Epitaxial crystallization of polyphenylene sulfide: Lattice effects on structures
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The investigation of dilute solution epitaxy of polyphenylene sulfide (PPS) has been carried out on a series of inorganic substrates, which cover a wide range of interatomic spacings, surface energies, and geometries. A variety of morphological arrangements of PPS on various substrates were observed. Both (100) and (110) epitaxial orientations were observed on monovalent alkali halides. While the (100) epitaxy is generally thought to grow upon nucleation on surface steps along the (100) direction, (110) epitaxy may also be nucleated on these surface steps. Changes in ionic lattice dimensions and ionic nature of the substrate have a substantial effect on the molecular packing of PPS and the resulting crystal orientations. Four new crystalline phases of PPS have resulted from the fold surface epitaxies on NaCl, KC1, KBr, and mica with favorable one- or two-dimensional lattice matching. This is a direct indication of the important role of lattice matching in inducing and defining the epitaxies and polymorphism of PPS.
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INTRODUCTION Epitaxial crystallization of polyethylene on (001) NaCl surfaces was first observed with optical microscopy by Willems1'2 and with electron microscopy by Fischer.3'4 From electron diffraction measurements Fischer concluded that the chain axis of polyethylene aligns parallel with the substrate surface; the preferential chain orientation of PE was in the (110) directions of the halide crystal. It was proposed that a lattice matching of chain repeat distance with the crystal spacing of the alkali halide was responsible for the observed morphology. The amount of disregistry between atomic or molecular spacings is generally measured by the quantity (C-Co)/Co, expressed as a percent, where C and Co are the lattice periodicities in the direction of interest of absorbed and substrate phase, respectively.5 Generally, epitaxial growth was found to be limited by an upper bound of about 15% disregistry. The existence of a two- or one-dimensional structure analogy of lattices seems valid for some of the types of epitaxy observed early. An example of this interpretation in the usual manner of epitaxy for an organic substance was the growth of anthracene on an inorganic substrate substance, such as NaCl. This example was well modeled on the basis of a two-dimensional structure analogy by Willems.6 However, some controversy does remain as to the requirement of a lattice match criterion in explaining the observed epitaxial growth. The role of matching crystal periodicities can most easily be assessed by crystallizing a given substance on a homologeous series of substrates with varying lattice spacings, or, conversely, by crystallizing related substances on the same substrate. Systematic investigations relevant to this issue have been undertaken for polymers deposited mainly on ionic substrates. Baer and J. Mater. Res., Vol. 4, No. 4, Jul/Aug 1989
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co-workers7 9 have indeed observed that polyethylene can grow epitaxially on a number of ionic substra
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