Atomic Force Microscopy Imaging and Fractal Analysis of Surface Roughness of Polyvinyl Alcohol-Chitosan Films

Polyvinyl alcohol (PVA)-Chitosan films were studied by thermogravimetric analysis (TGA), atomic force microscopy (AFM), ultrasonic force microscopy (UFM) and lateral force microscopy (LFM) and the results were compared to those of pure PVA and pure chitosan films. The surface of PVA-chitosan blends consists of small, rounded clusters, and random pores, but in contrast to the individual component films, inhomogeneous features were observed on the surface, in the form of relatively large higher clusters, which gave rise to lower elastic and frictional contrast. In addition, UFM and LFM revealed the presence of softer and lower friction regions which could not be distinguished in the topographic images. Such areas may be related to the formation of a new different phase as a result of PVA-chitosan interactions. TGA confirmed an increase in the thermal stability of the PVA-chitosan blend compared to the individual component films. The dependence of the root mean square (RMS) roughness on length scale was investigated. It was shown that the PVA-chitosan film has a higher RMS roughness for shorter measured lengths than the pure PVA samples, but the roughness for long measured lengths is lower than in pure PVA samples, which is explained by the lower Hurst number of the PVA-chitosan blend. The Hurst exponent and fractal dimension of the samples were obtained, which indicate a higher level of fractality and a more irregular shape of the surface topography for the PVA-chitosan films.