Nanomechanical properties of plasma treated ...

Plastics, Rubber and Composites Macromolecular Engineering

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Nanomechanical properties of plasma treated polylactic acid E. P. Koumoulos, M. Valentin, D. A. Dragatogiannis, C. A. Charitidis, I. Krupa & I. Novak To cite this article: E. P. Koumoulos, M. Valentin, D. A. Dragatogiannis, C. A. Charitidis, I. Krupa & I. Novak (2015) Nanomechanical properties of plasma treated polylactic acid, Plastics, Rubber and Composites, 44:8, 322-329 To link to this article: http://dx.doi.org/10.1179/1743289815Y.0000000023

Published online: 18 Dec 2015.

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Date: 10 January 2016, At: 03:08

Nanomechanical properties of plasma treated polylactic acid E. P. Koumoulos1, M. Valentin2, D. A. Dragatogiannis1, C. A. Charitidis*1, I. Krupa2 and I. Novak2

Downloaded by [Orta Dogu Teknik Universitesi] at 03:08 10 January 2016

In this work, a radio frequency discharged plasma generated in air atmosphere by pressure has been used to modify polylactic acid (PLA) surface. The results were evaluated through nanoindentation testing. Contact angle measurements revealed a gradual transition to a more hydrophilic state with increasing polarity after plasma treatment, while partial recovery to their untreated state during 10 day storage in air was evidenced. The results were evaluated through nanoindentation testing. All PLA samples exhibited an almost hard-like surface area where hardness and elastic modulus are enhanced. The activity of the plasma creates a higher cross-linking density within the material in the surface region. For higher displacements, both H and E tend to reach pristine PLA’s values. Hardness values reveal surface hardening due to plasma treatment except for 180 s etching time, where hardness is slightly decreased possibly due to surface deformation. The change of H/E slope reveals the strengthening of oxygen plasma etched PLA with 180 s of etching time with increasing displacement. Keywords: Plasma treating, Polylactic acid, Nanomechanical properties

Introduction Nowadays, polymeric materials are used for a wide variety of applications. The choice for a specific polymer is usually based on its bulk properties (e.g. mechanical, physical), which are mainly determined by the bulk chemistry and morphology of the processed material. For many applications, the polymer’s surface properties are of major importance. The desired surface properties can be obtained by modification of the surface chemistry and/or the surface structure of a polymer. Gas plasma treatment has proven to be a versatile tool for surface modification of polymeric materials.1–14 Gas plasma (glow discharge) is a partially ionised gas, which can be generated by an electrical discharge. Thus, a highly reactive environment is created with plasma species like electrons, ions, radicals and metastables. Besides being a dry (solvent free) and time efficient process, the main advantage of this technique is its confinement to the surface layer of a material. Using gas plasma treatment, a variety of advantageous, interfacial properties can be introduced at a polymer surface without affecting the desired bulk properties of a material like strength, toughness and biodegradability. Two types of gas plasma treatment, i.e. plasma etching and plasma polymerisation (deposition), are commonly

1

Department of Chemical Engineering, National Technical University of Athens, 9 Heroon, Polytechnioust., Zografos, Athens GR-157 80, Greece Polymer Institute, Slovak Academy of Science, Dubravskacesta 9, Bratislava 824236, Slovakia

2

*Corresponding author, email [email protected]

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Ñ Institute of Materials, Minerals and Mining 2015 Published by Maney on behalf of the Institute Received 22 March 2013; accepted 01 June 2015 DOI 10.1179/1743289815Y.0000000023

used for changing the surface chemistry of a polymer. In contrast to chemical surface modification of polymers by gas plasma treatment, tailoring of the surface structure (e.g. morphology, topography, roughness) as a means of changing the material’s surface properties has found far less applications. Structural and chemical surface modification of phase separated polymer systems (e.g. semicrystalline polymers, block copolymers, organic–inorganic hybrid materials) is of importance for tailoring a wide variety of interfacial properties. Surface characteristics like wettability, adhesion, printability, friction, fouling and biocompatibility largely determine the applicability of polymeric materials (e.g. thermoplastic objects, coatings, films, membranes, fibres, textiles, biomaterials) in various industrial areas (e.g. automotive, packaging, filtration, clothing, biomedical technology). Furthermore, nanostructuring of phase separated polymer systems has become increasingly important for the fast growing area of nanotechnology. Nanopatterned polymer templates have a huge potential as etching masks in nanolithographic processing.15–22 Preferential plasma etching is a promising method for surface nanostructuring of a phase separated polymer system.19,23,24 This method is based on the principle that different polymers show different etching rates when exposed to a gas plasma.25–27 In addition, a substantial difference in etching rate usually exists between the crystalline and the amorphous phase of a semicrystalline polymer.12,28 The nanoscale architecture of the structured surfaces obtained by preferential plasma etching is governed by the phase separation of the polymer system (crystalline lamellae/spherulites, cylinders or spheres in a continuous matrix, bicontinuous structure, etc.). Therefore, using preferential plasma etching as

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a surface modifying technique a diversity of nanostructured polymer and hybrid surfaces with various interfacial properties is feasible. Gas plasma treatment processes (glow discharges) are extensively used for the surface modification of polymeric materials (e.g. thermoplastic films, fibres, non-wovens, membranes, biomedical devices). The main advantage of this versatile technique is that it is confined to the surface layer of a material without affecting its bulk properties. Moreover, it is a dry (solvent free), clean and time efficient process with a large variety of controllable process parameters (e.g. discharge gas, power input, pressure, treatment time) within the same experimental set-up. However, due to the complexity of the plasma process and the variety of chemical and physical reactions that can occur, the exact chemical and structural composition of a plasma treated surface is hard to predict. Detailed descriptions of the theory of plasma treatment processes can be found in the literature.2,6,10,29–35 Basically, plasma etching can proceed through three different pathways. First, a polymer substrate is etched by chemical reaction of reactive plasma species (e.g. radicals, ions) with the surface, referred to as chemical etching. Second, ion bombardment on a polymer surface causes sputtering of the surface, which is a physical process. Finally, UV radiation from the plasma phase causes dissociation of chemical bonds, which leads to formation of low molecular weight material. In general, these three etching mechanisms occur simultaneously during the plasma treatment of a polymer and induce a flow of volatile (low molecular weight) products from the substrate to the plasma, causing a gradual weight loss of the treated polymeric material. Besides the three etching pathways discussed above, an intermediate type of plasma etching can be distinguished. Reactions between a polymer substrate and neutral species from the plasma phase (chemical etching) can be accelerated by ion bombardment (ion enhanced etching).36 For example, oxygen plasma etching of polymers proceeds at relatively low rates in the absence of energetic ions. The combined action of chemically reactive and accelerated ions results in anisotropical etching of the polymer surface. This combined process, which is also commonly referred to as reactive ion etching, is often used for patterning of surfaces in the semiconductor and (nano)lithographic industry.5,17,33,37–43 Anisotropicity (directionality) can only be achieved when the substrate is properly biased, i.e. when the substrate functions as the cathode with a (self-) bias voltage. This important feature distinguishes reactive ion etching from other plasma etching techniques. Reactive ion etching does not affect all polymers equally. Some polymers exhibit a higher etching rate than others, and the etching rate for a given polymer depends on the plasma conditions. Plasma treatment is mostly used to enhance the surface energy of a polymer. Oxygen or nitrogen containing groups are introduced on the surface of a (biodegradable) polymer when the material is exposed to cold plasma generated in O2, N2, air or NH3.43–45 These functionalities are polar hydrophilic groups, which are formed during the interaction of the plasma active species with the polymer molecules. Next to oxygen and nitrogen containing discharges, plasmas generated in

Nanomechanical properties of plasma treated PLA

pure helium or argon will lead to the creation of free radicals that can be used for cross-linking or grafting of oxygen containing groups when the surface is exposed to oxygen or air after the treatment. Finally, it should be mentioned that the induced surface characteristics are not permanent; the treated surfaces will tend to partially recover to their untreated state during storage in, e.g. air (so called hydrophobic recovery), and they will also undergo post-plasma oxidation reactions.46 ˚ min21) of a polymer largely The etching rate (A depends on the plasma treatment conditions (e.g. discharge gas, pressure, discharge power, substrate temperature and treatment time) and on the polymer’s chemical and physical properties. Based on a difference of etching rates, the most susceptible polymer segments in a phase separated polymer system (e.g. block copolymers, semicrystalline polymers, polymer blends) can be preferentially removed by plasma etching. In general, plasma etching will reach an equilibrium state (i.e. constant etching rate) in the course of treatment time. Initial fluctuations of the etching rate may arise due to changes of surface temperature (see above) and surface chemistry. For example, cross-linking in the surface layer of the substrate during plasma treatment could suppress the initial etching rate of a polymer. From previous studies, it is known that semicrystalline polymers can show preferential etching behaviour (see also previous section). In these cases, preferential etching of the amorphous phase was observed. This difference in etching rates allows the examination of the crystalline structure (e.g. lamellae, spherulites) after the amorphous phase has been removed.47

Experimental In this work, a radio frequency (rf) discharged plasma generated in air atmosphere by pressure 100 Pa has been used to modify PLA surface (Fig. 1). The modification by the capacitively coupled rf plasma was performed in a laboratory rf plasma reactor working at reduced pressure 100 Pa, consisting of two 240 mm brass parallel circular electrodes with symmetrical arrangement, 10 mm thick, between which rf plasma was created. The electrodes of rf plasma reactor are placed in a locked up stainless steel vacuum cylinder. The one is powered, and the other one is grounded together with a steel cylinder. The voltage of rf plasma reactor was 2 kV, frequency was 13.56 MHz, current intensity was 0.6 mA and the maximum power of the rf plasma source is 1200 W. Commercial plain amorphous PLA [poly(D,L-lactic acid)] film from NatureWorks PLA was purchased, without any coating material or prior treatments, with an average molecular weight of 28 000 Da. The PLA samples were modified by rf plasma at the power of 300W. Polylactic acid pressed samples were prepared from PLA granules in 190uC. Before plasma treatment, the samples were subjected to vacuum for 48 h, by temperature of 50uC. The rf plasma discharge was applied for 30, 60, 90, 120, 180 and 240 s. Contact angle measurement was performed using a prototype contact angle microscope, which is comprised of a moving metal table, a micropipette (5–50 mL), a light source with a diffuser and two cameras with |200 and |500 focus capacity. The two cameras are supported by a metallic frame, vertical to the plane of


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1 a schematic representation of plasma etching process and b rf plasma reactor chamber

the table. The |500 camera is used to study the surface morphology of the samples, and the |200 camera is used for the capture of the contact angle measurements. The position of the table for the contact angle measurement is set so that the sample is at a level to the |200 camera. The software used to operate the cameras is the Microviewer, and the images captured are processed in Adobe Photoshop and ImageJ software. The light source and diffuser are placed on a level and opposite to the |200 camera, in order to make the drop limits more visible. Before the sample measurement, a calibration picture is first obtained with the Rame´-Hart combo calibration device. The sample for measurement is then placed in front of the |200 camera. The edge of the micropipette should be as close to the surface as possible, without touching it, in order to avoid creating additional forces on the droplet, other than the interfacial tensions. The release of the droplet on the sample surface is recorded, and the frame taken is processed in Adobe Photoshop so that it is in greyscale mode and the droplet boundaries are easily visible. The drop analysis is carried out using the DropSnake plugin module, where the drop profile is set using points placed on the drop surface. The process is five times repeated per sample, for statistical purposes. Nanoindentation testing was performed with an HysitronTriboLab nanomechanical test instrument, which allows the application of loads from 1 to 10 000 mN and records the displacement as a function of applied loads with a high load resolution (1 nN) and a high displacement resolution (0.04 nm). The TriboLab employed in this study is equipped with a scanning probe microscope (SPM), in which a sharp probe tip moves in a raster scan pattern across a sample surface using a three-axis piezo positioner. In all nanoindentation tests a total of 10 indents are averaged to determine the mean hardness H and elastic modulus E values for statistical purposes, with a spacing of 50 mm, in a clean area environment with 45% humidity and 23uC ambient temperature. In order to operate under closed loop load or displacement control, feedback control option was used. All nanoindentation measurements have been performed with the standard three-sided pyramidal

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Berkovich probe, with an average radius of curvature of *100 nm.48 Based on the half space elastic deformation theory, H and E values can be extracted from the experimental data (load–displacement curves) using the Oliver–Pharr method,49 where derived expressions for calculating the elastic modulus from indentation experiments are based on Sneddon’s elastic contact theory (equation (1)) Er ¼

S p 1=2

ð1Þ

2bA1=2 c

where S is the unloading stiffness [initial slope of the unloading load–displacement curve at the maximum depth of penetration (or peak load)], Ac is the projected contact area between the tip and the substrate and b is a constant that depends on the geometry of the indenter (b ¼ 1.167 for Berkovich tip).50,51 Conventional nanoindentation hardness refers to the mean contact pressure; this hardness, which is the contact hardness Hc, is actually dependent upon the geometry of the indenter (equations (2)–(4)) Hc ¼

F Ac

ð2Þ

where Ac ðhc Þ ¼ 24; 5h2m þ a1 hm þ a1=2 h1=2 m þ ... 1=16 þ a1=16 hm

ð3Þ

and hc ¼ hm 2 1

Pm S

ð4Þ

where hm is the total penetration depth of the indenter at peak load, Pm is the peak load at the indenter displacement depth hm and 1 is an indenter geometry constant, equal to 0.75 for Berkovich indenter. Before indentation, the area function of the indenter tip was calibrated in a fused silica, a standard material for this purpose.52

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2 Images (SEM) (10610 mm2) of three main etching times are presented, i.e. a 0 s, b 120 s and c 180 s of etching time


Results and discussion In Fig. 2, SEM images of the three main etching times are presented, i.e. 0, 120 and 180 s of etching time, where significant differences of particle size distribution is revealed. Figure 3 shows higher magnification views of the samples, through SEM (presented in Fig. 2). Contact angle measurements revealed a gradual transition to a more hydrophilic state (Fig. 4 ) with increasing polarity after plasma treatment, while partial recovery to their untreated state during 10 day storage in air was evidenced, in accordance with similar work of the literature.46 In Fig. 5, the SPM image of surface for 180 s of etching time is presented; nanoindentation points were carefully selected after SPM imaging so as to obtain accurate results. The relation (input function) of displacement change to time for the materials examined in this work is plotted in Fig. 6 [schematic trapezoidal load–time P¼P(t) input function]. Hardness and elastic modulus values versus displacement (H, E) of PLA samples are presented below (Figs. 7 and 8), where surface affected hatched area is noted; all PLA samples exhibit an almost hard-like surface area where H and E are enhanced. As the tip penetrates further, both H and E tend to reach pristine PLA’s values. The plasma treatment created topographical change, which is due to the interactions of plasma species and thermal effect.53 However, the bulk

structure of the film remains almost unchanged as is suggested by thermal analysis and measurements of water vapour permeability measurements.53 Surface cross-linking is often used to enhance the performance of polymers. The activity of the plasma creates a higher cross-linking density within the material to a depth of a few thousand angstroms. The resulting increase in hardness and chemical resistance can be used to enhance performance in many applications. For example, silicone rubber components treated in inert gas plasma can be modified to form a hard ‘skin’ on the surface. This results in a substantial decrease in surface tack and coefficient of friction. Recently, a plasma immobilisation process has been developed to directly cross-link precoated molecules onto polymer surfaces. The molecules immobilised by this process can be organic compounds, surfactants, polymers or proteins, and do not require unsaturated double bonds in the molecules.54 Air plasma mainly adds oxygen atoms to the PLA surfaces. After plasma treatment in air, the concentration of C–O and O–C ¼ O groups increases, while the C–C and C–H functional groups decrease.46 In Fig. 9, the hardness versus etching time for oxygen plasma etched PLA, at *100 nm of displacement, is presented. Hardness values reveal surface hardening due to plasma treatment except for 180 s etching time, where hardness is slightly decreased possibly due to surface deformation. The ratio of hardness/elastic modulus is of significant interest in tribology. Higher stresses are expected in high

3 Images (SEM) (363 mm2) of three main etching times are presented, i.e. a 0 s, b 120 s and c 180 s of etching time


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4 Contact angle measurements for all etching times, immediately after etching and after 10 day storage

5 Scanning probe microscope imaging (30630 mm2) of plasma treated PLA surface

H/E, hard materials and high stress concentrations develop towards the indenter tip, whereas in the case of low H/E, soft materials, the stresses are lower and are distributed more evenly across the cross-section of the material.55,56 The high ratio of hardness/elastic modulus (H/E) is indicative of the good wear resistance in a disparate range of materials: ceramic, metallic and polymeric (e.g. c-BN, tool steel and nylon respectively),56,57 which are equally effective in resisting attrition for their particular intended application. In Fig. 10, the change of H/E slope reveals the strengthening of oxygen plasma etched PLA with 180 s of etching time with increasing displacement.

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Conclusions In this work, an rf discharged plasma generated in air atmosphere by pressure has been used to modify PLA surface. Contact angle measurements revealed a gradual transition to a more hydrophilic state with increasing polarity after plasma treatment, while partial recovery to their untreated state during 10 day storage in air was evidenced. The results were evaluated through nanoindentation testing. All PLA samples exhibited an almost hard-like surface area where hardness and elastic modulus are enhanced. The activity of the plasma creates a higher

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6 Schematic trapezoidal of load–time P5P(t) function for nanoindentation experiment

7 Hardness versus displacement for oxygen plasma etched PLA

8 Hardness versus displacement for oxygen plasma etched PLA

cross-linking density within the material in the surface region. For higher displacements, both H and E tend to reach pristine PLA’s values. Hardness values reveal surface hardening due to plasma treatment except for 180 s etching

time, where hardness is slightly decreased possibly due to surface deformation.The change of H/E slope reveals the strengthening of oxygen plasma etched PLA with 180 s of etching time with increasing displacement.


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9 Hardness versus etching time for oxygen plasma etched PLA, at ,100 nm of displacement

10 Hardness/modulus ratio for oxygen plasma etched PLA

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