Development of mechanical and interfacial characterization methods for polymer thin films

Polymer thin films have been developed in numerous industrial fields due to their cost efficiency and productivity. A primary step in developing new products is characterizing the properties of polymer thin films before implementing them in various applications. Many studies have been conducted to determine the physical properties of polymer thin films but there is still a need to further understand and characterize their mechanical and interfacial properties. Although there are standard testing methods to assess the mechanical and interfacial properties of thin films, they can be challenging to use due to the geometrical and physical limitations of polymer thin films. Hence, there is a need to develop a new approach for mechanical and interfacial characterization that has high sensitivity and can be broadly applied.

In this work, wrinkling and delamination of a glassy thin film on an elastomeric substrate, a well-defined and understood surface buckling instability, is adopted to investigate the interfacial and mechanical properties of a glassy polymer thin film on a soft elastomeric substrate. This new characterization tool utilizes the transition from thin film wrinkling to delamination (W2D) from the elastomeric substrate to subsequently measure the elastic modulus and the adhesion strength of a glassy polymer thin film, which is difficult to characterize simultaneously with conventional techniques due to the brittleness of glassy thin films. Furthermore, the dependency of the elastomer’s bulk mechanical properties on adhesion strength was investigated by expanding on the W2D technique. Further exploiting the W2D technique, the delamination propagation rate of a thin film debonding from a substrate is used to understand how the bulk mechanical properties of the substrate affect the adhesion energy, termed the strain energy release rate (G), of a thin film on an elastic substrate. Additionally, a new method for determining the axial fracture mechanism and axial modulus of cellulose nanocrystal (CNC) films has been developed by visualizing in-situ deformation and surface instabilities (wrinkling). Lastly, new experimental approaches were developed to understand the mechanical behavior of polymeric thin films using a common test method. The effect of temperature on the adhesion and mechanical behavior of polymeric thin films was studied by developing a double lap shear fixture and temperature stabilization platform. Through these newly developed or revised characterization approaches, measurement of the physical and mechanical properties of polymer thin films that are commonly difficult to measure have been overcome.