14 research outputs found
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 9â50 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 particleâs 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 DaoudâCotton 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 DNAâNanorod 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 behaviorî¸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 polyÂ(octylene
adipate) networks were synthesized, allowing for control of cross-link
density and network topology, including randomly cross-linked network
by free-radical polymerization, thiolâene 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<sup>â1</sup>, 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 baseâacid 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>-polyÂ(4-(phenylÂethynyl)Âstyrene)
(PS-<i>b</i>-PPES) diblock copolymers with a range of compositions
were prepared by reversible additionâfragmentation chain transfer
(RAFT) polymerization. Block copolymer/cobalt carbonyl adducts (PS<sub><i>x</i></sub>-PPES<sub><i>y</i></sub>[Co<sub>2</sub>(CO)<sub>6</sub>]<sub><i>n</i></sub>) were subsequently
prepared by reaction of Co<sub>2</sub>(CO)<sub>8</sub> with the alkyne
groups of the PPES block. Phase behavior of the block copolymer/cobalt
carbonyl adducts (PS<sub><i>x</i></sub>-PPES<sub><i>y</i></sub>[Co<sub>2</sub>(CO)<sub>6</sub>]<sub><i>n</i></sub>, 8% ⤠wt % PS ⤠68%) was studied by small-angle
X-ray scattering and transmission electron microscopy (TEM). As the
composition of PS<sub><i>x</i></sub>-PPES<sub><i>y</i></sub>[Co<sub>2</sub>(CO)<sub>6</sub>]<sub><i>n</i></sub> copolymers was shifted from PS as the majority block to PPES<sub><i>y</i></sub>[Co<sub>2</sub>(CO)<sub>6</sub>]<sub><i>n</i></sub> 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<sub><i>x</i></sub>-PPES<sub><i>y</i></sub>[Co<sub>2</sub>(CO)<sub>6</sub>]<sub><i>n</i></sub> diblock copolymers. Heating of PS<sub><i>x</i></sub>-PPES<sub><i>y</i></sub>[Co<sub>2</sub>(CO)<sub>6</sub>]<sub><i>n</i></sub> samples at relatively low temperatures
(120 °C) results in the formation of nanoparticles containing
crystalline cobalt and cobalt oxide domains within the PPES<sub><i>y</i></sub>[Co<sub>2</sub>(CO)<sub>6</sub>]<sub><i>n</i></sub> 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 FabryâPerot 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