PET/0.6 PHB melt, where PET is poly(ethylene terephthalate), PHB is p-hydroxybenzoic acid and 0.6 is the mole fraction of the second (liquid crystal, LC) component in the copolymer, was subjected to constant magnetic fields at several temperatures in the range 240–315°C, cooled to 25°C, and then studied by x-ray diffractometry, electron diffractometry, scanning electron microscopy (SEM), dilatometry (TMA) and thermogravimetry (TGA). Given the anisotropy of the magnetic susceptibility, significant orientation effects are found. The extent of orientation was evaluated as a function of the magnetic field strength, time of the field imposition and temperature; cyclic fields were also applied. Large changes in the extent of material orientation are connected to the phase transitions determined before [9]. The LC-rich islands [4] cause channeling of flexible sequences between the LC sequences, as predicted from an extension [20] of the statistical–mechanical theory of Flory [21]. In the phase region of coexistence of the smectic E, smectic B and isotropic phases there is a growth of the size of the islands—a consequence of increased mobility. However, in the next higher temperature region, where the smectic E phase is no longer present, one observes gradual dissolution of the islands along with the temperature increase. These conclusions are derived from both x-ray diffractometry and SEM. The phase transition temperatures from TMA agree with those found by other techniques. Linear expansivity is negative along the orientation direction, but it becomes ⬇ 0 after heating to 150°C. Electron diffractometry shows that the islands are elongated. TGA results show that the thermal degradation of the material in air plays a role only above 360°C. In general, the properties seem to depend on the extent of orientation but not on the nature of the field such as shear or magnetic.
Посилання на статтю:
Orientation of a longitudinal polymer liquid crystal in a constant magnetic field / Witold Brostow, Elena A. Faitelson, Mihail G. Kamensky, Vadim P. Korkhov, Yuriy P. Rodin // Polymer. – 1999. – N 40. – P. 1441–1449.
Orientation of a longitudinal polymer liquid crystal in a constant magnetic field - Завантажити.
