Swelling and Dissolution

Polymeric materials also experience deterioration by means of environmental interactions. However, an nndesirable interaction is specified as degradation rather than corrosion because the processes are basically dissimilar. Whereas most metallic corrosion reactions are electrochemical, polymeric degradation is physiochemical that is, it involves physical as well as chemical phenomena. Furthermore, a wide variety of reactions and adverse consequences are possible for polymer degradation. Polymers may deteriorate by sweUing and dissolution. Covalent bond rupture as a result of heat energy, chemical reactions, and radiation is also possible, typically with an attendant reduction in mechanical integrity. Because of the chemical complexity of polymers, their degradation mechanisms are not well understood. 

To cite a couple of examples of polymer degradation, polyethylene, if exposed to high temperatm-es in an oxygen atmosphere, suffers an impairment of its mechanical properties by becoming brittle, and the utility of poly(vinyl chloride) is limited because this material may become discolored when exposed to high temperatures, although such environments may not affect its mechanical characteristics. 

When polymers are exposed to liquids, the main forms of degradation are swelling and dissolution. With swelling, the liquid or solute diffuses into and is absorbed within the polymer the small solute molecules fit into and occupy positions among the polymer molecules. Thus the macromolecules are forced apart such that the specimen expands or swells. Fmthermore, this increase in chain separation results in a reduction of the secondary inter-molecular bonding forces as a consequence, the material becomes softer and more ductile. The liquid solute also lowers the glass transition temperature and, if depressed below the ambient temperature, causes a once-strong material to become rubbery and weak. 

In general, polymers are much more resistant to attack by acidic and alkaline solutions than are metals. For example, hydrofluoric acid (HF) corrodes many metals as well as etch and dissolve glass, so it is stored in plastic bottles. A qualitative comparison of the behavior of various polymers in these solutions is also presented in Tables 17.4 and 17.5. Materials that exhibit outstanding resistance to attack by both solution types include polytetrafluoroethylene (and other fluorocarbons) and polyetheretherketone. 

Concept Check 17.8 From a molecular perspective, explain why increasing crosslinking and crystallinity of a polymeric material will enhance its resistance to swelling and dissolution. Would you expect crosslinking or crystallinity to have the greater influence Justify your choice. Hint You may want to consult Sections 14.7 and 14.11. 

---

For safety purposes nitrocellulose is delivered to the factory in a wet state and before it is partially dissolved in a mixture of alcohol and ether the water must be removed since this prevents the process of swelling and dissolution. 

Some runs carried out under the same conditions but according to Procedure B gave the results represented by the points outside the curve, which indicate a higher conversion per hour. Such behavior might be explained by the fact that in the absence of water both swelling and dissolution of the elastomer into monomer are more rapid and complete hence, the subsequent dispersion of the reacting mass in water is favored. 

Generally, it must be kept in mind that the physicochemical characterization was carried out under conditions that are different from those of the surfactant rinse in the photolithographic process. In the process, the solution is added for about 20 s to a solution containing developer and dissolved photoresist. The physico-chemical characterization has been done under idealized conditions, i.e. mostly in equilibrium and in pure surfactant solution, but longtime effects as swelling and dissolution have to be taken into account. 

Siepmann, J., Kranz, H., Bodmeier, R., and Peppas, N., HPMC-matrices for controlled drug delivery A new model combining diffusion, swelling, and dissolution mechanisms and predicting the release kinetics, Pharmaceutical Research, Vol. 16, No. 11, 1999, pp. 1748-1756. 

The design of such battery cells is confronted with many problems such as the limited capacity in the first cycle (low currents must be used to obtain conducting material) or the instability over many cycles as a result of, e.g., swelling and dissolution processes. 

Coal-solvent interactions leading to swelling and dissolution of the coal (11, 12). 

In this chapter, we present results of the testing of a broad spectrum of polymers in carbon dioxide over a range of temperatures and pressures and evaluation of the effect of the high pressure carbon dioxide on the chemical/physical properties of materials tested. The testing was performed in a static manner with four controlled variables, namely temperature, pressure, treatment time and decompression time. The evaluation of the interaction of high pressure carbon dioxide with polymers included sorption and swelling behavior, solubility issue, plasticization and crystallization, and mechanical properties. The results of these evaluations are discussed in three sections Sorption, Swelling and Dissolution of Carbon Dioxide in Polymers at Elevated Pressure, Thermal Properties, and Mechanical Properties. ... 

SORPTION, SWELLING AND DISSOLUTION OF CARBON DIOXIDE IN POLYMERS AT ELEVATED PRESSURE... 

The sorption of carbon dioxide can result in swelling and/or dissolution of a polymer. The extent of either/or both swelling and dissolution depends on the solubility of carbon dioxide in the polymer and/or the solubility of the polymers in carbon dioxide. When the system pressure is reduced, absorbed carbon dioxide may nucleate into bubbles and cause the formation of foam, or small defects in the polymer structure that may significantly alter the mechanical properties of the material. In addition, carbon dioxide can extract the plasticizer, if present, in the polymer and thus cause embrittlement. In this chapter, we address such solubility issues. 

Yih-o Tu, "Miltiphase Stefan Problem Describing the Swelling and Dissolution of Glassy Polymer", IBM Symposium on Mathematics and Computation, October 6-7, 1976. Yorktown Heights, N.Y. 

When the Donnan potential and the excess osmotic pressure within the gel are sufficiently reduced by additional salt, the swelling and dissolution can be prevented but salt may also exert a salting-in effect which can compensate for the abolition of the electric and osmotic Donnan effects. When the salting-in threshold is very low, therefore, proteins caused to dissolve by shifting the pH away from the isoelectric point are not precipitated by addition of salt. This is true at pH 7 of tropomyosin, a relatively soluble protein for which the salting-in limit is particularly low and the salting-out limit is particularly high (Table VII). 

---