Atomic Force Microscopy Studies on Hydroxypropylguar Gels Formed under Shear

Atomic force microscopy (AFM) is used to image hydroxypropylguar gel structures formed under quiescent conditions as well as gels formed under varying levels of steady shear. Hydroxypropylguar (HPG) in aqueous solution is representative of a high molecular weight, flexible polysaccharide. HPG readily cross-links with borate ions to form a gel network. The steady-state rheological properties of borate cross-linked HPG gels are shown to be dependent on the shear rate applied during the gelation reaction. The storage modulus is used as an indication of the gel strength. The gel strength is shown to be enhanced under moderate shear conditions and is seen to decrease under high shear during gelation. AFM images show an aligned, ordered structure is present in the high gel strength material (low preshear rate), with a random network structure present in the gel formed under quiescent conditions. A material composed of globular structures is apparent in the gel formed under high shear. The variation in the observed microstructure and rheological data with preshear rate is discussed in relation to alignment of polymer molecules in shear fields.