Blob Size Controls Diffusion of Free Polymer in a Chemically Identical Brush in Semidilute Solution.

Zhenyu J Zhang,Jeppe Madsen,Matthew Mears,Mark Geoghegan,Graham J Leggett,Steven P Armes,Steve Edmondson

doi:10.1021/acs.macromol.8b01193

Zhenyu J Zhang, Jeppe Madsen + Show 5 more

Open Access

https://doi.org/10.1021/acs.macromol.8b01193

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Abstract

The diffusion of rhodamine-labeled poly(ethylene glycol) (r-PEG) within surface-grafted poly(ethylene glycol) (s-PEG) layers in aqueous solution at 18 °C was measured by fluorescence correlation spectroscopy. The diffusion coefficient of r-PEG within s-PEG was controlled by the grafting density, σ, and scaled as σ–1.42±0.09. It is proposed that a characteristic blob size associated with the grafted (brush) layer defines the region through which the r-PEG diffusion occurs. The diffusion coefficients for r-PEG in semidilute solution were found to be similar to those in the brushes.

Highlights

The diffusion of polymer chains in confined environments is an enduring subject in polymer science.[1−10] A deceptively simple problem concerns the diffusion of free chains within chemically identical brushes
fluorescence correlation spectroscopy (FCS) data and fits are shown in Figure 3 along with the diffusion coefficients obtained from fitting the autocorrelation data
This doublelogarithmic plot shows that a power law behavior can reasonably describe the diffusion, and the fit shows that the diffusion coefficient scales with grafting density as D ∝ σ−1.42

Summary

■ INTRODUCTION

The diffusion of polymer chains in confined environments is an enduring subject in polymer science.[1−10] A deceptively simple problem concerns the diffusion of free chains within chemically identical brushes (surface-grafted chains). A J.A. Woollam multiwavelength ellipsometer was used to measure the thicknesses (both dry and in water) of the PEG brush layers on the gold-coated silicon wafer. To understand the interaction of PEG in solution with the brush layer, force spectroscopy experiments were performed using a PEG-coated tip (σ = 0.016, assuming a similar brush growth to those on the gold-coated silicon surfaces). The PEG-coated tip readily penetrated a lightly grafted brush layer resulting in a 2 nm pulloff force (Figure 2b). When the PEG-coated tip was brought toward a more densely grafted layer (σ = 0.059), it was unable to penetrate, at least with an applied force of 15 nN, and instead a long-range repulsion was observed It can be concluded from these data that there is an energy barrier to PEG entering the brush layer. The number of molecules confined within the brush layer decreased by a factor of ∼6 as the grafting density increased

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES