The force amplitude was selected to assure that the tip completely leaves the surface after every indentation. The size of the region of interest (ROI) for each measurement ranging from 500 -3000 nm was chosen depending on the thickness of the interphase layer so the interphase layer would fill about 1/3 of the ROI. All force maps were taken at a resolution of 128 x 128 pixels, regardless of the physical dimensions of the ROI.