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Density studies in polymorphic liquid crystal, N-(p-n-heptyloxybenzylidene) p-n-pentylaniline is carried out in isotropic liquid, nematic, smectic A, smectic C, srnectic B and sinectic G phases. The temperature variation of density in all the mesophases confirms that isotropic liquid to nematic, nematic to sniectic A, smectic C to smectic B, smectic B to sniectic G transitions are of first order while smectic A to smectic C transition is of second order. The density variation across smectic C to sniectic B is greater than that of any other transition. Thermal expansion coefficients are also calculated.

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Phase transitions of the liquid crystal N‐(pn heptyloxybenzylidene)‐pn‐butylaniline (HpBBA) are investigated by specific‐volume and ultrasonic‐velocity measurements. A variation of the specific volume with temperature in all the mesophases confirms that the isotropic–nematic, nematic–smectic A, and smectic C–smectic G phase transitions are of first order, whereas the smectic A–smectic C one is of second order. All the phase transitions are confirmed by the variation of the ultrasonic velocity with temperature. The adiabatic compressibility (βad) and molar sound velocity (Rn) (Rao number) are deduced experimentally.

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