Structural relaxation of spin-cast glassy polymer thin films as a possible factor in dewetting

Abstract

Reiter [1] has recently reported a situation in which the dewetting of quasi-solid films is linked plastic deformation -rather than viscous flow -resulting from capillary forces. Herein we propose that, in thin films of some glassy polymers -especially poly(methyl methacrylate) (PMMA) -prepared by spin-casting from solvent, structural relaxation might impart sufficient stress to cause plastic deformation. We find that PMMA films decrease in thickness by several percent, which is sufficient to create significant stress in those cases in which the film is attached to a rigid substrate. The floating technique, which can take tens of minutes, might allow most of the structural relaxation to occur prior to dewetting experiments.

Thickness dependence of structural relaxation in spin-cast, glassy polymer thin films

“…The relaxation of volume (or thickness) in thin films of polymer glasses has been observed by several workers [7][8][9][10][11][12] in neat polymers [7,8] and in polymers containing residual solvent [9][10][11][12][13]. In these previous experiments, measurements of thickness have been made as a function of increasing temperature, and it was observed that the change in thickness (volume) due to relaxation occurs simultaneously with a change resulting from thermal expansivity.…”

“…In these previous experiments, measurements of thickness have been made as a function of increasing temperature, and it was observed that the change in thickness (volume) due to relaxation occurs simultaneously with a change resulting from thermal expansivity. Studies of isothermal relaxation in polymer thin films are much more rare [9,10,12]. The possible effects of molecular confinement and attraction to interfaces on the dynamics of relaxation in polymer thin films are largely unexplored.…”

“…Hence, after a solvent quench of a polymer solution, ␦ can be much higher than after a thermal quench, in which ␦ is typically 0.01. If a polymer glass is constrained in one or more dimensions, significant mechanical stress could be generated [10,11,14,15].…”

Solvent retention in thin polymer films studied by gas chromatography

“…The final thinning of the film is then due solely to solvent evaporation [2]. Most of the solvent evaporates during the last step of the spin-coating, but it is known that some solvent is still present in the films at the end of the spin-coating [1,3,4] and that the polymer chains are quenched in a nonequilibrated state [5][6][7][8]. Spin-coated films are therefore generally annealed above the glass transition temperature of the polymer to dry the films and let the polymer relax to equilibrium.…”

“…Spin-coated films are therefore generally annealed above the glass transition temperature of the polymer to dry the films and let the polymer relax to equilibrium. Whereas solvent loss during this drying step (with respect to the initial solvent content) has been extensively studied [3,4,[9][10][11][12][13][14][15], there is hardly any information available on the absolute amount of solvent left in the films.…”