Towards understanding hydrophobic recovery of plasma treated polymers: Storing in high polarity liquids suppresses hydrophobic recovery

Abstract

The phenomenon of hydrophobic recovery was studied for cold air plasma treated polyethylene films. Plasma-treated polymer films were immersed into liquids with very different polarities such as ethanol, acetone, carbon tetrachloride, benzene and carbon disulphide. Hydrophobic recovery was studied by measurement of contact angles. Immersion into high polarity liquids slows markedly the hydrophobic recovery. We relate this slowing to dipole–dipole interaction of polar groups of the polymer with those of the liquids. This kind of interaction becomes decisive when polar groups of polymer chains are at least partially spatially fixed.

Introduction

Plasma treatment is broadly used for modification of surface properties of polymer materials [1], [2]. The plasma treatment creates a complex mixture of surface functionalities which influence surface physical and chemical properties and results in a dramatic change in the wetting behaviour of the surface [3], [4], [5], [6], [7], [8], [9], [10], [11]. Not only the chemical structure but also the roughness of the surface is affected by the plasma treatment, which also could change the wettability of the surface [12]. Plasma treatment usually strengthens hydrophilicity of treated polymer surfaces. However, the surface hydrophilicity created by plasma treatment is often lost over time. This effect of decreasing hydrophobicity is called “hydrophobic recovery” [13], [14], [15], [16], [17], [18], [19], [20], [21]. The phenomenon of hydrophobic recovery is usually attributed to a variety of physical and chemical processes, including: (1) re-arrangement of chemical groups of the surface exposed to plasma treatment, due to the conformational mobility of polymer chains; (2) oxidation and degradation reactions at the plasma treated surfaces; (3) diffusion of low molecular weight products from the outer layers into the bulk of the polymer, (4) plasma-treatment induced diffusion of additives introduced into the polymer from its bulk towards its surface [19]. Occhiello et al. classified the complicated processes occurring under hydrophobic recovery according their spatial range, i.e. short-range motions within the plasma-modified layer, burying polar groups away from the surface and long-range motions, including diffusion of non-modified macromolecules or segments from the bulk to the surface [22]. A phenomenological model of hydrophobic recovery has been proposed recently by Mortazavi and Nosonovsky [20].

At the same time, the precise mechanism of this effect remains obscure. We demonstrate in our paper that dipole–dipole interaction of the plasma treated polymer and molecules of the surrounding medium plays an important role in the hydrophobic recovery.