Eur. Phys. J. Appl. Phys. 48, 30501 (2009) DOI: 10.1051/epjap/2009166 Regular Article THE EUROPEAN PHYSICAL JOURNAL APPLIED PHYSICS Accelerated nitridation and oxidation by plasma on polyethylene M.G. Olayo 1 , E. Col´ın 1 , G.J. Cruz 1, a , J. Morales 2 , and R. Olayo 2 1 Departamento de F´ısica, Instituto Nacional de Investigaciones Nucleares, Apdo. Postal 18-1027, D.F., CP 11801, Mexico 2 Departamento de F´ısica, Universidad Aut´onoma Metropolitana Iztapalapa, Apdo. Postal 55-534, D.F., CP 09340, Mexico Received: 29 October 2008 / Received in final form: 5 May 2009 / Accepted: 26 August 2009 Published online: 30 October 2009 – c  EDP Sciences Abstract. Glow discharges of oxygen and nitrogen were applied to low density polyethylene thin films in order to study accelerated oxidation and nitridation in the polymer. The studies were focused on the morphologic, crystalline and hydrophilic evolution promoted by plasma exposure. The particular chemical characteristics of the gases and the constant impact of high-energy particles on the surfaces produced different types of erosion. Oxygen plasmas produced the release of fragments from the polymeric surface which created fibered textures and nitrogen plasmas resulted in folded morphologies of nano and micro dimensions on polyethylene. The plasmas of both gases increased and decreased the crystallinity in the polymers, between 33% and 57%, with similar tendencies, differing only in the percentage of crystallinity. The plasma exposure produced a decrease in the contact angles of water on polyethylene in the first 30 min of plasma, from 70% in the untreated polymers, to 45% and 35% as a consequence of the polar groups added to the surface. PACS. 68.47.Mn Polymer surfaces – 52.77.Bn Etching and cleaning 1 Introduction Polyethylene (PE) is probably the most studied polymer because of its simple structure, composed only by -CH 2 - groups, which results in high hydrophobicity, low electric conductivity and low chemical activity. PE has applica- tions as packaging material, food protection films, electric insulating materials and implant prosthetics. These med- ical uses, in particular, have diversified continuously to include the replacement or modification of hips, articula- tions, bones, veins, eye tissues, etc. [1]. However, long term exposure to a specific chemical environment may affect the characteristics of the polymers. Oxidation and nitridation of low density polyethylene (LDPE) thin films are studied in this work using long exposures to plasmas of oxygen and nitrogen. The main promoters of chemical reactions in PE during these treat- ments are the free radicals produced on the surface be- cause of the continuous impact of electrons and ionized particles of these gases. Some short-life radicals usually neutralize with the plasma gases, producing the first step in the chemical reactions. However, long-life radicals usu- ally survive the plasma treatment and gradually neutralize among themselves or with the atmospheric gases after the treatment [2]. Oxygen plasmas produce chemical groups, such as C-O, C=O and O-C=O on the surface of PE [3]. However, a e-mail: guillermo.cruz@inin.gob.mx those groups have also been found in PE treated with plas- mas of Ar [4] or with low energy beams of Ar + [5]. The origin of oxygen atoms in these works resulted only in the atmospheric oxidation at the end of the treatment and not in the irradiation gases. These oxygenated groups created on the surface induce slightly polarized sites on the ho- mogeneous electric configuration of PE, which influences, among other properties, its water affinity. The interaction of water with PE is important because wettability can be considered the first step in the degradation of the polymer. Many studies have been done about the water affinity of PE modified by plasma. Behnisch treated PE with re- mote plasmas using different gases. The advancing contact angle with water was approximately 90 ◦ in the untreated samples; however, after 10 min, nitrogen and oxygen plas- mas reduced the contact angles to 38 ◦ and 42 ◦ , respec- tively [6]. Leroy treated PE powder with remote nitrogen plasmas in a fluidized bed reactor at 2.45 GHz and 600 W. The contact angles with water varied from 90 ◦ , before the treatment, to 51 ◦ in the first 20 min, and to 45 ◦ after 120 min [7]. Nitrogen plasma was also used by Wagner in the superficial modification of PE. The static contact an- gles with water varied from 75 ◦ , in the untreated samples, to 48 ◦ at 2.5 min and to 28 ◦ at 60 min [8]. The main re- duction happened in the first 10 min. HDPE treated with oxygen and ammonia rf plasmas was studied by Drnovska at l3.56 MHz and 100 W. The contact angle with water in the virgin substrates was 87 ◦ ; however, after 5 min of Article published by EDP Sciences