Melt strain hardening of polymeric systems increasing with decreasing elongational rate: Experimental results and discussion of models

Melt strain hardening is a special feature of polymer materials and polymeric systems relevant for applications and fundamental insights into rheological properties. Strain hardening is dependent on the molecular structure of materials and deformation parameters. This paper deals with literature results of materials exhibiting strain hardening increasing with decreasing elongational rate opposite to most of common polymers. Such a behavior is found for ionomers and polymers with hydrogen bonding, and electrostatic molecular interactions are discussed as the physical origin. Furthermore, layered composites of non-strain hardening polymers are presented that can be rendered strain hardening by introducing compatibilizers or increasing the effect of interfacial tension between two layers by using multilayer arrangements. The strain hardening behavior of polypropylene/high density polyethylene blends of various compositions is discussed. Measurements on ethylene/α-olefin copolymers showing strain hardening significantly increasing with decreasing rate in contrast to linear ethylene homopolymers are described, and it is hypothesized that this behavior is due to a heterogeneous molecular structure with large differences of molar masses of the components.