Physical aging of polyetherimide (PEI) was studied using a bellows dilatometer based on Zoller's design. A linear variable
differential transformer (LVDT) is used to measure the displacement of the bellows. The voltage output of the LVDT is interfaced
to a computer for automated data collection. Isothermal aging experiments were carried out at temperatures near the glass
temperature (206–209 ‡C) using a constant temperature oil bath maintained at the desired aging temperature. The time required
to reach equilibrium and the reduced curve produced by aging time-temperature superposition are given. The results compare
well with data obtained by capillary dilatometry for the same material.