Melt-recrystallized PTFE

Figure 5. F-19 REVS chemical shift lineshapes of melt recrystallized PTFE at...

Figure 9. Temperature dependence of the T, and T, BKI relaxation times of a 68% crystalline melt-recrystallized PTFE sample ( ) denote the short-relaxationtime component in a double exponential decay. T,times longer than 100 ms and Ttimes longer than 1 s are not shown since their experimental values could not be determined accurately. The expected temperature dependence below...

The chemical shift lineshape changes observed for the crystalline fraction of melt-recrystallized PTFE from /L14°C to w28°C (Figure 6) are due to reorientation about an axis essentially parallel to Ozz (which is tilted VL2° from the molecular chain axis). The temperature dependence of the line-shape changes can be simulated as an increase in the rate of rotational diffusion about this axis (Figure 6) and this process has an activation energy of 48+11 kcal/mole. In addition, the... 

Variable temperature (-150° to +350°) chemical shift spectra have been obtained for melt-recrystallized samples of PTFE. The NMR spectra were acquired using the REV-8 sequence which consists of a long train of closely spaced, high power rf pulses of fixed width, and varying in phase in a cyclical fashion. The cycle consists of eight ir/2 pulses. The total cycle time used was 43.2 psec with a shortest spacing between pulses of 3.6 usee. The nominal resonance frequency was 84.6... 

Figure 7. Temperature dependence of the rate of rotational diffusion about the chain axis of PTFE molecules in the crystalline regions of melt-recrystallized samples. Ex = 48 11 kcal/mot.

Melting and recrystallization behavior of virgin PTFE has been studied by dsc (69). A quantitative relationship was found between A/ and the heat of crystallization (AHc) in the molecular weight range of 5.2x 10 - 4.5x 10, where AHc is heat of crystallization in J/g, which is independent of cooling rates of 4-32°C/min. 

PTFE that has not been melted previously has a crystallinity of 92-98%, indicating a linear and non-branched molecular structure. Upon reaching 342°C, PTFE melts and changes from a chalky white color into a transparent amorphous gel. The second melting point of PTFE is 327°C, because it never recrystallizes to the same extent. 

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