Gaussian network theories

In this review, we have given our attention to Gaussian network theories by which chain deformation and elastic forces can be related to macroscopic deformation directly. The results depend on crosslink junction fluctuations. In these models, chain deformation is greatest when crosslinks do not move and least in the phantom network model where junction fluctuations are largest. Much of the experimental data is consistent with these theories, but in some cases, (19,20) chain deformation is less than any of the above predictions. The recognition that a rearrangement of network junctions can take place in which chain extension is less than calculated from an affine model provides an explanation for some of these experiments, but leaves many questions unanswered. 

The Gaussian network theory can, therefore, predict phase separation in partially swollen systems. Phase separation can be induced by increasing the degree of crosslinking or increasing the interaction parameter %. Examples are given in Fig. 16. 

Due to the dual filler and crosslinking nature of the hard domains in TPEs, the molecular deformation process is entirely different than the Gaussian network theories used in the description of conventional rubbers. Chain entanglements, which serve as effective crosslinks, play an important role in governing TPE behavior. The stress-strain results of most TPEs have been described by the empirical Mooney-Rivlin equation ... 

The Gaussian network theory fits the data less well than the Moon theoty. This could be partly due to the Mooney theory having two adjustable constants. 

As mentioned in 3.N.5, the Mooney equation will provide at large strains a better fit to the data than the Gaussian network theory. 

In 1940, well before the derivation of the Gaussian network theory which leads to eqn (3.N.S.1), Mooney derived, by mathematical arguments involving considerations of symmetry,... 

The large deformability as shown in Figure 21.2, one of the main features of rubber, can be discussed in the category of continuum mechanics, which itself is complete theoretical framework. However, in the textbooks on rubber, we have to explain this feature with molecular theory. This would be the statistical mechanics of network structure where we encounter another serious pitfall and this is what we are concerned with in this chapter the assumption of affine deformation. The assumption is the core idea that appeared both in Gaussian network that treats infinitesimal deformation and in Mooney-Rivlin equation that treats large deformation. The microscopic deformation of a single polymer chain must be proportional to the macroscopic rubber deformation. However, the assumption is merely hypothesis and there is no experimental support. In summary, the theory of rubbery materials is built like a two-storied house of cards, without any experimental evidence on a single polymer chain entropic elasticity and affine deformation. 

The two-network theory for a composite network of Gaussian chains was originally developed by Berry, Scanlan, and Watson (18) and then further developed by Flory ( 9). The composite network is made by introducing chemical cross-links in the isotropic and subsequently in a strained state. The Helmholtz elastic free energy of a composite network of Gaussian chains with affine motion of the junction points is given by the following expression ... 

The two-network method has been carefully examined. All the previous two-network results were obtained in simple extension for which the Gaussian composite network theory was found to be inadequate. Results obtained on composite networks of 1,2-polybutadiene for three different types of strain, namely equibiaxial extension, pure shear, and simple extension, are discussed in the present paper. The Gaussian composite network elastic free energy relation is found to be adequate in equibiaxial extension and possibly pure shear. Extrapolation to zero strain gives the same result for all three types of strain The contribution from chain entangling at elastic equilibrium is found to be approximately equal to the pseudo-equilibrium rubber plateau modulus and about three times larger than the contribution from chemical cross-links. 

From a theory of non-Gaussian network Hert-Smith34,3S derived dW/dlt = G exp [, (/, - 3)21 dW/dI2 =Gk2II2... 

Rusakov 107 108) recently proposed a simple model of a nematic network in which the chains between crosslinks are approximated by persistent threads. Orientional intermolecular interactions are taken into account using the mean field approximation and the deformation behaviour of the network is described in terms of the Gaussian statistical theory of rubber elasticity. Making use of the methods of statistical physics, the stress-strain equations of the network with its macroscopic orientation are obtained. The theory predicts a number of effects which should accompany deformation of nematic networks such as the temperature-induced orientational phase transitions. The transition is affected by the intermolecular interaction, the rigidity of macromolecules and the degree of crosslinking of the network. The transition into the liquid crystalline state is accompanied by appearence of internal stresses at constant strain or spontaneous elongation at constant force. 

In Chapter III, section 1 the elasticity theory for ideal Gaussian networks is reviewed. Since in actual practice dry rubbery networks virtually never adhere to these theories — even in the range of moderate strains — experimental verification of the theory has been undertaken... 

The exact result shown in Eq. (IV-4) indicates that appreciable errors are introduced if the Gaussian approximation is used below N = 6. The use of Eq. (IV-4) in a complete network theory (compare e.g. Eq. III-2) has not been undertaken because of mathematical difficulties. Instead, Treloar has shown that in a tetrahedral four chain arrangement around a central crosslink, the fluctuations in the crosslink... 

M. H. Wagner and J. Schaeffer, Constitutive Equations from Gaussian Slip-link Network Theories in Polymer Melt Rheology, Rheol. Acta, 31, 22-31 (1992). 

A number of workers have treated non-Gaussian networks theoretically in terms of this finite extensibility problem. The surprising conclusion is that the effect on simple statistical theory is not as severe as might be expected. Even for chains as short as 5 statistical random links at strains of up to 0.25, the equilibrium rubbery modulus is increased by no more than 20-30 percent (typical epoxy elastomers rupture at much lower strains). Indeed, hterature reports of highly crosslinked epoxy M, calculated from equilibrium rubbery moduh are consistently reasonable, apparently confirming this mild finite extmsibiUty effect. 

M.H.Wagner, J.Schaeffer, Constitutive equations from Gaussian slip-link network theories in polymer melt rheology, Rheol. Acta 31 (1992), 22-31. 

In this section some general considerations of interest regarding non-Gaussian statistical theory are made with the aim of bringing the simple network model discussed in Section 3,2 closer to a real network (2,4). 

Gaussian limit theory, 120 Gaussian network model (GNM), 230... 

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