Selective etching of semicrystalline polymers:: CF4 gas plasma treatment of poly(ethylene)

A series of poly(ethylene) (PE) films with different degrees of crystallinity was treated with a radiofrequency tetrafluoromethane (CF4) gas plasma (48-49 W, 0.06-0.07 mbar, and continuous vs pulsed treatment). The etching behavior and surface chemical and structural changes of the PE films were studied by weight measurements, X-ray photoelectron spectroscopy (XPS), static and dynamic water contact angle measurements, scanning electron microscopy (SEM), and atomic force microscopy (AFM). With increasing; crystallinity (14-59%) of PE, a significant and almost linear decrease of the etching rate was found, ranging from 50 Angstrom/min for linear low-density poly(ethylene) (LLDPE) to 35 Angstrom/min for high-density poly(ethylene) (HDPE). XPS analysis revealed that after CF4 plasma treatment the PE surfaces were highly fluorinated up to F/C ratios of 1.6. Moreover, CF4 plasma treatment of PE resulted in extremely hydrophobic surfaces. Advancing water contact angles up to 150 degrees were measured for treated LDPE films. Both SEM and AFM analysis revealed that pronounced surface restructuring took place during prolonged continuous plasma treatment (greater than or equal to 15 min). The lamellar surface structure of LDPE changed into a nanoporous-like structure with uniform pores and grains on the order of tens of nanometers. This phenomenon was not observed during plasma treatment of HDPE films. Apart from surface roughening due to selective etching, pulsed plasma treatment did not result in significant surface structural changes either. Therefore, the restructuring of continuously plasma-treated surfaces was attributed to a combined effect of etching and an increase of the surface temperature, resulting in phase separation of PE-like and poly(tetrafluoroethylene)like material, of which the latter is surface oriented.