Glass formation and stability of polystyrene-fullerene nanocomposites

The addition of nanoparticles (NP) to polymers is particularly interesting from a scientific and practical standpoint as their dimensions are comparable to the dynamic heterogeneity of glass-forming polymer matrices. Such additives can, therefore, potentially change the qualitative character of glass formation rather than just a change in the glass transition temperature. To gain insight into this phenomenon, we investigate a well characterized and common NP (fullerene nanoparticles whose diameter is approximately 1 nm) dispersed in a common amorphous polymer (polystyrene) by inelastic incoherent neutron scattering, small angle neutron scattering, calorimetric and dielectric spectroscopy methods. We find that the fullerene NP increase the glass transition temperature, slowing down the alpha relaxation dynamics associated with glass formation, while at the same time cause a softening of the material at high frequencies (as determined by the Debye-Waller factor). These effects are interpreted in terms of the particle modifying the polymer packing, causing an increase of the fragility of glass formation. Recent molecular dynamics simulations and experimental studies confirm that NP with attractive interactions for the polymer matrix should indeed increase the fragility of glass formation. Our measurements, in combination with previous work, demonstrate that that the fragility of glass formation can be tuned through the judicious choice of NP, an effect which has numerous implications for the mechanical properties, permeability and other basic properties of polymer nanocomposites.