Self-Adhesion of Polyethylene in the Melt. 2. Comparison of Heterogeneous and Homogeneous Copolymers

Abstract

The unexpectedly long time scale for development of melt adhesion strength in blown films of molecularly heterogeneous Ziegler−Natta ethylene copolymers previously led to speculation on the existence of a surface layer enriched in low molecular weight, highly branched fractions. In the present study the hypothesis was tested by performing the same melt adhesion experiments on metallocene ethylene copolymers. These copolymers should not develop a surface layer because the chains are homogeneous in branch content and the molecular weight distribution is relatively narrow. Melt adhesion of heterogeneous and homogeneous ethylene copolymers was strikingly different. Whereas the time dependence for melt adhesion of heterogeneous copolymers was 2 orders of magnitude longer than the time for complete interdigitation of the surface chains, ascribed to the presence of a surface layer that needed to resolve in order for bulk chains to reach the interface and for maximum adhesion strength to develop, full adhesion of homogeneous copolymers was achieved instantly on the experimental time scale. Furthermore, homogeneous copolymers could exhibit an order of magnitude higher peel strength in the melt due to the possibility for strain-induced crystallization. Tapping mode atomic force microscopy (AFM) confirmed the existence of an amorphous surface layer on heterogeneous copolymers. By probing to a maximum depth of 30−40 nm, as determined by the force-probe method, the AFM tip did not penetrate through the amorphous layer, which was previously estimated to be about 100 nm thick. In contrast, the surface of homogeneous copolymers exhibited the same densely packed lamellar morphology as the bulk. The amorphous surface layer of heterogeneous copolymers was replicated by blending homogeneous copolymers of different branch content. The blends reproduced the major melt adhesion characteristics of heterogeneous copolymers.