Mechanistic Insights into Shape-Controlled Synthesis of Polymer Nano/Microstructures Enabled by In Situ Long-Focal Range Microscopy

Liquid-templated polymerization could give rise to a variety of unprecedented shapes for polymer nano/microstructures without the need for microfabrication. Nevertheless, due to the highly dynamic reacting environment, it is currently challenging to control the shape of as-synthesized polymer structures due to a lack of fundamental understanding of the mass transfer and reaction kinetics during the polymerization in structured liquids. To bridge that knowledge gap, we developed a novel in-house assembly of long-focal range reflection-mode microscope to enable in-situ imaging of the molecular alignment of liquid crystals (LC) and observe the polymer morphology transitions in real time. Enabled by this capability, we synthesized a host of shape-controlled polymer particles ranging from nano spheres, hemispherical micro-domes, randomly oriented or directed macrogel, micro spheres, spheroids and micro discs by applying initiated chemical vapor deposition (iCVD) polymerization inside nematic LC-templates. The in-situ imaging has been pivotal for development of this technology on many fronts. Accessorized with the cross-polarization mode, it enabled us to identify that the process is mass transfer limited under the given set of conditions and further facilitated attainment of fastest polymerization rates within the structure-disrupting threshold of LC-templates giving rise to unique disk-shaped particles. Furthermore, we used in-situ bright field videos to characterize the morphology progression during the polymerization of divinylbenzene in nematic LC and decouple the effects of bulk nematic LC and the LC-solid interface on the particle shape evolution. Nematic LC medium being a poorer solvent lead to the formation of nanospheres in bulk in contrast to microspheres obtained in conventional isotropic solvents used for precipitation polymerization of divinylbenzene. With LC-solid interface capturing the continuous precipitation of nanospheres from the bulk, shaped-macrogel clusters guided by the LC-templates near surface are formed on the interface constituted from aggregating nanospheres. Further, we obtained microspheres, spheroids and micro discs shaped particles desolvating from the macrogel clusters formed at the LC-solid interface at later stages. Additionally, the in-situ imaging system also captured the effect of polymerization on the molecular alignment of the anisotropic medium, which, together with supplemental experiments revealed polymerization to cause lowering of LC-surface anchoring energy. Hence, the setup played a key role in enabling the development of LC-templated-polymerization for the synthesis of shape-controlled polymer nano/microstructures. This platform is at the center-stage for expanding the range of our synthetic capabilities for polymer structures with different shapes and sizes, which could benefit a wide cross-section of existing and future technologies in drug delivery, capacitors and batteries, membrane separation, biomaterials, to name a few.