Effect of large dosage irradiation in vacuo on polyethylene

The effect of large dosage irradiation in vacuo on polyethylene was investigated by the method of double irradiation. Polyethylene was first irradiated with γ-rays to a dosage of several hundred Mrad in vacuo at 298°K. The formed radicals were destoryed, and the polymer, changed chemically and morphologically, was irradiated again in vacuo at 77°K with a dose of 12.7 Mrad. The thermal decay of radicals produced in polyethylene by the second irradiation was investigated. The results show that many double bonds are formed in the sample irradiated to several hundred Mrad on the first irradiation. Alkyl radicals produced by the second irradiation react with these double bonds yielded by the first irradiation to form a large number of allyl radicals. These allyl radicals are much more stable than alkyl radicals radicals in vacuo at 298°K. In a sample not exposed to the first irradiation, almost all radicals formed by the second irradiation decay to form stable products. Crosslinks formed by a large amount of irradiation make the polyethylene matrix more rigid. This rigidity slows down the radical decay. These results suggest that the rate of radical accumulation in polyethylene irradiated in vacuo increases as the dosage increases.     

Effect of large dosage irradiation in air on polyethylene

The large dosage irradiation effect on polyethylene in air was examined by the method of double irradition. Polyethylene was first irradiated with γ-rays to a dosage of a few hundred Mrad in air at 298°K. The formed radicals were destroyed, and the polymer, changed chemically and morphologically, was irradiated again in vacuo at 77°K with a dose of 12.7 Mrad. The thermal decay of radicals produced in polyethylene by the second irrdiation was investigated. The results were compared with the sample not exposed to the first irradiation. The results were also compared with previous data of polyethylene irradiated in vacuo to a great extent and then treated in the same manner as that in the present experiment. The results show that scission of the polymer chain by oxidation makes the radical decay faster. Simultaneously, crosslinks formed in polyethylene irradiated in air to a great extent slow down the radical decay. Almost the same number of double bonds was formed in polyethylene irradiated in air as in polyethylene irradiated in vacuo. These double bonds react with alkyl radicals to form allyl radicals.     

Study of radical products of mechanically destructed poly(N-vinylene carbazole)

Free radicals generated by mechanical destruction of poly(N-vinylene carbazole) have been studied. Two types of chain-end radicals have been found in the system at 100K in accord with the polymer fracture theory. At 273K these radicals started to decay and new, more stable main-chain radicals have been observed. When O₂ was introduced into the system, the chain-end radicals disappeared rapidly giving rise to ROO· radicals, which decayed at 200K generating nitrogen centred cation radicals.     

Crystalline structure of polyethylene containing vinylene units in the main chain

Polyethylene containing trans- or cis-vinylene units in the main chain has been synthesized by means of partial hydrogenation of trans- or cis-polypentenamer (TP or CP), and trans-polyoctenamer (TO). Crystalline structure of the polymers has been investigated with wide-angle X-ray diffraction, FT-IR analyses, solid-state NMR measurements and quantum chemical calculations of the NMR chemical shifts. These results clear that the partially hydrogenated TP or CP samples having an average number of methylene units of 6.9-8.4 per vinylene unit forms not only orthorhombic crystal but hexagonal-like crystal.     

Thermal degradation of polyethylene in a nitrogen atmosphere of low oxygen content. III. Structural changes occuring in low-density polyethylene at oxygen contents below 1.2%

Samples of low-density polyethylene, free from additives, were kept at temperatures between 284° and 355°C under nitrogen containing 1.16% oxygen or less. Changes in molecular weight distribution (MWD) and degree of long-chain branching (LCB) were followed by gel chromatography (GPC) and viscosity measurements. Other structural changes were investigated by infrared spectroscopy and differential scanning calorimetry (DSC). Both chain scission and molecular enlargement occur simultaneously. Chain scission accounts for the formation of low molecular weight material and volatiles. Molecular enlargement reactions cause an increase in LCB and ultimately the formation of insoluble material. At lower temperatures (284°C) an increase in the high molecular weight end of the MWD is observed. The amount of olefinic unsaturation, carbonyl, and ether groups increase with degradation. Conjugated systems are formed. The formation of thin discolored and insoluble surface layers indicate that the attack of oxygen is diffusion controlled. The DSC thermograms undergo large changes at 333° and 355°C, increasing with time and oxygen content. A reaction scheme for the thermo-oxidative degradation of polyethylene is discussed. Both inter- and intramolecular hydrogen abstractions by peroxy radicals are suggested to occur. Thus, the formation of trans-vinylene and ether groups results from intramolecular abstraction, while internal carbonyl groups are formed by intermolecular abstraction. Chain scission will be accomplished by both routes and together with “back-biting” is suggested to accoun for the formation of volatiles. The formation of conjugated sequences causing discoloration is correlated with the formation of trans-vinylene groups. Because of the restricted accessability of oxygen under our conditions, the reactions discussed previously for pure thermal degradation¹ are also considered to be important. The molecular enlargement observed is thus proposed to be mainly due to the combination of alkyl radicals even when oxygen is present.     

Thermal degradation of polyethylene in a nitrogen atmosphere of low oxygen content. III. Structural changes occurring in low-density polyethylene at oxygen contents below 1.2%

Samples of low-density polyethylene, free from additives, were kept at temperatures between 284° and 355°C under nitrogen containing 1.16% oxygen or less. Changes in molecular weight distribution (MWD) and degree of long-chain branching (LCB) were followed by gel chromatography (GPC) and viscosity measurements. Other structural changes were investigated by infrared spectroscopy and differential scanning calorimetry (DSC). Both chain scission and molecular enlargement occur simultaneously. Chain scission accounts for the formation of low molecular weight material and volatiles. Molecular enlargement reactions cause an increase in LCB and ultimately the formation of insoluble material. At lower temperatures (284°C) an increase in the high molecular weight end of the MWD is observed. The amount of olefinic unsaturation, carbonyl, and ether groups increase with degradation. Conjugated systems are formed. The formation of thin discolored and insoluble surface layers indicate that the attack of oxygen is diffusion controlled. The DSC thermograms undergo large changes at 3333° and 355°C, increasing with time and oxygen content. A reaction scheme for the thermo-oxidative degradation of polyethylene is discussed. Both inter- and intramolecular hydrogen abstractions by peroxy radicals are suggested to occur. Thus, the formation of trans-vinylene and ether groups results from intramolecular abstraction, while internal carbonyl groups are formed by intermolecular abstraction. Chain scission will be accomplished by both routes and together with “back-biting” is suggested to account for the formation of volatiles. The formation of conjugated sequences causing discoloration is correlated with the formation of trans-vinylene groups. Because of the restricted accessability of oxygen under our conditions, the reactions discussed previously for pure thermal degradation¹ are also considered to be important. The molecular enlargement observed is thus proposed to be mainly due to the combination of alkyl radicals even when oxygen is present.     

Electrochemical oxidation of aliphatic hydrocarbons promoted by inorganic radicals. II. NO3 radicals

The anodic electrolysis of linear alkanes in tert-butanol/H₂O solutions containing HNO₃ and saturated with oxygen at P_{O 2}=1 atm results in the partial oxidation of the hydrocarbons to the corresponding isomeric ketones. The experimental data are in support of a mechanism in which the first steps are: (1) one-electron transfer from the nitrate ion to the anode to give a nitrate radical; (2) hydrogen abstraction by the nitrate radical on the aliphatic substrate to give an alkyl radical; (3) molecular oxygen addition by the alkyl radical and formation of peroxy radicals. These species decay by bimolecular reactions and a carbonyl function is formed. The isomer distribution is in excellent accord with a statistical H abstraction from the secondary C–H bonds.     

Free radicals trapped in polyethylene matrix: 1. Location of the alkyl radicals in irradiated polyethylene

Two types of alkyl radicals were found to be trapped in irradiated crystals grown from polyethylene solution. One of them corresponds to the broad sextet pattern of the e.s.r. spectrum and the other corresponds to the sharp sextet pattern. The free radicals attributed to the broad sextet began to disappear at a lower temperature than the temperature at which the free radicals attributed to the sharp sextet disappeared. When butadiene molecules were brought into contact with the specimen, the decay of the free radicals corresponding to the broad sextet was accelerated. When the specimen was subjected to fuming nitric acid treatment, no broad sextet was observed. The mat of the crystals, was aligned so that the c-axes of its crystallites were perpendicular to its surface. The broad sextet showed no anisotropy when the angle between the direction of applied magnetic field and that of the c-axis of the crystallite was varied. On the other hand, the sharp component of the spectrum showed apparent anisotropy. Consequently, it can be concluded that the broad component comes from the free radicals trapped in the lamellar surface and the sharp component is attributed to the free radicals trapped in the inner part of the crystallite. Hence, the locations of these two types of free radicals have been clarified with much more certainty than before.     

Crosslink mechanisms of high-solids alkyd resins in the presence of reactive diluents

The drying of alkyds was studied using NMR and mass spectrometry employing model compounds. Crosslinking occurs via oxidation of unsaturated fatty (linoleic) acids in the resin. Hydroperoxides formed as intermediates are degraded to alkoxy and peroxy radicals by cobalt catalyst. These radicals then recombine to form mainly ether and peroxy crosslinks. However, with conjugated fatty acids another mechanism prevails in which addition of radicals to the double bonds occurs. Despite the differences in mechanism the rates of both reactions were found to be similar. High-solids alkyd coatings may employ reactive diluents replacing traditional solvents (white spirit). The rate of incorporation of these reactive diluents into the actual paint film during drying can be followed with NMR. Allyl ether groups appear to react fastest whereas allyl esters show generally little reactivity. Using mass spectrometry it was found that incorporation involves recombination of radicals as well. However, reactive diluents with conjugated double bonds will be incorporated by radical addition.     

O-substituted phenolic antioxidant traps both alkyl and peroxy radicals

Phenolic antioxidants are well known for trapping peroxy radicals to prevent oxidative degradation in organic materials such as plastics. To eliminate such degradation it is also important to trap alkyl radicals, because autoxidation includes both peroxy and alkyl radicals as chain carriers. In general, alkyl radicals cannot be trapped because they react too quickly.This is a short news story only. Visit www.addcomp.com for the latest additives and compounding industry news.     

Spin trapping of propagating radicals with 2,4,6-tri-tert-butyl-1-nitrosobenzene in radical polymerization initiated with di-tert-butyl hyponitrite

To examine the polar effect on spin trapping with 2,4,6-tri-tert-butyl-1-nitrosobenzene (BNB), propagating radicals from monomers were allowed to react with BNB to provide anilino radicals and nitroxides of which the amounts were determined by ESR spectroscopy. The proportion of the anilino radical increased with an increase in the steric congestion and electron donating character of the radical. Poly(vinyl ether) radicals exclusively gave the respective anilino radicals. Poly(vinyl ester) radicals yielded large amounts (>50%) of the anilino radicals, although the nitroxides were the main products (>85%) from the poly(alkyl acrylate) radicals. The polymer radicals of methyl methacrylate and ethyl itaconate only yielded the anilino radicals. The polymer radicals from ethyl -chloroacrylate and acrylonitrile reacted preferentially with the respective monomers leading to polymer formation, and the ESR spectra of the adducts were not observed.     

Reaction mechanism of poly(vinyl chloride) degradation. Molecular orbital calculations

Semiempirical Molecular Orbital Calculations (MNDO AM1) support kinetic results concerning the molecular mechanism of thermal degradation of PVC and show that under special conditions radical and ionic mechanisms are also possible. The degradation of poly(vinyl chloride) is a complex chain dehydrochlorination that consists of an initiation process to generate an active intermediate followed by chain reactions that generate additional active intermediates with progressively increased numbers of double bonds. Each intermediate partitions between an intermediate with one more double bond and a stable conjugated polyene with the same number of double bonds. At low and moderate temperatures thermal degradation of PVC in an inert atmosphere is a succession of molecular concerted reactions. The initiation process is a 1,2-elimination through a four center transition state requiring a synperiplanar conformation. There are two main chain reactions: the first is a 1,4-elimination from allylic chlorine atoms and methylenes cis to a double bond through a transition state of six centers; the second is a 1,3-rearrangement of hydrogen atoms catalyzed by hydrogen chloride. The chain reaction is interrupted when a relatively stable trans double bond is formed and no hydrogen chloride is present to catalyze trans-cis isomerization or 1,3-rearrangement. Macro carbocations formed by heterolysis of carbon-halogen bonds in the presence of strong Lewis acids react much faster than does the original PVC in concerted elimination by 1,2-syn or 1,4-cis mechanisms, promoting a so-called catastrophic, very fast degradation. Macro radicals formed by thermal homolysis, irradiation or reaction with promoters can also promote very fast hydrogen chloride elimination because of a special mechanism consisting of a 1,2-rearrangement of a chlorine atom followed by a concerted 1,3-elimination through a five center transition state.     

Role of the unsaturated group in the radiation-induced crosslinking of polyethylene in the presence of chlorotrifluoroethylene

The effect of unsaturated groups on the crosslinking of polyethylene in the presence of chlorotrifluoroethylene (CTFE) has been studied. Butadiene- and isoprene-grafted polyethylenes with high concentrations of unsaturated groups were crosslinked more markedly than the original polyethylenes. The rate of gel formation increased with the increase in the initial concentration of unsaturated group in polyethylenes, while the rate of grafting of CTFE decreased. The concentration of unsaturated group decreased rapidly during the course of the reaction. The rate of decrease in each unsaturated group was in the order, vinylidene > vinyl >trans-vinylene. In the irradiation of polyethylene with a high concentration of unsaturated groups in CTFE, the number of grafted monomers was almost equal to the decrease in numbers of the unsaturated group. These results are discussed in relation to the reaction mechanism and the estimated activation energy of the elementary reaction     

Visible Light‐Induced Selective Generation of Radicals from Organoborates by Photoredox Catalysis

A new strategy for the generation of carbon‐centered radicals via oxidation of alkyl‐, allyl‐, benzyl‐ and arylborates by visible‐light‐driven single electron transfer (SET) photoredox catalysis has been established. The generated radicals smoothly react with TEMPO and electron‐deficient alkenes to afford C O and C C coupling products, respectively. In this radical initiating system, cyclic organo(triol)borates turn out to be useful radical precursors.     

Influence of intrachain double bonds on the properties of solution grown single crystals of polyethylene

The influence of intrachain trans double bonds, statistically distributed along a polyethylene chain, on the structure and behaviour of chain-folded single crystals has been studied. The dependence upon the percentage of double bonds of dissolution and thermal properties, external crystal habit, X-ray diffraction in the wide- and low-angle regions, of solution grown single crystals of ethylene-butadiene copolymers may be interpreted by assuming that a certain amount of CH₂CHCHCH₂ units are incorporated in the lattice as defects. The results of the bromination of single crystals of a sample of ethylene-butadiene copolymers show that about the 56% of double bonds, not accessible to bromine, are situated not on the fold surface but in the inner part of the crystals.     

Quantification of silane grafting efficacy,weak IR vibration bands and percentage crystallinity in post e-beam irradiated UHMWPE

Vinyl-tri-methoxy silane (VTMS) and vinyl-tri-ethoxy silane (VTES) were grafted onto ultra-high molecular weight polyethylene (UHMWPE) by irradiating the UHMWPE/silane hybrids with e-beam. The samples were irradiated under high moisture contents for total dose values of 30, 65 and 100 kGy, respectively. The synergistic effect of silane and irradiation on the grafting efficacy, concentration of weak bonds like trans-vinylene (–CH=CH–) and vinyl (–CH=CH₂) and percentage values of crystallinity were studied using FTIR spectroscopy. For the estimation of grafting reactions efficiency, absorption due to characteristic infrared absorption bands of –Si–CH– in the region ~800 cm^?1 was monitored and found that grafting efficacy of VTMS on UHMWPE was higher as compared to VTES and increased with irradiation. The relative amounts of grafting extension (R) for 100 kGy irradiated UHMWPE/VTMS and UHMWPE/VTES hybrids were found to increase 20 and 15 %, respectively. The concentration of trans-vinylene in UHMWPE was found to increase from 0.015 to 0.035 mmol/l due to synergistic effects of silane and irradiation. Moreover, crystallinity of UHWMPE was found to decrease from 65 to 55 % due to the abovementioned synergistic effects which was also confirmed with DSC tests. Furthermore, oxidation index values were measured to confirm the efficacy of silane as free radical quencher via silane grafting extension reactions.     

Polymorphism in single-acid triglycerides of positional and geometric isomers of octadecenoic acid

The polymorphism of 25 glycerol trioctadecenoates with double bonds ranging from Δ4-Δ17 was investigated by differential scanning calorimetry. Triglycerides withcis bonds in odd positions Δ7-Δ13 exhibited three intermediate melting (β′-) forms, but those withcis bonds in even positions, exceptcis Δ4, lacked β′-forms. Among thetrans compounds, only Δ11, 13, and 14 showed β′-forms. Thecis andtrans Δ5 triglycerides were unusual, because they readily assumed low melting (α-) forms that were not easily converted to high melting (β-) forms. β-Form mp of compounds in each series (cis ortrans) alternated depending upon double bond position; an even position correlated with high mp. Heats of fusion (ΔH_f) for β-forms, likewise, fluctuated with double bond position but nonuniformly;trans Δ6 had the highest ΔH_f (43 cal/g),cis Δ12 the lowest (21 cal/g).     

Well‐Defined End‐Functionalized Conjugated Polymers/Oligomers Exhibiting Unique Emission Properties through the End Groups: The Exclusive Synthesis by Combined Olefin Metathesis with Wittig‐type Coupling

Acyclic diene metathesis polymerization using ruthenium–carbene catalysts affords defect‐free, high molecular weight poly(arylene vinylene)s containing all trans olefinic double bonds. The exclusive end‐functionalization in the resultant poly(fluorene vinylene)s or poly(phenylene vinylene)s can be attained by treating the vinyl end groups using a molybdenum–alkylidene catalyst/reagent (through olefin metathesis) followed by addition of various aldehydes (Wittig‐type coupling). Some of these end‐modified conjugated materials display unique emission properties, which are different from the original ones, through an interaction (energy transfer or structural change in the excited state) between the conjugated main chain and the end groups [oligo(thiophene)s, F‐BODIPY, etc.]. Exclusive synthesis of well‐defined, all‐trans end‐functionalized oligo(2,5‐dialkoxy‐1,4‐phenylene vinylene)s [(oligo(phenylene vinylene), alkoxy = O(CH₂)₂OSiⁱ Pr₃, up to 31 repeat units] is demonstrated by adopting a stepwise synthetic approach (olefin metathesis and the subsequent Wittig‐type cleavage). It is clearly demonstrated that their optical properties (especially the fluorescence spectra including photoluminescence quantum yields) are strongly affected by the end groups as well as the conjugation repeat units.     

An investigation of vinyl–ester?styrene bulk copolymerization cure kinetics using Fourier transform infrared spectroscopy

A Fourier transform infrared (FTIR) spectroscopy technique was developed to investigate the effects of reaction temperature and reactant composition on the isothermal curing kinetics of commercial vinyl nester resins comprised of vinyl–ester monomer (dimethacrylate of diglycidyl ether of bisphenol A DGEBA) and styrene. This technique enables a more complete evaluation of the bulk copolymerization reaction of vinyl–ester styrene systems by monitoring the depletion of vinyl–ester and styrene double bonds independently. The results indicate that the rate of fractional conversion of styrene double bonds is initially less than that of vinyl–ester vinyl groups. However, styrene monomer continues to react after conversion of vinyl–ester double bonds has ceased. In addition, the overall extent of conversion was found to increase with increasing isothermal cure temperature, and it was observed that higher styrene concentration enhances final conversion of vinyl–ester double bonds and not styrene double bonds. Increasing styrene monomer concentration also resulted in lowering the apparent activation energy for the reaction of vinyl groups from both monomers as characterized by an empirical autocatalytic model used to fit the conversion results for styrene and vinyl–ester double bonds independently. The results of this work demonstrate that reaction temperature and resin composition significantly affect the cure behavior of vinyl–ester resins and provide insight into the development of the resulting network structure. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 76: 1572–1582, 2000     

Infrared spectroscopy and conformation of alkyl vinyl ketone monomer

The correlation between IR spectra and conformations was examined in alkyl vinyl ketone monomer. The stretching absorption bands of double bonds shift to lower wave number with the decrease in Taft's σ^* values of alkyl groups, and, conversely, the deformation band of the vinyl group shifts to the higher wave number. The inductive effect of the alkyl group is aparently three times larger in s-cis conformation than that in the s-trans. Absorption coefficients in the double-bond stretchings (A C?O and A C?C) were measured in each conformation. Both conformations have a nearly equal value of A C?O. The ratio (A s-cis/A s-trans) C?C is 3.4 ±1.0. The ratios (A′ C?O/A′ C?C) are 4.5 ±0.6 for the s-cis conformation and 15±2 for the s-trans conformation in all alkyl vinyl ketones studied.