Influence of dianhydride regiochemistry on thermomechanical and rheological properties of 3,3′- and 4,4′-polyetherimides

The design of linear high performance polyetherimides (PEIs) comprising 3,3′- and 4,4′-bisphenol-A dianhydride (bis-DA) and m-phenylene diamine (mPD) provided an opportunity to elucidate the influence of dianhydride regiochemistry on thermomechanical and rheological properties. This unique pair of regioisomers allowed the tuning of the thermal and rheological properties for high glass transition temperature polyimides for emerging engineering applications. Step-growth polycondensation enabled the production of high molecular weight PEIs for subsequent thermal and mechanical analysis. Monofunctional phthalic anhydride as an endcapper and monomeric stoichiometric imbalances enabled predictable number-average molecular weights ranging from 3,600 to 35,400 g mol⁻¹, as confirmed with size exclusion chromatography coupled with light scattering detection for absolute molecular weight determination. The selection of the dianhydride regioisomer influenced the weight loss profile as a function of temperature, entanglement molecular weight (M_e), glass transition temperature (T_g), tensile strain-at-break, zero-shear melt viscosity, average hole-size free volume (V_h), and the plateau modulus prior to viscous flow during dynamic mechanical analysis. The 3,3′- PEI composition interestingly exhibited a ~20 °C higher glass transition temperature than the corresponding 4,4′- PEI analog. Moreover, melt rheological analysis revealed a two-fold increase in M_e for 3,3′- PEI, which pointed to the origin of the differences in mechanical and rheological properties as a function of PEI backbone geometry.