THE INFLUENCE OF GAMMA RADIATIONS ON PHYSICO-CHEMICAL PROPERTIES OF SOME POLYMER MEMBRANES *

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1 CONDENSED MATTER THE INFLUENCE OF GAMMA RADIATIONS ON PHYSICO-CHEMICAL PROPERTIES OF SOME POLYMER MEMBRANES * CRISTINA-DELIA NECHIFOR 1, DANA-ORTANSA DOROHOI 1, CONSTANTIN CIOBANU 2 1 Al. I. Cuza University, Faculty of Physics, Iasi, Romania 2 Institute of Macromolecular Chemistry P. Poni, Iasi, Romania cd13_nechifor@yahoo.com Received September 25, 2008 In this paper we studied the way in which the surface morphology and the bulk properties are modified when some porous polymer membranes, obtained from poly (hydroxy-urethane) (PHU) and poly (vinyl alcohol) (PVA) were treated with gamma radiation. When different quantities of these two polymers were alloyed, we obtained membranes with various hydrophilic character, surface energy, resilience and initial elastic module. They are able to form hydro-gels viable for biomaterials. The gamma treatments of these membranes permit to obtain a stable surface in relation to the environmental conditions for small doses, but when the dose increases the gamma radiations induce changes in the chemical structure followed by degradation. Gamma radiations are usually used in biomedical field for sterilization. Therefore, in this study we intend to establish the properties modification induced by gamma radiations. Key words: PHU, PVA, roughness, contact angle, degradation, crosslinking. 1. INTRODUCTION PVA and PHU are considered to be biomaterials. A large number of biomaterials have been used in medical applications for: controlled drug delivery, orthopedic devices, sutures, cardiac pacemakers, and vascular grafts [1]. Natural polymers remain attractive primarily because they are economical, capable of chemical modifications, potentially degradable and compatible [1] with the human body. Biomedical polymers must be sterile [2] to avoid medical problems. The composition, microstructure, degree of porosity, or surface chemistry [2] may influence the bacterial contamination of the materials. Through the methods of sterilization, the exposure to gamma rays is one of the most employed [2] and common [3] techniques. The advantage of gamma irradiations is due to the fact that they are highly penetrating [2] and economically * Paper presented at the 9 th International Balkan Workshop on Applied Physics, July 7 9, 2008, Constanţa, Romania. Rom. Journ. Phys., Vol. 54, Nos. 3 4, P , Bucharest, 2009

2 350 Cristina-Delia Nichifor, Dana-Ortansa Dorohoi, Constantin Ciobanu 2 feasible for a large-scale terminal sterilization of products in sealed packages [6]. The minimum sterilization dose required by official authorities is 25 kgy [2]. Ionizing radiation processing at low doses plays an important role in the microbial decontamination of pharmaceutical and medicinal products [4]. Ionizing events cause a shower of secondary electrons that activate numerous chemical reactions [4], many of which induce oxidative degradations in the presence of oxygen. The aromatic polymers are more resistant to high-energy radiation than aliphatic polymers [5]. The presence of impurities and additives may enhance degradation and/or crosslinking [5]. Any molecule is more stable to radiation in the solid state than in the liquid state [7]. PVA has a simple chemical structure with pendant hydroxyl groups. The monomer unit is vinyl alcohol [8]. PVA is obtained by the vinyl acetate polymerization in alcoholic solutions [8] followed by partial hydrolysis. It is possible that not all acetate groups be substituted by OH radicals [9]. Consequently polymers with a different hydrolysis degree will be obtained [8]. The PVA melting temperature is 230 C for 100% hydrolyzed polymer and C for partial hydrolyzed polymer [10]. The PVA density is 1, 26 g/cm 3 for the amorphous phase at 25 C and it is 1, 35 g/cm 3 for the crystalline phase at the same temperature [11]. PHU is an aliphatic polymer from the polyurethane class. This is a principal polymeric class obtained by polymerization of diisocynate, a molecule containing two -NCO groups and two hydroxyl groups. PHU is an elastic polymer characterized by urethane groups, (-O-CO-NH). Because poly (hydroxyl-urethanes) are very good isolators, they are used in electronic and electrical industries for packing fragile objects [8], subaquatic cables, in constructions, in medicine for catheters and tubes. These materials are resistant to corrosion and abrasion, they have pores which absorb the humidity of the body and are recommended in hospitals. Polies (hydroxyurethanes) are also very durable materials. PVA is a hard and rigid polymer, with good mechanical properties, but it is not elastic, while PHU is an elastic polymer, but less hard. A combination of these two polymers will form a new material which merges both polymers properties. 2. MATERIALS AND METHODS The polymeric films studied in this paper were obtained by alloying PVA and PHU 20% viscous substances. The PHU concentrations were: 12, 16, 25, 32, 50 and 70 percents. PVA viscous substance was obtained by solving 200 g pure LOBA PVA (Austria) in 800 g deionized water at a temperature about C and than the gel was filtrated. PHU was synthesized through a well knowing method [12] in our laboratory, and then solved in deionized water.

3 3 Influence of gamma radiations on polymer membranes 351 These two polymers were alloyed in different concentrations, and then the mixture was deposed on clean glasses, on level surfaces. Films were dry 72 hours at room temperature and than dried at vacuum for 4 hours. These films were kept in exicator during the entire experimental time. The films were irradiated with a gamma source 137 Cs, for 2, 4 and 10 hours, at a distance of about 20 cm, in air. Caesium-137 is a radioactive isotope which is formed by nuclear fission. It has half-life of 30, 23 years, and decays by pure beta decay to a metastable nuclear isomer of barium-137. Barium-137 has half-life of 2, 55 minutes and is responsible for the gamma ray emission. The photon energy of Ba-137m is 662 kev. The polymeric films were analyzed by means of ATR-FTIR a Bruker Vertex 70 type spectrometer (U.S.), with a diamond crystal having a 6 mm 2 surface, with added software for spectral processing. The contact angles of the samples were evaluated with KSV CAM 101 Instruments Ltd., apparatus (U.S.), using the sessile drop technique with deionized water. Morphology of these films was studied with AFM technique at a resolution of 20 µm. The average roughness, Ra, the root mean square, Sq, the surface skewness, Ssk, the coefficient of kurtosis, Ska, and the high average, A, were calculated for each film. The same studies were made before and after gamma irradiation. 3. RESULTS AND DISCUSSIONS Morphologically, the untreated samples are porous, and the roughness coefficient depends on the PHU concentration and on the irradiated time. In figure 1 are represented the AFM 3D images for 12% PHU sample, unirradiated and irradiated for different time intervals. Figure 2 presents the dependence of some important physical scales which morphologically characterized the samples versus the irradiation time. These are: average roughness Ra, root mean square, Rms, surface skewness, Ssk, and coefficient of kurtosis, Ska. Average roughness, Ra, and root mean square, Rms are two physical scales describing the roughness degree of the sample. Surface skewness, Ssk, is a statistical parameter describing the asymmetry of average high distribution picks in histogram. For a symmetrical distribution (Gauss), Ssk=0. The value Ssk < 0, shows that the surface is porous, and when Ssk>0, the surface presents picks. The kurtosis coefficient, Ska is a statistical measure of the comparison of the measured profile and the Gaussian distribution characteristic of a perfectly random distribution of peak heights and valley depths. If Ska =3 there is a Gaussian distribution. When Ska < 3 picks prevail, and when Ska >3 pores prevail [13].

4 352 Cristina-Delia Nichifor, Dana-Ortansa Dorohoi, Constantin Ciobanu 4 Fig. 1. AFM 3D images 12% PHU sample: unirradiated and 2, 4, 10 hours gamma irradiated. The more important physical scales describing the roughness degree of the sample are presented in Table 1. Table 1 Important physical scales describing the roughness degree of the sample Physical scale Untreated Treated 2h Treated 4h Treated 10h High average (nm) Ra (nm) Rms (nm) Ssk Ska From Fig. 2 it can be seen that all these coefficients decreased for a small exposure time 0 4 hours, and after 4 hours they increased. We can say that the roughness of these materials decreases up to 4 hours treatment with gamma

5 5 Influence of gamma radiations on polymer membranes 353 radiations. This fact indicates that the sterilization process may affect the film porosity. These high-energy treatments result in free radicals generation, main chains scission and crosslinking [2], and subsequently alter the distribution of the chains sizes in the bulk polymer. The combined action of ionizing radiation and of oxygen on the polymers may rapidly lead to a severe deterioration of their properties [2]. The radiation damage and the oxidative degradation cause chemical changes in the polymer structure with the build-up of a variety of new functional groups as carbonyls, carboxyls, esters, hydroxyls [2, 3]. Fig. 2. Dependence of average roughness Ra, root mean square, Rms, and coefficient of kurtosis, Ska on the irradiated time. coefficient value Rms Ska Ra Exposure time, hours The ATR-FTIR spectra of unirradiated and different time irradiated samples are presented in Figs. 3 and 4. Absorbance, u.a cm cm -1 Sample I 12% PHU 1720 cm cm cm cm -1 10h irradiated 4h irradiated 2h irradiated Unirradiated Wave number, cm -1 Fig. 3. ATR-FTIR spectra for different irradiated time of the 12% PHU sample.

6 354 Cristina-Delia Nichifor, Dana-Ortansa Dorohoi, Constantin Ciobanu 6 From the ATR-FTIR spectra of gamma treated samples modification of the absorbancies were remarked: at 2940 cm 1 corresponding to asymmetric stretching vibrations of CH 2 groups ν (CH 2 ) asym ; at 1550 cm 1 stretching and bending vibrations from amide II [ν(nh)+δ(nc)]; at 1431 cm 1, 1258 cm 1 amide III bands; at 1087 cm 1 and 1029 cm 1 stretching vibrations of C-O bond from ester and urethane respectively. Important modifications appear during the gamma treatment in the range of cm 1 corresponding to stretching vibrations of the carbonyl groups (Figs. 3 and 4). Absorbance, a.u cm cm -1 Sample VI 70%PHU 1701 cm cm cm cm -1 10h irradiated 4h irradiated 2h irradiated Unirradiated Wave number, cm -1 Fig. 4. ATR-FTIR spectra for different irradiated time of the 70% PHU sample. ATR difference spectra for the sample containing 32% PHU are presented in Figure 5 for untreated and for different gamma irradiation time. The ATR subtracted spectra show negative and positive bands for 2 hours gamma treated samples. The disappearance of CH 2 groups at 2940 cm 1 band, and also amide I IV may indict the degradation of urethane structure for 2 hours gamma irradiation time. For long-term gamma irradiation the bands form are similar but their intensities are larger. This phenomenon suggests an increase of the gamma radiation effects on the polymer network. The lost of urethanic carbonyl and the appearance of free radicals due to scissions of N-C bonds may also be seen. The urethanic groups are influenced by the gamma radiation due to the possibility of the free radicals to form new bonds with NH or OH groups. The effect of irradiation on hydrogen bonding is observed in ATR data on the O H band. Figure 5 shows a higher intensity of the O H stretching band as the radiation doses increase. This fact supports the influence of gamma irradiation on the interchain hydrogen bonding mechanisms [2].

7 7 Influence of gamma radiations on polymer membranes 355 Absorbance, a.u h treatment h treatment h treatment Wave number, cm -1 Fig. 5. ATR difference spectra for the sample containing 32% PHU for untreated and for different gamma irradiation time. S I (12% PHU) S II (16% PHU) Table 2 The bands assignment of the PVA-PHU alloys Wave number, (cm 1 ) S IV S V S VI Intensity (32% (50% (70% PHU) PHU) PHU) S III (25% PHU) L I VI I VI I Band assignment L large; I intense; VI very intense; M medium; W weak; δ bending vibrations; ν stretching vibrations. νoh νnh urethanic associated through H bonds ν(ch 2 ) asym ν(ch 2 ) sym νc=o in ester Amide I νc=o in urethane I (ν C-N + δ N-H ) Amide II M I δ(ch 2 ) asym (scissoring) W W VI νc-n şi δ N-H asym Amide III VI νc-0 in ester I νc-o-c asym, M W W δ CH 2 (bending and scissoring) νc-o-c sym

8 356 Cristina-Delia Nichifor, Dana-Ortansa Dorohoi, Constantin Ciobanu 8 The existence of oxidative processes within the material is characteristic for sterilization by gamma irradiation, resulting in IR absorbance bands in the spectral range of cm 1, due to carbonyl-containing chemical species such as ketones, esters, aldehydes, and acids [3 5]. The gamma radiation has also effect on the intensity of the relaxation mechanism associated to the vibrations of the CH 3, CH 2 groups [2] (wave number range cm 1 ), Fig. 5. The urethane groups are influenced by the gamma radiation due to the possibility given to the free radicals in realizing new bonds with NH or OH groups. Gaussian deconvolution of carbonyl picks of the sample containing 12% PHU for untreated and treated for 2, 4 and 10 hours are shown in Fig. 6. The bands corresponding for two types of carbonyl structures of the mixture with 12% PHU: ester (from PVA) and urethanic respectively (Fig. 6) are present in the spectral range of 1733 cm 1 and 1713 cm 1. Fig. 6. The Gaussian deconvolutions of carbonyl picks for untreated and treated 2, 4 and 10 hours sample containing 12% PHU.

9 9 Influence of gamma radiations on polymer membranes 357 The forms and number of the curves are modified with the treatment time. For 2 hours gamma irradiation the 1735 cm 1 pick decrease and 1711 cm 1 pick increases. This indicates a better structuration of the esteric and urethanic carbonyl. For long-term gamma treatment the number of deconvolution curves increase. Inter- and intra-molecular rotations and translations take place when the macromolecules of the mixture components absorb gamma radiations. The polymer surface became more structured [8]. The lost urethanic carbonyl for 4 and 10 hours treatment suggests that the esteric carbonyl became more structured comparing to urethanic carbonyl which is more sensitive to gamma radiations. Gamma irradiation is a common method of sterilization which determines the free-radicals formation. Oxygen can diffuse into the polymer over time, reacting with the free-radicals from polymer matrix and it causes chain scission. This is accompanied by internal cross-linking reactions inducing changes in physical properties of the polymer [2]. The water contact angle for the untreated and irradiated samples is plotted as function of irradiation time in Fig. 7. The surface wettability expressed by the water contact angles, may play a major role in adsorption of proteins onto the surface, as well as in cell adhesion. Cell adhesion is generally better on hydrophilic surfaces. It is known that changes in the physico chemical properties, which influence the hydrophilicity will modulate the protein adsorption and further cell attachment [14]. Contact Angle, degree Irradiated time, hours Fig. 7. Contact angles vs. irradiated time. 50% PHU 16%PHU 12%PHU 25%PHU 70%PHU 32%PHU One can observe that the gamma radiation decreased the contact angle from approximately 90 to 57 (Fig. 7). This can be due to the fact that the complex branch attached to polyethylene chain (-CONH-CH 2 -CH 2 -NH-CO-O-CH 2 -CH 2 -

10 358 Cristina-Delia Nichifor, Dana-Ortansa Dorohoi, Constantin Ciobanu 10 OH) may be scission by the gamma radiations, followed by free radicals formation. Gamma irradiation increases the number of OH groups (Fig. 5) the polymer becoming hydrophilic. In Table 3 are given the water contact angles for untreated and gamma treated samples. Table 3 The water contact angle for untreated and gamma treated samples SAMPLE UNTREATED TREATED 2H TREATED 4H TREATED 10H 12% PHU % PHU % PHU % PHU % PHU % PHU CONCLUSIONS The new polymers obtained by alloying PVA and PHU in different concentrations were gamma treated and analyzed (energetically, spectrally, and morphologically) in this study. Gamma radiations treatment decreases the roughness of the samples. This fact indicates that the sterilization process affect the film porosity. The combined ionizing radiation and oxygen action on the polymers may rapidly lead to a severe deterioration of the biomaterial properties. Two processes generally take place when the polymers are irradiated by ionizing radiation: crosslinking and degradation. The ATR-FTIR spectra of gamma treated samples relieved modifications of the picks absorbancies, but were not remarked the appearance and the disappearance of new bands. The lost of urethanic carbonyl and the appearance of free radicals due to scissions of N-C bonds may also be seen. These high-energy treatments result in free radicals generation, main chains scission and crosslinking, and subsequently alter the distribution of chains size in the bulk polymer. Inter- and intra-molecular rotations and translations may take place when the macromolecules of the polymer mixtures absorb gamma radiations. The lost urethanic carbonyl for 4 and 10 hours treatment suggest that the esteric carbonyl became more structured comparing to urethanic carbonyl which is more sensitive to gamma radiations. The urethanic polymer groups are more affected by gamma radiations compare to the esteric groups. Porosity of the samples, the hydrophilic properties, and chemical structures are the important properties studied in this work for untreated and in air gamma irradiated samples.

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