Study On The Modified Thermosetting Systems And Composites Based On Cyanate Ester Resins

Cyanate ester resins have been used in the fields including encapsulants of electronic devices,structural materials of aerospace and high temperature adhesives because of their outstanding dielectric,thermal and adhesive properties.In general, the inherent brittleness is a major drawback with highly cross-linked thermosetting polycyanurates,some applications required improved fracture tougheness.In this work,cyanate ester resins were modified with thermoplastics and renewable resource, i.e.epoxidized soybean oil(ESO) respectively.In partâ… of the dissertation,the thermodynamic and dynamic factors during polymerization-induced phase separation and development of morphologies of high performance thermoplastics modified cyanate systems and its composites were studied in detail.The correlation between dynamic asymmetry and viscoelastic phase separation of thermoplastics modified cyanate systems was studied by differential scanning calorimeter(DSC),time-resolved light scattering(TRLS),scanning electron microscopy(SEM) and rheology measurement.The curing conversions at cloud points of poly(ether imide)(PEI) modified cyanate systems cured isothermally were determined and consequently obtained the phase diagram.The characteristic relaxation time and evolution time of phase structure vs.PEI content were plotted for the modified systems,which were similar with the phase diagram.The thermodynamic factors including volume fraction and molecular weight of thermoplastics modified cyanate systems were analyzed by monitoring the polymerization-induced phase separation of poly(ether sulfone) modified cyanate systems with optical microscope(OM) and TRLS.The results indicated that the phase separation process and final morphologies of the modified systems were determined by the location of systems in phase diagram.The phase separation of modified system adjacent to critical point started at lower curing conversion with shorter characteristic relaxation time during the early stage,in addition,secondary phase separation took place in these systems.The results also suggested that the thermodynamic factors could alter dynamic asymmetry as well as characteristic relaxation time.Moreover, the tensile properties and rheological behavior of modified systems were studied and discussed the relationship between morphologies and properties. The polymerization-induced phase separation of composites prepared by rod-like meso-scale particles,thermoplastics and cyanate was monitored by OM and TRLS, the results indicated that the rod-like meso-scale particles had a restriction effect on coarsening of phase structure.The rod-like meso-scale particles could be wrapped into thermoplastics-rich phase because of tangling during polymerization-induced phase separation,which intensified the dynamic asymmetry at middle and late stage of phase separation and therefore strengthened viscoelastic effect.In addition,the influence of rod-like meso-scale particles on morphologies and their location in composites were observed by SEM and TEM,then,the relationship between the morphologies and tensile properties of composites was discussed.In recent years,renewable resources have attracted continuously growing interest because of their advantage of eco-friendliness for industrial applications,of which the study on biobased polymers composed of renewable resources and polymers is a typical example.In partâ…¡of the dissertation,renewable resource,i.e.epoxidized soybean oil(ESO) was used to modify cyanate ester resin by preparing biobased cyanate resin and nanocomposites.Cyanate ester resin was modified with ESO at various contents.The morphologies, thermal-physical properties,tensile properties and water absorption of the modified systems were studied by SEM,DMA,tensile testing,etc.The experimental results suggested that the toughness of the modified systems had been improved remarkably with sacrifice of modulus and thermal stability.Therefore,biobased cyanate ester composites were prepared from ESO and in situ generated nano-silica via sol-gel process.The nano-silica existed in the forms of nanoparticles and silica networks in cyanate-rich phase and ESO-rich phase respectively.The biobased cyanate ester composites showed effectively improved properties compared with the systems without nano-silica,the modulus and thermal stability had been enhanced without sacrifice of elongation at break.