Enhanced surface hydrophobicity of poly(lactic acid) by Co$^{60}$ gamma ray irradiation

Contact angle measurements were performed on Co$^{60}$ gamma-ray irradiated poly(lactic acid) (PLA) samples at several doses, using the sessile drop method. It was found that irradiation alters the wettability of PLA. In particular, PLA wetting behavior changes from moderately hydrophilic at low dose ($< 100$ kGy), to hydrophobic after the samples were exposed above a threshold dose ($\approx 200$ kGy). At low doses, wettability follows the Wenzel relation but beyond a threshold dose the Cassie-Baxter regime takes place. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) show that surface roughness of the samples increases as the applied dose increases. PLA wettability changes are mainly a consequence of surface modifications as roughness promotes the formation of air pockets under a drop, which reinforces the hydrophobic nature of is surface. Co$^{60}$ gamma ray irradiation can be used to tailor wetting properties of PLA. The method might have also application to produce biphilic PLA. Finally it is very important to remark that reported contact angles values for PLA (by different authors) of untreated samples vary from 60 to 85 degrees. Reasons for this behavior are given in this article.