Surface modification via atmospheric cold plasma (ACP): Improved functional properties and characterization of zein film

In the present study, the nonhazardous atmospheric cold plasma (ACP) technique was introduced to improve the surface hydrophilicity and mechanical strength of zein film. The results indicated that the surface hydrophilicity was significantly increased after ACP treatment, with a drop of contact angle from 72.85 +/- 2.33 degrees (untreated) to 48.42 +/- 1.57 degrees (ACP-60 s). Increased surface roughness and enrichment of the oxygen- and nitrogen-containing groups were responsible for this modification. As the X-ray photoelectron spectroscopy (XPS) results shown, the O1 s and N1 s peak intensity was dramatically enhanced after 30 s ACP treatment with an increased O/C ratio from 0.64 to 1.09. Storage in ambient environment for an extra 24 h of treated film could further reduce the contact angle. With the treatment time extending, enhanced tensile strength (TS) and flexibility were observed in zein films, accompanied by improving thermal properties, which indicated the cross-linking within protein matrix. Moreover, ACP treatment could significantly decrease the water vapor permeability (WVP) due to the increasing tortuosity of the diffusion pathway after plasma treatment. The secondary structure in amide I band (1600-1700 cm(-1)) indicated that there was a transformation from beta-turn and alpha-helix into beta-sheet induced by ACP treatment. This research opens up new opportunities that ACP could be developed as a modification method of zein film for better utilization in packaging industry.