Mesophase cold crystallization

The thermal behavior of liquid crystalline main chain polymers is more complicated. Depending on thermal history, different transitions may occur. The DSC curves of samples quenched from the melt may show the glass transition (positive ACp), cold crystallization (exotherm), melting (endotherm)and the mesophase-isotropic liquid transition (endotherm)[ Fig. I6]. ... 

Further analysis of the two samples of Fig. 6.62 is done by TMDSC, as seen in the left graphs of Fig. 6.64. A comparison of the two reversing heat capacities shows that the cold crystallization and the transition mesophase-to-a-monoclinic crystals do not show, i.e., they are nonreversing. There remains, however, a substantial reversing contribution which is larger for the quenched iPP than for the lamellar, melt-cooled iPP. The upper limit of the devitrification of the RAF seems to occur at 320-330 K,... 

The effects of molecular orientation on the crystallization and polymorphic behavior of SPS and SPS/poly(2,6-dimethyl-l,4-phenylene oxide) (PPO) blends were studied with wide-angle X-ray diffraction (WAXD) and differential scanning calorimetry [37]. The oriented amorphous films of SPS and SPS/ PPO blends were crystallized under constraint at crystallization temperatures ranging from 140 to 240 °C. The degree of crystallinity was lower in the cold-crystallized oriented film than in the cold-crystallized isotropic film. It was inferred that the oriented mesophase was obtained in drawn films of SPS and that the crystallization of SPS was suppressed in that phase. The WAXD measurements showed that the crystal phase was more ordered in SPS/PPO blend than in pure SPS under the same annealing conditions. It was principally due to the decrease in the mesophase content. The crystal forms were found to be dependent on the crystallization temperature, blend composition, and... 

Dikshit, A. K., Kaito, A. Effects of the oriented mesophase on the cold crystallization of syndiotactic polystyrene and its blend with poly(2,6-dimethyl-l,4-phenylene oxide). /. Polym. Sci Part B Polym. Phys., 41(14), 1665-1675 (2003). 

Finally we mention that draw-induced mesophases have also been observed in some other semi-rigid chain polymers. Examples of such studies are cold drawing of glassy PEN [108,109] and oriented crystallization of PET/PEN copolymers [110, 111]. In all cases a smectic mesophase has been reported. In the absence of crystallization it can attain a degree of stability that resists decay from chain relaxation. This supports the view that the mesophase represents a thermodynamic state [101]. We expect that similar mesophases based on semi-rigid monomers can be observed in other polymers with intrinsic rigid building blocks, such as other aromatic polyesters and polyamides. 

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