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Solvent sensitive polymer composite structures
Optical Materials 36 (2014) 130–134
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Solvent sensitive polymer composite structures A. Chiappini a,⇑, C. Armellini a, A. Carpentiero a, L. Minati a, G.C. Righini b,c, M. Ferrari a a
CNR-IFN CSMFO Lab., Via alla Cascata 56/C, 38123 Povo, Trento, Italy Centro Fermi, Piazza del Viminale 1, 00184 Roma, Italy c CNR-IFAC, Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy b
a r t i c l e
i n f o
Article history: Available online 12 June 2013 Keywords: Colloidal crystal Elastomeric matrix Band gap Solvent sensitive
a b s t r a c t In this paper we describe a composite system based on polystyrene colloidal nanoparticles assembled and embedded in an elastomeric matrix (polymer colloidal crystal, PCC), in the specific we have designed a PCC structure which displays an iridescent green color that can be attributed to the photonic crystal effect. This effect has been exploited to create a chemical sensor, in fact optical measurements have evidenced that the composite structure presents a different optical response as a function of the solvent applied on the surface. In particular we have demonstrated that the PCC possess, for specific solvents: (i) high sensitivity, (ii) fast response (less than 1s), and (iii) reversibility of the signal change. Finally preliminary results on the PCC have shown that this system can be also used as optical writing substrate using a specific solvent as ink, moreover an erasing procedure is also reported and discussed. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Photonic band gap (PBG) crystals composed of spatially ordered dielectrics with lattice parameters comparable to the wavelength of visible light have received much attention due to their unique properties in controlling the propagation of light. In the specific, exploiting the features of these periodic structures, in the last two decades the PBG materials have been applied to several fields such as communications [1,2], sensors [3], templates [4,5], and color displays [6]. In this contest particular attention has been devoted to the formation of 3D Photonic Crystals (PCs) derived from self assembled colloids because of their relative easiness of preparation and the low cost associated with their manufacture [7,8]. In fact it is well known that monodisperse colloidal spheres of silica or polystyrene (PS) self-organize spontaneously into crystal structures at optical wavelength scales with long range periodicity and possess an optical pseudo PBG in the visible and NIR spectral range. In general the wavelength of light diffracted from a threedimensional colloidal crystal is expressed by the Bragg equation [5]; where any variation in factors such as lattice distance, particle size and refractive index contrast between the particles and the background medium can lead to change of the structural color. These properties have been exploited for the realization of systems sensitive to external stimuli working in the visible range, where the response of the PCs (color changes) can be visually tracked by naked eye. Generally two different approaches can be used for the fabrication of stimulus responsive materials: (i) the responsive ⇑ Corresponding author. E-mail address: [email protected] (A. Chiappini). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.04.030
materials (RMs) are directly prepared in the form of building blocks that can be used for the realization of the colloidal crystal and (ii) the RM are filled into the interstitial spaces of the CC to form a composite material that is optically and mechanically stable. To date the two above approaches have been used for the realization of several types of responsive PCs that can be used in different fields concerning the realization of color displays [9,10], biological and chemical sensors [11–14], physical sensors [15– 17], inks and paints [18–20] and many other optical components [21,22]. In this paper we describe the protocol developed for the fabrication of a simple, convenient and low-cost colloidal crystal embedded in an elastomeric matrix. Moreover an exhaustive structural and optical characterization has been carried out and discussed to better understand the properties of this composite system after each step necessary for the realization of the PCC structure. Optical measurements have shown that the polymeric structure presents a different optical response as a function of the solvent applied on the surface, indicating that this system can be used as a chemical sensor. Finally we have investigated and demonstrated that PCC materials can be used as rewriting substrates using a specific solvent as ink. 2. Experimental 2.1. Materials Styrene monomer, sodium dodecyl sulfate, potassium persuphate (KPS), sodium hydroxide, ethylene glycol, ter-butyl alcohol, methanol were purchased from Aldrich. Sylgard 184 and silicon
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fluids 1cSt and 10cSt were purchased from Dow Corning. Polyethylene terephtalate (PET) sheet was used as polymeric substrate. 2.2. Polystyrene spheres synthesis Latex spheres have been synthesized according to a single-stage polymerization process [23]. Briefly, the polymerization was carried out in a 500 ml glass reactor equipped with a stirrer, a reflux condenser and a heating jacket. All chemicals were used as received, only styrene monomer was washed with NaOH and water to remove the polymerization inhibitor. The standard procedure is as follows: water 245 ml, sodium dodecyl sulfate 0.081 g dissolved in 13.6 ml of water and styrene monomer 27.2 ml, were premixed in the reactor at the temperature of 80 °C for 2 min. and at a stirrer speed of 300 rpm. To start the polymerization an amount equal to 0.952 g of KPS dissolved in 13.6 ml of water was injected. After 4 h the polymerization was completed and after cooling down the colloidal solution was purified by repeated centrifugation/redispersion cycles. 2.3. Colloidal crystal template preparation and infiltration with PDMS A schematic outline of the procedure for the realization of polymeric the composite system is reported Fig. 1. Step 1 concerns the assembly of the PS NPs in a ordered fcc structures, this has been obtained using vertical deposition method [16]. First of all the polymeric substrate was treated by ozone in order to increase its hydrophilicity, then it was positioned vertically in 4 ml of water solution containing PS NPs in a dilution ratio 20:1. The entire apparatus was placed in a temperature controlled oven (45 °C) and submitted to evaporation. An iridescent and homogeneous colloidal crystal was obtained after 48 h. This method permitted to organize PS spheres on a 1.5 cm2 area (polymer substrate 30 15 0.25 mm) into a 3D ordered structure. Step 2 and Step 3 are about the infiltration of the bare opal by pouring a PDMS solution into the voids of the colloidal crystal. The elastomer was supplied as a kit with two separate components: base and curing agent; we mixed base and curing agent in a 3:1 ratio. After infiltration the PCC structure was cured for 4 h at 65 °C and then the excess elastomer was peeled-off from the crystal. The composite film obtained was a 3D lattice of PS spheres embedded in a PDMS matrix. 2.4. Characterization SEM measurements, acquired using a JEOL field emission microscope, where performed in order to determine the diameter of the
PS particles synthesized and to verify the dimension of the ordered domains of the crystalline structure. All samples were coated with gold by sputtering prior to observation. Digital pictures were captured using NIKON COOLPIX P510 camera. The reflection spectra of PCC on polymeric substrate were taken using a fiber-optic UV–VIS spectrometer (Ocean Optics, USB4000) with a beam spot of about 1 mm2 in area. 3. Results and discussion 3.1. Bare colloidal crystal and infiltrated opal Polystyrene (PS) colloids were self-assembled on a polymeric substrate by a simple and efficient vertical deposition method. During this process, spheres are organized in a face centered cubic (fcc) structure by the capillary forces that exist in the meniscus between the substrate and the colloidal suspension as previously reported [16]. In Fig. 2a is reported a typical SEM image of the top surface of the opal structure realized, where we can observe a long order periodicity and a hexagonal arrangement attributable to the h1 1 1i plane of the fcc structure [5]. In Fig. 2b is reported a photograph of the opal, where a clear green opalescence is evident. Optical characterization of the bare opal as well as the infiltrated opal was performed using a fiberoptic UV–VIS spectrometer [16]. In Fig. 3 are reported the reflectance spectra obtained on bare opal (black curve) and after infiltration and peeling off process (dotted and dashed curve respectively). Analyzing Fig. 3 (straight curve) we can notice the presence of a main peak centered at about 520 nm attributed to the diffraction peak due to the periodic arrangement of the nanospheres in a ordered structures. The wavelength of the diffracted light can be predicted using a modified form of Bragg’s law (Eq. (1)), this equation takes into account the reduced angle with respect to the normal that light experiences on entering a medium with average refractive index neff, i.e., taking into account Snell’s law of refraction [4]. 2
m kpeak ¼ 2 d<111> ðn2eff sin ðhÞÞ1=2
ð1Þ
where m is an integer (corresponding to the diffraction order), kpeak is the Bragg diffracted qffiffi wavelength (or PBG), d<111> is the interplanar distance (d<111> ¼ 23 D, where D is the diameter of colloidal particles), neff is the effective refractive index of the crystal, and h is the incidence angle. In our experimental conditions, the spectra were collected under normal incidence (h = 0°), and so Eq. (1) can be modified as
kpeak ¼ 2 d<111> neff
ð2Þ
Fig. 1. Scheme of the steps used for the fabrication of polymer composite structures: (i) assembly of PS NPs in a ordered fcc structure (ii) infiltration of the opal system with PDMS and (iii) peeling off of the extra amount of PDMS.
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Fig. 2. (a) SEM image showing the surface of the closely packed colloidal crystal (scale 1 lm) and (b) photograph of the opal where a green opalescence can be seen by naked eye (left to right). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Moreover, considering a refractive index of the PDMS equal to 1.40 and using the Bragg Law equations, we can clearly observe that there is a reasonable match between the measured position of the diffraction peak (dotted curve) with the predicted one, indicating that the infiltration step has been successfully obtained. Analogously, the peeling-off process was evaluated comparing the wavelength position of the dotted and dashed curves reported in Fig. 3; in fact analyzing these spectra we can clearly see that the position as well as the shape of the diffraction peak after the process (dashed curve) is unchanged, suggesting that the peeling off doesn’t affect the infiltration step. 3.2. Colorimetric detection of solvents using colloidal crystals
Fig. 3. Straight curve corresponds to the reflectance of the bare opal; dotted curve represents the reflectance spectrum after infiltration with PDMS; and dashed curve is about the reflectance spectrum after peeling off process.
The effective refractive index (neff) can be calculated using Eq. (3)
n2eff ¼ n2spheres f þ n2air ð1 f Þ
ð3Þ
that takes into account the indices of polystyrene (nspheres) and air (nair), and the volume fraction f (74% for ideal close packed spheres) occupied by the PS spheres according to what demonstrated in [23]. Now, considering the dimension of the NPs, obtained from SEM measurements, using Eq. (2) and comparing the experimental value obtained with the theoretical one, we can observe that there is a good agreement, confirming that effectively the PS nanoparticles are assembled in a ordered fcc structure. The evaluation of the infiltration process of elastomeric matrix (PDMS) in the voids of the opal structure was performed analyzing the position of the diffraction peak and considering Eq. (2) and (3). Analyzing the reflectance spectrum of the infiltrated bare opal with PDMS (Fig. 3 dotted curve) we can clearly notice a red shift of the diffraction peak, attributed to an increasing of the effective refractive index of the composite system as well as a reduction of full width at half maximum (FWHM) in agreement to that predicted by Vardeny [24].
The development of suitable polymeric systems is a crucial point for the realization of new flexible and low power displays as well as for the fabrication of low cost all optical chemical sensors. The main advantages concern their work of principle, in fact they do not need external power supply but they are based on a variation of their structural properties. First of all we have demonstrated that this structure can be used as chemical sensor, in fact we have verified that the PCC system presents a different response as a function of the organic solvent dropped on the surface. In the specific we used the following chemicals: ethylene glycol, ter-butyl alcohol, methanol, 1cSt and 10cSt silicon fluid, because they present different capability to swell the polymeric matrix as reported by Lee [25]. In Fig. 4 is shown the reflectance spectra acquired after spotting 0.5 ll of the above solvents onto the ‘‘sensor’’ surface, and monitoring the optical change of the diffraction peak; analyzing Fig. 4 we can observe a red shift in the wavelength position of the diffraction peak as a function of the different types of solvents used. In particular we can clearly observe that higher is the swelling ratio of PDMS with the solvent, bigger is the red shift of the diffraction peak of the PCC. This can be associated to a variation of the interplanar distance of the colloidal crystal due to the swelling of PDMS. This assertion is also confirmed analyzing the position of the diffraction peak after spotting ethylene glycol having a swelling ratio of 1, we can notice that in this case the position of the diffraction peak remains fixed at about 570 nm, indicating that, as expected, ethylene glycol cannot produce variation in the properties of the PCC structure and so any variation in the position of the diffraction peak can be detected. Moreover, it is important to highlight that the
A. Chiappini et al. / Optical Materials 36 (2014) 130–134
Fig. 4. Black line corresponds to the reflectance of the polymeric composite system. Reflectance spectra acquired after spotting 0.5 ll of different solvents onto the ‘‘sensor’’ surface, and monitoring the optical change of the diffraction peak: (yellow line) ethylene glycol, (green line) methanol, (blue line) ter-butyl alcohol and (red line) 1cSt silicon fluid. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
measurements were repeated several times, and the material was found to display highly reproducible changes in the optical spectra upon cycling. Furthermore, analyzing the spectra reported in Fig. 4 we can clearly notice that the PCC system possess a chromatic behavior as a function of the solvent used. In details, using silicon fluid, we can obtain an opalescence in the red region of the electromagnetic spectrum. This characteristic has been exploited in order to demonstrate that this structure can be used for the realization of a ‘‘reversible writing substrate’’. In Fig. 5 are reported a sequence of optical photographs showing the PCC system at different stages: (a) initial, (b) after writing, using a common office stamp, (c) after tens of seconds following the stamping step, and (d) after erasing. The stamping step was obtained through a commercial stamp and simulating the ‘‘standard
133
Fig. 6. Reflectance spectra of the PCC (black curve) before stamping step and (red curve) after erasing process. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
office operation’’. In our case the normal ink was replaced by silicon fluid (10cSt). From Fig. 5b we can clearly notice that it is possible to transfer on the polymeric colloidal crystal a symbol, represented in this case by the ‘‘S’’ letter; moreover we can clearly distinguish the ‘‘S’’ letter on the surface of the PCC with a resolution comparable with that achievable using a normal office ink. Additionally, as reported in Fig. 5c, we have shown that the symbol can be clearly seen after tens of second (30 s) without losing in resolution, indicating that effectively this type of composite material could be used for the realization of optical paper. However it is also important to point out that the quality in term of brightness and resolution of the ‘‘stamped letter’’ strongly depends on the amount of silicon fluid applied and present on the surface of the stamp used, but this parameter is under investigation and lies outside the aim of this work.
Fig. 5. Optical photographs showing the PCC system at different stages: (a) initial, (b) after writing, using a common office stamp, (c) after tens of seconds following the stamping step and (d) after erasing (clockwise).
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Fig. 5d represents an optical micrograph of the PCC sample after erasing process; the development of a suitable procedure to erase the ‘‘stamped’’ object is a crucial point for the fabrication of new and low cost displays working without the need of additional illuminating power. In the specific the erasing process consisted in adding an extra amount of silicon fluid that presented a lower molecular weight (1cSt) respect to what had been used as ink (10cSt). The low viscosity of the 1cSt permits to penetrate deep into the PCC structure and mix and dilute with the silicon fluid used as ink. The faster evaporation of the 1cSt silicon fluid respect to that of 10cSt, permits to clear and remove the stamped image, bringing the PCC to his original opalescence. This can be observed comparing the optical micrograph reported in Fig. 5a and d, in fact we can clearly notice that the erasing step does not produce any degradation of the opalescence of the polymeric composite structure. This assertion is also confirmed by optical measurements, in fact analyzing Fig. 6 we can observe that the position of the diffraction peak before and after erasing process is unchanged, indicating that stamp process as well as erasing step don’t change the optical properties of the PCC system. 4. Conclusions A low cost polymeric composite system, based on a PS colloidal crystal, has been fabricated and characterized from a structural and optical point of view; in particular we have designed a PCC structure whose optical response depends on its structural properties that change as a function of the solvent applied on the surface. In fact we have verified that higher is the swelling ratio of PDMS with the solvent, bigger is the red shift of the diffraction peak of the PCC. Moreover, we have demonstrated that the PCC possess: (i) high sensitivity, (ii) fast response (less than 1s), and (iii) reversibility of the signal change. Finally exploiting the chromatic behavior of the PCC system we have verified that for specific solvents (such as silicon fluid) it is possible to transfer on the surface of the PCC structure images with a resolution comparable with that achievable using a normal office ink, indicating that this type of composite materials can be used for the realization of writing substrates and for the development of color displays.
Acknowledgement We would like to thank CARITRO foundation for supporting SiMeCro research project. References [1] J.R. Lawrence, Y. Ying, P. Jiang, S.H. Foulger, Advanced Materials 18 (2006) 300– 303. [2] M.C. George, A. Mohraz, M. Piech, N.S. Bell, J.A. Lewis, P.V. Braun, Advanced Materials 21 (2009) 66–70. [3] S. Guddala, S. Kamanoor, A. Chiappini, M. Ferrari, N.D. Rao, Journal of Applied Physics 112 (2012) 084303–1-084303-7. [4] H. Fudouzi, Journal of Colloid and Interface Science 275 (2004) 277–283. [5] C. Lopez, Advanced Materials 15 (2003) 1679–1704. [6] A.C. Arsenault, D.P. Puzzo, I. Manners, G.A. Ozin, Nature Photonics 1 (2007) 468–472. [7] C. Gonçalves, L.M. Fortes, R.M. Almeida, A. Chiasera, A. Chiappini, M. Ferrari, Optical Materials 31 (2009) 1315–1318. [8] A. Blanco, E. Chomski, S. Grabtchak, M. Ibisate, S. John, S.W. Leonard, C. Lopez, F. Meseguer, H. Míguez, J.P. Mondia, G.A. Ozin, O. Toader, H.M. van Driel, Nature 405 (2000) 437–440. [9] J. Ge, S. Kwon, Y. Yin, Journal of Materials Chemistry 20 (2010) 5777–5784. [10] Y. Takeoka, M. Honda, T. Seki, M. Ishii, H. Nakamura, ACS Applied Materials & Interfaces 1 (2009) 982–986. [11] F. Fleischhaker, A.C. Arsenault, F.C. Peiris, V. Kitaev, I. Manners, R. Zentel, G.A. Ozin, Advanced Materials 18 (2006) 2387–2391. [12] W. Shen, M. Li, C. Ye, L. Jiang, Y. Song, Lab Chip 12 (2012) 3089–3095. [13] Z. Shen, Y. Yang, F. Lu, B. Bao, B. You, L. Shi, Journal of Colloid and Interface Science 389 (2013) 77–84. [14] J. Shin, S. Gu Han, W. Lee, Sensors and Actuators B 168 (2012) 20–26. [15] Y. Ying, S.H. Foulger, Sensors and Actuators B. 137 (2009) 574–577. [16] A. Chiappini, A. Chiasera, S. Berneschi, C. Armellini, A. Carpentiero, M. Mazzola, E. Moser, S. Varas, G.C. Righini, M. Ferrari, Journal of Sol–Gel Science and Technology 60 (2011) 408–425. [17] H. Fudouzi, T. Sawada, Langmuir 22 (2006) 1365–1368. [18] P. Kang, S.O. Ogunbo, D. Erickson, Langmuir 27 (2011) 9676–9680. [19] H. Fudouzi, Y. Xia, Advanced Materials 15 (2003) 892–896. [20] C.I. Aguirre, E. Reguera, A. Stein, Advanced Functional Materials 20 (2010) 2565–2578. [21] J. Ge, Y. Hu, T. Zhang, T. Huynh, Y. Yin, Langmuir 24 (2008) 3671–3680. [22] Xiao-yong Guo, Shu-chen Lü, Physical Review A 80 (2009) 043826-1–0438269. [23] A. Chiappini, C. Armellini, A. Chiasera, M. Ferrari, L. Fortes, M.C. Goncalves, R. Guider, Y. Jestin, R. Retoux, G. Nunzi Conti, S. Pelli, Rui M. Rui, G.C. Righini, Journal of Non-Crystalline Solids 355 (2009) 1167–1170. [24] M.N. Shkunov, Z.V. Vardeny, M.C. DeLong, R.C. Polson, A.A. Zakhidov, R.H. Baughman, Advanced Functional Materials 12 (2002) 21–26. [25] J. Ng Lee, C. Park, G.M. Whitesides, Analytical Chemistry 75 (2003) 6544–6554.
Contents lists available at SciVerse ScienceDirect
Optical Materials journal homepage: www.elsevier.com/locate/optmat
Solvent sensitive polymer composite structures A. Chiappini a,⇑, C. Armellini a, A. Carpentiero a, L. Minati a, G.C. Righini b,c, M. Ferrari a a
CNR-IFN CSMFO Lab., Via alla Cascata 56/C, 38123 Povo, Trento, Italy Centro Fermi, Piazza del Viminale 1, 00184 Roma, Italy c CNR-IFAC, Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy b
a r t i c l e
i n f o
Article history: Available online 12 June 2013 Keywords: Colloidal crystal Elastomeric matrix Band gap Solvent sensitive
a b s t r a c t In this paper we describe a composite system based on polystyrene colloidal nanoparticles assembled and embedded in an elastomeric matrix (polymer colloidal crystal, PCC), in the specific we have designed a PCC structure which displays an iridescent green color that can be attributed to the photonic crystal effect. This effect has been exploited to create a chemical sensor, in fact optical measurements have evidenced that the composite structure presents a different optical response as a function of the solvent applied on the surface. In particular we have demonstrated that the PCC possess, for specific solvents: (i) high sensitivity, (ii) fast response (less than 1s), and (iii) reversibility of the signal change. Finally preliminary results on the PCC have shown that this system can be also used as optical writing substrate using a specific solvent as ink, moreover an erasing procedure is also reported and discussed. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Photonic band gap (PBG) crystals composed of spatially ordered dielectrics with lattice parameters comparable to the wavelength of visible light have received much attention due to their unique properties in controlling the propagation of light. In the specific, exploiting the features of these periodic structures, in the last two decades the PBG materials have been applied to several fields such as communications [1,2], sensors [3], templates [4,5], and color displays [6]. In this contest particular attention has been devoted to the formation of 3D Photonic Crystals (PCs) derived from self assembled colloids because of their relative easiness of preparation and the low cost associated with their manufacture [7,8]. In fact it is well known that monodisperse colloidal spheres of silica or polystyrene (PS) self-organize spontaneously into crystal structures at optical wavelength scales with long range periodicity and possess an optical pseudo PBG in the visible and NIR spectral range. In general the wavelength of light diffracted from a threedimensional colloidal crystal is expressed by the Bragg equation [5]; where any variation in factors such as lattice distance, particle size and refractive index contrast between the particles and the background medium can lead to change of the structural color. These properties have been exploited for the realization of systems sensitive to external stimuli working in the visible range, where the response of the PCs (color changes) can be visually tracked by naked eye. Generally two different approaches can be used for the fabrication of stimulus responsive materials: (i) the responsive ⇑ Corresponding author. E-mail address: [email protected] (A. Chiappini). 0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.04.030
materials (RMs) are directly prepared in the form of building blocks that can be used for the realization of the colloidal crystal and (ii) the RM are filled into the interstitial spaces of the CC to form a composite material that is optically and mechanically stable. To date the two above approaches have been used for the realization of several types of responsive PCs that can be used in different fields concerning the realization of color displays [9,10], biological and chemical sensors [11–14], physical sensors [15– 17], inks and paints [18–20] and many other optical components [21,22]. In this paper we describe the protocol developed for the fabrication of a simple, convenient and low-cost colloidal crystal embedded in an elastomeric matrix. Moreover an exhaustive structural and optical characterization has been carried out and discussed to better understand the properties of this composite system after each step necessary for the realization of the PCC structure. Optical measurements have shown that the polymeric structure presents a different optical response as a function of the solvent applied on the surface, indicating that this system can be used as a chemical sensor. Finally we have investigated and demonstrated that PCC materials can be used as rewriting substrates using a specific solvent as ink. 2. Experimental 2.1. Materials Styrene monomer, sodium dodecyl sulfate, potassium persuphate (KPS), sodium hydroxide, ethylene glycol, ter-butyl alcohol, methanol were purchased from Aldrich. Sylgard 184 and silicon
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fluids 1cSt and 10cSt were purchased from Dow Corning. Polyethylene terephtalate (PET) sheet was used as polymeric substrate. 2.2. Polystyrene spheres synthesis Latex spheres have been synthesized according to a single-stage polymerization process [23]. Briefly, the polymerization was carried out in a 500 ml glass reactor equipped with a stirrer, a reflux condenser and a heating jacket. All chemicals were used as received, only styrene monomer was washed with NaOH and water to remove the polymerization inhibitor. The standard procedure is as follows: water 245 ml, sodium dodecyl sulfate 0.081 g dissolved in 13.6 ml of water and styrene monomer 27.2 ml, were premixed in the reactor at the temperature of 80 °C for 2 min. and at a stirrer speed of 300 rpm. To start the polymerization an amount equal to 0.952 g of KPS dissolved in 13.6 ml of water was injected. After 4 h the polymerization was completed and after cooling down the colloidal solution was purified by repeated centrifugation/redispersion cycles. 2.3. Colloidal crystal template preparation and infiltration with PDMS A schematic outline of the procedure for the realization of polymeric the composite system is reported Fig. 1. Step 1 concerns the assembly of the PS NPs in a ordered fcc structures, this has been obtained using vertical deposition method [16]. First of all the polymeric substrate was treated by ozone in order to increase its hydrophilicity, then it was positioned vertically in 4 ml of water solution containing PS NPs in a dilution ratio 20:1. The entire apparatus was placed in a temperature controlled oven (45 °C) and submitted to evaporation. An iridescent and homogeneous colloidal crystal was obtained after 48 h. This method permitted to organize PS spheres on a 1.5 cm2 area (polymer substrate 30 15 0.25 mm) into a 3D ordered structure. Step 2 and Step 3 are about the infiltration of the bare opal by pouring a PDMS solution into the voids of the colloidal crystal. The elastomer was supplied as a kit with two separate components: base and curing agent; we mixed base and curing agent in a 3:1 ratio. After infiltration the PCC structure was cured for 4 h at 65 °C and then the excess elastomer was peeled-off from the crystal. The composite film obtained was a 3D lattice of PS spheres embedded in a PDMS matrix. 2.4. Characterization SEM measurements, acquired using a JEOL field emission microscope, where performed in order to determine the diameter of the
PS particles synthesized and to verify the dimension of the ordered domains of the crystalline structure. All samples were coated with gold by sputtering prior to observation. Digital pictures were captured using NIKON COOLPIX P510 camera. The reflection spectra of PCC on polymeric substrate were taken using a fiber-optic UV–VIS spectrometer (Ocean Optics, USB4000) with a beam spot of about 1 mm2 in area. 3. Results and discussion 3.1. Bare colloidal crystal and infiltrated opal Polystyrene (PS) colloids were self-assembled on a polymeric substrate by a simple and efficient vertical deposition method. During this process, spheres are organized in a face centered cubic (fcc) structure by the capillary forces that exist in the meniscus between the substrate and the colloidal suspension as previously reported [16]. In Fig. 2a is reported a typical SEM image of the top surface of the opal structure realized, where we can observe a long order periodicity and a hexagonal arrangement attributable to the h1 1 1i plane of the fcc structure [5]. In Fig. 2b is reported a photograph of the opal, where a clear green opalescence is evident. Optical characterization of the bare opal as well as the infiltrated opal was performed using a fiberoptic UV–VIS spectrometer [16]. In Fig. 3 are reported the reflectance spectra obtained on bare opal (black curve) and after infiltration and peeling off process (dotted and dashed curve respectively). Analyzing Fig. 3 (straight curve) we can notice the presence of a main peak centered at about 520 nm attributed to the diffraction peak due to the periodic arrangement of the nanospheres in a ordered structures. The wavelength of the diffracted light can be predicted using a modified form of Bragg’s law (Eq. (1)), this equation takes into account the reduced angle with respect to the normal that light experiences on entering a medium with average refractive index neff, i.e., taking into account Snell’s law of refraction [4]. 2
m kpeak ¼ 2 d<111> ðn2eff sin ðhÞÞ1=2
ð1Þ
where m is an integer (corresponding to the diffraction order), kpeak is the Bragg diffracted qffiffi wavelength (or PBG), d<111> is the interplanar distance (d<111> ¼ 23 D, where D is the diameter of colloidal particles), neff is the effective refractive index of the crystal, and h is the incidence angle. In our experimental conditions, the spectra were collected under normal incidence (h = 0°), and so Eq. (1) can be modified as
kpeak ¼ 2 d<111> neff
ð2Þ
Fig. 1. Scheme of the steps used for the fabrication of polymer composite structures: (i) assembly of PS NPs in a ordered fcc structure (ii) infiltration of the opal system with PDMS and (iii) peeling off of the extra amount of PDMS.
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Fig. 2. (a) SEM image showing the surface of the closely packed colloidal crystal (scale 1 lm) and (b) photograph of the opal where a green opalescence can be seen by naked eye (left to right). (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article).
Moreover, considering a refractive index of the PDMS equal to 1.40 and using the Bragg Law equations, we can clearly observe that there is a reasonable match between the measured position of the diffraction peak (dotted curve) with the predicted one, indicating that the infiltration step has been successfully obtained. Analogously, the peeling-off process was evaluated comparing the wavelength position of the dotted and dashed curves reported in Fig. 3; in fact analyzing these spectra we can clearly see that the position as well as the shape of the diffraction peak after the process (dashed curve) is unchanged, suggesting that the peeling off doesn’t affect the infiltration step. 3.2. Colorimetric detection of solvents using colloidal crystals
Fig. 3. Straight curve corresponds to the reflectance of the bare opal; dotted curve represents the reflectance spectrum after infiltration with PDMS; and dashed curve is about the reflectance spectrum after peeling off process.
The effective refractive index (neff) can be calculated using Eq. (3)
n2eff ¼ n2spheres f þ n2air ð1 f Þ
ð3Þ
that takes into account the indices of polystyrene (nspheres) and air (nair), and the volume fraction f (74% for ideal close packed spheres) occupied by the PS spheres according to what demonstrated in [23]. Now, considering the dimension of the NPs, obtained from SEM measurements, using Eq. (2) and comparing the experimental value obtained with the theoretical one, we can observe that there is a good agreement, confirming that effectively the PS nanoparticles are assembled in a ordered fcc structure. The evaluation of the infiltration process of elastomeric matrix (PDMS) in the voids of the opal structure was performed analyzing the position of the diffraction peak and considering Eq. (2) and (3). Analyzing the reflectance spectrum of the infiltrated bare opal with PDMS (Fig. 3 dotted curve) we can clearly notice a red shift of the diffraction peak, attributed to an increasing of the effective refractive index of the composite system as well as a reduction of full width at half maximum (FWHM) in agreement to that predicted by Vardeny [24].
The development of suitable polymeric systems is a crucial point for the realization of new flexible and low power displays as well as for the fabrication of low cost all optical chemical sensors. The main advantages concern their work of principle, in fact they do not need external power supply but they are based on a variation of their structural properties. First of all we have demonstrated that this structure can be used as chemical sensor, in fact we have verified that the PCC system presents a different response as a function of the organic solvent dropped on the surface. In the specific we used the following chemicals: ethylene glycol, ter-butyl alcohol, methanol, 1cSt and 10cSt silicon fluid, because they present different capability to swell the polymeric matrix as reported by Lee [25]. In Fig. 4 is shown the reflectance spectra acquired after spotting 0.5 ll of the above solvents onto the ‘‘sensor’’ surface, and monitoring the optical change of the diffraction peak; analyzing Fig. 4 we can observe a red shift in the wavelength position of the diffraction peak as a function of the different types of solvents used. In particular we can clearly observe that higher is the swelling ratio of PDMS with the solvent, bigger is the red shift of the diffraction peak of the PCC. This can be associated to a variation of the interplanar distance of the colloidal crystal due to the swelling of PDMS. This assertion is also confirmed analyzing the position of the diffraction peak after spotting ethylene glycol having a swelling ratio of 1, we can notice that in this case the position of the diffraction peak remains fixed at about 570 nm, indicating that, as expected, ethylene glycol cannot produce variation in the properties of the PCC structure and so any variation in the position of the diffraction peak can be detected. Moreover, it is important to highlight that the
A. Chiappini et al. / Optical Materials 36 (2014) 130–134
Fig. 4. Black line corresponds to the reflectance of the polymeric composite system. Reflectance spectra acquired after spotting 0.5 ll of different solvents onto the ‘‘sensor’’ surface, and monitoring the optical change of the diffraction peak: (yellow line) ethylene glycol, (green line) methanol, (blue line) ter-butyl alcohol and (red line) 1cSt silicon fluid. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
measurements were repeated several times, and the material was found to display highly reproducible changes in the optical spectra upon cycling. Furthermore, analyzing the spectra reported in Fig. 4 we can clearly notice that the PCC system possess a chromatic behavior as a function of the solvent used. In details, using silicon fluid, we can obtain an opalescence in the red region of the electromagnetic spectrum. This characteristic has been exploited in order to demonstrate that this structure can be used for the realization of a ‘‘reversible writing substrate’’. In Fig. 5 are reported a sequence of optical photographs showing the PCC system at different stages: (a) initial, (b) after writing, using a common office stamp, (c) after tens of seconds following the stamping step, and (d) after erasing. The stamping step was obtained through a commercial stamp and simulating the ‘‘standard
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Fig. 6. Reflectance spectra of the PCC (black curve) before stamping step and (red curve) after erasing process. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
office operation’’. In our case the normal ink was replaced by silicon fluid (10cSt). From Fig. 5b we can clearly notice that it is possible to transfer on the polymeric colloidal crystal a symbol, represented in this case by the ‘‘S’’ letter; moreover we can clearly distinguish the ‘‘S’’ letter on the surface of the PCC with a resolution comparable with that achievable using a normal office ink. Additionally, as reported in Fig. 5c, we have shown that the symbol can be clearly seen after tens of second (30 s) without losing in resolution, indicating that effectively this type of composite material could be used for the realization of optical paper. However it is also important to point out that the quality in term of brightness and resolution of the ‘‘stamped letter’’ strongly depends on the amount of silicon fluid applied and present on the surface of the stamp used, but this parameter is under investigation and lies outside the aim of this work.
Fig. 5. Optical photographs showing the PCC system at different stages: (a) initial, (b) after writing, using a common office stamp, (c) after tens of seconds following the stamping step and (d) after erasing (clockwise).
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Fig. 5d represents an optical micrograph of the PCC sample after erasing process; the development of a suitable procedure to erase the ‘‘stamped’’ object is a crucial point for the fabrication of new and low cost displays working without the need of additional illuminating power. In the specific the erasing process consisted in adding an extra amount of silicon fluid that presented a lower molecular weight (1cSt) respect to what had been used as ink (10cSt). The low viscosity of the 1cSt permits to penetrate deep into the PCC structure and mix and dilute with the silicon fluid used as ink. The faster evaporation of the 1cSt silicon fluid respect to that of 10cSt, permits to clear and remove the stamped image, bringing the PCC to his original opalescence. This can be observed comparing the optical micrograph reported in Fig. 5a and d, in fact we can clearly notice that the erasing step does not produce any degradation of the opalescence of the polymeric composite structure. This assertion is also confirmed by optical measurements, in fact analyzing Fig. 6 we can observe that the position of the diffraction peak before and after erasing process is unchanged, indicating that stamp process as well as erasing step don’t change the optical properties of the PCC system. 4. Conclusions A low cost polymeric composite system, based on a PS colloidal crystal, has been fabricated and characterized from a structural and optical point of view; in particular we have designed a PCC structure whose optical response depends on its structural properties that change as a function of the solvent applied on the surface. In fact we have verified that higher is the swelling ratio of PDMS with the solvent, bigger is the red shift of the diffraction peak of the PCC. Moreover, we have demonstrated that the PCC possess: (i) high sensitivity, (ii) fast response (less than 1s), and (iii) reversibility of the signal change. Finally exploiting the chromatic behavior of the PCC system we have verified that for specific solvents (such as silicon fluid) it is possible to transfer on the surface of the PCC structure images with a resolution comparable with that achievable using a normal office ink, indicating that this type of composite materials can be used for the realization of writing substrates and for the development of color displays.
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