X-RAY CRYSTALLOGRAPHIC STRUCTURE OF Meta-AND Para-DIMETHYL ESTER OF PYROMELLITIC ACID

The polycrystalline powder of para- and meta-dimethyl ester of pyromellitic acid (PMDE)have been prepared by fractional crystallization, and their crystal structures have been de-termined by Wide-Angle X-ray Diffraction (WAXD). Both p-PMDE and m-PMDE wasfound to be orthorhombic crystal system, and their unit cell parameters a = 0.840nm,b = 0.707 nm, c = 1.136 nm and a = 1.032 nm, b = 0.835 nm, c = 0.714 nm, respectively.Space group all belongs to P_(mmm). p-PMDE has two molecules per unit cell with crystaldensity 1.388g.cm~(-3), while m-PMDE has two molecules per unit cell with crystal density1.522 g.cm~(-3). Indices of crystal diffraction peaks are also detailed in the present work.The difference in crystal structures between p-PMDE and m-PMDE has thus been usedto explain the curing behavior of isomerically pyromellitic dianhydride-based poly(amicester)s.