Polymer-solvent mixtures, stability

Phase diagrams of polymer-polymer blend systems and polymer-solvent mixtures can be constructed as a function of tanperature vs. composition. Two important factors need to be considered when constructing phase diagrams (1) equilibrium and (2) stability. It can be shown from the laws of thermodynamics and reversibility that the equilibrium sate of a closed systan is that state at which the total Gibbs free energy is a minimum with respea to aU possible changes at the given temperature and pressure. At equilibrium ... 

Monomer-soluble initiators are used in this polymerization technique. The monomer phase containing an initiator is dissolved in an inert solvent or solvent mixture including a steric stabilizer. The polymers or oligomer... 

Theoretical studies of the role of polymer additives lag behind their analogs in electrostatic stability since polymer molecules have considerably more configurational freedom and since the interaction of the polymer molecules with the solvent is an inseparable part of phenomena in polymer-colloid mixtures. We begin with some of the general issues and a thermodynamic analysis of the role of polymer on stability in Section 13.5. 

Solvent blending Solvent blending, also called solution intercalation in the case of clay and other nanolayers, involves both dispersing the nanoadditive and dissolving the matrix polymer in a solvent or a solvent mixture. Three parameters have been considered to be important, particularly for clays, in choosing the surface treatment of the nanoadditives with this process The structure of the modifier, its miscibility with the polymer, and its thermal stability. The miscibility of the modifier here has two meanings miscibility with both the final polymer and the solvent chosen to dissolve the polymer. The modifier structure and its miscibility are perhaps more important than the thermal... 

In addition to the molecular weight of the free polymer, there axe other variables, such as the nature of the solvent, particle size, temperature, and thickness of adsorbed layer which have a major influence on the amount of polymer required to cause destabilization in mixtures of sterically stabilized dispersions and free polymer in solution. Using the second-order perturbation theory and a simple model for the pair potential, phase diagrams relat mg the compositions of the disordered (dilute) and ordered (concentrated) phases to the concentration of the free polymer in solution have been presented which can be used for dilute as well as concentrated dispersions. Qualitative arguments show that, if the adsorbed and free polymer are chemically different, it is advisable to have a solvent which is good for the adsorbed polymer but is poor for the free polymer, for increased stability of such dispersions. Larger particles, higher temperatures, thinner steric layers and better solvents for the free polymer are shown to lead to decreased stability, i.e. require smaller amounts of free polymer for the onset of phase separation. These trends are in accordance with the experimental observations. 

This method, also known as the nanoprecipitation method, can be applied to numerous synthetic poly-mers. ° In general, the polymer is dissolved in acetone and the polymer solution is added into water. The acetone is then evaporated to complete the formation of the particles. Surface active agents are usually added to water to ensure the stability of the polymer particles. This easy technique of nanoparticle preparation was scaled up for large batch production. It leads to the formation of nanospheres. Nanocapsules can easily be prepared by the same method just by adding a small amount of an organic oil in the polymer solution.When the polymer solution is poured into the water phase, the oil is dispersed as tiny droplets in the solvent-non-solvent mixture and the polymer precipitates on the oil droplet surface. This method leads to the preparation of oil-containing nanocapsules... 

Polyphenylquinoxaline (PPQ) This resin is supplied by NASA Langley as monoether polyphenylquinoxaline in a solvent mixture 1 1 of practical grade m-cresol and mixed xylenes. The solid content is about 16.6% based upon final polymer weight. PPQ has demonstrated excellent elevated temperature stability and represents a different family of polymers for evaluation in this program. The chemistry of this polymer is shown below. 

The simple Flory-Huggins %-function, combined with the solubility parameter approach may be used for a first rough guess about solvent activities of polymer solutions, if no experimental data are available. Nothing more should be expected. This also holds true for any calculations with the UNIFAC-fv or other group-contribution models. For a quantitative representation of solvent activities of polymer solutions, more sophisticated models have to be applied. The choice of a dedicated model, however, may depend, even today, on the nature of the polymer-solvent system and its physical properties (polar or non-polar, association or donor-acceptor interactions, subcritical or supercritical solvents, etc.), on the ranges of temperature, pressure and concentration one is interested in, on the question whether a special solution, special mixture, special application is to be handled or a more universal application is to be foxmd or a software tool is to be developed, on munerical simplicity or, on the other hand, on numerical stability and physically meaningftd roots of the non-linear equation systems to be solved. Finally, it may depend on the experience of the user (and sometimes it still seems to be a matter of taste). 

TLC remains one of the most widely used techniques for a simple and rapid qualitative separation. The combination of TLC with spectroscopic detection techniques, such as FTIR or nuclear magnetic resonance (NMR), is a very attractive approach to analyze polymer additives. Infrared microscopy is a powerful technique that combines the imaging capabUities of optical microscopy with the chemical analysis abilities of infrared spectroscopy. FTIR microscopy allows obtaining of infrared spectra from microsized samples. Offline TLC-FTIR microscopy was used to analyze a variety of commercial antioxidants and light stabilizers. Transferring operation and identification procedure by FTIR takes about 20 min. However, the main drawbacks of TLC-FTIR are that TLC is a time-consuming technique and usually needs solvent mixtures, which makes TLC environmentally unsound, analytes must be transferred for FTIR analysis, and TLC-FTIR cannot be used for quantifying purposes. 

In the above descriptions we concentrated on situations where a polar background solvent was implicitly assumed. In apolar solvents double layer repulsion is diflhcult to achieve because dissociation, leading to charged surface groups, is less likely to occur and it becomes essential to stabilize colloids with polymers as to prevent instabilities. In the first decades after the establishment of the DLVO theory most papers on forces between colloidal particles focused on Van der Waals and double layer interactions. Forces of other origin such as polymeric steric stabilization [17], depletion [40] or effects of a critical solvent mixture [41] gained interest at a later stage. 

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