Morphological changes of poly(tetrafluoroethylene) by heat treatment on NaCl
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Masaki Tsuji Laboratory of Polymer Condensed States, Institute for Chemical Research, Kyoto University, Uji, Kyoto-fu 611, Japan (Received 21 April 1993; accepted 5 June 1993)
Fine granules of poly(tetrafluoroethylene) (PTFE) were heat-treated/annealed on NaCl near its melting temperature (Tm) and/or at a temperature (Tc) between upper and lower feet of the exothermic peak in the DSC cooling process from Tm. Morphological changes of the granules were examined in the bright- and dark-field modes by transmission electron microscopy. When the granules were heat-treated near Tm, microfibrils of 20-30 nm in width and fibrils of 70-120 nm in width came out of the granules. The microfibrils were also observed in the fibrils. The microfibrils formed by heat treatment near Tm seemed to be identified as microfibrils of 20-30 nm in width which were recognized outside the granules annealed at Tc. It is expected that such a microfibril will grow to be a band in the band structure observed on the surface of bulk PTFE. Since the 0015 dark-field images showed that the PTFE chains in such microfibrils and fibrils are set perpendicular to their fibril axis, the chains should fold back and forth repeatedly at both lateral side-surfaces of the microfibrils and fibrils.
I. INTRODUCTION There have been some experiments reported so far that examined the epitaxial behavior of organic or inorganic compounds on poly(tetrafluoroethylene) (PTFE) as a substrate.1'2 On the other hand, however, the epitaxial behavior of PTFE itself, which is heat-treated on a certain substrate, has scarcely been investigated. When PTFE 3 and one of its family, (tetrafluoroethylene)-(perfluoroalkyl vinyl ether) copolymer (PFA),4 were heat-treated for a few minutes above their melting temperatures on a carbon support-film for transmission electron microscopy (TEM), fibrillar or rod-like entities came out of the individual fine particles of the polymers. This phenomenon can be attributed to their low surface energy and cohesive energy. Nevertheless, it is of interest that such a phenomenon has never been observed on poly(vinylidenfluoride) (PVDF), which is also one of the PTFE family. The fibrils and rods in question, for example, of PTFE are placed randomly in their orientation on a carbon supportfilm, and accordingly it is not easy to investigate their fine structure, such as chain-axis orientation in them. Alkali halides, e.g., NaCl, KC1, KBr, and KI, have been generally and extensively utilized as substrates for epitaxial crystallization of organic and inorganic materials. These alkali halides have also been used as substrates to control the orientation of crystals of synthetic polymers by epitaxial crystallization from solution5 or from 2942
http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 11, Nov 1993
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vapor phase.6'7 In the previous paper,3 we reported the morphological changes of emulsion-grade granules of PTFE heat-treated near/above its melting temperature (340 ± 1 °C for virgin PTFE) on NaCl and KI as a substrate; a
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