High Performance Phenylethynyl-Terminated Imede Resins

Polyimides, as a kind of high performance materail, are desired for use as aerospace structural adhesives and polymer matrix/carbon fiber composites. However, the high performance/high temperature polymers typically exhibit high melt viscosities and insoluble in common organic solvent, as a consequence, the processability by compression and injection moulding for use as composite matrices and adhesives is often difficult or unachievable. In recent years, in an effort to address the problem of high melt viscosities, development of low molecular weight oligomers with phenylethynyl terminated group as cross-linking precursor for the preparation of high performance thermosets has attracted considerable attention. Since the glass transition temperature or melt temperature of oligomers are low, there is a desirable large melt processing window for these material between T_g(T_m) and T_cure.The relationship between the structure of macromolecule chains and processing performance, high temperature performance, the mechanical properties of phenylethynyl terminated polyimide has is studied. From the perspective of molecular design, a series of phenylethynyl terminated polyimide resins are synthesised with some diamines and dianhydrides. To find an oligomer with suitable molecular weight which provide an material with ideal cross-linking density, good processing performance and high thermal stability, the influence of molecular weight of polyimide oligomers was studied,too. The results show that: (1) the rigider molecular chain, the higher temperature the glass transition of the resin around curing, and the more the molecular chain orientation and formation of crystal, the worse processing performance. Example such as PI in the form of single-benzene monomers vs multi-benzene monomers, PI in form of the monomers whose reactive group in para-position vs in meta-position. PI containing naphthalene ring vs benzene ring; (2) the reaction of six-membered anhydrides with aromatic amines at high temperature is different from that of six-membered anhydrides with aromatic amines, not only form imide structure, but also form isoimide structure; (3) with the increase of the molecular weight of the oligomers, the crosslinking density decreased, while glass transition temperature of the oligomers increased, which results lower Tg of the cured resin, higer melting viscosity, worse processing performance, However, there is no obvious different of the the thermal oxidation stability.Since process technics have a great influence on performance of polyimide, to study the curing kinetics of the polyimide resin is very useful for further understanding the curing characteristics of the resin, formulating an appropriate process and optimizing the curing technics. In this thesis, the kinetics of the curing reaction of imide oligomers were studied by dynamic DSC, and the approximate gel temperature T_gel, curing temperature T_cured and post-processing temperature T_treat were acquired by T-Φextrapolation. The results show that: the cure kinetic model of phenylethynyl end-capped imide resins conform to n-order curing model. The kinetic models calculated with Melak method well describe the curing process and resulting fits are in a good agreement with the experimental data; the resin which has more flexible chains exhibit lower activation energy, characteristic temperatures,wider curing range and larger reaction order.