Rheology and Microstructure of an Unentangled Polymer Nanocomposite Melt

Abstract

The rheology and microstructure of 44 nm diameter silica particles suspended in unentangled polyethylene oxide (PEO) melts are studied through measurement of filled melt viscosity and X-ray scattering measurement of interparticle structure factors, S(q,φ c ), where q is the scattering vector and φ c is the silica volume fraction. The neat polymer melts are Newtonian over the probed shear range. Filled melts have a constant viscosity at low particle concentrations and shear thin at high concentrations. At the same particle volume fraction, filled melt viscosities increase with polymer molecular weight. By defining an effective particle volume, assuming polymer adsorption adds 2.9R g to the particle diameter, suspension viscosities of both molecular weights at all volume fractions resemble that of hard sphere suspensions. The particle osmotic compressibility and position and coherence of the first nearest neighbor shell suggest that the particles have radii larger than the core size due to polymer structuring near the particle surface. These results are interpreted as resulting from a modest strength of attraction between polymer segments and particle surfaces.

Rheological Behavior of Concentrated Hyperbranched Polymer/Silica Nanocomposite Suspensions

“…As an example, a 200-fold increase in viscosity was found when 3.4 wt % fumed silica was added to a cyanate ester. 21 Similar findings were reported for entangled 22 and nonentangled 23 polymer melts. The present work is devoted to the case of hyperbranched polymers 24 (HBP), which would a priori pertain to the nonentangled case, the difference being essentially the much higher molecular weight and size of HBP compared to nonentangled oligomers.…”

“…Immobilized layers were reported to increase the hydrodynamic radius of the particles in proportion with the radius of gyration of the molecules (e.g., ref 23). The impact of an immobilized polymer layer on the rheological properties of polymer/particle suspensions was described in earlier studies.…”

“…The fraction φ* can be considered equal to the maximum packing fraction φ cp = π/ √ 18 = 0.7405 for closepacked and φ rcp ∼ 0.64 for random close-packed monodisperse spheres 16 and decreases rapidly with increasing particle anisotropy. For the semidilute and concentrated regimes up to φ* an asymptotic relationship between the relative viscosity η r and φ was proposed by Krieger and Dougherty (so-called hard-sphere model 51 ) and is frequently used: 23,36,37,40,55,56 …”

Rheology and Microstructure of Entangled Polymer Nanocomposite Melts

“…25 However, in the entangled nanocomposite melts, the viscosity rises above that expected for hard spheres with effective sizes of D η = D c (1 þ 2.82R g /D c ), indicating that both pair and higher order interactions appear larger than expected of hard spheres of size D η .…”

“…Results for PEO400 and PEO1000 were reported previously and are shown here again for comparison with the entangled nanocomposites. 25 In Table 2, k is shown to increase with molecular weight meaning that the particle intrinsic viscosity is increasing as the molecular weight is raised.…”