Materials scienceDiffusion of a polymer ‘pancake’ (original) (raw)
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Surface Diffusion of Poly(ethylene glycol)
Macromolecules, 2002
We report direct measurement of the center-of-mass diffusion coefficient, D, of uncharged flexible linear chains adsorbed at the solid-liquid interface at dilute surface coverage. We find D ∼ N -3/2 (N is degree of polymerization) when N was varied by more than an order of magnitude (N ) 48, 113, 244, 456, and 693) and the scatter of the data was low. The experimental system was poly(ethylene glycol), PEG, adsorbed from dilute aqueous solution onto a self-assembled hydrophobic monolayer, condensed octadecyltriethoxysilane. The method of measurement was fluorescence correlation spectroscopy of a rhodamine green derivative dye that was end-attached to one sole end of the adsorbed PEG chains. The observed scaling implies the diffusion time τ ∼ N 3 if Rg ∼ N 3/4 as expected for a chain in good solvent in two dimensions (Rg is the radius of gyration), but a variety of other theoretical approaches to describe the dynamical scaling are also plausible. The multiplicity of plausible dynamical transport scenarios is compounded by the fact that polymer diffusion is sensitive to chain conformation on the surface which is not directly observable. Various theoretical scenarios are explored, and the need for new experiments, theory, and computer simulation studies to allow definitive interpretation of this observation of simple and clean fractional power law scaling is emphasized.
Single-Molecule Tracking of Polymer Surface Diffusion
Journal of the American Chemical Society, 2014
The dynamics of polymers adsorbed to a solid surface are important in thin-film formation, adhesion phenomena, and biosensing applications, but they are still poorly understood. Here we present tracking data that follow the dynamics of isolated poly(ethylene glycol) chains adsorbed at a hydrophobic solid−liquid interface. We found that molecules moved on the surface via a continuous-time random walk mechanism, where periods of immobilization were punctuated by desorption-mediated jumps. The dependence of the surface mobility on molecular weight (2, 5, 10, 20, and 40 kg/mol were investigated) suggested that surface-adsorbed polymers maintained effectively three-dimensional surface conformations. These results indicate that polymer surface diffusion, rather than occurring in the two dimensions of the interface, is dominated by a threedimensional mechanism that leads to large surface displacements and significant bulk−surface coupling.
Macromolecules, 1996
Porous microbeads of low cross-linked polystyrene (PS) grafted with poly(ethylene glycol) chains (PEG) are labeled at the free chain ends with 3-(1,6-diphenyl-1,3,5-hexatrienyl)propionic acid (DPH-PA) and 1-(dimethylamino)naphthalene-5-sulfonic acid (DANS), which probe the polarizability, polarity, and viscosity of their environments. The beads are investigated in a series of pure liquid phases and in acetonitrile/water mixtures by electronic absorption and steady state as well as time-resolved fluorescence spectroscopy. The extent of solvation in the polymer/liquid interphase is characterized from spectral shifts by introducing a solvation fraction which quantifies the relative amount of polymer and liquid in the solvation shell. The rotational mobility of DPH-PA is derived from time-resolved fluorescence anisotropy measurements. Depending on the liquid phase, the rotational correlation times vary strongly between τR) 500 ps and 100 ns. The shortest τR-values are found in liquids which are able to solvate both the fluorophore and the polymer, e.g. toluene and acetonitrile. Long correlation times are observed in the presence of aliphatic hydrocarbons, which do not solvate the bead and therefore cannot penetrate into the polymeric frame, and in presence of water, which solvates PEG but not PS and DPH. In contrast, water is able to weakly solvate DANS chemically bound to the polymeric frame.
Polymer Diffusion from Attractive and Athermal Substrates
Macromolecules, 2017
Given the exceedingly high interfacial area-to-volume ratios in polymer nanocomposites and the ability to manipulate the polymer/nanoparticle interfacial interactions, manipulating the chain dynamics at these interfaces has immense potential for impacting macroscopic properties. There, the polymer center-of-mass tracer diffusion coefficient (D) from attractive (hydroxylterminated) and athermal (phenyl-terminated or polymer-grafted) substrates was measured over a range of temperatures and tracer molecular weights using elastic recoil detection. The tracer polymer diffusion slows significantly relative to the bulk when polymers are in direct contact with an attractive substrate and exhibits a weaker molecular weight dependence, D ∼ M −1.4. For polymers without direct contacts on the attractive substrates and for athermal substrates, the diffusion coefficients are similar to the bulk case. The temperature dependence of these diffusion coefficients indicates that the slower diffusion at the interfaces is coupled to differences in polymer conformation and smaller fractional free volumes. These deviations from bulk are more pronounced for higher molecular weights and more attractive interfaces.
Fluorescent Labeling Can Alter Polymer Solution Dynamics
Macromolecules, 2015
There is a widespread assumption that modest chemical modification of a large polymer molecule, by adding a fluorescent label or the like, has no significant effect on polymer solution dynamics. We here report that light tagging of hydroxypropylcellulose (HPC) with fluorescein dye has a significant effect on the dynamic behavior of the chains, as measured using fluorescence photobleaching recovery (FPR) and dynamic light scattering (DLS). We compared unprocessed HPC, HPC processed in preparation for chemical modification, and HPC chemically modified by tagging with a fluorescein moiety. Addition of covalently bound fluorescein to the polymer eliminates the ultraslow (D ≈ 10 −10 cm 2 /s) relaxational mode found with DLS in the unlabeled HPC samples. Our findings explain the perceived discrepancies between DLS and FPR studies of HPC solutions. The discrepancies arise from differences in sample preparation. Comparison of FPR spectra (which measure the single particle diffusion process) and DLS spectra (which measure the relative motions of pairs of particles), each with its own intrinsic time and distance scales, provides insight into the nature of diffusion and the state of dissolution in dilute and nondilute HPC solutions.
Effect of microstructure on polymer chain dynamics in polyethylene glycol solutions
arXiv (Cornell University), 2020
Effect of microstructures and interactions on segmental dynamics in polyethylene glycol (PEG) solution in water is probed with macro-scale oscillatory rheology and micro-scale diffusion of a fluorescent probe. PEG solution fluorescence recovery after photobleaching (FRAP) curves have immobile fractions which increase with PEG concentration, for PEG volume fraction (c) > 0.2, indicating structuring. PEG solution micro-scale diffusion coefficients follow Rouse scaling ~− 0.54 for c < 0.8 (c*=0.03), resembling unentangled neutral polymers in good solvent. Small amount (0.01-1 wt%) nanoclay bentonite (B) in PEG matrix slows down probe diffusion 3-7 times, with heterogeneous dynamics. With 0.01-1 wt% carboxymethyl cellulose (CMC) in PEG matrix, probe diffusion is homogeneous with ~10% enhancement in diffusion time. The macroscale storage modulii (G') for PEG, PEG+B, and PEG + CMC solutions scale as viscous fluid-like power law ω α with α=2 for < −1 , with the terminal relaxation time, followed by short elastic plateau for −1 ≤ ≤ −1. For the regime > −1 , the scaling is a concentration-dependent power law ω α , with α greater than the Rouse scaling of 0.5 for 0.1<c<0.2 PEG solutions. We identify a time scale due to intermolecular interactions in PEG, such that for > −1 , Rouse scaling is recovered. Addition of CMC to PEG restores Rouse scaling. Addition of B gives contributions from both polymer matrix and network of B particles, leading to departure from pure Rouse behaviour. Static microtructural studies reveal clay aggregation due to depletion interactions on increasing the concentration of clay particles in PEG matrix, which leads to a non-monotonic concentration dependence of G' with c at 1 wt% B.
Physical Chemistry Chemical Physics, 2007
New low and high molecular weight poly(acrylic acid), PAA, 2000 g mol À1 and 450 000 g mol À1 , respectively, were tagged with pyrene (low and high contents of probe) and its behaviour in solution was investigated using absorption and fluorescence (steady-state and time-resolved) techniques. Fluorescence data shows that the degree and level of intramolecular association strongly depends on the molecular weight. With the short pyrene-labeled PAA chains in aqueous solution, the excimer-to-monomer fluorescence ratio I E /I M decreases with the increase of pH, oppositely to the increase in the I E /I M ratio with the increase in pH previously observed with the long chain PAA. Time-resolved data suggest that excimer formation with the short pyrene-labeled PAA polymers (ca. 28 acrylic acid monomers per chain) in water is largely due to excitation of Ground State Dimers, GSD. The increment of pH, and the consequent gradual ionization of the carboxylic groups in the chain, initially increases the fraction of GSD, possibly due to the occurrence of special micelle-like chain conformations, inside which the pyrene units are accommodated. A further increase of the pH above the pK a values, resulting in the full ionization of carboxylic groups, apparently destabilizes such chain conformations, which leads to a pH effect on the photophysical properties identical to that of the long chain polymers. In water, the dynamic data shows the existence of two excimers coexisting with two monomer classes. In methanol and dioxane (good solvents for the pyrene probe) at room temperature, where one excimer and two monomers are present, all rate constants could be obtained, as well as the fractions of ground-state species. It is thus shown that different types of interactions are produced with small-and long-sized PAA polymers, i.e., the size of the polymer matters.
Conformations of tethered polymer chains: a fluorescence energy transfer study
Polymer, 2001
Investigations have been carried out to probe the conformations of tethered poly(ethyleneglycol) (PEG) chains anchored on polystyrene (PS) latex particles in presence of an anionic surfactant, sodium dodecyl sulfate (SDS) and two inorganic salts, sodium and potassium chloride. For this purpose, PS latex particles were labeled with pyrene and mononaphthayl PEG ester. Distance dependent nonradiative energy transfer from naphthalene moieties to pyrene moieties was used as a ruler in this study. Results showed that the separation distance between the acceptor and the donor changes with external stimuli. Analysis of the results suggested that there is considerable contraction of the polymer chain upon interaction with salts and surfactant below its critical micelle concentration. The study also reveals that SDS interacts with PEG chains bellow its critical micellar concentration, probably due to binding of SDS counterions to PEG chains. q