| Poly(ethylene terephthalate) (PET) and polyethylene (PE) samples were modified by using oxygen capacitively coupled radio frequency plasma (CCP), inductively coupled radio frequency plasma (ICP), and combined capacitively and inductively radio frequency plasma (CCP and ICP). A correlation was persuaded between the surface morphology,chemical component and wettability. The mechanism of the wettability of polymers modified by radio frequency plasma was investigated, based on the optical emission spectra (OES) from the plasma. A graft polymerization of glycidyl methacrylate (GMA) on the pretreated polyethylene (PE) samples by oxygen capacitively coupled radio frequency (RF) plasma was carried out. The graft and adhesive conditions have been discussed to understand the mechanism of the improved adhesive performance of the grafted PE.The surface morphology and chemical component of the PET and PE samples modified by using the CCP, ICP, and combined ICP and CCP mode changed significantly. For the first 40 s, the surface roughness (Ra) of the PET samples modified by using the CCP of 200 W, ICP of 200 W, and combined ICP of 200 W and CCP of 100 W increased from 0.443 nm for the original to 0.453nm,0.447nm, and 1.005 nm, respectively. With further increasing the treatment time up to 300 s, the Ra increased to 2.779nm,0.862nm, and 4.062nm, respectively. The Ra of the PE samples changed from 0.137μm for the original to 0.108μm for the modified by the CCP of 200 W for 40 s. During the plasma modification, chemical reaction was predominant on the polymers in the first seconds. For the long time treatment, the depletion of the polar functional groups carried out due to the plasma etching by the activated gaseous species. With further increasing the treatment time up to 300 s, the C-C component increased and the C-0 and 0=C-0 components decreased, which was evidently observed for the polymers modified by combined ICP and CCP mode. The optical emission spectra showed there was a higher ionization rate in ICP mode than in CCP mode. There were stronger spectral lines of the ionic oxygen molecules in CCP mode than in ICP mode.The OES of combined ICP and CCP mode presented the stronger emission spectral lines than that in the CCP or ICP mode independently, because the combined CCP and ICP source demonstrated the plasma characteristics with high ion density, high excited species density, and high ion energy, resulting in enhanced surface chemical reaction and surface etching on the PET film.The wettability of the PET and PE samples modified by using the CCP, ICP, and combined ICP and CCP was improved significantly. In the CCP, ICP, and combined ICP and CCP processes, a rapid decrease of the advancing contact angle (θA) was respectively obtained for the first 40 s exposure. Then a slight increase of theθA was observed for the further 300 s exposure. However, a different tendency for the change of theθA of the surface in the ICP, and combined ICP and CCP processes, theθA increased after 40 s exposure. For the receding contact angle (θR) of the PET samples by the three RF plasma modes, the rapid decrease of the receding contact angle still occurred for the first 40 s exposure, following the slight decrease. The optimum wettability of the modified PET surface with the combined ICP of 200 W and CCP of 100 W for 40 s exposure was obtained as theθA decreased from 83.5°for the original to 37.5°for the modified.Accordingly, theθR decreased from 53.6°to 19.2°. The lowest static contact angle was obtained, decreasing from 97.0°for the original to 42.7°with the combined ICP of 200 W and CCP of 100 W for 20 s exposure. At a low surface roughness, the polar functional groups on the surface of the PET samples have significant effect on both the advancing contact angles and receding contact angles. When the surface roughness increased over a threshold, the advancing contact angle was mainly dependent on the polar functional groups on the surface, and the receding contact angles were particularly on the surface roughness.A plasma-induced graft polymerization of GMA samples took place on the PE modified by using the CCP of 200 W process. The wettability of the grafted PE samples was dependent on the grafting temperature, GMA concentration and grafting time.The optimum wettability of the graft polymerized PE surface with the concentration of 40 vol.% at the temperature of 70℃, and for the time of 24 hr was obtained as the static contact angle decreased from 97.0°for the original PE to 67.6°for the graft polymerized. After the graft polymerization, a strong absorption peak of C-O bonding was shown in Fourier transform infrared spectrum, indicating an introduction of epoxy groups on the graft polymerized surface. Correspondingly, the Ra increased from 0.137μm for the original PE to 1.660μm for the graft polymerized.The graft polymerization of GMA on the PE samples significantly improved the adhesive properties of PE. The maximal lap adhesive strength of the graft polymerized PE samples lapped using a mixture of epoxy resin and curing agent was achieved to about 1.6 MPa. At one side and another side of the fractured samples, obvious torn morphologies were shown with the alternate linear and abnormal parabolic patterns. In the rough edge zone of fractured surfaces, the torn fibrous morphology could be seen clearly. The fracture mode of the graft polymerized PE samples was essentially cohesive.The density of the interface for the clamped original PE, adhesive-bonded original PE and adhesive-bonded GMA grafted PE was detected by the ultrasonic testing. The peak ratio of reflection wave/surface reflection wave was 1.911 for the clamped original PE,1.53 for the adhesive-bonded original PE, and 0.237 for the bonded GMA grafted PE. The density of the interface for the clamped original PE, adhesive-bonded original PE and adhesive-bonded GMA grafted PE was compared. The density for the adhesive-bonded GMA grafted PE>the adhesive-bonded original PE>the clamped original PE. The plasma-induced graft polymerization of GMA significantly improved the adhesive properties of PE, which effectively promoted the adhesive spreading onto the grafted PE surface fully. The adhesive strengths of the graft polymerized PE samples were controlled predominately by chemical bonding in the curing process. Besides, the increase in surface roughness played the secondly role.
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