PTFE peroxy radicals

In Section 7.6.1, principal directions of the g-tensor of PTFE peroxy radicals were determined and rotation of the radicals around the PTFE chain axis, rather than around the 0-0 bond, was demonstrated. In this section, we describe a detailed study of the ESR spectra and the molecular motion of peroxy radicals when they are trapped in various regions with many kinds of heterogeneities in polymer matrices. Temperature dependent ESR spectra of peroxy radicals in the powdered polyethylene having a low crystallinity are shown in Fig. 7.32 [33]. 

In the case of PTFE ESR spectrum observed from the fragments siwed in liquid nitrogen is an as)mimetric spectrum, which indicates existence of single q>ecies of the peroxy radical. From analyses of the temperature variation of the q>ectrum it is concluded that the radical species responsible for the spectrum is not a peroxy chain radical,... 

Scissions of main-chains by the mechanical destruction of the polymers are experimentally proved by the analyses of the observed ESR spectra for the various pdy-mers PE, PTFE, PB, PP and PMMA. A pair formation of the radicals, (mechano-radicals), after the milling is clearly demonstrated and this pair formation is believed to be the direct evidence for tl mechano-radicals formed primarily by the medianical actions. A model for chain rupture in an amorphous pdymer was proposed. Excess electrons produced by the triboelectricity due to the friction, diich is always accompanied with the mechanical fracture, play an important role, with coexistence of oxygen, in the thermal conversion of the mechano-radicals. The characteristic behaviors of the mechano-radicals, the hi er reactivity with oxygen, complete photoconversion of the peroxy radical, indicate that the mechano-radicak are formed and trapped on the fresh surfaces produced by cleavage in the solid polymer. The polymerizations initiated at the low temperatures by the PTFE mechano-radicals were reported and the copol5mierization is experimentally evidenced. 

In general, alkyl radicals trapped in polymer materials convert to peroxy radicals when oxygen molecules are introduced. The disappearance of the double quintet due to the chain type fluoro alkyl radicals in polytetrafluoroethylene (PTFE) described... 

Fig. 7.27 ESR spectra of peroxy radicals in y-irradiated PTFE powders observed at room temperature (a) and 77 K (b). Measurements were made at 24 GHz. The figure is adapted from [28] with permission from John Wiley Sons Inc.

The ESR spectra for stretched PTFE films were also measured at room temperature and 77 K. The room temperature spectra obtained at various orientations of the stretch axis to the magnetic field are shown in Fig. 7.28. In addition to a strong signal with large anisotropy, a broad weak signal can be seen at g = 2.016. Since this peak is nearly isotropic and its g value is very close to the g value of the symmetric component of the powder samples, we can again attribute this peak to the peroxy radicals of PTFE. The value of 2.005, which was found with the field parallel to the stretch axis, is very close to g/ = 2.006 obtained for the powder spectrum, while 2.021 measured for the perpendicular direction, is close to gj. = 2.022, This means that the symmetry axis of the g tensor is parallel to the molecular chain axis at room temperature. 

The effect of gamma irradiation on the physical properties of PTFE film are shown in Table 52.10. The faUoff in physical properties is dramatic, even after irradiation in vacuo followed by exposure to air. The radicals produced by irradiation have been shown to have a long lifetime even after heating to 300 °C [103], By looking at the electron spin resonance spectrum, radical I is detected for irradiation in vacuo and peroxy radical II is detected after exposure to air [ 105]. 

Exposure to NO of PTFE powder containing a mixture of middle and terminal peroxy radicals and y-irradiation at room temperature in air leads to the formation of nitrogen-containing radicals whose ESR spectrum is displayed in Figure 3.1 ... 

Thus, to produce nitrogen-containing radicals distinguishable in their structure from radical XV one shonld initially condnct y-radiolysis of PTFE samples in air to accumulate peroxy radicals. The presence of RO radicals alone is insufficient. Some other prodncts arising in PTFE in the conrse of radiolysis in air must be present. Nitrogen-containing radicals form as a resnlt of the secondary reactions in which valence-satnrated compounds participate, transforming in an atmosphere of NO. 

In the absence of oxygen in the rigid PTFE matrix, the reverse reaction of these radical proceeds efficiently. In the presence of oxygen, the terminal alkyl macroradicals can be oxidised to form terminal peroxy radicals which interact with double bonds much slower. Under the action of NO on samples containing neighbouring terminal double bonds and peroxy radicals, the latter are converted into macromolecular nitrates and nitrites [47] ... 

Therefore, all the mechano-radical of PTFE react with oxygen in liquid nitrogen to form the peroxy end radical. Such high reactivity with oxygen was not found for the PTFE radicals produced by y-ifradiation. 

A similar procedure was used to obtain spin-labelled TEE-HEP [49]. The presence of hexafluoropropylene (HFP) groups in this polymer leads to disturbance of the structural ordering typical of PTFE to more complex dynamics of their motion. After y-irradiation of powders and films of TFE-HFP copolymer in air, there are three types of stable peroxy macroradicals in the samples end radicals CF -CF O, secondary middle-chain radicals CF -CF(00 )-CF2, and tertiary middle-chain radicals CF2-C(CF3)(00 )-CF2. In contrast to PTFE, prolonged exposure (>100 hours) of these samples in a NO atmosphere at room temperature does not lead to the formation of aminoxyl macroradicals. However, two types of macroradicals are formed if TFE-HFP is heated with evacuation after the decay of radicals in a NO atmosphere. At 90 °C, the ESR spectrum demonstrates the presence of tertiary alkyl macroradicals CF2-C (CF3)-CF2 formed upon decay of the tertiary nitroso compounds [57]. On further increasing of the temperature up to 180 "C, the tertiary alkyl macroradicals... 

However, in PTFE containing end peroxide radicals, no regeneration of these radicals after the removal of NO was observed, though the decomposition temperature was raised to 140 "C. Nevertheless, the reappearance of peroxide radicals in PTFE upon evacuating shows that the product of the R O reaction with NO is peroxy nitrate, and fluorinated compounds also exhibit the reversible reaction ... 

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