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Radiation effects on a linear model compound for polyethers
Polymer Degradation and Stability 96 (2011) 1225e1235
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Radiation effects on a linear model compound for polyethers C. Aymes-Chodur a, *, A. Dannoux b, V. Dauvois b, S. Esnouf c a
Université Paris-Sud 11, Laboratoire Matériaux et Santé - EA 401, IFR 141, Faculté de pharmacie- 5, rue J.B.Clément, Châtenay Malabry 92296, France CEA Saclay, DEN/DANS/DPC/SECR/LSRM, 91191 Gif/Yvette Cedex, France c CEA Saclay, DSM/IRAMIS/SIS2M/Laboratoire de Radiolyse, 91191 Gif/Yvette Cedex, France b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 February 2011 Received in revised form 18 March 2011 Accepted 13 April 2011 Available online 30 April 2011
Radiation effects on a model polyether e poly(tetramethylene) glycol (PTMG) induced by high energy radiation were investigated. To understand the degradation mechanism, electron paramagnetic resonance (EPR), Fourier transform infra-red spectroscopy (FTIR), electrospray and gas mass spectrometry (ESIeMS and gas-MS), were carried out to identify radicals and chemical modifications. Size exclusion chromatography (SEC) was used to follow the evolution of the distribution of molecular weight. On the basis of the results, a mechanism of degradation for PTMG is proposed. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Polyether Radiolysis EPR ESIeMS MSgas SEC
1. Introduction Materials based on organic polymers are widely used in the nuclear industry (jacketing cables, seals, plastic surfaces, gloves) where they are subjected to high energy radiation. Extensive research has been reported on radiation-induced oxidative degradation of polyethylene, polypropylene, polyvinyl chloride but much little attention has been directed to oxygen containing polymers such as polyethers. Nevertheless, oxidative degradation of poly(ethylene oxide) (PEO) that has numerous applications, for example, in pharmaceutical industry for hydrogels, excipients. is one of the polyether that was widely studied. In the 60e70 s, several authors have studied the crosslinking and the gelation properties of PEO irradiated in solution [1e4]. Later on, Zhang and Nedkov studied the gamma irradiation effect on the crystalline structure of high molecular weight PEO [5e8]. Decker [9] investigated the radiation-induced oxidation of solid PEO. The principal products identified were formate and hemiformal groups, hydroperoxides, and some volatile compounds mainly formaldehyde and carbon
* Corresponding author. Tel.: þ33 1 46 83 54 55; fax: þ33 1 46 83 59 63. E-mail address: [email protected] (C. Aymes-Chodur). 0141-3910/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2011.04.015
dioxide. Since there have been relatively few recent studies on changes induced by high energy radiation in PEO. Zainuddin et al. [10] analysed changes in molecular weight distribution after irradiation. They observed that in presence of oxygen, chain scissions are dominant. The radiation yield G(S) w 2,5 106 mol/J indicates a chain reaction. Under vacuum, crosslinking and scission are in competition and crosslinking dominates at high dose. Gardette and Morlat [11,12] studied the photo and thermooxidation of PEO in solid state and highlighted the formation of formates and esters with a ratio depending of the degradation type. Maggi et al. [13] investigated the effects of gamma irradiation on release drug delivery systems containing PEO or PVA. The authors used EPR spectroscopy to identify the nature of radicals induced by exposure to gamma rays. At low temperature dominant transient species is the hydrogen abstraction radicals eCH2eCH eOe. Warming to 298 K causes the formation of a novel signal attributed to the aldehyde radical eCH eCH(]O) that is a direct evidence of chain scissions. Finally the breaking of chains explains the observed changes of the dissolution and morphological performances of PEO tablets. Hassouna [14] studied the thermo and photo-degradation of PEO in aqueous solution and showed that, depending on the pH of the medium, formic acid was released in addition with the formation of formates and esters.
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Poly(methylene oxide) (POM) and poly(propylene oxide) (PPO) were also studied. Dole presented a review for these two materials [15]. Recently Fayolle analysed the thermo-oxidation kinetics of Poly(methylene oxide) (POM) [16]. The polyether investigated in this paper is poly(tetramethylene) Glycol (PTMG) also called Poly(tetramethylene Oxide). This material was frequently used in industry as soft segment for thermoplastic or crosslinked elastomers because of its low glass transition point, good rubbery, elasticity and high strength [17]. To our knowledge only Golden published data on the chemical and physical modifications of this polyether induced by high energy electrons or gamma rays [18]. It was most studied as soft segment for polyurethane. Yet Guignot et al. [19] studied the degradation of polyetherurethane induced by electron beam irradiation. The oxidation of polyether soft segments was analysed using size exclusion chromatography, Fourier transform infra-red spectroscopy and thermogravimetric analysis. Dannoux et al. [20] studied the degradation mechanism of a segmented aromatic poly(ethereurethane). They concluded that under oxidising atmosphere predominant degradation occurred at polyether soft segments that produces stable oxidative products as formates, alcohols, carboxylic acids. Moreover crosslinking is in competition with scission. The objective of this work is to obtain supplementary information about the degradation mechanism proposed by Dannoux et al.. Then we have investigated the radiation effects in absence and in presence of oxygen on model compounds of polyether using different techniques such as EPR spectroscopy, FTIR spectroscopy, Size exclusion chromatography and gas analysis. Electrospray mass spectrometry of irradiated oligomers was also carried out. This method of ionization has been developed to make characterization of intact polymers possible [21,22]. The principle is to produce ions from neutral parent molecules to which small cations or anions are attached. In absence of fragmentation, this method can potentially determine molecular weight distribution of polymers. In our case mass spectrometry was mainly used to analyse the new oligomers induced by radiation. Yet accurate mass analysis permits to identify the nature of the end groups and then to clarify the mechanism of scission. 2. Experimental 2.1. Materials Poly(TetraMethylene) Glycol (PTMG), also called poly (tetrahydrofuran) or poly(ether glycol) are provided by Sigma Aldrich under the commercial name TerathaneÒ. Its linear formula is H(OCH2CH2CH2CH2)nOH. According to the specifications of the manufacturer, the average molecular weight of the PTMG is 2000, then the average polymerization degree n is approximately 28. Its melting point is low (between 28 and 40 C). As samples are soft at ambient temperature, we have not been able to form films but we melt PTMG in glass tubes and froze them to obtain either 2 mm thick layers or cylinders of 4 mm in diameter. FTIR analysis of pristine PTMG reveals an intense band centred at 3480 cm1 characteristic of OH groups. This result indicates that oligomers are certainly hydroxyl-terminated. 2.2. Reagents Tetrahydrofuran (THF) used for SEC analyses, Tertiobutyl formate used for FTIR measurements and methanol (HPLC grade) used for MS QTOF are provided by Sigma Aldrich.
2.3. Irradiation Gamma irradiation were performed in IONISOS (Dagneux, France) using a 60Co g-source. Samples were exposed at room temperature at a dose rate of 0.7 kGy/h in open glass tubes or under inert atmosphere (helium) in sealed glass containers. In order to identify and quantify gases produced during irradiation, polymers were also irradiated in sealed glass tubes. Chromic dosimeters were used to measure the dose. No electronic correction was made. Irradiation doses (rigorously absorbed dose by air as a reference) were 94 and 380 kGy for oxidising conditions and 64 and 104 kGy for inert atmosphere. Low temperature irradiations were performed using the electron pulses of a Titan Beta, Inc. linear accelerator (Laboratoire de Radiolyse, Saclay, France). 10 ns pulses of 10 MeV electrons were used at a repetition rate of 10 Hz. Samples were irradiated in out gassed sealed Pyrex tube hold in liquid nitrogen dewar. The dose rate was 5 Gy/pulse approximately. After irradiation, the samples were kept in liquid nitrogen. 2.4. Size exclusion chromatography (SEC) The evolution of Mn and Mw with increasing doses was determined using PTMG dissolved in THF. The solutions were analysed by a PL-GPC 220 high temperature chromatograph (Polymer Laboratories, UK), equipped with a double detection that consisted of a refractometer (DRI) (Polymer Laboratories, UK) and a 220R high temperature differential viscometer (DV) (Viscotek, UK). The chromatographic system had its temperature set at 35 C. Flow rate of THF used as mobile phase was of 1 mL/min. The SEC system was controlled by the PL-GPC 220 ControlÒ software (version 2.01). The data acquisition and analysis were set by Viscotek software (Omnisec e version 4.0). Two Polymer Laboratories (PL) columns were set up in series: a 3 mm 100 Å and a 5 mm mixed C whose molecular weights linear range stood respectively up to 4000 and between 200 and 2 106 (data given by the supplier), their granulometry was of 3 and 5 mm, their length was of 300 mm and their internal diameter of 7.5 mm. Solutions containing 10 mg mL1 of sample in THF were injected by a 200 mL injection loop. Nine polystyrene (PS) standards (Polymer Laboratories) were used for universal calibration with molecular weights varying from 580 to 91,800. Mn and Mw were calculated using the following formula:
X Wi 2 Ji m P Dv Mn ¼ P h and Mw ¼ m sp;i Wi hsp;i i Dvi Ji
(1)
where m is the mass of polymer injected, hsp,i is the specific viscosity at retention volume vi (baseline-corrected response of DV detector), Ji ð ¼ ½hi Mi Þ is the value of the universal calibration curve, Wi is the baseline-corrected DRI chromatogram height at retention volume vi and Dvi increment retention time. Normally the calculation of Mn needs only the DV chromatogram, the mass injected and a calibration curve. In our case, the DRI chromatogram was used to evaluate m. Previously the differential index of refraction dn/dc of PTMG has been calculated by measuring the peak areas as a function of the concentration. Whatever the dose, dn/dc was equal to 0.06. To estimate Mw, both DRI and DV chromatograms are needed. Yet the interdetector volume was determined using narrow standards. Table 1 presents the value of Mn and Mw estimated for pristine PTMG. Mn value is higher than the value given by the supplier. This deviation may be attributed to the calibration. Harrison [23]
C. Aymes-Chodur et al. / Polymer Degradation and Stability 96 (2011) 1225e1235 Table 1 Changes of molecular weight of irradiated PTMG in air. Dose (kGy)
Mn (g/mol)
Mw (g/mol)
Ip ¼ Mw/Mn
0 94 380
2820 2890 3400
4640 5800 1000
1.65 2.01 2.94
compared different methods to determine molecular weight of PTMG. They concluded that universal calibration is not the best method to study oligomers. Indeed DV detector is insensitive to low molecular weight species whereas DRI response is sensitive to end groups at low molecular weights. These complications can bias the values of high Mn and low Mw. 2.5. Electron paramagnetic resonance (EPR) In order to trap transitory species, the samples were irradiated under vacuum at 77 K. EPR spectra were recorded at the X band (9.4 GHz) on a Bruker ER-200D ESR spectrometer equipped with a nitrogen flow cryostat. The irradiated samples kept in liquid nitrogen were rapidly transferred in the cryostat and analysed at 120 K. The electron spin resonance parameters were determined by means of an automated simulation programs using the LevenbergeMarquardt method. The hyperfine anisotropy of the Haeproton was not considered explicitly in the simulations. Yet we verified that the hyperfine anisotropy contributes only to the broadening of the spectra. 2.6. Fourier transform infra-red spectroscopy (FTIR) FTIR spectra have been collected on a Nicolet Magna IR-750Ô Spectrometer which is equipped with a DTGS KBr detector using a diamond Attenuated Total Reflection (ATR) device. To ensure a good reproducibility, the samples were preliminary melt then deposited on the diamond crystal. The spectra were saved at 4 cm1 of resolution with 128 scans. In order to evidence the degradation products formed during irradiation, a nitric oxide gas (NO) treatment was performed but this treatment was not fruitful certainly because of the initial high alcohol concentration [24]. 2.7. Gas analysis Gas analysis was performed with a quantitative gas mass spectrometer (R30) with direct inlet developed by CEA for chemical and isotopic analysis. The ionisation is made by electron impact, mass separation is performed with a magnetic sector and ion detection is made by Faraday cup and electron multiplier. The mass range is from 1 to 200 uma and the detection limit is about 1 ppm depending on the gas matrix and mass interference. The radiolytic yield, G-value of formation or consumption of a molecular product X is given by:
½X ½X0 ½X GðXÞ ¼ or GðXÞ ¼ D D
(2)
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equipped with an electrospray ionisation source. The spray needle voltage was set to 2.9 kV and nitrogen operating at 350 L/h was employed as both the drying and nebulizing gas. Samples were introduced at a delivery rate of 10 mL/min using a 250 mL glass syringe with a stainless steel needle (Hamilton Co.). The source temperature was set to 80 C and the sample cone voltage to 50 V. Spectra were acquired at 5 s/scan over a mass range of m/z 50e1500 with an acquisition time of 3 min. Samples were dissolved with THF to get solutions at 5.6.106 mol L1. The software used for acquisition and treatment was Mass LynxÔ (Waters). Using standard resolution, isotopically resolved charged polymers in the mass range up to m/z ¼ 1500 are measured. To aid the interpretation of the spectra and the data processing a computer program using Scilab was developed and the application of this procedure was automated.(see Appendix 1) Some experiments using combination of ESIeMS with HPLC as a separation technique were also carried on. Nevertheless the rate of degradation and the concentrations of new products were too low to obtain accurate results. 3. Results 3.1. Molecular mass distribution Fig. 1 shows the chromatograms obtained from RI and viscometer detectors for PTMG 2000 at different irradiation doses under air atmosphere. For the non irradiated sample, the molecular masses distribution (MMD) is broad and asymmetrical in the smaller MM range side. It reveals a maximum of the intensity at about 13 min of elution and oligomers distribution from 15 to 16 min in the case of the DRI detector. Table 1 gives the calculated MM from universal calibration for the different doses. We observe clearly an increase of Mn and Mw all together with the dose. MMD gets also larger, as illustrated by the polydispersity index (Ip) values. This broadening is characterized by the appearance of two shoulders at 12.4 and 14 min. For the highest dose of 380 kGy, the broadening of the MMD still increases and the lowest MMD at 12.4 min, corresponding to a reticulation phenomenon, becomes almost predominant. 3.2. Tentative determination of scission yield G(S) and crosslinking yield G(X) G(S) and G(X) were estimated considering the following Saito’s equations [25]:
1 1 ¼ þ 103 ðGðSÞ GðXÞÞ D Mn;D Mn;0
(3)
1 1 GðSÞ ¼ þ 103 2GðXÞ D Mw;D Mw;0 2 With D, the absorbed dose (Gy), Mn,0, Mw,0 and Mn,D, Mw,D being the mean molar masses in number and weight (g mol1) initially and at the dose D respectively. Finally, under oxidising atmosphere, we estimate:
[X] and D are given in mol/kg and Gy, respectively. The unit of G is mol/J.
G(S) ¼ 0.12 107 mol J1
2.8. Mass spectrometry
G(X) ¼ 1.72 107 mol J1
The mass spectrometric measurements were recorded in cation mode using a Q-ToF II spectrometer (Micromass, Manchester, UK)
The predominant crosslinking.
degradation
mechanism
is
clearly
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Fig. 2. FTIR Spectra of pristine and irradiated PTMG 2000. Enclosed is given the areas ratio between vibration bands set at 1725 and 2850 cm1 as a function of the dose.
%. In the inset of Fig. 2, the ratio between areas measured at 1725 cm1 and 2850 cm1 as a function of the dose is plotted. For the 380 kGy irradiated PTMG, a concentration of formate of 2.9 104 mol g1 was obtained. The radiolytic formate formation yield is estimated to 7.6 107 mol J1. As it was not possible to prepare thin films with controlled thickness, FTIR was also used to check that oxidation has proceeded homogenously during irradiation. The oxidation rate quantified by the concentration of formate was measured near the surface and inside the samples. We verified that interiors of the samples were oxidised but oxidation is more pronounced near the surface (w50%). So we supposed that oxidation was certainly limited by oxygen diffusion but there was no depletion inside the samples during irradiation. 3.4. EPR analysis Fig. 1. DRI (a) and viscometry (b) chromatograms obtained from PTMG irradiated at different doses.
3.3. FTIR analysis Fig. 2 shows the superposition of FTIR spectra obtained for irradiated at 380 kGy, and pristine PTMG 2000. In the wavelength range located between 3300 and 3600 cm1, vibrations can be attributed to intermolecular OH bonds. They do not seem to evolve after irradiation. On the contrary, as the dose increases, two sharp vibration bands located at 1726 and 1160 cm1 appear. Those bands are respectively attributed to the esters elongation vibrations of C] O and CeO bonds such as formates [26,27]. Infra-red investigations made by Decker after radiation-induced oxidation of PEO are very similar. They revealed a strong absorption centred at 1725 cm1. This band was attributed to low molecular weight compounds (formaldehyde, glycol aldehyde and formate of ethylene glycol) and formate groups attached to macromolecule. The formate concentration was tentatively estimated to reach the radiolytic yield (G). Tertiobutyl formate was used as reference to estimate the extinction coefficient of the band at 1726 cm1. Solutions of this formate in THF at different concentrations were prepared, Beer Lambert’s law was valid for concentrations ranging from 1 to 10 vol
Radiation degradation of polymers involves radical intermediates that can be observed by EPR spectroscopy. Fig. 3 presents the ESR spectrum observed in PTMG irradiated at 77 K. A five-line spectrum is overlapped by a broad signal. Annealing causes a reduction of the total intensity of the ESR spectra. The subtraction of the spectra before and after annealing at 250 K reveals the disappearing of a broad three-line spectrum while the intensity of the five-line spectrum remains almost the same. Yet the spectrum can be attributed to the superposition of a broad triplet and a five-line spectrum. Simulation reveals that quintet signal corresponds to a spin S ¼ 1/2 in interaction with three non-equivalent nuclear spin I ¼ 1/ 2 with coupling of 1.38 mT, 2.24 mT and 3.25 mT. The peak-to-peak linewidth DHpp is estimated to be 1.12 mT (see Fig. 3). Concerning the broad triplet, its signal is compatible with the superposition of a triplet and a singlet. The triplet can be account for in terms of the interaction of two equivalent protons with coupling of 20.6 mT (DHpp ¼ 1.27 mT). The shape of the singlet is Lorentzian and the peak-to-peak linewidth is 5.33 mT. 3.5. Gas analysis The main gases produced by irradiation of PTMG are H2, CO, CO2 and CH4. Table 2 compares the values obtained under inert
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Fig. 3. (a) EPR spectra of irradiated PTMG at 77 K, (b) annealed at 250 K, (c) simulation of spectrum (b). (d) represents the subtraction of spectra (a) and (b), (e) simulation of spectrum (d). (f) and (g) are the two components of the calculated spectrum (e).
atmosphere with those measured under oxidizing atmosphere [20]. The atmosphere has no effect on the formation of H2. Golden [18] obtained much lower H2 radiolytic yield (G(H2) ¼ 0.8/100 eV) in vacuum but the absorbed dose was very high (5 MGy). Obviously the radiolytic yields of CO2, CO increase in the presence of oxygen. Classically these gases are attributed to the decomposition of oxidation groups such as acids, ester and aldehyde. Under inert atmosphere, the formation of CO is associated with the breakdown of the CeO bond.
3.6. Mass spectrometry analysis It has been demonstrated that mass spectrometry in combination with soft ionisation techniques such as electrospray, which minimizes the fragmentation and produces molecular ions, can provide information about molecular weight of the individual oligomers [28]. Fig. 4 gives the mass spectra of PTMG 2000 irradiated at different doses. For pristine sample the most intense peaks correspond to the initial mass distribution of singly-charged oligomer ions. As the dose increases new peaks and distributions are clearly observed. For the highest dose the spectra is complicated and the interpretation is difficult because new peaks interfere with the initial distribution. In this case the assignment of all the peaks was clearly impossible. Nevertheless for the moderate dose of 94 kGy
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almost all new peaks corresponding to degraded molecules were identified. Fig. 5 is a ESIeMS spectrum of pristine PTMG that shows multicharged ions. Oligomers up to the þ1 through þ4 charge state are clearly resolved and the resolution is sufficient to observe the naturally occurring isotopes of the component molecules. Finally the initial spectrum is composed of approximately 700 peaks, which is rather complicated and is due to the presence of a large distribution of oligomers. Besides for each type of oligomers, the four peaks observed, corresponding to 1, 2, 3 and 4 þ charged ions, increases also the number of peaks. The polymerisation degree of PTMG chains spreads from 2 to 80. This result is in accordance with SEC measurements. Nevertheless, we have to notice that as the molecular mass of the polymer increases, the charge number increases too and as a consequence, their ionisation becomes more difficult. As mentioned above, only the spectra corresponding to the lowest dose (94 kGy) were analysed in detail. The mass difference Dm, the number-average molecular weight Mn and the total intensity for each new distribution were calculated. The results are presented in Table 3. To identify oligomers, we first suppose that they are linear. Then 1 þ M 2 . Moreaccording to eq. (5) (see Appendix 1), Dm ¼ Mend end over we consider that the dose is low enough to neglect double 1 zM . M represents the scission of initial oligomers. Then Mend H H molar mass of hydrogen. Finally the nature of the end group from 2 can be deduced. the value of Mend For example, it is easy to determine that the distribution corresponding to Dmz60 and Dmz74 are certainly associated to the formation of the end groups eOeCH2eCH2eCH3 eteOeCH2e CH2eCH2eCH3, respectively.Concerning the series Dmz46, two structures are plausible.
HOeðCH2 CH2 CH2 CH2 OÞn eCH2 eCH3
(A)
HOeðCH2 CH2 CH2 CH2 OÞn eCHO
(B)
Formates (B) were already identified by infra-red spectroscopy. Moreover their concentration is rather high after irradiation and they are clearly the dominant oxidation products. Then we suppose that the major contribution to this distribution is due to formates. The second more intense distribution corresponding to Dmz16 could be attributed to oligomers containing either an in-chain unsaturation group eCH]CHe or a crosslinking >CHeCH<. The formation of these two groups is associated with the production of H2. Although unsaturated groups have active infra-red band, we were not able to identify these groups using FTIR spectroscopy. As a consequence, the series Dmz16 is certainly characteristic of crosslinking. The intensities of supplementary peaks are very weak. Some are also present in both pristine and irradiated samples. Among these peaks we think that Dm ¼ 34 and 50 can be attributed to oligomers containing secondary hydroxyl groups (eC(OH)He) or hemiacetal groups (eOeC(OH)He). 4. Discussion
Table 2 Radiolytic yields values of H2, CO, CO2 CH4 from irradiated PTMG. The unit is 107 mol/J. Gas
Present work (under He)
H2 CO CO2 CH4 eO2
3.4 0.110 0.03 0.0040 e
0.2 0.003 0.01 0.0005
Dannoux, 2008 (under air) 3.9 0.35 1.35 e 13.35
On the basis of these results, a mechanism accounting for the degradation of PTMG under electron irradiation and under oxygen atmosphere can be proposed (see Scheme 1).
0.1 0.05 0.25
4.1. Formation of radicals
1.35
The EPR signal of PTMG irradiated at low temperature is the superposition of a triplet and a quintet. A signal close to the triplet
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Fig. 4. Comparison of the mass spectra of PTMG 2000 irradiated at different doses (20 scans): (a) pristine PTMG, (b) 94 kGy, (c) 380 kGy.
was already observed in polyoxymethylene and attributed to the terminal radical [29].
eOeCH2
(R1)
The simulation confirms that the signal results from the interaction between an odd electron and two equivalent hydrogen atoms. It reveals also the presence of an unstructured broad signal that can be attributed to radical clusters formed during irradiation at low temperature. The large broadening of the signal is a consequence of the elevated local concentration of radicals inside the clusters. The triplet separation was estimated to be 2.06 mT. This value is larger than those mentioned previously for polyoxymethylene [30]. This discrepancy can be attributed to the difficulty encountered by the above cited authors to isolate the triplet signal from others signals radicals especially those attributed to secondary radicals. Indeed they did not use simulation to deconvolute the signals. According to simulation the five-line spectrum can be accounted for an unpaired electron coupled to three non-equivalent protons: certainly one a-proton with coupling 1.38 mT and two nonequivalent b-proton with coupling 2.24 and 3.25 mT. Then it is attributed to radical (R2).
eOeCa HeCb H2 eR
(R2)
The a-proton hyperfine constant is consistent with the ones reported in the literature [13,31]. But there is a huge difference for b-proton. This discrepancy is attributed to a difference of steric conformation. For b-methylene group, coupling constants depend mainly of the angle between the projection of the CaeR (R is a substituent) bond and the axis of the 2pZ orbital. The reported hyperfine splitting suggests that this angle is close to 80 for PEO and comprised between 15 and 30 for PTMG. This difference could be either due to the nature of substituent or to the physical state of the
material. Indeed the radicals identified in PEO were rather stable even at room temperature. PEO being a crystalline polymer, we can suppose that these long life radicals are located inside or in the vicinity of the crystallites. In our case, the crystallinity of our PTMG is low. Then we can believe that the radicals are formed in the amorphous phase. Their high reactivity is consistent with this hypothesis. Finally it is important to notice that among the potential radicals that can be formed under irradiation, secondary (R3) or primary radicals and (R4) are clearly missing.
eCH2 eC HeCH2 e
(R3)
eCH2 eCH2
(R4)
Their signals (sextet and quintet with a total hyperfine splitting approximately of 15.5 and 11 mT respectively) are not observed in the EPR signal at low temperature or after annealing. This absence is disturbing as secondary radicals are believed to be primary species. This result involves either an energy transfer mechanism or a radical transfer mechanism. Although the efficiency of the intramolecular radical transfer at low temperature is still questionable eCH2eC HeCH2e 0 eCH2eCH2eC He We propose that this mechanism could be involved in the conversion of radicals (R3) or (R4) in radical (R2). 4.2. Formation of gases Dannoux et al. analysed the formation of gases in purified and unpurified PTMG under oxidising atmosphere [20]. The radiolytic yields for the main gases are presented in Table 2. The radiolytic oxygen consumption G(eO2) of PTMG is comprised between 12 and
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Table 3 Identified distributions of pristine and 94 kGy irradiated PTMG. Mn is expressed in g/mol and the total intensity is in arbitrary unit. Pristine
94 kGy
Dm
i
Mn
Intensity
Mn
Intensity
18 18 18 18 46 16 60 74 34 34 68 84 84 50 50 50 72 40 40 36 76 32 52 64
1 2 3 4 1 1 1 1 1 2 1 1 2 1 2 3 1 1 2 1 1 1 1 1
734 1589 2433 2967
5563 4134 2450 1048
696 1318 782 ? 1290 435 466 831 552 576
228 75 141 ? 127 74 107 65 81 40
727 1462 2219 2873 641 537 508 517 419
6709 5340 4191 2037 1130 245 281 146 127
442
33
535
37
543 1226 425 405 424 418 373
95 149 69 77 65 56 31
CO can be associated to the CeO bond breakage. As formates are the dominant oxidation products CO2 is attributed to their decomposition.
4.3. Formation of oxidation products The formation of formates is clearly the major oxidation products. This compound results from the decomposition by b scission of alkoxy radical (R5). Fig. 5. Spectrum of multi-charged ions produced by ESIeMS of PTMG 2000. (a) experimental spectrum of PTMG 2000 irradiated at 94 kGy, (b) theoretical isotopic pattern of HOe(CH2eCH2eCH2eCH2eO)15eH, 2 Naþ.
14 107 mol/J. These value could be compared to the estimated radiolytic yield of formation of formates G(formates)w 7.6 107 mol/J. Roughly formates account for 25% of the oxygen consumed. These results show that PTMG is more resistant to oxidation than others aliphatic polymers such as polyethylene, polypropylene or polyethylene oxide (G(eO2)>>105 mol/J at the same dose rate). This difference may be attributed to the higher mobility of radicals in low molecular PTMG that promotes recombination. H2 can be associated to the formation of radicals as well as unsaturation. G(H2) is close to the value measured for polyethylene and significant higher to those measured for polyethylene oxide or polypropylene oxide [32]. This difference was pointed by Dole [15] and attributed to the unsaturation formation mechanism from radical cation eCH2CH2þe. This mechanism could not occur in POM and is energetically unfavourable in PPO. Finally this result confirms that the number of CH2 groups between CeO ether linkage influences directly the mechanisms of primary radical formation.
Scheme 1. Simplified degradation mechanism of PTMG.
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Rt Fig. 6. Theoretical evolutions of PTMG molecular weight distribution at selected values of the ratio G(S)/G(X).The curves are plotted for different values of s ¼ 0 ðkS þ kC Þdt comprises between 0 and 104. It assumes that scissions and crosslinkings are random. s represents the total number of chains modified either by scission or crosslinking. The unit of s is mol/g.
Radical (R5) is either due to the decomposition of a hydroperoxide or the bimolecular recombination of secondary peroxy radicals. Since hydroperoxides are relatively stable at room temperature, the second route is privileged. In order to explain the relatively high rate of formate formation we assume that the majority of peroxy radicals do not terminate by the Russel mechanism. This hypothesis is corroborated by the fact that esters and hemiacetals that are the compounds generated by the bimolecular recombination of peroxy radicals according to Russel’s mechanism, are formed in low concentration. We did not succeed in quantifying hydroperoxides. But there is no evidence of oligomers containing secondary hydroperoxide (Dm w 50) nor primary hydroperoxide (Dm w 64, 78, 92) by ESIeMS. Only the structure eOeCH2eCH2eCH2eCH2eOOH (Dm w 106 ¼ 72 þ 34) can be confused with oligomers containing one secondary alcohol group eC(OH)He or hemiacetal group eOe C(OH)He (Dm w 34). Yet the intensity of the corresponding distribution (see Table 3) did not increase after irradiation. Then we can conclude that the formation of hydroperoxide is very weak. The absence of ROOH is certainly due to the high reactivity of low molecular compounds. In our material the chain length is rather low. The majority of the chains are not embedded which induces a high mobility and non restricted movements. Then unstable groups such as peroxide can react more easily with radicals or with each other. Finally we propose that in low molecular compounds the second order decomposition of hydroperoxides is enhanced and prevents their accumulation in the polymer.
FTIR studies did not reveal the presence after irradiation of a significant amount of acids, ketones nor esters. This result is in agreement with the data published by Decker [9]. Indeed his infrared investigations of irradiated PEO revealed only a small absorption at 1755 cm1 attributed to ester groups. The formation of alcohols seems also weak. Concerning primary alcohols, only a weak distribution corresponding to Dm w 76 can be attributed to the structure eOeCH2eCH2eCH2eOH. Yet the intensities of distributions of methyl end chain oligomers (H(OeCH2eCH2eCH2eCH2)neOeCH2eCH2eCH2eCH3 and H(Oe CH2eCH2eCH2eCH2)n-OeCH2eCH2eCH3) are intense. Primary alcohols and methyl end chain are assumed to have the same primary alkyl radical as precursor. Then we conclude that the hydrogen abstraction reaction by primary radical is more rapid than the reaction with oxygen. Again this non classical result could be attributed to the mobility of low molecular compounds. Scheme 1 resumes then the major route of degradation of PTMG under oxygen. The radicals and final products which are identified are in boxes. 4.4. Evolution of molecular weight For PEO, both Decker and Zainuddin measured high radiationchemical scission yields G(S) [9,10]. Moreover G(S) is dose rate dependant. These observations indicate the occurrence of chain reactions. This conclusion did not apply for PTMG. This discrepancy is ascribed to the physical state of materials that have been studied. Decker and Zainuddin studied polymer with rather high numberaverage molecular weight (Mn ¼ 1.2 105 and 2.2 105 g/mol respectively) while our material is composed of a distribution of short chain polymers almost oligomers. As a consequence the mobility
C. Aymes-Chodur et al. / Polymer Degradation and Stability 96 (2011) 1225e1235
and the crystallinity state are completely different. As for oxygen consumption, radical recombination is more efficient in low molecular compounds. Then chain reactions are limited. The scission yield deduced from the FloryeSaïto equations is disturbing. Indeed the G(S) value is lower than the radiolytic formate formation yield (7.6 107 mol J1). The equations used to estimate G(S) and G(X) are based on the hypothesis that scission and crosslinking are random processes. A rapid examination of the evolution of the size exclusion chromatograms with the dose shows that this assumption is certainly not fulfilled. In order to analyse this observation in closer detail, we tried to compare the observed evolution of the molecular weight distribution after irradiation with the theoretical expression. To calculate the evolution of given distribution subjected to random scission process, a generalized expression of the formula deduced by Yoon et al. was used (see Appendix) [33].
½P1
l1
½Pn
l1
¼
½P1 0
l1
ns
¼ e
1 1 es þ 1 2 e2s e2s þ 1 2 1
mn0
½Pn 0
l1
þ 2sinhs
s
e
1
mn0
2ðcoshs 1Þ n
n1 X i¼1
½Pi 0
!!
mn0
1
mn0
n1 X i¼1
ðn iÞ
½Pi 0
!
l1
l1
Pi represents a polymer molecule of i repeat unit and mn0 is the number-average degree of polymerization. [Pn] is the concentration of Pn at t and [Pn]0 the concentration of Pn at t ¼ 0. R s ¼ 0t kS dt. If chain scission occurs with equal probability, the scission rate constant kS is assumed to be the same for all the scission reactions. Then the relation between kS and G(S) is:
kS ¼ 103 GðSÞ w d w and d represent the molar mass of monomer and the dose rate, respectively. P l1 ¼ N i ¼ 1 i½Pi 0 is constant and equal to the initial concentration of monomer units. The molecular weight distribution of crosslinked chains is obtained by self-convolution of the initial distribution [Pn]. First the Fourier transform of the initial weight distribution FðxÞ ¼ J½½Pn ; x was calculated then the weight distribution of the crosslinked chain is equal to the inverse of the Fourier transform of F 2 ðxÞ. To calculate the molecular weight distribution, the following differential equation was solved:
d½Pn ¼ J1 ½FðxÞ; n kC ½Pn dt kC represents the crosslinking rate constant. In Fig. 6 the theoretical evolution of an initial distribution close to those measured for PTMG at selected values of the ratio G(S)/G(X) Rt are plotted for different values of s ¼ 0 ðkS þ kC Þdt comprised 4 between 0 and 10 . s represents the total number of chains modified either by scission or crosslinking. The unit of s is mol/g. The comparison of these simulations with SEC results (see Fig. 1) confirms that the shape of the curve is very different whatever G(S)/G(X). No simulation curve shows clearly the formation of rather well defined shoulder at low or high molar mass. Finally this result confirms that scission and crosslinking do not follow a random process. Moreover experimental data behave as if only a fraction of the distribution degrades and another fraction crosslinks. We believe that short chains degrade although long chains crosslink. Can the mobility or chain embedding explain such
1233
a difference? Obviously this point needs further investigations although it is difficult to analyse. 5. Conclusion On the basis of the characterization of radicals, gases and oxidation products, we have presented a degradation mechanism for gamma-irradiated PTMG. EPR studies confirm the selective formation of radical site: above 200 K, only radicals associated with the carbon a to the CeO bond are observed. The major oxidation product is formate, which is formed by non-terminating recombination of peroxy radicals. The originality of this work is the use of ESIeMS to identify degradation products. Since the ESIeMS spectrum of pristine PTMG oligomers was rather complicated, we developed a method to analyse the peaks generated by different degrees of polymerization and charge. The analysis of low dose irradiated polymer permits to isolate the major fragments. It reveals the signatures of formate and certainly crosslinking. Theoretical calculation of the evolution of molecular mass distribution shows that degradation does not occur randomly. Then conventional theory of FloryeSaïto was found to be not adapted to determine the yields of scission and crosslinking. We point out many of differences between the results obtained with oligomers and those obtained with others polyethers such as PEO or copolymers containing polyether. These differences were mainly attributed to the physical state of material which influences the reactivity of radicals. Finally we conclude that the study of model compounds can be a powerful tool to clarify the degradation mechanism especially if mass spectrometry can be used. But kinetic and macromolecular changes can be significantly different between low molecular compounds and polymers. Acknowledgements The authors are grateful to S. Legand, D. Durand, for their contribution to gas analysis, Marie-Odile Bachelier, S. Rouif (IONISOS) and J.P. Renault for their helpful collaboration during irradiation, B. Amekraz and C. Moulin for their scientific supports and encouragements and S. Rouzeau (Viscotek) and F. Sarlin (Polymer Laboratories e Agilent) for their kind SEC supports. Appendix 1 To aid the interpretation of the spectra and the data processing a computer program using Scilab was developed and the application of this procedure was automated. It permits the deconvolution of the initial distributions observed for each charge state. Then it calculates the number and weight averages molar masses of the molecular ion distribution for each charge states. Obviously the resulting Mn and Mw are charge state dependent. The intensities of the peaks were determined by integration over the isotopic pattern. For irradiated samples, the program subtracts the peaks of the initial distribution and analyses the remained series of peaks. In the case of linear oligomers, each ion consists of several units of tetramethylene ether (molecular weight MTMG ¼ 72.10032 g 1 mol1), two end groups (molecular weights of the end groups Mend 2 and Mend ) and cationizing species (molecular weight mcat). Therefore Kni the apparent mass-to-charge m/z for the peak position of an ion with a parent molecule characterized by a polymerization degree n can be written:
i. h 1 2 i þ mcat Kni ¼ n MTMG þ Mend þ Mend i designates the number of charges of the ion.
(5)
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For pristine PTMG, the series of peaks that are the most abundant are centred at m/z ¼ 473.43 and the equidistant interval between these peaks is of 72.10 amu. Performing linear regression, the masses of the end groups plus the mass of the cation is estimated:
I0 ¼ In;0 =ð1 pÞ4n
1 2 Mend þ Mend þ mcat z40:9
The mechanism of radiolytic degradation of a polymer can be described by the following series of reaction
(6)
Finally this result indicates that this series of peaks corresponds to sodiated hydroxyl-terminated oligomers. There was no evidence found for other cation adducts or end groups. Moreover we verify that the fragmentation is very limited. The procedure to interpret the mass spectra was the following: 1- the initial distribution was subtracted. The theoretical positions were calculated using equation (5). Then the corresponding peaks were identified and deleted. 2- the most intense peak was determined. Next we searched for the monoisotopic associated to this peak. Then, the charge i and the degree of polymerisation of the ion n were deduced. The peaks of the others oligomers that belong to the same family but with different degree of polymerisation were determined and deleted. The identified distribution was defined by the parameter:
Dm ¼ Kni mcat ði nÞ MTMG The mass difference Dm, the number-average molecular weight Mn and the total intensity for each new distribution were calculated. In all cases, we considered that the associated cation is sodium. The step 2 was reproduced since the intensities of the remaining peaks were significantly higher than the noise. The application of this procedure was automated. As a consequence, a major error was made: we missed a new distribution that interferes with an original one. Fig. 4 shows the series of peaks characteristic of the following double charged ion: HOe(CH2eCH2eCH2eCH2eO)15eH, 2Naþ The comparison of the experimental spectrum with the theoretical isotopic pattern reveals that the intensity of the isotopic peak due to 2 13C is too high. In addition this deviation is only observed for even degree of polymerisation. This result discloses the existence of a series of new peaks. Precisely this new distribution is characterized byDmz46. Its intensity was evaluated by subtraction of the initial distribution using the equation (7). Concerning isotopic effects, only 13C isotope was considered. Indeed natural abundance of 17O and 18O was too small to be resolved. For a given polymerisation degree, the mass difference between the apparent mass-to-charge corresponding to oligomers with no 13 C and with one 13C was used to determine ion charge: this difference equals 1 for i ¼ 1, ½ for i ¼ 2 and so on. When n increases, the relative intensity of the 13C isotopic pattern increases. The intensity of the peak (In,m) corresponding to an oligomer of polymerisation degree n and with m 13C are given by: m In;m ¼ I0 C4n pm ð1 pÞ4nm
Appendix 2
Pn /Pnr þ Pr where Pi represents a polymer molecule of i repeat unit, k is the rate constant of all elementary reactions. Scission is supposed to randomly occur so k is assumed to be the same for all the reactions. The temporal evolution of [Pn] (the concentration of Pn at t) is given by N X d½P1 ½Pi ¼ 2k dt i¼2 N X d½Pn ½Pi ¼ ðn 1Þk½Pn þ 2k dt i ¼ nþ1
Rt
We define s ¼
0
k dt, then the preceding equations become
d½l0 ¼ ðl1 ½l0 Þ ds d½P1 ¼ 2ðl0 ½P1 Þ ds n1 X d½Pn ½Pi ¼ ðn 1Þ½Pn þ 2 l0 dt i¼1
!
PN PN where ½l0 ¼ i ¼ 1 ½Pi and l1 ¼ i ¼ 1 i½Pi . l1 is constant and equal to the initial concentration of monomer units. Laplace transformation of these equations gives
l1 ½l0 0 ~ l0 ðpÞ ¼ þ p þ 1 pðp þ 1Þ 2½l0 0 2l1 ~ ðpÞ ¼ ½P1 0 þ þ P 1 p þ 2 ðp þ 1Þðp þ 2Þ pðp þ 1Þðp þ 2Þ p þ n 2 ½Pn1 0 ½Pn 0 ~ ~ n ðpÞ ¼ P P n1 ðpÞ ðp þ n þ 1Þ pþnþ1 ½l0 0 ¼ t ¼ 0. Then
½P1
l1
½Pn
l1
¼
PN
½P1 0
l1
¼ ens
i ¼ 1 ½Pi 0 .
½Pn 0 represents the concentration of Pn at
1 1 es þ 1 2 e2s e2s þ 1 2 1
mn0
½Pn 0
l1
þ 2sinhs
(7)
p represents the natural abundance of carbon-13 (1.103%) and I0 the total intensity. For a given ion, this relation was used to calculate the number of carbons. The total intensity I0 was determined either by integration of the complete isotopic pattern or deduced from the intensity of the first peak.
(8)
es 2ðcoshs 1Þ n 1
mn0
n1 X i¼1
½Pi 0
!!
mn0
1
mn0
n1 X
ðn iÞ
i¼1
½Pi 0
!
l1
l1
with mn0 is the number-average degree of polymerization. References [1] King PA, Ward JA. Radiation chemistry of aqueous poly(ethylene oxide) solutions I. Journal of Polymer Science: Part A: Polymer Chemistry 1970;8: 253e62.
C. Aymes-Chodur et al. / Polymer Degradation and Stability 96 (2011) 1225e1235 [2] Stafford JW. The radiation induced reactions of aqueous polyethylene oxide solutions II. Intrinsic viscosity changes in the pregel region. Die Makromolekulare Chemie 1970;134:71e86. [3] Stafford JW. The radiation induced reactions of aqueous polyethylene oxide solutions III. Radiation chemistry of polyethylene oxide in tert-butyl alcohol/ water systems. Die Makromolekulare Chemie 1970;134:87e97. [4] Ward JA. Radiation chemistry of aqueous poly(ethylene oxide) solutions. II. Highly purified samples. Journal of Polymer Science: Part A: Polymer Chemistry 1971;9:3555e61. [5] Nedkov E, Tsvetkova S. Effect of gamma irradiation on the crystalline structure of ultra high molecular weight poly(ethylene oxide). Radiation Physics and Chemistry 1994;43:397e401. [6] Nedkov E, Tsvetkova S. Effect of gamma irradiation on the melting characteristics on ultra high molecular weight poly(ethylene oxide). Radiation Physics and Chemistry 1994;44:251e6. [7] Nedkov E, Tsvetkova S. Structure of poly(ethylene oxide) hydrogels obtained by gamma irradiation. Radiation Physics and Chemistry 1994;44: 257e61. [8] Zhang L, Zhang W, Zhang Z, Yu L, Zhang H, Qi Y, et al. Radiation effects on crystalline polymers- I. Gamma-radiation-induced crosslinking and structural characterization of polyethylene oxide. Radiation Physics and Chemistry 1992;40:501e5. [9] Decker C, Mayo F, Richardson H. Radiation-induced oxidation of solid poly(ethylene oxide). I. Experimental results. Journal of Polymer Science: Polymer Chemistry Edition 1977;15:781e9. [10] Zainuddin O, Albinska J, Ulanska P, Rosiak JM. Radiation-induced degradation and crosslinking of poly(ethylene oxide) in solid state. Journal of Radioanalytical and Nuclear Chemistry 2002;253:339e44. [11] Gardette JL, Lemaire J. Oxydation photothermique d’élastomères de polyuréthannes thermoplastiques, 1. Propriétés des hydroperoxydes formés. Die Makromolekulare Chemie 1981;182:2723e36. [12] Morlat S, Gardette J-L. Phototransformation of water-soluble polymers: I: photo-and thermooxidation of poly(ethylene oxide) in solid state. Polymer 2001;42:6071e9. [13] Maggi L, Segale L, Machiste EO, Faucitano A, Buttafava A, Conte U. Polymersgamma ray interaction. Effects of gamma irradiation on modified release drug delivery systems for oral administration. International Journal of Pharmaceutics 2004;269:343e51. [14] Hassouna F, Morlat-Thérias S, Mailhot G, Gardette J-L. Influence of water on the photodegradation of poly(ethylene oxide). Polymer Degradation and Stability 2007;92:2042e50. [15] Dole M. The radiation chemistry of macromolecules. New York: Academic Press; 1972. [16] Fayolle B, Verdu J, Bastard M, Piccoz D. Thermooxidative ageing of polyoxymethylene. Part 1: chemical aspects. Journal of Applied Polymer Science 2008; 107:1783e92.
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[17] Shirasaka H. Polyurethane urea elastomer having monodisperse poly(oxytetramethylene) as a soft segment with a uniform hard segment. Macromolecules 2000;33:2776e8. [18] Golden JH. Degradation of Polytetramethylene oxide. II- ionising radiation. Makromoleculare Chemie 1965;81:51. [19] Guignot C, Betz N, Legendre B, Moël AL, Yagoubi N. Degradation of segmented poly(etherurethane) Tecoflex induced by electron beam irradiation: characterization and evaluation. NIM B 2001;185:100e7. [20] Dannoux A, Esnouf S, Amekraz B, Dauvois V, Moulin C. Degradation mechanism of poly(ether-urethane) EstaneÒ induced by high energy radiation. II. Oxidation effects. Journal of Polymer Science Part B Polymer Physics A 2008;46:861e78. [21] Desmazières B, Buchmann W, Terrier P, Tortajada J. APCI Interface for LC- and SEC-MS analysis of Synthetic polymers: advantages and limits. Analytical Chemistry 2008;80:783e92. [22] Terrier P, Buchmann W, Cheguillaume G, Desmazières B, Tortajada J. Analysis of poly(oxyethylene) and poly(oxypropylene) triblock copolymers by MALDIeTOF mass spectrometry. Analytical Chemistry 2005;77:3292e300. [23] Harrison CA, Mourey TH. Number-average molecular weight and functionality of poly(tetramethylene glycol) by multidetector SEC. Journal of Applied Polymer Science 1995;56:211e20. [24] Carlsson DJ, Brousseau R, Zhang C, Wiles DM. Polyolefin oxydation: quantification of alcohol and hydroperoxide products by nitric oxide reactions. Polymer Degradation and Stability 1987;17:303e18. [25] Saito O. Statistical theories of cross-linking. New York: Academic Press; 1972. [26] Wilhelm C, Gardette J-L. Infrared analysis of the photochemical behaviour of segmented polyurethanes: aliphatic poly(ether-urethane)s. Polymer 1998;39: 5973e80. [27] Lin-Vien D, Colthup NB, Fateley WG, Grasselli JG, editors. The handbook of infrared and Raman characteristic frequencies of organic molecules. Boston: Academic Press Inc.; 1991. [28] Watanabe T, Okabayashi M, Kurokawa D, Nishimoto Y, Ozawa T, Kawasaki H, et al. Determination of primary bond scissions by mass spectrometric analysis of ultrasonic degradation products of poly(ethylene oxide-block-propylene oxide) copolymers. Journal of Mass Spectrometry 2010;45:799e805. [29] Ranby B, Rabek JF. ESR spectroscopy in polymer research. Berlin Heidelberg New York: Springer-Verlag; 1977. [30] Yoshida H, Ranby B. Electron spin resonance studies on oriented polyoxymethylene. Journal of Polymer Science: Part A 1965;3:2289e302. [31] Butyagin PY, Dubinskaya AM, Radtsing VA. Electron spin resonance spectra, conformation, and chemical properties of free radicals in solid polymers. Russian Chemical Reviews 1969;38:290. [32] Roberts GP, Budzol M, Dole M. Radiation chemistry of poly(propylene oxide). Journal of Polymer Science Part A-2 1971;9:1729e45. [33] Yoon YS, Jin HJ, Chin IJ, Kim C, Kim MN. Theoretical prediction of weight loss and molecular weight during random chain scission degradation of polymers. Polymer 1997;38:3573e9.
Contents lists available at ScienceDirect
Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Radiation effects on a linear model compound for polyethers C. Aymes-Chodur a, *, A. Dannoux b, V. Dauvois b, S. Esnouf c a
Université Paris-Sud 11, Laboratoire Matériaux et Santé - EA 401, IFR 141, Faculté de pharmacie- 5, rue J.B.Clément, Châtenay Malabry 92296, France CEA Saclay, DEN/DANS/DPC/SECR/LSRM, 91191 Gif/Yvette Cedex, France c CEA Saclay, DSM/IRAMIS/SIS2M/Laboratoire de Radiolyse, 91191 Gif/Yvette Cedex, France b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 7 February 2011 Received in revised form 18 March 2011 Accepted 13 April 2011 Available online 30 April 2011
Radiation effects on a model polyether e poly(tetramethylene) glycol (PTMG) induced by high energy radiation were investigated. To understand the degradation mechanism, electron paramagnetic resonance (EPR), Fourier transform infra-red spectroscopy (FTIR), electrospray and gas mass spectrometry (ESIeMS and gas-MS), were carried out to identify radicals and chemical modifications. Size exclusion chromatography (SEC) was used to follow the evolution of the distribution of molecular weight. On the basis of the results, a mechanism of degradation for PTMG is proposed. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: Polyether Radiolysis EPR ESIeMS MSgas SEC
1. Introduction Materials based on organic polymers are widely used in the nuclear industry (jacketing cables, seals, plastic surfaces, gloves) where they are subjected to high energy radiation. Extensive research has been reported on radiation-induced oxidative degradation of polyethylene, polypropylene, polyvinyl chloride but much little attention has been directed to oxygen containing polymers such as polyethers. Nevertheless, oxidative degradation of poly(ethylene oxide) (PEO) that has numerous applications, for example, in pharmaceutical industry for hydrogels, excipients. is one of the polyether that was widely studied. In the 60e70 s, several authors have studied the crosslinking and the gelation properties of PEO irradiated in solution [1e4]. Later on, Zhang and Nedkov studied the gamma irradiation effect on the crystalline structure of high molecular weight PEO [5e8]. Decker [9] investigated the radiation-induced oxidation of solid PEO. The principal products identified were formate and hemiformal groups, hydroperoxides, and some volatile compounds mainly formaldehyde and carbon
* Corresponding author. Tel.: þ33 1 46 83 54 55; fax: þ33 1 46 83 59 63. E-mail address: [email protected] (C. Aymes-Chodur). 0141-3910/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2011.04.015
dioxide. Since there have been relatively few recent studies on changes induced by high energy radiation in PEO. Zainuddin et al. [10] analysed changes in molecular weight distribution after irradiation. They observed that in presence of oxygen, chain scissions are dominant. The radiation yield G(S) w 2,5 106 mol/J indicates a chain reaction. Under vacuum, crosslinking and scission are in competition and crosslinking dominates at high dose. Gardette and Morlat [11,12] studied the photo and thermooxidation of PEO in solid state and highlighted the formation of formates and esters with a ratio depending of the degradation type. Maggi et al. [13] investigated the effects of gamma irradiation on release drug delivery systems containing PEO or PVA. The authors used EPR spectroscopy to identify the nature of radicals induced by exposure to gamma rays. At low temperature dominant transient species is the hydrogen abstraction radicals eCH2eCH eOe. Warming to 298 K causes the formation of a novel signal attributed to the aldehyde radical eCH eCH(]O) that is a direct evidence of chain scissions. Finally the breaking of chains explains the observed changes of the dissolution and morphological performances of PEO tablets. Hassouna [14] studied the thermo and photo-degradation of PEO in aqueous solution and showed that, depending on the pH of the medium, formic acid was released in addition with the formation of formates and esters.
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Poly(methylene oxide) (POM) and poly(propylene oxide) (PPO) were also studied. Dole presented a review for these two materials [15]. Recently Fayolle analysed the thermo-oxidation kinetics of Poly(methylene oxide) (POM) [16]. The polyether investigated in this paper is poly(tetramethylene) Glycol (PTMG) also called Poly(tetramethylene Oxide). This material was frequently used in industry as soft segment for thermoplastic or crosslinked elastomers because of its low glass transition point, good rubbery, elasticity and high strength [17]. To our knowledge only Golden published data on the chemical and physical modifications of this polyether induced by high energy electrons or gamma rays [18]. It was most studied as soft segment for polyurethane. Yet Guignot et al. [19] studied the degradation of polyetherurethane induced by electron beam irradiation. The oxidation of polyether soft segments was analysed using size exclusion chromatography, Fourier transform infra-red spectroscopy and thermogravimetric analysis. Dannoux et al. [20] studied the degradation mechanism of a segmented aromatic poly(ethereurethane). They concluded that under oxidising atmosphere predominant degradation occurred at polyether soft segments that produces stable oxidative products as formates, alcohols, carboxylic acids. Moreover crosslinking is in competition with scission. The objective of this work is to obtain supplementary information about the degradation mechanism proposed by Dannoux et al.. Then we have investigated the radiation effects in absence and in presence of oxygen on model compounds of polyether using different techniques such as EPR spectroscopy, FTIR spectroscopy, Size exclusion chromatography and gas analysis. Electrospray mass spectrometry of irradiated oligomers was also carried out. This method of ionization has been developed to make characterization of intact polymers possible [21,22]. The principle is to produce ions from neutral parent molecules to which small cations or anions are attached. In absence of fragmentation, this method can potentially determine molecular weight distribution of polymers. In our case mass spectrometry was mainly used to analyse the new oligomers induced by radiation. Yet accurate mass analysis permits to identify the nature of the end groups and then to clarify the mechanism of scission. 2. Experimental 2.1. Materials Poly(TetraMethylene) Glycol (PTMG), also called poly (tetrahydrofuran) or poly(ether glycol) are provided by Sigma Aldrich under the commercial name TerathaneÒ. Its linear formula is H(OCH2CH2CH2CH2)nOH. According to the specifications of the manufacturer, the average molecular weight of the PTMG is 2000, then the average polymerization degree n is approximately 28. Its melting point is low (between 28 and 40 C). As samples are soft at ambient temperature, we have not been able to form films but we melt PTMG in glass tubes and froze them to obtain either 2 mm thick layers or cylinders of 4 mm in diameter. FTIR analysis of pristine PTMG reveals an intense band centred at 3480 cm1 characteristic of OH groups. This result indicates that oligomers are certainly hydroxyl-terminated. 2.2. Reagents Tetrahydrofuran (THF) used for SEC analyses, Tertiobutyl formate used for FTIR measurements and methanol (HPLC grade) used for MS QTOF are provided by Sigma Aldrich.
2.3. Irradiation Gamma irradiation were performed in IONISOS (Dagneux, France) using a 60Co g-source. Samples were exposed at room temperature at a dose rate of 0.7 kGy/h in open glass tubes or under inert atmosphere (helium) in sealed glass containers. In order to identify and quantify gases produced during irradiation, polymers were also irradiated in sealed glass tubes. Chromic dosimeters were used to measure the dose. No electronic correction was made. Irradiation doses (rigorously absorbed dose by air as a reference) were 94 and 380 kGy for oxidising conditions and 64 and 104 kGy for inert atmosphere. Low temperature irradiations were performed using the electron pulses of a Titan Beta, Inc. linear accelerator (Laboratoire de Radiolyse, Saclay, France). 10 ns pulses of 10 MeV electrons were used at a repetition rate of 10 Hz. Samples were irradiated in out gassed sealed Pyrex tube hold in liquid nitrogen dewar. The dose rate was 5 Gy/pulse approximately. After irradiation, the samples were kept in liquid nitrogen. 2.4. Size exclusion chromatography (SEC) The evolution of Mn and Mw with increasing doses was determined using PTMG dissolved in THF. The solutions were analysed by a PL-GPC 220 high temperature chromatograph (Polymer Laboratories, UK), equipped with a double detection that consisted of a refractometer (DRI) (Polymer Laboratories, UK) and a 220R high temperature differential viscometer (DV) (Viscotek, UK). The chromatographic system had its temperature set at 35 C. Flow rate of THF used as mobile phase was of 1 mL/min. The SEC system was controlled by the PL-GPC 220 ControlÒ software (version 2.01). The data acquisition and analysis were set by Viscotek software (Omnisec e version 4.0). Two Polymer Laboratories (PL) columns were set up in series: a 3 mm 100 Å and a 5 mm mixed C whose molecular weights linear range stood respectively up to 4000 and between 200 and 2 106 (data given by the supplier), their granulometry was of 3 and 5 mm, their length was of 300 mm and their internal diameter of 7.5 mm. Solutions containing 10 mg mL1 of sample in THF were injected by a 200 mL injection loop. Nine polystyrene (PS) standards (Polymer Laboratories) were used for universal calibration with molecular weights varying from 580 to 91,800. Mn and Mw were calculated using the following formula:
X Wi 2 Ji m P Dv Mn ¼ P h and Mw ¼ m sp;i Wi hsp;i i Dvi Ji
(1)
where m is the mass of polymer injected, hsp,i is the specific viscosity at retention volume vi (baseline-corrected response of DV detector), Ji ð ¼ ½hi Mi Þ is the value of the universal calibration curve, Wi is the baseline-corrected DRI chromatogram height at retention volume vi and Dvi increment retention time. Normally the calculation of Mn needs only the DV chromatogram, the mass injected and a calibration curve. In our case, the DRI chromatogram was used to evaluate m. Previously the differential index of refraction dn/dc of PTMG has been calculated by measuring the peak areas as a function of the concentration. Whatever the dose, dn/dc was equal to 0.06. To estimate Mw, both DRI and DV chromatograms are needed. Yet the interdetector volume was determined using narrow standards. Table 1 presents the value of Mn and Mw estimated for pristine PTMG. Mn value is higher than the value given by the supplier. This deviation may be attributed to the calibration. Harrison [23]
C. Aymes-Chodur et al. / Polymer Degradation and Stability 96 (2011) 1225e1235 Table 1 Changes of molecular weight of irradiated PTMG in air. Dose (kGy)
Mn (g/mol)
Mw (g/mol)
Ip ¼ Mw/Mn
0 94 380
2820 2890 3400
4640 5800 1000
1.65 2.01 2.94
compared different methods to determine molecular weight of PTMG. They concluded that universal calibration is not the best method to study oligomers. Indeed DV detector is insensitive to low molecular weight species whereas DRI response is sensitive to end groups at low molecular weights. These complications can bias the values of high Mn and low Mw. 2.5. Electron paramagnetic resonance (EPR) In order to trap transitory species, the samples were irradiated under vacuum at 77 K. EPR spectra were recorded at the X band (9.4 GHz) on a Bruker ER-200D ESR spectrometer equipped with a nitrogen flow cryostat. The irradiated samples kept in liquid nitrogen were rapidly transferred in the cryostat and analysed at 120 K. The electron spin resonance parameters were determined by means of an automated simulation programs using the LevenbergeMarquardt method. The hyperfine anisotropy of the Haeproton was not considered explicitly in the simulations. Yet we verified that the hyperfine anisotropy contributes only to the broadening of the spectra. 2.6. Fourier transform infra-red spectroscopy (FTIR) FTIR spectra have been collected on a Nicolet Magna IR-750Ô Spectrometer which is equipped with a DTGS KBr detector using a diamond Attenuated Total Reflection (ATR) device. To ensure a good reproducibility, the samples were preliminary melt then deposited on the diamond crystal. The spectra were saved at 4 cm1 of resolution with 128 scans. In order to evidence the degradation products formed during irradiation, a nitric oxide gas (NO) treatment was performed but this treatment was not fruitful certainly because of the initial high alcohol concentration [24]. 2.7. Gas analysis Gas analysis was performed with a quantitative gas mass spectrometer (R30) with direct inlet developed by CEA for chemical and isotopic analysis. The ionisation is made by electron impact, mass separation is performed with a magnetic sector and ion detection is made by Faraday cup and electron multiplier. The mass range is from 1 to 200 uma and the detection limit is about 1 ppm depending on the gas matrix and mass interference. The radiolytic yield, G-value of formation or consumption of a molecular product X is given by:
½X ½X0 ½X GðXÞ ¼ or GðXÞ ¼ D D
(2)
1227
equipped with an electrospray ionisation source. The spray needle voltage was set to 2.9 kV and nitrogen operating at 350 L/h was employed as both the drying and nebulizing gas. Samples were introduced at a delivery rate of 10 mL/min using a 250 mL glass syringe with a stainless steel needle (Hamilton Co.). The source temperature was set to 80 C and the sample cone voltage to 50 V. Spectra were acquired at 5 s/scan over a mass range of m/z 50e1500 with an acquisition time of 3 min. Samples were dissolved with THF to get solutions at 5.6.106 mol L1. The software used for acquisition and treatment was Mass LynxÔ (Waters). Using standard resolution, isotopically resolved charged polymers in the mass range up to m/z ¼ 1500 are measured. To aid the interpretation of the spectra and the data processing a computer program using Scilab was developed and the application of this procedure was automated.(see Appendix 1) Some experiments using combination of ESIeMS with HPLC as a separation technique were also carried on. Nevertheless the rate of degradation and the concentrations of new products were too low to obtain accurate results. 3. Results 3.1. Molecular mass distribution Fig. 1 shows the chromatograms obtained from RI and viscometer detectors for PTMG 2000 at different irradiation doses under air atmosphere. For the non irradiated sample, the molecular masses distribution (MMD) is broad and asymmetrical in the smaller MM range side. It reveals a maximum of the intensity at about 13 min of elution and oligomers distribution from 15 to 16 min in the case of the DRI detector. Table 1 gives the calculated MM from universal calibration for the different doses. We observe clearly an increase of Mn and Mw all together with the dose. MMD gets also larger, as illustrated by the polydispersity index (Ip) values. This broadening is characterized by the appearance of two shoulders at 12.4 and 14 min. For the highest dose of 380 kGy, the broadening of the MMD still increases and the lowest MMD at 12.4 min, corresponding to a reticulation phenomenon, becomes almost predominant. 3.2. Tentative determination of scission yield G(S) and crosslinking yield G(X) G(S) and G(X) were estimated considering the following Saito’s equations [25]:
1 1 ¼ þ 103 ðGðSÞ GðXÞÞ D Mn;D Mn;0
(3)
1 1 GðSÞ ¼ þ 103 2GðXÞ D Mw;D Mw;0 2 With D, the absorbed dose (Gy), Mn,0, Mw,0 and Mn,D, Mw,D being the mean molar masses in number and weight (g mol1) initially and at the dose D respectively. Finally, under oxidising atmosphere, we estimate:
[X] and D are given in mol/kg and Gy, respectively. The unit of G is mol/J.
G(S) ¼ 0.12 107 mol J1
2.8. Mass spectrometry
G(X) ¼ 1.72 107 mol J1
The mass spectrometric measurements were recorded in cation mode using a Q-ToF II spectrometer (Micromass, Manchester, UK)
The predominant crosslinking.
degradation
mechanism
is
clearly
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Fig. 2. FTIR Spectra of pristine and irradiated PTMG 2000. Enclosed is given the areas ratio between vibration bands set at 1725 and 2850 cm1 as a function of the dose.
%. In the inset of Fig. 2, the ratio between areas measured at 1725 cm1 and 2850 cm1 as a function of the dose is plotted. For the 380 kGy irradiated PTMG, a concentration of formate of 2.9 104 mol g1 was obtained. The radiolytic formate formation yield is estimated to 7.6 107 mol J1. As it was not possible to prepare thin films with controlled thickness, FTIR was also used to check that oxidation has proceeded homogenously during irradiation. The oxidation rate quantified by the concentration of formate was measured near the surface and inside the samples. We verified that interiors of the samples were oxidised but oxidation is more pronounced near the surface (w50%). So we supposed that oxidation was certainly limited by oxygen diffusion but there was no depletion inside the samples during irradiation. 3.4. EPR analysis Fig. 1. DRI (a) and viscometry (b) chromatograms obtained from PTMG irradiated at different doses.
3.3. FTIR analysis Fig. 2 shows the superposition of FTIR spectra obtained for irradiated at 380 kGy, and pristine PTMG 2000. In the wavelength range located between 3300 and 3600 cm1, vibrations can be attributed to intermolecular OH bonds. They do not seem to evolve after irradiation. On the contrary, as the dose increases, two sharp vibration bands located at 1726 and 1160 cm1 appear. Those bands are respectively attributed to the esters elongation vibrations of C] O and CeO bonds such as formates [26,27]. Infra-red investigations made by Decker after radiation-induced oxidation of PEO are very similar. They revealed a strong absorption centred at 1725 cm1. This band was attributed to low molecular weight compounds (formaldehyde, glycol aldehyde and formate of ethylene glycol) and formate groups attached to macromolecule. The formate concentration was tentatively estimated to reach the radiolytic yield (G). Tertiobutyl formate was used as reference to estimate the extinction coefficient of the band at 1726 cm1. Solutions of this formate in THF at different concentrations were prepared, Beer Lambert’s law was valid for concentrations ranging from 1 to 10 vol
Radiation degradation of polymers involves radical intermediates that can be observed by EPR spectroscopy. Fig. 3 presents the ESR spectrum observed in PTMG irradiated at 77 K. A five-line spectrum is overlapped by a broad signal. Annealing causes a reduction of the total intensity of the ESR spectra. The subtraction of the spectra before and after annealing at 250 K reveals the disappearing of a broad three-line spectrum while the intensity of the five-line spectrum remains almost the same. Yet the spectrum can be attributed to the superposition of a broad triplet and a five-line spectrum. Simulation reveals that quintet signal corresponds to a spin S ¼ 1/2 in interaction with three non-equivalent nuclear spin I ¼ 1/ 2 with coupling of 1.38 mT, 2.24 mT and 3.25 mT. The peak-to-peak linewidth DHpp is estimated to be 1.12 mT (see Fig. 3). Concerning the broad triplet, its signal is compatible with the superposition of a triplet and a singlet. The triplet can be account for in terms of the interaction of two equivalent protons with coupling of 20.6 mT (DHpp ¼ 1.27 mT). The shape of the singlet is Lorentzian and the peak-to-peak linewidth is 5.33 mT. 3.5. Gas analysis The main gases produced by irradiation of PTMG are H2, CO, CO2 and CH4. Table 2 compares the values obtained under inert
C. Aymes-Chodur et al. / Polymer Degradation and Stability 96 (2011) 1225e1235
Fig. 3. (a) EPR spectra of irradiated PTMG at 77 K, (b) annealed at 250 K, (c) simulation of spectrum (b). (d) represents the subtraction of spectra (a) and (b), (e) simulation of spectrum (d). (f) and (g) are the two components of the calculated spectrum (e).
atmosphere with those measured under oxidizing atmosphere [20]. The atmosphere has no effect on the formation of H2. Golden [18] obtained much lower H2 radiolytic yield (G(H2) ¼ 0.8/100 eV) in vacuum but the absorbed dose was very high (5 MGy). Obviously the radiolytic yields of CO2, CO increase in the presence of oxygen. Classically these gases are attributed to the decomposition of oxidation groups such as acids, ester and aldehyde. Under inert atmosphere, the formation of CO is associated with the breakdown of the CeO bond.
3.6. Mass spectrometry analysis It has been demonstrated that mass spectrometry in combination with soft ionisation techniques such as electrospray, which minimizes the fragmentation and produces molecular ions, can provide information about molecular weight of the individual oligomers [28]. Fig. 4 gives the mass spectra of PTMG 2000 irradiated at different doses. For pristine sample the most intense peaks correspond to the initial mass distribution of singly-charged oligomer ions. As the dose increases new peaks and distributions are clearly observed. For the highest dose the spectra is complicated and the interpretation is difficult because new peaks interfere with the initial distribution. In this case the assignment of all the peaks was clearly impossible. Nevertheless for the moderate dose of 94 kGy
1229
almost all new peaks corresponding to degraded molecules were identified. Fig. 5 is a ESIeMS spectrum of pristine PTMG that shows multicharged ions. Oligomers up to the þ1 through þ4 charge state are clearly resolved and the resolution is sufficient to observe the naturally occurring isotopes of the component molecules. Finally the initial spectrum is composed of approximately 700 peaks, which is rather complicated and is due to the presence of a large distribution of oligomers. Besides for each type of oligomers, the four peaks observed, corresponding to 1, 2, 3 and 4 þ charged ions, increases also the number of peaks. The polymerisation degree of PTMG chains spreads from 2 to 80. This result is in accordance with SEC measurements. Nevertheless, we have to notice that as the molecular mass of the polymer increases, the charge number increases too and as a consequence, their ionisation becomes more difficult. As mentioned above, only the spectra corresponding to the lowest dose (94 kGy) were analysed in detail. The mass difference Dm, the number-average molecular weight Mn and the total intensity for each new distribution were calculated. The results are presented in Table 3. To identify oligomers, we first suppose that they are linear. Then 1 þ M 2 . Moreaccording to eq. (5) (see Appendix 1), Dm ¼ Mend end over we consider that the dose is low enough to neglect double 1 zM . M represents the scission of initial oligomers. Then Mend H H molar mass of hydrogen. Finally the nature of the end group from 2 can be deduced. the value of Mend For example, it is easy to determine that the distribution corresponding to Dmz60 and Dmz74 are certainly associated to the formation of the end groups eOeCH2eCH2eCH3 eteOeCH2e CH2eCH2eCH3, respectively.Concerning the series Dmz46, two structures are plausible.
HOeðCH2 CH2 CH2 CH2 OÞn eCH2 eCH3
(A)
HOeðCH2 CH2 CH2 CH2 OÞn eCHO
(B)
Formates (B) were already identified by infra-red spectroscopy. Moreover their concentration is rather high after irradiation and they are clearly the dominant oxidation products. Then we suppose that the major contribution to this distribution is due to formates. The second more intense distribution corresponding to Dmz16 could be attributed to oligomers containing either an in-chain unsaturation group eCH]CHe or a crosslinking >CHeCH<. The formation of these two groups is associated with the production of H2. Although unsaturated groups have active infra-red band, we were not able to identify these groups using FTIR spectroscopy. As a consequence, the series Dmz16 is certainly characteristic of crosslinking. The intensities of supplementary peaks are very weak. Some are also present in both pristine and irradiated samples. Among these peaks we think that Dm ¼ 34 and 50 can be attributed to oligomers containing secondary hydroxyl groups (eC(OH)He) or hemiacetal groups (eOeC(OH)He). 4. Discussion
Table 2 Radiolytic yields values of H2, CO, CO2 CH4 from irradiated PTMG. The unit is 107 mol/J. Gas
Present work (under He)
H2 CO CO2 CH4 eO2
3.4 0.110 0.03 0.0040 e
0.2 0.003 0.01 0.0005
Dannoux, 2008 (under air) 3.9 0.35 1.35 e 13.35
On the basis of these results, a mechanism accounting for the degradation of PTMG under electron irradiation and under oxygen atmosphere can be proposed (see Scheme 1).
0.1 0.05 0.25
4.1. Formation of radicals
1.35
The EPR signal of PTMG irradiated at low temperature is the superposition of a triplet and a quintet. A signal close to the triplet
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Fig. 4. Comparison of the mass spectra of PTMG 2000 irradiated at different doses (20 scans): (a) pristine PTMG, (b) 94 kGy, (c) 380 kGy.
was already observed in polyoxymethylene and attributed to the terminal radical [29].
eOeCH2
(R1)
The simulation confirms that the signal results from the interaction between an odd electron and two equivalent hydrogen atoms. It reveals also the presence of an unstructured broad signal that can be attributed to radical clusters formed during irradiation at low temperature. The large broadening of the signal is a consequence of the elevated local concentration of radicals inside the clusters. The triplet separation was estimated to be 2.06 mT. This value is larger than those mentioned previously for polyoxymethylene [30]. This discrepancy can be attributed to the difficulty encountered by the above cited authors to isolate the triplet signal from others signals radicals especially those attributed to secondary radicals. Indeed they did not use simulation to deconvolute the signals. According to simulation the five-line spectrum can be accounted for an unpaired electron coupled to three non-equivalent protons: certainly one a-proton with coupling 1.38 mT and two nonequivalent b-proton with coupling 2.24 and 3.25 mT. Then it is attributed to radical (R2).
eOeCa HeCb H2 eR
(R2)
The a-proton hyperfine constant is consistent with the ones reported in the literature [13,31]. But there is a huge difference for b-proton. This discrepancy is attributed to a difference of steric conformation. For b-methylene group, coupling constants depend mainly of the angle between the projection of the CaeR (R is a substituent) bond and the axis of the 2pZ orbital. The reported hyperfine splitting suggests that this angle is close to 80 for PEO and comprised between 15 and 30 for PTMG. This difference could be either due to the nature of substituent or to the physical state of the
material. Indeed the radicals identified in PEO were rather stable even at room temperature. PEO being a crystalline polymer, we can suppose that these long life radicals are located inside or in the vicinity of the crystallites. In our case, the crystallinity of our PTMG is low. Then we can believe that the radicals are formed in the amorphous phase. Their high reactivity is consistent with this hypothesis. Finally it is important to notice that among the potential radicals that can be formed under irradiation, secondary (R3) or primary radicals and (R4) are clearly missing.
eCH2 eC HeCH2 e
(R3)
eCH2 eCH2
(R4)
Their signals (sextet and quintet with a total hyperfine splitting approximately of 15.5 and 11 mT respectively) are not observed in the EPR signal at low temperature or after annealing. This absence is disturbing as secondary radicals are believed to be primary species. This result involves either an energy transfer mechanism or a radical transfer mechanism. Although the efficiency of the intramolecular radical transfer at low temperature is still questionable eCH2eC HeCH2e 0 eCH2eCH2eC He We propose that this mechanism could be involved in the conversion of radicals (R3) or (R4) in radical (R2). 4.2. Formation of gases Dannoux et al. analysed the formation of gases in purified and unpurified PTMG under oxidising atmosphere [20]. The radiolytic yields for the main gases are presented in Table 2. The radiolytic oxygen consumption G(eO2) of PTMG is comprised between 12 and
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1231
Table 3 Identified distributions of pristine and 94 kGy irradiated PTMG. Mn is expressed in g/mol and the total intensity is in arbitrary unit. Pristine
94 kGy
Dm
i
Mn
Intensity
Mn
Intensity
18 18 18 18 46 16 60 74 34 34 68 84 84 50 50 50 72 40 40 36 76 32 52 64
1 2 3 4 1 1 1 1 1 2 1 1 2 1 2 3 1 1 2 1 1 1 1 1
734 1589 2433 2967
5563 4134 2450 1048
696 1318 782 ? 1290 435 466 831 552 576
228 75 141 ? 127 74 107 65 81 40
727 1462 2219 2873 641 537 508 517 419
6709 5340 4191 2037 1130 245 281 146 127
442
33
535
37
543 1226 425 405 424 418 373
95 149 69 77 65 56 31
CO can be associated to the CeO bond breakage. As formates are the dominant oxidation products CO2 is attributed to their decomposition.
4.3. Formation of oxidation products The formation of formates is clearly the major oxidation products. This compound results from the decomposition by b scission of alkoxy radical (R5). Fig. 5. Spectrum of multi-charged ions produced by ESIeMS of PTMG 2000. (a) experimental spectrum of PTMG 2000 irradiated at 94 kGy, (b) theoretical isotopic pattern of HOe(CH2eCH2eCH2eCH2eO)15eH, 2 Naþ.
14 107 mol/J. These value could be compared to the estimated radiolytic yield of formation of formates G(formates)w 7.6 107 mol/J. Roughly formates account for 25% of the oxygen consumed. These results show that PTMG is more resistant to oxidation than others aliphatic polymers such as polyethylene, polypropylene or polyethylene oxide (G(eO2)>>105 mol/J at the same dose rate). This difference may be attributed to the higher mobility of radicals in low molecular PTMG that promotes recombination. H2 can be associated to the formation of radicals as well as unsaturation. G(H2) is close to the value measured for polyethylene and significant higher to those measured for polyethylene oxide or polypropylene oxide [32]. This difference was pointed by Dole [15] and attributed to the unsaturation formation mechanism from radical cation eCH2CH2þe. This mechanism could not occur in POM and is energetically unfavourable in PPO. Finally this result confirms that the number of CH2 groups between CeO ether linkage influences directly the mechanisms of primary radical formation.
Scheme 1. Simplified degradation mechanism of PTMG.
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Rt Fig. 6. Theoretical evolutions of PTMG molecular weight distribution at selected values of the ratio G(S)/G(X).The curves are plotted for different values of s ¼ 0 ðkS þ kC Þdt comprises between 0 and 104. It assumes that scissions and crosslinkings are random. s represents the total number of chains modified either by scission or crosslinking. The unit of s is mol/g.
Radical (R5) is either due to the decomposition of a hydroperoxide or the bimolecular recombination of secondary peroxy radicals. Since hydroperoxides are relatively stable at room temperature, the second route is privileged. In order to explain the relatively high rate of formate formation we assume that the majority of peroxy radicals do not terminate by the Russel mechanism. This hypothesis is corroborated by the fact that esters and hemiacetals that are the compounds generated by the bimolecular recombination of peroxy radicals according to Russel’s mechanism, are formed in low concentration. We did not succeed in quantifying hydroperoxides. But there is no evidence of oligomers containing secondary hydroperoxide (Dm w 50) nor primary hydroperoxide (Dm w 64, 78, 92) by ESIeMS. Only the structure eOeCH2eCH2eCH2eCH2eOOH (Dm w 106 ¼ 72 þ 34) can be confused with oligomers containing one secondary alcohol group eC(OH)He or hemiacetal group eOe C(OH)He (Dm w 34). Yet the intensity of the corresponding distribution (see Table 3) did not increase after irradiation. Then we can conclude that the formation of hydroperoxide is very weak. The absence of ROOH is certainly due to the high reactivity of low molecular compounds. In our material the chain length is rather low. The majority of the chains are not embedded which induces a high mobility and non restricted movements. Then unstable groups such as peroxide can react more easily with radicals or with each other. Finally we propose that in low molecular compounds the second order decomposition of hydroperoxides is enhanced and prevents their accumulation in the polymer.
FTIR studies did not reveal the presence after irradiation of a significant amount of acids, ketones nor esters. This result is in agreement with the data published by Decker [9]. Indeed his infrared investigations of irradiated PEO revealed only a small absorption at 1755 cm1 attributed to ester groups. The formation of alcohols seems also weak. Concerning primary alcohols, only a weak distribution corresponding to Dm w 76 can be attributed to the structure eOeCH2eCH2eCH2eOH. Yet the intensities of distributions of methyl end chain oligomers (H(OeCH2eCH2eCH2eCH2)neOeCH2eCH2eCH2eCH3 and H(Oe CH2eCH2eCH2eCH2)n-OeCH2eCH2eCH3) are intense. Primary alcohols and methyl end chain are assumed to have the same primary alkyl radical as precursor. Then we conclude that the hydrogen abstraction reaction by primary radical is more rapid than the reaction with oxygen. Again this non classical result could be attributed to the mobility of low molecular compounds. Scheme 1 resumes then the major route of degradation of PTMG under oxygen. The radicals and final products which are identified are in boxes. 4.4. Evolution of molecular weight For PEO, both Decker and Zainuddin measured high radiationchemical scission yields G(S) [9,10]. Moreover G(S) is dose rate dependant. These observations indicate the occurrence of chain reactions. This conclusion did not apply for PTMG. This discrepancy is ascribed to the physical state of materials that have been studied. Decker and Zainuddin studied polymer with rather high numberaverage molecular weight (Mn ¼ 1.2 105 and 2.2 105 g/mol respectively) while our material is composed of a distribution of short chain polymers almost oligomers. As a consequence the mobility
C. Aymes-Chodur et al. / Polymer Degradation and Stability 96 (2011) 1225e1235
and the crystallinity state are completely different. As for oxygen consumption, radical recombination is more efficient in low molecular compounds. Then chain reactions are limited. The scission yield deduced from the FloryeSaïto equations is disturbing. Indeed the G(S) value is lower than the radiolytic formate formation yield (7.6 107 mol J1). The equations used to estimate G(S) and G(X) are based on the hypothesis that scission and crosslinking are random processes. A rapid examination of the evolution of the size exclusion chromatograms with the dose shows that this assumption is certainly not fulfilled. In order to analyse this observation in closer detail, we tried to compare the observed evolution of the molecular weight distribution after irradiation with the theoretical expression. To calculate the evolution of given distribution subjected to random scission process, a generalized expression of the formula deduced by Yoon et al. was used (see Appendix) [33].
½P1
l1
½Pn
l1
¼
½P1 0
l1
ns
¼ e
1 1 es þ 1 2 e2s e2s þ 1 2 1
mn0
½Pn 0
l1
þ 2sinhs
s
e
1
mn0
2ðcoshs 1Þ n
n1 X i¼1
½Pi 0
!!
mn0
1
mn0
n1 X i¼1
ðn iÞ
½Pi 0
!
l1
l1
Pi represents a polymer molecule of i repeat unit and mn0 is the number-average degree of polymerization. [Pn] is the concentration of Pn at t and [Pn]0 the concentration of Pn at t ¼ 0. R s ¼ 0t kS dt. If chain scission occurs with equal probability, the scission rate constant kS is assumed to be the same for all the scission reactions. Then the relation between kS and G(S) is:
kS ¼ 103 GðSÞ w d w and d represent the molar mass of monomer and the dose rate, respectively. P l1 ¼ N i ¼ 1 i½Pi 0 is constant and equal to the initial concentration of monomer units. The molecular weight distribution of crosslinked chains is obtained by self-convolution of the initial distribution [Pn]. First the Fourier transform of the initial weight distribution FðxÞ ¼ J½½Pn ; x was calculated then the weight distribution of the crosslinked chain is equal to the inverse of the Fourier transform of F 2 ðxÞ. To calculate the molecular weight distribution, the following differential equation was solved:
d½Pn ¼ J1 ½FðxÞ; n kC ½Pn dt kC represents the crosslinking rate constant. In Fig. 6 the theoretical evolution of an initial distribution close to those measured for PTMG at selected values of the ratio G(S)/G(X) Rt are plotted for different values of s ¼ 0 ðkS þ kC Þdt comprised 4 between 0 and 10 . s represents the total number of chains modified either by scission or crosslinking. The unit of s is mol/g. The comparison of these simulations with SEC results (see Fig. 1) confirms that the shape of the curve is very different whatever G(S)/G(X). No simulation curve shows clearly the formation of rather well defined shoulder at low or high molar mass. Finally this result confirms that scission and crosslinking do not follow a random process. Moreover experimental data behave as if only a fraction of the distribution degrades and another fraction crosslinks. We believe that short chains degrade although long chains crosslink. Can the mobility or chain embedding explain such
1233
a difference? Obviously this point needs further investigations although it is difficult to analyse. 5. Conclusion On the basis of the characterization of radicals, gases and oxidation products, we have presented a degradation mechanism for gamma-irradiated PTMG. EPR studies confirm the selective formation of radical site: above 200 K, only radicals associated with the carbon a to the CeO bond are observed. The major oxidation product is formate, which is formed by non-terminating recombination of peroxy radicals. The originality of this work is the use of ESIeMS to identify degradation products. Since the ESIeMS spectrum of pristine PTMG oligomers was rather complicated, we developed a method to analyse the peaks generated by different degrees of polymerization and charge. The analysis of low dose irradiated polymer permits to isolate the major fragments. It reveals the signatures of formate and certainly crosslinking. Theoretical calculation of the evolution of molecular mass distribution shows that degradation does not occur randomly. Then conventional theory of FloryeSaïto was found to be not adapted to determine the yields of scission and crosslinking. We point out many of differences between the results obtained with oligomers and those obtained with others polyethers such as PEO or copolymers containing polyether. These differences were mainly attributed to the physical state of material which influences the reactivity of radicals. Finally we conclude that the study of model compounds can be a powerful tool to clarify the degradation mechanism especially if mass spectrometry can be used. But kinetic and macromolecular changes can be significantly different between low molecular compounds and polymers. Acknowledgements The authors are grateful to S. Legand, D. Durand, for their contribution to gas analysis, Marie-Odile Bachelier, S. Rouif (IONISOS) and J.P. Renault for their helpful collaboration during irradiation, B. Amekraz and C. Moulin for their scientific supports and encouragements and S. Rouzeau (Viscotek) and F. Sarlin (Polymer Laboratories e Agilent) for their kind SEC supports. Appendix 1 To aid the interpretation of the spectra and the data processing a computer program using Scilab was developed and the application of this procedure was automated. It permits the deconvolution of the initial distributions observed for each charge state. Then it calculates the number and weight averages molar masses of the molecular ion distribution for each charge states. Obviously the resulting Mn and Mw are charge state dependent. The intensities of the peaks were determined by integration over the isotopic pattern. For irradiated samples, the program subtracts the peaks of the initial distribution and analyses the remained series of peaks. In the case of linear oligomers, each ion consists of several units of tetramethylene ether (molecular weight MTMG ¼ 72.10032 g 1 mol1), two end groups (molecular weights of the end groups Mend 2 and Mend ) and cationizing species (molecular weight mcat). Therefore Kni the apparent mass-to-charge m/z for the peak position of an ion with a parent molecule characterized by a polymerization degree n can be written:
i. h 1 2 i þ mcat Kni ¼ n MTMG þ Mend þ Mend i designates the number of charges of the ion.
(5)
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For pristine PTMG, the series of peaks that are the most abundant are centred at m/z ¼ 473.43 and the equidistant interval between these peaks is of 72.10 amu. Performing linear regression, the masses of the end groups plus the mass of the cation is estimated:
I0 ¼ In;0 =ð1 pÞ4n
1 2 Mend þ Mend þ mcat z40:9
The mechanism of radiolytic degradation of a polymer can be described by the following series of reaction
(6)
Finally this result indicates that this series of peaks corresponds to sodiated hydroxyl-terminated oligomers. There was no evidence found for other cation adducts or end groups. Moreover we verify that the fragmentation is very limited. The procedure to interpret the mass spectra was the following: 1- the initial distribution was subtracted. The theoretical positions were calculated using equation (5). Then the corresponding peaks were identified and deleted. 2- the most intense peak was determined. Next we searched for the monoisotopic associated to this peak. Then, the charge i and the degree of polymerisation of the ion n were deduced. The peaks of the others oligomers that belong to the same family but with different degree of polymerisation were determined and deleted. The identified distribution was defined by the parameter:
Dm ¼ Kni mcat ði nÞ MTMG The mass difference Dm, the number-average molecular weight Mn and the total intensity for each new distribution were calculated. In all cases, we considered that the associated cation is sodium. The step 2 was reproduced since the intensities of the remaining peaks were significantly higher than the noise. The application of this procedure was automated. As a consequence, a major error was made: we missed a new distribution that interferes with an original one. Fig. 4 shows the series of peaks characteristic of the following double charged ion: HOe(CH2eCH2eCH2eCH2eO)15eH, 2Naþ The comparison of the experimental spectrum with the theoretical isotopic pattern reveals that the intensity of the isotopic peak due to 2 13C is too high. In addition this deviation is only observed for even degree of polymerisation. This result discloses the existence of a series of new peaks. Precisely this new distribution is characterized byDmz46. Its intensity was evaluated by subtraction of the initial distribution using the equation (7). Concerning isotopic effects, only 13C isotope was considered. Indeed natural abundance of 17O and 18O was too small to be resolved. For a given polymerisation degree, the mass difference between the apparent mass-to-charge corresponding to oligomers with no 13 C and with one 13C was used to determine ion charge: this difference equals 1 for i ¼ 1, ½ for i ¼ 2 and so on. When n increases, the relative intensity of the 13C isotopic pattern increases. The intensity of the peak (In,m) corresponding to an oligomer of polymerisation degree n and with m 13C are given by: m In;m ¼ I0 C4n pm ð1 pÞ4nm
Appendix 2
Pn /Pnr þ Pr where Pi represents a polymer molecule of i repeat unit, k is the rate constant of all elementary reactions. Scission is supposed to randomly occur so k is assumed to be the same for all the reactions. The temporal evolution of [Pn] (the concentration of Pn at t) is given by N X d½P1 ½Pi ¼ 2k dt i¼2 N X d½Pn ½Pi ¼ ðn 1Þk½Pn þ 2k dt i ¼ nþ1
Rt
We define s ¼
0
k dt, then the preceding equations become
d½l0 ¼ ðl1 ½l0 Þ ds d½P1 ¼ 2ðl0 ½P1 Þ ds n1 X d½Pn ½Pi ¼ ðn 1Þ½Pn þ 2 l0 dt i¼1
!
PN PN where ½l0 ¼ i ¼ 1 ½Pi and l1 ¼ i ¼ 1 i½Pi . l1 is constant and equal to the initial concentration of monomer units. Laplace transformation of these equations gives
l1 ½l0 0 ~ l0 ðpÞ ¼ þ p þ 1 pðp þ 1Þ 2½l0 0 2l1 ~ ðpÞ ¼ ½P1 0 þ þ P 1 p þ 2 ðp þ 1Þðp þ 2Þ pðp þ 1Þðp þ 2Þ p þ n 2 ½Pn1 0 ½Pn 0 ~ ~ n ðpÞ ¼ P P n1 ðpÞ ðp þ n þ 1Þ pþnþ1 ½l0 0 ¼ t ¼ 0. Then
½P1
l1
½Pn
l1
¼
PN
½P1 0
l1
¼ ens
i ¼ 1 ½Pi 0 .
½Pn 0 represents the concentration of Pn at
1 1 es þ 1 2 e2s e2s þ 1 2 1
mn0
½Pn 0
l1
þ 2sinhs
(7)
p represents the natural abundance of carbon-13 (1.103%) and I0 the total intensity. For a given ion, this relation was used to calculate the number of carbons. The total intensity I0 was determined either by integration of the complete isotopic pattern or deduced from the intensity of the first peak.
(8)
es 2ðcoshs 1Þ n 1
mn0
n1 X i¼1
½Pi 0
!!
mn0
1
mn0
n1 X
ðn iÞ
i¼1
½Pi 0
!
l1
l1
with mn0 is the number-average degree of polymerization. References [1] King PA, Ward JA. Radiation chemistry of aqueous poly(ethylene oxide) solutions I. Journal of Polymer Science: Part A: Polymer Chemistry 1970;8: 253e62.
C. Aymes-Chodur et al. / Polymer Degradation and Stability 96 (2011) 1225e1235 [2] Stafford JW. The radiation induced reactions of aqueous polyethylene oxide solutions II. Intrinsic viscosity changes in the pregel region. Die Makromolekulare Chemie 1970;134:71e86. [3] Stafford JW. The radiation induced reactions of aqueous polyethylene oxide solutions III. Radiation chemistry of polyethylene oxide in tert-butyl alcohol/ water systems. Die Makromolekulare Chemie 1970;134:87e97. [4] Ward JA. Radiation chemistry of aqueous poly(ethylene oxide) solutions. II. Highly purified samples. Journal of Polymer Science: Part A: Polymer Chemistry 1971;9:3555e61. [5] Nedkov E, Tsvetkova S. Effect of gamma irradiation on the crystalline structure of ultra high molecular weight poly(ethylene oxide). Radiation Physics and Chemistry 1994;43:397e401. [6] Nedkov E, Tsvetkova S. Effect of gamma irradiation on the melting characteristics on ultra high molecular weight poly(ethylene oxide). Radiation Physics and Chemistry 1994;44:251e6. [7] Nedkov E, Tsvetkova S. Structure of poly(ethylene oxide) hydrogels obtained by gamma irradiation. Radiation Physics and Chemistry 1994;44: 257e61. [8] Zhang L, Zhang W, Zhang Z, Yu L, Zhang H, Qi Y, et al. Radiation effects on crystalline polymers- I. Gamma-radiation-induced crosslinking and structural characterization of polyethylene oxide. Radiation Physics and Chemistry 1992;40:501e5. [9] Decker C, Mayo F, Richardson H. Radiation-induced oxidation of solid poly(ethylene oxide). I. Experimental results. Journal of Polymer Science: Polymer Chemistry Edition 1977;15:781e9. [10] Zainuddin O, Albinska J, Ulanska P, Rosiak JM. Radiation-induced degradation and crosslinking of poly(ethylene oxide) in solid state. Journal of Radioanalytical and Nuclear Chemistry 2002;253:339e44. [11] Gardette JL, Lemaire J. Oxydation photothermique d’élastomères de polyuréthannes thermoplastiques, 1. Propriétés des hydroperoxydes formés. Die Makromolekulare Chemie 1981;182:2723e36. [12] Morlat S, Gardette J-L. Phototransformation of water-soluble polymers: I: photo-and thermooxidation of poly(ethylene oxide) in solid state. Polymer 2001;42:6071e9. [13] Maggi L, Segale L, Machiste EO, Faucitano A, Buttafava A, Conte U. Polymersgamma ray interaction. Effects of gamma irradiation on modified release drug delivery systems for oral administration. International Journal of Pharmaceutics 2004;269:343e51. [14] Hassouna F, Morlat-Thérias S, Mailhot G, Gardette J-L. Influence of water on the photodegradation of poly(ethylene oxide). Polymer Degradation and Stability 2007;92:2042e50. [15] Dole M. The radiation chemistry of macromolecules. New York: Academic Press; 1972. [16] Fayolle B, Verdu J, Bastard M, Piccoz D. Thermooxidative ageing of polyoxymethylene. Part 1: chemical aspects. Journal of Applied Polymer Science 2008; 107:1783e92.
1235
[17] Shirasaka H. Polyurethane urea elastomer having monodisperse poly(oxytetramethylene) as a soft segment with a uniform hard segment. Macromolecules 2000;33:2776e8. [18] Golden JH. Degradation of Polytetramethylene oxide. II- ionising radiation. Makromoleculare Chemie 1965;81:51. [19] Guignot C, Betz N, Legendre B, Moël AL, Yagoubi N. Degradation of segmented poly(etherurethane) Tecoflex induced by electron beam irradiation: characterization and evaluation. NIM B 2001;185:100e7. [20] Dannoux A, Esnouf S, Amekraz B, Dauvois V, Moulin C. Degradation mechanism of poly(ether-urethane) EstaneÒ induced by high energy radiation. II. Oxidation effects. Journal of Polymer Science Part B Polymer Physics A 2008;46:861e78. [21] Desmazières B, Buchmann W, Terrier P, Tortajada J. APCI Interface for LC- and SEC-MS analysis of Synthetic polymers: advantages and limits. Analytical Chemistry 2008;80:783e92. [22] Terrier P, Buchmann W, Cheguillaume G, Desmazières B, Tortajada J. Analysis of poly(oxyethylene) and poly(oxypropylene) triblock copolymers by MALDIeTOF mass spectrometry. Analytical Chemistry 2005;77:3292e300. [23] Harrison CA, Mourey TH. Number-average molecular weight and functionality of poly(tetramethylene glycol) by multidetector SEC. Journal of Applied Polymer Science 1995;56:211e20. [24] Carlsson DJ, Brousseau R, Zhang C, Wiles DM. Polyolefin oxydation: quantification of alcohol and hydroperoxide products by nitric oxide reactions. Polymer Degradation and Stability 1987;17:303e18. [25] Saito O. Statistical theories of cross-linking. New York: Academic Press; 1972. [26] Wilhelm C, Gardette J-L. Infrared analysis of the photochemical behaviour of segmented polyurethanes: aliphatic poly(ether-urethane)s. Polymer 1998;39: 5973e80. [27] Lin-Vien D, Colthup NB, Fateley WG, Grasselli JG, editors. The handbook of infrared and Raman characteristic frequencies of organic molecules. Boston: Academic Press Inc.; 1991. [28] Watanabe T, Okabayashi M, Kurokawa D, Nishimoto Y, Ozawa T, Kawasaki H, et al. Determination of primary bond scissions by mass spectrometric analysis of ultrasonic degradation products of poly(ethylene oxide-block-propylene oxide) copolymers. Journal of Mass Spectrometry 2010;45:799e805. [29] Ranby B, Rabek JF. ESR spectroscopy in polymer research. Berlin Heidelberg New York: Springer-Verlag; 1977. [30] Yoshida H, Ranby B. Electron spin resonance studies on oriented polyoxymethylene. Journal of Polymer Science: Part A 1965;3:2289e302. [31] Butyagin PY, Dubinskaya AM, Radtsing VA. Electron spin resonance spectra, conformation, and chemical properties of free radicals in solid polymers. Russian Chemical Reviews 1969;38:290. [32] Roberts GP, Budzol M, Dole M. Radiation chemistry of poly(propylene oxide). Journal of Polymer Science Part A-2 1971;9:1729e45. [33] Yoon YS, Jin HJ, Chin IJ, Kim C, Kim MN. Theoretical prediction of weight loss and molecular weight during random chain scission degradation of polymers. Polymer 1997;38:3573e9.