Aromatic Systems under Irradiation: Influence of the

Article pubs.acs.org/JPCB

Aliphatic/Aromatic Systems under Irradiation: Influence of the Irradiation Temperature and of the Molecular Organization M. Ferry,*,† E. Bessy,‡ H. Harris,‡ P. J. Lutz,‡ J.-M. Ramillon,† Y. Ngono-Ravache,† and E. Balanzat† †

CIMAP, Unité Mixte CEA-CNRS-ENSICAEN, BP 5133, 14070 Caen Cedex 5, France Université de Strasbourg, Institut Charles Sadron, CNRS UPR 22, 23 rue du Loess, BP 84047, 67034 Strasbourg Cedex 2, France

‡

ABSTRACT: With the aim of understanding the electronic excitation, charge or reactive species transfers occurring during irradiation, we studied the role of the aromatic content on ethylene/styrene random copolymers (PES) and on cyclohexane/benzene glasses (amorphous organic solids). Radiation-induced modifications were monitored in situ, at the molecular level, using Fourier transform infrared spectroscopy (FTIR). Irradiations were performed under a vacuum, and thanks to in situ measurements, oxidation was avoided. We followed both the CC bond creation in the aliphatic moiety and the destruction of the aromatic moiety. The influence of the irradiation temperature was investigated by irradiating samples at room temperature and at 11 K. At such a low temperature, long-range migration hardly occurs and its influence is considerably reduced or could even vanish. Therefore, low temperature irradiation gives insight on the relative influence of reactive species transport and electronic excitation and charge transport. We found that the effect of lowering the PES irradiation temperature from room temperature to 11 K is small, indicating a minor role for the reactive species transport. Moreover, the two chosen systems allow the examination of the relative magnitude of intra- and intermolecular transfers. We demonstrate that, under conditions where reactive species are almost frozen, intermolecular transfers are very efficient.

1. INTRODUCTION In very different systems, as for instance mixtures of molecular liquids, solid compounds of organic molecules, copolymers (random, block, or grafted), and polymer blends, there is solid published evidence that the radiation sensitivity of one moiety could be significantly affected by the presence of the other moiety. This effect is especially evident when aliphatic molecules are in intimate presence of aromatic molecules: the aliphatic moiety is stabilized1−3 by the aromatic one, whereas aromatic rings are sensibilized.4 For mixtures of molecular liquids, this radiation protection phenomenon is known since 1931, thanks to the study of C. S. Schoepfle and C. H. Fellows.5 These authors studied the gas emission during X-ray irradiation of different liquid organic mixtures under a vacuum and noticed that the total gas release in the cyclohexane/benzene mixture did not follow the law of averages. Thereafter, many other teams studied this phenomenon at room temperature by following the gas release in organic liquids and mixtures submitted to γ-rays or electrons.6−8 Apart from gas emission, the study of the formation of non-volatile molecules (or transient species) in cyclohexane/benzene liquid mixtures submitted to γ-rays6,7,9 or to e-beam pulse radiolysis10 also confirmed this radiation stabilization phenomenon. Proof of the effectiveness of this protection phenomenon in polymers was brought by J. B. Gardner and B. G. Harper11 when studying gas emission either in polyethylene/polystyrene physical blends or in polyethylene grafted styrene irradiated with electron beam, at room temperature. © 2013 American Chemical Society

Most of the aforementioned studies were based on the analysis of the hydrogen emission which is neither specific of a given component of the liquid mixture nor specific of a given moiety in polymers. Such an approach does not allow measuring the damage created in a given moiety. Only a few studies using experimental protocols that distinguish the damage specific to each moiety were reported in the literature.6−9,12 In this paper, we report on FTIR spectroscopy that enables the discrimination of each copolymer moiety and its behavior under ionizing radiation. The influence of the irradiation temperature on the radiation resistance of aliphatic/aromatic compounds or mixtures is poorly known, since studies at low temperature of aliphatic/ aromatic organic compounds are scarce. With the exception of those published by the team of the Karpov Institute of Physical Chemistry of Moscow,12−14 they are generally related to the analysis, by ESR (electron spin resonance), of the radicals created under γ-rays at 77 K either in cyclohexane/benzene mixtures9 or in ethylene/styrene random copolymers.15 One of the aims of the present paper is to study the irradiation temperature effect on two aliphatic/aromatic compounds. There are few doubts that the stabilization of aliphatic components in the presence of aromatic ones is due to energy transfer between the two components. The term energy transfers used in this work is wide and includes Received: June 25, 2013 Revised: October 29, 2013 Published: October 29, 2013 14497

dx.doi.org/10.1021/jp406260z | J. Phys. Chem. B 2013, 117, 14497−14508

The Journal of Physical Chemistry B

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corresponding alkane. The radiation protection effect when intramolecular and intermolecular transfers coexist (phenylalkanes) is about 20% higher than that observed when only intermolecular transfers are effective (benzene/alkane mixtures). In polymers, at room temperature, the same kind of studies were realized by following either dihydrogen emission or crosslinking formation in (1) aliphatic/aromatic random copolymers and block copolymers26 (intrachain and interchains transfers) and in (2) physical mixtures of aliphatic and aromatic polymers (pure interchain transfers).11,27 The conclusions show that radiation protection is effective in random copolymers but ineffective in block copolymers and in physical mixtures. The absence of interchain transfers in block copolymers and in physical mixtures can be assigned either to a real absence of interchain transfers or to a phase segregation effect that prevents moieties from being sufficiently close. In fact, it should be noticed that many pairs of polymers are immiscible,28 and so are polybutadiene/polystyrene and polyethylene/polystyrene. We have recently published results focused on the destruction of the benzene ring in ethylene/styrene random copolymers (PES) irradiated at room temperature.4 Here, we present a complete set of results concerning the destruction of the benzene ring of the aromatic moiety but also the creation of CC unsaturated bonds in the aliphatic moiety. Two aliphatic/aromatic systems were experimentally irradiated and studied at 11 K: PES and cyclohexane/benzene amorphous organic solids named cyclohexane/benzene glasses in the following. In PES, transfers can occur inside a given chain (intrachain) or between two chains (interchains). The use of cyclohexane/benzene glasses in this study aimed to set the effectiveness of intermolecular transfers. The ulterior motive, when comparing copolymers and cyclohexane/benzene glasses behaviors, was to determine the relative impact of interchain and intrachain transfers in copolymers. Indeed, instead of cyclohexane/benzene glasses, the use of solid mixtures of polyethylene (PE) and polystyrene (PS) would have been more adapted for studying pure interchain transfers. Unfortunately, these two polymers are hardly miscible and this should have led to a two-phase system with very limited interactions, thus quite limited transfers, between phases. The choice of the cyclohexane/benzene glasses to compare with PES copolymers was carefully balanced. Cyclohexane/ benzene mixtures were selected for the following reasons. First of all, these mixtures have been thoroughly studied, in the liquid phase, at room temperature.5−8 These two molecules are completely miscible. Moreover, benzene has the advantage of being the simplest aromatic molecule, whereas cyclohexane, due to the absence of a methyl group and thanks to its cyclic structure, can in a certain manner mimic polyethylene. Elsewhere, varying aromatic molar content (benzene or styrene, depending on the system) in aliphatic/aromatic materials gives information on the protection radius. Polyethylene/styrene copolymer (PES) samples were irradiated at room temperature and at 11 K, whereas cyclohexane/benzene glasses were solely irradiated at 11 K. In this paper, we provide, via an analysis of the influence of the irradiation temperature, a new insight on the relative contribution of reactive species transfers on one side and lonely charge transfers and electronic excitation on the other side.

- mass or reactive species transport (this term includes radicals and ions) - pure charge transfer (electrons and holes) - electronic excitation transfer. A priori, every three mechanisms can be encountered in the aliphatic/aromatic systems under study in this manuscript. In pure alkanes or in a non-polar polymer as polyethylene, an efficient charge generation occurs during irradiation. In pure non-polar polymers, rapid geminate recombination of the resulting ions with ejected electrons is observed, leading to a low or inexistent electron trapping probability. Pulse radiolysis studies of polymers containing arene probes have shown a rapid formation of the arene ionic components very shortly after the end of the pulse.16,17 Therefore, in polyethylene, adding styrene groups in the polymer backbone would lead to charge transfers to the benzene rings, very close to the observation of K. Okamoto18 during pulse radiolysis study of polystyrene in cyclohexane. On the basis of experimental studies and calculations, R. H. Partridge19 has shown that exciton migration in long-chain saturated hydrocarbons is very efficient and can occur on distances up to about 150 nm. Furthermore, in the presence of chemical defects (odd chemical groups) on the polymer backbone, the migrating excitons can be trapped within this defect.20 As a consequence, introducing styrene groups in polyethylene, as in ethylene/styrene copolymers, should induce exciton transfers to the benzene rings. The relative implication of the three mechanisms cited previously should be substantially different depending on the irradiation temperature. At very low temperature, molecular radical migration should be drastically reduced or annihilated. The fate of the proton, created with high efficiency during irradiation of simple olefin polymers or long-chain alkanes, can be questioned at low temperatures, but previous studies tend to show that its mobility is also decreased. Indeed, whatever the irradiation temperature, H° radical has never been observed;21 it reacts almost instantaneously, following its creation, either by hot abstraction or by tunneling abstraction by thermalized atoms. Radical transport in polymers occurs through H atom exchange. On their early work, D. C. Waterman and M. Dole22 have shown that the concentration of alkyl radicals created in polyethylene during 1 MeV electron irradiation at 4 K remained constant during annealing up to 77 K and then reduced to zero at room temperature. The same behavior, recently observed by our team,23,24 indicates the existence of a limited temperature value below which radical transport through H atom exchange is no longer permitted. Finally, concerning electronic excitation transfers, according to R. H. Partridge,20 exciton migration occurs very rapidly, roughly independently of temperature. On the basis of what has been described just before, varying the irradiation temperature should lead to separate the implication of reactive species transport from those of lonely charge and exciton transfers. In polymers or in alkylaryl molecules, transfers can happen through two levels: along a given chain (intrachain) or between two polymer chains (interchains). In cyclohexane/benzene systems, transfers are strictly intermolecular. Interesting studies were performed to discriminate these two levels of energy transfers in liquid, at room temperature, using either alkylbenzenes25 or phenylcyclohexane.5,9 In a rather complete study, A. Zeman25 compared the hydrogen emission in a series of phenylalkanes and in mixtures of benzene with the 14498


a

14499

PE* PES 2.2 PES 7.0 PES 19.0 aPS* PE* PES 2.2 PES 7.0 PES 19.0 aPS* cyclohexane/benzene glass 0% benzene cyclohexane/benzene glass 4.4% benzene cyclohexane/benzene glass 12.6% benzene cyclohexane/benzene glass 22.5% benzene cyclohexane/benzene glass 100% benzene

*PE, polyethylene; aPS, atactic polystyrene.

15

14

13

12

1 2 3 4 5 6 7 8 9 10 11

name

100

22.5

12.6

4.4

0 2.2 7.0 19.0 100 0 2.2 7.0 19.0 100 0

aromatic content (molar %)

11

11

11

11

11 11 11 11 11 300 300 300 300 300 11

irradiation temperature (K)

C

13

C

C

13

13

C

13

20

Ne Ne 20 Ne 20 Ne 18 O 20 Ne 20 Ne 20 Ne 20 Ne 18 O 13 C

20

projectile

0.96

0.96

0.96

0.96

13.6 13.6 13.6 13.6 8.3 13.6 13.6 13.6 13.6 8.3 0.96

Ei (MeV·A−1)

0.87

0.83

0.82

0.85

13.2 13.0 12.5 12.6 8.0 13.1 12.7 12.5 12.5 7.9 0.86

Eo (MeV·A−1)

6.9

7.5

7.6

7.6

3.9 3.9 3.9 3.8 3.4 3.9 3.9 3.9 3.8 3.4 7.6

LET (MeV·mg−1·cm2)

0.17

0.22

0.24

0.18

2.1 4.4 5.9 5.6 1.6 1.8 4.8 6.0 6.2 1.8 0.17

e (mg·cm−2)

2.9

2.6

2.6

2.6

5.1 5.1 5.1 5.2 5.1 5.1 5.1 5.1 5.2 5.1 2.6

flux (108 cm2·s−1)

13.5

4.5

4.5

4.5

11.3 6.8 6.8 6.8 19.9 4.5 4.5 11.3 5.6 20.1 4.5

maximum dose (MGy)

Table 1. Irradiation Conditions (Ei and Eo Are, Respectively, the Input and Output Energy of the Projectiles in the Films; LET Is the Linear Energy Transfer; e Is the Sample Thickness)a

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2. EXPERIMENTAL SECTION 2.1. Irradiation Conditions. Systems were irradiated using shift heavy ion beams. Ion beam irradiations were performed using the IRRSUD line facility (for cyclohexane/benzene glasses) or using the medium energy line facility (for PES) of the GANIL accelerator, Caen, France. Irradiations were performed either at room temperature or at 11 K, under a vacuum, at normal incidence. An x-, y-scanned beam was used to ensure a homogeneous irradiation field over the sample surfaces (typically 0.75 cm2). The target temperature was not monitored during irradiation. However, the fluxes (Table 1) were chosen in order to limit the power deposition on the sample to 0.5 mW·cm−2, thus avoiding any significant sample heating. The energy loss was calculated with SRIM, based on the TRIM code.29 In Table 1, PES sample thicknesses are expressed in mg·cm−2 because the densities are unknown. In any sample, the projectile range was by far much larger than the sample thickness. Elsewhere, the beam energy was high enough to ensure a relatively constant LET (Linear Energy Transfer) over the sample thicknesses. In most cases, the relative decrease of the projectile energy in the sample was well below 20%. Besides, in the aliphatic moiety, the mass stopping power is only about 9% higher than the corresponding one in the aromatic moiety. Hence, the KERMA (Kinetic Energy Released in MAtter) is higher in the aliphatic moiety by the same small amount of 9%. Concerning the absorbed dose, materials studied are random copolymers of ethylene and styrene. Meanwhile, there is an intimate mixture of both monomer moieties. Moreover, as the ion beams used in this study are of high energy (>10 MeV·A−1), secondary electron ranges are larger than the monomer units. Considering different absorbed doses in both moieties does make no sense. Consequently, for the absorbed dose calculation, the LET value used here is a mean value, taking into account the copolymer as a whole. The same reasoning is applied for cyclohexane/benzene amorphous glasses. Errors on the values have been estimated. Statistical errors, which are for a given sample and for a single beam condition, are small. We can estimate that statistical errors of defect concentration are at most a few %. The systematic errors are higher and are mainly due to the sample thickness and to the dose deposited in the material. They amount to 10%. 2.2. Setup. The results presented here correspond to a Fourier transform infrared spectroscopy (FTIR) analysis of the radiation-induced modifications. FTIR spectra were acquired in the transmission mode, at a resolution of 2 cm−1, and with 128 accumulations to improve the signal-to-noise ratio. Moreover, interference fringes were avoided by recording the spectra in the presence of a polarized light, at the Brewster angle (55°). To record the in-film modifications, a specific device allowed us to irradiate and to record the FTIR spectra without removing the sample from the cell.23 Consequently, the contact between the polymer and air was avoided; the post-irradiation oxidation did not occur and above all no annealing precedes the sample analysis. This device is equipped with a cryogenerator, which allows a gradual temperature lowering down to 11 K. The sample temperature is controlled by two sensors, a carbon resistance and a CLTS (compound linear thermal sensor). This sample irradiation temperature can vary from 11 K to room temperature. For direct comparison purposes, whatever the irradiation temperature, all FTIR spectra were recorded at 11 K.

Irradiations at 11 K are performed under vacuum conditions, at pressures below 10−7 mbar. At such a low temperature, even at low pressures, residual gases present in the irradiation cell tend to condense and accumulate over a long time on the sample surface. Under these conditions, only H2O molecules can condense on the sample surface. Characteristic infrared bands of water ices are found around 3400, 1600, and 600 cm−1 and do not interfere with bands used throughout the present study. 2.3. Ethylene/Styrene Random Copolymer Samples: PES. Materials based on ethylene−styrene random copolymers have attracted considerable attention in recent years due to their unique chemical structure and physicochemical properties that differ strongly from the pure homopolymers or from the blends of these polymers. The development of such copolymers has been limited for many years due to the absence of efficient olefin coordination polymerization catalyst. Ziegler−Natta and classical metallocene catalyst lead only to the formation of homopolymers or of copolymers with a very low content of comonomer. Thanks to the development, within the last 15 years, of the new homogeneous olefin polymerization constrained geometry catalysts (CGC),30 it is now possible by proper selection of the catalytic system and the reaction conditions to access a variety of styrene/ethylene copolymers with different compositions, structures, and properties.31 As such copolymers are not commercially available, we decided to synthesize them. The experimental procedure used to prepare the copolymers was very briefly introduced in our previous publication,4 and more details are given below. The ethylene/ styrene copolymerization was achieved using a Ti-based CGC 1, activated by MAO (methylaluminoxane) 2 (Figure 1).

Figure 1. CGC catalyst 1 and methylaluminoxane cocatalyst 2.

The procedure followed is the one of D. H. Lee,30 according to the generally accepted mechanism proposed by P. Cossee32 and E. J. Arlman.33 The copolymerization runs were carried out in a 250 mL Buchi reactor equipped with magnetic stirring, purged with argon and vacuum exchange. After addition of 75 mL of dried toluene, the proper amount of freshly distilled styrene, 0.235 g of MAO 2 in 2 mL of toluene, and 3 mg of the titanium-based CGC 1 in 2 mL of toluene were introduced. The reactor was pressurized to 2 bar with ethylene. The reaction medium was maintained under stirring, at room temperature, for 5 h. After this period of time, the copolymer was deactivated by addition of 3 mL of acidified methanol (methanol:HCl 80:20% v/v). The resulting copolymers were precipitated twice from their toluene solution into acidified methanol and dried in a vacuum. Copolymers of different styrene molar contents were obtained by varying the initial styrene concentration in the monomer feed. Toluene and styrene were distilled before use and kept under an inert argon atmosphere as described more in detail in the article of J. F. Lahitte et al.34 14500



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Vacuum dried samples were then characterized by liquid 1H nuclear magnetic resonance (NMR), differential scanning calorimetry (DSC), and Fourier transform infrared spectroscopy (FTIR). Using the 1H NMR spectra, we have quantified the styrene molar content in the samples. The 1H NMR spectra were recorded on a Bruker AvanceIII 400 MHz spectrometer, in deuterated para-xylene at 70 °C. As an example, the liquid 1 H NMR spectrum of PES 7.0% styrene molar content, dissolved in deuterated p-xylene at 70 °C and measured at 70 °C, is presented in Figure 2.

Figure 3. Melting peaks of PES copolymers varying with the styrene molar content.

thickness of the FTIR probed area was performed using the measured absorbance A of the 1600 cm−1 band related to the benzene ring CC stretching mode (quadrant mode) in the case of materials containing the benzene ring, and by using the measured absorbance A of the 1305 cm−1 band related to the CH2 wagging mode in the case of polyethylene. For calibration, different samples of well-defined surfaces were weighted and the corresponding absorbance measured. The mean value (A/ e) is obtained by a linear mean square fit that averages the thickness fluctuations. The results of sample characterization are summarized in Table 2. 2.4. Cyclohexane/Benzene Glasses. Cyclohexane/benzene glasses are obtained by gas condensation on a cold surface (11 K). For this purpose, the setup used for irradiation is modified by the addition of a gas ramp with multiple entries. A glass cell containing pure deaerated cyclohexane (or pure deaerated benzene) at room temperature is connected to a compartment with a known volume (V1 for benzene, V2 for cyclohexane) initially under a vacuum. This compartment is then filled with the saturation vapor over the liquid, up to the desired pressure for either benzene or cyclohexane (P1 for benzene, P2 for cyclohexane); each compartment is equipped with an absolute manometer. The two compartments containing respectively cyclohexane or benzene vapor are then connected and the mixture homogenization is ensured by a rotary system (Pt is the mixture pressure). After the desired mixing time, the gas ramp is open to the setup chamber and the gas mixture is condensed on an infrared transparent substrate (CsI), located at the coldest point of the cryostat, which is at 11 K. Here, cyclohexane is considered as the solvent and benzene is the solute. The pressures and thus the quantities of solvent and solute to be introduced into each compartment are calculated according to the molar content of solute desired in the cyclohexane/ benzene glass. The measurement of P1 and Pt pressures makes it possible to determine the molar content of solute in the cyclohexane/benzene glass, thanks to relation 2. This relation is obtained by assuming valid the ideal gas law. P1·100 molar benzene content % = V Pt· 1 + V2

Figure 2. Liquid 1H NMR spectrum, measured at 70 °C, of PES 7.0% styrene molar content dissolved in deuterated p-xylene at 70 °C. Assignments of spectrum peaks are given on the NMR spectrum: only protons corresponding to the peak studied are marked on the attached molecule.

Crystallinity contents and melting temperatures were determined with a Setaram DSC131 apparatus. The crystalline fraction was calculated using the following formula: x=

Δm Hpolymer Δm H100% crystalline polymer

(1) −1

The heating rate used in this study is of 5 °C·min . The parameter ΔmH100% crystalline polymer was taken equal to the one of the entirely crystalline polyethylene, that is, ΔmH100% crystalline polymer = 288 J·g−1.28 Unfortunately, we did not succeed in the determination of glass transition temperatures (Tg). Previous studies on the same types of polymers indicate that Tg remains below the room temperature for copolymers with styrene contents close to those used in this study.35,36 Thermograms are presented in Figure 3. They are composed of a unique melting peak, confirming the synthesis of ethylene/ styrene random copolymers, as reported in the literature for ethylene/styrene copolymerization using CGC.30,35,37,38 In fact, these systems showed their inability to homopolymerize styrene; thus, no polystyrene impurities should be formed during the copolymerization.31 It appears, as observed elsewhere, that the crystallinity decreases with the increase of the styrene molar content: the copolymer with 19% molar content in styrene is almost amorphous. Fourier transformed infrared spectra were acquired using a Nicolet Magna 750 spectrometer. Thin film thickness calibration was performed using FTIR spectroscopy. The films could indeed present some heterogeneity in the thickness over the entire surface. Thus, the determination of the exact

(

1

)

(2)

The cyclohexane/benzene glasses were characterized exclusively by FTIR spectroscopy in the transmission mode. This study allowed us to determine the sample structure and 14501



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Table 2. Polymer Sample Characteristics PE 1

H RMN

DSC FTIR

styrene molar content (%) ethylene molar content (%) crystallinity (%) melting temperature (°C) thickness calibration (cm2·mg−1)

aPS 100

100 88.0 131 0.055

0 0.240

PES 2.2

PES 7.0

PES 19.0

2.2 97.4 43.2 115 0.012

7.0 93.0 19.9 84 0.042

19.0 81.1 2.9 54 0.090

Table 3. Infrared Bands Studied (for Cyclohexane/Benzene Glasses Destruction, Notation Corresponds to the One of E. B. Wilson40) sample studied

behavior

band position (cm−1)

attribution

cyclohexane/benzene glasses

destruction creation destruction

3083 967 1477 678 1137

ν(C−H) - aromatic ring ω(C−H) - trans-vinylene ν19 (ring semicircle stretching) ν11 (in-phase out-of-plane aromatic C−H wag) T(CH2) - cyclohexene

PES

creation

concentrations vary with the benzene molar content in the cyclohexane/benzene glasses. This fact rules out an accurate quantification of the benzene destruction in the cyclohexane/ benzene glasses. Nevertheless, we will present qualitative evolution of the two infrared bands presenting the most different trends. For the creation of new chemical groups studies, we have focused on the creation of CC double bonds in the aliphatic moiety, i.e., trans-vinylene in PES and cyclohexene in cyclohexane/benzene glasses. These two defects are equivalent in the two systems and the evolution of their infrared band, as a function of the irradiation dose, is shown in Figure 4a for PES 7.0% styrene molar content irradiated at 11 K and in Figure 4b for the cyclohexane/benzene glass with 4.4% benzene molar content. For quantification, we used the Beer−Lambert law A = ε·1·C, where ε and C are the molar extinction coefficient (in L·mol−1· cm−1) and the molar concentration (in mol·L−1), respectively. We measured at 11 K the molar extinction coefficient values of cyclohexene in cyclohexane and cyclohexane/benzene (25% molar) matrixes.41 The use of ternary mixtures (cyclohexane/ benzene/cyclohexene) allows taking into account the influence of the presence of benzene in cyclohexane/benzene glasses on the ε value. We obtained very close values, for the band at 1137 cm−1, in both matrixes: ε(1137 cm−1) = 56 and 59 L·mol−1· cm−1 in cyclohexane and cyclohexane/benzene (25% molar), respectively. The ε value used to quantify trans-vinylene creation in polyethylene at room temperature is ε(967 cm−1) = 169 L· mol−1·cm−1.42 Since all spectra are collected at 11 K and since the molar extinction coefficient value is a function of the temperature, the knowledge of this value at 11 K is mandatory. This was achieved by acquiring, on the same sample, a first spectrum at room temperature and a second one at 11 K, all other conditions being equal. We will assume that ε(967 cm−1) does not depend on the styrene molar content. This seems confirmed by the fact that, in the glasses, we found almost no influence of benzene on ε(1137 cm−1). Therefore, the modification of the absorbance observed between room temperature and 11 K, for the 967 cm−1 line, is exclusively related to the variation of the ε value, as shown by relation 4. In this relation, ensuing from the Beer−Lambert law, εT is the molar extinction coefficient at the acquisition temperature T (ε11K at 11 K and εRT at room temperature) and AT is the band

thickness. Considering the band shapes and their very large widths, it appears that all the samples are amorphous. The procedure applied for cyclohexane/benzene glass thickness determination is different from the one used for PES thickness determination. At said upper, cyclohexane/ benzene glasses are obtained at 11 K by gas deposition on a substrate. Since their melting temperatures are lower than room temperature, the procedure used for PES films is no longer appropriate and the procedure used is the one presented in the following. To determine the glasses thicknesses, infrared spectra were recorded at the normal incidence, under a polarized beam. Under these conditions, interference fringes are observed on the spectra baselines. The difference in wavenumbers between two adjacent interference maxima, the interfringe, is related to the sample thickness through relation 3. In this relation, i, n, and e represent, respectively, the interfringe value, the sample refractive index, and the sample thickness. i=

1 2·n·e

(3)

The samples thicknesses are summarized in Table 1. 2.5. Data Analysis. The radiation-induced modifications appear in the FTIR spectra in two different ways. In the first one, infrared bands initially present in pristine samples decrease or increase in intensity, with eventually tenuous changes in width and position. The intensity decrease is due to the gradual transformation of the initial chemical groups into new ones. This effect is called hereafter “the overall or global destruction”. The second radiation-induced modification is the emergence of new bands, indicating the radiation-induced creation of new chemical groups called “defects”. All the infrared bands used to monitor the radiation-induced modifications in PES copolymers or in cyclohexane/benzene glasses are quoted in Table 3. Peak positions as well as the corresponding assignments are presented. To follow the destruction of the benzene ring from the styrene comonomer, we choose a unique infrared band in each sample type. In polystyrene, all the bands decrease in a similar way39 under ionizing radiations: the study of several bands in PES would have brought no additional information. In cyclohexane/ benzene glasses, the situation is more complex because all the bands do not decrease in a similar way. This difference indicates that the defects formed in benzene molecules and their 14502



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In the literature, the radiation chemical yields G are often expressed as moles of created defects per unit of energy deposited in the whole material, i.e., in the aromatic and aliphatic moieties. In this case, there is no indication on the moiety where the defect is created, i.e., in the aliphatic or in the aromatic moiety. The radiation chemical yield is then given by relation 7. G=

=

(6)

1 ) C(mol ·kg −moiety where the defect is created

D(Gy)

(8)

3. RESULTS 3.1. Double Bond Creation. In this section are presented results related to CC unsaturated bonds in PES and in cyclohexane/benzene glasses. Unsaturated bonds in PES, of the trans-vinylene type, are created in the ethylene moiety. The equivalent in cyclohexane/benzene glasses is the cyclohexene, created from the cyclohexane molecule. Figure 5 presents the evolution of the trans-vinylene concentration as a function of the irradiation dose, in PES irradiated either at 11 K (Figure 5a) or at room temperature (Figure 5b). Whatever the irradiation dose and the irradiation temperature, the trans-vinylene concentration decreases when the aromatic content in PES increases. Meanwhile, transfers involved in the radiation stabilization of the aliphatic moiety by benzene rings are effective even at 11 K. When comparing the evolution of the trans-vinylene concentration as a function of the irradiation dose, at room temperature on one side and at 11 K on the other side, the curve shapes are quite different. In the interval of irradiation doses used here, evolution is rather linear at 11 K (Figure 5a) but presents a saturation tendency at room temperature (Figure 5b). At room temperature, the introduction of the styrene adduct in the polyethylene has two main effects. The first effect is the reduction of the trans-vinylene creation at low doses and the second effect is the significant reduction of its concentration at saturation, at higher doses. Note that, in PES 2.2% styrene irradiated at room temperature, the trans-vinylene concentration is slightly higher than the one obtained in pure polyethylene.

As a matter of fact, the decrease of initial chemical groups and the formation of new chemical groups follow an exponential law with the irradiation dose. For “destruction”, this is expected from the Poisson law. In the case of new chemical groups, the exponential behavior comes from a zero order creation and a first order destruction of the corresponding defect. The evolution of the defects concentration as a function of the irradiation dose follows eq 5 for groups initially present in the polymer and eq 6 for new groups.

C = P4 + P5·(1 − exp(−P6·D))

(7)

1 ) C(mol ·kg −whole material 1 G= · D(Gy) y

absorbance at the same temperature (A11K at 11 K and ART at room temperature). We obtained ε(967 cm−1) = 260 L·mol−1· cm−1 at 11 K. A ε11K = εRT · 11K ART (4)

(5)

D(Gy)

In the absence of interaction between the two moieties, the radiation chemical yield follows the law of averages: G = yE·GE + yS·GS, with yi being the mass fraction of moiety i and Gi the radiation chemical yield of the defect studied in the pure moiety i. Thanks to the use of FTIR spectroscopy, we are able to follow defects specific to a given moiety. Consequently, the radiation chemical yields presented here are expressed in moles of defects created per unit of energy deposited in the moiety in which the defect is created. In this case, only one moiety being considered, the radiation chemical yield is constant when there is no interaction between the two moieties. The radiation chemical yield, in the considered moiety, is then related to the mass fraction y by relation 8.

Figure 4. (a) Evolution of the absorbance of the trans-vinylene band as a function of dose in PES 7.0% styrene irradiated at 11 K with 20Ne ions (LET = 3.9 MeV·mg−1·cm2). (b) Evolution of the absorbance of the cyclohexene band as a function of dose in the cyclohexane/ benzene glass 4.4% benzene irradiated at 11 K with 13C ions (LET = 7.5 MeV·mg−1·cm2).

C = P1 + P2·(exp( −P3·D) − 1)

1 C(mol ·kg −whole ) material

In these equations, Pi (i = 1, ..., 6) are free parameters to be determined. The use of P1 (P4) inserts a supplementary degree of freedom to C0 values. For ensuring SI units, C and M are expressed in mol·kg−1 and kg·mol−1, respectively. The exponential fit allowed calculating the radiation chemical yields at zero dose, G, by using the first order Taylor expansion of relations 5 and 6 depending on whether the defect considered is “destructed” or “newly created”. The benzene ring destruction yield is G(destruction) = P2·P3/M, with M being the molar mass of the aromatic monomer (M = 0.104 kg· mol−1 for styrene and M = 0.078 kg·mol−1 for benzene). The new groups creation yield is given by G(creation) = P5·P6. 14503



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Figure 6. Evolution of cyclohexene concentration (in mol· kg−1 whole material) in the cyclohexane/benzene glasses as a function of dose (in MGy). Samples were irradiated at 11 K with 13C ions (LETmean = 7.5 MeV·mg−1·cm2).

evolution of the cyclohexene created in the cyclohexane moiety as a function of the irradiation dose presents some tendency to saturation: the radiation chemical yield decreases with increasing dose. Figure 7 presents the evolution of trans-vinylene radiation chemical yields reduced to the ethylene moiety, as a function of

Figure 5. Evolution of the trans-vinylene concentration (in mol· kg−1 whole material) in the aliphatic moiety in ethylene/styrene random copolymers as a function of dose (in MGy): (a) samples were irradiated at 11 K with 20Ne ions; (b) samples were irradiated at room temperature with 20Ne ions (LETmean = 3.9 MeV·mg−1·cm2 at both irradiation temperatures).

This surprising fact is not really explained but can be due to an effect of dichroism on the trans-vinylene 967 cm−1 absorption band. As said before, FTIR analyses were performed with a polarized light and the polyethylene films used are highly crystalline and anisotropically textured. Therefore, depending on the film “orientation”, the 967 cm−1 absorption differs. However, if the film orientations are different during irradiation at 11K and at room temperature, the absorbance at a given concentration will be different. Concerning PES, the dichroism effect is less effective, since (1) their crystalline fraction decreases with increasing styrene molar content (see Figure 3) and (2) films are obtained with hot press and are thus nontextured. Nevertheless, we can assume that the radiation protection effect at room temperature is probably higher than what is observed in Figure 5. Indeed, according to results presented in the literature22 or previously obtained in our laboratory,23 the ratio between the trans-vinylene radiation chemical yield at room temperature and the trans-vinylene radiation chemical yield at low temperature is about 2. Thus, the trans-vinylene radiation chemical yield at room temperature in polyethylene should be higher, of the order of 2.4 × 10−7 mol·J−1. However, the important point to note is that, even if there is uncertainty on the initial point, the trans-vinylene concentration decreases when the styrene molar content increases. Figure 6 presents the evolution of the cyclohexene concentration as a function of the irradiation dose, in cyclohexane/benzene glasses with different benzene molar contents. These glasses were irradiated at 11 K. Whatever the irradiation dose, the cyclohexene creation decreases when the benzene content increases. It can be noticed that the radiation stabilization of cyclohexane by benzene, at a temperature as low as 11 K, is here also carried out in a very efficient way. In contrast to what is observed in PES irradiated at 11 K, the

Figure 7. Trans-vinylene radiation chemical yields reduced to the ethylene moiety in PES copolymers at zero dose, as a function of the styrene molar content. Irradiations were performed at room temperature and at 11 K with 20Ne ions (LETmean = 3.9 MeV·mg−1· cm2).

the styrene molar content, in random ethylene/styrene copolymers irradiated at room temperature and at 11 K with 20 Ne ions. At high styrene concentration (PES 7.0% and PES 19.0% styrene molar content), trans-vinylene radiation chemical yields are equivalent at room temperature and at low temperature, unlike what is observed at low styrene concentrations (pure PE and PES 2.2% styrene). In Figure 8 are compared the evolution, as a function of the benzene (styrene) molar content, of the normalized G(cyclohexene) in cyclohexene/benzene glasses on one hand and of the normalized G(trans-vinylene) in PES irradiated at 11 K on the other hand. The reference values used here are related to the cyclohexene creation in a pure cyclohexane glass and to the trans-vinylene creation in PE, irradiated under the same conditions. The evolutions of the normalized radiation 14504



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temperature, the aromatic moiety destruction increases when the styrene molar content in PES decreases. Figure 10 presents the evolution of the benzene ring “destruction” radiation chemical yield as a function of the

Figure 8. Comparison of the normalized radiation chemical yields of cyclohexene formation in cyclohexane/benzene glasses at 11 K with 13 C ions (G(reference) = 2.2 × 10−7 mol·J−1) and of trans-vinylene formation in ethylene/styrene random copolymers at 11 K with 20Ne ions (G(reference) = 1.2 × 10−7 mol·J−1). Figure 10. Aromatic moiety “destruction” radiation chemical yields reduced to the styrene moiety in PES copolymers at zero dose, as a function of the styrene molar content. Irradiations were performed at room temperature and at 11 K with 20Ne ions (LETmean = 3.9 MeV· mg−1·cm2 at both irradiation temperatures).

chemical yields at zero dose in the two systems, as a function of the benzene (styrene) molar content, are equivalent. In PES, transfers can occur either along a given polymer chain or between two polymer chains. In cyclohexane/benzene glasses, transfers happen between two different molecules. The similarity of the evolutions stated in Figure 8 implies a high efficiency of interchains and intermolecular transfers at 11 K. 3.2. Aromatic Moiety Destruction. Figure 9 presents the evolution of the aromatic moiety “destruction” as a function of the irradiation dose, in the different copolymers irradiated at 11 K in Figure 9a, and at room temperature in Figure 9b. It is observed that, whatever the irradiation dose and the irradiation

styrene content in PES copolymers, at 11 K and at room temperature. The destruction of the benzene ring, already observed at room temperature,4 is effective at 11 K. Thus, the sensitization of the aromatic moiety by the aliphatic one is maintained at very low temperatures. Elsewhere, at the exception of the PES 2.2% styrene, the aromatic moiety “destruction” radiation chemical yields are similar at 11 K and at room temperature. In cyclohexane/benzene glasses irradiated at 11 K, four infrared absorption bands were monitored as a function of the irradiation dose and as a function of the benzene molar content. Two bands present the most different trends: the one at 1477 cm−1 and the one at 678 cm−1. These bands are assigned, according to the notation of E. B. Wilson,40 to the ν19 (ring semicircle stretching mode) and to the ν11 (in-phase out-ofplane aryl C−H wag mode) vibrations, respectively. Between the pure benzene glass and the cyclohexane/benzene glass with 4.4% benzene molar content, the destruction of the benzene molecules is increased by a factor of 4 if one considers the band at 1477 cm−1 and by a factor of 15 if one considers the band at 678 cm−1. In spite of the differences in the trends of these benzene characteristic absorption bands, the aromatic moiety “destruction” in cyclohexane/benzene glasses at 11 K is clear and similar to that obtained in PES, confirming at low temperature the high sensitization of the aromatic moiety in the presence of the aliphatic one and the effectiveness of intermolecular energy transfers.

4. DISCUSSION 4.1. Irradiation Temperature Effect in PES Copolymers. Ionizing radiations induce excitation and ionization in organic materials. Energy deposited in the polymer by ionizing radiations is transferred throughout the material and can be trapped on various reaction sites. As said in the Introduction, the term energy transfers used in this work is large and includes, differing in their temperature dependence:

Figure 9. Evolution of the aromatic moiety destruction in aPS (atactic polystyrene) and in PES (expressed in (A − A0)/A0) as a function of dose (in MGy): (a) samples were irradiated at 11 K with 20Ne ions; (b) samples were irradiated at room temperature with 20Ne ions (LETmean = 3.9 MeV·mg−1·cm2 at both irradiation temperatures). 14505



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4.2. Irradiation Temperature Effect in Cyclohexane/ Benzene Mixture. Concerning cyclohexane/benzene mixtures, temperature effect is observed by comparing results presented in the literature at room temperature in liquids with those obtained in the present work, at 11 K, in amorphous glasses. The aim of this section is to compare the cyclohexene formation at room temperature,8 in cyclohexane/benzene mixtures, with our results on cyclohexene formation in cyclohexane/benzene glasses at 11 K. In addition, we include the hydrogen emission values obtained by different teams6−8 in cyclohexane/benzene mixtures irradiated at room temperature. Although H2 emission comes from the entire system, it is generally assigned to cyclohexane, due to the high difference in gas emission between cyclohexane and benzene. These results are gathered in Figure 11. Whatever the irradiation temper-

- mass or reactive species transport (this term includes radicals and ions) - lonely charge transfer (electrons and holes) - electronic excitation transfer. Lowering the irradiation and analysis temperatures was aimed to determine the relative contribution of each of these three types of transport mechanisms. At room temperature, since the PES samples are above their glass transition temperatures, all three types of transfer can occur. At the lowest irradiation temperature (11 K), all the polymers under study are well below their glass transition temperatures. At this temperature, all transfers implicating mass transport are drastically reduced or zero. Thus, results obtained at 11 K suggest the effect of electronic excitation and lonely charge transfers. Indeed, it has been shown in the literature that electrons and positive holes transfers from polyethylene to pyrene occur during irradiation at 77 K.17 Whatever the irradiation temperature, the stabilization of the ethylene moiety by the benzene ring is effective in ethylene/ styrene copolymers. Influence of the irradiation temperature on the trans-vinylene creation in the ethylene moiety is a function of the irradiation dose. At low doses, trans-vinylene concentration created in PES is more pronounced at room temperature than at low temperature. At high doses, evolution as a function of the irradiation dose of the trans-vinylene concentration remains linear at 11 K but presents a tendency to saturation at room temperature. These two facts can be linked to an enhanced radical migration at room temperature. At low doses, radical migration favors radical recombination and the subsequent trans-vinylene creation, whereas, at high doses, during migration, the radical can react either with another radical for trans-vinylene creation or with a trans-vinylene group, leading to its “destruction”. One of the reactions involved in trans-vinylene destruction corresponds to the transfer of H° toward the CC double bonds to form allyl radicals (eq 9), known to be more stable than alkyl radicals at room temperature.

Figure 11. Comparison of the normalized radiation chemical yields of cyclohexene (G(reference) = 3.4 × 10 −7 mol·J −1 8 ) and H 2 (G(reference) = 5.7 × 10−7 mol·J−1 6−8) in cyclohexane/benzene liquid mixtures irradiated at room temperature and of cyclohexene in cyclohexane/benzene glasses irradiated at 11 K (result obtained in this study, G(reference) = 2.2 × 10−7 mol·J−1).

ature, the cyclohexene creation decreases when the benzene content in the mixture increases. The radiation stabilization effect is thus effective at both temperatures. However, the decrease in the trans-vinylene G/G(reference) ratio is far more marked in liquid mixtures than in amorphous glasses for the lowest benzene contents. This difference is assigned to the more important diffusion of reactive species in liquids.43 4.3. Comparison between Intrachain and Interchain Transfer Efficiency. PES and cyclohexane/benzene amorphous glasses were irradiated and studied at 11 K. As explained in the Introduction, the use of cyclohexane/benzene amorphous glasses aimed to set the effectiveness of intermolecular transfers. The ulterior motive is to determine the relative impact of interchain and intrachain transfers in copolymers. As shown in Figure 8, double bond radiation chemical yields created in the aliphatic moiety decrease similarly as a function of the aromatic moiety content in both systems studied. This result indicates that, at an irradiation temperature of 11 K, the radiation protection effect is similar in the cyclohexane/ benzene amorphous glasses and in PES. Therefore, under conditions where reactive species transport is drastically reduced, transfers between two different chains in polymers are almost as effective as transfers inside a given chain. Although results compared here were obtained at different

CH 2CHCHCH 2 H•

→ CH•CHCHCH 2

(9)

Whatever the irradiation temperature, G(trans-vinylene) decreases rapidly at low styrene molar content and then levels off at higher styrene molar contents. This can be linked to the reduction of the mean distance between two aromatic units, at high styrene contents. The mean distance is calculated from the volume “occupied” by a styrene molecule, considering a homogeneous distribution of styrene adducts in the volume. This mean distance is estimated equal to 8.3 Å in PES 2.2% and 4.5 Å in PES 19.0%, compared to the value of 3.4 Å in atactic polystyrene. The transfers thus occur in the closest vicinity of the styrene unit. Besides, this low mean distance can explain the reduced difference observed between irradiation at room temperature and irradiation at 11 K, for trans-vinylene creation in PES with high styrene molar contents. Concerning the aromatic moiety behavior, the high benzene sensitization observed at 11 K, either in PES or in cylohexane/ benzene glasses, has previously been demonstrated at room temperature and discussed in our previous work.4 Consequently, for a detailed discussion on this effect, the reader is invited to refer to the cited reference. 14506



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(8) Stone, J. A.; Dyne, P. J. Radiation Chemistry of Cyclohexane: V. Dilute Solutions of Benzene in Cyclohexane. Radiat. Res. 1962, 17, 353−365. (9) Ohnishi, S.-I.; Tanei, T.; Nitta, I. ESR Study of Free Radicals Produced by Irradiation in Benzene and Its Derivatives. J. Chem. Phys. 1962, 37, 2402−2407. (10) Thomas, J. K.; Mani, I. Pulse Radiolysis of Benzene Cyclohexane Mixtures. J. Chem. Phys. 1969, 51, 1834−1838. (11) Gardner, J. B.; Harper, B. G. Radiation Protection of Polyethylene. J. Appl. Polym. Sci. 1965, 9, 1585−1591. (12) Mal’tseva, A. P.; Leshchenko, S. S.; Iskakov, L. I.; Karpov, V. L. Features of Radio-Chemical Processes in Ethylene-Styrene Copolymers. Polym. Sci. USSR 1976, 18, 1270−1279. (13) Terteryan, R. A.; Leshchenko, S. S.; Livshits, S. D.; Golosov, A. P.; Itsikson, L. B.; Monastyrskii, V. N.; Karpov, V. L.; Soboleva, N. S.; Mal’tseva, A. P.; Iskhakov, L. I. Radiation-Resistant Copolymers of Ethylene with Styrene. Sov. Plast. 1973, 7, 1−5. (14) Mal’tseva, A. P.; Golikov, V. P.; Leshchenko, S. S.; Karpov, V. L. IR and UV Spectroscopic Study of the Low-Temperature Radiolysis of Ethylene-Styrene Copolymers. High Energy Chem. 1977, 11, 283−287. (15) Mal’tseva, A. P.; Golikov, V. P.; Leshchenko, S. S.; Karpov, V. L.; Muromtsev, V. I. Free-Radical Processes during Low Temperature Radiolysis of Ethylene-Styrene Copolymers. High Energy Chem. 1977, 11, 189−193. (16) Biscoglio, M.; Thomas, J. K. Radiolysis of Polyethylene Films Containing Arenes: Bromopyrene Dissociation and Pyrene Binding in Polymer Films. J. Phys. Chem. B 2000, 104, 475−484. (17) Thomas, J. K. Fundamental Aspects of the Radiolysis of Solid Polymers, Crosslinking and Degradation. Nucl. Instrum. Methods Phys. Res., Sect. B 2007, 265, 1−7. (18) Okamoto, K.; Kozawa, T.; Miki, M.; Yoshida, Y.; Tagawa, S. Pulse Radiolysis of Polystyrene in Cyclohexane - Effect of Carbon Tetrachloride on Kinetic Dynamics of Dimer Radical Cation. Chem. Phys. Lett. 2006, 426, 306−310. (19) Partridge, R. H. Excitation Energy Transfer in Alkanes. II. Experimental Demonstration. J. Chem. Phys. 1970, 52, 2491−2500. (20) Partridge, R. H. Excitation Energy Transfer in Alkanes. III. Radiation Chemistry of Alkane Polymers. J. Chem. Phys. 1970, 52, 2501−2510. (21) Willard, J. E. The Radiation Chemistry of Organic Solids. In The Radiation Chemistry: Principles and Applications; Farhataziz, Rodgers, M. A. J., Eds.; VCH Publishers: New York, 1987; pp 395−433. (22) Waterman, D. C.; Dole, M. Infrared Spectrum of Polyethylene Irradiated at 4 K. J. Phys. Chem. 1971, 75, 3988−3992. (23) Mélot, M. Matériaux Organiques Irradiés à Très Basse Température et à Différents Pouvoirs d’Arrêt: Cas du Polyéthylène et des Molécules de Cyclohexane Isolées en Matrice. Spécialité Milieux denses et Matériaux, Université de Caen, 2003. (24) Mélot, M.; Ngono-Ravache, Y.; Balanzat, E. Very Low Temperature Irradiation of Aliphatic Polymers: Role of Radical Migration on the Creation of Stable Groups (O-127). Nucl. Instrum. Methods Phys. Res., Sect. B 2003, 208, 345−352. (25) Zeman, A.; Heusinger, H. Intramolecular Energy Transfer in γIrradiated Alkylbenzenes. J. Phys. Chem. 1966, 70, 3374−3376. (26) Basheer, R.; Dole, M. Effects of Copolymer Composition on the Formation of Ionic Species, Hydrogen Evolution, and Free-Radical Reaction in γ-Irradiated Styrene-Butadiene Random and Block Copolymers. J. Polym. Sci., Polym. Phys. Ed. 1984, 22, 1313−1329. (27) Witt, E. The Effect of Polymer Composition on RadiationInduced Crosslinking. J. Polym. Sci. 1959, 41, 507−518. (28) Brandrup, J.; Immergut, E. H.; Grulke, E. A. VI. Solid State Properties. In Polymer Handbook; John Wiley and Sons: New York, 1999; Vol. 2. (29) Ziegler, J. F. Particle Interactions with Matter; http://www.srim. org/. (30) Lee, D.-H.; Yoon, K.-B.; Kim, H.-J.; Woo, S.-S.; Noh, S. K. Copolymerization of Styrene and Ethylene with Mononuclear and Dinuclear Half-Titanocenes. J. Appl. Polym. Sci. 1998, 67, 2187−2198.

LET, any eventual LET effect should be discarded, since results obtained on cyclohexane/benzene glasses irradiated under 13C ion irradiations at a LET of 2.8 MeV·mg−1·cm2 (less complete than 13C ion irradiations at 7.6 MeV·mg−1·cm2 of this paper) have shown a similar effect. It can thus be concluded that results presented in the literature26,27 on the noneffectiveness of transfers in polymers mixture are due to the setup of phase segregation in these solid polymers mixtures.

5. CONCLUSION The aim of this study was to understand how aliphatic and aromatic groups interact under ionizing radiations and more specifically to determine first, the influence of the irradiation temperature and second, the influence of the system composition, on this radiation stabilization. Based on the comparison of trans-vinylene creation in PES at room temperature and at 11 K, it appears that excitation energy and charge transfers are very efficient at 11 K. However, although non necessary, reactive species transport contribution in the aliphatic moiety radiation stabilization by benzene rings at room temperature, should not be neglected. The comparison between PES copolymers and cyclohexane/ benzene glasses tends to show that, under conditions where reactive species are almost frozen, interchain transfers are very effective. As a consequence, the absence of interaction in polymer blends, quoted in the literature, resulted in the immiscibility of the polymers composing these blends. Thanks to the use of infrared spectroscopy, we have also shown an important effect of radiation sensitization of the aromatic moiety by the aliphatic one, whatever the irradiation temperature and the system studied: energy transfers to the aromatic moiety are carried out at its sacrifices.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +(33)1-69-08-27-34. Notes

The authors declare no competing financial interest.

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REFERENCES

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(31) Arriola, D. J.; Bokota, M.; Campbell, R. E.; Klosin, J.; LaPointe, R. E.; Redwine, O. D.; Shankar, R. B.; Timmers, F. J.; Abboud, K. A. Penultimate Effect in Ethylene-Styrene Copolymerization and the Discovery of Highly Active Ethylene-Styrene Catalysts with Increased Styrene Reactivity. J. Am. Chem. Soc. 2007, 129, 7065−7076. (32) Cossee, P. On the Reaction Mechanism of the Ethylene Polymerization with Heterogeneous Ziegler-Natta Catalysts. Tetrahedron Lett. 1960, 1, 12−16. (33) Arlman, E. J. Ziegler-Natta Catalysis II. Surface Structure of Layer-Lattice Transition Metal Chlorides. J. Catal. 1964, 3, 89−98. (34) Lahitte, J.-F.; Plentz-Meneghetti, S.; Peruch, F.; Isel, F.; Muller, R.; Lutz, P. J. Design of New Styrene Enriched Polyethylenes via Coordination Copolymerization of Ethylene with Mono- or α,ωDifunctional Polystyrene Macromonomers. Polymer 2006, 47, 1063− 1072. (35) Chen, H.; Guest, M. J.; Chum, S.; Hiltner, A.; Baer, E. Classification of Ethylene-Styrene Interpolymers based on Comonomer Content. J. Appl. Polym. Sci. 1998, 70, 109−119. (36) Sernetz, F. G.; Mülhaupt, R.; Waymouth, R. M. Influence of Polymerization Conditions on the Copolymerization of Styrene with Ethylene using Me2Si(Me4Cp)(N-tert-butyl)TiCl2/Methylaluminoxane Ziegler-Natta Catalysts. Macromol. Chem. Phys. 1996, 197, 1071− 1083. (37) Rodrigues, A.-S.; Carpentier, J.-F. Groups 3 and 4 Single-Site Catalysts for Styrene-Ethylene and Styrene-α-Olefin Copolymerization. Coord. Chem. Rev. 2008, 252, 2137−2154. (38) Chen, H. Y.; Chum, S. P.; Hiltner, A.; Baer, E. Comparison of Semicrystalline Ethylene-Styrene and Ethylene-Octene Copolymers based on Comonomer Content. J. Polym. Sci., Part B: Polym. Phys. 2001, 39, 1578−1593. (39) Ferry, M.; Ngono-Ravache, Y.; Picq, V.; Balanzat, E. Irradiation of Atactic Polystyrene: Linear Energy Transfer Effects. J. Phys. Chem. B 2008, 112, 10879−10889. (40) Wilson, J. E. B. The Normal Modes and Frequencies of Vibration of the Regular Plane Hexagon Model of the Benzene Molecule. Phys. Rev. B 1934, 46, 706−714. (41) Dos santos, D. Etude de Cibles Cryogéniques et Caractérisation par Spectroscopie Infrarouge, 2007. (42) Kock, R. J. d.; Hol, P. A. H. M.; Bos, H. F. Infrared Determination of Unsaturated Bonds in Polyethylene. Fresenius' J. Anal. Chem. 1964, 205, 371−381. (43) Indeed, a difference in diffusion does exist in polymers. The difference in the temperature effect between cyclohexane/benzene systems and PES polymers can be explained by the drastic diffusion differences between liquid alkanes and a PES solid polymer above its glass transition temperature. As an example, according to a J. K. Thomas work,17 at 22 °C, radical diffusion in PE is assumed to be 10−6 times the one in liquid. Another example of this concerns diffusion coefficients of molecular O2 in organic materials: 9.96 × 10−5 cm2·s−1 in liquid hexane44 at 23 °C and 1.7 × 10−7 cm2·s−1 in HDPE.28 (44) Kowert, B. A.; Dang, N. C. Diffusion of Dioxygen in n-Alkanes. J. Phys. Chem. A 1999, 103, 779−781.

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