graft copolymers of polypropylene films. 1. radiation-induced grafting of mixed monomers

Polymer International 42 (1997) 225È234

Graft Copolymers of Polypropylene Films.1. Radiation-induced Grafting of Mixed

Monomers

K. M. El-Salmawi,* A. M. El-Naggar, H. M. Said & A. H. Zahran

Radiation Chemistry Department, National Center for Radiation Research and Technology, PO Box 29, Nasr City, Cairo, Egypt

(Received 12 June 1996 ; revised version received 29 August 1996 ; accepted 23 September 1996)

Abstract : Radiation graft copolymerization of comonomer mixtures of acrylicacid (AAc) and styrene (S) onto polypropylene (PP) Ðlms by the mutual methodhas been investigated. The e†ect of di†erent factors that may a†ect the graftingyield, such as inhibitor concentration (MohrÏs salt), solvent composition (MeOHand radiation dose and dose rate, was studied. It was found that MohrÏsH2O),salt was very e†ective when the content of AAc in the comonomer mixtures waslow. However, the addition of 1É25 wt% of MohrÏs salt reduced homopolymerformation and enhanced the grafting process. Graft copolymerization in the pres-ence of a solvent mixture composed of methanol and water was found to a†ord ahigher grafting yield than in pure methanol, regardless of the composition of thecomonomer mixture used. However, the highest degree of grafting was obtainedat a solvent composition of 20% MeOH and a comonomer mixtureH2O : 80%of 20% AAc : 80% S. An attempt was made to determine each PAAc and PSfraction by di†erent methods in the graft copolymer obtained. Elemental analysisindicated that the PAAc fraction with respect to PS in the graft copolymerdecreased with increasing AAc ratio in the comonomer feed solution. The roughassessment of these fractions by IR spectroscopy showed similar trends. Thereactivity ratios of AAc and S monomers determined in the present graft copoly-merization system were found to be 0É45 and 1É3, respectively.

Key words : polypropylene Ðlms, radiation graft copolymerization, acrylic acid/styrene comonomer, reactivity ratios, IR, MohrÏs salt

INTRODUCTION

A literature survey indicates that a considerable numberof studies have been carried out on the radiation-induced grafting of individual vinyl monomers ontopolypropylene polymer by direct or pre-irradiationmethods.1h7 Also, the gamma pre-irradiation grafting of2(dimethylamine) and 2N-morphilino derivatives ofethyl methacrylate onto polypropylene fabric wasrecently investigated.8,9 The graft yield was studied as afunction of di†erent conditions such as monomer con-centration, grafting reaction time and temperature andpre-irradiation dose.

Grafting of polymers by mixtures of vinyl monomersis important since di†erent types of polymer chains con-

* To whom all correspondence should be addressed.

taining various functional groups can be introducedinto the polymer structure. Thus graft copolymers withdual properties can be obtained by controlling the con-ditions of grafting and selecting the appropriate pair, ormore, of vinyl monomers. The grafting of mixtures ofacrylic acid with acrylamide or methacrylic acid ontopolypropylene Ðbres by the radiation method has beenreported.10 It was found that the graft yield, when pureacrylamide is used, is smaller than when it is used inmixtures with acrylic acid and decreases when acrylicacid/methacrylic acid mixtures are used. Comonomermixtures of acrylamide and styrene were also graftedonto polypropylene by the radiation method in thepresence of di†erent solvents.11 Suitable diluents for thecomonomer, di†erent factors that a†ect the co-graftingprocess and some properties of the grafted material,such as electrical conductivity, were investigated.

225Polymer International 0959-8103/97/$09.00 1997 SCI. Printed in Great Britain(

226 K. M. El-Salmawi et al.

The synergism e†ect during graft copolymerization ofmixed vinyl monomers is very important, since it variesfrom one mixture to another and determines the extentof grafting yield from each feed monomer. The presentwork was undertaken to study the e†ect of di†erentfactors a†ecting the graft copolymerization of co-monomer mixtures of acrylic acid and styrene ontopolypropylene Ðlms by the direct radiation method. Anattempt was made to characterize the graft copolymersobtained by IR spectroscopy, elemental analysis andreactivity ratios.

EXPERIMENTAL

Materials

Polypropylene Ðlm. The polypropylene Ðlm was an iso-tactic polymer of homogeneous thickness (20 km),kindly supplied by Tecno Back Company, Cairo, Egypt.

Monomers and chemicals. Acrylic acid (Prolabo, France)and styrene (Koch Light Ltd, England) were of puregrade and were used without further puriÐcation. Thesolvents used, methanol (British Drug House, England)and benzene (Prolabo, France), were of pure grade.Ferrous ammonium sulphate. (MohrÏs salt), supplied byBritish Drug House, England) was used to reducehomopolymer formation.

Technical procedures

Graft copolymerization. Strips of polypropylene (PP)Ðlm of known weight were immersed in the di†erentgrafting solutions in glass quick-Ðt tubes. The graftingsolutions were composed of mixtures of acrylic acid(AAc) and styrene (S) monomers at di†erent ratios dis-solved in di†erent solvent compositions of methanoland water. MohrÏs salt at certain concentrations wasadded to the grafting solution. The contents of the glasstubes were deaerated by bubbling nitrogen gas for atleast 5 min and were then subjected to gamma irradia-tion. Irradiation to the required doses was carried outin the cobalt-60 gamma source of the National Centerof Radiation Research and Technology, Cairo, Egypt.The grafted PP samples were removed and thoroughlywashed and extracted with benzene to constant weightto remove the unreacted monomers and homopolymer.The total degree of grafting (TDG) was determined bythe percentage increase in weight as follows :

TDG(%)\ Wg [ W0/W0] 100 (1)

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

IR Spectroscopy. A Fourier transform infrared spectro-photometer (Unicom, model Mattson 1000) was usedover the range 400È6000 cm~1.

Elemental analysis. The grafted PP Ðlms with the co-monomer mixture under investigation were subjected toelemental analysis for carbon and hydrogen at a heatingrate of 10¡Cmin~1. This test was carried out at theMicroanalytical Center, Cairo University, using as astandard a poly(acrylic acid) polymer prepared bygamma irradiation. The oxygen weight in the sampleswas determined by subtracting the sum of carbon andhydrogen element weights from the initial weight of thesample.

RESULTS AND DISCUSSION

Effect of inhibitor (Mohr’s salt)

Although the direct method of grafting, in which thepolymer and monomerÈsolvent mixture are exposed togamma irradiation, is simple and involves a single step,it is associated with homopolymer formation. Therefore,a suitable inhibitor such as MohrÏs salt was added tothe grafting solution to minimize such homo-polymerization and force the reaction to the graftingside. Figure 1 shows the e†ect of MohrÏs salt concentra-tion on the total degree of grafting with two co-monomer mixtures, viz. 20% AAc : 80% S and 50%AAc : 50% S, onto PP Ðlms. The grafting conditionswere kept constant at solvent composition 20%

MeOH, comonomer mixture concentrationH2O : 80%10 wt% and radiation dose 2 Mrad. It can be seen thatthe total degree of grafting increases with increasing.MohrÏs salt concentration, reaching a maximum atabout 1É25 wt%. Further increase of the MohrÏs saltconcentration causes a sharp decrease in the TDG. Thisbehaviour was found to occur regardless of the co-monomer mixture used. However, the TDG in the case ofthe comonomer mixture 20% AAc : 80% S is muchhigher than in the case of the 50% AAc : 50% S co-monomer mixture. Thus, increasing styrene content in thecomonomer mixture increases the TDG. Also, the roleof MohrÏs salt as a homopolymer inhibitor is very e†ec-tive when the AAc content in the comonomer mixture isa minimum.

These Ðndings may be explained on the basis of chaintransfer to the backbone polymer from the growingacrylic acid ends and not from styrene ends in the pres-ence of Fe2` ions. For a system dealing with the graft-ing of acrylic acid and its mixtures with acrylamideinstead of styrene onto PP in the presence of Fe2` ions,opposite trends were observed.10 It was reported thatthe graft yield using acrylic acid in grafting is muchhigher than when using mixtures with acrylamide, evenat equal ratios of the two monomers. Also, it was sug-gested that the chain ends of acrylamide are scavengedvery e†ectively by Fe2` and decrease the total graftyield. Generally, the grafting process for comonomer

POLYMER INTERNATIONAL VOL. 42, NO. 2, 1997

Graft copolymers of PP Ðlms 227

Fig. 1. E†ect of MohrÏs salt concentration on the total degree of grafting (TDG) of di†erent acrylic acid (AAc)/styrene (S) co-monomer mixtures onto polypropylene Ðlms (PP) : 20% AAc : 80% S; 50% AAc : 50% S. Grafting conditions : comonomer…, >,

mixture conc. 10%; solvent composition, 20% MeOH; radiation dose, 2 Mrad.H2O : 80%

mixtures is critically controlled by factors other thanthe interaction of the radical species formed with theFe2` ions present in the grafting medium. The reacti-vity ratio of each monomer in the comonomer mixturemay be one of these factors. The proposed mechanismfor the grafting reactions and the inhibition of homo-polymer formation, thus increasing the grafting yield inthe presence of Fe2` ions, may be outlined as follows,taking acrylic acid as an example.

(1) Gamma photons produce radical sites on thepolypropylene backbone (P) and on the acrylicacid monomer (M) :

P, P~ ; M, M~

P~ ] M~ ] PM (graft copolymer)

P~ ] M ] PM~ (grafted chain)

(2) Homopolymerization is initiated either by therecombination of monomer macroradicals or by

and radicals formed from radiolysis ofH~ OH~the solvent :

M~] nM ] MnM; M~ ] M~ ] MM

(homopolymer)

OH~ or H~] M ] HOM~ (homopolymer)

(3) Termination by the Metal Fe2` cations proceedsas follows :

Fe2`] H`] PM~ ] Fe3` ] PM

(graft copolymer)

The sharp decrease in graft yield observed at rela-tively higher concentrations of MohrÏs salt may be dueto the di†usion of the inhibitor itself into the interiorregions of the PP Ðlms. Thus the inhibitor may interactwith the free radicals formed during irradiation andtherefore decrease the graft yield.

Effect of solvent and comonomer composition

Solvents are used in the grafting medium to swell thepolymer matrix and thus enhance the di†usion ofmonomers. Also, they are used as a diluent for themonomer. These functions, together with the otherfactors a†ecting grafting, will eventually lead to anincrease in the grafting yield. Table 1 shows the e†ect ofdi†erent solvent compositions of water and methanolon the total degree of grafting for comonomer mixturesof varied acrylic acid and styrene ratios onto poly-propylene Ðlm. The comonomer mixture concentrationwas kept constant at 10%, while the radiation dose anddose rate were kept constant at 2 Mrad and0É45 Mrads h~1, respectively. Every Ðgure in the table isthe average of three experiments.



TABLE 1. Effect of different solvent compositions of water and methanol on the total degree of grafting (%)

for comonomer mixtures of varied acrylic acid and styrene ratios onto polypropylene

Comonomer mixture, AAc : S (vol%)

10 : 90 20 : 80 30 : 70 40 : 60 50 : 50 60 : 40 70 : 30 80 : 20

Solvent

composition

H2O MeOH

(vol%) (vol%)

0 100 14·9 51 54·0 46·3 35·3 10·5 7·2 5·3

10 90 35·6 104·9 111·5 74·8 56·4 12·6 9·3 7·5

20 80 106·2 301·5 114·0 82·6 71·2 33·5 27·5 10·3

30 70 110·0 292·5 150·0 153·2 109·5 85·5 49·7 32·5

40 60 106·6 270·0 135·2 134·2 129·0 120·0 69·3 35·4

50 50 75 198·0 131·3 123·2 103·5 81·3 73·0 36·2

60 40 80·4 87·6 117·3 103·5 94·5 80·4 70·7 28·2

70 30 70·3 39·2 72·3 90·0 83·6 62·2 49·4 18·5

Grafting conditions : comonomer mixture conc. with respect to total grafting solution, 10 vol% ; Mohr’s conc., 1·25 wt% ;

radiation dose, 2 Mrad.

In general, and regardless of the comonomer mixtureused, the variation in the degree of grafting showssimilar trends. It increases with increasing water ratio inthe grafting solution to reach a maximum value at acertain solvent composition and then tends to decrease.For the comonomer mixture containing 10% AAc, themaximum TDG has no deÐnite value, but lies within arange of solvent compositions. However, the maximumTDG for the comonomer mixtures having 20% and30% AAc can be observed at solvent compositions 20%

MeOH and 30% MeOH,H2O : 80% H2O : 70%respectively. Also, it can be seen that the maximumTDG value increases with increasing AAc ratio up to20% and then tends to decrease, irrespective of thesolvent composition used.

Based on the above Ðndings several points may bemade.

(1) Overall, with the comonomer mixtures studied,solvent composition plays an essential role in determin-ing TDG.

(2) In pure methanol, an increase in AAc ratios from10% to 30% is accompanied by a rapid increase in theTDG from 15% to 54%. Further increase in AAccontent seems to cause a gradual decrease in TDG. Theinitial increase in TDG may be explained on the basisof the respective solubility parameters of the speciesused in the grafting bath. In this respect, acrylic acid ismore soluble in methanol than in water because of theclose values of their solubility parameters. Also poly-propylene polymer radicals may undergo maximumswelling in this mixture, which facilitates the di†usion ofthe comonomer solution to the active sites. At higherratios of AAc monomer in the comonomer mixture, thesynergism e†ect arises from the di†erence in reactivityratio of each monomer contributing to the grafting reac-tion. This in turn may lead to the decrease in TDG.Also, the swelling of polypropylene polymer in styrene

monomer is much higher than in acrylic acid owing tothe closer values of the solubility parameters :12 thesolubility parameter for styrene is 9É3, for acrylic acid itis 12É0, while for polypropylene it is 9É2 (cal cm~3)[email protected] may account for the higher TDG obtained athigher styrene ratios in the comonomer mixture.

(3) Using solvent compositions containing di†erentproportions of water and methanol, the solubilityparameter still has a great e†ect on the TDG, depend-ing on the comonomer mixture : the steady decrease inthe maximum TDG values observed by increasing AAcratios over 20% may be attributed to the tendency ofAAc monomer to homopolymerize rather than toinduce grafting reactions. Accordingly, the viscosity ofthe comonomer solution will increase, which decreasesthe di†usion into the polymer matrix. The e†ect of co-monomer concentration on the degree of grafting for acomonomer mixture composed of 20% AAc and 80% Sis shown in Fig. 2. It is clear that the TDG increaseswith increasing comonomer mixture concentration upto 10% and then tends to level o†. This observationalso indicates that, above this concentration, the vis-cosity of the comonomer mixture increases and hindersthe grafting reaction.

Effect of radiation dose and dose rate

Figure 3 shows the e†ect of radiation dose on thedegree of grafting of a comonomer mixture of 20%AAc : 80% S onto PP Ðlms at di†erent radiation doserates. The grafting yield increases rapidly with increas-ing radiation dose from 1 to 3 Mrad and then tends tolevel o†. Also, the lower the dose rate the higher thegrafting yield.

In the direct method of grafting, the number ofbranches of free radicals formed and their lengths areinÑuenced by radiation dose and dose rate, respectively.



Fig. 2. E†ect of varied concentrations of comonomer mixture composed of 20% AAc and 80% S (v/v) on the total degree ofgrafting onto PP Ðlms. Grafting conditions : solvent composition, 20% MeoH (v/v) ; radiation dose, 2 Mrad ; MohrÏsH2O : 80%

salt conc., 1É25 wt%.

The dose rate, which determines the rate of initiation ofpolymerization, will therefore a†ect the kinetics of thegrafting reaction and consequently the length of thegrafted chains.13 Therefore, the increased TDG may beattributed to the increased number of free radicalsformed on both the polymer backbone and the mono-mers. The levelling o† of TDG above 3 Mrad may bedue to the di†erence in the G value (the number ofactive sites formed per 100 eV) of the PP polymerandthe monomer species. The monomer macroradicals tendto recombine to form homopolymer and thus the graft-ing reaction stops. At lower dose rates, the rate of thegrafting reaction is higher, whereas the formation ofhomopolymer is a minimum.

Characterization of PP /PAAc /PS graft copolymer

The recorded values of the degree of grafting through-out this investigation represent the sum of the graftedPAAc and PS fractions in the resulting graft copolymer.The properties of the graft copolymer initially willdepend on the relative ratios of PAAc and PS in thegraft. Therefore, information about these fractions isvery important in terms of characterization of the

overall graft copolymer. There is no available scheme inthe literature describing an accurate relationshipbetween the ratio of the contents of the comonomermixtures in the feed solution and their fractions in theresulting graft copolymer in the case of hydrophobicpolymers. However, there are methods based on theidentiÐcation of the graft copolymer for grafting ofbinary mixtures of acrylamideÈacrylic acid andacrylonitrileÈacrylic acid into polyester Ðbre usingnitrogen estimation.14 Other methods are based on acomparison between the composition of the graftedchains determined according to the calculated reactivityratios and those reported in the literature.15 Spectro-scopic analysis, such as IR, has also been used toconÐrm and identify the grafted chains.16

Attempts have therefore been made to obtain infor-mation on the PP/PAAc/PS graft copolymers in orderto gain a better understanding of the synergism duringgraft copolymerization.

IR spectroscopic analysis

Figures 4 and 5 show the IR spectra of the original PPand PP Ðlms having di†erent TDG, obtained by using a



Fig. 3. E†ect of radiation dose on the degree of grafting ofcomonomer mixture of 20% AAc and 80% S onto PP Ðlms atdi†erent dose rates. Grafting conditions : solvent composition

and MohrÏs salt conc. are the same as in Fig. 2.

comonomer mixture composed of 10% AAc and 90% Sand radiation dose of 2 Mrad, but prepared at di†erentdose rates. An absorption band can be seen at about2700È3000 cm~1 due to the stretching of the wCHgroup present in PP polymer (Fig. 4). The intensity ofthis band increases with increasing TDG, which may beattributed to the addition of more wCH groups in the

graft copolymers from PAAs and PS (Fig. 5). The pres-ence of benzene rings in the graft copolymer can be con-Ðrmed from two regions. The weak absorption banddue to xCH stretching, which falls near 3030 cm~1 justto the left of the aliphatic wCH absorption and theabsorption band for acrylic bonding wCxCw givingrise to a series of four peaks between 1430 and1670 cm~1. The carboxylic groups due to the presenceof PAAc grafts are conÐrmed by a weak stretching bandat 1720È1750 cm~1, which is due to the carbonylgroups. A very distinctive absorption band begins atabout 3330 cm~1 and slopes into the aliphatic wCHabsorption and is indicative of the hydroxyl group.

The intensity of the absorption band correspondingto carbonyl groups is relatively weak and is not a†ectedby either increasing TDG or by changing dose rates.However, the width of the absorption bands due to PSincreases with decreasing dose rates. These Ðndingssuggest that these graft copolymers contain mainly PSand are initiated at low dose rates.

These spectra clarify the e†ect of dose rate on thesynergism between the two monomers during graftingunder equivalent conditions. The e†ect of changing AAcratio in the comonomer mixture on the respective PAAcand PS fractions was also investigated at constant con-ditions of radiation dose, dose rate, TDG and solventcomposition, as shown in Fig. 6. The intensity of thecharacteristic band of the carbonyl groups decreaseswith increase in the AAc content in the comonomermixture. This suggests that the PAAc fraction in thegraft copolymer is decreased.

Elemental analysis of PP graft copolymer

The thought behind using this technique is that bothPP polymer and the grafted PPAc and PS chainscontain the elements carbon, hydrogen and oxygen

Fig. 4. IR spectrum of pure PP Ðlm.



Fig. 5. IR spectra of di†erent graft copolymers prepared atdi†erent dose rates : (A) TDG, 240%; dose rate,0É45 Mrad h~1 ; (B) TDG, 448%; dose rate, 0É225 Mrad h~1 ;(C) TDG, 561%; dose rate, 0É1225 Mrad h~1. Grafting condi-tions : radiation dose, 2 Mrad ; comonomer mixture, 10%AAc : 90% S; comonomer mixture conc., 10% solvent com-position, 20% MeOH; MohrÏs salt conc.,H2O : 80%

1É25 wt%.

only. To check the accuracy of this test with respect tothe present system, PAAc homopolymer was subjectedto the same analysis. It was found that the percentage ofthe elements present in PAAc was very close to thatcalculated on the basis of the chemical structure

Table 2 presents the elemen-wCH2wCH(COOH)w.tal analysis of PP graft copolymers having di†erentTDG obtained by using di†erent comonomer mixtures.It should be noted that the weight percentage of oxygenwas determined as the di†erence between the totalweight and the sum of carbon and hydrogen weights.By increasing the AAc content in the comonomermixture from 20% to 60%, by increments of 10%, thePAAc graft yield in the graft copolymer was found to be59.17, 15.59, 10.89, 7.12 and 2.07%, respectively. ThePAAc fraction therefore decreases by increasing theAAc monomer ratio. This result is in accordance withthat observed through IR spectroscopy (Fig. 6).

Fig. 6. IR spectra of di†erent PP graft copolymers having anequal TDG of about 200% prepared by using di†erent co-monomer mixtures : (A) 10% AAc : 90% S; (B) 20%AAc : 80% S; (C) 30% AAc : 70% S. Grafting conditions :solvent composition, 20% MeOH; radiation dose,H2O : 80%2 Mrad; dose rate, 0É45 Mrad h~1 ; MohrÏs salt conc.,

1É25 wt.%.

When two monomers are copolymerized, the equa-tion, in terms of mole fractions, which relates their con-centration in the feed grafting solution and and(f1 f2)their fractions in the copolymer and is given by :17(F1 F2)

f1(1 [ 2F1)F1(1 [ f1)

\ r2] f 12(F1[ 1)F1(1 [ f1)2

r1 (2)

If the left side of this equation is plotted against thecoefficient the slope gives and the intercept Anr1, r1 r2 .attempt was made to estimate and from the experi-r1 r2mental results shown in Table 2. Table 3 shows the cal-culated mole fraction of styrene monomer in thegrafting solution its mole fraction in the resultingf1,graft copolymer and their application in eqn (2). TheF1mole fraction was calculated by using the data in theF1last column of Table 2 and taking into considerationthat the molecular weight of polystyrene is 104. Figure 7shows the plot of the data in Table 3. The calculatedslope was 1É3 and the intercept 0É45.(r1) (r2)

The reactivity ratios of the two monomers give abetter understanding of the synergistic, inÑuence duringgraft copolymerization, as well as the monomers capa-bility of reacting with the free radicals on the backbone



TA

BLE

2.

Ele

menta

lanaly

sis

of

poly

pro

pyle

ne

(PP

)gra

ftcopoly

mers

wit

hpoly

(acry

lic

acid

)and

poly

sty

rene

Com

onom

er

Weig

htof

Weig

htof

Weig

htof

Tota

lO

xygen

Weig

htof

Weig

htof

Gra

ftyie

ldG

raft

yie

ld

mix

ture

,PP

befo

rePP

after

gra

ftcopoly

mer

degre

eof

from

oxygen

PA

Ac

ofPA

Ac

ofPS

AA

c:S

gra

ftin

ggra

ftin

gPA

Ac

½PS

gra

ftin

ganaly

sis

(mg)

(mg)

(%)

(%)

(%)

(mg)

(mg)

(mg)

(%)

(%)

10

:90

73·2

151·1

77·9

106·5

4·8

7·2

516·3

122·2

884·2

1

20

:80

74·6

299·5

224·9

301·5

6·5

519·6

244·1

459·1

7242·3

30

:70

89·8

191·5

101·7

113·2

3·2

56·2

214·0

015·5

997·6

1

40

:60

70·3

128·4

58·1

82·6

2·6

53·4

07·6

610·8

971·7

1

50

:50

83·3

142·6

59·3

71·2

1·8

52·6

45·9

47·1

264·0

8

60

:40

73·2

97·7

24·5

33·5

0·6

90·6

71·5

22·0

731·4

3

70

:30

86·3

110·0

23·7

27·5

1·6

41·8

04·0

64·7

022·8

0

80

:20

80·7

89·0

8·3

10·3

1·2

01·0

72·4

02·9

87·3

2

Gra

ftin

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ns

are

the

sam

eas

inTable

2.



TABLE 3. Mole fraction of styrene in the comonomer feed

solutions and in the graft copolymer

Mole fraction Mole fraction of Application

of S in the S in the graft in eqn (2)

comonomer mixture copolymer

(f1) (F

1)

0·601 0·812 1·157 ¼0·525r1Ér

20·492 0·820 0·756 ¼0·206r

1Ér

20·392 0·860 0·541 ¼0·064r

1Ér

20·301 0·913 0·389 ¼0·018r

1Ér

2

Fig. 7. Determination of the reactivity of acrylic acid andstyrene monomers by applying eqn (2).

of the PP polymer chains. The higher reactivity ratio ofstyrene compared with that of acrylic acid indicates thatthe graft copolymer contains a larger proportion ofpolystyrene. This was observed, even though the molefractions of styrene monomer in the grafting solutionswere less than the mole fractions of acrylic acid. Thus,the presence of AAc in larger proportions in the graftingsolution enhances the grafting of PS onto PP polymerchains at the expense of its own graft copolymerization.Also, the presence of MeOH to a larger extent assolvent (80%) results in strong hydrogen bonding withAAc, thus minimizing the formation of homopolymer.The net result is that smaller amounts of the more reac-tive monomer are available for graft copolymerizationwith PP polymer.

Electrical properties of PP graft copolymer

The electrical conductivity of PP Ðlms grafted with dif-ferent comonomer mixtures of AAc and S was deter-mined and the results are given in Table 4. In general,the electrical conductivity increases with increasing

TABLE 4. Electrical conductivity of polypropylene films

grafted with different comonomer mixtures of acrylic acid

and styrene as a function of total degree of grafting

Total degree Comonomer mixtures, Electrical conductivity

of grafting AAc : S (ohmÉ1 cmÉ1)É16

(%) (vol%)

Ungrafted — 0·26

106·5 10 : 90 26·00

301·5 20 : 80 41·00

113·2 30 : 70 20·00

82·6 40 : 60 12·80

71·2 50 : 50 10·00

33·5 60 : 40 4·80

27·5 70 : 30 3·20

10·5 80 : 20 2·60

Grafting conditions are the same as in Table 2.



TDG and decreases with increasing AAc proportions inthe comonomer mixture.

The parameters which determine the electrical con-ductivity of a material are the number of charge carriersand their movement through the bulk of the polymer.Also, there is a good relationship between these param-eters and the chemical structure and morphology. Thesecharges originate from electrons or positive holes. Theopening of the structure of the polymer matrix inducedby grafting may enhance the mobility of these charges.Therefore, the observed decrease in electrical conduc-tivity with increasing acrylic acid ratio in the co-monomer mixture may be explained as follows.

(a) With the comonomer mixture 20% AAc : 80% S(maximum total degree of grafting), the highestelectrical conductivity value was obtained. Thisvalue, in accordance with the results shown inTable 2, corresponds to the highest graft yield ofPAAc fraction. The resulting electrical conduc-tivity is due to the migration of the negativelycharged PAAc grafts.

(b) The decrease in electrical conductivity onincreasing the AAc ratio in the comonomermixture is due to the decrease in PAAc graftyield. These electrical properties give support tothe inÑuence of synergism during graft copoly-merization and the reactivity ratios of acrylicacid and styrene monomers.

REFERENCES

1 Chapiro, A., J. Polym. Sci., 48, (1966) 109.2 Ang, C. H., Granett, J. L., Levot, R. & Long, M. A., J. Polym. Sci.,

Polym. L ett. Edn, 21 (1983) 257.3 Lokhande, H. T., Iog, A. G., Teli, M. D., Rao, M. H. & Rao, K. N.,

J. Appl. Polym. Sci., 33 (1987) 2753.4 Gupta, B. D., Tyagi, P. K., Ray, R. A. & Singh, H., Macromol. Sci.,

Chem., 27 (1990) 831.5 Hegazy, E. A., Polymer, 33 (1992) 96.6 Mehta, I. K., Kumar, S., Chauhan, G. S. & Misra, B. N., J. Appl.

Polym. Sci., 41 (1990) 1171.7 Rao, M. H., Rao, K. N., Teli, M. D., Jog, A. G. & Lokhande, H.

T., J. Appl. Polym. Sci., 33 (1987) 2743.8 Gawish, S. M., Kantouch, A., El-Naggar, A. M. & Mosleh, S., J.

Appl. Polym. Sci., 44 (1992) 1671È1677.9 Gawish, S. M., Kantouch, A., El-Naggar, A. M. & Mosleh, S., J.

Appl. Polym. Sci., 57 (1995) 45.10 Rao, M. H., Rao, K. N., Lakhande, H. T. & Teli, M. D., J. Appl.

Polym. Sci., 33 (1987) 2707.11 Taher, N. H., Dessouki, A. M. & Khalil, F. H., Radiation Phys.

Chem., 36, (1990) 785.12 Sperling, L. H., Introduction to Physical Polymer Science. John

Wiley & Sons, New York, 1986.13 Wilson, J., Radiation Chemistry of Polymers, Monomers and Plas-

tics. Marcel Dekker, New York, 1974, p. 483.14 Lokhande, H. T., Teli, M. D., Rao, K. N. & Rao, M. H., J. Appl.

Polym. Sci., 29, (1984) 1843.15 Sankholkar, S. & Deb, P. C., J. Appl. Polym. Sci., 39, (1990) 1681.16 El-Sawy, N. M., Hegazy, E. A., Rabie, A. M., Hamed, A. & Miligy,

G. A., Polym. Int., 33, 285È291 (1994).17 Brandrup, J. & Immergut, E. H., Polymer Handbook. John Wiley

& Sons, New York, 1966, Ch. 11, p. 224.


graft copolymers of polypropylene films. 1. radiation-induced grafting of mixed monomers

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