Session A4: Polymer Dynamics: An Honor Session for Sir Sam Edwards

Polymer dynamics, the Edwards tube model and neutron scattering.

Sam Edwards' papers with Masao Doi describing polymer dynamics and rheology based on the tube model were published just as high resolution quasi-elastic neutron scattering experiments achieved the resolution capable of observing directly some of the effects of the tube. The talk will summarize these experiments as well as later work on polymer inter-diffusion across interfaces observed by neutron reflection, where again the tube model is used to interpret the data.   

Recent advances with generalized entropy theory of glass-formation in polymers

The generalized entropy theory (GET) of glass-formation in polymers is a combination of the lattice cluster theory (LCT) for the configurational entropy density with the Adam-Gibbs (AG) theory for the structural relaxation time. A greatly simplified form of the GET (whose expression for the free energy is roughly double that of Flory-Huggins theory) accurately reproduces the four characteristic temperatures of glass-formation (the onset, crossover, glass transition, and Kauzmann temperatures) of the full GET to within 4K for a series of models of polymers composed of semi-flexible chains having the structure of poly(n-alpha olefins). The theory is now simple enough to be used in courses in polymer physics. Although the successes of the GET provide a strong validation of the final form of the AG theory provided the configurational entropy is used, the physical basis of the AG theory has remained an enigma. Hence, we have developed a new, more general, statistical mechanical derivation of AG theory that explains the previously perplexing observations that the string-like elementary excitations have the mass and temperature dependence of systems undergoing equilibrium self-assembly.   

The Ordinary-Extraordinary Transition in Dynamics of Solutions of Charged Macromolecules

Dynamic light scattering measurements on dilute salt-free polyelectrolyte solutions have shown over the past three decades that there are two distinctive diffusive modes: fast and slow. The diffusion coefficient deduced from the fast mode has been found to be essentially independent of molar mass and polymer concentration and it is merely a factor of four smaller than that of small electrolyte ion such as sodium or potassium ion. The diffusion coefficient deduced from the slow mode is much smaller suggestive of clumps of many chains although these chains are similarly charged. Upon addition of sufficient amount of small molecular salts, the fast and slow modes merge together and the deduced diffusion coefficient is within the expected value for uncharged polymers. We will present a theory for these observed behaviors based on the coupling between the polyelectrolyte chains and their counterions.   

Nanotribology of charged polymer brushes

Polymers at surfaces, whose modern understanding may be traced back to early work by Sam Edwards$^{\mathrm{1}}$, have become a paradigm for modification of surface properties, both as steric stabilizers and as remarkable boundary lubricants$^{\mathrm{2}}$. Charged polymer brushes are of particular interest, with both technological implications and especially biological relevance where most macromolecules are charged. In the context of biolubrication, relevant in areas from dry eye syndrome to osteoarthritis, charged polymer surface phases and their complexes with other macromolecules may play a central role. The hydration lubrication paradigm, where tenaciously-held yet fluid hydration shells surrounding ions or zwitterions serve as highly-efficient friction-reducing elements, has been invoked to understand the excellent lubrication provided both by ionized$^{\mathrm{3}}$ and by zwitterionic$^{\mathrm{4}}$ brushes. In this talk we describe recent advances in our understanding of the nanotribology of such charged brush systems. We consider interactions between charged end-grafted polymers, and how one may disentangle the steric from the electrostatic surface forces$^{\mathrm{5}}$. We examine the limits of lubrication by ionized brushes, both synthetic and of biological origins, and how highly-hydrated zwitterionic chains may provide extremely effective boundary lubrication$^{\mathrm{6}}$. Finally we describe how the lubrication of articular cartilage in the major joints, a tribosystem presenting some of the greatest challenges and opportunities, may be understood in terms of a supramolecular synergy between charged surface-attached polymers and zwitterionic groups$^{\mathrm{7}}$. 1. Dolan {\&} Edwards, \textbf{Proc. Roy. Soc. A}, \textbf{337}, 509 (1974). 2. Klein et al. \textbf{Nature, 370}, 634 (1994). 3. Raviv et al., \textbf{Nature}, \textbf{425}, 163 (2003). 4. Chen et al., \textbf{Science}, \textbf{323} 1698 (2009). 5. Peretz et al., to be published. 6. Tairy et al., \textbf{Macromolecules}, \textbf{48}, 140 (2015). 7. Seror et al., \textbf{Nature Communications}, \textbar 6:6497 \textbar (2015); Jahn et al., \textbf{Annual Reviews of Biomedical Engineering} (2016)