Plasma polymerization of TEMPO shows promise

Plasma polymerization is a single step process that does not require solvent and can be used to deposit ultrathin functionalized films onto surfaces. The majority of these experiments have used radiofrequency (RF) plasmas at low pressure, but this can produce multiple final products and is difficult to control.

Two ways to increase control is to reduce plasma power or to increase pressure. Barnes et al. tried both approaches using the monomer (2,2,6,6-tetramethylpiperidin-1-yl)oxyl, or TEMPO, since this substance can produce polymers with a high concentration of nitroxide groups.

“We took this approach, since the combination of low power and high pressure could result in greater functional retention than that observed using reduced power or higher pressure alone,” said author Michael Barnes.

The nitroxide functional groups behave like nitric oxide, NO, an important signaling molecule in biology. Incorporating NO-like functional groups in polymers could be useful for biofilm management, cancer treatment, or other uses.

The investigators produced TEMPO polymer films on aluminum substrates and studied them with a variety of techniques. X-ray photoelectron spectroscopic studies showed that combining high pressure and low power leads to collisional discharge and greater functional retention.

Films made with a power of 5 Watts at 5 Pascals of pressure retain 59% of nitroxide groups, but for the same power of 5 Watts while increasing the pressure to 25 Pascals, improves the retention to 75%.

The polymer deposit grows when intact monomer ions make a so-called “soft landing” on the substrate. This means their impact with the surface has low energy and does not rearrange the molecular structure, allowing these ions to contribute to the growth of the film.

Source: “Plasma polymerisation of (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) in a collisional, capacitively-coupled radio frequency discharge,” by Michael J. Barnes, Alexander J. Robson, Javad Naderi, Robert D. Short, and James W. Bradley, Biointerphases (2020) This article can be accessed at https://doi.org/10.1116/6.0000662 .

This paper is part of the Special Topic Collection: Biointerface Science in Australia 2020 – An issue in celebration of Hans Griesser’s career; learn more here .