Xyloglucan-Cellulose Nanocrystals Mixtures: A Case Study of Nanocolloidal Hydrogels and Levers for Tuning Functional Properties.

The development of fully biobased hydrogels obtained by simple routes and in the absence of toxic or environmentally harmful reagents is a major challenge in meeting new societal demands. In this work, we discuss the development of hydrogels made from cellulose nanocrystals (CNCs) and xyloglucan (XG), two non-toxic, renewable, and biobased components. We present three strategies to fine-tune the functional properties. The first one consists in varying the XG/CNC ratio that leads to the modulation of the mechanical properties of hydrogels as well as a better comprehension of the gel mechanism formation. The second relies on tuning the XG chains' interaction by enzymatic modification to achieve thermoresponsive systems. Finally, the third one is based on the increase in the hydrogel solid content by osmotic concentration. The high-solid-content gels were found to have very high mechanical properties and self-healing properties that can be used for molding materials. Overall, these approaches are a case study of potential modifications and properties offered by biobased nanocolloidal hydrogels.

pH-controlled breakup of fractal aggregates, microgels and gels formed by self-assembled amphiphilic triblock copolymers.

The degradation of (micro)gels and fractal aggregates based on self-assembled amphiphilic triblock copolymers has been investigated in water by confocal microscopy and light scattering respectively. The triblock copolymer consisted of a central hydrophilic poly(acrylic acid) (pAA) block and two hydrophobic end blocks that contained an equal amount of randomly distributed n-butyl acrylate (nBA) and AA units. These latter units helped at tempering the hydrophobic end blocks resulting in the control and the fine tuning of the dynamics of the self-assembled triblock through the pH. Starting from a pH where the dynamics is frozen, the rate of breakup of the macroscopic gels, microgels and of fractal aggregates was measured after increasing the pH to different values. The mechanism of the breakup was found to be independent of the pH, but its rate increased exponentially with increasing pH. The degradation proceeded through the release of the polymers from the bulk into the surrounding aqueous phase.

Elaboration and rheological characterization of nanocomposite hydrogels containing C60 fullerene nanoplatelets.

Nanocomposite hydrogels were elaborated that consisted of a physical network formed by an amphiphilic polymer in which C60 fullerene nanoplatelets were embedded. Characterization showed that the nanoplatelets within the polymer network were aggregated. The presence of these nanoplatelets led to an increase of the shear modulus of the hydrogels, that cannot be explained by a filler effect alone. The nanocomposite gels displayed similar rheological behavior, both in linear and non-linear domains, as neat hydrogels at higher polymer concentrations. We suggest that the particles reinforced the gels by forming additional connections between the polymer chains.

Self-Assembly in water of C60 fullerene into isotropic nanoparticles or nanoplatelets mediated by a cationic amphiphilic polymer.

HYPOTHESIS: To disperse high concentration of C60 fullerene in water, we propose to use an emulsification-evaporation process in the presence of an amphiphilic polymer whose chemical structure has been chosen for inducing specific interaction with fullerene The viscosity enhancement provided by self-assembly of the amphiphilic polymers in water should result in high stability of the suspensions. The organic solvent has also to been chosen so as to maximize the initial fullerene concentration. EXPERIMENTS: The concentrations of polymer and fullerene, the solvent type and the volume fraction of the organic phase have been varied. Their influence on the concentration of the fullerene dispersions and on the size and shape of the resulting nanoparticles have been investigated by UV-Visible spectroscopy, light scattering and cryo-transmission electron microscopy experiments. FINDINGS: The resulting nanoparticles consist of aggregates of C60 fullerene stabilized by the cationic polymer with morphologies/sizes tunable through fullerene and polymer concentration. At high fullerene concentration, nanoplatelets are obtained that consist in thin 2D nanocrystals. Their suspensions are very stable with time due to the viscosity of the dispersing aqueous medium. The concentration of fullerene nanoparticles dispersed in water is as high as 8 g/L which corresponds to an upper limit that has never been reached so far.

Synthesis and self-assembly of a penta[60]fullerene bearing benzo[ghi]perylenetriimide units.

A benzo[ghi]perylenetriimide (BPTI) derivative bearing a terminal azido group on the expanded π-conjugated backbone has been synthesized and characterized. This promising photo- and electroactive BPTI motif has been used to obtain an original penta(organo)fullerene as a promising multi-electron acceptor system. Our studies show its self-assembly resulting from aggregation via π-π stacking interaction in solution and in the solid state.

Salt-induced thermal gelation of xyloglucan in aqueous media.

The influence of Na2SO4 as a kosmotropic salt on the thermogelation of xyloglucan (XG) solutions was measured by rheology. The gelation occurred at lower temperatures and shorter times when the salt concentration was increased above 0.5 mol.L-1. For Na2SO4 concentrations equal to 1 mol.L-1, a not thermoreversible elastic hydrogel was obtained. Salts containing various types of anions were used, and it was observed that SO42-, HPO42- and H2PO4- promoted the formation of a gelled network. The gel structure was observed using confocal laser scanning microscopy and scanning electron microscopy. In XG containing SO42 at 1 mol.L-1aggregates and gels were formed by interconnected sub-micrometer XG particles. Increasing the concentration of SO42- led to conformation changes in the XG, from a twisted/helical to an extended/flat conformation, as observed using circular dichroism. The naturally occurring hydrophobic sequences promoted an economically feasible XG gelling that may produce thermo and kosmo-sensitive hydrogels.

Viscosity of mixtures of protein aggregates with different sizes and morphologies.

Protein aggregates were generated by thermal denaturation of whey protein isolates. Depending on the heating conditions, fractal aggregates of various sizes or microgels were obtained. The osmotic compressibility and correlation length of mixtures of fractal aggregates of different sizes were found to be close to the weighted averages of the individual components at the same concentration. The viscosity of these mixtures can be described by a logarithmic mixing law using the weight fraction and the viscosity of the individual components. The same mixing law describes the behavior of mixtures of fractal aggregates and microgels. The effect of the type of protein was investigated by mixing fractal aggregates formed by whey and soy protein isolates. It is suggested that the viscosity of the mixtures is determined by the cooperative movement over length scales much larger than the size of the aggregates.

Heat-induced gelation of mixtures of micellar caseins and plant proteins in aqueous solution.

The aim of this work was to investigate how the heat-induced gelation of micellar casein (MC)-plant protein mixtures in aqueous solution is affected by protein composition (MC/plant proteins = 100/0 to 0/100) and total protein content (4%, 6% and 8% w/w) at pH 5.8 and 6.0. Two types of plant proteins were used: soy proteins (SP) and pea proteins (PP). Storage moduli (G') were measured during heating ramps from 20 to 90 °C and heat-induced gelation was characterised by a sharp increase in G' at a critical temperature (Tc). The gel stiffness (Gel) was determined after 1 h at 90 °C and the microstructure before and after heating was investigated by confocal laser scanning microscopy (CLSM). Tc was found to increase with increasing the fraction of MC replaced by SP or PP, due to binding of calcium to the plant proteins. The effect was stronger for SP, which bound calcium more efficiently than PP. Tc decreased with decreasing pH, possibly caused by decreased electrostatic repulsion and increased calcium release from MC. Gel increased with increasing total protein content and did not depend significantly on the pH. Interestingly, Gel showed a minimum as a function of the plant protein fraction (40% for SP and 70% for PP) in the mixtures. It is concluded that MC and plant proteins did not co-aggregate in the mixtures during heating, and that each type of protein formed networks independently.

Polymer Probe Diffusion in Globular Protein Gels and Aggregate Suspensions.

Transport properties of macromolecules in dense aggregate suspensions and gels of proteins are important for usage of these biomaterials in areas such as pharmaceutics, food, and cosmetics. The mobility of polymers in protein gels has received some attention in the past, but the mobility in dense aggregate suspensions has not yet been investigated. In this study, self-diffusion of probe dextran chains was studied in suspensions of aggregates with different size and morphology and in gels using fluorescence recovery after photobleaching over a wide range of concentrations. Brownian diffusion of the probes was observed in aggregate suspensions as well as in weak gels formed just beyond the critical gel concentration. Diffusion of polymers in dense suspensions of protein aggregates depends not only on the concentration but also on the size and morphology of the aggregates. It is not directly related to the viscosity or the dynamic correlation length. Diffusion of polymers in protein gels is anomalous and occurs on logarithmic time scales. The recovery of the fluorescence for densely cross-linked gels was logarithmic with time, suggesting an exponential distribution of diffusion coefficients.

Specific effect of calcium ions on thermal gelation of aqueous micellar casein suspensions.

Aqueous suspensions of micellar casein (MC) gel when heated above a critical temperature that depends on the pH. The effect of adding CaCl2 and EDTA on thermal gelation was studied in order to assess the effect of increasing or decreasing the amount of bound Ca2+ on the process. The effect of adding NaCl was investigated in order to distinguish the effect of Ca2+ binding from the effect of screening of electrostatic interactions. Shear moduli were measured as a function of the temperature during heating ramps up to 90 °C. Gel formation was characterized by a sharp increase of the storage modulus at a critical temperature (Tc). In most cases, prolonged heating at 90 °C did not cause a significant increase of the gel stiffness. Addition of CaCl2 caused a decrease of Tc, whereas addition of NaCl caused an increase of Tc. Chelation of Ca2+ by EDTA led to a strong increase of Tc even if the fraction of chelated Ca2+ was small and the micelles remained intact. The gel stiffness was found to increase weakly with increasing CaCl2 concentration up to 20 mM. The gels persisted during cooling to 20 °C with an increase of the elastic modulus by a factor between 4 and 6 depending on the CaCl2 concentration. The results demonstrate that Ca2+ plays a crucial role in thermal MC gelation.

Xyloglucan gelation induced by enzymatic degalactosylation; kinetics and the effect of the molar mass.

Gelation kinetics of aqueous solutions of xyloglucan (XG) extracted from H. courbaril seeds were investigated, in-situ, during enzymatic removal of galactose units by oscillatory shear rheological measurements, at different XG and enzyme (ß-galactosidase) concentrations. Increasing the enzyme concentration (Cenz) led to an increase of the gelation rate. Master curves of the evolution of the storage shear modulus at different Cenz could be formed by time-Cenz superposition showing that Cenz influenced the kinetics, but not the gelation process and the final gel stiffness. The behaviour of gels formed by XG with different molar mass (Mw), prepared by endoglucanase hydrolysis, was evaluated as a function of the temperature. It was found that cooling led to a decrease of the crosslink density causing a decrease of the gel stiffness. The decrease of the crosslink density was sufficient to depercolate the network formed by relatively small XG with Mw=105gmol-1, but gels formed by XG with Mw≥8×105gmol-1 persisted down to 10°C. It is shown that the melting temperature and the gel stiffness at high temperatures can be controlled independently by varying the molar mass and the concentration of XG chains.

Exploiting Salt Induced Microphase Separation To Form Soy Protein Microcapsules or Microgels in Aqueous Solution.

Self-assembly of native glycinin at room temperature was investigated as a function of the pH and the NaCl concentration. Microphase separation leading to the formation of dense protein microdomains was observed by confocal laser scanning microscopy. Depending on the conditions, the microdomains coalesced into a continuous protein rich phase or associated into large clusters. Addition of ß-conglycinin inhibited phase separation and reduced the pH range in which it occurred. Microdomains of glycinin that were formed in the presence of 0.1 M NaCl transformed into hollow stable cross-linked microcapsules when heated above 60 °C with diameters between 3 and 30 µm depending on the protein concentration and a shell thickness between 1.0 and 1.4 µm. The microcapsules were stable to dilution in salt free water, whereas microdomains formed at room temperature redispersed. Microdomains formed in mixtures with ß-conglycinin did not transform into microcapsules, but they became stable cross-linked homogeneous microgels.

Micellar RAFT/MADIX Polymerization.

We apply the RAFT/MADIX technique to the micellar copolymerization of acrylamide and 2-acrylamido-2-methylpropanesulfonic acid sodium salt with a hydrophobic monomer, 4-tert-butylstyrene. The resulting polymers have well-controlled molecular weight distributions. In the presence of sodium dodecyl sulfate, the polymerization is better controlled by an oligo-acrylamide chain transfer agent (PAm7-XA1) than by Rhodixan A1. The associative character of the polymer is maintained under RAFT polymerization conditions and chains can be extended to form block copolymers with associative segments.

Structural, Viscoelastic, and Electrochemical Characteristics of Self-Assembled Amphiphilic Comblike Copolymers in Aqueous Solutions.

Self-assembly in aqueous solutions of an amphiphilic comblike polyelectrolyte (80C12) that consists of a polystyrene (PS) backbone onto which quaternary ammonium pendant moieties have been grafted has been investigated by light scattering and cryo-transmission electron microscopy measurements in the presence of KCl and methylviologen dication (MV2+) under conditions mimicking those for electrochemical measurements. Polymer chains self-assemble within branched cylindrical micelles that display viscoelastic properties, characterized by a relaxation time of 4 s. To tune this time, 80C12 was mixed with a polyoxyethylene nonionic surfactant (Brij C12E10). Relatively increasing the amount of the latter leads to a decrease in the relaxation time of the 80C12 solution. Correlatively, electrochemical experiments with a rotating disk electrode show a transition of the mass transport rate, which deviates from the classical Newtonian behavior in the same velocity domain. This result generalizes what has been already observed with solutions of linear polymers of high molecular weight and wormlike micelles based on surfactants subjected to elongational deformations. Moreover, the critical times derived from rheological and electrochemical experiments display the same trend.

Data on the characterization of native soy globulin by SDS-Page, light scattering and titration.

The data presented in this article are related to the research article entitled Structure of Self-assembled Native Soy Globulin in Aqueous Solution as a Function of the Concentration and the pH by N. Chen, M. Zhao C. Chassenieux, T. Nicolai (2016) [1]. Please refer to this article for interpretation of the data. The protein composition of soy protein isolate (SPI) was characterized by SDS-Page. The molar mass of native soy globulin aggregates formed at different protein concentrations was determined by light scattering as a function of the waiting time. The dependence of the pH on the net charge density of native soy globulins was determined for solutions containing 5 g/L or 2 g/L SPI.

Inhibition and Promotion of Heat-Induced Gelation of Whey Proteins in the Presence of Calcium by Addition of Sodium Caseinate.

Heat-induced aggregation and gelation of aqueous solutions of whey protein isolate (WPI) in the presence of sodium caseinate (SC) and CaCl2 was studied at pH 6.6. The effect of adding SC (0-100 g/L) on the structure of the aggregates and the gels was investigated by light scattering and confocal laser scanning microscopy at different CaCl2 concentration ([CaCl2] = 0-30 mM). The gelation process was studied by oscillatory shear rheology. At the whey protein concentrations studied here (34 and 60 g/L), no gels were formed in the absence of CaCl2 and SC. However, WPI solutions gelled above a critical CaCl2 concentration that increased with increasing SC concentration. In the absence of CaCl2, WPI gels were formed only above a critical SC concentration. The critical SC concentration needed to induce WPI gelation decreased weakly when CaCl2 was added. In an intermediate range of CaCl2 concentrations, gels were formed both at low and high SC concentrations, but not at intermediate SC concentrations. Finally, at high CaCl2 concentrations gels were formed at all SC concentrations. The gelation rate and the gel structure of the gels formed at low and high casein concentrations were very different. The effect of SC on the thermal gelation of WPI was interpreted by competition for Ca2+, a chaperon effect, and microphase separation.

Fast and effective quantum-dots encapsulation and protection in PEO based photo-cross-linked micelles.

Cadmium-based quantum dots (QDs) were easily, quickly and efficiently transferred from an organic medium to water without modification of their surface chemistry by the simple emulsion/solvent evaporation technique using micelles of amphiphilic diblock copolymers based on poly(ethylene oxide) and poly(2-methacryloyloxyethyl acrylate) (PEO-b-PMEA) as hosts. The resulting hybrid micelles were stabilized very rapidly by photo-cross-linking the hydrophobic core around the QDs. The encapsulation and photo-cross-linking process were shown to barely affect the photoluminescence properties. Grafting a short octyl chain at the end of the hydrophobic block enhanced both the colloidal stability of the QDs dispersed in water and prevented the quenching of their fluorescence by copper ions. Grafting a longer hexadecyl chain at the end of the PMEA block decreased the efficiency of the corona cross-linking and led to poorer stabilization and protection.

Interpenetrated Si-HPMC/alginate hydrogels as a potential scaffold for human tissue regeneration.

Interpenetrated gels of biocompatible polysaccharides alginate and silanized hydroxypropyl methyl cellulose (Si-HPMC) have been studied in order to assess their potential as scaffolds for the regeneration of human tissues. Si-HPMC networks were formed by reduction of the pH to neutral and alginate networks were formed by progressive in situ release of Ca(2+). Linear and non-linear mechanical properties of the mixed gels at different polymer and calcium concentrations were compared with those of the corresponding single gels. The alginate/Si-HPMC gels were found to be stiffer than pure Si-HPMC gels, but weaker and more deformable than pure alginate gels. No significant difference was found for the maximum stress at rupture measured during compression for all these gels. The degrees of swelling or contraction in excess water at pH 7 as well as the release of Ca(2+) was measured as a function of time. Pure alginate gels contracted by as much as 50 % and showed syneresis, which was much reduced or even eliminated for mixed gels. The important release of Ca(2+) upon ageing for pure alginate gels was much reduced for the mixed gels. Furthermore, results of cytocompatibility assays indicated that there was no cytotoxicity of Si-HPMC/alginate hydrogels in 2D and 3D culture of human SW1353 cells. The results show that using interpenetrated Si-HPMC/alginate gels has clear advantages over the use of single gels for application in tissue regeneration.

Blending block copolymer micelles in solution; Obstacles of blending.

Amphiphilic block copolymers can assemble into a variety of structures on the nanoscale in selective solvent. The micelle blending protocol offers a simple unique route to reproducibly produce polymer nanostructures. Here we expand this blending protocol to a range of polymer micelle systems and self-assembly routes. We found by exploring a range of variables that the systems must be able to reach global equilibrium at some point for the blending protocol to be successful. Our results demonstrate the kinetics requirements, specifically core block glass transition temperature, Tg, and length of the block limiting the exchange rates, for the blending protocol which can then be applied to a wide range of polymer systems to access this simple protocol for polymer self-assembly.

Structure and flow of dense suspensions of protein fractal aggregates in comparison with microgels.

Solutions of the globular whey protein ß-lactoglobulin (ß-lg) were heated at different protein concentrations leading to the formation of polydisperse fractal aggregates with different average sizes. The structure of the solutions was analyzed with light scattering as a function of the protein concentration. The osmotic compressibility and the dynamic correlation length decreased with increasing concentration and became independent of the aggregate size in dense suspensions. The results obtained for different aggregate sizes could be superimposed after normalizing the concentration with the overlap concentration. Dense suspensions of fractal protein aggregates are strongly interpenetrated and can be visualized as an ensemble of fractal 'blobs'. The viscosity of the heated ß-lg solutions increased extremely sharply above 80 g L(-1) and diverged at 98 g L(-1), mainly due to the sharply increasing aggregate size. At a fixed aggregate size, the viscosity increased initially exponentially with increasing concentration and then diverged. The increase was stronger when the aggregates were larger, but the dependence of the viscosity on the aggregate size was weaker than that of the osmotic compressibility and the dynamic correlation length. The concentration dependence of the viscosity of solutions of fractal ß-lg aggregates is much stronger than that of homogeneous ß-lg microgels. The behavior of fractal aggregates formed by whey protein isolates was similar.