Vapor-polymer solubility

The relative initial ratio of acrylonitrile to butadiene and degree of conversion of nitrile to amidoxime are directly related to the resultant film s solubility parameter and glass transition temperature. Ideally, the concentration of amidoxime functional groups would be maximized while the coating s solubility parameter is matched to the vapor to be detected and the glass transition temperature is kept below room temperature. In practice, the conversion limitations are set by the reaction conditions of limited polymer solubility, reaction temperature and time. Three terpolymers of varying butadiene, acrylonitrile and amidoxime compositions were prepared as indicated in Table 1. 

Nonlinear, pressure-dependent sorption and transport of gases and vapors in glassy polymers have been observed frequently. The effect of pressure on the observable variables, solubility coefficient, permeability coefficient and diffusion timelag, is well documented (1, 2). Previous attempts to explain the pressure-dependent sorption and transport properties in glassy polymers can be classified as concentration-dependent and "dual-mode models. While the former deal mainly with vapor-polymer systems (1) the latter are unique for gas-glassy polymer systems (2). 

Solubility Polymer solubility can be predicted by the Hildebrand solubility parameter. The Hildebrand solubility parameter, 8, is defined as the square root of the molar energy of vaporization, LE = AH - R, ... 

Interaction characteristics in polymer-related areas frequently make use of solubility parameters (16). While the usefulness of solubility parameters is undeniable, there exists the limitation that they need to be estimated either by calculation or from indirect experimental measurements. The thermodynamic basis of IGC serves a most useful purpose in this respect by making possible a direct experimental determination of the solubility parameter and its dependence on temperature and composition variables. Price (17) uses IGC for the measurement of accurate % values for macromolecule/vapor pairs, which are then used for the evaluation of solubility parameters for a series of non-volatile hydrocarbons, alkyl phthalates, and pyrrolidones. It may be argued that IGC is the only unequivocal, experimental route to polymer solubility parameters, and that its application in this regard may further enhance the practical value of that parameter. Guillet (9) also notes the value of IGC in this regard. 

Each term other than the constant contains a solvation parameter related to the vapor s solubility properties as a solute (R2, tt2h,1 2H, I/12H, or log L16) and a coefficient (/ s, a, b, or the letter l). All of these parameters except R2 are free energy related. The parameter scales were derived from measurements of complexation or partitioning equilibria. Grate et al. have applied this method successfully to evaluate polymer sensors on acoustic sensor arrays to study the sorption of target analytes in polymer films.314... 

The energy of vaporization is not accessible for polymers, but cohesive energy density of polymers can be determined from PVT-data. However, common ways for determining polymer solubility parameters use thermodynamic properties of polymer solutions and their relations to excess enthalpy or excess Gibbs energy per unit volume. These excess quantities are related to the (square) difference between the solubility parameters of solvents and polymers, i.e. (d -... 

Depending on the degree of affinity for moisture, plastic resins can be divided into two classes (1) hygroscopic and (2) nonhygroscopic. Moisture adsorption and/or absorption capability depends on the type of resins as well as the ambient temperature in which it is placed. In some instances, exposure of only few minutes can be detrimental. If the material is exposed to a certain temperature and relative humidity for a period of time, it will reach the equilibrium point, referred to as the equilibrium moisture content (EMC). Prior to drying it is important to know the permeability (product of the diffusion constant of water vapor-polymer system and the solubility coefficient) of polymer to water vapor since this dictates the condition for relative humidity for the safe storage of the polymer [16]. 

For polymers, solubility parameters cannot be calculated from heat of vaporization data because of their nonvolatility. Other methods of estimation must be used. 

The pressure at which plasticizing effects begin to overcome permeability depression depends primarily on the ratio of the partial pressure of the penetrant in the feed stream to the corresponding vapor pressure of the penetrant at the system temperature. As the pressure approaches the vapor pressure, solubility tends to increase rapidly, thereby inducing plasticization or clustering responses such as those already discussed for rubbery polymers. Even below the plasticization point, additional complexities not encountered with rubbery polymers are often observed in glassy materials. For example, low vapor pressure components such as water can cause depression of the permeability of other gases when water is... 

TABLE 12.5 Water Vapor Permeability, Solubility, and Diffusion Coefficients of PLA and Petroleum-Based Polymers at 23°C... 

Polymers are often polydisperse with respect to molecular weight. Whereas this is of minor importance for the solvent sorption in polymers (vapor-hquid equilibrium), this fact usually remarkably influences the polymer solubility (liquid-hquid equilibrium). Therefore, polydispersity needs to be accounted for in interpretation and modeling of experimental data. This can be done by applying continuous thermodynamics as well as by choosing a representative set of pseudocomponents. It was shown that a meaningful estimation of the phase boundary is possible when using only two or three pseudocomponents as soon as they reflect the important moments (Mn, Mw, Mz) of the molecular weight distribution. 

It is, therefore, seen that only when Fab—Faa and Fab—Fbb will coexistence or compatibility be possible. Obviously, if it is possible to obtain some measure of these forces, it should be possible to make predictions about polymer solubility. What then is a suitable measure of the forces holding like molecules together One would expect the latent heat of vaporization, L, to exceed the cohesion energy by an amount corresponding to the work done by evaporation, an amount approximating to RT, where R is the gas constant and T the absolute temperatore. Such a diagram of (L—RT) might be a sufficient measure if all of the molecules were of about the same size. 

Solubility parameters can be determined by direct measurements, indirect calculations, or correlations with other physical parameters. The solubility parameters of solvents usually can be determined directly by measuring the energy of vaporization. The solubility parameters of polymers can only be determined indirectly and may be affected by variations in their chemical constitutions, i.e., the number of crosslinks and the distribution of chain branches or substitutive groups along the polymer backbone. The methods presented in this section can be used to develop correlations of solubility parameters with other physical properties for specific commercial polymer products or to estimate the solubility parameters of new polymers. 

MethylceUulose reduces surface and interfacial tension. MethylceUulose forms high strength films and sheets that are clear, water-soluble, and oU-and grease-resistant, and have low oxygen and moisture vapor transmission rates (see Barrier polymers). 

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