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Structural Study of Hybrid Organic/Inorganic Polymer Gels Using Time-Resolved Fluorescence Probing† Elias Stathatos and P. Lianos* Engineering Science Department, University of Patras, 26500 Patras, Greece
Ursa Lavrencic Stangar and Boris Orel National Institute of Chemistry, Hajdrihova 19, 1001 Ljubljana, Slovenia
P. Judeinstein RMN en milieu Oriente´ , UPRESA CNRS 8074, Universite´ Paris Sud, Baˆ t. 410, 91405 Orsay Cedex, France Received March 1, 2000. In Final Form: May 23, 2000 Thin films made of hybrid organic/inorganic polymers, synthesized through the sol-gel method and composed of silica with grafted poly(ethylene oxide) or poly(propylene oxide) chains, are studied by timeresolved fluorescence-probing analysis. The fluorescence-probing technique is employed both to structurally characterize these nanoheterogeneous materials and to demonstrate the capacity of this technique to detect important differences in materials having approximately the same chemical structure. It is found that the films consist of interpenetrating networks of organic and inorganic domains constituting two distinguishable subphases, an organic and an inorganic one. The volume fraction of the organic phase is larger than that of the inorganic phase, and it increases with increasing poly(ethylene oxide) or poly(propylene oxide) chain length. In addition, hydrophobic interactions tend to organize the organic domain, forming organic clusters that can accommodate hydrophobic probes. This is particularly true for large polyether chains.
Introduction The study of sol-gel synthesized organic/inorganic nanocomposite gels comprising poly(ethylene oxide) chains grafted on silica nanoparticles is a fast-growing area with applications in both optics and optoelectronics. Such gels have been used as ion-conducting solid-state electrolytes1,2 and as hosts of luminescent cationic species.3,4 They also serve as interesting luminescent sources themselves.3-5 owing to the clustering induced by their double organic/ inorganic character. In addition, the organic content of the gels offers them a degree of mechanical flexibility and the possibility of index of refraction variation and control. Studies of the structures and dynamic aspects of such materials were conducted in the past by using a number of different techniques.6,7 These gels, in general, can be considered nanocomposite blends of organic and inorganic domains that form interpenetrating networks.6 Their disordered natures, the small sizes of their domains, and * Corresponding author. Tel. 30-61-997587. Fax: 30-61-997803. E-mail:
[email protected]. † Part of the Special Issue “Colloid Science Matured, Four Colloid Scientists Turn 60 at the Millennium”. (1) Dahmouche, K.; Atik, M.; Mello, N. C.; Bonagamba, T. J.; Panepucci, H.; Judeinstein, P.; Aegerter, M. A. Sol. Energy Mater. Sol. Cells 1998, 54, 1. (2) Groselj, N.; Gaberscek, M.; Opara Krasovec, U.; Orel, B.; Drazic, G.; Judeinstein, P. Solid State Ionics 1999, 125, 125. (3) Bekiari, V.; Lianos, P.; Judeinstein, P. Chem. Phys. Lett. 1999, 307, 310. (4) Ribeiro, S. J. L.; Dahmouche, K.; Ribeiro, C. A.; Santilli, C. V.; Pulcinelli, S. H. J. Sol-Gel Sci. Technol. 1998, 13, 427. (5) Carlos, L. D.; de Zea Bermudez, V.; SaFerreira, R. A.; Marques, L.; Assuncao, M. Chem. Mater. 1999, 11, 581. (6) Brik, M. E.; Titman, J. J.; Bayle, J. P.; Judeinstein, P. J. Polym. Sci., Part B: Polym. Phys. 1996, 34, 2533. (7) Lesot, P.; Chapuis, S.; Bayle, J. P.; Rault, J.; Lafontaine, E.; Campero, A.; Judeinstein, P. J. Mater. Chem. 1998, 8, 147.
the large differences between their dynamic behaviors necessitate a wide range of spectroscopic techniques for their proper characterization.6 One technique particularly adapted to the study of mesoscopic nanoheterogeneous systems is steady-state and time-resolved analysis of fluorescence quenching between hosted probes. This technique has been systematically used to study the structures and dynamics of organized molecular assemblies, such as micelles, microemulsions, and lipid vesicles. Hybrid organic/inorganic nanocomposites possess several of the characteristics of fluid gels. Both have domain sizes in the nanometer range, both are macroscopically homogeneous but microscopically biphasic and heterogeneous, and both possess extensive interfaces separating domains of different hydrophobic/hydrophilic characters. We thus concluded that fluorescence probing, which can provide valuable primary or complementary information regarding disordered systems at the mesoscopic level, would be potentially useful for the study of organic/inorganic nanocomposites. In the present work, we use fluorescence-probing processes, in both steadystate and time-resolved analysis, to investigate the structural and dynamic properties of six different nanocomposite films made of the same type of silica but containing poly(ethylene oxide) (PEO) or poly(propylene oxide) (PPO) chains of three different lengths. Our goals were, on one hand, to study variations of film structure and dynamics in relation to changes in the polyether chain length and, on the other hand, to present a time-resolved fluorescence-quenching model which can be particularly fit to a disordered system similar to the presently studied ones. Silica-based gels, prepared by the sol-gel method, were previously studied with fluorescent probes. A common practice was to introduce fluorophores into the gel
10.1021/la0002987 CCC: $19.00 © 2000 American Chemical Society Published on Web 09/06/2000
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Figure 1. Chemical structures of the poly(propylene oxide)- and poly(ethylene oxide)-containing precursors.
precursor and to monitor the evolution of the gel to optimize the macroscopic parameters influencing this evolution.8,9 In some other cases, the fluorophore was introduced into the matrix to create a sensor (e.g., an oxygen sensor).10 In the present work, our interest focuses on the capacities of the employed probes to detect important differences between materials of approximately the same chemical structure. The materials studied in the present work were fabricated in the form of thin films. The reason for this choice is that these films are used in our laboratories as solid electrolytes for applications in electrochromic2 and photoelectrochemical cells. It would thus be useful to fully characterize them in order to optimize the devices that are based on them. Experimental Section Materials. Poly(propylene glycol) bis(2-aminopropyl ether) of mol wts 4000, 2000, and 230 (2APPG-4000, 2APPG-2000, and 2APPG-230, Aldrich), O,O′-bis(2-aminopropyl)poly(ethylene glycol) of mol wts 1900, 800, and 500 (2APEG-1900, 2APEG-800, and 2APEG-500, Fluka), (3-isocyanatopropyl)triethoxysilane (ICS, ABCR Chemicals), pyrene (Fluka), coumarin-153 (C-153, Aldrich), poly(ethylene glycol) solvent (PEG-200, Aldrich), tetramethoxysilane (TMOS, Aldrich), and methanol and cyclohexane (spectroscopic grade, Merck) were used as received. Syntheses of Hybrid Precursors. Six different unhydrolyzed hybrid silicon precursors were prepared basically by using the procedure developed by Dahmouche et al.11 to synthesize poly(propylene glycol) (4000) and poly(ethylene glycol) (800) modified ethoxysilanes. 2APEG or 2APPG with different chain lengths and ICS (molar ratio [ICS]/[diamine] ) 2) were mixed in tetrahydrofuran (THF) under reflux (64 °C) for 6 h. The isocyanate group of ICS reacted with the amino groups of 2APPG or 2APEG (acylation reaction), producing urea connecting groups between the polymer units and the inorganic component. After evaporation of THF under vacuum, a viscous precursor was obtained, which was stable at room temperature for several months. We prepared similar hybrid precursors modified by both types of polymers with three different chain lengths for each type [poly(oxypropylene) 4000, 2000, and 230; poly(oxyethylene) 1900, 800, and 500]. The respective abbreviations names used in the present work are PP4000, PP2000, and PP230 for the poly(oxypropylene)-containing precursors and PE1900, PE800, and PE500 for the poly(oxyethylene)-containing precursors. The chemical structures of the precursors appear in Figure 1. Sol-Gel Syntheses and Film Depositions. Typically, 4.5 g of the precursor was mixed with 15 mL of methanol. After 5 min of stirring, 0.5 mL of 0.1 M HCl was added and the mixture was stirred for 30 min more. Finally, a glass slide, previously cleaned in sulfochromic solution, was dipped into the sol and (8) Ilharco, L. M.; Martinho, J. M. G. Langmuir 1999, 15, 7490. (9) Huang, M. H.; Dunn, B. S.; Soyez, H.; Zink, J. I. Langmuir 1998, 14, 7331. (10) Murtagh, M. T.; Shahriari, M. R.; Krihak, M. Chem. Mater. 1998, 10, 3862. (11) Dahmouche, K.; Atik, M.; Mello, N. C.; Bonagamba, T. J.; Panepucci, H.; Aegerter, M. A.; Judeinstein, P. J. Sol-Gel Sci. Technol. 1997, 12, 711.
withdrawn at a speed of 31 mm/min. A thin transparent and optically uniform film was obtained. All films were dried under vacuum for 3 h at room temperature. Fluorescent probes and quenchers were introduced by previous solubilization in the methanol used to prepare the precursor solutions. Sol-Gel Syntheses of the Pure Silica Matrixes. A 2.5 mL quantity of TMOS was added to 3 mL of methanol. To this solution was added 2.5 mL of acidified water (containing 0.005 mL of HCl) under stirring. Pyrene was then introduced, and portions of the mixture were placed in PMMA cuvettes, covered with perforated aluminum foil, and left to dry for 1 week at room temperature. Films were also made by dipping immediately after mixture of the components, as before. Experimental Methods. All measurements were made under ambient conditions. Absorption spectra were recorded with a Cary 1E spectrophotometer. Steady-state fluorescence measurements were made with a home-assembled spectrofluorometer using Oriel components in a standard configuration: 150 W xenon lamp; excitation monochromator, thermostated sample holder, and emission monochromator at right angles; and a computerdriven detection system. Time-resolved fluorescence decay profiles were obtained by the single-photon-counting technique using a homemade nanosecond hydrogen flash lamp and ORTEC electronics. An Oxford Instruments PC multichannel analyzer card was used for data storage over 1024 channels. The excitation pulse shape is shown in Figure 2. Pyrene decay profiles in the absence or in the presence of C-153 were recorded by exciting through a monochromator at 335 nm and by monitoring emission at a pyrene monomer wavelength by using an interference filter at 385 nm.
Model for Time-Resolved Analysis The fluorescence decay profiles of pyrene resulting from resonance energy transfer to C-153 were analyzed by a model of stretched exponentials given by the equation12,13
I(t) ) I0[exp(-t/τo)]{exp[-C1(t/τo) + C2(t/τo)2f]} 0 < f < 1 (1) while the first-order quenching rate constant was calculated by
K(t) ) 1/τo[fC1(t/τo)f-1 - 2fC2(t/τo)2f-1]
(2)
C1, C2 and f were calculated by fitting eq 1 (cf. example in Figure 2) to the experimental decay profile. τo, the decay time in the absence of quencher, was measured in separate experiments. Details regarding this model can be found in previous publications.13 The rate constant is time dependent in all cases where the model of eq 1 applies, since it then depends on the distance between the reacting species. Distance dependence creates timedependence, which becomes even more intense in the case of restricted geometries. Thus the value of K in eq 2 is time dependent. Figure 2b shows a typical example of the time evolution of K(t ). Since it is difficult to tabulate any K(t) value (12) Ferrer, M.; Lianos, P. Langmuir, 1996, 12, 5620. (13) Bekiari, V.; Ferrer, M.-L.; Lianos, P. J. Phys. Chem. 1999, 103, 9085.
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Figure 3. Pyrene monomer (1) and pyrene excimer (2) fluorescence spectra in a PP4000 film. The pyrene concentrations in the precursor solutions were 1 and 30 mM, respectively. Table 1. I1/I3 and IE/IM Values for Pyrene Incorporated into Six Films Made with PEO and PPO Precursorsa precursor or solvent
I1/I3
IE/IM
PE500 PE800 PE1900
1.69 1.57 1.53
0.74 0.69 0.48
PP230 PP2000 PP4000
1.61 1.38 1.30
0.67 0.63 0.64
cyclohexane pentanol methanol PEG-200 pure silica film
0.61 1.03 1.34 1.42 1.62
0.00
a
Figure 2. (a) Fluorescence decay profile of pyrene in the presence of C-153 in a PE500 film, together with the fitted curve and excitation pulse. (b) Variation of the corresponding reaction rate constant with time. The pyrene and C-153 concentrations in the precursor solution were 1 mM. for any value of time, we usually choose to tabulate only its value K1 at the first time channel and its value KL at the last recorded time channel. Thus, K1 represents the rate constant at short times and KL the rate constant at long times. In this respect, the ratio KL/K1 shows the importance of the quenching processes at long times as compared with those at short times. Both K1 and KL are, of course, first-order rate constants.
Results Data Based on the Vibronic Structures of Pyrene Fluorescence Spectra. Pyrene was dissolved in a precursor solution at a concentration of 1 mM. The resulting solution was then transferred into the film, producing a material with the spectroscopic characteristics of pyrene monomers. No aggregated species and no pyrene excimers (see also below) were thus detected at this probe concentration. Figure 3 shows the fluorescence spectrum of such a pyrene monomer. No deformation of the fluorescence spectrum was detected. Deformations may ensue from pyrene self-absorption due to absorption and fluorescence spectral overlap, as would occur in bulk solutions or gels at such concentrations. The thin film makes spectral deformation due to self-absorption, if any, difficult to detect. Spectral deformation may also be produced by pyrene aggregation or precipitation. No such phenomena were observed with the present hybrid films. The vibronic structure of the pyrene fluorescence spectrum
The pyrene concentrations in the precursor solutions were 1 mM for I1/I3 and 30 mM for IE/IM measurements. I1/I3 values for 10 µM pyrene in neat solvents are also shown for comparison.
is very sensitive to the probe environment.14,15 Thus the well-known I1/I3 index increases with the polarity of the pyrene microenvironment, providing information on both the properties of the hosting material and the localization site of the probe itself.16-18 I1/I3 values were measured for the six films made with the above PEO and PPO precursors and are shown in Table 1 (column 2). I1/I3 values measured in various characteristic solvents as well as in pure silica, made by the sol-gel method, are also shown in Table 1. Thus, when pyrene is incorporated in a hybrid film it finds itself in a very polar environment, more polar than that found in very polar solvents such as methanol or fluid poly(ethylene glycol) oligomers and comparable to that in pure silica. However, the localization site of the pyrene molecules is not the inorganic (i.e., silica) domain. Only in the case of the smallest PEO and PPO chains does the pyrene microenvironment approach that of pure silica. In the case of longer chains, particularly for PP2000 and PP4000, pyrene is apparently localized in the organic phase, where it senses polarity approaching that of methanol and PEG-200. Pyrene is a hydrophobic molecule with an easily polarizable π-electron cloud. For this reason, it has an affinity for organic subphases where hydrophobic (14) Ham, J. S. J. Chem. Phys. 1953, 21, 756. (15) Nakajima, A. Bull. Chem. Soc. Jpn. 1971, 44, 3272. (16) Kalyanasundaram, K.; Thomas, J. K. J. Phys. Chem. 1977, 81, 2176. (17) Lianos, P.; Georghiou, S. Photochem. Photobiol. 1979, 30, 355. (18) Koussathana, M.; Lianos, P.; Staikos, G. Macromolecules 1997, 30, 7798.
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domains can be created, showing at the same time a tendency to locate at hydrophobic/hydrophilic interfaces. It is obvious from the data in column 2 of Table 1 that when the PEO or PPO chains are sizable enough, they provide accommodation sites for pyrene molecules. Pyrene molecules are large, and it is highly unlikely that they can be solubilized by single chains, even when they are also large. It is more likely that the organic/inorganic polymers self-assemble to allow formation of organic domains comprising several PEO or PPO chains. Such a model is compatible with the structure of the organic domains reported in ref 6. Data Based on Pyrene Excimer Formation. When the pyrene concentrations in the organic precursor solution were increased (i.e., 30 mM), we observed excimer formation both in the precursor solutions and in the ensuing films. Column 3 of Table 1 shows the ratios of the maximum intensities of the excimer and monomer fluorescence bands while Figure 3 shows an example of a fluorescence spectrum where both bands appear. Again, the thin film prevents monomer fluorescence spectrum deformation by self-absorption. This is not the case in bulk solvents, and this is why no values for IE/IM are presented for them. The spectra of Figure 3 also show that no aggregated pyrene species exist in the films, despite the increase in pyrene concentration. Exclusion of aggregation and confirmation of (dynamic) excimer formation were also accomplished by analysis of pyrene monomer and excimer decay profiles (not shown; cf. ref 13). The values of column 3 in Table 1 are higher in cases where more excimer is formed, which is rather surprising. In pure silica matrixes, there is no excimer. This is expected, since pyrene molecules are dispersed and immobilized. Therefore, excimer formation, which is a diffusioncontrolled process, is impossible. One would then expect that, in the case of small PEO or PPO chains, where pyrene molecules would be hardly associated with any organic subphase, excimer formation would be very low if not zero. However, the obtained data show the opposite. There is more excimer where the chains are shorter and also more excimer in the case of the less bulky PEO chains. The only way to rationalize these data is to accept that the pyrene molecules are exclusively associated with the organic domain and that the overall volume fraction of the organic domain is proportional to the chain size. Once the organic domains are formed, pyrene molecules can be accommodated in the organic subphase, where they can form excimers. When the chain size is larger, the total volume of the organic subphase is also larger. In such a larger volume, the effective pyrene concentration decreases with immediate consequences to the quantity of excimer formed. It is also necessary to provide an explanation for the relatively high average value of IE/IM in the case of PPO chains and for the fact that this ratio changes very little with PPO chain size. This difference from the marked behavior of the PEO chains is obviously related to the essentially hydrophobic character of the PPO chains (cf. corresponding I1/I3 values). PPO chains provide a more favorable environment for pyrene solubilization owing to stronger hydrophobicity. It is also expected that PPO chains will tend to cluster due to hydrophobic interactions. Association of pyrene with such clusters facilitates excimer formation, hence the higher average IE/IM values and the small variation with chain size. In contrast, PEO chains, which are essentially hydrophilic, will have a weaker tendency to cluster and form hydrophobic domains, thus failing to localize pyrene molecules. The data given in the next paragraph provide more information on this last issue. The conclusions drawn from the present paragraph
Langmuir, Vol. 16, No. 23, 2000 8675 Table 2. Data Obtained by Time-Resolved Analysis of Pyrene Fluorescence in the Presence of C-153 Employing the Model of Eqs 1 and 2a precursor
f
K1 (106 s-1)
KL (106 s-1)
KL/K1
PE500 PE800 PE1900 PP230 PP2000 PP4000
0.50 0.54 0.61 0.35 0.48 0.55
28 23 23 39 29 19
1.7 1.6 2.0 1.7 1.4 1.3
0.06 0.07 0.09 0.04 0.05 0.07
a Pyrene and C-153 concentrations in the precursor solutions were in all cases 1 mM.
are that the volume fraction of the organic subphase grows with the chain size, obviously at the expense of the volume fraction of the inorganic subphase, and that the PPO chains tend to organize themselves by hydrophobic interactions. Energy Transfer between Pyrene and C-153 Incorporated in the Hybrid Films. The luminescence decay profiles of pyrene were recorded in the absence and in the presence of C-153. C-153 quenches pyrene fluorescence by resonance energy transfer due to a large spectral overlap between pyrene fluorescence and C-153 absorption.12 The fluorescence decay time τ0 of free pyrene was used to apply eq 1 and 2 and to analyze the pyrene decay profiles recorded in the presence of C-153. τ0 was ∼202 ns, varying slightly from one sample to the other. C-153 concentrations for efficient energy transfer and proper analysis ranged between 0.8 and 1.3 mM, as measured in the original precursor solution. The corresponding pyrene concentrations were 1 mM in all cases. For C-153 concentrations above 1.3 mM, quenching was extensive and the analysis failed to produce accurate and consistent results. The results of the time-resolved analysis are presented in Table 2 for samples corresponding to 1 mM C-153. Examples of a decay profile fitted with eq 1 the evolution of the quenching rate constant K(t) ( eq 2) with time are shown in Figure 2. Table 2, column 2, shows the values of the noninteger time exponent f. In the case of energy transfer between immobile donor and acceptor molecules, f reflects the dimensionality of the donoracceptor distribution. For random distribution, the geometry of the distribution reflects the geometry of the host environment. In this case, f ) D/6, where D is the dimensionality.19,20 The maximum value then that f can obtain is 0.5. However, as seen in Table 2, column 2, f takes, in our case, higher values, particularly for PEOcontaining samples. Such high f values mean that, in addition to pure energy transfer quenching, other factors also favor quenching. In most cases, such factors are due to molecular displacement in the excited pyrene state, which can bring reacting molecules closer and facilitate their interaction.13 Displacement can occur mainly through diffusion, and this is possible only in the presence of an organic subphase,13,21 which is supported by the data given in Table 2. The smallest-chain precursor gave the smallest f value, indicating that a very restricted environment is created for both donor and acceptor molecules in this case. In contrast, in the case of large PE and PP chains, f takes a value larger than 0.5, suggesting that the organic domain is then extensive enough to allow molecular diffusion, which is also supported bythe data in Table 2. As the (19) Levitz, P.; Drake, J.M.; Klafter, J. J. Chem. Phys. 1988, 89, 5224. (20) Nakashima, K.; Liu Y. S.; Zhang, P.; Duhamel, J.; Feng, J.; Winnik, M. Langmuir 1993, 9, 2825. (21) Bekiari, V.; Ferrer, M.; Stathatos, E.; Lianos, P. J. Sol.-Gel Sci. Technol. 1998, 13, 95.
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chain size increases, K1, the short-time reaction rate constant, decreases in favor of the long-time rate constant, KL, as reflected in the values of KL/K1. Indeed, highly restricted or aggregated reactants would favor short-time reactions, while diffusing reactants may more evenly distribute reaction rates over the whole time domain. However, the data of Table 2 also present an unexpected feature. According to the above model, PPO chains should produce larger f values than PEO chains. This is contrary to the date actually obtained.. A reasonable explanation for this phenomenon is the following: Both pyrene and C-153 are hydrophobic molecules, and they tend to associate with hydrophobic domains. Poly(propylene oxide)-containing samples should favor formation of such domains with greater facility than poly(ethylene oxide)containing samples. It is likely that micelle-like domains could be formed in the case of PPO chains. which wwould localize probes and thus restrict the dimensionality of their distribution. Such an explanation is compatible with the relatively high IE/IM values obtained with all PPO chains, as seen in Table 1. The conclusion then of this paragraph is that the organic phase grows with PEO or PPO chain size but in addition it is structured owing to hydrophobic interactions. Discussion The previous section leads to a certain number of conclusions. The behavior of the I1/I3 values proves the unquestionable existence of the organic subphase. The values of IE/IM indicate that the volume fraction of the organic subphase increases when the PEO or PPO chain lengthens and that polyether chains tend to organize themselves by hydrophobic interactions. The energytransfer data further indicate that the organic subphase is structured owing to hydrophobic interactions. It is obvious that these precursors create materials with interesting behaviors, creating a challenge for the physical chemist. Our data are consistent with the structural model shown in Figure 4, which is an evolution of an original model presented in ref 6 and is compatible with the model recently presented in ref 22. The model of Figure 4 is particularly fit to large PEO or PPO chains, and it is based on three major characteristics: (1) the silica clusters are highly dispersed and small, (2) the gel volume is mainly “organic”, and (3) the organic subphase forms clusters, i.e., organized domains mainly resulting from hydrophobic interactions. It is highly probable that the third feature (22) Dahmouche, K.; Santilli, C. V.; Pulcinelli, S. H.; Craievich, A. F. J. Phys. Chem. B 1999, 103, 4937.
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Figure 4. Schematic presentation of the structural model for the employed inorganic/organic materials: (1) silica clusters; (2) organic fragments; (3) organic cluster resulting from hydrophobic interactions.
will not be observed in the case of small PEO or PPO chains, but its existence is rather obvious in the case of PP4000. PPO chains are in any case hydrophobic entities and facilitate hydrophobic interactions. SAXS data recently presented by Dahmouche et al.22 lead to the conclusion that these nanocomposite materials are twophase systems composed of dispersed and spatially correlated siloxane nanoclusters linked at the ends of the polyether chains and forming a homogeneous continuous matrix.22 Dahmouche et al. also concluded that the polymer chains should be strongly folded and entangled, forming a nearly continuous matrix. Our data are compatible with the results of ref 22, and they qualify and complete the notion of a homogeneous matrix. The organization of the organic domain by hydrophobic interactions and the organization of the inorganic domain by inorganic polymerization reactions offer a degree of organization that is sufficient to create a matrix with an ordered arrangement of its mesoscopic components. It is obvious that fluorescence- probing techniques are of great value in studying these nanocomposite materials. Acknowledgment. This work is supported by the Greece-Slovenia Bilateral R&D Cooperation Program and by the COST Action 518 Program, of the DGXII of the European Commission. LA0002987
