Renjith Devasia

Solvent and kinetic penultimate unit effects in the copolymerization of acrylonitrile with itaconic acid

Abstract Copolymerization of acrylonitrile (AN) with itaconic acid (IA) in dimethylformamide (DMF) and DMF/water mixture was investigated at enhanced concentrations of the latter. Analysis of the copolymer composition revealed the existence of a marked penultimate unit effect with respect to radicals terminated in AN. The reactivity of IA was considerably less than that of AN, manifested as a negative reactivity ratio for the former. The rIA values ranging from −0.28 to −0.50 and rAN values ranging from 0.53 to 0.70, were obtained by Kelen–Tudos (KT) and extended KT methods. The penultimate reactivity ratios were determined by both linear and non-linear methods. The values ranged from r1=0.009 to 0.01, r1′=0.0015 to 0.0043, r2=0.54 to 0.69 and r2′=0.9 to 1.03. The reactivity of AN radical towards IA decreased about twofold when the latter formed the penultimate group. The penultimate model explained an acceptable rational feed-copolymer composition profile for the whole composition range. Addition of water decreased the reactivity of IA slightly. IA caused a decrease in the apparent copolymerization rate in agreement with the observed trends in the reactivity ratios; presence of water caused a further decrease in the rate of polymerization. A statistical prediction of monomer sequences based on reactivity ratios implied that IA existed as a lone monomer unit between the long sequences of AN units.

Copolymerization of acrylonitrile with itaconic acid in dimethylformamide: effect of triethylamine

The copolymerization of acrylonitrile (AN) in dimethylformamide (DMF) was retarded by the presence of itaconic acid (IA) comonomer. Addition of TEA helped overcome the retardation at enhanced concentrations of IA in the feed. The monomer reactivity ratios determined by both terminal and penultimate models revealed that the overall monomer reactivity’s are practically unaffected by the presence of TEA. The penultimate-unit effect for radicals terminated in AN was enhanced by the presence of TEA. Higher TEA concentrations helped regain the reactivities of AN and IA to AN-radical to the state in pure DMF. The penultimate model could explain the feed-copolymer composition profile for the whole range. Whereas IA systematically retarded the polymerization rate at all concentration regime in DMF, it increased the rate at higher IA concentration in DMF/TEA system. For a given IA concentration, the polymerization rate decreased as the solvent is enriched in TEA. The copolymers synthesized in the presence of TEA, manifested higher cyclization temperature and consequently lower char residue, attributed to the incorporation of TEA in the polymer by means of salt formation with IA moiety camouflaging the catalytic effect of the –COOH group in cyclization reaction. 13C-NMR studies confirmed the incorporation of the TEA molecules in the polymer chain.

Rheological Behavior of Dope Solutions of Poly(acrylonitrile-co-itaconic acid) in N,N-dimethylformamide : Effect of Polymer Molar Mass

The rheological behavior of dope solutions of poly(acrylonitrile‐co‐itaconic acid) or poly(AN‐co‐IA) is important from the point of view of deriving the spinning conditions for good quality special acrylic fibers. The viscosity of the resin dope is dictated by the polymer concentration, molar mass, temperature and shear force. The dynamic shear rheology of concentrated poly(AN‐co‐IA) polymer dope solutions in N, N‐dimethylformamide, in the molar mass (M¯v) range of 1×105 to 1×106 g/mol, was investigated in the shear rate (γ′) range of 1×101 to 5×104 min−1. An empirical relation between η and M¯v was found to exist at constant shear rate. The dope viscosity was dependent on the molar mass and the shear rate at a given temperature (T) and concentration. The polymer molar mass index of dope viscosity (m) was calculated as functions of concentration (c), shear rate and temperature. The m values increased with shear rate and temperature. A master equation relating m, with shear rate and temperature was derived for a given dope concentration. At higher shear rates, m tends to the value of 3.4, which is close to the molar mass index of viscosity reported for molten thermoplastics. m increased significantly with shear rate and nominally with temperature, while an increase in concentration decreased it. The onset of shear thinning of the dope shifted to a lower shear rate regime with an increase in polymer concentration and the molar mass. For a given value of molar mass, the increase in viscosity of the dope solution with polymer concentration was dependent on the shear rate.

Boron-modified phenol formaldehyde resin-based self-healing matrix for Cf/SiBOC composites

Cf/SiBOC was fabricated from 2D carbon fabric as reinforcement and slurry-containing boron-modified phenol formaldehyde (BPF) resin with silicon as matrix resin using reaction-bonded silicon carbide method. The processing involves synthesis of (BPF) resin by reacting various amount of boric acid with phenol formaldehyde resin, polymer to ceramic transformation at 1450°C under argon atmosphere, with and without silicon, thermal transformation of the polymer matrix composite into a ceramic matrix composite and evaluation of isothermal oxidation for ceramics and its composites at 1000, 1250 and 1500°C. The ceramic studies, confirmed the formation of B4C, SiC and SiB4 (SiBOC) mixed phase and the role of boron as a catalyst for graphitisation of free carbon present in the ceramic. Oxidation of Cf/SiBOC composite at various temperatures leads to the formation of borosilicate glass which heals the cracks, hindering the inwards diffusion of oxygen.

Processing and properties of SiCf/SiBOC ceramic matrix composites by polyborosiloxane impregnation and pyrolysis

SiCf/SiBOC Ceramic Matrix Composites (CMCs) were fabricated using polyborosiloxane as the matrix resin and Nicalon™ NL-202 silicon carbide fiber as the reinforcement via polymer infiltration/impregnation and pyrolysis process (PIP). Repeated PIP cycles resulted in CMCs with a density value of ≈ 2 g/cc and a maximum average flexural strength value of 108 MPa. Oxidation resistance of SiCf/SiBOC was compared with Cf/C and Cf/SiBOC at 1000°C. SiCf/SiBOC composite shows a better oxidation resistance due to the formation of a protective layer of amorphous borosilicate glass on oxidation.

POLYACRYLONITRILE PRECURSORS FOR CARBON FIBER WITH IMIDOCARBOX YLIC ACID UNITS: COPOLYMERIZATION OF ACRYLONITRILE WITH MALEIMIDOBENZOIC ACID

ABSTRACT 4-Maleimidobenzoic acid (MBA) was explored as a comonomer in polyacrylonitrile (PAN) precursors for carbon fiber. The copolymerization of acrylonitrile (AN) with MBA was carried out in DMF. The reactivity of MBA was considerably less than that of AN, which was manifested as a negative reactivity ratio for the former. The r MBA- values from −0.24 to −0.33 and r AN values of 1.07 were obtained by Kelen-Tudos and extended Kelen-Tudos methods. The penultimate reactivity ratios were determined by both linear and non-linear methods. The values were r 1=0.0093, r 1′=0.0132, r 2=1.063 and r 2′=1.625. The relative MBA concentration in the copolymer decreased drastically on enhancing its content in the monomer mixture. The penultimate model could satisfactorily explain the feed-copolymer composition profile for the whole composition range. MBA caused a decrease in the apparent copolymerization rate and molecular weight in agreement with the observed trends in the reactivity ratios. A statistical prediction of monomer sequences based on reactivity ratios implied that MBA existed as a lone monomer unit between the long sequences of AN units. This sequence distribution is suited for the efficiency of MBA in cyclisation reaction, which stabilizes PAN during its pyrolysis. Optimum thermal stabilization effect and char yield were observed for copolymers with around 3 mol% MBA in the chain.

Comparative Rheological Properties of Dope Solutions of High Molar-Mass Poly(acrylonitrile-co-itaconic acid) and Poly(acrylonitrile-co-methylacrylate-co-itaconic acid)

The rheological behaviour of a dope solution of poly(acrylonitrile-co-itaconic acid) copolymer was compared with that of a poly(acrylonitrile-co-methylacrylate-co-itaconic acid) terpolymer of identical molecular weight. The effects of temperature, concentration and shear force on the rheology of the polymer dope were examined. In both cases, the temperature dependence of zero shear viscosity conformed to the Arrhenius-Frenkel-Eyring equation. The free energy change ΔGv increased with concentration more significantly for the copolymer than the terpolymer. The Power Law model was fitted to the shear dependency of the viscosity, and the pseudoplasticity index (n) decreased with the dope concentration. The shear thinning behaviour was more pronounced for the terpolymer than the copolymer. The decreasing trend of the pseudoplasticity index with concentration and temperature was more pronounced in the case of the terpolymer than the copolymer. Empirical models were arrived at, which could predict the n value of the polymers of a given viscosity-average molecular weight at any concentration and temperature. The onset of shear thinning shifted to a lower shear regime with increase in solid concentration.