Deformation Behavior of Thin Films

The deformation behavior of amorphous polymers has been studied extensively, partly because the structure is rather simple as compared with semicrystalline polymers thus, the relationship between structure and properties can be established with relative ease. It is well known that two major micromechanisms are involved in the deformation and subsequent fracture of glassy polymers [1,2,13] (see Figs. 18.1 and 18.2). These are crazing and shear yielding, and both involve localized plastic deformation and some energy is dissipated during the deformation. In a craze, polymer chains are stretched along the stress direction and 

There has been considerable progress over recent years in the study of molecular mechanisms involved in crazing and shear yielding [1,2]. When sample preparation and test conditions (temperature, stress state, strain, strain rate, and thermal history) are chosen to be the same, the effect of molecular variables can be determined. It is now well established that (network) strand density plays an important role in determining the deformation mechanism and in affecting the craze-shear deformation transition. Polymers are 

The (theoretical) maximum extension ratio of a strand can be determined as (see Fig. 18.4) 

The idea described above for glassy amorphous homopolymers can be extended to include miscible amorphous polymer blends, such as PS/PPO. Furthermore, a low degree of covalent cross-links can be considered as equivalent to entanglements for controlling the deformation mode. The strand density of cross-linked polymers is defined as the sum of the entanglement density and the covalent cross-link density [18] as 

Although the results described above have been observed for thin films under tension and large-strain plastic deformation is usually observed only for localized regions - crazes or shear deformation zones - Meijer has demonstrated that macroscopic plastic deformation can be indeed observed for these amorphous polymers under tension, as far as the thickness of the deformed polymer is less than a critical value [3-8]. Thin 

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