MECHANISMS OF TOUGHENING BRITTLE POLYMERS

Heavily cross-linked thermosetting polymers such as epoxies and certai n flexible chain thermoplastic polymers which undergo crazing show bri ttle behavior in tension but can undergo quite significant plastic def ormation in compression. The cause of the tensile brittleness of the f ormer is a relatively low fracture toughness, while that of the latter results from premature fracture, precipitated in craze matter, due to acquisition of ill-adhering inorganic particulate inclusions of large r than micron size by the crazes during their growth. Both types of po lymers can be effectively toughened. A particularly attractive mechani sm of toughening the epoxies is by a crack-trapping process through th e use of a well-adhering set of tough fibers which force the brittle c rack to bow around the fibers. A very effective technique to toughen t he brittle crazable polymers is to decrease the craze flow stress belo w the critical threshold strength defined by the size of the inclusion s. This is accomplished by a novel mechanism of prepackaged, stress-ac tivated diluents that plasticize the polymer on demand during crazing. Examples of these two mechanisms are presented.