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 c onsecutively sealing and peeling a polymeric film at the same temperature, above the melting point, was developed. This made it possible to measure me lt adhesion over a nide range of temperatures, seal times, and peel rates. Taking advantage of the new experimental technique, the molecular mechanism s and kinetics of self-adhesion in heterogeneous ethylene copolymers with b road molecular weight distribution were investigated. It was found that the peel strength increased with the contact time in accordance with the conve ntional t(1/2) dependence until saturation at the maximum strength. The tim e to achieve maximum strength was significantly shorter for the copolymer w ith more homogeneous copolymer composition; however, the maximum strength w as 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 comple te 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 th e surfaces before the maximum adhesive strength could develop. The layer wa s thought to originate from surface segregation of the lower molecular weig ht, higher branch content fraction of the material.