Molecular Design And Properties Of Hyperbranched Benzoxazine Polymers

As an important component of composites,polybenzoxazines have been applied in aerospace and electronic packaging due to their high thermal and dimensional stability,low surface free energy and dielectric constant,good water resistance and flame retardancy,as well as excellent mechanical properties and flexible molecular design.However,the high curing temperature and brittleness of benzoxazine have limited its wider application.In this paper,a variety of triamine-diphenol and triphenol-diamine hyperbranched benzoxazine polymers and benzoxazine capped hyperbranched polyester were first designed and synthesized via A₂+B₃and AB₂ methods by using abundant petrochemical and bio-based raw materials.The hyperbranched polymers with specific functions were selected as modifiers to improve the flame retardancy or reduce the dielectric constant of epoxy resins while toughening them.A series of triamine-diphenol type hyperbranched benzoxazine polymers（HB-TAs and HB-TAFs）were synthesized by Mannich condensation reaction using bisphenol A（or bisphenol AF）,Jeffamine T-403 and paraformaldehyde as raw materials.HB-TAs and HB-TAFs possessed high weight-average molecular weights（M_w）in the range of 9700-11700 Da,and performed good solubility in commonly used polar solvents.Compared with typical bisphenol A-aniline and phenol-diaminodiphenylmethane polybenzoxazines,the triamine-diphenol hyperbranched polymer films exhibited better toughness,mainly due to the introduction of a large number of flexible alkyl ether chains,but also resulted in reduced thermal stability of the cured products.A series of trisphenol-diamine hyperbranched benzoxazine polymers（HB-TSis）were synthesized via the A₂+B₃ method by using 1,3-bis（3-aminopropyl）tetramethyldisiloxane（BATSi）,1,1,1-tris（4-hydroxyphenyl）ethane（THPE）and paraformaldehyde as raw materials.The causes of hyperbranched benzoxazine gelation phenomenon were analyzed for the first time,and the corresponding solutions to prevent it were proposed.The results of solubility experiments showed that,prolonging the reaction time or increasing the molar ratio of hydroxyl to amine groups are both beneficial to reduce or even avoid obtaining the gelation products.In most cases,increasing the reactant concentration can also play the same role.However,a lower reactant concentration was more likely to avoid the formation of gels when the molar ratio of hydroxyl to amino groups was 2:3.A sufficiently high vacuum was necessary to avoid the formation of polybenzoxazine gels during the drying of the samples.These results are instructive for the synthesis of other hyperbranched benzoxazines.In addition,the gelation-free HB-TSis exhibited high M_w of 6100-10100 Da.Their cured products,poly（HB-TSi）s,both performed well in terms of thermal stability with high char yield（Y_c）varying from 37.9 to56.2%,and flame retardancy with low the heat release capacity（HRC）varying from 49.9 to81.9 J/（g·K）,which can be used as non-combustible materials.Using p-phenylenediamine（PPA,replacing aliphatic diamine）,paraformaldehyde and THPE as raw materials,two new aromatic hyperbranched benzoxazine polymers were designed and synthesized via tuning the reaction time of the capping reagent（phenol）,namely HB-Tppa1（phenol added later）and HB-Tppa2（phenol added first）.The molecular weight of HB-Tppa2was smaller than that of HB-Tppa1 because the addition of phenol at the beginning of the reaction would limit the further growth of the polymer chain.Moreover,the results of differential scanning calorimetry（DSC）confirmed that the mass fraction of the benzoxazine ring for HB-Tppa1 is higher than that of HBTppa2.As a result,the thermal and flame retardant properties of poly（HB-Tppa1）were better than those of poly（HB-Tppa2）.Overall,in addition to the high glass transition temperature（T_g,≥320℃）,poly（HB-Tppa1）and poly（HB-Tppa2）also exhibited lower HRC values of 41.5 and 50.5 J/（g·K）,respectively,indicating that they can be used in flame retardant applications.Fully bio-based benzoxazines are attractive thermosetting resins,but the limited variety of raw materials is a serious challenge for their further development.Herein,a new sustainable hyperbranched benzoxazine polymer（namely HB-Rdfda）was first designed and synthesized using resveratrol,difurfuryldiamine derivative（5,5’-methylene-difurylamine,DFDA）and paraformaldehyde.Poly（HB-Ra）exhibited excellent thermal stability with a thermal decomposition temperature at 5%weight loss（T₅）of 374℃,a T_g of 348℃,and a ultralow coefficient of thermal expansion（CTE）of only 8.4 ppm/℃.In addition,the HRC value of poly（HB-Rdfda）was 39.6 J/（g·K）,meaning excellent flame-retardant performance.Hyperbranched polymers contain a large number of terminal reactive groups that can be post-functionalized by chemical modification.Therefore,in this paper,a hyperbranched polyester with abundant terminal hydroxyl structures was synthesized at room temperature via the AB₂ method using bio-based bisphenolic acid as raw material,N,N’-dicyclohexylcarbodiimide（DCC）as coupling agent,and 4-（N,N-dimethylamino）pyridinium p-toluenesulfonate（DPTS）as catalyst.A fully bio-based benzoxazine-capped hyperbranched polyester（HB-Dfa）was synthesized by adding furfurylamine and paraformaldehyde.HB-Dfa containing flexible alkyl chains and hyperbranched polyester structures exhibited good solubility in commonly used polar organic solvents.The dielectric constants（k）and dielectric loss（f）values of poly（HB-Dfa）ranged from 2.13 to 2.18,and from 0.0010 to 0.0232,respectively,in the frequency range of 2-18 GHz,which was significantly lower than that of conventional polybenzoxazine,commercial bismaleimide and epoxy resins.Selecting HB-TSi8 with good flame retardancy and HB-Dfa with low k value as modifiers,EP/TSi and EP/Dfa resin systems were prepared with epoxy resin（EP）as the matrix,respectively.Both HB-TSi8 and HB-Dfa could toughen the EP resin,and the toughening effect of HB-Dfa on EP resins was more significant.Compared with the pure EP resin,the elongation at break and impact strength of EP/Dfa-9%were increased by 52.0%and 120.0%,respectively.Scanning electron microscope（SEM）reveled that there is no phase separation in the EP/TSi and EP/Dfa resins,and the characteristic fracture morphology of in situ toughening was observed.Additionally,HB-TSi8 was able to enhance the flame retardancy of the EP resin,but the improvement was not significant.In contrast,HB-Dfa was able to effectively reduce the k value of EP resin.