Preparation and Characterization of High Frequency Low Dielectric Benzoxazine Resin
Author:Xu Qing Yu
Supervisor:yan chun jie
The twenty-first century is symbolized by the cloud computing,internet of things,and big data.With the development of the next generation high frequency and high speed integrated circuits,the low dielectric thermosetting resins are urgently required.Polybenzoxazines have received considerable attention in the field of the new generation high frequency and high speed integrated circuits,especially microwave communication system.Polybenzoxazines have a number of unique properties such as excellent mechanical properties,high char yield,near zero volumetric shrinkage/expansion upon polymerization,low water absorption,excellent resistance to chemicals and UV light,and high T_g even with rather low cross-link density.In particular,the excellent dielectric property with the relatively low and stable dielectric constant(D_k)value under the high frequency makes benzoxazines a good candidate for the next generation microelectronic industry.Typically,the dielectric constant of polybenzoxazines is approximately 3.5 at 1 MHz that is not satisfied with the requirement of microwave communication.Therefore,the design and preparation of the novel low dielectric benzoxazines and nanocomposites to meet the improved requirement is a stimulating fundamental field.The main contents of this thesis include the preparation and research of novel main chain benzoxazine copolymer and nanocomposites,characterization of the thermosetting resins by using advanced apparatus and new methods,and study the relationship among preparation method,chemical structure,curing behavior and high frequency dielectric property.Moreover,the mechanisms of both the effects of the solvent on the formation of main chain benzoxazine copolymer and carboxyl functional groups of functionalized graphene on the intermolecular interactions between benzoxazine and functionalized graphene are clarified.The creative achievements of this thesis are as follows.(1)The toluene/DMF heterogeneous solvent is firstly introduced to synthesize the main chain benzoxazine copolymer,and plays a key role in the preparation of aliphatic main chain copolymer oligomers.The mechanism of the solvent on the formation of main chain benzoxazine copolymer is investigated.(2)Novel benzoxazine/functionalized graphene nanocomposite resins are prepared via in situ intercalative polymerization.The effects of carboxyl functional groups of functionalized graphene on the curing behavior,intermolecular interactions and high frequency dielectric property of benzoxazine composites are clarified.The intermolecular interaction mechanism between benzoxazine and functionalized graphene is assumed.The main research contents are divided into the following parts.The novel mainchain benzoxazine copolymers are synthesized from aromatic or aliphatic difunctional diamine,aniline and bisphenol-A with paraformaldehyde through the Mannich reaction.The toluene and toluene/ethanol are employed as the nonpolar solvent and polar/nonpolar heterogeneous solvent,respectively,to clarify their effects on the chemical structure and high frequency dielectric property of main chain benzoxazine copolymers.The results demonstrate that aniline not only takes part in the formation of copolymer structure,but also lowers the molecular polar and molecular weight of precursor in comparison with those of main chain benzoxazines.Moreover,the present polar solvent has limited influence on the preparation of aliphatic main chain copolymer oligomers;on the contrary,it really plays a key role in the preparation of aromatic main chain copolymer oligomers.The polar solvent is assumed to not only decrease the crosslinking density of triazine network,but also reduce the reaction rate of the formation of oxazine groups,resulting in relatively lower molecular weight and higher purity of aromatic main chain copolymer oligomers.Accordingly,the aromatic main chain copolymers prepared in the optimized condition with toluene/ethanol as solvent occupy the minimum high frequency dielectric constant values(2.81,5 GHz;2.75,10GHz)and comparative high T_g(260℃).The toluene/DMF heterogeneous solvent is firstly introduced to synthesize the main chain benzoxazine copolymer based on 1,6-hexamethylendiamine and aniline.The effects of reaction solvents on chemical structure and high frequency dielectric property of prepared benzoxazines are investigated.FTIR and ~1H-NMR results prove that the formed chemical structure is a main-chain type benzoxazine copolymer prepolymer terminated by aniline.The curing behavior is investigated by non-isothermal DSC,and the apparent activation energy and reaction order are calculated.The benzoxazine copolymer prepolymer synthesized in toluene/DMF has more oxazine rings which needs more activation energy,and has higher glass transition temperature and lower dielectric constant than those prepared in other systems.All characterization results indicate that toluene/DMF as solvent is more suitable for the synthesis of benzoxazine copolymer prepolymer.A series of main chain benzoxazine copolymers based on 1,6-hexamethylendiamine and aniline are prepared in the toluene/DMF heterogeneous solvent.The different functional group molar ratios of 1,6-hexamethylendiamine and aniline are chosen to clarify their effects on chemical structure,curing behavior and high frequency dielectric property of prepared benzoxazine copolymers.With the decrease of aniline content,the benzoxazine copolymer prepolymers occupy the higher molecular weight and DPI values,suggesting the reduced structure regularity.When the functional group molar ratio of 1,6-hexamethylendiamine and aniline is 5:5,the main chain benzoxazine copolymer prepolymer has a significantly decreased PDI value(2.63),implying the increased structure regularity and purity.Accordingly,the main chain benzoxazine copolymer with the same the functional group molar ratio owns the least dielectric constant value(2.65,5 GHz),resulting from the increased structure regularity and purity,and the reduced molecular polar.Two series of benzoxazine based composites are prepared with graphene oxide and graphite via in situ intercalative polymerization.Based on isothermal and non-isothermal mode Differential Scanning Calorimetry results,benzoxazine with the addition of graphene oxide could not only decrease the cure temperature,but also increase the cure rate of benzoxazine.On the contrary,benzoxazine with the addition of graphite would delay the polymerization of benzoxazine monomer.It is hypothesized that carboxyl groups of graphene oxide act as weak organic acid which accelerate the ring opening procedure.Interestingly,benzoxazine/graphene oxide composite with 1wt%graphene oxide occupys the least time to form polymer networks,attributing to the catalytic effect and good dispersion of graphene oxide.Moreover,benzoxazine with 1wt%content of graphene oxide has relatively higher glass transition temperature and char yield,and low dielectric constant than those of the corresponding benzoxazine/graphite sample,which is due to relatively stronger intermolecular interactions between graphene oxide and benzoxazine.Novel benzoxazine/carboxylated graphene oxide composites are prepared via in situ intercalative polymerization.The curing behaviour,morphology and intermolecular interactions of carboxylated graphene oxide based nanocomposites are investigated and compared with those of a graphene oxide blend system to clarify the influence of carboxylic groups.Compared to graphene oxide,carboxylated graphene oxide with a large amount of carboxylic groups,relatively higher thermal stability,and exfoliated sheet morphology might be more easily disperse and react in the benzoxazine matrix.The carboxylated graphene oxide nanoplatelet based composites possess a different polymerization path from that of the graphene oxide based system,implying that carboxylic groups not only provide catalytic effects but also participate in the grafting polymerization reactions between carboxyl groups of carboxylated graphene oxide and phenolic hydroxyl groups of benzoxazine.A significant improvement of both the glass transition temperature and crosslinking network density of the carboxylated graphene oxide blend system further confirms that covalent bonding occur between filler and polymer chains,indicating that the carboxylated graphene oxide nanoplatelets have a stronger influence on the thermal property improvement of the nanocomposites than that of the graphene oxide blend system.Surprisingly,a very low amount(1 wt%)of carboxylated graphene oxide can affect the thermal properties of the composite remarkably,leading to a more than 30℃increase of T_g in comparison with pure benzoxazine,and the dielectric constant decrease from 3.53 to 2.87(5 GHz).The novel main chain benzoxazine copolymers and nanocomposites are designed and synthesized in this thesis.The relationship between structure and high frequency dielectric property is also clarified.The results of the researches mentioned above provide not only the effective paths for improving processing and high frequency dielectric properties of benzoxazine resins,but also the new way to the exploitation and utilization of benzoxazine.It is worth noting that the novel benzoxazine and nanocomposites combined with processing,thermal and high frequency dielectric properties merits could find some potential applications in microwave communication,aerospace,printed circuit board,and membrane.