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 later al fluorene groups with the intention of improving its fracture toughn ess. 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 hexafluouopropylidene group respectively. The thermoset produced upon hearing a thermostable polynadimide network having a glass trans ition temperature (T-g) close to 300 degrees C and a critical stress i ntensity factor equal to 0.9 MPa.m(1/2). The T-gs of the studied linea r polyimides were located above 340 degrees C in connection with the c hain-chain molecular interactions. After dissolving, the precipitated blend powders with different compositions were thermally polymerized u nder 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-ph ase blend was obtained by introducing the hexafluoropropylidene-contai ning polyimide, when a fully miscible system was formed with the secon dary alcohol-containing polyimide. The parallel increase in fracture t oughness seemed to be controlled by the degree of phase separation bet ween the blend components. The greater improvement resulted from the p artially fluorinated polyimide: the corresponding K-IC reaching 2.23 M Pa.m(1/2) with 20 wt% of linear component. Finally, the toughening eff ect due to the latter polymer was examined in relation to its average molecular weight. Almost no change was observed if the corresponding i nherent 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 therm omechanical stability of the blend was maintained at the same level as that of the initial polynadimide network.