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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 wellcross-linked gel separated by fluid domains. The TEM images confirm and extend rheological measurements.

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Hydroxypropyl guar (HPG) polymer solutions crosslinked with titanates (Ti) are used in hydraulic fracturing operations. The rheology of these gels arise from the complex interaction of polymer chemistry, metal ion chemistry, and flow history. We have employed several techniques (steady shear viscosity, dynamic oscillatory shear. Laser Doppler Anemometry, and freeze fracture electron microscopy) to develop a consistent picture of the factors controlling gel rheology. An impingement mixing technique has been developed to prepare uniform HPG gels and reproducible rheological data has been obtained. Dynamic oscillatory rheological measurements have been used as diagnostic probes of gel structure, and are shown to be useful in understanding chemical effects on gel structure. Laser Doppler Anemometry, a non-invasive technique for measuring velocity distributions, has been used to measure velocity profiles for gels under flow. Classical wall slip is not observed, rather distortions of the velocity profiles are observed for gels that have gelled under quiescent conditions. For gels made with impingement mixing linear velocity profiles are observed. Freeze fracture electron microscopy has been used to image gel structure and the results show the fracture of gel structure by shear. These observations demonstrate the mechanism of temperature stabilization of the viscosity of cross-linked gels. Temperature stability measurements of gel viscosity have been made using a novel reciprocating capillary viscometer.