Polymorphic structures of poly(tetramethylene adipate)

Abstract

Abstract X-ray diffraction and differential scanning calorimetry studies show that the linear polyester poly (tetramethylene adipate) (PTMA) can exist in two polymorphic forms in the solid state, which are designated α- and β-PTMA. The α form is obtained by annealing a stretched film at 52°C for several days. The unit cell is monoclinic with dimensions a = 6.70 Å, b = 8.00 Å, c = 14.20 Å (fiber axis) and β = 45.5°. The fiber repeat is approximately 0.5 Å shorter than that predicted for an extended planar zigzag conformation, as has been observed for some other related polyester structures. A second polymorphic form, designated β-PTMA, predominates in oriented films prepared from the quenched melt. Here the chains appear to have the planar zigzag conformation and are packed on an orthorhombic network with dimensions α - 5.05 Å and b = 7.36 Å but are not in register along the fiber axis. Spontaneous transformation of the β form to the a form occurs slowly at room temperatures, indicating the α form is the more stable structure for the bulk polymer.