V. Lakshmana Rao

Toughening of Diglycidyl Ether of Bisphenol-A Epoxy Resin Using Poly (Ether Ether Ketone) with Pendent Ditert-Butyl Groups

Poly(ether ether ketone) (PEEKDT), hydroxyl terminated poly(ether ether ketone) (PEEKDTOH) and fluorine terminated poly (ether ether ketone) (PEEKDTF) with pendent ditert-butyl groups were synthesized by the nucleophilic substitution reaction of 4,4′-difluorobenzophenone with 2,5-ditert-butylhydroquinone in N-methyl-2-pyrrolidone medium using anhydrous potassium carbonate as catalyst. Diglycidyl ether of bisphenol-A epoxy resin was blended with PEEKDT, PEEKDTOH, and PEEKDTF, and cured with 4,4′-diaminodiphenylsulfone (DDS). The polymers formed heterogeneous blends before curing, and upon curing the polymers got dispersed in the epoxy matrix. The mechanical properties of the cured blends were slightly lower than that of the unmodified resin. The fracture toughness increased with the addition of ditert-butyl PEEK into epoxy resin and the extent of improvement was dependent on the type of modifier used. Hydroxyl terminated polymers gave up to 40% increase in fracture toughness. The dynamic mechanical spectrum of the blends showed only a single Tg due to the proximity of the glass transition temperature of modified PEEK and DDS cured epoxy resin.

Synthesis and characterization of poly(ether sulfone) copolymers

Poly(ether sulfone) copolymers I–V were synthesized by the nucleophilic substitution reaction of 4,4-dichlorodiphenyl sulfone with varying mole proportions of 4,4-isopropylidene diphenol (bisphenol A) and 4,4-dihydroxydiphenyl sulfone (bisphenol S) using sulfolane as the solvent in the presence of anhydrous K2CO3. The polymers were characterized by different physicochemical techniques. The glass transition temperature was found to decrease with increase in the concentration of bisphenol A units in the polymers. All polymers were found to be amorphous. Thermogravimetric studies showed that all the polymers were stable up to 400°C with a char yield of about 36% at 900°C in a nitrogen atmosphere. 13C-NMR spectral analysis reveals that bisphenol S-based triads are preferentially formed compared to bisphenol-A triads, indicating greater reactivity of bisphenol S toward dichlorodiphenyl sulfone. The overall activation energy for the thermal decomposition of bisphenol A-based polymer (1) is much higher than that of bisphenol S-based polymer (II). This was attributed to the modification of the backbone of polymer I during the initial cleavage of the C—CH3 bond of the isopropyledene group. Polymer II decomposes by cleavage of the C—SO2 bond.

Microstructure and thermal degradation of poly(ether ketone sulfone) copolymers: 13C NMR and thermogravimetry studies

Microstructure of poly(ether ketone sulfone) copolymers I–V, derived from varying mol proportions of dihydroxy diphenyl sulfone (DHDPS, A) and dihydroxybenzophenone (DHBP, C) with stochiometric amounts of difluorobenzophenone (DFBP, B) was studied by 13C nuclear magnetic resonance spectroscopy. The results were interpreted in terms of the compositional triads BBB, BBA, ABA, BAB, and AAB because B and C moieties become indistinguishable in the copolymers. Feed ratios calculated from the triad intensities agree well with experimental values, validating the chemical shift assignments. The presence of AAB and BBA triads in polymer II (A : C = 1 : 0) indicates the occurrence of transetherification reaction during its synthesis. Thermal decomposition characteristics of the copolymers were studied by thermogravimetry. Activation energies for thermal degradation were calculated using Coats-Redferns method assuming the order of the reaction is 1 and was found to vary from 281 to 193 kJ mol−1. A good linear correlation was obtained between activation energy values and BBB triad intensities. These observations were rationalized by consideration of their decomposition mechanisms.