- Email: [email protected]
Temperature quenching of phosphorescence in polyethylene
European PolymerJournal. 1970. VoL 6, pp. 731-741. Pergamon Press. Printed in England.
TEMPERATURE QUENCHING OF PHOSPHORESCENCE IN POLYETHYLENE I. BOUSTEAD Physics Department, Royal Military College of Science, Shn~enham, Swindon, Wiltshire, England (Received 4 August 1969)
Abstract--Although the quenching of organic phosphorescence in both low molecular weight and polymer systems has been detected earher, no attempt appears to have been made to describe the effect quantztatively. This paper shows that impurity phosphorescence in polyethylene under u.v. excitation is quenched by four mechamsms ; three of these originate in the amorphous regions of the polymer and are associated with the molecular motions which cause mechanical relaxations, while the fourth is associated with the increasing motion of segments of the molecular chains in the crystalline regions of the material. A simple quantitative explanation is put forward and it is shown to describe effectively the experimental results. INTRODUCTION THE QUENCHING Of p h o s p h o r e s c e n c e in low m o l e c u l a r w e i g h t o r g a n i c s c i n t i l l a t o r s has b e e n k n o w n for s o m e t i m e ; (t> the effect has also b e e n o b s e r v e d q u a l i t a t i v e l y in p o l y e t h y l e n e . (a> A l t h o u g h similar effects are o b s e r v e d in the f l u o r e s c e n c e o f o r g a n i c c o m p o u n d s and have b e e n e x p l a i n e d q u a n t i t a t i v e l y , C3''L> there does n o t as yet a p p e a r to be a similar t r e a t m e n t for p h o s p h o r e s c e n c e . T h i s p a p e r r e p o r t s m e a s u r e m e n t s m a d e on a n u m b e r o f l o w a n d high density p o l y e t h y l e n e s , p r o v i d e s a simple q u a n t i t a t i , , e d e s c r i p t i o n a n d a t t e m p t s a c o r r e l a t i o n b e t w e e n the v a r i o u s q u e n c h i n g p r o c e s s e s a n d the s t r u c t u r a l c h a n g e s in the b u l k p o l y m e r . EXPERIMENTAL Phosphorescence measurements were made by the method of Lewis and Kasha. <5) The exciting radiation was the full spectrum of a high pressure mercury discharge lamp; the detector was an EMI photomultiplier 9665QB. The samples were held in a heavy brass holder, the base of which was drilled to take a nichrome heating wire operated from a low voltage transformer. The holder was made with two co-axial windows, one of which faced the exciting radiation, while the other faced the detector. The sample was drilled to take one junction of a copper-constantan thermocouple. Measurements were made by immersing the sample plus holder in liquid nitrogen inside the unsilvered quartz dewar of the rotating can phosphoroscope. When the heater was connected, the liquid nitrogen began to bo~l away and the sample plus holder began to warm at an approximately constant rate. Continuous measurements were made of phosphorescence intensity and sample temperature and these were plotted on to a Bryans X-Y recorder. Low density polyethylene samples were obtained from I.C.I. in the form of pellets and as commercially pressed sheets. The low density samples used are listed in Table 1. High density samples of Marlex 6000 and 6050 (manufactured by Phillips) and Rigidex 9 (manufactured by B.P. Chemicals) were also tested. Samples in the form of powders or pellets were pressed into 1/16 in. sheets in the laboratory at 15,000 Ib/in 2 at 125 ° for low density material and 150 ° for high density material. Pressing times were kept to a minimum to reduce any oxidation effects. C6) It was shown in an earlier paper
732
I. BOUSTEAD TABLE 1. PROPERTIES OF LOW DENSITY POLYETHYLENES USED [N THIS WORK
Name Alkathene 2 Alkathene 20 Alkathene 110 XRM 40 WVG 23 WSG 22 WNC 18 XJK 64 XHB 48 XDK 62
Average number molecular weight (x 10*)
Density
32 24 19 3 0-3 1 10 30 100 300
0- 921 0" 923 0" 914 0"923 0"913 0" 914 0'917 0-921 0"922 0" 922
RESULTS
Commercial low density polyethylene Figure I shows the effect of temperature quenching of impurity phosphorescence in DFD4400 low density polyethylene. This sample exhibits all the features found in varying degrees in all polyethylenes of both low and high density. Three regions
IO
A
B
t
G
8
g6 .4 I.._z O2 -i0
1 I00
! ISO 200 TEMPERATURE ~°K)
2SO
FIo. 1. Temperature quenching of phosphorescence in DFD4400 low density polyethylene. (labelled A, B and C in the diagram) can be distinguished; although the boundaries between them cannot be rigidly defined, each consists of 8 zone of almost constant intensity followed by a zone of increased quenching. Although all polyethylenes exhibit effects similar to those of Fig. 1, the relative importance of the different quenching mechanisms varies from sample to sample; Fig. 2 shows typical quenching curves for various low density polyethylenes. In Aikathene 2, Region A is more marked and Region C is weaker than in DFD4400. An almost identical effect is observed in WVG22. In Alkathene 20, the decay regions A
Temperature Quenching of Phosphorescence in Polyethylene
733
I$
F-
& >-
LIO
g S
o
Q.
O
I
I00
I
ISO TEMPERATURE
;ZOO
2';o
~°K)
Fro. 2. Phosphorescence quenching curves in low density polyethylene. A: WNC 18, B: Alkathene 20, C: Alkathene 2. and C are weak and most o f the quenching occurs m Region B ; a similar effect is also found in Alkathene 110 and XJK64. A curious effect is observed in W N C 1 8 and X H B 4 8 in that Region B shows an increase in emission intensity immediately prior to the expected quenching.
<,5, 4
" \ &
I00
150
200
250
TEMPERATURE (°K')
FIG. 3. Phosphorescence quenching cur-,es in high density polyethylene. A: Marlex 6050, B: Rigidex 9.
734
I. BOUSTEAD
Commercial high density polyethylene Figure 3 shows the quenching curves for Rigidex 9, Marlex 6000 and Marlex 6050. The Marlex samples give the same curve and the three regions A, B, C found earlier can still be distinguished, although less distinctly than in low density polymer. Rigidex however, presents an almost smooth curve with a weak slope discontinuity separating Regions B and C.
Hexane treated polyethylene It has been shown in an earlier paper (7> that, if polyethylene samples are allowed to stand for a few hours in low molecular weight saturated hydrocarbons (e.g. hexane) at room temperature, the phosphorescent impurities in the amorphous regions are leached out. Therefore phosphorescence measurements made on such samples detect the emission from impurities in the crystalline regions alone. Fig. 4 shows quenching I0
"-.,,,. ~ " ~
.",,c
~. D6
"\O un2 O I o.
0
L I00
| ISO TEMPERATURE ~°K)
Y
i 20(
FIG. 4. Phosphorescence quenching curves in A: hexane treated Rigidex 9, B: hexane treated Alkathene 2, C: n-tetradecane doped with traces of benzophenone, D: n-heptane doped with traces of benzophenone. curves for Alkathene 2 and Rigidex 9 treated in this way. (1) It is found that all hexanetreated low density samples give curves similar to that for Alkathene 2 and all high density samples give curves similar to Rigidex 9. Furthermore, if phosphorescent impurity is reintroduced into the amorphous regions by allowing hexane treated samples to stand in a hexane solution of the impurity, then the original three component curve is restored, and the effect is found to be independent of the nature of the additive. A parallel study has been carried out on the phosphorescence quenching characteristics of the low molecular weight linear paraffins containing a variety of phosphorescent additives• Two typical curves (for n-tetradecane and n-heptane doped with benzophenone) are also shown in Fig. 4. It has further been shown that the quenching curves in the linear paraffins are independent of the additives• The differences between the quenching curves of the various paraffins can be shown to follow the crystal structure of the paraffin matrix. For example, n-heptane is unique in possessing orthorhombic symmetry (8) while n-tetradecane which is typical of the even paraffins is
Temperature Quenching of Phosphorescence in Pobeth',lene
735
triclinic. The odd paraffins, excluding n-heptane, form hexagonal structures and produce a quenching curve similar to that for n-tetradecane but lying at somewhat lower temperatures. Since the molecules in the crFstalline regions of polyethylene pack in a manner similar to those in crystals of the linear paraffins,(9' l o~ it is expected that the quenching characteristics of polyethylene crystaIlites are similar to those of the n-alkanes. Comparison of the curves in Fig. 4 shows that quenching in low density polyethylene is similar to that in n-tetradecane while that in high density polymer is similar to n-heptane. The deviation of n-heptane at higher temperatures is probably due to its low melting point. DISCUSSION
Hexane treated polyethylene The quenching characteristics of the washed polymer will be considered first since this is the simplest form of quenching observed. Consider a system in which a small number of phosphorescent molecules are embedded in a non-phosphorescent matrix. During u.v. excitation of the system, depopulation of the lowest triplet state of the phosphorescent molecules may be either radiative, gi~ ing rise to phosphorescence, or non-radiative, in which case the excess energy is dissipated thermally. Taking the rate constants for these two processes as kp and k, respectively, the quantum efficiency for phosphorescence (qp) is given by q, --
ko
k,+k,'
(1)
The constant k~ can be written as the sum of a temperature dependent component, (k~)r, and a temperature independent component (k,.)o, i.e. /% = (k.)o ÷ (k,)r.
(2)
The constant (k.)o is necessary to account for such processes as internal conversion and vibrational relaxation, which depend on the intrinsic properties of the phosphorescent molecule and are therefore unlikely to be affected by temperature. The constant (k.)r consists of two principal components, viz. that due to the re-excitation of an electron from the triplet state back to the first excited singlet state and that due to energy loss by collision of a luminescent molecule with a non-luminescent matrix molecule. The first of these processes can be thought of as consisting of two steps, viz. thermal excitation to a high vibrational level of the triplet state followed by intersystem crossing to the corresponding singlet vibrational level. The rate determining step for such a mechanism is the probability of thermal excitation to a triplet vibrational level with energy at least equal to the singlet-triplet energy difference. Using the data of K a s h a , ~ l ) this energy is of the order of 0.9 eV for phenanthrene and 0.4 eV for benzophenone, two of the known emitting impurities in polyethylene. Thermal excitation to such energies at the temperatures used in this work is most unlikely; it must be concluded that the rate constant (k.)r is due primarily to collisional deactivation. Equations similar to (I) and (2) were found to explain the temperature dependence of the fluorescence quantum efficiency for liquid scintillators, o,~) In such systems,
736
I. BOUSTEAD
it was found that the rate constant (.k~)r could be written in the form of a Boltzmann function (L,)r = c . e x p ( - E/'/,'T) (3) where c is a constant and k is Boltzmann's constant. The significance of the energy E will be discussed later. Using this same Boltzmann form in the phosphorescence equations and combining Eqns. (1), (2) and (3), the quantum efficiency for phosphorescence can be written in the form qp =
kp (kn)o ,'-- kp q- c.exp(--
E/kT)"
(4)
Since the observed intensity (Jr) at a temperature T is proportional to the quantum efficiency, Eqn. (4) can be rewritten as 1
1
IF
/o
-- e x p ( -
E,'kT)
(5)
where the intensities are measured in arbitrary units and the various constants of Eqn. (4) have been absorbed into these units./o is the emission intensity at zero collisional quenching, i.e. at zero temperature. However, it was thought to be sufficiencly accurate to replace/o by/77, the intensity at liquid nitrogen temperature, to give 1
1
[T
I77
-- exp(--
E/kT).
(6)
The application of Eqn, (6) to the washed polyethylene samples gave good agreement with experiment as can be seen from Fig, 5 ; the gradient of the plot gave the energy E. It was found that for high density polyethylene E lies between 0.12 and 0.14 eV while for low density polyethylene E lies between 0- 15 eV and 0.17 eV. The corresponding values for the linear paraffins are 0- 14 for n-heptane and 0" 17 eV for r>tetradecane. The significance of E is of some importance. Since the phenomenon observed is due to collisions between the matrix molecules or segments of them and the impurity molecules, E must represent the energy which a segment of the matrix molecule must possess in order to collide effectively with a phosphorescent molecule. Thus the greater the mean separation of the matrix molecules, the greater will be the distance between the matrix molecules and the impurity molecules and, assuming the motion to be simple harmonic, the greater will be the value of E for effective collision. From crystallographic data on polyethylene, {~2) the mean separation in low density polyethylene is 6" 15 7k and in high density polyethylene is 4.56 A. Therefore, qualitatively, high density samples will be expected to have the lower value for E, as found by experiment.
Untreated polyethylene There are two striking differences between the phosphorescence quenching characteristics of treated and untreated polyethylene. First, all treated polyeth2,1enes show a single continuous quenching effect while untreated samples show three distinct quenching mechanisms; secondly, treated samples give only two forms of quenching curve (one for low density and one for high density material) while the untreated samples, although of the same general form, show considerable variation in the relative importance of the different effects.
Temperature Quenching of Phosphorescence in Polyeth?,lene
737
IO3 f
IO2
_-{ :a
"* IO 'O
I
5o
70
60
I/r
ao
x 10 -4
i
i
9o
tO0
(~cg -I )
FIG. 5. Experimental plot of ¢ = (1/lr)--(1/ITv) as a function of reciprocal of temperature for DFD4400 ~ashed ~,ith hexane.
/
\
! /"~\ B'
io / / ~-
z
~
a
b
P
I
/'
\
/
\
\ c'
I
\
1
\
/
/
/.
\ ~
\
.6
,_:4
/
,A o2
O~
i
/
--
', \K \
\
/
Ioo
,~
I
Jso
i
200 TEMPERATURE (*K)
2s o
FIG. 6. Phosphorescence quenching in DFD4-I00. Separation of the experimental curve into three components A, B and C. Curves A', B' and C" show the first derivative of A, B and C with respect to temperature. Bars a, b and c indicate the temperature regions over which relaxations have been detected by other methods (see text).
738
I. BOUSTEAD
It has been shown in an earlier paper rT) that the net effect of hexane treatment of polyethylene is the removal of impurities from the amorphous regions of the polymer. Hence the observed differences must be explained in terms of such an effect. It was mentioned above tkat the observed emission from hexane-treated samples was principally from the crystalline regions and therefore the emission in samples not so treated must include a component or components from the amorphous and folded regions of the polymer. It is expected therefore that the quenching curves in untreated samples are the sum of four components, three originating in the amorphous and folded regions, and one originating in the crystalline region. The resultant curve is, however, not necessarily a direct addition of the components since it was also found earlier (7) that the impurities usually present in the amorphous regions tended to quench the emission from the crystalline regions. In some samples (e.g. DFD4400) this quenching ~.as found to be total so that the observed quenching is solely that from the amorphous and folded regions while in others (e.g. Alkathene 20) a small component of the crystalline emission could be detected; direct addition of two quenching curves such as those shown in Figs. 1 and 4 can be shown to gi~e the form observed for Alkathene 20. Such an explanation does not however account for curves such as that for WNC18 (Fig. 2) which show a hump; an explanation of this effect is postponed. Quenching curves such as that shown in Fig. 1 may therefore be considered as representative of the phosphorescence quenching mechanisms present in the amorphous regions of polyethylene. This type of curve can be separated into three components, shown by the curves A, B and C in Fig. 6. The curved portions of these components obey an expression of the form of Eqn. (6) with values of E as summarized in Table 2. I f such an analysis of the composite curve is correct, then it is necessary to TABLE 2. TYPICAL VALUES OF ENERGY, E, DERIVED FROM PHOSPHORESCENCE QUENCITING. REGIONS A, B AND C REFER TO COMPONENTS AS SHOWN IN FIG. 6
Sample DFD4400 hexane treated Alkathene 2 hexane treated Rigidex 9 hexane treated Marlex 6000 hexane treated DFD4400 untreated Alkathene 2 untreated Alkathene 20 untreated
E(eV)
Region A B C Region A B C Region A B
0-15 0-13 0" 13 0" 12 0.25 0"28 0"51 0" 31 0"41 0"54 0.34 0"45
explain why each component only becomes effective at a particular temperature. Curves A', B' and C' in Fig. 6 show the first derivatives of A, B, C with respect to temperature; they show that the temperatures at which new quenching mechanisms come into play are approximately 95 °, 135 ° and 205°K with quenching maxima at 110 °, 185 ° and 240°K respectively. The onset of new quenching mechanisms at particular
Temperature Quenching of Phosphorescence in Pol?eth~lene
739
temperatures is similar to the onset of relaxation mechanisms at fixed temperatures due to the unfreezing of new molecular motions. The three bars marked a, b and c in the d m ~ a m show the temperature regions in which relaxation mechanisms have been detected in polyethylene by ~arious methods.(t:-~ :) The two relaxations centred on 243 ° and 147:K respectively are well established as the ,8 and y relaxations and are thought to be due to movement of branches and to rotation of segments of the molecules in the amorphous regions respectively. The transition centred on I l I : K has been observed by Sinnott (t'~) in dynamic modulus measurements while the low temperature N M R measurements of Fuschillo and Sauer (~6) could be interpreted to show the effect. If this correlation between phosphorescence quenching and molecular relaxation processes is valid, then it is interesting to note that the relaxations labelled a and c span the regions of maximum quenching while the transition labelled b lies at the onset of quenching. If it is remembered that the a, b and c bars cover the regions over which the loss curve is a maximum, then the molecular motions associated with each relaxation mechanism might be expected to lie a few degrees below the lowest temperature of each bar. Once unfrozen, the amplitude of the molecular motion might be expected to rise with temperature, obeying a Boltzmann type law of energy distribution. A relaxation will occur only when the molecular motion gains sufficient energy to cause the necessary reorganization of the solid and would therefore occur a few degrees higher than the temperature at which the molecular motion is unfrozen. It therefore appears from these results that phosphorescence quenching is very sensitive e~en to small molecular motion while mechanical relaxations do not come into play until a higher temperature. Using this same type of analysis on a number of other different polyethylene samples, values for the energies E in Eqn. (6) can be determined; the values are shown in Table 2. This correlation between molecular motions and phosphorescence quenching may also be used to advance a possible explanation for the hump on the quenching curves of WNC18 and XHB48 (Fig. 2). If at any relaxation there is a change in the light transmitting properties of the matrix, then a change in the emitted phosphorescence might be expected. To explain a rise in emitted intensity, it is necessary to postulate an increase in the transmission thus causing a greater proportion of the phosphorescent molecules to be stimulated. Such a small change has been observed in WNC18. (~s) Clearly, at some higher temperature the increasing molecular motion will cancel out this rise in intensity and the quenching curve will pass through a maximum and decrease again. Since the polyethylene/3 transition is associated with the degree of branching, the possibility was considered that the quenching curves might afford a means of estimating the degree of branching in a particular sample. However, direct comparison of two curves of different types of polymer does not allow a simple interpretation since the quenching effect of this transition depends to a large extent on the strength of the quenching produced by the lower temperature transitions. It will however be observed that the phosphorescence quenching in this temperature region is lower in the high density samples where the d e ~ e e of branching is lower than in the low density samples. Until more quantitative knowledge of the various effects is known, quantitative estimates of the degree of branching, using this technique, cannot be made.
740
I. BOUSTEAD CONCLUSIONS
It has been s h o w n t h a t q u e n c h i n g o f i m p u r i t y p h o s p h o r e s c e n c e in p o l y e t h y l e n e is the s u m o f f o u r d i s t i n c t q u e n c h i n g m e c h a n i s m s . T h r e e o f these are the m o l e c u l a r m o t i o n s a s s o c i a t e d w i t h the relaxation m e c h a n i s m s in the a m o r p h o u s regions o f the p o l y m e r , while the f o u r t h is a s s o c i a t e d with the e n e r g y o f m o l e c u l a r s e g m e n t s in the c r y s t a l l i n e r e ~ o n s . All f o u r q u e n c h i n g m e c h a n i s m s f o l l o w a B o l t z m a n n type o f temperature d e p e n d e n c e for q u e n c h i n g g i v i n g an i n t e n s i t y e q u a t i o n o f the f o r m
1
~r - - ~
1
= exp(-
E/kT)
w h e r e the p a r a m e t e r s are as defined earlier. It has also b e e n s h o w n t h a t the q u e n c h i n g is i n d e p e n d e n t o f the n a t u r e o f the i m p u r i t y a n d is a f u n c t i o n solely o f the p o l y m e r matrix. REFERENCES (I) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18)
P. Pringsheim, Fluorescence and Phosphorescence. Interscience, New York (1949). A. Charlesby and R. H. Partridge, Proc. R. Soc. A283, 329 (1965). E. J. Bowen and J. Sahu, J. phys. Chem. 63, 4 (1959). J. B. Birks, B. N. Srinivasan and S. P. McGlynn, J. ~,[ol. Spectrosc. 27, 266 (1965). G. N. Lewis and M. Kasha, J. Am. chem. Soc. 66, 2100 (1944). J. P. Luongo, J. Polym. Sci. 42, 139 (1960). i. Boustead and A. Charlesby, Europ. Polym. J. 3, 459 (1967). D. Fox, M. M. Labes and A. Weissberger, Physics and Chemistry of the Organic Solid State, Vol. 1. Interscience, New York (1963). D. C. Bassett and A. Keller, Phil. Mag. 6, 344 (1961). A. Keller, Phil. l~[ag. 6, 329 (1961). M. Kasha, Radial. Res. Suppl. 2, 243 (1960). J. Brandrup and E. H. Immergut, eds. Polymer Handbook. Interscience, New York (1966). J. A. Sauer and A. E. Woodward, Rev. mod. Phys. 32, 88 (1960). K. M. Sinnott, J. appL Phys. 37, 3385 (1966). N. N. Semenov, Proc. Conf. I.U.P.A.C. Montreal, Canada, 353 (1961). N. Fuschillo and J. A. Sauer, J. appl. Phys. 28, 1073 (1957). F. Danusso, G. Moraglio and G. Talamini, J. Polym. Sci. 21, 139 (1956). R. H. Partridge, Private communication.
R~sum~--Bien que l'on ait detect~ pr~c~demment l'extinction de la phosphorescence organique dans les syt~mes comprenant de faibles masses moMculaires et du polym~re, aucun essai ne parait avoir ~t6 tent~ jusqu'ici pour d~crire cet effet quatitativement. Cet article indique que la phosphorescence provoqu~e sous excitation u.v. par des impuret~s darts le polyethylene est ~teinte par quatre m~canisrues; trois de ceux-vi ont pour origine les r~gions amorphes du polym/~re et sont associ~s aux mouvements mol~culaires qui provoquent les relaxations m6caniques, cependant que le quatri~me est associ6 b. l'accroissement du mouvement de segments des chaines mol~ulaires dans les r~gions cristallines du produit. Une explication quantitative simple est propos6e et l'on montre qu'elle correspond effectivement aux r6sultats exp6rimentaux. Sommar,.'o---Sebbene d clecadimento della fosforescenza organica, sia in sistemi a basso peso molecolare che in polimeri sm stato gigt rilevato, non 6 stato fatto alcun tentativo per descrivere l'effetto quantitativamente. Questo lavoro mostra c h e l a fosforescenza per impurezza nel polietilene sotto eccitazione u.v. viene abbassata da quattro meccanismi; tre di questri hanno origine nelle regioni amorfe del polimero e sono associate con i moti molecolari che provocano i rilassamenti meccanici, mentre il quarto 6 associato con il crescente moto dei segmenti delle catene rnolecolari nelle regioni cristalline del matenale. Viene data una semplice spiegazione quantitativa e si mostra che descrive effettivamente i risultati sperimentali. Zusammenfassung--Obwohl die Ausl6schung organischer Phosphoreszenz sowohl in niedermolekularen als auch in Polymers~stemen schon frtiher entdeckt wurde, ist scheinbar noch nicht versucht
Temperature Quenching of Phosphorescence in Polyeth~ Iene
741
worden, diesen Effekt quantitatlv zu beschreiben. Diese Arbeit zelgt, dab die VerunreinigungsPhosphoreszenz in Pol:~thylen bei U.V. Anregung dutch vier Mechamsmen ausgeloscht wird; drei damon haben ihren Ursprung in den amorphen Bereichen des Polymeren und sind verbunden mit Molekularbewegungen, die mechamsche Relaxationen verursachen; der vlerte Mechanismus h~.ngt zusammen mit der zunehmenden Be~eglichkeit yon Segmenten der molekularen Ketten in den kristallinen Bereichen des Materials. Eine einfache quantitative Deutung wird vorgeschlagen und gezeigt, dab sich damit dle experimentellen Ergebnisse sehr gut beschreiben lassen.
TEMPERATURE QUENCHING OF PHOSPHORESCENCE IN POLYETHYLENE I. BOUSTEAD Physics Department, Royal Military College of Science, Shn~enham, Swindon, Wiltshire, England (Received 4 August 1969)
Abstract--Although the quenching of organic phosphorescence in both low molecular weight and polymer systems has been detected earher, no attempt appears to have been made to describe the effect quantztatively. This paper shows that impurity phosphorescence in polyethylene under u.v. excitation is quenched by four mechamsms ; three of these originate in the amorphous regions of the polymer and are associated with the molecular motions which cause mechanical relaxations, while the fourth is associated with the increasing motion of segments of the molecular chains in the crystalline regions of the material. A simple quantitative explanation is put forward and it is shown to describe effectively the experimental results. INTRODUCTION THE QUENCHING Of p h o s p h o r e s c e n c e in low m o l e c u l a r w e i g h t o r g a n i c s c i n t i l l a t o r s has b e e n k n o w n for s o m e t i m e ; (t> the effect has also b e e n o b s e r v e d q u a l i t a t i v e l y in p o l y e t h y l e n e . (a> A l t h o u g h similar effects are o b s e r v e d in the f l u o r e s c e n c e o f o r g a n i c c o m p o u n d s and have b e e n e x p l a i n e d q u a n t i t a t i v e l y , C3''L> there does n o t as yet a p p e a r to be a similar t r e a t m e n t for p h o s p h o r e s c e n c e . T h i s p a p e r r e p o r t s m e a s u r e m e n t s m a d e on a n u m b e r o f l o w a n d high density p o l y e t h y l e n e s , p r o v i d e s a simple q u a n t i t a t i , , e d e s c r i p t i o n a n d a t t e m p t s a c o r r e l a t i o n b e t w e e n the v a r i o u s q u e n c h i n g p r o c e s s e s a n d the s t r u c t u r a l c h a n g e s in the b u l k p o l y m e r . EXPERIMENTAL Phosphorescence measurements were made by the method of Lewis and Kasha. <5) The exciting radiation was the full spectrum of a high pressure mercury discharge lamp; the detector was an EMI photomultiplier 9665QB. The samples were held in a heavy brass holder, the base of which was drilled to take a nichrome heating wire operated from a low voltage transformer. The holder was made with two co-axial windows, one of which faced the exciting radiation, while the other faced the detector. The sample was drilled to take one junction of a copper-constantan thermocouple. Measurements were made by immersing the sample plus holder in liquid nitrogen inside the unsilvered quartz dewar of the rotating can phosphoroscope. When the heater was connected, the liquid nitrogen began to bo~l away and the sample plus holder began to warm at an approximately constant rate. Continuous measurements were made of phosphorescence intensity and sample temperature and these were plotted on to a Bryans X-Y recorder. Low density polyethylene samples were obtained from I.C.I. in the form of pellets and as commercially pressed sheets. The low density samples used are listed in Table 1. High density samples of Marlex 6000 and 6050 (manufactured by Phillips) and Rigidex 9 (manufactured by B.P. Chemicals) were also tested. Samples in the form of powders or pellets were pressed into 1/16 in. sheets in the laboratory at 15,000 Ib/in 2 at 125 ° for low density material and 150 ° for high density material. Pressing times were kept to a minimum to reduce any oxidation effects. C6) It was shown in an earlier paper
732
I. BOUSTEAD TABLE 1. PROPERTIES OF LOW DENSITY POLYETHYLENES USED [N THIS WORK
Name Alkathene 2 Alkathene 20 Alkathene 110 XRM 40 WVG 23 WSG 22 WNC 18 XJK 64 XHB 48 XDK 62
Average number molecular weight (x 10*)
Density
32 24 19 3 0-3 1 10 30 100 300
0- 921 0" 923 0" 914 0"923 0"913 0" 914 0'917 0-921 0"922 0" 922
RESULTS
Commercial low density polyethylene Figure I shows the effect of temperature quenching of impurity phosphorescence in DFD4400 low density polyethylene. This sample exhibits all the features found in varying degrees in all polyethylenes of both low and high density. Three regions
IO
A
B
t
G
8
g6 .4 I.._z O2 -i0
1 I00
! ISO 200 TEMPERATURE ~°K)
2SO
FIo. 1. Temperature quenching of phosphorescence in DFD4400 low density polyethylene. (labelled A, B and C in the diagram) can be distinguished; although the boundaries between them cannot be rigidly defined, each consists of 8 zone of almost constant intensity followed by a zone of increased quenching. Although all polyethylenes exhibit effects similar to those of Fig. 1, the relative importance of the different quenching mechanisms varies from sample to sample; Fig. 2 shows typical quenching curves for various low density polyethylenes. In Aikathene 2, Region A is more marked and Region C is weaker than in DFD4400. An almost identical effect is observed in WVG22. In Alkathene 20, the decay regions A
Temperature Quenching of Phosphorescence in Polyethylene
733
I$
F-
& >-
LIO
g S
o
Q.
O
I
I00
I
ISO TEMPERATURE
;ZOO
2';o
~°K)
Fro. 2. Phosphorescence quenching curves in low density polyethylene. A: WNC 18, B: Alkathene 20, C: Alkathene 2. and C are weak and most o f the quenching occurs m Region B ; a similar effect is also found in Alkathene 110 and XJK64. A curious effect is observed in W N C 1 8 and X H B 4 8 in that Region B shows an increase in emission intensity immediately prior to the expected quenching.
<,5, 4
" \ &
I00
150
200
250
TEMPERATURE (°K')
FIG. 3. Phosphorescence quenching cur-,es in high density polyethylene. A: Marlex 6050, B: Rigidex 9.
734
I. BOUSTEAD
Commercial high density polyethylene Figure 3 shows the quenching curves for Rigidex 9, Marlex 6000 and Marlex 6050. The Marlex samples give the same curve and the three regions A, B, C found earlier can still be distinguished, although less distinctly than in low density polymer. Rigidex however, presents an almost smooth curve with a weak slope discontinuity separating Regions B and C.
Hexane treated polyethylene It has been shown in an earlier paper (7> that, if polyethylene samples are allowed to stand for a few hours in low molecular weight saturated hydrocarbons (e.g. hexane) at room temperature, the phosphorescent impurities in the amorphous regions are leached out. Therefore phosphorescence measurements made on such samples detect the emission from impurities in the crystalline regions alone. Fig. 4 shows quenching I0
"-.,,,. ~ " ~
.",,c
~. D6
"\O un2 O I o.
0
L I00
| ISO TEMPERATURE ~°K)
Y
i 20(
FIG. 4. Phosphorescence quenching curves in A: hexane treated Rigidex 9, B: hexane treated Alkathene 2, C: n-tetradecane doped with traces of benzophenone, D: n-heptane doped with traces of benzophenone. curves for Alkathene 2 and Rigidex 9 treated in this way. (1) It is found that all hexanetreated low density samples give curves similar to that for Alkathene 2 and all high density samples give curves similar to Rigidex 9. Furthermore, if phosphorescent impurity is reintroduced into the amorphous regions by allowing hexane treated samples to stand in a hexane solution of the impurity, then the original three component curve is restored, and the effect is found to be independent of the nature of the additive. A parallel study has been carried out on the phosphorescence quenching characteristics of the low molecular weight linear paraffins containing a variety of phosphorescent additives• Two typical curves (for n-tetradecane and n-heptane doped with benzophenone) are also shown in Fig. 4. It has further been shown that the quenching curves in the linear paraffins are independent of the additives• The differences between the quenching curves of the various paraffins can be shown to follow the crystal structure of the paraffin matrix. For example, n-heptane is unique in possessing orthorhombic symmetry (8) while n-tetradecane which is typical of the even paraffins is
Temperature Quenching of Phosphorescence in Pobeth',lene
735
triclinic. The odd paraffins, excluding n-heptane, form hexagonal structures and produce a quenching curve similar to that for n-tetradecane but lying at somewhat lower temperatures. Since the molecules in the crFstalline regions of polyethylene pack in a manner similar to those in crystals of the linear paraffins,(9' l o~ it is expected that the quenching characteristics of polyethylene crystaIlites are similar to those of the n-alkanes. Comparison of the curves in Fig. 4 shows that quenching in low density polyethylene is similar to that in n-tetradecane while that in high density polymer is similar to n-heptane. The deviation of n-heptane at higher temperatures is probably due to its low melting point. DISCUSSION
Hexane treated polyethylene The quenching characteristics of the washed polymer will be considered first since this is the simplest form of quenching observed. Consider a system in which a small number of phosphorescent molecules are embedded in a non-phosphorescent matrix. During u.v. excitation of the system, depopulation of the lowest triplet state of the phosphorescent molecules may be either radiative, gi~ ing rise to phosphorescence, or non-radiative, in which case the excess energy is dissipated thermally. Taking the rate constants for these two processes as kp and k, respectively, the quantum efficiency for phosphorescence (qp) is given by q, --
ko
k,+k,'
(1)
The constant k~ can be written as the sum of a temperature dependent component, (k~)r, and a temperature independent component (k,.)o, i.e. /% = (k.)o ÷ (k,)r.
(2)
The constant (k.)o is necessary to account for such processes as internal conversion and vibrational relaxation, which depend on the intrinsic properties of the phosphorescent molecule and are therefore unlikely to be affected by temperature. The constant (k.)r consists of two principal components, viz. that due to the re-excitation of an electron from the triplet state back to the first excited singlet state and that due to energy loss by collision of a luminescent molecule with a non-luminescent matrix molecule. The first of these processes can be thought of as consisting of two steps, viz. thermal excitation to a high vibrational level of the triplet state followed by intersystem crossing to the corresponding singlet vibrational level. The rate determining step for such a mechanism is the probability of thermal excitation to a triplet vibrational level with energy at least equal to the singlet-triplet energy difference. Using the data of K a s h a , ~ l ) this energy is of the order of 0.9 eV for phenanthrene and 0.4 eV for benzophenone, two of the known emitting impurities in polyethylene. Thermal excitation to such energies at the temperatures used in this work is most unlikely; it must be concluded that the rate constant (k.)r is due primarily to collisional deactivation. Equations similar to (I) and (2) were found to explain the temperature dependence of the fluorescence quantum efficiency for liquid scintillators, o,~) In such systems,
736
I. BOUSTEAD
it was found that the rate constant (.k~)r could be written in the form of a Boltzmann function (L,)r = c . e x p ( - E/'/,'T) (3) where c is a constant and k is Boltzmann's constant. The significance of the energy E will be discussed later. Using this same Boltzmann form in the phosphorescence equations and combining Eqns. (1), (2) and (3), the quantum efficiency for phosphorescence can be written in the form qp =
kp (kn)o ,'-- kp q- c.exp(--
E/kT)"
(4)
Since the observed intensity (Jr) at a temperature T is proportional to the quantum efficiency, Eqn. (4) can be rewritten as 1
1
IF
/o
-- e x p ( -
E,'kT)
(5)
where the intensities are measured in arbitrary units and the various constants of Eqn. (4) have been absorbed into these units./o is the emission intensity at zero collisional quenching, i.e. at zero temperature. However, it was thought to be sufficiencly accurate to replace/o by/77, the intensity at liquid nitrogen temperature, to give 1
1
[T
I77
-- exp(--
E/kT).
(6)
The application of Eqn, (6) to the washed polyethylene samples gave good agreement with experiment as can be seen from Fig, 5 ; the gradient of the plot gave the energy E. It was found that for high density polyethylene E lies between 0.12 and 0.14 eV while for low density polyethylene E lies between 0- 15 eV and 0.17 eV. The corresponding values for the linear paraffins are 0- 14 for n-heptane and 0" 17 eV for r>tetradecane. The significance of E is of some importance. Since the phenomenon observed is due to collisions between the matrix molecules or segments of them and the impurity molecules, E must represent the energy which a segment of the matrix molecule must possess in order to collide effectively with a phosphorescent molecule. Thus the greater the mean separation of the matrix molecules, the greater will be the distance between the matrix molecules and the impurity molecules and, assuming the motion to be simple harmonic, the greater will be the value of E for effective collision. From crystallographic data on polyethylene, {~2) the mean separation in low density polyethylene is 6" 15 7k and in high density polyethylene is 4.56 A. Therefore, qualitatively, high density samples will be expected to have the lower value for E, as found by experiment.
Untreated polyethylene There are two striking differences between the phosphorescence quenching characteristics of treated and untreated polyethylene. First, all treated polyeth2,1enes show a single continuous quenching effect while untreated samples show three distinct quenching mechanisms; secondly, treated samples give only two forms of quenching curve (one for low density and one for high density material) while the untreated samples, although of the same general form, show considerable variation in the relative importance of the different effects.
Temperature Quenching of Phosphorescence in Polyeth?,lene
737
IO3 f
IO2
_-{ :a
"* IO 'O
I
5o
70
60
I/r
ao
x 10 -4
i
i
9o
tO0
(~cg -I )
FIG. 5. Experimental plot of ¢ = (1/lr)--(1/ITv) as a function of reciprocal of temperature for DFD4400 ~ashed ~,ith hexane.
/
\
! /"~\ B'
io / / ~-
z
~
a
b
P
I
/'
\
/
\
\ c'
I
\
1
\
/
/
/.
\ ~
\
.6
,_:4
/
,A o2
O~
i
/
--
', \K \
\
/
Ioo
,~
I
Jso
i
200 TEMPERATURE (*K)
2s o
FIG. 6. Phosphorescence quenching in DFD4-I00. Separation of the experimental curve into three components A, B and C. Curves A', B' and C" show the first derivative of A, B and C with respect to temperature. Bars a, b and c indicate the temperature regions over which relaxations have been detected by other methods (see text).
738
I. BOUSTEAD
It has been shown in an earlier paper rT) that the net effect of hexane treatment of polyethylene is the removal of impurities from the amorphous regions of the polymer. Hence the observed differences must be explained in terms of such an effect. It was mentioned above tkat the observed emission from hexane-treated samples was principally from the crystalline regions and therefore the emission in samples not so treated must include a component or components from the amorphous and folded regions of the polymer. It is expected therefore that the quenching curves in untreated samples are the sum of four components, three originating in the amorphous and folded regions, and one originating in the crystalline region. The resultant curve is, however, not necessarily a direct addition of the components since it was also found earlier (7) that the impurities usually present in the amorphous regions tended to quench the emission from the crystalline regions. In some samples (e.g. DFD4400) this quenching ~.as found to be total so that the observed quenching is solely that from the amorphous and folded regions while in others (e.g. Alkathene 20) a small component of the crystalline emission could be detected; direct addition of two quenching curves such as those shown in Figs. 1 and 4 can be shown to gi~e the form observed for Alkathene 20. Such an explanation does not however account for curves such as that for WNC18 (Fig. 2) which show a hump; an explanation of this effect is postponed. Quenching curves such as that shown in Fig. 1 may therefore be considered as representative of the phosphorescence quenching mechanisms present in the amorphous regions of polyethylene. This type of curve can be separated into three components, shown by the curves A, B and C in Fig. 6. The curved portions of these components obey an expression of the form of Eqn. (6) with values of E as summarized in Table 2. I f such an analysis of the composite curve is correct, then it is necessary to TABLE 2. TYPICAL VALUES OF ENERGY, E, DERIVED FROM PHOSPHORESCENCE QUENCITING. REGIONS A, B AND C REFER TO COMPONENTS AS SHOWN IN FIG. 6
Sample DFD4400 hexane treated Alkathene 2 hexane treated Rigidex 9 hexane treated Marlex 6000 hexane treated DFD4400 untreated Alkathene 2 untreated Alkathene 20 untreated
E(eV)
Region A B C Region A B C Region A B
0-15 0-13 0" 13 0" 12 0.25 0"28 0"51 0" 31 0"41 0"54 0.34 0"45
explain why each component only becomes effective at a particular temperature. Curves A', B' and C' in Fig. 6 show the first derivatives of A, B, C with respect to temperature; they show that the temperatures at which new quenching mechanisms come into play are approximately 95 °, 135 ° and 205°K with quenching maxima at 110 °, 185 ° and 240°K respectively. The onset of new quenching mechanisms at particular
Temperature Quenching of Phosphorescence in Pol?eth~lene
739
temperatures is similar to the onset of relaxation mechanisms at fixed temperatures due to the unfreezing of new molecular motions. The three bars marked a, b and c in the d m ~ a m show the temperature regions in which relaxation mechanisms have been detected in polyethylene by ~arious methods.(t:-~ :) The two relaxations centred on 243 ° and 147:K respectively are well established as the ,8 and y relaxations and are thought to be due to movement of branches and to rotation of segments of the molecules in the amorphous regions respectively. The transition centred on I l I : K has been observed by Sinnott (t'~) in dynamic modulus measurements while the low temperature N M R measurements of Fuschillo and Sauer (~6) could be interpreted to show the effect. If this correlation between phosphorescence quenching and molecular relaxation processes is valid, then it is interesting to note that the relaxations labelled a and c span the regions of maximum quenching while the transition labelled b lies at the onset of quenching. If it is remembered that the a, b and c bars cover the regions over which the loss curve is a maximum, then the molecular motions associated with each relaxation mechanism might be expected to lie a few degrees below the lowest temperature of each bar. Once unfrozen, the amplitude of the molecular motion might be expected to rise with temperature, obeying a Boltzmann type law of energy distribution. A relaxation will occur only when the molecular motion gains sufficient energy to cause the necessary reorganization of the solid and would therefore occur a few degrees higher than the temperature at which the molecular motion is unfrozen. It therefore appears from these results that phosphorescence quenching is very sensitive e~en to small molecular motion while mechanical relaxations do not come into play until a higher temperature. Using this same type of analysis on a number of other different polyethylene samples, values for the energies E in Eqn. (6) can be determined; the values are shown in Table 2. This correlation between molecular motions and phosphorescence quenching may also be used to advance a possible explanation for the hump on the quenching curves of WNC18 and XHB48 (Fig. 2). If at any relaxation there is a change in the light transmitting properties of the matrix, then a change in the emitted phosphorescence might be expected. To explain a rise in emitted intensity, it is necessary to postulate an increase in the transmission thus causing a greater proportion of the phosphorescent molecules to be stimulated. Such a small change has been observed in WNC18. (~s) Clearly, at some higher temperature the increasing molecular motion will cancel out this rise in intensity and the quenching curve will pass through a maximum and decrease again. Since the polyethylene/3 transition is associated with the degree of branching, the possibility was considered that the quenching curves might afford a means of estimating the degree of branching in a particular sample. However, direct comparison of two curves of different types of polymer does not allow a simple interpretation since the quenching effect of this transition depends to a large extent on the strength of the quenching produced by the lower temperature transitions. It will however be observed that the phosphorescence quenching in this temperature region is lower in the high density samples where the d e ~ e e of branching is lower than in the low density samples. Until more quantitative knowledge of the various effects is known, quantitative estimates of the degree of branching, using this technique, cannot be made.
740
I. BOUSTEAD CONCLUSIONS
It has been s h o w n t h a t q u e n c h i n g o f i m p u r i t y p h o s p h o r e s c e n c e in p o l y e t h y l e n e is the s u m o f f o u r d i s t i n c t q u e n c h i n g m e c h a n i s m s . T h r e e o f these are the m o l e c u l a r m o t i o n s a s s o c i a t e d w i t h the relaxation m e c h a n i s m s in the a m o r p h o u s regions o f the p o l y m e r , while the f o u r t h is a s s o c i a t e d with the e n e r g y o f m o l e c u l a r s e g m e n t s in the c r y s t a l l i n e r e ~ o n s . All f o u r q u e n c h i n g m e c h a n i s m s f o l l o w a B o l t z m a n n type o f temperature d e p e n d e n c e for q u e n c h i n g g i v i n g an i n t e n s i t y e q u a t i o n o f the f o r m
1
~r - - ~
1
= exp(-
E/kT)
w h e r e the p a r a m e t e r s are as defined earlier. It has also b e e n s h o w n t h a t the q u e n c h i n g is i n d e p e n d e n t o f the n a t u r e o f the i m p u r i t y a n d is a f u n c t i o n solely o f the p o l y m e r matrix. REFERENCES (I) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15) (16) (17) (18)
P. Pringsheim, Fluorescence and Phosphorescence. Interscience, New York (1949). A. Charlesby and R. H. Partridge, Proc. R. Soc. A283, 329 (1965). E. J. Bowen and J. Sahu, J. phys. Chem. 63, 4 (1959). J. B. Birks, B. N. Srinivasan and S. P. McGlynn, J. ~,[ol. Spectrosc. 27, 266 (1965). G. N. Lewis and M. Kasha, J. Am. chem. Soc. 66, 2100 (1944). J. P. Luongo, J. Polym. Sci. 42, 139 (1960). i. Boustead and A. Charlesby, Europ. Polym. J. 3, 459 (1967). D. Fox, M. M. Labes and A. Weissberger, Physics and Chemistry of the Organic Solid State, Vol. 1. Interscience, New York (1963). D. C. Bassett and A. Keller, Phil. Mag. 6, 344 (1961). A. Keller, Phil. l~[ag. 6, 329 (1961). M. Kasha, Radial. Res. Suppl. 2, 243 (1960). J. Brandrup and E. H. Immergut, eds. Polymer Handbook. Interscience, New York (1966). J. A. Sauer and A. E. Woodward, Rev. mod. Phys. 32, 88 (1960). K. M. Sinnott, J. appL Phys. 37, 3385 (1966). N. N. Semenov, Proc. Conf. I.U.P.A.C. Montreal, Canada, 353 (1961). N. Fuschillo and J. A. Sauer, J. appl. Phys. 28, 1073 (1957). F. Danusso, G. Moraglio and G. Talamini, J. Polym. Sci. 21, 139 (1956). R. H. Partridge, Private communication.
R~sum~--Bien que l'on ait detect~ pr~c~demment l'extinction de la phosphorescence organique dans les syt~mes comprenant de faibles masses moMculaires et du polym~re, aucun essai ne parait avoir ~t6 tent~ jusqu'ici pour d~crire cet effet quatitativement. Cet article indique que la phosphorescence provoqu~e sous excitation u.v. par des impuret~s darts le polyethylene est ~teinte par quatre m~canisrues; trois de ceux-vi ont pour origine les r~gions amorphes du polym/~re et sont associ~s aux mouvements mol~culaires qui provoquent les relaxations m6caniques, cependant que le quatri~me est associ6 b. l'accroissement du mouvement de segments des chaines mol~ulaires dans les r~gions cristallines du produit. Une explication quantitative simple est propos6e et l'on montre qu'elle correspond effectivement aux r6sultats exp6rimentaux. Sommar,.'o---Sebbene d clecadimento della fosforescenza organica, sia in sistemi a basso peso molecolare che in polimeri sm stato gigt rilevato, non 6 stato fatto alcun tentativo per descrivere l'effetto quantitativamente. Questo lavoro mostra c h e l a fosforescenza per impurezza nel polietilene sotto eccitazione u.v. viene abbassata da quattro meccanismi; tre di questri hanno origine nelle regioni amorfe del polimero e sono associate con i moti molecolari che provocano i rilassamenti meccanici, mentre il quarto 6 associato con il crescente moto dei segmenti delle catene rnolecolari nelle regioni cristalline del matenale. Viene data una semplice spiegazione quantitativa e si mostra che descrive effettivamente i risultati sperimentali. Zusammenfassung--Obwohl die Ausl6schung organischer Phosphoreszenz sowohl in niedermolekularen als auch in Polymers~stemen schon frtiher entdeckt wurde, ist scheinbar noch nicht versucht
Temperature Quenching of Phosphorescence in Polyeth~ Iene
741
worden, diesen Effekt quantitatlv zu beschreiben. Diese Arbeit zelgt, dab die VerunreinigungsPhosphoreszenz in Pol:~thylen bei U.V. Anregung dutch vier Mechamsmen ausgeloscht wird; drei damon haben ihren Ursprung in den amorphen Bereichen des Polymeren und sind verbunden mit Molekularbewegungen, die mechamsche Relaxationen verursachen; der vlerte Mechanismus h~.ngt zusammen mit der zunehmenden Be~eglichkeit yon Segmenten der molekularen Ketten in den kristallinen Bereichen des Materials. Eine einfache quantitative Deutung wird vorgeschlagen und gezeigt, dab sich damit dle experimentellen Ergebnisse sehr gut beschreiben lassen.