Stress swelling characteristics

The hydrophilicity of the hydrogel system can also be expressed by means of an interaction parameter (x). This parameter defines the interaction energy during the process of hydration. The parameter is determined experimentally from stress-strain cnrves and the swelling characteristics of the hydrogel using the Flory-Hnggins eqnation,... 

This example illustrates the simplified approach to film blowing. Unfortunately in practice the situation is more complex in that the film thickness is influenced by draw-down, relaxation of induced stresses/strains and melt flow phenomena such as die swell. In fact the situation is similar to that described for blow moulding (see below) and the type of analysis outlined in that section could be used to allow for the effects of die swell. However, since the most practical problems in film blowing require iterative type solutions involving melt flow characteristics, volume flow rates, swell ratios, etc the study of these is delayed until Chapter 5 where a more rigorous approach to polymer flow has been adopted. 

Thus, this consideration shows that the thermoelasticity of the majority of the new models is considerably more complex than that of the phantom networks. However, the new models contain temperature-dependent parameters which are difficult to relate to molecular characteristics of a real rubber-elastic body. It is necessary to note that recent analysis by Gottlieb and Gaylord 63> has demonstrated that only the Gaylord tube model and the Flory constrained junction fluctuation model agree well with the experimental data on the uniaxial stress-strain response. On the other hand, their analysis has shown that all of the existing molecular theories cannot satisfactorily describe swelling behaviour with a physically reasonable set of parameters. The thermoelastic behaviour of the new models has not yet been analysed. 

Another criterion for predicting if the transport in polymeric gels is controlled by diffusion (Fickian) or by relaxation, is to determine the diffusional Deborah number De), which is a ratio between the characteristic polymer relaxation time of the polymer (2) when it is subject to a swelling stress and a characteristic diffusion time (6), defined as the coefficient between the square of the sample thickness (h) and the coefficient of water diffusion in the polymeric gel... 

The length of time that fuel can be used in a reactor before it must be discharged depends on the characteristics of the reactor, the initial composition of the fuel, the neutron flux to which it is exposed, and the way in which fuel is managed in the reactor, as described in more detail in Chap. 3. Factors that eventually require fuel to be discharged include deterioration of cladding as a result of fuel swelling, thermal stresses or corrosion, and loss of nuclear reactivity... 

The reasons for popcorn polymer formation are still not very clear. Abnormally high concentration of free radicals was found to be characteristic of this material [65]. As early as in 1956, Pravednikov and Medvedev [66] explained the accumulation of free radicals by the rapture of stressed chains that fail to tolerate osmotic pressure. However, no new portions of free radicals were observed to appear during additional swelling of the popcorn itself. According to another opinion, popcorn arises as a result of immobihzation of a macroradical in a microgel or of chain raptures caused by chain oriental growing [67]. Perhaps, a delay, for one reason or another, or even impossibihty of chain termination is one of the reasons for the formation of the popcorn polymer. 

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. 

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