Characterization of PNIPAAm photografted on PET and PS surfaces

Aqueous solution of poly(N-isopropylacrylamide), or PNIPAAm, possess lower critical solution temperature (LCST) around 32-33 °C. Below this temperature the PNIPAAm is hydrophilic and above presents hydrophobic characteristics. In this work, virgin or oxidized films of PET and PS were submitted to surface modification by grafting of PNIPAAm. The film was immersed in aqueous solution of monomer and exposed to radiation from a low pressure Hg lamp (215 W) using KIO4 as photosensitizer. Virgin, oxidized and PNIPAAm-grafted surfaces were characterized by drop water contact angle (DWCA), SEM, AFM, nanoindentation and XPS techniques. When the grafted polymers are warmed to temperatures above LCST, significant change on DWCA indicates the hydrophilic-to-hydrophobic transition. SEM and AFM images showed a rising of defects and roughness on oxidized and grafted polymer surfaces, compared to the virgin ones. Results from nanoindentation showed that the oxidation process does not produce significant alteration on mechanical properties. However, the PNIPAAm-grafting into PET decreased the hardness and the elastic modulus of such surfaces. XPS analysis indicates significant alterations on atomic composition at surface of polymers, due to the PNIPAAm-grafting. The degrees of grafting were calculated from the relative compositions of nitrogen. The fraction of polymer surface covered by PNIPAAm was, at maximum 0.76. The grafting of PNIPAAm on PET and PS, as investigated in this work, enables the surfaces to be applied as biomaterials, for instance, as substrates for adhesion, growth and detachment of cells.