In a groundbreaking scientific study, researchers from the University of Bristol have successfully elucidated the elusive structure of block copolymer nanofibers by leveraging the potential of high-resolution cryo-electron microscopy. This pioneering study, published in Nature Materials on 22 May 2023, sheds light on the intricate core and corona architecture of these nanofibers, providing invaluable insights for their future applications.
Block copolymers, materials composed of chemically distinct polymer segments, have garnered significant interest due to their ability to self-assemble into complex structures. Among these structures, nanofibers with a crystalline core hold significant promise for diverse applications, ranging from drug delivery to nanoelectronics. However, a direct observation of the crystal lattice within these nanofibers has not been achieved.
In this study, the research team around Prof Ian Manners and lead author Dr Jia Tian employed cryo-transmission electron microscopy to examine vitrified solutions of nanofibers with a crystalline core of poly(ferrocenyldimethylsilane) (PFS) and a corona of polysiloxane grafted with 4-vinylpyridine groups. The cryo-TEM imaging unveiled that the PFS chains in the nanofiber's core pack into an 8-nanometer-diameter lattice exhibiting two-dimensional pseudo-hexagonal symmetry. Surrounding this crystalline core is a corona composed of 4-vinylpyridine strands, forming a 27-nanometer coating with a spacing of 3.5 nanometres between each strand. The combination of cryo-TEM imaging and molecular modelling allowed the researchers to propose a detailed molecular model for the solvated poly(ferrocenyldimethylsilane)-b-4-vinylpyridine nanofibers, providing unprecedented insights into their structure.
By directly observing the corona and crystalline core of nanofibers in vitrified solutions, the research team made a significant breakthrough. The precise determination of the lattice structure in the core, combined with the understanding of the corona's arrangement, has led to a comprehensive molecular model for these nanofibers. The molecular model suggests that the coronal chains extend from opposite faces with a brush density of approximately one P4VP strand for every four parallel PFS chains.
These remarkable findings provide a detailed glimpse into the structure of nanofibers with crystalline cores for the first time, offering valuable knowledge for their future development and applications. The high structural order observed within the core, as revealed by cryo-TEM imaging, is anticipated to be replicated in analogous fibre-like assemblies produced via living crystallization-driven self-assembly (CDSA). This epitaxial growth mechanism has the potential to yield fibre-like structures with exceptional properties, such as block copolymer nanofibers featuring crystalline π-conjugated polyfluorene or polythiophene cores, which have shown remarkable exciton diffusion lengths.
Looking ahead, the application of cryo-TEM to other nanofiber systems can facilitate structural comparisons and unlock key insights into their intrinsic properties, underpinning their potential use. By harnessing the newfound understanding of block copolymer nanofiber structures and assembly, scientists are now poised to make advancements in numerous fields, ranging from materials science to nanotechnology, opening up exciting avenues in medicine, energy, and electronics.