Session H4: Dillon Medal Symposium

John H. Dillon Medal: Tapered Block Copolymers: Tuning Self-Assembly and Properties by Manipulating Monomer Segment Distributions

The self-assembly of block copolymers (BCPs) presents unique opportunities to design materials with attractive chemical and mechanical properties based on the ability of BCPs to form periodic structures with nanoscale domain spacings. One area of recent progress in our group focuses on the behavior of tapered BCPs in which the segment distribution at the interface between blocks is synthetically varied to tune morphology, domain density profiles, thermal transitions as well as mechanical and transport properties. Two application targets for these materials are lithium-ion conducting membranes for batteries and nanostructured thin films for nanotemplates and barrier membranes. In the first target area, we found that the taper volume fraction and composition allow us to manipulate the self-assembly of salt-doped BCPs in a well-defined manner that permits optimization of morphology and ion-content. Additionally, we found that the tapered interfaces influence the glass-transition behavior of the ion-conducting block leading to significant changes in lithium-ion transport (ion conductivity). In the second target area, we found the taper content alters the rate of self-assembly as well as the rate of island/hole formation (and ultimate island/hole size) upon thermal annealing. Additionally, using reflectivity techniques, we probed the domain density profiles as a function of taper composition and linked these profiles to changes in domain spacing and glass transition temperature. Overall, these studies show the versatility of tapering to provide a unique handle for simultaneously optimizing multiple materials properties.   

Dodecagonal Quasicrystal Phase in a Diblock Copolymer Melt

Recent experiments with low molecular weight asymmetric poly(isoprene-$b$-lactide) (PI-PLA) diblock copolymers have established an equilibrium Frank-Kasper $\sigma $-phase at compositions between 18 and 22 percent by volume PLA, which transforms to a BCC phase followed by disordering with increasing temperature. This presentation will describe synchrotron small-angle x-ray scattering and dynamic mechanical spectroscopy experiments conducted following rapid temperature quenches from the disordered state to temperatures associated with the $\sigma $-phase. We document the development of a long-lived dodecagonal quasicrystalline (DQC) phase that transforms with time into the associated quasicrystal approximate $\sigma $-phase at a rate that is highly temperature dependent. Remarkably, the DQC does not form from either the $\sigma $-phase or BCC state. These finding will be discussed in the context of an apparent spontaneous structural transition that occurs when the disordered melt is supercooled below a threshold temperature coincident with the BCC to $\sigma $-phase order-order transition temperature.   

A Cool Way to Form High-Conductivity Two-Dimensional Polymers Using Ice

Ice surfaces are used as removable hard templates to form two-dimensional polyaniline (PANI) nanosheets. Distinctly high current flows of 5.5 mA at 1 V and a high electrical conductivity of 35 S/cm were obtained for the PANI nanosheets, which marked a significant improvement from the literature values on other PANIs reported over the past decades. These improved electrical properties of ice-templated PANI nanosheets were attributed to the long-range ordered edge-on p-stacking of the quinoid ring, ascribed to the ice surface-assisted vertical growth of PANI. The unprecedented advantages of the ice-templated PANI nanosheets are two-fold. First, the PANI nanosheet can be easily transferred onto various types of substrates via “float off” from the ice surfaces. Second, PANI can be patterned into any shape using predetermined masks, and this is expected to facilitate the eventual convenient and inexpensive application of conducting polymers in versatile electronic device forms.   

Molecular transport into and out of ionic-liquid filled block copolymer vesicles in water

We have developed a method to prepare stable, size-controlled block copolymer vesicles that contain ionic liquid in the interior, but that are dispersed in water. Such nanoemulsions are of interest as nanoreactors, because the mass transfer and cost limitations of ionic liquids are circumvented. However, a crucial question is whether target molecules ($e.g$., reagents and products) can enter and leave the vesicles, respectively, on a useful time scale ($i.e$., seconds or shorter). In this talk we will briefly describe methods to prepare such vesicles with narrow size distributions, using poly(styrene)-\textit{block}-poly(ethylene oxide) and poly(butadiene)-\textit{block}-poly(ethylene oxide) copolymers of various compositions. We will then present results of pulsed-field gradient NMR measurements of probe diffusion that yield independent measurements of the entry and escape rates for selected small molecules, as a function of membrane thickness and temperature.   

Influencing the structure of block copolymer micelles with small molecule additives

Amphiphilic block copolymer micelles in water are under broad exploration for drug delivery applications due to their high loading capacity and targeted drug delivery. We aim to understand the kinetic and thermodynamic processes that underlie the self-assembly of diblock copolymer micelle systems. The present work focuses on diblock copolymers containing poly(ethylene oxide) (a hydrophilic polymer) and polycaprolactone (a hydrophobic polymer), which spontaneously self-assemble into spherical micelles in water. Addition of a common good solvent (a co-solvent) for both of the constituting blocks, such as tetrahydrofuran (THF), reduces the interfacial tension at the core-corona interface. We are currently investigating the effect of this phenomenon on the micelle structural properties, using small-angle scattering and nuclear magnetic resonance. We have characterized the hydrodynamic radius, core radius, corona thickness, aggregation number, degree of swelling of the micelle core with the co-solvent, and unimer (free chain) concentration, as a function of the co-solvent concentration. Fundamental knowledge from these studies will inform design of drug delivery systems by allowing us to tailor micelle properties for optimal cargo loading.   

Computationally Guided Design of Polymer Electrolytes for Battery Applications

We develop an efficient computational framework for guiding the design of polymer electrolytes for Li battery applications. Short-times molecular dynamics (MD) simulations are employed to identify key structural and dynamic features in the solvation and motion of Li ions, such as the structure of the solvation shells, the spatial distribution of solvation sites, and the polymer segmental mobility. Comparative studies on six polyester-based polymers and polyethylene oxide (PEO) yield good agreement with experimental data on the ion conductivities, and reveal significant differences in the ion diffusion mechanism between PEO and the polyesters. The molecular insights from the MD simulations are used to build a chemically specific coarse-grained model in the spirit of the dynamic bond percolation model of Druger, Ratner and Nitzan. We apply this coarse-grained model to characterize Li ion diffusion in several existing and yet-to-be synthesized polyethers that differ by oxygen content and backbone stiffness. Good agreement is obtained between the predictions of the coarse-grained model and long-timescale atomistic MD simulations, thus providing validation of the model. Our study predicts higher Li ion diffusivity in poly(trimethylene oxide-alt-ethylene oxide) than in PEO. These results demonstrate the potential of this computational framework for rapid screening of new polymer electrolytes based on ion diffusivity.   

Theory and Simulations of Tapered Diblock Polymers

We study tapered block polymers, AB diblock polymers with a gradient region inserted between the pure A and B blocks such that composition smoothly transitions from A to B (or B to A in the case of inverse tapers). Phase diagrams were created using self consistent field theory (SCFT), and coarse-grained molecular dynamics (MD) simulations were used to study polymer conformations and diffusion, including diffusion of monomer-sized penetrants preferentially dissolved in one of the phases. As has been observed experimentally, we find that tapering makes the A and B blocks more miscible, decreasing domain spacing and shifting the order to disorder transition to lower temperatures. We predict a widening of the bicontinuous double gyroid region of the phase diagram for moderate length normal tapers versus diblocks, suggesting taper length can be used as a control parameter to obtain network phases even at high molecular weight, as may be desirable in transport applications. Additionally, in some inverse tapered systems, SCFT predicts phases not present in the standard AB diblock phase diagram, and MD simulations show how the chains fold back and forth across the interface. In these inverse tapered polymers, as segregation strength is increased, the competing effects of folding and stretching produces lamellae that have domain spacing nearly independent of temperature. We also find that diffusion of penetrants in normal tapers is significantly faster than that in inverse tapers, which is likely related to their unusual conformations.   

High-Tg Polynorbornene-Based Block and Random Copolymers for Butanol Pervaporation Membranes

Vinyl addition polymers of substituted norbornene (NB) monomers possess desirably high glass transition temperatures (Tg); however, until very recently, the lack of an applicable living polymerization chemistry has precluded the synthesis of such polymers with controlled architecture, or copolymers with controlled sequence distribution. We have recently synthesized block and random copolymers of NB monomers bearing hydroxyhexafluoroisopropyl and n-butyl substituents (HFANB and BuNB) via living vinyl addition polymerization with Pd-based catalysts. Both series of polymers were cast into the selective skin layers of thin film composite (TFC) membranes, and these organophilic membranes investigated for the isolation of n-butanol from dilute aqueous solution (model fermentation broth) via pervaporation. The block copolymers show well-defined microphase-separated morphologies, both in bulk and as the selective skin layers on TFC membranes, while the random copolymers are homogeneous. Both block and random vinyl addition copolymers are effective as n-butanol pervaporation membranes, with the block copolymers showing a better flux-selectivity balance. While polyHFANB has much higher permeability and n-butanol selectivity than polyBuNB, incorporating BuNB units into the polymer (in either a block or random sequence) limits the swelling of the polyHFANB and thereby improves the n-butanol pervaporation selectivity.   

Glassy Structural Trapping in Soft Multi-Face Colloids

Nanoparticles with soft, heterogeneously patterned surfaces often exhibit unique, multi-functional behaviors in response to environmental stimuli. The soft, polymeric nature of the particle surface, moreover, allows for the tailoring of both surface architecture and chemical composition towards particular applications. We have recently demonstrated that Precipitation-Induced Self Assembly (PISA) can be used to form soft Janus colloids as well as multi-faceted colloids in a scalable approach in which many colloidal characteristics can be controlled independently. Here, we present evidence not only of kinetic trapping in the formation of rapidly precipitated, multi-surface polymer particles; but also delineate the role of polymer vitrification in the determination of multi-faceted particle structures.   

Effect of Protein Supercharging on Interaction with Polyelectrolytes

Complexation of proteins with polyelectrolytes can lead to a liquid-liquid phase separation to generate a viscous complex coacervate phase rich in protein and polyelectrolyte. However, many proteins do not readily coacervate at conditions near neutral pH and physiological ionic strength. Here, protein supercharging is used to systematically explore the effect of protein charge on the complex coacervation with polycations. Four model proteins were chemically modified to generate a panel of proteins with varying surface charge, with both the average charge and charge distribution quantified by mass spectrometry. Proteins phase separated with the qP4VP and qPDMAEMA polycations when the ratio of negatively charged residues to positively charged residues was greater than 1.1-1.2. Efficient partitioning of the protein into the coacervate phase required larger charge ratio (1.5-2.0). The model proteins were also encapsulated in complex coacervate core micelles. Dynamic light scattering was used to assess the formation of micelles with POEGMA-$b$-qP4VP and revealed micellar hydrodynamic radii of approximately 25-30 nm. Small angle neutron scattering and transmission electron microscopy were used to confirm the formation of spherical micelles.   

Self-assembly of Open-Shell-containing Block Polymer Thin Films

Radical polymers, where a stable open-shell group is present on each repeat unit of a non-conjugated macromolecular backbone, are emerging as promising materials in organic electronic and magnetic applications. As such, designing molecular motifs that allow for the self-assemble of these open-shell species into nanostructured domains could be beneficial in a host of next-generation flexible electronic applications. In addition, the relatively flexible nature of their macromolecular backbone and ability to conduct charge in the amorphous state offer distinct advantages regarding their self-assembly relative to block polymers based on conjugated semiconducting polymers. Here, we demonstrate the controlled synthesis and self-assembly of diblock copolymers containing radical polymer moieties. In fact, we show that A-B diblock copolymers, where the A moiety contains nitroxide radical functionalities and the B moiety is a closed-shell, low glass transition temperature polymer, self-assemble into ordered structures with domain spacing values that are consistent with common coil-coil diblock copolymers (d $=$ 30 nm). In this way, we present a means by which to readily generate electronically-active macromolecules that self-assemble into nanostructured thin films with controlled morphologies over long ranges.   

Direct Immersion Annealing of Block Copolymer Thin Films

We demonstrate ordering of thin block copolymer (BCP) films via direct immersion annealing (DIA) at enhanced rate leading to stable morphologies. The BCP films are immersed in carefully selected mixtures of good and marginal solvents that can impart enhanced polymer mobility, while inhibiting film dissolution. DIA is compatible with roll-to-roll assembly manufacturing and has distinct advantages over conventional thermal annealing and batch processing solvent-vapor annealing methods. We identify three solvent composition-dependent BCP film ordering regimes in DIA for the weakly interacting polystyrene$-$poly(methyl methacrylate) (PS$-$PMMA) system: rapid short range order, optimal long-range order, and a film instability regime. Kinetic studies in the ``optimal long-range order'' processing regime as a function of temperature indicate a significant reduction of activation energy for BCP grain growth compared to oven annealing at conventional temperatures. An attractive feature of DIA is its robustness to ordering other BCP (e.g. PS-P2VP) and PS-PMMA systems exhibiting spherical, lamellar and cylindrical ordering. Inclusion of nanoparticles in these films at high concentrations and fast ordering kinetics study with neutron reflectivity and SANS will be discussed.   

Functional Thin Films from Aligned Block Copolymers and Blends

Block copolymer (BCP) self-assembly provides a simple, cost effective route to fabricating nanoscale patterns. Here we describe how we can modulate the alignment/orientation of BCP films using a modified solvent vapor anealing (SVA) method where the BCP is covered with an elastomer during SVA and controlled deswelling of the elastomer macroscopically produces a shear force that aligns the BCP domains. By proper selection of the BCP or BCP + functional precursors, functional nanopatterns can be obtained. Thin films of cylindrical forming polystyrene-block-polydimethylsiloxane (PS-b-PDMS) are shear aligned. High temperature calcination converts the PDMS to silica and removes the PS to yield the silica nanolines. The spacing of these features is effectively halved by the use of bilayer films. Sequential shear-alignment of two distinct layers can generate arbitrary line based nanostructured features such as a rhombic array, but the size of the features is defined by the BCP. Oligomeric phenolic resin can effectively modulate the size and morphology of amphiphilic BCPs even at high loadings (>70 wt %) due to hydrogen bonding. Alignment of these systems with hydrophobic cylinders can be converted to porous carbon films with anisotropic electrical conductivity.