Self-adhesion of polyethylene in the melt. 1. Heterogeneous copolymers

Development of self-adhesion above the melting temperature was studied in a group of linear low-density polyethylenes. An experimental procedure for consecutively sealing and peeling a polymeric film at the same temperature, above the melting point, was developed. This made it possible to measure melt adhesion over a nide range of temperatures, seal times, and peel rates. Taking advantage of the new experimental technique, the molecular mechanisms and kinetics of self-adhesion in heterogeneous ethylene copolymers with broad molecular weight distribution were investigated. It was found that the peel strength increased with the contact time in accordance with the conventional t(1/2) dependence until saturation at the maximum strength. The time to achieve maximum strength was significantly shorter for the copolymer with more homogeneous copolymer composition; however, the maximum strength was the same for all the materials at each test temperature. The saturation time in all cases was 2 orders of magnitude longer than the time for complete interdigitation of the surface chains. This was ascribed to the presence of a surface layer of about 100 nm that needed to resolve after sealing the surfaces before the maximum adhesive strength could develop. The layer was thought to originate from surface segregation of the lower molecular weight, higher branch content fraction of the material.