Polymer International Polym Int 48 :92–98 (1999)

Synthesis of permselective membranes byradiation-induced grafting ofN-vinylpyrrolidoneonto poly(tetrafluoroethylene–hexafluoropropylene–vinylidene fluoride) (TFB)filmsAM Des s ouki, NH Taher and MB El-ArnaoutyNational Center for Radiation Res earch and Technology, PO Box 29, Nas r City , Cairo, 11731, Egypt

Abstract : The radiation initiated grafting of N-vinylpyrrolidone (NVP) onto poly(tetraýuoroethylene–

hexaýuoropropylene–vinylidene ýuoride) (TFB) ülms has been investigated using a direct radiation

technique. Diþ erent solvents were used for diluting the monomer, and it was found that dioxane is

suitable for this grafting system. The inýuence of other grafting parameters, such as inhibitor,

monomer concentration and dose rate, on the rate and yield of grafting was studied. The dependence

of the grafting rate on the monomer concentration was found to be of the 1.1 order. Some physi-

cochemical properties, such as swelling, thermal behaviour, mechanical and electrical conductivity,

were investigated. A study was made to gain a better understanding of the observed water uptake

using IR spectroscopy and scanning electron microscope (SEM) analysis. The possibility of some prac-

tical uses, e.g. removal of heavy metals from solution by grafted membranes was investigated.

1999 Society of Chemical Industry(

Keywords: grafting ; ýuorinated polymer ; vinylpyrrolidone; application

INTRODUCTION

Fluorine-containing polymers are appropriatematerials for the preparation of membranes forseparation processes because of their good thermal,mechanical and chemical resistance. Hydrophilicmembranes have been prepared by radiation graftingof diþerent hydrophilic monomers onto diþerentpolymeric substrates.1h10 The nature and morpho-logical peculiarities of the trunk polymers, togetherwith the grafting reaction parameters, aþect themembrane properties of the grafted ülms.

In this study, the radiation grafting of NVP bya direct radiation technique onto poly-(tetraýuoroethylene–hexaýuoropropylene–vinylideneýuoride) (TFB) ülms, was carried out. The eþect ofgrafting conditions on the grafting process and inhi-bition of homopolymerization was studied. Someselected properties of the grafted ülms, such as wateruptake, electrical conductivity and mechanicalproperties, were investigated. Such non-ionic sup-ported hydrogels on ýuorinated polymers may be ofinterest for some practical use in which high electri-cal conductivity is not required, as in biomaterials.

EXPERIMENTAL

Materials

Poly(tetraýuoroethylene–hexaýuoropropylene–vinyl-idene ýuoride) ülms, 100km thick, supplied byHoechst (Germany), were washed with acetone anddried at room temperature. TFB is a crystallinepolymer of melting point 160–185¡C. All chemicalswere reagent grade and were used without furtherpuriücation.

Graft polymerization11h15

The strips of TFB ülm were washed with acetone,dried in a vacuum oven at 40¡C, weighed and thenput into glass ampoules with the monomer solutionusing dioxane as diluent. The glass ampoule contain-ing the monomer solution and ülms was deaerated bybubbling nitrogen gas for 5min and then subjectedto gamma irradiation at a dose rate that ranged from1.04 to 1.23Gy s~1. After irradiation, the graftedülms were washed thoroughly with hot distilledwater to extract the residual monomer and the homo-polymer occluded in the ülm. The ülms were then

* Corres pondence to : AM Des s ouki, National Centre for RadiationRes earch and Technology, POBox 29, Nas r City, Cairo 11731, Egypt

(Received 6 March 1998; revis ed vers ion received 29 June 1998;

accepted 13 Augus t 1998)

( 1999 Society of Chemical Industry. Polym Int 0959-8103/99/$17.50 92

Permselective NVP-TFB ülms

dried in a vacuum oven at 40–50¡C for 24h andweighed. The degree of grafting was determined bythe percentage increase in weight as follows :

Degree of grafting (%)\ Wg[ W0W0

] 100

where and represent the weights of initialW0 Wgand grafted ülms, respectively.

Thermogravimetric analysis (TGA)

The thermal behaviour was determined for a graftsample. A Shimadzu-50 thermal analyser (ShimadzuCo., Tokyo, Japan) was used for the TGA measure-ments. In the present study the nitrogen ýow had tobe kept at a constant rate of about 50ml min~1 toprevent oxidation of the polymer sample. Theheating rate was 10¡Cmin~1 up to 600¡C.

IR spectroscopy measurements

IR spectra were determined for the grafted andungrafted ülms using a Fourier transform infrared(FTIR) spectrophotometer (Pye Unicam, UK) therange of scanning being 400–4000cm~1.

Scanning electron microscopy (SEM)

The surface topography of the grafted and ungraftedülms was studied using a JEOL ISM-5400 scanningmicroscope (JEOL, Japan).

Atomic absorption

The concentration of metal ions Pb2` in solutionwas determined with a Pye Unicam 939 ýame atomicabsorption (AA) spectrometer (Pye Unicam, UK).

Semi-equilibrium dialysis (SED)

The permeability of the grafted ülms obtained wascalculated by using the semi-equilibrium dialysismethod.16 Dialysis is primarily a diþusion phenome-non. The grafted membranes were soaked thor-oughly in distilled water for approximately 15minbefore 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 wateron the other side (permeate side). Then the cellswere thermostated at 25¡C for 18–24h to attain equi-librium. The concentration of lead acetate which dif-fused through the membrane was determined byatomic absorption.

Other measurements such as water uptake, mecha-nical resistance and electrical conductivity werecarried out as described in a previous study.15

RESULTS AND DISCUSSION

Preparation of grafted films

Eþ ect of solvent and inhibitorIt is known that TFB is a non-porous material andalmost a hydrophobic polymer. It swells to someextent in organic solvents. Figure 1 shows the degreeof swelling as a function of swelling time for TFBülms at 25wt% of NVP in diþerent solvents. It wasobserved that the diþusion of monomer into thepolymer matrix is enhanced by the presence of dilu-ents such as butanone and dioxane. Table 1 showsthe eþect of diþerent solvents on the grafting processof NVP onto TFB ülms at an irradiation dose of10kGy. It was observed that the highest graftingyield was obtained when dioxane was used as adiluent for the monomers, and also better homoge-

Figure 1. Degree of s welling as a function of s welling time for

TFB films in 25wt% NVP in different s olvents at 25¡C: |, H2O; >,

mixture ; methanol ; benzene;], dioxane;methanol/H2O L, …,

butanone.K,

Type of Solvent Monomer Degree of Remarks

s olvent conc (wt%) conc (wt%) grafting (%)

Benzene 75 25 33.9 No homopolymer

Butanone 75 25 11.2 No homopolymer

Dioxane 75 25 38 No homopolymer

Methanol 75 25 7.11 Little homopolymer

Methanol/H2O (30 : 70) 75 25 3.6 Little homopolymer

H2O 75 25 2.3 Homopolymer formed

Table 1. Effect of s olvent on the

grafting yield of NVP onto TFB

films at irradiation dos e of 10 kGy

Polym Int 48 :92–98 (1999) 93

AM Dessouki, NH Taher, MB El-Arnaouty

nity was observed from the touch, appearance andshape of the grafted membranes.

It is known that in most radiation grafting pro-cesses homopolymerization occurs during reaction;this is a disadvantage of the direct radiation tech-nique, in which the polymer and the monomersolution are subjected to gamma-irradiation simulta-neously. In our case, when dioxane was used at radi-ation doses of less than 10kGy, no homopolymer wasformed. However, increasing the irradiation dose upto 30kGy caused the formation of some homo-polymer. In the present study, no inhibitor was used.

Eþ ect of monomer concentrationDiþusibility of the monomer diluent into thepolymer matrix has a great inýuence on the graftingprocess and grafting yield. It is well known that ýuo-rine containing polymers swell poorly in manymonomers or solvents. In such cases the graftingprocess proceeds by a front mechanism in which thegrafted layers initially formed on the ülm surface canswell in the monomer–diluent mixture resulting inmore progressive diþusion of monomer into the inte-rior regions of the ülm. Figure 2 shows the degree ofgrafting for TFB ülms in nitrogen gas as a functionof monomer concentration. It was found that thedegree of grafting increases as the monomer concen-tration increases, reaching a maximum value at50wt% of NVP, and then decreases gradually athigher monomer concentration. This behaviour maybe due to the rate of diþusion of the monomer intothe ülm, which is small in the case of high monomerconcentration, so that the trapped radicals canrecombine quickly and produce homopolymer thatincreases the viscosity of the solution: consequentlythe degree of grafting decreases. One additional pos-sible explanation is that the degree of swelling of the

Figure 2. Effect of monomer concentration on the degree of

grafting of TFB films at 25¡C and an irradiation dos e of 10 kGy.

Figure 3. Effect of irradiation time at a dos e rate of 1.23Gy s É1 on

the degree of grafting of NVP onto TFB films at various NVP

concentrations (wt%) : 10 ; 30 ; 50 ; 70.L, |, K, …,

polymer decreases as the NVP concentrationincreases.

Figure 3 shows the relationship between thedegree of grafting and irradiation time in nitrogenatmosphere for various monomer concentrations. Itwas observed that the grafting yield increases rapidlywith irradiation time for all monomer concentrations.The higher the monomer concentration, the higherthe degree of grafting obtained. Meanwhile, at theinitial stage of irradiation, the degree of graftingincreases with irradiation time at a given NVP con-centration. However, at higher irradiation time, thegrafting process tends to level oþ at a certain degreeof grafting. These results suggest that the diþusivityof the monomer is enhanced at high NVP concentra-tions, and consequently a high rate and high graftingyield are achieved. At higher doses, however, a geleþect takes place and the diþusivity of the monomeris hindered by the viscous medium formed, so thegrafting yield is limited. Also, the degree of graftingtends to level oþ at higher doses because of therecombination of free radicals without the initiationof new active sites.

The logarithmic relationship between the initialrate of grafting and monomer concentration is shownin Fig 4. Such plots give a linear relationship ; thedependence of the order of the grafting rate onmonomer concentration is found to be 1.1, i.e. analmost ürst order dependence is observed. It may beconcluded that the diþusivity of monomer into thepolymer matrix is enhanced at higher concentration,leading to much higher degrees of grafting.

94 Polym Int 48 :92–98 (1999)

Permselective NVP-TFB ülms

Figure 4. Logarithmic plots of initial grafting rate vers us NVP

concentration.

Eþ ect of dose rateThe eþect of irradiation time on the degree of graft-ing for various dose rates at a monomer concentra-tion of 50wt% was studied as shown in Fig 5. It canbe seen that for all dose rates used, the degree ofgrafting increases as the irradiation time increases.Meanwhile, the higher the dose rate, the higher thedegree of grafting obtained. It was also observed inFig 5 that there is an induction period whichdecreases, as the dose rate increases. These resultssuggest that at higher dose rate of irradiation, theconcentration of free radicals formed is higher for thepolymer and the monomer, compared with thoseformed at lower dose rates.

Figure 5. Effect of irradiation time on the degree of grafting of

NVP (50wt%) onto TFB films at various dos e rates (Gy s É1) :

0.27 ; 0.55 ; 1.23.L, K, |,

Figure 6. Percentage water uptake vers us degree of grafting for

grafted TFB films .

Characterization and properties of grafted TFB films

Water uptakeFigure 6 shows the eþect of the degree of grafting onthe water uptake for TFB-g-P(NVP) ülms. It can beseen that the percentage in water uptake increasesgradually with the degree of grafting for all graftedülms. Poly(vinylpyrrolidone) is, however, non-ionicin nature but has hydrophilic character. The graftedülms possess good hydrophilic properties andbecome ýat and smooth. Grafting improves thehydrophilic properties, and an increase in electricconductivity is also expected.

Figure 7. Electrical conductivity of grafted TFB films as a function

of the degree of grafting.

Polym Int 48 :92–98 (1999) 95

AM Dessouki, NH Taher, MB El-Arnaouty

Electrical propertiesConductivity measurements were carried out ongrafted TFB ülms after they had been dried in avacuum oven at 60¡C for 2h. The grafted TFB ülmsdid not contain any salts. Figure 7 shows the semi-logarithmic relationship between the electrical con-ductivity of TFB-g-P(NVP) ülms and the degree ofgrafting. It was found that as the degree of graftingincreases, an increase in the electrical conductivity isobtained, but that the conductivity tends to level oþabove 10% grafting. This may be caused by P(NVP)contained in the side chains of the TFB ülms whichacts as impurities, since the mobility of graftedchains is probably higher at lower degrees of graft-ing, inducing a smaller amount of crosslinking com-pared with that at higher degrees of grafting.However, at higher degrees of grafting, the cross-linked network structure restricts the mobility of thegrafted chains and no further increase in electricalconductivity is observed. The eþect of ionizing radi-ation on the polymer matrix during the graftingprocess must also be taken into consideration and itsinýuence on electrical conductivity included.

Mechanical properties of grafted TFB ülmsThe inýuence of radiation grafting on the structureof polymers has been found to diþer widely, depend-ing on whether the polymer crosslinks or degrades.The structural changes, together with the changes inmolecular weight, are usually reýected in the mecha-nical properties of the polymers. In the present

Figure 8. Change in tens ile s trength and elongation with degree

of grafting for grafted TFB films .

Figure 9. Thermal s tability of grafted TFB films with NVP as a

function of temperature at different degrees of grafting (%) :blank; 13 ; 33 ;], 71.…, K, L,

work, the tensile strength and elongation at breakpoint of the grafted ülms were investigated. Figure 8shows the changes in mechanical properties of thegrafted ülms ; it can be seen that the tensile strength

for all ülms gradually decreases as the degree ofTbgrafting increases, and that the percentage elongationat break point also decreases sharply for allEbgrafted ülms. These results show that at higherdegrees of grafting the ülms become hard and brittle.Generally, increasing the degree of grafting results inincreased rigidity and stiþness with concomitantdecrease of elasticity ; consequently the percentageelongation decreases.

Figure 10. IR s pectra of TFB films : 1, original TFB film;

2, grafted to 13% ; 3, grafted to 44%.

96 Polym Int 48 :92–98 (1999)

Permselective NVP-TFB ülms

Thermal analysisTGA was used to study the thermal stability of thegrafted and non-grafted TFB ülms. The thermal sta-bility varies with the degree of grafting and alsoaccording to the kind of monomer used. To studythe thermal stability of grafted TFB ülms, the per-centage of the remaining weight was plotted againsttemperature. Figure 9 shows the dynamic (TG) ther-mograms for ungrafted and grafted TFB ülms. Itshows that a TFB ülm is thermally stable up to400¡C. The grafting process, however, broughtabout a marked decrease in the thermal stability ofthe ülms. Therefore, the radiation-induced graftingof NVP onto TFB ülms leads to a decrease inthermal stability because of the degradation ofP(NVP) chains.

IR spectroscopyIR spectra of original and grafted ülms are shown inFig 10. Upon grafting NVP onto TFB ülms, certainchanges are encountered. The main characteristicchanges of the IR spectra of the graft copolymer arethe appearance of new bands at around 3400, 1665and 850cm~1. These bands are characteristic of thestructure of NVP. The strong broad band thatappears at 3400cm~1 is assigned to hydroxyl groupsand/or hydrogen bonding, which may indicate theexistence of a crosslinking network structure in thegraft copolymer.17 The appearance of a stretchingabsorption band at around 1665cm~1, which is cer-tainly due to the CxO groups of NVP conürms thepresence of P(NVP) graft chains in the TFB ülm.The band appearing at 850cm~1 is probably due tothe change in crystallinity and morphology of thepolymer caused by grafting and crosslinking.17 Inaddition, it may be concluded that the increase ingraft yield from 13% to 44% has a pronounced eþecton the shape, intensity and width of these bands.

Morphological structure of graft copolymer in TFB

films

Most polymer blends, grafts and blocks are generallyfound to phase-separate. SEM shows the superücialchanges of a polymeric matrix after graft poly-merization which cannot be seen with a normal lightmicroscope. Figure 11 shows the surface morphologyof the original and grafted TFB ülms with NVPmonomer. It is obvious that the grafted TFB ülm(44% grafting) is totally diþerent from the originalone. The formation of large pores with increasingdegree of grafting is observed.

Permeability measurements

The grafted TFB ülms can be used to decrease theconcentration of heavy metals, e.g Pb2`, in water,and thus may be applied for waste treatment andwater puriücation. Table 2 shows the eþect of thedegree of grafting of NVP onto TFB ülms on the

Figure 11. Scanning electron micrographs of the s urface of TFB

films s howing morphology: 1, original TFB film; 2, grafted to

13% ; 3, grafted to 44%.

Polym Int 48 :92–98 (1999) 97

AM Dessouki, NH Taher, MB El-Arnaouty

Degree of Solution conc Retained conc Permeate conc.

grafting (%) (ppm)]103 (ppm)]103 (ppm)

Blank 11.400 8.490 0.47

21.6 11.400 4.370 3.35

37.9 11.400 3.190 70.30

46.4 11.400 3.050 78.00

Table 2. Effect of grafting of NVP

onto TFB films on its permeability

towards lead acetate s olutions

extent of permeability of the membrane for leadacetate solutions. It can seen that the concentrationof permeate increases as the degree of graftingincreases. Also, the retentate does not contain thecomplement of the lead solution. This means that themissing lead ions are adsorbed on the membranesurface, which indicates the adsorption properties ofsuch grafted membranes. These results suggest thatthese grafted membranes are suitable for removal ofsome heavy metals such as Pb2` from waste water.

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