STRUCTURAL AND TRANSPORT PROPERTIES OF HYBRID ORGANIC INORGANIC SILICA MEMBRANES

The series of hybrid organic-inorganic silica materials have been prepared by introducing organic ligands materials based on sol-gel processing of alkoxysilanes for potential applications in membrane design for pervaporation. The materials were characterized using structural and dynamic techniques to gain information about the formation of microand mesoporous silicates. The dynamic interaction within silica matrices were investigated using FTIR, Raman Spectroscopy, Solid State NMR Spectroscopy, Physisorption and SEM. The transport properties of the hybrid materials were observed by monitoring the diffusion behavior of water and several selected solvents using Pulsed Field Gradient NMR. The self-diffusion of water and organic solvents in the hybrid silica materials were two to three orders of magnitude smaller than in the liquid bulk suggesting restricted diffusion at the pore surface. The effect of surface polarity also contributed to water and solvents diffusivities. The temperature dependence of diffusion was useful to derive the activation energy whereas the dependence on NMR observation time provided information on both tortuosity and pore connectivity of the hybrid silica materials. These materials have potential uses in gas and liquid separation such as pervaporation.


J. W. Haryadi i
Abstract--The series of hybrid organic-inorganic silica materia1s have been prepared by introducing organic Ugands materials based on sol-gel processing of alkoxysllanes for potential appUcations in membrane design for pervaporation.The materia1s were characterized using structural and dynamic techniques to gain information about the formation of micro-and mesoporous silicates.The dynamic interaction within silica matrices were investigated using FTIR, Raman Spectroscopy, SoUd State NMR Spectroscopy, Physisorption and SEM.The transport properties of the hybrid materials were observed by monitoring the diffusion behavior of water and several selected solvents using Pulsed Field Gradient NMR.The self-diffusion of water and organic solvents in the hybrid silica materials were two to three orders of magnitude smaller than in the liquid bulk suggesting restricted diffusion at the pore surface.The effect of surface polarity also contributed to water and solvents diffusivities.The temperature dependence of diffusion was useful to derive the activation energy whereas the dependence on NMR observation time provided information on both tortuosity and pore connectivity of the hybrid silica materials.These materials have potential uses in gas and Uquid separation such as pervaporation.

I. PENDAHULUAN
O rganically functionalized hybrid sol gel materials, obtained from the hydrolysis and co-condensation of TMOS or TEOS with other organosiloxanes (R x Si(OR)4x, where R is alkyl groups), has attracted increasing attention in recent years because of their potential applications in such fields as hard coating for optics, catalysis, selective membranes or molecular recognition [1][2][3].The major driving forces behind the intensive activities in these organic inorganic hybrid materials are the new and different properties of nanocomposites which have domain sizes typically in the range 1-100 nm, being the average size of the components or phase.In these materials, TEOS precursors function as building blocks to construct the framework while the organosiloxanes with non hydro1ysable organic groups contribute both framework silica units and the urganic surface functional groups.Various molecular structures are formed depending on the functionality of the organosiloxane and its proportion in the mixture.
The inorganic components can provide excellent mechanical and optical properties such as surface hardness, thermal and structural stability, transparency, and high re-fractive index.Introduction of organic moieties within the organic silicate framework may increase the flexibility of mesoporous films and fibres and reduce the brittleness of films or monoliths and are also helpful in the design of the surface polarity of the materials [3], [4].
In this paper, the structural properties of hybrid silica materials prepared from a low temperature sol gel process are discussed.Several techniques are used to characterise the materials namely, 29Si Solid State NMR, Infrared and Raman spectroscopy, gas physisorption, SEM and pulsed field gradient spin echo (pFGSTE) NMR.The main goal of the experiments is to characterize and if possible to predict the transport phenomena in the hybrid silica materials.This will enable design of new membrane material for gas separation and pervaporation applications.

II. METHODS AND MATERIALS
The procedure for the preparation of the hybrid silica materials was adopted from that in the literature with some modification [4].In general TEOS (tetra ethoxysilane) and MTES (methyl triethoxysilane) were prehydrolysed with deionized water in ethanol using HCl as catalyst at [MTES]:[TEOS]:[H20]:[EtOH]:[HCl] molar ratio of x:1-x:4:3:0.001where x ranged from 10 to 50 mol%, at about 70°C for 2-3 h.All samples in the petri dish were then cooled to room temperature and sealed with cellophane films containing several pinholes to allow for slow evaporation of solvent and reaction byproducts at room temperature IR spectra were recorded using a FTIR Nicolet A V A-TAR 320.Raman spectra were performed using a Renishaw 2000 Raman microscope with a 633 nm He-Ne laser.29Si cross polarization (CP) magic angle spinning (MAS) NMR spectra were measured on an Inova 300 Chemagnetics (Varian) instrument using a 7.5 rom zirconia rotor sample holder and rotation frequency of 2 KHz.Pulse width ~ JlS, variable contact time from 0.005 to 20 IDS, recycle delay 5 s and 1 H RF field of 62 KHz were selected.For the 29Si single pulse experiment (SPE), pulse width (l.2-l.6 Jls), RF decoupling power of 62 KHz and relaxation delay (80-120 s) were chosen to take into account the long T 1 relaxation times.Chemical shifts of silicon were referenced to kaolin which appears at -9l.5 ppm.PFGSTE NMR experiments were measured on an AMX 500 spectrometer (Bruker) using 5 rom NMR tubes.The specific surface area and porosity were measured using the BET method on a Micromeritics ASAP 2000 Gas Sorption Analyzer.The sample was placed in a glass tube and immersed in a bath of liquid nitrogen to control the temperature at 77 K. Outgassing was carried out at 120°C in a vacuum (10 JlID Hg) to drive off any adsorbed water in the sample prior to testing.
Chemically resistant Anodisc 25 aluminium oxide filters were used as ceramic supports.These membranes are flat and circular (21 mm diameter) having polypropylene support rings.They are 60 J.1m thick and have about 50 % surface porosity and 0.2 J.1m pore size.Colloidal silica (LUDOX HS-40, 40 wt"10 Si0 2 in water, pH =9) was acidified to pH 4 by adding HN0 3 50 v/v%.In the preparation of silica membranes, this colloidal silica was used to reduce the pore size of the substrate.
Silica membranes were formed by a spin coating deposition method.The silica sols (procedure 2.1.2.2) were deposited onto the treated substrate by using ClIEMA T TECHNOLOGY, KW-4 spin coater.Typically, 0.5 ml of the silica sols was deposited on the treated Anodisc filter by spin coating at spinning rate of 2000-2500 rpm for 10-20 s.Another coating of silica solon the same surface was added, using the same volume and spinning rate.Please use automatic hyphenation and check your spelling.Additionally, be sure your sentences are complete and that there is continuity within your paragraphs.Check the numbering of your graphics and make sure that all appropriate references are included.

A. Vibration spectroscopies
Infrared and Raman spectroscopies enabled functional groups to be detected within the silica matrices.The peak at 1250 cm-I indicates that the Si-C bond has been introduced and did not cleave in forming the silica hybrid matrices.The symmetric C-H stretching vibrations for the methyl groups appear at about 2925 and 2975 em-I The sUanol and silicon oxide bands exist at the surface of the silica as indicated at 960 cm-I and 1080 cm-I respectively.and 29Si nuclei as well as on the mobility of the corresponding groups.
In 29Si CP (Cross Polarisation) MAS NMR spectra, the signals due to various substituents on silicon can be assigned and the average structure of the functionalized gel characterized as represented in Figure 2. It can be seen that the hybrid silica materials contains five different silicon environments namely, CH r Si0 2 (OH) (T 2 ), CH 3 -Si0 3 (T 3 ), Si0 2 (OHh (Q2), Si0 3 (OH) (Q3) and Si0 4 (Q4).
Cross-polarization dynamics is usually described within the framework of spin thermodynamics considering two reservoirs corresponding to the abundant I-spin eH) and the rare S-spin ( 29 Si).This model predicts an exponential build-up of S-spin magnetization in the rotating frame, characterized by lITIS.If the loss of spinlocked proton magnetization in the rotating frame is taken into account by the relaxation rate IIT 1p , the time dependence of the S-spin magnetization is [5]: The polarization dynamics of Q4 units can give information on the proximity of T units in TEOSIMTES in hybrid derived samples.The corresponding parameters are summarized in Table I, together with parameters corresponding to TEOS and TEOSIMTES.T Si-H is related to the dipolar coupling and thus to the IH_ 29 Si distance.For Q4 units, the influence of mobility can be neglected as a first approximation.

B. CPMASNMR
The magic angle spinning (MAS) technique makes it possible to obtain well resolved 29Si NMR spectra for silica-based sol gel materials.In addition, the intensities of the 29Si peaks can be significantly enhanced by using the IH_ 29 Si cross-polarization technique (CP) when IH nuclei exhibit a good polarization transfer to 29Si.The efficiency of the polarization transfer between the protons and the 29Si nuclei depends strongly on the IH_ 29 Si dipolar coupling and thus on the distance between protons In TEOS derived silica gel, the T Si-H value is long due to poor protonated environments as also observed by Zumbulayadis [6].In the gels containing T units, the T Si -H values are reduced indicating an increased number of protons in the environment of Q4 units.The close proximity between Q4 units and T units is also supported by Fyfe et al [7], who observed TEOSIMTES derived gels by two dimensional 1H/ 29 Si correlation experiments which clearly demonstrated that the two components are not phase separated.The organic functional groups can be controlled in order to produce more hydrophobic character on the surface of the materials.CP MAS NMR can not be used to obtain quantitative data on the different Si sites due to the large distance between protons and some of the Q4 Si atoms [8].Therefore for quantitative data single pulse excitation (SPE) MAS NMR with a longer recycle delay (>3 x T,) are performed and the areas corresponding to each of the Si sites are obtained from deconvolution of the 29Si MAS NMR spectra.Figure 4 shows the methyl content of the silica materials as prepared (sol) and after gelation (gel) calculated from 29Si SPE MAS NMR measurement.A linear relationship is observed between methyl content in the sol and the gel suggesting the degree of condensation approaches 100 % based on the sample preparation.Permeability and selectivity are two important factors which govern the applications of the hybrid materials in separation processes.The permeability is controlled by the pore volume fraction whereas the selectivity is determined by the pore size and its distribution.Figure 4 displays nitrogen sorption measurements of various hybrid silica materials.A reversible type I isotherm that is ty- pical of a microporous structure is obtained.Table 2 also provides data derived from physisorption measurements of hybrid silica materials.

D. Water Diffusion measured by PFG NMR
The selectivity strongly depends on the relative solubility and diffusivity of solvent and solute in the materials.In order to understand the mechanism of the selectivity, the 389 ± 1 interactions between solvent, solute and the silica materials on the microscopic level should be obtained [7].The diffusional motion of a small molecule e.g.water in inhomogeneous materials such as porous materials depends on the structure of the materials and the diffusion time.The motion called restricted diffusion, shows individual time dependence related to each structure.The pulsed-field-gradient NMR or space-resolved NMR such as PFGSTE (Pulsed Field Gradient Stimulated Echo) NMR is a non invasive technique for studying transport phenomena at the microscopic scale.By applying a magnetic field-gradient, the NMR frequency of a hydrogen nucleus is determined by the position of the hydrogen containing molecule in the magnetic field.On this basis, the displacement of water molecules is detectable and the self-diffusion coefficient D, m 2 s• l , can be measured.Furthermore the method can be used to measure the diffusion coefficient of molecules in the porous materials at various diffusion times and to study the relationships between diffusion coefficient and porous material structure [9].Self-diffusion coefficients are calculated by measuring the decrease the NMR signal intensity with increasing magnetic field gradients.
The self-diffusion coefficient of bulk water (Do) is 2.29 X 10. 9 m 2 s• 1 which is the same as Holz's results of 2.29 x 10. 9 m 2 s-1 for PFGSE NMR calibration purposes (Cussler 1997).This therefore confirms the reliability of the PFG NMR experiments.
An Arhenius plot of the diffusion data observed at temperature from 288 to 328 K provides the activation energy, Y?a, from D = A exp{ -EaIRT} (2) where A is a constant, R the gas constant, and T the absolute temperature.Figure 5 displays the Arhenius plot for D of water in hybrid silica materials.While these are not perfectly linear, they can be used to estimate activation energy [10] for water diffusion as shown in Table 3.
Diffusion coefficients decreased as diffusion time increased which suggests the possibility of the restricted diffusion of water molecules in the pores.The result indicates that the effective restricted diffusion coefficients of water in the hybrid silica materials at 298 ± 0.5 K are three orders of magnitude smaller than in the neat liquid measured by PFGSTE NMR as shown in Table 3.These rather small diffusion coefficients, relative to bulk water could be attributed to the strong interaction of water molecules with the pore surface and the relatively small effective pore size

E. Morphology of hybrid silica membranes
Figure 7a and 7b show the anodisc-alumina support material which has surface pore diameters of about 0.2 J.UD and is 60 J.UD thick.Another membrane support made from polycarbonate, denoted Isopore, with pore diameters 0.5 J.UD was also considered.To achieve ideal membranes for gas separation or pervaporation, pore size modification is required.This follows the established method developed by Moadeb and Koros [11], through occlusion of the surface pores of the aluminium oxide with colloidal silica (Ludox 40), prior to deposition of the active layer consisting of microporous silica or hybrid silica.Figure 7c shows the anodisc after three deposition steps of the colloidal silica sols indicating that parts of the large pores were covered on the surface of the Anodisc membrane though some pinholes/smaller pores still existed with pore sizes of about 50-100 om.The pinholes might be caused by the nonuniformity of the occlusion process.Since a very small area of the membrane is viewed by the electron microscope, it is premature to conclude that the surface after three deposition steps is thoroughly covered and free of defects.
Figure 8 displays the results of spin coating at 2000 rpm for the polymeric sols ofTEOS-MTES hybrid silica membranes (TE8-MTI and TE5-MT5).No cracks were observed for these hybrid membranes whereas it is difficult to obtain non-cracked films for pure silica, TEOS, sols by spin coating.Therefore the preparation was only focused on the hybrid silica membranes.Furthermore, it also can be seen that there are still some fine gaps on the surface of the hybrid silica film although the spin coating step was repeated 10 times.These might be due to the non uniformity or incomplete intermediate layer formation of silica particles during plugging up of the pores.The viscosity of the sol and the water to silica ratio might also contribute to the inhomogeneous structure of the film during spin coating and drying of the silica films.
The spin coating was not prolonged since the thickness of the active layer (polymeric silica) had already exceeded 1 ~, Figure 9a, which is beyond the recommended active layer thickness (i.e. 100 om) for either gas separation or pervaporation applications.Furthermore, Figure 9b displays the bottom view of the alumina support showing no blocking of pores after the colloidal sol deposits and the spin coating process suggesting that the alumina support is not blocked by either silica polymer or silica particles and therefore the membranes can be trialled for pervaporation.

F. Contact Angle
Figure 10 shows the water contact angle (B) measurements at 298 K after spin coating of the hybrid silica membranes on the intermediate layer.The alumina support before coating and after deposition of the intermediate layer gives close to zero water contact angle due to hydrophilic character.After five to six times repetition of water contact angle measurements, it can be seen that by increasing the methyl content in the hybrid silica materials, the water contact angle, e, rises from (30 ± 2)° for TEOS (TE-GYO) to (110 ± 4t and (130 ± 3 )0 for TE8-MT2 and TE5-MT5 respectively, Figure 10.The hydrophobicity of the silica membranes increases systematically as the methyl content increases although the trend is not linear.
The results of water contact angle measurements are consistent with other o~servations of the hybrid silica xerogels.The TEOS (TE-GYO) sample contains a high proportion of silanol (Si-OH, QJ unit) groups which makes it more hydrophilic 'as I./,ompared with hybrid silica materials whose surfaces not only consist of silanol groups but also methyl groUps attached to Si (T J unit) as quantified by 29Si SPE MAS NMR.The proportion of the silanol groups decreases as the methyl content increases leading to more hydrophobic character on the surface of the hybrid silica gels.

IV. CONCLUSION
The characteristic of non-reactive organic groups in the pore surface of hybrid silica materials are observed from 29Si CP/MAS NMR of variable contact time experiments.Hybrid silica gel compositions can be quantified by 29Si SPE NMR.The total degree of condensation (DCT) increased as the amount of MTES increased.By comparing integrated intensities of the peaks with the Qn peaks in the 29Si SPE MAS NMR data, it was determined that the fraction of methyl modified silicon in the film was nearly identical to that of the precursors solution.
Introducing methyl groups into the silica network modified the silica surface properties to become hydrophobic, nevertheless microporosity could still be maintained.The BET surface area and porosity show significant variations but there are no clear trends with the amount of methyl groups whereas pore dimensions gradually increased as the methyl content increased.

TEOS
TEB-MTE2 TE5-MTE5 The surface polarity of the hybrid silica samples affects water diffusivity and the effect is more pronounced when the carbon chain length or amount of alkyl substituents increases.
The activation energy of water in the hybrid silica materials, derived from temperature dependence, is somewhat higher as compared with the bulk liquid and is again influenced by the hydrophobicity as well as the pore sizes of the samples.
The feasibility of fabrication of thin hybrid silica membranes with different organosilicate content on inorganic substrates is demonstrated.The 0.2 ~ pores of the alumina support membranes were reduced by occlusion of colloidal silica particles although there small pores of about 50 om diameter still exist.The spin coating of the polymeric sol of TEOS produced crack films whereas by introducing methyl groups during the sol-gel process, crack free films can be obtained.
The water contact angle increases as the methyl content increases from 0 to 50 mol % in the hybrid silica membranes confirming the more hydrophobic character on the bulk surface of the membranes.

o~~Fig. 4 .
Fig.4.Comparison of the methyl content (T (CH3-Si~» in the sols and the methyl content in the gel calculated from 29Si SPEMAS NMR measurements

Fig. 10
Fig. 10 Shape ofa water drop (-3 ilL) on the silica membranes with different Methyl contents.The dashed line indicates the film surface.The reflection of the drop is seen below the dashed line.

TABLE 1
SI Cp/MAs NMR EXPERIMENTS; PARAMETERS FOR PEAK DUE To Q4

TABLE 3
WATER SELF DIFFUSION IN HYBRID SILICA MATERIALS, II = 2 Ms AT 298 K.

TABLE 2 PORE
STRUCTURAL PARAMETERS AND SURF ACE AREAs OF PuRE SILICA AND HYBRID SILICA GELS SBET -BET surface area; PV = Pore Volume; PD= Average Pore Diameter; M-PV= Micropore Volume; MP-A= Micropore Area