Giovanni Romeo

Temperature‐Controlled Transitions Between Glass, Liquid, and Gel States in Dense p‐NIPA Suspensions

In this article we demonstrate that concentrated p-NIPA suspensions in a low temperature glassy state can liquefy and then solidify again as the temperature is raised across the LCST. Our system exhibits all the typical behavior of disordered colloidal suspensions, but the behavior is controlled by temperature. Below the LCST it shows the behavior typical of a colloidal glass, near the LCST it behaves like a liquid, while above the LCST it exhibits the properties typical of a colloidal gel. Moreover, we show that the elasticity of these suspensions exhibits critical-like behavior as a function of temperature both above and below the LCST, with a critical temperature that corresponds to the LCST. Our results thereby suggest interesting analogies between the glass and gel phases of these thermosensitive microgel particles.

Viscoelasticity and Structure of Polystyrene/Fumed Silica Nanocomposites: Filler Network and Hydrodynamic Contributions

We investigate the relationships between structure and linear viscoelasticity of a model polymer nanocomposite system based on a mixture of fumed silica nanoparticles and polystyrene. Alterations in the viscoelastic behavior are attributed to the structuring of primary silica aggregates. Above a critical filler volume fraction, a space-filling network builds up as the result of cluster aggregation, and the complex frequency-dependence of the moduli is simplified by splitting the viscoelasticity of the composites into the independent responses of the suspending polymer melt and the filler network. Specifically, we present a refinement of a two-component model recently proposed for attractive colloidal suspensions, in which hydrodynamic effects related to the presence of the filler are properly taken into account using the concept of shear stress equivalent deformation. Our approach, validated through the building of a master curve of the elastic modulus for samples of different composition, allows the estimation of the elasticity of samples in which the filler network is too tenuous to be appreciated through a simple frequency scan. In addition, the structure of the filler network is studied using both the percolation and fractal approaches, and the reliability of the critical parameters is discussed. We expect that our analysis may be useful for understanding the behavior of a wide variety of complex fluids where the elasticity of the components may be superimposed.

Origin of de-swelling and dynamics of dense ionic microgel suspensions.

A direct consequence of the finite compressibility of a swollen microgel is that it can shrink and deform in response to an external perturbation. As a result, concentrated suspensions of these particles exhibit relaxation dynamics and rheological properties which can be very different with respect to those of a hard sphere suspension or an emulsion. We study the reduction in size of ionic microgels in response to increasing number of particles to show that particle shrinkage originates primarily from steric compression, and that the effect of ion-induced de-swelling of the polymer network is negligible. With increasing particle concentration, the single particle dynamics switch from those typical of a liquid to those of a super-cooled liquid and finally to those of a glass. However, the transitions occur at volume fractions much higher than those characterizing a hard sphere system. In the super-cooled state, the distribution of displacements is non-gaussian and the dependence of the structural relaxation time on volume fraction is describable by a Volger-Fulcher-Tammann function.

Universal features of the melt elasticity of interacting polymer nanocomposites.

We study the structure and linear viscoelasticity of interacting polymer nanocomposites based on mixtures of poly(ethylene oxide) and fumed silica particles. The filler is dispersed within the polymer using two different techniques which lead to different dispersion states. The analysis of the dynamic response of our systems highlights the formation of a stress-bearing network above a critical volume fraction, Φ(c). Extending a two-phase model used to describe weakly interacting systems, we show that above Φ(c) the melt-state elasticity of the composites arises from the independent contributions of a polymer-particle network and a viscous matrix. We also find that, although Φ(c) depends on the initial state of dispersion, the network elasticity scales with volume fraction following a universal power-law, with an exponent ν ≈ 1.8. Such a scaling law has been recently predicted for the stress-bearing mechanism governed by polymer-mediated interactions.

Highly Efficient Surface-Enhanced Raman Scattering Substrate Formulation by Self-Assembled Gold Nanoparticles Physisorbed on Poly(N-isopropylacrylamide) Thermoresponsive Hydrogels

Employing thermoresponsive hydrogels as scaffolding material for noble metal surface loading might be useful for the fabrication of surface-enhanced Raman scattering (SERS) surfaces. Here, we report on a new, reproducible, and simple approach to engineer poly(N-isopropylacrylamide) (PNIPAAm) hydrogel surfaces optimized for physisorption of gold nanoparticles (AuNPs). The advantage of this approach consists of the simple mechanism by which AuNPs are adsorbed on hydrogel templates, without sophisticated chemical treatments for their conjugation with the hydrogel. The resulting PNIPAAm-40 nm AuNP modes demonstrate that this approach gives the capability to tune the interparticle distance and, therefore, to control and modulate SERS affinity upon temperature changing.

Elasticity of compressed microgel suspensions

We describe the elasticity of compressed microgel suspensions starting from a core–shell model for single particles. The mechanical properties of the inner core and of the outer corona are described via the mean field Flory theory and via the Alexander–de Gennes model for polymer brushes, respectively. The model successfully reproduces experimental measures of the elastic shear modulus up to a constant factor that we rationalize in the jamming perspective.

ELASTICITY AND DYNAMICS OF PARTICLE GELS IN NON‐NEWTONIAN MELTS

We investigate the relation between structure and viscoelasticity of model polymer nanocomposite systems based on a mixture of spherical nanoparticles and different polymer matrices. These composites exhibit a strong time‐dependence of the linear elastic and viscous moduli for filler volume fractions above a critical threshold. Despite the complexity of the rheological response, we can scale the viscoelastic properties of the hybrids by splitting their elasticity and dynamics into the independent responses of the suspending polymer melt and that of an elastic particle network. We show that the elasticity of the particle network exhibits critical behavior at the percolation threshold. Our analysis is expected to be useful for understanding the behavior of other complex fluids where the elasticity of the components may be superimposed.

Bimetallic Au/Ag nanoparticle loading on PNIPAAm–VAA–CS8 thermoresponsive hydrogel surfaces using ss-DNA coupling, and their SERS efficiency

Thermoresponsive hydrogels can be efficiently used as templates for bimetallic noble metal surface loading for the fabrication of plasmonic surfaces with a wide range of applications. Here, we report, for the first time, an easy approach for bimetallic Au/Ag surface loading by modifying poly(N-isopropylacrylamide) (PNIPAAm) hydrogel surfaces with ss-DNA. The advantages of this approach consist of the accuracy and the simplicity by which both gold and silver nanoparticles can be adsorbed by electrostatic interactions on hydrogel templates, without requiring sophisticated chemical treatment for their conjugation or the growth of nanoparticles on a hydrogel surface. The resulting patterns possess the capability of tuning the interparticle distance upon temperature changes, and thus their plasmonic properties. The aforementioned templates have been successfully used as SERS substrates for 5 × 10−7 M adenine detection.

Hindered Brownian diffusion in a square-shaped geometry

We study the spatial dependence of the mobility of microparticles diffusing close to an edge of a square microtube. Confocal particle tracking is used to measure the local diffusion coefficients of fluorescent latex 1.1μm particles suspended in an aqueous solution in a borosilicate square capillary of 50μm section side. Observations are made for a set of planes obtained by confocal sectioning of the capillary volume. The translational diffusion coefficients parallel to the axis channel and perpendicular to one of the walls are measured as a function of the distance from both the two channel walls concurring in an edge. A complete 3D spatial map of the colloid diffusion coefficients is thus obtained. Near the corner, the diffusion is hindered up to about 40% as compared to its bulk value. The three translational diffusion coefficients pertaining to the motions along the channel axis and within the channel cross-section turn out to be different from each other and differently affected by the confinement, i.e., we are in the presence of an anisotropic diffusion. The hindered diffusion phenomenon is also examined by finite element numerical simulations, and the numerical predictions fairly agree with the measured diffusion coefficients.

Effect of the Compounding Procedure on the Structure and Viscoelasticity of Polymer Nanocomposites

We investigate the relation between the structure and the viscoelastic properties of a model polymer nanocomposite system based on a mixture of inorganic particles and poly(ethyleneoxide). Hydrophilic fumed silica nanoparticles were used as fillers and PEO‐based nanocomposites were prepared by melt compounding and freeze‐dryng. In both cases, dynamic oscillatory measurements in the melt state highlighted an increase of the frequency‐dependent linear viscoelastic moduli with the filler content and a solid‐like behaviour above a critical volume fraction. The freeze‐dried samples exibhited a significant enhancement of the elasticity at lower filler contents as a result of the effect of particle dispersion on polymer chain dynamics.