Mesophase annealing behavior

Focusing attention on PTEB, it has been found that, similar to the case of PDTMB, the mesophase experiences a very slow transformation into the crystal. Thus, only the isotropization is observed in a sample freshly cooled from the melt [27]. However, after a long time at room temperature, the transformation mesophase-crystal is produced, owing to a glass transition temperature of about 14°C. Moreover, several endotherms were obtained before the final isotropization for a sample of PTEB annealed at 85°C for 12 days, i.e., PTEB shows enantiotropic behavior. The different endotherms may arise from polymorphism or melting-recrystallization phenomena [30]. 

In dynamic x-ray diffraction studies reported earlier (17). perpendicular equatorial and meridional arcs with the same d-spacing were observed in drawn fibers. Both pairs of arcs showed different transition temperatures. DSC of annealed samples showed a small endo therm at 120°C which occurred at the same temperature observed for the transition in the BP6L meridional diffraction arc. The endothermic transition of the annealed THF insoluble fraction corresponds to the 160°C transition of the BP6L equatorial diffraction arc. Both fractions exhibit mesophase behavior above the observed thermal transitions, and only a subtle textural change is evident at that temperature under crossed polars, indicating that these thermal transitions are due to trace amounts of crystallization. The BP6Li fraction displays characteristics of the smectic mesophase while the texture and x-ray observations of BP6Ls do not allow conclusive identification of the mesophase (presumably nematic). 

One example of such metastable enantiotropic behavior is provided by a side chain polysiloxane with the following transitions recorded on heating from the glassy state G 5 S 54 N 1121 [5]. However, a crystal which melts above 54" C (the S-N transition) develops upon annealing the mesophase above Tg, and displaces the smectic phase which becomes monotropic. 

In this section, copolymers of polyethylene are discussed, starting with low-density, branched polyethylene and culminating with crystallization, melting, and annealing of poly(ethylene-co-octene-l), also described as linear-low-density polyethylene, LLDPE. Furthermore, partial phase diagrams of poly(ethylene terephthalate-co-oxybenzoate), PETcoOB, andpoly(oxybenzoate-co-oxynaphthoate), POBcoON, are presented. The latter systems are examples of increasing chain stiffness by cocrystalUzation which leads to mesophase behavior (see Sect. 5.5). 

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... 

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