Rheological Behavior of Concentrated Hyperbranched Polymer/Silica Nanocomposite Suspensions

Abstract

The rheological behavior of two hyperbranched polymer/silica suspensions with different dispersion states, surface chemistries, and concentrations of the silica nanoparticles was investigated in terms of viscoelastic properties, activation energy for viscous flow, and yield stress. The viscoelastic properties of both types of suspensions were reduced to a master curve that was a function of the limiting viscosity and shear modulus. A liquid-to-solid transition and correlated activation energy change were found to occur for particle volume fraction in the range of 5-10% for well-dispersed systems and 20-25% for systems where silylated particles were agglomerated. The viscosity of the suspensions was found to be considerably higher than that predicted by the classical percolation model for concentrated particle suspensions; this was argued to result from an immobilized layer of polymer on the surface of the silica particles. The percolation model was therefore modified to include such confined layer in order to predict the viscosity as a function of filler fraction. In the case of silylated particles with weak interactions with the polymer, the model based on an immobilized layer of thickness in the range of 2-5 nm reproduced the data. In the case of well-dispersed particles with strong interfacial interactions, the immobilized layer was correlated to the average distance between adjacent particles. In this case the model predicted an exponential increase of the viscosity with particle fraction and that the whole matrix gelled at particle concentrations larger than 5 vol %, corresponding to a 7.5 nm thick immobilized layer.

UV-nanoimprint lithography and large area roll-to-roll texturization with hyperbranched polymer nanocomposites for light-trapping applications

“…2 was obtained with B¼5.25, which is among the highest values for B reported in former studies of particulate composites, including values close to 6 for polypropylenesilica nanocomposites [44]. Such a high value reflects the presence of strong interactions at the particle-polymer interface [24], exacerbated by the very high specific interface area (proportional to the particle volume fraction f and close to 40 m 2 /g for f¼0.1). Fig.…”

“…This finding is consistent with the fact that the HBP nanocomposites are yield stress fluids, which start to flow under a stress above a threshold 'yield' stress. The yield stress of the nanocomposite with 20 vol% of silica is equal to approximately 50 kPa [24], which is indeed lower than the lowest investigated pressure of 1 bar. A pressure as low as 50 kPa (0.5 bar) should therefore be sufficient to ensure complete filling of the texturized mold.…”

“…However, the considerable increase of viscosity of the HBP precursor monomer upon addition of nanoparticles may compromise the low-pressure replication process. For instance, the viscosity of HBP increased by 5 orders of magnitude upon addition of 10 vol% of 13 nm silica particles due to H-bond interactions [24]. The replication fidelity could also be compromised because the internal stress increases with nanoparticle fraction [25,26].…”

Low‐Stress Hyperbranched Polymer/Silica Nanostructures Produced by UV Curing, Sol/Gel Processing and Nanoimprint Lithography

“…The distance between nanoparticles in a suspension rapidly becomes smaller than the particle radius when the particle volume fraction goes beyond a few percent. [5] Further benefits of nanocomposites include a lower polymerization shrinkage with respect to the pure resin [6] and their transparency to visible and UV light, which is especially important if photopolymerization is used. As a result, nanocomposites are more and more used as photoresists, [7] thermally [8] and UV [9] imprintable resists of dimensionally stable and high precision nanostructures.…”

“…For instance, a 200-fold increase in viscosity was found when %1.6 vol% fumed silica was added to a cyanate ester, [10] and the increase was by more than five orders of magnitude when 20 vol% of silica nanoparticles were added to a hyperbranched polymer (HBP). [5] The liquid-to-solid transition is a major challenge for nanocomposite processing and is often overcome with the use of solvents. An alternative route to overcome processing problems of nanocomposites is the use of an organometallic liquid precursor, which forms an inorganic phase in situ in the polymer matrix through sol/gel condensation reactions.…”