Interaction between UV radiation and filled polytetrafluoroethylene (PTFE). I. Degradation processes

The first part of a series of two, this paper analyzes the degradation of pure and filled PTFE under high energy UV radiation. The polymer morphology is first investigated using differential scanning calorimetry, highlighting the respective nucleating efficiency of TiO₂ and CaF₂ during polymer crystallization. Then, the various polymers are exposed to excimer laser radiation and observed under an optical microscope. The results indicate that the degradation is closely connected with microstructural parameters. In pure PTFE, scattering by crystallites and reflection on piles of lamellae control the nature and extent of the degradation. In filled PTFE, nature and concentration of fillers are the most important features governing degradation. When absorbing particles are added to PTFE, the damage is restricted to the surface and photothermal processes can modify the degradation from heterogeneous to ablative, depending on the filler content. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 2057–2067, 1998     

An ESR study of PMMA mechanoradicals and of their interaction with oxygen

Analysis of ESR spectra of mechanoradicals from poly(methyl methacrylate) reveals that after mechanical degradation in vacuo at 77°K, the sample contains two types of primary radicals? CH₂? C(CH₃)(COOCH₃) (I) and CH₂? C(CH₃)(COOCH₃)? CH₂ (II) produced by the breaking of the polymer chain, and secondary radicals ? CH₂? C(CH₃)(COOCH₃)? CH? C(CH₃)? (COOCH₃)? CH₂? (III). With increasing temperature, radical I remains stable while II reacts with methylene hydrogen of the polymer chain giving rise to the secondary radical III, which decays and finally disappears as the temperature rises. After admission of oxygen at 113°K, the polymer radicals react with oxygen with formation of polymer peroxy radicals ROO. and diamagnetic dimers. With increasing temperature the latter dissociate again to the original polymer peroxy radicals which gradually decay, if the temperature is increased further. The present results are compared with earlier ones obtained on poly(ethylene glycol methacrylate) (PGMA).     

Effect of MeV protons on the phase behaviour and thermal stability of polytetrafluoroethylene

Thermomechanical spectroscopy analysis was used to study the influence of accelerated protons on the molecular-topological properties of polytetrafluoroethylene (PTFE). The study showed changes in a wide number of polymer parameters as a result of bombardment with 1, 2 and 4 MeV protons at fluences up to 2 × 10¹⁵ protons/cm². The basic topological process occurring under proton bombardment is amorphicity, as found for γ-irradiation of PTFE. The flow temperature of bombarded PTFE significantly decreases with increasing the fluxes and energy of the accelerated protons. The general process resulting from proton bombardment is cleavage of C-F bonds, leading to formation of “centered” radicals ～CF₂CF · CF₂～ and HF. The thermal stability of bombarded PTFE is below than that of virgin polymer. The rate of thermal destruction noticeably increases and the temperature of the initiation of effective thermal decomposition decreases after bombardment. The gaseous products generated during thermal destruction of the bombarded and virgin PTFE are similar.     

The radiation degradation of polytetrafluoroethylene resulting in low-molecular and functionalized perfluorinated compounds

The degradation of polytetrafluoroethylene (PTFE) by high energy radiation with a high dose and in presence of oxygen forms perfluorinated carboxylic acids, among other compounds. In an inert atmosphere (in nitrogen) mixtures of perfluorinated olefines and paraffins of different chain length ranges are obtained. This process represents a new alternative of the synthesis of special active components, such as fluorocarbon surfactants, fluorine containing textile finishing agents, special dielectrics and others. Irradiation of the resulting perfluorinated paraffins (in nitrogen) and of gaseous degradation products of PTFE leads to a significant increase in the yield of perfluorinated olefines. Reaction mechanisms are discussed. Recombinations of radicals obtained by irradiation form branched molecules. The reactions are diffusion controlled. Irradiation of high molecular PTFE leads to low branching. The degree of branching of molecules increases in correlation with a decrease of viscosity of the reaction medium. An apparatus conception pertaining to the process of a continuous degradation of PTFE to perfluoroolefines and perfluoroparaffins in the favourable chain length ranges from six to 14 carbon atoms, according to application, is described. An essential component of this conception is the use of a target which consists of a tempered thin layer which is moved into an inertly processed reactor for irradiation. The action of high energy radiation of relatively low dose in air in presence of reactive substances leads to a finely grained PTFE powder with functional groups, which can be mixed easily with liquids, solutions and polymers.     

Thermal degradation of mixtures of poly(methyl methacrylate) and silver acetate

The degradation behavior of silver acetate—PMMA blends at salt/polymer ratios of 1:1, 1:5, and 1:10 has been studied by using thermal volatilization analysis (TVA) as the principal technique. Degradation of the salt has also been examined; it gives a variety of products best explained by a series of reactions resulting from an initial cleavage of CH₃COO. radicals and silver atoms. Silver acetate, when present with PMMA during degradation, results in a severe destabilization of the polymer, which breaks down to monomer at a high rate at temperatures as low as 200°C. This effect is explained by diffusion of radicals from silver acetate decomposition into the polymer phase, in which they initiate chain scission and depolymerization.     

The effect of continuous CO<Subscript>2</Subscript> laser radiation on the thermal and molecular—Topological properties of polytetrafluoroethylene

The impact of high-intensity laser radiation on a polymer in vacuum is accompanied by the release of gaseous products of degradation and, in some cases, of clusters of the partially destroyed polymer. Polytetrafluoroethylene (PTFE) exhibits an abnormal behavior in this process: being exposed to continuous CO₂ laser radiation, it degrades at a high rate and its clusters have a fibrous form. Depending on the irradiation conditions, the fibrous fraction forms two types of product, “cotton wool” and “felt”. Polytetrafluoroethylene and its laser-modified “cotton wool“ product have a semicrystalline topological structure. The preliminary γ-irradiation of PTFE enhances the laser ablation process.     

Polytetrafluoroethylene: Effects of γ-radiation on fine structure

The structure of granular polytetrafluoroethylene has been studied by electron microscopy. On the basis of the texture of surfaces resulting from fracture a model of the structure is proposed which suggests that PTFE consists of extended chain crystals with both inter- and intra-lamellar noncrystalline regions. The effects of γ-radiation on the structure have been investigated by examining the texture of irradiated fracture surfaces and also the texture produced by post-irradiation fracture. The irradiations have been performed in vacuo and in oxygen. In both atmospheres PTFE undergoes degradation with a concurrent increase in crystallinity. However, the texture of the surfaces of high crystallinity PTFE, prepared by radiation, differs markedly to the texture of fracture surfaces of high crystallinity PTFE prepared by thermal annealing. It is proposed that radiation causes rupture of bonds in the interlamellar (chain fold) and intralamellar regions, resulting in the production of chain ends and interlamellar links. Due to scavenging of the free radicals, interlamellar linking is pobably a minor process with irradiation in oxygen. These chemical changes cause modifications to the extended chain lamellar crystals and consequently alterations to the physical properties of the polymer.     

Polymer radicals trapped in radical-initiated polytetrafluoroethylene

The electron spin resonance (ESR) spectra of polymer radicals found to be trapped in polytetrafluoroethylene (PTFE) polymerized with radical initiators were comparatively examined under various conditions and assigned. They are identified as the primary (propagating) radicals RCF₂CF₂·, which are transformed to primary peroxy radicals RCF₂CF₂OO· in the atmosphere. Studies of the rates of polymerization and postpolymerization and ESR measurements indicate that the radical content steadily increases during polymerization. The results are discussed in connection with the mechanism of polymerization of tetrafluoroethylene (TFE) and the unusual thermal stability of these radicals in PTFE prepared with initiator.     

Degradation of poly(vinyl chloride) by u.v. radiation—II: Mechanism

The mechanism of the light-induced degradation of solid poly(vinyl chloride) (PVC) has been investigated, and an overall reaction scheme has been developed, based on values of the quantum yields for the primary photoproducts. Only a very small fraction (0.2%) of the excited polyenes induces the degradation of PVC, primarily by photocleavage of the allylic CCl bond. The high instability of β-chloroalkyl radicals is responsible for the chain dehydrochlorination that leads to formation of polyenes. In the absence of O₂, chain scissions and crosslinking are postulated to originate mainly from α-chloroalkyl radicals through β-cleavage of CC bonds and radical coupling, respectively. In the presence of O₂, the chain dehydrochlorination still proceeds, together with an oxidative chain process which yields, via peroxy and alkoxy radicals, hydroperoxides, ketones and peroxide crosslinks. Cleavage of the polymer backbone results most probably from the decomposition of tertiary alkoxy radicals by a carbon-carbon β-scission process.     

XPS analysis of PTFE decomposition due to ionizing radiation

X-ray induced photoelectron spectroscopy (XPS) in combination with depth profiling has been used to investigate the structure and the degradation mechanism of PTFE bonded gas diffusion electrodes (GDE). The XP-spectra of these electrodes show distinctly separated binding states of the C1s electrons at E_b=292 eV and E_b=286 eV. These binding states are related to the carbon in the (CF₂)_n configuration (C1s_CF2) and the graphite (C1s_graphite), respectively. The C1s_CF2 signal decreases are induced by both X-ray exposure and ion etching. Simultaneously a decrease of the F1s signal has been approved. The intensity ratio of F1s to C1s_CF2 has increased during the experiment. These results indicate a decomposition of PTFE which creates CF fractions, leading to an excess intensity in the energetic range between the C1s binding states of the PTFE and the graphite. Although both the F1s and the C1s spectra are strongly modified by ionizing radiation, samples are comparable, when exposition doses are equal.     

TOF-SIMS evidence of intercalated molecular gases and diffusion-limited reaction kinetics in an alpha particle-irradiated PTFE matrix

The chemical evolution of poly(tetrafluoroethylene) (PTFE) that is brought about by increasing levels of irradiation with alpha particles is accompanied by the emergence and proliferation of functionalized moieties. Families of reaction products specifically identified in the alpha-irradiated polymer matrix include hydride-, hydroxide-, and oxide-functionalized fluorocarbons. The data also indicate the emergence of hydrogen peroxide (H2O2) and hydrazine (N2H4), but no distinct evidence suggesting the formation of perfluorinated amines, amides, or cyanogens is found. In this article we substantiate the speciation of emergent species and reveal evidence of intercalated molecular gases with which alpha particle-generated radicals may react to form the observed products. Furthermore, we present evidence to suggest that the kinetics of alpha particle-induced reaction is limited by the diffusion of radicals within the polymer matrix. That is to say, chemical additives in the polymer matrix are shown to be scavengers of H*, O*, and F* radicals and limit the rates of reaction that produce functionalized fluorocarbon moieties. Above a threshold dose of alpha particles, the concentration of radicals exceeds that of the scavenger species, and free radical diffusion commences as evidenced by a sudden increase in the yield of reaction products. Samples of PTFE were irradiated to alpha doses in the range of 10(7) to 5 x 10(10) rad with 5.5 MeV 4He2+ ions from a tandem accelerator. Residual gas analysis (RGA) was utilized to monitor the liberation of molecular gases from PTFE during alpha particle irradiation of samples in vacuum. Static time-of-flight SIMS (TOF-SIMS), equipped with a 20 keV C60+ source, was employed to probe chemical changes as a function of alpha particle irradiation. Chemical images and high-resolution mass spectra were collected in both the positive and negative polarities.     

Kinetics of thermal decomposition of PVC/fly ash composites

In this study, the kinetics and mechanisms of thermal degradation of Poly Vinyl Chloride (PVC) composites reinforced with class-F fly ash are studied experimentally and numerically using Flynn–Wall model. The addition of fly ash to the polymer matrix results in a decrease in the primary degradation temperature and an increase in the secondary degradation temperature. The metal oxides in the fly ash act as acid absorbers, which results in the destabilization of PVC during its dehydrochlorination process. However, they also react with the chlorine free radicals, which prevents the formation of HCl during degradation. In addition, it is observed that calcium and iron oxides, present in fly ash, are more reactive to the chlorine radicals rather than the silicon and aluminum oxides. The effect of fly ash chemical composition on the degradation of PVC composites was studied by comparing the thermal properties of composites containing two different classes of fly ashes, class-F and class-C, at similar levels. Thermal stability of the composites is found to be dependent on the chemical composition of fly ash. Higher dehydrochlorination rate is observed in the case of composites filled with class-F fly ash than those reinforced with class-C fly ash.     

ESR and chemical study of p-polyphenylene formed by using an AlCl3–CuCl2 catalyst

The free radicals in p-polyphenylene and the formation of free radicals in this polymer upon pyrolysis in vacuum have been studied by means of electron spin resonance. For an unpyrolyzed series of polymer samples, a linear relationship was observed between free radical concentration and increasing carbon content. The free radicals observed in the unpyrolyzed samples did not react with NO. When samples of polyphenylene were pyrolyzed, additional free radicals were produced which did react with NO. The growth of free radical concentration upon pyrolysis was observed to be closely related to the production of volatile products from the polymer. In the temperature range 250–600°C, HCl was the principal volatile species produced. Two mechanisms were involved in HCl production: a process with an activation energy of 7.1 kcal/mole which led to the production of stable free radicals; and a process involving 75 kcal/mole which was unconnected with the production of free radicals. From 600 to 700°C, H₂ was the principal volatile degradation product. The rate at which H₂ was evolved showed a second-order dependence on phenyl units bearing two or three substituents; this process had an activation energy of 79 kcal/mole. Electron spin resonance spectra indicated that this process led to the production of free radicals, and infrared spectra showed that a highly crosslinked product resulted.     

Modification of perfluorinated polymers by high-energy irradiation

This review surveys about the possibilities for the modification of perfluorinated polymers using high-energy irradiation: degradation, functionalization, branching, and cross-linking. The reaction mechanisms for the different reaction conditions are discussed. Electron irradiation of polytetrafluoroethylene (PTFE) with a very high dose leads to a complete degradation of the macromolecules to low-molecular products. In the presence of oxygen perfluorocarboxylic acids and in an inert atmosphere, mixtures of perfluorinated olefins and paraffins can be obtained. Virgin PTFE is disintegrated by high-energy irradiation in air with a lower dose into a micropowder modified with COOH groups. This powder can be homogeneously incorporated in other polymers. So, the special properties of PTFE can be made effective in these polymers. Micropowders functionalized with COOH groups and polyamides (PA) form by reactive extrusion PTFE-PA blockcopolymers which can be used as slide bearing materials. The copolymers poly(tetrafluoroethylene-co-hexafluoropropylene) (FEP) and poly(tetrafluoroethylene-co-perfluoropropyl vinyl ether) (PFA) irradiated in air show a significantly higher degree of COOH functionalization compared with PTFE. Irradiation of molten PTFE in an inert atmosphere leads to formation of different kinds of double bonds, CF₃ side groups, long-chain branches as well as cross-links. Irradiation of PFA in vacuum results in the generation of COF and COOH groups; in molten state also branches and cross-links are formed.The focus of the present paper is on the work that has been carried out at the Institute of Polymer Research Dresden.     

Inhibited degradation of nylon 66 in presence of nitrogen dioxide,ozone, air,and near—ultraviolet radiation

Degradation of nylon 66 films of different morphologies was studied in the presence of nitrogen dioxide, ozone, oxygen, and near-ultraviolet radiation (λ > 2900 Å). Films cast from formic acid solution showed normal random degradation, whereas films cast from benzyl alcohol solutions and dried at elevated temperatures under nitrogen showed very strongly inhibited random degradation. This inhibition may be due to protection of peptide groups by hydrogen bonding with benzaldehyde or benzoic acid or even to their chemical reactions at elevated temperatures. Oxygen was not rigorously excluded during preparation of the films. Degradation of nylon 66 films cast from formic acid solutions at room temperature containing benzaldehyde or benzoic acid, respectively, is also inhibited. The energy of activation for inhibited degradation in presence of nitrogen dioxide is relatively small, indicating that the process is either controlled by diffusion of polymer radicals from medium cages or by diffusion of gases into the polymer. The degradation kinetics can be expressed by “weak”-link random degradation. The weak links are in the present case unprotected peptide groups. The functional relationship between chain scission rate constants and NO₂ pressure is linear.     

The origin of the radiobiological damage in cells stored in cryostatic conditions

The radiation induced free radical damage in Chinese hamster lung fibroblast V-79 cells stored in DMEM culture medium containing 10% DMSO has been investigated by matrix EPR spectroscopy in connection with the H₂O/DMSO binary phase diagram. A major part of the indirect effect is due to radicals from the DMSO·3H₂O phase in the freezing medium, which are released on warming in the temperature range between 130 K and 160 K, that is, far below the eutectic melting temperature (210 K). The radicals trapped in the DMSO·3H₂O phase react with oxygen above 160 K giving reactive oxygen species (ROS) of the type of peroxyl radicals. A lower limit yield of 10–15% was calculated for this conversion. Scavenging experiments with a stable nitroxyl radical (tempol) have demonstrated that part of the DMSO·3H₂O radicals escape by mutual recombination on melting and are therefore available for inducing indirect cell damage. The same experiments performed with pure frozen water have shown that OH radicals are not available for inducing cell damage. The EPR measurements performed on H₂O/DMSO frozen mixtures suggest that the radiation induced radical forming process does not change when passing to the low dose range below 1 Gy, in agreement with the linear model.     

Confinement effects on the thermal stability of poly(ethylene oxide)/graphene nanocomposites: A reactive molecular dynamics simulation study

Nonisothermal and isothermal decomposition of poly(ethylene oxide) (PEO) loaded with different concentrations of pristine graphene (PG) and graphene oxide (GO) nanoplatelets were investigated using reactive molecular dynamics simulation. The onset of nonisothermal decomposition of the PG‐loaded PEO system was the highest among all systems, suggesting that introducing PG to the polymer improves its thermal stability (an effect that increases with an increase in the PG concentration). At low concentration, introducing GO to the polymer brings about a deterioration of the thermal stability of the polymer consistent with experimental findings. On average, the activation energy for the isothermal decomposition of PG‐loaded PEO system increases by 60% over that of the neat PEO system, while it decreases by 40% for the GO‐loaded PEO system. A time‐dependent analysis of the through‐thickness decomposition profile of the above systems reveals that the polymer confined between the PG sheets exhibit a higher thermal stability compared to the bulk polymer. However, an opposite effect is observed with the polymer confined between the GO sheets. The latter observation is attributed to accelerated polymer chain scission in confined regions due to the ejection of reactive hydroxyl radicals from the GO surface during the early stages of thermal decomposition. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1026–1035     

Free-radical degradation of solid poly(methyl methacrylate) initiated by photoreduction of the chloride complexes of Fe3+. Kinetics and mechanism

The kinetics have been studied quantitatively and a mechanism proposed for the free-radical degradation of solid PMMA initiated by photoreduction of the chloride complexes of Fe³⁺. The polymer was exposed in an inert atmosphere at 77 and 293 K to light of various spectral compositions with γ > 300 nm, not absorbed by the polymer. The rate of the build-up of the double isolated and conjugated bonds, formed as a result of thermal transformations of the PMMA macroradicals at 293 K, has been studied. It is shown that the isolated double bonds are generated at a rate equal to the initiation rate W, and the conjugated double bonds are generated at the rate of 0.3 W_i. A mechanism is proposed closely describing the observed regularities. It has been found that the degradation of PMMA irradiated at 77 K results from the thermal decomposition of macroradicals on heating the samples and is dependent upon the spectral composition of the light. The probability of degradation is 0.16 per photoreduced Fe³⁺ ion for light γ < 370 nm and decreases to only 0.009 for light with γ > 390 nm. It is concluded that macrochain breaking under these conditions is due to the thermal decomposition of the macroradicals ～CH₂C(CH₃)CH₂～ . At 293 K the photoinitiated PMMA degradation is a free-radical, but not a chain, process independent of the intensity and spectral composition of the light (in the wavelength range 313–390 nm), molecular mass of the polymer and film thickness. Degradation in an inert atmosphere is characterised by a probability factor per photoreduced Fe³⁺ ion (α) which increases with the degree of conversion of the initiators. The rate of degradation in an atmosphere of HCl is directly proportional to the initiation rate W_i. It is concluded that, at 293 K in an enert atmosphere, the rupture of macromolecules is due to the thermal decomposition of both the primary macroradicals ～C(CH₃)(COOCH₃)?H-C(CH₃)(COOCH₃)～ and the radicals ～CH₂-?(CH₃)CH₂～ formed by addition of low-molecular radicals to the radical reaction products in this system, i.e. the isolated middle double bonds.     

Thermal decomposition of PTFE in the presence of silicon,calcium silicide,ferrosilicon and iron

Simultaneous TG/DTA has been used to study the thermal decomposition of binary compositions containing polytetrafluoroethene (PTFE) with silicon (Si), calcium silicide (CaSi₂), ferrosilicon (FeSi) or iron (Fe) powders. In nitrogen and under dynamic heating program the thermal decomposition of Si/PTFE and CaSi₂/PTFE is an exothermic process. The other two compositions decompose endothermically. In each case the decomposition reactions show first-order kinetics but only iron does not change considerably the kinetics of PTFE depolymerization. The constants of the decomposition rate at 850 K for silicon containing reducers are about four times higher than those of PTFE and Fe/PTFE. This revised version was published online in July 2006 with corrections to the Cover Date.     

Simulations on the thermal decomposition of a poly(dimethylsiloxane) polymer using the ReaxFF reactive force field

To investigate the failure of the poly(dimethylsiloxane) polymer (PDMS) at high temperatures and pressures and in the presence of various additives, we have expanded the ReaxFF reactive force field to describe carbon-silicon systems. From molecular dynamics (MD) simulations using ReaxFF we find initial thermal decomposition products of PDMS to be CH(3) radical and the associated polymer radical, indicating that decomposition and subsequent cross-linking of the polymer is initiated by Si-C bond cleavage, in agreement with experimental observations. Secondary reactions involving these CH(3) radicals lead primarily to formation of methane. We studied temperature and pressure dependence of PDMS decomposition by following the rate of production of methane in the ReaxFF MD simulations. We tracked the temperature dependency of the methane production to extract Arrhenius parameters for the failure modes of PDMS. Furthermore, we found that at increased pressures the rate of PDMS decomposition drops considerably, leading to the formation of fewer CH(3) radicals and methane molecules. Finally, we studied the influence of various additives on PDMS stability. We found that the addition of water or a SiO(2) slab has no direct effect on the short-term stability of PDMS, but addition of reactive species such as ozone leads to significantly lower PDMS decomposition temperature. The addition of nitrogen monoxide does not significantly alter the degradation temperature but does retard the initial production of methane and C(2) hydrocarbons until the nitrogen monoxide is depleted. These results, and their good agreement with available experimental data, demonstrate that ReaxFF provides a useful computational tool for studying the chemical stability of polymers.