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

We report a study of dual stage crystallization and subsequent melting of P oly(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/crystalliza tion 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 v aried. DSC studies show there exist two endotherms when T-1 < T-2, and thre e endotherms when T-1 > T-2, for both PEEK and PEEK/PEI blend. Dual populat ions of crystals form during the first stage regardless whether T-1 < T-2 o r 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 availab le for the growth of additional crystals decreases, and the amount of cryst als formed during the second stage decreases. During melting, the average l ong period increases while the linear stack crystallinity decreases continu ously. The average crystal thickness also first increases, as the least per fect, 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 fo r the roll-off and decrease in crystal thickness during the final stages of melting. (C) 2001 Kluwer Academic Publishers.