Surface Modification and Autohesive Properties of Poly(tetrafluoroethylene) and Polyethylene by the Photografting Technique.

Abstract

Poly(tetrafluoroethylene) (PTFE) and low and high density polyethylene (LDPE and HDPE) plates were surface-modified by the photogafting of methacrylic acid (MAA) using a 400W high pressure mercury lamp. The autohesive strengths for MAA-grafted PTFE, LDPE and HDPE plates were estimated from the tensile shear strength measurements and their autohesive properties were discussed in relation to surface and bulk properties of the grafted layers formed on their substrates. The contact angles which were related with the wettabilities of the modified surfaces increased with an increase in the grafted amount, and then leveled off when the surfaces were wholly covered with grafted PMAA chains. The wettabilities for grafted PTFE plates were improved at a considerably low grafted amount, since the photografting of MAA onto the PTFE plates was most restricted to the outer surface region among the three types of substrates used in this study. The grafted LDPE and HDPE plates possessed high water-absorptivities. The autohesive strengths for grafted PTFE plates immersed in water reached the value equivalent to their adhesive ones. For the grated PTFE plates immersed in an aqueous HCl solution of pH 3 in place of water, the autohesive strength sharply increased in the range of lower grafted amounts. These results suggest 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. On the other hand, in the case of the autohesive strength measurements for Grafted LDPE and HDPE plates, the substrate breaking occurred at higher grafted amounts. The grafted amount at which the substrate failed for grafted HDPE plates was lower than that for grafted LDPE plate because the photografting onto the HDPE plates was more restricted to the outer surface region. It was made clear that the autohesive strength for grafted PTFE plates reached the value equivalent to the autohesive strength and the autohesive strengths for grafted LDPE and HDPE plates exceeded the ultimate tensile strength by heat-pressing the grafted samples with the grafted layers swollen in water.