Isothermal Crystallization of Poly(tetrafluoroethylene). Effect of Holding Time in the Melt on Calorimetric Measurements

Abstract

KEY WORDSPoly(tetrafluoroethylene) / Isothermal Crystallization / Calorimetry / Holding Time in Melt / Residual Order in Melt / Crystalline poly(tetrafluoroethylene) (PTFE) exhibits three solid phases at atmospheric pressure with first order solid-solid transitions occurring at 292 and 303 K. Depending on the experimental method and extrapolation used the equilibrium melting point lies between 600 and 620 K. 1 • 2 As-polymerized (virgin) and meltcrystallized PTFE are known to be highly crystalline; fully amorphous polymer has not been observed. The microstructure of melt-crystallized bulk PTFE is unusual in that it consists predominantly of thick lamellae. Fibrillar and striated band-like morphologies have been observed under certain crystallization conditions for polymer of suitable molecular weight 3 • 4 and incipient spherulitic structures with a large degree of imperfection were reported to occur at very slow cooling rates of the order of degrees Kelvin per hour. 3 In a previous calorimetric study it was concluded from non-isothermal data obtained at constant cooling rate that PTFE crystallizes one-dimensionally from pre-existing nuclei which were presumed to be impurities. 5 Neutron and X-ray diffraction, Raman spectroscopic, computational, and several other studies on molten PTFE all suggest that conformational, orientational, and local positional order persists after the polymer has been heated above the equilibrium melting point. 6 -12 The presumption that PTFE can retain some kind of order in the melt is in line with the outcome of a number of studies on the effect of melt holding time on the isothermal crystallization behaviour of polymers such as polyethylene, 13 poly(ethylene oxide), 13 poly(ethylene terephthalate), 14 -16 poly-( ethere ether ketone), 1 7 · 18 poly(phenylene sulfide), 19 • 20 and polycarbonate. 21 Many of the phenomena observed in these studies have been connected to self-nucleation by, for instance, Wunderlich in reference 22 which also provides quotations of similar studies on polymers not quoted above. The mechanism of self-nucleation, thereby microscopic remnants of polymer crystallites remain unmelted and can serve as nucleation sites on cooling below the melting temperature can be presumed to play a role also in the melt-crystallization of PTFE. EXPERIMENT ALAs-polymerized commercial grade polymers were supplied in powder form by ICI, UK (Fluon G 163) 1 To whom correspondence should be addressed. 23 between the number-average molecular weight and the heat of crystallization about 7 mg of polymer has been heated with a Perkin Elmer DSC-7 at a rate of 20 K min -1 from room temperature to 650 K. The sample has then been held at this temperature for 5 minutes before cooling with 20 K min -l back to room temperature. Heats of crystallization were obtained from the measured DSC exotherms and the molecular weight was estimated to be 44 x 10 5 gmol-1 , 22 x 10 5 gmol-1 , and 2 x 10 5 gmol-1 for G 163, TFl 750, and KU02, respectively. The peak melting temperatures Tm were obtain...

Manipulation of Athermal Nuclei in Aqueous Poly(ethylene oxide) by Scanning Activity Gravimetric Analysis

“…Memory effects in semicrystalline polymers have been documented for a variety of systems in which the rate of crystallization depends on the temperature and duration of a prior melting step. − This phenomenon is usually attributed to small remnants of the crystalline phase that persist for long times above the apparent melting temperature and then serve as athermal nuclei upon cooling. Although many believe that remnants survive exclusively because of sluggish kinetics, − , thermodynamic considerations may also play an important role. − …”

“…[5][6][7][8][9][10][11][12][13] This phenomenon is usually attributed to small remnants of the crystalline phase that persist for long times above the apparent melting temperature and then serve as athermal nuclei 14 upon cooling. Although many believe that remnants survive exclusively because of sluggish kinetics, [9][10][11][12][13]15 thermodynamic considerations may also play an important role. [16][17][18] We previously introduced scanning activity gravimetric analysis (SAGA) as a novel technique for investigating polymer crystallization from concentrated polymer solution under isothermal conditions.…”