposites in the forms of films and membranes can be produced by capillary rise infiltration (CaRI) by thermally annealing a bilayer of polymer and nanoparticle. CaRI induces imbibition of polymer into the interstices of the nanoparticle packing via capillarity. This method has been further extended to induce polymer infiltration into nanoparticle packings through solvent annealing and leaching from a elastomer network. While these methods provide powerful ways to produce highly loaded nanocomposites, they also provide a rich platform to study the behavior of polymers under extreme nanoconfinement. The chain dimension of the polymer, which depends on its molecular weight, can be comparable to or greater than the average pore size of the nanoparticle packing. In this talk, I will share our current understanding of the transport phenomena of polymers under such nanoconfinement using a combination of experimental and computational approaches. I will show that the dynamics of CaRI depends strongly on the confinement ratio as well as the molecular weight of the polymer. In particular, the effective viscosity of the polymer can decrease or increase depending on the extent of confinement and the molecular weight of the polymer. We also show that the structure and properties of the resulting nanocomposites also depend strongly on the processing parameters and the molecular weight of the polymer.
  

Biomimetic Nanocomposites

Materials with difficult-to-attain combination of multiple properties - mechanical, electrical, chemical, optical, thermal, and transport, – represent the quintessential bottleneck for nearly all technologies. Nanocomposites with molecular, nano-, meso-, and microscale levels of structural engineering can provide such properties, while intrinsic ability of nanoscale components to self-assemble make them suitable for scalable synthesis.

Biomimetic nanocomposites exemplified by nanostructured nacre-like assemblies, provide a generalized approach to engineer materials. Continuing this research, we learn that the unique mechanics of tooth enamel can be replicated combining out-of-plane nanoscale columns with molecular precision of layer-by-layer (LBL) assembly. These composites reveal remarkably high vibrational damping thought to be impossible for stiff materials.

The novel type of biomimetic nanocomposites are those based on aramid nanofibers (ANFs). They spontaneously assemble into three-dimensional percolating networks reminiscent of cartilage. The nanoscale structure of ANF composites reveal nanoscale porosity that can be controlled by nanofiber branching. The latest results from multiple groups demonstrate that ANF composites resolve some of the essential property bottlenecks for ion-selective membranes, dendrite-resistant electrolytes, and structural batteries.

One of the emerging fields for biomimetic nanocomposites are optical devices. The high strain and strong polarization rotation make possible metaoptical devices with kirigami composites with wide-angle diffraction gratings for LIDARs and highly efficient quarter wave plates for THz scanners. Chiroplasmonic composites with mirror-asymmetrical 3D shapes of gold nanoparticles represent the first dynamically reconfigurable metaoptical composites for visible, near-IR and THz ranges of electromagnetic spectrum.   

Polymer Processing at Liquid Crystal-Air Interfaces

Fluid interfaces are unique environments for materials processing because, as inherently open systems, they promote dynamic transport from adjoining phases and offer anisotropic structures that give rise to strong directional interactions during assembly. Liquid crystal interfaces add further prospects for producing materials with directional ordering or anisotropic morphology. For example, colloids assembled at liquid crystal-air and liquid crystal-water interfaces can form chains or hexagonal lattices. In this talk, we demonstrate liquid crystal-mediated synthesis and assembly of polymer colloids at liquid crystal-air interfaces. The polymer colloids are produced by polymerization of acrylic monomers in non-reactive liquid crystal mesogens. We examine mechanisms governing the simultaneous polymer growth and assembly as a function of reaction time, initial monomer concentration, and liquid crystal director orientation. Results outline design rules to control the nucleation and growth of morphologically enhanced polymer composites.   

Driving and manipulating polymer degradation in nanocomposites via photothermal heating of the particle

We are interested in thermally-driving chemical reactions in small volumes within a solid material, where diffusion of reactants and products are limited. Such experiments are achieved by photothermally heating metal nanoparticles incorporated within a polymer, which creates significant heat generation at the particle and an inhomogeneous steady state temperature distribution across the solid. Specifically, polymer far from any particle is cool while in contrast, local regions surrounding a particle experience temperatures up to few 100s deg. C. Utilizing polymer degradation as a test reaction creates a detectable product as a permanent record of the temperature profile and, if localized, forms defects which dramatically alter mechanical properties. In general, manipulating the connection between the fraction of chemical degradation and mechanical strength as an object deteriorates is important for plastic waste management where microfragmentation may either be harmful or beneficial depending on the remediation strategy. Polyethylcyanoacrylate (PECA) degrades by depolymerizing and in confinement the monomer will repolymerize to form oligomers. Photothermal heating of PECA exhibits heterogeneous degradation, including defect formation and synthesis of a carbonaceous by-product localized around each particle. Polyethylene (PE) degrades via thermo-oxidative processes that rely on the presence of oxygen and pre-existing defects in addition to heat; consequently, photothermal heating of PE demonstrates homogeneous degradation due to the distributed nature of the reaction pathway.   

Biological Blueprints Towards Next Generation Multiscale Composites

There is an increasing need for the development of multifunctional lightweight materials that are strong, tough, and reconfigurable. Natural systems have evolved efficient strategies, exemplified in the biological tissues of numerous animal and plant species, to synthesize and construct composites from a limited selection of available starting materials that often exhibit exceptional mechanical properties that are similar, and frequently superior to, mechanical properties exhibited by many engineering materials. These biological systems have accomplished this feat by establishing controlled synthesis and hierarchical assembly of nano- to micro-scaled building blocks that are integrated into macroscale structures. However, Nature goes one step further, often producing materials with that display multi-functionality in order to provide organisms with a unique ecological advantage to ensure survival.
In this work, we investigate a variety of organisms that have taken advantage of hundreds of millions of years of evolutionary changes to derive structures, which are not only strong and tough, but also demonstrate the ability to articulate as well as display multifunctional features including damage sensing and self-cooling. We discuss the mechanical properties and functionality stemming from these hierarchical features as well as how they are formed. From the investigation of synthesis-structure-property relationships in these unique organisms, we are now developing and fabricating cost-effective and environmentally friendly multifunctional engineering composites.