Session K4: From Nano to Meso: Assembly, Structure and Dynamics of Polymers and Polymer Nanocomposite Thin Films I - Industry Day

Directed Assembly of Nanofilled Polymer Thin Films

Facile directed self-assembly (DSA) of multicomponent thin films is important for potential technological applications. This requires a fine control of a complex interplay of processing parameters that need to be properly optimized for different organized structures. This talk will discuss some of our recent success towards realizing tunable DSA of soft matter multicomponent systems involving a dispersion of polymer-grafted nanoparticles in block copolymer or homopolymer matrices. DSA methods for such multicomponent films will be discussed. These include the use of zone-annealing with soft-shear to create highly anisotropic nanoparticle arrays, while direct immersion annealing (DIA) has been used to order nanoparticle filled films by dipping the films into controlled solvent quality solvent mixtures. A recently observed phenomena of confinement driven entropic order and phase segregation of polymer grafted nanoparticles in similar and dissimilar polymer matrices in melt state will be discussed. A high density of nano particles of different types ranging from metallic to inorganic to organic were patterned almost exclusively into channels via topographical soft confinement using entropic forces. Enthalpic interactions between the nanoparticle grafted layer and the polymer matrix could be used as a further handle to tune the directed assembly of the nanoparticles. The phenomena will be discussed in terms of confinement parameters, partition coefficient, free energy gain and entropic versus enthalpic interactions.   

Photothermal heating at the nano and meso scales within polymer nanocomposites

Metal nanoparticles strongly absorb specific wavelengths of visible/infrared light with no radiative relaxation by which to release this energy. As a result, the absorbed energy is efficiently converted to local heat (a photothermal effect [1]). With an effective cross-section of up to 10 times its physical size, each particle acts as a "super-sized" absorber even when embedded within a material environment, resulting in dramatic heating [2,3] originating at the particles. Polymer nanocomposites containing metal nanoparticles can then be probed [4] and altered by applying internal heat at nano- and meso- length scales. I'll discuss our recent studies [5] utilizing this effect, including internal annealing to increase crystallinity fraction in both films and nanofibers of poly(ethylene oxide) [6], in-situ curing of epoxy, and intentional degradation of starch-poly(ethyl cyanoacrylate) composites. The talk will highlight the unique features of a photothermal approach, such as the ability to couple energy quickly (as light) into low thermal conductivity environments and possible changes in thermal conductivity at the particle-polymer interface. [1] S. Maity et al., \textit{Polymer} \textbf{52}, 1674 (2011). [2] S. Maity et al., \textit{Adv. Funct. Mater.} \textbf{22}, 5259 (2012). [3] S. Maity et al., \textit{Part. & Part. Sys. Char.} \textbf{30}, 193 (2013). [4] S. Maity et al., \textit{Nanoscale} \textbf{6}, 15236 (2014). [5] D. B. Abbott et al., \textit{Macrom. Chem. & Phys.} \textbf{215}, 2345 (2014). [6] V. Viswanath et al.,\textit{Macromolecules} \textbf{ 46}, 8596 (2013).   

Evaporation-induced Nanoparticle Self-Assembly in a Polymer Matrix

A critical challenge in many applications of polymer nanocomposites is to control the dispersion of nanoparticles in a polymer matrix. We employ large-scale molecular dynamics simulations to study the assembly of nanoparticles as the solvent evaporates from a polymer solution containing nanoparticles. Results show that the organization of nanoparticles can be controlled by varying the strength of the polymer-nanoparticle interactions. When the nanoparticles and polymers strongly attract, as the solvent evaporates, a concentrated polymer film forms at the surface and entraps a layer of nanoparticles, which assemble into a close-packed hexagonal lattice. This dense film of polymers and nanoparticles dramatically reduce the rate of evaporation as the solvent has to transverse the film to reach the surface. If the nanoparticle-polymer interactions are weak, then as the solvent evaporates, the surface layer is almost entirely made of polymers. The nanoparticles are largely excluded from the surface and dispersed randomly in the region below the surface layer. In this case the slowing-down of the evaporation by the surface layer is less dramatic. Also of interest is the case of a nanoparticle solution in contact with polymers that are end grafted to a flat surface to form a polymer brush. For a relatively weak nanoparticle-brush attraction, after evaporation of the solvent the nanoparticles straddle the brush surface and form an ordered lattice. For a strong nanoparticle-polymer attraction, however, the nanoparticles are engulfed inside the brush and the packing quality diminishes because the lateral diffusion of the nanoparticles is suppressed. To better understand the nanoparticle-brush interactions, our calculations to quantify the free energy penalty of inserting a nanoparticle into a polymer brush will also be discussed.   

Influence of microstructure and environment on nanoparticle membrane and superlattice mechanical properties

Assembly of nanoparticles (NPs) offers a means to tailor materials, incorporating unique nanoscale electro-optical behavior with controllable, responsive mechanical properties. Encoding NPs with organic ligands provides a way to simultaneously drive assembly and control assembly properties. Atomistic molecular dynamics simulations of alkanethiol-coated gold nanoparticles are used to examine how coating chemistry, temperature, and assembly history affect the properties of two-dimensional nanoparticle membranes and three-dimensional nanoparticle superlattices. Specifically, NPs were coated with dodecanethiol and octadecanethiol ligands with COOH or CH$_3$ end groups and assembled into two-dimensional membranes at water vapor interface. Capping ligands with hydrogen-bond forming carboxyl groups rather than methyl groups more than doubles the membrane Young’s modulus from 1.5 to 3.6 GPa. The orientational order of the coating oligomers indicates that ligands strongly bundle and orient within the membrane. This effect inhibits ligand interdigitation, decreasing stiffness. Ligand structure is also highly temperature dependent, causing membranes to lose mechanical stability at about 400K. We observe that the interface asymmetry leads to a measurable stress asymmetry. Due to buckling, stresses in 2D membranes are typically quite small, however 3D superlattices can reversibly reach pressures of 8 GPa. Simulations show that at these pressures the ligand-core bond can be an important failure point, and experiments show that core sintering occurs at high pressure, creating novel 3D and quasi-2D structures. \\[4pt] Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S.~Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000.   

Polymer Melt Diffusion inside Nanoscale Cylindrical Pores.

Polymers in composites and inside porous media are frequently confined to spaces that are comparable to or even smaller than their mean end-to-end distances in the unconfined bulk state. Understanding the impact of nanoscale confinement on both polymer structure and dynamics is critical during processing and in applications. Anodized aluminum oxide (AAO) membranes with uniform cylindrical pores (diameters 18, 35, 55 or 80 nm) were filled with polystyrene (200 kDa) and then a thin layer of deuterated polystyrene was deposited on top. After annealing the concentration profile of the deuterated polymer was measured using elastic recoil detection and the center-of-mass polymer diffusion coefficient was determined. Melt diffusion is faster in AAO membranes with smaller pore diameters. This experimental finding is corroborated by coarse grain simulations with neutral interactions with the pore walls, although the increase is more pronounced in the simulations. Our simulations previously found that chain conformations slightly elongated parallel to the cylinder axis and compressed perpendicular to the cylinder and the number of entanglements per chain decreases as the cylinder diameter decreases. It is primarily the reduction in polymer entanglements that allows polymers to diffuse faster when the pore diameter is smaller in an athermal or weakly interacting system. Segmental dynamics have been measured using quasielastic neutron scattering. Polymer diffusion is cylindrical pores is now being studied at a fixed pore diameter as a function of molecular weight.