Investigation of the origin of tensile stress-induced rate enhancements in the photochemical degradation of polymers

To investigate the effect of tensile stress on the photochemical degradation efficiencies of polymers, a modified PVC polymer with Cp(CO)3Mo-Mo(CO)3Cp (Cp = eta5-C5H5) units along the backbone was synthesized. The polymer is photochemically reactive because the Mo-Mo bonds are photolyzed with visible light and the resulting radicals are captured with Cl atoms from along the polymer backbone. Of most importance from a mechanistic standpoint, the photochemical degradation reaction occurs in the absence of oxygen, which eliminates the kinetically complicating effect of rate-limiting oxygen diffusion. Tensile stress initially caused the quantum yield of polymer degradation to increase, but, after a certain point, additional stress caused a decrease in the quantum yield. This dependence of quantum efficiency on stress is consistent with a hypothesis in which stress affects the ability of the photochemically generated radicals to recombine. At low to moderate stress, the effect of stress is to increase the separation of the radicals (by recoil), thus decreasing their recombination probability. As the stress increases, however, segments of different chains align, which induces a higher degree of orientation and crystallinity in the polymer, which in turn makes diffusion more difficult. The efficiency of degradation is predicted to decrease accordingly because of decreased radical and/or trap mobility in the ordered polymer. Infrared and X-ray data are presented, showing that the degree of orientation and crystallinity in the polymer does indeed increase with increasing stress.     

Mechanistic Aspects of the Effects of Stress on the Rates of Photochemical Degradation Reactions in Polymers

Abstract

This review examines the mechanistic origins of the effects of stress on the photochemical degradation rates of polymers. Recent studies have shown that tensile and shear stresses accelerate the rate of the photochemical degradation of polymers. Conversely, compressive stress generally retards the rate of photochemical degradation. After an initial discussion of the photochemical auto‐oxidation mechanism, the three primary hypotheses that purport to explain how stress affects photochemical degradation are examined. The first hypothesis is attributed to Plotnikov, who proposed that stress changes the quantum yields of the reactions that lead to bond photolysis. The second hypothesis, attributed to a number of researchers, says that stress affects the ability of the geminate radical pairs, formed in the photochemical bond cleavage reactions, to recombine. The third hypothesis proposes that stress changes the rates of radical reactions subsequent to radical formation. A further attempt to account for the effects of stress on degradation rates is a modification of the so‐called Zhurkov equation that has been used rather successfully to predict the effects of stress on degradation rates in thermal reactions. This empirical equation relates the quantum yield of degradation to a composite activation barrier for the overall photochemical reaction. Following the discussion of these hypotheses, experimental mechanistic studies of stress effects are summarized, and what little data there is is shown to be consistent with the hypothesis that proposes that stress primarily affects the ability of photochemically generated radical pairs to recombine. By decreasing the efficiency of radical–radical recombination, the effect is to increase the relative efficiencies of the radicals' other reactions and hence the rate of degradation. In addition to stress, other factors can affect the rates of polymer photodegradation. These factors include the absorbed light intensity, the polymer morphology, the rate of oxygen diffusion in the polymer, and the chromophore concentration. Each of these parameters must be carefully controlled in mechanistic studies that probe the effects of stress on degradation rates.     

Degradable alternating polyperoxides from poly(ethylene glycol)‐substituted styrenic monomers with water solubility and thermoresponsiveness

Water soluble alternating copolymers were prepared by oxidative free radical copolymerization of 4‐vinylbenzyl methoxypoly(oxyethylene) ether (PEGSt) and molecular oxygen at 50 °C. NMR spectroscopy established alternate sequence of PEGSt and peroxy bonds ( O O ) along the polymer main‐chain. The obtained polymers show temperature induced hydrophilic to hydrophobic phase separation, confirmed by UV‐visible spectroscopy and dynamic light scattering. The cloud point temperature (T_CP) of the polymers can be tuned by changing the chain length of side‐chain poly(ethylene oxide) and incorporation of hydrophobic methyl methacrylate in the copolyperoxides. Exothermic degradation of these polyperoxides was confirmed by differential scanning calorimetry and the degradation products have been characterized by electron impact mass spectroscopy. Finally, N,N‐dimethylacrylamide was polymerized in the presence of these polyperoxides in toluene, highlighting their potential as polymeric free radical initiator during polymerization of vinyl monomers. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2018 , 56, 2030–2038     

CAROTENOIDS QUENCH EVOLUTION OF EXCITED SPECIES IN EPIDERMIS EXPOSED TO UV-B (290-320 nm) LIGHT

Abstract— Reactions involving singlet oxygen and other free radicals have been identified in epidermis containing either exogenous or endogenous photosensitizers. soaked in a singlet oxygen/free radical trap, and then exposed to visible or UV-A (320-400 nm) light. Such reactions can be quenched by the presence of the carotenoid pigments β-carotene and canthaxanthin which accumulate in epidermis after oral administration. We report here that the carotenoid pigments β-carotene. canthaxanthin and phytoene accumulating in epidermis can also quench to some degree those photochemical reactions involving singlet oxygen and free radicals that occur when epidermis is exposed to the sunburn spectrum of light (UV-B. 290–320 nm).     

Reducing the Exciton Binding Energy of Donor–Acceptor‐Based Conjugated Polymers to Promote Charge‐Induced Reactions

Exciton binding energy has been regarded as a crucial parameter for mediating charge separation in polymeric photocatalysts. Minimizing the exciton binding energy of the polymers can increase the yield of charge‐carrier generation and thus improve the photocatalytic activities, but the realization of this approach remains a great challenge. Herein, a series of linear donor–acceptor conjugated polymers has been developed to minimize the exciton binding energy by modulating the charge‐transfer pathway. The results reveal that the reduced energy loss of the charge‐transfer state can facilitate the electron transfer from donor to acceptor, and thus, more electrons are ready for subsequent reduction reactions. The optimized polymer, FSO‐FS, exhibits a remarkable photochemical performance under visible light irradiation.     

Influence of copper-phthalocyanine on the photodegradation of polycarbonate

Dyes and pigments are extensively used in polymer materials to confer colour-changing properties. However, these additives can significantly affect polymer stability against degradation. While the mechanism of stabilization of polymers by some pigments, such as carbon black, has been studied and is well known, the action of chromatic colorants, mainly in the sensitization of the degradation process, remains unclear. Cu-phthalocyanine dye can stabilize polymers against degradation as well as accelerate degradation in other situations. Cu-phthalocyanine incorporated into polycarbonate resulted in an acceleration of the degradation when the material was submitted to photochemical aging. The possible mechanism to explain the photodegradative behavior of PC containing Cu-phthalocyanine is based on the hypothesis that specific interactions among excited states of PC and Cu-phthalocyanine take place and enhance the formation of reactive species in polycarbonate. Excited states of Cu-phthalocyanine may abstract hydrogen atoms from methyl groups in polycarbonate, increasing the formation of free radicals P, which are the starting points for the sequential photo-oxidation reactions that lead to the degradation of the polycarbonate. Electron transfer sensitization is also a possible mechanism: the excited state of Cu-Ph abstracts an electron from PC to form the Cu-Ph radical anion and the PC radical cation. These reactive species in the presence of oxygen can cause oxidation of the aromatic ring.     

Electron spin resonance study of poly-3,3-bis-(chloromethyl)oxetane irradiated with ultraviolet light

When poly-3,3-bis(chloromethyl)oxetane has been irradiated at ?196°C in a nitrogen atmosphere with ultraviolet light, a triplet spectrum is observed. After warming the sample, both a doublet and a singlet ESR spectra are observed. These spectra are attributed to and ? CH₂? O, respectively. The formation mechanism of these free radicals is discussed. It is concluded that the main process of radical formation is the dissociation of chemical bonds from the excited state of the polymer produced through the energy absorption by irregular groups acting as sensitizers. In the presence of oxygen, the radical yield at ?196°C is greater than that in nitrogen atmosphere. This is attributed to the extra absorption of light by the charge transfer complexes of polymers with oxygen molecules. It is also proposed that participation of a charge transfer complex in photooxidation of ether is important in the primary radical formation step. When a polymer sample irradiated in vacuum with ultraviolet light is treated at ?78°C for a few minutes in the presence of air, peroxy radicals form. This shows that oxygen molecules diffuse very easily into this polymer, even at this low temperature.     

Regiospecific radical polymerization of a tetrasubstituted ethylene monomer with molecular oxygen for the synthesis of a new degradable polymer

A new class of degradable polymers is obtained from a diene monomer and molecular oxygen as the starting materials via a highly controlled radical copolymerization process. We now report the regiospecific copolymerization of a tetrasubstituted ethylene monomer with oxygen. Theoretical calculations support the highly selective propagations observed during the polymerization. The key steps are the regiospecific reactions of a peroxy radical to diene monomers and an allyl radical to molecular oxygen. The well-controlled molecular structure of the resulting polymer leads to the aldehyde-free degradation products during degradation by various stimuli, such as heating.     

Approaches for Making High Performance Polymer Materials from Commodity Polymers

A brief surrey of ongoing research work done for improving and enhancing the properties of commodity polymers by the author and author’s colleagues is given in this paper. A series of high performance polymers and polymer nanomaterials were successfully prepared through irradiation and stress-induced reactions of polymers and hydrogen bonding. The methods proposed are viable, easy in operation, clean and efficient. 1. The effect of irradiation source (UV light, electron beam, γ-ray and micr…     

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.     

Spectroscopic and electrical studies for the influence of ion beam and X-ray irradiation on poly-2-(N-propenamido-2-methylpropanesulfonic acid) and its polymer complex with cobalt chloride

The influence of argon ion beam and X-ray irradiations on poly-2-(N-propenamido-2-methylpropanesulfonic acid) (PPMPS) and its polymer complex with Co (II), (PPMPS-Co (II)), were studied using IR, UV/visible and d.c. electrical conductivity. After irradiation the polymer changed in color and become less soluble. The IR spectrum of irradiated PPMPS shows broadened bands at 3400 and 3550 cm^-1 which are assigned to stretch bands of NH and OH, respectively, as a consequence of intramolecular cyclization. Furthermore, a comparison of IR and UV/visible spectra of irradiated and non-irradiated PPMPS-Co(II), reveals that the main effect of irradiation was the degradation effect. Measurements of d.c. electrical conductivity for irradiated and non-irradiated polymers showed an increase of conductivity for the coordinated polymers compared to PPMPS. A relatively higher resistivity for the ion beam irradiated polymers and lower resistivity in case of X-ray irradiation have been observed. The increases of conductivity for the coordinated polymers compared to PPMPS were explained by the changes in glass transition temperatures (T_g) and activation energies for the different polymers.     

Study the effect of antioxidants on biological activity and on homopolypropylene; mechanical and physical properties

Exposure of polymers to temperature, atmospheric oxygen, or even light could result in some degradation of the polymer properties and features during processing (application), storage and end use. In hydrocarbon polymers, the polymer tend to free radical formation, eventually resulting in chain damage or crosslinking that leads to degradation. Antioxidants are used to terminate these chain reactions by removing radicals. Antioxidants are used in most hydrocarbon polymers including, polypropylene. a good addiction package must be existed to overcome the effect of degradation and save the polymer shape and characteristics. The practical experiment was carried out on a pure polypropylene (intermediate polypropylene resin without additives) and another practical experiment but with adding several types of additives with a certain concentration and study the behavior of polypropylene in all cases with successive extrusions. On other hand Flexible molecular docking on heme oxygenase, an important stress protein that is involved in cellular protection, antioxidant and anti-inflammatory activities, justified the antioxidant activity of the isolated compounds. From the binding energy 3114 and 1680 they could consider to be powerful and available antioxidant.     

Spectroscopic Investigation of Oxygen- and Water-Induced Electron Trapping and Charge Transport Instabilities in n-type Polymer Semiconductors

We present an optical spectroscopy study on the role of oxygen and water in electron trapping and storage/bias-stress degradation of n-type polymer field-effect transistors based on one of the most widely studied electron transporting conjugated polymers, poly{[N,N9-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,59-(2,29-bisthiophene)} (P(NDI2OD-T2)). We combine results obtained from charge accumulation spectroscopy, which allow optical quantification of the concentration of mobile and trapped charges in the polymer film, with electrical characterization of P(NDI2OD-T2) organic field-effect transistors to study the mechanism for storage and bias-stress degradation upon exposure to dry air/oxygen and humid nitrogen/water environments, thus separating the effect of the two molecules and determining the nature of their interaction with the polymer. We find that the stability upon oxygen exposure is limited by an interaction between the neutral polymer and molecular oxygen leading to a reduction in electron mobility in the bulk of the semiconductor. We use density functional theory quantum chemical calculations to ascribe the drop in mobility to the formation of a shallow, localized, oxygen-induced trap level, 0.34 eV below the delocalized lowest unoccupied molecular orbital of P(NDI2OD-T2). In contrast, the stability of the polymer anion against water is limited by two competing reactions, one involving the electrochemical oxidation of the polymer anion by water without degradation of the polymer and the other involving a radical anion-catalyzed chemical reaction of the polymer with water, in which the electron can be recycled and lead to further degradation reactions, such that a significant portion of the film is degraded after prolonged bias stressing. Using Raman spectroscopy, we have been able to ascribe this to a chemical interaction of water with the naphthalene diimide unit of the polymer. The degradation mechanisms identified here should be considered to explain electron trapping in other rylene diimides and possibly in other classes of conjugated polymers as well.     

Photocrosslinking of poly(methyl acrylate)s containing the 1,3-dioxolane structure as a pendant group

(2-Ethyl-2-methyl-1,3-dioxolan-4-yl)methyl- and (2-methyl-2-phenyl-1,3-dioxolan-4-yl) methyl acrylates were synthesized and polymerized. The photochemical behavior of the resulting polymers was investigated to determine whether the polymers pending on the 1,3-dioxolane structure were readily crosslinked with ultraviolet (UV) irradiation. The degree of crosslinking was estimated by the weight-loss method by immersion in acetone, with the result that the polymer with an aromatic substituent was more photocrosslinkable than the polymer that bore the aliphatic substituent. The catalytic effect on photocrosslinking of polymers was also studied by using benzoin and cobalt naphthenate. The infrared (IR) spectra of polymers irradiated in air that showed the new band at 3450 cm^?1 were attributed to a hydroxyl group; however, the spectra of polymers irradiated in vacuum displayed little absorption at 3450 cm^?1. To explain the mechanism of crosslinking model compounds were prepared and irradiated with UV light. It was concluded that crosslinking proceeds mainly from the fission of the 1,3-dioxolane ring and the coupling of the yielding radicals, together with autooxidation by atmospheric oxygen.     

Determination of primary bond scissions by mass spectrometric analysis of ultrasonic degradation products of poly(ethylene oxide‐block‐propylene oxide) copolymers

Ultrasonic degradation of poly(ethylene oxide‐block‐propylene oxide) copolymers consisting of a hydrophilic and a hydrophobic portion was studied with the aim to determine the location of bonds involved in the initial scission of the copolymers. LC–APCI‐IT‐MS and LC–APCI‐orbitrap‐MS were used for the detailed structural analysis of degradation products. The results indicated that initial bond scissions occurred principally at the boundary regions between backbones of polyethylene oxide (PEO) and polypropylene oxide (PPO) chains. Further structural analysis revealed the presence of oxygen adducts in the degradation products. Comparison with a thermal degradation carried out in helium atmosphere, one can conclude that the oxygen adducts are formed by radical reaction with water or dissolving oxygen molecules. The study demonstrated that chemical reactions as well as physical bond stress scissions are involved in the ultrasonic degradation of the copolymers. Copyright © 2010 John Wiley & Sons, Ltd.     

Stability and degradation of some electrically conducting polymers

The oxidative degradation reactions of polyacetylene, prepared from a soluble precursor polymer, are described and compared with those of more common polymers and of polyacetylene prepared by the conventional method. Because the band structure of the π-electron system of the polymer allows the formation of charge-transfer complexes with oxygen, the initial process in the pristine polymer is oxygen doping, with increasing conductivity. This is followed by irreversible degradation which is much faster than that of polyolefins or polydienes and faster than that of the crystalline polymer. Doping to low levels with electron acceptors removes the electrons involved in oxygen doping and the polymer becomes much more stable in air. Doping to high levels leads to new instabilities as the polymer reacts slowly with its counter-ions. Studies of polypyrrole and polythiophene show that these polymers are much more stable than polyacetylene but still undergo degradation reactions. The general features of their degradation mechanisms are discussed.     

Design of polyethers,thioethers, and amines with pendent iron moieties

Soluble organoiron polyethers, thioethers, and amines were synthesized via nucleophilic aromatic substitution reactions. The synthesis of these classes of organometallic polymers involved either the reaction of cyclopentadienyliron complexes of dichloroarenes with various oxygen and sulfur dinucleophiles or the reaction of ether‐ or amine‐containing diiron complexes with dithiols. Polymerization reactions with the diiron complexes gave rise to organoiron polymers with alternating ether/thioether or amine/thioether bridges. Removal of the iron moieties from the backbone of these polymers allowed for the production of the corresponding organic materials. Furthermore, the organometallic polymers had much higher solubilities than their organic analogues. Thermogravimetric analysis of the organoiron polymers indicated that the polymers lost their metallic moieties at approximately 200 °C, whereas degradation of the polymer backbones occurred around 500 °C. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 1216–1231, 2001     

Photodegradation of cellulose acetate fibers

The photodegradation of cellulose acetate fibers by ultraviolet light in vacuo at 77°K and at ambient temperature was studied. Three kinds of light sources with different wavelengths between 2353 and 6000 Å were employed. ESR studies at 77°K show that several kinds of free radicals are produced from cellulose diacetate (CDA) and cellulose triacetate (CTA) fibers when irradiated with light of wavelength shorter than 2800 Å. Among these methyl radicals formed decayed within 210 min at 77°K. When the temperature was raised above 77°K, radical transformation occurred at 87°K and most of the free radicals decayed at 193°K, whereas the cellulosic radicals were stable at this and even at higher temperatures. Ultraviolet spectroscopy studies revealed that the main chromophores are the carbonyl function of the acetyl group and acetal groups in the polymer. The photodegradation of the polymers at ambient temperature resulted in the formation of gaseous products (mainly CO, CO₂, and CH₄), together with the loss of bound acetic acid content and sample weight. Decreases in viscosity and reduction of tensile strength and elongation were also observed in the irradiated samples, revealing that the overt effects of ultraviolet light on cellulose acetate fibers are interpreted in terms of free-radical reactions ultimately leading to main-chain and side-group scissions, unsaturation, and the formation of small molecule fragments. Among these, main-chain scission took place predominantly in CDA fiber and side-group scission in CTA fiber. The mechanism of the fundamental photochemical degradation processes of cellulose acetate fibers is elucidated.     

Formation of organic monolayers on silicon via gas-phase photochemical reactions

A new method for the formation of molecular monolayers on silicon surfaces utilizing gas-phase photochemical reactions is reported. Hydrogen-terminated Si(111) surfaces were exposed to various gas-phase molecules (hexene, benzaldehyde, and allylamine) and irradiated with ultraviolet light from a mercury lamp. The surfaces were studied with in situ Fourier transform infrared spectroscopy, high-resolution electron energy loss spectroscopy, and scanning tunneling microscopy. The generation of gas-phase radicals was found to be the initiator for organic monolayer formation via the abstraction of hydrogen from the H/Si(111) surface. Monolayer growth can occur through either a radical chain reaction mechanism or through direct radical attachment to the silicon dangling bonds.     

Photodegradable polyamides

Unique photodegradable polyamides have been prepared by solution polymerization of diphenyl α-truxillate or δ-truxinate with hexamethylene or nonamethylenediamine, followed by melt polymerization. Photochemical properties of resulted polyamides were studied spectroscopically and are discussed in terms of photoreversibility and monomeric derivatives for comparison. Solution viscosity of the polymer was decreased by irradiation with a high-pressure mercury lamp. The polymers from α-truxillic acid depolymerize by the action of monochromatic light of 224 mμ to give rise to cinnamyl groups faster than those from δ-truxinic acid. On the photodegradation of polyalkylene-δ-truxinamide, two types of scission of cyclobutane were found: symmetrical scission, giving rise to two cinnamyl groups, and a symmetrical scission, giving rise to one trans-stilbene and one fumaramide linkage. The intense absorption peak due to cinnamyl groups at 272 mμ of partially photodepolymerized films diminished by the action of 304 mμ light; this can be accounted for by the recombination of cinnamyl groups into cyclobutane ring and some unidentified crosslinking reactions. The photochemical behavior of cyclobutane in the polyamides studied proved to be consistent with that of monomeric derivatives reported previously. The study on thermal properties of the polymers by DTA and TGA revealed that this type of polyamide is quite stable as high as 350°C unless significant steric effects are involved.