Dual Network Fluoropolymer DNF Model

A number of more advanced and general models attempting to predict the yielding, viscoplastic flow, time-dependence, and large strain behavior of fluoropolymers and other thermoplastics have recently been developed.in this section, we discuss the Dual Network Fluoropolymer (DNF) model. 

The DNF model incorporates the experimentally observed characteristics by using a micromechanism-inspired approach in which the material behavior is decomposed into a viscoplastic response, corresponding to irreversible molecular chain sliding due to the lack of chemical crosslinks in the material, and atime-dependent viscoelastic response. The viscoelastic response is further decomposed into the response of two molecular networks acting in parallel the first network (A) captures the equilibrium response and the second network (B) the time-dependent deviation from the viscoelastic equilibrium state. A onedimensional rheological representation of the model framework and a schematic illustrating the kinematics of deformation are shown in Fig. 11.6. 

In this constitutive framework, the total deformation gradient F is decomposed into viscoplastic and viscoelastic components F = F F. The viscoelastic deformation gradient acts on both the equilibrium network A, and on the time-dependent network F = F = F. The Cauchy stress acting on network A is given by the eight-chain representa- 

The volumetric response is governed by the bulk modulus K, which is taken to be the same for both networks A and B. The temperature dependence of the shear modulus (0) is captured using the same temperature dependence as was used for /. Using this framework, the total stress in the system is given 

The viscoelastic deformation gradient acting on network B is decomposed into elastic and viscous parts F = F F . The Cauchy stress acting on network B is obtained from the eight-chain network representation using the same procedure that was used for network  

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