Molecular Aspects of Fracture and Healing in Polymers

From a kinetics point of view, formation of new surfaces, fracture, and subsequent crack healing have a certain symmetry, with chains variously being pulled out or diffusing into the fracture surface. The symmetry is far from complete, however, because of chain breakage, the chemical and physical state of the surfaces at the time of crack healing, and so on (53-60). 

In general, the two surfaces being healed are from different sources. Examples of this include molding operations, film formation from latexes, and adhesion. This section examines the micromechanisms involved in fracture and healing in polymeric materials. The effect of chemical bonding and adhesion between a polymer and a substrate surface is considered in Section 11.6. 

When two otherwise identical polymer surfaces are brought into juxtaposition at r Tg and annealed, a healing process may take place. As a result the interface gradually disappears. Basic additional requirements are that the polymer be linear (or branched) and amorphous. The major mechanism involves the interdiffusion of polymer chain segments across the interface. For polymer chain interdiffusion, the reptation theory of de Gennes (54) and Edwards (55) 

Chain scission is important in the deformation and fracture of many polymers this micromechanism normally consumes a sizable fraction of the fracture 

Two types of deformation may be imagined to excite the energy wells. First, the 109° bond angle is opened (ideally) to 180°. Second, the carbon-carbon bond distance is elongated from 1.54 A to some larger value. Mark (65) assumed that stretching the C—C bond to 2.54 A was required to cause the bond to fail. 

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