Two-Stage Crystallization of Poly(Ether Ether Ketone) and its Blends with Poly(Ether Imide)
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HSIN-LUNG CHEN AND ROGER S. PORTER Polymer Science and Engineering Department, University of Massachusetts, Amherst, MA 01003 ABSTRACT Thermal mechanical analysis (TMA) has been used to study the crystallization behavior of poly(ether ether ketone) (PEEK) and its blends with poly(ether imide) (PEI). The two crystallization stages of PEEK are clearly distinguished by measuring the variation of film thickness with time during isothermal crystallization. Upon blending with PEI, the distinction of the two PEEK crystallization stages becomes obscure. This is attributed to the depressions in both nucleation density and spherulite growth rate upon blending with PEI. An Avrami analysis, modified by considering both primary and secondary crystallization, is used to extract the respective kinetic behavior of these two crystallization stages. The results indicate that the secondary crystallization proceeded slower than the primary crystallization in the diffusion-controlled crystallization region. On the other hand, these two crystallization stages proceeded at comparable rate in the thermodynamically-controlled crystallization region. It is also found in the diffusion-controlled crystallization that blending with PEI induced a larger depression in the secondary crystallization rate than in the primary crystallization rate. Explanations for these observations are proposed and discussed.
INTRODUCTION Poly(ether ether ketone) (PEEK) is a semicrystalline, aromatic polymer with good thermal and mechanical properties. The Tg of amorphous PEEK is 145 OC.1 The normally observed crystallinity and melting point of PEEK are about 35% and 335 oC, respectively. 1 Morphological 2 studies of crystalline PEEK have identified two distinct crystal populations of different thickness. 4 The thinner secondary crystals have been found to grow in the constraints imposed b•, the thicker primary crystals. The inhomogeneous morphology of PEEK has been considered to give rise to its well-known double melting endotherms observed by DSC. 5-7 Although these results have shown the existence of two lamellar morphologies in crystalline PEEK, other information, such as the respective crystallization kinetics and the origin of this two-stage crystallization behavior require further studies. It is known that blending with a miscible polymer may have significant effects on the crystallization behavior of the crystalline polymer. These include changes in thermodynamic driving force and molecular mobility associated with crystallization. The blends of PEEK and poly(ether imide) (PEI) have been the subject of several investigations. 8 -12 PEI is a linear, amorphous, aromatic polymer with a Tg of 215 oC. The blends of PEEK and PEI are miscible over the entire composition range. 8 -12 Miscibility of PEEK and PEI in the amorphous phase decreases the crystallization rate of PEEK. 8 ,9,11 Blending with PEI may provide a method to systematically control the crystallization rate of PEEK. The investigation of the effect of PEI on the crystallization behavior of PEEK is t
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