Ultrahigh Loading of Nanoparticles into Ordered Block Copolymer Composites

Phase selective, ultrahigh loading of nanoparticles into target domains of block copolymer composites was achieved by blending the block copolymer hosts with small molecule additives that exhibit strong interactions with one of the polymer chain segments and with the nanoparticle ligands via hydrogen bonding. The addition of d-tartaric acid to poly­(ethylene oxide-<i>block</i>-<i>tert</i>-butyl acrylate) (PEO-<i>b</i>-PtBA) enabled the loading of more than 150 wt % of 4-hydroxythiophenol-functionalized Au nanoparticles relative to the mass of the target domain (PEO + tartaric acid), which corresponds to greater than 40 wt % Au by mass of the resulting well-ordered composite as measured by thermal gravimetric analysis. The additive, tartaric acid, performs three important roles. First, as evidenced by small-angle X-ray scattering, it significantly increases the segregation strength of the block copolymer via selective interaction with the hydrophilic PEO block. Second, it expands the PEO block and enhances the number and strength of enthalpically favorable interactions between the nanoparticle ligands and the host domain. Finally, it mitigates entropic penalties associated with NP incorporation within the target domain of the BCP composite. This general approach provides a simple, efficient pathway for the fabrication of well-ordered organic/nanoparticle hybrid materials with the NP core content over 40 wt %

Rheological Study of Order-to-Disorder Transitions and Phase Behavior of Block Copolymer–Surfactant Complexes Containing Hydrogen-Bonded Small Molecule Additives

Dynamic mechanical measurements were used to investigate the effect of small molecule additives on the order-to-disorder transitions (ODTs) of Pluronic, poly­(ethylene oxide) (PEO)–poly­(propylene oxide) (PPO)–PEO triblock copolymer surfactant melts. The small molecule additives contain multiple functional groups (carboxyl or hydroxyl), which selectively interact with the PEO component of Pluronic via hydrogen bonding, thereby effectively increasing χ of the system and leading to microphase separation in otherwise disordered melts. The ODTs of these Pluronic/small-molecule-additive complexes can be detected by rheology since, upon increasing temperature, crossing the order-to-disorder transition temperature (<i>T</i>_ODT) results in a sharp decrease in the low frequency storage and loss moduli (<i>G</i>′ and <i>G</i>″, respectively). The crystallization of the PEO component is suppressed with increasing additive loading due to strong hydrogen bond interactions. The <i>T</i>_ODT is strongly composition dependent and increases up to 145 °C for 20 wt % loading of a particular additive. <i>T</i>_ODT is also found to vary widely but systematically with the number, position and hydrogen-bond-donating ability of the functional groups of the additive. Upon increasing temperature for high additive loadings, macrophase separation and crystallization of the additives can occur before the ODT is detected

Directed Assembly of Block Copolymer Templates for the Fabrication of Mesoporous Silica Films with Controlled Architectures via 3‑D Replication

Mesoporous silica films with cylindrical or spherical pores up to 40 nm in diameter were fabricated by replicating the morphologies of polystyrene-<i>b</i>-poly­(<i>tert</i>-butyl acrylate), PS-<i>b</i>-PtBA, copolymers using CO₂-assisted infusion and phase selective condensation of tetraethylorthosilicate within the polymer template. The template structures, including domain packing, orientation and spacing were controlled by adjusting the molecular weight, volume fraction and polydispersities of the block copolymers and by solvent annealing. Cylinder alignment was achieved in polymer templates through directed self-assembly (DSA). The structural details imparted to the template prior to precursor infusion were retained in the mesoporous films. In one example, aligned PS-<i>b</i>-PtBA templates were replicated to yield massively parallel arrays of cylindrical pores with pore diameters up to ∼20 nm. The ability to tune pore sizes in this range within aligned nanochannels is attractive for applications involving biomolecules

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Simple Ligand Exchange Reactions Enabling Excellent Dispersibility and Stability of Magnetic Nanoparticles in Polar Organic, Aromatic, and Protic Solvents

The use of magnetic nanoparticles (MNPs) in real-world applications is often limited by the lack of stable solutions of monodisperse NPs in appropriate solvents. We report a facile one-pot ligand exchange reaction that is fast, efficient, and thorough for the synthesis of hydrophilic MNPs that are readily dispersed in polar organic and protic solvents (polarity index = 3.9–7.2) including alcohols, THF, DMF, and DMSO for years without precipitation. We emphasize the rational selection of small-molecule ligands such as 4-hydroxybenzoic acid (HBA), 3-(4-hydroxyphenyl)­propionic acid (HPP), and gallic acid (GAL) that provide strong bonding with the MNP (FePt and FeO_<i>x</i>) surfaces, hydrophilic termini to match the polarity of target solvents, and offer the potential for hydrogen-bonding interactions to facilitate incorporation into polymers and other media. Areal ligand densities (Σ) calculated based on the NP core size from transmission electron microscopy (TEM) images, and the inorganic fractions of NPs derived from thermogravimetric analysis (TGA) indicated a significant (2–4 times) increase in the ligand coverage after the exchange reactions. Fourier transform infrared spectrometry (FTIR) and ¹H nuclear magnetic resonance (NMR) studies also confirmed anchoring of carboxyl groups on NP surfaces. In addition, we demonstrate a facile one-step in situ synthesis of FePt NPs with aromatic ligands for better dispersibility in solvents of intermediate polarity (polarity index = 1.0–3.5) such as toluene, chlorobenzene, and dichloromethane. The creation of stable dispersions of NPs in solvents across the polarity spectrum opens up new applications and new processing widows for creating NP composites in a variety of host materials

Formation of Helical Phases in Achiral Block Copolymers by Simple Addition of Small Chiral Additives

Helical superstructures were induced in poly­(ethylene oxide)-<i>b</i>-poly­(<i>tert</i>-butyl acrylate) (PEO-<i>b</i>-PtBA) achiral diblock copolymers (BCPs) through the simple addition of pure enantiomers of tartaric acid. Hydrogen bond interactions between tartaric acid and poly­(ethylene oxide) (PEO) block not only enhance the phase segregation strength of the PEO-based block copolymer but also transfer the chiral information from the additive into the achiral backbone to induce the conformational chirality. The helical phase was formed after thermal annealing with a pitch of ∼25 nm and confirmed by transmission electron microscopy (TEM) and TEM tomography. The handedness of helices can be easily selected by choice of the corresponding enantioisomer of tartaric acid

Additive-Driven Self-Assembly of Well-Ordered Mesoporous Carbon/Iron Oxide Nanoparticle Composites for Supercapacitors

Ordered mesoporous carbon/iron oxide composites were prepared by cooperative self-assembly of poly­(<i>t</i>-butyl acrylate)-block-polyacrylonitrile (PtBA-<i>b</i>-PAN), which contains both a carbon precursor block and a porogen block, and phenol-functionalized iron oxide nanoparticles (NPs). Because of the selective hydrogen bonding between the phenol-functionalized iron oxide NPs and PAN, the NPs were preferentially dispersed in the PAN domain and subsequently within the mesoporous carbon framework. Ordered mesoporous carbon nanocomposites with Fe₂O₃ NPs mass loadings as high as 30 wt % were obtained upon carbonization at the block copolymer composites at 700 °C. The morphology of the mesoporous composites was studied using small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and N₂ adsorption. The results confirmed high-fidelity preservation of morphology of the NP-doped block copolymer composites in the mesoporous carbon composites. The electrochemical performance of the mesoporous composite films improved significantly upon the addition of iron oxide NPs. The specific capacitance (<i>C</i>_g) of neat mesopororous carbon films prepared from PtBA-<i>b</i>-PAN was 153 F/g at a current density of 0.5 A/g, whereas films containing 16 and 30 wt % Fe₂O₃ present as well-dispersed NPs within the mesoporous carbon framework exhibited capacitances of 204 and 235 F/g, respectively. The well-defined mesoporous in the template carbon structure together with high loadings of iron oxide nanoparticles are promising for use in supercapacitor applications

Synthesis and Controlled Self-Assembly of UV-Responsive Gold Nanoparticles in Block Copolymer Templates

We demonstrate the facile synthesis of gold nanoparticles (GNPs) functionalized by UV-responsive block copolymer ligands, poly­(styrene)-<i>b</i>-poly­(<i>o</i>-nitrobenzene acrylate)-SH (PS-<i>b</i>-PNBA-SH), followed by their targeted distribution within a lamellae-forming poly­(styrene)-<i>b</i>-poly­(2-vinylpyridine) (PS-<i>b</i>-P2VP) block copolymer. The multilayer, micelle-like structure of the GNPs consists of a gold core, an inner PNBA layer, and an outer PS layer. The UV-sensitive PNBA segment can be deprotected into a layer containing poly­(acrylic acid) (PAA) when exposed to UV light at 365 nm, which enables the simple and precise tuning of GNP surface properties from hydrophobic to amphiphilic. The GNPs bearing ligands of different chemical compositions were successfully and selectively incorporated into the PS-<i>b</i>-P2VP block copolymer, and UV light showed a profound influence on the spatial distributions of GNPs. Prior to UV exposure, GNPs partition along the interfaces of PS and P2VP domains, while the UV-treated GNPs are incorporated into P2VP domains as a result of hydrogen bond interactions between PAA on the gold surface and P2VP domains. This provides an easy way of controlling the arrangement of nanoparticles in polymer matrices by tailoring the nanoparticle surface using UV light

Chiral Arrangements of Au Nanoparticles with Prescribed Handedness Templated by Helical Pores in Block Copolymer Films

Fabrication of films with plasmonic nanoparticles (NPs) arrays arranged in chiral configurations of prescribed handedness is highly attractive for the design of new functional materials; however, this remains a formidable challenge in nanotechnology. In this study, we demonstrated the controlled arrangements of gold (Au) NPs into helical structures templated by helical pores created in cross-linked block copolymer (BCP) films. d- and l-tartaric acid (TA) were used to direct the self-assembly of achiral poly­(1,4-butadiene)-<i>b</i>-poly­(ethylene oxide) BCPs into helical cylindrical morphologies with prescribed handedness, i.e., D or L. Helical pores were generated by BCP cross-linking followed by TA extraction. Helical Au NP arrays, subsequently arranged within the helical pores, exhibited the chiral optical response. The helical structures of NPs arrays and the resulting optical handedness were tunable simply by using either D- or L-porous templates. This simple strategy offers a straightforward pathway for the fabrication of chiral porous BCP films and helical NPs arrays with chiral optical properties

Polystyrene-<i>block</i>-poly(ethylene oxide) Bottlebrush Block Copolymer Morphology Transitions: Influence of Side Chain Length and Volume Fraction

A systematic study was conducted to investigate the morphology transitions that occur in polystyrene-<i>block</i>-poly­(ethylene oxide) (PS-<i>b</i>-PEO) bottlebrush block copolymers (BBCP) upon varying PEO volume fraction (<i>f</i>_PEO) from 22% to 81%. A series of PS-<i>b</i>-PEO BBCPs with different PEO side chain lengths were prepared using ring-opening metathesis polymerization (ROMP) of PEO–norbornene (PEO-NB) (<i>M</i>_n ∼ 0.75, 2.0, or 5.0 kg/mol) and PS–norbornene (PS-NB) (<i>M</i>_n ∼ 3.5 kg/mol) macromonomers (MM). A map of <i>f</i>_PEO versus side chain asymmetry (<i>M</i>_n(PEO-NB)/<i>M</i>_n(PS-NB)) was constructed to describe the BBCP phase behavior. Symmetric and asymmetric lamellar morphologies were observed in the BBCPs over an exceptionally wide range of <i>f</i>_PEO from 28% to 72%. At high <i>f</i>_PEO, crystallization of PEO was evident. Temperature-controlled SAXS and WAXS revealed the presence of high order reflections arising from phase segregation above the PEO melting point. A microphase transition temperature <i>T</i>_MST was observed over a temperature range of 150–180 °C. This temperature was relatively insensitive to both side chain length and volume fraction variations. The findings in this study provide insight into the rich phase behavior of this relatively new class of macromolecules and may lay the groundwork for their use as templates directing the fabrication of functional materials