Muonium as a Hydrogen-like Probe to Study Monomer Initiation

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M u o n i u m as a H y d r o g e n - l i k e P r o b e to S t u d y M o n o m e r Initiation K i n e t i c s 1

2

J. M. STADLBAUER, Β. W. NG, Y. C. JEAN, Y. ITO, and D. C. WALKER

Downloaded by CORNELL UNIV on August 17, 2016 | http://pubs.acs.org Publication Date: April 19, 1983 | doi: 10.1021/bk-1983-0212.ch004

University of British Columbia, Department of Chemistry and TRIUMF, Vancouver, B.C. V6T 1Y6 Canada The radioactive muonium atom(Mu) has the same ioni zation potential and Bohr radius as the hydrogen atom, but only 1/9 the mass. At the end of its 2.2µsec intrinsic life-time the positive muon (which acts as the Mu nucleus) decays into an ener getic positron. This decay can be observed using fast single particle counting methods. Because of this ease of detection and its hydrogen-like pro perties, Mu makes an excellent probe to directly study hydrogen atom reactions: for example, hydro gen atom initiation of monomer polymerization. We have examined the addition reaction kinetics of Mu with acrylamide, acrylic acid, acrylonitrile, methylmethacrylate and styrene, a l l in aqueous solution. Their second order rate constants were found to be, respectively, 1.9, 1.6, 1.1, 1.0 and 0.11 x 10 M S . As proof that Mu does add across the vinyl bond we have observed the muonium containing free radicals in the pure liquid monomers and obtained their hyperfine coupling constants. 10

-1

-1

First of a l l what is muonium, what is its source, how do we observe i t , and why is it useful? Muonium (Mu) is an atom comprised of a positive muon nucleus, (u ), and a bound elec tron. This bound electron can have its spin parallel or antiparallel to the muon nuclear spin resulting in 'triplet' and 'singlet' muonium atoms, respectively. The atom has a mass 1/9 that of the hydrogen atom, H; but because the reduced masses are +

1

Current address: University of Missouri-Kansas City, Department of Physics, Kan sas City, MO 64110. 2

Current address: University of Tokyo, Research Center for Nuclear Science and Technology, Tokyo, Japan. 0097-6156/83/0212-0035$06.00/0 © 1983 American Chemical Society Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

36

INITIATION

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n e a r l y the same, Mu has e s s e n t i a l l y the same Bohr radius and i o n i z a t i o n p o t e n t i a l as hydrogen. Muonium i s formed when a p o s i t i v e muon thermalizes i n a target and p i c k s up an e l e c t r o n from the stopping medium i n t o a bound s t a t e . Muons, both high energy (28 MeV) and low energy (4.1 MeV), are the product of p o s i t i v e pion decay, i n f l i g h t o r at r e s t , r e s p e c t i v e l y . π+

»μ+ + ν . μ The pion i t s e l f i s g e n e r a l l y produced


9

Be

+ lp

> 10

B e

+ ir+.

(1 through the r e a c t i o n (2

Intense high energy proton beams r e q u i r e d f o r these s t u d i e s are c u r r e n t l y a v a i l a b l e a t TRIUMF (Vancouver, Canada), LAMPF. (Los Alamos, U.S.A.), SIN ( V i l l i g e n , S w i t z e r l a n d ) , and KEK (Tsukuba, Japan). Under a transverse magnetic f i e l d the f r e e muon and other diamagnetic muonic species precess with a Larmor frequency of 0.0136 MHz/G, while t r i p l e t muonium precesses a t 1.39 MHz/G. The f a c t o r of 102 between these p r e c e s s i o n frequencies makes i t easy to d i s t i n g u i s h between the species present. The i n t r i n s i c l i f e time of the muon, 2.2 χ 10~^s, allows enough time to study the chemical r e a c t i o n s of muonium i n many chemical environments (Γ-4). When the muon decays i t e j e c t s a p o s i t r o n and two n e u t r i n o s . The p o s i t r o n s , with a maximum energy of 52 MeV, are e a s i l y detectable using nuclear physics s i n g l e - p a r t i c l e f a s t counting systems. The experimental data are tabulated as time histograms that are then computer analyzed. A d e t a i l e d d i s c r i p t i o n of the MSR (Muonium Spin Rotation) method i s a v a i l a b l e elsewhere ( 3 ) . Muonium has been observed i n pure hydrocarbons ( 5 ) , a l c o h o l s (6,7), and water (4). Because Mu r e a c t s slowly with these pure l i q u i d s , g i v i n g observable r e a c t i o n l i f e t i m e s of Mu up to 4us, they can be used as solvents to study various s o l u t e s of i n t e r e s t . As the f r e e t r i p l e t Mu atom r e a c t s with the s o l u t e i t s observed p r e c e s s i o n frequency i s damped and a decay constant, λ , can be obtained. The concentration dependence of the decay con stant provides second order chemical r a t e constants f o r Mu a d d i t i o n , a b s t r a c t i o n , s p i n conversion, and o x i d a t i o n - r e d u c t i o n r e a c t i o n s . When analogous hydrogen atom r a t e constants are a v a i l a b l e the k i n e t i c isotope e f f e c t can a l s o be c a l c u l a t e d . Muonium i s u s e f u l as a hydrogen-like probe to study the atomic and r a d i c a l r e a c t i o n s of hydrogen because Mu's r e a c t i o n decay constant i s d i r e c t l y observable, while most hydrogen atom data come from measured r e l a t i v e r a t e constants. The more we know of the r e a c t i o n s of hydrogen, the simplest and most abundant element i n the universe, the sooner we w i l l be b e t t e r able t o understand more complex atoms and t h e i r r e a c t i o n s .

Bailey et al.; Initiation of Polymerization ACS Symposium Series; American Chemical Society: Washington, DC, 1983.

4.

STADLBAUER

ET

37

Muonium in Monomer Initiation Kinetics

AL.

Experimental Chemicals. A c r y l o n i t r i l e (AN), methylmethacrylate (MMA), and styrène were s u p p l i e d by A l d r i c h and p u r i f i e d before use. A c r y l i c a c i d (AA) and acrylamide (AM) were purchased from Eastman. The a c r y l i c a c i d contained 200 ppm p-methoxyphenol as i n h i b i t o r . However, a t the s o l u t e concentrations of 5.0 χ 10~ M and 7.4 χ 10~*M the i n h i b i t o r ' s c o n c e n t r a t i o n would be ^ 10"^M and, t h e r e f o r e , could not c o n t r i b u t e s i g n i f i c a n t l y to the observed muonium decay. The water s o l v e n t was t r i p l y d i s t i l l e d , i n i t i a l l y from permanganate s o l u t i o n .


5

Apparatus. The schematic diagram i n Figure 1 shows the general experimental set-up f o r a low energy 'surface' muon beam. Right and l e f t p o s i t r o n d e t e c t o r s are made of p l a s t i c s c i n t i l l a t or 'paddles' connected to RCA 8575 p h o t o m u l t i p l i e r tubes by lengths of l i g h t pipe; the whole of which are wrapped to exclude outside l i g h t . Detectors can be s e t up f o r double or t r i p l e coincidence counting to help minimize background. Graphite degrader between the d e t e c t o r s helps maximize s i g n a l amplitudes. The Helmholtz c o i l s , which provide the e x t e r n a l magnetic f i e l d , are mounted t r a n s v e r s e l y to the beam. A s i g n a l from the TM ( t h i n s c i n t i l l a t o r - m u o n ) counter s t a r t s the Ins r e s o l u t i o n c l o c k . A s i g n a l from e i t h e r r i g h t or l e f t d e t e c t o r s i n coincidence mode stops the c l o c k i f t h i s s i g n a l appears between a set "gate" of 0 to 4μβ. I f the p o s i t r o n s i g n a l i s not c o i n c i d e n t or appears a f t e r 4ys i t i s not counted and the system s t a r t s over. U s u a l l y about 50,000 good events are counted per minute. MSR: Muonium Spin R o t a t i o n Measurements. For the k i n e t i c p o r t i o n of t h i s study a low energy (4.1 MeV) beam of muons from the M20 channel at TRIUMF was focused on a shallow T e f l o n c e l l with 0.007 cm Mylar as muon windows. These target c e l l s contained approximately 80 ml of sample s o l u t i o n which was bubbled w i t h high p u r i t y He to remove d i s s o l v e d oxygen. In the case of these v o l a t i l e a c r y l i c s o l u t e s the bubbling gas was f i r s t passed through a prebubbler c o n t a i n i n g a s o l u t i o n of the same concentration as the sample. Muonium p r e c e s s i o n s i g n a l s were observed at a f i e l d of 8G provided by Helmholtz c o i l s transverse to the beam and centered on the t a r g e t . Figure 2 shows some time-histograms of both raw and f i t t e d data. The data i s computer f i t t e d using MINUIT, a χ minimization program, to a nine parameter f u n c t i o n . Equation (3) shows that f u n c t i o n with two parameters, background and muon decay, subtracted out. 2

A ( t ) - Αμ cos(o) t + φ ) + A y

M

exp(-xt) cos(u) t - φ ) M

Μ

(3

where A^ and A J J are, r e s p e c t i v e l y , the muon and muonium s i g n a l amplitudes, and ω are t h e i r f r e q u e n c i e s , φ and φ^ t h e i r Μ

μ




1

Η

w Si

Ο

hd

>τΙ

H Ο Ο

>

H

g

— ιι

00



ÏÏ

s*

•H.

s*

5;

S

S*

ι

s'

ι

>

H

w

> w

H

C/î

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INITIATION

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i n i t i a l phases, while λ i s the muonium decay constant. Using equation (4), the obseved λ and the known s o l u t e c o n c e n t r a t i o n [S], the bimolecular r a t e constant, k , can be c a l c u l a t e d f o r the r e a c t i o n between Mu and the s o l u t e . M

λ = λ

0

+ k [S]

(4

M

The decay constant f o r the pure water s o l v e n t , λ , has a value o f ( 2 . 4 i 0 . 6 ) x l 0 s " 05-8). Two Xs, r i g h t and l e f t , a r e obtained per experiment and are p l o t t e d against s o l u t e concentra t i o n . The slope o f the best l i n e i s taken as k . 0

5

1


M

MRSR; Muonium R a d i c a l Spin R o t a t i o n . In order t o prove that Mu was indeed adding across the v i n y l double bond o f styrene and the l i q u i d a c r y l i c s , high energy muons (24 MeV) were made to s t r i k e g l a s s , round-bottomed f l a s k s c o n t a i n i n g neat samples. Two methods o f degassing were u t i l i z e d : i ) a freeze-pump-thaw c y c l e followed by vacuum s e a l i n g , and i i ) by bubbling the samples on l i n e using a s p e c i a l probe and ground g l a s s f i t t i n g s . Data were analyzed using a Fast F o u r i e r Transform (FFT) program t o o b t a i n the r a d i c a l frequencies and the hyperfine coupling constant, a , as described elsewhere (9,10). μ

Results Table I l i s t s the decay constants, λ, obtained f o r the d i f f e r e n t concentrations of the monomers s t u d i e d . These λs are the average of the l e f t and r i g h t values obtained f o r each concentra t i o n (11). Though s t a t i s t i c a l e r r o r s range from 5% t o 17%, ex perimental i r r e p r o d u c i b i l i t i e s i n t a r g e t geometry, f i e l d homoge n e i t y , detector t h r e s h o l d s , muon beam asymmetry and background r e s u l t i n a more probable e r r o r of ±25% ( 8 ) . T h i s l e v e l o f r e p r o d u c i b i l i t y i s q u i t e reasonable when compared to r a t e con stants obtained by competitive r a t e techniques and d i r e c t p h y s i c a l methods. F i g u r e 3 shows the FFT s p e c t r a o f styrene a t 1500G and 2500G. The l a r g e low frequency peak i s due to the muon while the two higher frequency peaks are due t o the r a d i c a l . A d d i t i o n o f these two r a d i c a l frequencies y i e l d s the muon hyperfine c o u p l i n g constant α . [Peaks a t 23 MHz are due t o the r a d i o frequency (RF) of the c y c l o t r o n . Peaks a t m u l t i p l e s of 125 MHz are due t o the counting system's c l o c k frequency]. These s p e c t r a show that the coupling constant f o r styrene i s

Muonium as a Hydrogen-like Probe to Study Monomer Initiation

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