DIELECTRIC-RELAXATION AS A PROBE OF INTERPHASE STRUCTURE

The electrical and thermal properties of two high-performance polymers, PEEK, poly(etheretherketone), and NEW-TPI semicrystalline thermoplastic polyimide, are reviewed and compared in this work. Dielectric relaxation was used as a probe of the crystal/amorphous interphase and is shown to be sensitive to the interphase structure in the temperature range above the glass transition. The dielectric relaxation intensity is related to the number density of dipoles that are relaxed at a given temperature. For NEW-TPI, the constrained amorphous phase is completely relaxed within thirty degrees above T-g, while for PEEK the interphase relaxes more gradually. Thermal analysis shows that PEEK contains a much larger fraction of constrained amorphous chains and a much smaller fraction of mobile amorphous chains, compared to NEW-TPI. PEEK crystallizes more rapidly above the glass transition, and a large fraction of PEEK crystallinity is attributed to secondary crystallization processes. In NEW-TPI, as a result of the increased chain stiffness, cold crystallization is slow and little or no crystals develop by secondary crystallization. These differences in crystallization behaviour of the two materials may be related to the differences in the formation of a rigid amorphous interphase.