Modification of polymer membranes by ion implantation

Modification of polymer membranes by ion implantation

Nuclear Instruments and Methods in Physics Research B 225 (2004) 483–488 www.elsevier.com/locate/nimb Modification of polymer membranes by ion implant...

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Nuclear Instruments and Methods in Physics Research B 225 (2004) 483–488 www.elsevier.com/locate/nimb

Modification of polymer membranes by ion implantation K. Dworecki a

a,*

, M. Drabik a, T. Hasegawa b, S. Waßsik

a

 ßtokrzyska Academy, Swie  ßtokrzyska 15, 25-406 Kielce, Poland Institute of Physics, Swie b Applied Physics Division, Miyazaki University, Miyazaki-shi 889-21, Japan Received 6 February 2004; received in revised form 7 May 2004

Abstract This paper presents an experimental study of the transport properties of ion implanted isotactic polypropylene (iPP) polymer membranes. The polymer samples have been irradiated with O7þ , S9þ and F6þ ions having the energy 10 keV/q with fluence up to 1016 ions/cm2 . In this experiments the incident ions were produced by the ECR ion source. The transport properties (diffusive permeability), changes of the chemical structure and wettability of ion implanted membranes were investigated. The transport properties were studied by measurements of the solute concentration profiles. The changes of the chemical structure were investigated by the differential scanning calorimetry (DSC) thermograms and the atom force microscopy (AFM) images measurements. Finally, the wettability – by the contact-angles methods. The results of the AFM imaging show that ion implantation induce changes in surface topography of polypropylene samples. The degree of crystallinity of iPP foils is decreased by ion implantation. The hydrophilicity of the modified iPP is increased considerably in comparison to the unimplanted foils. Also the hysteresis in the contactangle gives some indication about the surface roughness or composition. The behavior of the iPP membranes modified by ion implantation shows greater permeability for transported substance.  2004 Elsevier B.V. All rights reserved. PACS: 61.82.Pv; 81.40.Wx; 82.65.Fr; 85.40.Ry Keywords: Ion implantation; Polymer membrane; Permeability coefficient

1. Introduction Ion irradiation is an established tool for modifying the chemical structure and physical properties of the polymers [1–4]. The use of ion beam radiation is getting high impetus because both chemical composition and the related physical

*

Corresponding author. Tel.: +48-41-3496440; fax: +48-413496443. E-mail address: [email protected] (K. Dworecki).

properties of polymers can be modified in a controlled way by easy-to-control parameter like the ion fluence, i.e. the number of particles bombarding a given surface [5,6]. On the other hand, a lot of important properties of macromolecular solids, such as adhesion, friction, wetting, swelling and biological compatibility, are heavily influenced by surface composition and structure, which in turn differ from those of the bulk polymer. Main part of the effect of ionizing radiation on polymers is generally classified into main-chain scission (degradation) and cross-linking. For higher doses of

0168-583X/$ - see front matter  2004 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2004.05.024

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radiation, which are commonly obtained with high fluences of ions, carbonization can also take place [7]. The nature of these changes depends on the properties of the polymer, such as the composition and molecular weight, and on the mass and energy of the incident ions, as well as on the conditions of irradiation [8]. Polypropylene is well-known polymer because of its various commercial applications. It is commonly used in packing industry, for manufacture of disposable medical products, a large fraction of these products is now sterilized by radiation [9]. Recently, polypropylene films are used as a raw material for track membrane production [10]. We have chosen polypropylene for our present investigation because of a wide range of utilization and its simple structure consisting of only carbon and hydrogen atoms. In order to functionalize PP membranes with respect to the capability of solute permeation, we have chosen a ion implantation technique to change its transport properties. In this paper, we report on the results of the investigation of the polypropylene films irradiated with O7þ , S9þ and F6þ ions, which were obtained by using differential scanning calorimetry (DSC) [11], atomic force microscopy (AFM) [12], wetting contact-angle method [13] and laser interferometry [14].

2. Experimental procedures The isotactic polypropylene polymer samples of 12 lm thickness and area of 2 cm2 were irradiated with a variety of O7þ , S9þ , F6þ ions having the energy 10 keV/q and fluence up to 1:5  1016 ions/ cm2 , under vacuum at room temperature. In these experiments the impinging ions were produced using the ECR ion source at Heavy Ion Laboratory of Warsaw University, Poland. The ECR ion source at the Heavy Ion Laboratory is combined with the U-200P cyclotron system. To avoid thermal degradation of the sample the beam current was kept below 1.7 nA/cm2 . Theoretical calculation of ion ranges and absorbed dose were carried out using the SRIM-2003.20 code [15]. Simulation showed that the projected ranges of O7þ , S9þ , F6þ ions in the polymer in question were

251.8, 171.2 and 204.2 nm respectively. The conversion from fluence to absorbed dose is obtained by using the following relation: DðGyÞ ¼ 1:6  109 Uq1 DE=Dx; where U (cm2 ) is the ions fluence, q (0.90 g/cm3 ) the density of the PP samples and DE=Dx (274 eV/ nm for O7þ ions, 291.1 eV/nm for F6þ ions and 513.3 eV/nm for S9þ ions) is the energy loss in the film. The irradiated samples as well as unirradiated iPP foils were studied by different techniques: (i) differential scanning calorimetry (DSC), (ii) atomic force microscopy (AFM) and (iii) contact-angles method. The DSC measurements were performed using Perkin–Elmer TAC/7DC instrument. About 1–3 mg of the samples were scanned in the calorimetry furnace in the temperature range from )40 to 200 C, with a heating rate of 10 C/min for all the measurements. The AFM investigations of the surface morphology of samples were carried out with a commercial instrument (Park Scientific Instrument, Sunnyvale, USA) equipped with a homebuilt head with a laser deflection detection system. The measurements were performed with a nominal spring constant of 0.03 N/m in the contact mode AFM and in ambient atmosphere with a typical relative humidity of 50%. The contact-angle of liquid on solid is closely related to surface free energy and this parameter is useful in discussion of hydrophilicity and adhesivity of sample. The contact angles of water and aqueous solution of ethanol drops (4 ll) were measured according to a standard method [13] before and after the surface modification at room temperature. Advancing and receding angles were obtained by increasing or decreasing the drop volume. A NRL C.A Goniometer (A Rame-Hart, Mountain Laikes, NJ, USA) was used to determine the contact-angles. The changes of transport properties (permeation coefficient – Pm (m/s)) of each membrane were studied by employing a specific experimental device, which has been developed to study the transport of solute through polymer membrane. The experimental set-up is described in detail in [16,17]. Here, we only mention that it consists of the membrane system, the Mach–Zehnder interferometer including the He–Ne laser, TV-CCD camera, and the computerized data acquisition

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system. The membrane system under investigation makes the diffusion cell consisting of two vessels separated by the horizontally located membrane. The membranes, after irradiation, are arranged in a diffusion cell. Initially, we fill the upper vessel with the aqueous solution of ethanol while in the lower one there is pure water. As ethanol penetrated across the membrane, the ethanol concentration at the water side increased with time. A laser interferometer was used to monitor this change. The analysis of the interferograms allows to reconstructing the time-dependent concentration profiles of the substance transported across the membrane. Ethanol permeability of samples before and after irradiation was calculated from the following expression [16]: Pm ¼ Drx Cðx; tÞ=½DCðx; tÞ ; with D (0.9 · 109 m2 /s) being the diffusion coefficient of the ethanol in the water, DCðx; tÞ the difference of concentrations between upper and lower of membrane surfaces, and Cðx; tÞ the concentration profile of the ethanol in the lower vessel.

3. Results and discussion The differential scanning calorimetry curves of the unirradiated (virgin) and irradiated samples of iPP membranes implanted with O7þ , S9þ , F6þ ions are presented in Fig. 1. The uncertainties of the DSC measurement were estimated to be up to 0.04%. The endothermic transformation of virgin and irradiated films occurs in a wide temperature range 135–170 C, with melting peak temperature of about 162 C. For this single melting temperature point, the total heat of fusion Hf is evaluated to be equal to 104.5 J/g. This value is usually correlated with the polymer’s crystalline fraction ðvÞ which can be calculated by using the relation v ¼ Hf =Hsc , where Hsc is the melting enthalpy of a completely crystalline polymer equal to 190 J/g [11]. DSC curves of irradiated PP samples with given ions indicate a decrease of melting enthalpy as compared to the unirradiated sample, and hence smaller degrees of crystallinity. The measurements show that the values of the crystallinity fractions in PP are equal to 58.25% – for virgin and irradiated

Fig. 1. DSC curves of virgin and irradiated iPP samples with O7þ , S9þ , F6þ ions recorded for heating rate 10 C/min.

foils; 42.32% for O7þ , 47.15% for F6þ and 49.06% for S9þ ions, which was obtained by integrating the corresponding DSC curves in the respective temperature ranges. The broadening of the observed endotherm may be attributed to the formation of disordered material in the irradiated samples. Our results also indicate that glass transition for the irradiated iPP samples takes place at temperatures lower than those for virgin film. The DSC results show decrease of crystallinity in the irradiated polymer samples. The loss of crystallinity which was, resulted from scission processes of the main chains of the polymer. These results indicate on an increase in the content of amorphous phase at irradiated foils [11,18,19]. The effect of the absorbed dose on the change of melting temperature and degree of crystallinity of PP samples are presented in Fig. 2. From the figure one can see that the total energy deposited within implanted film changes the crystallinity and melting temperature. More information about the chemical modification of iPP was obtained by means of AFM measurements. In Figs. 3(a) and

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Fig. 2. Variation degree of crystallinity and melting temperature of virgin and irradiated PP samples as a function of absorbed dose.

(b) we present selected AFM topographic images of iPP samples before and after irradiation with O7þ , S9þ , F6þ ions. The unirradiated iPP foil surface is characterized by a granule-like network structure. The resulting granule diameters are approximately 100 nm. After O7þ ion irradiation, the iPP foil shows a remarkable change in morphology from the original surface, as small white mounds with diameter 30–70 nm and height up to 6 nm. Also, the topography of this sample shows large amount of pits of the order of 2–3 nm depth. These figures show us that after ion irradiation the polymer reorganizes in thin strands separated by free space. For the irradiated iPP foils, as shown in Fig. 3(b) the diameter of the strands in the net-like structure is less that for a virgin one. This thinning of the strands may result from the scission of the chains [19]. The modifications of the iPP foils, and virgin surfaces, were characterized by means of contactangle measurement with water and aqueous solu-

Fig. 3. Atomic force microscopy (AFM) topographic images to the iPP samples: (a) virgin, (b) irradiated with O7þ ions. The probe was scanned over an area of 3449 · 3449 nm2 . The images are recorded in height mode.

tion of ethanol drops. The mean values of contactangle obtained for these samples are presented in Table 1. The contact-angle measurements with water drops reveal, as expected, a very high hydrophobicity. The mean advancing angle Hadv 88 2 and mean receding angle Hrec 84 2 were measured. The iPP sample surfaces become more hydrophilic after its irradiation with ions. The values of contact-angle measured with aqueous solution of ethanol drops are lower than with water drops. In this case the surface of iPP samples, both virgin and ion irradiated, exhibit a significant increase in wettability. The increase of the hydrophilicity of the surface after ion irradiation indicate, that a change in the chemical composi-

Table 1 Contact-angle measurements of water and aqueous solution of ethanol (on concentration 1000 mol/m3 ) drops obtained for unmodified-virgin and implanted samples with O7þ , S9þ , F6þ ions Samples

Virgin O7þ S9þ F6þ

Advancing angle () Hadv

Receding angle () Hrec

Hysteresis () DH ¼ Hadv  Hrec

Water

Ethanol

Water

Ethanol

Water

Ethanol

88 ± 2 72 ± 2 68 ± 2 72 ± 2

76 ± 2 63 ± 2 58 ± 2 60 ± 2

84 ± 2 46 ± 4 51 ± 3 54 ± 3

68 ± 2 44 ± 3 45 ± 3 46 ± 2

4 26 17 16

6 19 13 14

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tion of the surface takes places. In general the hysteresis in the contact-angle DH ðDH ¼ Hadv  Hrec Þ gives some indication about the surface roughness or composition [20]. The virgin iPP samples are smooth, whereas the iPP surfaces irradiated with O7þ , S9þ , F6þ ions show average hysteresis of DH 26, 17 and 16 respectively. These results show that iPP foils irradiated with oxygen ion are more rough than those modified with S9þ , F6þ ions. The surface color of the samples implanted also varies from white to light brown. The permeability of a polymer membrane depends not only on the size of the molecules of the permeating solute, but it is also dependent on the forces holding the polymer chains together. The intermolecular cohesion is influenced by the morphology of the polymer, which can be changed by ion irradiation. The last part of our study is devoted to investigation of the influence of ion irradiation on the transport properties characterized by a coefficient of the diffusional permeability PP membrane. Ethanol permeability of membranes before and after ion implantation was determined by measuring the concentration profiles at membrane system for diffusive solute across the sample. Fig. 4 shows the concentration profiles of ethanol at lower vessel of a membrane system, as deter-

Fig. 4. Comparison of the measured concentration profiles of ethanol taken after a time of 1200 s in the lower vessel of the diffusion cell using different iPP membranes: – unirradiated and ion irradiated with:  – O7þ , O – S9þ , 4 – F6þ . The initial concentration of aqueous solution of ethanol in the upper vessel of diffusion cell was equal 1000 mol/m3 .

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mined by the interferometric method, which were received for virgin and irradiated samples. The values of the ethanol permeability of virgin and ion irradiated samples with O7þ , S9þ , F6þ ions were: (1.12 ± 0.15, 2.80 ± 0.20, 2.50 ± 0.20 and 2.25 ± 0.20) · 106 m/s respectively. Comparing this permeabilities we observe that the membranes produced by implantation of oxygen ion in iPP samples are characterized by larger permeability than those obtained by irradiation with S9þ , F6þ ions. Fig. 5 shows dependence of ethanol contactangle and ethanol permeability on absorbed energy dose for PP samples. We observe that iPP membranes samples become more permeable after ion implantation with a permeability that depends on the ion species and ion energy loss. The ion implantation leads to a change of the permeability, which results the changes of the composition and chemical structure of samples analyzed. The contact-angle is sensitive to surface properties, however the solute permeability is sensitive to surface and bulk properties, which are closely related to the chain structure and the system morphology. The chain structure may include the degree of polarity, interaction forces and tendency to crystallization and chain stiffness. The morphological properties are related to the crystallinity and free volume of the polymer matrix. This can be explained assuming a destruction of the crystallites, which act as ‘‘physical cross-links’’ [21].

Fig. 5. Dependence of the ethanol permeability coefficient and wetting angle in the PP membrane as a function of absorbed dose.

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4. Conclusions

References

The changes of iPP samples implanted with O , S9þ , F6þ ions were examined using DSC, AFM and contact-angle techniques. The DSC thermogramms, and AFM images indicate an increase in the amorphous phase content of the implanted polymer. Contact-angle measurements showed that the PP membranes became significantly more hydrophilic after the ion irradiation. These results show that the ion irradiation can induce substantial changes in the ethanol permeability of iPP membranes. The observed substantial increase of the diffusive permeability of the iPP samples implanted with O7þ , S9þ , F6þ ions suggests strong enhancement of the polymer degradation [22,23]. The results concerning the ethanol permeability are consistent with those obtained by DSC, AFM and contact-angle, which is a good explanation for the structural changes and transitional behavior in the polymer. The observed permeability of PP membranes strongly dependent on the crystallinity of the polymer. It seems possible that the ion irradiation of polymer favors a creation of the membranes with larger permeability for some solutes. The results also indicate that ion-implantation using the ECR ion source is a good and valuable technique for improving polymer surface properties, due to its low cost and large implantation areas.

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Acknowledgements The authors would like to thank to Prof. S. Chojnacki from the Heavy Ion Laboratory of Warsaw University, for his helpful suggestion, and delivering the PP samples, and Dr. I. Kuci nska from the Center of Molecular and Macromolecular Studies, Polish Academy of Science, Ł odz, for AFM photographs of samples.