Thermal and Photooxidation of Miscible Polymer Blends - American

32

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Thermal and Photooxidation of Miscible Polymer Blends L. Stoeber, Ε. M. Pearce*, and T. K. Kwei Herman F. Mark Polymer Research Institute and Department of Chemistry, Polytechnic University, Brooklyn, NY 11201 Thermal oxidation of miscible polymer

blends at 80 °C, 110 °C, and

140 °C were studied and compared with the photooxidation Pure polystyrene ified PS/PVME,

(PS)/

poly(vinyl methyl ether) (PVME)

the addition of an antioxidant to PS/PVME

at 30 °C.

blends, mod blends, and

other miscible polymer blend systems are discussed. The oxygen uptake by PS was negligible, whereas PVME and photooxidation

oxidized rapidly. During

thermal

the induction period was lengthened by the pres

ence of PS in the blend. The steady-state rate of oxidation of the blend was strongly

influenced by the segmental mobility

of the blend,

this mobility also governed the kinetics and morphology aration. The molecular weight of PVME

and

of phase sep

decreased more slowly as the

PS content in the blend increased. It is believed that the reaction be tween PVME

radicals and PS resulted in less reactive PS radicals, and

this phenomenon retarded oxidation. In thermal oxidation of

PS/PVME

blends, phase separation occurred at the end of the induction The steady-state rate of oxidation was proportional tent in the blend. Similarities blend

were found

systems as in poly(methyl

period.

to the PVME

con

in other miscible

polymer

methacrylate)/poly(ethylene

oxide)

blends. The addition of an antioxidant to PS/PVME

blends lowered the

rate of oxidation and lengthened the induction period. Phase occurred at the end of the induction period and caused a of the antioxidant between the different

separation

redistribution

phases.

RAPIDLY DEVELOPING F I E L D O F POLYMER B L E N D I N G is becoming i n creasingly important as a means to modify and extend desirable polymer prop erties to meet commercial needs. Polymer blends like the "impact" polystyrenes (PS), which possess outstanding properties without the brittleness of common PS, have attracted a great deal of scientific interest. Most of the T T H E

* Corresponding author

0065-2393/96/0249-0503$16.25/0 © 1996 American Chemical Society In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.


504

POLYMER DURABILITY

polymer blending research to date has been focused on heterogeneous poly mer blends, where one polymer is dispersed into the continuous phase of another polymer. The result is a heterogeneous blend in which the molecules are not intimately mixed on a molecular level. A fundamental aspect of this research is the question of miscibility and its control, because even in heter ogeneous blends it is common practice to add a graft or block copolymer as a compatibilizer to modify interfacial characteristics. More than 500 miscible or partially miscible blends are already known and that number is increasing steadily (1-5). The molecular interaction in miscible blends is on a much more intimate scale than that of heterogeneous blends. The mixing of the two dif ferent molecules is done at the segmental level and the resulting blends exhibit single-phase behavior. This interaction results in the appearance of entirely new chemical properties, unlike those of the separate components. These new sets of properties are often of great interest and require scientific explanation. Although homogeneous and heterogeneous polymer blends have become an important class of materials, our knowledge of their chemical stability is inadequate. There is information scattered in the literature about thermal degradation of heterogeneous polymer blends (6) and thermal or photooxidation of rubber-modified PS (7-9). But the oxidation of miscible polymer blends has received only occasional attention. Thermal and photooxidation studies of miscible blends have been a major interest at the Herman F. Mark Polymer Research Institute over the last decade. Our investigation involved thermal and photo oxygen uptake meas urements of polymer blends and the measurements of chemical changes and molecular weights of the component polymers. A number of miscible polymer blends, including PS/poly(vinyl methyl ether) ( P V M E ) (10-13), low molecular weight polystyrene (LPS)/PVME (JO, 11), 4-hydroxy-modified polystyrenes (MIPS, M2PS, and M7PS)/PVME (11), hexafluoroisopropanol-modified styrene-styrene copolymer (FPS)/PVME (11), poly(methyl methacrylate) (PMMA)/poly(ethylene oxide) (PEO) (14), poly(vinylidene fluoride) (PVDF)/ poly(vinyl acetate) (PVAc) (15), poly(4-hydroxystyrene) (PHOST)/poly(vinyl pyrrofidone) (PVP) (16), or P E O (17), were studied.

Miscibility Enhancement Through Hydrogen Bonding Hydrogen bonding between two dissimilar polymers can lead to miscibility or complexation. Miscibility enhancement is therefore often obtained through the blending of two polymers, one containing a hydrogen donor group and the other a hydrogen-acceptor group (18-20). The fact that the formation of hydrogen bonds can lead to miscibility raises the question of the minimum number of hydrogen bonds necessary to obtain miscibility (21 ) and the effect on the thermally induced phase separation. These cloud points (22) are be lieved to be sensitive to the magnitude of polymer-polymer interaction (23).

In Polymer Durability; Clough, Roger L., et al.; Advances in Chemistry; American Chemical Society: Washington, DC, 1996.


32.

STOEBER ET A L .

Thermal and Photooxidation of Miscible Blends 505

The prediction was borne out in the work by Pearce, Kwei, and co-workers (24-26). Copolymers were synthesized to contain small amounts of strong donors, such as a series of styrene copolymers containing varying amounts of p-(hexafluoro-2-hydroxyl isopropyl) styrene as the comonomer unit. Miscible blends containing different functional groups to increase miscibility have been used for thermal and photooxidation studies (Figure 1). The phase diagrams (Figure 2) of PS/PVME blends were studied extensively by many researchers (27-29). For thermal oxidation, Park and co-workers (II, 30) used 1, 2, and 7 mole% p-hydroxy-modifled PS (MIPS, M2PS, and M7PS, respectively) with P V M E . As the degree of modification increases, the phase separation tem peratures of the blends become higher. The M7PS/PVME blends do not show phase separation below 300 °C. The incorporation of the even stronger hydrogen-bonding donor of 2.5 mole% p-(hexafluoro-2-hydroxylisopropyl) (HHIS) group into the styrene co polymer raised the phase separation temperature above 300 °C. These blends stay miscible even after P V M E has undergone some oxidation. As shown in earlier work (18), the incorporation of only 0.1 mole% p-(hexafluoro-2-hydroxyl-isopropyl) group into the styrene copolymer raised the phase separation temperature minimum up to 160 °C; at 0.6 mole% H H I S , the temperature increased to 195 °C (Figure 3), The use of preoxidized P V M E , however, low ered the phase separation temperature of the blends drastically (Figure 4).

Thermal Oxidation of Miscible Polymer Blends Thermal and photooxidation of polymer blends are complicated processes in volving multicomponent systems simultaneously undergoing various free-rad ical reactions. In heterogeneous blends the interactions between radicals take place at the interface, whereas in miscible blends the chances of interpolymer (radical-polymer and radical-radical) reactions are increased if the polymer does not undergo phase separation during oxidation. Park et al. (10) showed in earlier studies of the thermal oxidation of mis cible blends of PS/PVME that the induction period of the P V M E oxidation was prolonged as the weight fraction of PS in the blend increased (Figures 5 and 6). However, the steady-state oxidation rates are almost proportional to the P V M E contents in the blends. The opaqueness of the blends after the induction period shows that all the blends have undergone phase separation after minor chemical changes during the induction period of oxidation. The steady-state region of oxidation is therefore almost proportional to the P V M E content of the blend. The same results were found in all PS/PVME blends measured at 140 °C. These blends undergo phase separation before oxidation. The induction periods as well as the steady-state region of oxidation are proportional to the P V M E content of



506

POLYMER DURABILITY

•polymers

Figure 1. Schematic diagram of hydrogen bonding between modified polystyren and counter polymers. (Reproduced with permission from reference 18. Copynght 1992 Elsevier Science Publishers.) 300-

2

I

»

On

P V M E Weight Fraction (%) Figure 2. Phase separation temperatures of PS/PVME, M1PS/PVME and M2PSI PVME blends; (O) PS/PVME; (Π) M1PS/PVME; (A) M2PS/PVME blends. (Reproduced with permission from reference 11. Copynght 1990 John Wiley & Sons, Inc.)



32.


STOEBER ET A L .

Wt. Fraction of PVME

Figure 3. Cloud points of the various modified PSs and PVME systems at t heating rate of 2 °C/min: (1) 0.64 MPS, (2) 0.22 MPS, (3) 0.096 MPS, and ( pure PS. (Reproduced with permission from reference 18. Copyright 1992 Elsevier Science Publishers.)

U ο

i δ

100

'

I

li 40

60

80

100

P V M E Weight Fraction (%)

Figure 4. Cloud point curves of blends of PS with preoxidized PVME. (Reproduced from reference 10. Copyright 1990 American Chemical Society.



508

POLYMER DURABILITY

Time (hours) Figure 5. Oxygen uptake curves of PVME and its blends with PS at 80 °C. (Reproduced from reference 10. Copynght 1990 American Chemical Society.)

2

Time (h)

Figure 6. Oxygen uptake curves of PVME and its blends with PS at 110 °C. (Reproduced from reference 10. Copyright 1990 American Chemical Society.)



32.

STOEBER ET A L .

Thermal and Photooxidation of Miscible Blends

509

the blends (Figure 7). After the steady-state region the oxygen absorption decelerates and reaches a saturated region where the oxygen absorptions are also proportional to the P V M E content in the blends. In contrast to the PS/PVME blends, the LPS/PVME blends and the hexafluoroisopropanol modified styrene-styrene copolymer (FPS)/PVME blends stayed miscible even during oxidation (Figures 8 and 9; Table I). The induc tion periods of FPS/PVME and LPS/PVME blends were longer than those of the PS/PVME blends, and their oxidation rates were lower. The steady-state oxidation rates were not proportional to the P V M E content of the blend be cause of the single-phase characteristic throughout the oxidation. The LPS/ P V M E blends had longer induction periods and lower oxidation rates than the FPS/PVME blends. The miscibility difference on the molecular level is believed to be responsible for that difference. The L P S polymer chain is be lieved to stay near the oxidized P V M E chain, whereas long sequences of sty rene units in FPS are thermodynamically unfavored to mix with the oxidized P V M E . The strong hydrogen bonds are believed to keep the two polymers in a macroscopically single phase. The different polymer blends are likely to have, therefore, a different scale of homogeneity (Figure 10).

Photooxidation of Miscible Polymer Blends The oxygen uptake of polymer blends during U V radiation provides major evidence for photooxidation. Although the mechanism of thermal- and pho tooxidation have great similarities, photooxidation was carried out at a lower temperature. The significantly reduced segment mobility at lower tempera tures was expected to have an effect on the rate of reaction and the phase separation behavior. The departures from the results obtained in thermal ox idation provide important clues to the understanding of mechanism of the oxidation process in miscible polymer blends. Chien and co-workers (13, 31) results of the oxygen uptake during U V radiation of PS, P V M E , and their blends are shown in Figure 11. PS absorbed less than 0.5 m L of oxygen/g in a period of 22 days, which could be regarded as negligible in the context of our investigation. On the other hand P V M E consumed, after a slow beginning, approximately 8.7 mL/g/day after 3.1 days. The total consumption was about 30 mL/g in 6 days. The induction period increased with the PS content in a regular manner. The oxidation rate instead undergoes a tremendous decrease between 30 and 50% PS in the blend. The rates of the thermal oxidation of the same polymer blends are proportional to P V M E content, and the departure between the thermal and photoprocesses provide important clues in the understanding of the mechanism involved (Fig ures 12 and 13). The films, containing 70% by weight of P V M E , turned slightly translucent after photooxidation for about 5 days. After 11 days those films showed two


POLYMER DURABILITY


600

100 P V M E Weight Fraction (%) Figure 7. Steady-state oxygen absorption rates of PS/PVME blends: bottom, 80 °C; middle, 110 °C; top, 140°C. (Reproduced from reference 10. Copyright 1990 American Chemical Society.)


32.

STOEBER ET A L .


t20-|

LPS/PVME=35/65


î

ί ' /

LPS/PVHB»50/50

/ I

LPS/PVME=65/35

/

/ 0.5

Time (hours)

Pi^ire 8. Oxygen uptake curves of blends with LPS at 140 °C. (Reproduced fro reference 10, Copyright 1990 American Chemical Society.)

LPS/PVHE=»3S/65

LPS/PVHE-50/50

"*tPS/PVKE«65/3S

6

8

Time (hours)

Figure 9. Oxygen uptake curves of blends with LPS at 110 °C. (Reproducedfro reference 10. Copyright 1990 American Chemical Society.)



fl

110 110 110 110 140 140 140 140

0 35 50 65 0 35 50 65

0.05

0.50

PVME 2.3 3.5 5.7 0.15 0.28 0.45

0.05* 0.05° 0.05°

LPS

0.17 0.23 0.28

1.5 2.0 3.6

FPS

0.93 1.23 1.45

6.8 7.0 6.3 1.20 1.53 1.72

16.8 10.4 7.5 498

124

MIPS M2PS PVME

Induction Period (h)

1.2 1.8 2.3

PS

313 237 132

81.5 54.8 35.0 239 130 58

31.9 17.1 8.5

LPS

215 108 45

37.5 18.3 8.4

FPS

208 56 20

18.8 7.0 3.8

4.4 3.9 2.8

MIPS M2PS

Oxidation Rate (mL/(g •h)] PS

78 27 10 NOTE: Values for induction period and oxidation rate were measured and calculated three times for each blend. Accuracy of these values is about ±5%. Phase separation occurred before oxidation. SOURCE: Reproduced with permission from reference 11. Copyright 1990 John Wiley & Sons, Inc.

Τ (°C)

PS (wt%)

Table I. Induction Periods and Oxidation Rates of PVME and Its Blends


32.

STOEBER ET A L .


( before oxidation

]

[ during oxidation

513

]

(PVHE)


(PS) • phase

separation

(ΡΥΠΕ).

(LPS)-

(PVWE)

—

(FPS) H-bond

Figure 10. Schematic diagram of the miscibility changes of the blends during oxidation. (Reproduced with permission from reference 11. Copyright 1990 John Wiley & Sons, Inc.) glass transition temperatures (T ). Similar to the blends observed in thermal oxidation at 110 °C, the blends at 30 °C underwent phase separation. The difference is that phase separation occurred after a higher value of oxygen consumption, namely, 7 mL/g. At 30 °C a more extensive chemical change of P V M E is required, due to a higher miscibility of the oxidized blend at low temperature. This demand was verified by measuring the cloud-point tem peratures of three blends that had been photooxidized. The 70% P V M E blend photooxidized for 3 days was still homogeneous at room temperature, whereas its cloud-point temperature was 110 °C (Figure 14). The 50 and 30% P V M E blends had even longer induction periods and slower oxidation rates than expected. The T values for the 70, 50, and 30% P V M E blends were about -19 °C, - 7 °C, and 15 °C, respectively. The lower segment mobility of the 30% blend, indicated by its high T , is reflected readily g

g

g


POLYMER DURABILITY


514

TIME, D A Y

Figure 11. Oxygen uptake curves of FVME and its blends. (Reproduced with permission from reference 13. Copynght 1991 John Wiley & Sons, Inc.). in its longer N M R spin-lattice relaxation time (32) at the temperature of the photooxidation. Therefore, the kinetics and the morphology of phase separa tion, as well as the oxidation rates, are strongly influenced by this change. The cross-propagation between the P V M E radicals and PS is still operative, as long as both phases obtain a substantial amount of PS. The less reactive PS radical is responsible for the decrease in the rate of oxidation.

Antioxidant K i m and co-workers (12, 33) used octadecyl 3,5-dHer£-butyl-4-hydroxyhydrocinnamate to show the effect of a hindered phenol as an antioxidant (34, 35) on the thermal oxidation of miscible blends (Figures 15 and 16). The antioxidant-containing polystyrene (AOPS)/PVME blends showed similar charac-



32.

STOEBER ET A L .


PVME,%

Figure 12. Induction periods (A) and steady-state oxygen absorption rates (Φ of PVME photooxidized PVME and its blends with PS. (Reproduced with permission from reference 13. Copyright 1991 John Wiley & Sons, Inc.)

TEMPERATURE (t)

Figure 13. Differential scanning calonmetry curves of photooxidized PVME an its blends with PS: 1, PVME as prepared; 2, PVME oxidized for 11 days; 3, 7 PVME as prepared; 4, 70% PVME oxidized for 11 days; 5, 50% PVME a prepared; 6, 50% PVME oxidized for 11 days; 7, 30% PVME as prepared 30% PVME oxidized for 11 days. (Reproduced with permission from referenc 13. Copynght 1991 John Wiley & Sons, Inc.)


POLYMER DURABILITY

516


150

0

20

40

PVME, %

60

80

100

Figure 14. Cloud point temperatures of photooxidized PS/PVME blends: 1, as prepared, 2-6, oxidized for 1, 2, 4, 5.5, and 7.5 days, respectively. (Reproduced with permission from reference 13. Copyright 1991 John Wiley & Sons, Inc.) teristics to the PS/PVME blends. The induction periods lengthened, the rates of oxidation decreased as the PS contents in the blends increased, and the saturation values were approximately proportional to the P V M E contents in the blend. The AOPS/PVME blends phase separated as well as the PS/PVME blends shortly before the end of the induction period. However, all AOPS/ P V M E blends showed longer induction periods and slower rates of oxidation to its corresponding antioxidant-free blends, as expected (Figure 17, Table I I ) . Although all blends containing antioxidants showed the expected effects of antioxidants, there were some unexpected deviations. As shown in Table I I , blends containing up to about 40% PS in the presence of antioxidant


32.

STOEBER ET AL.


90-j

80-j 70 Lu

lé


- 30 χ

ο

6

20

H

10 0 1

-τ 3

2

1

: 4

5

1 6

TIME (h)

1 7

1 8

» 9

10

Figure 15. Oxygen uptake curves of PS/PVME blends containing 10 wt% PS w various concentrations of Irganox 1076 at 110 °C. AO contents are as follow X, no antioxidant; O, 0.026 wt%; •, 0.052 wt%; A, 0.102 wt%. (Reproduc with permission from reference 12. Copyright 1992 Gordon and Breach Scien Publishers S.A.)

100 90 80 70

LU

60

Χ

Ο

xx*

ο

a

4(J 30 20-j 10 J

TIME (h)

Figure 16. Oxygen uptake curves of PS/PVME blends containing 20 wt% PS w various concentrations of Irganox 1076 at 110 °C. AO contents are as follo X, no antioxidant; O, 0.026 wt%; •, 0.052 wt %; Δ, 0.102 wt%; O, 0.204 wt (Reproduced with permission from reference 12. Copyright 1992 Gordon an Breach Science Publishers S.A.)


518

POLYMER DURABILITY

> ο ο ο


•

•

•

·

ο θ

3

ο °

Λ

ο α

ο

ο

ο

Φ

20

ο

α

0 0

h

10 °

*

S

A

Λ

0

185* 2

3

4

5

10

6

TIME ( h ) Figure 17. Oxygen uptake curves of AOPS/PVME blends at 110 °C. PS contents are as follows: Δ, 0 wt%; O, 10 wt%; Π, 20 wt%; O, 40 wt%. Filled symbols represent the blends without Irganox 1076.

Table II. Induction Periods and Oxidation Rates of PS/PVME and AOPS/PVME Blends Sample

Induction Composition (%) Period (h)

Oxidation Rate (mhl(g · h)]

100

0.6

128.5

PS/PVME

10/90

0.7

99.3

PS/PVME

20/80

0.9

83.9

PS/PVME

40/60

1.6

65.7

PS/PVME

60/40

3.2

36.1

PS/PVME

70/30

3.5

14.4

PS/PVME

80/20

6.5

10.3

AOPVME

100

5.2

43.1

PVME

AOPS/PVME

10/90

3.2

63.6

AOPS/PVME

20/80

4.0

54.8

AOPS/PVME

40/60

5.1

27.6

AOPS/PVME

60/40

36.0

5.1

AOPS/PVME

70/30

67.2

2.2

SOURCE: Reproduced with permission from reference 12. Copyright 1992 Gordon and Breach Science Publishers S.A.



32.

STOEBER ET A L .


showed shorter induction periods and higher oxidation rates than P V M E in the presence of the same amount of antioxidants. Upon further increase of the PS content of the blends to more than 60%, the induction periods in creased drastically and the oxidation rates decreased. The results for the 10, 20, and 40 wt% PS blends were unexpected, be cause the presence of PS in blends containing no antioxidants was believed to act as an inhibitor in lengthening the induction period. Phase separation in AOPS/PVME blends occurred after a similar amount of oxygen, 2 mL/g, was absorbed as in the case of the blends containing no antioxidants. Blends con taining up to about 40% PS phase separated relatively easily because the ex perimental temperature was quite close to the phase-separation temperature. Small changes in the chemical structure of P V M E alter the miscibility of the blends, and this change is believed to lower the solubility of the antioxidant in P V M E . The antioxidant is believed to redistribute to the PS phase during the initial stages of oxidative phase separation. The resulting P V M E - r i c h phase contains, therefore, less antioxidant than the nominal amount and undergoes relatively fast autoxidation.

Activation Energy of Oxidation Arrhenius plots of the steady-state oxidation rates as well as of the induction periods were made to show the differences in the activation energies of the blends before and after phase separation. The Arrhenius plots of the steadystate oxidation rates of the PS/PVME blends and P V M E have almost the same slopes (Figure 18). This result is again an indication that in the steady-state region of oxidation the activation energy of oxidation is almost the same. This result also suggests that the oxidation of P V M E proceeds independendy of PS, because phase separation has already occurred (Figure 19). Although the addition of L P S (or FPS) to P V M E generally raises the activation energy of oxidation, no further raise results from increasing the concentration of L P S (or FPS) in the blends. The Arrhenius plot shows that all blends have a higher activation energy of oxidation, but the value is the same for all blends. This similarity indicates that neither F P S nor L P S took part in the oxidation process direedy, but their addition generally increased the miscibility of the blends and resulted in higher activation energies, lower oxidation rates, and longer induction periods of the blends (Figures 20 and 21). The Arrhenius plots of the oxidation rates of the phenol group containing PS/PVME blends show that the blends have different activation energies de pending on the concentrations of the phenol groups. A higher concentration of phenol groups lowers the activation energies. The phenol groups are di rectly involved in the oxidation process. As a result, the presence of the phenol



2.4

2.3

2.6

1/T

: PS/PVME=65/35

•

_1

(K )

2.8

: PS/PVME=50/50

2.7

»-PS/PVME=35/65

0

ι PVME

A

φ

2.9 J

(xl(T )

S

α

ί 4

2

WO 5

2.3

2.4

1

140 *C Ι

3

.

-1

^

PVME

2.7

65/35

50/50

35/65

2.8

\FPS/PVME

(Κ )

«c

I

2.6

N.

1/TxlO

2.5

V

X.

no

Figure 18. Arrhenius plots of steady-state oxygen absorption rates for PVME (top) and its blends with PS. (Reproduced from reference Figure 19. Arrhenius plots of oxidation rates of FPS/ 10. Copyright 1990 American Chemical Society.) PVME blends.

• f t

•ft-


32.

STOEBER ET AL.

521


OH


I .Ο

II

«a 1 δ

1

I

1

1

Si

ο

il «s a>

co

.g

I ε

I


^

522

POLYMER DURABILITY

groups in PS has two contributions: an improvement of the miscibility of the blends, and an antioxidant effect.


Molecular Weight Changes The changes of the number-average molecular weight for the different PS/ P V M E blends after oxidation at 110 °C are shown for different periods of time (Figures 22 and 23). The number-average molecular weight of P V M E 90000

ι

30000-

3

20000 «

10000"

0

OU

l

1.5

5

2.5

3

Oxidatioti Time (h) Figure 22. Changes in the molecular weight of PS as a function of oxidation time at 110 °C. PS contents are 100, 65, 50 and 30 % from the top to the bottom curves. (Reproduced from reference 10. Copynght 1990 American Chemical Society).


32.

STOEBER ET A L .


523

90000


800001

Oxidation Time (h) Figure 23. Changes in the molecular weight of PVME as a function of oxidation time at 110°C (values below 2000 are inaccurate). PVME contents are as follows: O , 100%; Δ, 65%; O, 50%; •, 35%. (Reproduced from reference 10. Copynght 1990 American Chemical Society). decreased rapidly during oxidation. A 15-fold decrease in the molecular weight was observed throughout the induction period. PS showed almost no change, or even a slight increase, of molecular weight after oxidation. However, a twofold decrease was found in the case of PS in the blends, and this decrease is an average of one chain scission per polystyrene molecule (i.e., one scission per 820 repeat units). The continuous change of PS molecular weight can be explained through a reaction between the P V M E peroxy radicals and PS. The resulting PS radical is less reactive, as demonstrated by a prolonged induction period of the blend. Eventually the PS radical undergoes chain scission that



524

POLYMER DURABILITY

leads to a decrease in molecular weight. The decrease in molecular weight even happens after phase separation, which is thought to be due to boundaryregion contacts between domains. The antioxidant-containing PS/PVME blends also showed a significant de crease in molecular weight. However, during the induction period the stabi lized P V M E exhibited little change in molecular weight (Figures 24 and 25). The PS molecular weight almost did not change during an extensive ox idation for 15 days at 110 °C. The decrease in molecular weight occurred in 20 and 40% PS blends shortly after the induction periods. It is apparent that PS is directly involved in the oxidation process in both the AO-free and the AO-containing blends. The presence of the antioxidant and of PS lengthened the induction period. It is believed that the P V M E radical underwent a reaction with PS after the antioxidant was used up in the induction period, and this reaction led to a more stable PS radical that retards the oxidation process and undergoes chain scission.

Chemical Changes The structural change of P V M E as the component that undergoes oxidation is accompanied by the formation of carbonyl (32), double bonds (36), and hydroxy (hydroperoxide) groups (10). IR spectroscopy (37-40) was used by Chien et al. (13) and Park et al. (10) to show these changes (Figures 26 and 27). The chemical changes are visible during the oxidation. The vibrational peaks of C H , C H , and C H reduce, whereas growing intensities of - O H / - O O H , carbonyl, and C = C are of notice (Figures 28 and 29). Carbonyl groups are formed through β-scission of alkoxy radicals. A close relationship between the change of the number-average molecular weight and the concentration of carbonyl groups was observed by Iring et al. (41 ) during the oxidation of P E . The changes of the - O H / - O O H concentration of the oxidation of other polymers (42, 43) seem to be in general agreement with the results found for the oxidation of P V M E . The accepted interpretation is that the - O O H group increases during the induction period of the oxidation. As soon as the autoacceleration starts, the peroxides decompose into initiating radicals, a phenomenon that explains the downshift of the curve. However, deviations from this explanation are seen in the case of P V M E . The maximum value in the case of P V M E is 30 min, which is still within the induction period of P V M E , and autoacceleration has also begun. The similar temporal evolution of - O H / - O O H and C = 0 for PS/PVME blends and P V M E during oxidation is thought to be due to the preferential presence of P V M E at the surface (44, 45) in the blends. The lower surface energy component preferentially moves to the surface and dominates surface behavior. 2

3



32.

STOEBER ET AL.


Oxidation Time (h)

Figure 24. Number-average molecular weights of PVME in AOPS/PVME blends after oxidation. AO concentration, 0.05%. PVME contents: A, 40%; O, 80%; 60%; X, 40%. Uninhibited PVME represented by O. (Reproduced with permission from reference 12. Copyright 1992 Gordon and Breach Science Publishers S.A.)


526


POLYMER DURABILITY

4

6

8

10

Oxidation Time (h)

Figure 25. Number-average molecular weights of PS in AOPS/PVME blends afte oxidation. AO concentration, 0.05%. PVME contents: X, 40%; box, 60%; O, 8 (Reproduced with permission from reference 12. Copyright 1992 Gordon an Breach Science Publishers S.A.)


32.

STOEBER ET A L .



5 sin

Φ υ

e m «ο w o

m

SX

Thermal and Photooxidation of Miscible Polymer Blends - American

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