Rheological characterization of H-shaped poly(methyl methacrylate)s

Investigation of the extensional flow behavior and development of constitutive equations for well-characterized H-shaped polymers is important both from an industrial and academic viewpoints. For H-shaped polymer systems, little is known about the extensional flow behavior at constant strain rates. We synthesized H-shaped poly(methyl methacrylate)s (PMMAs) with narrow molecular weight distributions and various ratios of the molecular weight of the backbone, Mb, to that of the arm, Ma, by atom transfer radical polymerization. However, 1H NMR spectroscopy showed that the resulting PMMA samples contained substantial amounts of linear and star topologies. In spite of the limited purity of the H-shaped PMMAs obtained, the rheological properties in linear viscoelasticity and uniaxial extensional flow revealed interesting results. The zero-shear viscosity of model PMMAs was well described by an exponential relation of the molecular weight. Qualitative agreement was found between the experimental linear-viscoelastic behavior and predictions of the model of McLeish et al. (Macromolecules 32: 6734–6758, 1999) for H-shaped polymers. The elongational flow behavior was also analyzed by the molecular stress function (MSF) model. Surprisingly, the strain-hardening effect for all investigated model PMMAs was weaker than that for pom-pom polystyrene polymers.