Abstract
Polymer chains are attached to nanoparticle surfaces for many purposes, including altering solubility, influencing aggregation, dispersion, and even tailoring immune responses in drug delivery. The most unique structural motif of polymer-grafted nanoparticles (PGNs) is the high-density region in the corona where polymer chains are stretched under significant confinement, but orientation of these chains has never been measured because conventional nanoscale-resolved measurements lack sensitivity to polymer orientation in amorphous regions. Here, we directly measure local chain orientation in polystyrene grafted gold nanoparticles using polarized resonant soft X-ray scattering (P-RSoXS). Using a computational scattering pattern simulation approach, we measure the thickness of the anisotropic region of the corona and extent of chain orientation within it. These results demonstrate the power of P-RSoXS to discover and quantify orientational aspects of structure in amorphous soft materials and provide a framework for applying this emerging technique to more complex, chemically heterogeneous systems in the future.
Highlights
Polymer chains are attached to nanoparticle surfaces for many purposes, including altering solubility, influencing aggregation, dispersion, and even tailoring immune responses in drug delivery
A key feature of modern polymer-grafted nanoparticles (PGNs) research is a focus on the impact of graft density and how it affects the conformation of chains arranged radially about the particle[6,8,9]
Unlike conventional small-angle X-ray scattering (SAXS) with hard X-rays, and small-angle neutron scattering (SANS), polarized resonant soft X-ray scattering (P-RSoXS) imparts a unique sensitivity to molecular orientation through the interaction of near-edge X-ray absorption fine structure (NEXAFS) transition dipoles with incident X-ray polarization[14,15,16,17,18,19,20,21]
Summary
Polystyrene (PS) grafted gold nanoparticles (AuNP, radius ≈ 10 nm) were prepared and cast into monolayer films using established methods[7]. We account for random stereochemistry and amorphousness by assuming that all rotations about the backbone axis are possible Integration of this arrangement yields the uniaxial representation for the absorption (imaginary part) shown, which can be intuitively understood from Fig. 2d as all of the planes contribute to the direction parallel to the backbone axis, but they do not all contribute to the other two axis directions. To determine whether the P-RSoXS pattern anisotropy arises from the orientation in the corona of individual particles, or from the orientation correlations between the corona of neighboring particles, simulations were performed on morphologies consisting of different number of PGNs. Starting from only two particles spaced. The total SSE for A(q) were calculated at the two energies (284.7 and 285.2 eV) across 16,000 models having different parameters (raniso, S0, and d) for the anisotropic corona that were used to fit the simulations to the experiment. For the definition of raniso, S0, and d, see text
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