175 research outputs found

    Novel stabilisation of emulsions with polyelectrolyte complexes

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    The concept of a novel stabiliser of oil-water emulsions has been put forward, being the polyelectrolyte complex (PEC) formed between oppositely charged water-soluble polymers in cases where either polymer alone is incapable of stabilising an emulsion. Four oppositely charged synthetic polyelectrolytes (strong and weak) are selected, which allowed four polymer mixtures to be studied. The behaviour of their mixtures in water is correlated with that of emulsions after addition of oil.Aqueous polymer mixtures are investigated via dynamic light scattering to determine the size of the aggregates. Moreover, various optical techniques are used to identify the type of associative phase separation (precipitation or complex coacervation) and their shape. The effects of polyelectrolyte (PEL) mixing ratio, pH, [PEL] and salt content are studied in detail. In general, PEC particles are obtained as a result of a strong electrostatic interaction while complex coacervates arise from weak interactions. Around equal mole fractions of the two polymers, the zeta potential of the aggregates reverses in sign. Spherical complexes of diameters of few hundreds nanometres are obtained at low polyelectrolyte concentration. However, by increasing the initial [PEL], primary particles aggregate. Aggregated PEC particles have an irregular shape while coacervate droplets, which contain high amounts of water, are spherical and have no special internal structure, as observed from TEM images. Under specific conditions, coacervate droplets completely coalesce giving rise to the formation of the so-called coacervate phase. The effect of increasing the salt concentration is comparable in both PEC precipitates and coacervates and causes an initial destabilisation of the aqueous dispersion due to complex aggregation, followed by dissolution of the electrostatic complex at high salt concentrations.For the emulsion study, the same parameters as for aqueous PEC dispersions are evaluated, as well as the oil volume fraction (ϕo). The complete study is carried out with dodecane despite oils of different chemistry and polarity have also been considered throughout this thesis. The most stable emulsions to both creaming and coalescence are prepared with aqueous PEC dispersions containing complexes of almost neutral charge. By increasing the polyelectrolyte concentration, emulsions become more stable. However, at high [PEL], aggregation levels are relatively high and emulsion stability is slightly worse as big particles can easily be dislodged from the oil-water interface compared to smaller ones. From cryo-SEM images, close-packed particle layers are detected at drop interfaces as well as particle aggregation in the continuous phase. By increasing the oil volume fraction in the emulsion, the droplet diameter increases constantly up until a point where oil droplets appear to be deformed and the viscosity of the emulsion increases substantially. This suggests the formation of high internal phase emulsions (HIPEs), which is rare in particle-stabilised systems, where catastrophic phase inversion is the usual outcome. Taking advantage of the intrinsic fluorescence of the used PEL, confocal microscopy turns out to be a useful technique to visualise where PEC particles are placed upon homogenisation. At high oil volume fractions, particles are only detected around oil droplets, whereas at low oil volume fractions, excess particles remain at the continuous aqueous phase providing extra stability against coalescence. As for aqueous PEC dispersions, the concentration of salt has a remarkable effect on emulsion stability. For emulsions stabilised with PEC particles, by increasing the aggregation level, emulsions become completely unstable. However, at a relatively high salt content, emulsions re-stabilise due to adsorption of uncharged individual polymer molecules. Emulsions with coacervate droplets can be prepared by the addition of oil stepwise and multiple homogenisation steps. However, unlike PEC particles, the system is sensible to the oil type. The feasibility of the coacervate phase to spread at the oil-water interface is discussed in terms of the relevant spreading coefficients and predictions are compared with experiments for a range of oils. We encounter oils whose drops become engulfed by the coacervate phase as well as oils where no engulfing occurs.Therefore, from the findings obtained from four different polyelectrolyte combinations, we can claim that emulsion stability is given by the presence of PEC at the oil-water interface as individual PEL are not surface-active on their own. Despite this work being a complete starting point for the basic understanding of emulsions stabilised by mixtures of oppositely charged polymers, we are not yet in a position to predict definite rules of behaviour in both aqueous PEC dispersions and emulsions containing them. Further investigation of other polyelectrolyte combinations is required to develop a better understanding of this area

    Polyelectrolyte/Surfactant Mixtures: A Pathway to Smart Foams

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    This review deals with liquid foams stabilized by polyelectrolyte/surfactant (PS) complexes in aqueous solution. It briefly reviews all the important aspects of foam physics at several scales, from interfaces to macroscopic foams, needed to understand the basics of these complex systems, focusing on those particular aspects of foams stabilized by PS mixtures. The final section includes a few examples of smart foams based on PS complexes that have been reported recently in the literature. These PS complexes open an opportunity to develop new intelligent dispersed materials with potential in many fields, such as oil industry, environmental remediation, and pharmaceutical industry, among others. However, there is much work to be done to understand the mechanism involved in the stabilization of foams with PS complexes. Understanding those underlying mechanisms is vital to successfully formulate smart systems. This review is written in the hope of stimulating further work in the physics of PS foams and, particularly, in the search for responsive foams based on polymer-surfactant mixtures.Fil: Ritacco, HernĂĄn Alejandro. Consejo Nacional de Investigaciones CientĂ­ficas y TĂ©cnicas. Centro CientĂ­fico TecnolĂłgico Conicet - BahĂ­a Blanca. Instituto de FĂ­sica del Sur. Universidad Nacional del Sur. Departamento de FĂ­sica. Instituto de FĂ­sica del Sur; Argentina. Universidad Nacional del Sur. Departamento de FĂ­sica; Argentin

    Synthetic Approach towards P3EHT-b-PSS Conjugated and Ionic Block Copolymer

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    This work is focused on the design, synthesis and characterization of block copolymers (BCPs) containing both conjugated and polyelectrolyte blocks. This type of rod-coil BCP is considered to have microphase separation which is different compared to coil-coil BCPs. The conjugated block used for studies is regioregular poly(3-alkylthiophene) (P3AT), which can be synthesized by Grignard metathesis (GRIM) polymerization. The polyelectrolyte block used is polystyrene sulfonate with well-defined structure, which can be synthesized by atom transfer radical polymerization (ATRP). Chapter II is focused on evaluation of synthetic routes for P3EHT-b-PSS, including using bifunctional initiator to synthesize both blocks, and separately synthesizing end-functionalized polymer blocks for Cu-Catalyzed Azide-Alkyne Cycloaddition (CuAAC). The advantages and disadvantages of these synthetic routes were summarized to find the best to synthesize P3EHT-b-PSS. Chapter III is focus on the detailed synthesis of propynyl-terminated P3EHT and azido-terminated PNSS, which are the two blocks used for CuAAC to synthesize P3EHT-b-PNSS. The optimum conditions for synthesizing both blocks were found. This synthetic route can achieve better MW, PDI and end-group control compared to other reported synthetic attempts for similar end-group functionalities. CuAAC was also tried to synthesize P3EHT-b-PNSS by using previously mentioned polymer blocks

    Imidazole-phosphate polymers : Acid-base properties, association with oligonucleotides and oligosilicates

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    Water-soluble imidazole-phosphate copolymers were obtained by copolymerization of 1-vinylimidazole with vinyl acetate with subsequent phosphorylation under the action of phosphoric acid. The introduction of phosphate units into the vinylimidazole chain unexpectedly increased the basic properties of the polymer, the buffer capacity at pH 5-7 and activity in coordination with DNA oligonucleotides, which is important in gene therapy and genetic engineering. The condensation of silicic acid in the presence of new polymers leads to the appearance of composite nanoparticles, which are a model of silicon transport vesicles in nature, as well as a new precursor of silicon materials. (C) 2021 Elsevier B.V. All rights reserved.Peer reviewe

    Ionene and ionene alkyl sulfate stoichiometric complexes: Temperature and humidity sensitive materials

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    Stoichiometric polyelectrolyte-surfactant complexes represent a type of comb-shaped polymers, in which every polymer chain unit has an electrostatically bound "side chain". These complexes are water-insoluble. In the solid state they assemble spontaneously into mesogenic structures. The [X,Y]-ionenes ([(CH2)XN+(CH3)2(CH2)YN+(CH3)2]nBr-2n) investigated formed stoichiometric complexes with alkyl sulfates. The ionene alkyl sulfate complexes display mesogenicity, i.e. optically isotropic dry complexes underwent lyotropic and thermotropic phase transitions to the optically anisotropic phase (and vice versa) under controlled relative humidity. The optically anisotropic phases exhibited hexagonal textures as revealed by polarizing microscopy. A new feature is the lyotropic transition brought about by the uptake of water through the gas phase. The complexes were all sensitive to both humidity and temperature. In principle, the effects can be applied to measure humidity

    Stimuli-responsive poly(ampholyte)s containing L-histidine residues: synthesis and protonation thermodynamics of methacrylic polymers in the free and in the cross-linked gel forms

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    Methacrylate-structured poly(ampholyte)s were synthesized in the homopolymer and copolymer forms starting from the N-methacryloyl-L-histidine (MHist) and the N-isopropylacrylamide (NIPAAm). They were also obtained in the cross-linked (hydrogel) form, showing a close thermodynamic behaviour as that shown by the corresponding soluble free polymer analogues. Viscometric data revealed that the minimum hydrodynamic volume of the polymer at its isoelectric point (pH 5) shifted to lower pHs as the NIPAAm content increased, and beyond a critical low MHist content the reduced viscosity decreased, even at low pHs. The phenomenon was attributed to hydrophobic forces between the isopropyl groups outweighing the repulsive electrostatic interactions of the polymer in the positively charged form. A similar behaviour was shown by the corresponding hydrogel. The latter also revealed a different phase transition phenomenon induced by external stimuli (temperature, pH, ionic strength, electric current) when compared to the acrylate-structured analogues. The polyMHist, as well as the corresponding monomer, was found for two days to be non toxic against the mouse osteoblasts (MC3T3-E1)

    Hybrid Inorganic Core- Conjugated Polymer Shell Nanoparticles Prepared by Surface-Initiated Polymerization

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    Hybrid core-shell nanoparticles have attracted attention due to their unique characteristics which combine properties of the inorganic core and organic shell in a way that generates new properties which do not exist for the two individual parts. This makes them promising candidates for the design of stimuli-responsive materials, chemo- and biosensors, and various biomedical applications. In this dissertation research, we designed and prepared a series of hybrid environmentally responsive nanoparticles where fluorescent block copolymers, including various combinations of polythiophene (PT), poly(p-phenylene) (PPP), poly(3-hexylthiophene) (P3HT) and polyallene (PA), were grafted on the surface of inorganic nanoparticles using surface-confined Kumada catalyst-transfer polymerization. The studied inorganic core included silica and silica on gold nanoparticles. We found that the photophysical properties of the hybrid nanoparticles were strongly dependent on the proximity of the organic polymer shells to the inorganic surface (Au), polymer block sequence in the organic shell, and external stimuli such as solvent or pH. This dissertation primarily focuses on the development and preparation of well-defined hybrid inorganic core – organic polymer materials. This preparation stems from the well-defined and highly efficient surface-confined Ni(II) catalytic initiator which provided controlled chain-growth polymerization to form conjugated polymer shells. The role of inorganic core and other structural effects on the properties of the conjugated polymer shells were studied using both steady-state and time resolved transient spectroscopies. Better knowledge and understanding of these fundamental properties will enable rational control of these properties at the molecular level and will create the fundamental basis for the design of future optoelectronic and sensing materials

    The impact of global and local composition on the stability of Triple Helical DNA

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    It is common practise in antisense technology to view third strand binding to be controlled by the same principles which are found to determine the stability of the double helix. In contrast to this view based on a general consideration of the various forces contributing to the binding energy of the third strand it was proposed that the dominant contributions will originate from electrostatic interactions. These electrostatic contributions can be subdivided into sequence independent repulsive forces between the negatively charged backbones and into sequence dependent attractive forces between the positively charged protonated Hoogsteen cytosines and the backbone phosphates. The observable changes in the stability of triple helices should be a reflection of the number (global composition) and distribution (local composition) of cytosines in the third strand. To this aim two families of 38-mer oligonucleotides were synthesized, which have as a common design feature a linear array of 10 homopurine bases followed by 10 homopyrimidine bases as Watson & Crick complementary strand to the homopurine region and ending in a 10 homopyrimidine residue stretch which binds to the W&C helix via Hoogsteen base-pairing. This arrangement of homopurine and homopyrimidine sections with connecting pyrimidine linkers allows the formation of intramolecular triple helices of predetermined stoichiometry and strand orientation. Physical (UV-spectroscopy, CD-spectroscopy and fluorimetry) and biochemical techniques (P1-nuclease digestion) have been used to show that the oligonucleotides undergo a stepwise folding process from a random coil into a hairpin with 3'dangling tail and then into a intramolecular triple helix. This folding occurs as a function of pH and/or ionic strength. The effect of local and global composition on the stability of the three conformational transitions has been evaluated from a comparison of the melting temperatures and the behavior of the phase boundaries of the different oligonucleotides. As the result of this thesis the following general rules emerge: The stability of the third strand depends on the particular combination of sequence, pH and ionic strength. At physiological conditions (pH 7.1, 150 mM Naâș) thymines and cytosines contribute equally to the stability (global effect) provided that the cytosines are spaced by more than one thymine. (local effect). Below pH 7.1 (150 mM Naâș) the stability increases linearly with the number of cytosines and at pH above pH 7.1 ( 150 mM Naâș) it decreases. At ionic strength below 400 mM Naâș (pH 6. 75) the stability increases with the number of cytosine while above 400 mM Naâș (pH 6. 75) it decreases. Based on these results a rational approach for the design of oligonucleotide third strands and the choice of appropriate environmental conditions for the formation of a particular triple helix becomes feasible
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