Heterogeneity during Plasticization of Poly(vinylpyrrolidone): Insights from Reorientational Mobility of Single Fluorescent Probes

Abstract

While dynamics of single-molecule (SM) fluorescent probes have been used to investigate the structure and relaxation processes in polymers near the glass transition temperature (T), it is difficult to perform SM imaging at elevated temperatures which restricts such studies to a limited number of polymers for which T is close to room temperature (RT). Plasticization, solvent (or additive) induced lowering of T, offers an alternate avenue to access various effective temperatures in the glassy and rubbery phases of polymers under ambient conditions. By investigation of the reorientational propensity of individual Rhodamine 6G (Rh6G) probes, which is governed by rigidity/dynamics of the polymer cavities, we have explored the extent of spatiotemporal heterogeneity during moisture induced plasticization of poly(vinylpyrrolidone) (PVP), far below and near (below and above) bulk T. Lack of any probe reorientation suggests that the matrix remains extremely rigid up to a certain level of hydration, as expected for probes buried deep within the glassy state. At intermediate levels of hydration, SMs undergo a wide variety of rotational dynamics ranging from being static/wobbling motion to slow, hindered large-angle reorientation, as well as facile, intermittently hindered fast rotation, which reflects that swelling/softening of network cavities is spatiotemporally extremely diverse as the effective T approaches RT. SM probes exhibit temporally nonuniform rotational mobility even at relatively high moisture contents of the matrix beyond which probes can undergo translational motion, which indicates that relatively slow time scale polymer segmental motion can be operational for plasticized PVP (in the rubbery state). Our inferences are supported by the non-Gaussian nature of angular jump distributions for dipolar reorientation, similar to those reported for translational diffusion of SM tracers in polymers and cellular media, suggesting the existence of slow time-varying local environmental changes around individual probe molecules during plasticization.

Diffusing diffusivity: a new derivation and comparison with simulations

“…This kind of Brownian, yet anomalous diffusion of particles in complex fluids now seems to be a generic phenomenon and a large body of evidence from various fields including many physicochemical and socio-economic processes is available. These processes include hard sphere diffusion in colloidal suspensions, 29,32,33 liposome diffusion in entangled actin, 28 lateral diffusion of receptor molecules in cell membranes, 18 tracer diffusion in crowded media of either larger spheres 33,34 or polymers, 35,36 diffusive motion in supercooled liquids 37 and in systems close to glass and jamming transitions, 38,39 orientational diffusion of dye molecules in polymer thin films, 40,41 simulations of 2D colloids 42 and hard sphere fluids 43 , price dynamics, 44 and financial returns. 45 The salient features of this kind of diffusion processes have been summarised in a recent review by Metzler 46 and also in the paper by Cherstvy and Metzler.…”

Photobleaching of randomly rotating fluorescently decorated particles

“…Fluorophores within a glassy matrix, such as within a polymeric microbead, also exhibit limited wobble. 19 With volumetric fluorescent labeling within polymeric microbeads, the ideal conditions of our model should be accessible. Such microbeads of various sizes can separately probe local rotational and translational diffusion at length-scales comparable to the particle size.…”

Diffusing diffusivity: Rotational diffusion in two and three dimensions

“…We propose a similar "non-normal parameter" whose value would be zero, if the behavior of the probability distribution is similar to that for constant diffusivity, but is different from zero if the spread in diffusivity affects the probability distribution for rotational motion. Our results are of great interest, as there are experiments on rotational motion in crowded systems, 49,50 and should be useful in the analysis of such experiments. Further it should be possible to verify them in detail using the experimental setup proposed by Matse et al, 57,58 who investigated the Brownian motion of a colloidal particle near a wall.…”

“…In the present paper, we extend our model of diffusing diffusivity to the problem of rotational diffusion in a crowded, rearranging system. Several single-molecule experiments have been performed in the recent past in this regard, viz., the orientational diffusion of dye molecules in polymer thin films, 49,50 wobbling dynamics of dipoles in porous media, 51,52 diffusion of nano-doublers in living cells, 53 etc. Mazza et al 54 performed molecular dynamics simulations to probe the rotational and translational dynamics of water at low temperatures.…”