Polymer Gels - American Chemical Society

Chapter 5

Inorganic-Organic Hybrid Gel: Structural Characteristics and Formation Mechanism 1

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Hiroshi Urakawa , Yoshiaki Yuguchi , Yuko Ikeda , Kanji Kajiwara , Yoshitaka Hirata , and Shinzo Kohjiya 2

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Downloaded by EMORY UNIV on June 10, 2014 | http://pubs.acs.org Publication Date: October 15, 2002 | doi: 10.1021/bk-2002-0833.ch005

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Faculty of Engineering and Design, Kyoto Institute of Technology, Kyoto, Sakyo-ku 606-8585, Japan Institute for Chemical Research, Kyoto University, Uji 611-0016, Japan 2

Introduction Synchrotron radiation provides a powerful source for X-ray, which enables to observe small-angle X-ray scattering (SAXS) from solutions in a short time. SAXS is suitable for the observation of the spatial correlation in the order of 10 to 1000 Åowing to its wavelength. Thus SAXS has been successfully applied to the structural analysis of enzymes and colloidal particles in solution . The elec -tromagnetic profile (X-ray or light scattering) corresponds to the Fourier trans -form of the spatial correlation (density distribution function) of the scattering body, so that in principle any structure could be analyzed from the profile. In practice, the range of the observation is not wide enough to elucidate the whole structure of the scattering body, and the conventional method evaluates the size and shape approximately by assuming a homogeneous triaxial body or semi-flexible coil to represent an object . Here we apply the time-resolved SAXS to observe a structural change during the network formation in the sol-gel process. In order to analyze the observed scattering profiles quantitatively, the Flory-Stockmayer model was adopted to calculate the scattering intensity from the system undergoing gelation, and the results were discussed in terms of the parameters specifying the Flory-Stockmayer model. The Flory-Stockmayer model describes the gelation as trees growing over a space. In this context, gelation is equivalent to the cascade process, and is formulated with the generating function . Although the original generating function (the weight-fraction generating function) is based on a sim -ple enumeration of tree branches, the generating function can be extended to include the distance correlation as a path-weighted generating function . The particle scattering function is calculated from this path-weighted generating function for a randomly branched system . A tree-like model of Flory and 1

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© 2003 American Chemical Society In Polymer Gels; Bohidar, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

71 Stockmayer has an advantage of its simplicity, but the model corresponds to the state of ideal gas. The generating function for the real system will be given as a first approxi -mation by introducing the interaction between branches. The inter-branch inter -action is incorporated in the particle scattering factor as an interference term due to the finite size of the branch points and the repulsion between branch points.

Experimental


Preparation of Hybrid G e l Organic/inorganic hybrid gels were prepared by the sol-gel process from TEOS (tetraethoxysilane) and ET-PTMO (triethoxysilyl-terminated poly(oxytetramethylene)). ET-PTMO (M = 1.35 x 10 ) was synthesized from poly(oxytetramethylene) glycol by end-group modification. The sol-gel process and the preparation of hybrid gel are schematically shown in Figs. 1 and 2, re spectively. In the present sol-gel process, B u N H was employed for catalyst (Se ries B in Fig. 2), so that the reaction proceeded under the basic condition. The reaction mixture was prepared by mixing and stirring ET-PTMO, TEOS and ethanol (solvent) at room temperature for 2 min. Then hydrochloric acid and water were added to the reaction mixture and stirred for further 10 min. The catalyst was added and the mixed solution was stirred for 1 min. The resulted solution (the reaction mixture) was capsulated in the capillary cell (0 = 2 mm), inserted in the cell holder maintained at 50°C and the time-resolved small-angle X-ray scattering was observed immediately from the solution undergoing sol-gel reaction. 3

n

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1st Step Hydrolysis M(OR) Tetraalkoxymetal 4

(RO) M ' V W V W A W W M(OR) Trialkoxymetal-terminated Polymer 3

\

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| ]H*or OH" as Catalyst

M(OH)

(HO) M A/WWWVWW M(OH)

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2nd Step Pofycondensatton

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L

t

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Organic/Inorganic Hybrid Gel Figure 1. Sol-gel process (schematic)

In Polymer Gels; Bohidar, H., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2002.

72 Solvent (EtOH)

Series A •

ET-PTMO

H0 HCI 2

12min.

Sol-Gel Process

\ 3mtn.

—Stirring at Room Temp.-

•I Solution |

5 0 h m

-

TEOS Series B-

Solvent |HCI| H 0 (EtOH) BuNH 2

Hybrid Gel

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Figure 2 Preparation of hybrid gel

Small-angle X-ray Scattering Time-resolved S A X S was observed from a series of the mixed solution of ET-PTMO and TEOS undergoing sol-gel reaction with the S A X E S equipment installed at BL-10C in Photon Factory, Tukuba, Japan. The sample code and the reaction conditions are summarized i n Table 1. The S A X S measurement was started immediately after the cell was inserted in the cell holder block kept at 50°C. 98 measurements with an appropriate interval were repeated during the course of hydrosilylation, where each measuring time was 3 minutes. The excess scattering intensity at each reaction time was calculated by subtracting the scat tering intensity at time 0. Table 1. Concentration of each component in the reaction mixture (in terms of molar ratio) Sample EtOH H 0 HCI [(SiKEtJpTMo [(Si)-OEt] BuNH code SGGO 1.0 0.010 0 0 14 13 SGG1 14 0.028 1.0 0 13 0.010 SGG2 14 0.020 1.0 0 13 0.010 SGG3 14 0.015 1.0 0 13 0.010 SGG4 1.0 14 0.010 0.013 0 13 SGG5 1.0 14 13 0.010 0.013 0.23 SGG6 14 0.010 0.013 1.0 0.48 13 SGG7 0.60 14 0.010 0.015 1.7 13 SGG8 0.6 14 0.010 0.013 1.7 13 TCOS

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A n incident synchrotron-radiated X-ray was monochromatized to X = 1.488 A with a double-mirror monochromatizer, and focused to the position of the de tector by a focusing mirror. The scattered X-ray was detected by the one-dimensional position sensitive proportional counter (PSPC) of the effective


73 length 160 mm. The exact camera length was calibrated by using the diffraction peaks of collagen fiber (the long period = 670 A at the 6th, 9th, and 11th orders).


Theoretical background The Flory-Stockmayer model for polyfunctional polycondensation is repre sented by a tree, which corresponds to the generating function with a dummy variable 8 as shown in Fig. 3. The dummy variable corresponds to each branch point (node), and the prob ability that a branch bears further branches is given by the reactivity a. That is, the branch would not bear the next generation with the probability 1 - a. The generating function in Fig. 1 is summarized as x

W(0) = ^w 0 x

=d(l-a+au(0)Y f

(1)

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u(0) = 0(l-a+au(0)) (2) where w denotes the weightfractionof jc-mers in the system. Eq. (1) yields the weight average degree of polymerization before gelation as (\+d)/[\-(f-\)d[ by differentiating with respect to 0at 0=1. The result was first derived by Flory and defines a gel point at a = 1/(^-1) where the weight average degree of polym erization diverges. The scattering intensity is calculated by introducing the random spatial correlation between branch points as a function of the reaction probability a: x

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/(*) = A\q)(l + a)/[l-(f-Y)a] (3) 0 = expH>V/6) (4) where b denotes the mean-square distance between neighboring scattering units. A (q) is a scattering amplitude, and is equal to 1 when the scatterer is a point. Eq. (3) holds even when gelation takes place as far as l-(f-l)a oo, q

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Results and Discussion Both time-resolved SAXS profiles from the ET-PTMO/TEOS mixtures show the upturn at q -> 0 in the Kratky plots (q I(q) plotted against q with I(q) and q 2



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Figure 4. Time-resolved small angle X-ray scattering observedfromthe ET-PTMO/TEOS mixtures during the sol-gel process being the scattered intensity and the magnitude of scattering vector, respectively) when the sol-gel reaction proceeds. Two examples (SGG1 and SGG6) are shown in Fig. 4, where the reaction time of the initial upturn corresponds to the gel point of the system as expectedfromthe Flory-Stockmayer tree-like model. The scattering profiles were analyzed in terms of three parameters (/-1)ol, b and % by curve fitting. The analyzed results are demonstrated in Figs. 5 and 6, and the results are summarized in Table 2. The reaction time when (f-\)a ex ceeds 1.0 is thought to correspond to the gel point. After gelation takes place, (f-l)a hardly increases in all the series, indicating that further intermolecular reaction is suppressed by gelation, b and 2; increase with reaction before the gel point but remain almost constant with the value of (f-l)a becoming invariant (see Fig. 6). The result implies that the intermolecular reaction is also suppressed

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Kratky plot for SGG1 series

0.00

0.05

0.10

0.15

0.20

0.25

0.30

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q(A- ) Figure 5. Observed SAXS profiles and calculated profiles according to eqs. (3) and (7) for SGG1 as a function of the reaction time

6.01.2n

120 SGGS*^ 8.2 1.021 Eqs. (3) and (7); ">Eqs. (3) and (8) )

}


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in the wet state in the hybrid gel once gelation takes place. The result indicates the different situation from the network formation of polysiloxane , where the shrinkage of gel due to the intramolecular reaction was observed after gel point. The gelation model considered here is similar to the model proposed by Landry et a l . Their model comprises a liquid-like arrangement among non-interpenetrating fractal clusters, whereas in the present model non-interpenetrating fractal clusters are distributed and linked as described by the Flory-Stockmayer model. Here a correlation length £ specifies the non-interpenetrating fractal clusters according to eq. (7) or eq. (8) in the present instance. The evaluated parameters by curve fitting were summarized in Table 2. SGG0 (without TEOS under acidic condition) undergoes no gelation. When added no TEOS (SGG1-4), the resulted gel is rather homogeneous and the do main is characterized by a Gaussian type density distribution. Decreasing the amount B u N H (catalyst) reduces the gelation time, but no appreciable change was observed in the gel structure. The formation of relatively hard domain de scribed by the Debye-Bueche type correlation function was observed by adding TEOS in SGG5-8 synthesized under basic condition. The size of the domain increases and the gelation time reduces as increasing the amount of added TEOS. The domain is considered to be composed mainly of inorganic components as illustrated in Fig. 7. The gel structure is schematically shown in Fig. 7. In conclusion, TEOS promotes the formation of Si-rich domains under the basic condition, but the reaction is suppressed after gelation in the wet state. Fur ther reaction takes place where the intramolecular crosslinking induces the shrinkage of hybrid gel in the drying process. 13

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(a) Without adding TEOS (b) With adding TEOS Figure 7: Schematic structure of the PTMO/TEOS hybrid gel

Acknowledgements This work was performed under the approval of the Photon Factory Advisory Committee (Proposal No. 91-217). Y Y thanks JSPS for Research Fellowship for Young Scientists. The part of the work was financially supported by Grant-in-Aid for COE Research No. 10CE2003 (Ministry of Education, Science, Sports and Culture, Japan).

References 1.

See, for example, Glatter, O., Kratky, O., Small-Angle X-ray Scattering, Academic P., New York, 1982 2. See, for example, Flory, P. J., Principles of Polymer Chemistry, Cornell U.P., Ithaca, 1953 3. Good, I. J., Proc. Cambridge Phil., Soc., 1955, 51, 240-242 4. Gordon, M., Proc. Roy. Soc. (London), 1962, A268, 240-259 5. Kajiwara, K., Burchard, W., Gordon, M., Br. Polym. J., 1970, 2,110-115 6. Kajiwara, K., Gordon, M., J. Chem. Phys., 1973, 59, 3623-3632 7. Kajiwara, K., Kohjiya, S., Shibayama, M . , Urakawa, H. in Polymer Gels; De Rossi, D., Kajiwara, K., Osada, Y., Yamauchi, A., Eds.; Plenum: New York, 1991; pp3-19 8. Spouge, J. L. Macromolecules, 1983, 16, 121-127; Botet, R., Jullien, R., Kolb, M. Phys. Rev., 1984, A30, 2150-2152 9. De Gennes, P.-G. Scaling Concepts in Polymer Physics, Cornell U. P., Ithaca,1979 10. Freltoft, T., Kjems, J. K., Sinha, S. K. Phys. Rev., 1986, B33, 269-275


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11. Shibayama, M . , Kurokawa, H., Nomura, S., Muthkumar, M . , Stein, R. S., Roy, S. Polymer, 1992, 33, 2883-2890 12. Debye, P., Bueche, A. M. J. Appl. Phys., 1949, 20, 518-525 13. Yamanaka, S., Yuguchi, Y., Urakawa, H., Kajiwara, K., Kohjiya, S. J. Net -work Polym., 1999, 20, 157-163 14. Landry, M. R., Coltrain, B. K., Landry, C. J. T., O'Reilly, J. M. J. Polymer Sci., (Polym. Phys.), 1995, B33, 637-655


Polymer Gels - American Chemical Society

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