Crystallographic Equivalence of Ether and Ketone Groups in Poly(aryl ether ketone)s

Abstract

Crystallographic equivalence of ether and ketone in all para‐substituted PAEKs crystallized in Form I was discussed in this paper. In a word, crystallographic equivalence between ether and ketone groups is tenable when the polymer contains only phenyl rings in the repeat unit. If the polymer contains a diphenyl group in the repeat unit, two cases should be distinguished. In the case of PEDEKK and PEEKDK, crystallographic equivalence between ether and ketone linkages is untenable. However, in the case of PEDK and PEDEKDK, crystallographic equivalence between ether and ketone linkages is still tenable.

Critical Role of Diffraction Simulation in Establishing the Crystal and Molecular Structures of Poly(biaryl ether ketone)s

“…We have already proposed from DSC studies that aromatic poly(ether ketone)s containing biphenyl linkages can adopt similarly disordered structures ( 6 ), since random copolymers of the repeat units [−O−Ar−Ar−O−Ar−CO−Ar−Ar−CO−Ar−] n ( 3 , Ar = 1,4-phenylene) and [−O−Ar−Ar−CO−Ar−] n ( 4 ) do not differ significantly from their parent homopolymers, either in melting point or enthalpy of fusion . The same proposal of crystallographic ether/ketone disorder has also more recently been made for this family of polymers based on the similarity of their X-ray powder patterns, which were indexed in terms of a hypothetical orthorhombic unit cell . In the present paper, we analyze X-ray powder and fiber data for polymers 3 , 4 , and their 1:2 random copolymer ( 5 ) using diffraction simulation methods and show that although a disordered model (in fact, based on a monoclinic rather than an orthorhombic unit cell) satisfactorily accounts for the diffraction data for 3 and 5 , an at least partially ordered structure is present in oriented samples of polymer 4 .…”

“…In addition, simulation of fiber patterns from the two different cells clearly demonstrates the requirement for a monoclinic unit cell, as this symmetry accurately reproduces the splitting of the 111 and 111̄ reflections on the first layer line (Figures a and a), whereas the orthorhombic model suggested by Mo et al emphatically does not (Figure b). These observations highlight the dangers associated with assigning crystal systems and unit cells in the absence of reliable molecular models and reemphasize the importance of diffraction simulation in structural analysis of crystalline polymers. ,

8 Simulated X-ray fiber patterns for (a) the structure described here for polymer 4 (ordered version) and (b) the structure for polymer 4 proposed in ref . These patterns should be compared the experimental data shown in Figure .

…”

“…It is worth commenting at this point that the crystal system and unit cell established for polymers 3 and 4 in this work differ markedly from those recently proposed by Mo et al (orthorhombic; cell dimensions a ) 7.64, b ) 6.00, c ) 13.42 Å). 6 The crystallographic convention which places the c-axis parallel to the polymer chain direction means that the length of this axis must be a direct function of the polymer chain repeat distance. The c-axis of the orthorhombic cell assigned by these workers at 13.40 Å is however incompatible with the structural repeat distance determined here from single-crystal oligomer data.…”

“…This polymer does in fact crystallize in space group Pbcn with a disordered two-ring crystallographic repeat (1) rather than the sixring repeat (2) which would be required in a fully ordered crystal. 3 We have already proposed from DSC studies that aromatic poly(ether ketone)s containing biphenyl linkages can adopt similarly disordered structures (6), since random copolymers of the repeat units [-O-Ar-Ar-O-Ar-CO-Ar-Ar-CO-Ar-] n (3, Ar ) 1,4-phenylene) and [-O-Ar-Ar-CO-Ar-] n (4) do not differ significantly from their parent homopolymers, either in melting point or enthalpy of fusion. 5 The same proposal of crystallographic ether/ketone disorder has also more recently been made for this family of polymers based on the similarity of their X-ray powder patterns, which were indexed in terms of a hypothetical orthorhombic unit cell.…”

“…5 The same proposal of crystallographic ether/ketone disorder has also more recently been made for this family of polymers based on the similarity of their X-ray powder patterns, which were indexed in terms of a hypothetical orthorhombic unit cell. 6 In the present paper, we analyze X-ray powder and fiber data for polymers 3, 4, and their 1:2 random copolymer (5) using diffraction simulation methods and show that although a disordered model (in fact, based on a monoclinic rather than an orthorhombic unit cell) satisfactorily accounts for the diffraction data for 3 and 5, an at least partially ordered structure is present in oriented samples of polymer 4.…”