CALORIMETRIC AND INFRA-RED STUDY OF THE PHASE SITUATION IN SOLID MBBA

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Abstract
If MBBA is solidified by decreasing the temperature of the nematic phase, one obtains a metastable solid, whose specific heat vs. temperature curve has been measured by adiabatic calorimetry. This curve shows a sharp rise at melting. It also shows a quasi-second-order type of behaviour at about 210 K. If the metastable form of solid MBBA is kept for a few hours at a temperature not far below melting, it spontaneously transforms into the stable form, whose specific heat vs. temperature curve has also been measured. This curve lies below that of the metastable form and has a monotonic behaviour and a sharp rise at melting, which now occurs at a slightly higher temperature. Infrared and far-infrared spectra of the two modifications of solid MBBA were also measured. In the internal vibration wave number region the spectra are significantly different in the whole temperature range. The quasi-second-order phase transition discovered in the metastable MBBA by calorimetry does not show up in IR. Differences between the spectra of the two modifications are greatest in the lattice vibration region, i. e. between 20 cm-1 and 100 cm-1. Preliminary powder X-ray diffraction data, obtained for the two modifications, show patterns typical of crystalline materials. The X-ray pattern changes significantly when the metastable MBBA transforms into the stable modification

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