Toughening of a High-performance Bis-nadimide Thermoset by Blending with High-Glass Transition Temperature Linear Polyimides

A nadimide end-capped thermosetting oligomer was modified by blending with three homologous soluble linear polyimides containing bulky lateral fluorene groups with the intention of improving its fracture toughness. These linear polyimides were prepared by polycondensation between 4,4′-(9H-fluoren-9-yliden)-bisphenylamine (cardo structure) and three different bis-phthalic anhydride derivatives, containing between the bis-phthalic moities a secondary alcohol function, a carbonyl function or a hexafluoropropylidene group respectively. The thermoset produced upon heating a thermostable polynadimide network having a glass transition temperature (T_g) close to 300° and a critical stress intensity factor equal to 0.9 MPa.m^1/2. The T_gs of the studied linear polyimides were located above 340° in connection with the chain–chain molecular interactions. After dissolving, the precipitated blend powders with different compositions were thermally polymerized under pressure to give bulk specimens. The resulting morphologies were dependent on the chemical structure of the linear polyimide. As shown by the position of heat deflection temperatures, a well-defined two-phase blend was obtained by introducing the hexafluoropropylidene-containing polyimide, when a fully miscible system was formed with the secondary alcohol-containing polyimide. The parallel increase in fracture toughness seemed to be controlled by the degree of phase separation between the blend components. The greater improvement resulted from the partially fluorinated polyimide: the corresponding K_IC reaching 1.23 MPa.m^1/2 with 20 wt% of linear component. Finally, the toughening effect due to the latter polymer was examined in relation to its average molecular weight. Almost no change was observed if the corresponding inherent viscosity in N-methyl pyrrolidone solution was above 0.2 dl/g. In any case, owing to the high T_g of the linear component, the thermomechanical stability of the blend was maintained at the same level as that of the initial polynadimide network. © 1997 John Wiley & Sons, Ltd.