Molecular theory for Polymer Rheology: When the Mist Clears – and When it Swirls Back Again*

A day's unforgettable conversation with Bill Graessley was spent preparing for our jointly-written chapter for Stealing the Gold, a book dedicated to the innovative science of Sir Sam Edwards. The idea was that Bill would write about the phenomenology of polymer melt rheology before the advent of tube theory, and I would cover subsequent developments. I was struck by the years of experience and vast grasp of the data on melt flow and relaxation phenomena that Bill had garnered over the years. He described his encounted with the Doi-Edwards theory as 'the mist blowing away' or 'the fog clearing' - one of those moments where all the objects one has blundered into blindly suddenly take on their mutual relationship within a landscape clear to view.

In this presentation I shall attempt to explain why that would be, and what moments of clarity, and new falls of fog, have passed since. Successes and mysteries in linear and non-linear flows, of linear and non-linear polymers, are the oder of the day. The good news is that tube-theory based design algorithms are now in routine industrial use in product development. Furthermore, extensional rheology - the really important kind for processing - is understood molecularly, and can be engineered as a result, including a remarkable and counterintutive effect of solvent chains. The challenging news is that there are still inconclusive and discordant experiments, no-one knows what 'an entanglement' is yet and the industry is still selling and buying polymer process rheology on the 'Melt Flow Index' - an indictment of us all.