Visualization of inner stresses in networks on swelling

We took advantage of the photoelasticity technique for studying in detail conditions under which inner stresses emerge or, vice versa, fuUy disappear in polymeric networks, depending on their swelling state. Three types of polystyrene networks have been chosen for comparison, namely a spherical conventional gel-type styrene-0.5% DVB copolymer, the hypercrosslinked network obtained by crosslinking this copolymer with MCDE to X = 100%, as well as the styrene-5% DVB copolymer prepared in the presence of 30% (by volume) of toluene. Table 7.6 presents their characteristics. 

The single-phase uniformly crosslinked open network can be stressed in two opposite ways, on shrinkage and extension. As was discussed earlier, stresses of shrinkage appear in the dry hypercrosslinked material as a result of confrontation between the network rigidity and the attraction forces 

Accordingly, it is legitimate to assume that in the course of sweUing with toluene in a dry bead of a hypercrosslinked polymer the initial stress of shrinkage should be replaced by a final stress of extension. Indeed, as a droplet of toluene falls on the dry bead and the latter starts to swell, one can observe in the microscope the cross disappearing for a while at the moment the solvent reaches the bead center, and, then emerging again, in the form shown in Fig. 7.22. Here, it is important to emphasize that the transition of 

by using the photoelasticity effect, we managed to ascertain for the first time that the hypercrossHnked polystyrene network can experience inner stresses of shrinkage on drying, stresses of extension on sweUing in good solvents, and also can acquire a fuUy relaxed state, totally free from any stresses, when wetted with methanol. Dusek and Prins [163] have postulated a pecuHar reference state when polymeric chains do not exert any forces on network junction points (see also Chapter 1, Section 2.2), but this state of 

hypercrosslinked polystyrene networks gradually expand while their void volume becomes occupied with the following media  

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