Multicomponent solvents, polymers

Equilibrium phenomenon, operative in polymer solutions, in multicomponent solvents, and in polymer networks swollen by multicomponent solvents, that produces differences in solvent composition in the polymer-containing region and in the pure solvent which is in thermodynamic equilibrium with that region. 

Certain SEC applications solicit specific experimental conditions. The most common reason is the limited sample solubility. In this case, special solvents or increased temperature are inavoid-able. A possibility to improve sample solubility and quality of eluent offer multicomponent solvents (Sections 16.2.2 and 16.8.2). The selectivity of polymer separation by SEC drops with the deteriorating eluent quality due to decreasing differences in the hydrodynamic volume of macromolecules with different molar masses. The system peaks appear on the chromatograms obtained with mixed eluents due to preferential solvation of sample molecules (Sections 16.3.2 and 16.3.3). The multicomponent eluents may create system peaks also as a result of the (preferential) sorption of their components within column packing [144,145]. The extent of preferential sorption is often sensitive toward pressure variations [69,70,146-149]. Even if the specific detectors are used, which do not see the eluent composition changes, it is necessary to discriminate the bulk sample solvent from the SEC separated macromolecules otherwise the determined molecular characteristics can be affected. This is especially important if the analyzed polymer contains a tail of fractions possessing lower molar masses (Sections 16.4.4 and 16.4.5). 

Irani, C. A. Cozewith, "Lower Critical Solution Temperature Behavior of Ethylene Propylene Copolymers in Multicomponent Solvents," J. Appl. Polym. Sci., 31, 1879 (1986). 

Heil and Prausnitz proposed an FH/UNIQUAC-type model, which has been applied to a number of ternary liquid-liquid PS-mixed solvent systems. Satisfactory results were obtained for the ternary systems using solely two parameters per binary. These results show the potential of local composition concept (UNIQUAC, UNIFAC) for multicomponent polymer systems. The required binary parameters were estimated from solvent-polymer VLE data. 

The photoresist is sensitive to the incident radiation and it undergoes (photo) chemical transformations. Photoresists are complex formulations consisting of organic solvent, polymer, photoactive compound (PAC), base, and other chemicals that confer it desired properties. In one-component resists, the polymer is the photoactive compound, whereas in two- or multicomponent resists the photoactive compound undergoes photochemical transformation resulting in new species that interact with the radiation inert polymer, triggering transformations that alter the solubility in the exposed areas. 

Phase-Inversion Process. Most tortuous-pore membranes are made by a casting process known as "phase inversion." Figure 2.2 is a simplified schematic of a casting machine which makes cellulose ester membranes. Typically, a casting solution made up of the polymer and a multicomponent solvent system is metered onto a stainless steel belt or web. The belt passes through a series of environmental chambers usually containing water vapor at elevated temperatures. The more volatile solvents evaporate and the water vapor precipitates the polymer around the less volatile solvent which becomes the "pore-former." Subsequently, (not shown in Figure 2.2), after the membrane is formed, the residual solvents are washed out of the pores, surfactants are added, and the membrane is dried. 

Irani C.A., Cozewith C., Lower critical solution temperature behaviour of ethylene propylene copoloymers in multicomponent solvents, J, of Applied Polymer Science, 1986, (31), 1879-1899... 

Polymer chains in multicomponent solvents, when globule formation can be achieved by the re-distribution of solvent components between the globule interior and the outer solution. 

The solubility of macromolecules as a rule improves with the rising temperature. Solvent - polymer mixtures usually exhibit the upper consolute temperature or upper critical solution temperature, UCST, with a maximum on the plot of system concentration versus temperature. Above the critical solution temperature, polymer is fully soluble at any concentration. For practical work, the systems with UCST below ambient temperature are welcome. There are, however numerous polymer - solvent systems, in which the solvent quality decreases with increasing temperature. The plot of system concentration versus temperature exhibits a minimum. The phenomenon is called lower consolute temperature or lower critical solution temperature, LCST Polymer is only partially soluble or even insoluble above lower critical solution temperature. This unexpected behavior can be explained by the dominating effect of entropy in case of the stiff polymer chains or by the strong solvent - solvent interactions. The possible adverse effect of rising temperature on polymer solubility must be kept in mind when woiking with low solubility polymers and with multicomponent mobile phases. It may lead to the unforeseen results especially in the polymer HPLC techniques that combine exclusion and interaction retention mechanisms, in coupled methods of polymer HPLC (see section 11.8, Coupled Methods of Polymer HPLC). 

In practice, two-component solvent mixtures are employed as eluents and sample solvents inLC LC. One constituent of mixture supports elution of interactive polymer from the particular column, while another one induces its retention within column. To adjust polymer interactivity or to cope with the limited solubility of analyzed polymers, multicomponent solvents can be employed. Typical examples are mixtures of hexafluoropropanol with chloroform, which dissolve aromatic polyesters and some polyamides at ambient temperature. The sample solvents, eluents and barriers usually contain the same hquids but their composition is adjusted to fulfil their particular role Sample solvent must dissolve all its constituents and barrier must efficiently decelerate interactive macromolecules. Eluent serves either as a barrier in LC LCS, LC LCA and LC LCP or it promotes unhindered sample elution in LC LCD, LC LCU and LC LCI. 

Assuming a polymer thin film is prepared from a single-solute solution, fhree interaction pairs, namely, polymer-solvent, polymer-substrate, and solvent-substrate, determine the spreading and the stability of the film. By adding another polymer into the solution, six interaction pairs contribute to the final sfructure of the resulted film, which is much more complex. On the other hand, in order to get better performance or multifunctions, multicomponents (here, we focus... 

IRA Irani, C.A. and Cozewith, C., Lower critical solution temperature behavior of ethylene-propylene copolymers in multicomponent solvents, J. Appl. Polym. Sci., 31,1879, 1986. 86KUE Kuecuekyavruz, Z. and Kuecuekyavruz, S., Theta-hehaviour of poly(p-tert-butylstyrene)-b-poly(dimethylsiloxane)-h-poly(p-tert-hutylstyrene), M A rowo/. Chem., 187, 2469, 1986. 86RAE Raetzsch, M.T., Kehlen, H., Browarzik, D., and Schirutschke, M., Cloud-point curve for the system copoly(ethylene-vinyl acetate) + methyl acetate. Measurement and prediction by continuous thermodynamics, J. Macromol. Sci.-Chem. A, 23, 1349, 1986. 

Applications of solubility parameters include selecting compatible solvents for coating resins, predicting the swelling of cured elastomers by solvents, estimating solvent vapor pressure in polymer solutions for devolatilization and reaction systems (16), and predicting phase equihhria for polymer-polymer (107), polymer-binary (93), random copolymer (102), and multicomponent solvents (38, 98,108,109). 

The situation becomes most complicated in multicomponent systems, for example, if we speak about filling of plasticized polymers and solutions. The viscosity of a dispersion medium may vary here due to different reasons, namely a change in the nature of the solvent, concentration of the solution, molecular weight of the polymer. Naturally, here the interaction between the liquid and the filler changes, for one, a distinct adsorption layer, which modifies the surface and hence the activity (net-formation ability) of the filler, arises. Therefore in such multicomponent systems in the general case we can hardly expect universal values of yield stress, depending only on the concentration of the filler. Experimental data also confirm this conclusion [13],... 

There are two types of multicomponent mixtures which occur In polymer phase equilibrium calculations solutions with multiple solvents or pol ers and solutions containing poly-disperse polymers. We will address these situations In turn. 

Unfortunately, relatively little work has been done on the solution thermodynamics of concentrated polymer solutions with "gathering". The definitive work on the subject Is the article of Yamamoto and White (17). The corresponding-states theory of Flory (11) does not account for gathering. We therefore restrict our consideration here to multicomponent solutions where the solvents and polymer are nonpolar. For such solutions, gathering Is unlikely to occur. 

Among other approaches, a theory for intermolecular interactions in dilute block copolymer solutions was presented by Kimura and Kurata (1981). They considered the association of diblock and triblock copolymers in solvents of varying quality. The second and third virial coefficients were determined using a mean field potential based on the segmental distribution function for a polymer chain in solution. A model for micellization of block copolymers in solution, based on the thermodynamics of associating multicomponent mixtures, was presented by Gao and Eisenberg (1993). The polydispersity of the block copolymer and its influence on micellization was a particular focus of this work. For block copolymers below the cmc, a collapsed spherical conformation was assumed. Interactions of the collapsed spheres were then described by the Hamaker equation, with an interaction energy proportional to the radius of the spheres. 

The hydrophilic delivery system described in this review can be extended to drugs with a low water-solubility (e.g., doxorubicin). Such compounds may be incorporated in CT/TPP nanoparticles by means of dextran sulfate complex prior to entrapment [54] or by dissolving them in a polar solvent (acetone, ethanol or acetonitrile) as demonstrated for the relatively hydrophobic peptide cyclosporin A [26,81]. It is quite possible that this approach would work in a multicomponent polymer system as well. 

Multicomponent polymers systems such as polyblends, and block copolymers often exhibit phase separation in the solid state which results in one polymer component dispersed in a continuous phase of a second component. The morphological properties of these systems depend upon a number of factors such as the molar ratios of the components, the molecular weights, the thermal history of the system and, for solvent cast films, the solvent and drying conditions. 

In most situations the experimental system is more complicated than one (homodisperse) polymer adsorbing from a single solvent. In multicomponent systems preferential adsorption always plays a role. A common example is the adsorption of a polydisperse polymer, where usually long chains adsorb preferentially over short ones, even if the adsorption energy per segment is the same. 

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