pubs.acs.org/Langmuir © 2010 American Chemical Society
Time-Temperature Integrator Based on the Dewetting of Polyisobutylene Thin Films Annalisa Calo,† Pablo Stoliar,† Francesco Cino Matacotta,‡,§ Massimiliano Cavallini,*,†,§ and Fabio Biscarini†,§ † CNR-ISMN, Via P. Gobetti 101, 40121 Bologna, Italy, ‡CNR-ISM, Via Salaria km 29,300-00016 , Monterotondo Scalo, Roma, Italy, and §Scriba Nanotecnologie srl, Via P. Gobetti 52/3, 40129 Bologna, Italy
Received February 26, 2010. Revised Manuscript Received March 16, 2010 This work reports the application of a patterned thin film of polyisobutylene (PIB) irradiated with an electron beam as a time-temperature integrator, i.e., a device that is able to record the thermal history of a product. The device is fabricated by irradiation with an electron beam of regions of a PIB thin film to different doses of electrons. A different dewetting behavior occurs at these regions upon thermal exposure, depending on the dose. The experimental results are quantified by means of a model of dewetting based on nucleation and growth of holes in a strong slippage regime.
The study of devices able to record the thermal history of perishable products is an important field in industrial research, the conservation of food and drugs, or any product whose temperature must be maintained below a threshold throughout a shipping cycle).1 The most reliable devices are based on electronic sensors such as RFIDs or more complex but very efficient layouts. In these cases, the high cost, the complexity of some components, and the necessity of a battery are major limitations to their large-scale application. Other approaches to timetemperature integrators (TTI) are based on irreversible changes, usually expressed as a visible response, in the form of a mechanical deformation, color development,2 or color displacement.3 Proposed TTI are based on (i) the diffusion of a dyed ester along a stick4 or a viscoelastic material into a light-reflective porous matrix;5 (ii) enzymatic hydrolysis of a lipid substrate;6,7 (iii) patterned discotic liquid crystals;8 (iv) a solid-state polymerization reaction resulting in a highly colored polymer; and (v) reaction occurring in an extracellular bacterial medium.9 All of the proposed devices exhibit important advantages with respect to electronic sensors; however, their complexity dramatically increases when they are requested to monitor different temperatures instead of only a threshold temperature.4 In these cases, it is necessary to introduce in the same device different materials/components, with each one being sensitive to a specific critical temperature. The use of more than one material complicates the fabrication process, affecting the cost of production, especially in view of a possible scaling up.3 It would be desirable to *Corresponding author. E-mail:
[email protected]. (1) Ahvenainen, R. Novel Food Packaging Techniques; Woodhead Publishing Limited and CRC Press LLC: Cambridge, England, 2003; Chapter 2. (2) Kreyenschmidt, J.; Christiansen, H.; Hubner, A.; Raab, V.; Petersen, B. Int. J. Food Sci. Technol. 2010, 45, 208–215. (3) Taoukis, P. S.; Labuza, T. P. Novel Food Packaging Techniques; Woodhead Publishing Limited and CRC Press LLC: Cambridge, England, 2003. Chapter 6. (4) Manske, W. J. U.S. Patent 3,954,011, 1976. (5) Shimoni, E.; Anderson, E. M.; Labuza, T. P. J. Food Sci. 2001, 66, 1337– 1340. (6) Taoukis, P. S.; Labuza, T. P. J. Food Sci. 1989, 54, 783–788. (7) Guiavarc’h, Y.; Van Loey, A.; Zuber, F.; Hendrickx, M. Biotechnol. Bioeng. 2004, 88, 15–25. (8) Cavallini, M.; Calo, A.; Stoliar, P.; Kengne, J. C.; Martins, S.; Matacotta, F. C.; Quist, F.; Gbabode, G.; Dumont, N.; Geerts, Y. H.; Biscarini, F. Adv. Mater. 2009, 21, 4688–4691. (9) Tucker, G. S.; Brown, H. M.; Fryer, P. J.; Cox, P. W.; Poole, F. L.; Lee, H. S.; Adams, M. W. W. Innovative Food Sci. Emerging Technol. 2007, 8, 63–72.
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integrate monolithically, viz., on the same material, the capability to record different temperatures. Here, we describe a new application of an organic thin film as a time-temperature integrator that addresses this problem. This film consists of polyisobuthylene (PIB) that is locally exposed to different doses of electrons. Control of the critical temperature is achieved by tuning the radiation dose. The proposed device is based on the quantitative analysis of different dewetting rates observed upon thermal exposure of the PIB thin film previously patterned with controlled doses of electrons. Dewetting is a spontaneous process, which has been observed in thin films of polymers10,11 or molecular materials;12-14 its time evolution leads to the rupture of an initially continuous liquid or solid film. Dewetting can occur by the nucleation of dry spots (holes),15,16 ripening,17 spontaneous amplification of capillary waves (spinodal dewetting),18 or a combination of them.16 In our experiment, we used thin films of commercial PIB that were ∼30 nm thick deposited on a silicon oxide substrate by spin coating. PIB is an easily processable polymer that can be dissolved in apolar solvents, dispersed in polymeric matrices, and processed in rolling mills, presses, calenders, and extrusion machines. These properties cause PIB to be a suitable material for many industrial processes. When PIB interacts with 30 keV electrons, it mainly undergoes chain-scission reactions, which ultimately give rise to a permanent decrease in its molecular weight;19 PIB exhibits (10) Reiter, G. Langmuir 1993, 9, 1344–1351. (11) Bischof, J.; Scherer, D.; Herminghaus, S.; Leiderer, P. Phys. Rev. Lett. 1996, 77, 1536–1539. (12) (a) Cavallini, M.; Lazzaroni, R.; Zamboni, R.; Biscarini, F.; Timpel, D.; Zerbetto, F.; Clarkson, G. J.; Leigh, D. A. J. Phys. Chem. B 2001, 105, 10826– 10830. (b) Cavallini, M. J. Mater. Chem. 2009, 19, 6085–6092. (13) (a) Cavallini, M.; Facchini, M.; Albonetti, C.; Biscarini, F. Phys. Chem. Chem. Phys. 2008, 10, 784–793. (b) Cavallini, M.; Gomez-Segura, J.; Albonetti, C.; Ruiz-Molina, D.; Veciana, J.; Biscarini, F. J. Phys. Chem. B 2006, 110, 11607–11610. (14) Calo, A.; Stoliar, P.; Cavallini, M.; Sergeyev, S.; Geerts, Y. H.; Biscarini, F. J. Am. Chem. Soc. 2008, 130, 11953–11958. (15) Sharma, A.; Reiter, G. J. Colloid Interface Sci. 1996, 178, 383–399. (16) Seemann, R.; Herminghaus, S.; Jacobs, K. Phys. Rev. Lett. 2001, 86, 5534– 5537. (17) Brinkmann, M.; Graff, S.; Biscarini, F. Phys. Rev. B 2002, 66, 165430. (18) Xie, R.; Karim, A.; Douglas, J. F.; Han, C. C.; Weiss, R. A. Phys. Rev. Lett. 1998, 81, 1251–1254. (19) Hill, D. J. T.; Whittaker, A. K.; In Encyclopedia of Polymer Science and Technology; John Wiley & Sons: New York, 2005; p 5.
Published on Web 03/24/2010
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Figure 1. (a) Scheme of the irradiated zones of a PIB thin film. The intensity of gray is proportional to the electron dose, and the number indicates the exact dose expressed in μC cm-2. (b) AFM topography of holes formed at the beginning of dewetting. Optical images of the evolution of an irradiated PIB film at T = 60 °C after (c) 253, (d) 387, (e) 507, and (f ) 927 s. The bar is 50 μm.
the highest number of chain scissions per adsorbed energy unit (Gs = 11.1).20 Upon irradiation, a sequence of chemical reactions occurs and the polymeric chains are permanently broken into fragments with a reduced molecular weight (Mf). The control of the electron dose allows control of the molecular weight according to the expression21 1 1 Gs kzD ¼ þ FNA Mf Mn
ð1Þ
where Mn is the original number -average molecular weight, Gs is the number of chain scissions per adsorbed energy unit (eV-1), z is the thickness of the film, D is the deposited dose in term of charge deposited by the electron beam per surface area unit (μC cm-2), F is the mass density of the polymer, NA is Avogadro’s number, and k is a constant factor that indicates the energy deposited in the film per unit of charge deposited by the electron beam (eV μC-1). In our experiment, we irradiated five squared regions whose area is 50 50 μm2 with electron doses ranging from 5 to 25 μC cm-2 and a rectangle of 50 100 μm2 with a dose of 100 μC cm-2 used as a marker (Figure 1a). We monitored the evolution of the irradiated film upon thermal treatment by an optical microscope at low magnification (10x). Around room temperature, the PIB thin film spontaneously dewets by the nucleation and growth of holes forming asymmetric rims (Figure 1b) in about 104 s. At the end of the process, about 70% of the surface is made up of silicon oxide exposed at the air interface and the remaining 30% is covered by PIB reorganized in droplets. Dewetted zones exhibit a high contrast in bright-field optical microscopy and thus are well-recognizable. Under these conditions, we did not observe a relevant difference among the irradiated and the nonirradiated places during dewetting. (20) Perera, M. C. S.; Hill, D. J. T.; Polymer Handbook; Brandrup, J., Immergut, E. H., Grulke, E. A, Eds.; John Wiley & Sons: New York, 1999; pp II-481-489. (21) Fox, T. G.; Flory, P. J. J. Am. Chem. Soc. 1948, 70, 2384–2392.
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Figure 2. Time evolution of the percentage of the area free of PIB in each irradiated square at (a) 25 and (b) 60 °C. The continuous curves are data fits obtained using the model of dewetting described in the text.
By increasing the temperature the dewetting rate dramatically increases. Figure 1c-f shows the time evolution of an irradiated PIB film upon exposure at a constant temperature of 60 °C, as observed by optical microscope. After t = 253 s, the film exhibits many holes that are randomly distributed on the surface and no difference between irradiated and nonirradiated places is visible (except for the rectangular marker in Figure 1c). At t = 387 s, the generated pattern starts to become clearly distinguishable from the remaining part of the film (Figure 1d). After t = 507 s, almost all of the irradiated squares are very visible in the film (Figure 1e); the fraction of the area of the squares where the substrate is visible (i.e., the yellow regions in Figure 1e) is roughly proportional to the electron dose. At t = 927 s, all of the irradiated zones are completely dewetted (Figure 2f). Eventually, after a longer time (t>2 103 s) all of the film is dewetted. Figure 2 shows the time evolution of the area discovered by dewetting in each irradiated square at two different temperatures, (a) 25 and (b) 60 °C. From the experimental data, it clearly emerges that dewetting is faster in the places irradiated with higher doses with respect to those untreated or treated with lower doses. Furthermore, at a fixed time the percentage of the area free of PIB scales with the electron dose. It must be noted as for doses >20 μC cm-2, it decreases with respect to films irradiated with doses of ∼20 μC cm-2. This behavior depends on the enormous production of radicals that denaturated the material, allowing cross linking among the radical formed during the irradiation.19 DOI: 10.1021/la1008279
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Considering the asymmetric shape of the rims and the incomplete dewetting of the surface, our experimental results can be interpreted by assuming a model of dewetting based on the nucleation and growth of holes in a strong slippage regime.22 This mechanism is characterized by the formation of randomly distributed holes with circular (asymmetric) rims. As reported by Brochard-Wyart et al.,22 dewetting starts with a high-velocity regime characterized by strong slippage and then it changes to a regime where slippage continuously decreases, becoming negligible before the complete dewetting of the surface occurs. The switching point between the two regimes occurs when the rim radius reaches a characteristic value Rc that defines the characteristic time of the process τ as τ = K0Rc/σ*, where K0 is the friction coefficient of monomer and σ* is the critical share stress.22 On the basis of this model, the evolution of the fraction of the dewetted area (Ad) is 8 > 3 t - t0 4=3 2 > >fπRc ns g ; t τ. (b) Dependence of τ from the absolute temperature. The experimental data are fitted according to Arrhenius’ law.
Figure 4. Plot of t vs T according to the phenomenological law described in eq 5 with a defined Ad = 50% for different electron doses.
By setting Ad equal to a precise value (i.e., defining a percentage of the area that is free of PIB), we obtained a plot of the time of dewetting versus the temperature for each dose. Figure 4 shows an example where Ad is set to 50%. In summary we have presented a new application of a thin film of PIB where temperature-induced dewetting is modulated by means of controlled irradiation. The irradiation of specific areas of the film gives rise to a pattern with time-temperature integration functionality; this behavior is demonstrated by observing the evolution of the percentage of the film area discovered by Langmuir 2010, 26(8), 5312–5315
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dewetting in time and by modeling the dewetting according to the Brochard-Wyart theory. We demonstrated the feasibility of the fabrication method based on the electron beam (EBL, electron beam lithography) to generate repeated simple structures over relatively large areas using PIB as a testing material. However, the approach is general and can be extended to other active materials whose molecular weight is modified upon irradiation with electrons (i.e., cross-linking polymers23 or polymeric multilayers and blends for industrial packaging) or to other substrates (i.e., polymeric or soft substrates). Furthermore, the possibility of fine tuning the average molecular weight by irradiation allows the method to be applicable over wide temperature ranges, and for this reason, it could be exploited in commercial applications. The simplicity of the system and the possibility of reading the device response optically makes our approach extremely appealing as the basis for a new generation of low-cost TTI working over a wide range of temperature. (23) Al Akhrass, S.; Ostaci, R. V.; Grohens, Y.; Drockenmuller, E.; Reiter, G. Langmuir 2008, 24, 1884–1890.
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Experimental Methods As the substrate, we used a Si(100) wafer with a 200-nm-thick layer of thermally grown silicon oxide. Before film deposition, we dipped the substrate for 5 s into an aqueous HF solution (4%) and then we washed it with electronic-grade water (millipure quality). We prepared PIB thin films by spin coating from a 3.5 g/L solution of PIB (Mn =3.1 106, Aldrich product no. 181498) in toluene (HPLC grade) (time, 2 min; angular speed, 5000 rpm). We generated the pattern by irradiating square areas of the film with the electron beam of an SEM (scanning electron microscope, Hitachi 4000) with a pattern generator (JC Nabity Lithographic Systems, www.jcnabity.com). The acceleration voltage was 30 keV. We acquired the optical images with a Nikon eclipse 80i microscope equipped with a DS-Fi1 Nikon camera. We determined the percentage area discovered by dewetting by analyzing the optical images with Gimp software (www.gimp.org).
Acknowledgment. This work was supported by EU project STRP-STAG 033355 (A.C.). M.C. and P.S. are supported by ESF-EURYI-DYMOT.
DOI: 10.1021/la1008279
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