Temperature-Dependent Dynamics and Hysteresis in Polymer Brushes grafted onto nanoparticles

Polymer-grafted nanoparticles (PGNPs) area promising candidates for developing advanced polymer nanocomposite systems. We aim to better understand the structure and dynamics of the attached polystyrene (PS) brush to SiO₂ nanoparticles in solution. as a function of brush grafting density, molecular weight, and nanoparticle size. By cycling the temperature above and below the PS-brush/cyclohexane solution theta point (~33 ^oC), we observed a hysteresis in R_H in the first heating-cooling. The effect of hold-time at the end-point of the heat-cool cycle allowed us to analyze the PS-brush relaxation time. We hypothesize that the hysteresis primarily arises from the entanglement of the dense polymer brushes via intra- and inter- chain interactions. Initially, inter-brush entanglement dominates, leading to an increase in hydrodynamics size. As the system relaxes, the intra-brush entanglement becomes more significant, resulting in minor size fluctuations. Post-relaxation measurements confirm the reduction in size fluctuation, supported by the DLS measurements. While post-relaxation fluctuations are constrained by fluctuations in shear force and the deformation history of the brushes, the final relaxed brush height reflects a balance between the elastic restoring force and the osmotic pressure, which drives the brushes away from the substrate.