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We examined the packing structure of polystyrene-coated gold nanoparticles (Au@PS) as a function of grafting density. A series of Au@PS nanoparticles with grafting densities in the range of 0.51−1.94 chains nm −2 were prepared by a ligand exchange process using thiol-terminated PS and then selfassembled at a liquid−air interface. We observed a transition from disordered to bodycentered cubic (bcc) to face-centered cubic (fcc) arrangements with increasing grafting density, even though the ligand length-to-core radius ratio (λ) was as high as 3.0, a condition that typically favors nonclose-packed bcc symmetry in the self-assembly of hard nanoparticles. To explain this phenomenon, we define λ eff to include the concentrated polymer brush regime as part of the "hard core", which predicts that the softness of Au@PS nanoparticles is reduced from 1.53 to 0.14 in a theta solvent as the grafting density increases from 0.51 to 1.94 chains nm −2 . This new definition of λ can also predict the effective radii of nanoparticles using the established optimal packing model. The experimental findings are supported by a combination of coarse-grained molecular dynamics simulation and adaptive common neighbor analysis, which show that changes in grafting density can drive the observed transitions in nanoparticle packing. These studies provide new insights for controlling the selfassembled symmetries of polymer-coated nanocrystals using a simple ligand exchange process to tune particle softness.

Development of particles that change shape in response to external stimuli has been a long-thought goal for producing bioinspired, smart materials. Herein, the temperature-driven transformation of the shape and morphology of polymer particles composed of polystyrene-b-poly(4-vinylpyridine) (PS-b-P4VP) block copolymers (BCPs) and temperature-responsive poly(N-isopropylacrylamide) (PNIPAM) surfactants is reported. PNIPAM acts as a temperature-responsive surfactant with two important roles. First, PNIPAM stabilizes oil-in-water droplets as a P4VP-selective surfactant, creating a nearly neutral interface between the PS and P4VP domains together with cetyltrimethylammonium bromide, a PS-selective surfactant, to form anisotropic PS-b-P4VP particles (i.e., convex lenses and ellipsoids). More importantly, the temperature-directed positioning of PNIPAM depending on its solubility determines the overall particle shape. Ellipsoidal particles are produced above the critical temperature, whereas convex lens-shaped particles are obtained below the critical temperature. Interestingly, given that the temperature at which particle shape change occurs depends solely on the lower critical solution temperature (LCST) of the polymer surfactants, facile tuning of the transition temperature is realized by employing other PNIPAM derivatives with different LCSTs. Furthermore, reversible transformations between different shapes of PS-b-P4VP particles are successfully demonstrated using a solvent-adsorption annealing with chloroform, suggesting great promise of these particles for sensing, smart coating, and drug delivery applications.

Minimizing the use of Pt catalysts in proton exchange membrane fuel cells (PEMFC) is important, considering its high price and scarcity. Herein, we demonstrate novel catalysts for PEMFCs with exceptionally...

In this work, we develop mechanically robust and high-performance organic thin-film transistors (OTFTs) based on poly­(3-hexylthiophene) (P3HT) regioblock copolymers (block-P3HTs). These block-P3HTs consist of regioregular (rre) and regiorandom (rra) P3HTs, where the highly crystalline rre block allows efficient charge transport while the amorphous rra block provides mechanical robustness and interdomain connection. To examine the effects of the molecular architecture on the OTFT performance and stretchability, we prepare a series of block-P3HTs having different number-average molecular weight (M n) values of rra blocks (from 0 to 32 kg mol–1) and a fixed M n of rre blocks (11 kg mol–1). Thin films of all of the block-P3HTs exhibit a high charge-carrier mobility due to the formation of well-developed edge-on crystallites from the rre blocks confined within the rra domains, leading to a hole mobility of 1.5 × 10–1 cm2 V–1 s–1, which is superior to that of the rre P3HT homopolymer. In addition, the mechanical toughness of block-P3HT thin films is remarkably enhanced by the rra block. While the rre P3HT homopolymer thin film shows a brittle behavior with an elongation at break of only 0.3%, the elongation at break of the block-P3HT thin films increases by a factor of 100, yielding 30.2% with increasing M n of the rra block, without sacrificing the electrical properties. In particular, a noticeable enhancement of both elongation at break and toughness is observed between M n values of the rra block of 8 and 20 kg mol–1, indicating that the critical molecular weight of rra P3HT plays an important role in determining the mechanical response of the block-P3HT thin films. This study provides guidelines and strategies to improve the mechanical properties of organic electroactive materials without the disruption of optoelectrical properties, which is critical to fabricate high-performance soft electronics.