Thermal expansion of poly(ether‐ether‐ketone) (PEEK)

Thermal expansivities along the a, b, and c axes of the orthorhombic unit cell of poly (ether‐ether‐ketone (PEEK) crystals have been determined from 160 to 470 K by wide‐angle x‐ray diffraction. The expansivities along the a and b axes are positive while the expansivity along the c (chain) direction is negative, with values of \[ α _a^c = 11.3 × 10‐ 5 { K}^{ ‐ { 1}} ,{ }α _b^c = 4.6 × 10‐ 5 { K}^{ ‐ { 1}} , \] and \[ α _∥ ^c = ‐ 1.4 × 10‐ 5 { K}^{ ‐ { 1}} \] at 300 K. The negative α arises from the effective shortening along the chain axis caused by the torsional and bending motions of the chain, whereas the large and positive α and α reflect the weak interactions across the chains. Using a dilatometric technique we have also studied the expansion behavior of isotropic PEEK as a function of crystallinity (X = 0−0.34) and have measured thermal expansivities of oriented PEEK (draw ratio 1–4) parallel and perpendicular to the draw direction. The effects of crystallinity and orientation can be understood by treating semicrystalline PEEK as a composite consisting of crystallites embedded in an amorphous matrix.