Surface segregation of polydimethylsiloxane-polyether block copolymers in coatings driven by molecular architecture

Block copolymers containing polydimethylsiloxane (PDMS) and poly(ethylene oxide) (PEO) or poly(propylene oxide) (PPO) with varying molar masses were synthesized in a three-step pathway. The functional homopolymer blocks and final diblock copolymers were characterised using proton Nuclear Magnetic Resonance (1H NMR) and Matrix Assisted Laser Desorption/Ionization Time of Flight Mass Spectroscopy (MALDI-ToF-MS). These polymers were then incorporated in an industrially relevant solvent-borne coating formulation. Using X-ray Photoelectron Spectroscopy (XPS) and a combination of angle-resolved and depth profiling measurements, concentration profiles of the block copolymer in the top few nanometres of the cured coating were obtained. These amphiphilic molecules were found to be extremely surface active, and high levels of PDMS enrichment of the coating surface were observed at only minimal concentrations. The extent of segregation is sensitive to the exact mass of both the siloxane and polyether block, where an increase in the size of either part resulted in an overall decrease in surface enrichment. PDMS-PPO was found to be more compatible with the coating network than PDMS-PEO, as evidenced by the substantial lower surface enrichment of the former. The surface properties of the liquid and cured films were additionally characterised using surface tension and water contact angle measurements, which largely confirmed the trends observed with XPS. The characterisation of the complex and dynamic processes occurring during drying of the coating is key to provide the ability to effectively tune specific coating systems for required surface properties relevant for individual applications.