Atmospheric pressure surface modification and cross-linking of UHMWPE film by transporting discharge
Abstract
Ultra-high-molecular-weight polyethylene (UHMWPE) has been widely used because
of its excellent mechanical properties. However, despite these excellent characteristics,
UHMWPE needs some additional treatment to increase the surface activity, for better
wettability. Plasma treatment is a promising technique that enables the surface
modification of polymers without affecting the textural characteristics of the bulk material.
In the present work, the helium transporting discharge was applied to improve the surface
properties of UHMWPE film. The hydrophilicity of the UHMWPE film was characterized
by measuring the contact angle as a function of exposure time. The surface morphology
was analyzed using AFM and SEM. The functionalization of the plasma treated
UHMWPE film surfaces were characterized by ATR-FTIR, XPS and TOF-SIMS.
A schematic of the plasma reactors are shown in Fig.1. A transporting discharge was
generated by applying a homemade AC power supply at 5 kHz [1]. A high voltage power
supply was connected to the stainless needle electrode which was embedded in the center
of a HDPE tube with length of 70 cm allowing the working gas to flow through. In the jet
configuration (Fig.1(a)), the plasma jet was directed towards the substrate located at a
variable distance from the nozzle and placed on a grounded aluminum substrate holder. In
the glow configuration, another grounded electrode was wrapped around the tube outlet
and the sample is inside the tube (Fig.2(b)).
FTIR results show that besides surface oxidation which is very commonly identified in
the case of plasma treated polymer surfaces, in the case of the UHMWPE, we also observe
crosslinking which is also a process taking place with the formation of unsaturation.
Furthermore, we can point out that the glow plasma-treated sample inside the tube presents
for the same treatment time of 7 minutes more crosslinking than the jet one outside the
tube. The XPS results confirm those of FTIR i.e. besides oxidized groups such as C-O/COH, C=O and O-C=O (acid or ester groups) detected on the surface, C=C bondings can
be also detected at the surface. AFM and SEM images show a smoother surface after the
plasma treatment, where a lamellar structure can be identified. A comparison of the mass
spectra from untreated and treated sample showed significant differences of signal
intensities of ions that could be specifically related to the presence of oxygen-containing
and nitrogen-containing species in different setups.
[1] F. Sohbatzadeh, and A.V. Omran. Phys. Plasmas, 21, (2014), 11, 113510.