As visualized in Figure , at all molecular weights, the thicknesses of the thin films increase monotonically as solution concentration increases. The function's monotonic increase is due to the increase in viscosity and entanglement between polymer chains that occurs when the concentration of the polystyrene solutions increases , which causes the thickness of the thin films to increase . At lower molecular weights, the spin coated thin films exhibit a seemingly linear relationship between thickness and concentration. This relationship is in agreement with early works that found that the thickness of spin coated thin films increases linearly with concentration . However, at higher molecular weights, there is clearly a non-linear relationship between thickness and concentration. In fact, Hall et al. notes the tendency of Bornside's equations to underpredict film thickness for films spun from solutions with an initially high polymer concentration and overpredict film thickness for films spun from very dilute solutions, attributing these deviations to the assumption of the model that fluid flow and evaporation are decoupled.