Polymer CommunicationThe conformation of crystalline poly(phenylene sulphide)
Introduction
Polyphenylene sulphide, PPS,has attracted considerable interest as an engineering polymer because of its high modulus, tensile strength and good dimensional stability. Its high deflection temperature of about 500 K, flame retardancy and resistance to organic fluids determine many of its applications. These properties make it particularly useful in the electronics and automotive industries. It can be quenched from above the melting point to an amorphous sheet or crystallised to a high degree of crystallinity.
Central to a study of the morphology of PPS is the establishment of the degree of crystallinity and the unit cell dimensions. Unit cell dimensions were first reported by Tabor et al. [1] from which the crystalline density was calculated to be 1.43 g cm−3 with four chain segments per unit cell. The amorphous density was measured as 1.32 g cm−3. The dimensions of the unit cell were consistent with
- 1.
a planar zig-zag structure for the chain;
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the phenylene rings inclined at 45° to the plane of the zig-zag;
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a S–S separation along the chain of 0.627 nm;
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a C–S–C bond angle of 110°.
Garbarczyk [2] has questioned this structure on the basis that it could not account for the high electrical conductivity of PPS for which a planar alignment of the rings was preferred [3] and lower torsional angle would allow conjugation of the phenylene π with the sulphur p-electrons. Using a series of oligomers as models, Garbarcyk suggested, from an NMR spectroscopic study of dimers and trimers, that the C–S–C bond angle was in the range 103–107°. Reducing the bond angle allowed the angle of rotation of the phenylene ring to be reduced with alternate pairs being nearly coplanar with the plane of the zig-zag while the other rings were inclined at 60°. This was consistent with structure of PPS oligomers. He also suggested that Tabor et al.'s fibre samples had been distorted by heat treatment or by stretching of the sample that opened the C–S–C bond to 110°. Lovinger et al. [3], [4] have subsequently confirmed the crystallographic structure of PPS outlined by Tabor et al. on fibre samples.
There is some discrepancy in the precise conformation of the PPS chains in the crystalline phase and we have used X-ray diffraction studies on crystalline samples and molecular modelling to resolve this. Un-stretched stress-relieved bulk specimens were used in this study to eliminate any possibility of chain distortion.
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Experimental
PPS was obtained from Phillips Petroleum as Ryton, grade GR04, in the form of beige coloured pellets. The pellets were dried for 4 h at 150°C prior to moulding. Plaques, of different thickness, 0.1–3.00 mm, were made by compression moulding of the dried pellets between aluminium plates for 7 min at 220°C. Amorphous material was produced by quenching plaques, less than 1 mm thick, directly into ice/water. Crystalline material was produced either by slow cooling in the press or annealing
Crystallographic structure
X-ray diffraction patterns were measured for amorphous and semi-crystalline PPS samples as scattered intensities as a function of Bragg angle, 2θ, see Fig. 1. The scattered intensities from an amorphous sample was scaled for the amorphous content and subtracted from that of the crystalline sample to leave the pattern for the crystalline PPS. The program profit was used to determine the positions, width and intensities of all peaks in the patterns. The cell dimensions were determined by using
Conclusions
The unit cell dimensions of PPS have been confirmed by X-ray diffraction as and confirming the original work of Tabor et al. The cell symmetry was Pbcn with four molecules per unit cell. No change in the dimensions was observed on annealing or with increasing crystallisation temperature ruling out the possibility that these values could be attributed to a distorted structure from internal strain or heat distortion on rapid cooling. Computer calculated WAX
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