Polymer conformation and dynamics in crowded environments: A combined diffusion NMR and small-angle neutron scattering study

The effect of particles on the behavior of polymers in solution is important in a number of important phenomena such as the effect of “crowding” proteins in cells, colloid-polymer mixtures, and nanoparticle “fillers” in polymer solutions and melts. In this talk, I will present a study of the effect of spherical inert nanoparticles (which we refer to as “crowders”) on the diffusion coefficient and radius of gyration of polymers in solution using pulsed-field-gra- dient NMR and small-angle neutron scattering (SANS), respectively. In addition, the role of enthalpic crowder- crowder interactions on the crowding process is unknown: we can control this by varying charge on the crowder particle. Below a characteristic polymer concentration, which we identify as the overlap threshold concentration c⋆, the diffusion coefficients exhibit a plateau. Above c⋆, in a crossover region between the dilute and semidilute regimes, the (long-time) self-diffusion coefficients are found, universally, to decrease exponentially with polymer concentration at all crowder packing fractions, consistent with a structural basis for the long-time dynamics. When the polymer radius of gyration and crowder size are comparable, the polymer size is very weakly affected by the presence of crowders, consistent with recent computer simulations. We find that crowder charge only weakly affects polymer size and dynamics in the crowding limit, but that local macromolecular mobility depends strongly on molecular flexibility

Clusters in sedimentation equilibrium for an experimental hard-sphere-plus-dipolar Brownian colloidal system

In this work, we use structure and dynamics in sedimentation equilibrium, in the presence of gravity, to examine, $via$ confocal microscopy, a Brownian colloidal system in the presence of an external electric field. The zero field equation of state (EOS) is hard sphere without any re-scaling of particle size, and the hydrodynamic corrections to the long-time self-diffusion coefficient are quantitatively consistent with the expected value for hard spheres. Care is taken to ensure that both the dimensionless gravitational energy, which is equivalent to a Peclet number $Pe_g$, and dipolar strength $\Lambda$ are of order unity. In the presence of an external electric field, anisotropic chain-chain clusters form; this cluster formation manifests itself with the appearance of a plateau in the diffusion coefficient when the dimensionless dipolar strength $\Lambda \sim 1$. The structure and dynamics of this chain-chain cluster state is examined for a monodisperse system for two particle sizes

Contrasting the dynamics of elastic and non-elastic deformations across an experimental colloidal Martensitic transition

We present a framework to segregate the roles of elastic and non-elastic deformations in the examination of real-space experiments of solid-solid Martensitic transitions. The Martensitic transformation of a body-centred-tetragonal(BCT) to a body-centred-orthorhombic(BCO) crystal structure has been studied in a model system of micron-scale ionic microgel colloids. Non-affine fluctuations, i.e., displacement fluctuations that do not arise from purely elastic(affine) deformations, are detected in particle configurations acquired from the experiment. Tracking these fluctuations serves as a highly sensitive tool in signaling the onset of the Martensitic transition and precisely locating particle rearrangements occurring at length scales of a few particle diameters. Particle rearrangements associated with non-affine displacement modes become increasingly favorable during the transformation process. The nature of the displacement fluctuation modes that govern the transformation are shown to be different from those predominant in an equilibrium crystal. We show that BCO crystallites formed through shear may, remarkably, co-exist with those resulting from local rearrangements within the same sample

Combining Diffusion NMR and Small-Angle Neutron Scattering Enables Precise Measurements of Polymer Chain Compression in a Crowded Environment

The effect of particles on the behavior of polymers in solution is important in a number of important phenomena such as the effect of “crowding” proteins in cells, colloid-polymer mixtures, and nanoparticle “fillers” in polymer solutions and melts. In this Letter, we study the effect of spherical inert nanoparticles (which we refer to as “crowders”) on the diffusion coefficient and radius of gyration of polymers in solution using pulsed-field-gradient NMR and small-angle neutron scattering (SANS), respectively. The diffusion coefficients exhibit a plateau below a characteristic polymer concentration, which we identify as the overlap threshold concentration c⋆. Above c⋆, in a crossover region between the dilute and semidilute regimes, the (long-time) self-diffusion coefficients are found, universally, to decrease exponentially with polymer concentration at all crowder packing fractions, consistent with a structural basis for the long-time dynamics. The radius of gyration obtained from SANS in the crossover regime changes linearly with an increase in polymer concentration, and must be extrapolated to c⋆ in order to obtain the radius of gyration of an individual polymer chain. When the polymer radius of gyration and crowder size are comparable, the polymer size is very weakly affected by the presence of crowders, consistent with recent computer simulations. There is significant chain compression, however, when the crowder size is much smaller than the polymer radius gyration

Characterization of dynamics and internal structure of a mixed-surfactant wormlike micellar system using NMR and rheometry

We use complementary experiments—proton NMR diffusometry and relaxometry, deuterium NMR lineshapes, and rheometry—to construct a comprehensive picture of the microscopic structure of a mixed-surfactant wormlike micellar system composed of a zwitterionic surfactant and an anionic surfactant in brine. In this system, at some surfactant concentrations, the time for micellar breaking and recombination sb is not small compared with the micellar reptation time sR, weakening the condition to obtain a stress relaxation function with just one relaxation time at long times. FromNMRrelaxometry, we determine the overlap concentration. Deuterium NMR spectral lineshapes indicate the presence of a wide angular distribution in the orientational order. NMR diffusometry and rheology probe different timescales and yield complementary information indicating polymer-like behaviour at the corresponding lengthscales. Via NMR, surfactant diffusion coefficients are seen to decrease with increasing diffusion time, consistent with restricted diffusion within a reptating micelle. At the same time, comparison of measurements with protonated and deuterated surfactants strongly suggests that the measured short and long time diffusion coefficients correspond to intra-micellar and micellar diffusion, respectively. Fitting the diffusion results to a simple model, the average end-to-end micellar distance is estimated to be in the 1 mm range and only weakly dependent on concentration. The water diffusion measurements, on the other hand, imply a high degree of water structuring at the micellar surface. We also find that the wormlike micelles obeyed simple polymer-like scaling behaviors, with a crossover from Zimm-like (diffusion) to Rouse-like (rheology) exponents

Electrorheological responses of soft ionic colloids

N-isopropyl poly acrylamide microgel colloids exhibit strong electric-field-induced phase transitions, suggesting the possibility of a strong electrorheological response. [1] Electrorheological (ER) fluids draw researchers’ attention due to their huge potential for applications in mechanical devices, switches, valves and microfluidic chips. Please click on the file below for full content

Self organization of exotic oil-in-oil phases driven by tunable electrohydrodynamics

Self organization of large-scale structures in nature - either coherent structures like crystals, or incoherent dynamic structures like clouds - is governed by long-range interactions. In many problems, hydrodynamics and electrostatics are the source of such long-range interactions. The tuning of electrostatic interactions has helped to elucidate when coherent crystalline structures or incoherent amorphous structures form in colloidal systems. However, there is little understanding of self organization in situations where both electrostatic and hydrodynamic interactions are present. We present a minimal two-component oil-in-oil model system where we can control the strength and lengthscale of the electrohydrodynamic interactions by tuning the amplitude and frequency of the imposed electric field. As a function of the hydrodynamic lengthscale, we observe a rich phenomenology of exotic structure and dynamics, from incoherent cloud-like structures and chaotic droplet dynamics, to polyhedral droplet phases, to coherent droplet arrays

Deuterium NMR and rheology of microgel colloids at ambient and high pressure

Microgel colloids exhibit a polymer collapse transition resulting in a large reduction in colloid size at high temperatures or pressures. Our goal is to obtain a microscopic understanding of the internal structure and microscopic dynamics of microgels by examining the temperature and pressure dependence of the collapse transition. We have conducted a systematic study of how the nature of this collapse transition is affected by crosslink density (Cd). We used deuterium NMR (2H-NMR) to probe the microscopic dynamics of cross-linked poly-N-isopropylacrylamide (p-nipam) chains, in microgel colloids, as a function of temperature and pressure. Four differently crosslinked microgels colloids were synthesized with deuteron labels on the nipam backbone (d3- nipam). Corresponding macroscopic properties of unlabeled colloids having the same crosslink densities were characterized by dynamic light scattering (DLS) and rheology. Rheological characterization as a function of temperature (T) and particle concentration (c), and for 4 crosslink densities, showed that the microgel viscosity decreases as temperature is increased, and that in the high T/low c regime, there is a collapse of the viscosity as a function of T and c when plotted against volume fraction: this yields a measure of the water content in the particles as function of T. 2H-NMR spectra of the d3-nipam suspensions for all Cd indicated freely moving chains at low temperature and a nearly immobilized fraction above 35°C. This is consistent with DLS observations of a transition from swollen to collapsed colloids. 2H-NMR spectra for the dry powder indicated totally immobilized segments in the particle. Nipam segments in the collapse phase of the d3-nipam suspension were more mobile than those in the dry powder. This suggests significant amounts of water in the collapsed phase, a finding consistent with the rheology observations. For the highest two values of Cd, microgel spectra showed the presence of an immobilized fraction of segments even in the swollen phase. Variable pressure NMR (up to 90 MPa) showed a slight increase in transition temperature with pressure for all Cd values studied