High Performance Studies On Hyperbranched Polyborates Modified Phenolic Resins

High performance thermosets have been received abiding attention since the last century. Many of their favorable properties, including high glass transition temperature, high modulus are directly related to the underlying microstructure of high crosslinking density and rigid molecular network. However, this structure feature also results in an inherent brittleness of the materials. Different methods have been carried out in the past decades to increase the toughness of high performance thermosets, the difficulty lies in how to improve the toughness without sacrifice to such desired properties as modulus and glass transition temperature. Hyperbranched polymers offer a promising alternative for the toughening of high performance thermosets. Because of their highly branched molecular architecture, hyperbranched polymers have such unique physical and chemical properties as good rheology and solubility, lower solution and melt viscosity when compared to linear analogs. At presently, there are three main problems on the research of hyperbranched polymers: (1) The scarcity of the monomers made the bulk synthesis approach especially important from the view point of functional research and commercial applications. (2) Hyperbracnhed polymer has a broader polydispersity and a lower structural controllability in comparison with that of linear polymer. (3) The functionality of the end groups of hyperbranched polymer is in its infancy and there are may undiscovered superiorities. In this paper, we present such an approach to toughening phenolic resin with model hyperbranched polyborates(HB) that were synthesized with readily available monomers and have good thermal stability and scalability. Through characterization and study on the structure of HB and the curing process of blends of HB modified phenolic resins, we obtained the following results.1. Novel hyperbranched polyborates(HB) terminated with phenolic hydroxyl(HBp) and boric hydroxyl(HBb) functional end groups were successfully prepared via an A 2 +B3 method from the easily available reagents of 1,3-benzenediol and boric acid catalyzed with iron trichloride. The polymerization could be promoted by azeotropic distillation of water with benzene or xylene at 90℃and 165℃, respectively. It was found that the three stages synthesis process that is the polymerization was undertaking at 90℃, 165℃and 202℃for 8h, respectively was an ideal method both to decrease polydispersity and to suppress cyclization. HB have significantly higher glass transition temperatures(Tg> 220℃) and good solubility in a variety of common organic solvents. Thermogravimetric analysis exhibited the thermal stability of HB with T5 %of 428℃and 445℃and weight residues of 74.2% and 71.3% at 800℃in nitrogen atmosphere for HBp and HBb, respectively.2. HB modified phenolic resins were prepared by blending HB with phenolic resin in acetone solvent. Both Fourier transformed infrared spectrum and differential scanning calorimetry analysis revealed that the blends have good compatibility. It was found that HB modified phenolic resins have both a lower initial curing temperature and curing velocity in comparison with phenolic resins due to the dilute effect of HB. The T5 %s and the weight residues at 800℃under nitrogen atmosphere for HBp and HBb modified phenolic resins were improved 38℃, 59℃and 9.4%, 11.1%, respectively. The carbonization process strudy revealed that the improved thermal stability was due to the formation of B4C more than the boron acceleration of graphitization. Though it is difficult to accurately clarify and quantify the different states of boron atoms due to the lower boron content(0.4~0.8%), the presence of such a small amount of boron atom, different from that of physical mixing boron atoms, has great effect on enhancement of thermal stability and crystalline size of carbonized PR. The fracture surface study of HB modified phenolic resins revealed that the crack pining, the instantaneous in situ tropism and the multiple cracks are the main mechanisms that contributed to the improvement of the toughness. It was also found from the HB modified phenolic resins/carbon cloth composites that the fractures were the synergy of both the interfaces destroy and the resins destroy.3. The ring-opening temperature of polybenzoxazine was decreased because of the catalysis of phenolic hydroxyls of HB. Whereas the new phenolic hydroxyls produced by the ring-opening of benzoxazine promoted the ring-opening again. Though, the pyrolysis of phenoxy bonds made both T5 % and T1 0%of HB modified polbenzoxazine decreased to some extent, the weight residues of HB modified polybenzoxazines at 800℃under nitrogen atmosphere were improved in that the increased crosslinking density prevented the volatilization of amine of the cured product. On the other hand, boron atoms can form a protect coating on the surface of the carbonized materials, which can restrain the pyrolysis. The dynamic mechanical analysis revealed that the toughness was improved without the sacrifice of modulus and glass transition temperature. Morphologies of polybenzoxazine became two separated phases after adding HBb. The branched structures of HB not only can inhibit the propagation of the cracks but also can contribute to the transfer of stress, which jointly resulted an increased toughness by avoiding the local destroy.4. The"temperature field"and the"concentration flow"were built up by using the"field-flow"method. The obtained relation between both curing degree and velocity with time provide basis information for the control of phase structures. It was found that the initial curing temperature of 160℃was ideal for the enhancement of the toughness under isothermal curing conditions. While under anisothermal conditions, that's the samples were cured under 120℃, 160℃and 220℃for 4h, respectively, the morphologies were benefit for the increase of toughness. Moreover, the reactive feature of end groups, the core-shell structure and a numerous branched chains of HB can bridge and pin the cracks which were main toughening mechanisms of the unique hyperbranched structure.