ARTICLE pubs.acs.org/JPCA
Structural Evolution during Evaporation of a 3-Glycidoxypropyltrimethoxysilane Film Studied in Situ by Time Resolved Infrared Spectroscopy Plinio Innocenzi,*,† Cristiana Figus,† Masahide Takahashi,‡ Massimo Piccinini,§ and Luca Malfatti† †
DADU, Laboratorio di Scienza dei Materiali e Nanotecnologie (LMNT), Universita di Sassari and CR-INSTM, CNBS, Palazzo Pou Salit, Piazza Duomo 6, 07041 Alghero (Sassari), Italy ‡ Graduate School of Engineering, Osaka Prefecture University, Sakai, Osaka 5998531, Japan § Imaging Laboratory, Porto Conte Ricerche, SP 55 Porto Conte-Capo Caccia km 8,400, Loc. Tramariglio, 07041 Alghero (Sassari), Italy
bS Supporting Information ABSTRACT: Time resolved infrared spectroscopy has been applied to study in situ the evaporation process of a 3-glycidoxypropyltrimethoxysilane hybrid sol by casting a droplet on a ZnSe substrate; the analysis has been performed in the middle-infrared range and in the near-infrared range. The experiment has allowed following the structural changes induced by water evaporation and the formation of ordered structures within the cast film; the CH2 scissoring bands have been used as a fingerprint for the disorder to order transition of the hybrid. The experiment has been done using both a fresh sol and an aged sol which produce respectively an amorphous material and a crystalline hybrid material. The analysis has shown that the epoxy groups do not react during the evaporation while the silica structure shows only a slight condensation and an increase in open cage-like species. At the end of evaporation the hybrid has a “soft-like” state which allows structural rearrangements to self-order.
’ INTRODUCTION Self-organization in organic inorganic hybrids is as a kinetically dependent phenomenon which leads to the formation of layered type organic inorganic ordered structures.1 Selforganization2 by molecule-by-molecule self-assembly3 and molecular recognition through weak interactions4 are the possible driving processes. Weak intermolecular interactions, van der Waals or hydrogen bonding, between close hybrid supramolecular structures are at the base of self-organization. Bridged organically modified alkoxides, which are formed by a bridging organic spacer and a trifunctional silyl group at both the ends of the spacer (O1.5Si R SiO1.5, with R the organic spacer),5 8 are necessary for achieving self-ordered and organized structures. Several types of organic spacers in the bridged silsesquioxanes self-organize into crystalline structures.9,10 Self-organization through bridged silsesquioxanes in bulk is relatively easy to accomplish if the right spacer and synthesis conditions are used. Obtaining crystalline structures through self-organization in films is, instead, a more complex process because the fast evaporation of the solvent during film deposition makes the condensation of the inorganic species faster and self-organization more difficult. In previous works we have successfully prepared hybrid films containing nanoscale layered organosilica hybrid crystals, which have been formed through self-organization from organically modified alkoxides bearing an epoxy functionality.11,12 Self-organization has been achieved using 3-glycidoxypropyltrimethoxysilane r 2011 American Chemical Society
(GPTMS),13 which is a monofunctional organically modified alkoxide bearing an epoxy unit at the end of the organic functional chain.14 The chemistry of GPTMS is quite complex and several side reactions can be observed in different chemical environments;15 17 at high pH conditions the epoxy opening and silica condensation reactions are both slowed down. This favors the formation of bridged species via reaction of the epoxies that form dioxane bridging species.18 Transparent hybrid crystalline films with a large optical anisotropy have been prepared using a highly basic aqueous precursor sol containing NaOH as catalyst and GPTMS. Understanding self-organization and the chemical physical processes which occur during hybrid film synthesis is still an intriguing challenge. Evaporation during film deposition from a liquid phase is generally a fast process which triggers condensation reactions and at the very end the organization phenomena. The GPTMS precursor sol is a very simple one in term of composition because it contains only the alkoxide, water, and the catalyst; this means that water evaporation is the main phenomenon that occurs. We have, therefore, used a specific experimental setup to follow in situ the evaporation process of the hybrid sol using infrared spectroscopy. This method has been previously applied Received: May 9, 2011 Revised: August 1, 2011 Published: August 08, 2011 10438
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Figure 1. Instrumental setup for FTIR time resolved measurements on GPTMS water cast droplet.
to study in situ the evaporation phenomena at the ground of selforganization and self-assembly in the nanoscale.19,20 In the present case the time scale of the phenomenon is slow enough to allow recording spectra that can be used for studying the details of the process.
’ EXPERIMENTAL SECTION Sol Preparation. 3-Glycidoxypropyltrimethoxysilane (GPTMS; 97%, ABCR) was used as the organically modified alkoxide, sodium hydroxide (NaOH; Aldrich, 98%) was used as the basic catalyst, and distilled water was used for hydrolytic reactions; all the reagents were used as received without further purification. The molar ratio of the components was set to GPTMS:H2O: NaOH = 1:5:0.167. The precursor sol was prepared by dropwise addition of GPTMS (20 cm3) to an NaOH aqueous solution (8 cm3, 1.85 M, pH ∼14) under stirring; the sol became transparent in few minutes after the addition of GPTMS. The sol was left to react in an open vessel at 25 °C and 40% relative humidity for 45 min; after this time the sol was immediately used for the experiment by casting for the analysis a droplet on the ZnSe substrate. An aged solution has been obtained by keeping the sol in a sealed container at room temperature for 7 days. Time Resolved Fourier Transform Infrared (FTIR) Analysis. Time resolved in situ infrared (IR) analysis was performed using a Bruker Vertex 70 interferometer with a Globar source, coupled to a Bruker Hyperion 3000 IR microscope working in transmission mode and equipped with an MCT detector (250 250 μm size) cooled to liquid nitrogen temperature. The middle-infrared (MIR) measurements were performed in the 600 4000 cm 1 range with a resolution of 8 cm 1 using a KBr beamsplitter which was mounted in the interferometer. A 2-mm-thick, 25 10 mm double side polished ZnSe window was used as substrate; the background spectrum of the substrate was recorded as the average of 256 interferograms. Time resolved measurements were performed by averaging 64 interferograms per spectrum in an acquisition time of 10 s and a time interval of 60 s between beginnings of acquisition of consecutive spectra. The experimental equipment is shown in Figure 1; to control the atmosphere and relative humidity within the experimental chamber,
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the microscope was enclosed in a cabinet connected to an external gas source through a valve. The experiment was performed by casting a droplet of the solution on the ZnSe substrate. After measurement of the background spectrum, a small drop of GPTMS sol (∼1 μL) was cast on the substrate. Due to the high solution viscosity, the drop was manually spread all over the substrate surface; the upper aperture of the microscope was closed to select a 80 80 μm area near the border of the drop to avoid saturation of the signal. This procedure took 1 min before we could start acquiring the first spectrum after casting the drop. Time Resolved FT-NIR Analysis. The near-infrared (NIR) measurements were performed in the 3800 9000 cm 1 range with a resolution of 8 cm 1 and a CaF2 beamsplitter which was mounted in the interferometer. A silanol-free silica slide was used as substrate, and the background spectrum of the substrate was recorded as the average of 256 interferograms. Time resolved measurements were performed by averaging 64 interferograms per spectrum in an acquisition time of 10 s and a time interval of 15 s between beginnings of acquisition of consecutive spectra. The first spectrum was recorded 1 min after casting the drop. X-ray Diffraction Analysis. X-ray diffraction (XRD) patterns were collected in the angular range from 1 to 30° in 2θ, using a Bruker D8-Discover instrument in the grazing incidence geometry with a copper target tube (λ = 1.540 56 Å); the X-ray generator worked at a power of 40 kV and 40 mA.
’ RESULTS AND DISCUSSION To follow in situ the change of the hybrid structure during evaporation processes, we have used an infrared time resolved spectroscopic technique; the process of evaporation of the hybrid sol is not critical and it allows recording spectra with a very good signal-to-noise ratio. We recorded the spectra in all available wavenumber intervals of detection and then separately performed the analyses in selected ranges of interest (full spectra are available as Supporting Information). CH2 Bending Vibration Region. Figure 2a shows the FTIR absorption spectra in the 1500 1425 cm 1 range of the evaporating droplet which have been recorded as a function of time. Figure 2a has been obtained from the original FTIR data after cutting the spectra in the 1500 1425 cm 1 interval and calculating the baseline in the same range using a rubber-band correction with 64 baseline points (Software OPUS 6.5). The blue arrows in Figure 2a indicate the “direction of evaporation”; the blue spectrum at the bottom is the first one recorded at the beginning of the process. The spectra show three distinct bands peaking at 1478, 1458, and 1440 cm 1; the first two bands, in particular, are assigned to a scissoring (δ(CH2)) mode in the propyl chain while the 1440 cm 1 band is assigned to CH2 scissoring of the glycidoxy group.21 This region is quite important when the conformation of alkyl chains has to be studied in solid state materials22 because it gives some information about the packing and interaction with the neighborhood chains.23 In general, when the methylene chains order to an all-trans crystalline state, a shift to higher wavenumbers is observed; on the other hand, the presence of a liquid or soft state is associated with a downshift, broadening and decreasing in intensity of the band, which in turn is due to the decrease in the chain interactions. With the proceeding of the evaporation time (spectra in the sequence indicated by the arrows in Figure 2a), the bands associated with δ(CH2) bending show a significant change. A variation in the absolute and relative intensity of the bands is observed; at the 10439
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Figure 2. (a) Time resolved FTIR absorption spectra in the 1500 1425 cm 1 range during evaporation of a cast droplet of GPTMS water solution. (b) Time wavenumber absorbance FTIR spectra of a cast droplet of GPTMS water solution in the 1500 1425 cm 1 range. The absorbance intensity is shown in a false color scale.
Figure 3. (a) FTIR spectrum of GPTMS water droplet immediately after casting (black line). Fit of the spectrum (red line) with three Gaussian curves (green lines) and a linear baseline (blue line). (b) FTIR spectrum of GPTMS water droplet at the end of the evaporation process (black line). Fit of the spectrum (red line) with three Gaussian curves (green lines) and a linear baseline (blue line).
beginning of the process the band at 1480 cm 1 is the most intense, while with the proceeding of evaporation the band at 1458 cm 1 becomes predominant. The peak positions do not shift with time, while narrowing effects are more difficult to evaluate because of the presence of three different overlapping bands. We attribute the change of relative intensity of the 1458 and 1480 cm 1 bands during the evaporation to a different sensitivity to the change in the chemical environment; the 1458 cm 1 has a better and more detectable response. We also reproduced Figure 2a using a time (y-axis) wavenumber (x-axis) absorbance (false color scale) graphical visualization in the same range of wavenumbers which is shown in Figure 2b. The exact time of evaporation is reported in the y-axis; in this way it is possible to have a direct time correlation of the phenomenon. Using this graphical visualization, it is easier to follow the trend with evaporation of the CH2 bending bands at 1480 and 1458 cm 1; they increase in intensity as shown by the change of colors and the full width at half-maximum (FWHM) decreases. We have applied a fitting procedure to the first and last spectra recorded during the evaporation experiment; the result is reported in Figure 3. The curve fit has been obtained from the raw spectra using three Gaussian curves and a linear fit for the baseline. The first spectrum is shown in Figure 3a and the last one is shown in Figure 3b; the original curve is shown in black, the fit curve is shown in red, and the baseline is shown in blue. At the bottom of Figure 3 the three Gaussian components (green curves) obtained from the fit are shown. The fit confirms that the
Figure 4. Time resolved FTIR absorption spectra in the 4600 4480 cm range during evaporation of a cast droplet of GPTMS water solution.
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component peaking at 1458 cm 1 increases in intensity with the time, does not shift, and reduces the FWHM. We can use these data to make a small model of the hybrid structure which is forming with evaporation; the increase in intensity clearly indicates that the system evolves from a liquidlike to a softlike structure, which is still not yet fully interconnected and is in a wet state. This means that at the first stage of evaporation there is a liquidlike state, which gradually evolves to a soft-matter-like state. The highly basic environment promotes a 10440
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Figure 5. (a) Time resolved FTIR absorption spectra in the 5370 4930 cm 1 range during evaporation of a cast droplet of GPTMS water solution. (b) Time wavenumber absorbance FTIR spectra of a cast droplet of GPTMS water sol in the 5400 4900 cm 1 range. The absorbance intensity is shown in a false color scale.
Figure 6. Integrated absorbance of the 5190 (black line) and 4532 cm (blue line) bands as a function of evaporation time.
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fast hydrolysis of GPTMS, but it somehow hinders the polycondensation to a more extended stage. With the proceeding of evaporation and the partial formation of the inorganic network, the structure starts freezing and the motion of the alkyl chains decreases; this produces in turn a more interconnected structure and a chemical environment where the alkyl chains lose their mobility. Epoxy Content. The evaluation of the epoxy content in hybrid materials prepared from alkoxide bearing epoxy functionalities is critical for understanding the structural evolution as a function of the processing conditions.24 Vibrational spectroscopies are commonly used to measure the epoxy content in films because they are simple, fast, and not destructive. FTIR is the most common choice to study polycondensation of epoxy resins; alternatively, FT-Raman can be also employed,25 but it has the disadvantage of requiring much more expensive equipment and having some overlapping of the epoxy signals with other bands. The 950 cm 1 band of the epoxy26 is generally used with FTIR for measuring organic resins containing epoxy functionalities. In hybrid sol gel materials, because of the high overlapping in that region with the absorption band of silanols (Si OH stretching), the band at 3050 cm 1 is more commonly used,16,27 but also in this case the overlapping with the CH2 stretching region does not allow obtaining a very precise evaluation of the epoxy content. FTNIR is, instead, a much more reliable technique because the overtone or combination bands are generally well separated and
Figure 7. Time resolved FTIR absorption spectra in the 1240 940 cm range during evaporation of a cast droplet of GPTMS water solution.
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allow a quite precise quantitative evaluation. In the NIR range the epoxy gives a signal at 4550 cm 1 (combination band of the second overtone of the epoxy ring stretching at 916 cm 1 with the fundamental at about 2725 cm 1). We have followed the change of the epoxy band at 4550 cm 1 during evaporation in situ using an experimental setup of the microscope which has been designed for the NIR range; the results of the time resolved measure are shown in Figure 4. Figure 4 has been obtained by cutting the full spectra in the 4600 4480 cm 1 range and performing a baseline correction using a rubber-band algorithm with 64 points (OPUS 6.5); no smoothing has been performed. The spectra of epoxy show basically a small shift to lower wavenumbers with the proceeding of evaporation, with a small change in the full width at half-maximum; the areas of the epoxy signal do not change if we consider the variation in fwhm. This gives direct information on how, in this specific case, the evaporation affects the sol gel process. The epoxy does not react during this stage; the content of epoxy remains the same but the epoxies “sense” a change in the chemical environment during the evaporation which explains the shift to lower wavenumbers. During casting of the film water evaporates and around the epoxy groups the polarity changes; this is revealed by the shift to lower energies due to the progressive loss with time of the hydrogen bonded water. When water evaporates, it leaves a less polar environment around the epoxies which in this case act as a local probe of the polarity of the hybrid structure during evaporation. 10441
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Figure 8. Time resolved FTIR absorption spectra in the 1500 1430 and 1760 1560 cm GPTMS water solution (a and b, respectively).
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ranges during evaporation of a cast droplet of 7 days aged
Figure 9. X-ray diffraction pattern and FTIR absorption spectrum of an as-deposited amorphous GPTMS film (a and b). X-ray diffraction pattern and FTIR absorption spectrum of an as-deposited crystalline GPTMS film (c and d).
Water Evaporation. The time resolved experiment performed in the NIR allows also monitoring the water evaporation in a quite clear way using the ∼5200 cm 1 band which is assigned to the second-order combination of OH stretching (∼3350 cm 1) and OH bending vibrational modes (∼1650 cm 1), νs01(OH) + δ01(OH).28,29 The NIR absorption spectra taken as a function of evaporation time are shown in Figure 5a; the band peaks at 5190 cm 1 and is strongly asymmetric with a long tail at low wavenumbers and the arrow indicates “the direction of evaporation” with time. We also reported the spectra (Figure 5b) using a graphical visualization time (y-axis) wavenumber (x-axis) absorbance (false color scale) in the same range of wavenumbers; this visualization allows a direct correlation with evaporation time. The change in absorbance as a function of the time is shown in Figure 6 (black line, left scale in the figure); it decreases in intensity monotonically with time and the curve can be well fit by an exponential decay function. This trend can be correlated with that one of the epoxies which shows a simultaneous variation in intensity and position with time; the change in absorbance is given by the blue curve and right scale in Figure 6. The two curves
exhibit opposite trends: with the proceeding of the evaporation, the epoxy band increases in intensity with an exponential correlation with time. This finding supports the existence of a correlation and, therefore, our interpretation of the phenomenon. Silica Condensation with Evaporation. In sol gel materials the evaporation process triggers the sol gel transition by polycondensation of Si OH groups; this process is slowed down if very highly basic or acidic conditions are employed.18 This is the strategy which is used for achieving template-assisted self-assembly of mesostructured materials30 but also self-organization in hybrid materials from GPTMS. On the other hand, as we have shown in previous works, a highly basic condition favors a fast initial hydrolysis while polycondensation is slow;18 at the same time it also promotes the formation of cyclic species from GPTMS.31 Figure 7 shows the FTIR absorption spectra in the 1230 940 cm 1 range as a function of time; the arrows indicate the “direction of change” with the proceeding of evaporation. The spectra have been cut in the selected range and the baseline has been calculated by a rubber-band function using 64 baseline points (OPUS 6.5 software); no smoothing has been performed. 10442
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The Journal of Physical Chemistry A The spectra are characterized by the presence of different overlapped bands at 1200 cm 1, which is assigned to CH2 wagging, and at 1135, 1103, 1055, and 1035 cm 1, which are instead assigned to different silica modes. The main band around 1100 cm 1 is due to Si O Si symmetric stretching mode; this band has a shoulder at a higher wavenumber (1135 cm 1) which is ascribed to a broadened signature of the LO component of the TO3 asymmetric stretching mode. These components show a small increase in intensity (7%) with time. The other two components at lower wavenumbers are instead assigned to the formation of cyclic species, in accordance to previous findings and the literature.32 In particular, they are assigned to large strainless cage species Tn such as [RSiO1.5]n=8,10, or 12 silica structures33,34 (Tn denotes a cage structure with n SiO3/2 groups); this is well in accordance with the NMR data of the precursor sol.18 The band at 1055 cm 1 could be also assigned to CO stretch of methoxy groups, which should indicate the presence of unreacted alkoxies; however, we discharge this possibility because the band increases in intensity with the evaporation time, which is inconsistent with this assignment. The data in Figure 7 show, therefore, that with the proceeding of evaporation only a small increase of silica network condensation is observed, while the basic conditions promote the cyclization reactions which continue during the entire process. When more water evaporates, the chemical environment becomes more basic and favors the formation of cyclic species. The silica structure is formed, therefore, by an increasing number of Tn (n = 8, 10, and 12) cubelike species connected with short ladderlike bridging four-member rings. At this stage, as shown in Figure 4, the epoxies do not react and a very peculiar “soft” structure is formed; this special system allows forming, with the slow opening of the epoxies, a “crystalline-like” network. Amorphous vs Crystalline Solution. We have repeated our experiments using instead of a fresh sol an aged one, which is expected to produce a crystalline hybrid material. Aging of the sol allows the opening of the epoxies which react to give dioxane species and form a bridging species between two GPTMS molecules; these new bridged silsesquioxanes self-organize into an ordered layered structure. The time resolved FTIR spectra of the evaporating droplet of an aged sol are shown in Figure 8; Figure 8a shows the 1500 1430 cm 1 range with the CH2 scissoring bands and Figure 8b shows the 1760 1550 cm 1 range with the water bending mode. In this case while evaporation proceeds, as shown by the decrease of the water absorption band (Figure 8b), the spectra in Figure 8a do not change. This is a very important indication because it confirms the correlation of these specific FTIR bands with the presence of a highly packed structure. We have used this indication to create a specific signature of the hybrid structure by performing XRD analysis of the amorphous and crystalline droplets at the end of the evaporation; the results are shown in Figure 9. Parts a and b in Figure 9 show the XRD pattern and the FTIR absorption spectra of an amorphous film, respectively, while parts c and d are the XRD and FTIR spectra of the crystalline hybrid. The diffraction patterns are characterized by a broad peak around 6°, which has been previously found in hybrid materials with some local order.35,36 The XRD patterns of crystalline hybrids exhibit also a distinct feature which is a broad signal of weaker intensity around 9°; this signal is the distinct fingerprint of the formation of crystalline layered organic inorganic structures.37 The fresh sol gives, therefore, an amorphous hybrid while the aged sol gives a crystalline structure. The FTIR are also very peculiar: the
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1478 cm 1 band is the most intense when the hybrid is amorphous, while in an ordered system the spectrum is characterized by the rise of the 1458 cm 1 band which overlaps the other vibrational modes. This correlation will allow using the FTIR spectra of the CH2 scissoring mode for a quick assessment of the packing state of the hybrid network.
’ CONCLUSIONS We have studied in situ by time resolved FTIR spectroscopy the physical chemical changes during the evaporation of a hybrid solution of 3-glycidoxypropyltrimethoxysilane in highly basic conditions. The experiments have been realized using a fresh sol which produces amorphous hybrids and for comparison an aged sol which gives a crystalline material. The evaporation of water is a slow and continuous process which does not produce a drastic change in the structure which evolves from a wet to a soft-like state. The epoxy groups do not react during the process and remain closed; with water evaporation the chemical environment within the hybrid material becomes less polar as indicated by a variation of epoxy infrared spectra. On the other hand, while the silica network shows only a small condensation, the basic conditions with water evaporation promote the formation of cagelike silica structures. The FTIR bands assigned to scissoring vibrational modes give direct information on the change of the packing state of the structure. In particular, by comparing the spectra of an amorphous material and a crystalline material, it is possible to assign the spectra to the different structures. ’ ASSOCIATED CONTENT
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Supporting Information. FTIR spectra of fresh and 7 days aged GPTMS water sols immediately after casting and at the end of the evaporation process. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT C.F. acknowledges Regione Sardegna (RAS) for funding through the project PO Sardegna FSE 2007-20013 of L.R.7/ 2007 “Promozione della ricerca scientifica e dell’innovazione tecnologica in Sardegna”. Sardegna Ricerche is acknowledged for free access to analytical facilities. ’ REFERENCES (1) Besson, E.; Mehdi, A.; Reye, C.; Gaveau, P.; Corriu, R. J. P. Dalton Trans. 2010, 39, 7534–7539. (2) Lerouge, F.; Cerveau, G.; Corriu, R. J. P. New J. Chem. 2006, 30, 1364–1376. (3) Fujita, S.; Inagaki, S. Chem. Mater. 2008, 20, 891–908. (4) Arrachart, G.; Carcel, C.; Moreau, J. J. E.; Hartmeyer, G.; Alonso, B.; Massiot, D.; Creff, G.; Bantignies, L.; Dieudonne, P.; Wong Chi Man, M.; Althoff, G.; Babonneau, F.; Bonhomme, C. J Mater. Chem. 2008, 18, 392–399. (5) Boury, B.; Corriu, R. J. P.; Le Strat, V.; Delord, P.; Nobili, M. Angew. Chem., Int. Ed. 1999, 38, 3172–3175. 10443
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