British Polymer Journal 21 (1989) 467471

Effects of Synthesis Conditions on Rad iat ion = I nd uced G raft

Copolymerization of Methacrylic Acid -onto Poly(viny1 chloride) Films

Jagbir Singh, A. R. Ray & Harpal Singh*

Centre for Biomedical Engineering, Indian Institute of Technology, New Delhi-110016, India

(Received 17 May 1988; revised version received 19 September 1988; accepted 28 September 1988)

Abstract: Poly(viny1 chloride) (PVC) films were grafted with methacrylic acid (MAA) using a simultaneous-irradiation technique. The effect of solvent ratio (methanol-water) on grafting was studied and maximum grafting was found for an equivolume mixture of methanol and water. The graft level increased with increase of the plasticizer (dioctylphthalate) content in PVC. Grafting was found to be higher at low dose rate and increased linearly up to 0.25 Mrad for all dose rates and then levelled off. Grafting also increased continuously with increasing monomer concentrations up to 5.4 mol/litre, but a linear increase in grafting was observed only up to 2.5mol/litre. The dependences of rate of grafting on dose rate and monomer concentration were found to be 0.62 and 1.20 respectively.

Keywords: poly(viny1 chloride), radiation-induced grafting, dioctylphthalate, methacrylic acid.

1 INTRODUCTION

Poly(viny1 chloride) (PVC) is a widely used poly- meric material for the preparation of endotracheal tubes, blood bags and oxygenators. l ** Although PVC has excellent chemical resistance and good mechanical properties, it is highly hydrophobic. Radiation grafting is a suitable technique for changing the surface characteristics of the polymer and has been used to provide a composite material with a hydrophilic surface on a strong up port.^.^ PVC was grafted with methacrylic acid (MAA) with an aim to improve surface hydrophilicity and to have carboxylic groups for immobilization of drugs for sustained delivery. The present paper describes the radiation grafting of methacrylic acid on to PVC using different dose rates, doses, monomer con- centrations and solvents.

* To whom correspondence should be addressed.

2 EXPERIMENTAL

2.1 Materials

Food grade poly(viny1 chloride) (67ER092) manu- factured by Indian Petrochemicals Ltd, Baroda, India, was used for graft copolymerization studies. PVC films, with and without plasticizer (DOP), were cast on glass plates from a 6% total solution of either PVC resin or PVC resin and dioctylphthalate (DOP) in cyclohexanone and toluene (60/40, v/v). PVC films with 0, 10,20, 30 and 40% DOP content were thus prepared for grafting studies. All the films were dried in a vacuum oven at 60°C and stored in a desiccator over anhydrous calcium chloride. Most of the experiments were carried out with PVC films containing 30% DOP, unless otherwise specified. Methacrylic acid (MAA) obtained from E. Merck, was distilled at 80"C/10 mmHg before use. Analyti- cal grade methanol and laboratory grade cyclohexa-

467 British PoIymer Journal 0007-1641/89/$03.50 0 1989 Society of Chemical Industry. Printed in Great Britain

468 Jughir Singh, A . R. Ray, Harpd Sitigh

none and toluene supplied by Glaxo, India, were used without any further purification.

2.2 Radiation source

The irradiation of samples was carried out in a 60Co-gamma chamber (4000 Ci), supplied by Bhabha Atomic Research Centre, Bombay, India. Variation in dose rates was achieved by use of lead at tenuators of different thicknesses.

preferred to express our grafting results in mg/cm' as also adopted by other

During grafting studies it was observed that the degree of grafting varied up to 7.0% for the same grafting experiments run on different days, but the shape of the graft curves was always reproduced. From day to day however, the curves might appear shifted up or down with respect to the degree of graft axis. For any continuous curve presented in this paper, all experimental points are obtained for samples grafted simultaneously.

2.3 Preparation of graft copolymer 3 RESULTS AND DISCUSSION

Graft copolymerization was carried out in standard joint Corning tubes of 12.0 x 3.0cm size under a nitrogen atmosphere. In a typical procedure, a weighed amount (-0.2 g) of poly(viny1 chloride) film (6.5cm x 3.0cm x -0.006cm) was kept in a glass ampoule in equivolume mixture of methanol-water for 24 h. MAA monomer was added just before the exposure of samples and tubes were placed in the radiation chamber for the desired period. Most of the experiments were carried out under the following conditions unless otherwise specified: dose rate, 56 rdd/s; total dose 0.25 Mrad; monomer con- centration, 2.36 mol/litre; total volume of solvent, 14ml; methanol-water, 1: l ; and DOP content of PVC films, 30%. After the irradiation, the films were removed from the bulk external polymer and thoroughly washed with hot water and soaked in water at 50°C for several hours to remove adhering homopolymer. Further extraction of the homopolymer was done by Soxhlet extraction in methanol for 4 h and then films were washed with water and dried at 60°C at reduced pressure and weighed. Grafting was calculated as follows and expressed in mg/cm2:

Wg- wo A0

Grafting =

where W, is the weight of the grafted sample, Wo is the weight of the original sample, and A , is the area of the original sample.

The PVC films used in these experiments were prepared by a solution casting method. The films were not of uniform thickness. Therefore, in the situation in which grafting occurred only on the surface, the degree of graft expressed as a percentage would appear to vary with thickness of the substrate. We observed that the weight gain (grafting) on films of the same area having different thickness was almost the same. If these graft add on results are expressed in mg/cm2, the values are almost the same for films of different thickness. But when graft add on results are expressed in weight percent grafting, the values are quite different irrespective of almost the same total weight gain. To avoid the variation we

3.1 Effect of solvent composition

Figure 1 shows the variation of grafting with different methanol-water mixtures. In pure water, a

Fig. 1. Effect of methanol-water ratio on grafting of MAA on to PVC films: dose rate 56rad/s; dose 0.25 Mrad; monomer

concentration 2.36 mol/litre.

low amount of grafting (0075 mg/cm2) was ob- served. Addition of methanol had a marked effect on the grafting of MAA on to PVC. The level of grafting increased with increase in the percent of methanol in methanol-water mixtures and maxi- mum grafting was obtained in 1 : 1 methanol-water mixtures. The methanol had two major effects on the grafting kinetics: (1) homopolymerization was reduced, and (2) viscosity build-up in the surround- ing solution was retarded.' The percent grafting decreased after increasing the level of methanol in methanol-water mixtures beyond 50 percent and a negligible amount of grafting was found in 100% methanol. The higher concentration of methanol appears to inhibit not only grafting but also external homopolymerization.

3.2 Effect of dioctylphthalate (DOP)

The effect of dioctylphthalate (DOP) on PVC films casting (Fig. 2) was such that grafting of methacrylic acid increased as the percentage of DOP increased in

BRITISH POLYMER JOURNAL VOL. 21, NO. 6,1989

Radiation-induced graft copolymerization of methucrylic acid on to P VC,films 469

4J .- - . - iz 9 2.2- b-

m a m c 2.0- f c

X

Y

m w- ; 1.8- 4-8 c m 3 1.6

- 0 10 20 30 40

Dioctylphthalate (YO)

I I I I

Fig. 2. Effect of DOP content on grafting of MAA on to PVC films: dose rate 56 rad/s; dose 0.25 Mrad; monomer con-

centration 2.36 mol/litre; methanol-water 1 : 1.

PVC films. The glass transition temperature (T,) of rigid PVC is 85°C and decreases with increase in the plasticizer content in films, as reported in the literature8 Increased flexibility and molecular chain motion, and thus higher diffusion of monomer to the grafting site, might be the reason for higher grafting of MAA with increased plasticizer content in PVC films. This is in agreement with the results reported by Yao et al. on the grafting of acrylamide on poly(ethy1ene-vinyl acetate) copolymer films.' The exchange of DOP with MeOH/H,O/methacrylic acid mixture might also be one of the reasons for the higher grafting value.

3.3 Effect of dose rate and time

Grafting experiments were carried out at 26,56 and 106 rad/s with a total dose of 0.25 Mrad. The results are shown in Fig. 3. At a constant total dose of irradiation, grafting was found to be higher at low dose rates. Similar behaviour has been reported for other systems also.lO,'l It is a well established fact that the number of radicals formed during irradi- ation is directly proportional to the dose rate." During irradiation both monomeric and polymeric radicals are formed. The monomeric radicals may react with the polymer backbone (grafting) or with other monomer molecules (homopolymerization). ' Grafting at constant dose is higher a t low dose rates. This can be explained if we consider that at low dose rates, the creation of a certain number of radicals

0 , Dose rate (radls)

Fig. 3. Effect of dose rate on grafting of MAA on to PVC films. Monomer concentration 2.36 mol/litre; methanol-water 1 : 1;

dose 025 Mrad.

5'0t

Time (min)

Fig. 4. Effect of time on grafting of MAA on to PVC films at various dose rates. Monomer concentration 2.36 mol/litre; methanol-water 1 : l ; dose rate: .,26 rad/s; 0 , 5 6 rad/s; A, 106

rad/s.

takes a longer time than it does at a higher dose rate. Therefore, their instantaneous concentration is generally low and consequently the termination probability is also OW.'^'^^ Plots of the extent of grafting as a function of time at various dose rates (Fig. 4) pass through the origin, indicating that grafting proceeds without any induction period. Similar behaviour has been observed for poly- (ethylene-styrene) and poly(propy1ene-methacrylic acid) systems.10*16 The log-log plot of the rate of graft copolymerization (R,) versus dose rate of irradiation ( I ) (Fig. 5) gives a straight line with a slope of 0-62, meaning that:

R, K [Z]o'62 The dependence of rate of grafting on dose rate has been determined in several other systems and a wide range of dependence values from 0.31 to 0.9 have been reported. '' -

Results of the effect of total dose on degree of grafting (Fig. 4) show that the amount of grafting increases linearly with increase in the total dose up to 0-25-0.30 Mrad. In the initial stage of polymeriz- ation, the amount of monomer available is higher at

BRITISH POLYMER JOURNAL VOL.21, NO.6,1989

470

ul -l \ - ;

*- 2.L

'L 0 2 . 0 -

e -

0 x - m

.- - r 0

B 1.6-

a

0 1 . 2 -

0, - 0 -

c1,

Jaghir Singh, A. R. Ray, Harpal Singh

-

' ' I ' ' ' I '

e e e

0

1.0

0.0 0.0 2.0 L.0 6.0

Monomer concontrotion f m o l l L I

Fig. 6. Effect of monomer concentration on grafting of MAA on to PVC films: dose 0.25Mrad; dose rate 56rad/s;

methanol-water 1 : 1.

grafting sites on the polymer backbone. Hence monomer can diffuse very easily to the grafting sites and a smooth grafting reaction ensues. Further, the rate of homopolymerization is not much affected. Both these competing reactions proceed smoothly till a dose of 0.25-0.30 Mrad has been reached. Beyond this a slow decrease in graft efficiency indicates that there is an increase in the rate of homopolymerization as compared to the rate of grafting. Further increase in the content of grafted polymethacrylic acid may also act as a barrier against the diffusion of monomer into the polymer matrix.'0,20 As a consequence, the rate of homo- polymerization increases resulting in a decrease in the graft efficiency.

3.5 Effect of monomer concentration

The relationship between the level of grafting and monomer concentration in an equivolume mixture of methanol-water (Fig. 6) shows an increase in grafting with increase in monomer concentration. This can be attributed to the greater availability of

Fig. 8. Log-log plot of graft copolymerization rate ( R , ) versus monomer concentration.

monomer at grafting sites. However the graft level increased linearly only up to a monomer con- centration of 2.5 mol/litre.

The extent of grafting as a function of time at various monomer concentrations is shown in Fig. 7. The value of the rate of grafting (R,) at various monomer concentrations being calculated from this figure. The log-log plot of rate of grafting (R,) versus monomer concentration at a dose rate of 56rad/s is shown in Fig. 8. The rate of graft copolymerization is 1-20 as obtained from the slope of this plot. Therefore, the kinetic equation for observed dependences of rate of grafting on dose rate and monomer concentration may be rep- resented as

R, K [1]o""2[M]"20

Higher dependence of rate of grafting on monomer concentration has been reported by other workers also.' ' 9 ' 6 , 2 1 Th e exponents of 0.62 and 1.20 are slightly higher than the expected values. This may be due to a radical destruction reaction with an impurity or solvent, as reported elsewhere.22

REFERENCES

Time (min I

Fig. 7. Effect of time on grafting at various monomer concentrations at dose rate 56 rad/s. 0, 1.18 mol/litre; 0,

2.36 mol/litre; A, 2.96mol/litre; 0, 4.14 mol/litre.

1 Watson, W. F., Biomureriuls, 1 (1980) 41. 2 Gibbon, D. F., in Handbook of Engineering in Medicine and Biology

ed. D. G. Fleming & B. N. Feiubers. CRC Press, Ohio, 1976, p. 253. 3 Bruck, S. D., J. Biomed. Muter. Rex, 7 (1973) 387. 4 Hoffman, A. S., Radial. Phys. Chem., 9 (1977) 207. 5 Hoffman, A. S. & Ratner, B. D., J. Appl. Polym. Sci., 18(1974) 3183. 6 Lee, H. B., Shin, H. S. & Andrade, J. D., Polymer Prepr., 132) (1972)

7 Hoffman, A. S. & Kraft, W. G., Polymer Prepr., 132) (1972) 723. 8 Nass, L. I. & Heiberger, C. A. (eds), Encyclopedia of PVC, Vol. I .

9 Yao, K. D., Liu, Z. F., Gu, H. Q. & Fan, T. Y., J. Macvomal Sci.-

10 Gupta, B. D. & Mukherjee,A. K. ,J . Appl. Polym. Sci.,30(1985)2643. 11 Chapiro, A., Manakowski, Z. & Schmitt, N., Eur. Polym. J., 17(1981)

12 Chapiro, A,, Radiation Chemistry of Polymeric Systems. Wiley-

729.

Marcel Dekker Inc., New York and Basel, 1976, p. 393.

Chem., A24(10) (1987) 1191.

1161.

Interscience, New York, 1962, p. 129.

BRITISH POLYMER JOURNAL VOL.21, NO.6.1989

Radiation-induced graft copolymerizarion of methacrylic acid on to P VC ,films 47 1

13 Trivedi, I . M. & Mehta. P. C.. J . Appl. Polj7n7. Sci. 19 (1975) I . 14 Nikitina, T. S. & Bagdasarian, K. S. Sb., Rohor Radiar. Khim. Acad.

15 Vigo, F. & Uliana, C., J. Appl. Polym. Sci., 35 (1988) 1893. 16 Rabie, A. & Odian. G., J . Polyni. Sci.. Polyni. Chem. Ed., 15 (1977)

17 Chapiro, A. & Gouloubandi. R.. Eur. Po/ym. J. , 11 (1975) 241.

18 Furuhashi. A. & Kadonaga, M., J . Appl. Pol.ym. Sci., 10 (1966) 127. 19 Gouloubandi, R. & Chapiro, A,, Eur. Po/ym. J., 16 (1980) 957. 20 El-Azrnirly, M. A., Zahran. A. H. & Barakat, H. F., Eur. Polym. J., 12

(1976) 195. 21 Odian, G. & Rabie, A,, Polymer, 17 (1976) 171. 22 Clark, C. F. & Silverman. J., J . PoIj*m. Sci., Polym. Chrm. Ed., 15

( I 977) 269.

Sci. USSR, Moscu~. , 1955. 183.

469.

BRITISH POLYMER JOURNAL VOL. 21, NO. 6.1989