Transmission Electron Microscopy of Gel Network Morphology: Relating Network Microstructure to Mechanical Properties

Freeze-fracture TEM images of polymer network structure in aqueous (hydroxypropyl)guar gels are presented. With these images correlations between observed gel morphology and macroscopic chemical and mechanical properties can be directly determined. The images show that guar molecules associate, even without titanate cross-linker, to form an extended network, the structure of which depends on the electrolyte concentration. The titanate cross-linker does not appear to influence the microstructure but rather appears to weld together polymer molecules already associated by hydrogen bonding. Mechanical shear degrades the gel by breaking up the network inhomogeneously to form “fractured gel particles”, small domains of well-cross-linked gel separated by fluid domains. The TEM images confirm and extend rheological measurements.