Cohesive properties of Ultem and related molecules from simulations

The cohesive properties of many engineering plastics are difficult to determine experimentally, as the polymers are frequently insoluble, have high Tgs, and are sometimes poorly characterized. For difficult polymers such as these, molecular modeling can provide useful information of higher quality than might be obtained by other methods. Ultem® is a polyether imide having a very large and relatively stiff repeat unit. We have performed a series of simulations on Ultem and related molecules so as to evaluate the cohesive energy density of the polymer. Small molecular fragments (N-phenylphthalimide, 2,2-diphenylpropane, and diphenylether) of the repeat unit were studied to get approximate bounds to be expected for values of the solubility parameter for the polymer. Oligomers up to 4-mers were directly simulated, and the results were used to estimate the properties of the high polymer. These methods yield a value near 22.0 MPa1/2 for the solubility parameter of the polymer, lower than has been estimated from group additivity. The interfacial interactions between Ultem and a variety of low molecular liquids have also been evaluated in an effort to identify functional groups that might interact most favorably with the polymer for adhesive applications. These calculations are in good agreement with expectations from solubility parameters. Most significantly, the calculations are fully compatible with experimental observations.