Later Stages Of Degradation Research Articles

Organic thermal stabilizers for rigid poly(vinyl chloride) IX. N-Benzoyl- N′- p-substituted phenylthiourea derivatives

N-Benzoyl- N′- p-substituted phenylthiourea derivatives have been investigated as thermal stabilizers or co-stabilizers for rigid PVC at 180 °C in air. The results reveal the higher stabilizing potency of the investigated organic stabilizers as compared with some of the commercially known reference stabilizers, which is proved by their greater induction period values ( Ts), during which no detectable amount of hydrogen chloride gas could be detected, but with higher dehydrochlorination rate at the later stages of degradation. Complexation of these materials with the chlorides of Ni 2+ and Cd 2+ improved greatly the induction period values, however the dehydrochlorination rate is only slightly improved. On the other hand, blending these thermal stabilizers with some of the commercially known reference stabilizers in different proportions leads to a true synergistic effect. The induction period values and the degradation rate were remarkably improved. A probable ionic mechanism for the stabilizing action of N-Benzoyl- N′- p-substituted phenylthiourea derivatives is proposed. The stabilizing efficiency is attributed to the replacement of the labile chloride ions on the PVC chains by a relatively more stable moiety of the stabilizer.

Organic thermal stabilizers for rigid poly(vinyl chloride) VIII. Phenylurea and phenylthiourea derivatives

Phenylurea and phenylthiourea derivatives have been investigated as thermal stabilizers or co-stabilizers for rigid PVC in air, at 180 °C. The results reveal the higher stabilizing potency of the investigated organic stabilizers as compared with commercial reference stabilizers, which is proved by their greater induction period values, but with higher dehydrochlorination rate at the later stages of degradation. Complexation of these materials with the chlorides of Ni 2+ and Cd 2+ greatly improved the induction period values, however the dehydrochlorination rate is only slightly improved. On the other hand, blending the investigated thermal stabilizers with some of the commercially known reference stabilizers in different proportions leads to a true synergistic effect. The induction period values and the degradation rate were remarkably improved. A probable mechanism for the stabilizing action of phenylurea and phenylthiourea derivatives has been proposed.

Ortho pathway of benzoate degradation in Pseudomonas putida: induction of meta pathway at high substrate concentrations

The growth of Pseudomonas putida ATCC 49451 on various concentrations of sodium benzoate was examined under batch cultivation. At low initial concentrations (≤400 mgl −1), cell growth rate was described by the Monod model with parameters of maximum specific growth rate, μ m = 0.66 h −1 and saturation constant, K s = 10 mgl −1. At higher initial concentrations (≥500 mgl −1), product inhibition on specific growth and degradation rates was observed; specific growth rate decreased as sodium benzoate was degraded. A change in degradation pathway was observed with different initial concentrations of sodium benzoate. With initial concentrations up to 200 mgl −1, catechol was oxidized via the ortho ring cleavage with the induction of catechol 1,2-dioxygenase. The ortho pathway metabolites namely, catechol, cis, cis-muconate and muconolactone, were identified using gas chromatography/mass spectrometry (GC/MS) after trimethylsilyl (TMS) derivatization. When higher initial concentrations of sodium benzoate were used (≥300 mgl −1), the culture media showed absorbance at 375 nm (A 375) during the later stage of degradation after approximately 200 mgl −1 of sodium benzoate has been transformed. This was attributed to the accumulation of 2-hydroxymuconic semialdehyde (HMSA), which is an intermediate metabolite of the meta ring fission of catechol. It is concluded that cells grown on high enough concentrations of sodium benzoate simultaneously induced catechol 1,2-dioxygenase and catechol 2,3-dioxygenase, thus activating both the ortho and the meta catechol cleavage pathways. It was further found that catechol, a metabolic intermediate of sodium benzoate, did not effectively support the growth of P. putida when used as a sole substrate. Instead, a dark brown polymerization product was formed. We infer that the induction of the meta pathway is a consequence of the accumulation of 1,2-dihydro-1,2-dihydroxybenzoate (DHB) in the media.

Organic thermal stabilizers for rigid poly(vinyl chloride) V. Benzimidazolylacetonitrile and some of its derivatives

2-Benzimidazolylacetonitrile (BAN) and some of its conjugated unsaturated derivatives namely: 2-benzimidazolylbenzylidineacetonitrile (BBeAN), 2-benzimidazolyl-ω-phenylpropenylidineacetonitrile (BPAN) and 2-benzimidazolylbut-2-enylidineacetonitrile (BBuAN) have been investigated as organic thermal stabilizers for rigid PVC at 180 °C in air. Their stabilizing efficiencies are based on measuring the length of the induction period ( T s), the period during which no detectable amounts of HCl gas could be observed, and also from the rate of dehydrochlorination as measured by the continuous potentiometric titration on the one hand, and the extent of discoloration of the degraded polymer samples on the other. The stabilizing efficiencies of the investigated stabilizers were compared with that of dibasic lead carbonate (DBLC), dibasic lead stearate (DBLS), barium–cadmium–zinc stearate (Ba–Cd–Zn stearate) and dibutyltin maleate (DBTM) which are commonly used industrial stabilizers. The results reveal the higher stabilizing efficiency of the investigated materials relative to the reference stabilizers concerning the T s values. However, the rate of dehydrochlorination at the later stages of degradation for PVC stabilized with the investigated stabilizers is found to be relatively higher than that in the presence of reference stabilizers. These experimental findings indicate that almost all the stabilizers actions are exhausted during the induction period. The stabilizing efficiencies of the investigated stabilizers can be attributed to the replacement of the labile chlorine atoms on the polymer chains by a relatively more thermally stable groups derived from the stabilizer. This substitution reaction proceeds most probably through a radical mechanism. Moreover, the results also reveal the effect of the extent of conjugated unsaturates in the organic stabilizer molecule. The stabilizing potency of the investigated materials increases with the extent of their conjugated unsaturation sites. On the other hand, the results also reveal that while the extent of discoloration of thermally degraded PVC in the presence of BPAN stabilizer is comparable with most of the investigated reference stabilizers, the other derivatives showed a relatively higher extent of discoloration.

Hydrolytic degradation of tyrosine-derived polycarbonates, a class of new biomaterials. Part II: 3-yr study of polymeric devices

The kinetics and mechanisms of in vitro degradation of tyrosine-derived polycarbonates, a new class of polymeric biomaterials, were studied extensively at 37°C. These polymers carry an alkyl ester pendent chain that allows the fine-tuning of the polymer's material properties, its biological interactions with cells and tissue, and its degradation behavior. The polymer carrying an ethyl ester pendent chain, poly(DTE carbonate), has been established as a promising orthopedic implant material, exhibiting bone apposition when in contact with hard tissue. Tyrosine-derived polycarbonates are relatively stable and degrade only very slowly in vitro. Therefore, accelerated studies were conducted at 50 and 65°C to observe the behavior of polymers during the later stages of degradation. Varying the pendent chain length affected the rate of water uptake, initial degradation rate, and physical stability of the polymeric devices. During the 3-yr study, the polymer degraded by random chain cleavage of the carbonate bonds, accompanied by a relatively small amount of pendent chain de-esterification. No mass loss was observed during this period at 37°C, but mass loss was readily evident during the accelerated studies at 50 and 65°C. Thus, it is reasonable to assume that mass loss will occur also at 37°C, albeit only after extensive backbone carbonate cleavage and pendent chain ester hydrolysis. The dimension and surface area of the devices influenced the initial degradation rate, but did not significantly affect the overall rate of degradation. No evidence of “acid dumping” or the release of acidic residues found during the degradation of poly( d, l-lactic acid) were observed for this family of tyrosine-derived polycarbonates.

Effects of soil erosion on the floristic composition of plant communities on marl in northeast Spain

Abstract. This study explored the validity of three responses of vegetation to increased soil erosion: reduction of vegetation cover, number of species and reduced substitution of species.201 relevés, including edaphic and geomorphological data, were surveyed in the intensely eroded Eocene marls of the Prepyrenees (NE Spain). Changes in plant species’ presence in relevés from different degradation stages were compared. The level of vegetation degradation was defined as the total phanerogam cover which, in the studied area, was correlated to the degree of soil erosion. The considered trends were validated. Reduction of phanerogam cover and species number were gradual from low to high‐eroded areas. Vegetation degradation explained 48% of the species number variance. In the later stages of degradation a significant substitution of species was not observed, only a lower frequency of occurrence of several species that appeared in the whole set of relevés. Through the process of degradation, 47% of species displayed significantly reduced frequencies as degradation increased, none showed a significant increase in frequency.It is concluded that there are no characteristic species in these plant communities that survive in the severely eroded marls. Among the few species that had increased in frequency, most only colonised favourable micro‐environments.

Investigation of Oxidative Degradation in Polymers Using17O NMR Spectroscopy

The thermal oxidation of pentacontane (C50H102), and of the homopolymer polyisoprene, has been investigated using 17O NMR spectroscopy. By performing the oxidation using 17O-labeled O2 gas, it is possible to easily identify nonvolatile degradation products, even at relatively low concentrations. It is demonstrated that details of the degradation mechanism can be obtained from analysis of the 17O NMR spectra as a function of total oxidation. Pentacontane reveals the widest variety of reaction products and exhibits changes in the relative product distributions with increasing O2 consumption. At low levels of oxygen incorporation, peroxides are the major oxidation product, while at later stages of degradation these species are replaced by increasing concentrations of ketones, alcohols, carboxylic acids, and esters. Analyzing the product distribution can help in identification of the different free-radical decomposition pathways of hydroperoxides, including recombination, proton abstraction, and chain scission, as well as secondary reactions. The 17O NMR spectra of thermally oxidized polyisoprene reveal fewer degradation functionalities but exhibit an increased complexity in the type of observed degradation species due to structural features such as unsaturation and methyl branching. Alcohols and ethers formed from hydrogen abstraction and free radical termination reactions are the dominant oxidation products. In polyisoprene, the formation of esters and carboxylic acids is relatively minor, distinctly different from the oxidation of pentacontane. An approximately linear increase in these degradation functionalities is observed with increasing oxidation levels. These results demonstrate the promise of 17O NMR as a new technique for detailed investigation of oxidative polymer degradation.

Hydrolytic degradation of PLA/PEO/PLA triblock copolymers prepared in the presence of Zn metal or CaH 2

Various PLA/PEO/PLA triblock copolymers were prepared by ring-opening polymerization of L-lactide in the presence of poly(ethylene glycol), using CaH 2 or zinc metal as co-initiator. The degradation behavior of these copolymers was investigated in a pH=7.4 phosphate buffer at 37°C. Various techniques such as weighing, size exclusion chromatography, differential scanning calorimetry, X-ray diffractometry, and infra-red and 1H nuclear magnetic resonance spectrometries were used to monitor the changes in water absorption, weight loss, molar mass distribution, thermal properties, degree of crystallinity and composition. The results showed that introduction of PEO sequences considerably increased the hydrophilicity of the copolymers as compared with PLA homopolymers. However, the degradability of PLA blocks was not enhanced because of the phase separation between PLA and PEO blocks. Bimodal molar mass distributions were detected at the later stages of degradation, which were assigned to the formation of crystalline degradation byproducts within the bulk material.

Different pathways of colony-stimulating factor 1 degradation in macrophage populations revealed by wortmannin sensitivity.

Phosphatidylinositol 3'(OH)-kinase (PI 3-kinase) is activated on stimulation of macrophages with colony-stimulating factor 1 (CSF-1). We studied its potential role in the internalization and degradation of CSF-1 and its receptor in two primary populations of murine macrophages, namely bone marrow-derived macrophages (BMM) and resident peritoneal macrophages (RPM). Even though CSF-1 induced PI 3-kinase activity in both BMM and RPM, wortmannin, a potent inhibitor of PI 3-kinase activity, at concentrations that inhibited PI 3-kinase activity by 90% in these cells, had little or no effect on receptor internalization and degradation in either BMM or RPM or on CSF-1 degradation by BMM. Strong (more than 90%) inhibition was, however, observed for CSF-1 degradation by RPM. These findings suggest that both wortmannin-sensitive and wortmannin-insensitive pathways of ligand degradation exist in macrophages and that, although CSF-1 and CSF-1 receptor share the same endocytic pathway initially, they might be targeted to different compartments at later stages of degradation.

Degradation mechanism of TiZrVMnNi metal hydride electrodes

The degradation mechanism of TiZrVMnNi metal hydride electrodes was investigated because they rapidly degraded within 10 cycles despite having a high discharge capacity of 360∼464 mAh g −1 at the first cycle. It was observed that Ti-oxide formed and grew on the surface of the alloy powder in the early and later stages of degradation using the scanning electron microscopy (SEM) and the auger electron spectroscopy (AES) analyses. Also, it was found that the contact and the reaction resistances for the electrochemical hydrogenation reaction increased as the number of cycles increased using the EIS (Electrochemical Impedance Spectroscopy) analysis. It was suggested that the reason for the degradation of Ti-based hydride electrodes was because it was extremely difficult for hydrogenation to occur through the Ti-oxide film into the inner part of the alloy. This was due to a high polarization resistance during the electrochemical hydrogenation reaction, which included the resistance of the hydrogen diffusion through the Ti-oxide film and the resistance of the electron conduction between Ti-oxide on the surface of the alloy powder and external circuits, as well as between the Ti-oxide layers on the surface of the alloy powders.

Morphology and degradation behavior of aliphatic/aromatic copolyesters

AbstractRandom and segmented block copolymers of Poly(tetramethylene succinate) (PTMS) and Poly(tetramethylene terephthalate) (PTMT) were synthesized and characterized regarding sequence distribution and crystalline parameters. Biodegradation behaviors of the copolyesters were examined in activated sludge and enzyme solution in terms of CO2 evolution. It was found that the initial degradation was strongly dependent on the crystallinity in the random copolymer. However, the chemical compositions become a dominant factor at the later stage of degradation. In the case of block copolymers, the biodegradation rate was most significantly affected by size and degree of perfectness of PTMS crystallites.

The Cellulases Endoglucanase I and Cellobiohydrolase II of Trichoderma reesei Act Synergistically To Solubilize Native Cotton Cellulose but Not To Decrease Its Molecular Size

Degradation of cotton cellulose by Trichoderma reesei endoglucanase I (EGI) and cellobiohydrolase II (CBHII) was investigated by analyzing the insoluble cellulose fragments remaining after enzymatic hydrolysis. Changes in the molecular-size distribution of cellulose after attack by EGI, alone and in combination with CBHII, were determined by size exclusion chromatography of the tricarbanilate derivatives. Cotton cellulose incubated with EGI exhibited a single major peak, which with time shifted to progressively lower degrees of polymerization (DP; number of glucosyl residues per cellulose chain). In the later stages of degradation (8 days), this peak was eventually centered over a DP of 200 to 300 and was accompanied by a second peak (DP, (apprx=)15); a final weight loss of 34% was observed. Although CBHII solubilized approximately 40% of bacterial microcrystalline cellulose, the cellobiohydrolase did not depolymerize or significantly hydrolyze native cotton cellulose. Furthermore, molecular-size distributions of cellulose incubated with EGI together with CBHII did not differ from those attacked solely by EGI. However, a synergistic effect was observed in the reducing-sugar production by the cellulase mixture. From these results we conclude that EGI of T. reesei degrades cotton cellulose by selectively cleaving through the microfibrils at the amorphous sites, whereas CBHII releases soluble sugars from the EGI-degraded cotton cellulose and from the more crystalline bacterial microcrystalline cellulose.

Degradation of poly[bis(ethylamino)phosphazene] in aqueous solution

Three polymers of poly[bis(ethylamino)phosphazene] (PBEAP) containing different amounts of the residual P−Cl moieties, which had been hydrolyzed into P OH in the following sample purification processes, were prepared by substitution of the chlorines on poly(dichlorophosphazene) with ethylamine. Only the polymer which had the highest side-chain content of ethylamino groups (ca. 93°o) had a film-forming ability and a crystalline nature. The hydrolytic degradation of PBEAP in acidic solutions was investigated using the solution viscosity data obtained as a function of standing time. Acetic acid, 0.5 and 1N, pure acetic acid, and 2.2.2-trifluoroethanol were used as solvents. The degradation was composed of random breaking processes along the polymer chain, especially at the-N=P(OH)2-and-N=P(OH)(NHC2H5)-units, and an unzippering-like breaking process which was started at the chain ends produced by the former random breaking. The random breaking caused an abrupt decrease in viscosity at the beginning of the degradation, and on the contrary, the unzippering-like breaking appeared as a gradual decrease in viscosity at the later stages of degradation. The total rate of degradation depended on the concentration of the ethylamino groups.

Effect of weathering on UV-stabilized low density polyethylene films (LDPE) for a multilayer greenhouse cover

Multilayer films for greenhouses is a current trend in industry. LDPE films for the top layer, stabilized with different concentrations (0·1, 1·0 and 2·5%) of a nickel quencher, were produced in the laboratory by blow extrusion. The effect of natural weathering on the film properties was investigated over a period of 12 months. Significant changes in the mechanical properties were observed in the later stages of degradation. Films stabilized with 0·1% stabilizer crumbled after 12 months of natural weathering whereas films with higher concentrations retained their mechanical properties. We believe that inclusion of the UV stabilizer interferes with the crystallization process and the stabilizer particles accumulate in the amorphous matrix. Degradation of the imperfect crystalline region with its low oxygen permeability, proceeds via cross linking whereas chain scission predominates in the amorphous region with excess of oxygen. Films stabilized with 2·5% nickel quencher form a barrier against the transmission of UV radiation and the bottom layers are least affected by UV radiation. Recycled material can, therefore, be incorporated at high concentrations into these layers.

Degradation of aromatic polymers—III. Crosslinking and chain scission during photodegradation of polysulphones

Bisphenol A polysulphone (PSF) and polyethersulphone (PESF) were photodegraded homogeneously at room temperature in an inert and a normal atmosphere. The average molecular weights and the molecular weight distributions of degraded samples were measured by means of GPC to evaluate the quantum yields of chain scission and crosslinking. Simultaneous crosslinking and chain scission occur in the photodegradation of PSF and PESF. The ratio of the chain scission rate to the crosslinking rate was found to be 4.5 for PSF and 6.0 for PESF in the early stage of degradation. Quantum yields of crosslinking and chain scission were found to be of the order of 10 −4 for PSF as well as PESF and they are not greatly affected by the atmosphere during irradiation. It was also found that deceleration of reaction occurs in the later stages of degradation of both PSF and PESF. Photodegradation through addition to the benzene rings and H-abstraction by phenyl radicals formed by CS cleavage at diphenylsulphone moieties is proposed. The formation of phenol moieties by the photorearrangement of diphenylether moieties was revealed and related with the deceleration of reactions in the later stages of photodegradation of polysulphones, on the basis of i.r. analysis.

Photolysis of Poly[1-(4-substituted phenyl)-2-propen-1-one] in the solid phase

Photolysis of poly[1-(4-substituted phenyl]-2-propen-1-ones] in film of thickness 5–10 μm was followed by automatic viscometry, lowangle laser light scattering (LALLS) and gel permeation chromatography (GPC) under irradiation at 313 and 285·8 nm in air. The substituents in position 4 of the benzene ring were: H, fluoro, chloro, methoxy and ethyl. The monitoring of degradation by automatic viscometry and LALLS enabled determination of the Mark-Houwink parameters α and K for the above polymers in chlorobenzene at 25°C as follows. H: α = 0·76 ± 0·03, K = 3·8 × 10 −3; 4-F: α = 0·69 ± 0·02, K = 6·1 × 10 −3; 4-Cl: α = 0·79 ± 0·03, K = 1·4 × 10 −3; 4-C 2H 5: α = 0·85 ± 0·03, K = 1·3 × 10 −3; 4-OCH 3: α = 0·75 ± 0·06, K = 2·3 × 10 −3. The plots of main chain scission versus time were curved, which indicates inhibition at the later stage of degradation. The initial quantum yields of main chain scissions are about 0·08 except for the 4-methoxy derivative, in which case strong oxygen quenching causes the quantum yield to lie one order of magnitude lower. The unsaturated ketone formed during photolysis quenches the main chain scissions with Stern-Volmer constants which lie within the range 50–150 mol −1 for the polyketones under study.

Epitaxial polymerization of (SN)x: Chemical defects

Epitaxially polymerized (SN)x on various substrates has been studied by Fourier-transform infrared spectroscopy (FT-ir). Those substrates include Pt-shadowed KI, AgCl, NaCl, KCl, KI, KBr, RbI, and NaI. All samples of the polymer contained significant amounts of chemical defects, mostly as NH and S=O groups. The spectral variation observed with (SN)x polymerized on various substrates appeared to be due to differences in its degradation by water. There are at least two distinct degradation mechanisms. One is the degradation by water incorporated into the S2N2 crystals prior to the polymerization, and the other is the attack of moisture on the polymer. The nature of the chemical defects were studied by deuterium exchange and degradation by H2 18O as well as H2 16O. The band at 1400 cm−1 was assigned to the NH group while two bands at 1220 and 1090 cm−1 were assigned to S=O groups with different chemical environments. The band at 1220 cm−1 is related to a structure produced during the early stages of degradation in the preparative vacuum chamber, but the band at 1090 cm−1 is thought to arise from a structure formed at a later stage of degradation after sample preparation by atmospheric water.

Phagocytic activity of the stellate cells in the anuran pars intermedia.

In an attempt to study further the stellate cell and its functions, the ultrastructure of this cell type in the neurointermediate lobe of the bullfrog, Rana catesbeiana, was examined in both organ and dissociated-cell culture. The cytoplasmic activity of stellate cells from neurointermediate lobes incubated 3 1/2 or 5 1/2 h was greater than that of those in vivo. Mitochondria and bundles of cytoplasmic filaments were numerous, in addition to prominent, well-developed Golgi complexes with associated vesicles. The most striking ultrastructural feature was the presence of phagocytic vacuoles that contain cellular debris. The stellate cells were seen to form cytoplasmic processes that phagocytosed this extracellular debris identifiable as belonging to the secretory cells of the pars intermedia. The stellate cells from the dissociated-cell preparations were also seen to contain debris within phagocytic vacuoles. In those neurointermediate lobes transplanted for 3 1/2 to 4 days into the anterior chamber of the eye, the stellate cells demonstrated similar phagocytic ability, but the phagocytic vacuoles contained material that seemed to be at a later stage of degradation. In all three of these conditions, the stellate cells were not seen to release this cellular debris nor were they seen to undergo cell division. These glial-like stellate cells of the pars intermedia acted as macrophages in all three of these experiments. There is now, therefore, a need to determine under what conditions, if any, these stellate cells function in vivo as macrophages.

Isolation and some properties of two deoxyribonucleases from a snail, Achatina fulica.

1. Two main DNases were found in the dried liver extract of a snail, Achatina fulica. They were purified by the phosphocellulose batch method and by phosphocellulose column chromatography. The enzyme eluted earlier from the phosphocellulose column was designated as Achatina DNase-1 and the other as Achatina DNase-2. DNase-1 was purified further by QAE-Sephadex A-25 column chromatography (twice) just before use because of the instability of the purified enzyme. By these procedures, DNase-1 and 2 were purified 200- and 130-fold, respectively. 2. Divalent or monovalent cations had no marked effect on either enzyme. The showed pH optima of 4.8 (DNase-1) and 5.2 (DNase-2). Ionic strength was found to be critical for the maximal activity. The isoelectric points of DNase-1 and 2 were both 6.9. On heating at 70--75 degrees C for 5 min, each enzymic activity fell to half of the initial value. 3. The enzyme preparations degraded native DNA 1.5--2.5 times faster than heat-denatured DNA. They both degraded heat-denatured DNA endonucleolytically, to give oligonucleotides with 3'-phosphates. 4. The 3'-phosphoryl and 5'-hydroxy termini of the resulting oligonucleotides were analyzed. DNase-1 possessed marked specificity for dThd at 3'-termini and dAdo at 5'-termini in the early stages of degradation, but only for dAdo at 5'-termini in the later stages. DNase-2 showed some preference for purine nucleotides at both 3'- and 5'-termini in the later stages of degradation.

Thermal stability of segmented polyurethanes

The thermal stabilities of two series of segmented polyurethane fibres have been compared with their chemical structure. The polyurethanes were synthesized from trimethylene diamine; 4, 4′ diphenylmethane di-isocyanate and two polyether based macrodi-isocyanates. Thermal stability was measured by thermogravimetric analysis and differential thermal analysis. By comparing the changes in weight loss and DTA peaks with chemical structure it has been found possible to separate soft segment from hard segment degradation. In the initial stages stability increases as soft segment concentration increases while the reverse is true in the later stages of degradation. The 100% soft segment polymer (polytetrahydrofuran) and the 100% hard segment polymer appear to behave anomalously. It is suggested that the hard segment has a stabilizing influence on the degradation of the soft segment. The results are discussed in the light of various theories of polyurethane degradation.