Thermal stability of phosphonitrilic fluoroelastomers

Abstract

SynopsisThe thermal stability of the polymer prepared by substituting poly(dich1orophosphazene) with an excess of a 64/36 mole-% mixture of the sodium salts of 2,2,2-trifluoroethano1/2,2,3,3,4,4,5,5-octafluoropentanol were studied by thermogravimetric and gel permeation chromatography techniques. The primary mode of degradation of this polymer was shown to be via random scission. The overall degradation reaction corresponds to quasi-zero-order kinetics. INTRODUCTIONA number of such elements as sulfur, phosphorus, silicon, etc., have been investigated as potential inorganic polymer backbones with thermal stabilities in excess of 250°C. However, as Economy and Mason' have pointed out, the utility of these elements is limited because of their inability to form chains longer than ten atoms. Furthermore, the bond strengths of these elements indicate that polymers from them may be less stable than carbon chains.Heteroatoms appear to show significantly higher bond strengths and thus offer the possibility of obtaining polymers with improved thermal ~tabilities.~,~ These heteroatoms also show relatively high ionic character which can lead to dipole interactions with neighboring chains thus resulting in brittle, glass-like behavior. These chain interactions may be minimized by encasing the metal ion in an organic shell. In the case of silicones, the stearic effects of alkyl groups diminish these chain interactions. If one considers the bond strengths of these inorganic elements alone, one may overestimate the stabilities of the resulting polymers since these inorganic bonds have an availability of bonding orbitals which may provide low-energy pathways to degradation reactions. ' The stability of polymeric materials has been judged on the ability of the polymer to retain its mechanical properties over an extended service time. It is also possible to establish a stability criterion of some value by following the weight loss of a polymer as a function of temperature or time. Experimentally, such weight loss data can be obtained from thermogravimetric analysis (TGA). However, because significant property changes may occur without a noticeable weight loss, great caution should be exercized in drawing conclusions from these data.* It is possible that the rate of weight loss may represent the rate of sublimation or distillation of degradation products rather than the rate of degradation of the polymer. The latter problem may be overcome if the rate of evaporation of the fragments is extremely fast as compared with the rate of formation of fragments.5Information obtained from thermal weight loss data are quite valuable from the screening point of view. However, when combined with other information, such as molecular weight changes, this method of determining thermal stability may be extremely valuable and can serve as a basis for the calculation of the activation energies of the overall degradation reaction.

Kinetics of Thermal Degradation of Poly[bis(2,2,2-trifluoroethoxy)phosphazene]

“…Its application ranges from measurement of center-of-mass diffusion and rotational relaxation of both small molecules and macromolecules in solution14,15 to the kinetics of polymerization of polystyrene. 16 In this work we used DLS to study the rotational relaxation of chlorobenzene, henceforth referred to as CB, in poly(methyl methacrylate) (PMMA) over a range of temperature and CB concentration.…”

Synthesis of Polyphosphazenes by a Fast Perfluoroaryl Azide-Mediated Staudinger Reaction

“…The thermal decomposition temperatures were obtained from the differential thermogravimetric curve shown in Figure S24, from which the decomposition temperatures at the onset, the maximal rate, and the end of decomposition were obtained (Table ). Several factors influence the thermal stability of the polymer, including the structure of the monomer and the strength of the chemical bond. , Studies on the thermal degradation of polyphosphazenes conclude that the chain scission typically occurs randomly followed by both depolymerization and some degree of cross-linking. − The decomposition temperatures of polymers 3aa , 3ab , and 3ac synthesized from 1a having a diphenyl linker are in general higher than polymers 3ba and 3bb synthesized from 1b that has an ester linker (entry 1 vs 2, entry 3 vs 4, entry 5 vs 6). The ester bond typically decomposes around 263–284 °C as compared to the Ph–Ph bond which decomposes around ∼400 °C. , The highest thermal decomposition temperature was obtained from polymer 3ac (441 °C, entry 7), which was synthesized from the aromatic monomers 1a and 2c .…”

Mechanism of thermal degradation of some polyphosphazenes

“…On the basis of a terminal initiation-chain transfer mechanism, Boyd12 developed an equation for the rate of weight loss 1 dW W^dT = -M~*W/2 + 1 + 7]/[l + yñ(n°y0) ] (8) where W is the total number of repeat units in the sample (when multiplied by the molecular weight of a repeat unit (m0) it is the sample weight), kE is the rate constant for end-group initiation, x is the number-average degree of polymerization, ñ = n/n°, y is the reciprocal average zip w/w0_ length, y = y/y°, °is the transfer parameter, and superscript zero indicates initial values. Using Boyd's approach and making the assumption that the zip length is comparatively long, we are able to derive the following simplifying equation, relating fractional weight and degree of polymerization:…”