Study On Phase Structure And Performance Of Polyimi De-Siloxane/Epoxy System

Epoxy resin as one of a matrix resins is widely used in adhesives, paint, lightindustry, architecture, electronic electrical insulating materials and advanced compositematerials for its superior performances such as excellent bonding, wear-resisting,machinery, electric insulation, chemical stability and easy process molding. The threedimensional network structures of cured epoxy resins have the high crosslink density; theirmovement in main link segments is very difficult, and they are typical brittle material,which largely limit their application. Therefore, the work to toughening epoxy resin hasbeen very important.At present, the research of epoxy resin modification is mainly focused onincreasing epoxy resin toughness and keep thermal stability of epoxy resin at the same time.The epoxy resin materials has good heat resistance after modified by thermoplastic withhigh glass transition temperature, it is found that bicontinuous phase structure or inversionphase structure can achieve the best toughening effect by controlling the reaction inducedphase separation process.Polyimide can resist high temperature500℃in short time and can be used below300℃for long time because of the imide ring in the main chain. Therefore, usingpolyimide with excellent heat-resistant performance to modify epoxy resin is a hot topic inrecent years. By adding silicone into polyimide backbones, not only decrease theinterfacial energy of the system, improve the toughness of epoxy, expand the temperaturerange of materials on application, but also make the epoxy resin resist ultraviolet andatomic oxygen, which help the modified epoxy more suitable to be used as space materials,thus expanding the application field of epoxy resin.In this paper, polyimide (PI) with different content of silicon was used asmodifier, diglycidyl ether of bisphenol A (DGEBA) as raw material, methyltetrahydrophthalic anhydride (MTHPA) as the curing agent in the presence of accelerator N, N-dimethyl benzyl amine (DMBA) to prepare epoxy blends with excellent toughness.First, the phase diagram of the system without curing agent by thermally induced phaseseparation and the phase diagram of the system with curing agent by reaction inducedphase separation were obtained via testing the cloud point temperature using opticalmicroscope. The phase diagram of the system in two conditions all show the upper criticalsolution temperature (UCST). By contrast the phase diagram by thermally induced phaseseparation with the phase diagram by reaction induced phase separation, it is found that thetwo critical components were very close and they are all within the ratio of polyimide inepoxy20wt%-30wt%. The phase diagram curve of the system without curing agentappeared in the low-temperature region, while the phase diagram curve of the system withcuring agent tended to move to the high-temperature region. According to the result ofepoxy molecular weight tested by GPC, the molecular weight of cured epoxy was higherthan the molecular weight of uncured epoxy and it caused the phase separation temperatureof the system increasing. Using optical microscope to monitor the process of phaseseparation, phase separation take place in the system with different component and allfollow the spinodal decomposition mechanism. Three phase structures form: dispersedphase, bicontinuous phase and inversion phase after their morphology development andcoarsening. According to the final phase structure of the system observed by opticalmicroscope, the continuous phase structure dominated in the system when the PI content isbetween20wt%and30wt%and part of the system appeared inversion phase structure withhigher PI content. After the thermal gravimetric analysis of the system with the PI contentbetween20wt%and30wt%, it is found that the higher the silicone content, the higherresidual of the system the degradation temparature increased with the PI content too. Thefracture energy of the materials has also been measured which could be used tocharacterize the toughness, and the fracture energy increased from0.7for unmodifiedepoxy to2.4for modified system. All the experimental results above show that themodifier polyimide-siloxane can greatly improve the toughness of epoxy materials with theoriginal excellent performance.