PROBING BIOPOLYMER FILMS WITH SCANNING FORCE METHODS

GREG HAUGSTAD,* WAYNE L. GLADFELTER,* ELIZABETH B. WEBERG,** ROLF T. WEBERG** AND TIMOTHY D. WEATHERILL**

*University of Minnesota, Department of Chemistry and Center for Interfacial Engineering, 187 Shepherd Laboratories, 100 Union Street SE, Minneapolis, MN 55455

**E. I. du Pont de Nemours and Company, Medical Products Division, Brevard, NC 28712

ABSTRACT

Scanning force microscopy of thin gelatin films on mica reveals two distinct film components with characteristic frictional, morphological and adsorptive signatures. A high-friction film 1-4 nm thick completely wets the mica surface, while a low-friction minority component is present primarily as porous islands on top of the high-friction layer. Additional domains of extremely high friction within the majority component are detected in frictional images obtained with a blunt, gelatin-covered tip; no corresponding topographic differences are imaged. A high-force scanning procedure remarkably transforms the majority component into the low-friction phase if a sufficient amount of water is present in or on the film. The nanostructure of both components is imaged using a nanometer-scale asperity of gelatin attached to the SFM tip. The anticipated network structure of gelatin is observed on the high-friction majority phase. The low-friction phase is interpreted as moieties of intramolecularly-folded gelatin, with thickness (1.5±0.2 nm) equal to the diameter of the collagen-fold triple helix, containing substantial structural water.