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fluidised-bed reactor
Applied Surface Science 227 (2004) 268–274
Characterisation of polystyrene microspheres surface-modified using a novel UV-ozone/fluidised-bed reactor G.V. Lubarsky, M.R. Davidson, R.H. Bradley* Advanced Materials and Biomaterials Research Centre, School of Engineering, The Robert Gordon University, Aberdeen AB25 1HG, UK Received 10 November 2003; received in revised form 10 November 2003; accepted 2 December 2003
Abstract A fluidised-bed apparatus equipped with UVand oxygen sources has been developed and used for controlled surface oxidative modification of polystyrene microspheres. The composition and wettability of the microspheres modified using this method were characterised by X-ray photoelectron spectroscopy and a ‘‘floating’’ contact angle technique respectively. X-ray photoelectron spectroscopy data indicate an increase in surface oxygen concentration to 27 at.% after 1200 s UV-ozone treatment. This was accompanied by a decrease in the measured water contact angle from that of the unmodified surface, which gave a value of 87, to 248 after a similar UV-ozone treatment. Force–distance plots obtained from atomic force microscopy showed that the treatment resulted in an increase in the adhesion force measured between tip and surface which correlated well with the surface oxygen content. The experimental results demonstrate that this fluidised-bed apparatus is a convenient and reliable method for the modification of polymeric microparticles and that the resulting changes can be effectively studied and understood using the techniques applied here. # 2003 Elsevier B.V. All rights reserved. Keywords: Polystyrene microspheres; X-ray photoelectron spectroscopy; Water contact angle; Adhesion force
1. Introduction There is a current and growing interest in the use of polymer microspheres in a wide range of applications such as biomaterials. These include use as substrates for the immobilisation of enzymes [1–3]. One problem associated with the use of polymers in general is their low surface energy in their natural state. Although there are many methods for the controlled surface modification of polymers including plasma treatments, UV-ozone and a number of wet chemical *
Corresponding author. Tel.: þ44-1224-262822; fax: þ44-1224-262837. E-mail address: [email protected] (R.H. Bradley).
methods their application to particulate materials can often prove problematic. For example, the use of plasma treatments invariably requires a vacuum system which can present problems when using microspheres and powders. Aggregation of the particles results in uneven surface modification and their relatively large surface area means that treatment times of the order of several hours are often required [4]. Gawdzik and Sobiesiak have reported copper contamination of plasma treated microspheres which they attributed to sputtering from the electrodes inside the reactor the removal of which necessitated rinsing the microspheres in strong acids [5]. UV-ozone has been shown to be a highly successful method for the controlled modification of polymers
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.12.001
G.V. Lubarsky et al. / Applied Surface Science 227 (2004) 268–274
for applications ranging from adhesion improvement [6] to the production of surfaces for enhanced cell attachment [7]. UV-ozone also has many advantages over the alternative methods of surface modification namely: no vacuum systems are required, it can give precise control over the modification process and the absence of any wet chemistry means that there are no residual solvents or other contaminants. In this study we introduce a novel method for the surface modification of polymer particles by UV-ozone oxidation treatment in an oxygen-enriched fluidised-bed quartz reactor. The modification process recently developed in our laboratories has been studied by X-ray photoelectron spectroscopy, adhesion force measurement based on atomic force microscopy force–distance data and by floating contact angle.
2. Experimental 2.1. Polystyrene microspheres The materials used in this study were polydisperse polystyrene spheres (Brookhaven International Ltd., Stock Wood, UK) which have a distribution of sizes ranging from 100 to 500 mm. Before use the particles were washed in ethanol to remove any contaminants present on the surface. X-ray photoelectron spectroscopy data obtained from the as-received microspheres show up to 10 at.% silicon. This is thought
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to be present in the form of silicone oils often used to prevent agglomeration of the beads during manufacture and storage. After several washing cycles the solvent was removed by vacuum filtration and the microspheres were allowed to dry. The microspheres were considered clean when no residual silicon contamination was found during XPS analysis (i.e. <0.1 at.%). The cleaned microspheres (about 1.2 g) were divided for six equal groups. The first group was left untreated and were used as a control. Each of the remaining five groups was loaded separately into the reactor cell and treated for exposure times in the range 150–1200 s. 2.2. UV-ozone fluidised-bed apparatus The system for the surface modification of the microspheres was built around a quartz cell (SemiMicro Cell, Helma, Mulheim, Germany) and is shown in Fig. 1. An oxygen supply was connected to the cell inlet via a fine valve for precise flow control. The cell outlet tube was open to the atmosphere. A grounded metal brush was inserted into the cell via the outlet tube in order to minimise electrostatic charge formation on the moving particles and the internal surfaces of the cell. The assembled system was then placed into a compartment containing a high-intensity low-pressure mercury grid lamp which generates UV light at 184.9 and 253.7 nm (Jelight Company Inc., Irvine,
Fig. 1. Schematic diagram of the UV-ozone fluidised-bed apparatus used for the surface treatment.
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USA) [8]. These wavelengths are known to excite molecular oxygen to form ozone and to photosensitise polymer surfaces. Before performing any surface treatments the fluidising ability of the reactor was optimised using the quartz cell loaded with 0.2 g of microspheres. The cell angle and oxygen flow rate were the variable parameters in this process although there was no observed variation in the fluidising process with cell angle—probably because of the negligible influence of gravity and the comparatively short free path length of particles in the cell. Conversely, the properties of the fluidised-bed could be finely controlled by adjusting the oxygen flow rate. The bed was found to be completely homogeneous with a flow rate of about 50 ml min1. Generally, in these experiments the bed was fluidised by the reflection of the fast moving particles from the reactor walls rather than the pneumatic mobilisation. The reactor parameters used for surface treatment were: 458 for the cell angle (base of the cell with respect to the horizontal) and 50 ml min1 for the oxygen flow rate. 2.3. X-ray photoelectron spectroscopy Changes in chemical composition of the polymer microspheres were studied by X-ray photoelectron spectroscopy. XPS analyses were performed in a Kratos HSi spectrometer equipped with a five-channel detector and a monochromated aluminium Ka X-ray source operating at 150 W. All spectra were obtained in fixed analyser transmission mode with pass energies of 80 eV being used. Elemental compositions were calculated from the peak areas obtained from the survey spectra after a linear background subtraction. Surface charge build up on the insulating samples was minimised by the use of a low-energy electron flood gun. 2.4. Methods for surface energy characterisation Surface energy is a fundamental material parameter and is a measure of the energetic state of its surface. When two or more different phases are brought into contact an interface will be formed between them. At equilibrium the geometry of this contact will depend on the interfacial energies of the phases involved. In the case of a three-phase contact (e.g. liquid/vapour/
solid), the system can be described by the well-known Young’s equation [9] and be characterised by so-called contact angle measurements. The contact between two solid bodies in gaseous or liquid environments is generally known as an elastic contact. The mechanics of this type of contact can be described by the Johnson–Kendall–Roberts (JKR) theory [10]. In the present paper changes to the surface energy of the polystyrene microspheres resulting from UV-ozone treatments was assessed using a ‘‘floating’’ contact angle technique and by measuring the adhesion force between atomic force microscope tip and polystyrene surface from the force–distance data. 2.4.1. ‘‘Floating’’ contact angle measurements The floatation of sub-millimetre dimension spherical particles has been the subject of a number of theoretical works [11–13]. When a particle is placed at a water–air interface it is in a condition of equilibrium between the gravitational force and the sum of the surface tension and buoyancy. The relative contributions of gravitational and surface tension forces acting on a floating particle can be described by the Bond number: Bo ¼ DrgL2 =s, where Dr is density difference across the free surface, s the surface tension and L the characteristic length. When Bo is much less than unity the position of the microspheres at the water–air interface will be unperturbed by gravity and surface tension effects will dominate. The polystyrene microspheres used in this work had diameters of between 100 and 500 mm and consequently the Bond number is approximately 1 103. It was found that these particles caused no noticeable deformation of the meniscus, i.e. the fluid–vapour interface is flat. In this case it is possible to assess the surface properties of the particle by measuring the ‘‘floating’’ contact angle as shown schematically in Fig. 2. The critical particle positions are shown schematically in Fig. 2 for (a) hydrophobic, (b) hydrophilic particle. In the floating contact angles experiment the polystyrene microspheres were placed on the surface of water contained in a glass vessel which had been cleaned by rinsing with high-purity water. Images of their wetting behaviour were captured and measured in side-elevation using a contact angle analyser ˚ 200 (First Ten Angstroms, Portsmouth, VA). The FTA water used was purified using a commercial Milli-Q system containing ion-exchange and charcoal stages.
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adhesion force, FA, can be directly extracted from the cantilever deflection under the pull-off force, DC , from the relation: FA ¼ k DC , where k is the cantilever spring constant. All contact sites were imaged in contact mode following acquisition of force–distance data to verify that no damage to the surface had occurred. For the determination of the interface energy, 50 particles were randomly chosen from each group. Fig. 2. Scheme showing the different types of the particle behaviour at the liquid–vapour interface. Left and right images are illustrations of hydrophobic and hydrophilic surfaces, respectively.
3. Results and discussion 3.1. X-ray photoelectron spectroscopy
The deionised water had a conductivity less than 0:1 106 S m1 and was filtered through a nylon filter. For the contact angle measurements, 50 particles were randomly chosen from each treated group and the control group. 2.4.2. Adhesion force measurement A Digital Instruments Multimode SPM system, with an incorporated fluid cell, was used to measure the adhesion force between the AFM tip and the sample surface. All measurements were performed using 200 mm long V-shaped cantilevers with a nominal spring constant of 0.06 N m1 and tip radius of approximately 20–60 nm. Particles were attached to the substrate with epoxy resin. The AFM studies were carried out under water to prevent any interference from electrostatic or capillary effects. The adhesion force required to remove the tip from the sample surface can be extracted from the force– distance curve. To obtain a force–distance curve the AFM tip is extended to make contact with the surface and then retracted. During the retraction of the tip valuable information about the adhesive properties of the surface can be obtained. When the tip is being retracted from the surface it will not break contact until the applied force exceeds the tip–surface adhesion force. Thus, the pull-off force can be considered a measure of the adhesion force can be related to the surface energy [14]. According to Johnson–Kendall– Roberts (JKR) theory [10], there is a finite negative load required to separate two surfaces. This value is referred to as the pull-off force, given by: FC ¼ 3pRg, where R is the AFM tip radius and g the interface energy between the tip and the sample. The
The surface composition of the polystyrene microspheres was determined from the relative intensities of the XPS carbon 1s and oxygen 1s peaks obtained from the ‘survey scan’ spectra. The change in surface oxygen concentration as a function of UV-ozone treatment time is shown in Fig. 3. It can be seen that the unmodified polystyrene spheres already contain around 5 at.% oxygen which increased to around 27 at.% after 1200 s UV-ozone treatment. No peaks from elements other than carbon and oxygen were found after UV-ozone treatment indicating that only reaction between the polymer surface and oxygen is occurring. Saturation of the oxygen levels was found after 600 s. Previous studies carried out within
Fig. 3. The surface oxygen concentration of the polystyrene microspheres as a function of UV-ozone treatment time obtained from the XPS results.
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this group on the UV-ozone modification of thin films of polystyrene [8] have shown oxygen concentration to saturate after around 180 s. The longer treatment times needed to reach saturation in oxygen levels found in this study is attributed to the large surface area of the microspheres compared to thin film samples. 3.2. Contact angles measurements at the vapour–liquid interface Typical images of ‘‘floating’’ particles are shown in Fig. 4. It can clearly be seen that the liquid–vapor interface at the contact with the sphere is flat, i.e. the particle is in equilibrium. Furthermore, there are no interactions (attractive or repulsive) between particles due to lateral capillary forces. The origin of the lateral capillary forces is the deformation of liquid meniscus by the particle weight (or by the buoyancy of the particle) [13]. Typical images of particles from the control and from the modified groups are shown in Fig. 4b (right and left photos respectively). Contact angles measured from polystyrene particles from the same group were highly reproducible and showed a slight variation of 28 to 58 from particle to particle. The contact angle in the control group was found to be 87 2 and 25 5 in the group after 600 s treat-
Fig. 4. Images showing the ‘‘floating’’ particles at the water– vapour interface: (a) three particles with identical surface properties showing no interaction through the vapour–liquid interface through lateral capillary forces, (b) typical contact angles measurement on particles with different surface treatments.
ment. There was some difficulty found collecting systematic data from spheres treated for 1200 s because of the almost complete immersion of the particles. The presence of low molecular weight oxidized (LMWO) species has been observed in previous studies on the UV-ozone treatment of polystyrene [15]. These water-soluble components are known to affect the measured contact angle as a result of a reduction in the localized surface tension of the water. In the floating contact angle measurements described here the quantity of soluble organic on the surface of the microspheres combined with the relatively large volume of water used in each measurement (typically 2 ml) caused negligible changes in the surface tension of the water. Additionally, the contact angles did not change significantly from the value measured immediately after the initial contact with the water even after several hours flotation suggesting that no chemical modifications resulting from the immersion process had occurred. The variation in measured contact angle with time of treatment is observed in Fig. 5. The decrease in contact angle with treatment time is indicative of an increase in the surface energy of the polymer resulting from the incorporation of oxygen-containing functional groups. Another feature of this analysis is the increase in the variance in the contact angle measurement for the longer treatment times. This phenomenon can be attributed to the stochastic nature of the modification process.
Fig. 5. Plot showing changes in ‘‘floating’’ contact angles and adhesion forces measured on the UV-ozone modified spheres.
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Fig. 6. AFM force–distance curves obtained from polystyrene spheres after various treatment times.
3.3. Adhesion force measurements Typical force–distance curves measured between a polystyrene particle and the AFM tip in water are shown in Fig. 6. There is a clearly an increase in cantilever deflection under the pull-off force (DC) and thus adhesion force with increasing treatment time. All experiments were conducted with the same tip and under the same conditions, e.g. applied force etc. This allows us to tentatively conclude that any increases in the calculated adhesion force found in this study are due only to increases in the surface energy of the
Fig. 7. Correlation between the surface oxygen concentration and adhesion force obtained from XPS and AFM experiments.
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polystyrene. Fig. 5 shows the variation in adhesion force between the Si3N4 AFM tip and the polystyrene microspheres treated for 0, 150, 300, 450, 600 and 1200 s. It can be seen that the adhesion force increases with UV-ozone treatment time with saturation occurring after about 600 s. In common with the contact angle measurements, an increase in distribution of the adhesion force data was observed with increasing treatment time, as shown in Fig. 5. The relationship between the adhesion force (estimated from the force–distance curves) and the oxygen content of the microspheres is shown in Fig. 7. At short treatment times the increased adhesion between the tip and surface is thought to be a result of the incorporation of polar, oxygen-containing functional groups into the polymer surface. At longer treatment times increased adhesion force is attributed to the presence of high surface energy, highly oxidised, low molecular weight material caused by extensive scission of the polymer chain.
4. Conclusions A new and controllable fluidising bed apparatus equipped with UV and oxygen sources was built for the surface modification of polymer microspheres. Spherical polystyrene material was successfully modified in the reactor. The modified surface properties of single particles were characterised by three independent techniques: X-ray photoelectron spectroscopy, floating contact angle and adhesion force measurement using the atomic force microscope. XPS analysis shows that the unmodified microspheres contain less than 5 at.% oxygen. UV-ozone treatment results in a rapid increase in oxygen concentration to 27 at.% after 20 min. The wettability of the microspheres was determined by floating water contact angle. The measured floating contact angle decreased from an original value of 87–248 after 20 min treatment time. The observed decrease in contact angle correlates with the oxygen content of the film. The atomic force microscope was used to investigate the effect of UV-ozone treatment on the adhesion force between tip and sample. The adhesion force was found to monotonically increase with surface oxygen concentration. The UV-ozone fluidised-bed described in this paper was found to be a suitable method for controlled
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surface modification of polymeric microspheres. The experimental results demonstrate that the treatment results in measurable changes in surface composition, wettability and surface energy. References [1] V. Balmus¸, H. Ayhan, E. Pis¸kin, Chem. Eng. J. 65 (1997) 71– 76. [2] V. Balmus¸, K. Kesenci, E. Pis¸kin, React. Funct. Polym. 38 (1998) 1–9. [3] C.-W. Wu, Jing-Gang Lee, Wen-Chien Lee, Biotechnol. Appl. Biochem. 27 (1998) 225–230. [4] N. Inagaki, S. Tasaka, H. Abe, J. Appl. Polym. Sci. 46 (1992) 595–601. [5] B. Gawdzik, M. Sobiesiak, Appl. Surface Sci. 214 (2003) 52– 57.
[6] I. Mathieson, R.H. Bradley, Key Eng. Mater. (Adv. Eng. Mater.) 99 (1995) 185. [7] D.O.H. Teare, N. Emmison, C. Ton-That, R.H. Bradley, Langmuir 16 (2000) 2818–2824. [8] D.O.H. Teare, N. Emmison, C. Ton-That, R.H. Bradley, J. Colloid Interf. Sci. 234 (2000) 84–89. [9] T. Young, Miscellaneous works, in: G. Peacock (Ed.), vol. 1, Murray, London, 1855, p. 418. [10] K.L. Johnson, K. Kendall, A.D. Roberts, Proc. Roy. Soc. A 324 (1971) 301–309. [11] F. Boury, H. Marchais, J.P. Benoit, J.E. Proust, Biomaterials 18 (1997) 125–136. [12] J. Mingins, A. Scheludco, J. Chem. Soc. Faraday Trans. 75 (1975) 1–6. [13] P.A. Kralchevsky, K. Nagayama, Langmuir 10 (1994) 23–36. [14] H.-Y. Nie, M.J. Walzak, B. Berno, N.S. McInyre, Langmuir 15 (1999) 6484–6489. [15] D.O.H. Teare, C. Ton-That, R.H. Bradley, Surf. Interf. Anal. 29 (2000) 276–283.
Characterisation of polystyrene microspheres surface-modified using a novel UV-ozone/fluidised-bed reactor G.V. Lubarsky, M.R. Davidson, R.H. Bradley* Advanced Materials and Biomaterials Research Centre, School of Engineering, The Robert Gordon University, Aberdeen AB25 1HG, UK Received 10 November 2003; received in revised form 10 November 2003; accepted 2 December 2003
Abstract A fluidised-bed apparatus equipped with UVand oxygen sources has been developed and used for controlled surface oxidative modification of polystyrene microspheres. The composition and wettability of the microspheres modified using this method were characterised by X-ray photoelectron spectroscopy and a ‘‘floating’’ contact angle technique respectively. X-ray photoelectron spectroscopy data indicate an increase in surface oxygen concentration to 27 at.% after 1200 s UV-ozone treatment. This was accompanied by a decrease in the measured water contact angle from that of the unmodified surface, which gave a value of 87, to 248 after a similar UV-ozone treatment. Force–distance plots obtained from atomic force microscopy showed that the treatment resulted in an increase in the adhesion force measured between tip and surface which correlated well with the surface oxygen content. The experimental results demonstrate that this fluidised-bed apparatus is a convenient and reliable method for the modification of polymeric microparticles and that the resulting changes can be effectively studied and understood using the techniques applied here. # 2003 Elsevier B.V. All rights reserved. Keywords: Polystyrene microspheres; X-ray photoelectron spectroscopy; Water contact angle; Adhesion force
1. Introduction There is a current and growing interest in the use of polymer microspheres in a wide range of applications such as biomaterials. These include use as substrates for the immobilisation of enzymes [1–3]. One problem associated with the use of polymers in general is their low surface energy in their natural state. Although there are many methods for the controlled surface modification of polymers including plasma treatments, UV-ozone and a number of wet chemical *
Corresponding author. Tel.: þ44-1224-262822; fax: þ44-1224-262837. E-mail address: [email protected] (R.H. Bradley).
methods their application to particulate materials can often prove problematic. For example, the use of plasma treatments invariably requires a vacuum system which can present problems when using microspheres and powders. Aggregation of the particles results in uneven surface modification and their relatively large surface area means that treatment times of the order of several hours are often required [4]. Gawdzik and Sobiesiak have reported copper contamination of plasma treated microspheres which they attributed to sputtering from the electrodes inside the reactor the removal of which necessitated rinsing the microspheres in strong acids [5]. UV-ozone has been shown to be a highly successful method for the controlled modification of polymers
0169-4332/$ – see front matter # 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2003.12.001
G.V. Lubarsky et al. / Applied Surface Science 227 (2004) 268–274
for applications ranging from adhesion improvement [6] to the production of surfaces for enhanced cell attachment [7]. UV-ozone also has many advantages over the alternative methods of surface modification namely: no vacuum systems are required, it can give precise control over the modification process and the absence of any wet chemistry means that there are no residual solvents or other contaminants. In this study we introduce a novel method for the surface modification of polymer particles by UV-ozone oxidation treatment in an oxygen-enriched fluidised-bed quartz reactor. The modification process recently developed in our laboratories has been studied by X-ray photoelectron spectroscopy, adhesion force measurement based on atomic force microscopy force–distance data and by floating contact angle.
2. Experimental 2.1. Polystyrene microspheres The materials used in this study were polydisperse polystyrene spheres (Brookhaven International Ltd., Stock Wood, UK) which have a distribution of sizes ranging from 100 to 500 mm. Before use the particles were washed in ethanol to remove any contaminants present on the surface. X-ray photoelectron spectroscopy data obtained from the as-received microspheres show up to 10 at.% silicon. This is thought
269
to be present in the form of silicone oils often used to prevent agglomeration of the beads during manufacture and storage. After several washing cycles the solvent was removed by vacuum filtration and the microspheres were allowed to dry. The microspheres were considered clean when no residual silicon contamination was found during XPS analysis (i.e. <0.1 at.%). The cleaned microspheres (about 1.2 g) were divided for six equal groups. The first group was left untreated and were used as a control. Each of the remaining five groups was loaded separately into the reactor cell and treated for exposure times in the range 150–1200 s. 2.2. UV-ozone fluidised-bed apparatus The system for the surface modification of the microspheres was built around a quartz cell (SemiMicro Cell, Helma, Mulheim, Germany) and is shown in Fig. 1. An oxygen supply was connected to the cell inlet via a fine valve for precise flow control. The cell outlet tube was open to the atmosphere. A grounded metal brush was inserted into the cell via the outlet tube in order to minimise electrostatic charge formation on the moving particles and the internal surfaces of the cell. The assembled system was then placed into a compartment containing a high-intensity low-pressure mercury grid lamp which generates UV light at 184.9 and 253.7 nm (Jelight Company Inc., Irvine,
Fig. 1. Schematic diagram of the UV-ozone fluidised-bed apparatus used for the surface treatment.
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USA) [8]. These wavelengths are known to excite molecular oxygen to form ozone and to photosensitise polymer surfaces. Before performing any surface treatments the fluidising ability of the reactor was optimised using the quartz cell loaded with 0.2 g of microspheres. The cell angle and oxygen flow rate were the variable parameters in this process although there was no observed variation in the fluidising process with cell angle—probably because of the negligible influence of gravity and the comparatively short free path length of particles in the cell. Conversely, the properties of the fluidised-bed could be finely controlled by adjusting the oxygen flow rate. The bed was found to be completely homogeneous with a flow rate of about 50 ml min1. Generally, in these experiments the bed was fluidised by the reflection of the fast moving particles from the reactor walls rather than the pneumatic mobilisation. The reactor parameters used for surface treatment were: 458 for the cell angle (base of the cell with respect to the horizontal) and 50 ml min1 for the oxygen flow rate. 2.3. X-ray photoelectron spectroscopy Changes in chemical composition of the polymer microspheres were studied by X-ray photoelectron spectroscopy. XPS analyses were performed in a Kratos HSi spectrometer equipped with a five-channel detector and a monochromated aluminium Ka X-ray source operating at 150 W. All spectra were obtained in fixed analyser transmission mode with pass energies of 80 eV being used. Elemental compositions were calculated from the peak areas obtained from the survey spectra after a linear background subtraction. Surface charge build up on the insulating samples was minimised by the use of a low-energy electron flood gun. 2.4. Methods for surface energy characterisation Surface energy is a fundamental material parameter and is a measure of the energetic state of its surface. When two or more different phases are brought into contact an interface will be formed between them. At equilibrium the geometry of this contact will depend on the interfacial energies of the phases involved. In the case of a three-phase contact (e.g. liquid/vapour/
solid), the system can be described by the well-known Young’s equation [9] and be characterised by so-called contact angle measurements. The contact between two solid bodies in gaseous or liquid environments is generally known as an elastic contact. The mechanics of this type of contact can be described by the Johnson–Kendall–Roberts (JKR) theory [10]. In the present paper changes to the surface energy of the polystyrene microspheres resulting from UV-ozone treatments was assessed using a ‘‘floating’’ contact angle technique and by measuring the adhesion force between atomic force microscope tip and polystyrene surface from the force–distance data. 2.4.1. ‘‘Floating’’ contact angle measurements The floatation of sub-millimetre dimension spherical particles has been the subject of a number of theoretical works [11–13]. When a particle is placed at a water–air interface it is in a condition of equilibrium between the gravitational force and the sum of the surface tension and buoyancy. The relative contributions of gravitational and surface tension forces acting on a floating particle can be described by the Bond number: Bo ¼ DrgL2 =s, where Dr is density difference across the free surface, s the surface tension and L the characteristic length. When Bo is much less than unity the position of the microspheres at the water–air interface will be unperturbed by gravity and surface tension effects will dominate. The polystyrene microspheres used in this work had diameters of between 100 and 500 mm and consequently the Bond number is approximately 1 103. It was found that these particles caused no noticeable deformation of the meniscus, i.e. the fluid–vapour interface is flat. In this case it is possible to assess the surface properties of the particle by measuring the ‘‘floating’’ contact angle as shown schematically in Fig. 2. The critical particle positions are shown schematically in Fig. 2 for (a) hydrophobic, (b) hydrophilic particle. In the floating contact angles experiment the polystyrene microspheres were placed on the surface of water contained in a glass vessel which had been cleaned by rinsing with high-purity water. Images of their wetting behaviour were captured and measured in side-elevation using a contact angle analyser ˚ 200 (First Ten Angstroms, Portsmouth, VA). The FTA water used was purified using a commercial Milli-Q system containing ion-exchange and charcoal stages.
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adhesion force, FA, can be directly extracted from the cantilever deflection under the pull-off force, DC , from the relation: FA ¼ k DC , where k is the cantilever spring constant. All contact sites were imaged in contact mode following acquisition of force–distance data to verify that no damage to the surface had occurred. For the determination of the interface energy, 50 particles were randomly chosen from each group. Fig. 2. Scheme showing the different types of the particle behaviour at the liquid–vapour interface. Left and right images are illustrations of hydrophobic and hydrophilic surfaces, respectively.
3. Results and discussion 3.1. X-ray photoelectron spectroscopy
The deionised water had a conductivity less than 0:1 106 S m1 and was filtered through a nylon filter. For the contact angle measurements, 50 particles were randomly chosen from each treated group and the control group. 2.4.2. Adhesion force measurement A Digital Instruments Multimode SPM system, with an incorporated fluid cell, was used to measure the adhesion force between the AFM tip and the sample surface. All measurements were performed using 200 mm long V-shaped cantilevers with a nominal spring constant of 0.06 N m1 and tip radius of approximately 20–60 nm. Particles were attached to the substrate with epoxy resin. The AFM studies were carried out under water to prevent any interference from electrostatic or capillary effects. The adhesion force required to remove the tip from the sample surface can be extracted from the force– distance curve. To obtain a force–distance curve the AFM tip is extended to make contact with the surface and then retracted. During the retraction of the tip valuable information about the adhesive properties of the surface can be obtained. When the tip is being retracted from the surface it will not break contact until the applied force exceeds the tip–surface adhesion force. Thus, the pull-off force can be considered a measure of the adhesion force can be related to the surface energy [14]. According to Johnson–Kendall– Roberts (JKR) theory [10], there is a finite negative load required to separate two surfaces. This value is referred to as the pull-off force, given by: FC ¼ 3pRg, where R is the AFM tip radius and g the interface energy between the tip and the sample. The
The surface composition of the polystyrene microspheres was determined from the relative intensities of the XPS carbon 1s and oxygen 1s peaks obtained from the ‘survey scan’ spectra. The change in surface oxygen concentration as a function of UV-ozone treatment time is shown in Fig. 3. It can be seen that the unmodified polystyrene spheres already contain around 5 at.% oxygen which increased to around 27 at.% after 1200 s UV-ozone treatment. No peaks from elements other than carbon and oxygen were found after UV-ozone treatment indicating that only reaction between the polymer surface and oxygen is occurring. Saturation of the oxygen levels was found after 600 s. Previous studies carried out within
Fig. 3. The surface oxygen concentration of the polystyrene microspheres as a function of UV-ozone treatment time obtained from the XPS results.
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this group on the UV-ozone modification of thin films of polystyrene [8] have shown oxygen concentration to saturate after around 180 s. The longer treatment times needed to reach saturation in oxygen levels found in this study is attributed to the large surface area of the microspheres compared to thin film samples. 3.2. Contact angles measurements at the vapour–liquid interface Typical images of ‘‘floating’’ particles are shown in Fig. 4. It can clearly be seen that the liquid–vapor interface at the contact with the sphere is flat, i.e. the particle is in equilibrium. Furthermore, there are no interactions (attractive or repulsive) between particles due to lateral capillary forces. The origin of the lateral capillary forces is the deformation of liquid meniscus by the particle weight (or by the buoyancy of the particle) [13]. Typical images of particles from the control and from the modified groups are shown in Fig. 4b (right and left photos respectively). Contact angles measured from polystyrene particles from the same group were highly reproducible and showed a slight variation of 28 to 58 from particle to particle. The contact angle in the control group was found to be 87 2 and 25 5 in the group after 600 s treat-
Fig. 4. Images showing the ‘‘floating’’ particles at the water– vapour interface: (a) three particles with identical surface properties showing no interaction through the vapour–liquid interface through lateral capillary forces, (b) typical contact angles measurement on particles with different surface treatments.
ment. There was some difficulty found collecting systematic data from spheres treated for 1200 s because of the almost complete immersion of the particles. The presence of low molecular weight oxidized (LMWO) species has been observed in previous studies on the UV-ozone treatment of polystyrene [15]. These water-soluble components are known to affect the measured contact angle as a result of a reduction in the localized surface tension of the water. In the floating contact angle measurements described here the quantity of soluble organic on the surface of the microspheres combined with the relatively large volume of water used in each measurement (typically 2 ml) caused negligible changes in the surface tension of the water. Additionally, the contact angles did not change significantly from the value measured immediately after the initial contact with the water even after several hours flotation suggesting that no chemical modifications resulting from the immersion process had occurred. The variation in measured contact angle with time of treatment is observed in Fig. 5. The decrease in contact angle with treatment time is indicative of an increase in the surface energy of the polymer resulting from the incorporation of oxygen-containing functional groups. Another feature of this analysis is the increase in the variance in the contact angle measurement for the longer treatment times. This phenomenon can be attributed to the stochastic nature of the modification process.
Fig. 5. Plot showing changes in ‘‘floating’’ contact angles and adhesion forces measured on the UV-ozone modified spheres.
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Fig. 6. AFM force–distance curves obtained from polystyrene spheres after various treatment times.
3.3. Adhesion force measurements Typical force–distance curves measured between a polystyrene particle and the AFM tip in water are shown in Fig. 6. There is a clearly an increase in cantilever deflection under the pull-off force (DC) and thus adhesion force with increasing treatment time. All experiments were conducted with the same tip and under the same conditions, e.g. applied force etc. This allows us to tentatively conclude that any increases in the calculated adhesion force found in this study are due only to increases in the surface energy of the
Fig. 7. Correlation between the surface oxygen concentration and adhesion force obtained from XPS and AFM experiments.
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polystyrene. Fig. 5 shows the variation in adhesion force between the Si3N4 AFM tip and the polystyrene microspheres treated for 0, 150, 300, 450, 600 and 1200 s. It can be seen that the adhesion force increases with UV-ozone treatment time with saturation occurring after about 600 s. In common with the contact angle measurements, an increase in distribution of the adhesion force data was observed with increasing treatment time, as shown in Fig. 5. The relationship between the adhesion force (estimated from the force–distance curves) and the oxygen content of the microspheres is shown in Fig. 7. At short treatment times the increased adhesion between the tip and surface is thought to be a result of the incorporation of polar, oxygen-containing functional groups into the polymer surface. At longer treatment times increased adhesion force is attributed to the presence of high surface energy, highly oxidised, low molecular weight material caused by extensive scission of the polymer chain.
4. Conclusions A new and controllable fluidising bed apparatus equipped with UV and oxygen sources was built for the surface modification of polymer microspheres. Spherical polystyrene material was successfully modified in the reactor. The modified surface properties of single particles were characterised by three independent techniques: X-ray photoelectron spectroscopy, floating contact angle and adhesion force measurement using the atomic force microscope. XPS analysis shows that the unmodified microspheres contain less than 5 at.% oxygen. UV-ozone treatment results in a rapid increase in oxygen concentration to 27 at.% after 20 min. The wettability of the microspheres was determined by floating water contact angle. The measured floating contact angle decreased from an original value of 87–248 after 20 min treatment time. The observed decrease in contact angle correlates with the oxygen content of the film. The atomic force microscope was used to investigate the effect of UV-ozone treatment on the adhesion force between tip and sample. The adhesion force was found to monotonically increase with surface oxygen concentration. The UV-ozone fluidised-bed described in this paper was found to be a suitable method for controlled
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