Ref HAL: hal-03270652_v1
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Temperature is one of the basic parameters often required to characterize a system. A great demand has arisen for localmeasurements, especially in liquids or complex biological environments. Various approaches have been proposed to study thetemperature at the nano-scale level. Some of them are based on the spectroscopic properties of carbon nanotubes (CNT) usedas sensors.
Raman spectroscopy is indeed a powerful technique to identify single-walled carbon nanotubes (SWNT) and to study theirstructure, defects and electronic properties through the measurement of specific Raman signatures (RBM, D, G and 2D bands).On the other hand, individual SWNT or small bundles emit light in the near infrared and the photoluminescence (PL) spectra isvery sensitive to the quality of the dispersion and the dielectric environment of the nanotubes. In particular, when SWNT aredispersed in aqueous solutions, the PL energies are sensitive to the nature of the surfactants or polymers, to theirconcentration, and to the way they adsorb on/wrap around the nanotubes.
In this work we show that the PL/Raman spectra of SWNT dispersed with sodium dodecyl sulfate (SDS) is very sensitive to thetemperature (figure 1) in a large range of SDS concentrations. We discuss the influence of the chiral angle of the SWNT onthese PL changes, and the origin of the changes in terms of SDS reorganization at the surface of the nanotubes. Similarchanges are obtained with increasing laser power (figure 2), showing the local heating of the nanotubes. These results pavethe way for the development of SWNT-based nano-thermometers.

Ref HAL: hal-03269436_v1
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Polymer nanocomposites (PNCs) are made by the dispersion of hard fillers in a polymer matrix, where the fillers are usually added to enhance mechanical properties. Such materials attract industrial interest and their investigation is essential for the improvement of their performances. The latter are strongly related to the structural properties of PNCs: the dispersion state of nanoparticles (NPs) in the polymer melt is governed by the mixing protocols (e.g., solid phase mixing or solvent casting) and thermodynamics of the system (i.e., particle miscibility) which influence both filler-filler and filler-polymer interactions.The present study concerns nanocomposites obtained using hydrophilic silica NPs dispersed in two different polymer matrices: 1) poly(2-vinylpyridine) forming attractive NP/polymer interfaces or 2) a weakly interacting styrene-butadiene (SB) rubber. In the first system, the dynamical properties of the interfacial layer surrounding NPs are obtained using broadband dielectric spectroscopy (BDS). Moreover, we propose an original approach combining small-angle scattering experiments of X-rays and neutrons and simulations to investigate the static properties. Only in ideally well-dispersed systems using pre-adsorption of the polymer chains, a static interfacial layer with a gradient of density extending over a thickness of ca. 2 nm is evidenced based on the analysis with a form-free density profile. This gradient is found to be generated by out-of-equilibrium packing and folding of the preadsorbed layer.In the second polydisperse and disordered SB system, we recently proposed a reverse Monte-Carlo analysis of the scattering data giving access to NP aggregate mass distributions. In opposition to the previous system, BDS experiments are not conclusive regarding the impact of the industrial filler on the segmental relaxation. Therefore, we used neutron spin-echo spectroscopy (NSE) and demonstrate – here for the first time – that incoherent NSE based on fully hydrogenated compounds can be used to resolve small modifications of the segmental dynamics of PNCs.

Ref HAL: hal-03259378_v1
DOI: 10.1021/acs.macromol.1c00275
WoS: WOS:000651530900031
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Polymers with dynamic (transient) bonds, often called associating polymers, have been attracting significant attention in recent years due to their unique viscoelastic properties, self-healing ability, and recyclability. Nevertheless, understanding the mechanisms and the factors controlling their macroscopic properties remains limited due to the higher complexity introduced by the dynamic bonds. In this study, small-angle X-ray scattering (SAXS), broadband dielectric spectroscopy (BDS), and rheology were applied to unravel the structure and dynamics of telechelic associating polymers with different molecular weights. SAXS measurements revealed phase separation of the functional end groups with an average cluster size of ∼2–3 nm and the distance between clusters controlled by the chain length. Borrowing the interfacial layer model analysis of BDS data from the polymer nanocomposite field, we demonstrated the presence of an interfacial polymer layer with a thickness of ∼0.7–0.9 nm surrounding these clusters. Rheological measurements showed quantitatively that the presence of the interfacial layer significantly alters the viscoelastic behavior of these materials, indicating the crucial role of the interfacial layer in defining the macroscopic mechanical properties of the studied telechelic materials. The presented results emphasize that phase separation of the functional groups in associating polymers leads to very significant changes of the viscoelastic properties, opening a promising avenue in the design of novel functional materials.

Ref HAL: hal-03254324_v1
PMID 34037061
Ref Arxiv: 2105.09244
DOI: 10.1039/d1sm00438g
WoS: 000653939000001
Ref. & Cit.: NASA ADS
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5 Citations
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We investigate freely expanding viscoelastic sheets. The sheets are produced by the impact of drops on a quartz plate covered with a thin layer of liquid nitrogen that suppresses shear viscous dissipation as a result of the cold Leidenfrost effect. The time evolution of the sheet is simultaneously recorded from top and side views using high-speed cameras. The investigated viscoelastic fluids are Maxwell fluids, which are characterized by low elastic moduli, and relaxation times that vary over almost two orders of magnitude, thus giving access to a large spectrum of viscoelastic and elastocapillary effects. For the purposes of comparison, Newtonian fluids, with viscosity varying over three orders of magnitude, are also investigated. In this study, d(max), the maximal expansion of the sheets, and t(max) the time to reach this maximal expansion from the time at impact, are measured as a function of the impact velocity. By using a generalized damped harmonic oscillator model, we rationalize the role of capillarity, bulk elasticity and viscous dissipation in the expansion dynamics of all investigated samples. In the model, the spring constant is a combination of the surface tension and the bulk dynamic elastic modulus. The time-varying damping coefficient is associated to biaxial extensional viscous dissipation and is proportional to the dynamic loss modulus. For all samples, we find that the model reproduces accurately the experimental data for d(max) and t(max).

Ref HAL: hal-03245508_v1
DOI: 10.1016/j.matlet.2021.129611
WoS: WOS:000639094100055
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Downsizing electronic and electric equipment requires the optimization of electric field distributions in order to avoid localized dielectric breakdown (also called partial discharges). This paper presents a novel dielectric composite material aimed at grading electrical local surface stress. This functional material has a conductivity which increases by several orders with the applied electric field giving the ability to distribute the field by itself. It is prepared for the first time by dispersing graphite nanoplatelets in a polymer and may be used as a resistive or capacitive field grading material in electronic and electrical applications. Mechanisms at the origin of the nonlinear behavior are discussed.

Ref HAL: hal-03194024_v1
DOI: 10.1039/d0nj05933a
WoS: WOS:000623596600015
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Pesticide sensing is an important object of study due to its increasing use worldwide. Herein, we report a SERS study of 4-nitrophenol (PNP), which is product of neutralization processes of various pesticides such as Paraoxon, and can be used as a target molecule for monitoring. PNP is also widely used in the chemical industry and due to its high toxicity is considered a concerning pollutant. The sensing was carried out with a reduced graphene oxide nanocomposite functionalized with cysteamine and Ag nanoparticles (rGOSHAg), and compared with raw reduced graphene oxide and a commercial SERS substrates (SERStrate (TM)). A mechanistic evaluation was also carried out, focused in the degradation of PNP caused by the different exciting laser lines, evidencing the PNP dimerization in substrates containing Ag NPs (under 532 nm laser), which has important outcomes for sensing purposes. The nanocomposite rGOSHAg presented the highest sensitivity towards PNP, detecting concentrations as low as 10(-6) mol L-1 and with a high potential for field applications and real-time measurements of molecules commonly present in pesticides and industrial contaminants.

Ref HAL: hal-03189697_v1
DOI: 10.3390/polym13071153
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PNIPAm microgels synthesized via free radical polymerization (FRP) are often considered as neutral colloids in aqueous media, although it is well known, since the pioneering works of Pelton and coworkers, that the vanishing electrophoretic mobility characterizing swollen microgels largely increases above the lower critical solution temperature (LCST) of PNIPAm, at which microgels partially collapse. The presence of an electric charge has been attributed to the ionic initiators that are employed when FRP is performed in water and that stay anchored to microgel particles. Combining dynamic light scattering (DLS), electrophoresis, transmission electron microscopy (TEM) and atomic force microscopy (AFM) experiments, we show that collapsed ionic PNIPAm microgels undergo large mobility reversal and reentrant condensation when they are co-suspended with oppositely charged polyelectrolytes (PE) or nanoparticles (NP), while their stability remains unaffected by PE or NP addition at lower temperatures, where microgels are swollen and their charge density is low. Our results highlight a somehow double-faced electrostatic behavior of PNIPAm microgels due to their tunable charge density: they behave as quasi-neutral colloids at temperature below LCST, while they strongly interact with oppositely charged species when they are in their collapsed state. The very similar phenomenology encountered when microgels are surrounded by polylysine chains and silica nanoparticles points to the general character of this twofold behavior of PNIPAm-based colloids in water.