(Nanowerk Spotlight) In the quest to create materials that are both lightweight and strong, scientists have long been inspired by the intricate structures found in nature. From the lattice-like bones of birds to the porous, yet sturdy stems of plants, evolution has optimized these biological materials to achieve remarkable properties. However, replicating such designs in synthetic systems has proven challenging, particularly at the nanoscale.
Enter block copolymers, a class of self-assembling polymers that have emerged as a powerful tool for creating nanostructured materials. By carefully tuning the composition and processing of these polymers, researchers have been able to generate a wide variety of complex, ordered structures with feature sizes down to just a few nanometers. Among these, the gyroid – a convoluted, periodic network with a high surface area and unique optical properties – has garnered significant attention.
However, while block copolymer gyroids have shown promise in applications like catalysis, energy storage and photonics, they have been limited by a narrow range of accessible pore sizes and compositions. Overcoming these limitations requires innovative chemical strategies that can expand the design space while maintaining the delicate nanostructure. At the same time, translating these lab-scale materials into practical devices demands fabrication methods that are both scalable and compatible with industrial processing.
Now, a team of researchers from National Sun Yat-Sen University, Texas A&M University and National Tsing Hua University have developed a new approach that overcomes these limitations. Their work, published recently in Small (“Flexible Block Copolymer Metamaterials Featuring Hollow Ordered Nanonetworks with Ultra-High Porosity and Surface-To-Volume Ratio”), demonstrates block copolymer gyroids with record-low volume fractions, ultra-high porosities and a unique hollow tubular structure. Critically, the materials are generated in a single step and maintain their structure even in thick, flexible films.
Schematic representation of a dual-extractable DG-forming nanocomposite for fabricating various ordered nanoporous network structures. a) Illustration of the PMMA-b-P2VP(m) nanocomposite. b) Microphase-separated DG structure formed from the copolymer/mesogen nanocomposite with dual-extractable characteristics. c) PG, d) NG, and e) HG nanoporous structures obtained after removal of the photodegradable PMMA component (green), the solvent-extractable liquid crystalline mesogen component (one of the two red matrix components), or both extractable PMMA and mesogen components, respectively. (Reprinted with permission from Wiley-VCH Verlag)
The key advance in this work is a novel “dual-extractable ternary nanocomposite” metamaterial. The researchers started with a block copolymer called PMMA-b-P2VP, which consists of polymethyl methacrylate (PMMA) and poly-2-vinylpyridine (P2VP) blocks. They then blended in a liquid crystalline “mesogen” additive that selectively associates with the P2VP domains.
A mesogen is a type of molecule that plays a crucial role in the formation of liquid crystals. These molecules are characterized by their ability to arrange themselves into well-ordered structures, despite being in a fluid state. Mesogens typically possess a rigid, rod-like or disk-shaped core surrounded by flexible chains. This unique structure allows them to exhibit dual properties: the mobility of a liquid and the order of a solid. In the context of block copolymer gyroids, a mesogen additive is introduced to selectively interact with specific polymer blocks. This interaction significantly influences the self-assembly process, leading to the formation of complex nanostructures.
By associating with the polymer domains, mesogens enhance the structural integrity and functionality of the resulting materials, enabling the creation of nanostructures with unprecedented properties.
The resulting three-component mixture self-assembles into a gyroid structure where the PMMA block forms two intertwined networks surrounded by a P2VP/mesogen matrix. Crucially, both the PMMA and mesogen can be selectively removed in a single step. UV irradiation degrades the PMMA while immersion in methanol extracts the mesogen. This dual-extractable property enables three distinct porous gyroid structures to be generated from a single precursor.
If only the PMMA is removed, a “positive” gyroid (PG) results, with an air network in a polymer matrix. Extracting just the mesogen yields a “negative” gyroid (NG), with polymer networks in air. Finally, removing both components generates a unique “hollow” gyroid (HG) where the P2VP forms two ultra-thin tubular networks.
The HG structure is particularly remarkable, with a porosity of 77%, far higher than typical PG (24%) or NG (53%) structures. This translates into an enormous surface-to-volume ratio, over 8 times higher than the PG and 4 times the NG. Despite being nearly 80% air, the HG maintains substantial mechanical strength, with a specific modulus nearly double the NG and over four times the non-porous polymer.
These exceptional properties arise from two key factors. First, the mesogen additive greatly expands the accessible gyroid composition window. While the block copolymer alone could only form gyroids over a narrow range of block ratios, the mesogen increases the conformational asymmetry between the blocks, stabilizing the gyroid even at a record-low 24% PMMA volume fraction.
Second, the gyroid’s interconnected tubular structure allows it to maintain mechanical integrity even at ultra-high porosity. The researchers compare the HG to a nanoscale space frame, with stresses distributed uniformly through the crisscrossing struts. Even a 4 μm thick HG film can be bent to a radius of just 2 mm without cracking.
The HG’s combination of ultra-high surface area, nanoscale feature size and flexibility opens up exciting opportunities. The interconnected pore network could enable rapid diffusion and high catalytic activity for applications like fuel cells or chemical production. The high porosity and low refractive index are ideal for antireflective coatings and optoelectronic device packaging.
The generality of the dual-extractable nanocomposite approach suggests it could be extended to other block copolymers and mesogens, potentially yielding an even wider range of nanostructures. The efficient, single-step fabrication process is also amenable to scaling up for practical applications.
With their unprecedented combination of high surface area, low density and mechanical flexibility, these hollow gyroid materials establish a new benchmark for porous nanostructures. This groundbreaking study not only expands the capabilities of block copolymer self-assembly but provides a platform for creating advanced functional materials that balance structure, properties and processability like never before. As research in this field continues, we can expect to see these ultra-porous, hollow structures enable transformative applications across catalysis, separations, energy storage and optoelectronics.
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