Low energy oxygen ion beam modification of the surface morphology and chemical structure of polyurethane fibers

NIM B Beam Interactions with Materials & Atoms

Nuclear Instruments and Methods in Physics Research B 243 (2006) 63–74 www.elsevier.com/locate/nimb

Low energy oxygen ion beam modification of the surface morphology and chemical structure of polyurethane fibers K.H. Wong

a,*

, M. Zinke-Allmang a, W.K. Wan b, J.Z. Zhang c, P. Hu

d

a

b

Department of Physics and Astronomy, The University of Western Ontario, London, Ont., Canada N6A 3K7 Department of Chemical and Biochemical Engineering, The University of Western Ontario, London, Ont., Canada N6A 3K7 c Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China d Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Received 24 November 2004; received in revised form 15 July 2005 Available online 15 September 2005

Abstract Energetic O+ ions were implanted into polyurethane (PU) fiber filaments, at 60 and 100 keV with doses of 5 · 1014 and 1 · 1015 ions/ cm2, to modify the near-surface fiber morphology. The implantations were performed at room temperature and at
197
C, a temperature well below the glass transition temperature for this system. At room temperature, the lower energy implantation heats the fibers primarily near their surface, causing the fiber surface to smoothen and to develop a flattened shape. At the higher energy, the ion beam deposits its energy closer to the fiber core, heating the fiber more uniformly and causing them to re-solidify slowly. This favors a cylindrical equilibrium shape with a smooth fiber surface and no crack lines. The average fiber diameter reduced during 100 keV implantation from 3.1 to 2.3 lm. At
197
C, the ion implantation does not provide enough heat to cause notable physical modifications, but the fibers crack and break during subsequent warming to room temperature. The dose dependence of the crack formation along the fiber intersections is presented. The ion beams further cause near-surface chemical modifications in the fibers, particularly introducing two new chemical functional groups (C–(C@O)–C and C–N–C).  2005 Elsevier B.V. All rights reserved. PACS: 79.20.R; 81.65; 61.82.P; 79.60; 61.16.B Keywords: Polyurethane fibers; Ion implantation methods; Scanning electron microscopy (SEM); X-ray photoelectron spectroscopy (XPS); Surface morphology; Surface chemical modification

1. Introduction Ion beam implantation techniques have proven to be successful tools to modify the electrical and optical properties of semiconductors, and the mechanical properties of alloys. So far, they have not been systematically applied to unconventional materials, such as biomaterials, even though it is known that they may assist in creating cell compatibility [1] or cell attachment [2] due to chemical modifications. The past lack of focus on these materials *

Corresponding author. Tel.: +1 519 661 2111x86436; fax: +1 519 661 2033. E-mail address: [email protected] (K.H. Wong). 0168-583X/$ - see front matter  2005 Elsevier B.V. All rights reserved. doi:10.1016/j.nimb.2005.07.224

can partially be contributed to physical damage observed at the surface due to ion beam heating as the ions collide with the target atoms. The directions of ion beam modification research in unconventional materials are currently focused on polymer conductivity [3] and tribological properties of polymer surfaces [4]. For biological and medical applications, we expect the lower bond strengths within macromolecules to allow the formation of free radicals, which in turn stimulates chemical reactions [5]. These processes should therefore enable the formation of new functional groups. Such chemical modifications are interesting as they may lead to improved surface properties, including enhanced cell adhesion of a scaffold in tissue engineering [6], and the

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prevention of biological infection [7]. In some cases, changing the hydrophilic properties of polymers has been reported [8]. Improvements of the bio- and cell-compatibility of soft biomaterials is highly desirable, but other physical parameters deserve similar attention, such as the mechanical properties, permeability and porosity [9], stress/strain properties [10], and surface wear resistance [11] as these factors play a significant role in the bioactivity. A particular material of interest for medical applications is polyurethane. It is generally accepted that polyurethane is biocompatible, and it has been used to fabricate both external and implantable medical devices. Recent studies have further suggested that ion implantation is capable to improve the anticoagulability, red cells attachment, and the wettability of polyurethane [12,13]. In this work, the implantation of oxygen ion beams at two different energies, 60 and 100 keV, and two different doses is studied. We compare the virgin and ion beam modified fibers for physical alterations due to mechanical stress and the deposition of thermal energy during implantation. 2. Experiment 2.1. Materials Polyurethane (PU) was synthesized at Tsinghua University from polyethylene glycol (PEG) and poly(e-caprolactone) (PCL) as a multi-block polyurethane (PU) through a one-step condensation co-polymerization process using hexamethylene diisocyanate (HDI) as a coupling agent (PEG to PCL ratio is 6:1). The chemical structure of the monomer unit of PEG–PCL multi-block polyurethane is shown in Fig. 1. The polymer was converted into fiberform by an electro-spinning process. The fibers were deposited onto a grounded electrode surface to form non-woven sheets of 0.63 ± 0.02 mm thickness. The polyurethane fibers produced in this fashion are highly non-uniform in diameter and shape. They display further a domain structure with the aggregation of hard (PCL sections) and soft (PEG sections) segments. The fraction of space occupied by single strands in the fiber was determined from SEM micrographs, such as shown in Fig. 2. We found a value of 60% for the first layer coverage, from which an effective density of 0.43 g/cm3 ±10% was calculated. 2.2. Ion beam implantation The PU fiber sheet was mounted on a nickel sample holder and placed inside a high vacuum system with the pressure maintained at 9.0 · 10
9 Torr during implantation. Oxygen ion beams at energies of 60 and 100 keV were implanted, using the 1.7 MV tandetron accelerator at The

Fig. 2. SEM micrograph of a virgin polyurethane fiber sample. The length scale is indicated at the left in the lower bar.

University of Western Ontario. Two dosages were used, 5 · 1014 and 1 · 1015 ions/cm2, and the implantation was done with a beam current of 0.5 lA to avoid excessive heating, but to provide a sufficiently short implantation time of about 5 and 10 min, respectively. The nickel sample holder was kept at either room temperature (25
C) or cooled with liquid nitrogen to a temperature of
197
C. The maximum penetration depth of 60 and 100 keV oxygen ions in polyurethane is 1.2 and 1.8 lm, respectively, as determined from TRIM simulations [14]. With a mean fiber strand diameter of 3.1 ± 0.7 lm, the ions do not penetrate through any individual fiber strand completely. Thus, the depth profile of the collision transfer energy through recoil (nuclear loss energy profile as a function of depth) is the most critical parameter as it causes distinguishable results for the two ion beam energies. As the recoiled atoms slow down, most of the kinetic energy is converted into heat depositing thermal energy inside the fiber. 2.3. Sample analysis with SEM and XPS The morphology of the polymer fiber sheets in this work are examined with a Leo 1530 scanning electron microscopy (SEM), from LEO Electron Microscopy Ltd. The polymer samples are highly non-conductive. To avoid charging of the surface, a conductive layer (5–7 nm) of gold is coated onto the fiber using a sputter coater. While this layer may obscure the true outer surface morphology, it is important to note that the diameter of the fibers is >1 lm and the primary objective of this study are the structure and shape of the fiber network. The conductive coating for SEM is much thinner. A relatively low 2 kV beam voltage is used in this work to provide higher surface sensitivity on the polymer fibers.

Fig. 1. Chemical structure of a monomer unit of PEG–PCL multi-block polyurethane.

K.H. Wong et al. / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 63–74

The elemental chemical composition near the sample surface is analyzed with an XPS system from Kratos Axis Ultra. The Al-Ka monochrome (210 W) source tests the samples in the hybrid mode (slot) with 90
take-off angles. The survey scan has a pass energy of 160 eV and the highresolution scan has 20 eV. The sampling depth is no more than 15 nm into the surface. Data analysis and multi-peak fitting was performed with the software package CasaXPS. 3. Experimental results 3.1. The virgin surface morphology

65

bardment. Three maximum annealing temperatures were used: 100, 115 and 130
C. Each is held for 8 min. The resulting morphologies are shown in Fig. 4(a)–(c), respectively. Fig. 4(a) shows the polyurethane sample annealed at 100
C. Note that the heating source provides radiation in the visible range, which does not penetrate into the fiber. This distinguishes the thermal annealing from the ion beam annealing process where energy is primarily deposited below the fiber surface. During the annealing, the viscosity of the partially liquefied polymer decreases sufficiently to allow the top part of the fiber to flatten. We will discuss

Using SEM, the virgin sample morphology is established in Fig. 2 before choosing the energy for ion implantation. We chose arbitrarily 280 fibers within a 120 lm · 75 lm area in a set of SEM micrographs. The fiber diameter distribution is shown in Fig. 3 as open bars. The mean value is 3.1 ± 0.7 lm with error attributed to the counting statistics. The diameter of the fibers has the values between 1.6 and 4.4 lm; with approximately 25% of the fibers displaying a diameter between 3.0 and 3.4 lm. More than 80% of the fibers have a diameter exceeding 2.8 lm. The SEM micrograph in Fig. 2 also shows that most of the fibers are not circular in shape and that many smaller sized fiber strands are entangled to form large diameter fibers, contributing a non-uniform surface morphology. 3.2. Thermal annealing of the virgin fiber Ion implantation generates a significant amount of heat inside or near the surface of a micro-sized fiber. To distinguish later effects due to the ion beam and to calibrate the thermal energy deposition, thermal annealing of samples at various temperatures was undertaken. The heating rate during annealing was set at 20
C/s. This is a rather rapid rate applied to simulate the heating effect during ion bom-

80

100 keV O+

Number of fiber

60

Original Sample

40

20

0 1.2

1.6

2.0

2.4

2.8

3.2

3.6

4.0

4.4

4.8

Diameter (µm) Fig. 3. Fiber diameter distribution of a virgin sample (open bars) and a sample after 100 keV O+ implantation to a dose of 5 · 1014 ions/cm2 at room temperature (solid bars). The data are based on 280 fibers selected arbitrarily.

Fig. 4. SEM micrographs of a sample after thermal annealing for 8 min at (a) 100
C, (b) 115
C and (c) 130
C in a nitrogen gas ambient.

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later that this is a surface tension driven result for a fiber, which has been softened only for part of its surface. As the surface energy of the polymer does not vary significantly between the solid and the softened state, the liquefied polymer wets the solid core and bottom parts of the fiber. All physically entangled fibers and fibers touching at intersections are now fused together. Fig. 4(b) represents the sample at 115
C after 8 min annealing. The higher temperature annealing lowered the viscosity of the polymer, further softening the single strands deep down to the core. As a result, the same effects observed at 100
C become enhanced with polymer fibers strongly flattened throughout the surface layer. This sample illustrates that annealing from the surface progressively flattens the samples, irrespectively whether or not the samples are liquefied throughout or whether the sample cores remained solid. Note also the large number of cracks in the strands after cooling to room temperature, which was not observed after annealing at 100
C. Fig. 4(c) shows the sample after annealing at 130
C for 8 min. The fiber structure is now completely destroyed at the surface: the thermal energy fully liquefies all near-surface polymer strands causing them to flow together to form almost a closed sheet. Thus, the polymer has completely flattened, i.e. a continuation of the effect observed at 100 and 115
C. There is still some residual fiber structure visible underneath the polymer sheet. Thus, the complete destruction of the first layer precedes the liquefying of the second layer, which still provides mechanical support.

Fig. 5. SEM micrographs of a sample after 60 keV O+ implantation to a dose of (a) 5 · 1014 and (b) 1 · 1015 ions/cm2 at room temperature.

3.3. O+ implantation at room temperature (60 keV) The morphological sample structure after oxygen ion implantation at 60 keV is illustrated in Fig. 5(a) and (b); Fig. 5(a) is the result of the smaller dose of 5 · 1014 ions/cm2 and Fig. 5(b) is a representative micrograph at 1 · 1015 ions/cm2. Comparing these two figures with Fig. 2, we note that the accumulation of the implantation energy causes the polymer to soften and to liquefy in both cases. Studying Fig. 5(a) first, we observe a notable and consistent change in the shape of the topmost layer of fibers. Compared to the virgin sample, each fiber is flattened with a slightly upwards-bending curvature towards the visible fiber edges. The loss of cylindrical symmetry is later attributed to the near-surface ion beam bombardment. The fiber displays no crack formation upon removal from the ion beam chamber (room temperature transfer to the microscope). At the higher dose (Fig. 5(b)), the morphology of the top layer of the fiber has been altered in a similar fashion as shown in Fig. 5(a), but the sample displays extensive crack formation. Since the ion current in the experiment remained the same (as the implantation time was doubled to achieve the higher dose), we note that the crack formation cannot be attributed to the ion flux. However, the final temperature profile of each fiber differed for the samples in

Fig. 5(a) and (b) because of the longer ion beam exposure in the latter case. Under these conditions, we note that crack formation is most likely occurring during re-solidification when ion implantation has ceased because individual fibers undergo a larger variation in temperature and, correlated, a larger variation in volume density due to the difference of thermal expansion coefficients in the PEG and PCL domains. The uneven contraction introduces a larger strain during the heating and cooling cycle. 3.4. O+ implantation at room temperature (100 keV) A clearly different morphological modification of the polyurethane fibers is observed when we use a 100 keV O+ ion beam with an implantation to the lower dose of 5 · 1014 ions/cm2 at room temperature. The resulting morphology is shown in Fig. 6(a), and is compared to a higher dose implantation at the same energy in Fig. 6(b). Fig. 6(a) shows that the ion beam at 100 keV did not cause any major damage (structural deterioration) at the fiber surface. Instead, after cooling, the sample morphology is more uniform with fiber shapes displaying a near-cylindrical shape (equilibrium shape). The average fiber diameter has decreased as quantitatively shown in Fig. 3 (solid bars), with mean fiber diameter of 2.4 ± 0.8 lm. The error margin is again due to counting statistics. This means that

K.H. Wong et al. / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 63–74

Fig. 6. SEM micrographs of a sample after 100 keV O+ implantation to a dose of (a) 5 · 1014 and (b) 1 · 1015 ions/cm2 at room temperature.

most fiber diameters shrank by about 25%. Note that this effect was not observed for any of the thermally annealed samples in Fig. 4. The fibers in Fig. 6(a) shows no signs of cracking, consistent with the low dose sample at the lower implantation energy and the lowest temperature thermal annealing. Result of the 100 keV O+ implantation at the higher dose of 1 · 1015 ions/cm2 is shown in Fig. 6(b). It displays several of the same morphological changes as Fig. 6(a) but shows significant crack formation. Thus, we identify the higher dose as ‘‘overdosing’’ with respect to desirable morphological changes as crack formation does not improve the fiber morphology. 3.5. Oxygen implantation at
197
C Ion implantations were also performed at
197
C, because this temperature is well below the glass transition temperature of polyurethane. Once the polymer is below its glass transition temperature, the fibers become very hard and brittle. Without any ion beam treatment, fibers display no difference from their original morphology after returning back to room temperature. Fig. 7 shows the resulting morphology after ion implantation of a dose of 1 · 1015 ions/cm2 at 60 keV (Fig. 7(a)) and 100 keV

67

Fig. 7. SEM micrographs of a sample after (a) 60 keV and (b) 100 keV O+ ion implantation to a dose of 1 · 1015 ions/cm2 at a sample temperature of
197
C.

(Fig. 7(b)). Extensive disruptions of the fiber strands with formation of wide gaps are observed for both energies. 3.6. XPS results Fig. 8 shows XPS survey spectra of a virgin polyurethane sample (lower part) and a sample after 100 keV O+ implantation of 5 · 1014 ions/cm2 at room temperature (upper part). The elemental composition of the virgin sample is confirmed with the main peaks of the constituent atoms of the molecule: C1s (285.0 eV) with 78.4%, O1s (532.1 eV) with 18.3%, N1s (399.8 eV) with 2.2% and 1.1% for other impurities. From the stoichiometric formula we know that every two nitrogen atoms represent a single urethane functional group with four oxygen atoms. Therefore, the remaining 13.9% of oxygen atoms are either located in the PEG or the PCL sections. The observed impurities account for contaminations attached to the fiber surface during the experiments, mostly containing silicon. The XPS survey spectrum of the implanted polyurethane sample shows no significant change in the elemental composition compared with the virgin sample. The same observations apply to the XPS survey spectra for the other samples prepared at 60 and 100 keV O+ with beam doses

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Intensity (arb. unit)

O 1s

N 1s

C1s

100 keV O+ implanted

Untreated

1000

800

600

400

200

0

Binding energy (eV) Fig. 8. XPS survey scan spectra of virgin polyurethane and after oxygen ion implantation at 100 keV. (Three major peaks are labeled as C1s, O1s and N1s, they are located at 285.0, 532.1 and 399.8 eV, respectively.)

of 5 · 1014 and 1 · 1015 ions/cm2. This shows that the implanted O+ ions do not account for a surface doping effect, because they are not stopped within 15 nm depth (XPS sampling depth) at either energy. Fig. 9 shows the high-resolution XPS carbon 1s state spectra of the virgin polyurethane sample (bottom) and for the sample implanted at 100 keV with 5 · 1014 ions/ cm2 oxygen at room temperature (top). Three peaks are identified in the spectrum and are labeled A, B and C for the virgin sample. The lowest intensity of the spectrum is assigned to peak A (3.4%), which is identified [15] as either O–(C*@O)–N or O–(C*@O)–C at 289.2 eV, which are two different functional groups in the urethane or PCL sections. (Note that the * notation indicates the atom from which the signal originates.) The binding energies of these two functional groups overlap and form a single peak. A simi-

N 1s

C

C1s

Intensity (arb. unit)

C Untreated

100keV O + Implanted

B

D

A

E

F

100 keV O+ Implanted

Intensity (arb. unit)

lar overlap occurs for peak B (36.4%), which represents the C*–O and C*–N bonds (286.5 eV) in the molecule. Based on the PEG–PCL multi-block polyurethane chemical structure in Fig. 1, the two C*–N bonds are the only connection between two urethane functional groups and the methylene backbone. Therefore, of the 36.4% assigned to peak B, less than 3.4% accounts for the C*–N bond. The functional groups represented in peak A also contain the signal from C*–O functional groups in the PEG backbone. Thus, the C*–O bonds in the PEG sections cannot account for more than 30%. The remaining 60.2% is assigned to peak C at 285.0 eV, which represents the signal from either C*–C or C*–H bonds, which are part of the backbone of the PEG and PCL sections and occur in the urethane groups. The XPS carbon 1s spectrum of the ion implanted sample in the upper part of Fig. 8 displays several changes: the ion beam implantation destroys some of the urethane and ester (PCL) functional groups as the contribution of peak A drops from 3.4% to 2.3%. Peak B also drops from 36.4% to 26.6%, implying that some of the C*–O or C*–N bonds in the PEG sections are destroyed. The fraction of peak C has increased from 60.2% to 67.4%. A new peak at 287.8 eV, labeled peak D, is detected and represents a new functional group that is created during the O+ ion implantation. This peak is associated with a C–(C*@O)– C bond. This bond accounts for 3.8% in the sample. Fig. 10 shows the high-resolution XPS spectra for the nitrogen 1s state of the virgin polyurethane sample (lower part) and the same implanted sample as shown in Fig. 10 (upper part). For the virgin sample, all nitrogen atoms in the polyurethane molecule are located in the urethane groups forming an O–(C@O)–N* functional structure. This is labeled peak E at 400.0 eV in the spectrum and accounts accordingly for 100%. After oxygen ion implantation a new peak at 399.0 eV (labeled F) has appeared.

B

E Untreated

A

289

287

285

283

Binding energy (eV) Fig. 9. XPS carbon 1s core level spectra of virgin polyurethane and a sample ion implanted with 100 keV oxygen. (The peak A, B, C and D are located at 289.2, 286.5, 285.0 and 287.8 eV, respectively.)

402

401

400

399

398

Binding energy (eV) Fig. 10. XPS nitrogen 1s core level spectra of virgin polyurethane and a sample ion implanted with 100 keV oxygen. (The peak E and F are located at 400.0 and 399.0 eV, respectively.)

K.H. Wong et al. / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 63–74

100 keV O+ implanted H

Intensity (arb. unit)

only by 0.3 eV, which our analytical method does not allow to separate. Thus, the new functional group detected from the carbon 1s state spectrum cannot be detected in this spectrum because of the overlap of the two signals.

G

O1s

69

I

4. Discussion G

4.1. O+ implantation at room temperature (60 keV) Untreated

535

534

Fig. 12 summarizes in five conceptual steps the development of the final morphology for the lower energy implantation. We discuss the frames of the figure in detail, illustrating that the ion beam softens a crescent-shaped volume near the top surface of the fiber, which then flattens due to surface tension and ion beam hammering. A more detailed analysis of the thermal energy distribution allows us to predict a final convex–concave morphology as observed in Fig. 5. The detailed development of the morphology is based on TRIM simulations of the implantation process. In Fig. 13, the TRIM simulation for a 60 keV O+ beam is displayed with the curve labeled C representing the electronic energy loss and the curve labeled D representing the nuclear stopping contribution. The figure shows that both processes are completed (i.e. the ions are stopped) within 1.2 lm below the fiber surface, which represents only about 1/3 of the total cross-section of a typical individual fiber strand. Thus, both types of energy deposition occur primarily near the top surface (the surface exposed to the ion beam) as indicated in part A of Fig. 12. The nuclear stopping causes polymeric chain scissions and energy transfer to recoil atoms. As the recoiled atoms slow down, most of the kinetic energy is converted to heat causing an increase in thermal energy inside the fiber. This process represents the dominant energy transfer to the sample and is used to determine the energy distribution in the fiber. A detailed TRIM simulation is presented in Fig. 14, where a cylindrical sample shape has been assumed and a uniform ion beam deposition from the top is considered. This model is based on the mean fiber diameter of a virgin sample determined from Fig. 3. Due to its geometry, the central part of the fiber receives a maximum thermal energy while sections further to the left and right receive a decreasing

I

H

533

532

531

530

Binding energy (eV) Fig. 11. XPS oxygen 1s core level spectra of virgin polyurethane and a sample ion implanted with 100 keV oxygen. (The peak G, H and I are located at 532.6, 533.7 and 531.6 eV, respectively.)

This peak corresponds to a new C–N*–C functional group with a significant ratio of 31.1%. This means that 31.1% of the near-surface urethane functional groups have been modified. Fig. 11 shows the corresponding high-resolution XPS spectra for the oxygen 1s state for the same samples. For the XPS spectrum of the virgin sample, the peak labeled G (532.6 eV) dominates the spectrum with 75.8%. This peak represents the signal originating from the functional group C–O*–C in the PEG sections. Peaks labeled H (533.7 eV) and I (531.6 eV) have comparable intensities, because peak H is due to signals from C–(C@O)–O* and N–(C@O)–O* groups, and peak I is due to signals originating from the other oxygen atom in the same functional groups: C–(C@O*)–O and N–(C@O*)–O. The observed ratios in the spectrum confirm this analysis with 11.5% and 12.6%, respectively. The spectrum of the implanted sample matches the one for the virgin sample, including the ratios of the individual peaks. Note that peak G was earlier interpreted as the signal from C–O*–C bonds, but it also includes C–(C@O*)–C because both peaks differ

60 keV O+ at room temperature

A

B

C

D +

E

Fig. 12. Schematic model of the ion beam heating effect due to 60 keV O implantation. (A) Virgin sample, (B) ion beam soften the top crescent of the fiber, (C) heat redistribution, ion beam hammering and ion energy deposition cause flattening of the fiber top, (D) continuous ion beam exposure causes deeper sections of the fiber to be softened, and (E) the fiber solidifies with a concave–convex shape.

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the top surface is similar toward the surface (where radiation loss establishes a thermal sink) and toward the deeper sections of the fiber. However, the cross-sectional area for heat to pass toward the surface is larger than toward the fiber core due to the crescent shaped profile in Fig. 14. Thus, heat flow favors softening toward the surface versus softening of deeper parts of the fiber; a liquefying section near the surface of the fiber develops with upwards-curved edges at the sides. In the next step toward sketch C in Fig. 12 we apply the Young–Dupre equation, which connects the surface tension terms of a liquid drop on a solid surface:

8

A

eV/Angstrom/Ion

6

B C

4

D

2

cðsvÞ ¼ cðdsÞ þ cðdvÞ cos h.

0 0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

1.8

Depth (µm) Fig. 13. Electronic energy loss of (A) 100 keV and (C) 60 keV as well as nuclear energy loss of (B) 100 keV and (D) 60 keV O+ ions in multi-block polyurethane versus penetration depth as calculated with TRIM.

amount of energy. Thus, the ion beam energy heats and liquefies first the polymer in a shallow depth below the top end of the fiber (Fig. 12, sketch B). We now consider the dissipation of the deposited energy. Heat conduction is governed by FourierÕs law, which states that the heat flow is proportional to the temperature gradient and the cross-sectional area through which the heat flows. In Fig. 14, the color-scale represents the profile of the deposited energy and therefore allows estimating the temperature gradients in the polyurethane, which has a low thermal conductivity. Note that the color-scale in Fig. 14 is not linear to highlight the non-linear energy distribution of Fig. 13. The thermal gradient from the elliptical core near

In which c(ds) is the interfacial energy between drop and substrate, c(dv) is the surface tension drop to vacuum, and c(sv) is the surface tension of substrate to vacuum. h is the contact angle between the substrate and the drop. In our case, the liquefied material of the fiber and the solid fiber substrate are chemically very similar, and therefore the interfacial energy c(ds) is negligible (c(ds)  c(sv) ffi c(dv)). The Young–Dupre equation then predicts a contact angle h near 0
(wetting). Thus, the liquefied material will spread out across the solid (non-flowing) substrate to minimize its surface. The resulting shape closely resembles the surface of the solid part of the fiber forming a flat surface at the top of the fiber, which is exposed to the ion beam (sketch C in Fig. 12). This is the reason why the softening of the fiber surface from one direction (ion beam or thermal heat source) causes the top section of the fiber to flatten. The two ion doses applied in our study did not cause a significant progressing of this process beyond this point, however, the three samples studied at various annealing temperatures

Fig. 14. A cross-section recoiled energy distribution profile of a cylindrical fiber implanted with 60 keV oxygen ions based on TRIM data. The color bar ˚ . (For interpretation of color in this figure readers are referred to web version of this represents a non-linear scale and refers to values in units of eV/A article.)

K.H. Wong et al. / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 63–74

in Fig. 4 illustrate that this process continues as sustained energy deposition across the flattened fiber surface eventually reaches deeper and deeper. This slow increase in the fraction of liquefied polymer in the fiber results in a progressive flattening as indicated in sketch D of Fig. 12. While this step is not observed with the ion beam, we note that the ion implantation favors the flattening of the surface further due to the so-called hammering effect [16,17], which operates even before the fiber is sufficiently heated to show liquid flow-related effects. In the hammering effect, the ion beam transfers continuously momentum to the sample; the softened film responds to this momentum transfer by spreading sideways. The final convex/concave shape indicated in sketch E of Fig. 12 is a result of the thermal cooling after the implantation step is completed. At that point, the liquefied polymer returns to its solid form, which is accompanied by a volume contraction. With the interface liquefied/solid remaining stable, the surface of the liquefied material will carve in to accommodate the volume change. The greatest change occurs where the liquefied layer is thickest, i.e. at the top of the fiber as defined by the plane perpendicular to the direction of the ion beam. It is noteworthy that the thermal annealing in Fig. 4 does not develop the same degree of distinct convex/concave structure. This must be primarily associated with the unique energy distribution of the ion implantation compared to a more surface-centered heating in thermal annealing. One of the objectives of this study was to find the threshold for crack formation due to excessive heating in the fiber network. Cracks will alter the degradation mechanism of a biodegradable material, originally designed to degrade via surface erosion. Cracks add extra attack-points for surface erosion on the fibers, which may particularly accelerate the destruction of the fabric network because most cracks are observed at fiber intersections. This observation is caused by the fact that the entire fiber network undergoes thermal expansion and contraction processes simultaneously. As the volume changes during re-solidification, each fiber in the network contracts in directions toward fixed pivot points; therefore, the stress applied on each fiber by connected fibers differs from the original values. The change in volume pushes or pulls the intersection during the solidification process and creates the cracks. Note that samples implanted at the lower dose of 5 · 1014 ions/cm2 (Fig. 5(a)) show no crack formation. In turn, samples prepared with an ion dose of 1 · 1015 ions/cm2, i.e. samples that were exposed to the ion beam for twice the time (Fig. 5(b)), display cracking at many fibers. Since the energy of the ion beam is not changed, the high dose sample encounters the same morphological changes during the first 5 min of implantation as the one at the lower dose (sketched in Fig. 12). Therefore, most of the observed surface morphological changes are similar to those observed at the lower dose. However, the extra dose represents an additional amount of heat deposited in the fiber, which therefore will cool

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from a higher initial temperature when the ion implantation process is terminated. Greater temperature variations cause greater stress in the fiber. When this stress exceeds the elastic limit, cracks form. This happens predominantly near intersections, where more heat has to be dissipated and where stress occurs in four rather than two directions. 4.2. O+ implantation at room temperature (100 keV) The most important difference when using a 100 keV oxygen beam is that the energy loss is spread more evenly over a range of almost 1.6 lm. This is illustrated in the respective TRIM simulations as shown in Fig. 13. In that figure, the nuclear energy loss at 100 keV (curve labeled B) has a peak that occurs significantly deeper than for the 60 keV implantation (curve labeled D) and therefore closer to the center of the fibers. Fig. 15 provides a simulation of the deposited amount of recoiled energy as a function of depth in a typical cylindrical fiber. Note that the major heating occurs from the surface to almost the center of the fiber. Again, the figure uses a non-linear color-scale for the deposited energy: the bulk of the energy is deposited along about 2/3 to 3/4 of the full width of the fiber strand. Applying FourierÕs law in this case predicts a different evolution of the heat dissipation. The shallow gradient toward the surface but the steep gradient into the fiber favors heat flow toward the center. Also, the area through which heat flows toward the center is now a larger fraction of the entire area enveloping the center of initial heat deposition; thus, both factors, the cross-sectional area for heat flow and the driving force of the temperature gradient favor heat dissipation toward the core of the fiber. As a result, the 100 keV beam interacts stronger with the core of the fibers in the size range larger than 2.0 lm diameter (which represents about 90% of fibers near the surface of the virgin sample). The mechanism proposed in analogy to the discussion on the low energy implants is illustrated in Fig. 16. The development of the final morphology is divided into five sketches labeled L–P. Initially, ions penetrate into the virgin polymer (labeled L). As the number of ions increases, the fiber temperature rises in a zone well beneath the fiber surface, as suggested by the simulation in Fig. 15 (equivalent to sketch M in Fig. 16). A softened region emerges deep in the fiber surrounded by a cooler and harder surface shell as the temperature gradient favors the heat to flow toward the fiber core. As a result, the whole fiber core liquefies, which differs from the preferential near-surface liquefaction at 60 keV (compare sketch N in Fig. 16 with sketches C and D in Fig. 12). As the heat flows outwards to soften the shell in the higher energy implantation, eventually the whole fiber turns into a softened state (labeled O in Fig. 16). In this stage, a re-shaping occurs toward the equilibrium shape with minimum surface energy (labeled P). The equilibrium shape is a cylindrical structure with a smooth surface. In particular, fiber strands formed

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Fig. 15. A cross-section recoiled energy distribution profile of a cylindrical fiber implanted with 100 keV oxygen ions based on TRIM data. The color bar ˚ . (For interpretation of color in this figure readers are referred to web version of this represents a non-linear scale and refers to values in units of eV/A article.)

100 keV O+ at room temperature

L

M

N

O

P

Fig. 16. Schematic model of the core heating effect for oxygen ion implantation at 100 keV at room temperature. (L) Virgin sample, (M) initial heat deposition by ion beam, (N) ion beam heating and heat redistribution cause softening of the fiber core, (O) continuous ion beam exposure causes entire fiber to soften, and (P) solidified fiber structure after implantation with reduced fiber diameter.

initially from physical entanglement during electro-spinning are now completely liquefied and fused together to form single fibers. Morphological effects due to the hammering effect, such as compaction near the surface of the fibers during the implantation, no longer lead to observable alterations because the whole fiber has softened prior to cooling during the post-implantation stage. During the re-solidification process after the implantation, the fiber core continuously dissipates heat outwards as the heat has to escape through the polymer surface. This is a slow process due to the low thermal conductivity of the polymer and the low rate of loss of surface heat in the vacuum system. This process is sufficiently slow to allow the fiber to reach the small specific volume and high density characterizing its equilibrium: this is established in a quantitative form in Fig. 3 which shows that the mean diameter of the fibers shrinks from 3.1 to 2.4 lm after the 100 keV O+ implantation at room temperature. As the fiber diameter shrinks by about 25%, the fiber network structure

encounters a high stress. This stress stretches and tightens every fiber in the network as seen in Fig. 6(a). The mechanism shown in Fig. 16 applies also to the 100 keV implantation at the higher dose of 1 · 1015 ions/ cm2. Like in the case of implantation at 60 keV, cracks are observed after implantation. 4.3. O+ implantation at
197
C When the polymer is cooled below its glass transition temperature, the fibers become hard and brittle. In this stage, the hydrogen bonds between PCL sections of different chains aggregate to form a denser domain. There are no hydrogen bonds between PEG sections; therefore, the volume shrinkage is not uniform. This effect further enhances the already non-uniform density distribution in the domain structure of the polyurethane sample. As a result, the specific volume of the fiber decreases very unevenly below the glass transition. Neck formation along a fiber requires the

K.H. Wong et al. / Nucl. Instr. and Meth. in Phys. Res. B 243 (2006) 63–74

sample to exceed a critical stress. This occurs when the fibers in the network pull strongly against each other as they shrink. A neck forms in a region of greatest stress concentration or in a region with higher density of polymer chains entangled together. Due to the large mass of the nickel sample holder in the implantation stage, the cooling process takes more than 20 min to reach
197
C; thus, the volume shrinkage occurs slowly and the sample maintains its original crack-free morphology (as tested with a sample which was cooled and brought back to room temperature without ion implantation). Fig. 7(a) and (b) shows the resulting morphologies after ion implantation at
197
C for the two different implantation energies. The extensive crack formation and subsequent spreading of the fiber ends is consistent with the models we introduced above. The oxygen ions provide sufficient energy to break chemical bonds in the backbone of the polymer chain. At neck regions this weakening due to the ion beam increases the likelihood of breakage because the damage to each chain is irreversible as each end of a broken polymer chain pulls away from the breaking point due to the stress induced into the fiber network by cooling. This effect is self-enhancing as additional stress is then transferred to unbroken chains. Once the total number of unbroken chains cannot sustain the stress transferred to them, an entire strand brakes. The ends of the strand separate macroscopically, which prevents the strands to reconnect during warming to room temperature. A fiber network contains regions where the network is better able to accommodate stress and polymer chains break less likely or can be repaired easier. Therefore, cracks occur relatively close to each other and are usually concentrated in certain regions on the surface.

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4.4. Chemical analysis It is important to understand the difference in focus of this section compared to the previous discussion: the sampling depth of the chemical analysis technique applied in this study (XPS) is about 25 nm; i.e. we no longer study entire fiber strands but only a shallow surface zone. The first change this causes is that we focus now on the near-surface electronic energy loss of the ion beam rather than the energy distribution profile along the entire ion range inside the fiber. The electronic energy loss interaction at the fiber surface increases proportional to the energy and dose of the ion beam. At the same time, the passing ions are not seen as oxygen atoms implanted (as they travel much deeper into the fiber) but as ionizing particles with an ionization cross-section correlated to the electronic energy loss density ˚ . Table 1 is the summary of the peak ratios in units of eV/A from the XPS spectra taken from samples implanted with 60 and 100 keV O+ beams at two different doses; the table also includes for comparison the thermally annealed sample at 115
C. Note that the thermally treated sample shows no chemical modifications. The ion beam modified samples in turn display two new functional groups, C–(C*@O)–C and C–N*–C. These are visible in the C1s and N1s highresolution spectra, respectively. The ratios of both functional groups show a tendency to increase with the total electronic energy deposited at the surface. Thus, chemical alterations near the surface occur as an exclusive result of the ion beam, not as an artifact of the thermal energy deposited in the implantation process. For the samples implanted at
197
C we note that the specific volume of the polyurethane is lower in the glassy state. Thus, at the low temperature polymer molecules

Table 1 Summary of the chemical modifications found in the C1s and N1s state high-resolution XPS spectra for a virgin sample, a sample thermally annealed at 115
C and the ion beam modified samples with ion implantation at room temperature Peak

Functional groups Virgin sample (%) 115
C annealed (%) 60 keV O+ 5 · 1014 ions/cm2 (%) 60 keV O+ 1 · 1015 ions/cm2 (%) 100 keV O+ 5 · 1014 ions/cm2 (%) 100 keV O+ 1 · 1015 ions/cm2 (%)

A

B

C

D

E

F

O–(C*@O)–N or O–(C*@O)–C 3.4 3.2 3.0 2.7 2.3 2.1

C*–N or C*–O 36.4 37.9 31.4 28.2 26.6 25.2

C*–C or C*–H 60.2 58.9 62.5 65.6 67.4 68.5

C–(C*@O)–C 0 0 3.1 3.5 3.8 4.3

O–(C@O)–N* 100 100 80.8 73.6 68.5 61.6

C–N*–C 0 0 19.2 26.4 31.3 38.4

Table 2 Summary of the chemical modifications found in the C1s and N1s state high-resolution spectra for the ion beam modified samples with implantation at
197
C Peak

Functional groups (at
197
C) 60 keV O+ 5 · 1014 ions/cm2 (%) 60 keV O+ 1 · 1015 ions/cm2 (%) 100 keV O+ 5 · 1014 ions/cm2 (%) 100 keV O+ 1 · 1015 ions/cm2 (%)

A

B

C

D

E

F

O–(C*@O)–N or O–(C*@O)–C 3.3 2.8 2.8 3.1

C*–N or C*–O 27.0 27.7 27.3 26.9

C*–C or C*–H 66.1 66.2 66.5 66.8

C–(C*@O)–C 3.6 3.3 3.4 3.2

O–(C@O)–N* 70.2 60.2 68.5 61.6

C–N*–C 29.8 39.8 35.9 41.8

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are packed denser causing a stronger electronic energy loss in a shallower depth zone than at room temperature. Table 2 summaries the C1s and N1s state high-resolution XPS spectra of the samples implanted at
197
C. The ratio of C–N*–C bonds, obtained from the ion beam induced peak in the nitrogen 1s state spectrum, shows that the higher electronic energy loss at the surface causes an increased occurrence of this chemical modification, i.e. the values are higher than for the sample implanted at room temperature with the same energy and dose. However, from the carbon 1s state spectrum we find that the corresponding ratios stay constant for the different implantation energies and doses, even though new C–(C*@O)–C functional groups are still formed. It is not clear at this point why the ion beam has a higher efficiency to chemically modify the nitrogen chemical functional group in the polyurethane at lower temperatures, but does not display that efficiency in modifying carbon based bonds. 5. Conclusions Ion beam implantation techniques were applied to modify the near-surface morphology of a polyurethane-based multi-block polymer. Modifications of this type are interesting because they allow us to concurrently alter physical and chemical parameters to improve the surface properties for biological or medical applications. We studied the ion beam interaction with the fiber material with respect to thermodynamic and mechanical processes. We found that individual fiber strands and the global surface morphology can be altered as a function of ion beam energy and ion beam dose. In particular, re-shaping towards a higher uniformity of diameters was achieved. Using an oxygen beam at 100 keV with a low dose of 5 · 1014 ions/cm2 at room temperature, the morphological improvements were accomplished with no significant crack formation along the fiber strands. XPS data highlight a key difference be-

tween ion beam modification and thermal annealing of the fiber network: two new functional groups, C– (C*@O)–C and C–N*–C, are created in the polyurethane during the ion interaction with the polymer chains. Acknowledgments This work was supported by the Natural Sciences and Engineering Council of Canada (NSERC). We acknowledge technical assistance in the XPS analysis by Ross Davidson (Surface Science Western). References [1] X. Chen, X.F. Zhang, Y. Zhu, J.H. Zhang, P. Hu, Polym. J. 35 (2) (2003) 148. [2] D.J. Li, F.Z. Cui, H.Q. Gu, Biomaterials 20 (1999) 1889. [3] M. Ramakrishna Murthy, E. Venkateshwar Rao, Bull. Mater. Sci. 25 (October) (2002) 403. [4] H. Dong, T. Bell, Surf. Coat. Technol. 111 (1999) 29. [5] J.C. Pivin, Nucl. Instr. and Meth. B 84 (1994) 484. [6] Y. Suzuki, M. Kusakabe, J.S. Lee, M. Kaibara, M. Iwaki, H. Sasabe, Nucl. Instr. and Meth. B 142 (1992) 65. [7] J. Davenas, P. Thevenard, F. Philippe, M.N. Arnaud, Biomol. Eng. 19 (2002) 263. [8] C. Riccardi, R. Barni, E. Selli, G. Mazzone, M.R. Massafra, B. Marcandalli, G. Poletti, Appl. Surf. Sci. 211 (2003) 386. [9] P.B. Malafaya, R.L. Reis, Bioceramics 15 (2003) 39. [10] I. Banik, A.K. Bhowmick, Rad. Phys. Chem. 58 (2000) 293. [11] H. Dong, T. Bell, Surf. Coat. Tech. 111 (1999) 29. [12] D.J. Li, J. Zhao, H.Q. Gu, M.Z. Lu, F.Q. Ding, Q.Q. Zhang, Nucl. Instr. and Meth. B 82 (1993) 57. [13] D.J. Li, F.Z. Cui, Q.L. Feng, J. Zhao, Chin. Phys. Lett. 14 (1997) 531. [14] J.F. Ziegler, J.P. Biersack, U. Littmark, The Stopping and Range of Ions in Solids, Pergamon, New York, 1985. [15] G. Beamson, D. Briggs, High Resolution XPS of Organic Polymers, John Wiley & Son, New York, 1992. [16] S. Klaumu¨nzer, M. Rammensee, S. Lo¨ffler, H.C. Neitzert, J. Mater. Res. 6 (1991) 2109. [17] S. Rooda, L. Cliche, M. Chicoine, R.A. Masut, Nucl. Instr. and Meth. B 106 (1995) 80.