Real-time small angle X-ray scattering study of two-stage melt crystallization of PEEK

Abstract

We report a study of dual stage crystallization and subsequent melting of Poly(etherether ketone) (PEEK) and an 80/20 blend with Poly(etherimide) (PEI) using differential scanning calorimetry (DSC) and real-time small angle X-ray scattering (SAXS). The treatment scheme involves annealing/crystallization at T ₁ followed by annealing/crystallization at T ₂, where either T ₁ < T ₂ or T ₁ > T ₂. The holding time during isothermal melt treatment was varied. DSC studies show there exist two endotherms when T ₁ < T ₂, and three endotherms when T ₁ > T ₂, for both PEEK and PEEK/PEI blend. Dual populations of crystals form during the first stage regardless whether T ₁ < T ₂ or T ₁ > T ₂. In the high-to-low temperature sequence, holding at the second stage causes an additional third population of crystals to grow, creating a third endotherm. As the first stage holding time increases, space available for the growth of additional crystals decreases, and the amount of crystals formed during the second stage decreases. During melting, the average long period increases while the linear stack crystallinity decreases continuously. The average crystal thickness also first increases, as the least perfect, thinnest crystals melt. Eventually, the crystal thickness levels off and begins to decline with increasing temperature. Melting of the thickest, most perfect crystals occurs most probably from the surfaces accounting for the roll-off and decrease in crystal thickness during the final stages of melting.