Vapor-polymer solubility interactions

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. 

Some implicit databases are provided within the Polymer Handbook by Schuld and Wolf or by Orwoll and in two papers prepared earlier by Orwoll. These four sources list tables of Flory s %-function and tables where enthalpy, entropy or volume changes, respectively, are given in the literature for a large number of polymer solutions. The tables of second virial coefficients of polymers in solution, which were prepared by Lechner and coworkers (also provided in die Polymer Handbook), are a valuable source for estimating the solvent activity in the dilute polymer solution. Bonner reviewed vapor-liquid equilibria in concentrated polymer solutions and listed tables containing temperature and concentration ranges of a certain number of polymer solutions." Two CRC-handbooks prepared by Barton list a larger number of fliermodynamic data of polymer solutions in form of polymer-solvent interaction or solubility parameters." ... 

For vapors that can interact with the polymer matrix, for example, hydrocarbons or chlorinated hydro-caibons in poWethylene, both the diffusivity and the solubility rapidly increase with increasing vapor concentration." - In a similar fashion, liquids that are able to swell the membrane have enhan transport rates. Permeabilities for vapors that swell the membrane can easily be orders of magnimde greater than those for gases that do not chemically interact with the membrane."... 

Solubility-Selective Membrane The solubility coefficient reflects how many gas molecules can be sorbed in polymer membranes. It depends on the condensability as well as the physical interactions of the penetrants with the polymer membrane. Solubility is determined by the concentration of the sorbed gas per unit polymer volume. Generally, the concentration as a function of pressure at constant temperature shows a sorption isotherm with a characteristic shape that is concave to the pressure axis. The solubihty-selectivity term in Eq. (24.3) is thermodynamic in nature and is governed by the relative polymer-penetrant interactions and the relative condensabihty of the penetrants. Solubility-selectivity terms contribute significantly to separations of condensable vapors and polar molecules. 

Absorption of a solute liquid or vapor into a polymer film can profoundly affect the viscoelastic behavior of the polymer. The magnitude of this effect depends on the nature of the solute/polymer interactions and on the amount of solute absorbed. The solute/polymer interactions can range fttun simple dispersion to hydrogen-bonding and other specific interactions. The extent of absorption can be described by the partition coefficient, AT, which quantifies the thermcxlynamic distribution of the solute between two phases (K = coiKentration in polymer divided by die concentration in the liquid or vapor phase in contact with the polymer). It has long been known that acoustic wave devices can be used to probe solubility and partition coefficients (53,67). Due to the relevance of these topics to chemical sensors, more comprehensive discussions of these interaction mechanisms and the significance of the partition coefficient are included in Chapter 5. 

The deviation from the near-constant rule also occurs if the gas interacts specifically with polymer molecules. This is seen in the case of water vapor permeability. The molecular size of H2O is approximately the same as that of O2 however, the solubility of H2O is much greater than that of O2 in orders of magnitudes and varies greatly depending on the nature of polymers. Consequently, there is no near-constant value of oc that is found for many gas pairs, and the permeability ratio of H2O/O2 for polymers spreads in orders of magnitude. 

Analogous results have been found for other polymers. Crazing at crack tips exhibits exactly the same behavior. In order to evaluate the acceleration of crazing by action of a solvent, the interaction between the polymer and the solvent can be quantified by means of the solubility parameter, 5, defined as the cohesive energy density where AHy is the vaporization... 

IGC can be used to determine various properties of the stationary phase, such as the transition temperatures, polymer—polymer interaction parameters, acid-base characteristics, solubility parameters, crystallinity, surface tension, and surface area. IGC can also be used to determine properties of the vapor-solid system, such as adsorption properties, heat of adsorption, interaction parameters, interfacial energy, and diffusion coefficients. The advantages of IGC are simplicity and speed of data collection, accuracy and precision of the data, relatively low capital investment, and dependability and low operating cost of the equipment. 

Further analysis of the above results may be obtained by studying the solubility parameters of the polymers and plasticizers. Solubility parameters provide a measure of the extent of Interaction possible between chemical species. To determine the solubility parameters, the structure of the smallest repeat unit was considered and the contribution of each atomic group to the total energy of vaporization and molar volume was summed over the molecular structure. The ratio Is calculated as the cohesive energy. The calculated solubility parameters of the polymers and plasticizers are shown In Table II. 

These three approaches have found widespread application to a large variety of systems and equilibria types ranging from vapor-liquid equilibria for binary and multicomponent polymer solutions, blends, and copolymers, liquid-liquid equilibria for polymer solutions and blends, solid-liquid-liquid equilibria, and solubility of gases in polymers, to mention only a few. In some cases, the results are purely predictive in others interaction parameters are required and the models are capable of correlating (describing) the experimental information. In Section 16.7, we attempt to summarize and comparatively discuss the performance of these three approaches. We attempt there, for reasons of completion, to discuss the performance of a few other (mostly) predictive models such as the group-contribution lattice fluid and the group-contribution Flory equations of state, which are not extensively discussed separately. 

Accurate description of barrier films and complex barrier structures, of course, requires information about the composition and partial pressure dependence of penetrant permeabilities in each of the constituent materials in the barrier structure. As illustrated in Fig. 2 (a-d), depending upon the penetrant and polymer considered, the permeability may be a function of the partial pressure of the penetrant in contact with the barrier layer (15). For gases at low and intermediate pressures, behaviors shown in Fig. 2a-c are most common. The constant permeability in Fig.2a is seen for many fixed gases in rubbery polymers, while the response in Fig. 2b is typical of a simple plasticizing response for a more soluble penetrant in a rubbery polymer. Polyethylene and polypropylene containers are expected to show upwardly inflecting permeability responses like that in Fig. 2b as the penetrant activity in a vapor or liquid phase increases for strongly interacting flavor or aroma components such as d-limonene which are present in fruit juices. 

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