Unusual Transformation of Polymer Coils in a Mixed Solvent Close to the Critical Point

Abstract

We have discovered unusual behavior of polymer coils in a binary solvent (nitroethane+isooctane) near the critical temperature of demixing. The exceptionally close refractive indices of the solvent components make the critical opalescence relatively weak, thus enabling us to simultaneously observe the Brownian motion of the polymer coils and the diverging correlation length of the critical fluctuations. The polymer coils exhibit a collapse-reswelling-expansion-reshrinking transition upon approaching the critical temperature. While the first stage (collapse-reswelling) can be explained by the theory of Brochard and de Gennes, the subsequent expansion-reshrinking transition is a new unexpected phenomenon that has not been observed so far. We believe that this effect is generic and attribute it to micro-phase separation of the solvent inside the polymer coil.

Deconstruction of biomass enabled by local demixing of cosolvents at cellulose and lignin surfaces

“…The presence of nanoclusters in mixed solvents has been shown to drive other interesting phenomena. For example, the complex behavior of polymer coils in a binary solvent near the critical temperature of demixing has been attributed to microphase separation of the solvent inside the polymer coil ( 43 ). It is therefore becoming apparent that meso/nanoscale fluctuations of molecular fluids can drive major changes in the dimensions and organizations of macromolecular solutes.…”

Physical origin of the expansion of polymer coils in a binary solvent in the vicinity of its demixing critical point

“…Our experiments have clearly shown that addition of polymer does not decrease the critical temperature as assumed by Brochard and de Gennes, but instead increases the critical temperature, thus increasing the two-phase domain. Specifically, we found that, in the investigated range of polymer concentrations c, the difference between the critical temperature T c (c) at polymer concentration c and the critical temperature T c (0) of the pure binary solvent, when scaled by the degree of polymerization N, shows a universal linear dependence on c with a positive coefficient 17 . In fact, addition of polymer causing an increase of the two-phase domain, appears to be a general phenomenon in phase separating polymer solutions [13][14][15][16] .…”

“…However, the concentration of monomers inside the coil is significantly higher than the concentration in the bulk dilute solution. The concentration in terms of molecular fraction of monomers inside the undisturbed coil, where R 2 h ∝ N 17 , can be esimated as 17 , the system inside the coil will enter the two-phase region at…”

“…Recently, we studied the behavior of dilute polymer chains (polystyrene and poly-butyl methacrylate) of various molecular weights near the liquid-liquid critical point of a binary solvent (nitroethane+isooctane) by dynamic light scattering 17 . The mixture nitroethane+isooctane is special in that the difference between the refractive indices of the two liquid components is exceptionally small, so that the critical opalescence is weak while still measurable 18,19 .…”

“…The experimental results are summarized in Fig. 1 showing the hydrodynamic radius R h of the polymer coils as a function of the correlation length ξ of the critical fluctuations in the bulk solution, scaled in terms of the hydrodynamic radius R 0 h of the polymer coils far away from the critical a) Electronic mail: sengers@umd.edu temperature 17 . The polymer coils begin to collapse when ξ/R 0 h ≈ 1, reach a minimum size when ξ/R 0 h ≈ 2, and then expand to a maximum value R h much larger than the original unperturbed value R 0 h and finally shrinking again, a phenomenon not predicted by Brochard and de Gennes 2 .…”