Peroxide crosslinking of linear low-density polyethylenes with homogeneous distribution of short chain branching

Extraction and swelling experiments were carried out in order to measure the crosslinkability by dicumyl peroxide (DCP) of new homogeneous linear low-density polyethylenes (LLDPEs) with different molecular weight, branch content, and length of branches. Considering that the tertiary radicals are most probable macroradicals, our results indicate that tertiary carbons in LLDPEs could be either sites for scission reaction of effective coupling, depending upon degree of branching and to a lesser extent on degree of crosslinking. In the range of branching between 16 and n CH₃/1000 C (n ? 31 for low DCP concentration and decreases with DCP content approaching the value of 23 for the range above ca. 6 DCP moles per number average PE mole), the tertiary carbons seem to be rather the sites for effective coupling. In this range of branching an increase of crosslinkability with branch concentration and molecular weight of PE is due to the increase of crosslinking efficiency, but the scission probability is very low. At branch content exceeding the value of n CH₃/1000 C, the dominant reaction on tertiary carbons is scission, leading to a small decrease of crosslinkability with increasing branch content. The possible mechanisms favoring tertiary carbons for either coupling or scission are discussed. © 1995 John Wiley & Sons, Inc.     

ESR study on chemical crosslinking reaction mechanisms of polyethylene using a chemical agent. III. Effect of amine type antioxidants

The effect of antioxidant on the reaction mechanism of chemical crosslinking of polyethylene with dicumyl peroxide (DCP) at high temperatures was investigated by electron spin resonance (ESR). The crosslinking reactions were induced by the alkyl radicals in polyethylene (PE) formed by the thermal decomposition of DCP above 120°C. The type and the content of radicals were much changed for amine type antioxidants on PE crosslinking. It was confirmed that the radicals originated from DCP decomposition reacted preferentially with the amine type antioxidants to produce the nitroxyl radical and that the antioxidants retarded the initiation reaction of the PE crosslinking reaction. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 349–356, 1999     

ESR study on chemical crosslinking reaction mechanisms of polyethylene using a chemical agent—II. The effect of phenolic antioxidants

The effect of antioxidant on the reaction mechanism of chemical crosslinking of polyethylene (PE) with dicumyl peroxide (DCP) at high temperatures was investigated by electron spin resonance (ESR). The antioxidant reacts with the alkyl radicals in PE formed by the thermal decomposition of DCP above 120°C, and disturbs the crosslinking. A phenolic type antioxidant produced the phenoxy radical by the reaction with alkyl radicals formed in PE. It is suggested that the selection of a suitable antioxidant for PE crosslinking can be made by ESR analysis. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35 : 2431–2439, 1997     

Changes in sol fraction during peroxide crosslinking of linear low-density polyethylenes with homogeneous distribution of short chain branching

FTIR measurements of sol fractions were carried out in order to assess side reactions during dicumyl peroxide (DCP) crosslinking of linear low density polyethylenes (LLDPEs) with homogeneous distribution of short chain branching. The changes of methyl and olefinic unsaturation concentration were measured as a function of degree of branching and DCP content. The changes of methyl and vinylidene concentration indicate that the scission probability has greater significance for the samples with initial branch content above 31 CH₃/1000 C and increases with increasing DCP concentration. The analysis of trans-vinylene concentration indicates that the probability of disproportionation also increases with increasing DCP content. The changes of methyl concentration in the sol fraction of the sample containing initially ca. 23 CH₃/1000 C with increasing DCP content could be interpreted using the dependence of crosslinking efficiency on branch content found previously for some range of branch concentration. © 1995 John Wiley & Sons, Inc.     

ESR study on chemical crosslinking reaction mechanisms of polyethylene using a chemical agent. IV. Effect of sulfur‐ and phosphorous‐type antioxidants

The effect of an antioxidant on the reaction mechanisms of chemical crosslinking of polyethylene (PE) with dicumyl peroxide (DCP) at high temperatures was investigated using electron spin resonance (ESR). For sulfur‐ and phosphorous‐type antioxidants, changes of radical species and their contents during the PE crosslinking reaction were observed. It was confirmed that these antioxidants reacted preferentially with radicals yielded by decomposed DCP, restraining the crosslinking of PE by the increased antioxidant content. The compound of DCP and antioxidant decomposed to form 2‐phenyl isopropyl radicals. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3092–3099, 2000     

ESR study on chemical crosslinking reaction mechanisms of polyethylene using a chemical agent

The reaction mechanisms of thermal crosslinking of polyethylene with dicumyl peroxide (DCP) at high temperatures were investigated by electron spin resonance (ESR). The crosslinking reactions were induced by the alkyl radicals formed by the thermal decomposition of DCP above 120°C. The kinetics of the free radical reaction were followed during crosslinking reactions at temperatures between 145 and 180°C. © 1997 John Wiley & Sons, Inc.     

Preparation of polyethyleneimine nanogels via photo-Fenton reaction

Photo-Fenton reaction was used to prepare polyethyleneimine nanogels (M-PEIs) from PEI pre-polymer as potential non-viral gene delivery vectors. Competition between crosslinking and scission of PEI pre-polymer in assembled cluster made the size of particles homogeneous. Morphologies of the nanoparticles in every step showed the existence of self-assembly in aqueous solution. The results indicated that the crosslinking of PEI pre-polymer was mainly initiated via hydrogen abstraction by hydroxyl radicals generated in the systems.     

Photochemical reactions of high polymers. XIV. Syntheses and photochemical reactions of copolymers bearing O-acyloxime groups

Copolymers bearing pendant O-acyloxime groups were synthesized by two methods: copolymerizations of oxime acrylate (methyl β-naphthyl ketone oxime acrylate or benzophenone oxime acrylate) and styrene, condensation of acrylic acid—styrene copolymer with oximes (benzophenone oxime, p-nitrobenzophenone oxime, methyl β-naphthyl ketone oxime, benzalacetone oxime or 9-fluorenone oxime). The photochemical behavior of the O-acyloxime copolymers changed markedly with the irradiation conditions: irradiation of benzene solutions led to degradation in air and crosslinking under nitrogen, while irradiation of solid films in air resulted in simultaneous degradation and crosslinking. Photolysis of methyl β-naphthyl ketone oxime acetate, a model for the O-acyloxime copolymer, in benzene solution under nitrogen resulted in scission of the N? O bond. The same reaction was observed in the irradiation of the O-acyloxime copolymers. It is suggested that formation of free radicals on the polymer chains via scission of the N? O bond is followed by decarboxylation. In the absence of oxygen, crosslinking of the polymer by recombination of the free radicals competes with degradation via β-scission. In the presence of oxygen, autoxidative degradation predominates.     

Photochemistry of ring substituted polystyrenes—Part III: Photolysis of poly(para-methoxystyrene)

The photodegradation of films (4 × 10^?4 cm thick) of poly(p-methoxystyrene) with 254 nm radiation under high vacuum at 25°C has been studied. The principal gaseous product is hydrogen, but smaller quantities (in decreasing yield) of methane, methyl alcohol and ethane are also formed, indicating that fission of bonds in the para methoxy group is also involved. Ultraviolet and visible spectra of degraded films indicate the presence of unsaturated groups and of coloured species. Solubility data indicate that crosslinking and chain scission occur simultaneously. Rates of chain scission of a number of p-substituted styrenes are compared and a reasonable correlation between these and the electron donating character of the para group is observed, explainable in terms of the stabilising effect of such groups on the radicals formed during chain scission. Rates of crosslinking are greater than those for polystyrene and this is attributable to the participation of the substituted phenoxy radicals (formed by CH₃O fission) in addition reactions. Quantum yields for the gaseous products and for chain scission and crosslinking have been determined and a mechanism has been advanced to account for the experimental data.     

Electron spin resonance study on chemical crosslinking reaction mechanisms of polyethylene using a chemical agent. V. Comparison with polypropylene and ethylene‐propylene copolymer

We studied the chemical reaction process of polypropylene (PP), ethylene‐propylene copolymer (EPM), and ethylene‐propylene‐diene copolymer (EPDM) crosslinking induced by dicumyl peroxide (DCP) using electron spin resonance (ESR). Free radicals appeared at an elevated temperature of around 120 °C and the behavior and kinetics of the reaction process were observed at 180 °C. The radical species detected in PP were alkyl type radicals, formed by the abstraction of hydrogen atoms from the tertiary carbon of polymer chains. For EPDM containing a diene component, the radicals were trapped at double bonds in this diene component to form allyl radicals. The resolutions of these spectra were extremely clear; hence, isotropic spectra of these polymer radicals were obtained. We measured the ESR at high temperatures and confirmed that the process of crosslinking induced by DCP was a free radical reaction. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 3383–3389, 2000     

In-situ monitoring by fibre-optic NIR spectroscopy and rheometry of maleic anhydride grafting to polypropylene in a laboratory scale reactive extruder

The graft copolymerisation of maleic anhydride (MAH) onto polypropylene (PP) has been studied in situ by interfacing a laboratory scale mini-extruder/processor with a near-infrared (NIR) spectrometer via a fibre optic link. Apparent viscosity was measured simultaneously with the NIR spectra. The graft reaction was carried out at several temperatures with and without the initiator, dicumyl peroxide (DCP). Analysis of the NIR spectra showed the reaction to be first order with respect to MAH, and that the graft reaction rate was not affected by the presence of DCP. However, DCP caused a large increase in chain scission reactions leading to a dramatic drop in apparent viscosity. In the absence of DCP, scission reactions occurred at a slower rate and the apparent viscosity remained steady, after an initial drop, for about 15 min during the early part of the reaction. We suggest that a possible explanation for this might be a “repair mechanism” involving the reaction of two PP macro-radicals with a single MAH moiety, thus maintaining the molecular weight. Once the concentration of MAH drops, this reaction becomes less significant and scission reactions dominate.     

Electron spin resonance study on chemical‐crosslinking reaction mechanisms of polyethylene using a chemical agent. VI. Effect of α‐methyl styrene dimer

The effect of α‐methyl styrene dimer (AMSD), which is used as a scorch retarder, on the reaction mechanisms of the chemical crosslinking of polyethylene (PE) with dicumyl peroxide (DCP) at high temperatures was investigated using electron spin resonance. When AMSD was added to PE containing DCP, the AMSD radical was observed; however, the PE alkyl radical or allyl radical presence was not detected. At 145 °C, crosslinking was obstructed as a result of the reaction between AMSD and alkyl radicals. As the temperature increased, AMSD fragmented to form 2‐phenyl‐2‐propyl and double bonds in PE. This generation of double bonds, however, accelerated crosslinking at 180 °C and was more effective than when AMSD was not present. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 2151–2156, 2001     

Simulation‐Assisted Evaluation of Acetylene Effect on Macromolecular Crosslinking Rate under Polyethylene Irradiation

Gel content data on acetylene accelerated radiation crosslinking (R.A. Jones, J. Polym. Sci., Part B: Polym. Phys. 1994 , 32, 2049) have been used to illustrate a new method to determine macromolecular crosslinking and scission yields. The sol‐gel analysis data have been plotted as log(sol) versus log(dose) resulting in quite linear plots having different slopes. The linear approximations with the least squares method resulted in gel‐points with a high accuracy. Computer simulations have shown the plot slope to be dependent on relative rate of competitive macromolecular scission. The scission/crosslink ratios have been found from the plot slopes using simulation software GelSim6. As a result acetylene gas has been found to be accelerating both crosslinking and scission rates: 60 times and 130 times, respectively. Obviously, the radiation yield of radicals is increased due to acetylene inhibiting the recombination of primarily induced radicals in a cage.

Gel content vs. irradiation dose data plotted as log(sol) versus log(dose). Numbers near the curves indicate concentrations of acetylene gas (mmol · kg⁻¹).     

Studies on diphenylguanidine–accelerated sulfur vulcanization of styrene butadiene rubber in the presence and absence of dicumylperoxide

Diphenylguanidine (DPG) raises the rate of decomposition of dicumylperoxide (DCP) and the crosslinking maxima due to DCP is lowered to some extent by DPG. When the molar proportion of DPG–S is approximately 1:1, no additive results of crosslink formation (as reported for NR) are observed. Zinc oxide and stearic acid increase the rate of crosslinking as well as the crosslinking maxima. In the present study it appears most probable that DPG-accelerated sulfuration of SBR is an ionic process. At a constant level of DCP and sulfur crosslink density increases when the amount of DPG is increased; a constant level of DCP and DPG crosslinking increases with rising sulfur concentration. An increase in the concentration of DPG or sulfur leads to greater formation of the complex as well as a change in its composition. The effect of DPG is more pronounced, for by reducing the number of sulfur atoms more sulfur is available for crosslinking. Sulfur absorbs little SO₂ or H₂S; if it is already saturated, there is no perceptible effect. DPG does absorb H₂S or SO₂ and the rate and maxima of crosslinks is increased. The effect of SO₂ is higher because of the higher K_a values of H₂SO₃ and consequently higher concentration of HSO₃^?. Reversion is a free-radical process inhibited by free DPG present in the system. In the presence of zinc oxide and stearic acid the reaction follows a polar mechanism as well as a radical mechanism.     

The role of hydroperoxides in photo-oxidative degradation of cis-1,4-polybutadiene

Hydroperoxides undergo various types of homolytic reactions on exposure to u.v. radiation. Free radicals formed from the photodecomposition of the hydroperoxide group (OOH) are oxy (HO.) and peroxy (HOO.) radicals which participate in further reactions. In cis-1,4-polybutadiene, they may initiate free radical oxidations. Cleavage of alkoxy (RO.) radicals and crosslinking of polymer radicals through polymer peroxides in the presence of air in solid film nearly balance. Most polymer radicals produced in the absence of oxygen undergo cross-linking but form peroxy radicals (POO.) in its presence. This paper presents results on the photodecomposition of tert-butyl hydroperoxide, cumyl hydroperoxide and 2,5-dimethyl-2,5-dihydroperoxyhexane in cis-1,4-polybutadiene in film and in solution.     

Synthesis of Analogues of (1 → 6)‐Branched (1 → 3)‐Glucohexaose

Abstract

β‐D‐Galp‐(1 → 3)‐[β‐D‐Galp‐(1 → 6)‐]α‐D‐Glcp‐(1 → 3)‐β‐D‐Glcp‐(1 → 3)‐[α‐D‐Manp‐(1 → 6)‐]D‐Glcp 16 and β‐D‐Galp‐(1 → 3)‐[β‐D‐Glcp‐(1 → 6)‐]α‐D‐Glcp‐(1 → 3)‐β‐D‐Glcp‐(1 → 3)‐[α‐D‐Manp‐(1 → 6)‐]D‐Glcp 18 were synthesized as the analogues of the immunomodulator β‐D‐Glcp‐(1 → 3)‐[β‐D‐Glcp‐(1 → 6)‐]α‐D‐Glcp‐(1 → 3)‐β‐D‐Glcp‐(1 → 3)‐[β‐D‐Glcp‐(1 → 6)‐]D‐Glcp through coupling of trisaccharide donors 8 and 13 with trisaccharide acceptor 14 followed by deprotection, respectively.     

Bienzymatic analytical microreactors for glucose, lactate, ethanol, galactose and l-amino acid monitoring in cell culture media

A new alternative method for bioprocess monitoring based on bienzymatic analytical microreactors integrated in a flow injection analysis (FIA) system is described. Glucose-, alcohol-, lactate-, galactose- and l-amino acid oxidases (GO, AO, LacO, GalO and LAAO) and horseradish peroxidase (HRP) are immobilized on controlled pore glass (CPG) and used for the development of glucose, ethanol, lactate, galactose and amino acid sensors. The analytical methodology is based on HRP catalysed reaction of hydrogen peroxide produced by oxidases with phenol-4-sulfonic acid and 4-aminoantipyrine. The immobilized enzymes are characterized and used for preparation of the packed bed analytical microreactors. Shelf life and operational stability of the microeactors are determined. GO/HRP, AO/HRP and LAAO/HRP microreactors showed excellent shelf life, they could be stored and reused for more than 6 months with no or very little activity loss, while GalO/HRP and LacO/HRP could be stored for shorter periods of time (10-20 days). Operational stability of GO and LacO microreactors was very good: an equivalent to 16,900 FIA injections of 25 μl to a LacO microreactor resulted in loss of half of its activity, immobilized GO was so stable that it was impossible to evaluate enzyme halflife. Immobilized GalO and LAAO lose their operational activity much faster: approximately 1400 and 8000 FIA injections of the respective substrate solution in a FIA set-up resulted in 50% activity loss. The methods with all the described microreactors were successfully validated using off-line samples from S. cerevisiae, E. coli and mesenchymal stem cell cultures with HPLC as the reference method.     

Rheologic studies on chemical cross-linking kinetics for LDPE

Crosslinking reaction of LDPE resin in the presence of dicumyl peroxide(DCP) was studied by isothermal rheological measurements at different temperatures and non-isothermal differential scanning calorimetry(DSC) technique with different heating rates.The kinetic parameters of crosslinking reaction were calculated by both rheological and DSC measurements.The results reveal that with the increase of DCP contents,the apparent activation energy,E_a,ranges from about 140 kj/mol to 170 kj/mol and the order of crosslinking reaction,n,approaches unity.The influence of measurement frequency,ω,on crosslinking reaction was also investigated.It can be found that n does not change with the increase ofω, and E_a decreases slightly with the increase ofω.     

Studies of accelerated sulfur vulcanization of styrene–butadiene rubber by N-cyclohexylbenzothiazol-2-sulfenamide in the presence and absence of dicumyl peroxide

This study embodies the results and discussion of a comprehensive and systematic investigation of the mechanism of sulfuration of styrene–butadiene rubber accelerated by N-cyclohexylbenzothiazole-2-sulfenamide (CBS) with and without activators. Dicumyl peroxide (DCP) has been taken as a chemical aid to distinguish between free-radical and polar mechanisms of sulfuration. The rate constant for DCP decomposition in presence of CBS and the reduction in crosslink density by CBS have also been studied. With a constant amount of DCP and sulfur the crosslink density increases with increasing CBS concentration. In the presence of ZnO and stearic acid, crosslinking proceeds faster than in a similar system without these ingredients, and with DCP the crosslinks are found to be formed nearly additively as confirmed by methyl iodide treatment of the vulcanizates. In the absence of DCP, the crosslinking is characterized by an induction period, even in presence of ZnO and stearic acid. In the presence of sulfur, the 2-mercaptobenzothiazole (MSH) or amine or amine salt form crosslinkins by ionic reaction.     

聚苯乙烯/聚乙烯的反应性挤出共混

多官能团单体;聚苯乙烯/聚乙烯的反应性挤出共混