Nonoriented polymers samples

E reduction at large for compositions UHMPE/Al is due to not molecular mechanisms, but macroscopic ones (interfacial boundaries polymer matrix-filler fi acture) [16]. The linear dependence K2(Xj jj) is obtained, which at = 1 is extrapolated to = 0. This means, that polymer Ifacture is impossible without chain preliminary drawing. In other words, this assumes definite nonzero failure strain for nonoriented polymers samples. As it has been shown in Ref [22], the greatest value X j ( ) can be determined... 

It was also found [6], that in this case experimentally determined values of athermic fracture stress turn out to be essentially (2 3 times) smaller than theoretically calculated ones. A small values k 0.2 1.0) is one more important feature of nonoriented polymers fracture in impact tests. This means, that the stress on breaking bonds is essentially lower than nominal fracture stress of bulk sample. And at last, it was found out [7], that the value k reduces at testing temperature growth and the transition from brittle fiactuie to ductile (plastic) one. These effects explanation was proposed in Refs. [4-7], but development of fractal analysis ideas in respect to polymers lately and particularly, Alexander and Orbach woik [8] appearance, which introduced the fraction notion, allows to offer the major treatment of polymer fracture process [9, 10], including the dilaton concept [1-3] as a constituent part. 

Let us consider the reasons of the adduced in Table 7.1 values k < 1 for nonoriented polymers. As it has been shown in chapter two, the value characterizes molecular mobility (deformability) level of the indicated chain part [17]. At = 1.0 this mobihty is suppressed completely and at =2.0 it reaches the greatest possible level, typical for rubber-like state. Molecular mobility intensification results to corresponding stress relaxation intensification, applied to chain part between entanglements and, as consequence, to its reduction lower than macroscopic sample fracture stress [18], Such treatment assumes availability of the correlation between parameters K and This assumption is confirmed by the plot of Fig. 7.1, where the dependence for 10 polymers, pointed out in table 7.1, is adduced... 

As in the case of crystallinity, the sample is assumed to be composed of the oriented and nonoriented fractions, defined by / and 1-/, respectively. Using the observed dichroic ratio of a partially oriented polymer film or fiber, the fraction of oriented material is given by [19]... 

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