Synthesis of grafted polyethylene by ion beam modification

Polymer

Degradafion

and Stability

58 (1997)

143-147

0 1997 Elsevier Science Limited Printed in Northern Ireland. All rights reserved PII:

ELSEVIER

s0141-3910(97)00038-4

Synthesis of grafted polyethylene modification

0141-3910/97/$17.00

by ion beam

V. horG&,” V. Rybka,” I. Stibor,’ V. Hnatowicz’ J. VaclW & P. Stopkad ’ Department of Solid State Engineering, Institute of Chemical Technology, I66 28 Praque, Czech Republic ’ Department of Organic Chemistry, Institute of Chemical Technology, 166 28 Praque, Czech Republic ’ Institute of Nuclear Physics, Czech Academy of Science, 250 68 Rei, Czech Republic ‘Institute of Inorganic Chemistry, Czech Academy of Science, 160 00 Praque, Czech Republic

(Received 4 December

1996; accepted 6 January 1997)

The chemical reactivity of polyethylene modified by irradiation with 40 keV Ar’ ions to fluences of 1 X 1012-1 X lOI cm-* was studied. Ion beam modified polyethylene was exposed to the solutions of CH,=CH-COOH, CH,=CH-CN and Br, and the chemical and structural changes were examined using IR-, UV-VIS-spectroscopies, electronparamagnetic resonance and Rutherford back-scattering techniques. The above agents were found to react with radicals and conjugated double bonds created by the ion irradiation. Radical and additive reactions in the ion beam modified surface layer may lead to the creation of a grafted surface layer up to 150 nm thick. 0 1997 Elsevier Science Limited

1 INTRODUCTION

In these studies the polymer based on allylcarbonate (e.g. C-39) was irradiated with 10 MeV/u Au ions to a fluence of 1 X 10” cm-2 and subsequently chemically etched. Then an amino acid containing monomer was graft polymerized onto the membrane via radiation-induced polymerization. The grafting was thus accomplished via a group in the monomer unit on the polymer surface and in etched ion tracks. In this study, polyethylene irradiated with ion beam and a grafting of derivatives of acrylic acid and bromine through degradation products (namely radicals and double bonds) produced by the ion irradiation in the polymer surface layers is studied.

Irradiation with heavy energetic ions induces irreversible changes in a polymer surface layer which in turn results in dramatic changes in physical and chemical properties.’ The thickness of the modified surface layer as well as the concentration of the radiation defects produced, depend on the ion charge, mass and energy. Typically, polymer layers hundreds of nanometres thick are affected by irradiation with lo’-lo2 keV ions. The character of very complicated degradation processes taking place in ion beam irradiated polymers is still not fully understood. The irradiation leads to scission of macromolecular chains and to the creation of radicals24 which later may annihilate by crosslinking of polymer segments5 by oxidation from ambient atmosphere in the ion implanter6v7 or via the creation of conjugated double bonds.6 Part of the radicals may be detected using electron paramagnetic resonance which is sensitive to unpaired electrons.4~X,y Grafting of polymers modified by ion implantation has been studied by Omichi’“*” in connection with the preparation of ‘intelligent’ membranes.

2 EXPERIMENTAL 2.1 Material and modification The present experiments were performed using commercially available polyethylene (PE) films 15 pm thick with a density of O-945 g cm-“. The films were irradiated with 40 keV Ar’ ions to fluences from 1 X 10” to 1 X lOI cm-*. The ion 143

144

V. &wrc’ik

irradiation was performed at room temperature and the ion current density was kept below 50 nA cm-’ to avoid polymer thermal degradation. The PE films were irradiated from both sides to amplify expected degradation phenomena and to facilitate optical measurements. Immediately after the ion irradiation, the PE films were exposed for 12 h at room temperature to the three following agents: 1. 3 vol.% water solution of acrylic acid CH,=CH-COOH (specimens denoted as COOH throughout the paper) 2. 3 wt% Br solution in petrol ether (Br specimens) 3. 2 vol.% solution of acrylonitrile CH,=CH-CN in petrol ether (CN specimens). Water and petrol ether were chosen as solvents to prevent dissolution of the modified PE surface layer. 2.2 Measurements After thorough washing and drying of specimens, the concentration of unpaired electrons (i.e. the concentration of radicals) N, was determined by electron paramagnetic resonance (EPR) using a Carl Zeiss 220 spectrometer. For this purpose the irradiated and pristine PE samples (2 X 2 cm in area) were rolled up into a quartz ampoule, which was placed in the spectrometer resonator. The EPR spectra were registered at room temperature, in a 337 mT magnetic field using 10 mW microwave energy. The concentration of unpaired electrons per cubic centimetre was determined using a Mn*+/ZnS standard. The concentration of the unpaired electrons, N,, was determined only on the samples irradiated to fluences above 1 X 1014-1 X lOI cmp2 (untreated samples) and above 5 X lOI and 1 X 10’” cm-* (chemically treated samples). In other cases the concentration N, was below the available sensitivity. The IR spectra were measured with a Nicolet 740 FTIR spectrometer and a PerkinElmer spectrometer was used for the measurement of UV-VIS spectra. Differential IR spectra were obtained by subtracting the IR spectra of as-implanted specimens from those measured after chemical treating. The concentration depth profiles of incorporated oxygen and bromine were obtained using a standard Rutherford backscattering technique (RBS) with 2 MeV alphaparticles (for experimental details see e.g. 12).

et al. Table 1. The number of radicals N, (in 10” cm-‘) in asirradiated PE and after treatment with acrylic acid (CH,=CH-COOH), acrylonitrile (CH,=CH-CN) and with Br, as a function of the ion fluence Fluence of Ar’ ions (cry’) Reactions

Implantation + CH,=CH-COOH + CH,=CH-CN + Br,

I x 1fY

5 x IO’”

1 x 10J5

0.76

1.50 1.oo

585 457 4.77 4.74

-

1.06 1.18

3 RESULTS AND DISCUSSION As has been mentioned above, the radicals and other new structures, such as conjugated double bonds, created in a polymer by ion irradiation, exhibit enhanced reactivity. On the other hand, it is well known that acrylic acid and its derivatives and halogens (e.g. bromine) react easily with both radicals and conjugated double bonds. The measured concentrations of radicals iV, in the ion irradiated PE samples before and after treatment in acrylic acid, acrylonitrile and after bromination are summarized in Table 1. It is seen that in the as-implanted samples the concentration of radicals increases with increasing ion fluence. After chemical treating, the radical concentration declines, the decline being most pronounced for the treatment with acrylic acid and smallest for the bromination. The difference indicates the higher reactivity of acrylic acid with radicals. The present data for the PE samples irradiated with 40 keV Ar’ ions can be compared with data obtained earlier for irradiation with 150 keV Ar’ ions.4 The higher concentration of radicals observed in the present case may be connected with the different role of nuclear and electronic energy loss at various ion energies. The concentration of conjugated double bonds and the conjugation number can be examined using UV-VIS spectroscopy.‘” It is known that a higher absorbance at a certain wavelength corresponds to a higher concentration of double bonds and an absorbance increase at longer wavelengths indicates an increase in the number of conjugated double bonds. The UV-VIS spectra measured on the PE samples irradiated with Ar’ ions and chemically treated are shown in Fig. 1. The ion irradiation results in an increase in both the number of double bonds and the conjugation length. Subsequent chemical treating of the PE samples leads to a decrease in the number of

Synthesis of grafted polyethylene by ion beam modification

145 PE/Ar/COOH

__ PE/Ar PE/Ar/COOH, ~ _ PE/Ar/Br

PE/Ar

CN

I

Wave lenght

-

(nm)

Fig. 1. UV-VIS spectra of the PE specimens irradiated with 40 keV Ar’ ions to different fluences (shown in the figure) and subsequently treated with acrylic acid. acrylonitrile and bromine. The spectrum of pristine polymer is also shown for comparison as curve labelled PE.

double bonds and their conjugation length. The most significant decrease is observed after bromination while no significant difference is found for the samples exposed to acrylic acid and acrylonitrile. The present results (see Fig. 1) indicate that the chemical agents add to double bonds, produced by preceding ion irradiation, the effect being most pronounced for bromine. Similar addition of chlorine onto double bonds in ion beam modified polyimide was observed earlier.14 Chemical reaction with free radicals is another important process which should be considered. From the comparison of data summarized in Table 1 with Fig. 1 it is evident that acrylic acid mostly reacts with radicals while addition prevails in bromination. In any case, the groups -CH,-CH,-COOH, -CH,-CH,-CN and -Br are expected to be grafted as a result of the chemical treatment in the PE surface layer modified by the ion irradiation, The presence of these groups or elements and their depth distribution were determined via IRspectroscopy and the RBS method. A differential IR spectrum obtained as the difference between the spectrum of the PE sample treated in acrylic acid and that of an as-irradiated one is shown in Fig. 2. It is seen that the chemical treatment leads to an absorbance increase in the region 1710-1765 cm--’ which is characteristic of the presence of -COOH, -CO and ester groups. Obviously, these groups are the products of chemical reactions of acrylic acid with the radiation damaged PE. A similar differential spectrum for the sample treated in acrylonitrile is

1650

r

1

I

I

I

I

I

I

I

r I I

I

I I,

, , , , , , , , ,

1706

II 3:i0

1750 1800 Wavenumbers (em-‘) Fig. 2. Differential IR-spectra obtained by subtracting spectra of as-irradiated specimens from those irradiated subsequently treated with acrylic acid. The numbers are ion fluences applied.

the and the

shown in Fig. 3. In this case the presence of the -CN group due to the chemical reaction with acrylonitrile is manifested by an increase in absorbance in the interval 2200-2270 cm-‘. The presence of bromine in the samples exposed to the Br solution was not proved by examination of the IR spectra in the interval 500-600 cm-’ which is characteristic for the C-Br bond. The incorporated Br atoms are, however, clearly seen in the RBS spectra. Bromine depth profiles determined from the RBS spectra are shown in Fig. 4 and area densities of the Br atoms, incorporated in the surface layer ca 400 nm thick, are summarized in Table 2 together with some other relevant data which will be discussed below. One can see from Fig. 4 that the Br atoms penetrate to a depth of about 150 nm,

-. PE/Ar/CN

#8

2180

”

PE/Ar 1 I

t

I

I

I

I

I

2220 Wavenumbers

I

I1

1

I

,

I

2260 (cm-‘)

Fig. 3. The same as in Fig. 2 but for the samples acrylonitrile.

I,

I,,

,

,

2300 treated

with

V. hm?ik

146 6,

I

Depth

(nm)

Fig. 4. The concentration depth profiles of bromine incorporated in the PE samples irradiated to different ion fluences (indicated in the figure).

which is much larger than the theoretically predicted thickness of the radiation damaged surface layer. According to TRIM code (version 91 see Ref. 15), the projected range and the range straggling of 40 keV Ar’ ions in PE are 65 and 9 nm, respectively. A maximum of the Br area density is observed for the PE sample irradiated to the fluence of 1 X 1014 cmd2. For higher fluences the area1 density decreases significantly, due probably to a higher portion of crosslinked and carbonized fractions in the PE surface layer Table 2. Area densities of oxygen (No) and bromine (iv.,) atoms in the surface layer 400 run thich measured by the RBS method on the as-irradiated samples (PEIAr) and the samples subsequently treated with acrylic acid (PEIArICOOH) and with bromine (PE/Ar/Br). In the last part (PE/Ar/COOH - PEIAr) the differences between the oxy gen contents in the as-implanted and the acrylic acid treated specimens are given, which ilhtstrate the effect of chemical treatment

et al.

preventing penetration and incorporation of Br atoms.lh The RBS method also enables us to determine the content and depth distribution of oxygen which is acquired from the ambient atmosphere in the ion implanter during the ion irradiation and is chemically bound to the polymer chain. The measured area densities of incorporated oxygen are summarized in Table 2. It is evident that the bromination of the irradiated PE leads to a decrease in the concentration of oxygen compared with as-irradiated PE. No significant absorbance increase in the 1710-1760 cm-’ band was observed after the reaction of the modified PE/Ar with acrylonitrile or bromine. It may therefore be concluded that the grafting of radiation degraded PE is not accompanied by significant oxidation with aerial oxygen or oxygen dissolved in water. The oxygen depth profiles are bell-shaped as illustrated in Fig. 5, where the oxygen depth profiles in the PE samples irradiated to the fluence of 1 X 1015cm-’ and measured before and after the treatment in acrylic acid are compared. It is seen that the oxygen penetrates to a depth of about 150 nm which, similar to the above discussed case of Br incorporation, significantly exceeds the thickness of the radiation-damaged surface layer. The RBS results indicate an increase in oxygen content after the treatment in acrylic acid, similar to IR-spectroscopy (see Fig. 2). It is seen from Fig. 5 that a significant increase in oxygen concentration is observed throughout the 150 nm depth and it may therefore be concluded that the acrylic acid

Fluence of Ar’ ions (cm-‘)

PElAr 1 x lOI 5 x lOI 1 x 10” PE/Ar/COOH 1 x lOI 5 x 10IJ 1 x 10” PE/Ar/Br 1 x lOi 5 x lOI 1 x 10” PE/Ar/COOH 1 x 10’4 5 x lOI 1 x 10”

35.5

-

47.9 465 43.5 65.2 55.3

-

30.7 32.9 26.9

7.7 5.7 2.6

- PE/Ar 8.0 17.3 8.8

Fig. 5. The concentration depth profile of oxygen incorporated in the PE irradiated with 40 keV Ar’ ions to the fluence of 1 X 10” cm--* and in the same sample but subsequently exposed to acrylic acid.

Synthesis of grafted polyethylene by ion beam modification

penetrates the entire modified surface layer. The increase of the oxygen content after the treatment in acrylic acid is also documented at the bottom of Table 2, where the differences in oxygen content between treated acrylic acid and as-irradiated specimens are given for three different ion fluences. Grafting of the polymer surface layer results in an increase of surface polarity which in turn may significantly affect polymer biocompatibihty.” Despite the rather high experimental uncertainties it is clear that the maximum effect occurs at an ion fluence of 5 X 10’” cm-‘, similar to the case of Br incorporation discussed above. 4 CONCLUSION The present follows:

results

147

acrylic acid, acrylonitrile or bromine it is possible to prepare grafted surface layers containing chemically bounded -CH,-CH,COOH and -CH,-CH,-CN groups and -Br.

ACKNOWLEDGEMENTS This work was partly supported by the Grant Agency of Czech Republic under the contract No.202-96-0077 and by grant of Institute of Chemical Technology under the contact No. 126 IS 6101. REFERENCES

can

be

summarized

as

1. The concentration of radicals produced by the ion bombardment is a decreasing function of the ion energy. 2. Reaction of the ion beam modified PE with chemical agents leads to a decrease of the radical concentration, so that a radical reaction mechanism plays an important role in the system. The effect is most pronounced for the treatment in acrylic acid. 3 . Addition _ to double bonds (produced by preceding ion irradiation) also takes place, the effect being strongest for the bromination. leads, under the present 4. The ion irradiation experimental conditions, to significant oxidation of the PE surface layer. Treatment of the ion irradiated specimens in acrylic acid leads to another increase in oxygen content. 5 The depth distributions of the incorporated bromine and oxygen extend well beyond the surface layer modified by the ion irradiation. The amount of incorporated atoms achieves a maximum for the ion fluence of 5 X lOI cm ’ and it declines for higher fluences, the effect being probably due to higher concentration of crosslinked and carbonized fractions. 6. Present experiments show that by the treatment of PE modified by ion irradiation with

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