Effects of Geometric Confinement on Caging and Dynamics of Polymer-Tethered Nanoparticle Suspensions

Well-dispersed polymer-tethered silica nanoparticles exhibit soft glassy rheology and caging behaviors due to chain interpenetration driven by entropic attraction and geometric confinement. In this study, we use small-angle X-ray scattering and rheology to investigate caging dynamics of silica nanoparticles densely grafted with poly(ethylene glycol) methyl ether (mPEG) dispersed in PEG oligomers (mPEGm). By systematically varying the volume fraction, φc, of the silica cores in mPEGm hosts, we are able to manipulate the extent of geometric confinement and, by means of vibrational spectroscopy (infrared and Raman), probe how molecular chain conformations are influenced by tethering and confinement. Rheological measurements reveal that the hairy particle suspensions manifest soft glassy dynamics, analogous to those reported in solvent-free hairy nanoparticles, down to particle volume fractions as low as 0.065. A simple geometric model is proposed to explain the emergence of caging and soft glassy rheology of the materials. We show that this model correctly explains the low yield strains observed in the materials and may also be used to rationalize the disappearance of yielding at φc < 0.065. Within the soft glassy regime, the hairy nanoparticle suspensions exhibit a peculiar flow curve with an upturn at a low shear rate. And, stress relaxation experiments show a significant unrelaxable stress upon flow cessation. Spectroscopic analysis suggests that tethered PEG chains adopt more trans conformations compared to untethered chains, providing molecular evidence of confinement-induced chain interpenetration in hairy nanoparticle soft glasses.