Mechanical properties and characterization of the crystalline structure in annealed thermotropic poly(azomethines)

Fibers of a poly(azomethine) having hydroxyl groups within the mesogenic core have been melt extruded from the nematic phase. The polymer obtained by condensation of 1,10-bis[(4-formyl-3-hydroxyphenyl)oxy]decane and 2-methyl-1,4-phenylenediamine has a T-g of 54 degrees C and exhibits a nematic phase from 178 to 341 degrees C. Due to an observed increase in the melt viscosity upon heating above 200 degrees C, the processing temperature (190 degrees C) has been closely controlled above T-m. Fibers have been subjected to thermal annealing at 100 degrees C, i.e., above T-g and below T-m, either in a relaxed state or under an external force along the longitudinal fiber axis. Fibers have been studied by simultaneous TG-DTA, DSC, and X-ray scattering, and the mechanical properties have been measured. As-spun fibers show good thermal stability and good orientation along the fiber axis; the values of the elastic modulus and tensile strength are 8.9 GPa and 252 MPa, respectively, and the elongation at break is 3.3%. The mechanical properties improve after annealing in a relaxed state. After 90 min at 100 degrees C, the values of the elastic modulus and tensile strength increase to 11 GPa and 283 MPa, respectively, and the elongation at break is 2.5%. Longer heat treatments do not seem to significantly affect the properties; however, a gradual but remarkable increase in the degree of crystallinity is promoted. A highly crystalline phase is achieved after annealing on a stressed state. It consists of a primitive triclinic lattice where the chains are packed side-by-side with the alkyl chains in an all-trans conformation. After 12 h at 100 degrees C, an increase in the molecular weight is determined from the inherent viscosity values, but relaxation of the molecular orientation is not observed. All of these effects lead to an improvement in the tensile properties: the values of the elastic modulus and tensile strength increase to 18.4 GPa and 388 MPa, respectively, and the elongation at break is 2.2%.