Solvents, polymers dissolved multicomponent

Other multicomponent polymer solutions of interest consist of chemically different polymers dissolved in a solvent or a single polymer in a mixture of different solvents. Their phase equilibrium behavior is important in relation to many practical problems, but it is much more complex and difficult to tinalyze and predict than that of quasi-binary systems. 

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. 

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. 

For multicomponent systems, experiments with synthetic methods yield less information than with analytical methods, because the tie lines cannot be determined without additional experiments. A common synthetic method for polymer solutions is the (P-T-m ) experiment. An equilibrium cell is charged with a known amount of polymer, evacuated and thermostated to the measuring temperature. Then flie low-molecular mass components (gas, fluid, solvent) are added and the pressure inereases. These eomponents dissolve into the (amorphous or molten) polymer and the pressure in the equilibrium eell deereases. Therefore, this method is sometimes called pressure-decay method. Pressure and temperature are registered after equilibration. No samples are taken. The composition of the liquid phase is often obtained by weighing and using the material balance. The synthetic method is particularly suitable for measurements near critical states. Simultaneous determination of PVT data is possible. Details of experimental equipment can be found in the original papers compiled for this book and will not be presented here. 

An Interpenetrating polymer network, IPN, Is defined as a combination of two polymers In network form, at least one of which Is synthesized and/or crossllnked In the Immediate presence of the other. While closely related to other multicomponent polymer materials such as the polymer blends, grafts, and blocks, the IPN s can be distinguished from them In two ways (1) An IPN swells, but does not dissolve In solvents, and (2) creep and flow are suppressed. The IPN topology Is compared with that of other multicomponent polymer structures In Figure 1. 

The methods of separation and identification of multicomponent polymers are far different from the methods described previously for the statistical type of polymer. First, only the blends are separable by extraction techniques. The remainder are bound together by either chemical bonds or interpenetration. The interpenetrating polymer networks and the conterminously grafted polymers are insoluble in all simple solvents and do not flow on heating. The graft and block copolymers, on the other hand, do dissolve and flow on heating above T/and/or Tg. 

Up to this point, the discussion has concerned systems with one or more polymer species dissolved in a pure solvent. However, thermodynamic behavior in the more general case of systems with multiple solvents involves additional effects that are of considerable interest. Here, for simplicity, only a three-component system of a polymer (component 2) in two low molecular weight solvents (components 1 and 3) is considered but an exhaustive analysis of the general multicomponent case is available. The classification of solvents and solute is of course arbitrary, but it has utility with respect to specific experimental measurements. For instance, the operative principle in an osmotic measurement is that solvents pass through the membrane that confines a polymeric solute. The obvious effect of the additional thermodynamic degree of freedom in the three-component system is the possibility of selective interaction (preferential solvation or binding ) of the solute with a solvent component. In an osmotic experiment this will be manifested by equilibrium solvent compositions that are different in the solute and solvent compartments of the osmometer. It is possible, though not always very useful, to formally redefine a solute component so as to include in it any excess (or deficiency) of a solvent component required to make the free solvent mixture appear... 

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