Polymer Degradation and Stability 13 (1985) 313-326

Kinetics of Photo-sensitized and Photo-stabilized Photo-degradation of Isotactic Poly(1-Butene)*

R. P. Singh

Division of Polymer Chemistry, National Chemical Laboratory, Pune 411 008, India

(Received: 10 June, 1985)

ABSTRACT

This investigation describes the results of the 3-(o-carboxyphenyl)-l- phenyltriazene-N-oxide sensitized and copper(II) bis-3-(o-carboxy- phenyl)-l-phenyltriazene-N-oxide stabilized photo-degradation of iso- tactic poly(1-butene) film in air, at a temperature at which formation of" volatiles is negligible and with a light intensity flux of 2.38 x 10 - 9

einstein s- 1 cm - 2. The course of the degradation and stabilization was determined by means of light scattering and spectrophotometric techniques. The extent of photo-degradation was followed by carbonyl formation, gel content and activation energy. Infra-red, ultraviolet spectra and light scattering data have been employed to substantiate a mechanism of degradation.

INTRODUCTION

The polymeric materials in common use in industry, in military applications, in space technology and in household articles are susceptible to oxidative degradation when exposed to natural and induced environmental conditions. The net result of degradation is a decrease in the molecular weight and loss of mechanical properties until, ultimately, the polymer becomes useless. Because of the many advantages

* NCL Communication No. 3602. 313

Polymer Degradation and Stability 0141-3910/85/$03.30 © Elsevier Applied Science Publishers Ltd, England, 1985. Printed in Great Britain

314 R. P. Singh

of polymers, considerable efforts have been made to improve their weatherability, keeping in mind their low cost. A large number of compounds and their mixtures have been suggested for the thermal and/or photo-chemical stabilization 1 -3 of polymers.

In the present work, therefore, evidence concerning the kinetics by which 3-(o-carboxyphenyl)-l-phenyltriazene-N-oxide [CPT] sensi- tizes and copper(II) bis-3-(o--carboxyphenyl)-l-phenyltriazene-N-oxide [CCPT] stabilizes isotactic poly(1-butene) [IPB] is described (Scheme 1).

~ - ~ N--OH "(-A) H~t H " " " ~ 0

x~_._// l / N----N N--N /

~ C O O H ~ / C O O H

CPT

~ N~O. II ,~Cu/2 N--N v,,,,,

~ CO0

+by

('-A) Heat

CCPT Scheme 1

- - N - - O \ I ,~Cu/2

N=N" ~ , . ..CO0

The purpose of this study was to determine the kinetic parameters such as energy of activation, AE, and frequency factor A during 253.7nm light irradiation of IPB film from 267 to 313 K in Ihe absence and presence of CPT and CCPT, and to find a stabilizer which may act simultaneously as a light and heat stabilizer, an antioxidant and a free-radical scavenger. These kinetic parameters could throw some light on the mechanism of random chain scission and formation of the cross-linked residue. It was found, during the investigation, that CCPT behaved as an oxidation retarder whereas CPT merely enhanced the oxidation of IPB. Infra-red and ultraviolet spectra have been utilized for the qualitative deter- mination of the degradation products. Although the kinetics of thermal degradation of atactic poly(1-butene) have been extensively studied by

Kinetics of photo-sensitized and photo-stabilized isotactic poly(1-butene) 315

Stivala et al., 4 little attention has been paid to photo-oxidative degradation and stabilization of the isotactic form 5 -s in the solid state.

EXPERIMENTAL

Preparation of polymer film and photo-irradiation

Isotactic poly(1-butene) [IPB] was obtained by courtesy of Mobil Chemical Co., Metuchem, New Jersey, USA, and the atactic portion removed according to the procedure of Natta et al. 9 The polymer sample was washed with ether and dried (40-60 °) in vacuo. The ligand, 3-(0- carboxyphenyl)-l-phenyltriazene-N-oxide [CPT], was synthesized by coupling diazotized anthranilic acid with freshly prepared phenyl- hydroxylamine.~° The chelate, copper(II) bis-3-(o-carboxyphenyl)-l- phenyltriazene-N-oxide [CCPT], was obtained by coupling cupric chloride with 3-(o-carboxyphenyl)-l-phenyltriazene-N-oxide. Film (0.15mm) preparation, incorporation of the stabilizer into the film matrix, photo-irradiation of the film with 253.7 nm monochromatic light and subsequent dissolution of the stabilized and unstabilized I PB films in cyclohexane have been described elsewhere. 5

Methods of characterization

Refractive index increment (dn/dc) The refractive index increment (dn/dc) with IPB concentration and the optical constant (H) for the solution in cyclohexane were determined using a Brice-Phoenix differential refractometer (Phoenix Precision Instrument Co., Philadelphia, USA), the values at 298 K being 0.0743 and 0-694 × 10 -6, respectively, at 546nm.

Light scattering studies Known volumes of solutions of IPB, with and without CPT and CCPT, were centrifuged to 15 000 rpm for about an hour to remove suspended impurities from the solution. Light scattering measurements were carried out to evaluate the weight average molecular weight ()f/w), with a light scattering photometer designed and calibrated by the present author. 1 x

Spectrophotometric measurement Infra-red spectra of irradiated films were recorded by a Perkin-Elmer (Model 21) infra-red spectrophotometer in the absence and presence of

316 R. P. Singh

0.1 wt ~ CPT and 0-1 wt ~o CCPT at 293 K. The ultraviolet spectra of lPB films irradiated for different times at 293 K were recorded by a SP-7000 automatic ultraviolet-visible spectrophotometer.

Hydroperoxide group formation Polymer hydroperoxides produced during irradiation of IPB film were determined by an iodometric method. 12

Gel content measurement The gel content is the percentage of IPB insoluble after 48 h at ambient temperature in cyclohexane with occasional agitation and determined by evaporating a filtered aliquot to dryness. The time to failure is the time to the start of gel formation and was determined by extrapolating the steep plots of gel contents versus irradiation time to zero gel content.

RESULTS AND DISCUSSION

Plots of h~t w versus t show a rapid decrease in )f/w which then slows down, suggesting that the initial drop in/ff/~ is due to scission of bonds at various weak links which may be distributed along the polymer chain. The bond scission may be random. In the case of random bond scission, the rate of /f/w decrease at a given time should be proportional to the square of 37/~. 13,~ 4 Therefore, in order to clarify the type of bond scission, the rate of decrease of A~t with time, d~l~/dt, was calculated using the equation

d37/w ~ t , 0 - ~ t dt = t ' (1)

where )~tw, t and _~tw, o are weight-average molecular weight at irradiation times t and zero, respectively. The plots of d~tw/dt versus (Mw,,)2 are shown in Fig. 1 and were found to be linear. These plots suggest that the weak links are randomly distributed within the polymer chain and that two kinetically independent units are taking part in the scission of bonds in the polymer.

Plots of degree of degradation, ct, versus time, t, for I PBfilms irradiated in the absence and presence of 0.1Wt~o CPT and 0 . 1 w t ~ CCPT at various temperatures with a light flux of 2.38 x 10-9 einstein s-1 cm-2 are shown in Fig. 2. ~ can be obtained by assuming that the rate of

Kinetics of photo-sensitized and photo-stabilized isotactic poly(1-butene) 317

I~ ~ ~ 0 o.I oJ o.I ~1 ~')

÷ ÷ ÷ ÷ ÷ ÷ • ÷ ÷

O_ I~. Q. Q_ Q.. O_ 0 , 0 _ I1_ Q. Q. ft- Q. Q_

I-- I--" Q .

÷ ÷ ÷

I i l ~ I

o ~ o ~ 0

~p CL-~) M~_p

i l ~ I ! I

,...,

0

.3

.e. E e.,

c.q

~ . 2

e~

o ~

b x ~ E -

0~. . . ~ ~, o

' ~ N 0

N x :

e-,

0 ~

318 R. P. Singh

5 0

4 5

4 0

35

30

X

20

15

10

5

Y

/

I • ..."

/.." /

f

............ .I. ........... • . . . ' "

• . .It"" T," x

F . . .A ....A...... o . . . .

/ . . .$ . . V ~ •

[ . L i ' " " / / ~ /

.... .11 ............... II / ... '"

,/ I1"

o . . . . . . . . . . . . . o

/ , : / (7" ../.. .......

...I)

t l l . / "

/ . .~-_~___

" IPB ] ..... • ..... I P B * C P T ~ 267K - - e - - IPB * CCPTJ

IPB ] ..... o ..... I P B . C P T ~ 273K

t - ~ - IPB* CCPT J

- . . . . . ,PB / I P B * C P T ~ 283K !

- - I ~ - IPB* CCPT J

A IPB 1 .... • ..... I P B * C P T i ~ 293K - - ~ - - I P B . CCPT)

x IPB } .... X--.. IPB* CPT 303K - - X - - IPB* CCPT

• IPB t ..... T- .... IPB* CPT 313K

- - ~ - - IPB * CCPTJ

o

Fig. 2.

8 16 24 32 4 0 4 8 T ime ( x 1 0 ~ s )

Plots of degree of degradation (~t) versus irradiation time for IPB with and without O-lwt ~ CPT and O.lwt ~ CCPT.

Kinetics of photo-sensitized and photo-stabilized isotactic poly(1-butene) 319

breaking of links is proportional to the number of links present at any time t. Thus

d(pw,, - s) dt = k l(pw'' - s) (2)

where s links undergo scission out of a total of Pw,, = P w , o - 1 links. Integration of eqn (2) gives the degree of degradation:

= 1 - e k't (3)

The values of a for the irradiated films are lower in the presence of CCPT and higher in the presence of CPT compared with the corresponding values of the base films. These results indicate that CPT enhances-- whereas CCPT retards--the photo-degradation of IPB. The initial increase in ~ value subsequently reaches a saturation limit. Jellinek and Flagsman 15 concluded that oxygen attacks only the weak links in such cases. At longer periods of degradation, in both the absence and presence of C PT or CCPT, there are indications of simultaneous rupture of weak and normal links and cross-linking but the latter predominates.

For random chain degradation, ~ = k~t; thus, the initial slope of the a versus t plot allows one to calculate k 1, the specific rate constant. The activation energies of the chain scission process in I PB in the absence and presence of 0 . 1 w t ~ CPT and 0 . 1 w t ~ CCPT were estimated from Arrhenius plots (Fig. 3). The method of least squares can be used to evaluate the slope ( - A E / R ) and the intercept (In A) from these data. The values of frequency factor (A) and activation energy (AE) are substituted in the following equations 16 with AE in calories and A in s- 1.

k 1 = 5 . 4 0 5 × l O - 3 e x p ( - 6 0 8 0 / R T ) s -1 I P B + 0 - 1 w t ~ C P T (4) k 1 = 5 . 6 8 2 × l O - 3 e x p ( - 6 6 2 0 / R T ) s - l IPB (5) k 1 = 6 . 4 6 1 x l O - 3 e x p ( - 7 3 3 0 / R T ) s -1 I P B + 0 . 1 W t ~ o C C P T (6)

It has already 1 v been reported that polymer degradation is a zero order reaction with respect to polymer and oxygen concentrations, in which case the values of k 1 are nearly constant with time. Simha and Wall is have also pointed out that the first order law is not applicable to the random chain scission process.

The higher values of AE (7.33 kcal mol - 1) in the presence of CCPT and the lower values (6.08 kcal mol - 1) in the presence of CPT indicate that the former retards, whereas the latter enhances, the rate of photo- degradation of IPB.

320 R. P. Singh

6 t -17

-18 v ¢..

-20

Fig. 3.

=- I P B . . . . • e - . . . . . I P B + C P T - - e - - I P B + C C P T

"-.

~ . ~ * " . . . ,

" ' . ~ e . . . . ....

" ~ ~ " ' ~ . .

"~,,

-21 I I I I I I I 3.1 3 .2 3 .3 3 . 4 3 . 5 3 . 6 3 . 7

T -1 ( x 1 0 - 3 K "1 )

lnkl as a function of lIT in the photo-degradation of IPB.

In support of this view, the weight-average chain scission s, defined 19 as in eqn (7), is shown in Fig. 4 as a function of time at various temperatures:

[ 2_l(e-, Pw,o -- L s2 J + s - 1) (7)

where Pw,0 and pw,t are weight-average degrees of polymerization at times zero and t, respectively. The linearity of the plots of s versus t give an indication that no increase in the degree of branching occurs at the beginning of degradation such as might arise from cross-linking.

Figure 5 shows the changes in dissymmetry ratio, Za, the ratio of scattered intensity at 45 ° to that at 135 °, versus time of irradiation of IPB films in the absence and presence of CPT and CCPT in air for different times at 293 K. Z n decreases with increasing time in both the absence and presence of 0.1 wt % CPT and 0.1 wt % CCPT but the Zn values for the irradiated IPB films are lower in the presence of CPT and higher in the

Kinetics of photo-sensitized and photo-stabilized isotactic poly(1-butene) 321

Fig. 4.

9-5 o .." ""-- ....... o Q.. ,

.,..

o . . "

o .."

,,, ........ ~ _ _ _ g " ~ / / :. /

.:" o

/

/

o'

/ A ...... j . ,,--

/ &...'"' / ...,"

! " ~ / / " ~ ~ -- --A ! :' 1 ~ .,T ~/ ~ / .........

/ .:::

/ / /

/ ~ .... • / • ........

/ / ---

__ .0 -_

A

- - 4 k - -

_ _ o ~ _

~PB 1 IPB • CPT [ 2 6 7 K

/

IPB * CCPT J

IPB

IPB * CPT 2 7 3 K IPB * CCPT

IPB IPB ÷ CPT 2 8 3 K

IPB * CCPT

IPB

IPB * CPT 293K

IPB * CCPT

]PB 1 I P B * C P T [ 313K

IPB • CC PT J

le",~ ~ I I I I I I 0 8 16 2 4 32 4 0 4 8

T i m e ( x 103 s)

Changes in weight-average chain scission (s) during 253-7 nm irradiation of IPB film at 293 K with and without the stabilizers.

presence of CCPT compared with the corresponding values of base film. This means that CPT enhances, whilst CCPT retards, the photo- degradation of IPB.

The changes in hydroperoxide concentrations during the photo- oxidative degradation of IPB were determined by iodimetry.12 Figure 6 shows that the hydroperoxide content of the polymer containing CC PT is lower, whereas that of the polymer containing CPT is higher, than that of base IPB. These results suggest that CCPT stabilizes the IPB by destroying the unstable reaction intermediates, i.e. carbonyl and hydroperoxide, to give relatively stable products such as alcohols. On the

322 R. P. Singh

F i g . 5 .

3,C

-.-.~ -- IPB

.... • ...... IPB ,,. CPT

~.5 l l l ~ ill JPB. CCPT

2 ' 0 t - : ' " . ~ ' x •

"'""...~.. ~ IL~ JI . . . . -e-- ~ 1 ~

.... i . . . . . . . ° . . . . . . . . . . . . . •

1.5~ i i i ....... °"'"'i'"'"'° ,

0 8 16 24 32 40 48 T ime (x103 s)

Changes in dissymmetry ratio of I PB film with time of irradiation with 253.7 nm light at 293 K in air.

10"I I

10-2" i

I0"

~0-~.1~.; .P . / / / o I" :7 I

,o-JT I ,' f , ' ~ / 1 1 I II/g' 7 f l I?/

1 0

";'I//" 1 I

. . I . . l " - - - - I .............. il ............... P .... ..11" ......... •

• IPB } . . i _ - -_ • "o ...... • .... IPB+ CPT 267K 7 ..//i~. I I - - -~L . ~1.. " --e--IPB,CCPT

I L / / I ~ / J I¢" - - " - , l l " " ' i l , IPB 1 , / ~ - - - ~ - ~ -, ..... .it ..... IPB*CPT J 293K

.,~/ " , , " l i - - i - - IPB+CCPT

~ l ; IPB ..... t l .... IPB*CPT / 313K - - l l - - IPB÷ CCPT

I I I I 0 8 16 24 32 40 48

T ime ( x 103 s)

Fig. 6. Hydroperoxide group content of IPB film during 253"7 nm irradiation in air at 293 K.

Kinetics of photo-sensitized and photo-stabilized isotactic poly(l-butene) 323

F i g . 7 .

14 - ; IPB 1 .... • .... IPB÷CPT ~ 273K

1 2 - - - I - - IPB+CCPTJ • • IPB ] ...-

.... A- .... IPB+CPT i' 293K ...-"" - - • - - IPB *CCPTJ ..-"

1 C - ; IPB ] .... ...li / / / 1 1

.... ! . - - I P B ÷ C P T ~' 313K ...'" / ..A i - - I - - I P B ÷ CCPT,I i ' " ' " ~ .......... U - - "" • " ' "

_ ." .. .. ~ A _ _ ~ , l k

• " _ _ _ D . . . . 4 ,

0 8 16 24 32 40 48 Time (x 10 :} s)

Gel c o n t e n t o f I P B f i lm d u r i n g 253.7 n m i r r a d i a t i o n of I P B fi lm a t 293 K in air .

other hand, CPT enhances the degradation. The photo-degradation rate was also determined by means of carbonyl index measurement at 1721 cm - 1. It was found that the carbonyl index of the irradiated film is higher in the presence of CPT and lower in the presence of CCPT compared with the corresponding values of base IPB.

The photo-degradation was also studied by gel content measurement. The percentage gel content was plotted as a function of irradiation time in the absence and presence of 0.1 wt ~o C PT and 0-1 wt ~o CC PT as in Fig. 7. These plots show that gel contents increase in the presence of added CPT and decrease with CCPT in comparison with the base IPB at each temperature. This is a clear indication that CCPT acts as a photo- stabilizer and CPT as a photo-sensitizer.

Kinetics and mechanism o f photo-degradation-stabil ization

The photo-degradation of polymers (PH) is a flee-radical chain scission process 5 comprising initiation, propagation, hydrogen transfer and

324 R. P. Singh

termination reactions. Under steady-state conditions, the overall rate of photo-oxidation is

d[~t2] /¢R '~ \1 /2 ---kv~) [PH] (8)

and the overall activation energy, Eat,, of photo-oxidation is

Eac ' = Ep + ½E i - ½E t (9)

where El, Ep and E t are the activation energies for the initiation, propagation and termination steps, respectively.

However, the situation is different in the presence of a stabilizer (SH). The termination step, instead of being a combination of alkylperoxy radicals, involves reaction of an alkylperoxy radical with the stabilizer. If the stabilizer is effective, the termination rate should suppress the propagation rate and the overall photo-oxidative reaction may be stopped until the stabilizer is exhausted. Under steady-state conditions, the overall rate of photo-oxidation in the presence of a stabilizer is

d[O2] = kp (Ri) [PH] (I0) dt kin h [SH]

and the overall activation energy is

Eac t = Ep -b E i - Ein h (11)

where Ein h is the activation energy for the termination step in the presence of a stabilizer and is much lower than E t in eqn (9).

May et al. 21 found that addition of a stabilizer to a polymer delays the onset of photo-degradation and increases the activation energy of oxidative degradation; therefore, the higher the activation energy the more effective is the stabilizer.

Under ultraviolet irradiation the primary oxidation products of the polymer (i.e. peroxy and carbonyl groups) are easily decomposed into different types of free radicals, depending upon the polymer structure and the reaction conditions, and these newly formed groups initiate the oxidative chain scission process. The CPT has one readily available free carboxy group in its backbone, due to which it sensitizes the degradation. The ultraviolet irradiation is absorbed by carbonyl groups, causing their excitation, and some of the electronically excited carbonyl groups may transfer energy to dissolved molecular oxygen, forming singlet oxygen.22 The degradation is subsequently accelerated by the reaction of singlet

Kinetics of photo-sensitized and photo-stabilized isotactic poly(1-butene) 325

oxygen with even the saturated carbon atoms in the polymer chain to form hydroperoxides. 23'24

The photo-stabilization mechanism of IPB by CCPT involves both interference with the propagation reaction of the oxidative chain and decomposition of the polymer peroxides and hydroperoxides. In addition to hydrogen atom transfer, a series of other reactions can influence the process of inhibition by CCPT through electron transfer (eqn (12)) and formation of complexes (eqn (13)):

POO + C C P T ~ P O O - + C C P T + (12)

P O 0 + CCPT --* [ P O O - . . . CCPT + ] (13)

The stabilized free radical of CCPT would be expected to function as a free radical trap and thus terminate the chain. The metal chelate may also remove the energy by a quenching mechanism. The excitation energy of the polymer is transferred from the macromolecule to CCPT, which has an unsaturated group and dissipates the accumulated harmful energy as harmless radiation or heat through its resonating structures. Due to the energy difference of d-orbitals in CCPT, it may also absorb the ultraviolet light energy and thus initial light absorption processes are prevented. The CCPT also dissipates the energy by a molecular rearrangement without destroying IPB bonds or its own structure. Thus, the chelate acts as an ultraviolet absorber, light shielding agent and peroxide decomposer. In addition to this, CCPT inhibits the initiation process by acting as a free radical scavenger and by the formation of inert charge-transfer complexes.

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90, 93 (1981). 8. R. P. Singh and A. Syamal, J. Mater. Sci., 16, 3324 (1981). 9. G. Natta, P. Pinno, P. Corradini, P. Danusso and G. Moraglio, J. Am.

Chem. Soc., 77, 1708 (1956).

326 R. P. Singh

10. A. K. Majumdar, Anal. Chem. Acta, 40, 299 (1968). 11. R. P. Singh, PhD Thesis, Kurukshetra University, 79 (1979). 12. C. D. Wagner, R. H. Smith and E. D. Peter, Anal. Chem., 19, 976 (1947). 13. N. Grassie, Chemistry of high polymer degradation process, Butterworth,

London (1956). 14. R. H. Boyd, Thermal stability of polymers, Vol. I (R. T. Conley (Ed.)),

Dekker, New York, 75 (1970). 15. H. H. G. Jellinek and F. Flagsman, J. Polym. Sci., 8, 711 (1970). 16. S. Glasstone, H. Eyring and K. Laider, The theory of rate processes,

McGraw-Hill, New York, London (1941). 17. H. L. Bhatnagar and M. M. Singh, Ind. J. Chem., 6, 218 (1968). 18. R. Simha and L. A. Wall, J. Phys. Chem., 56, 707 (1952). 19. I. Sakurada and S. Okamura, Z. Phys. Chem., A187, 289 (1940). 20. J. A. Howard, Rubber Chem. Technol., 47, 976 (1974). 21. W.R. May, L. Bsharak and D. B. Merrifield, IandEC Prod. Res. Dev., 7, 57

(1968). 22. M. L. Kaplan and P. G. Kelleher, J. Polym. Sci., 9, 565 (1971). 23. M. L. Kaplan and P. G. Kelleher, J. Polym. Sci., 8, 3163 (1970). 24. T. Mill, K. C. Irwin and F. R. Mayo, Rubber Chem. Technol., 41,296 (1968).