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Swelling of cross-linked polystyrene beads in toluene
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Microelectronic Engineering 85 (2008) 1261–1264 www.elsevier.com/locate/mee
Swelling of cross-linked polystyrene beads in toluene R. Zhang, T. Cherdhirankorn, K. Graf, K. Koynov *, R. Berger * Max Planck Institute for Polymer Research, Mainz, D-55128, Germany Received 7 October 2007; received in revised form 14 January 2008; accepted 17 January 2008 Available online 8 February 2008
Abstract Ultraviolet light irradiation of PS microbeads can be used for controlling the solvent uptake and release upon exposure to its vapor. The irradiation leads to additional fracture and crosslinking of PS chains thus reducing the amount of solvent uptake significantly. Confocal fluorescence microscopy showed that this effect is pronounced in a shell of the PS microbeads having a thickness in the range of 1– 2 lm. In addition, this shell shows considerable fluorescence when excited between 300 nm and 500 nm that is attributed to lower molecular weight fragments of PS. Fluorescence correlation spectroscopy studies of small tracers diffusion in a microbead core have revealed that the swelling of the core is not altered by the UV exposure, and the UV crosslinking effects are limited to the shell only. Chemically and UV cross-linked PS beads keep their shapes even after numerous swelling cycles in saturated toluene vapors. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Microbeads; Swelling; Fluorescence correlation spectroscopy; Diffusion coefficient; Micromechanical cantilever sensor; UV crosslinking; Polystyrene
1. Introduction Control of solvent annealing conditions and understanding of associated effects in polymers are required in the fabrication of polymer-based thin film devices. A typical behavior of polymers is their ability to swell in an environment where solvents for polymer are present. Swelling however is influenced by both material and solvent properties. Recently, polystyrene surfaces have been micro-structured permanently by local irradiation of ions and heating [1] or by UV light and solvent exposure [2]. In order to understand the role of swelling and deswelling in the fabrication of arrays of microvessels made in polystyrene plates [3,4], we have studied the swelling of micron-sized polystyrene (PS) beads in toluene vapor via mass loading by means of micromechanical cantilever sensor (MCS) technique. We found that for 4–8% chemically cross-linked PS a mass increase of up to 180% can be observed. Furthermore preliminary studies indicated that ultraviolet (UV) *
Corresponding authors. E-mail addresses: [email protected] (K. Koynov), berger@ mpip-mainz.mpg.de (R. Berger). 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.01.090
light irradiation may create additional crosslinking of the PS microbead shell which strongly influence the swelling behavior [5]. Control of the latter would allow tailoring the amount of encapsulation and release of substances. In addition, local UV exposure of selected areas could allow therefore fine-tuning of the shape of polymer components. In the area of micro and nanofabrication or structuring of polymer materials repeated swelling of elements upon exposure to solvents may lead to fatigue of fracture of parts. Thus we have concentrated in this contribution on the characterization of lm-sized PS particles with very high spatial resolution using confocal laser scanning microscopy (CLSM) with the intention to visualize cracks in the UVtreated PS beads. In addition we used fluorescence correlation spectroscopy (FCS) [6] to study small tracers diffusion in the microbead core in dependence of UV exposure. The method is based on detecting the fluctuations of the fluorescent light intensity caused by fluorescent tracers diffusing through a small observation volume of less than 1 lm3 formed by the focus of a confocal microscope. Due to minimal requirements on sample amounts and its high sensitivity, FCS has found widespread applications, probing quantities such as diffusive behavior [7–11], hydrogel
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R. Zhang et al. / Microelectronic Engineering 85 (2008) 1261–1264
swelling [12], reaction kinetics [13], or intracellular particle concentrations [14]. While most of these studies were conducted in aqueous environments, more recently FCS was successfully applied to study polystyrene self-diffusion in dilute [15] and semidilute [16] solutions and small tracer diffusion in polymer melts [17]. Here, we present a comprehensive study of the effect of UV irradiation on the structure and swelling properties of PS microbeads. The results obtained by micromechanical cantilever sensors (MCS) are supplemented by confocal laser scanning microscopy (CLSM) to image the microbeads and fluorescence correlation spectroscopy (FCS) to study the local density of the PS in the microbeads. 2. Experimental PS beads (diameter ranging from 1 to 50 lm, density = 1.05 g/cm3, Duke Scientific Corporation, Palo Alto, CA) were used in this work. The PS beads were exposed to ultraviolet (UV) light under ambient conditions to induce additional crosslinking. The UV source, a pencil-style lamp with build in 254 nm filter (LOT-Oriel GmbH, Darmstadt, Germany), was placed at a distance of 0.5 cm to the PS bead resulting in a radiant intensity of 2.1 mW/cm2. For MCS measurement, an individual PS bead was placed at the apex of the MCS (OMCL-AC160TS, Olympus, Japan) and the resonance frequencies were monitored continuously at a time interval of 0.3 s (SCENTRIS, Veeco, CA, USA) as reported in [5]. For the CLSM and FCS experiments, the PS beads were distributed on a glass slide (diameter of 25 mm, thickness of 0.16 mm, Menzel, Germany) mounted in an Attofluor steel cell. Toluene liquid was added in to the cell and left for 1 h before the measurements to allow swelling of PS beads. The confocal microscopy experiments were performed on a commercial setup (Carl Zeiss, Jena, Germany) consisting of an inverted microscope model Axiovert 200, a fluorescence correlation spectroscopy module ConfoCor 2 and laser scanning unit LSM510. In all experiments a Zeiss plan neofluar 40/0.9 multi immersion objective was employed.
We exposed a selected PS bead (diameter of 13 ± 1 lm) to ultraviolet (UV) light for 10 h from all sides under ambient conditions to induce additional crosslinking. After UV exposure time the first swelling cycle differs significantly from the subsequent ones (Fig. 1). The frequency change at the saturation state is 3.45 kHz (272.65–269.2 kHz) for the first and 2.85 kHz (272.05–269.2 kHz) for the second cycle. The difference in magnitude is attributed to residual solvent molecules in the PS from the first toluene exposure step. Fitting of the recorded data of mass changes by an exponential law (Dm ¼ a þ b ekðtcÞ ) showed that the swelling takes approximately six times longer in the first cycle compared to all subsequent ones. A plausible reason is that during UV exposure, the bead is cross-linked and/or oxidized predominantly on the surface, resulting in a less permeable hard shell through which the toluene molecules must penetrate. During swelling of the PS bead upon the first exposure to saturated toluene vapor this shell is disrupted irreversibly. In the second swelling cycle this leads to the observed faster swelling process. In order to confirm the hypothesis of cracks we have performed CLSM imaging of the swollen PS l-beads before and after UV irradiation as described in the next section. 3.2. CLSM and FCS studies of swollen PS microbeads To observe directly the influence of UV light irradiation on the PS particles, chemically cross-linked and additional UV cross-linked PS beads were imaged by CLSM. At first we exposed a 4–8% chemically cross-linked PS beads to toluene liquid. The beads did not show any fluorescence and they could be observed only in transmission or reflection mode (Fig. 2a) of the confocal microscope. Clearly the PS bead is symmetric and with smooth surface. Next, similar 4–8% chemically cross-linked PS beads were exposed to
3. Results and discussions 3.1. PS microbead swelling after UV exposure Exposure of PS bead to toluene vapor leads to diffusion of toluene molecules into PS and to swelling of the bead that can even be visualized by optical microscopy pictures [5]. Photoirradiation in the presence of oxygen can induce degradation of PS, which on the microscopic level is characterized by macromolecular chain splitting, creation of low mass fragments, production of free radicals, oxidation, and crosslinking. Consequently, macroscopic properties of the polymer, such as mechanical strength, color, electrical conductivity, and swelling are affected.
Fig. 1. Frequency response of a PS bead attached to a resonating micromechanical cantilever after exposure to UV for 10 h. Upon exposure to saturated toluene vapor (at t = 4 min) vapor, the PS bead swells thus causing a decrease in resonance frequency. At 20 min the saturated toluene vapor is exchanged by dry N2 gas. Thus the toluene is removed from the PS and the resonance frequency increases again. This exposure sequence is repeated (t = 34 min and 40 min). In the second exposure sequence the swelling kinetics is significantly faster compared to the first one.
R. Zhang et al. / Microelectronic Engineering 85 (2008) 1261–1264
20 ~24μm
glass slide
a Fluorescence Intensity [a.u.]
UV light for 5 h. After UV irradiation, the sample showed considerable fluorescence with broad excitation (up to 500 nm) and emission bands. This is most likely due to the appearance of fluorescent species in the PS as result of the UV induced chemical reactions. Using the CLSM described above we have performed optical sectioning of the microbeads by recording fluorescent images at different altitudes. For excitation the 488 nm line of an Argon laser was used and the fluorescence was detected after long pass emission filter LP505. A typical image taken close to the top of a PS bead is shown in Fig. 2b. The picture clearly shows that cracks (dark ribbons in the bead) are formed on the surface of the PS bead. Furthermore our studies revealed that the fluorescence is originating from the surface of the bead only and the core is not fluorescent. These observations confirm that UV exposure induces the additional cross-linked and/or oxidized shell on the PS bead, which can be broken by the swelling of the internal part of the PS particle upon exposure to toluene. We found that the thickness of this hard shell is in the order of 1–2 lm. For comparison we have measured the absorption coefficient a of thin PS films (120 nm) using a UV–Vis spectrometer (Perkin Elmer Model Lambda 900) and found that a 13,000 cm1 at the wavelength of 254 nm. This value corresponds to penetration depth of 0.8 lm which is in a good accordance with the CLSM data. The experiments described above, can not show how the UV irradiation affects (if at all) the microbead core. In order to address this question we have used fluorescence correlation spectroscopy to study the local swelling in the microbead. The method is described in greater details elsewhere [12]. In short the diffusion coefficient of small fluorescent tracers (dye molecules) diffusing in dense polymer environment is measured and related to the local density. Here we added the tracers to the toluene in which the PS beads were immersed. The typical dye concentration was in the order of 107 M/L. Furthermore we have excited the tracers at 633 nm and collect the fluorescence after LP650 emission filter to avoid background caused by the fluorescent shell of the UV irradiated microbeads. First we have scanned the entire microbead with the focus of
the confocal microscope and measured the average fluorescent intensity versus the distance to surface of the glass slide at which the bead was deposited. Typical results are shown in Fig. 3a. As can be seen the fluorescent intensity is practically the same in the center of the bead and outside the bead i.e. in pure toluene, reflecting similar dye molecules concentration. Furthermore stronger intensity is observed from the bead surface probably because some dye molecules adsorbed there. We have compared the diffusion of the tracer in pure toluene and in the center of a 4–8% chemically cross-linked PS bead swollen in toluene. The corresponding normalized autocorrelation functions are shown in Fig. 3b. The characteristic diffusion times which show how long the tracer stays in the confocal observation volume are sD1 = 36 ± 1.8 ls in the pure toluene (dotted curve) and sD2 = 1810 ± 90 ls in the center of the bead (solid curve). They corresponds to diffusion coefficients of 2 106 and 4 108 cm2/s, respectively. The approximately 50 times slower diffusion inside the microbead is caused by the interactions of the tracers with the dense network of cross-linked PS chains. A stronger
15
10
5
Iin
0 -40
-20
0
Iout
20
40
60
z [µm]
b
1.2 1.0
[G(τ)-1] / [G(0)-1]
Fig. 2. Confocal laser scanning microscopy (CLSM) images of PS beads in toluene. Excitation wavelength is 488 nm and emission is collected after a 505LP filter. (a) 4–8% chemically cross-linked bead imaged in reflection. (b) Fluorescence image of a 4–8% chemically cross-linked bead after exposure to UV for 5 h.
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0.8 0.6 0.4
D3 D1 D2
0.2 0.0 -5 1x10
-4
1x10
-3
10
-2
10
-1
10
0
10
τ [s] Fig. 3. (a) Fluorescence intensity caused by the tracer molecules vs. distance from the glass substrate surface. (b) Autocorrelation curves of tracer molecules diffusing in: pure toluene (dot curve), in chemically 4–8% cross-linked PS particle (solid curve), and in the particle exposed to UV (dash curve). The vertical lines indicate the respective diffusion times: sD1 36 ls, sD2 1810 ls and sD3 1900 ls.
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crosslinking should lead to less swelling and consequently even smaller diffusion coefficients. We have therefore measured the tracer diffusion in a 4–8% chemically cross-linked PS beads exposed to UV light for 2 h and then swollen in toluene. The corresponding normalized autocorrelation curve is shown in Fig. 3b (dashed curve). As can be seen the diffusion time of the dye molecules in the center of this particle sD3 = 1900 ± 95 ls and the corresponding diffusion coefficient 3.8 108 cm2/s are very similar to those measured in non UV irradiated PS microbead. This indicates that the UV irradiation has no influence on the swelling properties of the microbead core. This is probably a result of the rather short penetration depth of the UV light in the PS particles. Further studies including longer UV irradiations, various degrees of chemical crosslinking and FCS measurements performed at different positions in the bead are needed in order to probe an eventual gradient of the UV crosslinking from the surface to the center of the PS beads. 4. Conclusion Our study shows that UV light irradiation of PS microbeads can be used for controlling the solvent uptake and release upon exposure to its vapor. We found that the UV light irradiation of PS microbeads leads to additional crosslinking and the formation of a harder shell with typical thickness in the order of 1–2 lm. Quite remarkably this shell shows considerable fluorescence. Furthermore chemically and UV cross-linked PS beads keep their shapes even after numerous swelling cycles in saturated toluene vapors. Local UV exposure of selected areas could allow fine-tuning of the shape of polymer components. Such elements might be used to uptake or release molecules or as microreactors.
Acknowledgements We thank Hans-Ju¨rgen Butt and Andreas Best for the fruitful discussions and continuous support. T.C. thanks the European Community for financial support in the framework of a Marie Curie Host Fellowship for Early Stage Research Training. References [1] Y. Karade, K. Graf, W.H. Bru¨nger, A. Dietzel, R. Berger, Microelectronic Engineering 84 (2007) 797. [2] M. Shahinpoor, Journal of Intelligent Material Systems and Structures 6 (1995) 307. [3] E. Bonaccurso, H.J. Butt, K. Graf, European Polymer Journal 40 (2004) 975. [4] E. Bonaccurso, K. Graf, Langmuir 20 (2004) 11183. [5] R. Zhang, K. Graf, R. Berger, Applied Physics Letters 89 (2006) 223114. [6] M. Eigen, R. Rigler, Proceedings of National Academy of Sciences of the United States of America 91 (1994) 5740. [7] R. Rigler, P. Grasselli, M. Ehrenberg, Physica Scripta 19 (1979) 486. [8] N.G. Walter, P. Schwille, M. Eigen, Proceedings of National Academy of Sciences of the United States of America 93 (1996) 12805. [9] B.S. Kim, O.V. Lebedeva, K. Koynov, H.F. Gong, G. Glasser, I. Lieberwirth, O.I. Vinogradova, Macromolecules 38 (2005) 214. [10] K. Koynov, G. Mihov, M. Mondeshki, C. Moon, H.W. Spiess, K. Muellen, H.-J. Butt, G. Floudas, Biomacromolecules 8 (2007) 1745. [11] J. Zhao, S. Granick, JACS 126 (2004) 6242. [12] M. Giannelli, P.W. Beines, R.F. Roskamp, K. Koynov, G. Fytas, W. Knoll, Journal of Physical Chemistry C 111 (2007) 13205. [13] D. Magde, E. Elson, W.W. Webb, Physical Review Letters 29 (1972) 705. [14] P. Cluzel, M. Surette, S. Leibler, Science 287 (2000) 1652. [15] H. Zettl, W. Hafner, A. Boker, H. Schmalz, M. Lanzendorfer, A.H.E. Muller, G. Krausch, Macromolecules 37 (2004) 1917. [16] R. Liu, X. Gao, J. Adams, W. Oppermann, Macromolecules 38 (2005) 8845. [17] A. Best, T. Pakula, G. Fytas, Macromolecules 38 (2005) 4539.
Microelectronic Engineering 85 (2008) 1261–1264 www.elsevier.com/locate/mee
Swelling of cross-linked polystyrene beads in toluene R. Zhang, T. Cherdhirankorn, K. Graf, K. Koynov *, R. Berger * Max Planck Institute for Polymer Research, Mainz, D-55128, Germany Received 7 October 2007; received in revised form 14 January 2008; accepted 17 January 2008 Available online 8 February 2008
Abstract Ultraviolet light irradiation of PS microbeads can be used for controlling the solvent uptake and release upon exposure to its vapor. The irradiation leads to additional fracture and crosslinking of PS chains thus reducing the amount of solvent uptake significantly. Confocal fluorescence microscopy showed that this effect is pronounced in a shell of the PS microbeads having a thickness in the range of 1– 2 lm. In addition, this shell shows considerable fluorescence when excited between 300 nm and 500 nm that is attributed to lower molecular weight fragments of PS. Fluorescence correlation spectroscopy studies of small tracers diffusion in a microbead core have revealed that the swelling of the core is not altered by the UV exposure, and the UV crosslinking effects are limited to the shell only. Chemically and UV cross-linked PS beads keep their shapes even after numerous swelling cycles in saturated toluene vapors. Ó 2008 Elsevier B.V. All rights reserved. Keywords: Microbeads; Swelling; Fluorescence correlation spectroscopy; Diffusion coefficient; Micromechanical cantilever sensor; UV crosslinking; Polystyrene
1. Introduction Control of solvent annealing conditions and understanding of associated effects in polymers are required in the fabrication of polymer-based thin film devices. A typical behavior of polymers is their ability to swell in an environment where solvents for polymer are present. Swelling however is influenced by both material and solvent properties. Recently, polystyrene surfaces have been micro-structured permanently by local irradiation of ions and heating [1] or by UV light and solvent exposure [2]. In order to understand the role of swelling and deswelling in the fabrication of arrays of microvessels made in polystyrene plates [3,4], we have studied the swelling of micron-sized polystyrene (PS) beads in toluene vapor via mass loading by means of micromechanical cantilever sensor (MCS) technique. We found that for 4–8% chemically cross-linked PS a mass increase of up to 180% can be observed. Furthermore preliminary studies indicated that ultraviolet (UV) *
Corresponding authors. E-mail addresses: [email protected] (K. Koynov), berger@ mpip-mainz.mpg.de (R. Berger). 0167-9317/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.mee.2008.01.090
light irradiation may create additional crosslinking of the PS microbead shell which strongly influence the swelling behavior [5]. Control of the latter would allow tailoring the amount of encapsulation and release of substances. In addition, local UV exposure of selected areas could allow therefore fine-tuning of the shape of polymer components. In the area of micro and nanofabrication or structuring of polymer materials repeated swelling of elements upon exposure to solvents may lead to fatigue of fracture of parts. Thus we have concentrated in this contribution on the characterization of lm-sized PS particles with very high spatial resolution using confocal laser scanning microscopy (CLSM) with the intention to visualize cracks in the UVtreated PS beads. In addition we used fluorescence correlation spectroscopy (FCS) [6] to study small tracers diffusion in the microbead core in dependence of UV exposure. The method is based on detecting the fluctuations of the fluorescent light intensity caused by fluorescent tracers diffusing through a small observation volume of less than 1 lm3 formed by the focus of a confocal microscope. Due to minimal requirements on sample amounts and its high sensitivity, FCS has found widespread applications, probing quantities such as diffusive behavior [7–11], hydrogel
1262
R. Zhang et al. / Microelectronic Engineering 85 (2008) 1261–1264
swelling [12], reaction kinetics [13], or intracellular particle concentrations [14]. While most of these studies were conducted in aqueous environments, more recently FCS was successfully applied to study polystyrene self-diffusion in dilute [15] and semidilute [16] solutions and small tracer diffusion in polymer melts [17]. Here, we present a comprehensive study of the effect of UV irradiation on the structure and swelling properties of PS microbeads. The results obtained by micromechanical cantilever sensors (MCS) are supplemented by confocal laser scanning microscopy (CLSM) to image the microbeads and fluorescence correlation spectroscopy (FCS) to study the local density of the PS in the microbeads. 2. Experimental PS beads (diameter ranging from 1 to 50 lm, density = 1.05 g/cm3, Duke Scientific Corporation, Palo Alto, CA) were used in this work. The PS beads were exposed to ultraviolet (UV) light under ambient conditions to induce additional crosslinking. The UV source, a pencil-style lamp with build in 254 nm filter (LOT-Oriel GmbH, Darmstadt, Germany), was placed at a distance of 0.5 cm to the PS bead resulting in a radiant intensity of 2.1 mW/cm2. For MCS measurement, an individual PS bead was placed at the apex of the MCS (OMCL-AC160TS, Olympus, Japan) and the resonance frequencies were monitored continuously at a time interval of 0.3 s (SCENTRIS, Veeco, CA, USA) as reported in [5]. For the CLSM and FCS experiments, the PS beads were distributed on a glass slide (diameter of 25 mm, thickness of 0.16 mm, Menzel, Germany) mounted in an Attofluor steel cell. Toluene liquid was added in to the cell and left for 1 h before the measurements to allow swelling of PS beads. The confocal microscopy experiments were performed on a commercial setup (Carl Zeiss, Jena, Germany) consisting of an inverted microscope model Axiovert 200, a fluorescence correlation spectroscopy module ConfoCor 2 and laser scanning unit LSM510. In all experiments a Zeiss plan neofluar 40/0.9 multi immersion objective was employed.
We exposed a selected PS bead (diameter of 13 ± 1 lm) to ultraviolet (UV) light for 10 h from all sides under ambient conditions to induce additional crosslinking. After UV exposure time the first swelling cycle differs significantly from the subsequent ones (Fig. 1). The frequency change at the saturation state is 3.45 kHz (272.65–269.2 kHz) for the first and 2.85 kHz (272.05–269.2 kHz) for the second cycle. The difference in magnitude is attributed to residual solvent molecules in the PS from the first toluene exposure step. Fitting of the recorded data of mass changes by an exponential law (Dm ¼ a þ b ekðtcÞ ) showed that the swelling takes approximately six times longer in the first cycle compared to all subsequent ones. A plausible reason is that during UV exposure, the bead is cross-linked and/or oxidized predominantly on the surface, resulting in a less permeable hard shell through which the toluene molecules must penetrate. During swelling of the PS bead upon the first exposure to saturated toluene vapor this shell is disrupted irreversibly. In the second swelling cycle this leads to the observed faster swelling process. In order to confirm the hypothesis of cracks we have performed CLSM imaging of the swollen PS l-beads before and after UV irradiation as described in the next section. 3.2. CLSM and FCS studies of swollen PS microbeads To observe directly the influence of UV light irradiation on the PS particles, chemically cross-linked and additional UV cross-linked PS beads were imaged by CLSM. At first we exposed a 4–8% chemically cross-linked PS beads to toluene liquid. The beads did not show any fluorescence and they could be observed only in transmission or reflection mode (Fig. 2a) of the confocal microscope. Clearly the PS bead is symmetric and with smooth surface. Next, similar 4–8% chemically cross-linked PS beads were exposed to
3. Results and discussions 3.1. PS microbead swelling after UV exposure Exposure of PS bead to toluene vapor leads to diffusion of toluene molecules into PS and to swelling of the bead that can even be visualized by optical microscopy pictures [5]. Photoirradiation in the presence of oxygen can induce degradation of PS, which on the microscopic level is characterized by macromolecular chain splitting, creation of low mass fragments, production of free radicals, oxidation, and crosslinking. Consequently, macroscopic properties of the polymer, such as mechanical strength, color, electrical conductivity, and swelling are affected.
Fig. 1. Frequency response of a PS bead attached to a resonating micromechanical cantilever after exposure to UV for 10 h. Upon exposure to saturated toluene vapor (at t = 4 min) vapor, the PS bead swells thus causing a decrease in resonance frequency. At 20 min the saturated toluene vapor is exchanged by dry N2 gas. Thus the toluene is removed from the PS and the resonance frequency increases again. This exposure sequence is repeated (t = 34 min and 40 min). In the second exposure sequence the swelling kinetics is significantly faster compared to the first one.
R. Zhang et al. / Microelectronic Engineering 85 (2008) 1261–1264
20 ~24μm
glass slide
a Fluorescence Intensity [a.u.]
UV light for 5 h. After UV irradiation, the sample showed considerable fluorescence with broad excitation (up to 500 nm) and emission bands. This is most likely due to the appearance of fluorescent species in the PS as result of the UV induced chemical reactions. Using the CLSM described above we have performed optical sectioning of the microbeads by recording fluorescent images at different altitudes. For excitation the 488 nm line of an Argon laser was used and the fluorescence was detected after long pass emission filter LP505. A typical image taken close to the top of a PS bead is shown in Fig. 2b. The picture clearly shows that cracks (dark ribbons in the bead) are formed on the surface of the PS bead. Furthermore our studies revealed that the fluorescence is originating from the surface of the bead only and the core is not fluorescent. These observations confirm that UV exposure induces the additional cross-linked and/or oxidized shell on the PS bead, which can be broken by the swelling of the internal part of the PS particle upon exposure to toluene. We found that the thickness of this hard shell is in the order of 1–2 lm. For comparison we have measured the absorption coefficient a of thin PS films (120 nm) using a UV–Vis spectrometer (Perkin Elmer Model Lambda 900) and found that a 13,000 cm1 at the wavelength of 254 nm. This value corresponds to penetration depth of 0.8 lm which is in a good accordance with the CLSM data. The experiments described above, can not show how the UV irradiation affects (if at all) the microbead core. In order to address this question we have used fluorescence correlation spectroscopy to study the local swelling in the microbead. The method is described in greater details elsewhere [12]. In short the diffusion coefficient of small fluorescent tracers (dye molecules) diffusing in dense polymer environment is measured and related to the local density. Here we added the tracers to the toluene in which the PS beads were immersed. The typical dye concentration was in the order of 107 M/L. Furthermore we have excited the tracers at 633 nm and collect the fluorescence after LP650 emission filter to avoid background caused by the fluorescent shell of the UV irradiated microbeads. First we have scanned the entire microbead with the focus of
the confocal microscope and measured the average fluorescent intensity versus the distance to surface of the glass slide at which the bead was deposited. Typical results are shown in Fig. 3a. As can be seen the fluorescent intensity is practically the same in the center of the bead and outside the bead i.e. in pure toluene, reflecting similar dye molecules concentration. Furthermore stronger intensity is observed from the bead surface probably because some dye molecules adsorbed there. We have compared the diffusion of the tracer in pure toluene and in the center of a 4–8% chemically cross-linked PS bead swollen in toluene. The corresponding normalized autocorrelation functions are shown in Fig. 3b. The characteristic diffusion times which show how long the tracer stays in the confocal observation volume are sD1 = 36 ± 1.8 ls in the pure toluene (dotted curve) and sD2 = 1810 ± 90 ls in the center of the bead (solid curve). They corresponds to diffusion coefficients of 2 106 and 4 108 cm2/s, respectively. The approximately 50 times slower diffusion inside the microbead is caused by the interactions of the tracers with the dense network of cross-linked PS chains. A stronger
15
10
5
Iin
0 -40
-20
0
Iout
20
40
60
z [µm]
b
1.2 1.0
[G(τ)-1] / [G(0)-1]
Fig. 2. Confocal laser scanning microscopy (CLSM) images of PS beads in toluene. Excitation wavelength is 488 nm and emission is collected after a 505LP filter. (a) 4–8% chemically cross-linked bead imaged in reflection. (b) Fluorescence image of a 4–8% chemically cross-linked bead after exposure to UV for 5 h.
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0.8 0.6 0.4
D3 D1 D2
0.2 0.0 -5 1x10
-4
1x10
-3
10
-2
10
-1
10
0
10
τ [s] Fig. 3. (a) Fluorescence intensity caused by the tracer molecules vs. distance from the glass substrate surface. (b) Autocorrelation curves of tracer molecules diffusing in: pure toluene (dot curve), in chemically 4–8% cross-linked PS particle (solid curve), and in the particle exposed to UV (dash curve). The vertical lines indicate the respective diffusion times: sD1 36 ls, sD2 1810 ls and sD3 1900 ls.
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R. Zhang et al. / Microelectronic Engineering 85 (2008) 1261–1264
crosslinking should lead to less swelling and consequently even smaller diffusion coefficients. We have therefore measured the tracer diffusion in a 4–8% chemically cross-linked PS beads exposed to UV light for 2 h and then swollen in toluene. The corresponding normalized autocorrelation curve is shown in Fig. 3b (dashed curve). As can be seen the diffusion time of the dye molecules in the center of this particle sD3 = 1900 ± 95 ls and the corresponding diffusion coefficient 3.8 108 cm2/s are very similar to those measured in non UV irradiated PS microbead. This indicates that the UV irradiation has no influence on the swelling properties of the microbead core. This is probably a result of the rather short penetration depth of the UV light in the PS particles. Further studies including longer UV irradiations, various degrees of chemical crosslinking and FCS measurements performed at different positions in the bead are needed in order to probe an eventual gradient of the UV crosslinking from the surface to the center of the PS beads. 4. Conclusion Our study shows that UV light irradiation of PS microbeads can be used for controlling the solvent uptake and release upon exposure to its vapor. We found that the UV light irradiation of PS microbeads leads to additional crosslinking and the formation of a harder shell with typical thickness in the order of 1–2 lm. Quite remarkably this shell shows considerable fluorescence. Furthermore chemically and UV cross-linked PS beads keep their shapes even after numerous swelling cycles in saturated toluene vapors. Local UV exposure of selected areas could allow fine-tuning of the shape of polymer components. Such elements might be used to uptake or release molecules or as microreactors.
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