Abstract
Understanding the effects of polymer brush architecture on particle interactions in solution is requisite to enable the development of functional materials based on self-assembled polymer-grafted nanoparticles (GNPs). Static and dynamic light scattering of polystyrene-grafted silica particle solutions in toluene reveals that the pair interaction potential, inferred from the second virial coefficient, A2, is strongly affected by the grafting density, σ, and degree of polymerization, N, of tethered chains. In the limit of intermediate σ (∼0.3 to 0.6 nm–2) and high N, A2 is positive and increases with N. This confirms the good solvent conditions and can be qualitatively rationalized on the basis of a pair interaction potential derived for grafted (brush) particles. In contrast, for high σ > 0.6 nm–2 and low N, A2 displays an unexpected reversal to negative values, thus indicating poor solvent conditions. These findings are rationalized by means of a simple analysis based on a coarse-grained brush potential, which balances the attractive core–core interactions and the excluded volume interactions imparted by the polymer grafts. The results suggest that the steric crowding of polymer ligands in dense GNP systems may fundamentally alter the interactions between brush particles in solution and highlight the crucial role of architecture (internal microstructure) on the behavior of hybrid materials. The effect of grafting density also illustrates the opportunity to tailor the physical properties of hybrid materials by altering geometry (or architecture) rather than a variation of the chemical composition.
Highlights
Grafted nanoparticles (GNPs) are made of a solid inorganic core covered by polymer chains tethered to the core surface
We have shown that the details of the internal microstructure significantly affect the interactions of grafted nanoparticles under the same conditions in solution
The measurements of the second virial coefficient reveal that sufficiently dense brush architectures can trigger attractive interactions even in systems for which dissolution would be expected on the basis of polymer/solvent composition
Summary
Grafted nanoparticles (GNPs) are made of a solid inorganic core covered by polymer chains tethered to the core surface. The negative second virial coefficient is not the consequence of bare core−core attraction, as determined by the value of A, but it is the grafted layer-mediated microstructure with predominantly stretched chain conformation (fraction of the dry region68) that plays the key role (e.g., in DP130) This simple model does predict a negative value of A2,th for GNP DP130 with the lowest N and highest σ, it serves here only as a qualitative confirmation of the intriguing experimental findings. There is an uncertainty in the determination of the dry CPB and wet brush SDPB regimes since the calculations in Figure 3 are very sensitive to the choice of monomer size (we used an effective size based on the cross-sectional area, following ref 49) These limitations could explain the differences between the measured and calculated values of the second virial coefficient for DP2480 (Figure 4b). It is important to investigate the effects of bulkiness and polarity of the monomers using chemically different grafted polymer chains and/or cores
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