Shell Architecture Strongly Influences the Glass Transition, Surface Mobility, and Elasticity of Polymer Core-Shell Nanoparticles

Despite the growing application of nanostructured polymeric materials, there still remains a large gap in our understanding of polymer mechanics and thermal stability under confinement and near polymer-polymer interfaces. In particular, the knowledge of polymer nanoparticle thermal stability and mechanics is of great importance for their application in drug delivery, phononics, and photonics. Here, we quantified the effects of a polymer shell layer on the modulus and glass-transition temperature (T_g) of polymer core-shell nanoparticles via Brillouin light spectroscopy and modulated differential scanning calorimetry, respectively. Nanoparticles consisting of a polystyrene (PS) core and shell layers of poly(n-butyl methacrylate) (PBMA) were characterized as model systems. We found that the high T_g of the PS core was largely unaffected by the presence of an outer polymer shell, whereas the lower T_g of the PBMA shell layer decreased with increasing PBMA thickness. The surface mobility was revealed at a temperature about 15 K lower than the T_g of the PBMA shell layer. Overall, the modulus of the core-shell nanoparticles decreased with increasing PBMA shell layer thickness. These results suggest that the nanoparticle modulus and T_g can be tuned independently through the control of nanoparticle composition and architecture.