Polymer International 46 (1998) 150È156

Gamma Radiation Induced GraftCopolymerization of Acrylamide onto

Poly(vinyl chloride) Films

E. M. El-Nesr,1 A. M. Dessouki1 & E. M. Abdel-Bary2,*

1 National Center for Radiation Research and Technology, Nasr City, P.O. Box 29, Cairo, Egypt2 Chemistry Department, Faculty of Science, Mansoura University, Mansoura, Egypt

(Received 4 June 1997 ; revised version received 20 September 1997 ; accepted 24 November 1997)

Abstract : Grafting of acrylamide (AAm) onto poly(vinyl chloride) (PVC) Ðlmsusing gamma radiation has been carried out. The e†ects of di†erent parameterson the graft yield have been investigated. These parameters include radiationdose, monomer concentration, solvent composition, types of inhibitors andacidity of the medium. In addition, the e†ects of the multifunctional monomersN,N@-methylene allyl amide (MAAm) and 2,4,6-triallyloxy-1,3,5 triazine (TARA)on the graft yield have been investigated. It has been found that methanol/watermixture in the ratio (1 : 1) is the proper diluent mixture for enhancing the graftcopolymerization process. The presence of sulphuric acid in the reaction mediumadversely a†ects the graft yield more than acetic acid. The presence of MAAm inthe reaction medium markedly increases the graft yield, while TARA has practi-cally no e†ect on the grafting process. Grafted Ðlms have been characterized andevidence of grafting has been obtained using thermal and spectroscopic analysistogether with swelling measurements in water. 1998 SCI.(

Polym. Int. 46, 150È156 (1998)

Key words : gamma radiation ; grafting ; polyvinyl chloride ; acrylamide ; multi-functional monomers

INTRODUCTION

Gamma radiation induced graft copolymerization ofacrylamide (AAm) onto di†erent substrates such as lowdensity polyethylene (LDPE),1,2 polytetraÑuoroethylene(PTFE),3 nylon 6,4 Ðlter paper,5 urethane and siliconerubber,6 cellulose acetate7 and cotton fabric8 hasattracted the attention of many investigators. Thee†ects of di†erent parameters such as radiation dosemonomer concentration and inhibitor concentration onthe graft efficiency of acrylamide onto ethene and vinylacetate copolymer (EVA), LDPE and their blends hasbeen investigated in our previous work.9 Medical gradepoly(vinyl chloride) (PVC) sheets were surface modiÐedby grafting a combination of 2-hydroxyethyl meth-acrylate (HEMA) and N-vinyl pyrrolidone (NVP), or

* To whom all correspondence should be addressed.

vinyl pyridine (4-VP) alone using gamma radiation.10This treatment helps to retard the migration of plastici-zer from the PVC matrix. However, little attention hasbeen paid to the grafting of PVC in comparison withpolyoleÐn substrates.11 For this reason we extended ourwork to study the parameters a†ecting the graft copoly-merization of acrylamide onto poly(vinyl chloride),because it is known that the grafting of hydrophilicmonomers such as acrylamide is interesting for medialapplications.

EXPERIMENTAL

Materials

Poly(vinyl chloride) (PVC) Ðlms of thickness 80lm werekindly provided by the Medial Supplier El-Nasr Co.,Egypt. Acrylamide (AAm), purity 99% (Merck,

1501998 SCI. Polymer International 0959È8103/98/$17.50 Printed in Great Britain(

Graft copolymerization of AAm onto PV C Ðlms 151

Germany), N,N@-methylene allylamide (MAAm) (Merck,Germany) and 2,4,6-triallyloxy-1,3,5 triazine (TARA)(Aldrich) were used as received. The other chemicals,were of reagent grade and were used as received.

Techniques

Graft copolymerization. Strips of commercial PVC Ðlmsplasticized with 20% dioctyl phthalate were washedwith acetone, dried at 50¡C in a vacuum oven, weighed,and then immersed in the monomer solution. Ammon-ium ferrous sulphate (MohrÏs salt) and other inhibitorswere introduced into the reaction mixture to avoid theformation of homopolymers by direct irradiation graft-ing. The reaction mixture was deaerated by bubblingnitrogen gas for 4È7 min, sealed and then subjected to aCo-60 gamma source at a dose rate of 0É69 Gy s~1. Thegrafted Ðlms obtained were removed and washed thor-oughly with hot distilled water and soaked overnight inwater to ensure the extraction of residual monomersand homopolymers from the Ðlms. The Ðlms obtainedwere then dried in a vacuum oven at 50È60¡C for 24 hand weighed.

The degree of grafting was determined by the percent-age increase in weight as follows :

Degree of grafting (%)\ Wg [ W0W0

] 100 (1)

where and represent the weights of initial andW 0 W ggrafted Ðlms, respectively.

Swelling measurements. The clean and dried PVC Ðlms(grafted and ungrafted) of known weights wereimmersed in distilled water and methanol for 24 h at18¡C. The Ðlms were removed, blotted with absorbentpaper and immediately weighed. The percentage swell-ing, either in water or methanol was calculated asfollows :

Swelling (%)\ Ws [ W0W0

] 100 (2)

where and represent the weights of initial andW0 Wsswollen Ðlms, respectively.

Infrared spectroscopy. Infrared spectra were obtained inthe region 4000È400 cm~1 using a Mattson 5000 FTIRspectrometer (Maltson, Cambridge, UK).

T hermal analysis. Thermal analysis of the grafted Ðlmswas carried out using a PerkinÈElmer 7 di†erentialscanning calorimeter (Perkin-Elmer, Norwalk, CT,USA) and a Schimadzu TGA-30 (Schimadzu, Kyoto,Japan) at a heating rate of 20¡Cmin~1 under nitrogenatmosphere.

RESULTS AND DISCUSSION

During grafting of plasticized PVC, one should becareful in the selection of the diluent and monomer.This is because the plasticizer may be partially extractedfrom plasticized PVC if it dissolves in either themonomer or the diluent. For instance, it is difficult tograft plasticized PVC with vinyl acetate monomer in thepresence of methyl alcohol as diluent, because adecrease in the weight of the Ðlm has been observedwith increasing vinyl acetate concentration.12 This isbecause both methyl alcohol and vinyl acetatemonomer are able to extract plasticizer from the Ðlms.For this reason the e†ect of solvent has been investi-gated.

Effect of solvent

A mixture of methyl alcohol and water has been used asdiluent for the graft copolymerization reaction. Theaddition of methanol to water is based on the fact thatmethyl alcohol in binary combination with other sol-vents usually enhances the grafting process of di†erentmonomers onto di†erent polymers e.g. styrene onnylon, polyethylene and polypropylene13,14 or meth-acrylic acid,15 or methyl methacrylate onto poly-propylene.16 The results for methyl alcohol/watermixtures in di†erent volume ratios are presented in Fig.1. It can be seen that the graft yield in the presence of100% water or 100% methanol is very low when com-pared with their combination in a volume ratio of 1 : 1.The graft yield using this binary mixture is 65% com-pared with 3É7% in the presence of methanol and 10É1%in the presence of water. The low graft yield in the pres-

Fig. 1. E†ect of solvent composition in water/methanolmixture on the graft yield : AAm, 25 wt%; MohrÏs salt,

2É5 wt%; total dose, 20 kGy.

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152 E. M. El-Nesr, A. M. Dessouki, E. M. Abdel-Bary

ence of 100% methanol can be explained in terms of theease of extraction of the plasticizer with methanol fromthe substrate. In contrast, the high graft yield for thebinary mixture can be attributed to the known roleplayed by methanol in enhancing the graft copolymer-ization process.

Effect of monomer concentration

E†ect of monomer concentration has been investigatedin water as diluent. The selection of water only is basedon the fact that water is a good solvent for themonomer and also for the homopolymer that may beformed. In addition, water does not extract the plastici-zer. The results obtained are given in Fig. 2. It can beseen that no grafting occurs on increasing the monomerconcentration up to 10%, after which the graft yieldincreases. The maximum graft yield achieved in thiscase is 20%. It is interesting to note that no grafting hasbeen observed under these conditions when PVC Ðlmsfree from plasticizer have been used. These Ðlms wereprepared by casting from PVC solutions in cyclo-hexanone. This can be attributed to the ease of di†usionof the monomer into the matrix, because in plasticizedÐlms the chains are more Ñexible and consequentlymonomer di†usion increases.

Effect of irradiation dose

The e†ect of irradiation dose, as one of the importantparameters a†ecting the graft yield has been investi-gated, and the results are given in Fig. 3. It can be seenthat the graft yield increases with increasing irradiationdose up to 20 kGy. Further increase in the radiationdose beyond this value decreases the graft yield. Thisbehaviour can be attributed to increase in homo-polymer formation at high radiation doses leading to an

Fig. 2. E†ect of AAm concentration on the graft yield :diluent, water ; MohrÏs salt 2É5 wt%; total dose, 10 kGy.

Fig. 3. E†ect of radiation dose on the graft yield : AAm, 25%in water ; MohrÏs salt, 2É5 wt%.

increase in the viscosity of the medium and, consequent-ly, a decrease in the graft yield. Also, the grafting ofunplasticized PVC under these conditions is not suc-cessful as mentioned earlier.

Effect of inhibitors

It is known that inhibitors are usually added duringgrafting to minimize homopolymer formation.17 In pre-vious sections, MohrÏs salt has been used as a conven-tional inhibitor. In principle, transition metals shouldbe able to minimize homopolymer formation.18 Hencethe inhibition e†ects of iron(III) chloride, copper(I) chlo-ride, and some cobalt salts such as cobalt acetate, cobaltsulphate, cobalt chloride and cobalt carbonate havebeen investigated. The concentration of the inhibitorswas kept constant at 2É5 wt%. The results obtained aregiven in Table 1. It can be seen that the graft yieldranges between 24% and 33% when MohrÏs salt,

TABLE 1. Effect of type of inhibitor on the graft

yielda

Type of inhibitor Graft yield (%)

Mohr’s salt 33

Iron(III) chloride 24

Copper chloride 30·7

Cobalt chloride 24·3

Cobalt sulphate 0·38

Cobalt acetate 0·36

Cobalt carbonate 1·25

a AAm 25% ; irradiation dose, 10 kGy; water/methanol

ratio, 1 : 1.

POLYMER INTERNATIONAL VOL. 46, NO. 2, 1998

Graft copolymerization of AAm onto PV C Ðlms 153

iron(III) chloride, copper chloride and cobalt chloridehave been used as inhibitors. Thus iron, copper andcobalt ions participate in the inhibition of homo-polymer formation. The mechanism of inhibition of thehomopolymerization process in the presence of ironions (as an example) can be explained as follows :19

Pw(M)x* ] Fe3` ]Pw(M)

x`] Fe2` (3)

Pw(M)x* ] Fe2` ]Pw(M)

x~] Fe3` (4)

where P is the polymer chain, M is acrylamidemonomer and x is the degree of polymerization. Thismechanism is valid for other transition metals.

However, cobalt acetate, cobalt sulphate and cobaltcarbonate have practically no e†ect on the inhibition ofhomopolymer formation and the graft yields obtainedin these cases are very low. For cobalt acetate andcobalt carbonate, the ionization constants are very lowbecause these are the salts of weak acids. For cobaltsulphate, as the salt of a strong acid, the solubility islimited and hence the cobalt ion concentration is verylow. In a trial to make this salt soluble in water, a fewdrops of concentrated sulphuric acid were added to thereaction mixture. However, no grafting was observed.

From the results above, one can conclude that tran-sition metals are able to inhibit homopolymerization,assuming that the salt is soluble and highly ionizable.

Effect of acids

The e†ect of acids on grafting of acrylonitrile onto woolÐbre under mutual irradiation has been studied.20 Thereactivity of the various acids towards graft copolymer-ization was found to be in the order : H2SO4 [HNO3[ HCl[ HClO4[HOAC.

Also, it was found21 that in methanol, N-2-vinyl-pyrrolidone (NVP) systems used for grafting of siliconerubber, the radiation graft yields were inhibited byadding sulphuric acid. In contrast, acid enhanced theradiation graft yields of 4-vinyl pyridine (4-VP). Doserate and monomer concentrations did not change theseresults.21 This contradiction led us to study the e†ectsof di†erent concentrations of sulphuric acid as a strongacid and acetic acid as a weak acid on the graft yields ofAAm on plasticized PVC Ðlms. The results obtained areshown in Fig. 4. It can be seen that the graft yielddecreases with increasing acid concentration. Thus, thegraft yield in the presence of sulphuric acid decreasesfrom 64% to 22% on increasing its concentration to2 wt%. For acetic acid the graft yield decreases from64% to 40% on increasing its concentration to 2 wt%.Beyond this concentration, no e†ect on the graft yieldhas been observed. This behaviour can be explained byassuming that the hydrogen ion concentration in thecase of sulphuric acid is much higher than for aceticacid at the same concentration. The proton producedmay attack the monomer as follows :

Fig. 4. E†ect of acid concentration in wt% of 1 M acid on thegraft yield : AAm concentration, 25 wt%; methanol/watermixture, 75 wt%; weight ratio 1 : 1 ; total dose, 20 kGy. (…)

Sulphuric acid, acetic acid.(=)

This reaction depends on the strength of the acid. Thereaction product restricts the movement of themonomer in the reaction medium and decreases thereactivity of the monomer towards grafting ; conse-quently, the graft yield decreases. From these resultsone can conclude that the enhancement of grafting byacids should be carefully handled because this e†ectdepends on the type of monomer used.

Effect of di- and tri-functional monomers

Although increasing monomer concentration increasedgraft level, surface uniformity diminished. Garnett andco-workers22,23 have already used a wide range ofmultifunctional monomers to accelerate both radiationand photografting. Therefore attempts were made toincrease the graft yield at low monomer concentrationsby using multifunctional monomers. For this purposeN,N@-methylene allylamide (MAAm) and 2,4,6-triallyl-oxy-1,3,5 triazine (TARA) were used as difunctionaland trifunctional monomers. Preliminary experiments

POLYMER INTERNATIONAL VOL. 46 , NO. 2, 1998

154 E. M. El-Nesr, A. M. Dessouki, E. M. Abdel-Bary

Fig. 5. E†ect of MAAm concentration on the graft yield :AAm concentration, 25 wt%; MohrÏs salt, 2É5 wt%; total dose,

20 kGy.

showed that while 1 wt% of (MAAm) resulted in agraft yield of 192%, (TARA) gave only a graft yield of21% under the same reaction conditions. Addition ofsulphuric acid to the reaction mixture, as mentionedbefore, decreased the graft yield in the case of (MAAm)from 192% to 40%.

A relation between the concentration of (MAAm) andgraft yield is shown in Fig. 5. It can be seen that thegraft yield increases from 64% to 215% by increasingthe concentration of (MAAm) up to 0É8 wt%. Abovethis value a slight decrease is observed. The low graftyield of TARA may be attributed to the low reactivityof the allyloxy group compared with the allyl group. Inaddition, the steric e†ect of the triazine ring makes itdifficult for TARA to contribute to graft copolymer-ization of acrylamide.

Swelling measurements of grafted PVC Ðlms in DMF

Fig. 6. E†ect of grafting yield on the swelling of PVC Ðlms :in water, in methanol.(…) (=)

Fig. 7. TGA diagram of PVC with di†erent degrees of graft-ing : (curve 1) none, (curve 2) 12É2%, (curve 3) 56%, (curve 4)

178%.

in the presence of TARA showed that the Ðlm dissolvedcompletely after 24 h, while grafted Ðlms in the case ofMAAm did not dissolve. The calculated soluble fractionof PVC Ðlms grafted with AAm in the presence ofMAAm reaches about 16%. This indicates that MAAmnot only increases the graft yield, but also leads to theformation of a crosslinked structure. The reactionmechanism may be as follows :

where M is AAm and A is MAAm. The chemical struc-ture of M and A is :

Characterization

Swelling measurements. Swelling of the samples in meth-anol and distilled water has been carried out and theresults obtained are shown in Fig. 6. It can be seen thatthe swelling of hydrophobic PVC markedly increasedwith increasing graft yield. This is because polyacryla-mide grafted onto PVC is a hydrophilic polymer and issoluble in water in its free state. The increase in wateruptake conÐrms our assumption mentioned earlierregarding the role played by water where the di†usion

POLYMER INTERNATIONAL VOL. 46, NO. 2, 1998

Graft copolymerization of AAm onto PV C Ðlms 155

Fig. 8. DSC diagrams of PVC with di†erent degrees of graft-ing : (a) 12É2%, (b) 56%, (c) 178%.

of AAm is facilitated during grafting. In contrast, theswelling in methanol with graft yield is much less thanin water. This means that methanol makes only a slightcontribution to monomer di†usion inside the matrix ; itsrole is only due to its contribution during radiolysis.

T hermal analysis. Poly(vinyl chloride) is not a thermallystable polymer and degrades under many environ-mental conditions such as ultraviolet light, heat, andgamma radiation. For this reason it was interesting tostudy the e†ect of grafting on the thermal characteristicsof PVC. Thus, thermal gravimetric analysis has beencarried out and the results obtained are given in Fig. 7.It can be seen that PVC Ðlms degrade at about 200¡C.The extent and rate of loss in weight decreases withincreasing graft yield. This behaviour cannot be attrib-uted to the extent of dehydrochlorination under thee†ect of gamma radiation during grafting, because allsamples were irradiated to the same dose. It can,however, be explained by regarding the Ðlm as a PVCsandwich element between two layers of polyacrylamide(PAAm) grafted onto the surface. These two layersprevent or reduce the direct attack of heat on the PVC.Such behaviour has been observed for polyethylenegrafted with polyacrylic acid.24

Di†erential scanning calorimetric analysis has alsobeen carried out and the results are given in Fig. 8. Itcan be seen that grafted PVC shows an endothermicpeak with *H values depending on the degree of graft-ing. Accordingly, this endothermic peak is mainly dueto chemical transformation of amide groups in PAAmchains grafted onto the PVC. Evidence of this type oftransformation is the disappearance of this endothermicpeak on a second heating run.

Spectroscopic analysis. Spectroscopic analysis usinginfrared has been carried out to detect the grafting ofacrylamide onto the Ðlms. The appearance of absorp-tion bands at 1665 cm~1 characteristic of the amidegroups and at 3350 cm~1 characteristic of asymmetricstretching of groups is evidence for the grafting ofNH2acrylamide onto the Ðlms investigated.

CONCLUSIONS

(a) A methanol/water mixture in the ratio 1 : 1 is theproper diluent mixture for enhancing the graftcopolymerization process.

(b) The presence of sulphuric acid in the reactionmedium adversely a†ects the graft yield more thanacetic acid.

(c) The presence of N,N@-methylene allylamide(MAAm) in the reaction medium markedly increasesthe graft yield, while 2,4,6-triallyloxy-1,3,5 triazine(TARA) has practically no e†ect on the graftingprocess.

(d) Grafting of PVC Ðlms with acrylamide improves thethermal stability of the Ðlms in proportion to thegraft yield.

(e) The water uptake of the grafted Ðlms markedlyincreases with increasing graft yield.

POLYMER INTERNATIONAL VOL. 46 , NO. 2, 1998

156 E. M. El-Nesr, A. M. Dessouki, E. M. Abdel-Bary

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POLYMER INTERNATIONAL VOL. 46, NO. 2, 1998