Benzoxazine-bismaleimide blends: Curing and thermal properties

A blend of bisphenol A based benzoxazine (Bz-A) and a bismaleimide (2,2-bis[4(4-maleimidophenoxy) phenyl] propane (BMI), was thermally polymerised in varying proportions and their cure and thermal characteristics were investigated. The differential scanning calorimetric analysis, supplemented by rheology confirmed a lowering of the cure temperature of BMI in the blend implying catalysis of the maleimide polymerisation by benzoxazine. FTIR studies provided evidences for the H-bonding between carbonyl group of BMI and -OH group of polybenzoxazine in the cured matrix. The cured matrix manifested a dual phase behaviour in SEM and DMTA with the minor phase constituted by polybenzoxazine dispersed in an interpenetrating polymer network (IPN) of polybenzoxazine and cured BMI. The IPN possessed improved thermal stability over the constituent polybenzoxazine. A benzoxazine monomer possessing allyl functional groups, 2,2′-bis(8-allyl-3-phenyl-3,4-dihydro-2H-1,3-benzoxazinyl) propane (Bz-allyl) was reactively blended with the same bismaleimide in varying stoichiometric ratios (Bz-allyl/BMI), where the curing involved mainly Alder-ene reaction between allyl- and maleimides groups and ring-opening polymerisation of benzoxazine. The rheological analysis showed the absence of catalytic polymerisation of BMI in this case. The overall processing temperature was lowered in the blend owing to the co-reaction of the two systems to form a single-phase matrix. The cured resins of both Bz-A/BMI and Bz-allyl/BMI blends exhibited better thermal stability than the respective polybenzoxazines. The T_g of the IPN was significantly improved over that of polybenzoxazine (Bz-A). However, the co-reaction resulted in a marginal decrease in the T_g of the system in comparison to the polybenzoxazine (Bz-allyl).     

Thermal degradation characteristics of polysulfones with benzoxazine end groups

Thermal degradation behaviors of phenol and benzoxazine end-capped polysulfone macromonomers (PSU-OH and PSU-P-a) and pre-cured PSU-P-a in the absence and presence of aniline and phenol based benzoxazine monomer (P-a) were investigated via pyrolysis mass spectrometry. A significant increase in thermal stability of both polysulfone and polybenzoxazine chains upon polymerization of benzoxazine end groups was determined compared to phenol-ended polysulfones and aniline based monofunctional polybenzoxazine. Thermal stability of both chain segments depends on concentration of benzoxazine monomer and the chain length of the polysulfone chain.     

Synthesis, curing kinetics and thermal properties of bisphenol-AP-based benzoxazine

A novel bisphenol-AP-aniline-based benzoxazine monomer (B-AP-a) was synthesized from the reaction of 4,4′-(1-phenylethylidene) bisphenol (bisphenol-AP) with formaldehyde and aniline. The chemical structures were identified by FT-IR, ¹H and ¹³C NMR analyses. The polymerization behavior of the monomer and the types of hydrogen bonding species were monitored by differential scanning calorimetry (DSC) and FT-IR. The curing kinetics was studied by isothermal DSC and the isothermal kinetic parameters were determined. The thermal properties of cured benzoxazine were measured by DSC and thermogravimetric analysis (TGA). The bisphenol-AP-aniline-based polybenzoxazine (poly(B-AP-a)) exhibited higher glass transition temperature (T_g) and better thermal stability than corresponding bisphenol A-aniline-based polybenzoxazines (poly(BA-a)). The T_g value of poly(B-AP-a) is 171 °C. The temperatures corresponding to 5% and 10% weight loss is 317 and 347 °C, respectively, and the char yield is 42.2% at 800 °C. The isothermal curing behavior of B-AP-a displayed autocatalysis and diffusion control characteristics. The modified autocatalytic model showed good agreement with experimental results.     

Porous structure of polybenzoxazine-based organic aerogel prepared by sol–gel process and their carbon aerogels

New organic aerogels were successfully prepared from a new class of phenolic resins called polybenzoxazines synthesized via conventional thermal curing reaction of a benzoxazine monomer using xylene as a solvent. Without the need for using supercritical conditions to remove the solvent during the process, the carbon aerogels were obtained with a much shortened time. From two different concentrations of benzoxazine solution, 20 and 40 wt%, the resulting polybenzoxazine aerogels, having densities of 260 and 590 kg/m³, respectively, were obtained after the curing process. The subsequent carbon aerogels were prepared by the carbonization of polybenzoxazine aerogels. The corresponding carbon aerogels exhibited a microporous structure with pore diameters less than 2 nm, the densities of 300 and 830 kg/m³, and surface area of 384 and 391 m²/g, respectively. The texture of the carbon aerogels was denser than that of their organic aerogel precursor, as evidenced by scanning electron microscopy. The transformation of the polybenzoxazine aerogel to the carbon aerogel was clearly observed using fourier transform infrared spectroscopy.     

Polybenzoxazine–silica (PBZ–SiO2) hybrid nanocomposites through in situ sol–gel method

New type of Polybenzoxazine–silica (PBZ–SiO₂) hybrid nanocomposites was prepared through in situ sol–gel method. Benzoxazine was synthesized using bisphenol-A, trans-4-aminocyclohexanol hydrochloride and formaldehyde solution through Mannich condensation reaction and was characterized by FT-IR, ¹HNMR and ¹³CNMR spectroscopy. The methodology adopted in the present study involves to formation of hydrogen bond interaction between the benzoxazine monomer and the silica matrix, followed by the ring opening polymerization of benzoxazine monomer through thermal curing to obtain a red brown transparent PBZ–SiO₂ hybrid. The formation of hybrid nanocomposites was confirmed by FT-IR. Thermal and morphological properties of the hybrid materials were investigated by the differential scanning calorimetry (DSC), thermo gravimetric analysis (TGA), scanning electron microscopy (SEM). The PBZ–SiO₂ hybrids show improved thermal properties and glass transition (Tg) temperature. The nitrogen porosimetry study was carried out to confirm the nanometer level integration of polybenzoxazine in the PBZ–SiO₂ hybrid nanocomposites.     

Studies on the mesophase formation mechanism of polybenzoxazines: polymerisation induction

ABSTRACT

To study the mesophase formation mechanism of polybenzoxazine, a novel linear benzoxazine oligomer bearing cholesteryl side groups [poly(PC-AC)] was designed and synthesised through thermally activated ring-opening polymerisation of a monofunctional benzoxazine monomer (PC-AC). The PC-AC was obtained by Mannich condensation reaction using mesomorphic amine of cholesteryl 4-aminobenzoate, p-cresol and paraformaldehyde as starting materials. During the isothermal polymerisation of PC-AC monomer, the phase evolution from a crystal phase to an isotropic molten phase and then to a liquid crystal (LC) phase was observed. Since it is PC-AC oligomers that form the LC phase, ‘polymerisation-induced LC’ mechanism is put forward. We believe that the structure factors including the confined formation of intramolecular hydrogen bonding and the side chain position of mesogenic units also play an important role in the formation of the LC phase. Furthermore, poly(PC-AC) exhibits a smectic mesophase. This work provides new insight into the LC phase formation mechanism of polybenzoxazines. This is very helpful to guide the rational design and synthesis of polymers with high thermal conductivity and high-temperature resistance.     

Synthesis and properties of new polybenzoxazines containing (substituted) cyclohexyl moieties

A series of new polybenzoxazines were synthesized based on diphenols containing (substituted) cyclohexyl moiety and were characterized by FT‐IR, ¹H‐NMR, and ¹³C‐NMR spectroscopy. These new benzoxazine monomers exhibited better processability with lower peak cure temperature and a wide cure controllable window (CCW) as manifested in differential scanning calorimetric analysis. The cure analysis was performed by FT‐IR spectroscopy. Glass transition temperature of new polybenzoxazines varied from 170 to 205°C. The cyclohexyl bridge groups facilitated ring opening, resulting in polymer with improved thermal stability in comparison to bisphenol A‐based benzoxazine as assessed by the various thermal analyses. The water contact angles of polybenzoxazines containing (substituted) cyclohexyl moieties were higher than that of bisphenol A‐based polybenzoxazine, implying their higher hydrophobicity. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis and characterization of polybenzoxazine containing phosphorus

The novel benzoxazine monomer containing phosphorus has been synthesized based on multifunctional amine route from bis(4-aminophenyl)phenylphosphate,p-cresol and formaldehyde.Subsequently,the benzoxazine monomer was thermo-cured into polybenzoxazine containing phosphorus.The chemical structures were identified by nuclear magnetic resonance(NMR),Fourier transform infrared spectroscopy(FT-IR).The curing reaction was monitored by differential scanning calorimetry(DSC) and FT-IR.The thermal and flame-retardant properties of obtained polybenzoxazine were evaluated by dynamic mechanical thermal analysis(DMA),thermal gravimetric analysis(TGA) and oxygen index meter, respectively.The results show that the novel polybenzoxazine has high limiting oxygen index(38.1) and glass transition temperature(232℃).     

Optical,electrochemical, and thermal behavior of polybenzoxazine copolymers incorporated with tetraphenylimidazole and diphenylquinoline

Two new polybenzoxazine copolymers were synthesized by polymerizing conventional benzoxazine monomer with varying weight percentage of tetraphenylimidazole and diphenylquinoline. The tetrasubstituted imidazole was synthesized through Debus‐Radziszewski imidazole synthesis method, and quinoline derivative was synthesized through Friedlander quinoline synthesis, and their structure was confirmed through FTIR, ¹HNMR, and MASS spectral analysis. New polybenzoxazine copolymers were synthesized by polymerizing conventional benzoxazine monomer with varying weight percentage of (10, 20, and 30%) of phenolic tetraphenylimidazole and diphenylquinoline. The polybenzoxazines cocured with 10, 20, and 30 wt% of imidazole derivative showed a band gap of 2.27, 2.08, and 2.2 eV, respectively, and the quinoline derivative incorporated at 10, 20, and 30 wt% in to polybenzoxazines exhibited a band gap of 2.26, 2.3, and 2.03 eV, respectively. The polybenzoxazines cocured with phenolic imidazoles and quinolines have high glass transition and thermal degradation stability in addition to significant improvement in optical and electrochemical properties than that of conventional bisphenol‐based polybenzoxazines.     

Thermal behaviour of cardanol-based benzoxazines

A novel benzoxazine monomer (Bz-C) based on agrochemical renewable resource—cardanol (by-product of cashew nut tree, Anacardium occidentale) was synthesized. Bz-C, a liquid monomer, was used as reactive diluent for the solventless synthesis of bisphenol-A benzoxazine monomer (Bz-A). Benzoxazine monomer based on cardanol and bisphenol-A in 3:1, 1:1 and 1:3 blend ratio were prepared by this method. The resins had Brookfield viscosity at 316 K in the range of 145–81,533 mPa s. The resins were characterized by ¹H-NMR, FTIR and elemental analysis. Curing characteristics were studied by DSC analysis. Thermal stability of cured resins was found to improve with increase in Bz-C content in the blends.     

Synthesis,polymerization, and thermal properties of benzoxazine based on bisphenol‐S and allylamine

A bifunctional benzoxazine monomer, 6,6′‐bis(3‐allyl‐3,4‐dihydro‐2H‐benzo[e][1,3]oxazinyl) sulfone (BS‐ala), was synthesized from bisphenol‐S, allylamine and formaldehyde via a solution method. The chemical structure of BS‐ala was confirmed by ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy, Fourier transform infrared (FTIR) spectroscopy, and elemental analysis. The polymerization behavior of BS‐ala was investigated by FTIR, solid‐state ¹³C NMR, and differential scanning calorimetry (DSC). The oxazine ring opening polymerization is prior to the addition polymerization of allyl group, and the exothermic peaks corresponding to the two reactions appear partially overlapped in the DSC curve. The storage modulus of the resultant polybenzoxazine at 25°C is about 3.9 GPa, and the glass transition temperature is 254°C. The 5% and 10% weight loss temperatures of the polybenzoxazine are about 335°C and 361°C in both air and nitrogen, respectively. The char yield is about 58% at 800°C in nitrogen, whereas almost no residue is remained at 700°C in air. Copyright © 2012 John Wiley & Sons, Ltd.     

Preparation,characterization, and polymerization of maleimidobenzoxazine monomers as a novel class of thermosetting resins

New series of benzoxazine‐based monomers, namely maleimidobenzoxazines, were prepared with hydroxyphenylmaleimide, formalin, and various amines (e.g., aniline, allylamine, and aminophenyl propargyl ether). The structure of the novel monomers was confirmed by IR, ¹H NMR, and elemental analysis. The monomers were easily dissolved in many common organic solvents. Differential scanning calorimetry of the novel monomers showed exotherms at different temperature ranges that corresponded to the polymerization regime of benzoxazine and maleimide along with other functionalities such as allyl or propargyl, if any. IR was studied to follow the progress of the curing reaction of maleimidobenzoxazine after various thermal treatments. The thermal cure of the monomers at 250 °C afforded a novel network structure that combined polybenzoxazine and polymaleimide. The dynamic mechanical analyses showed that the storage moduli of the thermosets derived from maleimidobenzoxazine were kept constant up to high temperatures. The glass‐transition temperatures were as high as 241–335 °C. Moreover, thermogravimetric analyses revealed that the thermosets did not show any weight loss up to about 350 °C, with char yields ranging from 62 to 70% at 800 °C. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 1424–1435, 2006     

Study of hydrogen bonding and thermal properties of polybenzoxazine and poly‐(ε‐caprolactone) blends

The thermal properties of physical blends containing benzoxazine monomer and polycaprolactone (PCL) were monitored by DSC and Fourier transform infrared spectroscopy (FTIR). The ring‐opening reaction and subsequent polymerization reaction of the benzoxazine were facilitated significantly by the presence of a PCL modifier. Hydrogen‐bond formation between the hydroxyl groups of polybenzoxazine and the carbonyl groups of PCL was evident from the FTIR spectra. Only one glass‐transition temperture (T_g) value was found in the composition range investigated, and the T_g value of the resulting blend appeared to be higher in the blend with a greater amount of PCL. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 736–749, 2001     

Recalculation of monomer reactivity considering the effect of monomer partitioning in microemulsion

From the copolymerisation data for the microemulsion polymerisation of both partially water soluble monomers ethylacrylate (EA)-methylmethacrylate (MMA) the concentration of the monomer at the polymerisation loci was calculated. While the reported copolymerisation data for the microemulsion copolymerisation of styrene (Sty)-methylacrylate (MA), monomer pair with different solubility and polarity and Sty-butylacrylate (BA), monomer pair with similar solubility and different polarity was used to calculate the monomer concentration at loci. It was inferred from the non-constancy of f^′_A/f_A and f^′_B/f_B ratio that the preferential site of polymerisation might be the emulsifier layer in case of Sty-MA and Sty-BA. While, based on the monomer fraction in the copolymer and the constancy of f^′_A/f_A and f^′_B/f_B it was concluded that microemulsion polymerisation for monomer pair EA-MMA, having similar solubility and polarity, conforms more to bulk polymerisation, where there is no preferential site of polymerisation. The loci concentration rather than feed value was used to recalculate the reactivity at the site of polymerisation. Also the loci concentration was calculated assuming their sum at the polymerisation site equal to unity.     

Preparation and properties of novel benzoxazine and polybenzoxazine with maleimide groups

A benzoxazine compound with a maleimide group, 3‐phenyl‐3,4‐dihydro‐2H‐6‐(N‐maleimido)‐1,3‐benzoxazine (HPM‐Ba), was prepared from N‐(4‐hydroxyphenyl)maleimide, formaldehyde, and aniline. The chemical structure of HBM‐Ba was identified by FT‐IR, ¹H‐NMR, and elemental analysis. HPM‐Ba showed a melting point of 52–55 °C and good solubility in common organic solvents. HPM‐Ba showed a two‐stage process of thermal polymerization. The first stage arose from the polymerization of maleimide groups, and the second one was the ring‐opening reaction of benzoxazine groups. Fusible polymaleimides with a Tg of around 100 °C could be obtained by thermally polymerizing HPM‐Ba at 130 °C. Further polymerizing the polymaleimides at 240 °C resulted in a completely cured resin showing a Tg at 204 °C. Good thermal stability and self‐extinguishing behavior was observed with the cured polybenzoxazine resins. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5954–5963, 2004     

Synthesis and thermal decomposition of a novel hyperbranched polyphosphate ester used for flame retardant systems

A novel hyperbranched polyphosphate ester (HPPE) was synthesized via the polycondensation of bisphenol-A as an A₂ monomer and phosphoryl trichloride as a B₃ monomer at 100 °C, without gelation. The initial molar ratio of A₂ to B₃ was set to be 1.5:1. The final product was precipitated from methanol. ³¹P NMR spectroscopy was used to monitor the reaction. The formed HPPE was characterized by FTIR and ¹H NMR to confirm its end groups. Differential scanning calorimetry data revealed that the cured bisphenol-A epoxy resin with HPPE as a curing agent possessed improved glass transition temperature. Dynamic mechanical thermal analysis also showed the increase in the glass transition temperature. The thermal degradation properties and flame retardancy were investigated by thermogravimetric analysis and limiting oxygen index (LOI). The results showed that the incorporation of HPPE into bisphenol-A epoxy resin increased its thermal stability and char yield during the decomposition by raising the second stage decomposition temperature. The LOI value increased from 23 to 31 when HPPE, instead of bisphenol-A, was used as a curing agent.     

Significant enhancement of thermal stability in the non-oxidative thermal degradation of bisphenol-A/aniline based polybenzoxazine aerogel

Previously we reported synthesis of a new type of organic aerogel from phenolic resins called polybenzoxazines and their transformation into carbon aerogels. Here, we further investigate the thermal degradation behaviors of both bulk polybenzoxazines and polybenzoxazine aerogels using Fourier transform infrared spectroscopy (FTIR), thermal gravimetric analysis (TGA)/FTIR, and gas chromatography/time of flight-mass spectroscopy (GC/TOF-MS). The activation energy (E_a) of the decomposition step was determined using the Kissinger method. It was found that the polybenzoxazine aerogels exhibit much higher degradation temperatures and char yields than the bulk. The decomposition temperatures at 10% weight loss and the char yields at 800 °C of the bisphenol-A/aniline based polybenzoxazine aerogel increased up to 24% and 97% higher, respectively, than the corresponding bulk values. Kinetic investigation indicated that the decomposition reaction of bulk polybenzoxazine exhibits three stages, whereas that of the polybenzoxazine aerogel features four stages with much higher overall activation energy. The enhanced thermal stability of the aerogel is ascribed to its highly porous structure, which increases the residence time of the primary decomposition products, and hence generates greater opportunity to form secondary reactions.     

Rheokinetic investigations on the thermal polymerization of benzoxazine monomer

The thermal cross-linking polymerization of bisphenol-A based benzoxazine was monitored at different isothermal curing temperatures by rheological analysis. An autocatalytic kinetic model was proposed for the curing of benzoxazine monomer. Gel point (t_gel) was determined from tan δ maximum and also from intercept of storage and loss modulus and the corresponding activation energy was estimated. All kinetic parameters including reaction orders and kinetic constants were determined. The autocatalytic effect was very significant. A master equation for the cure rate was generated from the kinetic data that was numerically integrated to predict the cure-profile. The theoretical cure prediction matched reasonably well with the experimental results. The high autocatalytic effect was attributed to the presence of phenolic OH groups generated by the ring-opening polymerization; which aids further ring-opening polymerization.     

Investigations on the cure chemistry and polymer properties of benzoxazine-cyanate ester blends

Thermosetting polymer blends composed of bisphenol A based benzoxazine (BA-a) and cyanate ester (BACY) were prepared via co-curing of benzoxazine with cyanate ester. DSC results manifested a multiple curing pattern with associated heat of reaction implying a co-reaction between oxazine moiety and cyanate group. The catalysis during the co-curing of blend was ascribed to the cycloaddition reaction between the two groups followed by the ring-opening of benzoxazine and cyclotrimerisation of cyanate ester. The spectral and analytical data supported the possibilities of further polymerization through the insertion of the phenolic OH of polybenzoxazine to cyanate group to form the intermediate iminocarbonate, which further induce curing of cyanate ester to form polycyanurate. A co-reacted network composed of triazine ring as a part of polybenzoxazine matrix is postulated. The co-reaction temperature diminished with increase in cyanate ester content in the blend. A single T_g was observed in DMTA of the cured matrix that implied a linked homogeneous matrix containing both triazine and polybenzoxazine. This was substantiated by the TGA, DTA and SEM behavior of the cured polymer. The modulus of the cured blend was higher than those of the component resins of the blend. The co-reaction with cyanate ester enhanced the high temperature stability of polybenzoxazine.     

Synthesis of a novel bridged nucleoside bearing a fused-azetidine ring, 3′-amino-3′,4′-BNA monomer

A novel bridged nucleic acid monomer, 3′-amino-3′-deoxy-5-methyl-3′-N,4′-C-methyleneuridine, was successfully synthesized via a useful and convenient azetidine ring formation under Staudinger's conditions. A ¹H NMR experiment and a PM3 calculation revealed that the sugar moiety of the novel bridged nucleic acid monomer, 3′-amino-3′,4′-BNA, was restricted to S-type conformation.