Role of free volume in molecular mobility and performance of glassy polymers for corrosion-protective coatings

Glassy polymer coatings on metal substrates rely on their mechanical, physical, and transport properties for corrosion protection. These properties are influenced by free volume between polymer chains that plays a critical role in coating performance. Free volume permits local segmental mobility that affects mechanical properties, such as yield and adhesion, and also permits diffusion of small molecules (water, ions, and oxygen) through the coating to the substrate. Fractional free volume is calculated from the specific volume and Bondi group contribution theory for the occupied volume. Free volume distribution (free volume element size and number) is measured by positron annihilation lifetime spectroscopy (PALS) using the ortho-positronium (oPs) probe, which detects free volume elements in glassy polymer coatings. The oPs probe is sensitive to phenomena that affect both the mechanical and barrier properties of coatings including physical ageing, water sorption, and plasticisation, making PALS a suitable allied testing method for corrosion-protective coatings.