Polymer microcrack

Polymer Microcrack longitudinal, nm Diameter transverse, nm iVcr, cm Density calculated Ap/p measured... 

Polyamides, like other macromolecules, degrade as a result of mechanical stress either in the melt phase, in solution, or in the soHd state (124). Degradation in the fluid state is usually detected via a change in viscosity or molecular weight distribution (125). However, in the soHd state it is possible to observe the free radicals formed as a result of polymer chains breaking under the appHed stress. If the polymer is protected from oxygen, then alkyl radicals can be observed (126). However, if the sample is exposed to air then the radicals react with oxygen in a manner similar to thermo- and photooxidation. These reactions lead to the formation of microcracks, embrittlement, and fracture, which can eventually result in failure of the fiber, film, or plastic article. 

Mechanical and Chemical Properties. Colorants, especially pigments, can affect the tensile, compressive, elongation, stress, and impact properties of a polymer (5). The colorants can act as an interstitial medium and cause microcracks to form in the polymer colorant matrix. This then leads to degradation of the physical properties of the system. Certain chemicals can attack colorants and there can be a loss of physical properties as well as a loss of the chromatic attributes of the colorant. Colorants should always be evaluated in the resin in which they will be used to check for loss of properties that ate needed for the particular appHcations. 

Fig 23 11 Crazing in a linear polymer molecules are drawn out as in Fig. 23.10, but on a much smaller scale, giving strong strands which bridge the microcracks. 

In investigations of the failure of fiber compositions (PETP — short glass fibers) [251] it was found that the main process responsible for composite failure under load is the rupture at the matrix-fiber interface. The author of [251] observed formation of microvoids in loaded samples, both at the interphases and in the bulk. The microvoids, or cavities) grow in size and become interconnected by microcracks, and this results in fiber separation from the binder. However, when the matrix-fiber bond is strong enough, the cavities appear mostly in the bulk of matrix, the failure of the specimen does not over-power cohesion and traces of polymer remain on the fibers. 

Hiemstra, D.L. and Sottos, N.R. (1993). Thermally induced interfacial microcracking in polymer matrix composites.. /. Composite Mater. 27, 1030-1051. 

Figure 2.7. SEM image of the fracture surfaces for the nanocomposites containing 0.5 wt% of CNTs. The arrows indicate that the nanotubes were to be broken with their ends still embedded in the polymer matrix or they were bridging the local microcracks in the nanocomposites. Reproduced from reference 35 with permission from Elsevier.

Sources of strength loss in such fibers are mainly surface damage due to contamination and, possibly, the presence of microcracks (e.g. bubbles, etc.). Polymer surface coalings are used to minimize the damage. Fibers are also proof tested to breaking strains in the range of 0.5-1%. 

Zhurkov et al. used light scattering and low angle X-ray scattering (LAXS) to study the time-dependent formation of microcracks or cavities in stressed polymers. The concentrations of microcavities were found to be as high as 10 - 10 cm , uniformly distributed throughout the specimen, thougji more numerous close to the specimen surface. 

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