Harnessing Multiple Surface Deformation Modes for Switchable Conductivity Surfaces

Surface deformation modes, such as wrinkling, creasing, and cracking, enable a plethora of surface morphologies under mechanical loading, which have been widely exploited to provide flexibility and stretchability to electronic devices. As each phenomenon offers a distinct set of potential advantages, controlling the types and spatial locations of deformation modes is key for their successful application. In this study, we demonstrate a method to simultaneously harness multiple surface deformation modeswrinkles, creases, and cracksin patterned multilayer films. The wrinkling of metal-coated stiff patterned films provides flexibility and stretchability, while the reversible formation of creases in the intervening regions of the bare elastomer is used to template the formation of patterned cracks in the metal. While conventional cracks can be difficult to precisely control, the patterned cracks demonstrated here remain straight over long distances and show tunable lateral spacings from hundreds of micrometers to centimeters. Finally, the reversible opening and closing of these cracks under mechanical loading provides mechanically gated electrical switches with small and tunable critical switching strains of 0.05–0.18 and high on/off ratios of >107, enabling the preparation of mechanical NAND and NOR logic gates each composed of multiple patterned switches on a single elastomer surface

Cocontinuous Nanostructures by Microphase Separation of Statistically Cross-Linked Polystyrene/Poly(2-vinylpyridine) Networks

Cocontinuous polymeric nanostructures have drawn considerable attention due to their ability to combine distinct, percolation-dependent properties of two different polymer domains. Randomly end-linked copolymer networks (RECNs) have previously been shown to support the formation of disordered cocontinuous nanostructures across wide composition windows in a robust way. However, achieving highly efficient linking of telechelic polymers with excellent end-group fidelity often requires complex synthetic routes. As an alternative, we study here statistically cross-linked copolymer networks (SCCNs) composed of polystyrene and poly(2-vinylpyridine) (PS and P2VP) with cross-linkable allyl pendent groups that are conveniently synthesized by controlled radical copolymerization. Via selective extraction of P2VP, coupled with gravimetry, small-angle X-ray scattering, and electron microscopy, we find disordered cocontinuous phases across wide composition ranges (up to ≈ 35 wt %), approaching values previously determined for RECNs. Remarkably, even for samples that appear to exhibit full percolation, a substantial fraction of P2VP (≈ 20–30 wt %) cannot be removed, which we ascribe to short strands between nearby cross-linkers that are physically embedded within PS domains. The resulting PS porous monoliths with residual surface P2VP layers enable facile surface modification to resist protein adsorption and templating of porous gold nanostructures

Wormlike Micelles with Microphase-Separated Cores from Blends of Amphiphilic AB and Hydrophobic BC Diblock Copolymers

Wormlike Micelles with Microphase-Separated Cores from Blends of Amphiphilic AB and Hydrophobic BC Diblock Copolymer

Wormlike Micelles with Microphase-Separated Cores from Blends of Amphiphilic AB and Hydrophobic BC Diblock Copolymers

Wormlike Micelles with Microphase-Separated Cores from Blends of Amphiphilic AB and Hydrophobic BC Diblock Copolymer

Wormlike Micelles with Microphase-Separated Cores from Blends of Amphiphilic AB and Hydrophobic BC Diblock Copolymers

Wormlike Micelles with Microphase-Separated Cores from Blends of Amphiphilic AB and Hydrophobic BC Diblock Copolymer

Synthesis of End-Functionalized Polystyrene by Direct Nucleophilic Addition of Polystyryllithium to Bipyridine or Terpyridine

We describe a new approach to synthesize 2,2′-bipyridine- or 2,2′:6′,2′′-terpyridine-terminated polystyrene that relies on anionic polymerization and direct end-capping with the desired pyridyl species. End-functionalization occurs by nucleophilic addition of the living polystyryllithium chain to the 6-position of the pyridine ring, followed by termination and oxidative rearomatization. By using an excess of the pyridyl species to avoid coupling of two living chains through addition to the same bipyridine or terpyridine unit, this technique yielded samples consisting of 77−93% singly end-functionalized chains. The functionality of the polymers was determined by nuclear magnetic resonance spectroscopy and chromatographic separation, while molecular weight and polydipersity were determined by size exclusion chromatography. The crude products were easily purified to near-quantitative functionalization by short column chromatography, and the excess pyridyl species could be efficiently recovered and reused. Even though the addition of the polystyrene chain to the 6-position provides some steric hindrance to the ability of pyridyl end-caps to serve as ligands, the terpyridine-functionalized products were found to form bis complexes readily upon addition of 0.5 equiv of iron(II) chloride to a solution of the polymers, as determined by ultraviolet−visible spectrophotometry and nuclear magnetic resonance spectroscopy

Impact of Composition and Placement of Hydrogen-Bonding Groups along Polymer Chains on Blend Phase Behavior: Coarse-Grained Molecular Dynamics Simulation Study

In this paper, we study symmetric polymer blends comprised of two polymer chemistries, one containing hydrogen-bonding (H-bonding) acceptor groups and another containing H-bonding donor groups to predict the blend morphology (i.e., two-phase, ordered/lamellar, disordered, disordered microphase-separated, and bicontinuous microemulsion or BμE) for varying compositions (i.e., fraction of monomers containing hydrogen-bonding groups along the polymer chain) and placements of hydrogen-bonding groups along the polymer chains. We use molecular dynamics (MD) simulations with a previously developed coarse-grained (CG) model that captures relevant macromolecular length and time scales and both the attractive directional interactions between H-bonding acceptor and donor groups and isotropic polymer–polymer interactions. We first validate our CG MD simulation approach by reproducing the published theoretical phase diagram for end-associating polymer chains at varying H-bonding strengths vs polymer segregation strengths. We also show that with increasing H-bonding strength, end-associating blends with short-chain lengths transition from two-phase to BμE or from disordered blends to BμE depending on the polymer segregation strength and finally to disordered microphase morphologies. End-associating blends with longer-chain lengths transition from two-phase to ordered lamellar phase at high polymer segregation strengths and from two-phase to disordered microphase-separated state at low polymer segregation strengths. Next, we study blends with the center placement of a single H-bonding group in each polymer chain as well as random and regular placements of multiple H-bonding groups per polymer chain. Regardless of the number and placement of H-bonding groups, with increasing H-bonding strength, the fraction of associated H-bonding groups increases with the system transitioning from blends of unassociated polymers to a mixture of associated copolymers and unassociated polymers and finally to a melt of fully associated supramolecular copolymers. At intermediate strengths of H-bonding, we observe BμE morphologies in all systems with end, center, random, and regular placements of H-bonding group(s). At high strengths of H-bonding, the blend morphology is disordered microphase-separated with domain sizes being smallest for the center placement, followed by the end, regular, and then random placements. We find that this variation in the placement of H-bonding groups leads to a greater change in domain sizes than with variation in the strength of the isotropic polymer–polymer interaction at constant H-bonding attraction. These trends in disordered microphase domain sizes with varying compositions and placements of H-bonding groups are linked to the supramolecular copolymer architecture formed upon the association of the two homopolymer chemistries. The polymers with the center placement of H-bonding groups form miktoarm star copolymers upon association, which show smaller domain sizes compared to diblock copolymers formed by polymers with end placement at the same molecular weight; in contrast, the polymers with random and regular placements of multiple H-bonding groups form nonlinear copolymer architectures with dispersity in block length leading to larger domain sizes. Overall, our work establishes design rules for incorporating H-bonding functional groups along polymer chains to achieve precisely tuned morphology and control over the disordered microphase domain sizes

Measuring the Elastic Modulus of Thin Polymer Sheets by Elastocapillary Bending

We describe bending by liquid/liquid or liquid/air interfaces as a simple and broadly applicable technique for measuring the elastic modulus of thin elastic sheets. The balance between bending and surface energies allows for the characterization of a wide range of materials with moduli ranging from kilopascals to gigapascals in both vapor and liquid environments, as demonstrated here by measurements of both soft hydrogel layers and stiff glassy polymer films. Compared to existing approaches, this method is especially useful for characterizing soft materials