Effect of Polymer Chemistry on Chain Conformations in Hairy Nanoparticle Assemblies

NK Hansoge and S Keten, ACS MACRO LETTERS, 8, 1209-1215 (2019).

DOI: 10.1021/acsmacrolett.9b00526

Matrix-free, polymer-grafted nanoparticles, called hairy nanoparticle assemblies (aHNPs), have proven advantageous over traditional nanocomposites, as good dispersion and structural order can be achieved. Recent studies have shown that conformational changes in the polymer structure can lead to significant enhancements in the mechanical properties of aHNPs. To quantify how polymer chemistry affects the chain conformations in aHNPs, here we present a comparative analysis based on coarse-grained molecular dynamics simulations. Specifically, we compare the chain conformations in an anisotropic cellulose nano particle grafted to four common polymers with distinct chemical groups, fragility, and segmental structures, that is, poly(methyl methacrylate) (PMMA), polystyrene (PS), polycarbonate (PC), and polybutadiene (PB). We observe that semiflexible glassy polymers such as PMMA and PS have a higher critical chain length (N-cr), the transition point where the polymer conformation changes from concentrated to semidilute brush regime. Flexible rubbery polymers (PB) can overcome the N-cr barrier at relatively lower molecular weights. We have used theoretical scaling laws based on Daoud-Cotton theory to uncover a direct correlation between empirical constants and physical parameters, such as persistence length and monomer excluded volume. Furthermore, we carried out a systematic study to understand the role of backbone rigidity and side- group size of polymer, and it revealed that the backbone rigidity significantly affects N-cr but the side-group size doesn't seem to have an appreciable effect on N-cr. We find that normalization of the monomer radial distribution curves using N-cr and other key molecular parameters collapses the curves for 110 distinct model aHNP systems studied. Our work paves the way for systematic quantification of these molecular design parameters to accelerate the design of polymer-grafted nanoparticle assemblies in combination with universal scaling relationships.

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