Nanostructured materials and nanocomposites

A Comparison of Interpolyelectrolyte Complexes (IPECs) Made from Anionic Block Copolymer Micelles and PDADMAC or q-Chitosan as Polycation

Özge Azeri, Dennis Schönfeld, Bin Dai, Uwe Keiderling, Laurence Noirez and Michael Gradzielski

Block copolymers synthesized via Atom Transfer Radical Polymerization from alkyl acrylate and t-butyl acrylate and the subsequent hydrolysis of the t-butyl acrylate to acrylic acid were systematically varied with respect to their hydrophobic part by the variation in the alkyl chain length and the degree of polymerisation in this block. Depending on the architecture of the hydrophobic part, they had a more or less pronounced tendency to form copolymer micelles in an aqueous solution. They were employed for the preparation of IPECs by mixing the copolymer aggregates with the polycations polydiallyldimethylammonium chloride (PDADMAC) or q-chit. The IPEC structure as a function of the composition was investigated by Static Light and Small Angle Neutron Scattering. For weakly-associated block copolymers (short alkyl chain), complexation with polycation led to the formation of globular complexes, while already existing micelles (long alkyl chain) grew further in mass. In general, aggregates became larger upon the addition of further polycation, but this growth was much more pronounced for PDADMAC compared to q-chit, thereby leading to the formation of clusters of aggregates. Accordingly, the structure of such IPECs with a hydrophobic block depended largely on the type of complexing polyelectrolyte, which allowed for controlling the structural organisation via the molecular architecture of the two oppositely charged polyelectrolytes.

https://doi.org/10.3390/polym15092204

The desalting/salting pathway: a route to form metastable aggregates with tuneable morphologies and lifetimes

Sumit Mehan, Laure Herrmann, Jean-Paul Chapel, Jacques Jestin, Jean-Francois Berret and Fabrice Cousin

We investigate the formation/re-dissociation mechanisms of hybrid complexes made from negatively charged PAA2k coated g-Fe₂O₃ nanoparticles (NP) and positively charged polycations (PDADMAC) in aqueous solution in the regime of very high ionic strength (I). When the building blocks are mixed at large ionic strength (1 M NH₄Cl), the electrostatic interaction is screened and complexation does not occur. If the ionic strength is then lowered down to a targeted ionic strength Itarget, there is a critical threshold Ic = 0.62 M at which complexation occurs, that is independent of the charge ratio Z and the pathway used to reduce salinity (drop-by-drop mixing or fast mixing). If salt is added back up to 1 M, the transition is not reversible and persistent out-of-equilibrium aggregates are formed. The lifetimes of such aggregates depends on Itarget: the closer Itarget to Ic is, the more difficult it is to dissolve the aggregates. Such peculiar behavior is driven by the inner structure of the complexes that are formed after desalting. When Itarget is far below Ic, strong electrostatic interactions induce the formation of dense, compact and frozen aggregates. Such aggregates can only poorly reorganize further on with time, which makes their dissolution upon resalting almost reversible. Conversely, when Itarget is close to Ic more open aggregates are formed due to weaker electrostatic interactions upon desalting. The system can thus rearrange with time to lower its free energy and reach more stable out-of-equilibrium states which are very difficult to dissociate back upon resalting, even at very high ionic strength.

Contact LLB : Fabrice Cousin (LLB/MMB)

"The desalting/salting pathway: a route to form metastable aggregates with tuneable morphologies and lifetimes"
S.Mehan, L. Herrmann, J.-P. Chapel, J. Jestin, J.-F. Berret and F. Cousin, Soft Matter 17 (2021) 8496-8505.

Design of concentrated colloidal dispersions of iron oxide nanoparticles in ionic liquids: Structure and thermal stability from 25 to 200 °C

J.C.Riedl, M.Sarkar, T.Fiuza, F.Cousin, J.Depeyrot, E.Dubois,G.Mériguet,R.Perzynski and V.Peyre

Some of the most promising fields of application of ionic liquid-based colloids imply elevated temperatures. Their careful design and analysis is therefore essential. We assume that tuning the structure of the nanoparticle-ionic liquid interface through its composition can ensure colloidal stability for a wide temperature range, from room temperature up to 200 °C.

https://doi.org/10.1016/j.jcis.2021.08.017

Iron Oxide/Polymer Core–Shell Nanomaterials with Star-like Behavior

Virginie Vergnat, Benoît Heinrich, Michel Rawiso, René Muller, Geneviève Pourroy and Patrick Masson

Embedding nanoparticles (NPs) with organic shells is a way to control their aggregation behavior. Using polymers allows reaching relatively high shell thicknesses but suffers from the difficulty of obtaining regular hybrid objects at gram scale. Here, we describe a three-step synthesis in which multi-gram NP batches are first obtained by thermal decomposition, prior to their covalent grafting by an atom transfer radical polymerization (ATRP) initiator and to the controlled growing of the polymer shell. Specifically, non-aggregated iron oxide NPs with a core principally composed of γ-Fe₂O₃ (maghemite) and either polystyrene (PS) or polymethyl methacrylate (PMMA) shell were elaborated. The oxide cores of about 13 nm diameter were characterized by dynamic light scattering (DLS), transmission electron microscopy (TEM), and small-angle X-ray scattering (SAXS). After the polymerization, the overall diameter reached 60 nm, as shown by small-angle neutron scattering (SANS). The behavior in solution as well as rheological properties in the molten state of the polymeric shell resemble those of star polymers. Strategies to further improve the screening of NP cores with the polymer shells are discussed.

https://doi.org/10.3390/nano11092453

Iron Oxide/Polymer Core–Shell Nanomaterials with Star-like Behavior

Thermodiffusion anisotropy under a magnetic field in ionic liquid-based ferrofluids

T. Fiuza, M. Sarkar, J. C. Riedl, A. Cebers, F. Cousin, G. Demouchy, J. Depeyrot, E. Dubois, F. Gelebart, G. Meriguet, R. Perzynski and V. Peyre

Ferrofluids based on maghemite nanoparticles (NPs), typically 10 nm in diameter, are dispersed in an ionic liquid (1-ethyl 3-methylimidazolium bistriflimide – EMIM-TFSI). The average interparticle interaction is found to be repulsive by small angle scattering of X-rays and of neutrons, with a second virial coefficient A2 = 7.3. A moderately concentrated sample at F = 5.95 vol% is probed by forced Rayleigh scattering under an applied magnetic field (up to H = 100 kA m1 ) from room temperature up to T = 460 K. Irrespective of the values of H and T, the NPs in this study are always found to migrate towards the cold region. The in-field anisotropy of the mass diffusion coefficient Dm and that of the (always positive) Soret coefficient ST are well described by the presented model in the whole range of H and T. The main origin of anisotropy is the spatial inhomogeneities of concentration in the ferrofluid along the direction of the applied field. Since this effect originates from the magnetic dipolar interparticle interaction, the anisotropy of thermodiffusion progressively vanishes when temperature and thermal motion increase.

https://doi.org/10.1039/d0sm02190c

Controlled Synthesis of Gold Nanoparticles in Copolymers Nanomolds by X-ray Radiolysis

We show by X-ray and neutron small-angle scattering that gold nanoparticles with controlled sizes and morphologies can be obtained by the metallic reduction of AuCl₄^– ions trapped in 3D organic molds by X-ray radiolysis. The molds are spherical frozen micelles of polystyrene-b-poly(dimethylaminoethyl methacrylate) (PS-b-PDMAEMA) block copolymer in acidic aqueous solution with a PS spherical core surrounded by a corona of PDMAEMA chains in good solvent. The behavior of micelles is controlled by the [AuCl₄^–]/[DMAEMA] ratio R_{AuCl₄^–/DMAEMA}. At low gold concentration, AuCl₄^– ions condense on the positively charged DMAEMA moieties without changing the behavior of the PDMAEMA chains. At intermediate gold concentration, the ions induce a progressive contraction of the corona’s chains and dehydration of micelles. At large gold concentration, the corona becomes a fully dry phase loaded with gold ions, which induces micelle aggregation. Radiolysis of the solution by an intense X-ray beam produces different types of gold nanoparticles with respect to R_{AuCl₄^–/DMAEMA} and irradiation time. At R_{AuCl₄^–/DMAEMA} = 0.033, irradiation produces in the first step gold clusters in the micelle corona which in the second step merge to form nanoparticles of a similar size to that of the micelle. Conversely, at R_{AuCl₄^–/DMAEMA} = 0.33, micelles do not operate as templates but only as nucleation zones and large nanoparticles grow outside the micelles.

http://doi.org/10.1021/acs.langmuir.0c00554

Controlled Synthesis of Gold Nanoparticles in Copolymers Nanomolds by X-ray Radiolysis

Coupling NMR to SANS: Addressing at once structure and dynamics in soft matter

de Oliveira-Silva, Rodrigo; Bélime, Agathe; Le Coeur, Clémence; Chennevière, Alexis; Helary, Arnaud; Cousin, Fabrice; Judeinstein, Patrick; Sakellariou, Dimitrios; Zanotti, Jean-Marc

In soft condensed matter, Small Angle Neutron Scattering (SANS) is a central tool to probe structures with characteristic sizes ranging from 1 to 100 nm. However, when used as a standalone technique, the dynamic properties of the sample are not accessible. Nuclear Magnetic Resonance (NMR) is a versatile technique which can easily probe dynamical information. Here, we report on the coupling of a low-field NMR system to a SANS instrument. We show that this original set-up makes it possible to obtain structural information and to simultaneously extract in situ on a same sample, long-range translational diffusion coefficient, T1T1 and T2T2 nuclear spin relaxation times. Such a feature is of major interest when a sample experiences a transient physical state or evolves rapidly. We illustrate the capabilities of alliancing these experimental methods by following the critical temperature-induced phase separation of a concentrated Poly(Methacrylic Acid) solution at its Lower Critical Solution Temperature. The characteristic size related to the domain growth of the polymer-rich phase of the gel is monitored by the evolution of the SANS spectra, while the dynamics of the sol phase (H₂O and polymer) is simultaneously characterized by NMR by measuring T1T1, T2T2 and the diffusion coefficient. Great care has been taken to design a cell able to optimize the thermalization of the sample and in particular its equilibration time. Details are given on the sample cell specifically designed and manufactured for these experiments. The acquisition time needed to reach good signal-to-noise ratios, for both NMR and SANS, match: it is of the order of one hour. Altogether, we show that in situ low-field NMR/SANS coupling the NMR is meaningful and is a promising experimental approach.

DOI

Covalently bonded multimers of Au25(SBut)18 as a conjugated system

Annelies Sels, Giovanni Salassa, Fabrice Cousin, Lay-Theng Lee, Thomas Bürgi,
Nanoscale 26 (2018) 12754.

Aromatic dithiol linkers were used to prepare aggregates of Au₂₅(SR)₁₈ clusters (SR: thiolate) via ligand exchange reactions. Fractions of different aggregate sizes were separated by size exclusion chromatography (SEC). The aggregates were characterized by UV-vis absorption spectroscopy, matrix assisted laser desorption ionization (MALDI) mass spectrometry, nuclear magnetic resonance (NMR) spectroscopy (including diffusion-ordered spectroscopy, DOSY) and small angle X-ray scattering (SAXS).

At 2:1 cluster: dithiol ratio, small aggregates (dimers, trimers) and also larger aggregates consisting of 10-20 Au₂₅ clusters were formed, according to DOSY, besides unreacted (monomeric) Au₂₅(SR)₁₈. MALDI mass spectrometry shows signals consistent with dimers and trimers (doubly charged). The SAXS curves for the small aggregates can be well fitted by a pearl-necklace model. For the bigger aggregates the SAXS curves evidence a characteristic separation distance between the clusters within the aggregates, which is imposed by the length of the linker. The SAXS curves of these larger aggregates can be well fitted with a core-shell sphere model with a sticky hard-sphere structure factor, in agreement with closely packed aggregates.

The absorption spectra of smaller aggregates resemble the one of individual Au₂₅(SR)₁₈ clusters; however, and most importantly, the larger aggregates show completely different, less structured spectra with a new band emerging at 840 nm. We assign this drastic change in the absorption spectra and the new band to the electronic coupling between the clusters through the all aromatic linker. In accordance with this view, aggregates formed with a linker containing methylene groups, thus breaking conjugation, do not show the band at 840 nm. By addition of monothiols to the larger aggregates their size can be reduced through an “unlinking” reaction. This reaction also affects the band at 840 nm, which moves to higher energy when reducing the aggregate size, as would be expected within a particle in a box model. The electronic coupling between the clusters through the linker is the basis for future applications in nanoelectronics.

http://dx.doi.org/10.1039/C8NR01902A

Covalently bonded multimers of Au25(SBut)18 as a conjugated system

How Necklace Pearls Evolve in Hydrophobic Polyelectrolyte Chains under Good Solvent Addition: A SANS Study of the Conformation

Souha Ben Mahmoud, Wafa Essafi, Annie Brûlet and François Boué

The chain conformation in sulfonated polystyrene PSSNa of a degree of sulfonation 0.34 ≤ f ≤ 1, i.e., of various hydrophobicity, is followed in mixtures of water and increasing content of tetrahydrofuran (THF), a good solvent of the hydrophobic polystyrene moieties (which improves the solvent quality of the mixture). This is achieved by measuring the chain form factor by small-angle neutron scattering using the zero average contrast method (ZAC). Polymer concentrations 0.17 and 0.34 M correspond in our case to the semidilute regime or its limit with dilute regime depending on the chain conformation. The main result is the monitoring with added THF of the pearl necklace conformation. This heterogeneous structure, made of wormlike chain parts (strings) and pearls, was observed formerly in water: when f decreases, the string contribution decreases, and the pearls size, characterized by a maximum in Kratky q2S1(q) representation, slightly increases. Here we see that in the presence of increasing content of added THF (i) the pearls contribution decreases, as expected, and (ii) their size does not change by more than 10% in most cases (30% at the most). Among different modeling, the most complete has been done following the pearl necklace models of Schweins, Huber et al. and Lages, Huber et al.; beyond the size and distribution of pearls, it addresses the radius of gyration, the correlation distance between spheres, weakly visible, and, importantly their number N. The values of Rg, as well as the modeling, suggest that while the sphere size varies a few, N decreases clearly with added good solvent. A link with the simulation of Liao et al. can be found. A second result, important in practice, is that all modeling of the scattering establishes that THF addition makes vanish an excess of low q scattering due to large compact spheres present in water. A third result is obtained after a “THF treatment” procedure, consisting of adding THF and then removing it by evaporation: (i) the spherical aggregates are washed out, and (ii) the pearl necklace conformation of the chain returns to the one in water solution before treatment. Therefore, the pearl necklace conformation of the hydrophobic polyelectrolyte in aqueous solution appears to be in an annealed equilibrium state resulting from hydrophobic attraction and electrostatic repulsion.

DOI: 10.1021/acs.macromol.8b00990

Melt Chain Conformation in Nanoparticles/Polymer Nanocomposites Elucidated by the SANS Extrapolation Method: Evidence of the Filler Contribution

Anne-Sophie Robbes, Fabrice Cousin , Florian Meneau, and Jacques Jestin

We probe by SANS the conformation of polymer chains of the matrix in various nanocomposites based on the same building blocks, namely spherical magnetic nanoparticles of maghemite (γ-Fe₂O₃) as fillers and polystyrene (PS) for the matrix. Given that the nanoparticles can be arranged in oriented chains during the processing by an external magnetic field and/or grafted by tethered PS chains with a grafting density of ∼0.15 chains/nm², very different organizations of the nanofillers were tested according to different particle–polymer interactions: (i) homogeneous isotropic dispersion of aggregates of bare nanoparticles; (ii) chains of bare nanoparticles oriented along one direction over the whole sample; (iii) perfect dispersion of grafted nanoparticles; (iv) homogeneous isotropic dispersion of large aggregates of grafted nanoparticles; and (v) chains of large aggregates of grafted nanoparticles objects oriented along one direction over the whole sample. Measurements were performed by the extrapolation to the zero concentration method made possible by the fact that γ-Fe₂O₃ has the same neutron scattering length density (SLD) as a deuterated polystyrene, so that the nanoparticles scattering is matched in a deuterated PS matrix, whether they are grafted or not. This robust method enables to check that only the polymer chain form factor is effectively probed in a very accurate way. This allows us to show some deviations of the radius of gyration induced by the nanoparticles: (i) for the case of very weak interaction between the polymer and the bare particles, the radius of gyration is swollen by 16% whatever the filler dispersion and orientation; (ii) for the athermal interaction between grafted particles and polymer, the radius of gyration is either unchanged when particles are individually dispersed or compressed of almost 11% when particles are forming overlapped clusters. Despite the remaining relatively small deviations, this is to our best knowledge the first unambiguous experimental evidence on a single system of the influence of the well-known nanofiller dispersion onto the mean chain conformation in nanocomposites for different polymer–particles interactions ranging from attractive to repulsive.

https://pubs.acs.org/doi/10.1021/acs.macromol.7b02318

Melt Chain Conformation in Nanoparticles/Polymer Nanocomposites Elucidated by the SANS Extrapolation Method: Evidence of the Filler Contribution