Role of Particle Size and Polymer Length in Rheology of Colloid–Polymer Composites

Photopolymerization Approach to Advanced Polymer Composites: Integration of Surface-Modified Nanofillers for Enhanced Properties

The incorporation of functionalized nanofillers into polymers via photopolymerization approach has gained significant attention in recent years due to the unique properties of the resulting composite materials. Surface modification of nanofillers plays a crucial role in their compatibility and polymerization behavior within the polymer matrix during photopolymerization. This review focuses on the recent developments in surface modification of various nanofillers, enabling their integration into polymer systems through photopolymerization. The review discusses the key aspects of surface modification of nanofillers, including the selection of suitable surface modifiers, such as photoinitiators and polymerizable groups, as well as the optimization of modification conditions to achieve desired surface properties. The influence of surface modification on the interfacial interactions between nanofillers and the polymer matrix is also explored, as it directly impacts the final properties of the nanocomposites. Furthermore, the review highlights the applications of nanocomposites prepared by photopolymerization, such as sensors, gas separation membranes, purification systems, optical devices, and biomedical materials. By providing a comprehensive overview of the surface modification strategies and their impact on the photopolymerization process and the resulting nanocomposite properties, this review aims to inspire new research directions and innovative ideas in the development of high-performance polymer nanocomposites for diverse applications.

Brittle and ductile yielding in soft materials

Many soft materials yield under mechanical loading, but how this transition from solid-like behavior to liquid-like behavior occurs can vary significantly. Understanding the physics of yielding is of great interest for the behavior of biological, environmental, and industrial materials, including those used as inks in additive manufacturing and muds and soils. For some materials, the yielding transition is gradual, while others yield abruptly. We refer to these behaviors as being ductile and brittle. The key rheological signatures of brittle yielding include a stress overshoot in steady-shear-startup tests and a steep increase in the loss modulus during oscillatory amplitude sweeps. In this work, we show how this spectrum of yielding behaviors may be accounted for in a continuum model for yield stress materials by introducing a parameter we call the brittility factor. Physically, an increased brittility decreases the contribution of recoverable deformation to plastic deformation, which impacts the rate at which yielding occurs. The model predictions are successfully compared to results of different rheological protocols from a number of real yield stress fluids with different microstructures, indicating the general applicability of the phenomenon of brittility. Our study shows that the brittility of soft materials plays a critical role in determining the rate of the yielding transition and provides a simple tool for understanding its effects under various loading conditions.

Crosslinking konjac-glucomannan with kappa-carrageenan nanogels: A step toward the design of sacrificial materials

The challenge in designing sacrificial materials is to obtain materials that are both mechanically stable and easily dissolvable. This research aimed to meet this challenge by fabricating a new polymer-nanogel hydrogel based solely on hydrogen bonds between two polysaccharides. The study focused on hydrogels formed from soluble konjac-glucomannan and nanogels synthesized from kappa-carrageenan. This novel hydrogel exhibited self-healing and shear-thinning properties due to its weak physical interactions. The hydrogel dissolved simultaneously with its swelling. Changes in temperature or nanogel concentration, or the addition of potassium ions, altered the swelling and dissolution rates. Furthermore, adding KCl to the as-prepared hydrogel increased its compression and tensile moduli and its strength. The new formulation opens numerous possibilities as a potential sacrificial material for different applications since it is mechanically stable yet rapidly dissolves in physiological conditions without applying high temperatures or using chelating agents.

Fast and reversible crosslinking of a silk elastin-like polymer

Elastin-like polymers (ELPs) and their chimeric subfamily the silk elastin-like polymers (SELPs) exhibit a lower critical solvation temperature (LCST) behavior in water which has been extensively studied from theoretical, computational and experimental perspectives. The inclusion of silk domains in the backbone of the ELPs effects the molecular dynamics of the elastin-like domains in response to increased temperature above its transition temperature and confers gelation ability. This response has been studied in terms of initial and long-term changes in structures, however, intermediate transition states have been less investigated. Moreover, little is known about the effects of reversible hydration on the elastin versus silk domains in the physical crosslinks. We used spectroscopic techniques to analyze initial, intermediate and long-term states of the crosslinks in SELPs. A combination of thermoanalytical and rheological measurements demonstrated that the fast reversible rehydration of the elastin motifs adjacent to the relatively small silk domains was capable of breaking the silk physical crosslinks. This feature can be exploited to tailor the dynamics of these types of crosslinks in SELPs. STATEMENT OF SIGNIFICANCE: The combination of silk and elastin in a single molecule results in synergy via their interactions to impact the protein polymer properties. The ability of the silk domains to crosslink affects the thermoresponsive properties of the elastin domains. These interactions have been studied at early and late states of the physical crosslinking, while the intermediate states were the focus of the present study to understand the reversible phase-transitions of the elastin domains over the silk physical crosslinking. The thermoresponsive properties of the elastin domains at the initial, intermediate and late states of silk crosslinking were characterized to demonstrate that reversible hydration of the elastin domains influenced the reversibility of the silk crosslinks.

Anomalous Terminal Shear Viscosity Behavior of Polycarbonate Nanocomposites Containing Grafted Nanosilica Particles

Viscosity controls an important issue in polymer processing. This paper reports on the terminal viscosity behavior of a polymer melt containing grafted nanosilica particles. The melt viscosity behavior of the nanocomposites was found to depend on the interaction between the polymer matrix and the nanoparticle surface. In the case of polycarbonate (PC) nanocomposites, the viscosity decreases by approximately 25% at concentrations below 0.7 vol% of nanosilica, followed by an increase at higher concentrations. Chemical analysis shows that the decrease in viscosity can be attributed to in situ grafting of PC on the nanosilica surface, leading to a lower entanglement density around the nanoparticle. The thickness of the graft layer was found to be of the order of the tube diameter, with the disentangled zone being approximately equal to the radius of gyration (Rg) polymer chain. Furthermore, it is shown that the grafting has an effect on the motion of the PC chains at all timescales. Finally, the viscosity behavior in the PC nanocomposites was found to be independent of the molar mass of PC. The PC data are compared with polystyrene nanocomposites, for which the interaction between the polymer and nanoparticles is absent. The results outlined in this paper can be utilized for applications with low shear processing conditions, e.g., rotomolding, 3D printing, and multilayer co-extrusion.

The role of solvent molecular weight in shear thickening and shear jamming

The application of stress can drive a dense suspension into a regime of highly non-Newtonian response, characterized by discontinuous shear thickening (DST) and potentially shear jamming (SJ), due to the formation of a frictionally stabilized contact network. Investigating how the molecular weight of the suspending solvent affects the frictional particle-particle interactions, we report on experiments with suspensions of fumed silica particles in polyethylene glycol (PEG). Focusing on the monomer-to-oligomer limit, with n = 1 to 8 ethylene oxide repeat units, we find that increasing n enhances shear thickening under steady-state shear and even elicits rapidly propagating shear jamming fronts, as assessed by high-speed ultrasound imaging of impact experiments. We associate this behavior with a weakening of the solvation layers surrounding the particles as n is increased, which thereby facilitates the formation of frictional contacts. We argue that for n larger than the monomer-to-oligomer limit the trend reverses and frictional interactions are diminished, as observed in prior experiments. This reversal occurs because the polymeric solvent transitions from being enthalpically bound to entropically bound to the particle surfaces, which strengthens solvation layers.

Universal relation between the density and the viscosity of dispersions of nanoparticles and stabilized emulsions

The effective viscosity of nanoparticle dispersions has been investigated experimentally quite a lot and various behaviours have been observed. Many models have been proposed to predict the effective viscosity, but these are mainly empirical ones, correlations with a tuning parameter or based on fastidious molecular interactions simulations. In this work, we propose a new fully physics-based analytical expression for the effective viscosity implementing theories from extended thermodynamics, including nano-confinement effects, nanoparticle-fluid interactions, density effects, size effects and nanoparticle volume fraction. We validate this model against several different types of nanoparticle dispersions and emulsions and explain the different behaviours using the same model. It appears that the density ratio of the nanoparticles with respect to the fluid plays a crucial role affecting the viscosity. The nanoparticle-fluid interactions become increasingly important for smaller nanoparticle sizes. From these comparisons, we arrive at a general simplified expression for the effective viscosity of nanoparticle dispersions, where it is observed that there is a direct universal relation between the nanoparticles and fluid densities and the nanodispersion viscosities. The validity of such a relation has been explicitly demonstrated.

Controlling the shear thickening behavior of suspensions by changing the surface properties of dispersed microspheres

To investigate the effect of the surface properties of dispersed particles on the shear thickening behavior of their corresponding suspensions and further control this characteristic, three kinds of suspensions were prepared by mixing SiO₂, SiO₂-NH₂, and SiO₂-COOH microspheres with a poly(ethylene glycol) fluid medium, and their rheological behavior was analyzed carefully. Compared to the SiO₂ microsphere suspension, the SiO₂-NH₂ and SiO₂-COOH microsphere suspensions show a weaker thickening behavior and a greater critical shear rate due to the aggregation tendency caused primarily by the organic chains. Moreover, the rheological behavior of the three suspensions display different dependencies on the pH value, which is comprehensively determined by the interaction between the microspheres and the medium. Moreover, the critical shear stress of suspensions with different pH values could be predicted by the Wagner model, which basically proves that the interaction between the particles significantly influences the beginning of thickening. The thickening degree could be interpreted by friction theory. The critical volume fraction corresponding to the onset of discontinuous shear thickening is determined by the friction coefficient between the particles, which is greatly affected by the pH value.

Polymer-free electrospinning of tannic acid and cross-linking in water for hybrid supramolecular nanofibres

Electrospinning is the process of choice allowing the preparation of nanofibrous materials from a solution usually based on a high molar mass polymer. The solution must bring enough chain entanglements to avoid any breaking or Rayleigh instability of the electrospun jet resulting thus in the deposition of a continuous and regular solid nanofibre. It has been however shown that some few non-polymeric molecules can be electrospun without using a carrier polymer. We demonstrate here the case of tannic acid. Indeed, it was possible to electrospin this molecule solubilised in a mixture of water and ethanol as well as in pure water. Rheology, dynamic light scattering and cryo-TEM highlight the formation of tannic acid aggregates in solution. Above a critical concentration, these aggregates form a supramolecular interconnected network strong enough to allow the electrospinning of a continuous and regular nanofibre. The resulting nanoweb is mechanically stable and can be handled and wrapped. Furthermore, as opposed to the other small molecules for which polymer-free electrospinning was also demonstrated, tannic acid nanowebs can be efficiently cross-linked in water either by oxidative reaction with sodium periodate or, most interestingly, with FeIII by a combination of oxidative reaction and the formation of coordination complexes. The proposed electrospinning and cross-linking strategy is easy, of low cost, and scalable and uses non-toxic solvents as well as biocompatible and biofunctional molecules. Furthermore, thanks to the chelation capacity of tannic acid having the ability to coordinate with a wide variety of metals, hybrid smart nanowebs can be envisaged for diverse applications such as biomedical, catalysis as well as environment.

Lubrication and plasticization behavior of large-size micro-spherical structured SiO2 for natural rubber

In many rubber dynamic applications such as tires and seals, imparting excellent flex fatigue properties and processing behavior are of prime importance. Research in this direction has been done based on a judicious choice of polymer type or a blend thereof and the compounding ingredients. In this study, the effect of micro-spherical SiO₂ on the flex fatigue properties and processing behavior of natural rubber (NR) is studied. Two different particle sizes of spherical SiO₂ (N90, average diameter: 200 nm and N98, average diameter: 120 nm) were used to optimize the flex fatigue properties and processing behavior, and the mechanism is investigated. In this blend, 5 phr loading of N90 was effective in imparting the best overall combination of properties. This work was aimed at providing some theoretical basis and application basis for the use of micro-spherical SiO₂ in the rubber industry.

In many rubber dynamic applications such as tires and seals, imparting excellent flex fatigue properties and processing behavior are of prime importance. Micro-spherical N90 and N98 play boundary lubrication action during rubber processing.

Drying-mediated patterns in colloid-polymer suspensions

Drying-mediated patterning of colloidal particles is a physical phenomenon that must be understood in inkjet printing technology to obtain crack-free uniform colloidal films. Here we experimentally study the drying-mediated patterns of a model colloid-polymer suspension and specifically observe how the deposit pattern appears after droplet evaporation by varying particle size and polymer concentration. We find that at a high polymer concentration, the ring-like pattern appears in suspensions with large colloids, contrary to suppression of ring formation in suspensions with small colloids thanks to colloid-polymer interactions. We attribute this unexpected reversal behavior to hydrodynamics and size dependence of colloid-polymer interactions. This finding would be very useful in developing control of drying-mediated self-assembly to produce crack-free uniform patterns from colloidal fluids.

Intrinsic nonlinearities in the mechanics of hard sphere suspensions

The onset of nonlinear responses in near hard sphere suspensions is characterized as a function of oscillatory frequency and strain amplitude. At low frequencies where the viscous behavior dominates, the onset of nonlinearities is driven by increases in rate of strain. At high deformation frequency, where suspension mechanics is dominated by an elastic response, the nonlinear responses occur when deformation exceeds a characteristic strain. This strain is associated with the transient confinement of particles by nearest neighbors and its volume fraction dependence is through cage parameters derived from the high frequency elastic modulus. The onset of nonlinear responses takes on a universal behavior when deformation frequency is normalized by the characteristic time governing the shift from viscous to elastic behavior indicating that this transition is associated with transient particle localization and is expected to be observed for all volume fractions where pair interactions are important.

Effect of Temperature on the Shear-Thickening Behavior of Fumed Silica Suspensions

Shear-thickening fluids (STFs) can be subjected to a significant temperature variation in many applications. Polymeric or oligomeric fluids are commonly used as suspending media for STFs. Because the viscosities of polymeric fluids are strongly temperature-dependent, large temperature changes can profoundly affect the shear-thickening responses. Here, the effect of temperature on the shear-thickening behavior of four low-molecular-weight polymeric glycols/fumed silica suspensions is reported. The dispersed-phase volume fraction, its surface chemistry, and the chemical compositions of the suspending media were varied. These factors influence the viscosity and the interactions between the suspended particles and the suspending media. Fumed silica particles with two different silanol-group surface densities were suspended in the polymeric glycols, where these silanol surface groups formed hydrogen bonds with the suspending media's glycols and internal oxygen atoms. Steady-shear experiments were performed over a temperature range spanning approximately 100 °C. The critical shear rate for the onset of shear thickening decreased with decreasing temperature. The critical shear rates were inversely proportional to the viscosity of the pure suspending media over these same temperature ranges. The response of STFs to varying both the temperature and shear rate investigated here will help to design application-specific STFs. Mitigation of a hypervelocity (6.81 km/s) impact on an aluminum facesheet sandwich composite filled with one of these STFs was demonstrated.

Impact of small changes in particle surface chemistry for unentangled polymer nanocomposites

We report microstructural and rheological consequences of altering silica particle surface chemistry when the particles are suspended in unentangled polyethylene glycol with a molecular weight of 400. The particle surfaces are altered by reacting them with isobutyltrimethyoxysilane. Levels of silanization are chosen so that the particles remain dispersed in the polymer at all volume fractions studied. Our studies indicate that at the levels studied, silanization does not alter the hydrodynamic thickness of the absorbed polymer layer thickness. Rheological properties are not sensitive to levels of silanization up to particle volume fractions where the average particle separation h ∼ 6Rg (4.8 nm). At these volume fractions, composite microstructure undergoes changes associated with jamming of soft particles (decorrelations in the first peak of the particle structure factor and the onset of a non-diffusive mechanism that dominates particle density fluctuations at short times.) In the region of volume fractions where h/Rg < 6, the zero-shear rate viscosity of the composites is extremely sensitive to level of silanization with a decrease in the zero-shear rate viscosity by four orders of magnitude observed for the highest levels of silanization studied in comparison to the bare particles.

Temperature induced gelation transition of a fumed silica/PEG shear thickening fluid

An interesting gelation transition of fumed SiO₂/PEG shear thickening fluid induced by elevating the temperature.

Bound Polymer Layer in Nanocomposites

There has been considerable interest in characterizing the polymer layer that is effectively irreversibly bound to nanoparticles (NPs) because it is thought to underpin the unusual thermomechanical properties of polymer nanocomposites (PNC). We study PNCs formed by mixing silica nanoparticles (NPs) with poly-2-vinylpyridine (P2VP) and compare the bound layer thickness δ determined by three different methods. We show that the thickness obtained by thermogravimetric analysis (TGA) and assuming that the bound layer has a density corresponding to a dense melt clearly underestimates the real bound layer thickness. A more realistic extent of the bound layer is obtained by in situ measurements of the interaction pair potential between NPs in PNCs via analysis of TEM micrographs; we verify these estimates using Dynamic Light Scattering (DLS) in θ solvent. Our results confirm the existence of long-ranged interactions between NPs corresponding roughly in size to the radius of gyration of the bound chains.