Synthesis and properties of phenylethynyl-terminated, star-branched, phenylquinoxaline oligomers

The primary objective of this work was to prepare readily melt and solution processable phenylquinoxaline (PQ) oligomers that could be thermally crosslinked to solvent-resistant resins. Thus, a mixture of 2-(4-hydroxyphenyl)-3-phenyl-6-fluoroquinoxaline and 3-(4-hydroxy-phenyl)-2-phenyl-6-fluoroquinoxaline (HPFQ) was used to prepare star-branched PQ oligomers end-capped with 4-fluoro-4'-phenylethynylbenzophenone (FPEB). 1,1,1-Tris(4- hydroxyphenyl)ethane (THPE) was used as the branching unit. The oligomer number-average molecular weights ((M) over bar(n)s) as determined by size exclusion chromatography (SEC) were close to the calculated values of 2922, 4698, 6474, and 13,578 g/mol, and their intrinsic viscosities ranged from 0.16 to 0.57 dl/g (m-cresol at 30 degrees C). The oligomers, which were quite soluble in common organic solvents, had glass transition temperatures (T(g)s) that ranged from 181 to 233 degrees C (DSC, Delta T = 20 degrees C/min). They also underwent an exothermic cure with maxima from 377 to 443 degrees C. The T(g)s of the cured oligomers ranged from 259 to 284 degrees C depending on the oligomer (M) over bar(n) and the curing conditions. The oligomers had low melt viscosities, e.g. an oligomer (SPQ-46) with an (M) over bar(n), of 4816 g/mol (SEC) had a melt viscosity of 150 Pa s at 348 degrees C. A cured thin film of SPQ-46, which was insoluble in common organic solvents, had a room-temperature (RT) tensile strength of 100 MPa, a RT modulus of 2358 MPa, and a RT elongation of 5.9%. A cured sample of SPQ-46 displayed a RT titanium-titanium lap shear tensile strength of 35.2 MPa. SPQ-46/carbon fiber(IM-7) composites were prepared that displayed a RT flexural strength of 1902 MPa, a RT modulus of 1.38 GPa and a RT open hole compressive strength of 433 MPa. (C) 2000 Elsevier Science Ltd. All rights reserved.