The effect of electron beam irradiation on dynamic shear rheological behavior of a poly (propylene‐co‐ethylene) heterophasic copolymer

In this study the effect of electron beam irradiation on rheological properties of a poly (propylene‐co‐ethylene) heterophasic copolymer is evaluated. Using dynamic viscoelastic measurement in the linear viscoelastic range of deformation, it is observed that the complex viscosity and dynamic modulus of polypropylenes were decreased by increasing the irradiation dose. Polypropylene heterophasic copolymers consist of ethylene propylene rubber phase dispersed in polypropylene homopolymer matrix. The high energy electron beams simultaneously affect both isotactic polypropylene (iPP) matrix and ethylene propylene dispersed phase. The molecular chains of polypropylene homopolymer phase breakdown to smaller species, those are prone to degradation and branching as well. Increase in the melt flow rate behavior and shifting the cross‐over point to higher frequencies and increase in melt strength are due to this phenomenon. At the same time, the ethylene propylene phase of the polypropylene copolymer cross‐links due to irradiation, and a significant effect on the rheological behavior of samples are observed. The mathematical modeling of complex viscosity behavior revealed the conformity of experimental data with modified Carreau equation. Copyright © 2010 John Wiley & Sons, Ltd.     

Degradation kinetics of electron beam irradiated poly(propylene-co-ethylene) heterophasic copolymer

This study considers the effects of electron beam radiation on degradation kinetics of a poly(propylene-co-ethylene) heterophasic copolymer. Polypropylene heterophasic copolymers are composed of ethylene–propylene rubbery phase dispersed in crystalline polypropylene homopolymer matrix. Electron beam radiation can affect both polypropylene homopolymer matrix and ethylene–propylene dispersed phases simultaneously. Both phases undergo degradation and crosslinking reactions, but degradation is more probable in the polypropylene homopolymer matrix. The aim of this work is to study kinetics of degradation in this material. A high power electron accelerator irradiated raw samples under nitrogen atmosphere. The samples are analyzed using TGA in non-isothermal mode, and the degradation kinetic parameters were determined using Kissinger, Flynn–Wall–Ozawa and Coats–Redfern methods. The kinetic parameters resulted from these methods are compared. Results of kinetics studies show that orders of degradation reactions occurring in nitrogen atmosphere are all less than one. It indicates degradation takes place due to thermal dissociation of the chemical bonds.     

Effect of gamma-irradiation on the foaming behavior of ethylene-co-octene polymers

Ethylene-co-octene polymers containing different branching levels were irradiated in air and under vacuum at 25, 50 and 100 kGy. Gel fraction measurements, thermal analysis and rheology were used to assess the effect of the treatment on polymer structure modifications. The copolymer with 24 wt% octene was shown to be more sensitive to gamma rays and degradation was observed in some cases. Cross-linking in the amorphous phase also occurred as a consequence of irradiation and affected the foaming behavior of these materials.     

About crosslinking of low molecular weight ethylene‐propylene(‐diene) copolymer‐based artificial latices

Crosslinking of artificial latices based on ethylene–propylene copolymers (EPM) and/or ethylene–propylene–diene copolymers (EPDM) has not thoroughly been studied yet. Moreover, crosslinking of EPM and/or EPDM particles is a prerequisite for the formation of a shell using seeded emulsion polymerization of, for example, methyl methacrylate (MMA), as described elsewhere. Therefore, the aim of this article is to improve the general understanding of the chemistry involved in the crosslinking process. This work especially emphasizes the influence of the initiation method, that is, a peroxide or a pulsed electron‐beam, on crosslinking efficiency. All crosslinking efficiencies were obtained after extraction of the soluble polymer by tetrahydrofuran. The incorporation of the coagent, that is, divinylbenzene, into the EPM/EPDM phase was studied on a microscopic level by solid‐state ¹³C and ¹H nuclear magnetic resonance (NMR). Crosslinking of a low molecular weight EPM/EPDM latex requires the presence of a coagent, for example, divinylbenzene, 1,6‐hexanediol diacrylate, or poly(1,2‐butadiene). The efficiency of crosslinking initiated by a pulsed electron‐beam was improved to a great extent by the presence, in the aqueous phase, of potassium nitrosodisulfonate, also referred to as Fremy salt. Matrix Assisted Laser Desorption/Ionization–Time of Flight–Mass Spectrometry (MALDI‐TOF‐MS) was used to determine the influence of electron‐beam irradiation on the chemical stability of surfactants. It was demonstrated that sodium dodecyl benzene sulfonate (SDBS) is not degraded by the irradiation, and is therefore the surfactant of choice for the stabilization of EPM/EPDM‐based latices subjected to electron‐beam irradiation. © 2005 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 3600–3615, 2005     

Determination of scission, crosslinking and branching parameters of electron beam irradiated methacrylate-acrylamide copolymer

The aim of this work was to investigate the impact of electron beam irradiation at different dose rates on the molecular structure of linear methacrylate-acrylamide copolymer. In the first part, the radiation chemical yields of scission (G_s) and crosslinking (G_x) have been determined after irradiation for various initial molecular weights CL1 (40?000 g/mol), CL2 (90?000 g/mol) and CL3 (425?000 g/mol). Based on this calculation, solvent (ethanol) was found to increase the impact of irradiation especially at low concentration of copolymer. In the second part, the presence of branching in samples before and after e-beam irradiation was explored, and branching calculation was performed.     

Electron beam irradiation effects on poly(ethylene terephthalate)

Changes in poly(ethylene terephthalate) subjected to electron beam irradiation at doses up to 15 MGy and dose rate of 1.65 MGy/h, were investigated by differential scanning calorimetry, molecular weight measurement, X-ray photoelectron spectroscopy, and scanning electron microscopy. Irradiated samples showed a decrease of molecular weight with a minimum at 5 MGy, which is attributed to chain scission of the macromolecules and then an increase at further doses due to branching and some degradation effect. Irradiation in air is not an important factor because the high dose rate of irradiation inhibits oxygen diffusion in the samples.     

Toughened polypropylene with balanced rigidity (I): preparation and chemical structure of toughening master batch

The title toughening master batch (TMB) was synthesized in a low‐viscosity reaction system by using dynamic vulcanization technique starting from polypropylene (PP) as the matrix resin and ethylene–propylene or butadiene–styrene elastomer as the toughening agent through a polymer–bridge conjunction derived from a monomer containing a carbonate group in the presence of a free radical initiator. The chemical structure of the TMBs and the effects of technological conditions on structural parameters were investigated using fractional extraction and infrared spectroscopy. The prepared TMBs consisted of unreacted PP, unreacted elastomers, graft copolymer of PP and/or elastomers containing branched chains formed by bridging agent, and crosslinked copolymer of PP and/or elastomers in conjunction with polymer bridge chains derived from bridging agent. Results showed that the PP existed in graft and crosslinked forms was in the range of 3–21 wt% and that of the elastomer toughening agent was in the range of 50–70 wt%, grafting and bridging efficiency of bridging agent was in the range of 62–88 wt%, graft copolymer content in the total TMB was in the range of 0.18–3.65 wt% and crosslinked copolymer content was in the range of 22–42 wt%. Copyright © 2000 John Wiley & Sons, Ltd.     

Radiochemical aging of ethylene–propylene–diene monomer elastomers. I. Mechanism of degradation under inert atmosphere

This article is devoted to the study of electron‐beam‐induced degradation under argon atmosphere of an ethylene–propylene–diene monomer (EPDM, based on 5‐ethylidene 2‐norbornene) and an ethylene–propylene rubber (EPR) containing the same molar ratio of ethylene/propylene. The chemical structure modifications of polymeric samples were analyzed by ultraviolet–visible and IR spectroscopies. Crosslinking reactions were deduced by measuring the changes in gel fraction and the degree of swelling in n‐heptane. Irradiation of EPDM and EPR created trans‐vinylene, vinyl, vinylidene, and dienic‐type unsaturations. The radiochemical yields for unsaturation formations in EPDM and EPR were similar. Degradation also involved crosslinking and the production of molecular hydrogen. The comparison between EPDM and EPR showed that the diene (in which a double bond is consumed with a high radiochemical yield) contributes to the increase in rate and intermolecular bridges density. Mechanisms are proposed to account for the main routes of EPDM degradation. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1239–1248, 2004     

Cytotoxicity of nonionic amphiphilic copolymers

The purpose of this study is to ascertain the relationship between the structure of an amphiphilic nonionic polymer and its toxicity for cells (cytotoxicity) growing in a culture. To this end, 16 polymers of different architectures and chemical structures are tested, namely, linear triblock copolymers of poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide) (Pluronics); diblock copolymers of propylene oxide, ethylene oxide, and hyperbranched polyglycerol; alternating and diblock copolymers of ethylene oxide and dimethylsiloxane; and two surfactants containing linear (Brij-35) or branched (Triton X-100) aliphatic chains. Polymer-cell interaction is assayed in a culture medium in the absence of serum. Effective concentrations of the polymers causing 50% cell death, EC₅₀, vary within three orders of magnitude. Toxic concentrations of the alternating copolymer, Triton X-100, and Brij-35 are lower than their CMC values. In contrast, all block copolymers, regardless of their chemical structures, become toxic at concentrations above the CMC; that is, they acquire cytotoxicity only in the micellar form. The EC₅₀ values of the copolymers depend on their hydrophilic-liphophilic balance (HLB) through the following empirical formula: EC₅₀ × 10⁶ = 8.71 × HLB^2.1. This relationship makes it possible to predict the cytotoxic concentration region of a block copolymer of a known structure.     

Characterization of electron beam irradiation blends based on metallocene ethylene‐1‐octene copolymer

Binary blends using metallocene ethylene‐1‐octene copolymer as matrix were prepared and subjected to electron beam (EB) irradiation (50, 100, and 200 kGy). Gel content and melt flow index values indicated that the blends were crosslinking. Fourier transform infrared‐ATR spectroscopy was used to study the crosslinking and oxidative degradation of the blends via tertiary carbon and carboxyl group formation, respectively. Thermal and mechanical properties were studied showing that the crystallinity of both matrix and dispersed phase decreased with irradiation dose, and that the thermoplastic elastomers with good mechanical properties may be obtained by EB irradiation. Chain branching and scission were also detected at all irradiation doses, although at the highest doses (200 kGy) a crosslinking reaction was the most predominantly observed effect. The successive self‐nucleation annealing technique was used to determine the EB irradiation effects on crystallization of some blends in which crosslinking and chain branching take place, modifying the chain's structure and therefore crystalline regions in the matrix and the dispersed phase. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2432–2440, 2007     

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     

Characterization of ethylene–propylene block copolymers by proton magnetic resonance

A high-resolution proton magnetic resonance compositional analysis has been developed for propylene polymers containing 0–40 wt.-% ethylene as either homopolymer or copolymer blocks. The test is independent of tacticity and provides qualitative information on copolymer sequencing and propylene chain structure. The analysis was developed using a series of standard reference polymers synthesized to contain various ratios of C¹⁴-tagged ethylene and propylene. The compositional standards were established by radiotracer analysis for C¹⁴ and by preparing weighed physical mixtures of homopolymers. Spectra were obtained at 200 ± 10°C. by placing externally heated polymer solutions into a conventional probe of a Varian A-60 proton spectrometer. All measurements were made on ± 10% polymer solutions in diphenyl ether. Analyses are accurate to about ± 10% at higher ethylene concentrations. The method is sensitive, with less precision, to below 1% for ethylene either as blocks or homopolymer.     

Chemical degradation of crosslinked ethylene-propylene-diene rubber in an acidic environment. Part I. Effect on accelerated sulphur crosslinks

The time-dependent chemical degradation of accelerated sulphur cured ethylene propylene diene rubber containing 5-ethylidene-2-norbornene as diene in an acidic environment (20% Cr/H₂SO₄) was investigated. Two different rubbers with a similar ethylene to propylene ratio and diene content but with a significant difference in molar mass and level of long chain branching were used in the study. The molecular mechanisms of the chemical degradation occurring at the surface were determined using surface analysis (X-ray photoelectron spectroscopy and attenuated total reflectance Fourier transform infrared spectroscopy). The results reveal formation of several oxygenated species on the surface as a consequence of the acid attack. Furthermore, the crosslink sites of the exposed rubber samples are also found vulnerable to hydrolytic attack as evidenced by the decrease in crosslink density. The extent of surface degradation was strong enough to affect the bulk mechanical properties. Changes in mechanical properties were also monitored through determining retention in tensile strength, (%) elongation at break, modulus at 50% elongation, and change in micro-hardness. A negative correlation is also established between retention in modulus at 50% elongation and decrease in crosslink density. Scanning electron microscopy reveals the topographical damage at the surface due to the aqueous acid induced chemical degradation. The results indicate that the chemical degradation proceeds mainly via hydrolysis of crosslinks but upon prolonged exposure, the oxygenated species tend to combine with each other. The effect of molar mass and level of long chain branching also influences the chemical degradation.     

Combination of TREF, high-temperature HPLC, FTIR and HPer DSC for the comprehensive analysis of complex polypropylene copolymers

A novel, powerful analytical technique, preparative temperature rising elution fractionation (prep TREF)/high-temperature (HT)-HPLC/Fourier transform infrared spectroscopy (FTIR)/high-performance differential scanning calorimetry (HPer DSC)), has been introduced to study the correlation between the polymer chain microstructure and the thermal behaviour of various components in a complex impact polypropylene copolymer (IPC). For the comprehensive analysis of this complex material, in a first step, prep TREF is used to produce less complex but still heterogeneous fractions. These chemically heterogeneous fractions are completely separated by using a highly selective chromatographic separation method—high-temperature solvent gradient HPLC. The detailed structural and thermal analysis of the HPLC fractions was conducted by offline coupling of HT-HPLC with FTIR spectroscopy and a novel DSC method—HPer DSC. Three chemically different components were identified in the mid-elution temperature TREF fractions. For the first component, identified as isotactic polypropylene homopolymer by FTIR, the macromolecular chain length is found to be an important factor affecting the melting and crystallisation behaviour. The second component relates to ethylene–propylene copolymer molecules with varying ethylene monomer distributions and propylene tacticity distributions. For the polyethylene component (last eluting component in all semi-crystalline TREF fractions), it was found that branching produced defects in the long crystallisable ethylene sequences that affected the thermal properties. The different species exhibit distinctively different melting and crystallisation behaviour, as documented by HPer DSC. Using this novel approach of hyphenated techniques, the chain structure and melting and crystallisation behaviour of different components in a complex copolymer were investigated systematically.

Bulk and surface modification of elastomers by electron beam irradation: Structure-property relationship

Bulk and surface modification of ethylene propylene diene monomer (EPDM) and fluoroelastomer by electron beam irradiation was investigated. The structure of the modified elastomers was analysed with the help of Infrared (IR) spectroscopy, Electron Spectroscopy for Chemical Analysis (ESCA) and gel content. Mechanical and dynamic mechanical properties of bulk modified fluoroelastomers and surface energy and frictional coefficient of the surface modified EPDMs were measured. The properties were correlated with the structure developed.     

Crystal phase transformations within propylene/hexene-1 copolymers films as induced by direct current discharge treatment

The effect of a direct current discharge on the films of polypropylene and copolymers of propylene and hexene-1 synthesized with an isospecific catalytic system, rac-Me₂SiInd₂ZrCl₂-polymethylaluminoxane, was investigated. The treatment of isotactic polypropylene films by the discharge did not affect the ratio of crystalline phases in the polymer to a measurable extent. However, for the plasma treated films of copolymers of propylene and hexene-1 (the hexene-1 content of 1-2 mol%), a structural transformation of γ-modification into α-modification has been noticed. The observed phase transition has no apparent relation to any changes in microstructure of the copolymer chain because melting temperature values and the stereoregularity parameters of the samples remained practically unchanged. An experimental investigation of the specific influence exerted by individual components of a direct current discharge on the crystalline structure of copolymers has been undertaken. The exposure to a quantum component of the discharge did not induce any changes in the phase composition of the irradiated samples. The heating of the samples led to a negligible change of their phase composition. It has been determined that the surface of polypropylene and propylene/hexene-1 copolymer films facing the cathode in the course of the direct current discharge treatment had an accumulated negative charge Q > 10 nC/cm² which persisted for a long time afterwards. It has been suggested that the electrical field of a negative discharge may be the main cause of the γ-into α-phase transition in propylene/hexene-1 copolymers under the plasma effect. To verify this assumption, a propylene/hexene-1 copolymer film was charged under electron beam with energy of 4 keV. The electron beam treatment of the film resulted to the negative charge value of 11 nC/cm². The electron beam irradiation has induced the phase transition which was quite similar to the transition observed as the result of plasma treatment. So, it may be concluded that the phase transition from crystal γ-modification to α-modification under the effect of direct current discharge which has been investigated for copolymers of propylene and hexene-1 is induced by electric field of the negative charge accumulated at the surface layers of the films of the copolymers.     

A study of a simulated low Earth environment on the degradation of FEP polymer

Spacecraft flying in a low Earth orbit environment require thermal blankets to provide protection from direct solar heat from the sun. Fluorinated ethylene propylene copolymer is one of the major components of these thermal blankets. In this study, the effect of a simulated low Earth orbit environment on FEP was investigated. UV and VUV degradation of fluorinated ethylene propylene (FEP) copolymer was studied using ESR and XPS. The ESR study revealed the formation of a terminal polymer chain radical. The stability of this radical has been investigated under different environments. An XPS study of FEP film exposed to VUV and atomic oxygen showed that oxidation takes place on the polymer surface. The study revealed also that the percentage of CF₂ in the polymer surface decreased with exposure time and the percentage of CF, CF₃ and carbon attached to oxygen increased. SEM micrographs of FEP film exposed to VUV and atomic oxygen produced a rough surface with regular undulations similar to sand dunes. © 1998 John Wiley & Sons, Ltd.     

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.     

Reaction of chlorine-containing polymers with living polymers

A graft polymer was prepared by means of the coupling reaction of chlorinated ethylene–propylene terpolymer with living polystyrene, obtained with a sodium–naphthalene complex as initiator, under various conditions; the grafting efficiency and the percentage of grafting are discussed. Poly(chloroprene), chlorinated butyl rubber, poly(vinyl chloride), poly(epichlorohydrin), and epichlorohydrin–ethylene oxide copolymer were also used as chlorine-containing polymers. The grafting efficiencies were found to be in the following order: chlorinated butyl rubber > poly(epichlorohydrin) > epichlorohydrin-ethylene oxide copolymer > chlorinated ethylene-propylene terpolymer > poly(chloroprene) > poly(vinyl chloride). A graft polymer was obtained from the reaction between chlorinated ethylene–propylene terpolymer and living poly(isoprene), with butyllithium in benzene. The undesirable metal–halogen interchange reaction was considerable.     

The chemical structure dependence of interaction strength between polymers and mobility of polymer chains in the polymer interface

The structural difference of the interface between polypropylene pair (PP/PP) and polypropylene and poly(ethylene-alt-propylene) pair (PP/EAP) was investigated using molecular dynamics and molecular mechanics, under a two-dimensional periodic boundary condition (2DPBC). From the position of the end group of each polymer chain, the influence of the chemical structure and the mobility of the polymer chain were considered. In the PP/PP system, the end group is mainly located in the middle region of each polymer layer. In the PP/EAP system, the PP end group and the ethylene end group of EAP are also located in the middle region of each polymer layer, but the propylene end group of EAP is located very near the interface. These results suggest that the mobility of the middle part of the polymer chain is not so small, and that the similarity in the chemical structure, which is a measure of the interaction strength between polymers, plays an important role in the early stage of adhesion. An easy and efficient estimation method of the interaction strength between polymer pairs is also proposed, and the influence of the component sequence in copolymer on the interaction strength is systematically estimated.