Surface Diffusion of Poly(ethylene glycol)

Abstract

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.

Polymer Surface Diffusion in the Dilute Limit

“…4, D for the same polymer samples is compared for the two quartz surfaces, asreceived and thermally annealed. Over the range of M investigated in -previous experiments on this question [13,14], data for the rougher surface are consistent with the D ~ M -3/2 predicted by simulations [8][9][10][11][12]15], while data for diffusion on the smoother surface obey D ~ M -1 observed previously only for diffusion on a fluid surface ( DNA on supported phospholipid bilayers [24,25]). As we observe D ~ M -1 on both a crystalline surface (mica) and on an aperiodic surface (quartz) shows that explanation should not be sought in surface-specific structure or lack of structure, though this is the premise of some theoretical approaches [15].…”

“…It goes beyond the limitations of a previous study from this laboratory [13,14] where a narrow range of M was considered and questions of system-specificity were presented by the choice of an aqueous system. Here we consider scaling with M over an exceptionally large span of molecular weight while investigating polystyrene, whose nonpolar character renders it a model system in which to investigate generic effects.…”

Fluorescence correlation spectroscopy in polymer science

“…96,168 In this system, the polymer chains adsorb to the surface, thus exhibiting a at 'pancake' conformation. Using PEG of different molecular weights (between 2 and 31 kg mol À1 ), it was found that the diffusion coefficient scales with the number of chain segments according to a strikingly strong power-law scaling with an exponent of À3/2.…”

Computer simulation of soft matter at the growth front of a hard-matter phase: incorporation of polymers, formation of transient pits and growth arrest

“…A single polymer on a surface, although it may never desorb, can diffuse on this surface. This diffusion has been studied, and found to be much slower than in the bulk of the solution [29][30][31] .…”