Mechanical study of the ferroelastic smectic A to smectic C phase transition

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Abstract
We measure the instantaneous stress/strain relationship associated with layer compression in the smectic liquid cristal butyloxy phenylnonyl oxybenzoate (40P 90B) across the smectic A → C phase transition. The temperature dependence of the critical stress and strain required to induce mechanically one phase from the other, allows the Landau parameters of the transition to be determined. The critical compressional stress of molecular buckling in phase A is unexpectedly 30 % larger than the critical dilative stress in the C phase which realigns the molecules normal to the layers. This could be accounted for by a departure from the mean field, or by a non symmetric influence of defects