In this paper, we examine the melt rheology of well-defined, model polymers where the long chain branching (LCB) is precisely known from the synthesis. All of these are made by the hydrogenation of polybutadiene, but they vary greatly in the level and type of LCB present. We find that all polymers that have LCB show a greater degree of shear thinning than linear chains. This applies both to those with a single branch (stars) and also to those with multiple branches per chain (such as combs). However, only molecules with multiple branches induce extensional thickening in a sample. Only a small amount of these comblike molecules, on the order of 5%, are needed to show this effect. We also show here how a new method of treating the shear data, the so-called Van Gurp−Palmen analysis, can give a more easily interpreted form of the results that can reveal the length and amount of branches in a sample. The insights generated from this work show the importance of access to well-defined polymers with several kinds of branching architecture for the development of a deeper understanding of polymer rheology.

In this paper we report experimental velocity and concentration profiles for suspensions possessing a bidisperse distribution of particle size undergoing pressure-driven flow through a parallel-wall channel. In addition to the overall concentration distributions determined by implementing the modified laser Doppler velocimetry method described in Part 1 (Lyon & Leal 1998), concentration profiles for the particles of each size were measured by sampling the position of marked tracer particles across 60% of the channel gap. Non-uniform overall particle concentration distributions and blunted velocity profiles were found at bulk particle volume fractions of 0.30 and 0.40, which were equal to the monodisperse data of Part 1, within experimental uncertainty. The large-particle concentration profiles were non-uniform down to a large-particle bulk volume fraction of 0.075, while non-uniform distributions of the small particles were only found when the volume fraction of small particles in the bulk was greater than or equal to 0.20. Experiments in which at least half the suspended particulate volume was occupied by large particles revealed enrichment of the large particles in the centreline region of the channel. This size segregation was found to increase as the total number of suspended particles decreased. Finally, the data from experiments in which a uniform small-particle concentration profile was measured were compared with suspension balance model (McTigue & Jenkins 1992; Nott & Brady 1994) predictions for parameter values that corresponded only to the large particles. While close agreement with the large-particle concentration profiles was found, this comparison also reflected the fact that the small particles bring the suspension viscosity to a regime that is more sensitive to the particle concentration, rather than simply providing an increment in background viscosity to the suspending liquid.

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