Chapter 5
Radiolytic Formation and Decay of transVinylene Unsaturation in Polyethylene 1
2
Downloaded by NORTH CAROLINA STATE UNIV on October 29, 2012 | http://pubs.acs.org Publication Date: April 13, 1993 | doi: 10.1021/bk-1993-0527.ch005
B. J. Lyons and W. C. Johnson 1
22 Hallmark Circle, Menlo Park, CA 94025-6683 (retired) Raychem Corporation, 300 Constitution Drive, Menlo Park, CA 94025
2
This paper is the first of several in which relations, derived from various mechanisms of growth and decay of trans-vinylene unsaturation, are compared with a selection of experimental measurements, some already published and here re-examined and some newly made. Understanding the reaction mechanism and kinetics of, and the possibility of interfering reactions with, the growth and decay of fra/is-vinylene unsaturation in polyethylene exposed to ionizing radiation is of interest for both practical and theoretical reasons. For practical reasons because, if the dependence on dose can be accurately predicted, rraiw-vinylene concentration measurements can be used as a method of dosimetry. Theoretically because, although there is general agreement on the transvinylene formation process, there are several suggested mechanisms of decay and no clear indication why any particular mechanism should be preferred. One of us (WCJ) discovered some years ago that the variation of fra/tf-vinylene unsaturation with dose could be represented very accurately in the dose range up to about 1000 kilograys by an empirical equation, binomial in form. This paper is the first in a series in which we will examine the calibration and use of this relation in dosimetry, derive this binomial as a special case of a polynomial relation from consideration of scavenging of free radicals by radiolytically formed mz/w-vinylene groups, compare the binomial and polynomial relations with previously published and new measurements and comment generally on the use of measurements of fra/w-vinylene unsaturation concentration in radiation dosimetry. A number of investigations of the growth of fra/w-vinylene unsaturation have been carried out over the years {Dole, Milner and Williams (7), Lyons and Crook (2) and others summarized in (2)}. There seems to be little if any disagreement as to the formation reaction but widely different opinions as to the decay reaction. These authors agree that the formation reaction is a primary process possibly involving ionization followed by the detachment of a molecule of hydrogen in a one step process.
0097-6156/93/0527-0062$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.
5. LYONS & JOHNSON
\r*ns-Vinylene Unsaturation in Polyethylene
63
Downloaded by NORTH CAROLINA STATE UNIV on October 29, 2012 | http://pubs.acs.org Publication Date: April 13, 1993 | doi: 10.1021/bk-1993-0527.ch005
We concur, noting however that there may also be a contribution, most likely amounting to 5% or less, from the recombination of adjacent alkyl free radicals on the same polymer chain. This contribution is more important in the crystalline phase and therefore larger in high density polyethylenes. Thus, this contribution may largely account for the difference in the G(rraAW-vinylene) (G(V1)) values noted between high (HDPE) and low (LDPE) density polyethylenes. As mentioned above, the growth of fra/w-vinylene unsaturation in low density polyethylene over a dose range up to about 1000 kGy can be fitted accurately to a binomial of the form: 2
Ar + D = B[V1] + C[V1]
0)
where A, Β and C are proportionality constants, D allows for an initial non-zero rra/w-vinylene molar concentration ([Vl] ) at zero dose, and [VI] and [Vl]oo represent molar fra/w-vinylene concentrations after r kilograys and at infinite dose. As the relations derived in references (i) and (2) can be reduced to this simple equation for values of [VI] less than, say, 0.3[Vl]oo, that is doses up to approximately 500 kGy, measurements in this range do not in themselves enable us to discriminate between the various mechanisms which have been suggested. 0
However, in the course of deriving a theoretical basis for this empirical equation one of us (BJL) noted that expansion of the published relations gave polynomials of quite different character to each other. Thus comparison of such expansions with the experimental results over a much wider dose range might serve to distinguish between them. Theory Dole and others considered the fra/w-vinylene decay reaction to involve excitation of the fra/w-vinylene group followed by further reaction with a neighboring polymer chain to form a crosslink. This analysis leads to the relation (7): Πdt
= k l - k"[Vl]
(2)
where k' and k" are the reaction constants for formation and decay of trans-vinylene groups and I is the radiation intensity. This equation, after integration and insertion of limits, becomes: m
[VIL
' - [Vi]
=
i - e-"
