Thermal expansion of the crystal lattice of novel thermoplastic polyimides

Measurements of the coefficient of thermal expansion (CTE) of the crystalline lattice of two semicrystalline thermoplastic polyimides are reported. NEW‐TPI and LARC‐CPI polyimides were studied using elevated temperature wide‐angle x‐ray scattering (WAXS) from 25°C. To 325°C. To examine possible shifts in the c‐axis, a novel approach developed in our lab was used to create highly oriented samples. Films were treated in 1‐methyl‐2‐pyrrolidinone (NMP) at the reflux temperature, washed and then dried under constraint. The films treated in this manner were highly oriented, with c‐axes preferentially aligned along the film normal. The advantage of this orientation is that it allows numerous reflections of type (00l) to be examined for temperature shifts of the c‐axis using reflection mode WAXS. No systematic shift of the c‐axis lattice parameter as a function of temperature was observed in either NEW‐TPI or LARC‐CPI. The c‐axis thermal expansion is concluded to be smaller than 8 x 10−6/°C for LARC‐CPI, and for NEW‐TPI may be weakly negative. From WAXS of unoriented films, systematic shifts in the a and b lattice parameters were deduced as a function of temperature. The linear CTEs relating these unit cell parameters at temperature T to their values at 0°C are: ©1995 John Wiley & Sons, Inc.