Predictions of polymer chain conformations and molecular weight scaling in solvents of varying quality and in the melt are well established.1
They have been verified by measurements that have relied primarily on scattering techniques.2
In this work, we take a different approach to directly measure polymer chain size and scaling using polymer-grafted nanoparticle monolayers imaged with electron microscopy. Our results are compared to predictions that include the effect of tethering to a surface.3
Poly(ethylene oxide)-tethered gold nanoparticles (PEO-AuNPs) were prepared by grafting thiol-terminated PEO to Au NPs. The grafted NPs were then transferred into organic solvent and cast into a monolayer that self-assembled into a hexagonal close-packed (HCP) structure. The HCP lattice constant and the interparticle edge-to-edge distance were analyzed in detail by transmission electron microscopy (TEM) and small angle X-ray scattering (SAXS). The average edge-to-edge distance between adjacent Au NPs was found to be approximately equal to the PEO coil size. Using PEO of different molecular weights, the particle spacing was found to scale with the expected square root of the degree of polymerization. Solvent polarity was used to change the PEO chain conformations from random coil to collapsed globule, and these conformational changes were observed using the Au NP lattice spacing. The interparticle distances of the Au NP films formed under various experimental conditions have been found to agree well with theoretical predictions. This demonstrates a direct method to study the scaling behavior of polymer ligands grafted to nanoparticle surfaces.
1. Rubinstein, M.; Colby, R. H., Polymer Physics; Oxford University Press: USA, 2003, p 454.
2. Sperling, L. H., Introduction to Physical Polymer Science; Wiley Interscience: USA, 2006.
3. Zhulina, E. B.; Borisov, O. V.; Pryamitsyn, V. A.; Birshtein, T. M., Coil Globule Type Transitions in Polymers 1. Collapse of Layers of Grafted Polymer-Chains. Macromolecules 1991, 24, 140-149, DOI: 10.1021/ma00001a023.