Fracture molecular origins

BPA-PC is one of the toughest amorphous pure polymers and that is the reason why many studies have been performed to elucidate the molecular origin of this fracture behaviour, in particular concerning the role of the ft transition analysed in detail in [1] (Sect. 5). 

The main experimental techniques used to study the failure processes at the scale of a chain have involved the use of deuterated polymers, particularly copolymers, at the interface and the measurement of the amounts of the deuterated copolymers at each of the fracture surfaces. The presence and quantity of the deuterated copolymer has typically been measured using forward recoil ion scattering (FRES) or secondary ion mass spectroscopy (SIMS). The technique was originally used in a study of the effects of placing polystyrene-polymethyl methacrylate (PS-PMMA) block copolymers of total molecular weight of 200,000 Da at an interface between polyphenylene ether (PPE or PPO) and PMMA copolymers [1]. The PS block is miscible in the PPE. The use of copolymers where just the PS block was deuterated and copolymers where just the PMMA block was deuterated showed that, when the interface was fractured, the copolymer molecules all broke close to their junction points The basic idea of this technique is shown in Fig, I. 

Hg. S. Cold snap fracture surfaces showing effect of molecular weight of modifier on morphology. Modifiers in top and bottom micrographs are nominally 100/i mcthyltrifluoropropyl siloxane. Original SEM magnification, 300 x... 

The linear relation GC°=T observed in Fig. 12 is not sufficient evidence that would unambiguously support Eq. (6) and reveal the interfacial nature of the transition, because a bulk phenomenon may also produce such a temperature dependence. For instance, one might think of melt fracture and write down oc=Gyc that would be independent of Mw where yc would correspond to the critical effective strain for cohesive failure and modulus G would be proportional to kBT. Previous experimental studies [9,32] lack the required accuracy to detect any systematic dependence of oc on Mw and T. This has led to pioneers such as Tordella [9] to overlook the interfacial origin of spurt flow of LPE. It is in this sense that our discovery of an explicit molecular weight and temperature dependence of oc and of the extrapolation length bc is critical. The temperature dependence has been discussed in Sect. 7.1. We will focus on the Mw dependence of the transition characteristics. 

ESR spectrum observed after the simultaneous fracture of PTFE with MMA at 77 K was found to be the quartet-quintet which is undoubtedly attributed to the propagating radical of PMMA. No trace of the PTFE radical was mixed in the observed spectrum. It is believed from both experimental and theoretical reasons that no radical is produced by a mechanical destruction of any solid consisting of a low molecular organic compound 31). Thus, the radicals produced by this mechanical action originate from not MMA but the PTFE polymers, although both solids of MMA and PTFE were fractured amultaneously. Accordingly the polymerization of MMA, which was proved by ESR, had been initiated not by the MMA radical but... 

Distinction between higher ranks. A distinction between higher ranks of products, difficult to make by mere inspection of concentration histories, is possible, but may remain ambiguous if the network contains steps of higher molecularities, as must be so if reactants combine to form larger product molecules. However, if the carbon skeleton of the original reactant remains intact or is fractured, the network may well consist exclusively of first-order or pseudo-first order steps. 

Now let me try to get some insists into molecular mechanisms of the mechanic fracture in polymers 73). The l teilin model was originally proposed in ex]danation of the mechano-radical formatirm in the hi y stretched fibre. However, one can apply the Peterlin model to the fracture ptenomena in crystalline polymers, because large deformations proceed always in advance of a mechanical fracture. Thus, the tie molecules are assumed to be only parts vtdiich are broken in the case of destmction of bulk polymers. The fact that no mechano-radical is formed from the polymer having no tie mdecules even after the milling supports the interpretation mentioned ove. However, for amorjdious pd[ymers such as PMMA and PB, formation of the mechano-radkals is not attributed to the ruptures of the tie molecules, becau% neither the crystalline parts nor frie tie molecule exist in an amorphous polymer and no particular part of the polymer, on which the applied stress is concentrated, can be assumed in the amorphous polymer. It was found that the polymer chains are ruptured even in the case of an amorphous polymer, like PMMA, PB, and other elastomers, as mentioned in the Section III. The medianism other than the Peterlin model is needed to explain the bond scissions of polymer chains in the amorphous pdymers. 

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