Study of thickness dependent density in ultrathin water soluble polymer films

TL;DR

This study investigates how the density of ultrathin polyacrylamide films varies with thickness, preparation conditions, and spin coating parameters, revealing that thinner films have higher density due to chain orientation effects.

Contribution

It provides new insights into the relationship between film thickness, density, and chain orientation in ultrathin water-soluble polymer films, using X-ray reflectivity and scattering techniques.

Findings

Density increases as film thickness decreases.

Density saturates at higher spin speeds.

No correlation between shrinkage and density.

Abstract

Density of the polyacrylamide ultrathin films has been studied using X-ray reflectivity technique. Two sources (one powder and another aqueous solution) of polyacrylamide were used to prepare spin coated films on silicon substrate. Light scattering measurements show that the polymer chains were unentangled in a concentrated (4 mg/ml) as well as in a dilute (2 mg/ml) solution prepared from the powder, whereas the solution (4 mg/ml) prepared by diluting the solution source shows entangled chain morphology. Three sets of films of different thicknesses were prepared using the three solutions by spin coating on silicon substrates. Comparison of X-ray reflectivity data for as prepared and dry films reveals that the shrinkage of the films decreases with increasing thickness. Average electron densities of the films were found to follow a trend of higher density for thinner films with a maximum increase of about 12% compared to the bulk. The densities of all the films irrespective of the nature of entanglement and concentration of their source were found to increase with spin speed of coating and attain saturation at higher speed. Absence of correlation between shrinkage and density data and the fact that the densities of all the films follow a master curve irrespective of their origin suggest that the higher density of the films result from the higher orientation of chains as a consequence of an interplay between stretching and stronger attractive interactions of polar nature.