Wettabilities and Adhesive/Autohesive Properties of Poly(Tetrafluoroethylene) Surfaces Photografted with Hydrophilic Monomers
Poly(tetrafluoroethylene) (PTFE) plates grafted with methacrylic acid (MAA), acrylic acid (AA), and 2-(dimethylamino)ethyl methacrylate (DMA) were prepared by the combined use of the plasma treatment and photografting. The relation between the wettabilities and chemical compositions of grafted PTFE plates was followed up and their adhesive and autohesive properties were examined from tensile shear strength measurements. In addition, the location of failure was determiened from the analysis by ESCA of the failed surfaces. The wattabilities for all three kinds of grafted PTFE surfaces steeply increased with the grafted amount in the range below 0.5 μmol/cm2, followed by attaining to the individual constant values. The changes in the cos θ and the intensity ratios obtained by an ESCA analysis resembled closely each other in common with all grafted PTFE surfaces. The adheive strength for grafted PTFE plates using epoxy type adhesives was improved at a low grafted amount. The initial increase in the adhesive strength can be ascribed to more enhanced hydrophilic properties of PTFE surfaces by the phohograftings. On the other hand, the autohesive strength increased sharply around 1.0 μmol/cm2 for grafted PTFE plates heat-pressed in the presence of water. The autohesive strength for PTFE-g-PMAA plates reached the value equivalent to the adhesive strength at higher grafted amounts and the grafted amount at which the autohesive strength increased sharply could be decreased to 0.5 μmol/cm2 by immersing them in an aqueous solution of HC1 of pH 3. These results indicate that an increase in the autohesive strength can be caused not only by the entanglements of grafted PMAA chains but also by the formation of hydrogen bonding between carboxyl groups affixed to the grafted PMAA chains. The analysis of the failed surfaces of PTFE-g-PMAA plates by ESCA revealed that for adhesive strength measurements the location of failure was shifted from the inside of the grafted layer consisting of grafted PMAA chains and PTFE chains to the inside of the substrate as the grafted amount increased and the grafted samples whose failure occurred in the inside of the substrate showed a high adhesive strength. It was found that the failure occurred in the inside of the grafted layer regardless of the grafted amount.
KeywordsAcrylic Acid Plasma Treatment Adhesive Strength Strength Measurement Tensile Shear
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