Particularly in LC polymers, rubbery state to smectic, smc to nematic and LC to isotropic are always gives as peak type transitions in DSC (means that some enthalpy change takes place) accordingly we can say those are 1st order transitions.
Actually i taught that 1st order transitions enthalpy is measurable.... but some times i am not able observe these type transitions in DSC even though the transitions observed by optical polarizing microscopy. Its may be low enthalpy change...which depends on sensitivity of the instrument.
accordingly i cleared that even 1st order transitions also we cant observe which depends on material and its enthalpy change value......please suggest that is my assumtion is correct or not?
True LC phase transitions should be first-order. Many have small enthalpy changes, but for small molecules they should be visible on a DSC trace under normal conditions. For some polymers, you can run into the same issues you find with crystallinity. Re-ordering may be incomplete, May be slow, or may require some activation energy. Particularly with LC phases, the temperature window may be quite narrow, which means the heating or cooking rate may need to be slow, which means the signal to noise ratio may be too low. With microscopy you can change the temperature very slowly and avoid these issues. If you want confirmation, try light scattering or some other scattering method if DSC doesn't work. I wouldn't casually dismiss DSC results, however. You should try a variety of heating rates, bigger sample sizes, etc. before you just assume something is there that can't be detected, abd if you must make that assumption, be sure you can explain why and have good supporting evidence from other methods.
Liquid crystal transitions are first order in all cases. Liquid crystal transitions do not supercool into the liquid crystal phase below them in temperature, i.e. freezing and melting points must be identical in a well poised instrument. Many liquid crystal phases supercool into the solid phase. The smectic plase is notorious for forming a "glass" when supercooled into the true solid phase. Heating the glass produces some interesting thermograms. However, the smectic to nematic or smectic to liquid (if there is no nematic) is always sharp and 100% reversable. Also, the solid phase formed upon cooling the liquid crystal phase can be quite different from the solid phase formed from solvent crystalization. In many cases this has been confirmed by X-ray diffraction. Thus, do not expect the heat of melting to be the same on first and second heating for many larger molecules and polymers (if the solid at first heating was from a solvent recrystallization process). Also, beware of trapped solvent! Sloppy preps have produced many "unusual" solid phases.
For liquid crystalline materials there are many mesophases between the crystalline solid and the isotropic liquid state. Liquid crystals also exhibit a rich variety of phase transitions. These phase transitions can either be first-order or second-order where critical fluctuations may play an important role. In order to determine the order of the different kinds of transitions and to characterize fully the nature of the fluctuations at critical and multicritical points, different types of high-resolution experimental techniques are necessary.
Copied from: J. Thoen, Thermal investigations of phase transitions in thermotropic liquid crystals, International Journal of Modern Physics B 9 (1995) 2157-2218.
Nothing more to add.
Liquid crystal polymers appear to be a special case (polymers are like that). The lower molecular weight thermotropic liquid crystals are much better behaved. All transitions between liquid crystal phases are first order and self nucleating. Mixtures (which are the basis of modern LC displays) follow well known rules.
In general, if a transition involves a discontinuous change of structure at the point of transition, it is a first order phase transition.
A glass transition, should not involve any change of structure, only change of dynamics, no matter if the glass is formed from an isotropic or a LC phase. The glass should keep the structure of the liquid (or liquid crystal) from which it is formed, therefore it is not a first order phase transition. In DSC practice although one can observe peaks at glass transitions, they tend to disappear in the 2nd scan.
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