Kinetics of thermal depolymerization of trimethylsiloxy end-blocked polydimethylsiloxane and polydimethylsiloxane- N-phenylsilazane copolymer

• Published in: Science of the total environment 1988-07, Vol.73 (1), p.71-85
• Main Authors: Zeldin, M., Kang, D.W., Rajendran, G.P., Qian, B., Choi, S.J.
• Format: Article
• Language: English
• Subjects: Applied sciences; Exact sciences and technology; Inorganic and organomineral polymers; Physicochemistry of polymers; Properties and characterization
• ISSN: 0048-9697; 1879-1026
• DOI: 10.1016/0048-9697(88)90188-X

The thermal degradation of Me 3SiO end-blocked polydimethylsiloxane (eb-PDMS) and polydimethylsiloxane- N-phenylsilazane (eb-PDMS-NPhSz) copolymer was studied. For both polymers, relative degree of polymerization( DP / DP 0 ) as a function of conversion ( C = 1 − W/ W 0) data were obtained. For eb-PDMS with three different molecular weights, the results were consistent with a mechanism involving a rate-determining random siloxane bond cleavage initiation step followed by a rapid and complete depropagation of the active fragments evolving volatile cyclic oligomers. Rate constants for initiation were obtained at four temperatures from plots of DP −1 vs. time for eb-PDMS of M n = 6.83 × 10 4 . An Arrhenius activation energy of ∼ and is consistent with a SiOSi scission transition state. The degradation of eb-PDMS-NPhSz appears to follow the same depolymerization process evolving cyclic oligomers. Although DP / DP 0 vs. C data suggest a random cleavage-complete depolymerization mechanism, an Arrhenius plot suggests a more complex degradation mechanism. The role of impurities as degradation catalysts is discussed.