Synthesis Of Polyimide Oligomers And The High Performance Resin Matrices Therefrom

Epoxy resins are a versatile group of cross-linked polymers that have excellent chemical and corrosion resistance, good mechanical and thermal properties, good electrical insulating properties, outstanding adhesion to various substrates like glass and metals, low shrinkage upon cure and can be easily fabricated. Depending on the specific needs for certain physical and mechanical properties, combinations of epoxy resin and curing agents can usually be formulated to meet the market demands. However, in terms of structural applications requiring high impact and fracture strengths like the aerospace, due to the fact that epoxy resins cannot absorb and distribute stress because of their intrinsic brittleness, arising from their three dimensional cross-linked network structure, it limits their use as resin matrices. Due to this limitation, development of approaches for toughening the epoxy resins or lowering their high dielectric constant, without sacrificing modulus and glass transition temperature (Tg) while retaining their relative low cost, would lead to an increase in their application.This research project has used the method of blending polyimide oligomers with active carboxyl and hydroxyl functional groups with two types of epoxy resins (difunctional and tetrafunctional) with the objective of studying their compatibility and the effect of the resulting resin matrices on the performance properties. The project is divided into the following parts:(1) synthesis and characterization of4,4’-diamino-4"-hydroxytriphenylmethane (DAHTM) monomer using aniline hydrochlorate as the catalyst;(2) synthesis and characterization of carboxyl ic imide4,4’-diamino-4"-hydroxytriphenylmethane (CIDAHTM) and carboxylic imide2,2-bis[4-(4-aminophenoxy)phenyl propane (CIBAPP) polyimide oligomers;(3) preparation of the high performance resin matrices by incorporating the polyimide oligomers into the epoxy resins and carrying out the performance tests; and (4) preparation of a glass-fiber reinforced composite using the modified E51+TGDDM epoxy resin and testing its performance.To start with, the DAHTM monomer was synthesized by condensation reaction between aniline and4-hydroxyphenyl aldehyde as raw materials in the presence of aniline hydrochlorate catalyst. The FT-IR absorption peaks and melting point were used to characterise the monomer. Then CIDAHTM and CIBAPP imide oligomers were synthesized via two-step polycondensation of DAHTM and BAPP monomers, respectively and TMA anhydride. The main absorption peaks of the FT-IR spectra proved complete imidization and confirmed their structures.The synthesized oligomers were used to modify diglycidyl ether of bisphenol-A (DER331E51) and tetragl ycidyl-4,4’-diaminodiphenylmethane (TGDDM) epoxy resins, whereby different quantities of oligomer were blended with the epoxy resins. P-EP1,2,3and4were samples modified by CIDAHTM imide oligomer, while P-EP5,6,7,8and9were samples modified by CIBAPP imide oligomer. Samples modified with solid CIDAHTM imide oligomer had activation energy in the range of54.8～64.1kJ/mol; water absorption rate in the range of1.4%～1.8%; surface energy in the range of44.8～45.9mJ/m2and tensile shear strength in the range of10.7～24.6MPa. Sample modified with liquid CIDAHTM imide oligomer had activation energy in the range of38.9～53.8kJ/mol; water absorption rate of2.6%; surface energy in the range of48.9～61.5mJ/m2and tensile shear strength in the range of2.8～11.7MPa. On the other hand, samples modified with liquid CIBAPP oligomer had activation energy in the range of46.0～54.2kJ/mol; average water absorption rate of0.4%; surface energy in the range of43.6～53.3mJ/m2and tensile shear strength in the range of17.8～33.3MPa.From the modified resin matrices test results, all the samples had good hydrophobicity since their surface energy values were less than that of water and their water absorption rate, except the one modified with liquid CIDAHTM imide oligomer, was less than average absorption rates for most adhesives (about2to3%) under the85℃/85%RH wet condition storage. This showed that the polyimide oligomers improved the hydrophilic nature of all the epoxy resins. CIBAPP oligomer modified samples had the best rate of water absorption. Samples modified with CIBAPP imide oligomer had better mechanical properties than those modified with CIDAHTM imide oligomer. This shows that CIBAPP imide oligomer is more compatible with both difunctional and tetrafunctional epoxy resins. The best percentage ratio that gave the best mechanical properties for mixing E51:TGDDM was60:40.Since60:40percentage ratio of E51:TGDDM epoxy resin modified with liquid CIBAPP (20phr), gave the best mechanical properties, the resin matrix was used to prepare a glass-fiber reinforced composite and the performance tests were carried out. The results showed that the glass-fiber reinforced composite of modified epoxy resin had a dielectric strength of197kV/cm, volume resistance of2.1×1015Ω·cm, longitudinal and transverse stress of686MPa and631MPa, respectively, water absorption rate of0.18%, surface energy of43.6mJ/m2and average dielectric constant of4.9.