Radiation Effects on Polymers - American Chemical Society

Chapter 5

Pulse Radiolysis of Doped Polyethylene in Molten State Ortwin Brede

Downloaded by COLUMBIA UNIV on August 9, 2012 | http://pubs.acs.org Publication Date: November 12, 1991 | doi: 10.1021/bk-1991-0475.ch005

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

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

5.

BREDE

73 Pulse Radiolysis of Doped Polyethylene in Molten State

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 mm . 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 /

H2 + - C = C - C "

(5)

In the presence of a scavenger as, e.g., diphenylamine the PE alkyl radicals (per haps also the other primary radicals) react under generation of diphenylaminyl ra dicals. This happens in the millisecond time scale and can be derivedfromthe time-resolved part of theright-handtimeprofile of Figure 5 taken near the aminyl 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. In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.

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 distin guished 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-ΟΗ + PE^

-Φ-Ο- + H + PE > Φ-Ο * + PE

(7) (8)

As additional species phenolate anions were found in consequence of the dissocia tive 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)



5.

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Pulse Radiolysis of Doped Polyethylene in Molten State 79

Figure 6. Transient absorption spectra of PE samples containing 0.2 (o - imme diately 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 In Radiation Effects on Polymers; Clough, R., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1991.


80

RADIATION EFFECTS O N 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. (Adaptedfromref. 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 takenfromtransient 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' all transients were found to be generated within the electron pulse and no time-resolved formation could be observed. At a styrene concentration < 10" mol dm only the benzyl type 3

2

-3


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). (12a) [PE" + S T ] (12b) + ST

PE+ST

* FE-ST'

ST + ST

• ST-ST-

· PE-ST-ST-

(13)

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). II.

C-C

I +

+ ST

*ST

I I

+

+-C = Cι

(15) 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.


82

RADIATION EFFECTS ON POLYMERS

migration

(X'OU


{c-c

Radiation Effects on Polymers - American Chemical Society

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