Download - Oxidation of polyethylene surface by corona discharge and the subsequent graft polymerization

Oxidation of Polyethylene Surface by Corona Discharge and the Subsequent

Graft Polymerization

HIROO IWATA,* AKIO KISHIDA, MASAKAZU SUZUKI, YOSHIO HATA, and YOSHITO IKADA,? Research Center for

Medical Polymers and Biomateriuls, Kyoto University, 53 Kawahara-cho, Shogoin, Sakyo-ku, Kyoto 606, Japan

Synopsis

Oxidation of a polyethylene (PE) surface by corona discharge and the subsequent graft polymerization of acrylamide (AAm) were studied. The maximum amount of peroxides introduced by corona treatment at a voltage of 15 kV was about 2.3 X mol m-'. The decomposition rate of peroxide and the dependence of graft amount on the storage period of the corona-treated PE films showed that there were several kinds of peroxides, the labile one being mainly responsible for the initiation of graft polymerization. When the corona-treated film was brought into contact with a deaerated aqueous solution of AAm, graft polymerization took place more strongly with the treatment time, but was reduced after passing a maximum. Although the x-ray photoelectron spectroscopic analyses of the corona-treated PE films showed homogeneous oxidation of the outer polymer surface by corona discharge, optical micrcecopy on the cross section of the grafted film revealed the graft polymerization to be limited to a very thin surface region.

INTRODUCTION In the series of our studies we have introduced peroxides to polymer

substrates by irradiation with high energy radiations' or glow discharge2 for the consequent use as an initiator of graft polymerization. An advantage of the glow discharge is formation of peroxides restricted only on the surface. Thus the location of grafting is definitely limited to the surface region of the polymer material without any change of bulk properties, independent of the polymerization condition. In the irradiation pretreatment, peroxides are generated uniformly throughout the cross section of the thin polymer material, so that the graft polymerization is needed to carry out in a nonsolvent of the bulk polymer to restrict the grafting only in the surface region of the polymer. The reaction common to these two pretreatments is an introduction of peroxides to the polymer. Any other pretreatments that can introduce peroxides to polymer may be applicable to such graft polymerization as to modify the polymer surface.

*Permanent address: National Cardiovascular Center, Research Institute, Suita-shi, Osaka,

'To whom correspondence should be addressed at: Research Center for Medical Polymers and Japan.

Biomaterials, Kyoto University, 53 Kawahara-cho, Shogoin, Sakyo-ku, Kyoto 606, Japan.

Journal of Polymer Science: Part A: Polymer Chemistry, Vol. 26, 3309-3322 (1988) 0 1988 John Wiley & Sons, Inc. CCC 0360-6376/88/123309-14$04.00

3310 IWATA ET AL.

Numerous studies have been devoted to the use of corona discharge for surface modifications of polymers, mainly from the industrial point of vie^.^.^ The surface properties and the chemical compositioii of the polymers treated by corona discharge have been extensively studied by various kinds of physicochemical methods. Especially, analyses by x-ray photoelectron spectroscopy (XPS) have proved to be useful and have shown that the polymer surfac.es treated by corona discharge are effectively oxidized to yield various kinds of polar groups containing oxygen such as ether, ketone, peroxide, e t ~ . ~ - ' Corona discharge seems to be as effective as glow discharge and high-energy irradiation for introduction of peroxides to a polymer for the subsequent use of graft polymerization. However, very few works have been reported on the graft polymerization by utilizing the corona discharge treatment.", l1

The present work will describe the graft polymerization of acrylamide (AAm) onto a polyethylene (PE) surface activated by corona discharge.

EXPERIMENTAL

Materials

The substrate polymer used for graft polymerization is a high density PE film of 50-60 pm thickness, which was donated by Showa Denko Co., LTD., Japan (Sholex FWOFC). The film was purified by Soxhlet extraction with methanol for 20 h and stored in a desiccator before use. The AAm monomer was of electrophoresis grade and used as obtained similar to other chemicals.

Corona Discharge

The equipment used in the corona discharge treatment was developed by ourselves, referring to that of Blythe et aL5 A circular parallel-plate electrode cell was constructed with electrodes having a diameter of 75 mm. A pair of 2 mm thick glass plates, 130 mm in diameter, were used as insulator, being adjacent to each of electrodes, with two glass blocks as spacer to maintain a fixed air gap of 2 mm. In assembling the cell the film sample was placed on the lower glass plate and any trapped air was squeezed from underneath. All runs were done in dry air a t atmospheric pressure. The gap voltage was 10 or 15 kV, expressed as peak voltage. The dissipated electrical energy in the air gap was determined using the similar electrical circuit to that of Blythe et al.

Peroxide Decomposition

To determine the surface concentration of peroxide, corona-treated films were put in a 1.0 x lop4 mol L-' deaerated solution of l,l-diphenyl-2-picryl- hydrazyl (DPPH) in benzene and kept a t 70°C for 24 h to decompose the peroxides formed on and near the film surface. The DPPH molecules con- sumed were quantitated from the difference in txansrm 'ttance a t 520 nm between the virgin and the corona-treated films. The absorption coefficient of DPPH at 520 nm was 1.18 X lo4 L mol-' cm-'. Decompogition of peroxide a t 50°C with the lapse of time was determined uaing the game procedure mentioned above.

OXIDATION OF PE SURFACE BY CORONA DISCHARGE 3311

Contact Angle Measurement

Static contact angles of water on corona-treated films were measured at 25°C and 65% relative humidity with the &le drop method, and ten readings were averaged.

X-Ray Photoelectron Spectroscopy

A spectrometer ESCA 750 manufactured by Shimadzu Corp. Kyoto, Japan, was employed to carry out XPS measurements of corona-treated films using a MgKa x-ray source. For determining O/C and N/C stoichiometries, collect- ing factors of 2.9 and 1.5 (compared with Cls) were used for O,, and N,,, respectively. The C,, spectra bands were deconvoluted by computer into individual peaks. The employed peak function is

where a, p, and w are peak height, peak position, and full width at half maximum, respectively. The parameter f is equal to 1 for Gaussian function. When f approaches the limiting value, that is, 0, the function y wil l be asymptotic to Lorentzian function. The details will be published elsewhere.


To effect graft polymerization, the films treated by corona discharge were immersed in 10 w t S aqueous solution of AAm in glass ampules. After vigorous degassing, the ampules were sealed and kept at 50°C for 1 h. In redox-initiated polymerization, FeS04(NH4),S046H20 was added to the monomer solution to decompose peroxides and the polymerization was al- lowed to proceed at 18OC for 3 h. The AAm homopolymer formed was removed from the grafted film by extraction with water at 7OoC overnight under stirring.

The amount of PAAm grafted was determined with the ninhydrin method.2 Briefly, the grafted i3m.s were immersed in 2.5N HC1 at about 12OoC in an autoclave for 30 min to hydrolyze the -CONH, side groups of grafted PAAm to -COOH and NH,. After that, the hydrolysis mixture was neutral- ized with NaOH and further kept at about 12OOC for 5 min following an addition of ninhydrin of 0.12 mol L-' in an ethylene glycol monomethyl ether and acetic acid mixture. The light transmittance at 570 nm was measured for the resulting solution and the amount of liberated NH, was determined. By reference to the calibration curve obtained for the AAm monomer, the grafted amount of PAAm could be evaluated, if the amount was greater than about 1 pg No attempt was made to determine the yield of homopolymer.

optical Microecopy

Grafted films were put in 1N NaOH at 5 5 O C for 10 min to hydrolyze the grafted PAAm chains to poly(acrylic acid) (PAA). A fluorescent microscope

3312 IWATA ET AL.

was used to observe the cross-section of the hydrolyzed films after staining with a fluorescent dye, Auramin 0.

RESULTS

Treatment of PE by Corona Discharge

Contact Angle Change

As is well known, corona treatment of a film in air introduces polar groups containing oxygen onto the film surface and increases its water wettability with reduction in water contact angle. Figure 1 shows the change of contact angle of the PE film corona-treated by our equipment at a voltage of 15 kV. The contact a n g h were measured immediately after the corona treatment. The PE film without corona treatment exhibited a water contact angle of 93", indicating that hydrophobicity of the PE film was drastically decreased by corona treatment. It is seen that corona treatment for about 10 min was required for lowering the water contact angle of the PE film to the leveling-off value (40").

X P S study

Oxidation of the PE films treated by corona discharge but not yet grafted was studied by measuring core-level spectra of Cis, O,,, and N,, with XPS. Figure 2 gives the C,,, O,,, and N,, spectra for the PE films treated at a voltage of 15 kV for different periods of time. As is evident from the strong 0,, band and the high binding energy of C,, components due to oxidized carbon groups, a larger amount of oxygen atoms were introduced to the surface region of the film with the increasing exposure time. And as Briggs et al.12 observed, N,, spectra exhibited a well-developed peak at high binding energies probably due to -0N0, and NO, groups.

CORONA TREATMENT TIME,min

' 0 5 10 15 20 25 30 I I I ,

-

-

0 - 0

0 25 50 75 100 125 150 175

TOTAL DlSSl PAT E D ENERGY, J . cm-' Fig. 1. Decrease of the contact angle by corona treatment for the PE film. Contact angles were

measured immediately after corona treatment at a voltage of 15 kV.


2min

\ I . . . x//-\ original

10min

Smin

2min

5 4 0 53 5 530

nal

BINDING ENERGY, e V Fig. 2. Nls, C,,, and OIs XPS spectra of the PE film treated by corona discharge at a voltage

of 15 kV for various times.

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CORONA TREATMENT TIME, min

0 5 10 15 20 25 30

0 10 20 30 40 50 kV - 45 kV

lO0l------

25 50 75 100 125 150

TOTAL DISSIPATED ENERGY, J cm2

n oq 50 75 100 125 150 I 1 I I

'5

Fig. 3. O/C and N/C stoichiometries for the PE film treated by corona discharge at the voltages of 10 kV and 15 kV (A) 10 kV and (0) 15 kV for O/C, (A) 10 kV and (0) 15 kV for N/C.

Figure 3 shows the O/C stoichiometries of the films treated at voltages of 10 and 15 kV as a function of the dissipated total energy and the corona treatment time. The corona treatment at 15 kV gives O/C with a broad maximum around 10 min of treatment time. This result might suggest that polymer segments in the surface region were highly oxidized and broken into fragments during the prolonged treatment, and volatile oxidized products being ablated from the stdace and the polymer substrate underneath the highly oxidized layer appeared during the XFS measurement at an extremely low pressure. The O/C stoichiometries of the film treated at 10 kV increased monotonously with the treatment time within the time range examined in this study. The treatment condition at 10 kV was milder and hence introduced less oxygen on the PE surface than that at 15 kV, when compared at the Same treatment time.

The C,, profile consisted of a relatively broad unresolved shape with a distinctive asymmetry shifting to the high binding energy side as shown in Figure 2. The line-shape analysis revealed the presence of four components with binding energies of 285.0, 286.4, 287.8, and 289.3 eV. Referring to the works of Clark et al.13 and Dilks et al.,14 it is likely that these peaks may be assigned to carbon inC-0 groups for 286.4 eV, carbon in C=O or 0-C-0 groups for 287.8 eV, and carbon in 0-C=O groups for 289.3 eV, respectively. The variation of the relative intensities of individual Cls components with the corona treatment time is presented in Figure 4 for the spectra shown in Figure 2. The relative intensities of three components having



0 5 10 15 20 25 30 100- I I I I 1 I

0

Y 0- 25 50 75 100 125 150 175

TOTAL Dl SSl PAT ED EN E RGY, J - cm-*

Fig. 4. Percentage contribution of each CIS component to the total CIS spectrum: (0) 285 eV, (A) 286.5 eV, (0) 287.8 eV, (0) 289.3 eV.

higher binding energy are seen to change, passing a broad maximum, with the corona treatment time. This tendency is in good agreement with the increase of Ols peak intensities with the corona treatment time. The increase in the intensities of 286.4 and 287.8 eV peaks with the treatment time occul~l more rapidly than that of the peak at 289.3 eV. It seems that the oxidized groups which have peaks at 286.4 and 287.8 eV were further oxidized and transformed during corona discharge to more highly oxidized status corresponding to a peak at 289.3 eV by corona discharge and ozone which would be concomitantly produced during corona discharge in air.

The method of varying the photoelectron takeoff angle in XI'S is com- monly used to learn depth profiling of compositional variation. We also applied this technique to our corona-treated surface (results are not presented) and found the intensity ratio of Ols/Cls to be independent of the photoeledn>n takeoff angles, although they were widely varied. This in&- c a b that at least few tens angstroms of the outermost polymer surface was homogeneously modified by corona discharge.

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Peroxide Formation

The graft polymerization to be described later seems to proceed by the radical mechamam * . However, it is hard to assume that free radicals formed by corona discharge directly take part in graft polymerhtion, because no appre- ciable @aft polymerization took place at room temperature, and the free radicals formed during the corona discharge should be immediately captured by atmospheric oxygen to be transformed into oxygen-containing polar groups. Furthermore, as wil l be shown later, the dependence of the graft polymer formation on the concentration of a reducing agent supports that peroxides may be responsible to the initiation reaction of the graft polymerization onto the corona-treated surface.

Although a line-shape analysis of C,, revealed existence of four different individual components in C,, spectra, the amount of peroxides could not be determined by XPS analysis because the component with a binding energy of 286.4 eV would contain not only peroxide, but also ether and ester C-0 carbons. Thus, the amount of peroxides on the film surface was quantitatively evaluated by the DPPH method following a previous paper.2

In Figure 5, the surface concentration of generated peroxides is plotted as a function of the dissipated total energy and the corona treatment time. The voltage applied for the corona treatment was 15 kV. It can be seen that the dependence of the peroxide formation on the corona treatment time is not monotonous. Clearly, longer exposure does not help formation of a larger amount of peroxide. As the corona treatment was carried out in air, the peroxides initially introduced by corona treatment was continuously exposed to further corona discharge, probably resulting in production of highly oxidized groups-

CORONA TREATMENT TIME, min N

0 5 10 15 20 25 30 '5 2.51 I I 1 1 I I

TOTAL DISSIPATED ENERGY, J-cm-'

Fig. 5. Formation of peroxides on the PE film treated by corona discharge at a voltage of 15 kV.


w 0 z 1.0- 0 0 w

X

W

P

g ' 1 2 4 12 24 144 2 a

TIME, hr

e

Fig. 6. Change of peroxide concentration during storage at room temperature in air for the corona treated PE film.

The change of the peroxide concentration during storage in air at ambient temperature is given in Figure 6. The peroxide concentration gradually decreased during 24 h storage and then leveled off to about 60% of that measured immediately after the corona treatment. Kinetic studies of decomposition of peroxides performed at 5OoC are shown in Figure 7. The decomposition of peroxides upon heating did not follow a simple first-order reaction. A curvature was observed in the ht-order plot in the initial stage of the reaction, but then the curve became linear probably because the fairly labile peroxide had rapidly disappeared. The fact suggests that several kinds of peroxides must be formed by the corona treatment of PE film. Some of them are labile and easily decomposed even at room temperature, while the others should be more stable.

0 1 2 3 4 5

Reacticm Tm, tr Fig. 7. Kinetic analysis of peroxide decomposition at 50°C in benzene. The PE films were

treated by corona discharge at a voltage of 15 kV for 2 min. [Peroxide],: surface concentration of peroxide introduced onto PE surface; [Peroxide],: concentration of peroxide remaining after heating for t h.

3318 IWATA ET AL.


I I I 'p i 1 5 k V 0 5

I I I I I l O k V 0 5 1 0 15 2 0

1 I I I 1 I I I I l O k V

0 5 1 0 15 2 0

I I I

0 n I

30 40

TOTAL DISSIPATED ENERGY, J.cm-' Dependence of the graft amount on the dissipated total energy and on corona

treatment time for the PE films treated by corona discharge at the voltage of 10 kV and 15 kV; (0) 15 kV and (0) 10 kV.

Fig. 8.


AAm was graft polymerized onto the corona-treated films in aqueous solution at 50°C without any additive. The amount of PAAm grafted per square centimeter of film is plotted as a function of the dissipated total energy in Figure 8. A distinct maximum of the grafted amount is seen in the dissipated total energy region from 6 to 8 J Almost similar dependence of the grafted amount on the dissipated total energy was found for the different applied voltages. This fact implies that the dissipated total energy wil l determine the grafted amount of the corona-treated film. From the corona exposure time given in the abscissa of the upper part of Figure 8, it is obvious that the exposure time required for the maximum grafted amount becomes shorter with the increasing applied voltage.

It has been pointed out that, in the graft polymerization onto polymers irradiated in air, a redox reaction is advantageous in generating free radicals which initiate graft polymerization, because the redox reaction is able to minimize the production of homopolymer and allow the polymerization to proceed at low temperat~es. '~ Therefore, we studied graft polymerization onto a corona-treated PE film at 18°C using ferrous iron (F<II)) as a reducing agent. As is shown in Figure 9, the amount of graft polymers formed in the redox system increaaed with the increasing concentration of the reducing agent and after passing through a maximum, again decreased. We did not determine the homopolymer formed during the graft polymerization, but observed that the homopolymer formation monotonously decreased with the increasing concentration of reducing agent. The kinetic equation derived in


E a a n

I- LL 4. CL: 0

, I 12

10

-

-

8 -

6 -

4 -

2 -

- 0 104 IO-* 10-' I 10

FeW, mole I-' Fig. 9. Dependence of the graft amount on Fe(II) concentration for the PE film treated by

corona discharge at a voltage of 15 kV for 2 min.

our previous paper16 clearly explains these phenomena and supports the participation of the peroxide in the graft polymerization.

The reduction of the grafted amount during storage of the corona-treated film at ambient temperature is shown in Figure 10. It gradually decreases with the storage time. As mentioned above, the surface concentration of peroxide also monotonously decreased during storage. However, the dependence of the graft amount and the peroxide concentration on the storage time is clearly Merent from each other, as is apparent from the comparison of the results shown in Figure 6 with those of Figure 10. The amount of peroxides after 12 days storage is about 60% of that measured immediately after the corona treatment, whereas the grafted amount is about 20% after 12 days storage. As anticipated above, it seems that several kinds of peroxide species exist which have different rate constants of decomposition. The labile peroxide must mainly be responsible for the initiation of graft polymerization at MOC. The

E a

6 I a n W I- LL

U 0 a

N I

E, 0 a

1001 1

0 0 , , I

1 2 4 12 24 144 288

TIME, hr

Fig. 10. Influence of storage at mom temperature on the graft amount for the corona-treatd PE film.

3320 IWATA ET AL.

Fig. 11. Cross section of PE film grafted with PAAm after hydrolysis and staining (graft amount was 140 fig observed by fluorescent microscope.

peroxides which remained even after 12 days storage seem to be too stable to initiate graft polymerization at 50°C

Swface Structure of PAAm Grafted Film

Figure 11 demonstrates a photo of the crow section of a stained PE film with a graft amount of 140 pg For staining the grafted layer, the grafted PAAm chains were first hydrolyzed to PAA and then immersed in an aqueous solution of Auramin 0. It is clearly seen that the location where graft polymerization proceeded is restricted to the film surface, as expected. The thickness of the grafted layer in Figure 11 cannot be measured with accuracy but seems smaller than 4 pm.

DISCUSSION There have been quite a large number of works on 'the corona treatment of

polymers. m e y have been performed mostly to enhance their adhesive properties. Recently, the introduction of XPS analysis has made it possible to clarify what kind of functional groups is generated onto the polymer surface by corona treatment and it has become clear that peroxide moieties are introduced onto the polymer surface by corona treatment.6 In this work, the PE film treated by corona discharge was analyzed to identify species initiating


graft polymerization. The chemical structure of functional groups which initiated the graft polymerization could not be elucidated, but the peroxide which was labile at room temperature seemed to be the most plausible species playing a major role in this graft polymerization.

Sakata and Goringlo studied graft polymerization of vinyl monomers onto cellulose treated by corona discharge. They examined the redox initiation to identify the species initiating graft polymerization. Contrary to us, they concluded that the initiation species of graft polymerization was trapped free radicals. If peroxides participate in the redox initiation of graft polymerization, the amount of graft polymer formed must have a maximum at a certain concentration of the reducing agent, as we found. However, their results revealed that the grafted amount monotonously decreased with the increasing concentration of reducing agent. There have been a lot of works on the graft polymerization onto cellulose pretreated by high-energy ionizing radiati~n.'~ It is accepted that the initiating species of radiation graft polymerization of cellulose is trapped radicals, because cellulose chains are so tightly assembled with each other by the hydrogen bonding as not to react with environmental oxygen, leading to stabilization of trapped radicals.ls During polymerization procedure, cellulose is normally swollen by an aqueous medium or a monomer and hence the trapped radicals wil l be exposed to the monomer to initiate polymerizati~n.'~ Also in the study of Sakata and Goring, the trapped radicals might be formed onto cellulose by corona discharge, similar to pretreatment of cellulose with high-energy ionizing radiation. On the other hand, in the case of PE chains which have no polar groups, oxygen may easily penetrate into polymer chains. As a consequence, the radicals formed on PE must rapidly react with oxygen to be transformed to peroxide.

In a previous paper, we studied a glow discharge pretreatment on a PE film for graft polymerization and showed that peroxide was also species responsible for initiating graft polymerization. However, the amount of peroxide generated on the PE film surface upon glow discharge was 10 times less than that formed by corona discharge. On the other hand, there was no significant difference in the maximum grafted amount between these two different methods, if graft polymerization was carried out under the same condition. It is likely that the type of peroxide is different between these two pretreatment methods. In the corona treatment several kinds of peroxide is formed in the corona discharge treatment, because hydroperoxides are generated during discharge but may be rapidly transformed to more highly oxidized species such as peroxyester and peroxywbonate by ozone which is concomitantly produced during corona discharge in air.2o The amount of labile peroxide which may be responsible for graft polymerization is less than 40% of generated total peroxide. These facts might explain why the almost same grafted amount was obtained despite the difference in peroxide concentration on the PE films treated by different methods.

Finally we would like to emphasize that on the contrary to the previous two pretreatment methods, i.e., glow and y-ray treatments, the corona pretreatment can be carried out at the ambient pressure by an apparatus ordinarily used in the industrial field. The corona treatment needs neither an expensive apparatus such as used in the pray irradiation nor attention to keep the pressure low in the apparatus such as for the glow treatment.

3322 IWATA ET AL.

References 1. M. Suzuki, Y. Tamada, H. Iwata, and Y. Ikada,Physicochemical Aspects of Polymer

2. M. Suzuki, A. Kishida, H. Iwata, and Y. Ikada, MacromoZecuZes, 19, 1804 (1986). 3. C. Y. Kim, J. Evans, and D. A. I. Goring, J. Appl. Polym. Sci., 15, 1365 (1971). 4. M. M. Kadasb and C. G. Seefried, Plast. EM.. 41, (1985).

Surfaces, K. L. Mittal, Ed., Plenum, New York 1983, Vol. 2, p. 923.

5. A. R. Blythe, D. Briggs, C. R. Kendall, D. GIRance, and V. J. I. Zichy, Polymer, 19, 1273 (1978). . ,

6. L. J. Genenser, J. F. Ekmar, M. G. Mason, and J. M. Pochan, Polymer, 21, 1162 (1985). 7. D. Briggs and C. R. Kendall, Polymer, 20, 1053 (1979). 8. J. Amouroux, M. Goldman, and M. F. Revoil, J. Polym. Sci. Polym. Chem. Ed., 15, 991

9. H. Steinhauser and G. Ellinghorst, Angew. Makroml. C h . , 120, 177 (1984). (1977).

10. 1. Sakata and D. A. I. Goring, J. Appl. Polym. Sci., 20,573 (1976). 11. I. Noh, C.-H. Yoon, and I.-J. Hwang, NorJnrcnjip-sMop Kwahak Kkul Yonguso (Znha

12. D. Brigga, C. R. Kendall, and A. B. Wootton, Polymer, 24,47 (1983). 13. D. T. Clark, B. J. Cromarty, and A. H. R. Dilks, J . Polym. Sci. Polym. Chem. Ed., 16,

14. A. Dilks and A. Vanheken, Physicochemical Aspects of Polymer Surfaces, K. L. Mittal,

15. I. Ishigaki, T. Sugo, T. Takayama, T. Okada, J. Okamoto, and S. Machi, J. Appl. Polym.

16. H. Iwata, M. Suzuki, and Y. Ikada, High Molecular Compounds Short C o r n . (Vysokoml.

17. P. J. Baugh, 0. Hinojosa, and J. C. Arthur, J. Appl. Polym. Sci., 11, 1139 (1967). 18. J. C. Arthur, ACSSymp. Ser., 187,21(1987). 19. R. M. Reinhart, J. C. Arthur, and L. L. Muller, J . Appl. Polym. Sci., 24, 1739 (1979). 20. J. Peeling, M. Jazuu, and T. Clark, J. Polym. Sci. Polym. Chmn. Ed., 20,1797 (1982).

TaeMkyo), 7.99 (1980).

3173 (1978).

Ed., Plenum, New York 1983, Vol. 2, p. 749.

sci., n, 1043 (1982). soedin.), 27B, 313 (1986).

Received November 1, 1987 Accepted February 11,1988