A qualitative analysis of particle-induced viscosity reduction in polymeric composites

Non-Einstein Rheology in Segmented Polyurethane Nanocomposites

“…As mentioned in the Introduction section, the free-volume increase, constraint release, the entanglement density decrease, the selective adsorption, and the local friction drop-based effects such as interfacial slippage under shear and ball-bearing were reported as possible mechanisms for viscosity reduction with nanoparticles. − ,, The nanoparticle-induced free-volume enhancement was first reported by Mackay et al as a possible mechanism to elucidate the viscosity drop of filled polystyrene melts. In fact, the additional excluded free volume around the nanoparticles can be considered as a stimulant to increase the chain mobility and hence the viscosity drop.…”

“…Finally, the possible origin of the observed viscosity reduction regarding PU-425-H40 by including C 60 nanoparticles was searched in the mechanisms based on the local friction reduction under shear such as slipping and ball-bearing effects. In fact, it is well known that nanoparticles can contribute to the viscosity reduction of polymer melts via reducing local friction, which can be implemented under shear by either interfacial slipping or ball-bearing effects. ,,− The latter effect has been interpreted as the reduction of friction between the polymer chains under shear by a small amount of rotating spherical particles when the polymer matrix behaves close to a Newtonian fluid . Such ball-bearing effect resulting from the particle rotation can be intensified at higher frequencies and shear rates as well as at higher filler loadings. , On the other hand, it is difficult to prove the slip between the polymer chains and nanoparticles (molecular interfacial slippage) by experiments under shearing condition.…”

“…However, this interfacial slippage as the vector difference in surface velocities between the nanoparticle and polymer chain can induce a lower viscosity due to the lower friction between the surface of particles and surrounding polymer chains in comparison to the friction between polymer chains. , Noteworthy, the slippage at the interfaces between nanoparticles and matrix chains is quite different from the slippage among polymer chains in pure matrix. In addition, the molecular interfacial slippage can be enhanced in nanocomposites with very weak interactions between polymer chains and nanoparticles (noninteracting interfaces). , …”

“…This non-Einstein-type behavior regarding the viscosity of complex fluids was initially reported for the filled linear polystyrene (PS) through adding nanoparticles such as the intermolecular cross-linked single-chain PS, fullerenes (C 60 ), magnetite Fe 3 O 4 , and cadmium selenide. − The important design strategies to achieve this anomalous behavior were outlined in five requirements: (1) chemical similarity of components to create athermal nanocomposites as much as possible, (2) presence of entangled polymer chains in the matrix, (3) well dispersion of nanoparticles in the matrix, (4) matrix chains with gyration radius, R g , larger than the nanoparticle radius, r , and (5) interparticle half-distance, h , smaller than the R g values of matrix chains. Following these seminal works, many studies regarding the non-Einstein viscosity reduction have been done using various model systems, which could not be readily explained by conventional theories. − Besides, several molecular dynamic simulations were focused on the essential parameters determining the nanoparticle-induced viscosity reduction in the polymeric nanocomposites, which was either in agreement or in conflict with the experimentally found results. − …”

“…Indeed, regardless of the enthalpic interactions in nanocomposites, the nanoparticles with comparable size to the matrix chains can greatly alter the chain conformations in a confined geometry depending on their concentration. , Hence, the nanoparticles lead to a constraint release phenomena where entanglements as the constraints imposed on the matrix chain dynamic are released thanks to the movement of surrounding molecules constituting the entanglement mesh. On the other hand, the friction coefficient change in the vicinity of nanoparticles was introduced as an effective factor on the local dynamics (hence, the anomalous viscosity reduction) due to the dependence of the polymer chain diffusion coefficient as well as shear viscosity on the local friction. ,,,,, In fact, adding a certain amount of nanoparticles with the radius smaller than entanglement mesh size to respective polymeric matrix can lead to the local friction decrease and thereby enhanced flowability. It should be noted that the exact relationship between the above-mentioned mechanisms for anomalous viscosity reduction is not yet clear, opening new challenge for scientists.…”