Chemistry Reference
In-Depth Information
Figure 4.22. DSC thermograms of a semi-crystalline polyester,
4
.
1
, that has a
nematic phase above melting: (A) is the heating curve, (B) is the cooling curve.
O
C
O
C
O
C
O
C
O
CH
2
O
O
O
4
.
1
10
NO
2
The nature of the two endothermal processes has been revealed by study
with POM: that at 434
◦
K is the melting of the crystals and the formation
of a nematic phase, while at 467
◦
K is the isotropization of the mesophase.
If the polymer is cooled down from its isotropic liquid state the curve (B) is
obtained. In the cooling process the first exothermal peak occurs at 463
◦
K
which is the formation of the nematic phase as revealed by POM. The
second exothermal peak is centered at 387
◦
K corresponding to the crystal-
lization of nematic polymer. The jump in (B) of the glass transition is not
as clear as in the heating curve. This is understandable because the glass
transition is not a genuine thermodynamic transition.
One sees in the above example that the supercooling of the isotropic liq-
uid phase is significant (467
463 = 4 degrees), but it is much smaller than
that for the nematic phase (434
−
387 = 47 degrees). Significant degrees of
supercoolings are often observed for polymers but that for low mass systems
are often much smaller. There have been suggestions that the transition is
liquid-crystallization if the supercooling is no larger than 0.6 degrees, other-
wise it is crystallization. The suggestion does not apply in polymer systems
because much higher viscosities and much slower molecular motions are
involved in polymers than in low mass liquid crystals. In addition, the tran-
sition peaks of polymers are usually much broader than that of low mass
−
