Chemical and thermo-responsive characterisation of surfaces formed by plasma polymerisation of N-isopropyl acrylamide

Plasma polymerisation of N-isopropyl acrylamide (NIPAAm) presents an exciting route for the production of thermally responsive coatings on a wide variety of substrates for applications in tissue culture and microfluidics. One issue associated with the polymerisation of NIPAAm via plasma polymerisation is the limited volatility of the monomer and the subsequent requirement for monomer and reactor heating to create and maintain the vapour. It is already well established that power is critical in the balance between polymer functionality and coating stability in plasma polymers. However, little is known of how reactor and substrate temperatures may be used to influence the physico-chemical characteristics of polymers produced from such low-volatility monomers. In this paper, we examine the effects of a range of plasma deposition parameters on the functionality and stability of plasma-polymerised NIPAAm surfaces. X-ray photoelectron spectroscopy (XPS), near-edge X-ray absorption fine structure spectroscopy (NEXAFS), ellipsometry and contact angle goniometry have been used to examine coating chemistry, stability in aqueous environments, deposition rates and thermo-responsive behaviour. Our results indicate that plasma polymerisation at low powers and low temperatures enhances the ability of plasma-polymerised NIPAAm to display a wettability phase transition, but also contributes to instability of the coating to dissolution or delamination in water. Our spectroscopic measurements confirm that retention of the monomer structure is facilitated by low power and temperature deposition and reveal that conversion of the amide groups to amine and nitrile groups occurs during the polymerisation process, particularly at high discharge powers. Copyright (C) 2006 John Wiley & Sons, Ltd.