Nonequilibrium Polymer Rheology in Spin-Cast Films

A new experimental approach is introduced to probe the rheology of $\gtrsim 100\mathrm{nm}$-thick liquid polymer films. As-cast films were found to have a substantially reduced effective viscosity compared to annealed films. The reduced viscosity is explained in terms of nonequilibrium chain conformations giving rise to a reduced entanglement density caused by the rapid quenching of the film during spin coating. Unexpectedly long annealing times at high temperatures are required for the films to recover their bulk rheology.

Figure 1

(a) Schematic of the experimental setup. The polymer film is sandwiched between two parallel plates, which serve simultaneously as plate capacitor electrodes and as an optical cavity. (b) Image of the onset of an electric field driven surface instability with $\lambda \sim 7\mu \mathrm{m}$.

Figure 2

Characteristic destabilization time $\tau$ vs $\lambda$. (a),(c) and (b),(d) compare films before and after annealing for ${\mathrm{PS}}_{27.5}$ and ${\mathrm{PS}}_{113}$. The different symbols correspond to bare and SAM covered substrates in (a), samples of different thicknesses in (b), annealed and as-cast samples in (f). The dashed lines are predictions of Eq. (3) using the bulk viscosity, the solid lines are fits of Eq. (3) by variation of $\eta$ (Table ).

Figure 3

Model for the reduction in effective viscosity in spin-cast films. (a) Starting from a semidilute solution, (b) the chains start to entangle as the concentration increases. (c) The slowdown and collapse of the chains for high polymer concentrations prevents the full equilibration of the chains. Thermal annealing equilibrates the chains, restoring the melt entanglement density (d).

Figure 4

(a) Lateral stress vs strain rate induced by the film destabilization in as-cast (★) and annealed (●) ${\mathrm{PS}}_{113}$ films. (b) Characteristic wavelength vs the electric field, $E_p$, for as-cast (●) and annealed (○) ${\mathrm{PS}}_{113}$ films.