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Synchrotron x-ray diffraction from poly(3-octylthiophene) and revealed that the structures of these two polymers remain different under hydrostatic pressure despite their similar chain configuration. The a-axis of poly(octyl-thiophene) is considerably more compressible than its b-axis, and poly(octyl-thiophene) is more compressible along both these directions than poly(dioctyl-bithiophene). The differences in compressibility corroborate with the planar chain model of poly(octyl-thiophene) and the chain twist model of poly(dioctyl-bithiophene), but with a reduction in the chain twist angle under pressure for the latter. The spectral shift caused by variation of temperature (thermochromism) is compared with the spectral shift caused by pressure. From the shifts of the band edge and the behaviour of the overall peak shape it is argued that these two effects are associated with different structural aspects: whereas thermochromism is related to a two phase situation, the shift caused by pressure is related to an increased interchain wave function overlap.

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