Spatially Homogeneously Distributed Chain Scissions

The activation energies and activation volumes for the three polymer films investigated are listed in Table 8.2 together with the preexponential factorsKq. 

In these experiments the authors also observed that the concentration of end groups at break for a series of specimens did not depend on stress, temperature, or type of loading (Fig. 8.12). The apparent critical concentration of all newly formed end groups in PE-HD amounted to 3.9 10 cm or to — 2 10 bond ruptures 

Polymer Preexponential factor Kq (s ) Activation energy (kcal/mol) Activation volume kcal mm N mol l0-6m3 mol 

For polypropylene they derived concentrations of 6 10 cm broken bonds in the surface layer — as compared to 0.25 — 0.40 10 cm reported by Zhurkov et al. [17, 18] for the interior of the film. The latter value should be compared with the number of overloaded 5 nm segments. In Section I B a concentration of 8.5 10 cm overloaded segments had been determined from the area of the tail region of the stressed IR band . Such a comparison indicates that of No = 56 10 cm segments present in the interior material of the PP film 0.15 Nq are initially overloaded and 0.0045 Nq will have been broken at the point of macroscopic fracture. In another study on PP [25] a concentration of broken bonds of 3 10 cm is reported, i.e. a fraction of 35% of the overloaded bonds is broken throughout the total volume of the sample. In the latter case wholesale destruction of the polymer must have occurred since on the average each molecule in a (mono-disperse) Mn = 50000 g/mol sample would have been broken 2.5 times. In the former case the effect is ten times smaller, but Mn is still reduced to 40000 g/mol and 

If the number of breaking chain segments is as large as indicated by the end-group analysis and if each segment breaks at the limiting stress value derived from the analysis of deformed-IR bands then the accumulated molecular stresses would be comparable within an order of magnitude to the applied macroscopic stress. 

---

The ion track radius is also an important parameter in such reactions, reflecting the local spatial distribution of energy deposited by an incident ion and influencing the character of subsequent chemical reactions [8-12]. We recently reported on main-chain scission and crosslinking reactions in a variety of polymer systems and proposed chemical core sizes in ion tracks based on discussion of the non-homogeneous spatial distribution of reactions [9-15]. Intratrack crosslinking reactions are also of interest with respect to the potential for the direct formation of nano-structured materials, and materials exhibiting these reactions have been successfully visualized in recent years [11,13]. However, despite the extensive experimental and theoretical study undertaken to date, many factors in the relationship between the ion track structure and the chemical core radius remain unclear. This paper proposes a new formulation that determines the chemical core radius in an ion track based on the initial... 

---