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

Structural Diversity and Phase Behavior of Brush Block Copolymer Nanocomposites

Brush block copolymers (BBCPs) exhibit attractive features for use as templates for functional hybrid nanomaterials including rapid ordering dynamics and access to broad ranges of domain sizes; however, there are relatively few studies of the morphology of the BBCPs as a function of their structural variables and fewer still studies of BBCP composite systems. Here we report the structural diversity and phase behavior of one class of BBCP nanocomposites as a function of the volume fractions of their components and the side chain symmetry of the BBCPs. We conducted a systematic investigation of gold nanoparticle (NP) (∼2 nm) assembly in a series of poly­(<i>tert</i>-butyl acrylate)-<i>block</i>-poly­(ethylene oxide) (P<i>t</i>BA-<i>b</i>-PEO) BBCPs with a fixed side chain length of P<i>t</i>BA (<i>M</i>_n = 8.2 kg/mol) but with different PEO brush lengths (<i>M</i>_n = 5.0, 2.0, or 0.75 kg/mol) as well as volume fractions (<i>f</i>_PEO = 0.200–0.484). The gold NPs are selectively incorporated within the PEO domain via hydrogen bond interactions between the 4-mercaptophenol ligands of the gold NPs and the PEO side chains. A number of morphological transitions were observed and were dependent on the total volume fraction (<i>f</i>_NP/PEO) of NPs and PEO domain. Symmetric or asymmetric lamellar morphologies of NP arrays were readily created through simple variation of <i>f</i>_NP/PEO. Interestingly, a lamellar structure was obtained at a small <i>f</i>_NP/PEO of only 0.248 for nanocomposites based on BBCPs with comparable side chain lengths (MW_PEO/MW_PtBA = 0.63). In contrast, NP morphological transitions from wormlike through cylindrical to lamellar structures were observed with the increase of <i>f</i>_NP/PEO for nanocomposites based on BBCPs with a large difference in side chain length (MW_PEO/MW_PtBA = 0.09). Highly deformed cylinders were observed in the cylindrical morphology as clearly identified by high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM) tomography. This work represents a starting point for understanding BBCP composite phase behavior, and it provides new insight toward strategies for control over the microstructure of NP arrays assembled in BBCP templates, which is essential for functional materials design

Controlled Supramolecular Self-Assembly of Large Nanoparticles in Amphiphilic Brush Block Copolymers

To date the self-assembly of ordered metal nanoparticle (NP)/block copolymer hybrid materials has been limited to NPs with core diameters (<i>D</i>_core) of less than 10 nm, which represents only a very small fraction of NPs with attractive size-dependent physical properties. Here this limitation has been circumvented using amphiphilic brush block copolymers as templates for the self-assembly of ordered, periodic hybrid materials containing large NPs beyond 10 nm. Gold NPs (<i>D</i>_core = 15.8 ± 1.3 nm) bearing poly­(4-vinylphenol) ligands were selectively incorporated within the hydrophilic domains of a phase-separated (polynorbornene-<i>g</i>-polystyrene)-<i>b</i>-(polynorbornene-<i>g</i>-poly­(ethylene oxide)) copolymer via hydrogen bonding between the phenol groups on gold and the PEO side chains of the brush block copolymer. Well-ordered NP arrays with an inverse cylindrical morphology were readily generated through an NP-driven order–order transition of the brush block copolymer

Direct Imprinting of Scalable, High-Performance Woodpile Electrodes for Three-Dimensional Lithium-Ion Nanobatteries

The trend of device downscaling drives a corresponding need for power source miniaturization. Though numerous microfabrication methods lead to successful creation of submillimeter-scale electrodes, scalable approaches that provide cost-effective nanoscale resolution for energy storage devices such as on-chip batteries remain elusive. Here, we report nanoimprint lithography (NIL) as a direct patterning technique to fabricate high-performance TiO₂ nanoelectrode arrays for lithium-ion batteries (LIBs) over relatively large areas. The critical electrode dimension is below 200 nm, which enables the structure to possess favorable rate capability even under discharging current densities as high as 5000 mA g^–1. In addition, by sequential imprinting, electrodes with three-dimensional (3D) woodpile architecture were readily made in a “stack-up” manner. The height of architecture can be easily controlled by the number of stacked layers while maintaining nearly constant surface-to-volume ratios. The result is a proportional increase of areal capacity with the number of layers. The structure-processing combination leads to efficient use of the material, and the resultant specific capacity (250.9 mAh g^–1) is among the highest reported. This work provides a simple yet effective strategy to fabricate nanobatteries and can be potentially extended to other electroactive materials