Effects of diffusion in the oxidative degradation of vulcanized rubbers. I. Rate of chain scission in the steady state

The effects of diffusion in the oxidation of cis-1,4-polyisoprene vulcanizates were investigated by means of the stress relaxation method. It was assumed that the diffusion of oxygen is coupled with first-order oxygen consumption and that the rate of chain scission is proportional to the rate of oxygen consumption. The diffusion equation of this process was solved under the steady-state condition to give a simple relation between the rate of chain scission and the film thickness. The experimental results were in good agreement with the theoretical treatment. The true activation energy as well as the ratio of the rate of oxidation k to the diffusion constant D could be estimated.     

Rate of thermooxidation and polymer morphology

A model of the thermooxidation of semicrystalline polymers involving two processes simultaneously is described. The first process is initiated by oxygen dissolved in the interspherulitic amorphous pahse. The second is related to a slower rate of polymer chain scission m the interlamellar amorphous pahse, which is controlled by oxygen diffusion into the spherulite. The presented model describes an exampble of isotactic polypropylene and predicts the effect of polymer morphology on the process of thermooxidation. It leads to the conclusion that the initial rate of polymer chain scission depends on the amount of interspherulitic amorphous phase fraction but the slower rate, considered in steady state conditions of oxygen diffussion into the interlamellar amorphous phase, depends depends on spherulite size according to the derived equation.     

Thermal degradation of copolymers of styrene and acrylonitrile. III. Chain-scission reaction

The chain-scission reaction which occurs in copolymers of styrene and acrylonitrile has been studied at temperatures of 262, 252, and 240°C. Under these conditions volatilization is negligible, and chain scission can be studied in virtual isolation. At 262°C three kinds of chain scission are discernible, namely, at weak links which are associated with styrene units, “normal” scission in styrene segments of the chain and scission associated with the acrylonitrile units. The rate constants for normal scission and scission associated with acrylonitrile units are in the ratio of approximately 1 to 30. The molecular weight of the copolymer has no effect on the rates of scission. At 252°C the same general behavior is observed for the copolymers containing up to 24.9% acrylonitrile. The 33.4% acrylonitrile copolymer is anomalous, however. At 240°C the trends observed at 262°C appear to break down completely although individual experiments are quite reproducible. This behavior at the lower temperatures is believed to be associated with the fact that the melting points of the various copolymers are in this temperature range. Thus the viscosity of the medium, which should be expected to have a strong influence on the chain scission reaction, will be changing rapidly with temperature, copolymer composition, and molecular weight in this temperature range.     

Thermal degradation of copolymers of styrene and acrylonitrile. I. Preliminary investigation of changes in molecular weight and the formation of volatile products

By the use of thermal volatilization analysis (TVA), 292°C was chosen as a suitable temperature for a preliminary experimental survey of the thermal degradation of styrene–acrylonitrile copolymers. TVA also indicated that there is no fundamental change in reaction mechanism as the acrylonitrile content of the polymer is increased from zero to 33.4% although there is a progressive increase in the rate of volatilization. The increase in the rate of volatilization over that of polystyrene is directly proportional to the acrylonitrile content of the copolymer. From the changes in molecular weight which occur during the reaction it is clear that the primary effect of the acrylonitrile units on stability is to cause an increased rate of chain scission, but there is a small proportion of “weak links” which are associated with the styrene units and which are broken instantaneously at 292°C. The number of monomer molecules liberated per chain scission, the zip length, is about 40 for polystyrene in the initial stages of degradation and decreases only to the order of 20 even in copolymer containing 24.9% acrylonitrile. Thus the unzipping process is not severely affected by the acrylonitrile units; this is borne out by the fact that acrylonitrile appears among the products in very much greater concentrations than from pure polyacrylonitrile. The proportion of larger chain fragments (dimer, trimer, etc.) also increases with acrylonitrile content.     

The vacuum and oxidative pyrolysis of poly-p-xylylene

Poly-p-xylylene prepared by pyrolysis of di-p-xylylene has been degraded under vacuum and in the presence of oxygen as a function of temperature and oxygen pressure. The vacuum pyrolysis is mainly due to “abnormal” structures. Volatiles are initially produced quite slowly, but the reaction accelerates subsequently. Arrhenius equations were derived for various ranges of volatile formation. A mechanism has been formulated consisting of random chain scission followed by depropagation (dimers to pentamers); simulatanously another zip reaction produces hydrogen. The thermal, oxidative degradation has been studied above and below the softening point of the polymer as a function of oxygen pressure. A first-order reaction of volatile formation due to “abnormal” chain scission is followed by normal chain scission, which is also first order. The postulated mechanism leads initially to hydroperoxide formation. Arrhenius equations for volatile formation are different below and above the softening point. Oxygen consumption also follows a first-order reaction with an energy of activation of 31.5 kcal/mole.     

Mechanical degradation of polypropylene solutions under large pressure drops

The degradation of polypropylene (PP), dissolved in n‐alkanes at high temperatures and pressures, during the solution discharge to ambient conditions was experimentally studied. Molecular weight distributions (MWD) of the solubilized PP were measured by gel permeation chromatography. The MWD curves of PP obtained after discharge of the polymer solution shift to the low molecular weight side of the distribution and the polydispersity is reduced. In this work, a systematic study on the discharge products was performed to elucidate the degradation mechanism and the effects of temperature and concentration on this phenomenon. Initially, pure polymers, PP and polystyrene (PS) were studied varying the solution temperature. In a second stage, the effect of polymer concentration on chain scission was assessed using experiments on physical blends of PP/PS. In all cases, thermal and oxidative degradation were previously analyzed. Mechanical degradation was found to be the main chain scission mechanism. A negative linear functionality of the chain scission was found in both temperature and polymer concentration. To analyze the relationship between polymer degradation and molecular weight, the chain scission distribution function was calculated. On this basis, a critical molecular weight for the beginning of chain scission was obtained. This value is a function of temperature but remains constant with concentration. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 455–465, 2007     

Thermal oxidative degradation of poly-lactic acid

The study of molecular weight and rate of change of molecular weight during thermal degradation of PLA suggests the random nature of chain scission with activation energy of 28 kcal/mol. The rate of weight loss indicates the formation of the volatiles by chain end initiation. The melting temperature of the polymer initially decreases and afterwards increases with increase in time of heating due to chain stiffening.     

The Photolysis of Polyvinylpyrrolidone in Aqueous Solutions in the Presence of Oxygen and Cupric Ions

Abstract

The oxidative photolysis of polyvinylpyrrolidone with light of λ = 2537 Å has been studied over a range of oxygen pressures and polymer concentrations. The results show that chain scission and cross linking take place simultaneously. In the range where cross linking is a negligible component, a mechanism based on chain scission due to components which lead to chain scission without intervention of oxygen, and a component which leads to chain scission via hydroperoxide side groups, has been proposed. This mechanism accounts satisfactorily for all observed features of the reaction. The degree of degradation at any one time decreases with oxygen pressure. Cupric ions, with or without oxygen present, have very little influence on the degradation process. However, the UV spectra of PVP in the presence of cupric ions are different from those without them.     

PHOTOCHEMISTRY OF cis-POLYISOPRENE AND ITS SINGLET OXYGEN ADDUCT

Abstract— Reaction of singlet oxygen (¹Δ_g, ¹O₂) with cis -polyisoprene yields an allylic hydroperoxide with an olefinic double bond shifted in the polymer chain. The photochemical decomposition of the resultant hydro-peroxide and the subsequent polymer chain scission kinetics have been studied in the absence of oxygen. Quantum yields of hydroperoxide decomposition range from 3.1 to 8.4 in cyclohexane, depending on the initial amount of hydroperoxide in the polymer. On the other hand, the quantum yields for polymer chain scission are low, and vary with the frequency of the incident light. The ratio for number of polymer scissions per number of hydroperoxy groups decomposed is of the order of 10^-2. The polymer chain degradation is sensitized by the addition of ketones. Based on these data, a reaction mechanism for the overall photodegradation of the cis -polyisoprene initiated by singlet oxygen is proposed.     

Thermal degradation of copolymers of methyl methacrylate and methyl acrylate. II. Chain scission and the mechanism of the reaction

It has been established that one molecule of carbon dioxide is produced for each chain scission during degradation of methyl methacrylate–methyl acrylate copolymers with molar compositions in the ratios 112/1, 26/1, 7.7/1, and 2/1. Thus the relatively simple measurement of the production of carbon dioxide can be used to determine the extent of chain scission. In this way the relationships between chain scission and volatilization, zip length, copolymer composition, and the production of permanent gases have been established. The rate of chain scission is proportional to a power of the methyl acrylate content of the copolymer less than 0.5, from which it has been concluded that a significant proportion of the initial production of radicals and the subsequent attack of these radicals on the polymer chains is at random and not specifically associated with the methyl acrylate units. A mechanism for the overall thermal degradation process in this copolymer system is presented in the light of these observations.     

Two-dimensional chromatography applied to the study of the thermo-oxidative degradation of poly(styrene-b-butadiene) star block copolymers

Two-dimensional chromatography with gradient polymer elution chromatography in the first dimension and gel permeation chromatography in the second dimension was used to characterize a poly(styrene-b-butadiene) star block copolymer. The data evidence several populations that are clues left by the different steps in the sequential reaction employed to make the polymer. The sample was subjected to thermo-oxidative degradation at 180°C and was analyzed at different times during the process. After a relatively long induction period, the two-dimensional chromatograms show how the different populations are progressively degraded via random chain scission of the polybutadiene block to leave essentially polystyrene as the only soluble component. With longer thermal aging times, the polystyrene also degrades via chain scission.     

氧化镧对HDPE热氧化分解行为的影响

聚合物改性是聚合物结构与性能研究中的一个重要领域,而稀土元素具有4f0-145dl-106s2的电子构型,由于4f轨道的特殊性和5d轨道的存在,稀土离子具有丰富的电子能级,离子半径较大,电荷较高,又有较强的络合能力,可对多种聚合物的热稳定性产生影响[1,7].     

Kinetics of thermal degradation of poly(-caprolactone)

The thermal degradation/modification dynamics of poly(-caprolactone) (PCL) was investigated in a thermogravimetric analyzer under non-isothermal and isothermal conditions. The time evolution of the molecular weight distribution during degradation was studied using gel permeation chromatography. Experimental molecular weight evolution and weight loss profile were modeled using continuous distribution kinetics. The degradation exhibited distinctly different behavior under non-isothermal and isothermal heating. Under non-isothermal heating, the mass of the polymer remained constant at initial stages with rapid degradation at longer times. The Friedman and Chang methods of analysis showed a 3-fold change (from 18 to 55–62 kcal mol⁻¹) in the activation energy from low temperatures to high temperatures during degradation. This suggested the governing mechanism changes during degradation and was explained using two parallel mechanisms (random chain scission and specific chain end scission) without invoking the sequential reaction mechanisms. Under isothermal heating, the polymer degraded by pure unzipping of specific products from the chain end.     

DYE-SENSITIZED PHOTOOXIDATION OF 1,4-POLYDIENES

Abstract— The reaction of singlet oxygen with polydiene polymers produces hydroperoxides by the typical 'ene' type reaction. The observed chain scission process cannot be explained by the photodecom-position of hydroperoxide formed by visible light, because these hydroperoxides do not absorb light in this repion. Spectroscopic and EPR studies of the dye-solvent systems show the formation of reactive free radicals. which are probably responsible for the abstraction of hydrogen from the polymer molecules. The next step is the well known free radical oxidation mechanism which is responsible for the chain scission reactions.     

Aging and degradation of polyolefins. I. Peroxide-initiated oxidations of atactic polypropylene

Efficiencies of polymer radical production by thermal decomposition of di-tert-butylperoxy oxalate (DBPO) have been measured in bulk atactic polypropylene (PP) at 25–55°C; they range from 1 to 26%, depending on [DBPO], temperature, and presence of oxygen. Most of the polymer radicals thus produced disproportionate in the absence of oxygen but form peroxy radicals in its presence. Most of the pairs of peroxy radicals interact by a first-order reaction in the polymer cage. The fraction that escapes gives hydroperoxide in a reaction that is half order in rate of initiation. In interactions of polymer peroxy radicals, in or out of the cage, about one-third give dialkyl peroxides and immediate chain termination, two-thirds give alkoxy radicals. About one-third of the later cleave at 45°C; the rest abstract hydrogen to give hydroxy groups and new polymer and polymer peroxy radicals. The primary peroxy radicals from cleavage account for the rest of the chain termination. Cleavage of alkoxy radicals and crosslinking of PP through dialkyl peroxides nearly compensate. Up to 70% of the oxygen absorbed has been found in hydroperoxides. The formation of these can be completely inhibited, but cage reactions are unaffected by inhibitors. Concentrations of free polymer peroxy radicals have been measured by electron spin resonance and found to be very high, about 10^?3M at 58–63°C. Comparison with results on 2,4-dimethylpentane indicate that rate constants for both chain propagation and termination in the polymer are much smaller than those for the model hydrocarbon but that the ratio, k_p/(2k_t)^½, is about the same.     

Quantitative analysis of random scission and chain-end scission in the thermal degradation of polyethylene

The thermal degradation of polyethylene includes two different kinds of pathways. These are random and chain-end scissions which include β-scission on the chain end and radical transfer scission. We conducted a quantitative analysis on these pathways by Pyrolysis-GC/MS and computer simulation. Two different distributions of scission products of polyethylene were observed at different temperatures. They are determined by the relationship between rate of reaction and that of volatilisation. Furthermore, a characteristic distribution was observed in lower molecular weight. It could be explained by direct scission and one to five-step radical transfer scissions. The pathway possibilities calculated with the accumulated schemes showed that the direct scission and one-step-radical transfer increased with the temperature. This indicates that β-scission occurs on the chain end before the radical transfer because the rate of the β-scission becomes faster as the temperature rises.     

The thermo-oxidative degradation of polyolefines—Part 10. Correlation between the formation of carboxyl groups and scission in the oxidation of polyethylene in the melt phase

Films of low density polyethylene have been degraded under an oxygen atmosphere at temperatures above the semicrystalline melting point. Time, conversion and temperature dependence of carboxyl group formation and chain scission have been studied. After induction periods we found linear dependences both in function of time and conversion. One third of absorbed oxygen forms carboxyl groups and the absorption of 3·57 mmol oxygen per monomer unit is needed for one chain scission. Maximum rates of carboxyl formation and chain scission have Arrhenius temperature dependence with 33·5 kcal/mole activation energy. The number of carboxyl groups and chain scissions are always practically the same; we assume that the isomerisation of secondary alkyl peroxy radicals simultaneously causes chain scission and carboxyl formation.     

Thermal stability and degradation of chitosan modified by benzophenone

N-(biphenylmethylidenyl) chitosan polymer was prepared, characterized and thermal stability was compared with chitosan. Thermal degradation products of the modified polymer were identified by GC-MS technique. It seems that the mechanism of degradation of the prepared polymer is characterized by formation of low molecular weight radicals, followed by random scission mechanism along the backbond chain.     

The thermal degradation of poly(-(d)-β-hydroxybutyric acid): Part 2—Changes in molecular weight

Changes in molecular weight occur in poly(-(d)-β-hydroxybutyric acid) in the temperature range 170–200°C, at which latter temperature evolution of volatile products of degradation becomes significant. Two processes are involved in these changes in molecular weight. The more important is random chain scission at ester groups, which results in the formation of carboxyl and vinyl groups. Although this ultimately results in a drastic reduction in the molecular weight of the polymer, this is delayed in the early stages of the reaction by a condensation reaction between the terminal hydroxyl groups present in the original polymer and the terminal carboxyl groups, which were either originally present or formed in the chain scission process. This delay could have relevance to the industrial processing of this material.     

Gel permeation chromatography studies of chain scission and cross-linking during photo-oxidation of atactic polystyrene below and at tg

Photo-oxidative degradation of polystyrene in the form of film 20 μm thick was carried out in air using u.v. light of 254 nm at room temperature and at temperatures up to T_g. GPC was used to study changes of molecular weight distribution during the process. The GPC results were analysed using equations for an initially most probable distribution and non-uniform energy dissipation; the quantum yield values of chain scission and cross-linking of polystyrene during degradation were calculated. Initially, degradation progressed at high rate, connected with consumption of oxygen dissolved in the film. The slower subsequent degradation was connected with consumption of oxygen supplied during the reaction. An appreciable increase in the quantum yields for chain scission and cross-linking was observed just below and at T_g for the initial stage of photo-oxidative degradation. This increase of the quantum yield of photodegradation was caused by increased mobility of oxygen molecules in the film, connected with movement of polymer chain elements.