EVIDENCE OF A LATTICE INSTABILITY DURING THE THERMALLY ENHANCED REACTION OF MSE STUDIED BY CALORIMETRY, C-13-NMR, RAMAN-SCATTERING AND BIREFRINGENCE

A methyl p-dimethylaminobenzenesulfonate (MSE) polycrystalline powder or single crystal is converted to the zwitterionic product p-trimethyl ammonium benzenesulfonate (ZWT) by a chemical thermally induced reacti on. This reaction is associated to a first-order ferroelastic phase tr ansition in partially reacted MSE crystals. A C-13-NMR study made duri ng the in situ transformation MSE --> ZWT shows that the reaction is o f first order. A strong change appears in the low-frequency Raman spec trum of MSE monocrystals when passing the transition, but only a conti nuous evolution in the intensity of the high-frequency modes is observ ed. Birefringence observations permit the evolution of the optical axe s during the chemical reaction to be specified. The importance of stre ss induced by the reaction is discussed in relation to the ferroelasti c phase transition. (C) 1997 Elsevier Science B.V.