On the kinetics of polymer degradation in solution—III

European Polymer Journal. Vol I1 pp 761 to 766. Pergamon Press 1975 Printed in Great Britain

O N THE KINETICS OF POLYMER DEGRADATION IN S O L U T I O N - - I l l PULSE RADIOLYSIS STUDIES USING THE LIGHT-SCATTERING

ON POLYISOBUTENE DETECTION METHOD

G. BECK, D. LINDENAU and W. SCHNABEL Hahn-Meitner-Institut ftir Kernforschung Berlin GmbH, Bereich Strahlenchemie. 1 Berlin 39, Germany

(Received 14 April 1975) Abstract--Polyisobutene (PIB) dissolved in saturated hydrocarbons was degraded by irradiation with 2 ,us impulses of 15 MeV electrons. The time dependence of the change of light-scattering intensity (LSI) (due to the decrease of molecular weight) was measured. The half life r I,_,(LS) of the decrease of LSI was not influenced by the addition of cyclohexene, which on the other hand drastically reduced the extent of degradation, leading to the conclusion that the time of establishing bond scissions in the main chains is much shorter than rx;,(LS ) and the latter corresponds to disentanglement diffusion. This is furthermore evidenced by the following experimental findings: r L_,(LSt increases with decreasing PIB concentration, i.e. with decreasing macroviscosity. However, T~ _,(LS) increases linearly with increasing microviscosity as shown by measuring T~,2(LS) in different hydrocarbon solutions 1('~ C~,) r~ e(LS) is proportional to M°3'k i.e. significantly weaker dependence on N-'Ithan expected for translational diffusion. Thus, it is believed that disentanglement diffusion and translational diffusion may be distinguished As n-propanol was successively added to 17-hexane solutions of PIB. r~ _,ILSI increased, indicating that disentanglement diffusion is easier for Ioosel~ lhan for lighll.~ coiled macromoleculcs.

A. I N T R O D U C T I O N

We reported recently [1] first results concerning timeresolved light-scattering measurements with polyphenylvinylketone (PPVK) and polymeth> I mcthacrylate in solution. Main chain scissions ~c~c generated by irradiating polymer solutions either v,ith 15 MeV electron impulses (2 las) or with frequency doubled flashes (25 ns) from a ruby laser (347.1 rim). During that work, it turned out that, for PPVK in benzene. the light-scattering intensity (LSI) decreased after the flash due to chain scissioning with a half-life r~.2(LS ) of about 15--25 p,s, depending on the molecular weight. These values were not influenced by quenchers although the quantum yield of scissioning q5 (S) was significantly decreased. It was therefore assumed that the time for splitting a bond in the main chain is much shorter than "q~z(LS) observed. This conclusion was corroborated by work described in Part II [2]. As is generally assumed [3-5], the photolytic scissioning of PPVK occurs essentially via triplet states. Our measurements of triplet life-times based on T - T absorption yielded half-lives of *PPVK 3 in benzene of about 70 ns [2]. It was therefore concluded that zl/2(LS ) corresponds to diffusion. Furthermore, since %/2(LS) was only slightly dependent on the molecular weight of the polymer, it was assumed that z z,._,(LS) values found for PPVK in benzene correspond to disentanglement diffusion [1]. Since, to our knowledge, no other work was connected with this mode of macromolecular diffusion, we sought another suitable polymer which (a) undergoes main chain scissioning without simultaneous crosslinking,

(b) degrades in its main chains in a much shorter time than the fragments need for disentanglement and (cl whose solution possesses a reasonably high refractive index increment in order to keep the signal to noise ratio at a high level. Polyisobutene dissolved in saturated hydrocarbons appeared to fit all these conditions (a) Bulk PIB is known to be degraded in its main chains rather efl'ectivel,, under the influence of ionizing radiation [20]. One hundred eV yields of chain fractures G(S) between 1.5 and 5.0 were reported. Simuhaneous crosslinking has not been indicated: if it occurs at all. the i00 eV yield was estimated as GIXt < 0-05 [20]. Contrary to thermal degradation, depolymerization does not contribute to the degradation process initiated bv ionizing radiation at room temperature. Only small traces of isobutene were detected after irradiation with high doses. In dilute solution the degradation yield is enhanced significantly; G(S} values of about 20 were reported [6. 7]. The reaction mechanism leading to main chain ruptures involves radical intermediates as well as excited states. During this work it was found thai the high intensity irradiation applied at pulse radiolysis yields about the same CASt value as obtained with low intensity irradiation (e.g. irradiation with ;'-rays). (b) As ',,,'ill be shown below, evidence was obtained that the time for disentanglement is much longer than the time for a rupture in the main chain. (c) The refractive index increment of PIB in hydrocarbons is rather high ldn'dc > 0.1 cm 3 g t) [21]. We report on several series of measurements with PIB degraded by 15 MeV electrons trom an L-band linear accelerator (Vickers).

762

G. BECK, D. LINDENAUand W. SCHNABEL

Table 1. PIB samples used during this work

According to Eqn. (5) in Part I, the ratio ( U o - U ~ ) / ( U ~ - U L ) should be proportional to the absorbed dose

Sample

D in (ev g - t ) :

M

PIBIII

M

w

2 . 6 x 10 5

PIB II

2.1 x 10 6

n

U0 -

Uoo

2 . 1 4 x 10 5

-

i. 3 x 10 5

Uoo- UL

-

(2)

o¢ G(S)D,

since B. EXPERIMENTAL (a) Apparatus

Essentially the same equipment was used as described in Part I and we shall mention here only the improvements. The rectangular quartz cell (Suprasil) was connected to a flow system, which provided a fast exchange of the solution in the cell. The output from the photomultiplier was fed to a current-to-voltage converter with a rise time of less than 100 ns. The signal was digitized by a model 8100 transient recorder (Biomation) and transferred to one half of the memory of a model 1072 instrument computer (Nicolet). To improve the signal to noise ratio, 4--16 signals were added in the Nicolet computer. The corresponding number of C,~erenkovsignals was stored in the second half of the memory. Substraction yielded the corrected LSI signal. (b) Material

Polyisobutene samples were obtained by courtesy of BASF AG. They were three times reprecipitated from cyclohexane with methanol. Mw was measured by light scattering using a Sofica instrument and n-hexane (dn/dc = 0-167 g cm-3). M, was obtained by membrane osmometry in toluene at 37° using a high speed osmometer (Mechrolab). Values are given in Table 1. Oxygen free solutions were prepared by bubbling with Ar purified by Oxysorb (Messer-Griesheim). The solvents were commercial products of the highest obtainable purity and were fractionally distilled (n-pentane, p.a., Merck), n-hexane (p.a., Merck), heptane (p.a., Merck), n-nonane (> 99%, Aldrich), n-decane (gold label, Aldrich). n-undecane (> 99%, Aldrich), n-dodecane (99% Ferak), npentadecane (98%, Aldrich), ,-hexadecane (spectrophotometric grade, Aldrich). C. RESULTS (a) Characterization o f the observed signal

In all experiments, the light scattering intensity (LSI) decreased after the pulse and approached a limiting value after a certain time. Typical oscilloscope traces obtained in different solvents are shown in Fig. la. Based on the assumption that the change of LSI proceeds according to first order kinetics, Eqn. (1) was derived in Part I [1] ha

(u~- u ~ - ' - ( u , -

(U~ -

UL)- ' -

(Uo -

~-

G(S)mD I02N ,

(3)

where G(S) is the number of main chain scissions per 100 eV absorbed directly by the polymer, m is the molecular weight of the base unit and N the Avogadro number. Plots of results according to Eqn. (2) yielded straight lines. A typical example is shown in Fig. 2. It may therefore be concluded that the observed changes of the LSI signal are due to radiation-induced changes of the average molecular weight of the dissolved polyisobutene. However, it turned out that an estimate of the G(S) value from the slope of the straight line in Fig. 2 was rather inaccurate, since the molecular weight dependence of the second virial coefficient and the particle scattering factor have to be considered. For the interpretation of the time dependence of LSI signal change, knowledge of the number of scissions per macromolecule zM was very important. Therefore, small ampoules with PIB solutions were irradiated at the place of the cell of the flow system with single electron pulses. By dilution of samples irradiated with one pulse of about 40 krad to different concentration, the average molecular weight was determined in a conventional light scattering instrument (Sofica). Applying Eqn. (3) and the relationship ~ = DP~,ot - D P ~ , ot, G(S) values in the order of magnitude reported in the literature [6,7] were obtained. A typical value obtained at [PIB] = 4 x 10 - 2 base mol -~ in hexane was G(S) = 25 corresponding to ZM of about 1-3 sossions per macromolecule and pulse. Table 2 shows results of half-life determinations in n-hexane solutions of different concentrations at different doses (which cover the range of doses applied during this work). At a fixed concentration, the halflife does not depend on the absorbed dose respectively on zM which is proportional to the dose.

0,5

'®

u,.)-' UL)-

(1) 0coo U0 and U~o denote the signal voltage which is proportional to LSI before and a relatively long time after the pulse, UL is the signal voltage due to the LSI of the pure solvent. = designates the degree of degradation, i.e. the number of chain scissions per base unit. Plots of the results of Fig. la according to Eqn. (1) yield straight lines as shown in Fig. lb. = ha a~o -- ~t, = - k t .

@ ° ,%7

i , , 0,1 50 I00 150 200 t[,us] Fig. I. (a) Oscilloscope traces obtained by monitoring the change of LS intensity after irradiation with a pulse of 15 MeV electrons in different solvents. [PIB]: 0.027 base mol l - t Mw = 2-1 x 106, absorbed dose 4 x 10* rad, pulse length 2 ,us. (b) First order plots of data of Fig. l(a) according to Eqn. (1).

0

On the kinetics of polymer degradation in solution Table 3. The degradation of PIB in cycIohexane in the presence and absence of O:.M~ = 6.7 × t0 S[PIB] = 0.027base mol 1-~ absorbed dose

0.8 PIB in n-hexone

dj

;°)

763

o"

o.,U -U o

o9

"ru2(LS)

Uoo'UL

0

'

2

I

3

L

5

absorbed dose [104md]

Fig. 2. Plot of (U o - Uoo)/(Uo~ - UL) vs absorbed dose. PIB in n-hexane, ]~[w= 2'6 x 105. [ P I B ] : 0-018 base

air

0.44

+ - 0.04

+ 51 - 3

Ar

0.38

+- 0 , 0 4

51 +- 3

mol l- '. We expect zu2(LS ) depending on ZM. if ---Mis significantly greater than one. These results, therefore, demonstrate that the experiments were carried out under conditions where the splitting of macromolecules into not more than two fragments has to be considered. Corroborating earlier findings i-63, we did not observe an influence of oxygen on the degradation of PIB. Typical results obtained in cyclohexane are presented in Table 3.

(b) D~fluence of cyclohexene Unsaturated compounds like cyclohexene and diisobutene are known [6] to reduce the G(S)-values of PIB irradiated in solution. The question arose whether the addition of such a compound influences the half-life z~,2(LS ). Table 4 shows typical results: whereas the ratio ( U o - U~.)/(U~- UL) decreases significantly upon addition of cyclohexene, the half life r m ( L S ) remains unchanged within experimental error. We therefore conclude that the time for accomplishing a bond seission in the main chain is much less than the observed z,/2(LS). It follows that Zuz(LS ) of PIB in a hydrocarbon solution is due to diffusion as r ~,.2(LS) observed in the case of photolysis of PPVK in benzene solution. As was noted in Part I, q/2(LS) corresponds in such a case to the average time the fragments need to separate after chain rupture. When PIB molecules exist in hydrocarbon solutions as statistical coils, the fragments should be intramolecularly entangled to a certain degree at the moment of chain rupture. Therefore, r~ 2(LS) should correspond to the disentanglement relaxation time. In order to learn more about the kinetics of the disentanglement process, we varied certain experimental parameters such as polymer concentration, average molecular weight of PIB and solvent viscosity. Results are reported in the following sections.

Table 2. Dependence of r, 2(LS) on absorbed dose (D,: dose directly absorbed by the polymer). PIB in n-hexane [PIB l= 0 . 5 g / l Da (krad)

TI/2 (LS) (/as)

[PIB] = 1.0 g / l

IPIBI = 10.0 g / l

Da

T I/2(hS}

Da

(krad)

(Ms)

(krad)

(c) Influence of polymer concentration and molecular weight Ideal conditions for observation of the disentanglement relaxation time refer of coure to the case when actually all macromolecules are separated to such a degree that intermolecular interactions do not disturb the diffusion of the fragments after chain rupture. That means measurements should be carried out far below the critical coil concentration at which the sum of all coil volumes is equal to the total volume of the system. We tried to meet such a situation by doing measurements at concentrations as low as possible. The dependence of rt 2(LS) on polymer concentration was measured in the range from 0.1 to 10 g ] - I (1"8 × 1 0 - 3 1"8 x l 0 - I base mol 1-1). As sho~n in Fig. 3(a) and (b), rl 2(LS) increases remarkabl> with decreasing polymer concentration. This increase is more pronounced in hexadecane [ F i g 3(a)] than in n-hexane [Fig. 3(b)]. Obviously, there is also a significant effect of molecular weight. At small polymer concentrations, r~/2(LS) is longer for the higher molecular weight. Furthermore, r,/2(LS) decr_eases less drastically at M,, = 2-6 x 10 5 than a t M , , = 21 x 10 6 . It may be noted that the decrease of r~,2(LS) occurs while the macroviscosity of the polymer solutions increases. As shown in Fig. 4. the macroviscosity increases by a factor of about 7 by increasing the PIB concentration up to about 6 g 1- ~ in the case of the sample with 19[, = 2.1 × 10~. The increase of solution viscosity is less pronounced at Mw = 2.6 x 105. Therefore it is concluded that the macroviscosity does not exert a dominating influence on r~/,(LS), if it is operative at all. Macroviscosity should not affect r~ 2(LS} in dilute solutions if zt2(LS) is correlated Table 4. The influence of cyclohexene on the degradation of PIB m n-hexane. M~ = 2.1 x 10~',[PIB] = 0.018 base tool 1-1. absorbed dose: ca. 25 krad Voi,-% cyc]ohexene

u -u o co Uoo'UL

T U2(LSI (/us)

T 14~2(LS) (~ts)

0

0.55

23 + 1.5

1

0.26

24 -+ 1.5

35.0

+ 30 - 2

47.5

2 3 + 1.5

41.0

+ I0 - 1

2

0.25

25 +- 1,5

17.6

35+2.

24.0

24+1.5-

21.0

I0-+I

5

0.16

26 ~ 1.5

4.5

31+2-

~.5

23+1.5-

ii.0

9+1-

i0

0.14

26 -+ ]. 5

G. BECK,D. LINDENAUand W. SCHNABEL

764

2s0

20O

"•

0

,

=

m

,

2

i "

,

~

,

t

6

,

8

i 100

,

t

,

,"

I

t

6

I

.= . x i

,

2

i

1

t.

i

,

- -

l

150

,.r

- 2 i

6

'~

~10e'

(~ ,

0

8

-6

~ ,

0

, 2

11

.

10 ~ 0

,

8

i

1

I

i0 0

i

t

2

I

1

I

/.

6

i

I

SO

0

0

Fig. 3. ri:z(LS) as a function of polymer concentration, (a) in n-hexadecane at ]~r,, = 2.1 x 10 6, (b) in n-hexane at g]', = 2'1 x 106 and g~l'~= 2"6 x 10s. (c) and (d) plots of the reciprocal values of r(LS) from Fig. 4(a) and (b). with diffusional motions of the fragments produced by main chain rupture, rl,2(LS ) should, however, depend on the microviscosity of the solution.

(d) Influence of solvent viscosity The microviscosity could be varied easily in this case since PIB is soluble in a series of low molecular weight n-hydrocarbons. Figure 5 shows the results obtained at PIB = 1'5 g 1-~ (0.027 base mol I-~). r~ ,_,(LS)increases linearly with-tl (solvent) in the investigated region from about 20 ps in n-pentane to about 250 #s in n-hexadecane.

(e) Influence of coil density It appears feasible to assume that the rate of separation of the fragments depends on coil density in the way that tightly coiled fragments separate more slowly than loosely coiled ones. In order to perform such a change from a relatively loosely to a tightly coiled conformation, we added n-propanol to n-hexane solution, n-Propanol operates as a precipitant at concentrations above 20 vo!._~o: The root mean square end-t__o-end distance x/(M) of a polyisobutene sample of Mw = 1-9 x 105 was reported to decrease by about 30% on the addition of n-propanol to a heptane solution up to the precipitation point [8]. The addition of n-propanol to n-hexane clearly leads to an increase of rl/2(LS) as shown in Fig. 6. A remarkable effect of propanol on the half-life, however, can be detected only at alcohol concentrations above ca. 10%. The dependence of r,.2(LS) on the n-propanol content resembles, therefore, fairly well

2

:s

i

I

1

t

I

i

t

2 i/[cpl --'-'"

I

3

Fig. 5. Influence of microviscosity on the half-life of the light scattering signal, rl;,(LS) as a function of solvent viscosity. Solvents: n-hydrocarbons from n-pentane to nhexadecane. 191"w= 2-1 x 106, [PIB] = 0.027 base tool I-t; Do = 4 x 10'~(rad). the respective dependence of \ / ( ~ ) . It may be noted that the increase of rl,2(LS) cannot be due to the variation of microviscosity, since t1 (solvent) of the 80:20 mixture is equal to 0-38 cP compared with 0.29 cP for pure mhexane, i.e. )/ (solvent) increases by a factor of 1.3, but rl,2(LS) by a factor of more than 3. D. DISCUSSION

From the experimental findings reported above, it is concluded that the observed relaxation time r (LS)* corresponds to disentanglement diffusion. It appeared of interest to compare our results with estimates and experimental results of other authors concerning the time of entanglement between pairs of macromolecules which may be considered as the reverse of disentanglement. Graessley [9] assumed that the characteristic entanglement time is of the same order of magnitude as the maximum relaxation time for the mechanical response of long chain molecules in solution z'. Relationships for r' have been derived by 80 70 i

60

-

50

~

40

..J

=2.1,106

£:

20

8

~o

T=22 °

c,,

IPIBI(g/I)

t3t /

cgC~

8

° 0

2

4 6 [PlBl(gll)

10

8

10

Fig. 4. T h e viscosity of the s o l u t i o n a s a function of p o l y m e r c o n c e n t r a t i o n . Solvent : n-hexane at 22:. * ~(LS) = Tt.z(LS)/0"693.

i

t

i

i

I

i

i

10 volume froction (%) n - propanol

i

,

2O

Fig. 6. Influence of coil density on the half-life of the light scattering signal, zl .,(LS) as a function of the fraction of n-propranol in n-hexane-n-propanol mixtures, lq',,r= 2"1 X 106 [PIB]: 0"027 base mol I- t. D,~ = 4 × 104 (rad).

On the kinetics of polymer degradation in solution Table 5. ~(LSI) o values (extrapolated to [PIB] = O) compared with ~' calculated according to Eqn. (4) for solutions in n-hexane.

-Mw

1~11

v" (~.s)

106

465

99

41

2.6 x 105

ii0

3

26

2.

i

v(Ls) °

(~s)

(crn3[~) x

Bueche [10] a n d Rouse [11]. According to the theory of the latter author, at infinite dilution the following e q u a t i o n should hold: 6M[q]~s r' --

~z2R T

.

(4}

M denotes the molecular weight. [r/] is the intrinsic viscosity and r/~ the solvent viscosity. Recently it was reported [12] that the longest internal relaxation time for a polystyrene sample ofM,~ = 2.7 x 10 v in cyclohexane and b u t a n o n e obtained by quasielastic light scattering measurements a m o u n t s to a few ms. Calculations of ,' according to Eqn. (4) yielded fair agreement with those experimental values. In our case values calculated according to Eqn. ( 4 / a n d observed r(LS) values do not agree very well. as shown in Table 4. The significant difference between experimental and calculated values pertains to the molecular weight dependency of the relaxation time. Equation (4) yields z' m M ~69 (with [~/] oc M°4"~). i.e. a dependency which is much stronger than the observed [r (LSI ~c M ° 34]. Therefore it may be concluded that disentanglement diffusion c a n n o t be correlated in a simple v,a\ to the internal relaxation times of coiled macromolecules. Insight into the p h e n o m e n o n of increasing :{LS) with increasing dilution may be gained from theories concerning the concentration dependence of translational diffusion coefficients D, of macromolecules [13, 19], D, is postulated to depend upon both therm o d y n a m i c a n d hydrodynamic terms. A series expansion often used expresses D, as a function of polymer concentration c: D, = D,,o(1 + k ~ c + . . . . )

(3)

where k o = 2 BM - k: - T. k : = friction coefficient, i = = specific volume of polymer. Evidence for D r increasing with increasing polymer concentration has been provided for various polymers with relatively high molecular weight [13-18]. As is s h o w n in Fig. 3(c) a n d (d), plots of reciprocal half lives vs polymer concentration yielded straight lines. I~ therefore appears that a similar relationship holds

765

for the disentanglement diffusion and for the translational diffusion. However, an estimate based on z~,=(LS) values obtained by extrapolation of [ P I B ] --~ 0 yields %/2(LS) :~ M ° aa. As shown in Part 1. ~, _, proportional to a b o u t M ~ s is expected if the polymer consists of flexible chains with coil conformation and if the fragments separate after chain rupture by pure translational motion. The results obtained with PIB. therefore, corroborate findings concerning the molecular weight dependence of rw2(LS) for polyphenylvinylketone in benzene solution [1] and prove once more thal disentanglement diffusion might be discriminated from translational diffusion. Acknowh'd.qcment The authors arc grateful to BASF A(i for providing polymer >amples and to Fonds dot Chcmischen Induslrie for partial supporl ol thi> v, ork

REFERENCES I. G. Beck, J. Kiv.'i. D. kindenau and W Schnahel. l-mOl,. Polym. J. tO. 1069 119741. 2. G. Beck, G. Dobrov, olski..1. Kix,,i and W Schnabcl. Macromoh'cuh's 8, 9 11975i. 3. F. J. Golemba and J E Guillet..'~[a¢rtml, qc~uh'~ 5. 212 (1972t. 4 1 Lukfic~, P Hradloxid. Z 'Mtlll~lsek and D Bcih:,. ,I Polvm S('i, AI. 9. 6q {t9711 5 C. Da,,id. V~'. Delll~lrtcdu al/d (J Gcu>kcn,,. t:un,i, Polym. J. 6, 1406 (1970) 6 A Henglem, and C Schncidcl. Z. phl~ Ch~,m \/- 19, 367 {1959}. 7. K. 1. Lukhovitskii. \ . A. Tsingister. N. A Shdr:ldilla and v I L. Karpov. [ysokotllolck Soedin & 1932 11966t. 8. E. D. Kunst, Re~l Trac (him. P~s-Ba5 Bcl H 69. 125 ( 1050}. 9. W. w Graessle 3. J ~hcm Phl~ 43. 2ht,'6 (1~)6~,1 10. F. Bueche. J. chem. Phvs 22. 603 11954) 11. P. E. Rouse. J. chem PIn.~ 21. 12"72 {It)S'h 12 W.-N Huang and J [-. Frederick. \/~c~,,,,,lc~,/,., 7. ~4 (1974). 13 L Mandclkern and P J. Flol\..I ~hcm Phl~ 19. 9,,4 119511. 14. W. N. Vanderkooi. M. W Long and R A Mock. J. Polvm. Sci. 56, 57 {196,2}. 15. ]. Bisschops, J. Polvm S{i. 17. 81 (]955i 16. R M. Secor. A. I. C h . E . Jt il, 4.';2 ~1965l 17. T. A. King. A. Knox. and J D G Me,dam. P,,ll,lcJ 14. 293 {1973). 18. T. A. King. A Knox. \V. I Lee. and .I D ( ; \4c.\d~m~. Polymer 14. 151 (1973). 19. P. J. Florv. and V~L R. Krigbaum..I chc,i Phl'~ 18. 1086 ( 195()1. 20. G.G.A. B6hm. I n : The Rtidiation Ch~,mistrl Ol '~la~,,,u~lecules. (Edited b} M Dole). Vol [I. p 19t~ ..\c;tdcm~c Press. New York {1973i. 21. M. B Huglin. In: Light Scare'rim/ lro,l Pollm~, .'~oh~tions. (Ediled b3 M. B. Huglinl. p 16"~ fl Academic Press. London I19721

Zu~mmenfassung--ln ges~ttigten Kohlenwasserstoffen get6stes Polyisobuten (PIBt wurdc mit 2 Hslmpulsen von 15 MeV-Elektronen bestrahlt. Dabei wurde das Polymere abgebaut und die damit ~c~bundene Abnahme tier Intensidit des gestreuten Lichtes (LSI) als Funktion der Zeit registriert I)ic Halbwertszeit r, _,(LS)der Abnahme der LSI wurde dutch die Zugabe von Cyclohexcn nichl bccinltulq. obwohl das AusmaB des Abbaus drastisch vermindert wurde. Daraus folgt, dal3 die Zeit £ir &c Spaltung einer Hauptkettenbindung viel kLirzer sein muB als r~ 2tLS/ und dab T~ 21LS! dutch d~c

766

G. BECK, D. LINDENAU and W. SCHNABEL Entkn~iuelungsdiffusion bestimmt wird. Diese Schlul3folgerung wird durch andere experimentelle Ergebnisse best~itigt: rj ,(LS) nimmt mit abnehmender PIB-Konzentration, d.h. mit sinkender Makroviskosit,it, zu. Messungen in verschiedenen Kohlenwasserstoffen (C5-C16) ergaben, dal3 rl/z(LS ) mit zunehmender Mikroviskosit~it linear ansteigt, r~ _,(LS) ist porportional M °34, d.h. bedeutend schw~icher vom mittleren Molekulargewicht abh~ingig, als fiir die translatorische Diffusion erwartet wird. Es wird daher angenommen, dal3 man zwischen Entkn~iuelungs und Translationsdiffusion unterscheiden kann. Bei tier sukzessiven Zugabe yon n-Propanol zu n-Hexan-L6sungen yon PIB wurde ein Ansteigen yon ~2(LS) beobachtet. Dies deutet darauf hin. dab die Entkn~uelungsdiffusion langsamer erfolgt, wenn die Makromolekeln dicht gekn~iuelt sind und vice versa im Falle lose geknfiueller Makromolekeln schneller verl~iuft.