[acs symposium series] radiation effects on polymers volume 475 || pulse radiolysis of doped...

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Chapter 5 Pulse Radiolysis of Doped Polyethylene in Molten State Ortwin Brede Central Institute of Isotope and Radiation Research, Permoserstrasse 15, O-7050, Leipzig, Germany Pulse radiolysis experiments at 393 Κ with transparent, molten, but form-stable polyethylene samples enable the study of transient pro- cesses in PE under kinetically homogeneous conditions. As PE spe- cies cations of the olefin type and radicals were observed. Scavenger radicals were formed by PE exciton trapping and reactions of mole- cular alkyl radicals. It is reported on some applied PE pulse radiolysis studies which gave information for the antioxidants chemistry and some PE modification processes. The time-resolved study of transient processes in polymers is of interest in radiation chemistry but also more generally in physical and polymer chemistry. Such prosesses driven mainly by radical species are responsible, e.g., for the radiation-induced crosslinking and grafting, for the polymer ageing during manufacture and in the subsequent use, for the selection of stabilizers etc. Pulse radiolysis as the time-resolved technique of radiation chemistry enables the direct observation of reactive species as solvated or trapped electrons, ions, electronically excited states and radicals. In comparison to laserflashphotolysis, pulse radiolysis has the advantage of the energy absorption proportionally to the electron increments of the sample, i.e., of transient generation within the matrix. Hence, reactions of transients within the polymer and with added scanvengers can be analyzed. For the study of polyethylene (PE) two main difficulties exist that are caused by the semicristalUnity of the polymer at room temperature: PE is non-transparent and represents a non- homogeneous systemfromthe kinetical point of view. There- fore, until now PE as original material has been studied in pulse radiolysis with optical detection only in case of thin foils (1). To overcome the mentioned limiting factors we started with PE pulse radiolysis in molten state which will be briefly reported in this paper. 0097-6156/91/0475-0072506.00/0 © 1991 American Chemical Society Downloaded by COLUMBIA UNIV on August 9, 2012 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch005 In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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Page 1: [ACS Symposium Series] Radiation Effects on Polymers Volume 475 || Pulse Radiolysis of Doped Polyethylene in Molten State

Chapter 5

Pulse Radiolysis of Doped Polyethylene in Molten State

Ortwin Brede

Central Institute of Isotope and Radiation Research, Permoserstrasse 15, O-7050, Leipzig, Germany

Pulse radiolysis experiments at 393 Κ with transparent, molten, but form-stable polyethylene samples enable the study of transient pro­cesses in PE under kinetically homogeneous conditions. As PE spe­cies cations of the olefin type and radicals were observed. Scavenger radicals were formed by PE exciton trapping and reactions of mole­cular alkyl radicals. It is reported on some applied PE pulse radiolysis studies which gave information for the antioxidants chemistry and some PE modification processes.

The time-resolved study of transient processes in polymers is of interest in radiation chemistry but also more generally in physical and polymer chemistry. Such prosesses driven mainly by radical species are responsible, e.g., for the radiation-induced crosslinking and grafting, for the polymer ageing during manufacture and in the subsequent use, for the selection of stabilizers etc.

Pulse radiolysis as the time-resolved technique of radiation chemistry enables the direct observation of reactive species as solvated or trapped electrons, ions, electronically excited states and radicals. In comparison to laser flash photolysis, pulse radiolysis has the advantage of the energy absorption proportionally to the electron increments of the sample, i.e., of transient generation within the matrix. Hence, reactions of transients within the polymer and with added scanvengers can be analyzed.

For the study of polyethylene (PE) two main difficulties exist that are caused by the semicristalUnity of the polymer at room temperature: PE is non-transparent and represents a non- homogeneous system from the kinetical point of view. There­fore, until now PE as original material has been studied in pulse radiolysis with optical detection only in case of thin foils (1). To overcome the mentioned limiting factors we started with PE pulse radiolysis in molten state which will be briefly reported in this paper.

0097-6156/91/0475-0072506.00/0 © 1991 American Chemical Society

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In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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5. B R E D E Pulse Radiolysis of Doped Polyethylene in Molten State 73

Experimental

The experiments were performed at 393 Κ with transparent, molten, but higly viscous and form-stable polyethylene samples. Additives were admixed by heat-rolling at about 410 Κ or diffusion into the sample in the case of liquids. The samples were prepared by cutting of 4 mm thick PE plates to pieces of the dimensions of 4 χ 10 χ 20 mm3. As polymer matrices different PE types were used. Most of the experiments reported here were made with the Leuna LDPE A121FA having a cristallinity of 45 per cent (4).

The pulse radiolysis experiments were performed mainly with 40 ns pulses of 1 MeV electrons of an ELIT type accelerator (dose per pulse between 100 and 200 Gy) (2). In the/is-time scale some experiments were undertaken with the 3 MeV LINAC in Budapest (2.5 /<s pulses with a similar dose) (3).The detection system consisted of a boosted xenon lamp, a 1Ρ 28 photomultiplier, a grating monochro-mator and a 500 MHz real time oscilloscope. Details of the equipment are given elsewhere (2).

Species in Polyethylene

In the PE radiolysis as reactive transients hydrogen atoms, alkyl radicals, cations, trapped electrons and vibronically excited molecules have to be taken into account. But only a part of these species is expected to be observable under the conditions of pulse radiolysis of the molten polymer.

Figure 1 shows optical transient absorption spectra taken in pulse radiolysis of molten pure PE. The spectra were measured point by point in different runs and a relatively good reproducibility was reached. There is a broad spectral tail coming from the UV range decaying to the VIS and continued in the NIR (not demonstrated here). From the kinetics analyzed in representative spectral points (insets in Fig­ure 1) two general transient types could be distinguised - a short-living one (ri/2=500 ns) absorbing mainly in the VIS part and a long-living species (τι/2 £ 10 μ$) absorbing mainly in the UV.

Experiments with PE samples containing typical cation scavengers as aliphatic amines and ethers gave arguments for the interpretation of the short-living species as PE cations and the long-living transients as PE radicals of different structures (4). Figure 2 shows a typical result of such a cation scavenger action: in the presence of an oligomeric glycol ether the cation lifetime was reduced, the radical kinetics was not influenced and the radical yield dropped down.

It is reasonable that because of the non-uniformity of the polymer we can define only structure types of the observed species. In analogy to low-molecular olefin radical cations that have absorption bands at the UV border and in the red spectral range (5) the PE cations are interpreted to be of the olefin type, i.e., localized at a double bond.

As probable generation channels the charge transfer from the PE parent cations to alkene groups contained in the polymer (1) and the fragmentation of the molecular cations yielding olefin cations and molecular by-products (2) have to be taken into account.

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74 RADIATION EFFECTS ON POLYMERS

0.15

Ο

0.10Y

005

300 WO 500 λ in ml 600

Figure 1. Transient spectra in pure PE taken immediately after a 40 ns pulse (various points and the curve set are due to different runs) and after 3 /is (m). Insets show time profiles at λ = 330 and 500 nm for the different runs. (Adapted fromref.4)

. ι ι i l . CnH2n + 2 + C = C • CnH2n+2 + C -CT

I I . I I C n H 2 i i + 2 + - PE-C-C+ + H 2 ; CH4 ; C2H6 etc.

I I

(1)

(2)

Then, the mentioned scavenger reaction with an ether or an amine should be a deprotonation of the PE cation.

C * - C + + R 2 O -I I

R 2 0 H + + C = C - C -I I I

(3)

Radical Formation fay Exciton Trapping

The depletion of PE radical yield in presence of added substances (cf. Figure 2) is accompanied by the formation of scavenger radicals taking place in times much shorter than our best time resolution of about 10 ns. Tins is demonstrated on an

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5. BREDE Puke Radiolysis of Doped Polyethylene in Molten State 75

0.20

0.15

0.10

005

PE/[<H2-CH2-01n

0.1 Q5wt-%

300 AOO 500 XCnmJ 600 Figure 2. Optical absorption spectra in pure PE (non-marked lines) and samples containing 0.1 ( · , ο ,χ ) and 0.5 ( A , Λ ) per cent oligomer glycol ether taken immediately after the pulse (upper set) and after 3 /is (lower curves). Insets show time profiles at 330 and 400 nm (PE left-hand side). (Adapted from ref. 4)

example in that the optical absorption spectrum of the scavenger radicals lyes within our spectral observation range. In samples doped with benzophenone the well-known and very pronounced benzophenone ketyl radical spectrum is observed (Figure 3) also at very low additive concentrations 10"3 mol dm"3). This fast formation of radicals within the PE matrix cannot be explained by a molecular process. In accordance with a hypothesis developed by Partridge (6) for explaining results of matrix isolation studies the described phenomenon will be interpreted by trapping of strongly-coupled singlet excitons. These excitons should be very mobile energy packets moving through the polymer chain with a frequency higher than that of the nuclear vibration rate. The decay by trapping either within the macromolecule (at internal irregularities) or by impurities or additives. Some considered conditions of exciton trapping according to the Partridge model are skeched in Figure 4.

As alrealy mentioned, the exciton trapping by scavengers takes place in com­petition to the internal trapping inside of the macromolecule as formulated in (4).

PE ΡΕ* + H

S * + H + ΡΕ

(4a)

(4b)

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76 RADIATION EFFECTS ON POLYMERS

PE/BP 0.6

0.5 \ 4%I 4%i

0Λ j 1 1 1ps

ance

1 1 1ps

^0.3 \ ^ 0.2 - 1 1 1 ι

Ips I 0.1

1 - ι

1 1 ι Ips I

I I ι I 300 U00 500 hCnm] 600

Figure 3. Transient spectrum in PE doped with 0.2 wt.-% benzophenone taken immediately after the 40 ns pulse ( ± ). (o) = PE background absorptioa Insets show time profiles at 330 and 520 nm (PE pure left-hand curves). (Adapted from ref.4)

The external exciton scavenging seems to be a general phenomenon in all polyolefins and has been studied in a lot of cases (4,7-9). But in contrast to the interpretation of Partridge (6) as products only radicals were observed.

The ratio between internal and external trapping can be changed by introdu­cing additional internal traps in the PE erther by use of different PE types or by variation of the quality of the polymer, e.g., by oxidation of the PE. Hence, in respect to its number of internal exciton traps it is posible to characterize the quality of a PE by the determination of the yield of the scavenger radicals using, e.g., benzophenone in form of its ketyl radical as monitor (10).

Reactions of Massive PE Radicals

The PE radicals represented in Figure 1 by the long-living spectral tail include very different structures as, e.g., alkyl radicals in different positions, hetero-atom centered radicals etc.. Nevertheless, taking them formally as a unity it is possible to describe their kinetic fate.

In pure PE the radicals decay with a rate of 3 χ 104 s-1yielding a very long-lasting product observable by a residual optical absorption. This canbe seen by the left-hand time profile given in Figure 5. This effect is interpreted as a conversion of PE alkyl radicals into allylic ones (5) known to proceed at very low temperatures in the time scale of seconds. Because of our pulse radiolyses results obtained in molten PE the process has been formulated in one step, but may happen also in a more-step sequence for that arguments were given by Dole (11).

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5. BREDE Pulse Radiolysis of Doped Polyethylene in Molten State 77

Exciton migration in PE (R. H. Patridge 1970}

CH exc. (fractionation)

CC exc. (migration)

transfer to other chain

short branches

reaction with

(CH2)n

exciton scavenger

hetero group

trans vinytidene

less efficient traps

dissociation i> ionization

excitation

internal exciton traps

CH3 terminal group

Figure 4. Qualitative considerations on the PE consistency connected with the exitation transfer efficiency (modes of exciton trapping).

P E s 1 I I c - c - c I I I

— > H2 + -C=C-C" (5)

In the presence of a scavenger as, e.g., diphenylamine the PE alkyl radicals (per­haps also the other primary radicals) react under generation of diphenylaminyl ra­dicals. This happens in the millisecond time scale and can be derived from the time-resolved part of the right-hand time profile of Figure 5 taken near the ami-nyl absorption maximum (4).

ΡΕ* + Φ2ΝΙ^ Φ2Ν* + PE (6)

Similar effects are observable also using other types of scavengers (9,12).

Sterically Hindered Phenols as Scavengers

The observation of transients in molten PE can serve as a basis for analyzing the elementary reactions of antioxidant action within the polymer matrix. This was performed with a series of sterically hindered phenols (9). As an example the case of bis-(2-hydoxy-3-tert.-butyl-5-methylpenyl)sulfide will be elucidated.

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78 RADIATION EFFECTS ON POLYMERS

L 1ms

pure PE PB/1 wt-'ÂDPA

Figure 5. Time profiles taken in pure PE at A = 320nm (on the left) and ΡΕ containing 1 wt-% diphenylamine at A = 350 nm (on the right hand) withes-pulse radiolysis.

Ionizing irradiation generates in this sulfur-bridged phenol three different species which could be identified by comparative experiments in liquid alkane solutions (13). Figure 6 gives the transient absorption spectum of such a phenol / PE sample taken immediately after a 40 ns electron pulse. Three absorption maxima can be distin­guished identified as follows: - Amax = 340 nm, phenoxyl type radical (Φ-Ο '), - Amax = 390 nm, phenolate anion (Φ-Ο *") and - Amax = 480 nm, sulfidyl radical ( -S *-).

The phenoxyls are formed by different reaction channels as, e.g., PE exciton trapping (7) and reaction of massive PE radicals (8) that are analogeous to reactions (4b) and (6).

Φ-ΟΗ + ΡΕ* -Φ-Ο- + H + PE (7) Φ-ΟΗ + PE^ > Φ-Ο * + PE (8)

As additional species phenolate anions were found in consequence of the dissocia­tive electron attachment reaction (9).

Φ-ΟΗ + etr" * Φ-Ο ' +H (9)

Electrons are not directly observed in our experiments with molten PE. But they exist, certainly, in the picosecond time range and can be scavenged by additives forming anions. The neutralization of Φ-Ο" (see time profiles in Figure 6) delivers a further part of phenoxyls and also S-centered radicals (formed only by this path)

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5. BREDE Pulse Radiolysis of Doped Polyethylene in Molten State 79

Figure 6. Transient absorption spectra of PE samples containing 0.2 (o - imme­diately after pulse) and 0.5 wt-% of the S-phenol (o - immediately, A -after 3 /is, Ο -difference between ο and A ). Insets show time profiles for the 0.5 wt-% containing sample). (Adapted from ref. 9)

which can be explained by the mesomery of the phenolate anion as formulated in (10).

These sketched experiments should demonstrate the power of pulse radiolysis in kinetic studies in the antioxidant chemistry. Further details are given in (14).

PE Doped with Aromatic Olefins

Connected with considerations on the radiation-induced grafting of monomers onto PE (15) a pulse radiolysis study on PE samples doped with aromatic olefins was made (16). Figure 7 shows transient spectra of PE samples containing styrene (ST). The

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80 RADIATION EFFECTS ON POLYMERS

300 400 , , % 500 600

Figure 7. Transient absorption spectra taken from PE samples after lying 31 h in ST (part A) and 26 h in a 1:10 (v:v) mixture of ST and CCU (part B), resp. ( ο , · ) immediately after the 40 ns pulse, (-) non-marked curve after 3/is, (o) difference spectrum between ( α ) and ( - ), ( · ) anion spike decaying within 200 ns, ( Δ ) cationic part, ( χ ) cation spike taken up to 1 /*s. Insets show time profiles in PE / ST. (Adapted from ref. 16)

manifold of species could be cleared up by using carbon tetrachloride as additional scavenger and by comparing the results with those of liquid state experiments (17):

- Amax = 320 nm, benzyl type radicals (ST '), - Amax = 350 and 460 nm, dimer styrene radical cations and - Amax = 400 nm styrene radical anions.

It was found that carbon tetrachloride scavenges the electrons as the precursors of styrene anions and reduces the yield of benzyl radicals. In Figure 7, looking on the difference spectra taken from transient absorptions at different time delay and analyzing the transient kinetics the mentioned identification of the species could be further verified.

Also at a very low styrene concentration of about 0.05 mol dm'3 all transients were found to be generated within the electron pulse and no time-resolved formation could be observed. At a styrene concentration < 10"2 mol dm-3 only the benzyl type

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5. BREDE Pulse Radiolysis of Doped Polyethylene in Molten State 81

radicals could be found. This suggests that also these radicals are generated by the exciton trapping process as elucidated above.

(lia)

[PE+ST] (lib)

The radical pair formed in reaction (lib) can recombine efficiently (12a) or can be separated by diffusion (12b) where the latter one is very restricted within the polymer. Therefore, only a small amount of free PE' can survive being able to initiate a grafting in form of longer styrene chains (13).

[PE" + ST] (12a)

(12b)

+ ST P E + S T * FE-ST' · PE-ST-ST- (13)

ST + ST • ST-ST- ect. (14)

The main part of all the radicals underlyes deactivation according to (12a) or can initiate styrene homopolymerization (14) which was also found to dominate by steady-state experiments (15).

The experimentally supported considerations show that exciton scavenging in PE is a very efficient process. But because of the competition of internal and external trapping of excitons (11a, b) with increasing styrene concentration the PE radical formation via (11a) will be suppressed and, therefore, grafting onto the matrix becomes inefficient.

From the ionic part of the styrene transients information on the reactivity of the ionic precursors in the PE matrix can be obtained. In distinction to the liquid state, styrene cations were formed only at relatively high concentrations that speaks for a fast fragmentation of the PE parent ions as already formulated for by reaction (1). As found in liquid state experiments for alkene cations (5) also the PE olefin type cations undergo only a relatively slow charge transfer to styrene (15).

I I . I C - C + + ST *ST + +-C = C- (15) I I ι v '

In molten PE the trapped electrons have a lifetime much shorter than or electron pulse. But with high styrene concentrations they are scavenged, here as styrene anions.

etr" + ST * ST" (16)

In a similar way as for grafting the process of the oligobutadiene-sensitized PE crosslinking has been analyzed (18). As intermediates radicals and cations were found.

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82 RADIATION EFFECTS ON POLYMERS

migration (X'OU

{c-c<) pr l- )c-è; ;/?'./?' ' ()c-o-) • '

neutralization I J

happing (Κ)

•C-OH -C-0' -C t I I

yis-ronge

- o c - ε ­ι ι ι

7

5·'. ;c<

ns-range

Pi-S'

Figure 8. Radiation-induced primary processes in molten PE doped with a scavenger S. The processes proceed within the picosecond time range with the exception of the marked areas.

Conclusions

The pulse radiolysis experiments with molten PE samples enabled the observation of a lot of species and processes. Figure 8 gives a survey on the radiation-induced transient processes in the system PE / scavenger. Generally, PE excitons are precur­sors of the most part of matrix radicals as well as of the scavenger ones. As cations PE olefin ions were observed being products of the parent ion fragmentation. By electron scavenging and charge transfer product anions and cations are formed. Scavenger radicals are generated via exciton trapping (fast process) and reaction of massive PE radicals (ms time scale).

The observation of the mentioned species marked in Figure 8 by the dashed line areas may be of interest from the point of basic research (7) and also under more applied aspects as, e.g., the analysis of processes of polymer ageing, the description of modes of stabilizer action (14) and the better understanding of distinct processes of polymer modification

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5. B R E D E Pulse Radiolysis of Doped Polyethylene in Molten State 83

Acknowledgments. The author gratefully acknowledges financial support of the LEUNA WERKE-AG (Germany) and scientific consultation by Drs. L. Stephan andT.Taplick.

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(3) Hargittai, P.; Stenger, V.; Földiak, G.; ZfI-Mitteilungen (Leipzig) 1984, vol 98, p 514

(4) Brede, O.; Hermann, R.; Wojnarovits, L.; Stephan, L.; Taplick, T.; Radiat. Phys. Chem. 1989, vol 34, p 403

(5) Mehnert, R; Brede, O.; Cserep, Gy.; Radiat. Phys. Chem. 1985, vol 26, p 353 (6) Patridge, R. H.; J. Chem. Phys. 1970, vol 52, pp 2485, 2491, 2501 (7) Brede, O.; Hermann, R.; Helmstreit, W.; Taplick, T.; Stephan, L.; Makromol.

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