- Email: [email protected]
Structure of luminescent mono and di-urethanesil nanocomposites doped with Eu3+ ions
Nuclear Instruments and Methods in Physics Research B 199 (2003) 117–122 www.elsevier.com/locate/nimb
Structure of luminescent mono and di-urethanesil nanocomposites doped with Eu3þ ions K. Dahmouche
a,*
, M.C. Goncßalves b, C.V. Santilli a, V. de Zea Bermudez b, L.D. Carlos c, A.F. Craievich d
a
b
Instituto de Quımica, UNESP, CP 355, 14800-900 Araraquara, SP, Brazil Departamento de Quımica, Universidade de Tr as-os-Montes e Alto Douro, Quinta de Prados, Apartado 1013, 5000, Vila Real, Portugal c Departamento de Fısica, Universidade de Aveiro, 3810-193 Aveiro, Portugal d Instituto de Fısica, USP, P.O. Box 66318, 05315-970, S~ ao Paulo, SP, Brazil
Abstract A structure modeling of two families of sol–gel derived Eu3þ -doped organic/inorganic hybrids based on the results of small-angle X-ray scattering experiments is reported. The materials are composed of poly(oxyethylene) chains grafted at one or both ends to siloxane groups and are called mono- and di-urethanesils, respectively. A theoretical function corresponding to a two-level hierarchical structure model fits well the experimental scattering curves. The first level corresponds to small siloxane clusters embedded in a polymeric matrix. The secondary level is associated to the existence of siloxane cluster rich domains surrounded by a cluster-depleted polymeric matrix. Results show that increasing europium doping favors the growth of the secondary domains. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 81.20.F; 78.70.C; 61.10 Keywords: Hybrids; Sol–gel; SAXS; Luminescence
1. Introduction The sol–gel process has been successfully used in the last few years for the production of advanced multifunctional materials as monoliths, thin films, fibers and powders [1]. In particular, hybrid photonic materials for optical data storage, optical waveguides, sensors, electrochromic smart
*
Corresponding author. Tel.: +55-16-201-6765; fax: +55-16222-7932. E-mail address: [email protected] (K. Dahmouche).
windows, solid-state lasers and screen displays have been recently obtained. A particularly interesting family of photonic materials is composed of di-ureasil molecules. The incorporation of lanthanide ions onto the di-ureasil framework leads to the synthesis of multiwavelength nanohybrid emitters, whose emission spectra displays a large host band superposed on a series of sharp cation lines. It was recently shown by small-angle X-ray scattering (SAXS) that europium doped di-ureasils materials prepared by the sol–gel procedure exhibit a two-level hierarchical structure [2]. The primary structure level consists of siloxane nanoclusters formed by polycondensation
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 1 4 2 8 - 3
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K. Dahmouche et al. / Nucl. Instr. and Meth. in Phys. Res. B 199 (2003) 117–122
reactions between silicon-based species located at polymer chains ends. The secondary, coarse, level consists of high average electron density domains, in which siloxane clusters are embedded, surrounded by a cluster depleted low electron density (polymeric) phase. In this work we investigate the effect of europium doping on the structural features of two urethanesil xerogels. The samples were prepared by the sol–gel procedure starting from mono-urethanesil [m-Ut(350)] and di-urethanesil [d-Ut(300)] precursor solutions. In mono- and di-urethanesils the basic organic/inorganic molecular species are composed of methyl poly(oxyethylene) chains (with a molecular weight 350 and 300) covalently bonded through a urethane moiety to siloxane groups only at one chain ends or at both ends, respectively. 2. Experimental The synthesis of europium-doped mono- and di-urethanesils has already been described [3]. The SAXS study was performed using the synchrotron SAS beamline of LNLS (Campinas, Brazil). This beamline is equipped with an asymmetrically cut and bent silicon (1 1 1) monochromator that yiel) and horided a monochromatic (k ¼ 1:608 A zontally focused beam [4]. A position sensitive X-ray detector and a multichannel analyzer were used to record the SAXS intensity, IðqÞ, as a function of the modulus of the scattering vector q, q ¼ ð4p=kÞ sinðe=2Þ, e being the scattering angle. 3. Structural model and SAXS analysis A first examination of the experimental SAXS curves shown in Figs. 1 and 2, corresponding to dUt(350) and m-Ut(300) xerogels, respectively, lead us to propose a two-level structure model. This model is similar to that established in a previous investigation of europium doped di-ureasils [2]. We first consider a two-electron density model consisting of a diluted set of isolated colloidal clusters embedded in a homogeneous matrix. A semi-empirical equation for this model that satisfies Guinier and Porod asymptotic behaviors [5] at low and high q, respectively, is given by [6]
IðqÞ ¼ NP ðqÞ
pffiffiffi 3 4 ¼ G expðq2 R2g =3Þ þ B½ðerfðqRg = 6ÞÞ =q ; ð1Þ
where Rg is the average radius of gyration of the 2 clusters, G ¼ NV 2 ðqp qm Þ and B ¼ 2pðqp 2 qm Þ S, qp and qm being the average electron densities of the siloxane clusters and matrix, respectively, V the siloxane cluster volume, N the cluster number and S the interface area between the clusters and the matrix. A first analysis of our SAXS results lead us to the conclusion that the primary siloxane clusters concentrate in relatively large domains embedded in a cluster depleted matrix. This effect of phase separation is assumed to be responsible for the formation of a two-level hierarchical structure. The scattering intensity produced by relatively large domains with an average radius of gyration Rg2 composed of small clusters with a average radius of gyration Rg1 is given by [6] IðqÞ ¼ ½G1 expðq2 R2g1 =3Þ þ B1 ½ðerfðqRg1 =2:449ÞÞ3 =qZ1 S1 ðqÞ þ ½G2 expðq2 R2g2 =3Þ þ B2 expðq2 R2g1 =3Þ 3
Z
½ðerfðqRg2 =2:449ÞÞ =q 2 S2 ðqÞ;
ð2Þ
where S1 ðqÞ is the structure factor accounting for the spatial correlation between clusters described by S1 ðqÞ ¼ 1=½1 þ k1 hðqÞ, where k1 is a cluster packing factor and hðqÞ ¼ 3½sinðqdÞ qd cosðqdÞ= 3 ðqdÞ , d being the average intercluster distance. For clusters or domains with smooth and sharp interfaces Z1 ¼ Z2 ¼ 4. The secondary domains being non-spatially correlated, we have S2 ðqÞ ¼ 1. The cluster packing factor k1 is equal to 8v=v0 , where v is the average ‘‘hard-core’’ cluster volume and v0 the average spherical volume associated to each cluster. The model assumes that these spherical volumes are in contact. The maximum theoretical value of the packing factor, k1 ¼ 5:9, corresponding to the closest sphere packing. The mentioned structure parameters were determined by fitting of Eq. (2) to the experimental SAXS curves.
K. Dahmouche et al. / Nucl. Instr. and Meth. in Phys. Res. B 199 (2003) 117–122
119
Fig. 1. Experimental SAXS curves (symbols) and fitted function (continuous line) corresponding to d-Ut(300) hybrids with different (r ¼ ½Eu=½O) ratios. The curves are vertically displaced for clarity.
4. Results and discussion SAXS spectra of a non-doped d-Ut(300) hybrid and of samples containing two different Europium contents, r ¼ ½Eu=½O (O being the oxygens of the ether type) are shown in Fig. 1. For the non-doped sample (r ¼ 0) and for the weakly doped hybrid (r ¼ 0:005), the experimental SAXS intensity curves IðqÞ are well fitted by the theoretical function given by Eq. (2) that consider two structural levels with S1 ðqÞ 6¼ 1 and S2 ðqÞ ¼ 1. The experimental results for europium free and for weakly doped samples are consistent with a model of large domains composed of spatially correlated spherical siloxane nanoclusters embedded in a polymeric matrix. Such hierarchical structure was previously observed in europium-doped di-ureasils [2] and appears during gelation of these hybrids. The
structure parameters Rg1 , k1 , d1 , Rg2 and G2 =G1 , determined from the best fitting procedure, are reported in Table 1. In the europium free sample (r ¼ 0) the average radius of the primary nanoclusters is Rg1 ¼ 2:0 A and in the weakly doped hybrid (r ¼ 0:005) . This increasing trend in nanocluster Rg1 ¼ 2:1 A radius was confirmed by additional analyses of other samples with a higher europium content in for r ¼ 0:025. This dicating a growth up to 4 A result indicates that the addition of europium atoms favors the hydrolysis and condensation reactions between the siloxane species located at polymer chain ends. This effect was previously observed in other sol–gel derived materials containing different salts [1] and was assigned to a decrease in electrostatic repulsion between silicon species promoted by Eu3þ ions. The number of
120
K. Dahmouche et al. / Nucl. Instr. and Meth. in Phys. Res. B 199 (2003) 117–122
Fig. 2. Experimental SAXS curves (symbols) and fitted function (continuous line) corresponding to m-Ut(300) hybrids with different (r ¼ ½Eu=½O) ratios. The curves are vertically displaced for clarity.
Table 1 Structure parameters Eu doping (r)
Hybrid type
) Rg1 (A
) d1 (A
k1
) Rg2 (A
G2 =G1
0
d-Ut(300) m-Ut(300) d-Ut(300) m-Ut(300) d-Ut(300) m-Ut(300)
2.0 5.0 2.1 4.0 – –
15 22 15 27 – –
2.1 3.0 2.1 1.2 – –
29 24 82 85 85 104
8 1.4 390 550 870 680
0.005 0.2
silanols groups are usually particularly abundant in sol–gel materials prepared under neutral sol pH, as used in this study [1], favoring electrostatic repulsion in non-doped hybrids. The broad interference shoulders in Fig. 1 for 1 < q < r ¼ 0 and 0.005, in the range 0:15 A 1 , suggests that the siloxane nanoclusters 0:35 A in d-Ut(300) are weakly correlated. The average
distance between clusters (Table 1) is the same, , for non-doped and also for weakly d1 ¼ 15 A doped hybrids. The packing factor is rather low (k1 ¼ 2:1) for both samples. The average radius Rg2 of the secondary domains in d-Ut(300) is strongly affected by euro1 for pium doping, increasing from Rg2 ¼ 29 A 1 for samples europium free samples up to 82 A
K. Dahmouche et al. / Nucl. Instr. and Meth. in Phys. Res. B 199 (2003) 117–122
with r ¼ 0:005. High doping (r ¼ 0:2) does not induce further secondary domain growth, as can be seen in Table 1. This result reveals that a low europium doping (r ¼ 0:005) is sufficient to induce a strong effect on the segregation of siloxane primary nanoclusters leading to secondary domains much larger than europium-free hybrids. The increase in domain size (Rg2 ) by eventual unfolding of polymer chains promoted by europium doping can be discarded since the intercluster ) remains essentially the same distance (d ffi 15 A for europium free and weakly doped samples (Table 1). The higher G2 =G1 ratio for the weakly doped material (r ¼ 0:005) is consistent with the observed increasing trend of the average domain radius of gyration (Rg2 ). SAXS spectra corresponding to m-Ut(350) hybrids with three different ½Eu=½O ratios (r ¼ 0, 0.005 and 0.2) are shown in Fig. 2. Similarly to the case of d-Ut(300) hybrid, the experimental SAXS intensity curves IðqÞ for non-doped and low doped hybrids (r ¼ 0 and 0.005) can be well fitted by the theoretical function given by Eq. (2) considering two structural levels with S1 ðqÞ 6¼ 1 and S2 ðqÞ ¼ 1. The shape of the different SAXS curves for mUt(350) and d-Ut(350) (Figs. 1 and 2, respectively) and the trend of the parameters determined from the best fitting procedure (Table 1) indicate that the structure of both hybrids are similar. Since in m-Ut(350) the siloxane groups are grafted only at one molecular end, the average distance between clusters is less constrained by the chain length than in the case of di-urethanesils. This molecular feature is expected to induce a higher average distance, d1 , between siloxane clusters in m-Ut(350) as experimentally observed (Table 1). Instead of the increasing trend of the radius of gyration of siloxane cluster, Rg1 , for increasing europium doping observed for d-Ut(350), an opposite effect is apparent for m-Ut(350) hybrids. The dominant feature of the SAXS curves for both hybrids, m-Ut(350) and d-Ut(350), with high europium content (r ¼ 0:2) is a strong intensity concentrated at very small q produced by the secondary structural level. In addition, a nearly 1 is apconstant intensity at q higher than 0.2 A parent. The absence of a correlation shoulder in the high q range is probably due to the existence of
121
a large number of (high Z) europium atoms dispersed in the polymeric matrix (inside and outside the coarse domains). This invalidates the assumption of the two electron density model used for the derivation of Eqs. (1) and (2). The constant SAXS 1 can so be exintensity observed for q > 0:2 A plained as a dominant contribution from isolated europium atoms. This is consistent with the conclusion of previous independent optical analysis indicating that, in highly doped hybrids, europium atoms predominantly bond to ether-type oxygens of polymeric chains [7].
5. Conclusion The studied mono- and di-urethanesils exhibit a two-level hierarchical structure. The primary level consists of small and correlated siloxane nanoclusters embedded in a polymeric matrix. The secondary level corresponds to large domains in which the siloxane clusters are located surrounded by a depleted polymeric phase. A small addition of europium induces a strong enhancement of the segregation of siloxane clusters, which occurs during the synthesis process. The structural features observed for both (mono- and di-) urethanesils are similar to those previously reported for di-ureasils. On the other hand, the main difference between mono- and diurethanesils concerns the internal structure of the coarse domains. In mono-urethanesils siloxane clusters are larger and located at a higher average distance each from the other. These features are probably induced by the weaker structural constraints imposed by single end grafting in mono-urethanesils as compared to double grafting of di-urethanesils.
Acknowledgements The authors acknowledge the collaboration of LNLS staff during SAXS experiments and the financial support provided by the Brazilian (CAPES and PRONEX) and Portuguese (ICCTI and FCT) funding agencies.
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References [1] C.J. Brinker, G.W. Scherrer, Sol–gel Science, The Physics and Chemistry of Sol–gel Processing, Academic Press, San Diego, CA, 1990. [2] K. Dahmouche, L.D. Carlos, V. de Zea Bermudez, R.A. Sa Ferreira, C.V. Santilli, A.F. Craievich, J. Mater. Chem. 11 (2001) 3249. [3] V. de Zea Bermudez, L. Alcacer, J.L. Acosta, E. Morales, Solid State Ionics 116 (1999) 197.
[4] G. Kellermann, F. Vicentin, E. Tamura, M. Rocha, A.L. Barbosa, A.F. Craievich, I.J. Torriani, Appl. Cryst. 30 (1997) 880. [5] A. Guinier, G. Fournet, Small-Angle X-ray Scattering of Xrays, John Wiley and Sons, New York, 1955, p. 55. [6] G. Beaucage, T.A. Ulibarri, E.P. Black, D.W. Schaefer, in: J.E. Mark, C.Y.C. Lee, P.A. Bianconi (Eds.), Hybrid Organic–Inorganic Composites, Am. Chem. Soc. Symp. Proc., Vol. 585, ACS, Washington, 1995, p. 97. [7] M.C. Goncßalves, V. de Zea Bermudez, D. Ostrovskii, L.D. Carlos, Ionics 8 (1 and 2) (2002) 62.
Structure of luminescent mono and di-urethanesil nanocomposites doped with Eu3þ ions K. Dahmouche
a,*
, M.C. Goncßalves b, C.V. Santilli a, V. de Zea Bermudez b, L.D. Carlos c, A.F. Craievich d
a
b
Instituto de Quımica, UNESP, CP 355, 14800-900 Araraquara, SP, Brazil Departamento de Quımica, Universidade de Tr as-os-Montes e Alto Douro, Quinta de Prados, Apartado 1013, 5000, Vila Real, Portugal c Departamento de Fısica, Universidade de Aveiro, 3810-193 Aveiro, Portugal d Instituto de Fısica, USP, P.O. Box 66318, 05315-970, S~ ao Paulo, SP, Brazil
Abstract A structure modeling of two families of sol–gel derived Eu3þ -doped organic/inorganic hybrids based on the results of small-angle X-ray scattering experiments is reported. The materials are composed of poly(oxyethylene) chains grafted at one or both ends to siloxane groups and are called mono- and di-urethanesils, respectively. A theoretical function corresponding to a two-level hierarchical structure model fits well the experimental scattering curves. The first level corresponds to small siloxane clusters embedded in a polymeric matrix. The secondary level is associated to the existence of siloxane cluster rich domains surrounded by a cluster-depleted polymeric matrix. Results show that increasing europium doping favors the growth of the secondary domains. Ó 2002 Elsevier Science B.V. All rights reserved. PACS: 81.20.F; 78.70.C; 61.10 Keywords: Hybrids; Sol–gel; SAXS; Luminescence
1. Introduction The sol–gel process has been successfully used in the last few years for the production of advanced multifunctional materials as monoliths, thin films, fibers and powders [1]. In particular, hybrid photonic materials for optical data storage, optical waveguides, sensors, electrochromic smart
*
Corresponding author. Tel.: +55-16-201-6765; fax: +55-16222-7932. E-mail address: [email protected] (K. Dahmouche).
windows, solid-state lasers and screen displays have been recently obtained. A particularly interesting family of photonic materials is composed of di-ureasil molecules. The incorporation of lanthanide ions onto the di-ureasil framework leads to the synthesis of multiwavelength nanohybrid emitters, whose emission spectra displays a large host band superposed on a series of sharp cation lines. It was recently shown by small-angle X-ray scattering (SAXS) that europium doped di-ureasils materials prepared by the sol–gel procedure exhibit a two-level hierarchical structure [2]. The primary structure level consists of siloxane nanoclusters formed by polycondensation
0168-583X/02/$ - see front matter Ó 2002 Elsevier Science B.V. All rights reserved. PII: S 0 1 6 8 - 5 8 3 X ( 0 2 ) 0 1 4 2 8 - 3
118
K. Dahmouche et al. / Nucl. Instr. and Meth. in Phys. Res. B 199 (2003) 117–122
reactions between silicon-based species located at polymer chains ends. The secondary, coarse, level consists of high average electron density domains, in which siloxane clusters are embedded, surrounded by a cluster depleted low electron density (polymeric) phase. In this work we investigate the effect of europium doping on the structural features of two urethanesil xerogels. The samples were prepared by the sol–gel procedure starting from mono-urethanesil [m-Ut(350)] and di-urethanesil [d-Ut(300)] precursor solutions. In mono- and di-urethanesils the basic organic/inorganic molecular species are composed of methyl poly(oxyethylene) chains (with a molecular weight 350 and 300) covalently bonded through a urethane moiety to siloxane groups only at one chain ends or at both ends, respectively. 2. Experimental The synthesis of europium-doped mono- and di-urethanesils has already been described [3]. The SAXS study was performed using the synchrotron SAS beamline of LNLS (Campinas, Brazil). This beamline is equipped with an asymmetrically cut and bent silicon (1 1 1) monochromator that yiel) and horided a monochromatic (k ¼ 1:608 A zontally focused beam [4]. A position sensitive X-ray detector and a multichannel analyzer were used to record the SAXS intensity, IðqÞ, as a function of the modulus of the scattering vector q, q ¼ ð4p=kÞ sinðe=2Þ, e being the scattering angle. 3. Structural model and SAXS analysis A first examination of the experimental SAXS curves shown in Figs. 1 and 2, corresponding to dUt(350) and m-Ut(300) xerogels, respectively, lead us to propose a two-level structure model. This model is similar to that established in a previous investigation of europium doped di-ureasils [2]. We first consider a two-electron density model consisting of a diluted set of isolated colloidal clusters embedded in a homogeneous matrix. A semi-empirical equation for this model that satisfies Guinier and Porod asymptotic behaviors [5] at low and high q, respectively, is given by [6]
IðqÞ ¼ NP ðqÞ
pffiffiffi 3 4 ¼ G expðq2 R2g =3Þ þ B½ðerfðqRg = 6ÞÞ =q ; ð1Þ
where Rg is the average radius of gyration of the 2 clusters, G ¼ NV 2 ðqp qm Þ and B ¼ 2pðqp 2 qm Þ S, qp and qm being the average electron densities of the siloxane clusters and matrix, respectively, V the siloxane cluster volume, N the cluster number and S the interface area between the clusters and the matrix. A first analysis of our SAXS results lead us to the conclusion that the primary siloxane clusters concentrate in relatively large domains embedded in a cluster depleted matrix. This effect of phase separation is assumed to be responsible for the formation of a two-level hierarchical structure. The scattering intensity produced by relatively large domains with an average radius of gyration Rg2 composed of small clusters with a average radius of gyration Rg1 is given by [6] IðqÞ ¼ ½G1 expðq2 R2g1 =3Þ þ B1 ½ðerfðqRg1 =2:449ÞÞ3 =qZ1 S1 ðqÞ þ ½G2 expðq2 R2g2 =3Þ þ B2 expðq2 R2g1 =3Þ 3
Z
½ðerfðqRg2 =2:449ÞÞ =q 2 S2 ðqÞ;
ð2Þ
where S1 ðqÞ is the structure factor accounting for the spatial correlation between clusters described by S1 ðqÞ ¼ 1=½1 þ k1 hðqÞ, where k1 is a cluster packing factor and hðqÞ ¼ 3½sinðqdÞ qd cosðqdÞ= 3 ðqdÞ , d being the average intercluster distance. For clusters or domains with smooth and sharp interfaces Z1 ¼ Z2 ¼ 4. The secondary domains being non-spatially correlated, we have S2 ðqÞ ¼ 1. The cluster packing factor k1 is equal to 8v=v0 , where v is the average ‘‘hard-core’’ cluster volume and v0 the average spherical volume associated to each cluster. The model assumes that these spherical volumes are in contact. The maximum theoretical value of the packing factor, k1 ¼ 5:9, corresponding to the closest sphere packing. The mentioned structure parameters were determined by fitting of Eq. (2) to the experimental SAXS curves.
K. Dahmouche et al. / Nucl. Instr. and Meth. in Phys. Res. B 199 (2003) 117–122
119
Fig. 1. Experimental SAXS curves (symbols) and fitted function (continuous line) corresponding to d-Ut(300) hybrids with different (r ¼ ½Eu=½O) ratios. The curves are vertically displaced for clarity.
4. Results and discussion SAXS spectra of a non-doped d-Ut(300) hybrid and of samples containing two different Europium contents, r ¼ ½Eu=½O (O being the oxygens of the ether type) are shown in Fig. 1. For the non-doped sample (r ¼ 0) and for the weakly doped hybrid (r ¼ 0:005), the experimental SAXS intensity curves IðqÞ are well fitted by the theoretical function given by Eq. (2) that consider two structural levels with S1 ðqÞ 6¼ 1 and S2 ðqÞ ¼ 1. The experimental results for europium free and for weakly doped samples are consistent with a model of large domains composed of spatially correlated spherical siloxane nanoclusters embedded in a polymeric matrix. Such hierarchical structure was previously observed in europium-doped di-ureasils [2] and appears during gelation of these hybrids. The
structure parameters Rg1 , k1 , d1 , Rg2 and G2 =G1 , determined from the best fitting procedure, are reported in Table 1. In the europium free sample (r ¼ 0) the average radius of the primary nanoclusters is Rg1 ¼ 2:0 A and in the weakly doped hybrid (r ¼ 0:005) . This increasing trend in nanocluster Rg1 ¼ 2:1 A radius was confirmed by additional analyses of other samples with a higher europium content in for r ¼ 0:025. This dicating a growth up to 4 A result indicates that the addition of europium atoms favors the hydrolysis and condensation reactions between the siloxane species located at polymer chain ends. This effect was previously observed in other sol–gel derived materials containing different salts [1] and was assigned to a decrease in electrostatic repulsion between silicon species promoted by Eu3þ ions. The number of
120
K. Dahmouche et al. / Nucl. Instr. and Meth. in Phys. Res. B 199 (2003) 117–122
Fig. 2. Experimental SAXS curves (symbols) and fitted function (continuous line) corresponding to m-Ut(300) hybrids with different (r ¼ ½Eu=½O) ratios. The curves are vertically displaced for clarity.
Table 1 Structure parameters Eu doping (r)
Hybrid type
) Rg1 (A
) d1 (A
k1
) Rg2 (A
G2 =G1
0
d-Ut(300) m-Ut(300) d-Ut(300) m-Ut(300) d-Ut(300) m-Ut(300)
2.0 5.0 2.1 4.0 – –
15 22 15 27 – –
2.1 3.0 2.1 1.2 – –
29 24 82 85 85 104
8 1.4 390 550 870 680
0.005 0.2
silanols groups are usually particularly abundant in sol–gel materials prepared under neutral sol pH, as used in this study [1], favoring electrostatic repulsion in non-doped hybrids. The broad interference shoulders in Fig. 1 for 1 < q < r ¼ 0 and 0.005, in the range 0:15 A 1 , suggests that the siloxane nanoclusters 0:35 A in d-Ut(300) are weakly correlated. The average
distance between clusters (Table 1) is the same, , for non-doped and also for weakly d1 ¼ 15 A doped hybrids. The packing factor is rather low (k1 ¼ 2:1) for both samples. The average radius Rg2 of the secondary domains in d-Ut(300) is strongly affected by euro1 for pium doping, increasing from Rg2 ¼ 29 A 1 for samples europium free samples up to 82 A
K. Dahmouche et al. / Nucl. Instr. and Meth. in Phys. Res. B 199 (2003) 117–122
with r ¼ 0:005. High doping (r ¼ 0:2) does not induce further secondary domain growth, as can be seen in Table 1. This result reveals that a low europium doping (r ¼ 0:005) is sufficient to induce a strong effect on the segregation of siloxane primary nanoclusters leading to secondary domains much larger than europium-free hybrids. The increase in domain size (Rg2 ) by eventual unfolding of polymer chains promoted by europium doping can be discarded since the intercluster ) remains essentially the same distance (d ffi 15 A for europium free and weakly doped samples (Table 1). The higher G2 =G1 ratio for the weakly doped material (r ¼ 0:005) is consistent with the observed increasing trend of the average domain radius of gyration (Rg2 ). SAXS spectra corresponding to m-Ut(350) hybrids with three different ½Eu=½O ratios (r ¼ 0, 0.005 and 0.2) are shown in Fig. 2. Similarly to the case of d-Ut(300) hybrid, the experimental SAXS intensity curves IðqÞ for non-doped and low doped hybrids (r ¼ 0 and 0.005) can be well fitted by the theoretical function given by Eq. (2) considering two structural levels with S1 ðqÞ 6¼ 1 and S2 ðqÞ ¼ 1. The shape of the different SAXS curves for mUt(350) and d-Ut(350) (Figs. 1 and 2, respectively) and the trend of the parameters determined from the best fitting procedure (Table 1) indicate that the structure of both hybrids are similar. Since in m-Ut(350) the siloxane groups are grafted only at one molecular end, the average distance between clusters is less constrained by the chain length than in the case of di-urethanesils. This molecular feature is expected to induce a higher average distance, d1 , between siloxane clusters in m-Ut(350) as experimentally observed (Table 1). Instead of the increasing trend of the radius of gyration of siloxane cluster, Rg1 , for increasing europium doping observed for d-Ut(350), an opposite effect is apparent for m-Ut(350) hybrids. The dominant feature of the SAXS curves for both hybrids, m-Ut(350) and d-Ut(350), with high europium content (r ¼ 0:2) is a strong intensity concentrated at very small q produced by the secondary structural level. In addition, a nearly 1 is apconstant intensity at q higher than 0.2 A parent. The absence of a correlation shoulder in the high q range is probably due to the existence of
121
a large number of (high Z) europium atoms dispersed in the polymeric matrix (inside and outside the coarse domains). This invalidates the assumption of the two electron density model used for the derivation of Eqs. (1) and (2). The constant SAXS 1 can so be exintensity observed for q > 0:2 A plained as a dominant contribution from isolated europium atoms. This is consistent with the conclusion of previous independent optical analysis indicating that, in highly doped hybrids, europium atoms predominantly bond to ether-type oxygens of polymeric chains [7].
5. Conclusion The studied mono- and di-urethanesils exhibit a two-level hierarchical structure. The primary level consists of small and correlated siloxane nanoclusters embedded in a polymeric matrix. The secondary level corresponds to large domains in which the siloxane clusters are located surrounded by a depleted polymeric phase. A small addition of europium induces a strong enhancement of the segregation of siloxane clusters, which occurs during the synthesis process. The structural features observed for both (mono- and di-) urethanesils are similar to those previously reported for di-ureasils. On the other hand, the main difference between mono- and diurethanesils concerns the internal structure of the coarse domains. In mono-urethanesils siloxane clusters are larger and located at a higher average distance each from the other. These features are probably induced by the weaker structural constraints imposed by single end grafting in mono-urethanesils as compared to double grafting of di-urethanesils.
Acknowledgements The authors acknowledge the collaboration of LNLS staff during SAXS experiments and the financial support provided by the Brazilian (CAPES and PRONEX) and Portuguese (ICCTI and FCT) funding agencies.
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References [1] C.J. Brinker, G.W. Scherrer, Sol–gel Science, The Physics and Chemistry of Sol–gel Processing, Academic Press, San Diego, CA, 1990. [2] K. Dahmouche, L.D. Carlos, V. de Zea Bermudez, R.A. Sa Ferreira, C.V. Santilli, A.F. Craievich, J. Mater. Chem. 11 (2001) 3249. [3] V. de Zea Bermudez, L. Alcacer, J.L. Acosta, E. Morales, Solid State Ionics 116 (1999) 197.
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