Mechanical stress, initiation, graft

Mechanical syntheses are, of course, generally affected by the presence of radical acceptors (20,29). The yield on copolymers also increases with duration of mechanical stress. However, if the milling time is too long, the properties of the graft or block copolymers can be deteriorated by degradation of initial product. The nature of the balls and mill material can also influence the reaction (37) the mechanical activation of inorganic materials. The production of graft and block copolymers on freshly formed surfaces has been established in the lierature. 

In the presence of monomers, of course, graft and block copolymers are formed. The polymerization is initiated by the macro free radicals generated by mechanical stresses (block copolymers) or by free radicals obtained by intermolecular transfer (graft polymers), such as... 

When 4-(mercaptoacetamido)diphenylamine [60766-26-9] (39) is added to EPDM mbber and mixed in a torque rheometer for 15 minutes at 150°C, 87% of it chemically binds to the elastomer (24). The mechanical and thermal stress placed on the polymer during mixing mptures the polymer chain, producing radicals that initiate the grafting process. 

Many types of graft and block copolymers, including the polyurethanes, stress-soften on repeated extension, as shown in Figure 5.8 (Estes et ai, 1970). The mechanism of stress-softening is believed to arise due to the deformation (and perhaps disruption) of the hard domains during the initial application of stress. As a result, lower moduli and stress levels... 

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