Phase separations, polymer-colloid-solvent

Stability in mixtures of colloidal particles and polymer molecules, dispersed in a solvent, has been the subject of experimental and theoretical investigations for a long time and it has applications in diverse fields such as paint technology, wastewater treatment, emulsion polymerization, biology etc. It has now been well recognized that polymer molecules can be used to induce either stabilization or flocculation (phase separation) in colloidal dispersions. It is important to distinguish between polymers which are adsorbed on the particle surface and those that are free in solution because the two situations usually lead to qualitatively different effects. Stability imparted by adsorbed polymers is known as steric stabilization and the flocculation or phase separation caused by the free polymer is due... 

Several examples of polymer-polymer incompatibility in aqueous solution are given in Reference 3, whereas de Hek and Vrij (4) recently described a phase separation in a solvent in which both polymer and colloidal spheres were dissolved. Ph se separation can be suppressed by reducing the molecular weight of the polymers. Reduction of the salinity reduces the size of the surfactant micelles and indeed also the polymer-surfactant incompatibilities (5,6). Actually, reduction of the salinity of the polymer drive, even without direct reference to polymer/surfactant incompatibilities, has recently become a favorable recipe for successful micellar floods (7-9). 

In the micellar region the trend to decreasing colloid stability is arrested and a partial improvement, in line with the enhanced level of polymer adsorption, is noted until the conditions for gross phase separation are reached. Only the intermediate block copolymer BC 42 shows indications of discontinuities in behavior at the solvent composition for micelle formation. The results presented here do not show the sharp transition from stability to instability found experimentally (4,8,17) by Napper and generally expected on theoretical grounds. However, there are important differences in experimental methodology that must be emphasised. 

As the polymer concentration increases, interchain association inevitably occurs, but some amphiphilic chains can undergo a limited interchain association to form a stable mesoglobular phase that exists between microscopic single-chain globules and macroscopic precipitation. As expected, when the solvent quality changes from good to poor, intrachain contraction and interchain association occur simultaneously and there exists a competition between these two processes. Such a competition depends on the comonomer composition and distribution on the chain backbone and also depends on the rate of micro-phase separation. When intrachain contraction happens quickly and prior to interchain association, smaller mesoglobules are formed. A proper adjustment of the rates of intrachain contraction and interchain association can lead to polymeric colloidal particles with different sizes and structures. 

Nanostructures primarily result from polyelectrolyte or interpolyelectrolyte complexes (PEC). The PEC (also referred to as symplex [23]) is formed by the electrostatic interaction of oppositely charged polyelectrolytes (PE) in solution. The formation of PEC is governed by physical and chemical characteristics of the precursors, the environment where they react, and the technique used to introduce the reactants. Thus, the strength and location of ionic sites, polymer chain rigidity and precursor geometries, pH, temperature, solvent type, ionic strength, mixing intensity and other controllable factors will affect the PEC product. Three different types of PEC have been prepared in water [40] (1) soluble PEC (2) colloidal PEC systems, and (3) two-phase systems of supernatant liquid and phase-separated PEC. These three systems are respectively characterized as ... 

In recent years other colloid systems—such as microemulsions—have been found to exhibit a wide range of structures [81,82]. We can observe spontaneous phase separation, flocculation and formation of complex bicontinuous structures after the formation of these colloidal systems. It is not possible to form a colloidal system, whether in a polymeric matrix, in water, or in an organic solvent, without a supercritical input of energy, which is provided by turbulent flow conditions during the formation of microemulsions or melt fracture conditions [86] during the formation of colloidal systems in polymers. It seems that a general principle for the behaviour of multiphase systems has been found. 

Everett and Stageman (1978a) have also claimed that at the CFPT, surface attached polymer chains phase separate from the dispersion medium and collapse on to the surface of the colloidal particle, this process occurring in dispersion media that are better solvents than 0-solvents. To-date, there is no experimental evidence to support their speculations for those systems that display a strong correlation between the CFPT and the 0-point. 

Upon achieving the limiting size of the colloid particles and with increasing polymerization of the oligomer, the concentration of particles per unit volume of the solvent increases, causing loss of stabihty and producing aggregation with formation of a separate phase. Separation of surfactant from the polymer results in the abrupt increase of its concentration in the siuface layer when the system becomes incompatible a decrease in its siuface tension is observed. 

Nanoparticles can also be obtained from a polymer that has previously been prepared according to a totally independent method. The general principle is based on the solubility properties of the polymer. A diluted solution of the polymer is prepared and a phase separation is induced by addition of a non-solvent or by a salting-out effect. Once the proper conditions to form polymeric colloids are identified, the particles can be stabilised either by elimination of the polymer solvent by evaporation or by chemical cross-linking of the polymer. 

Consider the complexity involved in modeling steric stabilization with a diblock copolymer. The reservoir bulk solution of copolymer is usually dilute (<1 wt % polymer) and the copolymer and solvent equilibrate between the bulk and surface regions. However, as solvent quality is decreased to the LCST phase boundary, the bulk solution will also separate into polymer-rich and polymer-lean phases. In addition, many diblock copolymers form self-assembled aggregates such as micelles and lamellae, if the concentration is above the critical micelle concentration. Thus, stabilizer can partition among up to four phases as solvent quality or polymer concentration is changed. The unique density dependence of supercritical fluids adds another dimension to the complex phase behavior possible. In the theoretical studies discussed below, surfactant adsorption energy, solubility, and concentration are chosen carefully to avoid micelle formation or bulk phase separation, in order to focus primarily on adsorption and colloid stability. 

In dispersion polymerization, the monomer and the initiator are soluble in the continuous solvent phase, the polymer phase separates but is stabilized as a colloid with stabilizer additives. Polymerization proceeds to high degrees of polymerization and the end product is recovered as spherical particles. In emulsion polymerization, the initiator is preferentially dissolved in the continuous phase and not the monomer phase, and the monomer does not have high solubility in the continuous phase. 

Similar materials could be obtained by an emulsification method [253]. Nematic liquid crystal is emulsified into an aqueous dispersion of a water-insoluble polymer colloid (i.e., latex paint). An emulsion is formed which contains a droplet with a diameter of a few microns. This paint emulsion is then coated onto a conductive substrate and allowed to dry. The polymer film forms around the nematic droplets. To prepare an electrooptical cell a second electrode is laminated to the PDLC film [253]. In the phase separation and solvent-casting methods the chloroform solutions of liquid crystal and polymer are also used [254, 255]. The solution is mixed with the glass spheres of the required diameter to maintain the desired gap thickness and pipetted onto a hot (140 °C) ITO-coated glass substrate [255]. After the chloroform has completely evaporated another ITO-coated glass cover is pressed onto the mixture and then it is cooled down. Structural characteristics of the PDLC films are controlled by the type of liquid crystal and polymer used, the concentration of solution, the casting solvent, the rate of solvent evaporation, perparation temperature, etc. [254]. 

For polymer solutions, a decrease in the solvent thermodynamic quality tends to decrease the polymer-solvent interactions and to increase the relative effect of the polymer-polymer interactions. This results in intermolecular association and subsequent macrophase separation. The term colloidally stable particles refers to particles that do not aggregate at a significant rate in a thermodynamically unfavourable medium. It is usually employed to describe colloidal systems that do not phase separate on the macroscopic level during the time of an experiment. Typical polymeric colloidally stable particles range in size from 1 nm to 1 xm and adopt various shapes, such as fibres, thin films, spheres, porous solids, gels etc. 

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