Polyacrylic acid ultra-thin films: Influence of cross-linking structure via hyperthermal hydrogen-induced cross-linking

The facile technology of hyperthermal hydrogen-induced cross-linking (HHIC) has recently been employed to efficiently cross-link the typical surface modification material of poly acrylic acid (PAA) on polymer substrates, such as polylactic acid substrates, parylene C substrates and hydrophobic polypropylene substrates. This paper systematically studied the HHIC induced self-cross-linking of PAA without polymer substrates. It was found that the -H was cleaved not only from the -C-H, but also from the surface functional group -COOH, evidenced by both X-ray photoelectron spectroscopy (XPS) measurement and the water contact angle test. We explained the self-cross-linking process as a result of both -C-C- bonding and -COOC- bonding. Meanwhile, it was also found that PAA on different substrates can also be cross-linked by secondary electron generated by X-ray radiation during XPS analysis, which could be the main reason for wide variations of different reported XPS results.