Glassy polymers micromechanics

The aim of this chapter is to describe the micro-mechanical processes that occur close to an interface during adhesive or cohesive failure of polymers. Emphasis will be placed on both the nature of the processes that occur and the micromechanical models that have been proposed to describe these processes. The main concern will be processes that occur at size scales ranging from nanometres (molecular dimensions) to a few micrometres. Failure is most commonly controlled by mechanical process that occur within this size range as it is these small scale processes that apply stress on the chain and cause the chain scission or pull-out that is often the basic process of fracture. The situation for elastomeric adhesives on substrates such as skin, glassy polymers or steel is different and will not be considered here but is described in a chapter on tack . Multiphase materials, such as rubber-toughened or semi-crystalline polymers, will not be considered much here as they show a whole range of different micro-mechanical processes initiated by the modulus mismatch between the phases. 

Micro-mechanical processes that control the adhesion and fracture of elastomeric polymers occur at two different size scales. On the size scale of the chain the failure is by breakage of Van der Waals attraction, chain pull-out or by chain scission. The viscoelastic deformation in which most of the energy is dissipated occurs at a larger size scale but is controlled by the processes that occur on the scale of a chain. The situation is, in principle, very similar to that of glassy polymers except that crack growth rate and temperature dependence of the micromechanical processes are very important. 

Unfortunately, the initiation and evolution of crazes do not concern only the majority of thermoplastic glassy polymers, which exhibit brittle behavior. Crazes usually also constitute the dominant micromechanism for failure when many polymers generally considered tough are subjected... 

MICROMECHANICAL MODELLING OF RATE AND TEMPERATURE DEPENDENT FRACTURE OF GLASSY POLYMERS... 

Broutman, L. J. and Panizza, A. (1971) Micromechanics studies of rubber-reinforced glassy polymers, Int. J. Polymeric Mater., 1, 95 109. 

To address these limitations, a new constitutive model was developed for conventional and highly crosslinked UHMWPEs (Bergstrom, Rimnac, and Kurtz 2003). This model, which is inspired by the physical micromechanisms governing the deformation resistance of polymeric materials, is an extension of specialized constitutive theories for glassy polymers that have been developed during the last 10 years, is discussed later. 

This approach incorporates the stress-concentrating effect of cross-tie fibrils, widely observed in crazes in glassy polymers (compare Figure 14.14). In the absence of any stress-concentrating effect, that is, for a —> 0, a time-independent fibril failure criterion oy implies crack advance can never occur, because the stress in a given fibril can never exceed Oc- This result has been confirmed by more detailed micromechanical modeling, and is important in that it provides a direct link between the... 

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... 

More often, toughening mechanisms involved in polymer blends are influenced by the properties of the matrix material and also by the morphology of the blend. In the glassy state (glassy polymers), rubber toughening mechanisms can often involve three main micromechanical deformation mechanisms ... 

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