Characterization of Liquid Crystalline Polymers - Liquid Crystalline Polymers - page 224

The polymers in the 4 . 2 series with different R groups all are non-
crystalline but each form a nematic phase when heated to above Tg. The
nematic phase is thermally very stable so that no endothermic isotropization
transition is shown in the curve. Instead the signals for chemical decompo-
sition will occur if the heating is continued to 300 ◦ C or above depending
on R. In such cases the characterization of the mesophase relies more on
POM and X-ray scattering.
More recently in the study of another series of mesogen-jacketed liq-
uid crystalline polymers we found (Zhang et al ., 1996) that the frozen-in
nematic order (formed from evaporation of the solvent or when precipi-
tated from solution) in the non-crystalline polymers will disappear when
they are heated up to the temperature region of the glass transition. The
DSC curves of these polymers shows again only the glass transition, how-
ever instead of a jump a broad endothermic peak appears. Obviously in such
cases the glass transition and the transition of the nematic-to-amorphous
are superimposed.
(2) The glass transition may appear as an endothermic peak rather than
a jump in the curve. It is thus necessary to distinguish the glass transition
from the melting and other endothermic phase transitions. The endothermic
peak at glass transition is believed to be a result of annealing or storage
of the sample at temperatures below Tg (Volkenstein and Sharonov, 1961;
Qian et al ., 1989).
Cheng et al . (1990) carried out isothermal annealing experiments below
Tg of a series of the main-chain type liquid crystalline poly(azomethine)s.
The samples had been quenched rapidly from their nematic or isotropic
states before the annealing experiment. A jump was observed in the DSC
heating curves of the samples without annealing. However, they were able
to observe an endothermic peak of glass transition after only a short time
of annealing. For example, with the polymer having the Tg of 318 ◦ K,
the annealing at 295 ◦ K for 0.75 h had led to a small endothermic peak.
With increasing annealing time, the peak temperature shifts to a higher
location and the enthalpy of the peak increases. According to the authors
the enthalpy could reach a value of 0.891 kJ mol − 1 after an infinite time
of annealing. The results are well interpreted by the enthalpy relaxation
theory proposed by Kovacs et al . (1979).
(3) Like other crystalline polymers, liquid crystalline polymers can also
exhibit multiple melting peaks when studied with DSC. Multi-melting is
common to crystalline polymers including single crystals of polyethylene.
For example, Bair et al . (1967) found the single crystals of PE grown