Synthesis and characterization of polybenzoxazine networks nanocomposites containing multifunctional polyhedral oligomeric silsesquioxane (POSS)

A new class of polybenzoxazine/POSS nanocomposites with network structure is prepared by reacting multifunctional benzoxazine POSS (MBZ-POSS) with benzoxazine monomers (Pa and Ba) at various compositional ratios. Octafunctional cubic silsesquioxane (MBZ-POSS) is used as a curing agent, which is synthesized from eight organic benzoxazine tethers through hydrosilylation of vinyl-terminated benzoxazine monomer (VP-a) with octakis(dimethylsiloxy)silsesquioxane () using a platinum complex catalyst (Pt-dvs). Incorporation of the silsesquioxane core into polybenzoxazine matrix could significantly hinder the mobility of polymer chains and enhance the thermal stability of these hybrid materials. For these nanocomposites, increasing the POSS content in the hybrids is expected to improve its thermal properties with respect to the neat polybenzoxazine. The morphology feature is useful to explain the thermal property changes (T_g and T_d) and AFM images show that the presence of POSS aggregation in larger scales occurs at higher POSS contents. The reason of the heterogeneous phase separation may be from the less compatibility of the inorganic silsesquioxane core with organic benzoxazine species and the homopolymerization of MBZ-POSS. In the course of the formation of the polybenzoxazine/POSS hybrids, POSS particles were separated from the polybenzoxazine rich region, leading to POSS rich domains in the range of 50-1000 nm.     

Synthesis and characterization of benzoxazine resin-SiO2 hybrids by sol-gel process: The role of benzoxazine-functional silane coupling agent

Sol-gel process has been used to incorporate silica nanoparticles into polybenzoxazine matrix to form polybenzoxazine-silica hybrids. The hybrids have been synthesized by the reaction of bisphenol A/aniline type benzoxazine monomer (BA-a) and tetraethoxysilane as precursor for silica nanoparticles. Triethoxysilane-functional benzoxazine monomer was also prepared from γ-aminopropyltriethoxysilane, phenol and paraformaldehyde, and has been used as silane coupling agent to improve the adhesion between polybenzoxazine matrix as organic domain and silica nanoparticles as inorganic domain. Differential scanning calorimetry (DSC) and FT-IR are used to study the polymerization behavior of benzoxazine monomer in the presence of silica nanoparticles over different temperature range. Dynamic mechanical analysis indicates that the coupling agent is very effective to increase the T_g and the storage modulus of the hybrids. Thermogravimetric analysis also shows the thermal stability of neat polybenzoxazine matrix is improved by the inclusion of silica nanoparticles.     

Immobilization of polyvinylacetate macromolecules on hydroxyapatite nanoparticles

Concentration dependence of the storage modulus, E′, was investigated for polyvinylacetate (PVAc) filled with hydroxyapatite (HAP) nanoparticles. The filler volume fraction, v_f, varied from 0 to 0.05 and the E′ and loss tangent, tan δ, were measured below neat matrix T_g at −40 °C and above neat matrix T_g at +50 °C at 1 Hz. The T_g determined as the position of the maximum on the temperature dependence of tan δ increased by 14 °C compared to the neat PVAc (39 °C) by adding 5 vol.% of HAP. At −40 °C, the observed small increase of E′ with v_f was in agreement with the prediction based on the simple Kerner equation. At +50 °C, the increase of E′ with v_f observed was an order of magnitude greater than that predicted using the simple continuum mechanics model. An attempt was made to explain the observed deviation employing the hypothesis of immobilized entanglements.     

Synthesis and characterizations of a vinyl-terminated benzoxazine monomer and its blending with polyhedral oligomeric silsesquioxane (POSS)

Benzoxazine was synthesized through the Mannich condensation of phenol, formaldehyde, and primary amines through ring-opening polymerization. Polybenzoxazines are phenolic-like materials that possess dimensional and thermal stability, and they release no toxic byproducts during their polymerization. To further improve the thermal stability of polybenzoxazines, a hydrosilane-functionalized polyhedral oligomeric silsesquioxane (H-POSS) was incorporated into the vinyl-terminated benzoxazine monomer (VB-a) which we then subjected to ring-opening polymerization. In addition, we also prepared hybrids from a non-reactive POSS (IB-POSS) and VB-a. The glass transition temperature (T_g) of a regular polymerized VB-a (i.e. PVB-a) is 307 °C, while the hybrid containing 5 wt% of H-POSS is 333 °C. The IB-POSS modified PVB-a hybrids, in general, results in lower T_g than the pure PVB-a due to poor missibility.     

Ultrafine full-vulcanized powdered rubbers/PVC compounds with higher toughness and higher heat resistance

A new type of rigid PVC compound with higher toughness and higher heat resistance was prepared by using a new type of PVC modifier, ultrafine full-vulcanized powdered rubber (UFPR). The UFPRs used in this paper were butadiene nitrile UFPR-1 (NBR-UFPR-1) with particle size of about 150 nm and butadiene nitrile UFPR-2 (NBR-UFPR-2) with particle size of about 90 nm. Dynamic mechanical thermal analysis (DMTA) showed that glass transition temperature (T_g) of PVC in compounds increased from 77.52 °C of neat PVC to 82.37 and 85.67 °C, while the notched impact strengths increased from 3.1 kJ/m² of neat PVC to 5.2, 5.5 kJ/m², respectively. It can be found that both T_g and toughness of PVC have been improved simultaneously, and the smaller the particle size of NBR-UFPRs, the higher the T_g and the impact strength. The property could be attributed to larger interface and more interfacial interaction between NBR-UFPRs and PVC matrix. Transmission electron microscopy (TEM) showed that NBR-UFPRs could be well dispersed in PVC matrix.     

Structure and thermo-mechanical properties study of polyurethane-urea/glycidoxypropyltrimethoxysilane hybrid coatings

A series of organic–inorganic hybrid coatings were prepared using polyurethane (PU)-urea and glycidoxypropyltrimethoxysilane (GPTMS) To prepare this first acid terminated saturated polyester, having 230 hydroxyl value and acid value 25 mg/KOH, were reacted with coupling agent GPTMS at different concentrations in the presence of base catalyst and each of them were further reacted with isophorone diisocyanate (IPDI) at NCO/OH ratio of 1.6:1 for 4–5 h at 70–80 °C These prepolymers were casted on tin foil and cured at ambient conditions for 6 h and prepared the hybrid coating free films by amalgamation. These free films were stored in the room temperature for 40 days and used for further characterization. The coating without and with different concentrations of GPTMS were named as base polymer and hybrid coatings, respectively. FTIR spectroscopy was used for the structural analysis of the coatings. Thermogravimetric analysis (TGA) showed that thermal stability of the hybrids was significantly higher than the base polymer. The onset degradation temperature of the base polymer starts at 268.9 °C, while it ranges from 279.1 °C to 290.8 °C for the hybrids based on the concentration of GPTMS used. The glass transition temperature (T_g) and storage modulus as determined from DMTA were higher for hybrid coatings as compared to base polymer. T_g of base polymer was 42.3 °C while it varies between 65.8 °C to 83.5 °C for hybrids.     

A new synthetic approach for difficult benzoxazines: Preparation and polymerization of 4,4′-diaminodiphenyl sulfone-based benzoxazine monomer

The first successfully synthesized benzoxazine in high purity from 4,4′-diaminodiphenyl sulfone (DDS), paraformaldehyde and phenol using high boiling point nonpolar solvent is reported. The solution method for benzoxazine synthesis is modified by using a nonpolar solvent of high boiling point. For comparison, the synthesis of such difficult benzoxazine monomer was prepared in high boiling point polar solvent, dimethylsufoxide. ¹H NMR indicates the purity of the monomer prepared by this novel method to be quite high in comparison with that obtained using dimethylsulfoxide (DMSO). The thermally activated polymerization of the monomer affords polybenzoxazine with T_g at ca. 203 °C. The 5% and 10% decomposition temperatures of the polymer are 324 and 368 °C with 58% char yield, reflecting the excellent thermal stability than the typical polybenzoxazine based on bisphenol-A and aniline.     

The effect of interfacial chemistry on molecular mobility and morphology of multiwalled carbon nanotubes epoxy nanocomposite

We report on our attempts to understand the link between the nature of the CNT surface modification, dispersion in an epoxy resin and the resulting properties. Carboxylated and fluorinated nanotubes were used to synthesize nanocomposites by dispersing them separately in an epoxy resin. Dynamic mechanical analysis, using torsional deformation, was applied both parallel and perpendicular to the long axis of the multiwall nanotubes (MWNTs). Interestingly, for epoxy/MWNT (1 wt%) nanocomposites, the shear moduli in the glassy state were higher for the nanocomposites, and it's highest for the nanocomposites in which the nanotubes are parallel to the direction of applied torque. These nanocomposites also exhibited higher T_gs than the neat resin. In addition, the rubbery plateau modulus (between 150-200 °C) was higher by a factor of three for the nanocomposites. Master curves constructed using time-temperature superposition allowed us to probe low frequency dynamic moduli and further discern differences in the relaxation behavior. Samples containing fluorinated nanotubes exhibited the highest T_gs, longest relaxation times and highest activation energies relative to the carboxylated nanotube samples and the neat resin, indicative of stronger interactions. SEM and TEM studies confirmed the nanotube dispersion and alignment.     

Polymerization of linear aliphatic diamine-based benzoxazine resins under inert and oxidative environments

A series of linear aliphatic diamine-based benzoxazine monomers have been studied. Reaction times and purification procedures have been optimized for each individual diamine. The structure of these diamine-based benzoxazine monomers has been characterized by ¹H and ¹³C NMR, and infrared spectroscopy. The rate of polymerization has been studied by Fourier transform infrared spectroscopy as a function of the chain length of the aliphatic amines. The glass transition temperatures (T_g) of the polybenzoxazines from these monomers are also studied. The short chain amine polybenzoxazine exhibits the T_g of around 170 °C. The influence of the polymerization environment for these linear aliphatic diamine-based series of benzoxazine monomers has been studied under air and inert atmosphere. Differential scanning calorimetry is used to determine the melting points of these benzoxazines and the temperature of the peak polymerization exotherm. An anomalous polymerization behavior of ethylene diamine-based polybenzoxazine is also reported.     

Synthesis and properties of polybenzoxazole-clay nanocomposites

A novel polybenzoxazole (PBO)/clay nanocomposite has been prepared from a PBO precursor, polyhydroxyamide (PHA) and an organoclay. The PBO precursor was made by the low temperature polycondensation reaction between isophthaloyl chloride (IC) and 2,2-bis(3-amino-4-hydroxyphenyl)hexafluoropropane with an inherent viscosity of 0.5 dl/g. The organoclay was formed by a cation exchange reaction between a Na⁺-montorillonite (Na⁺-Mont) clay and an ammonium salt of dodecylamine. The PHA/clay was subsequently thermal cured to PBO/clay. Both X-ray diffraction and transmission electron microscope analyzes showed that the organoclay was dispersed in the PBO matrix in a nanometer scale. The in-plane coefficient of thermal expansion (CTE) of PBO/clay film decreased with increasing amounts of organoclay. The CTE of PBO/clay film containing 7 wt% clay was decreased by 21% compared to the pure PBO film. Both of the glass transition temperature (T_g) and the thermal decomposition temperature of PBO/clay increased with increasing amounts of organoclay. The thermal decomposition temperature and the T_g of PBO/clay containing 7 wt% clay increased to 12 and 16 °C, respectively.     

Reversible thickness control of polymer thin films containing photoreactive coumarin derivative units

The reversible control of the thickness of polymer thin films was investigated using (meth)acrylic polymers containing photoreactive coumarin derivative units in the side chain. Coumarin derivative units underwent dimerization and the reverse-dimerization by photoirradiation and were used as a reversible cross-linking point. The homopolymer of 7-methacryloyloxy-4-methylcoumarin (T_g = 194 °C) did not cause changes in film thickness after photoreactions. The homopolymer of 7-(2′-acryloyloxyethoxy)-4-methylcoumarin (AEMC) (T_g = 89 °C) decreased 19% of film thickness by photodimerization and 73% of the decreased thickness was recovered after the reverse-dimerization and the subsequent thermal annealing at 130 °C. The reverse-dimerization of the copolymer of AEMC and n-butyl acrylate (AEMC content = 19 mol%, T_g = 11 °C) resulted in 53% of recovery from the decreased film thickness without annealing. The mobility of polymer main-chain was revealed to be essential factor to change film thickness by photoreactions. Photodimerization of coumarin derivative units in low glass transition temperature (T_g) tended to proceed faster than in high T_g polymers and resulted in larger decrease in film thickness.     

Miscibility and viscoelastic properties of acrylic polyhedral oligomeric silsesquioxane-poly(methyl methacrylate) blends

We investigate the miscibility of acrylic polyhedral oligomeric silsesquioxanes (POSS) [characteristic size d≈2 nm] and poly(methyl methacrylate)(PMMA) in order to determine the effect of well-dispersed POSS nanoparticles on the thermomechanical properties of PMMA. Two different acrylic POSS species (unmodified and hydrogenated) were blended separately with PMMA at volume fractions up to ?=0.30. Both POSS species have a plasticizing effect on PMMA by lowering the glass transition temperature T_g and decreasing the melt-state linear viscoelastic moduli measured in small amplitude oscillatory shear flow. The unmodified acrylic-POSS has better miscibility with PMMA than the hydrogenated form, approaching complete miscibility for loadings ?<0.10. At a loading ?=0.05, the unmodified acrylic POSS induces a 4.9 °C decrease in the T_g of PMMA, far less than the 17.4 °C decrease in the glass transition temperature observed in a blend of 5 vol% dioctyl phthalate (DOP) in PMMA; however, the decrease in the glass transition temperature per added plasticizer molecule is nearly the same in the unmodified acrylic-POSS-PMMA blend compared with the DOP-PMMA blend. Time-temperature superposition (TTS) was applied successfully to the storage and loss moduli data and the resulting shift factors were correlated with a significant increase in free volume of the blends. The fractional free volume f₀=0.046 for PMMA at T₀=170 °C while for a blend of 5 vol% unmodified acrylic-POSS in PMMA f₀=0.057, which corresponds to an addition of 0.47 nm³ per added POSS molecule at ?=0.05. The degree of dispersion was characterized using both wide-angle X-ray diffraction (WAXD) and dynamic mechanical analysis (DMA). Diffraction patterns for both blend systems show clear evidence of phase separation at ?=0.20 and higher, but no significant phase separation is evident at ?=0.10 and lower. The storage modulus measured in DMA indicates appreciable phase separation for unmodified acrylic POSS loadings ?≥0.10, while no evidence of phase separation is present in the ?=0.05 blend in DMA.     

In-situ preparation of poly(2-hydroxyethyl methacrylate)-titania hybrids using γ-radiation

Hybrids of titania and poly(2-hydroxyethyl methacrylate) were prepared using ⁶⁰Co γ-radiation initiated polymerisation combined with sol-gel chemistry of titanium isopropoxide. Careful control of the rates of the sol-gel reaction was achieved with acetylacetone which ensured that the dispersion of titania nanoparticles remained stable and minimised aggregation during the polymerisation reaction. Essentially quantitative conversion was achieved with extremely few residual volatiles, as indicated by evolved gas FTIR and solid state NMR. The process yielded transparent orange hybrids, with titania loadings ranging from 3 wt.% to 15 wt.% (TGA). The pyrolysis onset temperatures increased by approximately 30 °C for the hybrids compared to the unfilled polymer. DSC indicated higher T_g’s compared with other pHEMA data in the literature, with a decrease in magnitude of the T_g occurring with increasing titania content. TEM shows high dispersion of particles in the 10-20 nm range, with modest aggregation. Equilibrium swelling showed that the hybrid hydrogels swelled less than half that of unfilled polymer, indicating that the titania is restricting the mobility of the chains through a cross-linking process.     

Structure and properties of polyacrylonitrile/single wall carbon nanotube composite films

Polyacrylonitrile (PAN)/single wall carbon nanotube (SWNT) composite films have been processed with unique combination of tensile strength (103 MPa), modulus (10.9 GPa), electrical conductivity (1.5×10⁴ S/m), dimensional stability (coefficient of thermal expansion 1.7×10⁻⁶/°C), low density (1.08 g/cm³), solvent resistance, and thermal stability. PAN molecular motion above the glass transition temperature (T_g) in the composite film is significantly suppressed, resulting in high PAN/SWNT storage modulus above T_g (40 times the PAN storage modulus). Rope diameter in the SWNT powder was 26 nm, while in 60/40 PAN/SWNT film, the rope diameter was 40 nm. PAN crystallite size from (110) plane in PAN and PAN/SWNT films was 5.3 and 2.9 nm, respectively. This study suggests good interaction between PAN and SWNT.     

Preparation and properties of polyhedral oligomeric silsesquioxane/epoxy hybrid resins

Octaaminophenyl polyhedral oligomeric silsesquioxane (OAPS) was synthesized using three‐step method and used to modify o‐cresol‐novolac epoxy resin (ECN) for printed circuit board. The influence of OAPS on the reactivity and the final properties of the hybrid networks were evaluated. The intercrosslinking reaction between ECN and OAPS was confirmed by Fourier transform infrared spectra. The ECN/OAPS hybrids have better impact strength, higher electrical resistivity and thermal stability, lower water absorption than the unmodified ECN. The volume resistivity and surface resistivity of the hybrids increase by an order of magnitude or more compared to the neat epoxy. The thermal stability of the hybrids improves by the incorporation of OAPS; the initial decomposition temperature and char yield show an increasing tendency up to 4 wt% loading of OAPS. The hybrids exhibit higher storage modulus and glass transition temperature (T_g) than the neat epoxy. The T_g of the hybrids greatly improves up to 153.3°C at 3 wt% content, much higher than 119.4°C of the neat epoxy. POLYM. COMPOS., 34:1753–1760, 2013. © 2013 Society of Plastics Engineers     

Mechanical and thermal properties of waterborne epoxy composites containing cellulose nanocrystals

Cellulose nanocrystals (CNCs) are reinforcing fillers of emerging interest for polymers due to their high modulus and potential for sustainable production. In this study, CNC-based composites with a waterborne epoxy resin matrix were prepared and characterized to determine morphology, water content, and thermal and mechanical properties. While some CNC aggregation was observed, the glass transition temperature (T_g) and modulus for the composites increased with increasing CNC content. Relative to neat epoxy, at 15 wt.% CNC the storage modulus increased by 100%, the T_g increased from 66.5 °C to 75.5 °C, and tensile strength increased from 40 MPa to 60 MPa, suggesting good adhesion between epoxy and CNC surfaces exposed to the matrix. Additionally, no additional water content resulting from CNC addition were observed. These results provide evidence that CNCs can improve thermomechanical performance of waterborne epoxy polymers and that they are promising as reinforcing fillers in structural materials and coatings.     

Synthesis of nitrile and benzoyl substituted poly(biphenylene oxide)s via nitro displacement reaction

A series of substituted poly(biphenylene oxide)s (PBPOs) was synthesized via nucleophilic nitro displacement reactions. High molecular weight PBPO's with nitrile groups were effectively synthesized from the polymerization of A-B type monomers with K₂CO₃ as a base in N-methyl-2-pyrrolidinone (NMP) at 140 °C. The polymers are completely amorphous, soluble in polar aprotic solvents, and formed flexible films on solution casting. Para-linked PBPO with nitrile groups showed excellent thermal properties such as high 5% weight loss temperature above 530 °C and T_g at 241 °C which is higher than those of commercially available PPO™ (T_g=210 °C). The pendent nitrile groups of PBPO were easily transformed to carboxylic acid groups by acidic hydrolysis.     

Soft matter electrolytes based on polymethylmetacrylate dispersions in lithium bis(trifluoromethanesulfonyl)imide/1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids

Ion transport in a polymer-ionic liquid (IL) soft matter composite electrolyte is discussed here in detail in the context of polymer-ionic liquid interaction and glass transition temperature. The dispersion of polymethylmetacrylate (PMMA) in 1-butyl-3-methylimidazolium hexafluorophosphate (BMIPF₆) and 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide (BMITFSI) resulted in transparent composite electrolytes with a “jelly-like” consistency. The composite ionic conductivity measured over the range −30 °C to 60 °C was always lower than that of the neat BMITFSI/BMIPF₆ and LiTFSI-BMITFSI/LiTFSI-BMIPF₆ electrolytes but still very high (>1 mS/cm at 25 °C up to 50 wt% PMMA). While addition of LiTFSI to IL does not influence the glass T_g and T_m melting temperature significantly, dispersion of PMMA (especially at higher contents) resulted in increase in T_g and disappearance of T_m. In general, the profile of temperature-dependent ionic conductivity could be fitted to Vogel-Tamman-Fulcher (VTF) suggesting a solvent assisted ion transport. However, for higher PMMA concentration sharp demarcation of temperature regimes between thermally activated and solvent assisted ion transport were observed with the glass transition temperature acting as the reference point for transformation from one form of transport mechanism to the other. Because of the beneficial physico-chemical properties and interesting ion transport mechanism, we envisage the present soft matter electrolytes to be promising for application in electrochromic devices.     

Cure kinetics and morphology of blends of epoxy resin with poly (ether ether ketone) containing pendant tertiary butyl groups

The cure kinetics and morphology of diglycidyl ether of bisphenol-A (DGEBA) epoxy resin modified with a poly (ether ether ketone) based on tertiary butyl hydroquinone (PEEK-T) cured with diamino diphenyl sulphone (DDS) were investigated using differential scanning calorimetry (DSC), scanning electron microscopy (SEM) and dynamic mechanical thermal analysis (DMTA). The results obtained from DSC were applied to autocatalytic and diffusion controlled kinetic models. The reaction mechanism broadly showed autocatalytic behaviour regardless of the presence of PEEK-T. At higher PEEK-T concentration, more diffusion controlled mechanism was observed. The rate of curing reaction decreased with increase in thermoplastic content and also with the lowering of curing temperature. The activation energies of the blends are higher than that of the neat resin. The blends showed a phase separated morphology. The dispersed phase showed a homogeneous particle size distribution. The T_g of the neat resin decreased with the decrease in cure temperature. Two T_g's corresponding to the epoxy rich and thermoplastic rich phases were observed in the dynamic mechanical spectrum. The storage modulus of 10 and 20 phr PEEK-T blends are found to be greater than the neat resin.     

Highly filled graphene‐benzoxazine composites as bipolar plates in fuel cell applications

This research emphasizes on the development of highly filled graphene‐polybenzoxazine composites and investigates thermal, electrical, and mechanical properties of the obtained composites for bipolar plate applications. The composition of graphene loading was achieved to be in the range of 10–60 wt%. The experimental results revealed that at the maximum graphene content of 60 wt% (44.8 vol%) in the polybenzoxazine, storage moduli at room temperature of the composites were considerably enhanced with the amount of the graphene, that is, from 5.9 GPa of the neat polybenzoxazine to about 25.1 GPa at 60 wt% of graphene. Glass transition temperatures (T_g) of the obtained composites were observed to be 174–188°C and the values substantially increased with increasing the filler contents. At 60 wt% of graphene content, thermal conductivity, as high as 8.0 W/mK, is achieved for the graphene‐filled polybenzoxazine. Furthermore, the flexural modulus and flexural strength of the composites were found to be as high as 18 GPa and 42 MPa, respectively. Water absorption of graphene filled‐composite is relatively low with the value of only about 0.06% at 24 h of water immersion. Additionally, electrical conductivity was measured to be 357 S/cm at maximum loading of the graphene. Therefore, the graphene‐filled composites based on polybenzoxazine are highly attractive as bipolar plates for polymer electrolyte membrane fuel cells applications. POLYM. COMPOS., 37:1715–1727, 2016. © 2014 Society of Plastics Engineers