Adhesion Properties and Stability of Polar Polymers Treated by Air Atmospheric Pressure Plasma

Abstract

This study continues the discussion on the surface modification of polymers using an atmospheric pressure plasma (APP) reactor in air. These results complement prior research focusing on nonpolar polymers. Polymers, such as polyethylene terephthalate, polyetheretherketone, and polymethyl methacrylate, containing structurally bonded oxygen are studied, representing a range of properties such as oxygen content, crystalline/amorphous structure, polarity, functionality, and aliphatic/aromatic structure. APP induces superior wetting properties on the hydrophilic polymer surfaces with rapid and uniform modification within 0.5 s of exposure. The amorphous structures undergo additional modification for longer exposure. Moreover, the aliphatic chain structures require longer plasma exposure to reach surface modification equilibrium. The polar polymers reach a limit level of modification corresponding to a minimum water contact angle of about 50°. The surface polarity increases on average by a factor of approximately two. The equilibrium values of the adhesion work attained after post-processing recovery fall within a limited range of about 100-120 mJ/m². The enhancement of surface functionality through the creation of oxidized groups primarily depends on the initial oxygen content and reaches a limit of about 40 at.% oxygen. The surface properties of the treated polar surfaces exhibit good stability, comparable to that of the previously tested nonpolar polymers.

Keywords: XPS; adhesion; atmospheric pressure plasma; contact angle; plasma treatment of polymers; polyetheretherketone; polyethylene terephthalate; polymethyl methacrylate.

Figure 1

Chemical structure of the repeat units of the tested polymers (theoretical initial oxygen content: PET—40 at.%, PEEK—15.8 at.%, and PMMA—40 at.%).

Figure 2

Curve-fitted diffractogram for PET.

Figure 3

Variation in WCA vs. ageing time for plasma-treated (a) PET, (b) PEEK, and (c) PMMA for different treatment times (0.5 s and 1.0 s).

Figure 4

Polar (γ_S^p) and dispersive (γ_S^p) contributions to the surface energy (γ_S) of PET, PEEK, and PMMA before and after the plasma treatment for different durations (0.5 s and 1.0 s).

Figure 5

Typical deconvolutions of the high-resolution C1 XPS spectra for PET (a), PEEK (b), and PMMA (c), untreated (left) and 1.0 s plasma-treated (right).

Figure 5

Typical deconvolutions of the high-resolution C1 XPS spectra for PET (a), PEEK (b), and PMMA (c), untreated (left) and 1.0 s plasma-treated (right).