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

Oligomeric and polymeric surfactants for the transfer of luminescent ZnO nanocrystals to water

International audienceThe water dispersion of luminescent nanocrystals (NCs) synthesized in organic solvent by encapsulation in a surfactant bilayer is a promising strategy for preserving the optical properties of NCs. The phase transfer of highly monodispersed ZnO NCs using the monomer, dimer, trimer and polymer of a series of alkyl ammonium surfactants is compared. Transfer yields over 60% could be obtained with the oligomers and the polymer. In contrast, we observed no measurable transfer using the single chain surfactant. NMR spectroscopy, including DOSY and NOESY, demonstrated that increasing the oligomerization number ameliorates the stability within the coating bilayer. The NCs exhibit a strong luminescence in water and show long term chemical and photo-chemical stability

Tuning the aggregation behavior of pH-responsive micelles by copolymerization

YesAmphiphilic diblock copolymers, poly(2-(diethylamino)ethyl methacrylate-co-2-(dimethylamino)ethyl methacrylate)-b-poly(2-(dimethylamino)ethyl methacrylate), P(DEAEMA-co-DMAEMA)-b-PDMAEMA with various amounts of DEAEMA have been synthesized by RAFT polymerization. Their micellization in water has been investigated by scattering measurements over a wide pH range. It appeared that the polymers self-assembled into pH sensitive star like micelles. For a given composition, when the pH is varied the extent of aggregation can be tuned reversibly by orders of magnitude. By varying the copolymer composition in the hydrophobic block, the onset and extent of aggregation were shifted with respect to pH. This class of diblock copolymer offers the possibility to select the range of stimuli-responsiveness that is useful for a given application, which can rarely be achieved with conventional diblock copolymers consisting of homopolymeric blocks.European Science Foundation (ESF), Engineering and Physical Sciences Research Council (EPSRC), BP (Firm), Birmingham Science City, Advantage West Midlands (AWM), European Regional Development Fund (ERDF

Polymeric drift control adjuvants for agricultural spraying

The movement of a pesticide or herbicide to an off-target site during agricultural spraying can cause injury to wildlife, plants and contamination of surface water. This phenomenon is known as spray drift and can be controlled by spraying during favorable environmental conditions, and by using low drift nozzles and drift control adjuvants (DCAs). Polymeric DCAs are the most common type of DCA and function by increasing the droplet size produced during spraying. There are, however, two main drawbacks of polymeric DCAs; they are prone to mechanical degradation during spraying which reduces their performance and they can produce oversized drops which reduces the efficacy of the spray. In this trend article, existing DCA technology is reviewed including the mechanism through which they function. This then provides a platform for the discussion of novel polymeric architectures which have currently not been applied in DCA formulations

Heat-induced gelation of mixtures of casein micelles with whey protein aggregates

International audienceGelation of aqueous solutions of casein micelles (MC) in mixtures with fractal aggregates of whey protein isolate (WPI) has been studied first by addition of different amounts of aggregates at a fixed concentration of micelles and then by substitution of micelles with aggregates at a fixed total protein concentration. Addition of small amounts of aggregates (1–5 g/L) to suspensions of micelles (20–60 g/L) at a fixed pH led to a strong decrease of the gelation temperature (Tg) and to an increase of the elastic modulus (G′) of the mixed gels. A minimum in Tg and a maximum in G′ as a function of the WPI fraction were obtained in mixtures with fixed total protein concentration and fixed pH and explained by the opposing influence of increasing WPI aggregate concentration and decreasing concentration of micelles. Increasing the total protein concentration at the same WPI fraction caused a decrease of Tg and increase of G′. Decreasing the pH at the same protein composition led to a decrease of Tg but caused an increase of G′. Tg decreased sharply if a small amount of CaCl2 was added, but if more than about 30 mM of calcium was added, Tg increased again. Gelation of mixtures did not depend significantly on the size of the WPI aggregates, but if fractal aggregates were replaced by microgels, weaker gels were produced and the temperature of gelation was higher. Potentiometric titration curves showed that protonation of the micelles and of the WPI aggregates in the mixtures was independent. It is concluded that MC and WPI aggregates co-aggregate during gelation and there is a transition between gelation at a critical temperature for pure MC suspensions and gelation controlled by an activation energy for pure WPI aggregate suspensions

Structures et propriétés rhéologiques d'hydrogels à dynamique contrôlée obtenus par l'auto-assemblage de copolymères à blocs amphiphiles

Les copolymères à blocs amphiphiles sont des macromolécules composées d au moins un bloc hydrophile lié chimiquement à un ou plusieurs blocs hydrophobes. En milieu aqueux, ils s auto-associent pour former des micelles dont les cœurs constitués des blocs hydrophobes sont protégés de l eau par une couronne constituée des blocs hydrophiles hydratés. La majorité des copolymères à blocs amphiphiles génèrent dans l eau des micelles gelées ne présentant aucun échange de chaînes entre elles. Ceci vient du fait que l énergie nécessaire pour extraire un bloc hydrophobe du cœur des objets est beaucoup trop importante. Par conséquent, les caractéristiques des micelles sont plus contrôlées cinétiquement que thermodynamiquement. Pour diminuer cette énergie nous avons incorporé des unités hydrophile acide acrylique (AA) dans le bloc hydrophobe de poly(acrylate de n-butyle) (PnBA). L incorporation de 50% molaire d unités AA dans le bloc hydrophobe conduit à la formation d agrégats pH-sensibles dans le cas du dibloc PAA-b-P(AA0.5-stat-nBA0.5) comme montré dans une étude antérieure. Cette thèse a consisté en une analyse quantitative de la dynamique d auto-association de copolymères dibloc et tribloc amphiphiles à base d acrylate de n-butyle et d acide acrylique dont les blocs hydrophobes contiennent 50% d unités hydrophiles réparties de manière statistique. Les copolymères à blocs ont été synthétisés par polymérisation radicalaire contrôlée par ATRP. L influence de la concentration, du pH, de la température et de la force ionique sur la structure et les propriétés mécaniques des systèmes auto-assemblés a été systématiquement étudiée. Par diffusion statique de la lumière nous avons montré la présence d une concentration d agrégation critique (CAC) au-dessus de laquelle, des micelles de type étoile (dibloc) ou fleur (tribloc) sont formées par auto-association des blocs hydrophobes. A plus fortes concentrations, des interactions répulsives de type volume exclu apparaissent entre les micelles étoiles. Pour les micelles fleurs, à l inverse des interactions attractives conduisent au pontage des fleurs jusqu à l obtention de réseaux tri-dimensionnels au-dessus de la concentration de percolation. Une attraction trop importante entre les fleurs peut même conduire à une séparation de phase à forte force ionique et bas pH. En diffusion dynamique de la lumière, nous avons montré que la formation des réseaux s accompagnait de l apparition d un mode lent dont l origine a été expliquée par un mouvement balistique d hétérogénéités relaxées dans les systèmes. La vitesse de relaxation de ces hétérogénéités s avèrent être dépendantes des propriétés mécaniques des hydrogels. La formation des réseaux et la dynamique d échange des chaînes ont été étudiées par rhéologie. La viscosité augmente régulièrement avec la concentration jusqu à la concentration de percolation où une augmentation brusque de la viscosité se produit et un temps de relaxation apparaît. Le temps de vie des ponts a été finement contrôlé et modulé sur plusieurs décades par modification du pH, de la température et de la force ionique. La formation in-situ des hydrogels nous a permis de mettre en évidence un phénomène de vieillissement des réseaux après leur formation avant d atteindre un état stationnaire. Ce phénomène s est traduit par une augmentation du temps de relaxation au cours du temps avant d atteindre une valeur plateau. Ceci nous a également permis de comprendre pourquoi il était possible de générer des réseaux homogènes, par vieillissement, possédant une dynamique extrêmement lente voir nulle.Amphiphilic block copolymers are macromolecules composed of at least one hydrophilic block chemically linked to one or several hydrophobic blocks. In water, these macromolecules self-assemble to form micelles composed of a hydrophobic core surrounded by a hydrated hydrophilic corona. The majority of amphiphilic block copolymers form frozen micelles in aqueous solution. This means that there is no dynamic exchange of chains between micelles because the energy necessary to extract a hydrophobic block from the core of micelles is too high. Consequently, the characteristics of the micelles are controlled kinetically and not thermodynamically. In order to decrease this energy, we have incorporated acrylic acid units (AA) in the hydrophobic block of poly(n-butyl acrylate) (PnBA). It was previously shown that the incorporation of 50% molar of AA units in the hydrophobic block led to generation of pH-sensitive micelles in the case of PAA-b-P(AA0.5-stat-nBA0.5) diblocks. This thesis presents of a quantitative analysis of the dynamics of self-assembled amphiphilic diblock and triblock copolymer based on acrylic acid units and n-butyl acrylate units. The hydrophobic blocks contained 50% of acrylic acids units incorporated randomly. The block copolymers were synthesized by controlled radical polymerization (ATRP). The influence of the concentration, pH, temperature and the ionic strength on the structure and the mechanical properties of the self-assembled systems was systematically studied. At low concentrations, static light scattering measurements showed the formation of star-like micelles (diblock) or flower-like micelles (triblock) above a critical aggregation concentration (CAC). At higher concentrations, purely repulsive excluded volume interactions between micelles appeared in the case of diblock copolymers. In the case of triblock copolymers bridging of flower-like micelles induced in addition attractive interactions leading to network formation above the percolation concentration. At high ionic strength and low pH, we showed that the attraction between flower-like micelles became sufficiently stong to induce phase separation. Dynamic light scattering measurements showed besides a fast mode due to cooperative diffusion, a second slow relaxation mode that appeared at the percolation concentration. The origin of this mode was explained by a balistic motion induced by the relaxation of heterogeneities inside the system. The velocity of heterogeneities was determined by the mechanical relaxation of the hydrogels. The formation of the network and the exchange dynamic of chains were studied by rheology. The viscosity of solutions increased sharply at the percolation concentration. The terminal visco-elastic relaxation time of the network is related to the lifetime of bridges. It could be controlled and tuned over several decades by varing of pH, temperature and the ionic strength. The in-situ formation of networks revealed an aging of networks after their formation before they reached their stationary state. Aging caused a slow increase of the relaxation time before reaching its steady value. This explains why it is possible to generate homogeneous networks even if the network at steady is kinetically frozen.LE MANS-BU Sciences (721812109) / SudocSudocFranceF

Polymer Probe Diffusion in Globular Protein Gels and Aggregate Suspensions

International audienc