Gamma ray induced graft copolymerization ofN-vinylpyrrolidone, acrylamide and their mixtures onto polypropylene films

Polymer International 45 (1998) 67È76

Gamma Ray Induced GraftCopolymerization ofN-Vinylpyrrolidone,

Acrylamide and Their Mixtures OntoPolypropylene Films

A. M. Dessouki,* N. H. Taher & M. B. El-Arnaouty

National Centre for Radiation Research and Technology, P.O. Box 29, Nasr City, Cairo, Egypt

(Received 11 April 1997 ; revised version received 15 July 1997 ; accepted 1 August 1997)

Abstract : The radiation initiated grafting of N-vinylpyrrolidone (NVP) acryl-amide (AAm) and their mixtures onto polypropylene (PP) Ðlms using a directradiation technique has been investigated. Di†erent solvents were used for dilut-ing the monomers and it was found that dioxane was suitable for this graftingsystem. The inÑuence of other grafting parameters such as inhibitor, monomerconcentration and dose rate on the rate of grafting and grafting yield wasstudied. The values of the coefficients relating the grafting rate to monomer con-centration and dose rate were found to be 1É4 and 1É5 for (NVP), and 1É6 and1É47 for (AAm), respectively. Also, the coefficients relating grafting rate to co-monomer concentration for 20/80 and 50/50 AAm/NVP mixtures were found to be1É6 and 1É7. Some physicochemical properties such as swelling, thermal behav-iour, mechanical and electrical conductivity were also investigated, and the possi-bility of some practical uses, e.g. the removal of heavy metals from solution, forthe grafted membranes is discussed. 1998 SCI.(

Polym. Int. 45, 67È76 (1998)

Key words : radiation grafting ; N-vinylpyrrolidone ; acrylamide ; polypropyleneÐlms

INTRODUCTION

Radiation induced grafting is a very efficient method forpreparing polymers with tailored properties. A greatdeal of work has been done during the last 25 years todevelop the best possible membranes using almost everyavailable polymeric material.1h10 In a previousstudy,11,12 the direct radiation grafting of methacrylicacid (MAA) and acrylic acid (AAc) onto polypropyleneÐlms was studied. The dependence of the grafting rateon monomer concentration was found to be of theorder of 1É2 and 1É1 for MAA and AAc, respectively.Some properties of the grafted Ðlms were investigated,

* To whom all correspondence should be addressed.

as were the preparation and selected properties ofneutral, cationic and anionic membranes obtained byradiation-induced grafting of some vinyl and acrylicmonomers onto di†erent polymeric substrates, usingdirect and post-irradiation techniques.13h18 Thesegrafted membranes showed great promise in practicalapplications as ion-exchange and reverse osmosis mem-branes.

In the present study, the preparation of hydrophilicmembranes obtained by the direct radiation inducedgrafting of NVP and AAm with dioxane and their mix-tures (20/80, 50/50 AAm/NVP) onto PP Ðlms has beenstudied. The e†ect of solvent, inhibitor, monomer con-centration and dose rate on the rate of grafting has beendetermined. Some selective properties of the grafted

671998 SCI. Polymer International 0959-8103/98/$17.50 Printed in Great Britain(

68 A. M. Dessouki, N. H. T aher, M. B. El-Arnaouty

Ðlms such as swelling, thermal behaviour, mechanicaland electrical properties have also been investigated,together with the possibility of using these membranesin the removal of heavy metals such as Pb2`.

EXPERIMENTAL

Materials

Polypropylene (PP) Ðlms 20km thick (Technoback Co.,Egypt) were washed with acetone and dried in avacuum oven at 40¡C. N-Vinylpyrrolidone (NVP), acryl-amide (AAm) and other chemical reagents were used asreceived.

Graft polymerization

A glass ampoule that contained strips of polymer Ðlmand monomer or comonomer solution was deaeratedwith bubbling nitrogen for 5 min and then subjected toc-rays at a dose rate ranging from 1É1 to 1É23 Gy s~1.The grafted Ðlms were washed thoroughly with hot dis-tilled water to extract residual monomer and occludedhomopolymer. They were then dried in a vacuum ovenat 60¡C for 24 h and weighed. The degree of graftingwas determined by the percentage increase in weight asfollows :

Degree of grafting (%)\ [(Wg/W0) [ 1]] 100 (1)

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

IR spectroscopic measurements

IR spectra were determined for the grafted PP Ðlmsusing a Fourier transform infra red (FTIR) spectrometer(Pye-Unicam, UK) over the range 400È4000 cm~1.

Thermal measurements

Thermal behaviour was determined for a graft sampleusing a DSC 7 di†erential scanning calorimeter (PerkinElmer, USA). A small quantity of PPÈg-PNVP, PPÈg-PAAm, or PPÈg-P(AAm/NVP) Ðlm (usually 5È10 mg)was weighed out. The Ðlm and reference were placed ina sample holder and heated at a constant rate of20¡Cmin~1 in nitrogen. Changes were recorded graphi-cally for measurements of either temperature or energydi†erential against temperature or time.

Semi-equilibrium dialysis (SED) measurements

The permeability of the grafted Ðlms was determined bythe semi-equilibrium dialysis method,19 which has been

used to infer solubilization equilibrium constants or,alternatively, activity coefficients of solutes solubilizedinto micelles of aqueous surfactant solutions. Methodshave been described for inferring the concentration ofmolecules of the organic solute and of the surfactant onboth sides of the dialysis membrane, under conditionswhere the organic solute is in equilibrium with both thehigh-concentration (retentate) and low-concentration(permeate) solutions. In the present work, the dialysiscell consisted of two chambers supported by originalmembranes (cellulose) having a 600 Da molecularweight cut-o†. The dialysis membranes were soakedthoroughly in distilled water for approximately 15 minbefore use. A solution containing a known concentra-tion of lead acetate was placed on one side of the pre-pared membrane (retentate side) and distilled water onthe other side (permeate side). Then the cells were thermo-statted at 25¡C for 18È24 h to attain equilibrium.The concentration of lead acetate which di†usedthrough the membrane was determined by using anatomic absorption technique.

Other measurements such as water uptake, andmechanical and electrical conductivity were carried outas described in previous papers.20h22

RESULTS AND DISCUSSION

Preparation of the membrane

E†ect of solvents. Solvents are used in radiation graftingexperiments to enhance the degree of accessibility ofmonomer to grafting sites within the polymer, broughtout by the ability of the added solvent to swell the basepolymer. Table 1 shows the e†ect of di†erent solventson the grafting process. It was observed that the Ðlmsswell in benzene, butanone and dioxane much morethan in the other diluents used. Furthermore, in

TABLE 1. Effect of solvent on the grafting of NVP

and AAm monomers onto PP films at an irradiation

dose 10 kGy

Solvent Monomer Degree of

concentration grafting (%)

(wt%)

NVP AAm NVP AAm

H2O 25 25 6·2 1·2

Methanol 25 25 13·7 3·8

Methanol/H2O mixture 25 25 4·9 5·5

(70/30)

Dioxane 25 25 181·0 37·0

Butanone 25 25 31·5 33·8

Benzene 25 25 296·0 —

POLYMER INTERNATIONAL VOL. 45, NO. 1, 1998

Copolymerization of NV P and AAm onto PP Ðlms 69

benzene, butanone and dioxane, no homopolymer wasformed, in contrast to other solvents in which the largeamount of homopolymer formed meant the sampleswere difficult to extract. This is in good agreement withreported results,7 that solutions of NVP in benzene orpyridine are good for grafting onto poly-tetraÑuoroethylene (PTFE) Ðlms. Therefore, dioxanewas chosen as diluent for this grafting system.

E†ect of inhibitor. Attempts were made to use selectiveinhibitors such as MohrÏs salt, copper(I) chloride,copper(II) chloride and ferric chloride to minimize thehomopolymerization occurring in the grafting system.All the inhibitors except were insoluble inFeCl3dioxane. Figure 1 shows the e†ect of concentra-FeCl3tion on the degree of grafting. It was observed that thedegree of grafting decreases as the inhibitor concentra-tion increases, reaching a minimum at 2É0 wt%.Although homopolymerization was e†ectively reduced,no signiÐcant grafting occurred. Therefore, in thepresent work the addition of inhibitors was unsuccessfuland the grafting process was carried out without usingany inhibitors.

E†ect of monomer concentration. Figures 2È5 show thee†ect of irradiation time on the degree of grafting atvarious monomer and comonomer concentrations. Itwas found that the degree of grafting increased withirradiation time for all NVP concentrations used. It canbe seen in Fig. 2 that for NVP concentrations rangingfrom 10 to 50 wt%, the higher the NVP concentrationthe higher the degree of grafting obtained. However, atconcentrations higher than 50 wt%, the degree of graft-ing decreased, especially at higher radiation doses com-pared with those obtained at lower concentrations. Thisbehaviour also occurred in the case of AAm at 25 wt%concentration as shown in Fig. 3. The decrease in thedegree of grafting at high monomer concentrations isprobably due to increase in the homopolymer formed atsuch concentrations, especially at longer irradiationtimes and as the viscosity of the reaction mediumincreased. This resulted in a decrease in the rate ofmonomer di†usion, and hence the grafting was lower.The dependence of initial rate of grafting on monomerconcentration was calculated from the linear part ofthese Ðgures, and is illustrated in the double logarithmicplots shown in Fig. 6. A linear relationship wasobtained and the values of the coefficient of dependenceof grafting rate on monomer concentration were foundto be 1É43 and 1É6 for NVP and AAm, and 1É6 and 1É7for AAm/NVP 20/80 and 50/50 mixtures, respectively.

These results suggest that this grafting systemdepends greatly on the di†usivity of the monomer solu-tion into the polymer matrix, and also on the amount ofradicals formed.

E†ect of dose rate. The e†ect of dose rate on the degreeof grafting at monomer concentration 50 wt% NVP and20 wt% AAm onto PP Ðlms as a function of irradiationtime was studied, the results are shown in Figs 7 and 8.The higher the dose rate the higher the initial rate ofgrafting obtained. After an induction period a pro-nounced acceleration e†ect was observed during graft-ing of NVP, as shown in Fig. 7, particularly when alower dose rate was used. As the dose rate increased, asigniÐcant decrease in induction period was observed.Figure 9 shows a logarithmic relationship between theinitial rate of grafting and dose rate. It was found thatthe initial rate of grafting increased linearly with doserate in double logarithmic plots ; the coefficients ofdependence of grafting rate on the dose rate were calcu-lated and found to be 1É5 and 1É47 for NVP and AAmmonomer, respectively. The results suggest that at highdose rates the concentration of free radicals is high forboth PP Ðlms and the monomer, compared with theconcentration at low dose rates.

Properties of the grafted membranes

Swelling behaviour. Figure 10 shows the e†ect of degreeof grafting on the percentage water uptake for thegrafted Ðlms. Percentage water uptake increases grad-ually with degree of grafting for all the Ðlms. Alkalinetreatment of the grafted PP Ðlms is required to intro-duce electrolytic groups and/or increase the hydrophi-licity of the graft copolymer. The water uptake wasmuch higher for KOH-treated than for untreated Ðlms.It can be seen from Fig. 10 that the treated grafted Ðlmshaving groups have more hydrophilic charac-CONH2ter than those containing free and NVP groupsCONH2or their mixture. This means that the water uptakedepends mainly on the type of group introduced intothe polymer matrix. Furthermore, the KOH-treatedgrafted Ðlms became much smoother and more planarthan the untreated ones.

IR spectroscopy. Infrared spectroscopic measurementsfor the grafted materials are shown in Figs 11È13. Ongrafting NVP and AAm onto PP Ðlms, certain changesoccurred in the IR spectra. New bands appeared around3400, 1660 and 1270 cm~1, which are characteristic forthe structure of NVP (Fig. 11). The band appearing at835 cm~1 is probably due to the change in crystallinityand morphology of the polymer caused by grafting andcrosslinking.14 The strong broad band that appears at3400 cm~1 is assigned to hydroxyl groups and/orhydrogen bonding, which may indicate the existence ofa crosslinked network structure in the graft copolymer.Also, the degree of grafting can be followed from theintensity of the band at 1660 cm~1 which indicates thepresence of the carbonyl group of NVP, AAm and itscomonomer.



Fig. 1. E†ect of concentration on the degree of graftingFeCl3of monomer (concentration 25 wt%) onto PP Ðlms : AAm;L,

NVP at 10 kGy.K,

Electrical conductivity. The electrical conductivity ofpolymeric materials depends mainly on the presence oflow molecular mass impurities which are the majorsource of conductivity. Figure 14 shows the semilog-arithmic relationship between the electrical conductivityand degree of grafting. It was found that as the degreeof grafting increased, an increase in the electrical con-

Fig. 2. Degree of grafting versus irradiation time in nitrogenfor grafting of NVP at various concentrations onto PP. NVP(wt%) : 10 ; 30 ; 50 ; 70. Film thickness 20km atL, |, K, …,

dose rate 1É1 Gy s~1.

Fig. 3. Degree of grafting versus irradiation time in nitrogenfor grafting of AAm at various concentrations onto PP. AAm(wt%) : 10 ; 15 ; 20 ; 25. Film thickness 20km atL, |, K, …

dose rate 1É1 Gy s~1.

Fig. 4. E†ect of irradiation time on the degree of grafting ofan AAm/NVP binary system of composition 20/80 wt% atvarious comonomer concentrations (wt%) : 10 ; 30 ;L, K, |,

50 ; 70. Dose rate 1É1 Gy s~1.…,



Fig. 5. E†ect of irradiation time on the degree of grafting ofan AAm/NVP binary system of composition 50/50 wt% atvarious comonomer concentrations (wt%) : 10 ; 30 ;L, K, |,

50. Dose rate 1É1 Gy s~1.

ductivity for the alkali treated and untreated Ðlms wasobserved. The change in the electrical conductivity inthe case of grafting with NVP may be due to poly-vinylpyrrolidone (PNVP) molecules in the side-chainsof the PP Ðlms acting as impurities ; hence, the mobilityof grafted chains is probably/higher at low degrees of

Fig. 6. Logarithmic plots of the initial grafting rate versusmonomer concentration : AAm; NVP; and comonomerK, …,concentration for AAm/NVP mixture compositions 20/80 (L)

and 50/50 Dose rate 1É1 Gy s~1.(È).

Fig. 7. Degree of grafting versus irradiation time for graftingof 50 wt% NVP concentration onto PP Ðlms at various dose

rates (Gy s~1) : 0É27 ; 0É55 ; 1É1.L, K, |,

Fig. 8. Degree of grafting versus irradiation time for graftingof 20 wt% AAm onto PP Ðlms at various dose rates (Gy s~1) :

0É27 ; 0É55 ; 1É1.L, K, |,



Fig. 9. Logarithmic plots of the initial grafting rate versusdose rates (Gy s~1) : NVP (50 wt%) ; AAm (20 wt%).K, L,

grafting because of the small amount of crosslinkingcompared with that at higher degrees of grafting. It wasalso found that the alkali-treated graft copolymer withAAm possessed higher conductivity compared with theother membranes having the same degree of grafting.

Fig. 10. Percentage water uptake as a function of degree ofgrafting for : PPÈg-PAAm after treatment ; PPÈg-…, |,PAAm; PPÈg-P(AAm/NVP, 20/80) after treatment ;K, L,

PPÈg-P(AAm/NVP 20/80) ; (H) PPÈg-PNVP.

Fig. 11. IR spectra of the original PP Ðlm and PPÈg-PNVP:curve 1, original ; curve 2, 16% grafting.

This may be due to the presence of free ions, whichenhance the electrical conductivity compared with theother monomer used (N-vinylpyrrolidone (NVP)) whichis non-ionic in nature.Mechanical properties. The changes in mechanicalproperties of the grafted Ðlms are shown in Figs 15 and16. It can be seen that, for all PP Ðlms, the elongation at

Fig. 12. IR spectra of the original PP Ðlm and PPÈg-PAAmhaving di†erent degrees of grafting : curve 1, original ; curve 2,

13% grafting ; curve 3, 37É5% grafting.



TABLE 2. Effect of grafting of NVP, AAm and their mixture (AAm/NVP,

20/80 wt%) on PP films on the permeability of lead acetate solutions

Degree of grafting (%) Solution conc. Retained conc. Permeate conc.

(ppm) Ã103 (ppm) Ã103 (ppm)

Blank 10·679 8·3 0·1

PP–g-PNVP

30 10·679 8·1 1·8

300 10·679 3·7 104·0

400 10·679 3·3 104·5

PP–g-PAAm

7·5 10·679 7·95 2·7

10 10·679 5·0 3·1

36 10·679 3·5 5·6

PP–g-P(AAm/NVP, 20/80)

166 10·679 3·7 70·0

208 10·679 3·1 78·0

244 10·679 1·3 79·0

break decreases sharply as the degree of graftingEbincreases, while the tensile strength at break at ÐrstTbincreases with degree of grafting to reach a maximum ataround 30% grafting and thereafter decreases withfurther increase in the degree of grafting. These resultssuggest that the grafted Ðlms having lower degrees ofgrafting become smooth, but at higher degrees of graft-ing they become hard and brittle and the elongation atbreak becomes lower.

T hermal behaviour. The grafting of side-chains to apolymer backbone has a pronounced e†ect on its physi-cal and mechanical properties. Although this techniquehas been widely used to modify polymer properties,there are relatively few studies which deal with the inÑu-ence of the side-chains on the melting of the graftedpolymer.23 The melting point and the heat of fusionwere determined by di†erential scanning calorimetry(DSC).

Fig. 13. IR spectra of PPÈg-P(AAm/NVP, 20/80) as a function of degree of grafting : curve 1, 14É5% grafting ; curve 2, 69%grafting.



Fig. 14. Electrical conductivity for the grafted PP Ðlms as afunction of degree of grafting : PPÈg-PAAm; PPÈg-|, …,PAAm after treatment ; PPÈg-P(AAm/NVP 20/80) ; PPÈL K,

g-PNVP.

Fig. 15. Change in percentage elongation at break withdegree of grafting of PP Ðlms : PPÈg-PAAm; PPÈg-K, L,

PNVP; PPÈg-P(AAm/NVP, 20/80).|,

Fig. 16. Change in tensile strength with degree of grafting ofPP Ðlms : PPÈg-PAAm; PPÈg-PNVP; PPÈg-K, L, |,

P(AAm/NVP, 20/80).

Fig. 17. DSC diagram of PPÈg-PNVP for di†erent degrees ofgrafting : curve 1, original PP Ðlm; curve 2, 20%; curve 3,

140%; curve 4, 209%.



Fig. 18. DSC diagram of PPÈg-PAAm for di†erent degrees of grafting : curve 1, 12%; curve 2, 25%; curve 3, 34%.

Fig. 19. DSC diagram of PPÈg-P(AAm/NVP 20/80) for di†erent degrees of grafting : curve 1, 26%; curve 2, 78%; curve 3, 150%.

It is known that PP is a semicrystalline polymer. Theheat of fusion of a sample is the heat absorbed when thecrystalline portion of the material melts ; hence it is pro-portional to the amount of crystalline material presentin the sample and can be used as a measure of thedegree of crystallinity. The melting points of the graftedPP were determined from the peak in the endotherm.Figures 17È19 show the DSC diagrams of grafted PPÐlms with NVP, AAm and their mixture, respectively. It

can be seen that the crystalline melt temperature ofTmthe Ðlm having 209% grafting increased by 8¡C com-pared with that of the original PP Ðlm (Fig. 17). Also, adecrease in the area under the peak with increasingdegree of grafting occurred, i.e. the heat of fusiondecreased. Meanwhile, a new broad peak appeared atabout 115¡C which may be related to PNVP which hasa softening point at about 140¡C. This peak is broadand its area increases with increasing degree of grafting.



This behaviour was also observed for grafted PP withcomonomer composition (20/80, AAm/NVP) as shownin Fig. 19. However, in the case of AAm monomer (Fig.18) the crystalline melt temperature is independent ofthe degree of grafting and no new peaks appear.

These results suggest that changes in the crystallinedomains occur with increasing degree of grafting. Fur-thermore, the molecular structure changes in the crys-talline regions and this inÑuences the crystalline melttemperature. Moreover, restriction of chain mobilityand disordering of the structure caused by crosslinkingmust be taken into consideration.

Permeability measurements. The grafted PP Ðlms can beused for decreasing the concentration of heavy metals,e.g. Pb2` and Hg2`, in water and for waste water treat-ment and water puriÐcation. Table 2 shows the e†ect ofpercentage grafting of NVP, AAm and their mixture(AAm/NVP, 20/80) on PP Ðlms, on the extent of thepermeability of the membrane to lead acetate solution.The concentration of permeate increases as the degreeof grafting increases. For PPÈg-PNVP having 30%grafts the permeability is about 1É8 ppm, whereas forPPÈPAAm with 36% grafts the permeability is about5É6 ppm for the same lead acetate solution. These resultssuggest that the grafted membranes are suitable for theremoval of some heavy metals from waste water.

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Gamma ray induced graft copolymerization ofN-vinylpyrrolidone, acrylamide and their mixtures onto polypropylene films