title: Kinetics of Thermal Conversion of Conjugated Polymers Investigated from Their Optical Absorption Spectra.
abstract: We report on a phenomenological, theoretical model to calculate the rate constants and activation energies for the thermal conversion reactions from poly(xylylidene tetrahydrothiophenium chloride) (PTHT) into poly(p-phenylenevinylene) (PPV) using the optical absorption spectra of spin-coated films. The probabilities of electron transitions were calculated considering Franck-Condon states with a Gaussian distribution of conjugated segments and molecular excitons. The dependence on the conjugation degree (n) for the energy gap, transition dipole moment, and electron-phonon coupling were obtained semiempirically using published data for PPV. Fitting was performed for the C-C stretching of the aromatic ring 1550 cm(-1), for it is considered the most optically active. The isotherms for consumption and formation of PPV segments were fitted using a first-order and consecutive reactions, respectively. With this modeling we could identify the most probable reactions, where the formation of longer PPV segments n ≥ 3 occurs only via reactions from smaller conjugated segments (i.e., n = 1 and 2). The activation energies tend to decrease with longer conjugation lengths. Significantly, the modeling allows us to predict the conversion temperature and chemical composition yielding a predefined distribution of conjugated segments, which can be applied to any polymer undergoing thermal conversion, decomposition, or photo-oxidation.
authors.affiliation: †Center for Biological and Natural Sciences, Federal University of Acre, P.O. Box 500, 69920-900 Rio Branco, Acre, Brazil.
authors.forename: Marcelo Castanheira
authors.initials: MC
authors.lastname: da Silva
authors.affiliation: ‡São Carlos Institute of Physics, University of São Paulo, P.O. Box 369, 13560-970 São Carlos, São Paulo, Brazil.
authors.forename: Osvaldo N
authors.initials: ON
authors.lastname: Oliveira
authors.affiliation: §Institute of Physics, Federal University of Uberlândia, P.O. Box 593, 38400-902 Uberlândia, Minas Gerais, Brazil.
authors.forename: Alexandre
authors.initials: A
authors.lastname: Marletta