Gel swelling

Gel Swelling. Starobinets et al. have studied interactions and differential absorption coefficients for poly(ethylene terephthalate) in mixed solvents by gravimetric analysis of swollen gels. 

Introduction.— This section collects remarks and observations relating to particular types of polymer systems and not appropriately included under previous headings. It concludes with a tabulation of individual systems encountered in the 1977/8 literature on which new data have been obtained. In this no attempt has been made to evaluate the quality of the data and as to their type, citations appearing previously in this Report (to which the source numbers refer) will need to be consulted. The table is intended only to give access to sources of thomo-dynamic data updating the more detaUed compilations which are available. These include The Polymer Handbook, which contains an extensive collection of 0 - temperatures by Elias and Butler a table of solubility parameters collected by Beerbower and Hansen the table of jmlymer-solvent interaction parameters included in the review by Orwoll and the review of polymer mktures by Krause.  

Polymer Solutions.— Polystyrene-cyclohexane remains the most thoroughly investigated system in polymer solution thermodynamics but even it has thus far resisted a fully quantitative description in terms of molecular models. This system has generated much of the motivation for theoretical study, although data are now available for many other linear polymer-solvent systems (see p. 324), including siloxane poljmer solutions.  

Earlier literature has suggested that polymer coils in critical regions undergo a contraction or so called collapse transition the statistical physics of which is covered in detail in a review by Lifshitz et Volkova et al. have found that poly(a-methyl styrene) coils in cyclohexane are compacted to about 2/5 of unperturbed dimensions in the critical region Tager et al. make similar observations for polystyrene in three solvents. 

Previous conclusions that poly(phenylene oxide)-toluene exhibits UCST liquid-liquid phase separation were seriously questioned - and it has now been demonstrated that phase separation occurs as a result of direct crystallization of the polymer (see Chapter 12). Phase separation by crystalliza- 

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Of particular importance for the application are the effects of the external compression and the ionic composition of the outer solution on the swelling degree. The reason is that hydrogels usually exist in mineralized aqueous solutions (soil solution) and are affected by compression, for example, produced by the surrounding particles of the soil. Even in the absence of any external load the compression develops due to the gel swelling in a constrained volume. 

When considering the effect of the external compression on the SAH swelling their response to mechanical forces is of primary interest. Such forces arise, for instance, when the gel swells in the cavity constraining its volume. In this case the gel swelling pressure (n) expanding the network is counteracted by an external pressure (p)... 

Complicated theories of ionic gel swelling [99, 113, 114] must inevitably take into account the real electrostatic interactions, the finite extensibility of chains, as well as the electrostatic persistence length effect. Their application is most advisable in the case of strongly charged hydrogels [114]. 

Chrambach this indicates that the effective protein size for gel filtration is larger than the effective size for gel electrophoresis. They concluded that this could not be accounted for by gel swelling, pH, or ionic strength effects. Biefer and Mason [36] found the constant a in Eq. (93) to be 0.93. They measured the conductance of cellulose acetate filter pads with porosities from 0.5 to 0.9 in solutions of 10 M KCl. 

J Singh, ME Weber. Kinetics of one-dimensional gel swelling and collapse for large volume change. Chem Eng Sci 51 4499-4508, 1996. 

RA Siegel. pH sensitive gels—Swelling equilibria, kinetics and applications for drug delivery. In J Kost, ed. Pulsed and Self-Regulated Drug Delivery. Boca Raton, FL CRC Press, 1990, pp 129-157. 

When the gel is placed so as to be separated from the electrodes, the pH of the solution affects the degree of deformation. In alkali and acid regions, both PAANa and PAA gels swell at the anode side on application of electric fields. 

Flory osmotic pressures at the anode and cathode sides have been calculated using Eqs. 3-10. In a system of i cations and j anions, the osmotic pressures are given by a summation of Eqs. 3-10. When dn/dt > 0 in an electric field, the gel swells. It shrinks when dn/dt < 0. When n at the anode side becomes larger than that at the cathode side, the gel bends toward the cathode. 

In addition, these authors have discussed the deformation of a PAANa gel placed in contact with the cathode in a dc electric field. Because there is no interaction between the gel and the cathode, the gel swells at the anode side, as observed in the gel separated from the electrodes. It is concluded that the gel also swells at the anode side under an electric field. 

Another promising class of hydrogels that exhibit responsive behavior is complexing hydrogels. Osada studied complex formation in PMAA hydrogels (Osada, 1980). In acidic media, the PMAA membranes collapsed in the presence of linear PEG chains due to the formation of interpolymer complexes between the PMAA and PEG. The gels swelled when placed in... 

Siegel, R. A., pH-Sensitive Gels Swelling Equilibria, Kinetics, and Applications for Drug Delivery, in Pulsed and Self-Regulated Drug Delivery (J. Kost Ed.), pp. 129-155. CRC Press, Boca Raton (1990). 

It can also be noted that all of the gels rise in enzyme activity at 30 as the number of 30 -40 cycles increases. (Compare Fig. 7 to Fig. 9 to Fig. 10) This may be due to scission of some crosslinks as the gels swell and shrink during the temperature cycling and/or to relative movements of the enzyme and polymer segments within the gel which provide more rapid access of substrate (asparagine) to the enzyme as well as more rapid diffusion of product (aspartate) away from the enzyme, with increasing number of cycles. 

Kuhn et al. [22] extended their studies to crosslinked networks of poly-(acrylic acid) (PAA) and found that these gels swelled to a high degree in alkaline environment and contracted rapidly when acid was added to the surrounding solution. Dilations and contractions of the order of 300% were observed. They were reversible and could be repeated at will. A contracting and expanding gel... 

Rigid adsorbents, like charcoal or silica gel, swell by only a small percent, but nonrigid adsorbents, like agar-agar, may swell to several times the original size. Provided the adsorption occurs slowly, the tigid solid may accommodate die stresses, but if suddenly exposed to high vapor pressure the material may shatter. 

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