Phase transitions in molecular crystal 4,4 '-bis(6-hydroxy-1-hexyloxy)biphenyl studied by molecular dynamics simulations and IR spectroscopy

Molecular dynamics (MD) simulations in an NPT ensemble combined with temper ature dependent IR spectroscopic measurements were used to study the phase transitions in a molecular crystal of a mesogenic diol -4,4'-bis(6-hydroxy- 1-hexyloxy)biphenyl (D). A molecule of D consists of two distinct parts: a stiff biphenyl group and two flexible ICH:),OH chains. The potential energy was calculated using the peff_300 force field in Cerius(2) modelling envir onment. The results of MD calculations revealed the changes of molecular ac id crystal structure at two phase transitions temperatures, T-1 similar to 365 K (crystal-crystal) and T-2 similar to 445 K (crystal-isotropic state); these temperature estimated by differential scanning calorimetry measureme nts correspond to those observed by IR spectroscopy. MD calculations showed that the structural changes occurring during the first and second phase tr ansitions could be attributed to changes in the conformation of the flexibl e part of D (connected with T-1 transition) and the stiff part of D (connec ted with T-2 transition. The distortions in the flexible parts of D molecul es above T-1-transition result in the break up of a regular network of hydr ogen bonds between diols and consequently a disturbance in the layer orderi ng of molecules The distortions in the stiff biphenyl group above T-2-trans ition indicate the premelting stage with a large departure from the three-d imensional molecular ordering. (C) 2001 Elsevier Science B.V. All rights re served.