Evidence of the Transition from a Flexible to Rigid Percolating Network in Polymer Nanocomposites

Abstract

The formation of a percolating filler network (PFN) in polymer nanocomposites is critical in explaining the substantial improvement of their mechanical properties in comparison to neat polymer systems. However, the structural mechanism underlying the formation and development of the PFN has remained under active debate. Here, we report a direct observation of microstructure development of the PFN in a model isoprene rubber filled with various nanoparticle fractions by means of nanoscale loss tangent (tan δ) imaging with amplitude-modulated atomic force microscopy. Tanδ imaging revealed, for the first time in real space, the formation of a flexible PFN when the filler volume fraction was increased to ∼16%, which subsequently developed to a rigid PFN at the fraction of ∼24%. Importantly, the observed transition agrees well with a marked increase in the mechanical reinforcement in macroscopic measurements.

Mechanical Properties of Polyisoprene-Based Elastomer Composites

“…As the filler fraction increases beyond 10 wt %, however, we observe significant differences in mechanical reinforcement among the different morphologies. 58 In the limits of high or low graft densities (samples A−F), we observe minimal reinforcement, even at the highest NP loading. These samples show a behavior similar to that predicted by the Guth−Gold model.…”

Quantitative Correlation between Hierarchical Nanofiller Structure and Rheology of Polymer/Fumed Silica Nanocomposites

“…It is well known that nanoparticles can affect the whole viscoelastic spectrum over a wide range of time (or frequency) scales; 22−27 however, the reinforcement was mostly discussed under specific states of polymers, namely, the glassy state, 26,28 the rubbery state, 26,27,29 and the viscous flow state. 16,22,24,25 In the rubbery zone of polymers, reinforcement mechanisms are complicated due to the interaction between nanoparticles and polymers.…”

“…23,24,37 Overlap of the glassy layer at high filler concentration results in bridging chains percolated glassy domains, significantly increasing the rubbery plateau modulus. 27,32,38 The conformation of the adsorbed chains also plays an important role. The loop conformation may induce additional entanglements with matrix chains and increases the rubbery plateau modulus due to higher entanglement density near particles.…”

“…It is well known that nanoparticles can affect the whole viscoelastic spectrum over a wide range of time (or frequency) scales; − however, the reinforcement was mostly discussed under specific states of polymers, namely, the glassy state, , the rubbery state, ,, and the viscous flow state. ,,, In the rubbery zone of polymers, reinforcement mechanisms are complicated due to the interaction between nanoparticles and polymers. ,,,,− The spatial confinement and the attractive interaction lead to a bound polymer layer, , where segmental dynamics are greatly suppressed. , The constrained polymer around particles with a shift of the glass transition temperature ( T g ) in the vicinity of particles could increase the effective radius of particles and contributes to the enhancement of the plateau modulus in PNCs via the increment of effective filler volume fraction. ,, Overlap of the glassy layer at high filler concentration results in bridging chains percolated glassy domains, significantly increasing the rubbery plateau modulus. ,, The conformation of the adsorbed chains also plays an important role. The loop conformation may induce additional entanglements with matrix chains and increases the rubbery plateau modulus due to higher entanglement density near particles. , The effect of interfacial chain conformation was found to rely on the effective specific surface area of particles, , which can be readily evaluated for well-dispersed nanoparticles.…”

Gelation Dynamics during Photo-Cross-Linking of Polymer Nanocomposite Hydrogels

“…The mechanical properties of polymer hydrogels can be further tailored by incorporating nanomaterials to form polymer nanocomposite hydrogels. , Mechanical reinforcement emerges from the adsorption of polymer chains to nanomaterial surfaces. − Nanomaterial–polymer interactions increase the density of effective cross-links throughout the polymer nanocomposite hydrogel, thereby increasing the bulk elastic modulus. − Interestingly, nanocomposite stiffness is independent of nanomaterial size and governed by the volume fraction ϕ nano according to the Guth–Gold model: G ∞ ′ G 0 , ∞ ′ = 1 + 2.5 ϕ nano + 14.1 ϕ nano 2 where G ∞ ′ and G 0,∞ ′ are the elastic moduli of polymer networks with and without added nanomaterials, respectively.…”