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

Abstract

The thermal degradation of Me3SiO end‐blocked polydimethylsiloxane (eb‐PDMS) and polydimethylsiloxane‐N‐phenylsilazane (eb‐PDMS–NPhSz) copolymer was studied. For both polymers relative degree of polymerization (DP/DP0) as a function of conversion (1 – W/W0) data were obtained. For eb‐PDMS 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 (k) for initiation were obtained at four temperatures from plots of DP−1 vs. time. An Arrhenius activation energy of approximately 80 kcal/mol was determined 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/DP0 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.

Modeling hydrophobic recovery of electrically discharged polydimethylsiloxane elastomers

“…At this severe discharge condition, the LMW species are produced in quantities that is sufficient to cause full hydrophobic recovery [30] of the oxidized PDMS. These LMW species may be produced in situ by the siloxane bond rearrangement occurring via a transition state [45][46][47][48][49], and the degradation products may be comprised of similar sized cyclic siloxane fluids [31].…”

“…4) show the absence of the fluorinated carbon species on the surface before discharge, but its presence (e.g., the appearance of the peak at 293-294 eV region is indicative of the CF 3 groups) on the surface after discharge due to the diffusion of the in situ produced LMW species by which the oxidized PDMS elastomers recover their hydrophobicity. The fluorine-containing species may be produced by the cleavage of the Si-C bond of 3,3,3-trifluoropropylmethylsiloxane fluid (MW = 950) present in the polymer network, further degradation of which could result in the formation of 1,1-difluoropropene and cyclic PDMS fluids [46][47][48][49][50]. In particular, the 1,1-difluoropropene (MW = 76) has a molecular weight which is lower than that of hexamethylcyclotrisiloxane (D 3 , MW = 222) and octamethylcyclotetrasiloxane (D 4 , MW = 296) and may thus have a higher diffusivity than D n (n 3).…”

Studies on the silicone resins cured with polymethylsilazanes at ambient temperature

“…Radhakrishman12 reported an E a of 170 kJ/mol for the thermal degradation of the hydroxyl‐terminated polydimethylsiloxane; Kang et al13 reported an E a of 334 kJ/mol for the thermal decomposition of trimethylsiloxy end‐capped polydimethylsiloxane. It is quite interesting that the E a for thermal decomposition of the heat‐cured silicone resin is close to that for the hydroxyl‐capped polydimethylsiloxane, whereas the E a for the silicone resin cured with 5% SN‐1 is very close to that for the trimethylsiloxy‐capped polydimethylsiloxane.…”

A new class of silicone resins for coatings

“…10 In an inert atmosphere, depolymerization occurs with the loss of volatile products-mostly low-molecular weight cyclic oligomers-but is often catalyzed by traces of acids, bases, water, or residual catalysts used in the polymer's original production. 34 Typically, depolymerization occurs near 400°C for reasonably pure polydimethylsiloxane. 35 …”