Scanning Force Microscopy Characterization of Viscoelastic Deformations Induced by Precontact Attraction in a Low Crosslink Density Gelatin Film

Greg Haugstad*
Center for Interfacial Engineering, 187 Shepherd Laboratories
University of Minnesota, Minneapolis, MN 55455

Wayne L. Gladfelter
Department of Chemistry
University of Minnesota, Minneapolis, MN 55455

Richard R. Jones
Sterling Diagnostic Imaging
Brevard, NC 28712

Abstract

Scanning force microscopy (SFM) is used to investigate novel perturbation/response phenomena in a soft polymer network. Topics addressed include (i) the volume of film affected by tip-sample contact and (ii) the time-evolving residual signature of this contact. An outward deformation of nanometer-scale, soft, hydrated gelatin films is induced by the close proximity of the SFM tip. A dome-like defect is created, centered at the site of approach and exceeding the tip-sample contact zone in diameter by as much as three orders of magnitude. The stretching of the film changes the stiffness of the polymer network and its frictional character. A precise correspondence of height and frictional force is quantified in histograms of the number of image pixels versus height or frictional force, and as a function of lateral distance from the center of approach. Relaxation of the dome is observed on a time scale of minutes with stretched exponential time dependence, consistent with a distribution of relaxation times. Film age also affects the size of the doming region: an increase to a maximum volume is observed, followed by a decrease to nanometer scale dimensions with age. This apparently reflects competing increases of long and short-range order that determine film cohesion. Five stages of gelatin film aggregation are experimentally distinguishable, differing in the extent of cohesion generated by progressive intermolecular coordination (e.g. crystallinity).