Surface Modification of Thermoplastics and Elastomers for Heterogeneous Bonding

Abstract

Bonding of heterogeneous substrates, specifically polymers, is an important requirement for industries involved with the manufacture and use of products such as adhesives, coatings, and medical devices. Attempts at bonding via welding, chemical treatment, and use of adhesives are often hindered by the hydrophobic nature and chemical inertness of polymers, with many applications requiring prior surface treatment to improve compatibility between substrates. One way to modify a polymer surface is plasma treatment. This can be effective at producing hydroxyl groups on the surface of polymers which provide reactive sites for surface modification. However, plasma treatment alone cannot successfully bring together two heterogeneous substrates, a silane tethering agent is also required to chemically bond the hydroxylated surfaces with each other. In this project, the main aim was the heterogeneous bonding of elastomers to the surface of various thermoplastics by plasma treatment and using functional silanes as an intermediate between the two substrates. Specifically, polydimethylsiloxane (PDMS) would be bonded to polycarbonate (PC), polypropylene (PP), and nylon with the help of vinyl, glycidoxy, and amino-functionalised silane coupling agents. Alongside this, chemical treatment of PDMS was attempted in order to permanently produce a hydrophilic surface. This was carried out by bulk modification with polyethylene glycol, a well-known hydrophilic polymer. Fourier-transform infrared spectroscopy (FTIR) and water contact angle were used to determine the hydrophilic nature of the surface as well as observe the introduction of hydroxyl, aldehyde, and carboxylic groups upon plasma treatment. Surface morphology of the plasma-and silane-treated substrates was investigated through Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Density functional theory (DFT) calculations were employed to optimise ground state structures and frequencies to identify bond strength between the silane linkers and plasma-treated polymers. These characterisation techniques led to the successful demonstration of heterogeneous bonding between the thermoplastics and PDMS, as well as establishing a simple method to produce PDMS that can retain its hydrophilicity for over one week.