Effects of Molecular Weight below the Entanglement Threshold on Interfacial Nanoparticles/Polymer Dynamics

This work deals with effects of polymer molecular weight, <i>W</i>_m, below the entanglement threshold, <i>W</i>_m,e, on molecular dynamics of polydimethyl­siloxane (PDMS) adsorbed onto silica particles, employing differential scanning calorimetry (DSC) and two dielectric techniques: broadband dielectric spectroscopy (BDS) and thermally stimulated depolarization currents (TSDC). The rigid amorphous polymer fraction at interfaces, RAF_int, was found suppressed for larger <i>W</i>_m by all techniques in qualitative agreement with each other. Results on RAF_int were supported by evaluating, for the first time, the coverage of hydroxyls at the surfaces of nanoparticles by polymer chains (<i>S</i> relaxation). The mobility of interfacial polymer (α_int relaxation) was followed by BDS and TSDC, showing suppression of dynamics and cooperativity with decreasing <i>W</i>_m. We suggest that interfacial polymer fraction and dynamics are dominated by the concentration of polymer–particle contact points, the latter increasing for smaller <i>W</i>_<i>m</i> due to more free chain ends, as expected below <i>W</i>_m,e. Furthermore, adopting models that describe multiple conformations for polymers adsorbed on solid surfaces, we explain our results in terms of promotion of tail/loop-like conformations in the particle–polymer interfacial layer for shorter/longer polymer chains, respectively. The model was further checked by employing surface modification of initial silica, which resulted in smoothening of nanoparticle surface and led to further suppression of RAF_int and interfacial polymer dynamics