An official website of the United States government
The incorporation of nanoparticles (NPs) into polymers is one of the most effective strategies for reinforcing polymeric materials. The impact of NP reinforcement is exemplified by automobile tires, where it enhances a broad range of performance metrics, including fracture toughness, wear resistance, wet traction, rolling resistance, and fatigue durability. Reinforcement arises from the coupled evolution of hierarchical structures and hierarchical dynamics of polymers and NPs across multiple length and time scales. A comprehensive understanding of reinforcement mechanisms remains elusive. The advancement of PNCs depends on fundamental research investigating the hierarchical structures and dynamics of polymers and NPs by employing integrated measurements, analysis, and predictions. In this Perspective, the emphasis is placed on elucidating the hierarchical dynamics of PNCs at the nano-to-mesoscale through the integration of neutron spin echo (NSE) and X-ray photon correlation spectroscopy (XPCS). We highlight recent advances in understanding the hierarchical dynamics of bound polymer, nanoparticle networks, and cross-linked polymer networks. NSE combined with isotope labeling provides unique access to the segmental and chain dynamics─including entanglement dynamics─of labeled polymer chains across length scales from subnanometers to tens of nanometers and time scales from picoseconds to several hundred nanoseconds. In contrast, XPCS provides unique access to the collective dynamics of hierarchical structures of NPs in a polymer matrix over relevant time scales (submilliseconds to 1000s of seconds) and length scales (nanometers to hundreds of nanometers). Furthermore, recent advancements in in operando XPCS have enabled the investigation of the nonequilibrium evolution of structures and dynamics in PNCs under industrially relevant processing conditions. Together with complementary computational and experimental approaches, these advanced neutron and X-ray scattering techniques establish quantitative hierarchical structure–dynamics–property relationships that provide a molecular foundation for the rational design of next-generation PNCs.
more »
« less
This content will become publicly available on June 16, 2027
Characterizing emergent multiscale dynamics in colloidal nanoparticle gels
Colloidal gels assembled from nanoparticles (NPs) are a versatile class of soft network-based materials capable of rich dynamic, mechanical, and even optical or magnetic responses to stimuli. Their behaviors are governed by dynamics of heterogeneous structures coupled across multiple length and timescales. Observable dynamics range from nanoparticle diffusion and clustering to mesoscopic cluster dynamics and interactions to localized or collective network relaxations. Understanding how these hierarchically organized processes relate to macroscopic network properties remains a broad and unresolved problem in soft matter physics. The mechanisms of gel formation can depend sensitively on the pathway and the nature of NP interactions, thus far preventing a unified theoretical bridge between nanoscopic interactions, structural evolution, and network dynamics. Indirect measurement of dynamics using light-scattering techniques provides an experimental means to quantify underlying particle and network motion. The rich dynamic behavior of NP gels warrants consideration of a broad range of models to help interpret nonlinear relaxation phenomena such as anomalous diffusion, nonergodicity, and intrinsically nonequilibrium or mechanically driven dynamics. X-ray photon correlation spectroscopy (XPCS) has emerged as a powerful tool for probing nanoscopic motion in nanoparticle gels but alone cannot resolve the full spatiotemporal spectrum of dynamics that drive gelation, aging, and network mechanical properties. While rheo-XPCS enables simultaneous probing of nanoscale and bulk mechanical responses, complementary light scattering, microscopy, or simulations can extend spatiotemporal characterization and, consequently, understanding of NP gel network physics. Implementing a modular model platform with tunable primary nanoparticle features allows systematic variation of nanoscopic characteristics that drive emergent gel responses and inform the development of theoretical models for a wide range of soft, dynamic, nanostructured materials. Gels formed from particles with unique structural proxies, such as electromagnetic coupling in plasmonic NPs, provide additional metrics for model validation and offer opportunities to develop computational methods for the efficient and accurate replication of NP gel properties. The rapid expansion of XPCS capabilities at fourth-generation light sources, combined with complementary tools and robust model systems, positions the field to move beyond descriptive fundamental studies toward the design of nanoparticle gels with adaptive and programmable behaviors.
more »
« less
- Award ID(s):
- 2323482
- PAR ID:
- 10699256
- Publisher / Repository:
- Physical Review E
- Date Published:
- Journal Name:
- Physical Review E
- Volume:
- 113
- Issue:
- 6
- ISSN:
- 2470-0045
- Page Range / eLocation ID:
- 061001
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
More Like this
-
-
Supramolecular polymer gels are an evolving class of soft materials with a vast number of properties that can be tuned to desired applications. Despite continuous advances concerning polymer synthesis, sustainability or adaptability, a consistent understanding of the interplay between structure, dynamics, and diffusion processes within transient networks is lacking. In this study, the hierarchy of several relaxation processes is investigated, starting from a microscopic perspective of a single sticker dissociation event up to the center-of-mass diffusion of a star-shaped polymer building block on different length scales, as well as the resulting macroscopic mechanical response to applied external stress. In addition to that, a second focus is placed on the gel micro-structure that is analyzed by light scattering. Conversion of the dynamic light scattering (DLS) inverse length scale into real space allows for a combination of relaxation times with those obtained by forced Rayleigh scattering (FRS). For these investigations, a model-type metallo-supramolecular network consisting of narrowly dispersed tetra-arm poly(ethylene glycol)-terpyridine macromolecules that are interconnected via complexation with zinc ions is chosen. Assembling the obtained activation energies reveals that all complex dissociation-governed relaxation processes exhibit similar activation energies.more » « less
-
We develop a simple model to probe the ion transport and mechanical properties of low volume fraction colloidal nanoparticle gels. Specifically, we study the influence of the morphology of gels on ion diffusion and the corresponding roles of affinity to and enhanced ion transport along nanoparticle surfaces. We employ kinetic Monte Carlo simulations to simulate ion transport in the colloidal gels, and we perform nonequilibrium molecular dynamics to study their viscoelastic behavior. Our results indicate that in the presence of enhanced diffusion pathways for ions along the particle surface, morphology has a significant influence on the diffusivity of ions. We demonstrate that some gel morphologies can exhibit simultaneously enhanced ion transport and mechanical properties, thus illustrating a strategy to decouple ion transport and mechanical strength in electrolytes.more » « less
-
Single particle tracking (SPT) of PEG grafted nanoparticles (NPs) was used to examine the gelation of tetra poly(ethylene glycol) (TPEG) succinimidyl glutarate (TPEG-SG) and amine (TPEG-A) terminated 4-armed stars. As concentration was decreased from 40 to 20 mg mL −1 , the onset of network formation, t gel , determined from rheometry increased from less than 2 to 44 minutes. NP mobility increased as polymer concentration decreased in the sol state, but remained diffusive at times past the t gel determined from rheometry. Once in the gel state, NP mobility decreased, became sub-diffusive, and eventually localized in all concentrations. The NP displacement distributions were investigated to gain insight into the nanoscale environment. In these relatively homogeneous gels, the onset of sub-diffusivity was marked by a rapid increase in dynamic heterogeneity followed by a decrease consistent with a homogeneous network. We propose a gelation mechanism in which clusters initially form a heterogeneous structure which fills in to form a fully gelled relatively homogenous network. This work aims to examine the kinetics of TPEG gelation and the homogeneity of these novel gels on the nanometer scale, which will aid in the implementation of these gels in biomedical or filtration applications.more » « less
-
null (Ed.)Colloidal gels represent an important class of soft matter, in which networks formed due to strong, short-range interactions display solid-like mechanical properties, such as a finite low-frequency elastic modulus. Here we examine the effect of embedded active colloids on the linear viscoelastic moduli of fractal cluster colloidal gels. We find that the autonomous, out-of-equilibrium dynamics of active colloids incorporated into the colloidal network decreases gel elasticity, in contrast to observed stiffening effects of myosin motors in actin networks. Fractal cluster gels are formed by the well-known mechanism of aggregating polystyrene colloids through addition of divalent electrolyte. Active Janus particles with a platinum hemisphere are created from the same polystyrene colloids and homogeneously embedded in the gels at dilute concentration at the time of aggregation. Upon addition of hydrogen peroxide – a fuel that drives the diffusiophoretic motion of the embedded Janus particles – the microdynamics and mechanical rheology change in proportion to the concentration of hydrogen peroxide and the number of active colloids. We propose a theoretical explanation of this effect in which the decrease in modulus is mediated by active motion-induced softening of the inter-particle attraction. Furthermore, we characterize the failure of the fluctuation–dissipation theorem in the active gels by identifying a discrepancy between the frequency-dependent macroscopic viscoelastic moduli and the values predicted by microrheology from measurement of the gel microdynamics. These findings support efforts to engineer gels for autonomous function by tuning the microscopic dynamics of embedded active particles. Such reconfigurable gels, with multi-state mechanical properties, could find application in materials such as paints and coatings, pharmaceuticals, self-healing materials, and soft robotics.more » « less
