Swollen state relaxation

In a sense, it is much simpler to consider swelling and deswelling phenomena when a gel is initially in the swollen state. In this case the polymer is already in the rubbery state and glass-to-rubber relaxation ceases to be a rate determining factor. 

FIGURE 16.9 Schematic illustration of the different states of gels and their mesh chain conformations A is the free mesh chain solution, B is the newly formed gel in the relaxed state, C is the equilibrium swollen state after contacting with solvent molecules, and D is the gel in the dried state. (From Huang, Y., Szleifer, I., and Peppas, N.A., Macromolecules, 35, 1373, 2002. With permission.)... 

With respect to sweUing in non-solvents, toluene-modified styrene— DVB copolymers have much in common with hypercrosslinked polystyrenes [330]. Both are prepared in accordance with the same basic principle, the formation of rigid networks in strongly solvated state. It will be shown in detail in Chapter 7 that rigid expanded networks possess a relaxed favorable conformation only in their swollen state and, therefore, exhibit a marked tendency to acquire this state by sweUing and incorporating any liquid, even non-solvating one. 

To show the reverse effect of varying A we have chosen somewhat different conditions where the critical point is embedded in the folium and the oscillations of the swollen state are more of a relaxation type. This is shown in Figure 7. The amplitudes of the oscillations increase as A decreases. It therefore seems that the product of the gel relaxation time x by the characteristic frequency co of the chemical oscillations, P = tot, provides the crucial parameter that determines the amplitude of these oscillations. 

Methanol-treated PIM-1, that was soaked in methanol for at least a day, to flush out any residual casting solvent and allow relaxation of chains in the swollen state, then dried under vacuum to constant weight. 

Extended immersion of the PIM-PI membrane in alcohol obviously affects the properties of permeation. After immersion in alcohol, the residual solvents can be removed and polymer chain relaxation may be achieved in the swollen state. It was observed that both ethanol and methanol treatment have similar effects on gas permeability. PIM-PI membranes treated with methanol or ethanol for several days show higher gas permeabilities than the films cast directly from chloroform, even though these films are subjected to removal of the chloroform in a vacuum at room temperature for a few days until a constant weight of the film is achieved (Table 5.3). These effects were also observed in PIM-1, though PIM-1 has the higher permeability coefficient of the films. These phenomena indicate local interactions between low molecular weight alcohol and some binding sites on the PIM structure. It should be noted that an increase of permeability is always accompanied by a decrease in permselectivity. 

The long decay time (j ) of the other component is t5q>ical of semidiluted polymer solutions. A T value continuously increases with an increasing solvent content. This component is apparently originated from the relaxation of network defects which are disentangled from network chains in a swollen state. At the equilibrium swelling degree, the relative fraction of the 7 relaxation component could be used as a measure of the fraction of highly mobile network defects. The... 

Nonetheless, one observation is accounted for by a simple empirical extension of the above theory, the undamped propagation of a high-density polymer pulse as illustrated in Figure 9.5. In this experiment, the whole cylinder is initially in the swollen F state. An acid perturbation is made at the bottom end of the cylinder. This induces a transition to the collapse state of the gel, which starts to invade the swollen upper part. Yet, after some time, the collapsed part of the cylinder slowly reswells and relaxes back to the stable swollen state. The phenomenon can be explained by a chemomechanical version of the propagation of an excitability wave [13]. After the initial acid perturbation, the core of the cylindrical OSFR switches to the acid-core reacted state. The acid core contaminates the neighboring unreacted swollen part by diffusion, which, in turn, shrinks, following the acid wave. But the collapse of the gel undershoots the critical size Rinf below which the reacted acid state is stable. Diffusion from the boundary overtakes the reaction, the composition... 

In a subsequent communication, Elliott and coworkers found that uniaxially oriented membranes swollen with ethanol/water mixtures could relax back to an almost isotropic state. In contrast, morphological relaxation was not observed for membranes swollen in water alone. While this relaxation behavior was attributed to the plasticization effect of ethanol on the fluorocarbon matrix of Nafion, no evidence of interaction between ethanol and the fluorocarbon backbone is presented. In light of the previous thermal relaxation studies of Moore and co-workers, an alternative explanation for this solvent induced relaxation may be that ethanol is more effective than water in weakening the electrostatic interactions and mobilizing the side chain elements. Clearly, a more detailed analysis of this phenomenon involving a dynamic mechanical and/ or spectroscopic analysis is needed to gain a detailed molecular level understanding of this relaxation process. 

Solid-state NMR magnetisation relaxation experiments provide a good method for the analysis of network structures. In the past two decades considerable progress has been made in the field of elastomer characterisation using transverse or spin-spin (T2) relaxation data [36-42]. The principle of the use of such relaxation experiments is based on the high sensitivity of the relaxation process to chain dynamics involving large spatial-scale chain motion in elastomers at temperatures well above the Tg and in swollen networks. Since chain motion is closely coupled to elastomer structure, chemical information can also be obtained in this way. 

If the penetrant enters the glassy matrix faster than the polymer can adapt itself by volume relaxation, the solvent front advances linearly with time. This behaviour is called case-II diffusion or relaxation-controlled diffusion. It is a special case of anomalous diffusion, where the mean square particle displacement is proportional to t. It commonly applies to polymers in the glassy state [Wei2]. Here the system 1,4-dioxane/PVC is an example (Fig. 10.1.8(b)). Due to the softening of the material behind the diffusion front, the polymer relaxation in the already swollen matrix is fast enough to adapt to a new situation created by further solvent uptake. Therefore, solvent ingress as well as swelling behind the diffusion front is Fickian. 

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