The extensibility of macromolecules in solution; A new focus for macromolecular science

The paper is a summary of ongoing work in this laboratory laid on foundations of about 10 years standing. It concerns the extensional and aligning effect which appropriately designed elongational flow-fields have on linear macromolecules in solution. In the case of flexible molecules the chains can be fully stretched out, and the corresponding conformational relaxation time, thus determined, provides information on the molecular weight (amongst others providing a new method for determination of the molecular weight distribution), on the coil dimension as relevant to most recent theories, on the draining characteristics of the coil, and on the difference in extension and retraction characteristics. Further, it can provide information on chain flexibility, utilised here in the case of polyelectrolytes where this flexibility can be systematically varied and thus followed. It also signals the onset of associations, geometric entanglements in particular, opening a new window on entanglement behaviour. Also it offers a uniquely definitive method for the study of flow induced chain scission. In addition, these studies reveal how chain extension reacts back and modifies the flow-field producing it with relevance to rheology and fluid transport. Beyond this all, the work shows explicit connections with gelation and absorption phenomena, to the study of which it promises to contribute. In the realm of rigid rod molecules it indicates how elongational flow can promote liquid crystal formation and, more comprehensively, it provides a definitive method for the determination of rotational diffusion as a function of a number of variables. The most salient conclusion in the latter area is the realisation that rigid rods are incomparably less restricted by their neighbours in their rotational frreedom, and are thus correspondingly more orientable than predicted by theory.

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