Molecular failure processe

The main experimental techniques used to study the failure processes at the scale of a chain have involved the use of deuterated polymers, particularly copolymers, at the interface and the measurement of the amounts of the deuterated copolymers at each of the fracture surfaces. The presence and quantity of the deuterated copolymer has typically been measured using forward recoil ion scattering (FRES) or secondary ion mass spectroscopy (SIMS). The technique was originally used in a study of the effects of placing polystyrene-polymethyl methacrylate (PS-PMMA) block copolymers of total molecular weight of 200,000 Da at an interface between polyphenylene ether (PPE or PPO) and PMMA copolymers [1]. The PS block is miscible in the PPE. The use of copolymers where just the PS block was deuterated and copolymers where just the PMMA block was deuterated showed that, when the interface was fractured, the copolymer molecules all broke close to their junction points The basic idea of this technique is shown in Fig, I. 

A second model was proposed by Benkoski et al. [16] based on the idea that chain friction and pull-out, rather than chain scission are the important molecular scale failure processes. It is assumed that the chain failure force is given by fc = N/mono, where N is the number of monomers in a loop that crosses the interface, and that... 

Another important aspect of the fatigue of all materials is the statistical nature of the failure process and the scatter which this can cause in the results. In a particular sample of plastic there is a random distribution of microcracks, internal flaws and localised residual stresses. These defects may arise due to structural imperfections (for example, molecular weight variations) or as a result of the fabrication method used for the material. There is no doubt that failure... 

Bueche and Halpin (126, 203, 215-217) have developed a fracture theory for amorphous rubbers. Their model pictures rupture as the result of the propagation of tears or cracks within the material. The growth of a tear is viewed as a process in which molecular chains at the tip of the tear stretch viseoelastieally, under the influence of a high stress concentration, until they rupture. The failure process is a non-equilibrium one, developing with time and involving consecutive rupture of molecular chains. The principal result of the theory is embodied in the equation... 

Shogenov, V. N. Kozlov, G. V. Mikitaev, A. K. The prediction of failure process parameters of rigid-chain polymers. High-Molecular Compounds. B, 1989, 31(11), 809-811. 

Adhesives are a very diverse and complex group of materials. They can manifest themselves in many shapes and forms—they can be viscous liquids, powders, or cured products. Analysis or characterization is an essential step in working with adhesives. As a rule, such efforts are directed toward a specific purpose that may focus on structural determination, curing reaction, size of the molecule, material design at a molecular level, process control, or failure analysis. In this chapter we provide a general review of several physical methods frequently used for analysis of adhesives. In view of the prolific literature on the subject as well as the space constraints, it is not intended to give a comprehensive treatment of the theory and experimental aspects. The examples chosen for this review are illustrative and not exhaustive. 

Phenomenological aspects of fibre fracture have been discussed elsewhere (see Kausch, 1987 for example). It suffices to remind that the details of the failure process are highly complex and depend upon many factors such as polymer structure, environment, type of loading and time. Molecular fracture does not occur to the same extent in all polymer fibres and the micromechanisms differ in different types of fibres. 

The cause of brittle fracture in polymers is the inability of the material to quickly dissipate by molecular relaxation processes the internal stresses generated as a result of the imposed deformation. Brittle fracture occurs when the time to failure is the same order of magnitude (or faster) than the speed of the relaxation process that dominates the mechanical behavior in the temperature range of interest. The relevant relaxation processes are the first T < Tg secondary relaxation (p or y). A qualitative criterion for determining whether the relaxation... 

This envelope may be used to describe relaxation, creep or constant strain rate measurements. A change in strain rate or temperature only shifts a point along the failure envelope, which is thus dependent only on the structural characteristics of the elastomer. The ultimate properties of rubbers are mainly governed by their viscoelastic properties, and reduced master curves can be obtained for tensile strength and strain as a function of time to break. The failure process is a non-equilibrium one, developing with time and involving the consecutive rupture of the molecular chains. The ultimate properties can then be predicted from creep measurements. ... 

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