NEW-TPI thermoplastic polyimide: thermal analysis and small-angle X-ray scattering

Abstract

Isothermal cold crystallization of the thermoplastic polyimide NEW-TPI has been investigated from 300 to 360°C. The fastest crystallization took place at 330°C where the time to maximum exothermic heat flow was 148 s. A single Avrami exponent could be used to describe the bulk crystallization kinetics for degrees of conversion from 0.02 up to 0.95, indicating that most crystallization takes place at the growth front by a single mechanism, nearly up to the completion of crystallization. The Avrami exponent is 3.5 for T c < 330° C, and decreases as T c increases. A regime II to III transition is indicated by a slope change seen in ln( l t 1 2 ) + U R (T−T ∞) vs. 1 TΔTf . These results indicate that NEW-TPI cold crystallization can be modelled according to heterogeneous nucleation and three-dimensional crystal growth. From T m vs. T c analysis, the infinite-crystal melting point of NEW-TPI is estimated to be 400°C. With regard to the glass transition, T g was affected by cold crystallization in a systematic but minor way. The heat-capacity increment of semicrystalline NEW-TPI at T g shows a small negative deviation from that predicted on the basis of the degree of crystallinity. The absence of secondary crystallization processes has been used to explain the different glass transition behaviour and small negative heat-capacity deviation in NEW-TPI. Real-time small-angle X-ray scattering (SAXS) was used to monitor the structure change as a function of cold crystallization time at 300°C. Systematic change of long period, lamellar thickness, invariant and volume fraction of crystallinity were obtained from the one-dimensional electron density correlation function analysis. At T c=300° C, lamellar thickness as a function of t c ranges from 37 to 52 Å, or about 1.5–2.0 times the monomer repeat unit. The thickness of the interphase region was determined from SAXS to be about 17 Å independent of cold crystallization time at 300°C.