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Abstract
The second order phase transition at 195 K which occurs in bis (p-toluene sulfonate) diacetylene polymer crystals has been studied by optical reflection and transmission, Raman and far-infrared spectroscopy. Order parameter fitting to the data suggests that at low temperatures the transition has a two dimensional character while closer to the transition it appears to be three dimensional. There are indications of a broad transition region which might be expected for a predominately two dimensional phase transition in which fluctuations dominate the behaviour of the system. Analysis of new X-ray crystallographic data gives some support to a structural model in which the transition would have a substantial two dimensional character.