FRACTURE - Fractal Mechanics of Polymers - page 152

the molecular mobility level can be characterized by the value of fractal
dimension D ch of the chain part between its fixation points (chemical cross-
linkings, entanglements, clusters and so on). The value D ch is determined
with the help of the Eq. (2.16). The dependence of D ch values, calculated by
the indicated mode, on K st is adduced in Fig. 7.10. As one can see, the value
D ch reaches maximum at K st = 1.20, that is, at this K st value epoxy polymer
possesses the greatest molecular mobility. The molecular mobility intensi-
fication results to the supplied mechanical energy dissipation growth and,
as consequence, to polymer deformability enhancement [20]. From the Eq.
(2.16) it follows, that the only cause of D ch extreme growth is C ¢ decrease,
that is, we return again to the necessity of cross-linking polymers molecular
characteristics change in their curing process accounting for these materials
properties correct description.
The Eq. (7.13) was used for the studied epoxy polymers strength (frac-
ture stress) s f calculation. In Fig. 7.11 the comparison of strength values
s f , obtained experimentally and calculated according to the Eq. (7.13), for
sulfur-containing epoxy polymers is adduced. As the adduced comparison
shows, the good conformity of theory and experiment is obtained.
FIGURE 7.10 The dependence of fractal dimension of chain part between clusters D ch on
K st value for sulfur-containing epoxy polymers [49].