Atomic force microscopy studies-on hydroxypropylguar gels formed under shear

Atomic force microscopy (AFM) is used to image hydroxypropylguar gel struct ures formed under quiescent conditions as well as gels formed under varying levels of steady shear. Hydroxypropylguar (HPG) in aqueous solution is rep resentative of a high molecular weight, flexible polysaccharide. HPG readil y cross-links with berate ions to form a gel network. The steady-state rheo logical properties of berate cross-linked HPG gels are shown to be dependen t on the shear rate applied during the gelation reaction. The storage modul us 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 unde r high shear during gelation. AFM images show an aligned, ordered structure is present in the high gel strength material (low preshear rate), with a r andom network structure present in the gel formed under quiescent condition s. A material composed of globular structures is apparent in the gel formed under high shear. The variation in the observed microstructure and rheolog ical data with preshear rate is discussed in relation to alignment of polym er molecules in shear fields.