TEM Observation of Nano-Scale Hydrogel Network Structure

Abstract

Hydrogel, containing large amount of water inside its polymer network, is one of the most attractive biomaterials, because of its similarities with living tissues such as, high water content, low friction and flexibility. These unique features come from complicated polymer network structure, and until now great effort have been put in to clarify nano scale structure of hydrogel. Especially, many researchers tried to investigate by using scattering techniques. However, scattering data just presents limited information in reciprocal space and “real” polymer network structure is still not well understood due to difficulty on direct observation of hydrogel polymer. In this study, we adopt an idea to stain polymer network by using inhomogeneous nucleation, nucleation starts from the functional group of additive. If we carried out inhomogeneous nucleation in the hydrogel, mineral crystals easily forms complex with hydrogel network and this mineral distribution should represent the original polymer structure. Therefore, the purpose of this study is to stain hydrogel polymer network with mineral and to observe its nano-scale structure by using transmission electron microscope (TEM). Double network hydrogel was synthesized by two step UV radical polymerization.1 2-acrylamido-2-methyl propanesulfonic acid (AMPS), which contains sulfonic functional group, was used for 1st polymer network. Then, mineral bernalite (Fe(OH)3), whose nucleation accelerates by sulfonic group, was mineralized into DN gel. Fig.1 shows TEM image of mineralized DN gel and its schematic image. Nano scale hydrogel polymer network structure was successfully observed and this structure well agreed with previous prediction from neutron scattering method. This is the world’s first direct observation of polymer network. Mineralized crystal size was several nm in diameter, which is same scale of polymer blob. It indicates that we observed mineral crystals surrounded by individual polymer blobs. The detailed results will be presented in conference. In conclusion, we succeeded direct observation of hydrogel polymer network using TEM. This is a very challenging study not only for understanding hydrogel, but also for visualization polymer network, which is one of biggest dream of polymer scientists.