Direct Observation of Polymer Surface Mobility <i>via</i> Nanoparticle Vibrations

Understanding the mechanical properties of polymers nanoparticles (NPs) is essential to optimizing their performance and processing. The glass transition temperature (T_g) plays a crucial role in processing these and other polymeric materials. Polymer NPs show T_g deviations similar to that of polymer thin films, which has been attributed to the mobility of a surface mobile layer with reduced T_g. However, due to the lack of an experimental means, the existence of a surface mobile layer has yet to be directly verified. We studied the vibrational dynamics of polystyrene NP clusters by Brillouin light scattering. Since eigenmodes of the NP cluster are controlled by the elastic moduli and the interactions between NPs, the temperature dependent variation in the spectra allows us to not only measure T_g of NPs, but also to verify the presence of a surface mobile layer. Below T_g, the contact adhesion is enhanced due to an active surface mobile layer which introduces unique thermal variation of eigenmodes. The calculated phonon dispersion supports that this abnormal thermal transition is induced by sudden increase in adhesion area among neighboring NPs at a temperature T_s that is lower than the bulk T_g.