Some open challenges in polymer physics*

Abstract

Three subjects in polymer and soft matter physics are outlined in the present article. The first relates to concepts of an ideal glass transition. We describe work on ultra-stable glass, either a 20-million-year-old amber or a vapor deposited amorphous fluoropolymer, which examines the temperature dependence of the dynamics in a window between a low fictive temperature and the glass transition temperature. From this "finesse" of the problem of the geological time scales in sub-glass temperature systems strong evidence that the divergence of the relaxation times at finite temperature from WLF-types of extrapolation is not correct. The second topic is polymer nonlinear viscoelasticity as it relates to dynamic heterogeneity. We show results from mechanical hole burning experiments that suggest that dynamic heterogeneity arises from the nature of specific relaxation mechanisms rather than heterogeneous changes in, for example, the fictive temperature. Included is a discussion of large amplitude oscillatory shear testing and how it relates to characterization of nonlinear viscoelastic materials. The last topic addressed is the rheology of circular macromolecules. Here, we take a historical perspective and describe important new results from several laboratories that seem inconsistent. New works from our own studies on circular DNA are also discussed.

Persistence Length and Encounter Frequency Determination from Fluorescence Studies of Pyrene-Labeled Poly(oligo(ethylene glycol) methyl ether methacrylate)s

“…8,9 The relaxation time of a polymer can be determined from rheological or NMR measurements among other techniques. 10 While effective, the above-mentioned techniques suffer from stringent requirements that limit their widespread applicability. For instance, scattering techniques typically require that polymer samples be prepared with a narrow molecular weight distribution, 11−13 which cannot always be accomplished.…”

“…The mechanical properties of any polymeric material can be characterized by how much and how quickly it can deform after application of stress since the extent of deformation of a plastic depends on its relaxation time. − In turn, the properties experienced at the macroscopic level by the polymeric material, such as its ease of deformation, reflect the behavior of the polymer chains at the molecular level. , These properties are represented by the ability of a chain to bend, described by its persistence length ( l p ), and the time scale over which bending occurs, represented by the relaxation time (τ r ) of the polymer. Traditionally, l p has been determined by scattering techniques , or conformation plots generated by gel permeation chromatography (GPC). , The relaxation time of a polymer can be determined from rheological or NMR measurements among other techniques . While effective, the above-mentioned techniques suffer from stringent requirements that limit their widespread applicability.…”

Linear Viscoelastic Properties of Putative Cyclic Polymers Synthesized by Reversible Radical Recombination Polymerization (R3P)

“…Cyclic polymers stand out from other topologies by their lack of free ends, which are central to models of relaxation of entangled linear and branched polymers. − Models developed to describe the dynamics of cyclic polymers include the double-folded closed ring model, double-folded lattice animal (DFLA) model, , fractal loopy globule (FLG) model, decorated loop model, and the Rouse ring model . It has been known for more than 35 years that the viscoelastic behavior of cyclic polymers − is very different from that of their linear counterparts. However, establishing quantitative, reproducible linear viscoelastic properties of cyclic polymers has been limited by uncertain purity (particularly studies in the 1980s) and by the limited range of molecular weights available for highly pure cyclic polymers prepared using liquid chromatography at the critical condition (LCCC) − that, in principle, separates cyclic molecules from linear chains.…”

Editorial: In honor of Gregory B. McKenna's contributions to the field of polymer science

“…In this special issue, Dr. McKenna himself contributes a trendsetting article [ 1 ] which presents three current directions in polymer physics: the question of the divergence of relaxation time at a finite temperature (in which amber plays a key role), dynamic heterogeneity, and the dynamics of circular macromolecules. While there is evidence of a clear answer for the first question and there is significant progress concerning the second, for the last topic, many unanswered questions remain.…”