J. MATER. CHEM., 1994, 4(12), 1855-1860 Relaxation Behaviour of NLO Chromophores Grafted in Hybrid Sol-Gel Matrices 6. Lebeau,aJ. Maquet,” C. Sanchez,*a E. Toussaere,b R. Hierleband J. Zyss a Laboratoire de Chimie de la Matiere Condensee, URA CNRS7466, Uniwersite Pierre et Marie Curie, 4 place Jussieu 75252 Paris Cedex 05, France Centre National d’Etudes des Telecommunications, France TELECOM, 796 Avenue Henri Rawera, BP 707, 92 225 Bagneux Cedex, France Poled hybrid siloxane-oxide coatings synthesized through hydrolysis and condensation from tetramethoxysilane (TMOS) and N-[3-triethoxysilylpropyl]-2,4-dinitrophenylamine (TSDP) precursors have been characterized via second-harmonic generation (SHG), differential scanning calorimetry (DSC) and 29Simagic-angle spinning nuclear magnetic resonance (MAS NMR) measurements.Depending on chemical composition the SHG values range between 2.5 and 10 pm V-’. The matrix rigidity seems to have a strong influence on the relaxation behaviour of the optically non-linear chromophores grafted in these hybrid networks. The glass-transition temperatures of the hybrid coatings increase with the TMOS content and the thermal curing time. These modifications can be explained by an increase in the condensation and degree of crosslinking of siloxane and silica species. The sol -gel process offers new approaches to the synthesis of hybrid materials in the field of optics.’-3 The chemistry involved in the sol-gel process is based on hydrolysis and condensation of metal alkoxides, leading to the formation of metal-oxo based macromolecular The sol-gel process presents two main advantages.First, rheological properties of sols allow the easy deposition of coatings onto glass, semiconductors, ceramics or polymeric substrate^.^ The second advantage results from the various characteristics of sol-gel process (metallo-organic precursors, organic solvents, low processing temperatures) that allow the introduction of ‘fragile’ organic molecules into an inorganic network.’-12 Inorganic and organic components can be mixed in virtually any ratio to obtain the targeted properties, making these hybrid nanocomposites extremely versatile in their composi- tion, processing, and optical and mechanical properties. 11,12 As shown in the pioneering work performed by Avnir et organic dyes can be incorporated into an oxide gel matrix without any risk of thermal damage.Extensive work on dyes embedded in silica, aluminosilicates or transition-metal oxide based matrices made by sol-gel procedures has been per- formed over the past ten years.8,10312-21 Th ese dyes can play an important role in applications such as tunable lasers, luminescent solar concentrators or materials for non-linear optics12 and related devices. Most of the sol-gel optics research devoted to NLO materials was first related to third-order processes which are compatible with the isotropy of amorph- ous sol-gel matrices.’.2 Organic molecules inside amorphous sol-gel matrices are in general randomly oriented, thus ruling out the emission of the second harmonic.As the second-order non-linearities are achieved only in a non-centrosymmetric environment, we first demonstrated that orientation of the organic chromophores could be performed in hybrid sol-gel by using electrical field induced second harmonic (EFISH) or Corona electrical field poling techniques. Organic molecules such N-[3-triethoxysilylpropyl]-2,4-dinitrophe-nylamine (TSDP) were chemically bonded to the oxide back- bone of gels. The chemical bonding of the dye to the sol-gel matrix allowed the dye concentration to be increased without any crystallization o~curring.~~-~~ The sol-gel matrices were synthesized by copolymerization of silicon alkoxysilanes [TSDP and SiHCH,(OEt),] and zirconium propoxide pre- cursors.2J The sols were deposited as transparent coatings and exhibited (after corona poling) an SHG response of 1.6 pm V-’.22-24 Even though in this first generation of sol-gel matrices relaxation of the organic chromophores occurred over several hours, these results indicated the feasibility of using poling techniques with hybrid inorganic sol-gel matrices that are more ionic than classical polymers.Consequently a wealth of opportunity for the synthesis of optical sol-gel devices with efficient second-harmonic properties was exposed. Over the past two years there has been increasing interest in second-order NLO materials synthesized cia sol-gel chemis-try.26-33 The optimization of the second-order NLO rcsponse of hybrid sol-gel matrices with grafted chromophores is currently under investigation by several research groups.Several strategies are used to improve the NLO response of the hybrid coatings. (i) The intrinsic NLO response of the dye can be increase by using chromophores such as N-(4-nitropheny1)-~-prolinol (NPP) or ‘disperse red one’ (DR1) derivative^^'-^^ which exhibit higher non-linearities than nitro- aniline derivatives. (ii) The chromophore relaxation can be controlled by increasing the matrix rigidity. This point is, without doubt, the most important to be solved in order to be able to make efficient NLO devices. The modification of the binary composition (siloxane-crosslinker), the nature of the M(OR)4 crosslinking alkoxide [SiR,(OR),-,/M (OR),: R’=any NLO chromophore; M=Zr, Si, Ti.,.], and the pro- cessing of these hybrid materials in the presence of pdymers with well known mechanical properties such as methyl metha- crylates or polyimides, are the most commonly used st1 ategies to minimize dye rela~ation.~~-~’ This paper describes the optical, chemical and thermochem- ical characterizations of hybrid sol-gel coatings with second- order NLO properties of a simple binary (siloxane-silica) hybrid sol-gel system in which the organic chromophores (TSDP) are incorporated inside hybrid coatings through hydrolysis and co-condensation between TSDP and Si( OMe), (TMOS).Recently the effect of the processing parameters on the NLO behaviour of a similar system [TSDP/Si(OEt),] were reported.26 29Si liquid-state nuclear magnetic resonance (NMR) was used to investigate the initial stages of poljmeriz- ation.26 It was postulated that after thin-film deposition no further cross-linking could occur.26 We expected of course that the degree of condensation and crosslinking observed in the sol state could be very different from those of thc.dried coating which is the NLO material. The present work aims to shed some light on the relationship between the chemistry, thermal treatment and NLO properties of these hybrids by coupling in situ temperature-dependent NLO measurements, DSC calorimetry measurements and 29Si MAS NMR per- formed in the solid state on the different hybrid NMR liquid-state experiments were also performed on the sols before film deposition.Experimental Synthesis of Hybrid NLO Materials TSDP was used as the siloxane network precursor carrying the NLO chromophore (Fig. 1) while TMOS was used as a crosslinking reagent to increase the network rigidity. Sol-gel coatings with different TSDP :TMOS molar ratios (1, 0.8, 0.6, 0.4, 0.2) were prepared as follows. The precursors, TSDP and TMOS, were mixed in THF and co-hydrolysed with acidic water (HC1; pH 1). The H20 :Si molar ratio was 2: 1. The solution was stirred for 30 min and the resulting sols were aged for several hours. From these sols, hydrophobic transparent films several micrometres thick, were produced without cracks or failure. Two procedures were used to process the coatings.They were prepared on ordinary soda-lime glass sheets previously cleaned and dried by simple deposition (for NMR and DSC experiments) or by spin-coating (for NLO, DSC and some NMR experiments). In the first process an appropriate amount of solution was poured onto the glass sheet and allowed to gel before being dried at room tempera- ture. Thin films were also spin-coated onto clean glass sub- strates. The spinning rate was 3000 rpm and the deposition took 30 s. The resulting hybrid xerogels were then charac- terized. In this paper, samples are labelled Tx-Qy, where T stands for TSDP, Q for the silicon added from TMOS. x and y are the molar percentage (x+y=lOO) of the different precursors. These hybrids can be described as block copoly- mers" made from siloxane and silica units which can be labelled by (T -T),-(Q -Q)bwhere: PP PPT-T = -gi-O-Si-0-and Q-Q = -Si-O-Si-0-Rh bb Optical Characterization A standard Corona34 poling technique was used to orient the chromophores in the hybrid sol-gel matrices.24 The samples were placed on a temperature-regulated heating substrate 2 cm from a tungsten needle and a high voltage was applied between the needle and the plate (10 kV).In a typical experi- ment a poling temperature of 120°C and a poling time (at this temperature) of 30 min were used. The current reached typical values in the range of several PA. The optical set-up was modified to measure in situ the time and temperature dependence of the second-harmonic response.Second-harmonic measurements were performed by the Maker fringe method?' A Y-cut quartz crystal was used as the reference, allowing for calibration of the measurements. 0.46 pm V-' was taken as a reference value for the optical non-linear coefficient of the quartz. The measurements were Fig. 1 Structure of the N-[3-triethoxysilylpropyl]-2,4-dinitrophenyl-amine (TSDP) NLO chromophore J. M14TER.CHEhI., 1994, VOL. 4 performed by using an irradiating pulsed Nd3' :YAG laser beam operating at 1.34 pm (10 ns per pulse, 10 mJ per pulse, 10 Hz) with both fundamental and harmonic wavelengths removed from the resonances of the non-linear chromophores. The thicknesses of the spin-coated films meavured by trans- mission spectroscopy ranged from 2 to 3 pm.The refractive indices were measured by spectroscopic ellipsometry (using the Sellmeier method36) and by a simple method based on the fringe pattern of the transmission spectrum.37 The mean refractive index for the hybrid coatings obtained from hydrolysis and co-condensation of TSDP and TMOS was 1.62. Chemical Characterization Elemental analysis (Si, N, C, H) was performed by the Service d'Analyse du CNRS on each sample in order to check if the initial chromophore :silicon ratio was conserved in each of the hybrid xerogels after drying and curing. '9Si liquid-state NMR data were recorded on a Bruker AM 250 spectrometer working at 49.69 MHz. MAS NMR spectra were recorded on a Bruker MSL 400 spectrometer.The frequency was adjusted to 79.5 MHz for 29Si nuclei. The pulse width and relaxation delays were 2.5 ps and 60 s for "Si MAS NMR. The solid samples were spun at 5 kHz. Tetramethylsilane (TMS) was used as the reference. For silicon NMR data T,, (m=0-3) and Q, (n=0-4) notations have their usual meaning. In T, notation38 T refers to the 0x0 trifunctional R-SiO, central unit and m represents the number of Si-0"-Si bridging oxygens attached to the central unit. The Q, notation rep- resents the silicon-oxygen tetrahedron; Q refers to the 0x0 quadrifunctional central Si04 unit and n indicates the number of other silicate structures attached to the central unit. DSC experiments were performed on a Perkin-Elmer 7 series instrument, using ca.15 mg of each xerogel. The experi- ments were carried out under cyclic conditions. First the temperature was increased from -20 to 145 -C to clear the sample memory. Then the sample temperature was set to -20 "C. Heating and cooling rates for each run were fixed at 10°C min-'. The data were collected during the third run (the second temperature increase) and were treated using the Perkin-Elmer thermal analysis software. The determination of the glass-transition temperatures, q,was performed using an integration area as large as 40°C for each measurement. The accuracy of the Tg values is f2"C depending on the capability to adjust the tangents of the curve\' steps. Results and Discussion NLO Measurements Second-harmonic measurements (d33 valuesI performed on poled coatings for several TSDP/TMOS compositions are listed in Table 1.The d,, values are higher than those for TSDP-SiHCH,(OEt,)-Zr(OPr"), and close to those reported for the TSDP-TEOS2s hybrid systems. The d,, values decreases with the initial TSDP-TMOS ratio, except for the sample T60-Q40. This singularity is assigned to differences in the sequencing of the (T-T), and (Q-Q),,units Table 1 Second-harmonic measurements values) measured on poled coatings for several TSDP-TMOS compositions TSDP-TMOS d,,,'pm V-' 1oo:o 10 80:20 5.63 60 :40 2.5 40:60 3 J. MATER. CHEM., 1994, VOL. 4 in this hybrid coating. Optimisation of the curing time and poling conditions should lead to a better sequencing. The room-temperature decay of the SHG intensity was studied as the poling voltage was turn off, for pure TSDP and for T80-Q20 binary systems.In both cases the decay occurred over several months. After 250 days the measured d,, values decreased, respectively, to 18% (T100) and 30% (T80-Q20) of the initial d3, values. This shows that the crosslinking of the siloxane network by silica units improves but does not inhibit the chromophore relaxation. Fitting the time depen- dence of the d,, value to an exponential law [d3,(t)=d3, (0)exp(-t/z)] leads to a relaxation time z of ca. 3100 h for the T80-Q20 system. This shows that hybrid organic-inorganic systems (TSDP-TMOS) have a much longer NLO chromophore relaxation time than grafted organic polymers with a similar glass-transition temperature (q=30-70 "C, uide infra) such as butoxymethacrylate (z=500 h).39 The in situ temperature dependence of the SHG values were measured as follows. The hybrid coatings were first poled at 120°C for 30min, then the samples were cooled and the electric field was turn off when room temperature was reached.The d,, values were then measured and monitored us. tempera-ture. The heating rate was 10 'C min-'. Fig. 2 shows the temperature dependence of the SHG signal for Tx-Qy hybrid coatings. The relaxation of the NLO chromophores is favoured by short- and long-range order polymeric chain motions. These degrees of freedom are gener- ally allowed by increasing temperature. Thus chromophore relaxation is characterized by the strong decay observed in the SHG response as temperature is increased (Fig.2). The temperature at which the SHG decreases increases with the amount of crosslinking reagent Q. For a given composition a sample poled and cured for longer times (several hours) exhibits a displacement of this transition towards higher temperatures, e.g. this transition occurs at 20-35°C for a freshly deposited and poled TlOO coating while it reaches 60-65 -C for a TlOO sample poled and cured for 2 h. DSC Experiments DSC studies were performed on the different Tx-Qy hybrid films. Effect of TMOS Content Fig. 3 shows DSC traces of air-dried hybrid coatings with different TSDP :TMOS molar ratios. A relaxation phenom- enon occurs between 20 and 100°C and corresponds to the glass transition of the gel.A plot of Tg us. TMOS molar concentration is shown in Fig. 4.Tgis shifted to higher values n20 r I 20 40 60 80 T/"C Fig.2 In situ evolution of the SHG response uersus temperature for coatings with different Tx :Qy compositions: (a) 40 :60, (b) 60 :40, (c) SO : 20 and (d) 100:0 0.0 1 I I I I I ! 1 I1 0 20 40 60 80 100 T/"C Fig. 3 DSC traces of air dryed hybrid coatings with different TSDP: TMOS molar ratio: (a) 100: 0, (b)90: 10, (c) 80: 20, (ti) 60: 40. (e)40 :60 and (.f)20 :80. 'O0I80 *,t Fig. 4 Glass-transition temperature (T,)as a function of the TMOS molar concentration with increasing TMOS concentration. This is due to the increased molecular weight and restricted segment niobility of Tx-Qy hybrid systems as a consequence of the cross-linking reaction.This point will be discussed in the NMR section. The inflection point of the curves in Fig. 2 yields the temperature of depolarization (the temperature for which the SHG decreases) The values of the depolarization temperature and the Tgmeasured by DSC for several coatings with different Tx-Qy compositions are listed in Table 2. For a given Tx-QjI composition both temperature values are in good agreement. Effect of Thermal Curing DSC studies were also performed in order to characterize the evolution of the mechanical properties the Tx-Qy film as a function of temperature. For an air-dried TlOO sample the Tg is of ca. 18-25'C. After thermal curing, without and with poling, increases up to 32 and 59"C, respectively.Strong variations in 7; were also observed for the Tx-Qy xerogel coatings depending on their thermal history. Systematic DSC experiments were car- ried out to study the influence of the thermal treatment on Tg. Table 2 Depolarization temperature and T, values of air-dried hybrid coatings with different TSDP-TMOS molar ratios TSDP :TMOS GI, c depolarisation temperature/-C 1oo:o 25 27 80 :20 38.5 38 60 :40 59 56 40 :60 61 60 J. MATER. CHEM., 1994. VOL. 4 The evolution of the glass transition for T80-Q20, T60-Q40 for TlOO and QlOO. This indicates a higher degree of conden- and T40-Q60 samples has been carefully studied after different sation in binary systems than in pure sol-gel polymers.This successive curings. Before each measurement the sample was phenomenon is particularly marked in the 29Si MAS NMR cured for 30min at 120°C. The glass transition is clearly spectrum of a T40-Q60 film which only shows T2(-58 ppm), shifted to higher temperatures with increasing curing time. T3 (-65 ppm), Q3 (-100 ppm) and Q4 (-108 ppm) reson- After curing at 120"C,Tg for each sample reached a maximum. ances. Table 3 reports the evolution of the concentration of T The value of this maximum increased with the TMOS content, and Q units us. sample composition. From the relative inten- i.e. values of 45, 73 and 84°C are, respectively, reached with sity of each T and Q resonance, the concentration of each T80-Q20, T60-Q40 and T40-Q60 samples.Moreover, the species can be measured. Fig. 6 represents a plot of the curing time at which the maximum is reached decreases with evolution of T-unit concentration with the TMOS content the T: Q ratio. This time is ca. 75 h for a T80-Q20 coating and shows that the number of T3 species (fully condensed) and decreases to 6 h and 3 h, respectively, for T60-Q40 and increases with the addition of TMOS. The mean degree of T40-Q60. The evolution of the rigidity of these hybrid condensation, C, can be calculated for each set of T or Q matrices could be described by time-temperature transform-components by taking C(T)=Ci iti/3 and C(Q)=Gj jqj/4. ti ation models. However, in order to complete the relationship and qj are the concentration measured by NMR for Ti and the chemical composition Qj species (i andj the number of Si-0-Si bridging oxygen between the matrix rigidity (q),and the degree of condensation, the different hybrid Tx-Qy i=O to 3; j=O to 4).The degree of condensation for T and coatings were characterized by high-resolution solid-state Q units us. TMOS concentration are plotted in Fig. 7. For (MAS) NMR. the binary Tx-Qy systems the degree of condensation increases with the TMOS content. 29SiMAS NMR Fig. 5 shows 29Si NMR MAS spectra for air-dried hybrid coatings prepared with different TSDP :TMOS ratios from sols aged for several hours. The isotropic chemical shifts observed in 29Si spectra are dependent on the degree of h Their assignment has z.condensation of silicate stru~tures.~~-~~ 601-/been made in agreement with the literature. The 29Si MAS NMR spectrum of a TlOO coating shows three components due to T, (-51 ppm), T, (-57.4 ppm) and T, (-64.9 ppm) units.Those of a QlOO coating shows three components corresponding to Qz (-91.6 ppm), Q3 (-100.7 ppm) and Q4 (-108.7 ppm) units. The 29Si NMR spectra of the Binary T-Q hybrid samples show both T and Q components. In the (a12ollzlIzLTx-Qy binary samples, the relative intensities of the reson- 00 20 40 60 80 ances due to TI and Q2 units are lower than those observed TMOS content (%) Fig.6 Evolution of T units concentration with TMOS content. (a) Q TI, (b)Tz and (c) T3 I\ 1OOr -20 -60 -1 00 -1 40 6 0 20 TMOS40 content60(%) 100 Fig. 5 29Si NMR MAS spectra for air dryed coatings prepared with different TSDP :TMOS ratios: (a) 0: 100, (b) 20: 80, (c) 40 :60, (d) Fig.7 Degrees of condensation (C)for T and Q units wrstls TMOS 60:40, (e)80:20, (f) 90: 10, (g)1OO:O concentration Table 3 Evolution of the concentration of T and Q units measured by 29Si NMR for several TSDP-TMOS compositions 100:o 9.7 54.2 36.1 0 0 0 75.5 0 90: 10 0 20.3 63.2 1.4 8.2 6.9 91.9 83.3 80: 20 0 17.3 59.7 0.9 14.2 7.9 92.5 82.6 60 :40 0 9.1 52.6 0.4 23.5 14.4 95.1 84.1 40 : 60 0 5 46.5 0.7 30.5 17.3 96.8 83.6 20: 80 0 3.3 34 0.9 31.1 30.7 97.1 86.8 0:100 0 0 0 5.2 67.6 27.2 0 80.5 J. MATER. CHEM., 1994, VOL.4 The T40-Q60 hybrid network reached C(T) =97%, much higher than those of the pure TlOO hybrid xerogel [C(T)= 75%]. Obviously the addition of TMOS greatly increases the degree of condensation of the siloxane network. This pheno- menom is related to the increase of Tg observed with increasing TMOS content. Efect of Sol Ageing Time bcfore Coating Deposition When TSDP and TMOS precursors were co-hydrolysed in acidic conditions, the second one hydrolysed more rapidly than TSDP.26b,43 The hydrolysis rate of the latter is known to be slowed down by the steric hindrance provided by the propyldinitrophenylamino s~bstituent.~~,~~However, as soon as silanol groups are formed from the TMOS precursors, condensation occurs between these silanols and other TMOS or TSDP precursors. On the course of the reactions both precursors hydrolyse and condense, leading to different T and Q species, as evidenced by the 29Si NMR study performed in the sol state.As an example, for a T60-Q40 sol aged for 8 h the 29Si NMR spectra exhibit the characteristic resonances of TI (6= -49.5 ppm), cyclic T, (6= -51.1 ppm), linear T2 (6=-58.5 ppm), T, (6 =-67 ppm), Q2 (6=-92 ppm), Q, (6= -99.7 ppm) and Q4 (6= -109 ppm). In the aged T60-Q40 sol Toand Qounits are not present and T,, Q3 and Q4 resonances are weak. This indicates that a complete hydrolysis but a low extent of condensation reaction occurs in the sol state even after long ageing times. After coating deposition, as shown in Fig. 6 and Table 3, the structure of the xerogels (T60-Q40 is taken as an example) is mainly composed of T2, T,, 4, and Q4 silicon units. This difference between the sol state and xerogel state demonstrates that the main condensation and crosslinking reactions occur upon solvent removal. However, even if condensation reac- tions are mostly complete upon drying, the extent of reaction in the sol state (which depends on ageing time) has an influence on the final structure of the coating.29Si NMR data recorded from air-dried hybrid coatings (T60-Q40) deposited after short (sample A) or long (sample B) ageing times are gathered in Table 4. The extent of reaction is clearly different for both samples. Some T, units are present in sample A and the T, :T, and Q4 :Q3 ratios are smaller in this sample than in sample B.C(T) is lower in coatings deposited after a short ageing time (sample A) than for those deposited after a long ageing time (sample B), while their degrees of condensation C(Q) are quite close. For other Tx-Qy compositions the same tendency is observed. All coatings processed from freshly made sols are made with T-Q copolymers that have a lower degree of condensation than those present in coatings deposited from aged sols. For a set of coatings deposited from freshly made sols, Tg increases with the TMOS content. However, note that for a given Tx-Qy composition, the difference between Tgvalues measured for coatings A and B is quite small (Table 4) even if both samples exhibit a different C(T).This result raises several questions: Is Tg mainly gov- erned by the amount of Q species, their degree of condensation and their degree of crosslinking with T-species units? As 29Si MAS NMR do not provide enough resolution to estimate the ratio between linear and cyclic T species, the C(T) values of two samples can give numerically the same number but could represent a different distribution between linear and cyclic units. The crosslinking behaviour of these species with silica units is of course different and will lead to different thermo- mechanic behaviour. The size of the polymeric (T-T), and (Q-Q)b segments and their sequencing should be an important parameter that probably governs the mobility of the segments and consequently Tg and the relaxation properties of such hybrid materials. Obtaining hybrid copolymers made from (T-T),-(Q-Q)b segments with low a and b values should lead to behaviour closer to the glass state in terms of mechanical properties (ix.less chromophore rela~ation).~' "Si CP MAS NMR and NMR relaxation measurements are being carried out in order to shed some light on this point.Conclusion Compared to the tremendous amount of work and time devoted to polymeric NLO materials, NLO materials made by sol-gel processes are still in their infancy. As a consequence, the sol-gel materials described in this work have Tgvalues in the range 30-70"C, well below the state of the art obtained with pure organic polymeric materials based on p~lyimides~~ which are highly non-linear and stable for hundreds of hours at temperatures >100"C.However, the results obtained with the sol-gel materials can be improved by carrying out a multiple covalent bonding of azo dye derivatives which have a high non-linear response.46 Polyimide-inorganic sol-gel composites recently showed an excellent temporal stahility at 120OC31 Moreover the use of transition-metal 0x0-pcllymers (M=Zr, Ti, V, Fe) as crosslinking agents of higher func- tionality should modify the mechanical properties of these hybrid materials. The presence of transition-metal 0x0 species may also offer the possibility of modifying the refractive index of the coating, or creating materials that carry both optical and redox or magnetic properties.Sol-gel chemistry is versa-tile, but the quality and reproductibility of the resulting materials are strongly linked to the chemical and processing conditions of preparation. Therefore a fundamental under- standing of the relationship between chemistry and 21 given property such as NLO chromophore relaxation is crucial for the improvement of these systems. This study concerns the synthesis and characterization of hybrid siloxane-oxide compounds with second-order NLO properties. Depending on the chemical composition the SHG values ranged between 2.5 and 10 pm V-'. The thermal treatment of the hybrid coatings has a strong influence on the relaxation behaviour of the NLO chromophores grafted into these hybrid networks. These compounds are made uia hydrolysis and co-condensation of TSDP (T units) and 7'MOS (Q units) precursors.In such systems gelation most likely occurs through the crosslinking of linear or branched T siloxane polymers with Q silica species. These hybrid net works are made of (T-T), units co-condensed with (Q-Q)bspecies as evidenced by 29Si MAS NMR. Depending on a and h they can be described as copolymers between T and Q species (low a and b) or as nanocomposites made of silica domains Linked to siloxane polymers (higher a and b)." The TSDP :TMOS ratio, proton concentration, hydrolysis ratio, sequence of mixing the reagents and ageing time of the sol are the chemical parameters that should directly influence a and b values. For Table 4 29Si NMR data and T, values for two air-dried hybrid (T60-Q40) coatings deposited after short (sample A) or long (sample B) ageing times T60-Q40 8 (T2) 6 (T3) 8 (Q3) 6 (44) C(Ti ("/.I C(Q) T,/'C sample A sample B 23.1 9.1 34.4 52.6 27.8 23.9 14.7 14.4 86.6 95.1 83.6 84.1 41.5 59 1860 J.MATER. CHEM., 1994, VOL. 4 a given set of chemical parameters this study shows that Tg increases with TMOS content, as evidenced by DSC experi- ments. To a first approximation this behaviour can be related to the increase in the degree of condensation and crosslinking 9 10 11 12 H. Schmidt and B. Seiferling, Muter. Res. Soc. Sy'np., 1986,73, 10. B. Dunn and J. I. Zink, J. Mater. Chem., 1991, 1, 003. C. Sanchez and F. Ribot, Proc. 1st Eur. Worhshop on Hybrid Organic-Inorganic Materials, New J.Chem., 1994. 18, 1007. R. Reisfeld, SPIE Proc. Sol-Gel Optics, ed. J. D Mackenzie and of the polymers as shown by 29Si MAS NMR. However, the mechanical properties of hybrid siloxane-oxide materials and thus the relaxation behaviour of chromophores grafted into these matrices is strongly dependent on their thermal history. The physical behaviour of these organic-inorganic hybrid 13 14 15 D. R. Ulrich, 1990, 1328,29. J. C. Pouxviel, S. Parvaneh, E. T. Knobbe and B. Dunn, Solid State Ionics, 1989,32,646. D. Avnir, V. R. Kaufman and R. Reisfeld, J. Ann-Crjst. Solids, 1985,14, 395. C. Sanchez, SPlE Proc., Sol-Gel Optics, ed. J. D Mackenzie and T-Q copolymers could be described by time-temperature transformation (TTT) models, such as those proposed for thermoset^.^^ Chemical crosslinking is not complete at the gelation or even after room-temperature air drying as shown by 29Si NMR experiments. When the samples are aged and cured the chemical reactions continue closer towards com- 16 17 18 D.R. Ulrich, 1990, 1328,41. C. Guizard, J. C. Achdou, A. Larbot, L. Cot, G. Le Flem, C. Parent and C. Lurin, SPIE Proc., Sol Gel Optics, ed. J. D. Mackenzie and D. R. Ulrich, 1990, 1328,208. P. N. Prasad and B. A. Reinhardt, Chem. Muter. 1990,2,660. S. Dirk, F. Babonneau, C. Sanchez and J. Livage. J. Muter. Chem., 1992,2,239. pletion. Consequently, given sufficient time and temperature to allow the mobility of the species, a network-forming system continues to crosslink far after gelation.The increase in the density of crosslinks modifies the thermomechanical properties of the hybrid, as illustrated by the changes observed in Tp 19 20 21 B. Dunn, E. Knobbe, J. M. Mc. Kiernan, J. C. Pouxviel and J. I. Zink, Muter. Res Soc. Symp. Proc., 1988, 121, 331. J. C. Pouxviel, B. Dunn and J. I. Zink, J. Phys. Chem., 1989, 93,2134. J. M. Boulton, J. Thompson, H. H. Fox, I. Gorodisher, G. Teowee, P. D. Calvert and D. R. Uhlmann, Muter. Res. Soc. Sjwp., 1990, upon thermal curing. Consequently, care must be taken when comparing physical properties of hybrid organic compounds. Even if such hybrid coatings have been synthesized following the same route (i.e. same precursors, same chemical conditions) small differences in the ageing time of the sols and in the thermal treatment may lead to large modifications of their chemical structure and consequently different relaxation behaviour.Another processing parameter that should have a high importance is the electrical field used to poled the NLO chromophores. 22 23 24 25 26 180,987. G. Pucetti, 1.Ledoux, J. Zyss, P. Griesmar and <'.Sanchez, Polym. Prepr., 1991,32, 61. J. Zyss, G. Pucetti, J. Ledoux, P. Griesmar, J. Livage and C. Sanchez, Eur. Pat. 11 52, March 1991. (a)E. Toussaere, J. Zyss, P. Griesmar and C. Sachez, Nonlinear Opt., 1991, 1, 349; (b) C. Sanchez, P. Griesmar, E. Toussaere, G. Puccetti, 1. Ledoux and J. Zyss., Nonlineur Opt., 1992,4, 245. P. Griesmar, C. Sanchez, G. Pucetti, I. Ledoux and J. Zyss, Mol. Eng., 1991, I, 205. (a) J. Kim, J. L. Plawsky, R.LaPeruta and G M. Korenowski, Chem. Muter., 1992, 4, 249; (b) J. Kim, J. L. Plawsky, E. Van Wagenen and G. M. Korenowski, Chem. Muter., 1993,5.1118. Accelerated field-induced curing must likely occur in these hybrid TSDP-TMOS materials. The high electrical field pro- vided during poling must favour crosslinking and interpen- etration of both polymeric T and Q networks. As mentioned by Haruvy and Webber,27*4g this dramatic curing effect should 27 28 29 Y. Haruvy and S. E. Weber, Muter. Res. Soc. Symp. Proc., 1992, 271, 297. F. Chaput, D. Riehl, Y. Levy and J. P. Boilot, C'hem. Mater., 1993, 5, 589. Y. Zhang, P. N. Prasad and R. Burzynski, Chem. Muter., 1992. 4, 851. involve bond and chain segment orientations, ion migration and surface corona electrolysis.Consequently, this effect should depend on the siloxane :water ratio, the polymer length and the initial crosslinking and degree of polycondensation reached in the sol state. Thus, for a given temperature, voltage 30 31 32 Y. Nosaka, N. Tohriiwa, T. Kobayashi and N. Fuji, Chem. Mater., 1993,5,930. S. Marturunkakul, J. I. Chen, R. J. Jeng, S. Sengupta, J. Kumar and S. K. Tripathy, Chem. Mater., 1993,5, 743. K. Izawa, N. Okamoto and 0.Sugihara, Jpn. J. Appl. Phys., 1993, 32, 807. and substrate, the efficiency of the field-induced curing must 33 L. Kador, R. Fischer, D. Haarer, R. Kaieman, S. Briick, depend on the ageing time of the polymeric precursor solution and on the ageing time of the hybrid coatings before poling occurs. Further studies are underway to shed some light on this difficult phenomenon. 34 35 H.Schmidt and H. Durr, Adv. Muter., 1993,5,270. K. D. Singer, M. G. Kuzyk, W. R. Holland, J. E. Sohn, S. J. Lalama, R. B. Comizzolli, H. E. Katz and M. L. Schilling, Appl. Phys. Lett., 1988,53, 19. P. D. Maker, R. W. Terhune, N. Nisennoff and C. M. Savage, Phys. Rev. Lett., 1962, 8, 21. DRET is gratefully acknowledged for financial support. 36 J. A. Dobrowski, F. C. Ho and A. Waldorf. Appl. Opt.. 1983, 22, 3191. 37 J. C. Manifacier, J. Gasiot, J. P. Fillard, J. Php. E, 1976,9, 1002. References 38 R. H. Glaser, G. L. Wilkes and C. E. Bronnimann, J. Non-Cryst. Solids, 1989,113, 73. Sol-Gel Optics I, ed. J. D. Mackenzie and D. R. Ulrich, Proc. SPIE 1328, Washington, 1990. Sol-Gel Optics Il, ed.J. D. Mackenzie, Proc. SPIE 1758, Washington, 1992. Sol-gel Optics, Processing and Applications, ed. L. C. Klein, Kluwer, Boston, 1993. C. J. Brinker and G. Scherrer, Sol-Gel Science: the Physics and Chemistrjl of Sol-Gel Processing, Academic Press, San Diego, 1989. (a)J. Livage, M. Henry and C. Sanchez, Prog. Solid State Chem., 1988, 18, 259; (b)C. Sanchez and J. Livage, New J. Chewi., 1990, 14, 513. 39 40 41 42 43 44 45 H. T. Man and H. N. Yoon, Adv. Muter., 1992.4, 159. E. Lippmaa, M. Magi, A. Samoson, G. Engelhardt and A. R. Grimmer, J. Am. Chem. Soc., 1980,102,4889. G. Engelhardt, H. Jancke, E. Lippmaa and A. Samoson, J. Orgunomet. Chem., 1981,210,295. M. Magi, E. Lippmaa, A. Samoson, G. Engelhardt and A. R. Grimmer, J. Phys. Chem., 1984,88, 1518. B. Lebeau, personal communication. Silune Coupling Agents, Plenum Press, Edwin P. New York, 2nd edn., 1991. R. F. Shi, M. H. Wu, S. Yamada, Y. M. Cai and A. F. Garito, Appl. Phys. Lett., 1993, 63, 1173; J. Appl. Polym. SCL, 1983. 28, 2567. C. Sanchez, F. Ribot, S. Doeuff, in Organometallic Polymers with Special Properties, ed. R. M. Laine, Kluwer, Dordrecht, 1992, p. 267. The Sol-Gel Process in Sol-Gel Technolog!! for Thin Films, Fibers, Preforms, Electronics and Specialty Shapes, ed L. C. Klein, Noyes, 46 47 48 B. Lebeau, C. Guermeur and C. Sanchez, Muter. Res. Soc. Symp. Proc., 1994,346, 3 15. B. Enns and J. K. Gillham, J. Appl. Polym. Sci., 1983,28, 2567. Q; Hibben, E. Lu, Y. Haruvy and S. E. Webber, Chern. Muter., 1994,6, 76 1. 1988. D. Avnir, D. Levy and R. Reisfeld, J. Phys. Chem., 1984,88, 5956. Paper 4/02634I; Receiced 3rd May, 1994