Session G42: Padden Award Symposium

Get in Line: Orienting Cylinders in Block Copolymer Thin Films via Shear

Block copolymer thin films have garnered much attention for their potential as nanolithographic masks. For many patterning applications, however, the need to impart well-defined order to the microdomains is paramount. One method to achieve long-range orientational order in thin films is through the use of shear. The ease with which orientation is achieved as well as the ultimate quality of alignment is strongly influenced by the copolymer composition and molecular weight, film thickness, and shearing conditions, as revealed using a series of cylinder-forming poly(styrene)-poly(hexylmethacrylate) (PS-PHMA) copolymers. Quality of in-plane alignment is assessed via atomic force microscopy, quantified through an orientational order parameter ($\psi_{2})$. The ease with which alignment is achieved is determined by measuring $\psi_{2}$ as a function of applied shear stress and comparing the results to a melting-recrystallization model which allows for the determination of two key alignment parameters: the critical stress needed for alignment ($\sigma_{c})$, and an orientation rate constant ($\Gamma )$. The $\psi _{2}$ vs. stress behavior, including the time-dependence, is well captured by the model. For monolayers, as PS weight fraction or overall molecular weight increases, $\sigma_{c}$ also increases, while $\Gamma $ greatly decreases. As the number of cylinder layers is increased, $\sigma_{c}$ decreases to a plateau. The ultimate quality of alignment is studied by comparing $\psi_{2}$ vs. lattice defect density for well-aligned films; $\psi_{2\, }$is limited by both undulations in the cylinder trajectories as well as the presence of isolated dislocations.   

How Mechanical Deformation of Polymers during Vitrification Alters the Subsequent Stability of the Glass

How stress and mechanical deformation impart mobility to polymer glasses have been studied primarily for materials where the glassy state was formed stress free. Here, we investigate the stability of polymer glasses where a constant stress is applied during the formation of the glassy state (thermal quench). Previously we found that physical aging is strongly dependent on the conditions during glass formation, including cooling rate and (often unintended) stress [Macromolecules 2012, 45, 1701]. We constructed a unique jig to apply a known stress to free-standing films during the thermal quench. We used ellipsometry to measure the physical aging rate of polystyrene films by quantifying the time-dependent decrease in film thickness that results from an increase in average film density during aging. As the magnitude of stress during vitrification increases, the physical aging rate quickly transitions over a small range of stresses to a faster aging rate, indicating the resulting glass is less stable [Soft Matter 2014, 10, 1572]. To explore this unique finding, we have constructed a computer-controlled apparatus to measure and apply stress and strain to polymer films during vitrification in order to characterize the temperature-dependent stress build up.   

Lithium dendrite growth through solid polymer electrolyte membranes

Replacing the graphite-based anode in current batteries with a lithium foil will result in a qualitative increase in the energy density of lithium batteries. The primary reason for not adopting lithium-foil anodes is the formation of dendrites during cell charging. In this study, stop-motion X-ray microtomography experiments were used to directly monitor the growth of lithium dendrites during electrochemical cycling of symmetric lithium-lithium cells with a block copolymer electrolyte. In an attempt to understand the relationship between viscoelastic properties of the electrolyte on dendrite formation, a series of complementary experiments including cell cycling, tomography, ac impedance, and rheology, were conducted above and below the glass transition temperature of the non-conducting poly(styrene) block; the conducting phase is a mixture of rubbery poly(ethylene oxide) and a lithium salt. The tomography experiments enable quantification of the evolution of strain in the block copolymer electrolyte. Our work provides fundamental insight into the dynamics of electrochemical deposition of metallic films in contact with high modulus polymer electrolytes. Rational approaches for slowing down and, perhaps, eliminating dendrite growth are proposed.   

DNA-programmable Nanoparticle Self-Assembly and Crystallization via Multi-Scale Modelling \& Simulation

In the past decades, DNA hybridization has proven promising to rationally guide nanoparticles to assemble into 1D, 2D and 3D structures, lattices and recently, faceted single crystals. In this sense, a gold nanoparticle coated by a dense shell of DNA behaves as a ``programmable atom equivalent.'' Using a scale-accurate coarse-grained model with explicit DNA chains, we identify that the key ingredient for achieving successful 3D crystallization is in the kinetics of DNA hybridization. We predict phase diagrams and propose suitable DNA linker sequences for optimal assembly. We determine the equilibrium shape of single crystals by computing surface energies. Surface energy fluctuations are further estimated for different surface orientations, and are shown to be critical in determining the equilibrium shape of a crystal. In addition, we apply a colloidal model with implicit DNA chains to study the kinetics of crystallization into faceted single crystals.   

Chain exchange in triblock copolymer micelles

Block polymer micelles offer a host of technological applications including drug delivery, viscosity modification, toughening of plastics, and colloidal stabilization. Molecular exchange between micelles directly influences the stability, structure and access to an equilibrium state in such systems and this property recently has been shown to be extraordinarily sensitive to the core block molecular weight in diblock copolymers. The dependence of micelle chain exchange dynamics on molecular architecture has not been reported. The present work conclusively addresses this issue using time-resolved small-angle neutron scattering (TR-SANS) applied to complimentary S-EP-S and EP-S-EP triblock copolymers dissolved in squalane, a selective solvent for the EP blocks, where S and EP refer to poly(styrene) and poly(ethylenepropylene), respectively. Following the overall SANS intensity as a function of time from judiciously deuterium labelled polymer and solvent mixtures directly probes the rate of molecular exchange. Remarkably, the two triblocks display exchange rates that differ by approximately ten orders of magnitude, even though the solvophobic S blocks are of comparable size. This discovery is considered in the context of a model that successfully explains S-EP diblock exchange dynamics.   

Mechanics of helical mesostructures from polymer-nanoparticle hybrids

We describe the fabrication and mechanics of polymer and nanoparticle (NP)-based high-aspect ratio mesostructures, which we refer to as ribbons, with nm-scale cross-sections and up to cm-scale lengths. When placed into a fluid like water, interfacial tension associated with the ribbons' intrinsic geometric asymmetry balances the elastic cost of bending, turning ribbons into helices with tunable preferred curvature. This universal, elastocapillary-based mechanism enables the reversible formation of helices from a variety of polymer and NP compositions, as demonstrated with specific examples of poly(methyl methacrylate), CdSe quantum dots, and gold NPs with polystyrene-azide or undecene ligands. Using custom-designed characterization methods, we quantitatively show that helices are highly stretchable with force-displacement relationships described by a nonlinear spring of finite extensibility. At small strains, these helices generate nN forces, affording mesostructures with a stiffness similar to single polymer chains (ca. 10$^{\mathrm{-6}}$ N/m), and when fully stretched, they display properties similar to synthetic polymer nanofibers. These mesostructures offer a novel platform for engineering tunable materials with a broad range of mechanical properties and organic or inorganic functionality.   

Simple, generalizable route to highly aligned block copolymer thin films

Macroscopic alignment of block copolymer domains in thin films is desired for many applications, such as cell responsive surfaces or optical polarizers. Alignment generally requires specialized tools that apply external fields, shear force gradient, or produce topological patterned substrates. This requirement limits the broad academic application of aligned BCPs. Here, we describe a simple modification of commonly utilized solvent vapor annealing (SVA) process for macroscopic alignment of BCPs. Adhering a flat, crosslinked elastomer pad to the BCP film leads to differential swelling between the elastomer pad and BCP to produce a shear force that aligns the ordered BCP domains. The role of elastomer properties, solvent quality, drying rate and degree of segregation of the block copolymer will be discussed to provide generalized rules for alignment with this technique. Cylindrical nanostructures formed in polystyrene-block-polydimethylsiloxane can be transformed into arrays of silica lines and increasing the thickness from a monolayer to bilayer can effectively halve the spacing of the lines. These results illustrate a generalized method for BCP alignment and a potential route for the generation of complex hierarchical assembled structures.   

Nanoscale physical properties of polymer glasses formed by solvent-assisted laser deposition

High-energy, low-density nanostructured polymer glasses are formed via the solvent-assisted laser deposition technique MAPLE (Matrix Assisted Pulsed Laser Evaporation). During film deposition, micro- to nano-size polymer/solvent clusters are ejected via laser ablation from a frozen dilute polymer solution. During flight to the substrate under vacuum, the clusters experience rapid cooling and solvent stripping, forming polymer nanoglobules. Bulk polymer films are formed via the gradual assembly of these spherical-like nanostructured building blocks (i.e. nanoglobules). The MAPLE process thus enables investigation of the exceptional properties of glasses formed under extreme processing conditions. In the bulk state, we probe the effect of process parameters and chemical identity of the thermal behavior of a series of methacrylate polymers. We also employ multiple techniques to directly measure the properties of the polymer nanoglobules and connect the results to the global film properties. This talk will address nanoscale dilatometry via AFM, in which the volume of an individual polymer nanoglobule is tracked as it is heated through its glass transition, as well as Flash DSC analysis of the thermal properties of nanogram size MAPLE-deposited polymer glasses. We then discuss these findings in the context of the material's unconventional route to the glassy state.   

Chains, Rings, and Dendrites of Active Colloidal Polymers

In order to better understand active polymeric matter, colloidal polymers are imaged, in situ in real time, obtaining not only temporal and spatial information about each ``monomer'' in these living polymers but also about the time-dependent and orientation-dependent correlations between them. Our reversible colloidal polymer system is assembled from self-propelled monomeric Janus particles with dynamic ``plug and play'' self-assembly and programmed direction-specific interactions between the particles. Enabling this, AC voltage induces dipoles on the monomeric Janus particles that link them into chains while also generating active phoretic motility. Unique features of this system relative to conventional Brownian polymers are emphasized.