Melting behavior of high-temperature polymers

The melting behavior of three representative semirigid polymers, poly(aryl ether ether ketone), poly(ethylene terephthalate), and poly(ethylene naphthalenate), has been studied. Analysis of experimental results indicates that the melting process is morphologically the reverse of the isothermal crystallization process with respect to primary and secondary structural elements. On this basis, it is hypothesized that melting of all three polymers occurs in three distinct steps, assuming that spherulites are composed of dominant lamellae and subsidiary branches. The latter might have originated either from material rejected from the dominant lamellae or from noncrystalline molecular sections in the vicinity of the dominant lamellae. Experimental results emerging from differential scanning calorimetry, polarized optical microscopy, and small-angle X-ray scattering can be explained in terms of such a morphology, for which two possible models are suggested.