1. Introduction
Poly(ether ether ketone) (PEEK) is a high performance
thermoplastic material with high glass transition temperature
and high melting temperature [1]. It exhibits very good
thermal stability [2], high chemical resistance and excellent
mechanical properties [3]. These properties make PEEK an
attractive matrix material in carbon fiber reinforced
composites. The thermal stability experiments have indicated
that PEEK is stable in nitrogen up to 400 8C, holding
for 15 min [4]. After this treatment, the subsequent crystallization
behavior (isothermal or non-isothermal) was
independent of the prior thermal history. Detailed kinetics
analyses of the isothermal and non-isothermal growth-rate
data indicated that PEEK exhibited an unmistakable regime
II ! III transition temperature ðTII!IIIÞ at 296 ^ 1 8C [5,6].
A time–temperature-transformation diagram superposed on
the continuous-cooling-transformation curves could be used
to predict the regime of crystallization [7]. If the crystals
were formed in regime III fashion (below 296 8C), they
contained more defects and had a lower crystallinity.
Isothermal crystallization is a good way to understand
the crystallization kinetics. Differential scanning calorimetry
(DSC) is a major tool to study the crystallization kinetics
of polymers. The well-known Avrami equation describes
two crystallization stages. Velisaris and Seferis [8] proposed
a model of dual crystallization mechanisms to quantitatively
express the crystallization rates of PEEK in terms of a linear
combination of two competing processes in parallel. The
initial linear portion (slope ¼ n1) corresponds to the
primary crystallization process, and the secondary crystallization
process (slope ¼ n2) dominates after the leveling
off point. Day et al. [9,10] found a lower n2 value after about
70–80% conversion from the amorphous state to the
crystalline state. This ascribed the secondary crystallization
to the filling within the spherulites. Hay and Kemmish [11]
observed that the primary crystallization corresponded to
the development of spherulites ðn1 ¼ 3Þ and contributed
about 80% of the overall process, while the secondary