B. B. Troitskii

Inhibition of thermo-oxidative degradation of poly(methyl methacrylate) and polystyrene by C60

Abstract Investigations were made on the effects of C60 on the degradation of poly(methyl methacrylate) (PMMA) and PSt in the stream of helium having 0.06% of oxygen and in dynamic oxygen by differential scanning calorimetry (DSC) method and under oxygen at 282 and 238°C for the case of PMMA and PSt, respectively, by thermogravimetric method. It was shown that C60 retards the degradation of the polymers. By thermogravimetry it was demonstrated that in the presence of fullerene, induction periods on the curves of dependence of weight loss of polymers on time of degradation, rise substantially. By DSC method it was illustrated that C60 considerably increases the temperatures of the onset of polymers degradation. The degradation of PMMA and PSt at elevated temperatures in the presence of small amounts of oxygen is assumed to be the purely thermal degradation initiated by decomposition of small quantities of formed polymeric hydroperoxides in addition. The suggestion was made that in this case, the retarding effect of C60 is connected with its interaction with macroradicals R• with formation of less active compounds. The correlation between the rate of the depropagation reaction of R• and the rate of their interaction with oxygen was estimated in the PMMA degradation at different temperatures and concentrations of oxygen in the polymer. The retarding influence of oxygen on the PMMA depropagation rises with an increase in oxygen concentration in the polymer and with a decrease in the degradation temperature. It was supposed that in the thermo-oxidative degradation of PMMA at elevated temperatures, the inhibiting effect of C60 is due to its interaction with R• and oxygen-containing radicals with formation of less active compounds. In the case of PSt retarding influence of fullerene is mainly connected with its reaction with oxygen-containing radicals, but the interaction of C60 with R• must not be ruled out.

Retardation of thermal degradation of PMMA and PVC by C60

Abstract Investigations were made on the effects of C60 upon the rate of depolymerization of PMMA films during the thermal degradation in vacuo at 300 and 332 °C by thermogravimetric method and upon the rates of dehydrochlorination of PVC during the thermal degradation in vacuo with continuous and effective removal of HCl and in the presence of volatile products at 200 °C. It was shown that C60 retards the thermal depolymerization of PMMA and the autocatalytic thermal dehydrochlorination of PVC in the presence of HCl and does not decrease the rate of PVC nonautocatalytic degradation with removal of HCl. The suggestion was made that the retarding effect of C60 is connected with its interaction with macroradicals, which generate in the thermal degradation of PMMA and PVC, with formation of less active compounds. In the case of PVC, there is the probability of interaction of C60 with chlorine atoms too.

Investigation of stabilizing effectiveness of alkyl(aryl)tin maleates and thioglycolates in the thermal degradation of poly(vinyl chloride)

Abstract Investigations were made on the effects of alkyl(aryl)tin salts of maleic and thioglycolic acids and their esters upon the rate of dehydrochlorination and crosslinking, and on the absorption spectrum of PVC during degradation in vacuum at 190°. It was shown that these stabilizers cause dehydrochlorination of PVC to be eight times less than the rate of polymer degradation in vacuo with removal of HC1. The stabilizers are arranged according to their stabilizing powers. The interaction of the stabilizers with 2-chlorobutane (a model for normal units of PVC) was studied at 180°. According to the rate of this interaction, the compounds are arranged in inverse order compared with that for the stabilizing efficiency. Thermal degradations of the stabilizers themselves were investigated. Experimental data concerning the thermal degradation of PVC in the presence of organo-tin salts were considered from the point of view of simultaneous reactions of the stabilizers with HC1, chloroallylic fragments and normal units of the macromolecules.

Mathematical models of the initial stage of the thermal degradation of poly(vinyl chloride)—3. The thermal degradation of poly(vinyl chloride) in the presence of HCl☆

Abstract Several mathematical models for the initial stage of the thermal degradation of PVC in the presence of HCl which, are concerned with the different mechanisms of the autocatalytic degradation of PVC have been considered. In two cases agreement of the theoretical kinetic curves of PVC dehydrochlorination with the experimental ones was observed: (i) HCl accelerates the thermal degradation of normal units of macromolecules by the molecular mechanism; (ii) autocatalytic degradation of PVC is the branched chain reaction with degenerative branching of chain. The products of the thermal degradation of PVC (polyene and HCl) interact with each other and produce new active species which cause additional initiation of the thermal degradation of normal units of PVC by a chain mechanism. The mathematical models for the thermal degradation of PVC in the presence of HCl and active dienophil (maleic anhydride) have been proposed. The results obtained and the literature data concerning the thermal degradation of PVC in the presence of HCl have allowed the supposition to be drawn that the autocatalytic degradation of PVC is a branched chain reaction with degenerative branching of chain.

Degenerated branching of chain in poly(vinyl chloride) thermal degradation

Abstract The theory of the autocatalytic thermal degradation of PVC in the presence of HCl as the branched chain reaction with the degenerated branching of the chain is developed. The role of the thermally excited states of polyenes and polyenyl carbocations in reactions of the degenerated branching of chain has been considered. Literature data concerning dependences of energies for S0→S1 and S0→T1 electron transitions and of energy barriers for rotation about double bonds in polyenes and polyenyl carbocations on n have been discussed. Using literature data concerning the thermal cis–trans rearrangement of all-trans-β-carotene, the rate constants of this reaction have been calculated at 180° and 200°. These rate constants have been compared with rate constants of dehydrochlorination of different fragments of PVC macromolecules and those of cyclization of polyene fragments. It has been concluded that at 180–200°C polyenes having eight or more conjugated double bonds are highly reactive. Reactivity of polyenyl carbocations is greater because energy for excitation of them in the triplet state is less than that of polyenes. It has been shown that activation energies for the thermal degradation of chlorohydrocarbons having double bonds in the presence of HCl are in the field of triplet states of polyenyl carbocations. It has been concluded that in the autocatalytic thermal degradation of PVC in the presence of HCl, reactions with participation of polyenes and polyenyl carbocations excited in the triplet state make the principal contribution to the degenerated branching of chain. HCl is shown to be the catalyst of these reactions, which proceed with a slow rate in the nonautocatalytic degradation of the polymer in the absence of HCl.

Effect of tacticity on thermal degradation of poly(vinyl chloride)

Abstract The tacticity of different samples of PVC suspension has been measured by 13C NMR spectroscopy. The dehydrochlorination of PVC samples under vacuum with continuous removal of HCl by freezing has been studied at 200 °. The values of the effective constants of dehydrochlorination rates of unstable fragments and normal units of PVC, the values of the initial and the stationary rates of degradation and the total concentrations of unstable groups in PVC samples have been obtained. It has been shown that only the contents of isotactic nonads may be compared with the values of the concentrations of unstable fragments in the samples. There is no correlation between the initial rate of PVC degradation and the content of isotactic triads or nonads. A correlation exists between the total concentration of unstable fragments and the initial rate of degradation of PVC samples. The obtained results give evidence that the tacticity of macromolecules has no significant influence on the initial stage of the thermal degradation of PVC and that the branching groups with the chlorine atom near the tertiary carbon atom and the internal chloroallylic fragments may be the main unstable groups of PVC macromolecules.

Investigation of autocatalytic thermal degradation of poly(vinyl chloride) by ESR spectroscopy

Abstract The dehydrochlorination of PVC under vacuum with continuous removal of HCl and in the presence of HCl, has been studied at 200 °C. The dependences of the intensity of the ESR signal of degraded PVC on time of degradation and HCl loss conversion have been obtained. In PVC degradation in the presence of HCl, they have an autocatalytic character. At the same percentage of dehydrochlorination, the spin concentration is significantly more in the PVC degradation in the presence of HCl than with removal of HCl. The conclusion has been made that the formation of paramagnetic centers in degraded PVC is intensified by the presence of HCl. It has been shown that during autocatalytic PVC degradation, H 2 practically does not form. The obtained results are discussed in terms of a theory of the autocatalytic thermal degradation of PVC as a branched chain reaction with degenerate branching of the chain.

Investigation of Fullerenes as High Temperature Stabilizers of Poly(methyl methacrylate) and Polystyrene

Abstract Investigation has been made on the effects of fullerenes C60 and C70 on the degradation of PMMA and PS under helium and oxygen by a DSC method. The dependences of the temperature of the onset of the thermal and thermo-oxidative degradation of the polymers on concentration of C60 and C70 have been obtained. The temperature limits of effective inhibition of the polymers by fullerenes have been determined. The temperature limit depends on the chemical structure of polymer, namely it is considerably greater in the degradation of more stable polymer. In the thermo-oxidative degradation of the polymers with addition of fullerenes, the temperature limit of inhibition of less stable PS is much less and of more stable PMMA is considerably greater than that in the presence of well-known antioxidants. It was concluded that fullerenes are more effective high temperature inhibitors of the thermo-oxidative degradation of PMMA than well-known antioxidants. The suggestion was made that the thermo-oxidative degradation of polymers initiates the oxidation of fullerenes.

Mathematical models of the initial stage of the thermal degradation of poly(vinyl chloride)—IV. Influence of stereoisomery

Abstract Two mathematical models for the initial stage of the thermal degradation of PVC initiated by decomposition of normal units and the internal chloroallylic fragments have been proposed. Solution of the systems of differential equations was carried out according to Eulers method with the computer. The theoretical curves for PVC dehydrochlorination at 200, 240 and 250 ° were calculated. They coincide with the experimental ones up to 30–40% conversion. The results give evidence of the validity of the equations which have previously been deduced for the initial stage of PVC degradation initiated by decomposition of normal units and unstable fragments such as the internal chloroallylic groups, the branching fragments with chlorine atom near tertiary carbon atom or sum of these fragments in all proportions. The mathematical model for the PVC degradation with the assumption that PVC dehydrochlorination is initiated by decomposition of normal units and the unstable conformer of the definite isotactic stereoisomer, has been proposed. It has been shown that in this case there is no agreement between the experimental kinetic curve for PVC dehydrochlorination at 200 ° and the theoretical ones. The conclusion has been made that at elevated temperatures the PVC thermal degradation is not the stereoselective reaction and that the internal chloroallylic fragments and branching groups with a chlorine atom near the tertiary carbon atom may be the major unstable fragments which have a significant influence on the early stage of PVC degradation.

Investigation of the Stabilizing Action of Mixtures of Fullerene C60 with Known Antioxidants in the Thermo-oxidative Degradation of Polystyrene

Abstract Investigations have been made on the effects of double mixtures of C60 with well-known antioxidants (amines, phenols, S- and P-containing compounds, Ph3Sb) on the thermo-oxidative degradation of PS by DSC method under oxygen. The influence of mixtures on the temperature of the onset of polymer degradation, T 0, has been studied. The effects of concentration of C60 on the value of the temperature limit of inhibition of PS thermo-oxidative degradation by known antioxidants, T 0 max, have been investigated. It has been shown that mixtures of C60 with Ph3Sb and phenyl-β-naphthylamine have a synergistic effect upon T 0. At a total concentration of 1.4 × 10−2mol/kg, the maximum value of T 0 is observed at a molar C60: compound ratio of 3:4 and 1:2.5 for the mixtures of C60 with Ph3Sb and phenyl-β-naphthylamine, respectively. Addition of C60 to these compounds leads to an increase in T 0 max for Ph3Sb and phenyl-β-naphthylamine. The suggestion was made that in the case of mixtures of C60 with Ph3Sb, the synergistic effect is Associated with different mechanisms of stabilizing action of the components on PS degradation, namely C60 reacts with alkyl and oxygen-containing radicals with the formation of more stable compounds while Ph3Sb decomposes polymeric and fullerene-containing peroxides and hydroperoxides. The decomposition of Ph3Sb was investigated by DSC method under oxygen. The temperature of the onset of Ph3Sb degradation was shown to be 362.5°C. It was concluded that further investigations are necessary for understanding the synergistic action of mixture of C60 with phenyl-β-naphthylamine in the thermo-oxidative degradation of PS.