Network chemical stress relaxation

The effects of chemical stress relaxation (sometimes referred to as chemorheo-logy) depend on whether the network scissions occur at the original cross-links or are randomly disposed at all points along the network strands." " They also depend on whether the broken bonds join again to leave the same number of network... 

The equilibrium 20 is important not only in the synthesis of linear polysiloxanes but also in their applications. The effects of water vapor on inducing chain cleavage at high temperature are not only reduced molecular weights but also a dramatic increase in the rates of chemically induced stress relaxation at 250 °C in cross-linked poly(dimethylsiloxane) networks under load (70). Slow hydrolytic bond cleavage in cross-linked networks is seen even in studies of stress relaxation in air at room temperature, and appreciable rates of stress relaxation in the loaded networks are measured at temperatures as low as 70 (7i). The stress relaxation is greatly accelerated... 

These studies pointed out that a comprehensive model of thermoset cure for stress calculations must account for a large number of processing influences and material properties. Mass transfer (22.), chemical kinetics, network structure formation, and material property development are essential ingredients. Induced strains must be accurately calculated, as must stress relaxation. Properties dependencies on temperature are significant and must be accounted for, as must the inter-relationship between reaction kinetics and diffusion. 

A well defined way of obtaining kinetic data for chemical reactions which lead to chain scission in bulk network polymers is by stress relaxation. The formalism of this method has been developed by Tobolsky and others (24-26), and can be summarized as follows. 

Figure 5-16. Physical stress relaxation at 25 °C in a crosslinked network made by a zinc oxide cure of a halobutyl rubber. The elastomer has been soaked in oil to speed the relaxation. The line is the fit to the data using the Chasset-Thirion equation with E =0.018 MPa, t= 1.04 s and m = 0.31. The nature of the polymer and the cure makes for a chemically stable structure otherwise, the plot would begin to curve downward at long time.

Figure 10.7 (8) illustrates the stress relaxation of a poly(dimethyl siloxane) network, silicone rubber, in the presence of dry nitrogen. The reduced stress, o(t)/(T(0),is plotted,so that under the initial conditions its value is always unity. Since the theory of rubber elasticity holds (Chapter 9), what is really measured is the fractional decrease in effective network chain segments. The bond interchange reaction of equation (10.2) provides the chemical basis of the process. While the rate of the relaxation increases with temperature, the lines remain straight, suggesting that equation (10.2) can be treated as the sole reaction of importance. 

If a mixture of two polymeric species which are almost identical chemically except that only one of them carries cross-linkable reactive groups is subjected to cross-linking, a network is formed through which are threaded unattached macromolecules. From viscoelastic measurements, the motions of these molecules through the network can be deduced, as illustrated in Fig. 14-11 for stress relaxation of ethylene-propylene terpolymer networks containing 25% of unattached linear copolymer with essentially the same chemical composition. A network containing no unattached species undergoes very little relaxation in the time scale covered. 

If the chemical structure of a polymer is altered during a viscoelastic experiment—in particular, if a cross-linked network is subjected to a reaction which increases or decreases the number of network strands while it is being investigated in the rubbery zone of viscoelastic behavior—the apparent mechanical properties will be profoundly influenced. For example, scission of the network strands will cause stress relaxation at constant strain" (Fig. 14-12) or creep under constant stress. Formally, if a single first-order chemical reaction is responsible, the relaxation may be described by a single relaxation time which is the reciprocal of the chemical rate constant, instead of the broad spectra which are characteristic of the usual mechanical processes. 

FIG. 14-12. Stress relaxation of a polyester rubber (Vulcollan A) due to chemical scission of network strands, at four temperatures as shown. (Offenbach and Tobolsky." )... 

The one general class of polymers that fall outside this concept is the thermoplastic elastomers, one example of which was discussed previously. These materials can be processed (and reprocessed) at high temperature, yet they maintain properties of cured rubber at use temperatures. This system functions by the formation of either hard plastic, crystalline, or ionic domains that, at use temperature, act as cross-link sites because multiple chains are involved in the domains. Upon heating, the integrity of these domains breaks down, and the polymer chains can easily flow past one another. It should be noted that at use temperatures these systems have a three-dimensional network. Such systems tend to show more creep and stress relaxation than cured systems, as the network is formed via weaker secondary effects rather than primary chemical bonds. These problems become more severe as the use temperature is increased because ultimately the network cannot remain intact at processing temperatures. For any network, its structure is important in defining the performance of the... 

In paper [43] acceleration of the stress relaxation process was found at loading of epoxy polymers under the conditions similar to those described above (Figure 6.8, curves 2-4). The authors [43] explained the observed effect by the partial rupture of chemical bonds. In order to check this conclusion in paper [39] repeated tests on compression of samples, loaded up to the cold flow plateau and then annealed at T < T, were carried out. It has been established that in the diagram o-e tooth of yield is restored. This can occur at the expense of the restoration of unstable clusters, since the restoration of failed chemical bonds at T < is scarcely probable. In this connection it is also necessary to note that yield tooth suppression as a result of preliminary plastic deformation was observed earlier for linear amorphous polymers, for example, polycarbonate [44], for which the chemical bonds network is obviously absent. 

Although many different processes can control the observed swelling kinetics, in most cases the rate at which the network expands in response to the penetration of the solvent is rate-controlling. This response can be dominated by either diffu-sional or relaxational processes. The random Brownian motion of solvent molecules and polymer chains down their chemical potential gradients causes diffusion of the solvent into the polymer and simultaneous migration of the polymer chains into the solvent. This is a mutual diffusion process, involving motion of both the polymer chains and solvent. Thus the observed mutual diffusion coefficient for this process is a property of both the polymer and the solvent. The relaxational processes are related to the response of the polymer to the stresses imposed upon it by the invading solvent molecules. This relaxation rate can be related to the viscoelastic properties of the dry polymer and the plasticization efficiency of the solvent [128,129],... 

Major disadvantages of chemical strengthening by ion exchange include costs, (in time and materials). In addition, not all compositions may be strengthened to useful depth profiles and the temperature used permits stress release by continued diffusion of alkali or structural relaxation of the network. 

The work discussed here has related not only to structure relationships but also to means of protection of the macromolec-ular material and the protective functions of these materials. There are many modes of failure, by chemical reaction, failure by fracture, environmental stress cracking and creep. Further there are complicating interactions arising from chemical reaction during relaxation of polymer networks, and in multiphase polymer systems and cos osites, failure at interfaces by adhesive failxire or stress-stress dilation. 

There are no infinite mechanical relaxation times for reversible gels. If the macroscopic coherent network present at any instant is deformed rapidly by some applied strain, the stress stored in this deformation can relax through the chemical reaction, even if the mechanical relaxation time of the network (had the cross-links been permanent) would have been infinite. 

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