Swelling and shrinking

The degree of swelling and shrinking is important for design of ion-exchange columns, especiaUy for the location of the distributors used to disperse incoming fluids, and coUect outgoing ones, evenly over the cross-sectional area of the resin bed. Once placed, these distributors are not adjustable. The upper distributor should be above (the lower one below) the resin bed, even in the bed s swoUen form. 

ASE is a measure of the extent to which the swelling and shrinking tendency has been reduced at an equUibrium condition. 

Chemical modification of the cotton fiber must be achieved within the physical framework of this rather compHcated architecture. Uniformity of reaction and distribution of reaction products are iaevitably iafiuenced by rates of diffusion, swelling and shrinking of the whole fiber, and by distension or contraction of the fiber s iadividual stmctural elements duting finishing processes. 

Influence parameters are especially the press temperature and the moisture content in the mat. Additional parameters are the wood density, the porosity, the swelling and shrinking behavior of the wood, the structure at the surface and the wetting behavior. During the press cycle several mechanisms are in operation ... 

In a film, the cooperative effort of the different molecular motors, between consecutive cross-linked points, promotes film swelling and shrinking during oxidation or reduction, respectively, producing a macroscopic change in volume (Fig. 18). In order to translate these electrochemically controlled molecular movements into macroscopic and controlled movements able to produce mechanical work, our laboratory designed, constructed, and in 1992 patented bilayer and multilayer103-114 polymeric... 

Even when they have a partial crystallinity, conducting polymers swell and shrink, changing their volume in a reverse way during redox processes a relaxation of the polymeric structure has to occur, decreasing the crystallinity to zero percent after a new cycle. In the literature, different relaxation theories (Table 7) have been developed that include structural aspects at the molecular level magnetic or mechanical properties of the constituent materials at the macroscopic level or the depolarization currents of the materials. 

Microporous, spongelike gels may swell and shrink by a convection-controlled process. That is, the rate-limiting step may be the rate at which the solvent is absorbed or expelled from the micropores of the sample rather than... 

Y Li, T Tanaka. Kinetics of swelling and shrinking of gels. J Chem Phys 92 1365-1371, 1990. 

These highly porous glasses retain a rigid and exposed interfacial surface area (typically 300-1000 m g ), whereas conventional organic polymer beads swell and shrink in different solvents, often with unpredictable effects on catalysis Functionalization of a monolithic (largest dimension 1 mm) gel affords a bulk catalyst sample. This obviates the need for filtration to recover the catalyst tweezers can be used instead ... 

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. 

The kinetics of swelling and shrinking processes was observed under various conditions it was found that characteristic relaxation times are in the order of... 

A gel system that swells and shrinks in response to specific ions or molecules has many potential applications, such as sensors, drug delivery devices and actuators. There are two possible approaches to attain this end. One approach is to utilize host molecules as the receptor part, the other utilize enzymes. 

Fig. 9. Plot of normalized approach to equilibrium mass against the square root of time for a temperature-sensitive 10 x 4 PNIPAAm gel sheet swelling and shrinking between 10 and 25 °C-Shown are the curve fits to the kinetic data of theory developed from Fick s law of diffusion in a polymer-fixed reference frame [149]. The equilibrium degree of swelling is 17.0 at 10 °C and 11.1 at 25 °C the diffusion coefficients obtained from the curve fits are 2.3 x 10 7 cm2/s for swelling and 3.6 x 10 7 cm2/s for shrinking [121]...

Fig. 11. Diffusion coefficients for PNIPAAm gels obtained from swelling and shrinking experiments over small temperature intervals. The gels were initially equilibrated at 10 °C, then wanned in increments to 32 °C, where the process was reversed and the gels were cooled until they returned to 10 °C. The temperature axis is the temperature at which the volume change occurred (bath temperature). The trends are interpreted using polymer scaling concepts [121]. The lines drawn are to guide the eye...

---