Association and relaxation of supra-macromolecular polymers.

This paper describes the structures created by assembling functionalised entangled polymers and the effect these have on the rheology of the material. A polybutadiene (PBd) linear polymer precursor of sufficient molecular weight to be entangled is used. This is end functionalised with the self-associating group 2-ureido-4pyrimidinone (UPy). Interestingly, despite the relatively high molecular weight of the precursor diluting the UPy concentration, the effect on the material's properties is significant. To characterise the assembled microstructure we present linear rheology, extensional non-linear rheology and small angle X-ray scattering (SAXS). The linear rheology shows that the functionalised PBd assembles into large macro-structures where the terminal relaxation time is up to seven orders of magnitude larger than the precursor. The non-linear rheology shows strain-hardening over a broad range of strain-rates. We then show by both SAXS and modelling of the extensional data that there must exist clusters of UPy associations and hence assembled polymers with branched architecture. By modelling the supra-molecular structure as an effective linear polymer, we show that this would be insufficient in predicting the strain-hardening behaviour at lower extension-rates. Therefore, in this flow regime the strain-hardening is likely to be caused by branching. This is backed up by SAXS measurements which show that UPy clusters larger than pair-pair groups exist.

Age-dependent modes of extensional necking instability in soft glassy materials.

We study the instability to necking of an initially cylindrical filament of soft glassy material subject to extensional stretching. By numerical simulation of the soft glassy rheology model and a simplified fluidity model, and by analytical predictions within a highly generic toy description, we show that the mode of instability is set by the age of the sample relative to the inverse of the applied extensional strain rate. Young samples neck gradually via a liquidlike mode, the onset of which is determined by both the elastic loading and plastic relaxation terms in the stress constitutive equation. Older samples fail at smaller draw ratios via a more rapid mode, the onset of which is determined only by the solidlike elastic loading terms (though plastic effects arise later, once appreciable necking develops). We show this solidlike mode to be the counterpart, for elastoplastic materials, of the Considère mode of necking in strain-rate-independent solids.

In Silico Molecular Design, Synthesis, Characterization, and Rheology of Dendritically Branched Polymers: Closing the Design Loop.

It has been a long held ambition of both industry and academia to understand the relationship between the often complex molecular architecture of polymer chains and their melt flow properties, with the goal of building robust theoretical models to predict their rheology. The established key to this is the use of well-defined, model polymers, homogeneous in chain length and architecture. We describe here for the first time, the in silico design, synthesis, and characterization of an architecturally complex, branched polymer with the optimal rheological properties for such structure-property correlation studies. Moreover, we demonstrate unequivocally the need for accurate characterization using temperature gradient interaction chromatography (TGIC), which reveals the presence of heterogeneities in the molecular structure that are undetectable by size exclusion chromatography (SEC). Experimental rheology exposes the rich pattern of relaxation dynamics associated with branched polymers, but the ultimate test is, of course, did the theoretical (design) model accurately predict the rheological properties of the synthesized model branched polymer? Rarely, if ever before, has such a combination of theory, synthesis, characterization, and analysis resulted in a "yes", expressed without doubt or qualification.