Effects of ion- and electron-beam treatment on surface physicochemical properties of polylactic acid

Applied Surface Science 422 (2017) 856–862

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Effects of ion- and electron-beam treatment on surface physicochemical properties of polylactic acid I.V. Pukhova a,b,∗ , K.P. Savkin b , O.A. Laput c , D.N. Lytkina a , V.V. Botvin a , A.V. Medovnik d , I.A. Kurzina a a

National Research Tomsk State University, 36 Lenin Ave, Tomsk 634050, Russia Institute of High Current Electronics, 2/3 Akademichesky Ave., Tomsk 634055, Russia c National Research Tomsk Polytechnic University, 30 Lenin Ave, Tomsk 634050, Russia d Tomsk State University of Control Systems and Radioelectronics, 40 Lenin Ave., Tomsk 634050, Russia b

a r t i c l e

i n f o

Article history: Received 13 February 2017 Received in revised form 8 June 2017 Accepted 8 June 2017 Available online 10 June 2017 Keywords: Polylactic acid Ion implantation Electron-beam treatment Surface modification

a b s t r a c t We describe our investigations of the surface physicochemical and mechanical properties of polylactic acid modified by silver, argon and carbon ion implantation to doses of 1 × 1014 , 1 × 1015 and 1 × 1016 ions/cm2 at energies of 20 keV (for C and Ar) and 40 keV (for Ag), and by electron beam treatment with pulse-width of 100–300 ␮s in 50 ␮s increments at a beam energy 8 keV. Carbonyl bonds ( C O) related IR peak was reduced after ion and electron beam irradiation. Molecular weight of PLA decreases twice and does not depend on the nature of the bombarding particles. The microhardness of treated samples decreases by a factor of 1.3, and the surface conductivity increases by 6 orders of magnitude after ion implantation, and increases only modestly after electron beam treatment. Atomic force microscopy shows that surface roughness increases with irradiation dose. Samples irradiated with Ag to a dose of 1 × 1016 ions/cm2 show the greatest roughness of 190 nm. © 2017 Elsevier B.V. All rights reserved.

1. Introduction Polylactic acid (PLA) is a biocompatible and biodegradable polymer used in biomedicine [1] primarily for implant manufacture. Biodegradable materials based on polylactic acid are widely used in biomedicine and tissue engineering because of their biocompatibility and their degradation to lactic acid in biological media. However PLA-based materials application for implants creation is limited by their adhesion characteristics and lack of functional groups for interaction with cellular media. To obtain materials with modified surface properties for specific applications including biomedical, ion/electron beam treatment [2–4,7,8], plasma processing [9], chemical grafting [10], as well as the combination of these methods [11] and excimer laser treatment [12] could be used as modification techniques. The use of ion-beam surface modification for the synthesis of new materials, modification of surface structure, formation of composite materials, and for generating predetermined surface patterns, etc., is a well-developed technology. Ion- and electron-

∗ Corresponding author at: National Research Tomsk State University, 36 Lenin Ave, Tomsk, 634050, Russia. E-mail address: [email protected] (I.V. Pukhova). http://dx.doi.org/10.1016/j.apsusc.2017.06.112 0169-4332/© 2017 Elsevier B.V. All rights reserved.

beam irradiation of polymers is widely used for polymer treatment due to their environmental friendliness and wide range of treatment conditions. The shallow ion penetration depth can modify the polymer surface functional properties while maintaining the original bulk properties of the material [2]. The chemical and physical processes leading to modification of the structural and physicochemical properties of polymer materials have been studied. It is known that ion irradiation techniques can significantly modify the chemical and functional property of materials. The ion irradiation caused scission of molecular chains in the samples and emission of volatile products from the surfaces. The exposure dose increasing leads to the ion average projected range enhancing and the irradiated part density decreasing [3]. Polymer chain scission accompanied by surface oxidation processes and new functional group formation, which contribute to the material hydrophilicity, occurs as a consequence of ion implantation [4]. It has been shown [5] that plasma treatment of polylactic acid surfaces causes roughness and increase in contact angle, and, in turn, increased roughness was found to improve PLA biocompatibility [6]. Electron beam treatment of PLA surfaces has been shown to cause polymer chain length transformation, with the molecular weight and degree of crystallinity decreasing proportionally to the exposure dose increase [7,8].

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Plasma treatment is often used as pre-treatment technology and the way of surface activation followed by distinct processes of coating, e.g. chemical modification by grafting of organic functional groups [9] and deposition of plasma polymerized acrylic acid on PLA surface [10]. It was found that applying of such combined methods of surface modification promote the wettability improvement, surface roughness increment, chemical composition alteration. Authors of [9] confirm that their approach permits to create spatially distributed properties on the sample, for instance, hydrophilic and bio-adhesive from one side and hydrophobic and bio-repulsive from the other side that is highly desired for the medical (implants) application. On the contrast, Y. Zhao, et al. [10] focus on food packaging industry to protect economically and effectively the food quality by usage of optimal combination of different gases in their methods of PLA modification. Chemical modification of PLA microspheres by aminolysis and grafting-coating [11] demonstrate the possibility to create the material with ability to support the attachment and proliferation of chondrocytes. These results show that the collagen-coated PLA microspheres are promising candidate for cell microcarriers. Other way of PLA surface modification is excimer laser treatment described in [12], where the surface wettability, morphology and roughness changes as well as mass loss by ablation are investigated. It is revealed that the contact angle decreasing is mainly associated with number of laser pulses increasing. The excimer laser has a strong effect on the polymer ablation; the mass loss is strongly dependent on the laser fluence and number of pulses. The literature shows that ion- and electron-beam treatment of PLA leads to a reduction in molecular weight and degree of crystallinity, increased hydrophilicity, and bioresorption. However, the mechanisms occurring in PLA under ion beam irradiation have not been investigated; there are no data of PLA surface properties after Ag, C, or Ar ion implantation. Thus exploration of the effects of various kinds of ions and ion implantation conditions on PLA functional characteristics is of interest. The aim of the work described here was to study the influence of implantation of various ion kinds (silver, argon, carbon) at exposure doses of 1 × 1014 , 1 × 1015 , and 1 × 1016 ions/cm2 , and electron beam treatment with pulse-width of 100–300 ␮s in 50 ␮s increments and beam energy 8 keV, on the surface physicochemical, functional and biological properties of PLA.

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the implantation beams include Ag2+ , Ar+ and C+ ions. Since the ion source extraction voltage was always 20 kV, the ion beam energies were 40 keV, 20 keV and 20 keV, respectively. For Ar+ ion generation we used the same ion source but somewhat modified to form a hollow cathode glow discharge mode [17]. Implantations were carried out to accumulated doses of 1 × 1014 , 1 × 1015 , and 1 × 1016 ions/cm2 . The implantation dose rate and average power density at the PLA target were adjusted by the ion beam current and pulse repetition rate, and were 1 × 1011 ions/(cm2 sec) and 0.5 mW/cm2 , respectively. The samples were mounted on a water-cooled target holder whose temperature did not exceed 20 ◦ C. A working pressure of 1 × 10−6 Torr was maintained by an oil-free high-vacuum cryogenic pump. 2.3. Electron beam treatment Electron beam processing was carried out using a repetitivelypulsed, forevacuum-pressure, plasma-cathode electron beam source based on a hollow-cathode glow discharge [18]. A working pressure of 5–10 Pa (37.5 − 75 × 10−3 Torr) was maintained by admitting air into the vacuum chamber. Electron beam irradiation was carried out by a series of 10 pulses with pulse duration 100 − 300 ␮s in increments of 50 ␮s at an energy of 8 keV and current density 4.5 A/cm2 . 2.4. Characterization techniques

2. Experimental

Chemical structure of implanted PLA was investigated by infrared spectroscopy using a single-attenuation total reflection attachment to a Nikolet 5700 IR-spectrometer. The molecular weight (MW) and molecular weight distribution (MWD) were determined by gel permeation chromatography on Agilent 1200 LC Infinity chromatograph with refractometer detector (AgilentTechnologies, USA)(eluent – chloroform). Surface morphology was studied by atomic force microscopy (AFM) using an NTEGRA Aura scanning probe microscope in tapping mode. Water and glycerol contact angles were measured by a sessile drop technique using a Kruss Easy Drop instrument. Surface energy calculation was done using the Owens-Wendt equation [19]. Microhardness was measured with a Nanotest 600 hardness testing instrument at a load of 0.5 mN. Surface electrical resistivity (also called sheet resistance) was measured using an E6-13A teraohmmeter and calculated from the equation

2.1. Preparation of PLA samples

 = Rb/l,

(1)

where R is the measured resistance, b = 3 mm is the distance between the contacts, and l = 10 mm is the electrode length. Graflex plates pressed tightly to a polymer sample were used as the contacts (electrodes). Surface resistivity is measured in units of Ohms/square (Ohm/䊐) [20,21].

PLA samples were prepared by dissolving polylactic acid ([OCН(CНз)-CO-]n) with molecular weight of 250 000 g/mol in chloroform at room temperature in a 7% solution [13]. The solvent was then removed by drying at room temperature in a Petri dish to form material with thickness ∼1 mm, then the PLA plates were cut into samples with area 10 × 10 mm2 .

3. Results and discussion

2.2. Ion implantation

3.1. Physicochemical properties of PLA

Ion implantation was done using a facility incorporating our MevvaV.Ru vacuum arc ion source [14,15]. This implantation facility operates in a repetitively pulsed mode with repetition rate 10 Hz and pulse duration 250 ␮s. Ion kinds used in the present work were Ag, Ar and C. Charge state distributions of the ion beams were measured by a time-of-flight mass-to-charge spectrometer [16]. In this kind of ion source, gaseous ions are singly ionized and hence we used Ar+ ions. Metal ions are in general multiply ionized; for carbon the charge state of the extracted ion beam is singly ionized C+ , while for silver the mean charge state of the extracted beam is 2+. Thus

IR-spectroscopic investigations reveal that the spectra of the initial and the ion- or electron-irradiated PLA samples are identical, with only absorption bands corresponding to the PLA functional group vibrations; see Fig. 1. Valence vibrations with wavenumbers 2944 cm−1 (symmetrical vibration) and 2996 cm−1 (asymmetrical vibration) correspond to the CН3, CН functional groups. Moreover, there are valence vibrations of the carbonyl group ( C O) with wavenumber 1759 cm−1 and the ( C( O) O) group with wavenumbers 1456, 1186, 1093, 1045 cm−1 . There are also ( C O C ) functional group bending vibrations with wave

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destruction of PL during ion implantation can be represented by the scheme: 3.2. Morphology and contact angle of PLA

Fig. 1. Infrared spectra of initial and ion implanted with dose of 1 × 1016 ion/cm2 and electron beam treated with pulse duration of 300 ␮s PLA samples.

Fig. 2. Molecular weight of initial and ion/electron beam treated polylactic acid.

number 872 cm−1 . New chemical bond formation following both ion implantation and electron beam processing is not detected. The intensities of the ␯( C O) carbonyl group valence vibration bands of the initial and ion/electron treated samples were compared by the baseline method. The results show that the C O band intensity reduces by ∼20%, which may be associated with decarbonylation and decarboxylation processes of the PLA macromolecular chains. The IR-spectroscopy results are fully consistent with X-ray photoelectron spectroscopy data shown in [22]. Polymer destruction processes that are accompanied by CO and/or CO2 exhalation and molecular weight decrease may occur following ion implantation and electron beam treatment. The molecular weight decreases twice after irradiation regardless of the ion kinds or exposure dose (Fig. 2) – the energy of the bombarding ions or electrons is sufficient to break the polymer chain. In addition, polymer chain rupture occurs, leading to the formation of free electrons. Silver ions implanted into a near-surface layer are restored to the metallic state by these electrons and form clusters in a subsurface layer [23,24]. A possible mechanism for the

The ion implantation or electron beam treatment should changes in surface properties such as wettability, which plays an important role in the biochemical processes that occur on the ostein–liquid border of living organisms. Contact angle measurements of the PLA surface were carried out by the sessile drop technique for two liquids: polar – water, and dispersive – glycerol. Wetting, as a phenomenon occurring at the contact of a liquid with the surface of a solid body, is characterized by a contact angle  whose vertex lies at the three phase contact point, with one side being the solid–liquid surface and the other side the tangent to the surface of the wetting liquid [25]. Poly-L-lactide is an inherently hydrophilic material; the initial PLA contact angle is 77◦ . Medical application of PLA includes many areas, from implant and tissue-substrate fabrication to materials with antimicrobial properties. Depending on the particular area field of implementation, the PLLA surface must have specific properties, including the hydrophobic or, on the opposite, hydrophilic feature. Fig. 3(a) shows water contact angle of initial and ion/electron treated polylactic acid. For Ar-implanted PLA the contact angle remains approximately equals to its initial value (77◦ ), but after C implantation it reaches the value 84◦ . Ag-implantation with a dose of 1 × 1016 ions/cm2 has the lowest contact angle, 62◦ . However, electron beam treatment is more effective for PLA wettability modification than ion implantation. The minimal contact angle of 48◦ occurs for 150 ␮s pulse duration electron beam treated PLA, and pulse duration increasing leads to a slight increase in contact angle (up to 55◦ ). Thus electron beam irradiation results in a reduction in PLA water contact angle and hence hydrophilic behavior improvement. The contact angles for PLA wetted with glycerol, plotted for various bombarding kinds and for similar exposure dose and pulse duration as for water, are shown in Fig. 3 (b). The initial glycerol contact angle of PLA is 65◦ . For Ar-implanted samples, the changes of glycerol contact angle are in the framework of the measurements error. With Ag implantation to a dose of 1 × 1014 ions/cm2 there is no significant change of wettability; a dose of 1 × 1015 ions/cm2 increases the contact angle to 67◦ , followed by a reduction to 61◦ for 1 × 1016 ions/cm2 . When PLA samples are irradiated with carbon ions at the maximum dose of 1 × 1016 ions/cm2 , the glycerol contact angle increases to 72◦ . Note that electron beam treated samples show lower contact angle than implanted samples, both for water and for glycerol. When PLA is electron beam treated with pulse duration of 100 ␮s, the contact angle reaches the lowest value of 45◦ . Increased pulse duration leads to a contact angle of 58◦ at 300 ␮s. Thus electron beam processing is more effective for PLA glycerol receptivity improvement than ion implantation. Surface energy has two components – dispersion (Van-derWaals Force and other non-specific interactions) and polar (strong interactions and hydrogen bonding). Changes in the surface energy of implanted materials are due to changes in the ratio of the polar to dispersive parts. Table 1 shows that the total surface energy does not change significantly, but increases by a factor of 1.5 with electron beam treatment. Note that for Ag implantation the dispersive part decreases with a simultaneous increase in the polar component (see Table 1). With Ar implantation the ratio of polar to dispersive component is not substantially changed; this is probably due to the inertia of the implanted argon. Carbon implantation of PLA at a dose of 1 × 1014 ions/cm2 decreases the dispersion component, and the polar part increases; then the polar part increases with increasing dose. Thus the ratio of the polar and dispersion components is changed, and

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Table 2 Roughness (Ra ) of initial and ion/electron treated PLA samples. PLA sample

Initial

Ag 1 × 1016 ions/cm2

Ar 1 × 1016 ions/cm2

C 1 × 1016 ions/cm2

electron beam 300 ␮s

Roughness (Ra ), nm

126

190

159

161

173

the total surface energy increases. We find that the dispersion component of surface energy decreases as a result of electron beam processing, with simultaneous increase in the polar components. It is known that the wettability of a surface is influenced primarily by two parameters: the presence of hydrophilic bonds and the surface roughness. Fig. 4 shows AFM images for the initial and ion/electron irradiated PLA samples. As can be seen from Table 2, the surface roughness increases in the following order: initial → C-implanted → electron-beam treated → Ar-implanted → Ag-implanted. According to Fig. 4(e), cratering is observed for electron beam treatment but not for ion implantation; electron beam bombardment may more effectively generate destructive processes, followed by chain scission. The roughness of the 300 ␮s pulse duration electron beam irradiated sample is 173 nm; the average diameter of craters is 116 nm. Clearly the surface energy increase is due to changes in surface roughness. Roughness and surface energy increase may enhance cellular adsorption on PLA surfaces. 3.3. Functional properties of PLA

Fig. 3. Contact angle of initial and ion/electron treated polylactic acid when wetted with (a) water, (b) glycerol. Table 1 Surface energy and its polar and dispersion components, for initial and ion/electron treated polylactic acid. PLA sample

Initial Ar 1 × 1014 ion/cm2 Ar 1 × 1015 ion/cm2 Ar 1 × 1016 ion/cm2 Ag 1 × 1014 ion/cm2 Ag 1 × 1015 ion/cm2 Ag 1 × 1016 ion/cm2 C 1 × 1014 ion/cm2 C 1 × 1015 ion/cm2 C 1 × 1016 ion/cm2 electron beam 100 ␮s electron beam 150 ␮s electron beam 200 ␮s electron beam 250 ␮s electron beam 300 ␮s

Surface energy, mN/m Dispersion

Polar

Total

21.96 ± 0.72 25.69 ± 0.94 17.13 ± 0.81 24.27 ± 0.36 13.5 ± 1.18 15.38 ± 0.54 9.79 ± 0.35 12.56 ± 0.65 7.71 ± 0.50 21.93 ± 0.35 17.88 ± 0.8 1.18 ± 0.58 2.3 ± 0.25 4.93 ± 0.58 13.52 ± 0.49

10.06 ± 0.52 9.55 ± 0.53 16.3 ± 0.64 9.09 ± 0.15 19.44 ± 0.97 14.43 ± 0.57 20.51 ± 0.59 21.49 ± 0.60 29.33 ± 0.96 6.58 ± 0.18 30.31 ± 0.82 49.05 ± 4.07 51.75 ± 1.26 41.75 ± 1.86 39.82 ± 0.82

32.02 ± 1.24 35.24 ± 1.47 33.44 ± 1.45 33.36 ± 0.51 32.94 ± 2.15 29.8 ± 1.10 30.3 ± 0.94 34.05 ± 1.26 37.04 ± 1.46 28.51 ± 0.53 48.19 ± 1.62 50.23 ± 4.65 54.05 ± 1.51 46.67 ± 2.44 53.34 ± 1.31

Functional properties determine the suitability for use and operation of materials for a predetermined lifetime, as well as affecting their processability in manufacturing. Physico-mechanical properties, such as microhardness and surface resistivity, are important for implant quality assessment. The microhardness values of polylactic acid at a load of 0.5 mN in the initial state and after ion and electron beam treatments are shown in Fig. 5(a). The microhardness decreases by a factor of 1.3 compared to the initial PLA. A minimal value of 0.32 GPa occurs for Ar-implantation at 1 × 1016 ions/cm2 and for 300 ␮s pulse duration electron beam treated PLA. This microhardness reduction of the surface layer of ion/electron treated samples may be associated with increased amorphous fraction to a depth of 60 nm (ion path length in the polymer), as well as with degradation processes caused by polymeric chain rupture. In addition, intense thermal effects may occur under ion/or electron beam irradiation, as well as ion implantation processes and electron-nuclear interactions. These processes could lead to a restructuring of the material and reduce the fractional content of ordered arrangement of polylactide macromolecules. Surface energy modification can significantly affect the bioavailability and surface cell absorption. Biodegradable implants, being in the environment of a living organism, are actively involved in physiological processes with hundreds of reactions that have different structure, kinetics and thermodynamics. Implants may have greater or lesser wettability, ability to adsorb cells that participate in electrochemical processes, and bioresorption characteristics. There is a huge amount of easily dissociated biochemical compounds and salts in the body, and information on the surface resistivity of pure PLA and Ag- and C- ion implanted PLA is of great interest. An important consequence of ion implantation into dielectric polymers is that the surface electrical conductivity increases. In the implantation process there are large releases of localized energy density, and the PLA is exposed to radiothermolysis resulting in mass scission of C H and some fraction of −C C bonds. This causes gradual polymer dehydrogenation and surface layer doping with carbon; a carbon-rich layer and polynuclear structures are

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Fig. 4. AFM images of PLA surface: (a) initial, (b) Ag-, (c) Ar- and (d) C- implanted with dose of 1 × 1016 ion/cm2 (e) electron beam treated with pulse duration of 300 ␮s.

formed. For each atom of carbon there is one unpaired electron that is able to contribute to ␲-binding. The surface electrical conductivity of ion-implanted polymers is not due to doping by the implanted ion kinds (except in special cases), but is caused by ␲electron cluster formation and is only mildly affected by ion kinds. The electrical conductivity is determined primarily by the energy transfer from the bombarding ions to the polymer matrix. Fig. 5(b) shows the measured PLA surface resistivity as a function of implanted dose for Ag, Ar, C ion implantation and against pulse width for electron beam treatment. The initial surface resis-

tivity is 2.1 × 1013 Ohms/square. Electron beam treatment has no significant effect on the PLA electrical conductivity. However, Ar implantation results in the surface resistivity decreasing linearly with increasing dose, by about six orders of magnitude at 1 × 1016 ions/cm2 . Implantation of Ag and C ions at doses of 1 × 1014 and 1 × 1015 ions/cm2 results in a slight decrease of one order of magnitude, which may be only measurement error; further increase of implantation dose to 1 × 1016 ions/cm2 leads to three orders of magnitude decrease ( = 4.5 × 1010 Ohm/sq.) for C, and a fur-

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activate significantly different radiation and radiation-pyrolytic processes by ion implantation (crosslinking, degradation of polymer chains at low implantation dose, and carbonization of the polymer at high implantation dose) provides a means to control the mechanical, optical and conductivity properties of the polymer surface or thin polymer film. The mechanism for sub-surface electrical conductivity formation may be based on direct charge transfer between conducting particles (clusters) formed by embedded silver or carbon atoms. On the other hand, “unconjugated” carbon atoms in the ion-modified sub-surface layer are inevitably formed due to macromolecule degradation by high energy bombardment. Thus changes in the surface resistivity may be the result of the cumulative effects of interstitial metal atoms and unconjugated carbon.

4. Conclusion

Fig. 5. (a) Microhardness and (b) Surface electrical resistivity of PLA as a function of dose for Ag, Ar and C ion implantation, and of pulse duration for electron beam treatment.

ther five orders of magnitude for Ag-implanted samples ( = 5 × 107 Ohm/sq.). With the accumulation of ion irradiation dose in the implanted layer, a conducting island system separated by potential barriers develops. The resistivity of the ion-implanted layer is strongly dependent on the carbonized phase structure (size of the carbon clusters, nature of their binding into aggregates, presence of heteroatoms). Thus the electrical resistivity of various polymers subjected to ion implantation under the same conditions can differ by several orders of magnitude. The conductivity of the ion-implanted layer is determined by the implantation conditions, in particular the energy of the implanted ions. Usually the polymer conductivity increases with implanted ion energy; the highest conductivity is observed when the bombarding ions transfer energy mainly by “electronic stopping” (ion slowing due to inelastic collisions with bound electrons). Thus the ability to

We have studied the effects of ion implantation and electronbeam treatment on the physicochemical and mechanical properties of polylactic acid (PLA). Silver, argon and carbon ion implantation was performed to doses of 1 × 1014 , 1 × 1015 and 1 × 1016 ions/cm2 at energies of 20 keV (for C and Ar) and 40 keV (for Ag), and electron beam treatment was performed with a series of ten pulses with a duration of 100–300 ␮s in increments of 50 ␮s and beam energy of 8 keV. IR spectroscopy showed that new chemical bonds are not formed by ion implantation and electron beam treatment. It was established that the C O band intensity reduces by ∼20% accompanied by CO and/or CO2 exhalation. The molecular weight decreases by a factor of two after ion implantation and is independent of ion kinds and implantation dose. The surface roughness increases with dose and maximum roughness (190 nm) is formed by Ag-implantation at 1 × 1016 ions/cm2 and 40 keV. Both ion and electron beam processing contribute to improved surface wettability when wetted by both polar and dispersion liquids (water and glycerin, respectively), as measured by a sessile drop technique. Electron beam treatment led to greater reduction in the contact angle than ion implantation. The surface energy of electronirradiated PLA is greater than for ion-implanted samples; changes in the total surface energy by ion and electron-beam irradiation are due to modification of the ratio of components. The reduction in the surface energy dispersion component occurs predominantly with a simultaneous increase in the polar component. The microhardness of ion and electron treated samples drops by a factor of 1.3, which can be attributed to amorphization and polymer destruction processes. Surface electrical conductivity increases by up to 6 orders of magnitude after ion implantation, and increases only modestly after electron beam treatment; surface conductivity enhancement by ion implantation is caused by free electron generation, together with decarbonization and destruction of PLA as well as conducting cluster formation. Our investigations reveal that ion implantation and electron beam processing provide good tools for polylactic acid surface physicochemical and mechanical property modification.

Acknowledgements This work was supported by the Russian Foundation for Basic Research (RFBR) Project # 15-08-05496, as well as The Tomsk State University competitiveness improvement programme (project # 8.2.06.2017). Special thanks are extended to the Centre for Collective Use NR TSU, the Research Centre “Nanomaterials and Nanotechnologies” NRTPU, and personally to V.P. Sharkeev, K.V. Oskomov, and N.V.Ryabova for their assistance and support.

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