Makromol. Chem. 185,103 - 111 (1984) 103

Evaluation of kinetic parameters resulting from photostabilized degradation of poly(2,6-dimethyl-l,4-phenylene oxide) films in the absence and presence of zinc bis(0,O ‘-diisopropyldithiophosphate)

Ramesh Chandra

Department of Chemistry, Delhi College of Engineering, Delhi-llOOO6, India

(Date of receipt: September 23, 1982)

SUMMARY: Light scattering was used to evaluate the resulting changes in molecular weight, root-mean-

square radius of gyration, degree of polymerization, number of chain scissions per single chain length, degree of degradation, specific rate constant, energy of activation, enthalpy, entropy and free energy of activation for the photo-oxidative degradation of poly(2,6-dimethyl- 1,4-phenylene oxide) (poly(oxy-2,6-dimethyl-l ,Cphenylene)) film, both in the presence and absence of 0,l wt.-To zinc bis(0,O’-diisopropyldithiophosphate) (ZnDTP) as photostabilizer, undergoing random chain scission and crosslinking processes in the temperature range 313 -363 K with light of wavelength 365 nm. The inhibitive action of ZnDTP was discussed from the viewpoint of the amount of gel fraction.

Introduction

Earlier publications - 5 ) on the degradation of poly(2,6-dimethyl-l ,Cphenylene oxide)” (PPO) have focussed on the changes in chemical and physical structure occurring during the thermal (100 - 250 “C) degradation in which PPO is usually in the form of a blend with polystyrene. The kinetic parameters of the photostabilized degradation of PPO have not, so far, been elucidated.

The purpose of this study was to determine the kinetic parameters such as Arrhenius energy of activation (a), frequency factor (A), enthalpy (AH*), entropy (AS*), and free energy of activation (MI) during 365 nm light irradiation of PPO film between 313 and 363 K in the presence and absence of 0,l wt.-Yo zinc bis(0,O’- diisopropyldithiophosphate) (ZnDTP) which could throw some light on the mech- anism of random chain scission and the formation of a crosslinked residue.

(Zn DTP)

a) Systematic IUPAC nomenclature: poly(oxy-2,6-dimethyl-l ,Cphenylene).

0025-1 16X/84/$03.00

104 R. Chandra

Experimental part

The sample of poly(2,6-dimethyl-l ,Cphenylene oxide) (PPO) was obtained through a copper- pyridine complex catalyzed, oxidation of 2,6-dimethylphenol by the polymerization techniques reported by Hay6). Prior to use the polymer was purified by three reprecipitations from benzene solution by methanol and dried overnight at 60°C under vacuum. It was found from the spectroscopic analysis that the synthesized PPO sample was free from the traces of catalyst impurity (Cu(I), Cu(I1)). The mechanisms of degradation of the polymer in the presence of metallic impurities were described previously7). The polymer had a number-average molecular weight of 2,07 * lo', a weight-average molecular weight of 6,69. lo', a softening point of 260 - 264 "C, a specific gravity of 1,06, and an intrinsic viscosity in chloroform at 25 "C of 0.94 dl/g.

Zinc bis(O.0'-diisopropyldithiophosphate) (ZnDTP) was prepared and purified according to the published procedure*).

The preparation of the polymer film (0,12 mm), the method of incorporation of the metal chelates into the matrix of the films, the procedures of photo-irradiation, the subsequent dissolution of the films, and the characterization by molecular weight have been described elsewhere9).

The light flux at 365 nm being 1,78. lo-" einstein * s - l . cm-2 a) from the source at the polymer film surface was determined by potassium ferrioxalate actinometry lo).

The kinetics of the photostabilized degradation of PPO were followed by the changes of the weight-average molecular weight (H,) of the polymer sampleietermined with a light scattering photometer. Zimm plots have been employed to determine M, as a function of time and the root-mean-square radius of gyration

The procedure for the calculation of the weight-average degree of polymerization (4), the number of chain-scissions (s) per single chain length, the degree of degradation (a), the specific rate constant (k), and the changes in the energy of activation (AE), and the enthalpy (AH"), entropy (AS * ) and free energy of activation (AF* ) of the processes were explained previous-

For the determination of the gel content the samples (0,25 g) of the PPO films were treated with dichloromethane (25 ml) in centrifuge tubes. The tubes were capped to prevent spillage or volatilisation and centrifuged for half an hour at 50000 r. p. m. using an ultracentrifuge. The solutions were removed by pipetting. Approximately 0,5 ml was left containing the gel and this was allowed to evaporate slowly at ambient temperature for 24 h before adding further dichloromethane (25 ml) and the phase separation was repeated. The gel remaining was dried for up to 72 h at ambient temperature, then removed and weighed.

ly").

Results and discussion

Fig. 1 shows the plots of mw as a function of the time of irradiation, for PPO films with and without 0,l wt.-Yo ZnDTP at different temperatures in air. The plots demonstrate a rapid decrease of the initial I%?,,, which then slows down suggesting that the initial rapid drop in mw is due to random scission of bonds at various links. After a long period, there are visible opalescences, due to cross-linking with macrogel formation by the fragment of the polymer molecules, particularly in the absence of ZnDTP. It is seen that, as the temperature of the degradation increases, the indica- tions for the formation of cross-linked products appear at shorter time intervals. The values of mW at various temperatures are higher with 0,l wt.-Yo ZnDTP as compared to the corresponding values of neat PPO with a light flux of 1,78.10-" ein-

a) In SI units: 8,38 * mol-' . s- ' . cm-2.

Evaluation of kinetic parameters resulting from photostabilized degradation of. . . 105

Fig. 1 . of the weight-average molecular weight (MW) of poly(2,6-dimethyl-l,4- phenylene oxide) (PPO) film irradiated with light of wave- length 365 nm in the absence and presence of 0,l wt.-Yo zinc bis(0,O ’-diisopropyldi- thiophosphate) (ZnDTP) at different temperatures in air. 313 K: PPO (curve l), PPO + ZnDTP (2); 323 K: PPO (3), PPO + ZnDTP (4); 333 K: PPO (5), PPO + ZnDTP (6); 343 K: PPO (7), PPO + ZnDTP (8); 353 K: PPO (9), PPO + ZnDTP (10); 363 K: PPO (11), PPO + ZnDTP (12)

Variation with time t 7 t

0 1 8 12 16 20 2L 11

f/ h

stein * SKI - cm-2 for different time intervals. This indicates that the presence of 0,l wt.-Yo ZnDTP photo-stabilized the PPO film.

This type of Mw vs. time behaviour where the curves reach a minimum and then tend to increase has been observed by Jellineki2) also for the degradation of butyl rubber. This clearly indicates that crosslinking and degradation go on simultaneously. At longer periods of degradation, crosslinking predominates. Such an observation would be missed altogether, if measurements of the viscosity were to be used as a method of following the degradation, since the intrinsic viscosity is an index of the volume pervaded by a macromolecule. The viscosity method is comparatively insensitive to changes in molecular weights brought about by cross-linking and result- ing in branched chain polymer molecules.

as a function of the time of degradation and temperature. The values of this parameter lie within the range of 1715 - 1365 A for PPO samples, undegraded and progressively degraded in the temperature range of 313 -363 K. In the case of PPO samples con- taining 0,l wt.-% ZnDTP, the variation of (r2)1/2 values with time of degradation and temperature is quite interesting. The values with 0,l wt.-Vo ZnDTP are decreased only from 1715 to 1562 A.

From Tab. 1, it can also be seen that at each temperature the rate of chain-scission (s) is lower with 0,l wt.-Vo ZnDTP as compared to the corresponding values of neat PPO.

The degree of degradation (a) is defined as a = S/P~,~. The values of a vs. time are shown in Fig. 2 with and without 0,l wt.-Yo ZnDTP and various temperatures in air. Inspection of the plots reveals that at each temperature the values of a of the irradiated samples are lower in the presence of 0,l wt.-Vo ZnDTP as compared to the corresponding values of neat PPO. In the initial stages of the photodegradation, the

Tab. 1 contains the values of the root-mean-square radius of gyration

Tab.

1.

Phot

osta

biliz

ed d

egra

datio

n of

poly(2,6-dimethyl-l,4-phenylene ox

ide)

(PPO

) film

s in

the

abse

nce

and

pres

ence

of 0

,l w

t.-Y

o zi

nc b

is(0

,O'-d

i- iso

prop

yldi

thio

phos

phat

e) (Z

nDTP

) in

air a

t diff

eren

t tem

pera

ture

s (lig

ht o

f w

avel

engt

h 365

nm; flux 1,78 *

lo-" e

inst

ein

ssl

* cm

-2);

Mw

= w

eigh

t- av

erag

e mol

ecul

ar w

eigh

t; =

root

-mea

n-sq

uare

radi

us o

f gyr

atio

n; p

w,!

and Fw

,o

wei

ght-a

vera

ge d

egre

e of

pol

ymer

izat

ion

at ti

me

t and

at t

= 0

, re

sp.;

s =

num

ber o

f ch

ain-

scis

sion

s; a

= d

egre

e of

deg

rada

tion;

k =

spe

cific

rate

con

stan

t

atio

n in

I@

s

PPO

, 313 K:

0.0

6,690

14,4

6,250

28.8

5.808

43,2

5,623

57,6

5,432

72,O

5,338

86,4

5,312

PPO

, 323 K:

0,O

6,690

14,4

5,952

28,8

5,438

43,2

5,112

57,6

4,804

72,O

4,631

86,4

4,589

PPO

, 333 K:

0,O

6,690

14,4

5,720

28,8

5,011

43,2

4,754

57,6

3,985

72,O

3,793

86,4

3,748

1715

1664

1 628

1621

1599

1591

1577

1715

1 657

1645

1617

1596

1588

1570

1715

1650

1635

1613

1584

1573

1562

1 ,Oo

o 0,934

0,868

0,846

0,812

0,798

0,794

1 ,O

oo

0,889

0,813

0,764

0,718

0,693

0,683

1,Ooo

0,855

0,729

0,728

0,596

0,567

0,537

0,Ooo

0,O

oo

0,223

0,041

0,447

0,080

0,558

0,100

0,670

0,120

0,731

0,131

0,762

0,137

0,Ooo

0,O

oo

0,355

0,064

0,659

0,119

0,883

0,159

0,883

0,159

1,228

0,221

1,279

0,229

. 2,663

4,166

0,Ooo

0,O

oo

0,508

0,091

1,046

0,188

1,833

0,329

2,041

0,367

2,092

0,376

PPO

0,l

6,690

6,577

6,454

6,369

6,275

6,229

6,176

wt.-

Yo

ZnD

TP,

1715

1 675

1664

1 648

1635

1 620

1610

313 K:

1 ,Oo

o 0,O

oo

0,000

0,983

0,055

0,009

0,965

0,110

0,019

0,952

0,139

0,022

0,938

0,192

0,034

0,930

0,221

0,039

0,923

0,247

0,044

PPO

+ 0

,l w

t.-%

ZnD

TP, 323 K:

6,690

1715

1 ,ooo

0,Ooo

0,O

oo

6,438

1 668

0,969

0,083

0,015

6,322

1650

0,945

0,166

0,029

6,269

1 639

0,937

0,194

0,035

6,074

1 628

0,908

0,302

0,054

5,974

1613

0,893

0,355

0,06

4 5,920

1 602

0,889

0,384

0,069

PPO

+ 0,

l w

t.-Y

o Zn

DTP

, 333 K:

6,690

1715

1 ,00

0 0,O

oo

0,Ooo

6,359

1 660

0,951

0,142

0,025

6,109

1653

0,913

0,284

0,051

5,926

1631

0,886

0,383

0,069

5,719

1 628

0,855

0,494

0,089

5,573

1 624

0,853

0,582

0,105

5,527

1 601

0,826

0,605

0,109

+ 6,076

9,549

15,625

Tab

. 1.

C

ontin

ued

irrad

i- at

ion

in I

@ s

PPO

, 343

K:

0,O

6,69

0 14

,4

5,33

8 28

,8

4,40

1 43

,2

3,74

9 57

,6

3,29

2 72

,O

3,11

5 86

,04

3,09

7

PPO

, 353

K:

0,O

6,69

0 14

,4

4,62

9 28

,8

3,37

2 43

,2

2,74

3 57

,6

2,48

2 72

,O

2,29

5 86

,4

2,31

5

PPO

, 36

3 K

: 0,O

6,

690

14,4

3,

492

28,8

2,

629

43,2

2,

248

57,6

1,

813

72,O

1,

852

86,4

1,

901

A

1715

1

642

1624

1

581

1 57

7 15

59

1 54

9

1715

1

617

1 59

9 1

570

1 47

2 1

428

1 49

0

1715

1

577

1 56

8 1

460

1440

1

363

1 36

5

P,,,/

P,,o

1 0,79

8 0,

636

0,53

0 0,

492

0,46

5 0,

463

1 ,O

oo

0,69

2 0,

504

0,41

0 0,

371

0,34

3 0,

346

1 ,Oo

o 0,

522

0,39

3 0,

336

0,27

0 0,

261

0,28

4

0,Ooo

0,O

oo

0,73

1 0,

131

1,38

0 0,

248

2,68

1 0,

481

2,92

5 0,

525

2,94

5 0,

531

0,Ooo

0,O

oo

1,23

1 0,

221

2,51

7 0,

452

3,53

6 0,

635

4,13

2 0,

742

4,59

4 0,

825

4,52

2 0,

812

0,Ooo

0,O

oo

2,36

7 0,

425

3,73

9 0,

615

4,74

4 0,

852

6,21

0 1,

112

6,48

4 1,

164

5,86

2 1,

053

15,-

JO

30,0

90

PPO

+ 0

,l w

t.-%

ZnD

TP,

343

K:

6,69

0 17

15

1 ,O

oo

6,12

0 1

648

0,91

5 5,

756

1 64

6 0,

860

5,40

5 1

622

0,80

8 5,

062

1614

0,

756

4,91

9 1

464

0,73

4 4,

916

1 46

4 0,

734

PPO

+ 0

,l w

t.-%

ZnD

TP,

353

K:

6,69

0 17

15

1 ,O

oo

5,97

4 I

650

0,89

3 5,

402

1 64

2 0,

807

4,93

7 1

621

0,73

8 4,

496

1613

0,

672

4,20

1 1

602

0,62

8 4,

088

I 60

0 0,

611

PPO

+ 0

,l w

t.-%

ZnD

TP,

363

K:

6,69

0 17

15

1 ,O

oo

5,63

2 16

46

0,84

2 4,

944

1 63

5 0,

739

4,30

8 16

17

0,64

4 3,

796

1 59

9 0,

567

3,64

3 1

591

0,54

5 3,

701

1 59

6 0,

553

0,00

0 0,

264

0,39

2 0,

681

0,92

0 1,

051

1,05

1

0,m

0,

352

0,68

2 1,

041

1,34

3 1,

594

1,10

3

0,00

0 0,

546

1,04

3 1,

551

2,03

2 2,

184

2,12

3

0,00

0 0,

047

0,07

1 0,

122

0,16

5 0,

188

0,18

8

0,00

0 0,

063

0,11

9 0,

187

0,24

1 0,

286

0,30

6

0,00

0 0,

091

0,18

7 0,

279

0,37

5 0,

392

0,38

1

28,6

45

42,5

3

65,1

01

108 R. Chandra

values of a increase rapidly with time and a point of inflexion is reached after which a starts decreasing. Such a sharp rise in a in the initial stages of the degradation indicates a random breaking of bonds in the polymer.

It can be seen that initially the rate of scission is proportional to the number of links present at any time. The initial slope of the a vs. t curve (Fig. 2) gives a method of

1.c

0.8

N 0.6

U

0.1

0.2

0 t / h

Fig. 2. Variation with time t of the degree of degradation a of poly(2,6-dimethyl- 1 ,Cphenylene oxide) (PPO) film irradiated with light of wavelength 365 nm in the absence and presence of 0,l wt.-% zinc bis(0,O'-diiso- propyldithiophosphate) (ZnDTP) at different temperatures in air. (Numbering of curves as described in legend Fig. 1)

evaluating the specific rate constant k. If the plot is non-linear, Jellinek12) has indicated the possibility of more than one rate constant being operative. From Fig. 2, it can be seen that the plots of a vs. t are linear at initial stages, proving that the degradation is taking place by the random breaking of one type of bond initially. In order to avoid complications arising from branching and crosslinking at high degrees of degradation for PPO, in the absence of 0,l wt.-% ZnDTP, two average values of k were evaluated from the slopes of a vs. t curves. Thus, a distinct feature of these curves is a linear increase in the value of a with time and convex curves near the maxima followed by a decrease in the values of a with time. Therefore, 1) one may calculate k from the relationship a = kt, assuming that random degradation predominates over crosslinking or 2) one may analyse the data for both cross-linking

Evaluation of kinetic parameters resulting from photostabilized degradation of. . . 109

and random degradation. The problem is comparatively simple in the presence of 0,l wt.-To ZnDTP where very small cross-linking is taking place during the irradiation of the PPO films.

The average k values at the different temperatures are substituted in the conven- tional Arrhenius equation: k = A e exp [ - AE/(R T)] to obtain the energy of activa- tion (AE) and frequency factor (A) . The values of AE and A calculated from the plots of logk vs. 1/T by the method of least squares are satisfied by the equations (Fig. 3):

PPO(313-353K): k = 3,94-10-2exp[-36,95/(RT)] (1)

PPO (353 - 363 K): k = 5,79 . exp [ - 70,56/(R 771 (2)

PPO + 0,l wt.-Yo ZnDTP(313 -363 K): k = 22,02. 10-2exp[-55,34/(RT)] (3)

with the activation energy in kJ - mol-I and the frequency factor in SKI. Fig. 3 shows

-15

-16

a d -

-17

- I f

-11

1 0 3 w WK-' Fig. 3

c 3 .G 1 - c C

t / h Fig. 4

Fig. 3. Arrhenius plot for the photodegradation of poly(2,6-dimethyl-l ,Cphenylene oxide) (PPO) film, irradiated with light of wavelength 365 nm (flux: 1,78. lo-" einstein . s - I * cm-2) in the absence (0) and presence ( 0 ) of 0,l wt.-Yo zinc bis(0,O'-diisopropyldithiophosphate) Fig. 4. Variation with time t of gel content of poly(2,6-dimethyl-l,4-phenylene oxide) (PPO) films, irradiated with light of wavelength 365 nm (flux: 1,78 . einstein . s - ' . cm-2) at 333 K (60OC) in the absence (0) and presence ( 0 ) of 0,l wt.-Yo zinc bis(0,O'-diisopropyldi- thiophosphate)

110 R. Chandra

for neat PPO a distinct change of the AE values. The results point to an acceleration of PPO cross-linking, due to photo-oxidation which occurs at about 353 K.

Earlier work") on the photo-oxidation of PPO gives evidence for the formation of hydroperoxide groups, the point of initial attack being the methyl groups of the PPO chain. The observed value of the effective activation energy (AE) of the formation of carbonyl groups from the initial rate of oxidation at low concentration of pendant -CH,OOH of PPO in the temperature range 313 -353 K is 37 kJ/mol. This value is in good agreement with that observed by Gugumus and Marchal13) for radiation induced degradation at low temperature. The subsequent free radical reaction led to the formation of various functional groups observed by IR and UV spectroscopy and to crosslinking.

The determination of the insoluble fraction of the gel fraction offers a simple method for the evaluation of the degree of crosslinking of the irradiated PPO samples. The curves in Fig. 4 show that neat PPO has a relatively strong tendency for cross-linking. The sample containing 0,l wt.-Vo ZnDTP is, however, appreciably more resistant to this gel formation process. It is evident that the cross-linking process is probably less hindered than the chain-scission process. The gel fraction in neat PPO increases quite rapidly in the early stages of photo-oxidation.

It is also seen" that the photodegradation of the polymer is of zero order with respect to polymer and oxygen concentrations. Simha and Wall14) have pointed out that the first order law is not applicable for such a process. For a zero order reaction AE may be equated to the heat of formation of the activated complex AH*, without any appreciable error. If a zero order law is assumed, then according to the theory of absolute reaction rate

k = RT/(NAh)exp(AS*/R)exp(-AH*/(RT) (4)

and

AE P A H *

where h is Planck's constant and NA is the Avogadro number.

Tab. 2. Values of the kinetic parameters for the photostabilized degradation of poly(2,6-di- methyl-l,4-phenylene oxide) (PPO) films in the absence and presence of 0,l wt.-Yo zinc bis(0,O'-diisopropyldithiophosphate) (ZnDTP) in air. (Light of wavelength 365 nm; flux 1,78. lo-" einstein . s - l . cm-2); A = frequency factor; AS*, AH* and AF* entropy, enthalpy and free energy of activation, resp.

System A - A S * AH* AF* Temp. S - ' J . K - I . mol-i kJ . mol-1 kJ . mol-i range -

in K

PPO 3,94* 10-2 279,14 36,95 153,20 313 -353 PPO 5,78.10-4 268,27 70,56 177,86 353 - 363 PPO + ZnDTP 22,02. 276,40 55,34 165,86 313 -363

Evaluation of kinetic parameters resulting from photostabilized degradation of. . . 1 1 1

The approximate free energy of activation (AF*) for the degradation process may be determined at any convenient temperature using the relation:

AF” = AH* - TAS* (5 )

Values of the frequency factor A , the changes in enthalpy (AH*), the entropy (AS * ) and free energy (AF*) of activation at 400 K for the degradation of PPO with and without ZnDTP are presented in Tab. 2.

P. S. Kelleher, B. D. Gesner, Polym. Eng. Sci. 10, 38 (1970) 2, R. A. Jerussi, R. A., J. Polym. Sci. 9, 2009 (1971) 3, W. M. Prest, J . Polym. Sci. 10, 1639 (1972) 4, A. R. Shultz, B. M. Gendron, J. Appl. Polym. Sci. 16, 461 (1972) 9 M. T. Shaw, J. Appl. Polym. Sci. 18, 449 (1974) 6, A. S. Hay, J. Polym. Sci. 58, 581 (1962) ’) R. Chandra, Eur. Polym. J. 17, 567 (1981) 8, V. P. Wystrach, E. 0. Hook, E. 0. Christapher, J. Org. Chem. 21, 705 (1956) 9, R. Chandra, R. P. Singh, Makromol. Chem. 181, 1637 (1980)

lo) J. G. Calvert, J. N. Pitts, Jr., “Photochemistry” Wiley, New York 1966

12) H. H. G. Jellinek, J. Polym. Sci. 4, 1 (1949) 13) F. Gugumus, J. Marchal, J. Polym. Sci., Part C 16, 3963 (1968) 14) R. Simha, L. A. Wall, J. Phys. Chem. 56, 707 (1982)

R. Chandra, B. P. Singh, Eur. Polym. J. 18, 199 (1982)