Rheological impacts of particle softness on wetted polymer-grafted silica nanoparticles in polymer melts

Well-characterized dispersions of silica nanoparticles, stabilized by end-tethered poly(dimethylsiloxane) (PDMS) chains in PDMS melts have been investigated to better understand the impact of the graft layer on bulk properties. The relative "softness'' of completely wet, dispersed particles is examined by varying the melt molecular weight (P = 2-13 kg/mol) and particle core radius (a(c) - 16-600 nm). As P decreases at constant graft molecular weight (N - 25 kg/mol) and graft density (Sigma approximate to 0.17 chains/nm(2)), enhanced stretching of the brush is supported by polymer scaling theory predictions. Using rheology, particle interactions for systems of varying particle volume fraction (i.e., interparticle separation distance) have been studied. Through steady shear and oscillatory rheological experiments, we have observed a liquid-solid transition that depends on particle softness, or the ratio of the brush height to particle core radius. At particle volume fractions above the liquid-solid transition, an increase in mechanical properties, namely storage modulus and relative viscosity, in lower molecular weight melts occurs due to higher interparticle repulsions from greater stretching of the brush. Moreover, plateau moduli, G'(infinity), for particle concentrations above the liquid-solid transition, scale with the interparticle separation distance with a dependence corresponding to a repulsive interaction potential. Ultimately, elucidation of the role of the graft layer on nanoparticles dispersed in polymer melts will aid in the formulation of nanomaterials in applications such as coatings, optics, catalysis, and plastics.