The investigation of the structural change and the wetting behavior of electron beam irradiated PTFE film

2.1 Materials

Industrial PTFE (0.05 mm in thickness) used in this work was obtained from Dongguan Hong Wei Plastic Product Co. (Dongguan, China).

2.2 Electron beam irradiation

Irradiation experiments were performed on a 30 Kev, 1 A electron accelerator SOLO (Russia). The specimens were set in a vacuum vessel (20 mbar) at room temperature and irradiated with dose of 0–60 kGy, respectively. The PTFE films were irradiated in steps in order to avoid the increase of temperature.

2.3 Fouriter transform infrared spectroscopy (FTIR)

FTIR was performed using a US Nicolet 380 apparatus in absorbance mode at a resolution of 4 cm^-1.

2.4 X-ray diffraction (XRD)

The crystallite size of the pristine and EB irradiated PTFE was evaluated by the XRD technique. XRD was carried out between 2θ=15–30° with CuKα radiation at a generation voltage of 40 kV in a Rigaku D/max-RB (Japan) diffractometer. A generator current of 40 mA was used and the scan step was 0.02.

2.5 Field emitting scanning electron microscope (SEM)

A JEOL JSM-5600LV (SEM) equipped with energy dispersive spectrometer (EDS) was used to examine surface morphologies and distribution of the main element of PTFE films. All the samples were bonded the sample holder with a piece of glue that is electrically conductive and were sputter coated with gold. EDS was directly carried out on it.

2.6 Contact angle measurements

Dynamic contact angle measurements were carried out under ambient conditions with a DCAT21 contact angle meter and tensionmeter (Germany). The measurement of the contact angle was done with rectangle samples (20×10×0.05 mm) according to the Wilhelmy method in the water. The speed of the lift motor was 0.5 mm/s. Position to weight analysis and contact angle computation were performed using the software DCAT. For ensuring the accuracy, the measurements were performed with the number of five measuring cycles, and then the average value was regard as the final contact-angle result.

3.1 FTIR spectroscopic analysis

Chemical changes of PTFE after exposure to electron beam (EB) were monitored by FTIR spectroscopy. Figure 1 shows the FTIR spectra of prinstine PTFE and irradiated PTFE. In the FTIR spectra of PTFE irradiated with a dose of 60 kGy, a new distinctive absorption bands around 1880 cm^-1 can be observed, which can be assigned to carbonyl fluoride groups(-COF). The band at 2360 cm^-1 is a combination band associated with the CF₂ backbone (13). The intensity of this absorption becomes weaker with the increasing of irradiation dose, which indicates the scission of the C-F bond happened. On the other hand, the intensity of the absorption at 1800 cm^-1 increases along with the increasing of EB irradiation dose. This band is assigned to carboxylic acid groups (-COOH). Irradiation-induced degradation of PTFE in the presence of air results in C-F and C-C scission during electron beam irradiation. C-F scission results in the formation of secondary radicals, whilst C-C scissions produce primary free radicals. These free radicals react with atmospheric oxygen to yield stable perfluoroalkylperoxy radicals. Besides these peroxy radicals, carbonyl fluoride groups are also formed which will be hydrolyzed in the presence of atmospheric moisture to form carboxylic acid groups during and after the irradiation (14). Moreover, as shown in Figure 1, there are very strong CF₂ asymmetrical and symmetrical stretching bands appear in the region of 1350±1000 cm^-1. Apparently, it is due to the non-transparency of the PTFE film with 0.05 mm thickness in the irradiation.

Figure 1:

FTIR spectra of pristine and irradiated PTFE.

3.2 XRD analysis

The loss of crystallinity for a PTFE sample irradiated by EB is confirmed by XRD measurements. Figure 2 shows the XRD intensity profiles of the pristine PTFE and irradiated samples at different doses. The XRD profile of the pristine PTFE shows a sharp peak at 2θ=18.2°. As the EB dose is increased, the intensity of the peak decreases indicating the evolution of PTFE towards a more disordered state. Such behavior is inconsistent with that in other studies (15). No significant shift of the peak position is observed, which implies that the lattice parameters do not change dramatically.

Figure 2:

XRD of pristine and irradiated PTFE.

The Scherrer equation gives the relation between the crystallite size (L) and FWHM (b) of an XRD peak

b=KλL/cosθ

where K is the wavelength of X-ray beam, L is the crystallite size, K is a constant usually equal to 1 and b is FWHM of the peak in radians. The crystallite size of the pristine PTFE is calculated to be 331 Å and for 60 kGy EB irradiated film this size decreases to 263 Å. Furthermore, crystallite size decreases with the increasing the irradiate dose till 60 kGy. The detail is shown in Table 1.

3.3 Structure morphology analysis (SEM and EDS)

After irradiation, both the chemical structure and the surface morphology of PTFE are changed. Figure 3 is the SEM graphs of the pristine PTFE and the PTFE irradiated by EB with 60 kGy dose. The pristine film shows smooth surface (Figure 3A) whereas the irradiated film (Figure 3B) shows rough surface. The EDS of the films (Table 2) shows the elemental composition and the atomic ratio of PTFE film before and after irradiation. Atomic ratio of fluorine to carbon F/C is found to be 1.83 before irradiation compared to that of 1.38 after irradiation. The irradiated PTFE film has an increase of O content and a drop of F percentage. It is assumed that a portion of activated points reacted with oxygen in the air. As a result, the O/C atomic ratio increases from 0.19 to 0.55 distinctly. It is clear from the FTIR spectra, as shown in Figure 1, that the bands around 2360 cm^-1, 1880 cm^-1 and 1800 cm^-1 represent the degradation, scissions and oxidation reaction occurred in the rough region.

Figure 3:

SEM of pristine and irradiated PTFE film (A) pristine PTFE film; (B) PTFE film irradiated with 60 kGy dose.

3.4 Wetting properties

It is important for a high-performance hydrophilic surface to sustain the hydrophilicity in the practical application. Figure 4 illustrates the changes of water contact angle for the pristine and the irradiated PTFE film at different doses at room temperature. The change of water contact angle for the irradiated sample shows a sharp decrease when increasing the irradiation dose from 10 to 20 kGy, followed by a slight decrease as the irradiation dose further increases. The values of the water contact angle for the pristine and the irradiated film with a dose of 60 kGy are 81.392° and 57.456°, respectively. The result proves that the groups with strong hydrophilicity are successfully introduced onto the surface of the PTFE film. However, degradation and oxidation of polymers are competitive processes and are considered as a function of depth and dose when the polymer is irradiated by electron beam. It needs further investigation and clarification of any changes in physical properties of PTFE when it is irradiated with various doses electron beam in future.

Figure 4:

Changes of water contact angle of pristine and irradiated PTFE film.