23 research outputs found

    Tunable Upper Critical Solution Temperature of Poly(<i>N</i>‑isopropylacrylamide) in Ionic Liquids for Sequential and Reversible Self-Folding

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    We demonstrate sequential folding of micropatterned polymer actuators by tuning the upper critical solution temperature (UCST) of poly­(<i>N</i>-isopropylacrylamide) (PNIPAM) copolymers in the ionic liquid (IL) 1-ethyl-3-methylimidazolium bis­(trifluoro-methylsulfonyl) imide. Incorporation of comonomers having different hydrogen-bonding capacities, acrylic acid and methyl acrylate, is shown to shift the UCST of PNIPAM to higher and lower temperatures, respectively. Relying on the ability to tune the transition temperature through copolymerization along with the wide thermal range afforded by the IL as a solvent, we fabricated a photopatterned self-folding device which shows reversible and sequential bending of three sets of hinges. Such sequential and reversible bending of microactuators offers potential for the design of complex self-folding origami and soft robots

    Measuring the Elastic Modulus of Thin Polymer Sheets by Elastocapillary Bending

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    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

    Anisotropic and Interconnected Nanoporous Materials from Randomly End-Linked Copolymer Networks

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    Microphase separation within randomly end-linked copolymer networks (RECNs) provides access to disordered bicontinuous morphologies over a wide composition range of the constituent network strands. Here, we rely on end-linking of telechelic hydroxyl-terminated polystyrene (PS) and poly­(d,l-lactide) (PLA) chains of equal molecular weight, with a tetrafunctional isocyanate cross-linker in a good solvent for both strands, followed by solvent removal to induce microphase separation, and finally etching of the PLA phase to yield nanoporous materials. Transmission electron microscopy (TEM) tomographic reconstructed 3D images along with gravimetric measurements and small-angle X-ray scattering (SAXS) indicate the formation of highly interconnected structures over a range of ∼40–70 vol % of PLA, while N<sub>2</sub> adsorption measurements indicate narrowly distributed pore sizes that can be tuned by varying the strand molecular weights. Stretching of the PS/PLA copolymer networks above the glass transition temperatures of both components prior to etching the PLA phase provides a straightforward means to introduce controlled anisotropy into the 3D interconnected porous materials

    Tailoring Ultrasound-Induced Growth of Perylene Diimide Nanowire Crystals from Solution by Modification with Poly(3-hexyl thiophene)

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    Tailoring nanocrystalline morphologies of organic semiconductors holds importance for organic electronics due to the influence of crystal characteristics on optoelectronic properties. Soluble additives that control crystal growth are commonly found in a variety of contexts such as biomineralization, pharmaceutical processing, and food science, while the use of ultrasound to modify crystal nucleation and growth has been routinely employed in producing crystals of food ingredients, biomolecules, pharmaceuticals, and inorganic materials. However, both methods have been applied to the growth of organic semiconductor crystals only in limited fashion. Here, we combine these two approaches to show that colloidally stable nanowire suspensions of a n-type small molecule, perylene diimide (PDI), can be prepared with well-controlled structures by sonocrystallization in the presence of a p-type polymer, poly(3-hexyl thiophene) (P3HT), as a soluble additive. By preferentially adsorbing on lateral crystal faces, P3HT dramatically reduces PDI crystal growth rate in the lateral directions relative to that along the nanowire axis, yielding nanocrystals with widths below 20 nm and narrow width distributions. With the use of uniform short PDI nanowires as seeds and extension with metastable solutions, controlled growth of PDI nanowires by “living crystallization” is demonstrated, providing access to narrowed length distributions and tailored branched crystal morphologies

    Thermally Reversible Aggregation of Gold Nanoparticles in Polymer Nanocomposites through Hydrogen Bonding

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    The ability to tune the state of dispersion or aggregation of nanoparticles within polymer-based nanocomposites, through variations in the chemical and physical interactions with the polymer matrix, is desirable for the design of materials with switchable properties. In this study, we introduce a simple and effective means of reversibly controlling the association state of nanoparticles based on the thermal sensitivity of hydrogen bonds between the nanoparticle ligands and the matrix. Strong hydrogen bonding interactions provide excellent dispersion of gold nanoparticles functionalized with poly­(styrene-<i>r</i>-2-vinylpyridine) [P­(S-<i>r</i>-2VP)] ligands in a poly­(styrene-<i>r</i>-4-vinyl phenol) [P­(S-<i>r</i>-4VPh)] matrix. However, annealing at higher temperatures diminishes the strength of these hydrogen bonds, driving the nanoparticles to aggregate. This behavior is largely reversible upon annealing at reduced temperature with redispersion occurring on a time-scale of ∼30 min for samples annealed 50 °C above the glass transition temperature of the matrix. Using ultraviolet–visible absorption spectroscopy (UV–vis) and transmission electron microscopy (TEM), we have established the reversibility of aggregation and redispersion through multiple cycles of heating and cooling

    Ionoelastomers at Electrified Interfaces: Differential Electric Double-Layer Capacitances of Cross-Linked Polymeric Ions and Mobile Counterions

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    Ionoelastomers (IEs), consisting solely of cross-linked networks of polymerized ionic liquids (PILs), have gained attention for use in various electrochemical applications due to their unique combination of the electrochemical properties of ionic liquids (ILs) and the solid-state elastic properties of the polymer networks. However, the structure of the electric double layer (EDL) of IEs at electrified surfaces is not well understood, especially when considering that one of the ionic species is covalently bound to the cross-linked polymer network. Herein, we investigate the differential EDL capacitances of cross-linked polymeric ions and counterions in IEs. A pair of IEs with opposite polarity are prepared; the polycationic IE with cross-linked imidazolium cations and free bis(trifluoromethyl sulfonyl)imide (TFSI) anions and the polyanionic IE with cross-linked sulfonimide anions and free 1-ethyl-3-methylimidazolium (EMIM) cations. By applying two electrodes with a large contrast in surface area, the EDL capacitances of the cross-linked polymeric ions and the mobile counterions can be decoupled. We find that the EDL capacitances of the fixed ions are lower than that of the counterions in both the polyanion and polycation IEs, resulting in a highly asymmetric capacitance response depending on the sign of the applied voltage. Without cross-linking, we observe similar EDL capacitances for polymeric ions and counterions, suggesting that the elastic energy of the cross-linked networks in IEs restricts the freedom of polymeric ions to rearrange at the electrified surfaces, thereby reducing the EDL capacitance

    Effects of Stiff Film Pattern Geometry on Surface Buckling Instabilities of Elastic Bilayers

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    Buckling instabilitiessuch as wrinkling and creasingof micropatterned elastic surfaces play important roles in applications, including flexible electronics and microfluidics. In many cases, the spatial dimensions associated with the imposed pattern can compete with the natural length scale of the surface instabilities (e.g., the wrinkle wavelength), leading to a rich array of surface buckling behaviors. In this paper, we consider elastic bilayers consisting of a spatially patterned stiff film supported on a continuous and planar soft substrate. Through a combination of experimental and computational analyses, we find that three surface instability modeswrinkling, Euler buckling, and rigid rotationare observed for the stiff material patterns, depending on the in-plane dimensions of the film compared to the natural wrinkle wavelength, while the intervening soft regions undergo a creasing instability. The interplay between these instabilities leads to a variety of surface structures as a function of the pattern geometry and applied compressive strain, in many cases yielding contact between neighboring stiff material elements because of the formation of creases in the gaps between them

    Stress-Induced Orientation of Cocontinuous Nanostructures within Randomly End-Linked Copolymer Networks

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    Randomly end-linked copolymer networks (RECNs) provide a robust route to self-assembled cocontinuous nanostructures. Here, we study the orientation of cocontinuous polystyrene/poly­(d,l-lactide) (PS/PLA) RECNs induced by uniaxial stretching above the glass transition temperatures of the components. Small-angle X-ray scattering (SAXS) reveals that the domains initially undergo nonaffine stretching at low strain (ε < 0.4), followed by domain rotation at larger strains, yielding a “soft elastic” response and providing a high degree of orientation. Transmission electron microscopy (TEM) tomography confirms that stretching leads to topological changes in the nanostructure, corresponding to reorganization of domain interfaces. The combination of orientation at the molecular and nanostructural levels provides substantial improvements in yield strength, toughness, and stiffness. In addition to possibilities for improving mechanical properties, cocontinuous nanostructures with controlled levels of orientation have potential in a variety of contexts including directional ion transport and energy absorption

    Orthogonal Ambipolar Semiconductor Nanostructures for Complementary Logic Gates

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    We report orthogonal ambipolar semiconductors that exhibit hole and electron transport in perpendicular directions based on aligned films of nanocrystalline “shish-kebabs” containing poly­(3-hexylthiophene) (P3HT) and <i>N,N′</i>-di-n-octyl-3,4,9,10-perylenetetracarboxylic diimide (PDI) as p- and n-type components, respectively. Polarized optical microscopy, scanning electron microscopy, and X-ray diffraction measurements reveal a high degree of in-plane alignment. Relying on the orientation of interdigitated electrodes to enable efficient charge transport from either the respective p- or n-channel materials, we demonstrate semiconductor films with high anisotropy in the sign of charge carriers. Films of these aligned crystalline semiconductors were used to fabricate complementary inverter devices, which exhibited good switching behavior and a high noise margin of 80% of 1/2 V<sub>dd</sub>. Moreover, complementary “NAND” and “NOR” logic gates were fabricated and found to exhibit excellent voltage transfer characteristics and low static power consumption. The ability to optimize the performance of these devices, simply by adjusting the solution concentrations of P3HT and PDI, makes this a simple and versatile method for preparing ambipolar organic semiconductor devices and high-performance logic gates. Further, we demonstrate that this method can also be applied to mixtures of PDI with another conjugated polymer, poly­[2,5-bis­(3-tetradecylthiophen-2-yl)­thieno­[3,2-<i>b</i>]­thiophene]) (PBTTT), with better hole transport characteristics than P3HT, opening the door to orthogonal ambipolar semiconductors with higher performance

    Characterization of Heterogeneous Polyacrylamide Hydrogels by Tracking of Single Quantum Dots

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    Single particle tracking (SPT) microscopy is a powerful experimental technique for characterizing mobility within complex media. In this paper, we study the motion of single core/shell CdSe/ZnS quantum dots (QDs) within synthetic polyacrylamide (PAAm) hydrogels to provide insight into the structure of these heterogeneous gel networks. Subdiffusive mean-square displacements (MSD) and non-Gaussian van Hove functions are observed for gels with a range of cross-linker contents, which we interpret in terms of transient caging events of QDs due to the presence of denser regions of the network. Experimentally determined caging time distributions follow power-law behaviors for short times, consistent with the predictions of a simple random trap model for the confining energy landscape. Over the range of composition studied, greater cross-linker concentrations are found to yield an increase in the frequency of long trapping events, corresponding to larger characteristic trapping energies and therefore a lower overall mobility
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