Thermal decomposition siloxanes

Investigation of the thermal decomposition of the novel architectures of POSS macromonomers and POSS siloxane co-polymers was reported.598 Novel, thermally stable polysiloxanes with bulky side-chain tris(trimethylsilyl)hexyl, and reactive glycidoxypropyl substituents, were recently described.599... 

At the other end of the temperature spectrum, with high thermal stability of siloxane-modified poly(arylene carbonates) also a desired property, the onset of thermal decomposition (40) for polymers 1-12 was found to be in the range of 385-456°C (as determined from TGA curves obtained by heating polymer samples in nitrogen at a heating rate of 20°C/min.). There does not appear to be any pronounced trend in regard to variation of the thermal stability with structure in polymers 1-12. The small differences in the values of T for these polymers can be due... 

Thus, N-pyrimidine phthalimide reacted with hexylamine at room temperature to form an amide-amide. The initial amide-amide formation proceeded more rapidly in chloroform as compared to dimethylsulfoxide (DM SO). However, the ring closure reaction to the imide was favored by the more polar, aprotic DMSO solvent, yielding the imide in nearly quantitative yield after 3 hours at 75 °C. The authors were able to utilize this synthetic approach to prepare well-defined segmented poly(imide-siloxane) block copolymers. It appears that transimidi-zation reactions are a viable approach to preparing polyimides, given that the final polyimide has a Tg sufficiently low to allow extended excursions above the Tg to facilitate reaction without thermal decomposition. Additionally, soluble polyimides can be readily prepared by this approach. Ultimately, high Tg, insoluble polyimides are still only accessable via traditional soluble precursor routes. 

The 1273 K treated sample (figure 14.9a) shows 2 bands, at 3740 and 2290 cm 1. These may be assigned to surface hydroxyls and silane groups, respectively. Upon thermal decomposition of the alkyl chain, hydrogen gas is formed. The evolved gas may react with the silica surface siloxanes, according to reaction (E). 

Plasma-polymerized materials differ significantly from those polymerized by conventional methods in their surface properties, and surface tension values do not correspond. This difference may be due to the highly cross-linked nature of plasma polymers or to the incorporation of other entities from the carrier gas. These effects are more important than the intrinsic differences in backbone fiexibility. Wrobel (88) presents ATR-IR (attenuated total reflection infrared) spectroscopic data indicating that silazanes and silanes cross-link more readily than do siloxanes under plasma conditions. Wrobel and his co-workers (89) have also used contact angles to study the thermal decomposition of plasma-polymerized organosilicon polymers. 

Fig. 2. Thermal decomposition of Tsi-siloxanes with various degrees of cross-linking (TGA measurements — heating rate 20 °C/min).

Table 16.1.1. Summary of reports on thermal decomposition of poly(siloxanes) [6],...

Thermal decomposition of poly(alkylsiloxane) copolymers is similar to that of poly(dimethylsiloxane), the main compound generated from the siloxane moiety being cyclic siloxanes. The results for a Py-GC/MS analysis of a sample of poly(dimethyl-siloxane-co-methylphenylsiloxane) with phenyl/methyl mole ration 1/1 are shown in Figure 16.1.2. The polymer idealized formula is [-Si(CH3)20-]x [-Si(CH3)(C6H5)0-]y where x = y. The pyrolysis was done at 600° C in He with the analysis done in the same conditions as for other examples previously discussed (see Table 4.2.2). The peak identification for the chromatogram was done using MS spectral library searches only and it is given in Table 16.1.4. 

Besides poly(siloxanes), a few other silicone containing polymers have been synthesized. One such polymer is poly(dimethy-1,4-silphenylene). Thermal decomposition of this polymer at 400° C in argon [3] generates methane, dimethylsilane, benzene, dimethylphenylsilane, HSi(CH3)2-C6H4-SiH(CH3)2, H(Si(CH3)2-C6H4)2-H, dimethyl-diphenylsilane, and C8H4-(Si(CH3)2-C6H4)2H. 

The results of the thermal decomposition of branched chain methylsiloxane polymers and polycyclics indicate that the decomposition is similar to that observed with PDMS. The structures of the degradation products from (T2D6) and (T2D9) show that they originate from the cyclolinear polymers through the loop mechanism. Indeed, considering the bicyclic compounds of series I in Table 13 they can only be formed via a concerted intramolecular mechanism. Products in series II could result from three consecutive rearrangement steps which would explain the presence of the three-mem-bered cyclic siloxane... 

The thermal decomposition of linear and branched polydimethyl-siloxanes and their derivatives - has been studied extensively. All of these papers were published... 

Blazso, M., G. Garzo, and T. Szekely, Py-GC studies on Poly(dimethyl siloxanes) and Poly(dimethyl alkylene siloxanes), Chromatographia, 5 485-492 (1972). Blazso, M., G. Garzo, K. Andrianov, N. Makarova, A. Chernavski, and L. Petrov, Thermal decomposition of cyclo-Unear methylsUoxane polymers, J. Organometal. Chem., 165 273 (1979). 

BaUistreri, A. Montaudo, G. Lenz, R. W., Mass Spectral Characterization and Thermal Decomposition Mechanism of Alternating Silarylene-Siloxane Polymers. Macromolecules 1984,17,1848-1854. 

Garzo et al also studied the thermal decomposition of branched chain methylpolysiloxane resins and used mass spectrometry to identify up to 17 low molecular weight methyl siloxane reaction products. These workers list retention indices on three different stationary phases, which enable conclusions to be drawn regarding the structure of the individual pyrolysis products and hence of the resin degredation process. 

The industrial process of the anionic equilibration of cydic siloxanes is usually carried out in bulk at devated temperature. The choice of initiator is critical. Some contaminations, originating from the initiator, particularly those of add, basic or ionic natme, dramatically reduce thermal stability of polysiloxanes. Thermolabile silanolates, such as Me4N OSi= or Bu4P OSi=, are convenient initiators, because they can be easily removed from polymer by thermal decomposition. " Silanolate centers must be neutralized to avoid decomposition of polymer. MesSiCl is commonly used for this purpose, as it introduces the inert MesSi groups to the chain ends however, it may not react sufficiently fast with strongly aggregated silanolates. 

Above all of the polym shown in the transition diagram can be in the mesomorphic state. This is primarily indicated by the presence of a narrow and intense reflection of 29 = 8-12° in the diffiaction patterns charactmstic of the mesomorphic state of siloxane polym. The isotropic transition temperature T apparently decreases in the region of intense thermal decomposition in polymers enriched with phenyl substituents. In contrast to the heat of fusion, the dependence of the heats of the isotnq)ic transition on the concentration of phenyl substituents is nonmonotonic. Incorporation of one phenyl substituent results in a sharp decrease in Qi 10 to 3.8 J/g, and a slight decrease in the heat is then observed with an increase in the number of phenyl substituents. 

The diffusion coefficient required to account for penetration of about 500 A is of the order of 10 cm /sec. (for a 200 s. bake). For small molecules (such as the monomeric resist components) this is a quite reasonable number, either above or below 7 (cf. measurements on camphorquinone in polycarbonate 18), which are in the range of 10 — 10 above Tg and 10 — 10 below). Tg of PVA is 85C (29). The situation is less clear with polymer diffusion. PPSQ apparently does not have a distinct Tg (20), but remains assy up to its thermal decomposition temperature (this report is for high material). Wang, et al., ( ) obtained diffusion coefficients of 10 cmVsec for high Af poly(but methacrylate) in latex particles, at temperatures well above Tg. It is possible that the low Af, siloxanes used here diffuse at substantially higher rates. Thus the postulated diffusion is plausible albeit not assured. 

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