Synthesis,characterization, and mechanical properties of an injection moldable polyimide based on 1,4-diaminobutane

A partially aliphatic polyimide was synthesized in a two-step process in N-methylpyrrolidone (NMP) from 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA) and 1,4-diaminobutane (DAB). Weight average molecular weights ranging from 35,000 to over 160,000 were obtained by adding different amounts of the chain stopper phthalic anhydride, or by using a slight excess of BTDA. Rheological measurements, supported by a size exclusion chromatography (SEC) analysis before and after injection molding, point to branching or even partial crosslinking during molding of poly(BTDA-DAB) if reactive amine endgroups are present. By reacting these amine endgroups with benzoyl chloride, branching is limited and a more Newtonian flow is observed. Both non-endcapped and endcapped reactor powders are ∼60% crystalline (Tg = 170 to 175°C, Tm = 290 to 300°C), but according to wide and small angle X-ray scattering (WAXS and SAXS) studies on test bars, both the more linear polyimides and their branched counterparts are fully amorphous after injection molding. Nevertheless, poly(BTDA-DAB) shows interesting mechanical properties like a bending modulus of ∼3.9 GPa, a tensile strength of ∼130 MPa, a tensile elongation at break of ∼13%, and a notched Izod impact of ∼4 kJ/m².