Understanding the role of grafted polystyrene chain conformation in assembly of magnetic nanoparticles

Polystyrene-grafted iron oxide nanoparticles have been shown to phase separate into ordered morphologies of strings, well-dispersed particles, and spherical aggregates at low graft densities in polymer matrices. In this work, small-angle neutron scattering experiments are performed to reveal the role of grafted chain conformation on nanoparticle assemblies. We demonstrate that chains grafted at low densities follow Gaussian statistics at any dispersion states. These results suggest that grafted chains are not distorted but remain Gaussian when particles are aggregated into strings. Small-angle x-ray and neutron scattering results show that matrix chains do not influence the formation of strings, but have a significant impact on the size and internal structure of aggregated particles. We conclude that spherical aggregates of nanoparticles with low polymer graft densities are akin to interpenetrating networks in which free matrix chains bridge the fractals of particles and control the cluster density.

Figure 1

Schematic illustration of oleic acid and oleylamine covered ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles and their PS-grafted forms after ligand exchange reaction.

Figure 2

TEM images of hPS-grafted ${\mathrm{Fe}}_3{\mathrm{O}}_4$ NPs in 84 kg/mol d-PS matrices at low and intermediate graft densities (a) 100-0.071, (b) 100-0.016, (c) 43-0.096, and (d) 43-0.057; and SAXS profiles (e)–(g) of the corresponding composites in two different matrices. Red solid lines are the simulated model. Dispersion of the same particles in lower d-PS matrices can be found in Fig. 7. SAXS curves of the sample 43-0.057 are displayed in Fig. 5 for further discussion.

Figure 3

Measured SANS profiles of 100 kg/mol PS-grafted ${\mathrm{Fe}}_3{\mathrm{O}}_4$ NPs forming strings at 0.016 (black sphere symbols) and 0.071 (blue square symbols) ${\mathrm{chains}/\mathrm{nm}}^2$. Two sets of data are presented for particles dispersed in 51 kg/mol (open symbols) and 84 kg/mol (filled symbols) d-PS matrices. Red solid lines are the Pederson fits. Curves are shifted vertically for clarity.

Figure 4

SANS profiles of 43 kg/mol PS-grafted nanoparticles with $0.096{\mathrm{chains}/\mathrm{nm}}^2$ in 84 (filled symbols) and 32 kg/mol (open symbols) d-PS matrix. Red solid lines are the Pederson fits. Curves are shifted vertically for clarity.

Figure 5

(a) SAXS and (b) SANS profiles of 43 kg/mol PS-grafted nanoparticles with $0.057{\mathrm{chains}/\mathrm{nm}}^2$ in 84 (filled symbols) and 32 kg/mol (open symbols) d-PS matrices. Red solid lines are the Beaucage fits. Three-level fitting is applied within shaded $q$ regions for the sample in 84 kg/mol matrix. Curves are shifted vertically for clarity.

Figure 6

(a) SANS of d-PS (84kg/mol) filled with bare particles (green symbols) and the fits for the surfactant layer (black dotted line) and void scattering (blue solid line). (b)–(e) SANS profiles before (solid lines) and after (open symbols) void subtraction in (b) 100-0.071, (c) 100-0.016, (d) 43-0.096, and (e) 43-0.057 samples.

Figure 7

Dispersion of PS-grafted ${\mathrm{Fe}}_3{\mathrm{O}}_4$ nanoparticles dispersed in short matrix. Matrix molecular weight is 51.6 for (a) 100-0.071 and (b) 100-0.016; and 32.2 kg/mol for (c) 43-0.096 and (d) 43-0.057.