(450h) Structure and Interaction of Solvent-Free Bidisperse Polymer Brushes Investigated Using Mean-Field Theory and Molecular Simulations

Solvent-free polymer-functionalized nanoparticles form a special type of colloidal system composed of inorganic cores self-suspended by their tethered polymeric coronas. In the absence of intervening solvent molecules, the fluidity of the system is completely provided by the space-filling polymers. Experimentally, polydispersity in the particle size and variations in the polymer grafting density have shown to alter the relative space-filling capability of neighboring brushes, which further impacts the system structureand dynamics.^1,2 In order to theoretically predict the effects of brush polydispersity (including variations in both chain length and surface grafting density) on the interparticle interactions in such nanoparticle systems, we consider two opposing bidisperse brushes grafted to surfaces, and formulate a semi-analytical density-functional theory for the brush distribution and the interbrush energy. Specifically, we investigate two bidisperse systems corresponding to different physical scenarios: One for opposing brushes uniformly mixed with two species at a fixed grafting density, and the other for opposing brushes with distinct chain lengths and grafting densities. The space-filling capabilities of the neighboring brushes differ not only by their ratio of radii of gyration for the composing polymers but also by their ratio of grafting densities. We show that the interbrush energy is affected not only by the typical steric repulsion as the brushes are compressed, but also by the extent of interpenetration or mixing of the two brushes as the interwall separation increases. The theoretical model is compared with coarse-grained molecular dynamics simulations at similar bidisperse conditions to elaborate the monomer-monomer packing configurational and enthalpic interactions. The limiting accessible parameter space of polymer sizes and grafting densities is explored for the bidisperse brush system. We envision that such an information would be useful for guiding the design of experimental self-suspended nanoparticles.

References:

[1] H.-Y. Yu, S. Srivastava, L. A. Archer, and D. L. Koch, Soft Matter 10, 9120 (2014).

[2] A. Agrawal, H.-Y. Yu, S. Srivastava, S. Choudhury, S. Narayanan, and L. A. Archer, Soft Matter 11, 5224 (2015).