Mesoporous Polymer Frameworks from End-Reactive Bottlebrush Copolymers

Reticulated nanoporous materials generated by versatile molecular framework approaches are limited to pore dimensions on the scale of the utilized rigid molecular building blocks (<5 nm). The inherent flexibility of linear polymers precludes their utilization as long framework connectors for the extension of this strategy to larger length scales. We report a method for the fabrication of mesoporous frameworks by using bottlebrush copolymers with reactive end blocks serving as rigid macromolecular interconnectors with directional reactivity. End-reactive bottlebrush copolymers with pendant alkene functionalities were synthesized by a combination of controlled radical polymerization and polymer modification protocols. Ru-catalyzed cross-metathesis cross-linking of bottlebrush copolymers with two reactive end blocks resulted in the formation of polymer frameworks where isolated cross-linked domains were interconnected with bottlebrush copolymer bridges. The resulting materials were characterized by a continuous network pore structure with average pore sizes of 9–50 nm, conveniently tunable by the length of the utilized bottlebrush copolymer building blocks. The materials fabrication strategy described in this work expands the length scale of molecular framework materials and provides access to mesoporous polymers with a molecularly tunable reticulated pore structure without the need for templating, sacrificial component etching, or supercritical fluid drying

Tunable Nanoparticle Arrays at Charged Interfaces

Structurally tunable two-dimensional (2D) arrays of nanoscale objects are important for modulating functional responses of thin films. We demonstrate that such tunable and ordered nanoparticles (NP) arrays can be assembled at charged air-water interfaces from nanoparticles coated with polyelectrolyte chains, DNA. The electrostatic attraction between the negatively charged nonhybridizing DNA-coated gold NPs and a positively charged lipid layer at the interface facilitates the formation of a 2D hexagonally closed packed (HCP) nanoparticle lattice. We observed about 4-fold change of the monolayer nanoparticle density by varying the ionic strength of the subphase. The tunable NP arrays retain their structure reasonably well when transferred to a solid support. The influence of particle’s DNA corona and lipid layer composition on the salt-induced in-plane and normal structural evolution of NP arrays was studied in detail using a combination of synchrotron-based <i>in situ</i> surface scattering methods, grazing incidence X-ray scattering (GISAXS), and X-ray reflectivity (XRR). Comparative analysis of the interparticle distances as a function of ionic strength reveals the difference between the studied 2D nanoparticle arrays and analogous bulk polyelectrolyte star polymers systems, typically described by Daoud–Cotton model and power law scaling. The observed behavior of the 2D nanoparticle array manifests a nonuniform deformation of the nanoparticle DNA corona due to its electrostatically induced confinement at the lipid interface. The present study provides insight on the interfacial properties of the NPs coated with charged soft shells

One-Shot Synthesis and Melt Self-Assembly of Bottlebrush Copolymers with a Gradient Compositional Profile

Morphological control plays a central role in soft materials design. Herein, we report the synthesis of a gradient bottlebrush architecture and its role in directing molecular packing in the solid state. Bottlebrush copolymers with gradient interfaces were prepared via one-shot ring-opening metathesis polymerization of <i>exo</i>- and <i>endo</i>-norbornene-capped macromonomers. Kinetic studies revealed a gradient compositional profile separating the two blocks along the backbone. Side-chain symmetric gradient bottlebrush copolymers exhibited a strong tendency to assemble into cylindrical microstructures, in contrast to their block copolymer analogs with sharp interfaces. Such exquisite architectural control of the interfacial composition affords a delicate handle to direct macromolecular assembly

Linear Mesostructures in DNA–Nanorod Self-Assembly

The assembly of molecules and nanoscale objects into one-dimensional (1D) structures, such as fibers, tubules, and ribbons, typically results from anisotropic interactions of the constituents. Conversely, we found that a 1D structure can emerge <i>via</i> a very different mechanism, viz, the spontaneous symmetry breaking of underlying interparticle interactions during structure formation. For systems containing DNA-decorated nanoscale rods, this mechanism, driven by flexible DNA chains, results in the formation of 1D ladderlike mesoscale ribbons with a side-by-side rod arrangement. Detailed structural studies using electron microscopy and <i>in situ</i> small-angle X-ray scattering (SAXS), as well as analysis of assembly kinetics, reveal the role of collective DNA interactions in the formation of the linear structures. Moreover, the reversibility of DNA binding facilitates the development of hierarchical assemblies with time. We also observed similar linear structures of alternating rods and spheres, which implies that the discovered mechanism is generic for nanoscale objects interacting <i>via</i> flexible multiple linkers

Advancing Reversible Shape Memory by Tuning the Polymer Network Architecture

Because of counteraction of a chemical network and a crystalline scaffold, semicrystalline polymer networks exhibit a peculiar behaviorreversible shape memory (RSM), which occurs naturally without applying any external force and particular structural design. There are three RSM properties: (i) range of reversible strain, (ii) rate of strain recovery, and (iii) decay of reversibility with time, which can be improved by tuning the architecture of the polymer network. Different types of poly­(octylene adipate) networks were synthesized, allowing for control of cross-link density and network topology, including randomly cross-linked network by free-radical polymerization, thiol–ene clicked network with enhanced mesh uniformity, and loose network with deliberately incorporated dangling chains. It is shown that the RSM properties are controlled by average cross-link density and crystal size, whereas topology of a network greatly affects its extensibility. We have achieved 80% maximum reversible range, 15% minimal decrease in reversibility, and fast strain recovery rate up to 0.05 K^–1, i.e., ca. 5% per 10 s at a cooling rate of 5 K/min

Chemically Enhancing Block Copolymers for Block-Selective Synthesis of Self-Assembled Metal Oxide Nanostructures

We report chemical modification of self-assembled block copolymer thin films by ultraviolet light that enhances the block-selective affinity of organometallic precursors otherwise lacking preference for either copolymer block. Sequential precursor loading and reaction facilitate formation of zinc oxide, titanium dioxide, and aluminum oxide nanostructures within the polystyrene domains of both lamellar- and cylindrical-phase modified polystyrene-<i>block</i>-poly(methyl methacrylate) thin film templates. Near-edge X-ray absorption fine structure measurements and Fourier transform infrared spectroscopy show that photo-oxidation by ultraviolet light creates Lewis basic groups within polystyrene, resulting in an increased Lewis base–acid interaction with the organometallic precursors. The approach provides a method for generating both aluminum oxide patterns and their corresponding inverses using the same block copolymer template

Two-Dimensional DNA-Programmable Assembly of Nanoparticles at Liquid Interfaces

DNA-driven assembly of nanoscale objects has emerged as a powerful platform for the creation of materials by design via self-assembly. Recent years have seen much progress in the experimental realization of this approach for three-dimensional systems. In contrast, two-dimensional (2D) programmable nanoparticle (NP) systems are not well explored, in part due to the difficulties in creating such systems. Here we demonstrate the use of charged liquid interfaces for the assembly and reorganization of 2D systems of DNA-coated NPs. The absorption of DNA-coated NPs to the surface is controlled by the interaction between a positively charged lipid layer and the negatively charged DNA shells of particles. At the same time, interparticle interactions are switchable, from electrostatic repulsion between DNA shells to attraction driven by DNA complementarity, by increasing ionic strength. Using in situ surface X-ray scattering methods and ex situ electron microscopy, we reveal the corresponding structural transformation of the NP monolayer, from a hexagonally ordered 2D lattice to string-like clusters and finally to a weakly ordered network of DNA cross-linked particles. Moreover, we demonstrate that the ability to regulate 2D morphology yields control of the interfacial rheological properties of the NP membrane: from viscous to elastic. Theoretical modeling suggests that the structural adaptivity of interparticle DNA linkages plays a crucial role in the observed 2D transformation of DNA-NP systems at liquid interfaces

Phase Behavior of Alkyne-Functionalized Styrenic Block Copolymer/Cobalt Carbonyl Adducts and <i>in Situ</i> Formation of Magnetic Nanoparticles by Thermolysis

A series of polystyrene-<i>block</i>-poly­(4-(phenyl­ethynyl)­styrene) (PS-<i>b</i>-PPES) diblock copolymers with a range of compositions were prepared by reversible addition–fragmentation chain transfer (RAFT) polymerization. Block copolymer/cobalt carbonyl adducts (PS_<i>x</i>-PPES_<i>y</i>[Co₂(CO)₆]_<i>n</i>) were subsequently prepared by reaction of Co₂(CO)₈ with the alkyne groups of the PPES block. Phase behavior of the block copolymer/cobalt carbonyl adducts (PS_<i>x</i>-PPES_<i>y</i>[Co₂(CO)₆]_<i>n</i>, 8% ≤ wt % PS ≤ 68%) was studied by small-angle X-ray scattering and transmission electron microscopy (TEM). As the composition of PS_<i>x</i>-PPES_<i>y</i>[Co₂(CO)₆]_<i>n</i> copolymers was shifted from PS as the majority block to PPES_<i>y</i>[Co₂(CO)₆]_<i>n</i> as the majority block, the morphology was observed to shift from lamellar with larger PS domains to cylindrical with PS as the minority component and then to spherical with PS as the minority component. These observations have been used to map out a partial phase diagram for PS_<i>x</i>-PPES_<i>y</i>[Co₂(CO)₆]_<i>n</i> diblock copolymers. Heating of PS_<i>x</i>-PPES_<i>y</i>[Co₂(CO)₆]_<i>n</i> samples at relatively low temperatures (120 °C) results in the formation of nanoparticles containing crystalline cobalt and cobalt oxide domains within the PPES_<i>y</i>[Co₂(CO)₆]_<i>n</i> regions as characterized by TEM, X-ray diffraction (XRD), and X-ray scattering

Structural and Optical Properties of Self-Assembled Chains of Plasmonic Nanocubes

Solution-based linear self-assembly of metal nanoparticles offers a powerful strategy for creating plasmonic polymers, which, so far, have been formed from spherical nanoparticles and cylindrical nanorods. Here we report linear solution-based self-assembly of metal nanocubes (NCs), examine the structural characteristics of the NC chains, and demonstrate their advanced optical characteristics. In comparison with chains of nanospheres with similar dimensions, composition, and surface chemistry, predominant face-to-face assembly of large NCs coated with short polymer ligands led to a larger volume of hot spots in the chains, a nearly uniform E-field enhancement in the gaps between colinear NCs, and a new coupling mode for NC chains due to the formation of a Fabry–Perot resonator structure formed by face-to-face bonded NCs. The NC chains exhibited stronger surface-enhanced Raman scattering in comparison with linear assemblies of nanospheres. The experimental results were in agreement with finite difference time domain simulations

Shapeshifting: Reversible Shape Memory in Semicrystalline Elastomers

We present a general strategy for enabling reversible shape transformation in semicrystalline shape memory (SM) materials, which integrates three different SM behaviors: conventional one-way SM, two-way reversible SM, and one-way reversible SM. While two-way reversible shape memory (RSM) is observed upon heating and cooling cycles, the one-way RSM occurs upon heating only. Shape reversibility is achieved through partial melting of a crystalline scaffold which secures memory of a temporary shape by leaving a latent template for recrystallization. This behavior is neither mechanically nor structurally constrained, thereby allowing for multiple switching between encoded shapes without applying any external force, which was demonstrated for different shapes including hairpin, coil, origami, and a robotic gripper. Fraction of reversible strain increases with cross-linking density, reaching a maximum of <i>ca</i>. 70%, and then decreases at higher cross-linking densities. This behavior has been shown to correlate with efficiency of securing the temporary shape