Crystallization of Poly(l-lactic acid) on Water Surfaces via Controlled Solvent Evaporation and Langmuir-Blodgett Films.

Solvent evaporation is one of the most fundamental processes in soft matter. Structures formed via solvent evaporation are often complex yet tunable via the competition between solute diffusion and solvent evaporation time scales. This work concerns the polymer evaporative crystallization on the water surface (ECWS). The dynamic and two-dimensional (2D) nature of the water surface offers a unique way to control the crystallization pathway of polymeric materials. Using poly(l-lactic acid) (PLLA) as the model polymer, we demonstrate that both one-dimensional (1D) crystalline filaments and two-dimensional (2D) lamellae are formed via ECWS, in stark contrast to the 2D Langmuir-Blodgett monolayer systems as well as polymer solution crystallization. Results show that this filament-lamella biphasic structure is tunable via chemical structures such as molecular weight and processing conditions such as temperature and evaporation rate.

Fabrication of Surface Polymer Brushes Via Thin Film Crystallization and Solvent Annealing.

Polymer single crystals are used as templates to synthesize polymer brushes, known as the "polymer-single-crystal-assisted-grafting-to" (PSCAGT) approach. Polymer brushes with controlled grafting densities and spatial tethering locations are demonstrated. Previous works focused on solution crystallization, which involves large amounts of organic solvent, and the grafting density can only be tuned by varying crystallization temperatures. In this work, thin film crystallization is utilized to fabricate 2D polymer crystals on flat surfaces. Subsequent chemical tethering leads to polymer brushes that retain the original morphology of the crystals with high fidelity. Furthermore, it is shown that the grafting density of the polymer brushes fabricated using this method depends on the chain end distribution on the top/bottom surfaces of the crystal, which can be facilely controlled by annealing the crystals at various nonsolvent media. This work broadens the scope of the PSCAGT method and provides a new route to achieve polymer brushes with controlled structures.

Crystallization-driven Nanoparticle Crystalsomes.

Nanoparticle (NP) assembly has been extensively studied, and a library of NP superstructures has been synthesized. These intricate structures show unique collective optical, electronic, and magnetic properties. In this work, we report a bottom-up approach for fabricating spherical gold nanoparticle (AuNP) assemblies that mimic colloidosomes. Co-crystallization of lipoic acid-end-functionalized poly(ethylene oxide) (PEO) and AuNPs in solution via a self-seeding method led to the formation of hollow spherical NP assemblies named nanoparticle crystalsomes (NPCs). Due to the spherical shape, the translational symmetry of PEO crystals is broken in NPCs, which can be attributed to the competition between NP close packing and polymer crystallization. This was confirmed by tuning the NPC morphology via varying the self-seeding temperature, crystallization temperature, and PEO molecular weight. We envisage that this strategy paves the way to attaining exquisite morphological control of NP assemblies with broken translational symmetry.

Adaptable Multivalent Hairy Inorganic Nanoparticles.

We report a polymer brush-based approach for fabricating multivalent patchy nanoparticles (NPs) with the number of nanodomains (valency) from 6 to 10, potentially from 1 to 10, by exploiting the lateral microphase separation of binary mixed homopolymer brushes grafted on NPs with a radius comparable to the polymer sizes. Well-defined mixed brushes were grown on 20.4 nm silica NPs by two-step surface-initiated reversible deactivation radical polymerizations and microphase separated laterally upon casting from a good solvent, producing multivalent NPs on 2D surfaces. A linear relationship between valency and average core size for the corresponding valency was observed. The mixed brush NPs exhibited abilities to form "bonds" through the overlap of nanodomains and to change the valency when interacting with adjacent NPs. This method could open up a new avenue for studying patchy NPs.

Designing Comb-Chain Crosslinker-Based Solid Polymer Electrolytes for Additive-Free All-Solid-State Lithium Metal Batteries.

Developing solid polymer electrolytes (SPEs) is a promising approach to realize practical dendrite-free lithium metal batteries (LMBs). Tuning the nanoscale polymer network chemsitry is of critical importance for SPE design. In this work, we took lessons from the rubber chemistry and developed a series of comb-chain crosslinker-based SPEs (ConSPEs) using a preformed polymer as the multifunctional crosslinker. The high-functionality crosslinker increased the connectivity of nanosized cross-linked domains, which led to a robust network with dramatically improved toughness and superior lithium dendrite resistance even at a current density of 2 mA cm-2. The uniform and flexile network also dramatically improved the anodic stability to over 5.3 V versus Li/Li+. Additive-free, all-solid-state LMBs with the ConSPE showed high discharge capacity and stable cycling up to 10 C rate, and could be stably cycled at 25 °C. Our results demonstrated that ConSPEs are promising for high-performance and dendrite-free LMBs.

Frustrated Layered Self-Assembly Induced Superlattice from Two-Dimensional Nanosheets.

Here we reported a hierarchical self-assembly approach toward well-defined superlattices in supramolecular liquid crystals by fullerene-based sphere-cone block molecules. The fullerenes crystallize to form monolayer nanosheets intercalated by the attached soft hydrocarbon cones. The frustration caused by cross-sectional area mismatch between the spheres and the somewhat oversize cones leads to a unique lamellar superlattice whereby each stack of six pairs of alternating sphere-cone sublayers is followed by a cone double layer. While such areal mismatch problems in soft matter are usually solved by interface curvature, the lamellar superlattice solution is best suited to systems with rigid layers. Meanwhile, formation of the superlattice significantly improves the material's transient electron conductivity, with the maximum value being among the highest for π-conjugated organic materials. The design principle of solving steric frustration by forming a superlattice opens a new avenue toward self-assembled optoelectronic materials.

Directed Nanoparticle Assembly through Polymer Crystallization.

Nanoparticles can be assembled into complex structures and architectures by using a variety of methods. In this review, we discuss recent progress of using polymer crystallization (particularly polymer single crystals, PSCs) to direct nanoparticle assembly. PSCs have been extensively studied since 1957. Mainly appearing as quasi-two-dimensional (2D) lamellae, PSCs are typically used as model systems to determine polymer crystalline structures, or as markers to investigate the crystallization process. Recent research has demonstrated that they can also be used as nanoscale functional materials. Herein, we show that nanoparticles can be directed to assemble into complex shapes by using in situ or ex situ polymer crystal growth. End-functionalized polymers can crystallize into 2D nanosheet PSCs, which are used to conjugate with complementary nanoparticles, leading to a nanosandwich structure. These nanosandwiches can find interesting applications for catalysis, surface-enhanced Raman spectroscopy, and nanomotors. Dissolution of the nanosandwich leads to the formation of Janus nanoparticles, providing a unique method for asymmetric nanoparticle synthesis.

Fabrication of 2D Block Copolymer Brushes via a Polymer-Single-Crystal-Assisted-Grafting-to Method.

Block copolymer brushes are of great interest due to their rich phase behavior and value-added properties compared to homopolymer brushes. Traditional synthesis involves grafting-to and grafting-from methods. In this work, a recently developed "polymer-single-crystal-assisted-grafting-to" method is applied for the preparation of block copolymer brushes on flat glass surfaces. Triblock copolymer poly(ethylene oxide)-b-poly(l-lactide)-b-poly(3-(triethoxysilyl)propyl methacrylate) (PEO-b-PLLA-b-PTESPMA) is synthesized with PLLA as the brush morphology-directing component and PTESPMA as the anchoring block. PEO-b-PLLA block copolymer brushes are obtained by chemical grafting of the triblock copolymer single crystals onto a glass surface. The tethering point and overall brush pattern are determined by the single crystal morphology. The grafting density is calculated to be ≈0.36 nm-2 from the atomic force microscopy results and is consistent with the theoretic calculation based on the PLLA crystalline lattice. This work provides a new strategy to synthesize well-defined block copolymer brushes.

Anomalous Ostwald Ripening Enables 2D Polymer Crystals via Fast Evaporation.

We demonstrate by molecular simulations that the Ostwald ripening of crystalline polymer nuclei within the fast-evaporation-induced 2D skin layer is retarded at suitable temperatures and evaporation rates. Such an anomalous ripening can be attributed to the interplay between the thermodynamically driven diffusion of noncrystalline fragments toward the growing nuclei and the diffusive current away from the free surface caused by the densification in the nonequilibrium skin layer. The growth orientation of the nuclei inside the skin plane can be adjusted during this anomalous ripening process, which is beneficial for fabricating 2D polymer crystals.

Terraced and Smooth Gradient Polymer Brushes via a Polymer Single-Crystal Assisted Grafting-To Method.

Gradient polymer brushes provide a spatial gradient change in molecular characteristics of the brush, and such a change can be utilized to study structure-property relationships in a combinatorial fashion. In this study, a bottom-up method was used to synthesize gradient polymer brushes with a predesigned and precisely controlled grafting density gradient and brush pattern. A polymer single-crystal assisted grafting-to (PSCAGT) method was employed where end-functionalized polymers were grown into two-dimensional polymer single crystals. The latter were chemically coupled to a solid substrate to form well-defined polymer brushes. To tune the grafting density, end-dissimilar polymers were used to co-crystallize into one single crystal. Programmed single-crystal growth was introduced to synthesize brushes with two different gradient architectures, that is, terraced and smooth gradient with pyramid patterns. This work demonstrates that the PSCAGT method offers a unique means to tune polymer brush nanostructure.

Unique Supramolecular Liquid-Crystal Phases with Different Two-Dimensional Crystal Layers.

We report herein a series of tetrablock-mimic azobenzene-containing [60]fullerene dyads that form supramolecular liquid crystals (LCs) from phase-segregated two-dimensional (2D) crystals. The unique double-, triple-, and quadruple-layer packing structure of fullerenes in the 2D crystals leads to different smectic supramolecular LC phases, and novel LC phase transitions were observed upon changes in the fullerene packing layer number in the 2D crystals. Interestingly, by combining the LC properties with 2D crystals, these materials show excellent electron mobility in the order of 10-3  cm2 V-1 s-1 , despite their relatively low fullerene content. Our results provide a novel method to manipulate 2D crystal layer thickness, with promising applications in optoelectronic devices.

Precisely Assembled Cyclic Gold Nanoparticle Frames by 2D Polymer Single-Crystal Templating.

In recent decades, extensive studies have been devoted to assembling nanoparticles (NPs) into various ordered structures to achieve novel optical properties. However, it still remains a challenging task to assemble NPs into cyclic one-dimensional (1D) shapes, such as rings and frames. Herein, we report a directed assembly method to precisely assemble NPs into well-defined, free-standing frames using polymer single crystals (PSCs) as the template. Preformed poly(ethylene oxide) (PEO) single crystals were used as the template to direct the crystallization of block copolymer (BCP) poly(ethylene oxide)-b-poly(4-vinylpyridine) (PEO-b-P4VP), which directs the gold NPs (AuNPs) to form AuNP frames. By controlling the PSC growth, we were able to, for the first time, precisely tune both the size and width of the AuNP frame. These novel AuNP frames topologically resemble NP nanorings and cyclic polymer chains, and show unique surface plasmon resonance (SPR) behaviors.

Fluorination of epitaxial oxides: synthesis of perovskite oxyfluoride thin films.

While the synthesis of ABO3 perovskite films has enabled new strategies to control the functionality of this material class, the chemistries that have been realized in thin film form constitute only a fraction of those accessible to bulk chemists. Here, we report the synthesis of oxyfluoride films, where the incorporation of F may provide a new means to tune physical properties in thin films by modifying electronic structure. Fluorination is achieved by spin coating a poly(vinylidene fluoride) (PVDF) solution onto oxygen-deficient films. The film/polymer bilayer is then annealed, promoting the diffusion of F into the film. We have used this method to synthesize SrFeO(3-α)Fγ films, as confirmed by X-ray photoemission spectroscopy and X-ray absorption spectroscopy.

From multi-ring to spider web and radial spoke: competition between the receding contact line and particle deposition in a drying colloidal drop.

Deposition morphologies of inkjet-printed colloidal drops are examined under various drying conditions, particle volume fractions, and particle sizes. Concentric multi-rings, radial spokes, spider web, foam, and island-like depositions are observed as a result of the competition between the receding contact line and particle deposition during drop drying. Experimentally measured multi-ring spacing, δR, shows good agreement with the model predicted linear correlation with the local ring radius R. The results also show that the instability near the contact line leads to the radial spoke and saw-toothed structures. The resulting wavelength of the radial structures, λ, satisfies λ ~ (3)√R and λ ~ 1/(3)√[1-RH], where RH is the relative humidity. A dimensionless parameter ξ, defined as the radial deposition growth rate to contact line velocity ratio, has been identified to determine the conditions under which the entire contact line can be pinned to leave a continuous ring deposit. Increasing the particle size while keeping the volume fraction the same is found to suppress the formation of the multi-ring deposition, due to a smaller number of particles available to pin the receding contact line.

Compliant Solid Polymer Electrolytes (SPEs) for Enhanced Anode-Electrolyte Interfacial Stability in All-Solid-State Lithium-Metal Batteries (LMBs).

Practical application of high energy density lithium-metal batteries (LMBs) has remained elusive over the last several decades due to their unstable and dendritic electrodeposition behavior. Solid polymer electrolytes (SPEs) with sufficient elastic modulus have been shown to attenuate dendrite growth and extend cycle life. Among different polymer architectures, network SPEs have demonstrated promising overall performance in cells using lithium metal anodes. However, fine-tuning network structures to attain adequate lithium electrode interfacial contact and stable electrodeposition behavior at extended cycling remains a challenge. In this work, we designed a series of comb-chain cross-linker-based network SPEs with tunable compliance by introducing free dangling chains into the SPE network. These dangling chains were used to tune the SPE ionic conductivity, ductility, and compliance. Our results demonstrate that increasing network compliance and ductility improves anode-electrolyte interfacial adhesion and reduces voltage hysteresis. SPEs with 56.3 wt % free dangling chain content showed a high Coulombic efficiency of 93.4% and a symmetric cell cycle life 1.9× that of SPEs without free chains. Additionally, the improved anode compliance of these SPEs led to reduced anode-electrolyte interfacial resistance growth and greater capacity retention at 92.8% when cycled at 1C in Li|SPE|LiFePO4 half cells for 275 cycles.

Carbon nanotube-directed polytetrafluoroethylene crystal growth via initiated chemical vapor deposition.

Initiated chemical vapor deposition (iCVD) has been shown to be suitable for blanketing surfaces with thin polymer coatings of ≈1-2 nm and greater. In this work, iCVD coatings of polytetrafluoroethylene (PTFE) deposited on carbon nanotube (CNT)-based surfaces show CNT-templated PTFE single crystal growth. While the coating forms disoriented agglomerates when deposited on an amorphous carbon background, "shish-kebab" structures are observed when grown on single-walled carbon nanotubes (SWCNT) as well as CNT buckypaper. It is shown that the shish-kebab structure is composed of PTFE lamellae arranged with the chain backbones running parallel to the SWCNT axis. This result allows one to control not only the surface chemistry using PTFE but also the coating surface topology.

Tuning ion conducting pathways using holographic polymerization.

Polymer electrolyte membranes (PEMs) with high and controlled ionic conductivity are important for energy-related applications, such as solid-state batteries and fuel cells. Herein we disclose a new strategy to fabricate long-range ordered PEMs with tunable ion conducting pathways using a holographic polymerization (HP) method. By incorporating polymer electrolyte into the carefully selected HP system, electrolyte layers/channels with length scales of a few tens of nanometers to micrometers can be formed with controlled orientation and anisotropy; ionic conductivity anisotropy as high as 37 has been achieved.

Helical Crystals in Aliphatic Copolyesters: From Chiral Amplification to Mechanical Property Enhancement.

We demonstrate herein a bottom-up strategy for achieving helical crystals via chiral amplification in copolyesters by incorporating a small amount of (d)-isosorbide into semicrystalline polyester, poly(ethylene brassylate) (PEB). During bulk crystallization of poly(ethylene-co-isosorbide brassylate)s, the molecular chirality of isosorbide in the amorphous region is transferred to PEB crystal chirality and amplified by the formation of right-handed helical crystals. Increasing isosorbide content or reducing crystallization temperature leads to thinner PEB lamellae crystals, strengthening chiral amplification by forming superhelices with a smaller helical pitch. Moreover, the superhelices with smaller helical pitch (larger chiral amplification) endow aliphatic copolyesters with enhanced modulus, strength, and toughness without sacrificing elongation-at-break. The principle outlined here could apply to the design of strong and tough materials.

Multilayered Solid Polymer Electrolytes with Sacrificial Coating for Suppressing Lithium Dendrite Growth.

The practical application of lithium-metal batteries (LMBs) is hindered by the lithium dendrite formation during cycling. In this work, we report a multilayered solid polymer electrolyte (SPE) formed by sandwiching a comb-chain cross-linker-based network SPE (ConSPE) film with a linear poly(ethylene oxide) (PEO) SPE coating. Benefiting from the drastically different lithium dendrite resisting properties of the ConSPE and linear PEO SPE, the lithium dendrite growth in the multilayered SPEs could be tuned, with the linear PEO SPE effectively serving as a sacrificial layer to accommodate the lithium dendrite growth. Symmetrical lithium cells with the multilayered SPE exhibited an extended short-circuit time ∼4.1 times that for the single-layer ConSPE at a high current density of 1.5 mA cm-2. Li/LiFePO4 batteries with multilayered SPEs delivered superior cycling performance at extremely high C-rates of 2C and 10C. Our multilayered SPE architecture, therefore, opens up a new gateway for advancing SPE design for future LMBs.

Porous Crystalsomes via Emulsion Crystallization and Polymer Phase Separation.

Crystalsomes are crystalline capsules that are formed by controlling polymer crystallization to break translational symmetry. While recent studies showed that these crystalline capsules exhibit interesting mechanical properties, thermal behavior, and excellent performance in blood circulation, the closed capsule is undesired for drug delivery applications. We report the formation and characterization of porous crystalsomes where porosity is rendered on the crystalline shells. A miniemulsion is formed using two amphiphilic block copolymers (BCP). The competition between controlled crystallization and phase separation of the BCPs at the emulsion surface leads to multiphase crystalsomes. Subsequently removing one BCP produces porous crystalline capsules.