Solid Crystalline Polymorphism in M-21

Abstract

This paper reports on the different solid crystalline (SC) forms of 4-cyano-4'-heptyloxy biphenyl (M-21) as revealed through differential scanning calorimetric, polarizing microscopic and infrared spectroscopic investigations. There are three solid crystalline modifications, namely SC_I,SC_II,SC_III, with the thermal phase diagram.

The appearance and preponderance of the SC forms depend critically on the manner in which the liquid crystalline melt solidifies. Each SC phase has its CN stretching band split into two components because of Davydov or correlation splitting. The position, separation and relative intensities of the two components characterize each SC phase. The systematic study of the variations in these three features with temperature leads to interesting information about the intermolecular ordering forces and the spatial arrangement of the molecules in the unit cell. It is shown that the intermolecular interactions of the dipolar nature play a dominant role in the SC phases but contribute negligibly in stabilizing the nematic phase. The angle between the correlated molecules in the unit cell changes in the order SC_I > SC_II > SC_III > nematic = 0, resulting in parallel arrangement of molecules in the nematic phase.