Crack healing in polymers

Once intimate contact is achieved, bonding of the ply interfaces can occur. The mathematical relationships between interply bond formation and processing temperature and time were discussed in Section 7.3. The analyses are based on the theories explaining strength development of a polymer-polymer interface and crack healing in polymers. 

This process has been used for self-healing in polymer composites via release of a polymerizable healing agent that would bridge cracks after reaction in the presence of appropriate catalysts. 

The randomization stage refers to the equilibration of the nonequilibrium conformations of the chains near the surfaces and in the case of crack healing and processing, the restoration of the molecular weight distribution and random orientation of chain segments near the interface. The conformational relaxation is of particular importance in the strength development at incompatible interfaces and affects molecular connectivity at polymer-solid interfaces. 

One of the most important and useful applications of the reptation concept concerns crack healing, which is primarily the result of the diffusion of macromolecules across the interface. This healing process was studied particularly by Kausch and co-workers [76]. The problem of healing is to correlate the macroscopic strength measurements to the microscopic description of motion. The difference between self-diffusion phenomena in the bulk polymer and healing is that the polymer chains in the former case move over... 

Re-mendable polymers can achieve multiple healing cycles, but require external intervention in the form of heat treatment and applied pressure. It is a powerfiil method of crack reparation for polymers that exhibit thermally reversible reactions for cross-linking linear chains. For example, a Diels-Alder polymerization product of reaction between a multi-furan and multi-maleimide has shown to be feasible and a recovery of about 57% of the original fracture load has been obtained. Other types of organic adhesives are not capable of repolymerizing, and alternative self-healing procedures should be employed. 

Diffusion in polymer-polymer systems is important in many practical applications, such as crack healing, polymer blending, welding, adhesion, elastomer tack, polymer fusion, and many others. However, diffusion coefficient data is lacking due to the complexity of the polymer diffusion process. Typical techniques employed are infrared spectroscopy, spectroscopic ellipsometry, and optical schlieren. Only a few studies of interdiffusion in polymer pairs are reported. Depending on the temperature and other physical parameters of the system, Fickian and Case II diffusions (see Section 4.3) can be observed with varying relative importance (Jabbari and Peppas, 1993). 

C.3 Mutual Dijfusion of Polymers in Contact (Adhesion). Diffusion theory is one of the theories of adhesion. According to this theory, the molecules of two polymers in mutual contact diffuse across the interface (Fig. 4.27), so that after some time the interface does not exist. The strength of the joint will then depend on the distance the macromolecules have interpenetrated each other. Crack healing is also based on the theory of diffusion. 

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