Amorphous phase, viscous flow

Although PBT fiber also has a plateau region in the stress-strain curve [4], the crystalline chains do not respond to external strain in the first few percent of deformation. They increased in length only when the strain is above 4% (see Figure 11.13). Therefore, initial macroscopic deformation involved viscous flow of the amorphous phase. Furthermore, PBT undergoes strain-induced crystal transformation at moderately low strains of 15-20% [75], The differences in their microscopic crystalline chain deformation explained why PTT has a better elastic recovery than PBT even though both have contracted chains and knees in their stress-strain curves [4, 69],... 

Fig. 1.19.2. Isoplethal section of the solid-liquid state diagram for R = 0.1. Section fields 1, solution (viscous-flow state) 2, ice 3, ice and amorphous phase (mechanical properties...

Viscous flow in an amorphous material, requiring a liquid phase. 

Linear Polymers Long chains are necessary to confer the mechanical properties of fibers, plastics, and elastomers that make polymers so valuable. Fibers such as cellulose and polyester arc semicrystalline materials in which the same chemical stmeture exists in both rigid microcrystalline and flexible amorphous phases. Plastics may be either semicrystalline, such as poly(ethylene terephthalate) (the same polyester of fibers is also the PET of beverage bottles), or completely amorphous and glassy, such as polystyrene or poly(methyl methacrylate) (PMMA, Plexiglas or Lucite ). Elastomers are completely amorphous and flexible and would flow as a viscous mbbery liquid except that the polymer chains are cross-hnked to prevent macroscopic flow but allow reversible stretching. As an example, poly(dimethylsiloxane)... 

For solid phase sintering, there are fonr ways of diffusion i) surface diffusion, ii) volnme diffusion (often called lattiee diffusion), iii) vapor phase transport (evaporation-eondensation), and iv) grain boundary diffusion the boundaries are very disturbed areas, which allow diffusion short-circuits . For liquid phase sintering, we must add dissolution-reprecipitation effects or a vitreous flow. Finally, for pressure sintering the pressure exerted allows the plastic deformation of the crystallized phases and the viscous flow of the amorphous phases. 

Hence, the cluster model of pol5nners amorphous state structure and the model of WS aggregates friction at translational motion in viscous medium [24] combination allows to describe solid-phase polymers behavior on cold flow (forced high-elasticity) plateau not only qualitatively, but also quantitatively. In addition the cluster model explains these polymers behavior features on the indicated part of diagram a - , which are not responded to explanation within the frame woiks of other models [14]. 

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