Dona Mathew

Cyanate Ester Resins, Recent Developments

The search for advanced, high performance, high temperature resistant polymers is on the rise in view of the growing demand for polymer matrix composites that are to meet stringent functional requirements for use in the rapidly evolving high-tech area of aerospace. Cyanate esters (CEs) form a family of new generation thermosetting resins whose performance characteristics make them attractive competitors to many current commercial polymer materials for such applications. The chemistry and technology of CEs are relatively new and continue to evolve and enthuse researchers. The CEs are gifted with many attractive physical, electrical, thermal, and processing properties required of an ideal matrix resin. These properties are further tunable through backbone structure and by blending with other polymer systems. The structure-property correlation is quite well established. Several new monomers have been reported while some are commercially available. The synthesis of new monomers has come to a stage of stagnation and the present attention is on evolving new formulations and processing techniques. The blends with epoxy and bismaleimide have attracted a lot of research attention and achieved commercial success. While the latter is now known to form an IPN, the reaction mechanism with epoxy is still intriguing. Extensive research in blending with conventional and high performance thermoplastics has led to the generation of key information on morphological features and toughening mechanisms, to the extent that even simulation of morphology and property has now become possible. Despite the fact that the resin and its technology are nearly two decades old, the fundamental aspects related to curing, cure kinetics, reaction modeling, etc. still evince immense research interest and new hypotheses continue to emerge.

Phosphazene–triazine cyclomatrix network polymers: some aspects of synthesis, thermal- and flame-retardant characteristics

Hydroxy phenyl-substituted cyclotriphosphazenes were synthesized by reacting hexachlorocyclotriphosphazene with sodium phenolate and monosodium bisphenolate. The derivatives, consisting of a mixture of multi-substituted and partly chain-extended cyclophosphazenes, with overall functionality close to the targeted values, were transformed into the cyanatophenyl derivative. Thermal curing of the latter gave phosphazene–triazine cyclomatrix network polymers with varying ratios of phosphazene and triazine rings in the matrix. Although they manifested diminished Tg, the cured polymers were more thermally stable and provided higher char residue in comparison to the polycyanurate derived from bisphenol-A dicyanate. The activation energies for thermal decomposition of the cyclomatrix networks increased with both phosphazene content and crosslink density, and showed a direct relationship with their thermal stability. The presence of phosphazene was conducive for enhancing the flame retardancy of the network. The flame retardancy improved with increase in crosslink density and char-yielding property of the polymer, which implied that the flame-retardant action was operative in the condensed phase. © 2000 Society of Chemical Industry

Bisphenol A dicyanate–novolac epoxy blend: Cure characteristics, physical and mechanical properties, and application in composites

Reactive blends of bisphenol A dicyanate (BACY) and a novolac epoxy resin (EPN) were investigated for their cure behavior and the mechanical, thermal, and physical properties of the cocured neat resin and glass-laminate composites. Contrary to the apparent observation in DSC, the dynamic mechanical analysis confirmed a multistep cure reaction of the blend, in league with an established reaction path for similar systems. The cured matrix was found to contain both polycyanurate and oxazolidinone networks that existed in discrete phases exhibiting independent glass transitions in dynamic mechanical analysis (DMA). The flexible and less crosslinked oxazolidinone network contributed to enhanced flexural strength at the cost of the tensile strength of the neat resin. The increased resin flexibility was, however, not translated to the glass-laminate composite for which the flexural strength decreased with the oxazolidinone content, although the latter was conducive for rendering a stronger interphase. The presence of oxazolidinone adversely affected the thermal stability of the cured resin and the high-temperature performance of both neat resin and the composites.

Catalysis of the cure reaction of Bisphenol A dicyanate. A DSC study

The kinetics of the thermal cure reaction of Bisphenol A dicyanate (BACY) in presence of various transition metal acetyl acetonates and dibutyl tin dilaurate (DBTDL) was investigated using dynamic differential scanning calorimetry (DSC). The cure reaction involved a pregel stage corresponding to around 60% conversion and a postgel stage beyond that. Influence of nature and concentration of catalysts on the cure characteristics was examined and compared with the uncatalyzed thermal cure reaction. The activation energy (E), preexponential factor (A), and order of reaction (n) were computed by the Coats–Redfern method. A kinetic compensation correction was applied to the data in both stages to normalize the E values. The normalized activation energy showed a systematic decrease with increase in catalyst concentration. The exponential relationship between E and catalyst concentration substantiated the high propensity of the system for catalysis. At fixed concentration of the catalyst, the catalytic efficiency as measured by the decrease in E value showed dependency on the nature of the coordinated metal and stability of the acetyl acetonate complex. Among the acetyl acetonates, for a given oxidation state of the metal ions, E decreased with decrease in the stability of the complex. A linear relationship was found to exist between activation energy and the gel temperature for all the systems. Manganese and iron acetyl acetonates were identified as the most efficient catalysts. In comparison to DBTDL, ferric acetyl acetonate proved to be a more efficient catalyst. The activation parameters computed using the Coats–Redfern method agreed well with the results from two other well known integral equations.

A robust, melting class bulk superhydrophobic material with heat-healing and self-cleaning properties

Superhydrophobic (SH) materials are essential for a myriad of applications such as anti-icing and self-cleaning due to their extreme water repellency. A single, robust material simultaneously possessing melt-coatability, bulk water repellency, self-cleanability, self-healability, self-refreshability, and adhesiveness has been remaining an elusive goal. We demonstrate a unique class of melt-processable, bulk SH coating by grafting long alkyl chains on silica nanoparticle surface by a facile one-step method. The well-defined nanomaterial shows SH property in the bulk and is found to heal macro-cracks on gentle heating. It retains wettability characteristics even after abrading with a sand paper. The surface regenerates SH features (due to reversible self-assembly of nano structures) quickly at ambient temperature even after cyclic water impalement, boiling water treatment and multiple finger rubbing tests. It exhibits self-cleaning properties on both fresh and cut surfaces. This kind of coating, hitherto undisclosed, is expected to be a breakthrough in the field of melt-processable SH coatings.

Free radical copolymerisation of N-(4-hydroxy phenyl) maleimide with vinyl monomers: solvent and penultimate-unit effects

Abstract N-(4-hydroxy phenyl) maleimide (HPM) was copolymerized with butyl acrylate (BuA), methyl methacrylate (MMA) and styrene in dimethyl formamide (DMF) and dioxane. The terminal model reactivity ratios were estimated using the method of Kelen–Tudos. In all cases, the penultimate model reactivity ratios were estimated by the Barson–Fenn method and by a computer-assisted non-linear regression analysis. The copolymerization behaviour was found to be dependent both on the electronic nature of the comonomer and on the solvent medium for a given monomer pair. The terminal model reactivity ratios revealed the statistical nature of copolymerization between HPM and BuA. The r1 (HPM=M1) value near zero and r2 value close to unity indicated the tendency for MMA to form its sequences between HPM molecules in the copolymer at lower concentrations of the latter in the feed and to form alternating copolymer in HPM-rich feeds. The electron-rich styrene formed nearly alternating copolymer with HPM at all practical feed compositions, as evidenced from near-zero values of both r1 and r2. On changing the solvent from dioxane (THF in the case of styrene) to DMF, the apparent reactivity of HPM increased in the case of BuA and decreased in cases of both MMA and styrene. The increased reactivity of HPM in DMF in the former case can be due to possible aggregation of this monomer and its association with the propagating radical through hydrogen bonding. This association is interrupted in dioxane due to reciprocal H-bonding of HPM with this solvent. The enhanced reactivity in DMF can also be caused by the DMF-stabilized polarized and hence more electrophilic structure of HPM monomer or radical which has reduced preference for the electron-deficient BuA or its radical. This polarized structure has enhanced preference for the electron-rich MMA and styrene or their corresponding radicals which accounts for the reduced reactivity in these cases. The same polarized structure of HPM caused penultimate-unit effects for radicals terminated with MMA or styrene in DMF, although no such effect was observed in the case of BuA. In DMF, the reactivities of these radicals towards their corresponding monomers were several-fold enhanced when the penultimate group was HPM. In the cases of both MMA and styrene copolymerized with HPM in DMF, reasonable feed-copolymer composition profiles could be predicted only by the penultimate model. The monomer sequence distribution in the copolymers were calculated by the statistical method taking into account the penultimate-unit effect. The calculation showed that the penultimate-unit effect caused HPM-styrene system to deviate from the otherwise expected alternating copolymerization which caused some styrene diads in the polymer chain.

Long-living, stress- and pH-tolerant superhydrophobic silica particles via fast and efficient urethane chemistry; facile preparation of self-recoverable SH coatings

Superhydrophobic (SH) and water-rolling oligomer wrapped silica particles (OWS) were synthesized using a one-step method by employing the quick and efficient silanol–isocyanate surface reaction. The presence of urethane/allophanate linkages and oligomer formation over the silica surface was confirmed using FTIR and MALDI-TOF-MS analyses. The thin coating of the particles displayed a static contact angle of >160°, a roll-off angle of ∼3° and exhibited a micro-nano structure as shown in the FESEM images. The OWS particles are tolerant to variations in pH either side of the pH scale, from 1 to 13, exhibiting water roll-off properties when exposed to harsh acidic and basic conditions. The pH tolerance was observed after mechanical damage. The rapidity and efficiency of the method was demonstrated with a low extent of grafting also. The superhydrophobic particles are long-living as they retained SH properties in different pH conditions even after one year of continuous exposure to ambient conditions. Furthermore, the SH particles with an incorporated cross-linked poly(dimethylsiloxane) coating displayed excellent pH and stress resistance. Surprisingly, the SH coating exhibits self-recoverable superhydrophobicity without an external stimulus (under ambient conditions) or heat treatment after water impalement, pH, boiling water and stress tests.

Pendant cyanate functional vinyl polymers and imido- phenolic-triazines thereof: synthesis and thermal properties

Abstract Maleimide-incorporated vinyl polymers, based on butyl acrylate (BuA), methyl methacrylate (MMA) and styrene (STY) with pendant phenol functions were synthesised by free radical copolymerization of respective monomers with N-(4-hydroxy phenyl) maleimide. The phenol functions of the linear polymers were transformed to the corresponding cyanato phenyl derivatives. Thermal curing of these polymers through the cyclotrimerization of the pendant cyanate groups led to crosslinked imido-phenolic-triazines. The thermal stabilities of the phenol precursors depended on the backbone structure. Presence of maleimide improved the thermal stability of BuA- and STY-based copolymers, whereas, it was detrimental for MMA-based ones. The early chain unzipping in the case of the latter has been attributed to the steric interaction between the α-methyl group of MMA and maleimide moiety. Although the presence of imide and triazine groups in the crosslinked network was conducive for decreasing the rate of thermal degradation at higher temperatures, they triggered an early onset of decomposition because of steric factors. The nonisothermal kinetics of thermal decomposition was studied and the relevant kinetic parameters like activation energy (E) and pre-exponential factor (A) have been calculated for various phenol-functional precursors and the corresponding imidophenolic triazines. Although the trend in values of E with composition was not indicative of the thermal stability of the triazine-network polymers due to the prevalence of kinetic compensation effect, the values served to estimate the rate constant for decomposition of various polymers, which exhibited a direct correlation with their thermogravimetric behaviour.

Graphene oxide induced fast curing of amino novolac phthalonitrile

Graphene oxide (GO) possessing hydroxyl and epoxy groups was synthesized by a modified Hummers process and was found to conform to the empirical formula C0.801[epoxy]0.190[OH]0.009. GO was examined as a curative for amino novolac phthalonitrile (APN). It was found to facilitate the crosslinking reaction by acting as a reactant for the amino group. Additionally, epoxy and OH groups in GO react with the nitrile groups of APN providing another pathway for facilitating the curing of APN. Epoxy groups in GO react with the nitrile groups of APN in the presence of OH groups (generated by the epoxy–amine reaction), facilitating the crosslinking at a lower curing temperature. Blending with GO decreases the overall curing temperature of APN. The co-reaction of the two systems led to a single phase matrix whose failure mode changed from brittle to ductile with no penalty in the thermal stability.

Effect of polymeric additives on properties of glass–bisphenol A dicyanate laminate composites

The polycyanurate network matrix derived from the thermal, dibutyl tin dilaurate catalyzed polymerization of bisphenol A dicyanate was modified in their glass–laminate composites with different linear polymeric additives bearing pendant phenol, cyanate, and epoxy functions. The mechanical properties and fracture energy for delamination of the glass–laminate composites were estimated as functions of the backbone structure and concentration of the various additives. The effect of altering the nature or concentration of the functional group for a given backbone structure of the additive was examined in some cases. Except for the epoxy functional acrylic polymer, all other systems adversely affected the fracture energy for delamination of the composites due to either plasticization or embrittlement of the matrix. With the exception of the styrene-hydroxyphenyl maleimide (SPM) copolymer, the other modifiers impaired the mechanical properties and adversely affected the thermomechanical profile of the composites. In the cases of the phenol functional acrylic polymer and its cyanate derivative, plasticization of the matrix by the partly phase-separated additive, which eased the fiber debonding, appears to be responsible for the impairment of the mechanical properties. The high glass transition temperature SPM copolymer enhanced the resin–reinforcement interaction through dipolar interactions induced by the hydroxyl groups, which resulted in amelioration of the mechanical properties. However, its possible coreaction and formation of a brittle, homogeneous phase with the polycyanurate was conducive for poor damage tolerance. The SEM analysis of the fractured composites showed that in the elastomers fiber debonding is the major cause for delamination. Although the presence of SPM led to a stronger interphase, failure occurred either in the brittle matrix or through fiber breakage.