Distribution of block copolymers in drying polymer films

Hypothesis: Block copolymers (BCP) consisting of a polar block and a surface active apolar block are widely used for surface functionalization of polymer films. The characteristics of the copolymer blocks determine whether surface segregation and/or phase separation occurs, for a given bulk mixture. This data can be used to find the optimal BCP composition where high surface enrichment is obtained without accumulation of phase separated BCP in the bulk. Methods: The distribution of poly(ethylene oxide)-polydimethylsiloxane (PEO-PDMS) BCP in a polymer formulation relevant for coating applications is systematically investigated. The surface segregation is studied in liquid formulations with surface tension measurements and dried films with X-ray photoelectron spectroscopy (XPS), whereas phase separation is quantified using turbidity measurements. The results are compared with Scheutjens-Fleer self-consistent field (SF-SCF) computations, which are also applied to determine the effect of film drying on BCP phase stability and surface segregation. Findings: Longer PDMS blocks result in lower interfacial tension of the liquid polymer mixture, whereas for the cured films, the largest PDMS concentration at the interface was obtained for intermediate PDMS block lengths. This is explained by the observation that phase separation already occurs at very low BCP concentrations for long PDMS blocks. The SCF predictions qualitatively agree with the experimental results and reveal that the BCP distribution changes significantly during film drying

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

Design of dual hydrophobic–hydrophilic polymer networks for highly lubricious polyether-urethane coatings

Bio-lubricated surfaces found in nature have inspired the design of low friction polymer coatings for biomedical applications. This work presents a systematic study of the relation between the network structure parameters and the macroscopic friction properties of highly lubricious dual hydrophobic/hydrophilic polyurethane (PU) coatings in an aqueous environment. Chemically cross-linked PU coatings were prepared by adding poly(ethylene glycol) mono-methyl ether (mPEG) as hydrophilic dangling chains, or poly(ethylene glycol) (PEG)-diol as hydrophilic elastically active network chains, to poly(propylene glycol) (PPG)-PU coating formulations. The friction behaviour of the water swollen coatings was measured using a custom-made water immersed tribology setup. Addition of the PEG segments or mPEG dangling chains to hydrophobic PPG coatings greatly enhances the lubricious properties of the coatings. These dual hydrophobic/hydrophilic diol PU network exhibit a surface with a lower coefficient of friction compared to reference coatings from either individual precursors, demonstrating a large synergistic effect between the hydrophobic PPG and the hydrophilic PEG in the coatings. Based on network structure and surface chain considerations it is hypothesized that the existence of a thin and softer hydrated surface layer on top of a less hydrated, more rigid, coating bulk layer gives rise to the observed enhanced lubricious properties, hereby mimicking to some extent bio-lubricated systems, such as cartilage