(532j) Critical Cracking Thickness of Drying Polymer Films

Polymer coatings are ubiquitous and may be applied to protect surfaces and act as functional part of devices. It is important that the coatings do not fill during lifetime. While a number of investigations have studied the shrinkage stresses in drying polymer films, few have explored the mechanics of failure in such films. Here, we present a simple model, which considers the drying polymer film as a porous solid, and determine conditions under which drying films of polymer solutions can crack. The model accounts for the properties of the polymer film and substrate and predicts the tensile stress developed in the drying film. As the tensile stress increases and the stored elastic energy of the system equals the surface energy required to create new surfaces, the film relaxes by nucleating a crack. The model calculations predict a critical thickness below which the film does not crack. The predicted critical cracking thickness is compared with experiments performed on drying films of silicone resin on six different substrates with the value of Young's modulus spanning over six decades. The predicted trend matches measurements.