Molecular Origins of Temperature-Induced Jamming in Self-Suspended Hairy Nanoparticles

Abstract

Suspensions of solvent-less, polymer-grafted nanoparticles have previously been reported to exhibit enhanced elasticity, glassy dynamics, and jamming as temperature is increased. This so-called thermal jamming behavior is not predicted by current models for soft glassy materials and is opposite to what is normally observed for polymer melts. Here we report results from a detailed study of structural, dynamic, and rheological transitions in solvent-less silica–poly­(ethylene glycol) methyl ether (SiO2–PEGME) hybrid particles aimed at understanding the molecular origins of the phenomenon. We find that interdigitated PEGME corona chains enforce coupling between SiO2 cores analogous to cross-links in associating polymer networks leading to elastic behavior at low strains and small deformation rates. Pullout and slippage of interdigitated corona chains is thought to produce yielding of the materials in nonlinear shear flow and stretched exponential relaxation following small amplitude step shear. We show with the help of small-angle X-ray scattering, density functional theoretical calculations, and a constitutive model previously reported for deformation and flow of physically associating polymer networks, that thermal jamming of solvent-less polymer-grafted nanoparticles originates from enhanced cross-linking of the cores produced by increased stretching and interpenetration of particle-tethered polymer molecules to satisfy the space-filling constraint on tethered polymer chains.

Transient inhomogeneous flow patterns in supercooled liquids under shear

“…As these stress relaxation events become more frequent, they balance the elastic response accompanied by a decrease of the accumulated stress to level o↵ to a stationary value. Similar stress strain curves are also observed for polymer gels and hairy nanoparticles 43,44 where the drop in the stress-strain curve is associated with the breaking of bonds in the microstructure as a response to the applied force. Likewise, many complex fluids exhibit a well-defined microstructure (e.g.…”

Particle localization and hyperuniformity of polymer‐grafted nanoparticle materials

“…. This increase in the long range correlations in for seems superficially similar to “thermal jamming” of GNPs , where increased particle localization occurs with increasing T and causes particle jamming. However, in the present system, this structural behavior results in a speeding up rather than slowing down the structural relaxation so that this is not equivalent to thermal jamming.…”

Structural considerations for physical hydrogels based on polymer–nanoparticle interactions

“…This characteristic G″ maximum is commonly seen in a variety of soft glassy materials (e.g., pastes, emulsions, and soft particle glasses) and, in the case of polymer-nanoparticle composites, has been attributed to the local yielding ("uncaging") of jammed particles. 25,[33][34][35][36][37][38] Analogously, in the PNP hydrogels, uncaging occurs when jammed polymer-nanoparticle units locally yield and dissipate stress. The area under the G″ peak can be estimated as the energy dissipated per unit volume for these uncaging events.…”

“…The area under the G″ peak can be estimated as the energy dissipated per unit volume for these uncaging events. 25,38 The existence of a G″ peak in gels comprising HPMC-C 12 , and not in those comprising HPMC or HPMC-C 6 , implies the increased hydrophobicity of C 12 pendant groups induce a jamming transition. This G″ peak can be normalized and fit to a log-normal curve to calculate an energy of uncaging (U) of ∼24 kJ m −3 (Fig.…”

“…[19][20][21]23 PNP hydrogels form when addition of nanoparticles to polymers results in crosslinking and corresponding enhancements in elasticity, which is commonly experimentally observed as an increase in the shear storage modulus (G′), an increase of relative elasticity (i.e., a decrease in tanIJδ), as defined as the ratio between the shear loss and storage moduli, G″/G′), longer bulk relaxation times (τ), and the emergence of a functional yield stress (σ y ). 24 The origins of these changes can be attributed to two main contributions: [24][25][26] (1) The formation of transient networks which are driven by energetically favourable polymer-nanoparticle interactions leading chains to bridge between particles, and (2) jamming of polymernanoparticle unit components (defined here as a particle and the locally bound polymer corona). Although this generalization is useful for broad discussions of PNP hydrogels, the relative contribution of each mechanism is highly dependent on formulation variables such as particle size, polymer molecular weight and hydrodynamic size, PNP interaction strength, concentration of each component, and total occupied volume fraction.…”