| Cyanate ester resins are a kind of high-performance resins with outstanding integrated properties. They possess good thermal resistance, very low dielectric constant and dielectric loss tangent, low moisture absorption, small thermal expansion coefficient and no low molecular precipitation during the curing process, etc. Especially, the dicyclopentadiene groups in dicyclopentadiene bisphenol dicyanate esters(DCPDCE) possess excellent dielectric and water-resistant properties, which can further reduce the dielectric constant and loss as well as water absorption of cyanate ester resins. So the DCPDCE resins have more competitiveness than others in aerospace and electronic fields. But the cured DCPDCE resin contains high cross-linked network, which makes the cured system very brittle. In recent years,many methods have been developed to modify cyanate ester resins and the fracture toughness can be improved dramatically. However, the improvement of mechanical properties is generally based on decreasing the other properties of original cyanate ester resins, such as dielectric and thermal properties, which largely reduces its application value in industry.Therefore, there is a great interest to develop a new method to overcome the key shortcomings of original cyanate ester resin at the same time without declining its original outstanding properties. Graphene oxide has special two-dimensional molecular structure and excellent mechanical, thermal and electrical properties. The poor dispersion of graphene oxide in the polymeric hosts and interfacial interaction are the main problems to use graphene oxide in nanocomposites. Hyperbranched polymers have low intrinsic viscosity, high heat resistance,high rheological variation and extremely high density of functional groups, which offers a new way to improve dispersion of graphene oxide in matrix and control interfacial interaction between graphene oxide and matrix. Therefore, several types of hyperbranched polymers grafted graphene oxide sheets were designed and synthesized, and then the functionalized graphene oxide sheets were used to modify DCPDCE resin with the aim to toughen DCPDCE matrix and achieve excellent thermal and dielectric properties of DCPDCE resin. The main research work of this dissertation was illustrated as follows:1. The hyperbranched polysiloxane with epoxy groups functionalized graphene oxide(HPE-GO) was synthesized by the dehydration condensation reaction. Subsequently, the resultant HPE-GO was incorporated into DCPDCE resin to prepare composites employing the method of melting mixing. The addition of HPE-GO can improve the impact and flexural strengths, thermal stability and moisture resistance as well as dielectric properties. Themechanism can be summarized as two points. On one hand, the hyperbranched polysiloxane grafted on the surface of graphene oxide nanosheets can restrain the restacking of graphene oxide nanosheets in matrix, On the other hand, the hyperbranched polysiloxane with epoxy groups were expected to form the chemical bonding between graphene oxide and DCPDCE resin, thus increasing the interfacial interaction.2. The hyperbranched polysiloxane with vinyl groups functionalized graphene oxide(HPD-GO) was synthesized by the hydrosilylation reaction, and the effects of HPD-GO on the mechanical, dielectric, thermal and water resistent properties of DCPDCE resin were investigated. The results show HPD-GO has higher modification efficiency than HPE-GO.When HPD-GO is 0.6 wt%, the impact and flexural strengths of corresponding composites were increase by 69 % and 51 %, respectively. The increases in mechanical properties can be understood from the higher grafting rate of HPD-GO, which can greatly exfoliate the graphene oxide nanosheets layers, thus improving the dispersion of graphene oxide in matrix.Moreover, there exist more reactive groups in the molecule of HPD-GO, which can react with DCPDCE resin. As a result, the interfacial bonding strength was further improved. In the fracture process, the homogeneously dispersed graphene oxide can effectively distribute stresses. The increases in water resistance properties can be attributed to the good barrier effect of HPD-GO. The thermogravimetric analysis results show that the HPD-GO/DCPDCE composites have higher initial thermal decomposition temperature and char yields, this is because the “bend effect†of graphene oxide can prevent degradation products volatile escape.Therefore, the thermal stability of composites is improved.3. The hyperbranched cyclotriphosphazene polymer functionalized graphene oxide(HPC-GO)was synthesized by the repeated grafting reactions. Subsequently, the resultant HPC-GO was incorporated into DCPDCE resin to prepare composites. The XPS, FTIR, XRD and TEM results show that the hyperbranched cyclotriphosphazene polymer has been successful grafted on the surface of graphene oxide, and the graphene oxide has been reduced to some extent,which results in the different dielectric properties of the resultant composites. The dielectric constant is slightly higher, but the dielectric loss is lower. The impact and flexural strengths of the corresponding composites were lower than that of HPD-GO/DCPDCE composite, which may be attributed to the fact that the flexibility of P-N cyclic ring in hexachlorotriphosphazene is worse than that of Si-O-Si chains of hyperbranched polysiloxane.In addition, the thermal stability and moisture resistance of HPC-GO/DCPDCEnanocomposties are superior to that of pure DCPDCE resin.4. A hyperbranched polyhedral oligomeric silsesquioxanes(POSS) polymer functionalized graphene oxide was synthesized by nucleophilic substitution reaction. Subsequently, the resultant HPP-GO was used to modify DCPDCE resin. The-NH2 groups in the molecule of HPP-GO can react with-NCO in DCPDCE, leading to lower cure temperature and activation energy. Also, the HPP-GO/DCPDCE composites exhibit significantly improved mechanical properties, higher heat resistance, lower dielectric constant and loss and better moisture resistant. The increases in mechanical properties can be understood from the fact that the POSS cage structure grafted on the surface of graphene oxide possesses excellent properties and steric hindrance effect. The dielectric constants and losses are decreased because the POSS chains in HPP-GO possess hollow structure and low polarity. In addition, the thermal stability of modified system has also been improved because the HPP-GO possesses highly steric hindrance and the interfacial bonding strength between fillers and matrix is very strong.5. The interface bonding performances of HPE-GO/DCPDCE, HPD-GO/DCPDCE,HPC-GO/DCPDCE and HPP-GO/DCPDCE composites were studied by dynamic mechanical thermal analysis, and the effects of interfaces on the mechanical, dielectric and thermal properties of the composites were also discussed. A view was put forward that there exist interface layers of covalent connection and soft hyperbranched polymers. The covalent connection is the basis to improve the interface bonding strength, while the hyperbranched polymers layers play the role of toughening, cushion and other more complex ones. The final properties of the composites will depend on the two kinds of interfaces. |
