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Viscoelastic and dielectric relaxation behavior of substituted poly(p-phenylenes)

The thermal, dynamic mechanical, and dielectric relaxation behavior of poly(p-phenylenes) substituted with benzoyl and 4-phenoxybenzoyl groups and of a copolymer of benzoyl-1,4-phenylene and 1,3-phenylene were investigated. These amorphous materials were found to have flexural moduli higher than any other reported unoriented thermoplastic. Benzoyl-substituted poly(p-phenylene) which possesses the greatest rigid-rod character also displayed the highest modulus. The temperature dependence of viscoelastic and dielectric relaxation times were well described by the Williams-Landel-Ferry and Vogel-Fulcher equations, respectively. The temperature sensitivity or «fragility» of viscoelastic shift factors α T and apparent dielectric relaxation times τ * was greatest for the copolymer. The glass transition temperature of these polymers varied as a function of the total free volume. The polymer containing the 4-phenoxybenzoyl side group exhibited the lowest T g due to internal plasticization, but the highest Vogel energy attributable to the large intramolecular bond rotational barrier associated with the bulky pendant group. The shape of frequency plane dielectric relaxation spectra of the substituted polyphenylenes could be accurately fitted to the Kolrausch-Williams-Watts (KWW) correlation function. The KWW stretched exponential term β displayed a very weak dependence on molecular structure and remained constant within experimental error for each material over the temperature range studied. Since β should be proportional to the degree of intermolecular coupling, the structural variation in «fragility», albeit small, of the substituted polyphenylenes may not be fully described by the coupling model for relaxation. Assuming the macroscopic expansion coefficient is proportional to the free volume expansion coefficient, the temperature dependence of τ * and α T may be explained more simply from free volume considerations. The 4-phenoxybenzoyl-substituted polymer displayed the highest expansion coefficient and largest total free volume, but the benzoyl-1,4-phenylene/1,3-phenylene copolymer exhibited the largest relative change in free volume with temperature, which explained the greater «fragility» of the copolymer compared to either homopolymer