Crystal Morphology and Phase Identification in Poly(aryl ether ketone)s and Their Copolymers. 3. Polymorphism in a Polymer Containing Alternated Terephthalic Acid and Isophthalic Acid Isomers

Crystal structures and morphology of a polymer containing alternated terephthalic acid (T) and isophthalic acid (I) linked phenylene units in poly(ether ketone ketone) [PEKK(T/I)1 have been investigated via transmission electron microscopy (TEM), electron diffraction (ED), wide angle X-ray diffraction (WAXD), and differential scanning calorimetry (DSC) experiments. When the polymer is crystallized at high temperatures (at and above 310 °C), only one orthorhombic crystal unit cell (form I) is found, and the crystal dimensions are a = 0.778 nm, b = 0.610 nm, and c = 3.113 nm. At the crystallization temperature below 200 °C, another orthorhombic crystal unit cell (form II) forms, having the dimensions of a = 0.417 nm, b = 1.108 nm, and c = 3.113 nm. Even at this low-temperature range, form II always coexists with form I except in the case of solvent-induced crystallization. A possibility of the existence of a new form (form III) is identified in a temperature range between 200 and 300 °C, and usually coexists with form I. This form also possesses an orthorhombic unit cell with dimensions of a = 0.641 nm, b = 0.610 nm, and c = 3.504 nm. Transformations from form II to both forms I and III require melting and recrystallization. Relatively regular-shaped lamellar crystals in PEKK(T/I) can be observed.