Modified Sepiolite Nanoclays in Advanced Composites for Engineering Applications

Clay-polymer nanocomposites (CPNs) containing a small weight fraction of nanoclay are known to display enhanced mechanical and thermal properties compared to neat polymers. However, the preparation and application of such nanocomposites remain challenging owing to the difficulties in dispersing nanoclays in polymer matrices. This study focuses on two surfactant-modified organophilic sepiolite clays to demonstrate the simplicity of the modification process, as well as on the use of a benzoxazine monomer (i.e., a CPN matrix precursor) itself as the modifier. Our in-house modified bespoke sepiolites achieve much better dispersion in a benzoxazine matrix, compared to the pristine clay, revealing their potential for applications as nanoenhancers for advanced composites.

Review on the Accelerated and Low-Temperature Polymerization of Benzoxazine Resins: Addition Polymerizable Sustainable Polymers

Due to their outstanding and versatile properties, polybenzoxazines have quickly occupied a great niche of applications. Developing the ability to polymerize benzoxazine resin at lower temperatures than the current capability is essential in taking advantage of these exceptional properties and remains to be most challenging subject in the field. The current review is classified into several parts to achieve this goal. In this review, fundamentals on the synthesis and evolution of structure, which led to classification of PBz in different generations, are discussed. Classifications of PBzs are defined depending on building block as well as how structure is evolved and property obtained. Progress on the utility of biobased feedstocks from various bio-/waste-mass is also discussed and compared, wherever possible. The second part of review discusses the probable polymerization mechanism proposed for the ring-opening reactions. This is complementary to the third section, where the effect of catalysts/initiators has on triggering polymerization at low temperature is discussed extensively. The role of additional functionalities in influencing the temperature of polymerization is also discussed. There has been a shift in paradigm beyond the lowering of ring-opening polymerization (ROP) temperature and other areas of interest, such as adaptation of molecular functionality with simultaneous improvement of properties.

Synthesis of novel allylamine-fluorene based benzoxazine and its copolymerization with typical benzoxazine: curing behavior and thermal properties

Poly(t-BF-sa-a) showed much better thermal properties and glass transition temperatures than traditional fluorene-based benzoxazine monomers.

Study on the modification of hexamethylenediamine-phenol-based polybenzoxazine resins by halloysite nanotubes

In this study, hexamethylenediamine-phenol-based benzoxazine monomer (Ba-h) was synthesized using 1,6-hexamethylenediamine, phenol, and paraformaldehyde. Halloysite nanotubes (HNTs) were treated with silane coupling agent and succinic anhydride, respectively. Then the pristine and grafted HNTs were used as nano-fillers to modify Ba-h at different mass ratios ranging between 0 and 2.5 wt% via solution processing. Nuclear magnetic resonance (NMR) spectroscopy and Fourier transform infrared spectroscopy confirmed the molecular structure of the monomer. The curing behaviors of the nanocomposites were examined by differential scanning calorimetry (DSC). The results of DSC, thermogravimetric analysis, and dynamic mechanical analysis displayed the obvious improvement in the thermal stability of the nanocomposites modified by the grafted HNTs. The results of impact and flexural tests revealed the improvement in the mechanical property of the nanocomposites modified by the pristine HNTs. These enhancements are related to the homogeneous dispersion of HNTs based on the scanning electron microscopy and transmission electron microscopy results.

Photoisomerization and thermal degradation kinetics of poly(ether–ester)s containing azomethine moiety in the main chain

Poly(ether–ester)s containing azomethine group in the main chain were synthesized by solution polycondensation of 4,4′-bis(3-hydroxypropyloxy)- N-benzylidene aniline with adipoyl and terephthaloyl diacid chlorides. The synthesized poly(ether–ester)s were characterized by Fourier transform infrared and proton, and carbon-13 nuclear magnetic resonance spectroscopic techniques. Thermal properties were studied using thermogravimetric analysis (TGA) and differential scanning calorimetry. Thermal degradation kinetics of poly(ether–ester)s were characterized by TGA at various heating rates (5°C min−1, 10°C min−1, and 20°C min−1). The apparent activation energy for the degradation of both the polymers was determined by three different non-isothermal model-free kinetics methods (Friedmann, Flynn–Wall Ozawa, and Kissinger–Akahira–Sunose). The photoisomerization property was examined with ultraviolet (UV) spectroscopy, and the polymer PEE1 showed a rate of trans to cis isomerization ranging 10–20 s, whereas reverse process took around 100 min in solution. UV studies suggested that this material may be used in the field of rewritable applications.

Preparation and characterization of novel polybenzoxazine hybrid materials

The novel polybenzoxazine hybrid materials were prepared using 6,6′-(9H-fluorene-9,9-diyl)bis(3-allyl-3,4-dihydro-2H-benzo-1,3-oxazine) (VF9Bp), 4,4′-bisphenol silane as raw materials. The chemical structure of the raw materials was confirmed by proton, carbon, and silicon nuclear magnetic resonance and Fourier transform infrared (FTIR) spectroscopies. The structural changes from benzoxazines to their corresponding polybenzoxazines were studied by FTIR. Thermal characterization indicated that the glass transition temperature, thermal stability, and dimensional stability of the novel polybenzoxazine hybrid materials were all superior to those of conventional polybenzoxazine.

Innovative approach for the synthesis of benzoxazine-modified montmorillonite

The synthesis of a benzoxazine (BZ) monomer inside the montmorillonite (MT) layers was done using a diamino-modified MT as the amine component, phenol, and formaldehyde. The formation of the BZ rings within the MT layers was investigated by thermogravimetric analysis, X-ray photoelectron spectroscopy, proton nuclear magnetic resonance spectroscopy, and X-ray diffraction, this being enhanced when dioxane was used as the solvent. The morphology of the hybrid BZ monomer was investigated by scanning electron microscopy.

Studies on structurally different diamines and bisphenol benzoxazines

The compounds resorcinolbisbenzoxazine, quinolbisbenzoxazine, p-phenylene diaminebisbenzoxazine, and m-phenylenediaminebisbenzoxazine are prepared. The structural and thermal characterizations of the materials are done using Fourier transform infrared spectroscopy (FTIR), proton nuclear magnetic resonance (1H NMR) and 13C NMR, and differential scanning calorimetry. Both FTIR and NMR studies reveal the presence of oxazine rings in the synthesized monomers. The curing exotherm of these bisbenzoxazines is much influenced by the nature of the aromatic unit present in the chosen compound and also on the type of the benzoxazine unit in the system. Curing kinetics is performed using Flynn–Wall–Ozawa, Vyazovkin, and Friedman methods. The apparent activation energies ( Ea-C) for the thermal curing of the synthesized monomers varied and are dependent on the nature and the position of the functional groups present in the compound. The highest Ea-C values are noted for the compounds having the functionalizations being para oriented.

Studies on structurally different benzoxazines

Bisbenzoxazines were prepared by the condensation of the respective bisphenols bisphenol A (BA), indane bisphenol (IBP), and spirobiindane bisphenol (SBI) with paraformaldehyde and aniline. The apparent activation energies for the polymerization curing process ( Ea-C) and the degradation process ( Ea-D) were calculated using Flynn–Wall–Ozawa, Vyazovkin, and Friedman methods. The variation in Ea-C noted for the thermal curing of different bisbenzoxazines is attributed to the operation of different mechanisms for the curing process. The variation of the Ea-D for the degradation of spirobiindane benzoxazine polymerized at high temperature was different from the other materials investigated and is attributed to its complex structure. The volatile products obtained during the thermal degradation of the polymers were analyzed using thermogravimetric–Fourier transform infrared analyses. Aniline was found to be the major product and was released during the primary degradation. At higher temperatures, breakage of the isopropylidine, indane, and biindane structural entities were favored.

Electron beam curing of functionalized N-(4-hydroxy phenyl)maleimide monomers

Electron beam (E-beam) curing by different total radiation doses (100, 200, 300 and 400 kGy at 10 kGy per pass) of functionalized maleimides (20%) [ N-(4-acryloyloxy phenyl)maleimide, N-(4-methacryloyloxy phenyl)maleimide and N-(4-cyanato phenyl)maleimide] is carried out using N-vinyl-2-pyrrolidone as the reactive diluent (80%). The swelling studies indicate the presence of highly cross-linked structure in the polymer derived from cyanate-functionalized maleimide cured using 400 kGy electron beam (E-beam) irradiation. The thermal stabilities of all the polymers are studied using thermogravimetric analyzer. The materials cured using the highest radiation dose (400 kGy) are found to have better thermal stability compared to other materials cured using lower doses (100, 200 and 300 kGy at 10 kGy per pass). The degradation kinetic studies using the Vyazovkin method showed that the activation energies for the thermal degradation of polymeric materials got by E-beam irradiation of the monomers at 400 kGy are higher. Of all the materials investigated, liquid composition having N-(4-cyanato phenyl)maleimide (20%) and N-vinyl-2-pyrrolidone (80%) cured by E-beam radiation (400 kGy) showed better thermal stability.

High-temperature matrix resins based on bispropargyl ethers (BPEs) – part 2

Structurally diverse bispropargyl ethers (BPEs) using resorcinol, quinol, 4,4′-dihydroxy biphenyl, bisphenol-A, 4,4′-dihydroxy diphenyl ketone, 4,4′-dihydroxy diphenylsulphone, trimethyl indane bisphenol and tetramethyl spirobiindane bisphenol were synthesized, and it was separately blended with 4,4′-bismaleimido diphenyl methane (BMIM) in mole ratios (0.5:0.5). The structural characterizations of the materials are carried out using Fourier transform infrared and nuclear magnetic resonance studies. Differential scanning calorimetric analysis shows that the difference in the melting characteristics and decrease in the curing window for different BPEs were caused by the incorporation BMIM. Addition of BMIM to BPEs reduces the heat liberated during the thermal curing. The curing kinetics is investigated using Flynn–Wall–Ozawa, Vyazovkin and Friedman methods. Amongst the various BPEs investigated, the activation energy ( Ea) values obtained from spirobiindane BPE (SPIPE) behaves entirely in a different manner compared to all other materials investigated. The apparent Ea values for SPIPE was observed to increase with increasing extent of reaction up to 0.2–0.5 and then the Ea values decrease with increasing extent of reaction up to 0.55–0.8. The Ea for the polymer blends obtained by the incorporation of BMIM with different BPEs shows nearly linear increase in the Ea value, and the rate of increase is influenced by the structure of the aromatic spacer present in between the propargyl groups.

Studies on thermal, mechanical, electrical, and morphological behavior of organoclay-reinforced polybenzoxazine–epoxy nanocomposites

Organoclay-reinforced polybenzoxazine–epoxy nanocomposites were prepared via in situ polymerization and thermal, thermomechanical, mechanical, electrical, and morphological properties were characterized by standard methods. Two types of skeletal-modified benzoxazines namely 1,1-bis(3-methyl-4-hydroxyphenyl)cyclohexane benzoxazine and bis(4-maleimidophenyl) benzoxazine were synthesized by reacting paraformaldehyde and 4,4′-diaminodiphenylmethane with 1,1-bis(3-methyl-4-hydroxyphenyl)cyclohexane and N-(4-hydroxyphenyl)maleimide, respectively. Diglycidyl ether bisphenol A epoxy resin was modified with 5, 10, and 15 wt% of benzoxazines and were cured using 4,4′-diaminodiphenylmethane at appropriate conditions. The occurrence of chemical reaction between benzoxazines and epoxy resin was ascertained by Fourier transform infrared spectra. Epoxy and benzoxazines-modified epoxy systems were further reinforced with 1, 3 and 5 wt% of organically modified montmorillonite (OMMT). Thermal behavior of matrices was characterized by differential scanning calorimetry, thermogravimetric analysis, and dynamic mechanical analysis. Mechanical properties were studied as per ASTM standards. The organoclay (OMMT)-reinforced polybenzoxazine–epoxy nanocomposites exhibited lower values of glass transition temperature and dielectric constant/dielectric loss with enhanced values of thermal stability, char yield, impact strength, and storage modulus than those of neat matrix. The morphology of organoclay-reinforced benzoxazine-modified epoxy matrix was studied by scanning electron microscopy, x-ray diffraction, and transmission electron microscopy analyses.

Kinetics of benzoxazine polymerization studied by Raman spectroscopy

Raman spectroscopy was used to monitor the curing process of a benzoxazine monomer. The reaction follows an autocatalytic mechanism. The autocatalytic model described in Kamal equation was employed. The activation energy and the pre-exponential factor were determined as: Ea = 60.52 kJ mol−1 and A = 6.09 × 105 min−1. The overall reaction order was found to be 1.731 ( n = 1.078; m = 0.653). The kinetic model characterized by these values is in good agreement with the experimental values.