Radiation grafting of acrylamide onto starch-filled low density polyethylene

Radiat. Php.

Chem.

Vol. 49, 0

Pergamon

No. 4, pp. 497-501. 1997 1997 Elsevier Science Ltd

Printed in Great Britdin. All rights reserved 0969-806X197$!7.00 + 0.00

PII: s0969-806x(%)001504

RADIATION GRAFTING OF ACRYLAMIDE ONTO STARCH-FILLED LOW DENSITY POLYETHYLENE ROUHALLAH

BAGHERIT,

FRANAK

NAIMIAN

and NASSRIN

SHEIKH

Chemical Engineering Department, Polymer Group, Isfahan University of Technology, Isfahan, 1. R Iran and Gamma Irradiation Center, Atomic Energy Organization of Iran, Tehran, Iran (Receired 24 May I995; revised 19 July 1996)

Abstract-Acrylamide (AAm) was grafted on the surface of starch-filled low density polyethylene (SLDPE) and low density polyethylene (LDPE) films by the mutual irradiation technique at doses from 0.75 to 5 kGy. The effect of dose, solvents and dihydroxybenzoquinone on the degree of grafting was studied by Fourier transform infrared spectroscopy and the weight measurement method ofextracted films at a constant monomer concentration (10% w/w). An ultraviolet spectrophotometer was also used to elucidate the results of the above methods. Grafting on SLDPE and LDPE samples reaches a maximum followed by a slight decrease with increasing dose. A higher degree of grafting was obtained on SLDPE samples compared with that on LDPE. An induction period was observed in the case of the samples prepared in tetrahydrofuran (THF) as the solvent compared with those in chloroform. Addition of benzene to chloroform and THF (50% v/v) accelerates the rate of AAm grafting on the samples. Dihydroxybenzoquinone inhibits the grafting reactions of the samples especially in the THF solutions. The water uptake measurement of the samples correlates with the degree of grafting. 0 1997 Elsevier Science Ltd

EXPERIMENTAL

INTRODUCTION

Radiation-induced graft polymerization is one of the methods for surface modification of polymers, leading to materials with new properties (Kabanov et al., 1991). Extensive work on the fundamentals of radiation grafting of vinyl monomers onto various types of polymers has already been performed (Chapiro, 1959, 1962; Charlesby, 1960; Garnett, 1979). Radiation-induced grafting of acrylamide (AAm) onto polyethylene (PE) is very important, as modified PE is of interest for various applications such as biomaterials, membranes and coatings (Hoffman and Ratner, 1979; Postnikov et al., 1980; Ikada et al., 1983; Rucinska et al., 1985). Grafting of AAm onto cornstarch has also been studied under different conditions (Fanta et al., 1979; Hayakawa et al., 1970). The present work is an account of the effect of radiation dose, organic solvents and an inhibitor, dihydroxybenzoquinone (HZQ), on the grafting process of AAm onto starch-filled low density polyethylene (SLDPE) and low density polyethylene (LDPE). The aim of this work is to prepare biocompatible materials by grafting a hydrophillic monomer onto the surface of the polymer. Low doses of gamma radiation (0.755 kGy) were used for grafting to minimize possible degradation or cross-linking of the backbone of the polymer. tAuthor

10 whom correspondence

should be addressed.

Materials

The polymer used in the present study was LDPE in granular form containing no additives, supplied by National Petrochem&al Company of Iran, identified as LF045. Cornstarch was supplied by Glocosine Co. Ltd, Iran and vacuum oven dried at 75°C for 8 h before use. Acrylamide monomer and inhibitor were supplied by Merck Co., Germany, and used without further purification. Solvents were also obtained from Merck Co. and distilled before use. Mixing

A pre-mixing was done by tumbling of cornstarch (5% w/w) with LDPE granules. The compounded sample was poured in the chamber of a HAAKE (HBI system 90) torque rheometer. Processing was carried out at 150°C for 5 min with rotor speed of 60 rev min _ ‘. The processed samples were quenched in cold water to prevent further thermal oxidation. Film preparation

The films were prepared by compression moulding technique using a COLLIN press with plate size of 20 x 20 cm. Films of 0.15-0.16 mm thickness were moulded at 160°C and 2 x 10’Pa for 2 min using a special grade of cellophane as a mould release agent. Then the films were quenched in cold water and the clear films without any defect were used for the grafting process. 497

Rouhallah Bagheri et al. Table PI Code Solvents Code Solvents

CHCI, ESI CHCI,

E2 THF ES2 THF

1. Codes used for arafted extracted films”

E3 THF/CsHb ES3 THF/CaHb

E6 CHCl,/C,H, + H,Q ESS THF/GHe + H,Q

E4 CHCl,/C,H, ES4 THF + H,Q

ES CHCII + H,Q

‘: Monomer concentration was 10% (w/v) in all the samples; solvents in the mixtures were 50% each (v/v); the inhibitor concentration was O.S”k (w/v).

Grafting procedures

Weighed strips of SLDPE and LDPE films were placed in 5 ml solution of 10% w/v AAm in CHClj, tetrahydrofuran (THF) or mixtures of 50% v/v C6H, with one of these solvents, in 10 ml glass bottles. Without any pre-soaking, the glass bottles were exposed to cobalt-60 gamma radiation for the desired period of time at room temperature. For this purpose a Gammacell 220, Atomic Energy of Canada, Ltd., with a dose rate of 7 kGy/h, was used. After irradiation, the grafted films were washed thoroughly with hot distilled water to remove external homopolymer. The samples were soaked overnight in distilled water to extract the residual monomer and homopolymer in the films. Then al1 the samples were vacuum oven dried at 70°C to a constant weight. A Mettler analytical balance, AE 160, was used for weighing the films. The average weight of three samples was recorded. The degree of grafting was determined as the percentage of increase in weight:

leve1 is followed by a slight decrease in sample El (max. at 3.5 kGy). It can be seen that ES1 shows slightly slower rate of grafting at the start of the reaction up to 0.75 kGy compared with El, however the grafting reaches a maximum at 1.5 kGy which is higher than the grafting leve1 for the El. The heterogeneous nature of starch in relation to the polymer structure and tight coiling of its molecules may retard the generation of active site on the surface of the polymer at the beginning of irradiation. Diffusion of more AAm during irradiation into the starch molecules increases the grafting onto these molecules (Weaver et al., 1977; Fanta et al., 1978). This causes higher degree of grafting at lower doses (1.5 kGy) in comparison with the El sample. The absente of an induction period indicates that there is no significant inhibition or diffusional barrier at the beginning of the grafting process. 10

,

, 10

Degree of grafting = [( W, - W,)/ W, x 100 where W, and W, represent the weights of the unmodified and the vacuum oven dried grafted samples, respectively. The degree of water uptake was also calculated by the following equation: Water uptake percentage = [(W, - W,)/ WJ x 100 where W, and W, represent the weights of wet and vacuum oven dried grafted films, respectively. The infrared (IR) spectra were obtained using a Bruker IFS45 Fourier transform IR (FTIR) apparatus. The carbonyl index measurements of amide groups were carried out by measuring the ratio of the carbonyl absorbance at 1666 cm - ‘, relative to a reference peak at 1895 cm-‘. A Pye Unicam 88005 ultraviolet-visible (UV-vis) spectrophotometer was used to measure UV absorbance of samples at different doses. Table 1 shows the codes used to identify the prepared samples for analysis throughout this work. E and ES represent grafted LDPE and SLDPE respectively, and the index number indicates the type and composition of the solvent used for preparation of the AAm solution. It can be seen from Table 1 that solvents may also contain an added inhibitor.

- 0 0.5

1 1.5 2 2.5 3 3.5 4 4.5 5 -

Dose (kGy) h 100 g

90-

2

80-

g2

70-

q

60 -

-5oQI v 405 30-

(b) A_

c *-ic

2

20 -

3

lO-

uo 0

““““’ 0.5 1

1.5

2 2.5

3

3.5

4

4.5

5

Dose (kGy) RFSULTS AND DISCUSSION

The amount of AAm grafting onto SLDPE and LDPE in CHC& as a function of absorbed dose is presented in Fig. l(a). The rapid rise in the grafting

Fig. 1. (a) The degree of AAm grafting on to LDPE and SLDPE in CHQ as a function of absorbed dose. (c) The percentage of water uptake of grafted sample versus absorbed dose. (b) Variation of amide group samples with irradiation dose.

Radiation grafting onto polyethylene 10

7

9

_

Ca) 0 E2

Dose (kGy)

Dose (kGy) Fig. 2. (a) The degree of AAm grafting onto LDPE and SLDPE in THF as a function of absorbed dose. (b) Variation of amide group of E2 and ES2 samples with irradiation dose.

Figure 2(a) shows that grafting of AAm onto E2 and ES2 samples begins after an induction period. An increase followed by a decrease in the degree of

grafting is realized. The existente of an induction period may be attributed to the unfavourable partition of monomer between the polymer surface and the solvent. The effect of benzene on the grafting of AAm on the polymer surface is depicted in Fig. 3. Comparison of Fig. 3 with Fig. I(a) and Fig. 2(a) indicates that benzene accelerates the overall rate of AAm grafting onto the surface of the samples. This effect leads to a higher leve1 of AAm grafting which is more obvious in the case of the E4 sample. The observed effect of benzene may be attributed to the reduction in the rate of AAm homopolymerization (Taher et al., 1990). Thus as a result of lower viscosity of the C,H, solution during irradiation, more monomer diffusion of the AAm to the polymer sample can be expected. Figure 4 shows the spectra of grafted film containing no monomer, homopolymer and LDPE film. The spectrum of the grafted film (a) indicates the existente of peaks at 3359 cm- ’ (N-H stretching)

499

and 1666 cm-’ (carbonyl amide) which confirms grafting of AAm onto LDPE. The IR spectra of samples at different doses of irradiation indicate that with increasing irradiation dose, the peaks at 3359 cm - ’and 1666 cm - ’increase to a maximum and then decrease. Changes in the carbonyl index of the amide groups of E 1, ES 1. E2 and ES2 samples as a function of absorbed dose are shown in Fig. I(b) and Fig. 2(b). It can be observed from these figures that the increase in AAm grafting with irradiation dose up to a maximum corresponds reasonably with the results obtained by the weight measurement method (see Fig. l(a) and Fig. 2(a)). The effect of dihydroxybenzoquinone on grafting of AAm solutions onto the LDPE and SLDPE samples is shown in Table 2. The data in this table indicate that the inhibitor reduces significantly the grafting of AAm onto the E5 and E6 samples. NO grafting on the ES4 and ES5 is observed up to 5 kGy dose. E5 and E6 samples showed AAm grafting in the presence of H2Q at 5 kGy, which may be explained by higher rate of radical generation than radical scavenging. The inhibitor shows slightly better activity in THF which might be due to its stronger solvent activity. Dihydroxybenzoquinone, as an effective inhibitor, may scavenge some of the radicals produced in the initiation step at the surface layer of the polymer during irradiation. This prevents grafting of AAm onto the polymer surface (Table 2). The percentage of water uptake of grafted El and ES1 samples as a function of absorbed dose is shown in Fig. l(c). The content of water in the grafted samples increases with radiation dose. Comparison of Fig. I(c) and Fig. I(a) shows that the change in the water content of the samples correspond to the amount of grafting. The higher leve1 of water content in SLDPE samples may also be due to the tendency of the cornstarch to absorb water. The water uptake of samples E2, E3, E4, ES2 and ES3 as a function of absorbed dose is shown in Table 3. The results in this 25 , 0 E3 .; 20 Cz : 0Oo 15 M B 0

10

0 a; a

5

0 0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

5

Dose (kGy) Fig. 3. The percentage of AAm grafting onto E3, ES3, and E4 samples as a function of absorbed dose.

Rouhallah Bagheri et al.

500

(b)

2000 Wavenumber Fig.

4. FTIR

spectra

Table 2. Degree of AAm grafting onto LDPE presence of H,Q

and SLDPE

Dose- (kGy)

3.5

5

1.77 0 0

3.34 2.57 0 0

ES E6 ES4 ES5

0.75

1.5

0 0 0 0

0.43 0 0 0

film and (b) LDPE film.

of: (a) grafted

table correspond to those shown in Fig. 2(a) and Fig. 3. The explanation of the results for these samples is similar to that given for the data in Fig. l(c). Figure 5 shows typical UV spectra of the grafted sample (ESl) at four different doses. The reference is an unmodified LDPE film of 0.16 mm thickness. It can be seen that the variation of the absorbance at 220 nm due to AAm grafting with irradiation corresponds with the weight measurements and the FTIR results. Energy absorption from ionizing radiation is uniformly distributed over al1 the components of the substrate. Thus any substance added to the monomer wil1 be radiolysed and thereby contribute to chain initiation. This is the case in our experiments in which AAm is dissolved in solvents. It has been found that in the simplest cases free radicals are produced in the monomer and the solvent without any interference (Chapiro, 1979). Since CHCI, is much more susceptible to radical generation than THF during irradiation, AAm grafting in CHCI, occurs from the beginning of irradiation which increases to a higher leve1 in low doses (Ranby and Rabek, 1975); see Fig. l(a), Fig. 2(a) and Fig. l(c). Homopolymerization, homografting and crosslinking (or degradation) would occur in the AAm solutions as a result of the effect of these radicals during irradiation (Charlesby, 1980). These side reactions als0 account for an upper limit of grafting in the

cm-’

for each sample. The decrease in grafting leve1 after the maximum cannot be reasonably explained by degradation of the grafted polymer. This might be due to the formation of osmotic cells, proposed by Hoffman et al. (1959) and Cohn et al. (1984). The results presented in Fig. l(a) and Fig. 2(a) indicate that the samples prepared in THF show an induction period in contrast with those in CHCl,. This may also be attributed to higher stability of THF against irradiation. The higher leve1 of AAm grafting onto ES2 sample, compared with ESl, may also be explained by the better THF interaction with cornstarch during irradiation. CONCLUSION

The following conclusions can be drawn from this work: 1. AAm monomer in organic solvents can be grafted to LDPE and SLDPE using the direct irradiation method. 2. The leve1 of grafting reaches a maximum, followed by a decrease in some cases. 3. A higher grafting leve1 is obtained in CHCl, compared with THF, at lower doses, owing to more radical generation by CHCl+ 4. The different degrees of partitioning of monomer between the polyethylene surface and the solvent may be a factor in determining the different grafting behaviour observed. Table 3. Water uptake

percentage

Dose (kGy)

0.75

1.5

3.5

E2 ES2 E3 ES3 E4

0.4

4 3.5 1.4 5.9 22.7

6.3 ll.6 ll.5 14.1 17.6

0

of grafted extracted

films 5

4

8.8

10.4 31.8 9.1

Radiation 1.4 ,

grafting I

1.2 5 kGy * 3.5 kGy . 1.5 kGy + 0.75 kGy

0

I 0.8

c

0 E 0.6 Ti t; 0.4 2

0.2 0 -0.2 -0.4 180

200

220

240

Wavelength Fig.

5. UV spectra

of grafted

260

280

300

(nm) sample

(ESI) at different

doses of irradiation. 5. Grafting on SLDPE reached both a maximum at a lower dose and also a higher degree of grafting as compared with LDPE. 6. The water uptake of the samples increased with radiation dose corresponding with the degree of grafting. 7. The decline in the graft leve1 after reaching the maximum may be explained by the formation of osmotic cells. 8. Dihydroxybenzoquinone inhibited the grafting reactions on the samples, especially in the THF solutions. Acknowledgemenrs-The authors would like to thank Dr M. Sohrabpour, Directer of GIC, for supporting this work. Thanks also to the Polymer Research Centre of Iran for using torque rheometer.

onto polyethylene

501 REFERENCES

Chapiro, A. (1959) Journal of Polymer Science 29,321, Chapiro, A. (1962) Radiation Chemistry of’ Polymerie Systems. Interscience, New York. Chapiro. A. (1979) Journal of Radiation Physics and Chemistry 14, 101. Charlesby, A. (1960) Atomic Radiation and Pofymers. Pergamon, Oxford. Charlesby, A. (1980) Radiation Physics and Chemis/ry 15, 3. Cohn, D., Hoffman, A. S. and Ratner, B. D. (1984) Jour& of Applied Polymer Science 29, 2645. Fanta, G. F.. Burr, R. C., Doane, W. M. and Russell, C. R. (1978) Staerke 30, 237. Fanta, G. F., Burr, R. C. and Doane, W. M. (1979) Journul qf Applied Polymer Science 24, 20 15. Garnett, J. L. (1979) Radiarion Physics and Chemisq, 14, 79. Hayakawa, K., Yamakita. H. and Kawase, K. (1970) Radioisoropes 19, 8 1. Hoffman, A. S., Gilliand, E. R., Merrill. E. W. and Stochmayer, W. H. (1959) Journal of Pol_vmer Science 34, 46 1. Hoffman, A. S. and Ratner, B. D. (1979) Radiation Physics and Chemisny 14, 83 1. Ikada. Y., Suzuki, M. and Tamada, Y. (1983) American Chemieal Society Polymer Preprinis 24, 19. Kabanov, V. Ya., Aliev, R. E. and Kudryavtsev, Radiarion Physics und Chemistry 37, Val. N. (1991) 175. Postnikov, V. A.. Lukin, N. Yu., Maslov, B. V. and Plate, N. A. (1980) Polymer Bulletin 3, 75. Ranby, B. and Rabek, J.F. (1975) Phorodegradurion, Pholo-oxidation and Photosrabilizarion of‘ Pol.vmers. Chap. 6. Wiley-lnterscience, London. Rucinska, A.. Rosiak. J. and Pekala, W. (1985) Rndiurion Ph_vsics and Chemistry 24, 339. Taher, N. H., Dessouki, A. M. and Khalil, F. H. (1990) Radiation Physics and Chemistry 36, 785. Weaver, M. O., Montgomery, R. R., Miller, L. D., Sohns, V. E., Fanta. G. F. and Doane, W. M. (1977) Staerke 29, 413.