| Epoxy resins are traditionally toughened by the incorporation of reactive rubbers, such as carboxyl-terminated butadiene-acrylonitrile (CTBN) copolymers and amino-terminated butadiene-acrylonitrile (ATBN) copolymers, in which phase separation occurs between epoxy resins and copolymers during cure and a multiphase network is thus formed. Although toughness enhancement in these networks is very effective, in most instances this enhancement is achieved at the expense of stiffness and high-temperature performance. In this research, attempts have been made to toughen epoxy resins using functionalized aromatic polyimides which possess excellent thermal stability and mechanical properties. It is hoped that the high temperature performance of these toughened networks will not be sacrificed.; The solubility of a series of polyimides was first investigated in terms of the structure-property relationships. The solubility of polyimides in different solvents and epoxy resins can be controlled via dianhydride modification. The isopropylene linkages in the polyimide backbones are most likely responsible for the decrease of modulus and yield stress as well as the increase of T{dollar}sb{lcub}rm g{rcub}{dollar} of the polyimides. Property additivity has been established on modifying the mechanical properties of polyimide via the synthesis of copolyimide.; A difunctional epoxy resin, EPON 828 and curing agent 4,4{dollar}spprime{dollar}-diamino diphenylsulfone (DDS) are chosen as a model matrix. First, a series of polyimides are synthesized from 1,3-bis(3-aminophenoxy) benzene, APB, and 4,4{dollar}spprime{dollar}-oxydiphthalic anhydride, ODPA, and they are used as tougheners. The particle dispersed phase is found in the toughened networks regardless of the molecular weights of polyimide. A nearly unchanged toughness in these toughened networks is observed and this may be attributed to the fast phase separation kinetics due to the poor miscibility between the polymide and epoxy resins.; A polyimide synthesized from (Bisphenol-A dianhydride and 4,4- (1,4-phenylene-bis-(1-methyl-ethylidene)) bisanline (BisADA/Bisp) has been found to have a similar solubility parameter to the epoxy resin and demonstrated a successful toughness improvement by using high molecular weight tougheners. Ductile drawing of the polyimide (BisADA-Bisp) matrix in the phase inversed morphology is considered to be the toughening mechanism, in most cases, for this toughened network. Fracture energy (G{dollar}sb{lcub}rm IC{rcub}{dollar}) is increased about five times with 20%(w/w) loading without sacrificing the desired properties such as glass transition temperature (T{dollar}sb{lcub}rm g{rcub}{dollar}), modulus and thermal stability.; The effects of functional end-capped group, molecular weight and weight loadings of tougheners on curing and phase separation are investigated via differential scanning calorimetry (DSC) and small angle light scattering (SALS) experiments. X{dollar}sb{lcub}rm cp{rcub}{dollar}, which is defined as the conversion of the curing reaction at the beginning of phase separation, is found to be decreased with increasing molecular weight and/or weight loading of the polyimide tougheners. This decrease of X{dollar}sb{lcub}rm cp{rcub}{dollar} may be ascribed to the high viscosity environment and/or the decrease of concentration of reactive end groups by adding amino or phenyl terminated polyimide into the curing mixtures. |
