Case Study 2 - XPS Analysis, Including Depth Profiling, of Polymer Brushes

Polymer brushes describe polymer chains tethered to a surface at one end at such a high concentration they extend away from the surface. Such a polymeric system has possible applications in the field of surface lubrication, biofouling resistant surface, antibacterial surfaces and in biosensors.

 

For optimum performance in these applications, particularly in bio-applications, polymer brushes may need to be derivatised. Frequently, derivatisation is carried out in organic solvents and leads to a low degree of functionalisation.

 

XPS at the SSAC has been used to support and confirm the work of Dr E. Johnson and Prof S. Armes (reference 1) with hydrophilic polymer brushes made from the methacrylic monomer GEO5Ma which can subsequently be oxidised in situ to give aldehyde functionality using an aqueous solution of sodium periodate. This was confirmed by XPS analysis of the sample surface before and after oxidation of the polymer brush and also in comparison with an analysis of PGEOGMA homopolymer dissolved in aqueous solution after oxidation.

Reaction scheme for growth of PGEO5MA polymer brush, and oxidation by sodium periodate for form aldehyde group, taken from reference

The carbon 1s scans show the expected oxidised carbon functionality and shows very similar spectra for oxidised polymer brushes and oxidised homopolymer.  The extent of functionalisation was determined from the XPS data using the %carbon component concentrations which suggested a degree of aldehyde functionalisation of 94%.

 

These hydrophilic aldehyde functionalised polymer brushes were then derivatised with histidine followed by a reductive amination using NaCNBH3 using a one-pot synthesis. XPS was able to confirm the increase in nitrogen concentration due to histidine take-up. The N:O ratio from the XPS study compared to that expected from a fully derivatised film suggests 81% conversion to PHisGEO5MA which agreed well with the % conversion determined by ellipsometry from the increase in film thickness. The zwitterionic nature of the histidine group post reductive amination can be seen from the two nitrogen environments in the XPS high resolution spectrum.

XPS depth profiling using a large gas cluster ion source, Ar3000+, was used to confirm derivatisation had occurred all the way through the polymer brush by showing the elemental concentration, including nitrogen, was approximately constant throughout the thickness of the brush, up until the silicon wafer interface is reached. 

In this was XPS was able to confirm the aldehyde functionalisation of the methacrylate polymer brush had taken place and to what extent, and similarly confirm and quantify the extent of histidine deposition after reactive amination and demonstrate the derivatisation had taken place uniformly throughout the polymer brush thickness.

Reference

Hydrophilic Aldehyde-Functional Polymer Brushes: Synthesis, Characterization, and Potential Bioapplications

Emma E. Brotherton, Edwin C. Johnson, Mark J. Smallridge, Deborah B. Hammond, Graham J. Leggett, and Steven P. Armes

Macromolecules 2023, 56, 2070−2080