Irradiation of Polymeric Materials - ACS Publications - American

Chapter 4

Downloaded by UNIV OF GUELPH LIBRARY on October 29, 2012 | http://pubs.acs.org Publication Date: April 13, 1993 | doi: 10.1021/bk-1993-0527.ch004

Temperature Effects on the Radiation Degradation of Poly(isobutene) and Poly(α-methylstyrene) David J. T. Hill, T. T. Le, James H. O'Donnell, M. C. Senake Perera, and Peter J. Pomery Polymer Materials and Radiation Group, Department of Chemistry, The University of Queensland, Brisbane, Queensland 4072, Australia The effect of temperature on the radiation chemistry of poly(isobutene) and poly(α-methyl styrene) has been studied over the temperature range at which the polymers begin to degrade thermally. The radical intermediates have been investigated using ESR spectroscopy and their chemical yields and reactivities have been examined using thermal annealing techniques. The low molecular weight products of radiolysis at 300 Κ have been investigated and their yields measured. Hydrogen was the major low molecular weight product of radiolysis together with monomer, but no significant depropagation of the chain radical was found to occur at 300 K. Molecular weight studies at 300 Κ showed that the polymers undergo predominantly chain scission at room temperature, and the yields of scission and crosslinking have been determined at this temperature. At higher temperatures both polymers were found to undergo radiation induced depolymerization, and in these temperature regions, the values of G(S) and G(—M) have been determined. High energy radiation causes chemical and physical changes in polymers due to the formation of reactive intermediate species such as radicals, ions and excited states. Small volatile molecules may also be formed as the result of the cleavage of polymer side chains. For example, hydrogen and low molecular weight olefins are formed on the radiolysis of polyethylene . Side chain cleavage can also result in the formation of chain radicals which may undergo chain crosslinking reactions, as occurs in polyethylene and poly met hylacrylate . Main chain scission may also occur, either as a direct consequence of the energy absorption or as a subsequent step in a sequence of reactions involving the loss of the side chain, as in poly(methyl methacrylate) . The chain scission and crosslinking reactions have obvious consequences for the molecular weight changes in the polymer. Scission would be expected to predominate over crosslinking in polymers which contain a tetrasubstituted carbon atom in each monomer unit in the chain, such as in poly(methyl methacrylate) for example. In this polymer it has been suggested that the polymer main chain may undergo /J-scission following the removal of the ester side chain, with concomitant 1

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0097-6156/93/0527-0050$06.00/0 © 1993 American Chemical Society

In Irradiation of Polymeric Materials; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

4. HILL ET AL.

Temperature Effects on Radiation Degradation

51

4

formation of a side chain scission radical . In cases where this chain scission mechanism occurs, the general sequences of reactions would be: Ri CH — i

Ri —

2

CH2

—i

Ri —

—

*

CH2

i.2

À2

—i

Ri —

CH2

— i — -f

R2*

i.2


J β-scission Ri

Ri

- C H - < ^ + CH = i 2

2

(1)

L The process shown in reaction scheme (1) results in the formation of a radical which is identical to the chain propagation radical for vinyl polymerization. Because free radical vinyl polymerizations are reversible, spontaneous depolymerization of this chain radical may take place if the prevailing conditions of temperature and pressure favour the reaction. In this paper some aspects of the effects of temperature on the radiation chemistry of poly(isobutene), PIB, and poly(a-methyl styrene), PaMS, under vacuum will be discussed, with particular attention being directed to the radiation induced depolymerization of these materials at elevated temperatures. Experimental Polyisobutene was obtained from Aldrich Chemical Co and was found to have an M = 4.33 χ 10 . The polymer was purified by reprecipitation from chloroform/met hanol. Poly (α-methyl styrene) was prepared by cationic polymerization using BF3-ether in dichloromethane as the initiator. The polymer was reprecipitated using dichloromethane/methanol, and found to have M = 1.44 χ 105. Irradiations were performed using a Co gamma source with a dose rate of 3-5 kGy hr . The studies were carried out in sealed tubes under vacuum at temperatures where depolymerization does not occur, and under vacuum with continuous pumping where depolymerization does take place. ESR studies were performed using a Bruker ER-200D X-Band spectrometer fitted with a variable temperature cavity. During the annealing experiments, samples were allowed to equilibrate at each temperature for ten minutes before the radical spectra were obtained. Gas analyses were performed as described previously using a Hewlett Packard 5730A Chromatograph fitted with a chromosorb column. Molecular weight measurements were made using a Waters Associates Chromatograph fitted with five ultrastyragel columns (10 , 10 , ΙΟ , 10 , 10 A) and a refractive index detector. Samples for GPC analyses were not exposed to air before any radicals present were allowed to decay by thermal annealing. 5

w

w

60

_1

e

s

4

3

2

Results and Discussion For any vinyl polymerization of the type shown in reaction (2) below, the free energy change can be determined from the enthalpy and entropy changes for the polymerization reaction.


52

IRRADIATION OF POLYMERIC MATERIALS

(2)

Free radical vinyl polymerizations characteristically have a large negative entropy change associated with the chain propagation reaction. Thus, as the temperaturerises,the propagation process becomes less thermodynamically favourable, and at a certain critical temperature called the ceiling temperature, T , the free energy change will be zero . Above the ceiling temperature, depolymerization is the thermodynamically favoured reaction. For a given monomer, the ceiling temperature will depend on the polymerization conditions, such as the polymer concentration and pressure. The ceiling temperatures for some well—known vinyl polymerizations are given in Table 1, together with the glass transition temperatures for the polymers. The data in Table 1 provide information about the relative propensity for the various polymers to undergo depolymerization. For example, while PIB has a much lower glass transition temperature than PaMS, PaMS has a lower ceiling temperature, and thus will undergo depolymerization at temperatures at which PIB remains stable, even though the PIB chains have greater mobility. At very low pressures of monomer, the ceiling temperatures would be lower than those given in Table 1. 5


c

Table 1. The ceiling temperature, T , and glass transition temperatures, T , for representative vinyl polymerizations c

g

Monomer

T^C

a>T /oC c

isobutene methacrylonitrile methyl methacrylate α methyl styrene

-70 108 114 180

175 145 198 66

The ceiling temperatures are for polymerization of pure liquid monomer at 1 atmosphere pressure. It is important to note that for depolymerization to occur, the chain-end propagation radical must be present. Thus polymers may be stable above their ceiling temperature when no propagation radicals are present. Indeed, the thermal cleavage of main-chain bonds in carbon based polymers to form propagation radicals typically occurs at temperatures in the range 200-300°C., which is considerably greater than T for many polymers. Because high energy radiation can result in the cleavage of main—chain bonds and result in the formation of propagation radicals, high energy radiation can induce the depolymerization reaction at temperatures at which a polymer may be thermally stable. c

Low Temperature Studies. Investigations of the radiolysis of polymers at 77 Κ allows some of the important chemical intermediates to be identified and subsequent thermal annealing of these radicals allows their reactivities to be ascertained. ESR studies of the radiolysis of ΡΙΒ*·* and PaMS at 77 Κ have been reported and the radical intermediates corresponding to radicals I and Π were found to be present in both polymers. 8


4. HILL ET AL.

Temperature Effects on Radiation Degradation •CH

- CH 2

CH

2

53

3

1

A-

-CH-A

A I

II


In addition to these two radicals, the radical anion III and the cyclohexadienyl radical IV was found in PaMS. CH -CH -i

CH

3

-CH -

-

2

3

2

H

ΠΙ

IV

It is notable that there was no evidence for the formation of any methyl radicals in these polymers at 77 Κ, which suggests that there is little if any cleavage of the methyl side—chains in the polymers. In addition, there was no evidence for the formation of a significant proportion of propagation radicals at 77 Κ in either of these polymers. The total radical yields (G-values) at 77 Κ are 2.1 for PIB and 0.105 for P a M S . The lower G—value for PaMS is a consequence of the protective influence of the aromatic group. Annealing of PIB after irradiation at 77 Κ results in a slow decay of the radicals over the temperature range 77 — 180 K, as shown in Figure 1. Both of the radicals present were found to decay simultaneously , because the profile of the spectrum did not change over this region. However, as the glass transition temperature is approached, the rate of decay of both radicals increases rapidly, and at 300 Κ the radical concentration has dropped to zero. There was no evidence for the formation of propagation radicals during the annealing process. Annealing of PaMS after irradiation at 77 Κ results in the decay of the anion radicals in the temperature range 100 — 200 Κ and of the cyclohexadienyl radicals in the region 200 - 300 Κ, with a consequential decrease in the total radical concentration, as shown in Figure 2. Annealing of the irradiated polymer to 300 Κ on irradiation of the polymer at this temperature yields an 7

8

7

8

CH

3

V In Irradiation of Polymeric Materials; Reichmanis, E., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 1993.

54

IRRADIATION OF POLYMERIC MATERIALS


τ

1

Γ

0 «—ι ι ι ' ' ' ι l_i ι I 120 140 160 180 200 220 240 260 280 300

Temperature

Figure 1.

(K)

The variation in the radical yield with temperature for PIB after irradiation at 77 Κ. 16

*0

100

150

200

250

300

350

400

Temp / Κ

Figure 2.

The variation in the radical yield with temperature for PaMS after irradiation at 77 Κ.


4. HILL ET AL.

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Temperature Effects on Radiation Degradation

ESR spectrum with clear evidence for the presence of a significant proportion (~ 10%) of propagation radicals , V, as shown in Figure 3. Annealing to above 330 Κ results in a rapid decrease in the concentration of radicals, which falls to zero at approximately 400 K, as demonstrated in Figure 2. The G—value for radical formation for irradiation at 300 Κ has been reported to be 0.05 , which is approximately half of that for irradiation at 77 K, and is close to the value expected on the basis of the annealing studies summarized in Figure 2. The low molecular weight, volatile products of the radiolysis of PIB and PaMS at 300 Κ are summarized in Table 2. In both polymers, the major gaseous product is hydrogen, which suggests that scission of carbon—hydrogen bonds is a major degradation pathway. This observation is consistent with the observed radicals being carbon centred radicals formed by the loss of hydrogen. In PaMS some hydrogen atoms are scavenged by the aromatic ring to form cyclohexadienyl radicals. The yield of methane isrelatively small for both polymers, and no benzene was found on radiolysis of PaMS, which indicates that little side-chain cleavage occurs in these systems. Monomer was also formed on radiolysis of both polymers, but the observed yields of monomer are relatively small and not consistent with depolymerization playing a significant role in the radiation chemistry of the polymers at 300 K. 8


8

Table 2. G values of low molecular weight volatile products formed on radiolysis of PIB and PaMS at 300 Κ

a b

Product

PIB a

PaMS b

hydrogen methane ethane monomer

1.68 0.67 0.08 0.62

0.063 0.003 —

0.30

From reference 7 From reference 8

In polymers which do not undergo depolymerization, the G—values for scission and crosslinking can be determined from molecular weight studies in which the number average (M ) and weight average (M ) molecular weights are measured over a range of absorbed doses. The relationships which describe the dose dependence of the two molecular weight averages are : n

w

11

( à > = (à„)o +

1 0 3 7 x

= ( M > +

1 0

' l

Irradiation of Polymeric Materials - ACS Publications - American

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