Abstract <p>Polymer films coated on soft substrates find applications in diverse areas. Understanding the mechanical behavior of the film-substrate composite is important for achieving films that can sustain large strains without failure. To this end, we perform tensile stress experiments for thin films of a model polymer, ethyl cellulose, coated on a soft substrate, polydimethylsiloxane. Drying of wet polymer films in high humidity environment results in porous films that are opaque in appearance while those dried in low humidity conditions lead to transparent films without pores. The porous films show lower tensile strength compared to the transparent films. While the tensile strength of the opaque films was invariant with film thickness, the strength of the transparent film increased with decreasing film thickness. We explain the observations in terms of the Griffith’s fracture criteria wherein flaw size, proportional to the thickness of the film, sets the tensile strength. The results connect the polymer film drying process and microstructure evolution with its mechanical properties and fracture behavior.</p> Graphical abstract <p></p>