K. N. Ninan

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.

Synthesis and properties of polyether nitrile copolymers with pendant methyl groups

Abstract Polyether nitrile and polyether nitrile copolymers with pendant methyl groups were prepared by the nucleophilic substitution reaction of 2,6′-dichlorobenzonitrile with hydroquinone (HQ) and with varying mole proportions of HQ and methyl hydroquinone (MeHQ) using N-methyl pyrrolidone solvent in the presence of anhydrous K2CO3. The polymers were characterised by different physico-chemical techniques. The crystallinity of the polymers was found to decrease with increase in concentration of the MeHQ units in the polymer. Thermogravimetric studies showed that all the polymers were stable up to 450 °C with a char yield above 50% at 900 °C in N2 atmosphere. The glass transition temperature and activation energy of the polymers was found to increase with increase in concentration of the MeHQ units in the polymer.

Styrene–butadiene rubber/natural rubber blends: Morphology, transport behavior, and dynamic mechanical and mechanical properties

Blends of styrene–butadiene rubber (SBR) and natural rubber (NR) were prepared and their morphology, transport behavior, and dynamic mechanical and mechanical properties were studied. The transport behavior of SBR/NR blends was examined in an atmosphere of n-alkanes in the temperature range of 25–60°C. Transport parameters such as diffusivity, sorptivity, and permeability were estimated. Network characterization was done using phantom and affine models. The effect of the blend ratio on the dynamic mechanical properties of SBR/NR blends was investigated at different temperatures. The storage modulus of the blend decreased with increase of the temperature. Attempts were made to correlate the properties with the morphology of the blend. To understand the stability of the membranes, mechanical testing was carried out for unswollen, swollen, and deswollen samples.

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

Thermal characteristics of addition-cure phenolic resins

Abstract The thermal and pyrolysis characteristics of four different types of addition-cure phenolic resins were compared as a function of their structure. Whereas the propargyl ether resins and phenyl azo functional phenolics underwent easy curing, the phenyl ethynyl- and maleimide-functional ones required higher thermal activation to achieve cure. All addition-cure phenolics exhibited improved thermal stability and char-yielding property in comparison to conventional phenolic resole resin. The maleimide-functional resins exhibited lowest thermal stability and those crosslinked via ethynyl phenyl azo groups were the most thermally stable systems. Propargylated novolac and phenyl ethynyl functional phenolics showed intermediate thermal stability. The maximum char yield was also given by ethynyl phenyl azo system. Non-isothermal kinetic analysis of the degradation reaction implied that all the polymers undergo degradation in at least two steps, except in the case of ethynyl phenyl azo resin, which showed an apparent single step degradation. The very low pre-exponential factor common to all polymers implied the significance of volatilisation process in the kinetics of degradation. Isothermal pyrolysis studies led to the conclusion that in the case of nitrogen-containing polymer, the pyrolysis occurs via loss of nitrogenous products, which is conducive for enhancing the carbon-content of the resultant char. FTIR spectra of the pyrolysed samples confirmed the presence of C–O groups in the char. XRD analysis of the partially carbonised polymers did not give any indication of crystallites except in the case of ethynyl phenyl azo system.

Kinetics of Alder-ene reaction of Tris(2-allylphenoxy)triphenoxycyclotriphosphazene and bismaleimides — a DSC study

Tris(2-allylphenoxy)triphenoxycyclotriphosphazene was reacted with three bismaleimides (BMI), viz. bis(4-maleimido phenyl)methane (BMM), bis(4-maleimido phenyl)ether (BME) and bis(4-maleimido phenyl)sulphone (BMS) via the Alder-ene reaction. The differential scanning calorimetric analysis of the blend manifested two distinct exotherms. The low temperature exothermic reaction was attributed to the Wagner-Jauregg reaction following the ene reaction and the strong exotherms at around 250–270°C to the cross-linking Diels–Alder reactions of the initially formed adducts. The Kinetic parameters, viz. activation energy (E) and pre-exponential factor (A) of the reactions were evaluated by Kissinger and Ozawa methods using the variable heating rate method. The kinetic data revealed that the Wagner-Jauregg reaction was disfavoured by electron-withdrawing nature of the BMI. The Diels–Alder reaction was facilitated by the electron-withdrawing nature of the bismaleimide. The activation energy for the first exothermic stage decreased and for the second major step increased on enhancing the stoichiometry of BMI in the blend for a given pair. The activation parameters served to predict the isothermal cure profiles of the blends and deduce the possible network structure under the given conditions of cure temperature and stoichiometry.

Synthesis and characterization of polyamides and poly(amide–imide)s derived from 2,2-bis(4-aminophenoxy) benzonitrile

A series of polyamides and poly(amide–imide)s were prepared by the direct poly-condensation of 2,2-bis(4-aminophenoxy) benzonitrile [4-APBN] with aromatic dicarboxylic acids and bis(carboxyphthalimide)s in N-methyl-2-pyrrolidone [NMP] with triphenyl phosphite and pyridine as condensing agents. The synthesis of 4-APBN involves a nucleophilic displacement reaction in dipolar aprotic solvent with the alkali metal salt of p-aminophenol and an activated aromatic dichloro compound. Bis(carboxyphthalimide)s were prepared by condensation of 4,4′-diaminodiphenylsulfone, 3,3′-diaminodiphenylsulfone, 4,4′-diaminodiphenylether, 4,4′-diaminodiphenylmethane, 3,3′-diaminobenzophenone, and trimellitic anhydride at a 1:2 molar ratio. The inherent viscosities of the resulting polymers were found to be in the range of 0.31–0.93 dl/g and glass transition temperatures between 235 and 298 °C. All polymers were soluble in aprotic polar solvents such as dimethylsulfoxide and NMP. The results of thermogravimetry revealed that all the polymers showed no significant weight loss before 400 °C. Wide-angle X-ray diffractograms revealed that all polymers were found to be amorphous except for the polyamide derived from isophthalic acid and polyamide–imides derived from diaminodiphenylether and diaminobenzophenone based bis(carboxyphthalimide)s.

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.

Molecular transport of aromatic hydrocarbons through crosslinked styrene-butadiene rubber membranes

Abstract Molecular transport of aromatic hydrocarbons throu gh crosslinked SBR has been carried out in the temperature range (25–65°C). SBR has been vulcanized by four different vulcanizing techniques viz., conventional, efficient, dicumyl peroxide and a mixture consisting of sulfur and peroxide. SBR vulcanized with EV system showed highest solvent uptake tendency and that vulcanized with peroxide showed the lowest. The influence of penetrant size on sorption behaviour of SBR has been examined. The thermodynamic constants such as standard entropy, standard enthalpy and first-order kinetic rate constant have been evaluated. A correlation between theoretical and experimental sorption results was evaluated. The polymer—solvent interaction parameter (χ) was also computed from the diffusion data.

Bis propargyl ether resins : synthesis and structure-thermal property correlations

Abstract Bis propargyl ethers of bisphenol-A, (BPA), bisphenol ketone (BPK) and bisphenol sulfone (BPS) were synthesised and characterised. These monomers were thermally polymerised to the corresponding poly (bischromenes). The cure behaviour, as monitored by differential scanning calorimetry, depended on the structure of the monomer. The non isothermal kinetic analysis of the cure reaction using four integral methods revealed that the presence of electron-withdrawing group did not favour the cyclisation reaction leading to formation of chromene, which precedes the polymerisation and this is in conformation to the proposed mechanism of polymerisation. Thus, the cure temperature and activation energy for the reaction increased in the order BPA