ISSN 00181439, High Energy Chemistry, 2015, Vol. 49, No. 3, pp. 213–215. © Pleiades Publishing, Ltd., 2015. Original Russian Text © T.S. Demina, M.Yu. Yablokov, A.B. Gilman, A.I. Gaidar, T.A. Akopova, A.N. Zelenetskii, 2015, published in Khimiya Vysokikh Energii, 2015, Vol. 49, No. 3, pp. 242–244.
SHORT COMMUNICATIONS PLASMA CHEMISTRY
Vacuum Deposition of Chitosan Thin Films by Electron Beam Sputtering T. S. Deminaa, M. Yu. Yablokova, A. B. Gilmana, A. I. Gaidarb, T. A. Akopovaa, and A. N. Zelenetskiia a
Enikolopov Institute of Synthetic Polymer Materials, Russian Academy of Sciences, ul. Profsoyuznaya 70, Moscow, 117393 Russia email:
[email protected] b Research Institute for Advanced Materials and Technologies (NII PMT), ul. Malaya Pionerskaya 12, Moscow, 115054 Russia Received September 18, 2014; in final form, December 18, 2014
DOI: 10.1134/S0018143915030078
eposition of thin layers of natural materials onto the surface of hydrophobic synthetic polymers is used for enhancement of their biocompatibility. For example, the surface of matrices for regenerative medicine is frequently coated with chitosan, the deacetylation product of the natural polysaccharide chitin [1–3]. As a rule, a chitosan thin film is applied by casting a solu tion on a polymer surface after its preliminary modifi cation, for example, by plasma treating to create active sites responsible for high adhesion of the film to the substrate [4–6]. There are reports in the literature on the gasphase deposition of polytetrafluoroethylene (PTFE) on pol ished singlecrystal silicon substrates by irradiation of a polymer sample (target) with an electron beam of a 0.1–2.5 keV electron energy [7, 8]. It was shown that the first layers of the deposited film had the same com position and structure as those of parent PTFE, whereas a considerable amount of double bonds and oxygencontaining groups in the structure of the film was observed at its thickness of ≥0.3 µm. This method was also used for the deposition of lowdensity poly ethylene thin films on silicon and PTFE substrates [9]. It was found that the nature of the substrate has a noticeable effect on the crystallinity and chemical stricture of the films. In this study, we first used the electronbeam sput ter deposition of a polymer in a vacuum for preparing chitosan thin films on a poly(L,Llactide). Chitosanpoly[(1 → 4)2amino2deoxyβD glucose]derived by solidstate synthesis from crab shell chitin [10] was used. The chitosan molecular weight was 60 kDa and the degree of acetylation was 0.1 according to potentiometric titration and elemen tal analysis data. The target for electronbeam sputter ing was prepared by compressing a chitosan powder at a pressure of 0.1 MPa to have the target disk of 15 mm in diameter and 5 mm in thickness.
The substrates were films of poly(L,Llactide) (PLLA), a biodegradable, biocompatible, thermo plastic aliphatic polyester that is widely used for the manufacturing of resorbable medical articles, such as surgical sutures and pins. Semicrystalline PLLA with a molecular weight of 160 kDa and mp 165°C (available from Sigma) was used without further purification. Film samples of ~100 µm in thickness were prepared by casting a 5% solution in CH2Cl2 into Petri dishes. The films were dried at room temperature under equi librium conditions until complete dryness (~2 days). A chitosan film was applied by the electronbeam sputter deposition of the polymer in a vacuum from the active gas phase on a PLLA substrate using the procedure and setup detailed in [8]. The electron source was an electron gun with a heated filament cathode, which made it possible to form beams with a current density of 5–100 A/m2, an electron energy of 0.5–2 keV, and a cross section of (5–10) × 10–4 m2. The initial pressure of residual gas in the vacuum chamber was ∼10–3 Pa, and the substrate surface tem perature was ~300 K. The current density during the experiment was 20 A/m2, the electron energy was 1.4 keV, and the average film deposition rate was ~1 Å/s. The thickness of the chitosan film deposited under the aforementioned conditions was 80 ± 15 nm as measured with an MMI4 microinterferometer [11]. After the deposition of the chitosan film, the PLLA substrate surface took a light yellow color characteris tic of chitosan. The film was insoluble in a 4% acetic acid aqueous solution (conventional solvent for chito san), thereby suggesting that the deposited chitosan film has a crosslinked structure. Thus, a chitosan thin film was immobilized on a hydrophobic support using a solventfree method. The surface properties were characterized by values of the contact angle (θ) measured with an Easy Drop DSA100 instrument (KRUSS, Germany) and the
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DEMINA et al.
Contact angles (θ) of water and glycerol, works of adhesion (Wa) of water and glycerol, total surface energy (γ), and its polar (γp) and dispersion (γd) components for PLLA films, chitosan films obtained by casting from solution, and PLLA films coated which chitosan deposited by electronbeam sputtering θ, deg
Sample
γ, mJ/m2
Wa, mJ/m2
of water
of glycerol
of water
of glycerol
γ
γp
PLLA film
78
68
87.9
87.2
30.0
10.8
19.2
Chitosan film cast from solution
68
61
100.1
94.1
35.4
19.0
16.4
EBSdeposited chitosan film on PLLA
65
70
103.6
85.1
39.6
35.8
3.8
software suite Drop Shape Analysis V.1.90.0.14 using two test liquids, (deionized) water and glycerol (error ±1°). From the experimental values of θ, the work of adhesion (Wa), the total surface energy (γ), and its polar (γр) and dispersion (γd) components were calcu lated according to the procedure described in [12]. The table presents the data for the PLLA film, the chi tosan film deposited on the PLLA substrate using the electronbeam sputtering (EBS) procedure and the chitosan film cast from solution as described in [13, 14]. It is seen that the EBS deposition of chitosan
(а)
20 µm
(b)
20 µm SEM of the surface of (a) the initial PLLA film and (b) the chitosan film deposited on PLLA by electron beam sput tering.
γd
enhanced surface hydrophilicity and decreased the contact angle of water from 78° typical of initial PLLA to 65° after chitosan deposition. Note that θ of water on the chitosan film cast from solution is 68°. Coating with chitosan leads to a certain increase in Wa and γ (by ~30%), a growth in the polar component of sur face energy by a factor of 3.3, and an about fivefold decrease in the dispersion component. The surface morphology of the deposited chitosan film was studied using a Zeiss EVO 40 scanning elec tron microscope equipped with an XFlash 1106 sili con drift detector (SDD). To ensure charge draining, the sample surface was decorated with a thin conduct ing gold film. The microscope chamber was evacuated to an operating vacuum of 6 × 10–4 Pa, and measure ments were made in the highresolution mode at an accelerating voltage of 20 kV, a minimal probe current of 15–50 pA, and a minimal working distance of 5– 15 mm. The microphotograph in the figure shows that the initial PLLA film has a nonuniform semicrystal line structure (image a). The chitosan film applied by EBS deposition repeats the substrate morphology, fill ing the defect areas, and facilitates the formation of a more homogeneous surface structure (image b), a development that agrees well with the decrease in the dispersion component of surface energy (see table). Note that the EBSdeposited film can differ from the film of native chitosan in not only surface morphology, but also the chemical structure. Since its formation results from the electron beaminduced partial bond scission in the chitosan macromolecule, this mecha nism calls for further investigation. We believe that not only the scission of glycoside bonds connecting the polymer constituting units, but also the degradation of the pyranose rings themselves occur in the chitosan structure. The deposition of chitosan fragments in both cases should be accompanied by their simulta neous crosslinking. This conclusion is confirmed by the fact that the layer obtained is insoluble in the con ventional solvent for chitosan (4% acetic acid aqueous solution). In summary, chitosanbased thin films have been obtained for the first time using vacuum deposition by electronbeam sputtering. It has been shown that the deposition of chitosan on the surface of a poly(L,L lactide) film leads to the formation of a more homoge HIGH ENERGY CHEMISTRY
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neous and hydrophilic surface of the material as com pared with the neat substrate. The authors are grateful to V.P. Kazachenko for fruitful discussion.
9. Liua, Z., Rogachev, A.V., Zhou, B., Yarmolenko, M.A., Rogachev, A.A., Gorbachev, D.L., and Jiang, X., Progr. Org. Coat., 2011, vol. 72, no. 3, p. 321.
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11. Semenov, I.V., Gil’man, A.B., Yablokov, M.Yu., Surin, N.M., Shchegolikhin, A.N., Shmakova, N.A., and Kuznetsov, A.A., High Energy Chem., 2011, vol. 45, no. 2, p. 157. 12. Wu, S., Polymer Interfaces and Adhesion, New York: Marcel Dekker, 1982. 13. Demina, T.S., Yablokov, M.Yu., Gilman, A.B., Akop ova, T.A., and Zelenetskii, A.N., High Energy Chem., 2012, vol. 46, no. 1, p. 60. 14. Demina, T., ZaytsevaZotova, D., Yablokov, M., Gil man, A., Akopova, T., Markvicheva, E., and Zelenetskii, A., Surf. Coat. Technol., 2012, vol. 207, no. 1, p. 508. Translated by S. Zatonsky
