Higher polythionic acids – the precursors for formation of thallium sulphide layers on polyethylene surface

Abstract

The method of thallium sulphide thin films deposition on low density polyethylene (LDPE), based on applying solutions of higher polythionic acids (H₂S_nO₆, n = 21, 33, 45), was developed and investigated. The concentration of sulphur in the LDPE film increases with the increase of temperature of higher polythionic acid solution and its sulphurity, as well as the duration of retention of LDPE in its solution is prolonged. The sulphur concentration in the PE film under the same duration of sulphurisation and repetition of treatment cycles insignificantly changes. When the sulphurised LDPE was treated with a thallium Tl(I) salt solution, sulphur present in the polymer reacts with thallium ions and the layers of thallium sulphides, Tl_xS_y, of different composition are formed. The stoichiometric composition of the layer changes from Tl_0.1S to Tl_2.6S. The amount of thallium increases in the sulphide layers when the number of the cycles of LDPE sulphurisation in the H₂S₃₃O₆ solution and treatment of sulphurised polymer with the Tl(I) salt solution is increased. The stoichiometric composition of Tl_xS_y coats of one treatment cycle samples changes from Tl_0.5S to Tl_1.1S. The X-ray diffraction analysis of the polyethylene, treated with the solution of higher polythionic acid, H₂S₃₃O₆, and then with the Tl(I) salt solution, confirmed the premise that on the LDPE surface the layers of thallium sulphides, Tl_xS_y, were formed. Five phases of thallium sulphides were identified: TlS, Tl₂S, Tl₂S₂, Tl₄S₃ and Tl₂S₅. The TlS phase prevailed in diffractograms. The chemical and phase composition of the Tl_xS_y layers surface (up to 1 nm depth) is identified by X-ray photo-electronic spectroscopy analysis. The analysis data confirmed that the compounds of TlS, Tl₂S, Tl₂SO₄, Tl(OH)₃, S₈ and Tl₂O₃ were formed on the surface of the layers. The morphology of thallium sulphide layers and the mechanism of their formation are determined by means of scanning atomic force and electronic microscopy. Two processes take place during the formation of the layer: desorption of sulphur and reaction of thallium ions from the thallium(I) salt solution with sulphur present on the LDPE surface. The layer of thallium sulphides is formed on the surface of the sulphur layer. Deposition of thallium sulpfide films from the solution of higher polythionic acid on the LDPE films is not homogeneous, it has the separated island type morphology. The identified regularities let purposively choose the conditions for sulphurisation of LDPE films by using the solution of higher polythionic acid as the agent of polymer sulphurisation in order to compose the layers of thallium sulphides, Tl_xS_y, of the desired chemical and phasic composition on the surface of this polymer by means of the sorption–diffusion method.