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

Abstract 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.