486
SHIH-TVEI C H E S
THE P H O T O C H E l I I C ~ ~IIEGR~4Da4TIOS L (IF POLITSTITKESE SHIH-KEI CHLS' l ) e p a i ~ i t t i c ~ os i t C ' h c i t ! i s t r y , c.iiii,cc;,sii?j o j I l l i t i o i s > [-i,hrtnci, Illinois
Reccii,ed I.'tbi.iinr!l 2 6 , 1948
The typical polymerization reactions of unsaturated molecules ai'e usually explained by assuming a free-radical mechanism. Since unsaturated molecules such iis styrene shox strong ultraviolet absorption, one niay expect that polymerization can be promoted hy photorhemical action. There is ample evidence for this conclusion in the literature. K h e n the polymerization has been conipleted, the unsaturation \vi11 ha\-e disappeared from the aliphatic chain so that the ultraviolet absorption \vi11 he greatly reduced. Thus Smaliula (19) showed that the iiltraviolet ahsorption of polystyrene is that of the benzene ring, the structure being such that the rings are connected hy saturated cartmn chains and insulated effectively against conjugation between rings. The absorption of ultrariolet light hy the polymer \vi11 result in n different reaction from that ohtainetl \\.lien the monomer is present and the 111osr probahle one is u tlegrndation reaction, essentially the reverse of pol~-merization. For example:
H
CsHj H
('6Hj
polymcr-~-~--~-~-~pol~-niei~
H H
+ hv
+
H I I H
CBHj H
polymer- C-12---
H H
CFH~
C-C-polymer
H
+
H
'
The dehydrogenated polymer may now undergo a rearrangement which result: in two fragments, one of which is a free radical. 1
On Icave f r o m 1:ition:il Sun T:rt deli t-iiiversity, C'hiiin
polynie1.- C- C= CHL
H
ant1 -C- polymer
f€ r .
The ixdicJ \\-ill be self-propugating. 1lie hydrogen atom liberated may likev-ise initiate a self-propagating chain. Since the polymer may hreak at m\point fhe.se proce+es. the general result ivill be thc degradation of the polymer. If t\vo radicals collide they ii-ill terminate the chain by reacting with each other. Ii the concentration of radicals is very high, this reaction may become i~iiportaniin establishing the steady-state concentration of radicals. At steady statc the r:?te of furmation of the rntlic.als miist he eqiial t o the rate of disappearance. For t h r rletermination of niulectilar \\-eights of polymers, L: generally accepted proc'etlni e i.; to relate the average mo1ecul:tr \\-eight of the dissolved polymer to the intrinsic viscosity of the solution, ab siiggestetl hy Icraemer (12). Hence \\-e can get wme information atmiit the tlegr:idation of polystyrene from determinations of inti%isic viscosity. If q is the viscosity of a solution of concentration c in grcinis per 100 ml., i~nc! is that of the pure solvent. the ciurmtity ( q - 770) 70 4ii.e.: \ Y ~ X Tis I;no\\-n :is the specific visco..;ity of the solution, v.,). The limiting r a t infinite tliliition is cniletl t h e intrin$ic i-iacosity [TI.
'l'hc poly-tyrene weti ]\-:is ohtained from the Bakelite Company (sms-13135). is :I roloi~less,transparent solid. ,Ifter dissolving in lienzene, it \\-as precipinte(1 \ \ - i ~ l i:in excess of methanol, filtered off, i\-ashed \\-ith methanol, ancl dried n a v:icinini. ;1highly transparent, colorless po\\-dcr ivas obtained. The sol;ent iised \vas thiophene-free benzene, manufactured by -\. Ilaigger antl Coni,any. -\lthough it T V : ~ indicated ti>+ chromntographic fractionation (1) that \-hen liquicl lienzene is irradiated in the presence of air \\-ith iiltraviolet radiation if 2 3 7 . 5 at least fi1-e tliflferent substances are formed, these suhstances are in -cry smd1 amount and Lire as,5iinied to tie harmless to our present experiment. I-ranyl oxalate \\-as made ti>- niising C.P. uranyl nitrate and reagent quality )salic acid. -1fter it hi~tllieen thoroughly I\ashecl ivith distilled \\-atel and conluctivity water antl centrifuged, it \vas dried at 110°C. for 4 hi. The oxalic acid isecl foi, ;ictinonietric measurement \\-as of reagent quality and \\-as tn-ice recrysnllized. centrifuged, ancl dried over calcium chloride. The sodiuni oxalate, )otassiuni permanganate, potassium iodide. ancl iodine used \\-ere all chemically lure and h n d been carefull\- repurifietl. -411 the aqueous solutions \\-ere nuicle rom conducti~ity\\.ater, filtered through sintered glass. if necessary, ancl kept n botrles of resistant glnas col-ered I\-ith hlacl; enamel.
[t
488
SHIH-TTEI CHES
B. Light soiirce I n this experiment. the helical mercury lamp manufactitred hy the Han Chemical and 3Ianufactiuhg Company ~ v a . wed. It is in the form of a h c grid of 13 turns. 54 nun. in inner diameter ant1 covering a length of 202 mm potential of GO00 T'. I\ a3 applied acro-s the lamp electrotleh through a transfor The power conwniption v a s 13.5 u-att-. By actual test with the artinom 133 watts inpiit gave 4.NS X 10"' quanta (253i.3 ).; per second.
c'. Irradiation The quartz r e a h o n tube v a s 31.1 mni. 111 outer diameter and L4~imn length. It was connected TI ith n small Pyrex tithe at one end, 11 hich berved handle and to fn the 3tirrer. During irradiation the reaction tuhe n-21.: i \\ ith 83 ml. of solution of polybtyrene in benzene antl I\ a s kept concentric the axis of the helical gl.id lamp. To reduce the intensity of the incident ultraviolet radiation tlie re:tction n as mounted vertically I\ ithin an outer quartz tube T\ hich n as 44 mm. in ( diameter and 40 mm. in inner diameter and was filled with filter solution. ing irradiation, both the outer tube and the inner reaction tube were kept centric u-ith the aris of the lamp. The filter solution used was an aqueous tion of iodine and potassium iodide ( 5 ) . The concentrations used are givt table 1. The behavior of the filter solution deviates some\\ hat from Lanihert Beer's law. Xlthough great care hac1 been talien in the purification of the stances used, the filter boltition is not entirely stable t o the light, hence solution \\-as supplied after every 10 min. of irradiation. The solution I\ as conbtttntly stirred to obtain maximum absorption.
I>. -1ctinometric mcasrtrenwnt The total nunibei~ot quanta incident to the reaction tube under different ditions were determined by the use of uianyl oxalate and oxalic acid :is the nometer. The reaction tube \\as filled with a solution 0.012 -11in uranyl 01 and O.OG1 -11 in oxalic acid. -4fter a known period of irradiation an aliquot tion of the solution \\as titrated I\ ith potassium permanganate solution \\hicI been standardized v i t h sodium oxalate to he 0.1111 S . The quaiittiin yiel this actinometric solution had heen determined hy Forhes (6, 9, 13) antl hi \i orkers to he approximately 0.63. The actinometric data used in determ the various inteniities of the incident ultraviolet are tabulated in tuhle 1. The total emissivity of the lamp ac3 mentioned above \\-as determined ir same \yay, except that the lamp was totally immersed in the actinometric tion.
E. Vzscosity Tkcositg determinations were carried out with an WIT-ald viscometer. actly 5 ml. of the solution was introduced into the large bulb of the viscometl means of a pipet. The apparatus I\ as then wspended in a thermostat a t
PHOTOCHEMICAL DEGRSDATIOS OF POLYSTYRENE
489
(& 0.10OC.). -4fter the viscometer and contents had acquired the temperature of the bath, the level of the liquid rose above the upper graduation mark; the liquid was then allowed to flow back through the capillary, and the time required for the surface of the liquid t o pass from the upper to the lower mark was noted by means of a stop watch. For each sample, the mean value of three reliable readings was taken. TABLE 1 Data of actinometric rnensiiretncnts IO
I\ TENSITY i/ \ ~-
-
I1
-
Period of irradiation 4so. 2 4SO.1 480.0 (sec 1 450 0 480.3 Volume of actiiionietric , 85.02 85.01 55.01 s5.01 solutionused (nil.) 85 0 ('oncentration of filter j I k i t h o u t fil solution (g ,100 nil ) t e r solution and 0.1s5 g. I? 1 . 2 3 T g . I? 0.291 g. I? 0.366g. 1, in outer quartz tube n it hou t 0 . 2 9 2 g . K I 3 . 3 7 5 g . K I 0.460 g. K I 0.562 g. IiI outer 1 tube V ol ume of a ct i nom e t 1 i c 20.00 20.00 solution titrated (ml.) 20 00 20.00 20.00 Volume of Ii?.lnO, solution required before 22.09 22.09 22.09 22.09 22.09 irradiation (nil.) Folume of K?.In04 solution required after irra20.45 21.29 9.70 19.03 21.65 diation (ml.) (umber of q u a n t a incident t o t h e reaction 5.526 X 1016 1.438 X 10ls ci. 771 X lox8J.376 X 10l8 ).207 X 10'8 tube per second . . 1 1 ntensity ratio, Z,I C 1 11 _1_ I 4.05 7.56 15.51 28.14
'
~
*Z
=
~
observed intensity of incident ultraviolet
Io = original intensity of incident ultraviolet =
2 x (22.09 - 9.iO) X 0.1111 = 85 - X loo0 20
1 1 1 X 6.0228 X l W 3 X - X __ 2 0.63 480.0 -
= 5.826 X 1Ol8
RESULTS A S D DISCUSSION
-4. Viscosity data
The results of viscosity determinations on about 140 samples under various oncentrations (c), various intensities of the incident ultraviolet light ( I ) ,and arious periods of irradiation ( t ) are summarized in table 2.
490
.
TABLK 2 Florc t i m e , t f (in seconds), at various calues of c, I , and t ( i n secotids) ~~
~
I (SECOSDS).. . . . , . ,. . . . . . . ... . . .. . . , . . , ,
(1) c = 1 g./lOOml. ( 2 ) c = O.i5g./100nil. (3) c=0.5g./lOOml. (4) c = 0.25g./1001111. ( 5 ) c = 0.125g./lOOml.
0
1200
2400
3600
4600
72I)l!
832.8 630.6 460.3 322.0 263.9
696.2 540.2 100.!) 299.5 253.1
616.5 488.2 375.5 285.3 246.1
567.4 453.5 258.1 276.6 2q2.3
541.1 435.3 313.4 271.2 239.5
489. : 395 2 320.6 260.8 234 7
-
(B) I = I1 = (1/4.05) X Io = 1.438 X 1OI8 (1 I c = 1 g./100 ml. (2) c = 0 . i 5 g./lOO nil. (3) c = 0.5g./lOOnil. (4) c = 0.25g./100ml. ( 5 ) c = O.l25g./lOOml.
832.8 630.6 460.3 322.0 263.9
, (C) I
=
I?
(1) c = 1 g./100ml. ( 2 ) c =O.i5g./lOOml. (3) c = O.5g./100ml. (4) c = 0 . 2 5 g./100 1111. ( 3 c = 0.125 g./lOO 1111.
801.3 607.4 444.6 315.2 260.7
= (1/7.56) X
832.8 630.6 460.3 322.2 263.9
i71.5 584.5 430.7 308.9 258.0
743.0 564.6 418.0 303.2 253.2
717. 2 671 8 546.5 512.3 409.9 ' 392.5 299.1 291.5 240.6 253.2
I O = 0.771 X 101s 78S.9 598.9 422.6 314.0 260.3
810.1 614.1 451.5 317.7 262.0
769.8 584.7 433.9 310.4 255.7
753.2 573.5 427.2 307.1 257.3
718.1 552.7 414.5 300.7 254,s
695 0 604 9 444.5 315 1 260 1
778 594 440 312 258
(U) I = 13 = ( l / l 5 . 5 1 ) X I O = 0.376 X 10ls ~
b32 S 630 G 460 3 322 2 263.9
(1) c = 1 g./100 ml. 12) c = 0.75g./10011il I3 I c = 0 5 g./lOO 1111 (4) c = 0 . 2 5 g./100 nil ( 5 I c = 0.125 g./lOO 1111.
S22 623 454 320 262
813 617 451 318 261
0 7
7 3
8
S 0 9 8 0
-__
_.
(L) I (1) c = 1 $./lo0 ml. (2) c = 0 . 7 5 g./100 1111. ( 3 ) c = 0 . 5 g./100 1111. (4) c = 0.25g./1001111. ( 5 ) c = 0.125 g./lOO nil.
The values of
803 611 447 316 261
1 2 3 5 9
T~~
=
IC
= (1/28 14)
S32.S 630.6 460.3 322.2 263.9
x
826.6 625.0 456.5 320.6 263.2
Io
,
= 0.207
520.6 621.4 454.1 319.4 262.6
x
8 3
0 0
-
~
10'8 814.5 617.1 451.0 318.1 262.1
were calculated by the relation
Jvhere t,,, = time of flair of pure benzene = 212.5 do = density of piire benzene at 26.4"c'. = 0.8G88 tl = density of solution of polystyrene in benzene
809.6 799.6 612.9 604.8 448.0 442.9 317.0 , 314.9 261.6 , 260.7 ~
PHOTOC"13IIC.iL
49 1
DEGR-iD-iTIOS O F POLTSTTHESE
The densities of the solutions a t different concentrations are listed belon-: SOLETIOS
d
Polution 1 (1 g. 1100 m l . ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . Holution 2 (0.75 g./lOO nil.). . . . . . . . . . . . . . . . . . . . . . . . . Solution 3 (0.5 g.,;lOO m l . ) . . . . . . . . . . . . . . . . . . . . . . . Solution4 ( 0 . 2 5 g . ~ I O O n i l . ). . . . . . . . . . . . . . . . . . Solution5 (0.125g./lOOml.). . . . . . . . . . . . . . . . . . . . .
0.8698 0.8695 0.5692 0.8690 0.5689
On irradiation the density changes very slightly; TI e may consider it as constant. In determining intrinsic viscosities, the values of log T~~ c were calculated a n d plotted against various value. of c according to Martin's equation (15): 1%
TSP
c = log [r7]
+ k[T]C -
/
f
sxnsmj.,. . . . . . . . . . . . . .
I ~~
~
-.
~. ~.
ir. . . . . . . . . . . . . . . li.. . . . . . . . . . . . . . . . . . . . . . I?.. . . . . . . . . . . . . . . . . . . . .
1.S4 4 4
I:... . . . . . . . . . . . . . . . . . . .
1.S2-I I .S24
14.. .......................
-
1200
2100
3600
4800
7200
1.452 1.713 1,756 1.7S-l
1.207 1.621 1.695 1.754 1.778
1.062 1.533 1.638 1.726 1.755
0,962 1.442 1,588 1.703 1,741
0.793 1.320 1.198 1.661 1.714
ii
-
I.7RS
____._____
The straight lines so obtained were extrapolated to zero concentration to get the values of log [TI. From the values of log [ q ] ,the values of [77] a t various periods of irradiation (i) and various intensities of incident ultraviolet ( I ) n-ere calculated and tabulated in table 3 .
B. Kinetics The relation betn-een [v] and t at various values of I is shown in figure 1. The intrinsic viscosity decreases iirst appreciably and then slo\\-ly v-ith increase of time of irradiation, n-hich decrease is related to the intensity of incident ultraviolet. This indicates that the polystyrene molecules are degraded by ultraviolet light of wave length 2537.5 8. The free radicals produced reunite with each other nhen they collide and the extent of degradation is proportional t o the intensity of the incident radiation. -1steady state may be reached in n-hich the iate of disappearance of the free radicals is equal to the rate of formation. By plotting the Yalues of [TI againqt the product of t and I from the data obtained by the writer, we get the curves shon-n in figure 2 . The five curves deviate from one another slightly. If 11e plot [TI against I:t or I t (where n varies from 0 t o 2 3 ) , the deviations betu-een the five curves obtained are rather large. Althoiigh llesrohian and Tobolqky I l i ) have demonstrated the existence of
492
SHIH-WE1 CHEN
simultaneous thermal degradation and polymerization of polystyrene and styrene a t 100°C. and other elevated temperatures, in the present experiments the rate
1.8
t.6 .
1.4 -
1.111 12 -
1.0 -
0.8 I
0
1
1200
2400
t 3600
FIG. 1. Itelstion of
[SI
4800
6000
7200
t o t at various I
of the thermal reaction of degradation of polystyrene and that of recombination of the degraded fragments were found t o be negligible. Hence $ye may conclude that the rate of degradation of polystyrene by ultraviolet light of wave length
PHOTOCHEJIIC.~L DEGRADSTIOS OF POLTSTTRESE
493
2537.5 8. is proportional to the first power of the intensity of the incident radiation. This can be espreased by
FIG.2. Relation of
[ql t o
1X
t at various I
where d[q] is proportional to the extent of degradation.
If dR/dt be the rate
494
hHIH-JVPZI C H E S
oi forniation of the free raclical, then equation 3 can he rewritten a s :
The rate of disappearance of the free radicals produced may hc expreesecl as:
Hence for a steady state the concentration of the radicals should be proportional i o the square root of the light intensity. It \\-as hoped that this dependence could be demonstrated. But in this ~ o r we k did not find the steady state, prohably because the range of intensities availahle \\-as not snfficient t o demonstrate this effect.
C. Calculation o j molccttlar weiyht I n expressing the relationship between intrinsic viscosity and average molec311lar weight of high polymers, IIarl.; (14) has pointed out that the equation S,Xa (6) is theoretically better fouiidetl than dtaitdinger's rule (201, vsD c = 2131,and seems to be necessary to cover satisfactorily the experimental findings in the range of high degrees of polymerization where Ii and n are two constants characteristic of a certain solute-solvent system. Flory (S) in a very careful and thorough investigation of polyisobutylene fractions has sho\\-n that equation 6 establishes B satisfactory relationship hetu-een 11 and [q] over a very wide yange of both variables. JIarl.; ( 2 ) also pointed out that the wide variation in value from system t o system in a molecular weight range above about 30,000 indicates the necessity for the use of equation 6. For molecular weights belox 5000 a small additive terni ( 2 , 4, 10) is necessary to reach agreement with esperiment. Sumerical values for these two constants, Ii and a , h a r e been estahYished by comparative osmotic and viscosity iiieasurements on strictly fractional samples over a n-ide range of degrees of polymerization for some polymers (3, 4, 8, 11, 16). For the polystyrene-benzene system, these values as ivorlted out by Kemp and Peters (11) and Bartovics :tntl >lark (4)are K = 3.6 x 10-5 and (I = 1.0. From these values the average niolecului, \\-eight of polystyrene used in this experiment \vi11 be: [q] =
11. Ca1citlatio)i oj quantuni yicld From the average molecular weight of polystyrene so obtained, it ib possiblc t o calciilate approximately the quantinn yield of the photocbemical degradation of polystyrene by ultraviolet light of \\:+ye length 2337.5 -1. Take solution 5 and intenity I , as the example.
1'IfOTOCHElIIC.i.L
DEGRhD.i.TIOS
495
OF I'OLYSTTRESE
The average ma3s of the polystyrene molecule5 used in this experiment = 1 1.824 2 X 1.67339 X lo-?' X 2;010 X X 1 O j g., Ti-here 1.67339 X p. is the 3 .A ma*s of the hydrogen atom. 0.125 'YIie weight of polystyrene used for irradiation = -~ X 83 = O.lOti2.5 g . , 100 since the volunw of solution used to fill the reaction tube = 83 ml. Siimber of rnolrctiles of polystyrene used before irradiation
0.10625
=
1.2(ij x 10"
Siinber of niolecules of polystyrene after 3300 wc. irradiation - ___
-___
0.10623
2 X 1.67'339 X lo-" X
Smnber of inolecules increase after 3600 sec. irradiation
1753 2.016 x 3.G
---'-
=
0.048
x
10'
x
Under the present conditions, the reunion of the free radicals produced is con+idereti to be negligible, and all the incident ultraviolet is assumed to he completely absorbed. Since the over-all quantum yield is defined as Suniber _of _ molecules finally decomposed or formed -_______ Sumber of quanta absorhed the tnw-all quantuni yield of .the photoochemical degradation of polystyrene by iiltr:ii.iolet light of TI ave length 2537.5 -1.will be
1
Lssuming the niwt probable degradation to be one molecule degraded into two The primary process in this reaction is the absorption of cz certain nuniber of ight quanta by I: normal molecule of polystyrene, which is thereby activated or sonl-erted into a n escited molecule. Since the quantum yield is so low, it is es)ected that most of the escited molecules undergo deactivation after absorption nd that only n small fraction of escited molecules are degraded into fragments. 'his deactivation may occur by emission of fluorescent radiation or by transfer f the excess energy t o the molecule of solvent during a collision. I n the liquid tate, each molecule is so closely surrounded by other molecules that it is, in effect, iffering continunl collisions. K i t h truly continuous abqorption a molecule rohnbly dissocintes in a time less than the period of one vibration, about
496
SHIH-lVEI CHEN
sec. This is short conipared to the mean interval between collisions with other molecules a t ordinary gas pressures, but becomes of the same magnitude as the time between collisions in a liquid. Hence the latter factor of deactivation seems more prominent than the former one. SUMMARY
1. When exposed t u ultraviolet light of ware length 2537.5 'I., polystyrene molecules are degraded. 2. The degradation rate of polystyrene by ultraviolet light is proportional to the first power of the intensity of the incident radiation:
3. The average nioleculltr weight of polystyrene used in this experiment has been determined to be approximately 50,000. 4. The over-all quantum yield is found to be approximately 1/15,500. 5 . It is suggested that most of the excited molecules undergo deactivation after absorption. Acknowledgment is made to Professors b7.H. Rodebush and F. T. Wall for suggestions and advice in studying this problem and also t o Professor T. E. Phipps for assistance in the construction of the apparatus. REFERESCES XLLSOPP, C. B., .ai)SZE( I , B.: J . Soc. Cheni. l n d . 63,30-1 (1944). BADGLEY, W.J . , . ~ S D~ I . ~ R13.: K , J . Phys. Colloid Chem. 61,58 (1947). B A K E R , \v. o.,E'CLLER,C. s . , h K D HEISS,J. H., JR.:J. Ani. Cheni. SOC.63,3316 (1941). H ,. : J. Ani. Chem. Soc..66, 1901 (1943). BARTOVICS, A Y D ~ I A R K BOWEX, 1:. J.: J . Cheni. Soc. 1936, i 6 . BRACKETT,F.P., -4511FORBES, G . s . : J. - h i .C'heni. soc. 66, 4459 (1933). ELLIS,C., WELLS,-4..i.,.GD HEYROTH, F. I?.: The Cheniica2riction of C'ltraciolet rtays, pp, 251-3. Reinhold Publishing Corporation, S e w York (1941). 18) FLORY, .'1 J.: J. Ani. Cheiii. Soc. 66, 3 i 2 (1943). ( i . S . , A S D HEIDT,I,. J.: J . Ani. Chcni. Soc. 66,2363 (1934). (9) FORBES, (10) FORBYCE, li., .4su HIBBEHT, 11.: ?J. h n i . Cheiii. Soc. 61,1912 (1939). (11) KEMP,A . Ii., . u i ~ PETER>, H . : Ind. I h g . Chein. 34, 1097 (1942). (12) KRAEYER, E. 0.: Ind. k!ng. Chem. 30, 12oo (1935). (13) LEIGHTOS, W. G . , ASI)FORBES;, G . S.:J. .hi. Chcin. Soc. 62, 3139 (1930). (14) MARK,H . : D w f e s t e Korper, p. 103. Y. Hirzcl, Leipzig (1938). (15) MARTIN,.-i. F. : Paper presented at the 103rd lleeting of t h e hinerican Chemical Society, which \\-as held in Memphis, Tennessee, April, 1942. : J . * h i . Cheni. Soc. 6 4 , 2 i i (1942). (16) MEAD,R . J., . 4 S D F V O S S , R (17) & h 3 S R O B L 4 S , I t . , .4XD TOBOI , .I.:J. Ani. Chem. Soc. 67, i s 5 (1945). (1s) SOYES, W.-4.: J R . ,.isn LEIGHTOS, P. A , : Tho Phofochernistry of Gases, p . 147. Reinhold Publishing Corporation, S e w Tork (1941). (19) S M A K C L A , .%.: z. :ingcK. Chem. 47, i i i (1934). (20) STAUDIXGER, 13. : Die hochniolekularen organischen I7erbindungen. J. Springer, Berlin (1) (2) (3) (4) (5) (6) (7)
(1934).
