CROSS-LINKING CHARACTERIZATION OF A POLYIMIDE - AFR700B

Crosslinking in AFR700B polyimide was studied using several techniques . Crosslinking could not be detected using Fourier transform infrared spectroscopy (FTIR) and differential scanning calorimetry (DSC). Swell ing in n-methyl pyrroli done [NMP) showed network formation occurs at cure temperatures greater than or equal to 325 degrees C (617 degrees F), but the reaction kinetics could not be determined from the data. D ynamic mechanical analysis [DMA] studies have shown that the storage m odulus (G') increased with increasing cure temperature up to 350 degre es C (662 degrees F), and was constant above 350 degrees C (662 degree s F), indicating that crosslinking via the reverse Diels-Alder reactio n was occurring. A broad secondary transition was seen in the loss mod ulus (G '') curves, centered between 150 and 180 degrees C (302 and 35 6 degrees F). This secondary transition appeared at a cure temperature of 300 degrees C (572 degrees F) and became more dominant with increa sing temperature and time. This secondary transition was not seen for cures <300 degrees C (572 degrees F) when the chains have not crosslin ked. Therefore, this is likely due to a crosslink bond rather than one in the backbone. At 1 atm, crosslinking followed second-order kinetic s based on the increase in glass transition temperature (T-g). The Arr henius plot of the rate constant showed a break in the slope, possibly indicating a change in reaction mechanism. At 1.38 MPa (200 psi), the T-g data was too scattered to determine the kinetics.