Breaking of the Bancroft rule for multiple emulsions stabilized by a single stimulable polymer

International audienceWe investigated emulsions of water and toluene stabilized by (co)polymers consisting of styrene (S) and 2-(dimethylamino)ethyl methacrylate (DMAEMA) monomer units with different compositions and structures such as a PDMAEMA homopolymer, a P(S-co-DMAEMA) random copolymer and various PS-b-PDMAEMA and PS-b-(S-co-DMAEMA) block copolymers. The model system is used to study the fundamental conditions under which the different kinds of polymer-stabilized emulsions (direct oil in water, inverse water in oil and multiple emulsions) are stabilized or destabilized by pH change (at constant temperature). Polymer properties like chain conformation at the toluene-water interface as probed by SANS and neutron reflectivity at the liquid-liquid interface, the oil-water partitioning of the polymer chains (Bancroft's rule of thumb) as determined by UV spectroscopy and interfacial tensions measured by the rising and spinning drop techniques are determined. Overall, results evidence that the curvature sign, as defined by positive and negative values as the chain segments occupy preferentially the water and toluene sides of the interface respectively, reliably predicts the emulsion kind. In contrast, the Bancroft rule failed at foreseeing the emulsion type. In the region of near zero curvature the crossover from direct to inverse emulsions occurs through the formation of either unstable coexisting direct and inverse emulsions (i) or multiple emulsions (ii). The high compact adsorption of the chains at the interface as shown by low interfacial tension values does not allow to discriminate between both cases. However, the toluene-water partitioning of the polymeric emulsifier is still a key factor driving the formation of (i) or (ii) emulsions. Interestingly, the stabilization of the multiple emulsions can be tuned to a large extent as the toluene-water polymer partitioning can be adjusted using quite a large number of physico-chemical parameters linked to polymer architecture like diblock length ratio or polymer total molar mass, for example. Moreover, we show that monitoring the oil-water partitioning aspect of the emulsion system can also be used to lower the interfacial tension at low pH to values slightly higher than 0.01 mN m-1, irrespective of the curvature sign

Adsorption dynamics of hydrophobically modified polymers at an air-water interface

The adsorption dynamics of a series of hydrophobically modified polymers, PAAαCn, at the air-water interface is studied by measuring the dynamic surface tension. The PAAαCn are composed of a poly(acrylic acid) backbone grafted with a percentage α of C8 or C12 alkyl moieties, at pH conditions where the PAA backbone is not charged. The observed adsorption dynamics is very slow and follows a logarithmic behavior at long times indicating the building of an energy barrier which grows over time. After comparison of our experimental results to models from the literature, a new model which accounts for both the deformation of the incoming polymer coils as well as the deformation of the adsorbed pseudo-brush is described. This model enables to fit very well the experimental data. The two fitting parameters give expected values for the monomer size and for the area per adsorbed polymer chain.This article is uploaded in "arXiv.org" https://arxiv.org/abs/1706.0710

New substituted polymethylenes by free radical polymerization of bulky fumarates and their properties

The free radical polymerization of fumarates (DRFs) bearing sterically crowded ester groups was investigated. Various new monomers were synthesized and their polymerization behaviour was looked into. In bulk polymerizations at 60 degrees C initiated with 2,2'-azobisisobutyronitrile (AIBN), their polymerization reactivity depends sensitively on the structure of both alkyl ester groups. Besides the polymerization of simple alkyl fumarates, the extension to substituents bearing functional groups was explored. Functional groups have been found to be barely tolerated, successful polymerization requiring the use of particular initiators. The polymers were characterized by H-1-NMR and C-13-NMR spectroscopies, and wide-angle X-ray diffraction (WAXD), and some thermal properties were examined. (C) 2000 Elsevier Science Ltd. All rights reserved

Growth mechanism of polymer membranes obtained by H-bonding across immiscible liquid interfaces

Complexation of polymers at liquid interfaces is an emerging technique to produce all-liquid printable and self-healing devices and membranes. It is crucial to control the assembly process but the mechanisms at play remain unclear. Using two different reflectometric methods, we investigate the spontaneous growth of H-bonded PPO-PMAA membranes at a flat liquid-liquid interface. We find that the membrane thickness h grows with time t as h~t^(1/2), which is reminiscent of a diffusion-limited process. However, counter-intuitively, we observe that this process is faster as the PPO molar mass increases. We are able to rationalize these results with a model which considers the diffusion of the PPO chains within the growing membrane. The architecture of the latter is described as a gel-like porous network, with a pore size much smaller than the radius of the diffusing PPO chains, thus inducing entropic barriers that hinder the diffusion process. From the comparison between the experimental data and the result of the model, we extract some key piece of information about the microscopic structure of the membrane. This study opens the route toward the rational design of self-assembled membranes and capsules with optimal properties

Growth mechanism of polymer membranes obtained by H-bonding across immiscible liquid interfaces

Complexation of polymers at liquid interfaces is an emerging technique to produce all-liquid printable and self-healing devices and membranes. It is crucial to control the assembly process but the mechanisms at play remain unclear. Using two different reflectometric methods, we investigate the spontaneous growth of H-bonded PPO-PMAA membranes at a flat liquid-liquid interface. We find that the membrane thickness h grows with time t as h~t^(1/2), which is reminiscent of a diffusion-limited process. However, counter-intuitively, we observe that this process is faster as the PPO molar mass increases. We are able to rationalize these results with a model which considers the diffusion of the PPO chains within the growing membrane. The architecture of the latter is described as a gel-like porous network, with a pore size much smaller than the radius of the diffusing PPO chains, thus inducing entropic barriers that hinder the diffusion process. From the comparison between the experimental data and the result of the model, we extract some key piece of information about the microscopic structure of the membrane. This study opens the route toward the rational design of self-assembled membranes and capsules with optimal properties

Growth mechanism of polymer membranes obtained by H-bonding across immiscible liquid interfaces

Complexation of polymers at liquid interfaces is an emerging technique to produce all-liquid printable and self-healing devices and membranes. It is crucial to control the assembly process but the mechanisms at play remain unclear. Using two different reflectometric methods, we investigate the spontaneous growth of H-bonded PPO-PMAA membranes at a flat liquid-liquid interface. We find that the membrane thickness h grows with time t as h~t^(1/2), which is reminiscent of a diffusion-limited process. However, counter-intuitively, we observe that this process is faster as the PPO molar mass increases. We are able to rationalize these results with a model which considers the diffusion of the PPO chains within the growing membrane. The architecture of the latter is described as a gel-like porous network, with a pore size much smaller than the radius of the diffusing PPO chains, thus inducing entropic barriers that hinder the diffusion process. From the comparison between the experimental data and the result of the model, we extract some key piece of information about the microscopic structure of the membrane. This study opens the route toward the rational design of self-assembled membranes and capsules with optimal properties

Formulation d'émulsions multiples stables, stimulables et biocompatibles

International audienceLes émulsions eau-dans-huile-dans-eau (E/H/E) constituent des vecteurs intéressants pour l'encapsulation et le relargage contrôlé de principes actifs. Nous avons développé un copolymère amphiphile (PDMS-b-PDMAEMA) capable de former ce type d'émulsions en une seule étape, et de les stabiliser pendant plus d'une année, bien qu'elles soient connues pour leur manque de stabilité lorsqu'elles sont formulées plus classiquement en deux étapes. Le copolymère utilisé est aussi sensible au pH et à la force ionique, ce qui nous permet, en faisant varier ces deux paramètres, de former des émulsions directes, E/H/E ou inverses, et de provoquer la déstabilisation contrôlée de nos émulsions grâce à une variation de pH. Enfin, notre polymère est biocompatible et nous envisageons donc de développer des applications pour l'homme. Notre but final serait d'utiliser notre système pour coencapsuler une molécule hydrophile et une molécule hydrophobe au sein de la même émulsion double, pour les protéger de l'oxydation durant le stockage, et pouvoir les relarguer de façon contrôlée lors de leur arrivée dans l'estomac où le pH diminue. Nous présenterons des premiers tests d'encapsulation d'une molécule modèle, le saccharose, ainsi que d'un antioxydant venant du thé vert, la catéchine. Deux modes d'émulsification seront aussi décrits: la voie mécanique, classique, et la voie microfluidique

Reactivity of Si-H and Si-vinyl end functionalized siloxanes toward PBT: A model system study

Based on a previous model compound study, we investigated the melt reactivity of Si-H and Si-vinyl end-functionalized siloxane oligomers towards polybutylene terephthalate (PBT) in the presence of platinum(0)-catalyst. We clearly observed grafting of the Si-H functional oligomer onto PBT and have also indications for grafting of the Si-vinyl oligomer. The grafting efficiencies and kinetics of both siloxane model compounds are compared and roughly quantified by infrared spectroscopy. This result indicates that the same reactive groups that are used to vulcanize silicone gums at high temperature can also be used to favor compatibility with a polyester matrix, opening perspectives toward polyester-siloxane thermoplastic elastomers

Neutron reflectivity measurements at the oil/water interface for the study of stimuli-responsive emulsions

International audienceStable stimuli-responsive emulsions between oil and water are formed with an amphiphilic block copolymer bearing polystyrene (PS) and poly(dimethylaminoethyl methacrylate) (PDMAEMA) moieties. Different kinds of emulsions like direct, multiple or inverse ones are reproducibly formed as a function of chemical parameters such as p H and salt concentration. To test the correlation between the different nature of the emulsion and the conformation of the polymer chain at the interface, neutron reflectometry at the oil/water interface was carried out. An original sample cell was built and the procedure to get reliable results with it on the FIGARO reflectometer at the Institut Laue-Langevin is described. Results show that for direct emulsions, the copolymer is much more extended on the water side than on the oil side. In the case where multiple emulsions are stabilized, the conformation is strongly modified and is compatible with a more equilibrated extension of the chain on both sides. The inverse case shows that the extension in oil is stronger than in water. These results are discussed in term of polymer brushes (charged or neutral) extension with respect to salt addition and hydrophobic interactions