Epitaxial crystallization of polyphenylene sulfide on sodium chloride
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I. INTRODUCTION Epitaxial crystallization can be defined, most generally, as the oriented overgrowth of a crystalline phase upon the surface of another crystalline substrate. The orientation is highly specific and can affect both structure and morphology of the crystallizing substance.1 Examples of epitaxy of low molecular weight substances, especially transition metals, are well documented, and applications have manifested themselves in the semiconductor industry.23 Polymer crystallization by epitaxial interaction was originally reported for the crystallization of linear polyethylene on sodium chloride from a dilute xylene solution.4'5 Anisotropic rodlike crystals were observed which were distinctly oriented in the substrate (110) directions, following rows of like charge. Electron diffraction showed that the polymer chains were oriented perpendicular to the long axis of the rodlike crystals, and parallel to the substrate surface. Those rodlike crystallites were identified as lamellae that have their basal planes perpendicular to the NaCl (001) plane, and the rodlike appearance is the result of looking "edge on" at the thinnest dimension. Since that initial observation, many different linear polymers have been epitaxially crystallized onto alkali halides.6"12 In virtually every case, the rodlike morphology remains the same. Little effort has been made in the past to study the effect of various epitaxial parameters on the growth kinetics, crystalline structure, and morphology of possible epitaxial deposits. One of our objectives is to investigate the effects of solution concentration, crystallization temperature, and time on the crystal structure, orientation, and morphology of PPS. Polyphenylene sulfide has gained considerable importance as a specialty polymer. It has excellent chemical resistance coupled with good thermal, mechanical, and insulating properties at high temperatures.1314 Perhaps, most 996
http://journals.cambridge.org
J. Mater. Res., Vol. 4, No. 4, Jul/Aug 1989
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importantly, PPS has drawn considerable attention from physicists and electrical engineers because it can be made conducting through doping with charge transfer agents such as arsenic pentafluoride.1517 Excellent conductivity was obtained with a mixture of arsenic trifluoride and arsenic pentafluoride.18 PPS has the advantage that it can be melt or solution fabricated to the desired form before the doping operation while most other conducting polymers are intractable. The degree of crystallinity seems to be a critical parameter in the doping of PPS.16 It is therefore the second objective of this work to produce a highly oriented crystalline thin PPS film which may promote an enhancement of PPS conductivity. Epitaxy is a unique technique which enables us to produce thin films of polymers with high orientation and crystallinity.
II. EXPERIMENTAL Two commercial polyphenylene sulfides (Ryton V-l and GR01) were provided by Phillips Petroleum Co. V-l PPS has a weight-average molecular weight of about 25 000, and GR01 P
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