Study of the direct detection of crosslinking in hydrocarbons by 13C-NMR. II. Identification of crosslink in model compound and application to irradiated paraffins

A ¹³C-NMR investigation was carried out in aid of direct detection of crosslinks in hydrocarbons with the future objective of studying radiation-induced crosslinking in polyethylene by a direct method. The resonance signal due to a tertiary carbon atom appropriate to a crosslink far remote from molecular ends has been identified in a definitive manner with the aid of the H-shaped model compound 1,1,2,2-tetra(tridecyl)ethane synthetized in Part I of this study. This identification was then put to use in the examination of the irradiated linear paraffins n-hexadecane and n-eicosane, where it enabled the detection of radiation-induced crosslinks. This crosslinking could then be associated with corresponding changes in molecular weight (dimer, trimer formation) as revealed by discrete peaks in the gel-permeation chromatograms of the same samples and randomness of the crosslinking process in the liquid state of these compounds being inferred.     

Stress relaxation of natural rubber during irradiation

The site of chain scission and crosslinking in vulcanized natural rubber irradiated with 4 MeV electrons has been determined by analysis of stress relaxation data. Sulfur and peroxide vulcanizates of different crosslink densities were prepared and the crosslink densities determined from stress-strain measurements. Stress relaxation was measured during irradiation using modified commercial relaxometers. The specimens were maintained in an atmosphere of nitrogen to minimize oxidative side effects. Scission is deduced to take place in the vicinity of crosslinks, since the rate of continuous stress relaxation is independent of crosslink density. Scission may be associated both with crosslinks initially present and with those subsequently introduced by irradiation. Crosslinking by radiation is largely a random process. However, there is a crosslinking reaction dependent to a slight extent on crosslink density as well as a small contribution from random scission reactions. G values for the random reactions are given.     

Crystallinity and the effect of ionizing radiation in polyethylene. I. Crosslinking and the crystal core

An extensive study has been carried out on the effect of ionizing radiation on polyethylene in the form of solution-grown single crystals to follow up preceding investigations which had indicated that the radiation-induced effects along the fold surfaces could be significantly different from those within the crystal interior. In this first part of the series, radiation-induced crosslinking was investigated in the case of “crystal core” material. This material, obtained by removal of the fold surface by oxidative degradation with ozone, consists of isolated foldfree crystal layers of dicarboxylic acids of uniform length, the length itself depending on the fold length of the starting crystal. This “core material” was irradiated by γ rays and the effect of crosslinking was followed by GPC analysis by recording the development of dimer, trimer, etc., peaks in the chromatograms. For a given dose, the fractional amount of crosslinked material is independent of the chain length. This, together with other effects described, provides unambiguous evidence that crosslinking occurs at the chain ends or, in other words, that there is no crosslinking within the interior of the paraffinoid lattice. Further, no permanent scission was observed to occur within the lattice, at least in amounts detectable by the present molecular weight measurements. The obvious significance of these effects for radiation studies on paraffinoid substances in general, beyond those of the present long-chain dicarboxylic acids, is discussed prior to utilizing them in the study of chain-folded polyethylene crystals in the following parts of the series.     

Effect of Chain Length of Polyethylene Glycol and Crosslink Density (NCO/OH) on Properties of Castor Oil Based Polyurethane Elastomers

The equilibrium swelling study of polyurethanes (PU) was carried out in various solvents in order to calculate their solubility parameter. The kinetics of swelling and sorption have also been studied in 1,4‐dioxane at 30°C. The PU was synthesized by reacting a novel polyol (castor oil derivative and epoxy based resin, EpxR) and one of the polyethylene glycols (PEG 200, PEG 400, PEG 600) with different weight compositions, with a toluene diisocyanate (TDI) adduct (derived from toluene diisocyanate and R60 polyol). Different NCO/OH ratio viz. 1, 1.3 and 1.7 were employed in the study. The results were found to vary with the weight composition of polyol components, as well as the crosslink density of the samples. The sorption behavior is also found to vary with the molecular weight of polyethylene glycol employed in the preparations of the polyurethanes. Kinetic studies of swelling revealed that the sorption is anomalous in nature. The diffusion coefficient (D) increased with an increase in the NCO/OH ratio and decreased with an increase in chain length of polyethylene glycol. The sorption coefficient (S) decreased with an increase in crosslink density (NCO/OH) and increased with increasing polyethylene glycol (i.e., PEG 200, PEG 400, and PEG 600) moieties in the polyurethanes. The molecular weight between two crosslink points was calculated using the Flory Rehner equation (24), and hence, the number of chains per unit volume (N) and degree of crosslinking (ν) in all the samples were determined.     

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.     

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     

Chemical degradation of crosslinked ethylene-propylene-diene rubber in an acidic environment. Part II. Effect of peroxide crosslinking in the presence of a coagent

An investigation on the time-dependent chemical degradation of ethylene-propylene diene rubber containing 5-ethylidene-2-norbornene as diene cured by peroxide crosslinking in the presence of a coagent in an acidic environment (20% Cr/H₂SO₄) has been made. Two types of rubber, with comparable monomer composition, but having significant differences in molar mass and levels of long chain branching were tested. Dicumyl peroxide and triallylcyanurate under similar conditions were used for curing the rubbers. The molecular mechanisms of chemical degradation at the surface were studied using X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy, which demonstrate that several oxygenated species evolve during exposure. The primary process of degradation is hydrolytic attack on the crosslink sites, which is manifested by a decrease in crosslink density. The surface degradation is found to be strong enough to alter the bulk mechanical properties as observed by the change in retention in tensile strength, elongation at break, modulus at 50% elongation and, the change in micro-hardness. Retention in modulus at 50% elongation is found to follow a negative linear correlation with decrease in crosslink density. With higher molar mass and level of long chain branching more crosslinking occurs and thus comparatively more hydrolytic attack ensues. Scanning electron microscopy shows that the surface topography is significantly altered upon exposure and supports the notion of the dependence of degradation on the crosslinking density of the samples. Importantly, the coagent used in this study is shown to enhance the chemical degradation through formation of weaker sites for hydrolysis. The results also show that upon prolonged exposure the resulting oxygenated species tend to combine with each other.     

Simulations on radiation-cured polyethylene

Simulations of the simultaneous crosslinking and chain scission reactions that are induced in bulk amorphous polyethylene by irradiation have been implemented with a computer. A detailed study of the various defect structures in the network is reported, to show that dangling ends have the highest population among these defects, especially at higher crosslink densities. The calculated sol fractions compare very favorably with previously published experimental studies. We find that there is about 1 chain scission for every 9 crosslinked units, i.e., one scission per 4.5 crosslinks. This is fewer scissions than had previously been deduced by application of the Charlesby-Pinner method.     

γ-radiolysis of ketone polymers. III. Copolymers of ethylene and carbon monoxide

The γ-radiolysis of copolymers of ethylene and carbon monoxide (CO-PE) containing 0–9 wt-% of CO has been studied in the solid phase under vacuum with respect to hydrogen production, scission, and crosslinking. In this regard, the introduction of carbonyl groups into polyethylene increased both the scission and crosslinking efficiency. However, a theoretical statistical treatment for random scission and crosslinking, which was used to calculate G(s) and G(x) yields, predicted gelation with irradiation dose; this was confirmed experimentally. The increased G(x) values are attributed to the localization of absorbed energy in the carbonyl group arising from preferential trapping of thermalized secondary electrons. A stoichiometric balance is shown to be consistent with the reduction of one carbonyl group to a secondary alcohol per excess crosslink observed. The G(H₂) yield is reduced by the presence of ketone groups to a level which is in close agreement with the yield obtained for the model ketone 12-tricosanone. This quenching of H₂ production is explained by intramolecular electronic energy transfer along the chain to the carbonyl group with concomitant intermolecular charge transfer between neighboring molecules.     

Kinetics of crosslinking copolymerization of styrene with symmetric divinyl compounds

Gel formation usually occurs in crosslinking systems during polymerization. The critical conversion in this process is dependent on the reactivity ratios of the different kinds of double bond in the system and on the tendency to cyclization. In this article a procedure for estimating the gel point for small amounts of a symmetric divinyl compound is shown in measurements of the degree of polymerization at different conversions. A method is also given to relate the reactivity ratio of the pendent double bonds to the amount of cyclization and to the degree of multiple crosslinking. In m-divinylbenzene/styrene the results indicate that about 10% of the pendent double bonds is consumed by cyclization due to backbiting of the primary chain, whereas an initial effective crosslink results in about seven multiple crosslink bonds.     

Crosslinking chemistry and network structure in organic coatings. I. Cure of melamine formaldehyde/acrylic copolymer films

Crosslinking chemistry and network formation in hydroxy and carboxy functional acrylic copolymer resins cured with representative melamine-formaldehyde crosslinking agents have been studied by infrared spectroscopy. Network formation in these systems is dominated by two reactions, the condensation of the hydroxy (or carboxy) functionality of the acrylic resin with melamine alkoxy groups to form acrylic-melamine crosslinks, and the condensation of melamine hydroxy groups to form melamine-melamine crosslinks. The extents of these reactions have been studied as functions of acrylic resin composition, melamine type and concentration, and cure time and temperature. For melamines with just methoxy functionality, the extent of formation of acrylic-melamine crosslinks increased steadily with cure temperature. For melamines with substantial hydroxy functionality, the extent of formation of acrylic-melamine crosslinks increased rapidly then leveled off with increasing cure temperature. The formation of melamine-melamine crosslinks increased slowly with increasing cure temperature. From these data and a statistical model, effective crosslink densities were calculated. The crosslink densities correlated well with solvent resistance.     

Characterization of ring‐opening polymerization of genipin and pH‐dependent cross‐linking reactions between chitosan and genipin

In this study, a novel chitosan‐based polymeric network was synthesized by crosslinking with a naturally occurring crosslinking agent—genipin. The results showed that the crosslinking reactions were pH‐dependent. Under basic conditions, genipin underwent a ring‐opening polymerization prior to crosslinking with chitosan. The crosslink bridges consisted of polymerized genipin macromers or oligomers (7 ～ 88 monomer units). This ring‐opening polymerization of genipin was initiated by extracting proton from the hydroxyl groups at C‐1 of deoxyloganin aglycone, followed by opening the dihydropyran ring to conduct an aldol condensation. At neutral and acidic conditions, genipin reacted with primary amino groups on chitosan to form heterocyclic amines. The heterocyclic amines were further associated to form crosslinked networks with short chains of dimmer, trimer, and tetramer bridges. An accompanied reaction of nucleophilic substitution of the ester group on genipin by the primary amine group on chitosan would occur in the presence of an acid catalysis. The extent in which chitosan gels crosslinked with genipin was significantly dependent on the crosslinking pH values: 39.9 ± 3.8% at pH 5.0, 96.0 ± 1.9% at pH 7.4, 45.4 ± 1.8% at pH 9.0, and 1.4 ± 1.0% at pH 13.6 (n = 5, p < 0.05). Owing to the different crosslinking extents and different chain lengths of crosslink bridges, the genipin‐crosslinked chitosan gels showed significant difference in their swelling capability and their resistance against enzymatic hydrolysis, depending on the pH conditions for crosslinking. These results indicated a direct relationship between the mode of crosslinking reaction, and the swelling and enzymatic hydrolysis properties of the genipin‐crosslinked chitosan gels. The ring‐opening polymerization of genipin and the pH‐dependent crosslinking reactions may provide a novel way for the preparation and exploitation of chitosan‐based gels for biomedical applications. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 1985–2000, 2005     

REACTION MECHANISM OF BENZOPHENONE-PHOTOINITIATED CROSSLINKING OF POLYETHYLENE

The radical intermediates, the crosslink microstructures, and the reaction mechanism of benzophenone (BP)-photoinitiated crosslinking of low-density polyethylene (LDPE) and model compounds (MD) have been reviewed in detail.The spin-trapping electron spin resonance (ESR) spectra obtained from the LDPE/BP systems with spin-trap agents showthat two kinds of polymer radical intermediates are mainly formed: tertiary carbon and secondary carbon radicals. The spin-trapping ESR studies of MD/BP systems give further evidence that photocrosslinking reactions of PE predominantly takeplace at sites of tertiary carbon, secondary carbon, and especially allylic carbon when available. The high resolution ~(13)C-NMR spectra obtained from LDPE and MD systems show that the crosslink microstructures have H- and Y-type links andthat their concentrations are of the same order. The fluorescence, ESR ~(13)C and ~1H-NMR spectra from the PE and MDsystems demonstrate that the main photoreduction product of BP(PPB) is benzpinacol formed by the recombination of twodiphenylhydroxymethyl (K·) radical intermediates. Two new PPB products: an isomer of benzpinacol with quinoid structure,1-phenylhydroxymethylene-4-diphenylhydroxymethyl-2, 5-cyclohexadiene and three kinds of α-alkyl-benzhydrols have beendetected and identified. These results provide new experimental evidence for elucidating the reaction mechanism in the BP-photoinitiated crosslinking of polyethylene.     

Synthesis and characterization of polyfunctional crosslinking agents for cyanate resins

An efficient crosslinking monomer for a mixed cyanate/epoxy resin system, bisphenol-A-monocyanate monoglycidyl ether 3 , has been synthesized and characterized. The intermediate compound, the monoglycidyl ether of bisphenol-A 2 , was also isolated and purified by extraction and chromatographic separation using a silica gel column. The cyanate functional group in the crosslinking monomer 3 can be cured easily by heat to form a triazine structure 8 , but the epoxy functional group in the crosslinking monomer 3 can not be cured without affecting the cyanate group because the latter is more reactive both under heat and basic conditions. A practical approach for the application of the crosslinking monomer 3 is discussed and tested. Most interestingly, under heat curing, a very tough and strong resin material was produced from this crosslinking mixed resin mixture. By using a secondary amine, diethylamine, as a curing agent, the cyanate groups in the crosslinking monomer 3 react to form the structures 11 or 12 , depending on the molar ratio of monomer 3 to diethylamine. A bifunctional crosslinking agent for a mixed cyanate (thermoset) and polyolefin (thermoplastic) resin system, 2-allylphenyl cyanate 16 , has also been synthesized and characterized. Like 3 , 2-allylphenyl cyanate 16 easily forms the crosslinking triazine compound 17 upon heating. 17 is a crystalline solid with mp = 110–111°C. As a crosslinking agent, 2-allylphenyl cyanate 16 reacts not only with itself, but also with other cyanates to form heterogeneous triazine rings, exemplified by triazines 18 and 19 . Even though it does not self polymerize through the allyl double bond, it can copolymerize with an other olefinic monomer, such as methyl methacrylate, to form a crosslinked and insoluble polymer. © 1995 John Wiley & Sons, Inc.     

Elastic properties of crosslinked poly(vinyl alcohol) gels: Network topology

Current network theory exhibits inconsistencies which show up particularly clearly in deformation of networks prepared by crosslinking a polymer in solution. A check of theory can be obtained if one knows precisely the number of crosslinks in the network and if a range of deformations is applied to the network. In an effort to explore this problem we have examined the relation of shear modulus to crosslink density, primary molecular weight, and polymer concentration for a series of poly(vinyl alcohol) gels at low to intermediate concentrations. Aqueous poly(vinyl alcohol) solutions were crosslinked to form infinite networks using terephthalaldehyde. We find a large discrepancy with these poly(vinyl alcohol) gels between measured shear modulus and that calculated from classical elasticity theory assuming quantitative reaction of crosslinking. The ratio of measured to calculated modulus is independent of crosslink density for a given primary molecular weight and concentration. It shows linear dependence on polymer concentration prior to crosslinking and extrapolates to a critical concentration which is consistent with the effective sizes of the polymer molecules.     

Polyethylene networks crosslinked in solution: Preparation,elastic behavior,and oriented crystallization. I. Crosslinking in solution

A study has been made of the crosslinking of linear polyethylene in solution. Networks containing a low number of trapped entanglements and elastically ineffective chain ends were prepared by crosslinking high molecular weight linear polyethylene in 1,2,4-trichlorobenzene solutions with dicumyl peroxide at 120°C. No syneresis was observed during crosslinking except at high peroxide concentrations. The networks were characterized by swelling measurements, infrared spectroscopy, and differential scanning calorimetry. The crosslinking efficiency, calculated from swelling, was found to be proportional to the square of the polymer volume fraction. The proportionality constant was 0.8, indicating close to unit efficiency for undiluted polymer. Chemical modification of the polyethylene chains by attachment of peroxide and solvent fragments was of the order of one foreign unit per elastically active network chain, depending on peroxide and polymer concentration. Sol–gel analysis indicated that no chain scission occurred. These results are shown to be consistent with a “cage” mechanism for crosslinking. The possible topological consequence of this mechanism, preferential crosslinking of entanglements, is discussed. The concentration of trapped entanglements was also found to be proportional to the square of the polymer volume fraction. The proportionality constant corresponds to a molecular weight between entanglements of 4000 for the undiluted polymer, which is close to the value of 4200 found for networks prepared from the undiluted polymer. Since the results obtained are based mainly on the use of the swelling equation, different aspects of the applicability of this equation for the evaluation of the crosslinking process are discussed. As regards the reference dimensions, which should be known for a quantitative application of the elastic theory, the results strongly support the use of the dimensions of the network chains after completion of crosslinking.     

Crosslink network evolution of BIIR/EPDM blends during peroxide vulcanization

Crosslink network evolution of brominated butyl rubber (BIIR)/ethylene–propylene–diene-monomer rubber (EPDM) blends during peroxide vulcanization is studied at a meso-scale level. In this work, EPDM is added as a co-agent to increase the crosslink density of BIIR vulcanization. With increasing EPDM content from 0 to 20 phr, the maximum torque of BIIR/EPDM compounds during vulcanization increases by 73%, reaching to 3.40 dNm. Vulcanization kinetic study shows that addition of EPDM favors to the crosslinking of BIIR compound. Meanwhile, the addition of 20 phr EPDM contributes to an increase in the crosslink density of BIIR/EPDM(80/20) vulcanizate, avoiding downward trend at post-cure period in comparison with BIIR only. Crosslink network evolution of BIIR/EPDM blends is divided into three periods during peroxide vulcanization at 150 °C. The role of EPDM in the crosslink network evolution is studied by proton nuclear magnetic resonance, and a “network patching” mechanism is proposed in which EPDM is implied to work as patch on damaged crosslink network resulted from the degradation nature of BIIR.     

Modeling the effect of crosslinking in methyl-pendant poly(p-phenylene benzobisthiazole)

Molecular mechanics and dynamics simulations have been performed on methyl-pendant PBZT to study the effects of intermolecular crosslinking. Several possible crosslinked structures were investigated. The effect of crosslinking was found to be strongly dependent upon crosslink type and, in some instances, crosslink density. A significant axial stress is predicted to occur upon the formation of phenyl-to-phenyl type crosslinks. This provides a reasonable explanation for the experimental observation of transverse cracks in the skin of crosslinked, MePBZT fiber. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36: 3057–3064, 1998     

The modulus of randomly-crosslinked poly(dimethylsiloxane) networks

In several published studies, randomly crosslinked networks were prepared from poly(dimethylziloxane) by the selective crosslinking of vinyl side chains with a silicon-hydride crosslinking agent. Stress-strain measurements on these elastomers gave values of the elongation modulus in the limits of small and large deformations which exceeded those predicted by the Flory-Erman theory. Although these unexpectedly large values at the small-strain limit have frequently been attributed to contributions from trapped entanglements, the present analysis interprets them as simply arising from contributions from short chains inadvertently introduced from the silicon-hydride crosslinking agent. In this interpretation there is a bimodal distribution of network chain lengths and, possibly, of crosslink functionalities as well. The present analysis gives results in good agreement with experiment.