Polytetrafluoroethylene transition temperature

Polytetrafluoroethylene transitions occur at specific combinations of temperature and mechanical or electrical vibrations. Transitions, sometimes called dielectric relaxations, can cause wide fluctuations in the dissipation factor. 

Fig. 7. Phase diagram for polytetrafluoroethylene. (Low temperature transitions according to Beecroft and Swenson, melting according to McGeer and Duus)...

Transition Temperature - This is a temperature (19°C for 100% homopolymer of tetrafluoro-ethylene) at which the unit crystalline cell of polytetrafluoroethylene changes from triclinic to hexagonal. 

For at least two polymers, polytetrafluoroethylene 25, 62) and poly- trans-l,4-butadiene) 4, 25), relaxation processes accompanying crystal-crystal phase transitions are found. Sharp NMR line narrowing has been observed at the transition temperature for various normal paraffins 44)-... 

Figure 3.16 Some experimental dynamic components, (a) Storage and loss compliance of crystalline polytetrafluoroethylene measured at different frequencies. [Data from E. R. Fitzgerald, J. Chem. Phys. 27 1 180 (1957).] (b) Storage modulus and loss tangent of poly(methyl acrylate) and poly(methyl methacrylate) measured at different temperatures. (Reprinted with permission from J. Heijboer in D. J. Meier (Ed.), Molecular Basis of Transitions and Relaxations, Gordon and Breach, New York, 1978.)...

No general rules about the entropy of transitions, as were found for liquid and plastic crystal transitions, can be set up for condis crystals. Two typical examples may illustrate this point. Polytetrafluoroethylene has a relatively small room-temperature transition-entropy on its change to the condis state and a larger transition entropy for final melting. Polyethylene has, in contrast, a higher condis crystal transition entropy than melting entropy (see Sect. 5.3.2). 

The specific heat of polytetrafluoroethylene obtained by Marx and Dole (1955) over the room temperature transition range was the same for the polymer in fiber form as in the undrawn form. 

Slichter, (1959) found that a narrowing of the proton resonance line width occurred 10—20° C above Tg in the case of polyisobutylene and natural rubber but closely at Tg in the case of atactic polypropylene. The two room temperature transitions in polytetrafluoroethylene, which are so clearly visible in the specific heat-temperature curve, Fig. 12, were found by Slichter (1958a) in a fluorine nucleus magnetic resonance study to cause a drop in the second moment with rise of temperature, but the whole effect occurred over the wide temperature range of 225 to 320° K. The slight bulge" in the specific heat-temperature curve of polymethyl methacrylate from 130 to 180° K, Fig. 13, might possibly be correlated with the drop in the NMR second moment between 150° and 200° K found by Powles (1956). 

The rheology of lubricated polytetrafluoroethylene compositions was studied by Lewis and Winchester. The mechanism appeared to be a combination of permanent and elastic deformations in the region just before the orifice of the die in the extruder. As a result of permanent deformation, the polymer particles are partially transformed into long fibers. The relative amounts of permanent and recoverable deformation were related to the rate and temperature of extrusion and the geometry of the extruder. Plastic deformation is favored by extruding at temperatures above the 19 and 30° transitions (Snelling and Lontz). 

Rigby, H. A., and C. W. Bunn A room-temperature transition in polytetrafluoroethylene. Nature (London) 164, 583 (1949). 

The identification of polymer blends is illustrated by the DTA curve in Figure 7.48. Chiu (154) studied a physical mixture of seven commercial polymers high-pressure polyethylene (HPEE), low-pressure polyethylene (LPPE), polypropylene (PP), polyoxymethylene (POM), Nylon 6, Nylon 66, and polytetrafluoroethylene (PTFE). Each component shows its own characteristic melting endothermic peak, at 108,127,165,174,220,257, and 340°C, respectively. Polytetrafluoroethylene also has a low-temperature crystalline transition at about 20°C. The unique ability of DTA to identify this polymer mixture is exceeded by the fact that only 8 mg of sample was employed in the determination. 

EFFECT OF LOW PRESSURES ON THE ROOM TEMPERATURE TRANSITIONS OF POLYTETRAFLUOROETHYLENE. 

This technique has found the following applications in addition to those discussed in Sections 10.1 (resin cure studies on phenol urethane compositions) [65], 12.2 (photopolymer studies [66-68]), and 13.3 (phase transitions in PE) [66], Chapter 15 (viscoelastic and rheological properties), and Section 16.4 (heat deflection temperatures) epoxy resin-amine system [67], cured acrylate-terminated unsaturated copolymers [68], PE and PP foam [69], ethylene-propylene-diene terpolymers [70], natural rubbers [71, 72], polyester-based clear coat resins [73], polyvinyl esters and unsaturated polyester resins [74], polyimide-clay nanocomposites [75], polyether sulfone-styrene-acrylonitrile, PS-polymethyl methacrylate (PMMA) blends and PS-polytetrafluoroethylene PMMA copolymers [76], cyanate ester resin-carbon fibre composites [77], polycyanate epoxy resins [78], and styrenic copolymers [79]. 

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