January 21, 2020 by Luke James
Researchers have used bond percolation to synthesize a highly ordered and flexible three-dimensional network made from a polymer gel.
Different applications like filtration, drug release, optics, sensing, and electronics benefit from 3D networks with nanometer-sized pores. Although new and developing technologies help in building more precise and intricate structures, each object has a limitation in its overall practical size, and ordered three-dimensional nanostructures are challenging to construct on a large scale.
Polymer gels are soft materials with 3D nanometer-sized networks that can synthesize rapidly by connecting long polymer chains with solvent cross-linkers. However, they have a substantial defect level, such as loops, non-uniform pore sizes, dangling ends, and entanglements, due to the stochastic cross-linking reaction. Neutron or photon scattering techniques detect these spatial defects as robust small-angle scattering, nonergodic concentration fluctuations, and stationary laser speckles.
Researchers attempted to fix these defects by:
synthesizing them from monodisperse polymer chains
restricting adverse intramolecular effects
using a homogenous cross-linking process
cross-linking polymer chains with movable cross-linkers
Because of stochastic reaction, gels are inherently disordered. In a study by University of Tokyo, researchers attempted to fabricate gels with low levels of defects. In the study, the researchers broke the preconception by introducing a scheme to fabricate gels with low spatial defects under a synthesis structure.
They used the bond percolation model, which assumed a uniformly pre-packed space of mutually exclusive pieces. Percolation occurred due to the cross-linking of nearby units that resulted in an exemplary and highly ordered structure. They achieved the ideal bond percolation by using monodisperse star polymers as a space-filler. They, then, dissolved them in an excellent solvent to ensure uniformity and tight filling of the spaces.
Optical image of fully developed transparent gel contained in a glass tube. Image Credit: Xiang Li, Institute for Solid State Physics, The University of Tokyo.
The researchers also used dehydrated acetonitrile as a solvent due to its excellent affinity to the polymers used. Furthermore, they removed the nanobubbles and dust by filtering the solutions using ultrafine syringe filters. After adding the cross-linkers, the polymer solution transitioned into sol-gel after about an hour at room temperature.
Gels and glass have a disordered structure; however, the results of the experiment showed how the bond percolation gel differed from a glass. Tightly packed star polymers have no detected spatial defects, and if there are gaps, they are nonergodic by nature, so the polymer chains can’t exchange position with the other units. However, the researchers didn’t observe the nonergodicity in their gel because the polymer chains filled the entire space.
The flexible, highly ordered polymer gels are beneficial to speckle-free optical fibers, controlled drug release, powerful actuators, and high-performance sensing and filtration. Moreover, because the fabrication of ordered polymer networks didn’t rely on specific polymer chemistry and cross-linking reaction, other applications are also possible. The study further provides an ideal platform for the exploration of the fundamental physics of polymer networks, which have been unclear due to the varied defects of traditional gels.
