Figure 2-5(a) shows the lamellar morphology of PS-b-PMMA after staining with RuO4. The periodic lamellar thickness of PS-b-PMMA is about 50 nm. The dark region is the PS domain, owing to staining, and the PS volume fraction of PS-b-PMMA is 0.55, which falls into the ordered lamellar phase region[4] (a PS volume fraction between 0.34~0.62).
The TiO2-TMAC/PS-b-PMMA morphology is shown in Figure 2-5(b). In Fig. 2-5(b), the presence of TiO2 in the dark spots is confirmed by EDS (Figure 2-5(c)), the size of TiO2 aggregates (dark spots) is about 15-20nm. The Ti band peak indicates the
existence of TiO2 at the PS domains, while the presence of Cu peaks is caused by the Cu grid used in the sample preparation. In Figure 2-5(d), the gray phase is the PS domain, which is a result of staining with RuO4, while the light phase is the PMMA domain.
Dark TiO2 nanoparticles are found to disperse in the gray domain (PS domain) in the lamellar PS-b-PMMA. That the TiO2-TMAC nanoparticles can be dispersed in the PS domain corresponds to the fact that both the cetyl trimethyl ammonium chloride (TMAC), containing 10 methylene units, and the polystyrene domain are hydrophobic and miscible.
The presence of TiO2-TMAC in the PS domain is further supported by differential scanning calorimetry (DSC) results. Figure 2-6 reveals that the glass transition temperature (Tg) of the PS domain in TiO2-TMAC/PS-b-PMMA increased by 9℃, as compared to that of neat PS-b-PMMA (104℃ vs 95℃). This increase might be attributed to TiO2 aggregates, which hinder the molecular movement of the PS domain, indicating that TiO2-TMAC aggregates are located at the PS domain. Since the heat capacity of glass transition of PMMA is much smaller than that of PS (0.03 watts/g vs.
0.065 watts/g), the Tg of PMMA is indetectable in this case.[27] Therefore, the presence of TiO2 in the PMMA phase of PS-b-PMMA can only be confirmed by other means. The Fourier-transform infrared (FTIR) spectra of TiO2/PS-b-PMMA
nanocomposites are shown in Figure 2-7. The peaks at 1741 cm-1 and 1726 cm-1 result from the carbonyl groups of the PMMA domain in neat PS-b-PMMA. The carbonyl band of TiO2-TMS/PS-b-PMMA shifts to lower wavenumbers (from 1726 to 1714 cm-1) as compared to that of PS-b-PMMA. This indicates the possibility that TiO2 is present in the PMMA domain since hydrogen bonding between the remainder of the dangling -OH groups on the surface of TiO2 and the carbonyl groups of the PMMA domains causes the carbonyl band to shift to smaller wavenumbers. Figure 2-8 shows a transmission electron microscopy image of TiO2-TMS/PS-b-PMMA. That the TiO2
nanoparticles are dispersed rather uniformly in the PMMA phase is consistent with the
fact that TMS contains methacrylate structures. The difference in the modes of dispersion of TiO2 in PS and in PMMA domains can be manifested by the bonding difference between the surfactants and TiO2. In the TiO2-TMAC/PS-b-PMMA case, the polar-ionic bondings between TiO2 surfaces and TMAC are weak and hence allow TiO2
nanoparticles to rearrange to form aggregates during the solvent removal process.
Whereas, in the TiO2-TMS/PS-b-PMMA case, TMS is bonded to TiO2 surfaces covalently, and this type of bondings is well maintained during the solvent removal process. The covalently tethered TMS prevents TiO2 from aggregating, resulting in a better dispersion.
A schematic drawing of the formation of these two types of dispersion of TiO2 in the PS and PMMA block is presented in Figure 2-9.
Figure 2-10 shows the photoluminescence of the TiO2/PS-b-PMMA nanocomposites as excited by 260nm UV light. A mild 410 nm luminescence peak, caused by the
band-to-band transition,[24]is displayed by the TiO2 nanoparticles modified by TMS.
For neat PS-b-PMMA, the 320 nm luminescence peak is resulted from the PS domain.
In the case of TiO2-TMAC/PS-b-PMMA, only a broad and weak 326 nm luminescence peak appeared. Whereas, in the case of TiO2-TMS/PS-b-PMMA, there are two luminescence peaks (323 nm and 400 nm) being present. The stark difference in the two cases can be interpreted by the morphological evidences as discussed in the previous paragraph. When TiO2-TMAC forms aggregates in the PS domain, a large portion of the excitation light is absorbed by PS domain, which also luminescences at shorter
wavelength, resulting a small portion of excitation light reaching TiO2 aggregates. This results in non-luminescence by TiO2-TMAC in the PS domains. On the other hand, since TiO2-TMS nanoparticles dispersed more uniformly in the PMMA domain, both TiO2-TMS nanoparticles and PS domain can luminescence independently. This luminescence phenomenon is consistent with our previous argument on the distribution of TiO2 nanoparticles in different blocks.
2-4 Conclusion
The dispersion of TiO2 nanoparticles can be controlled in one of the two blocks of lamellar PS-b-PMMA by using hydrophobic or hydrophilic surfactants, as revealed by transmission electron microscopy, differential scanning calorimetry and
Fourier-transform infrared spectroscopy. The modes of dispersion of TiO2 nanoparticles in different blocks are determined by the type of bondings between the surfactant and the nanoparticles. The photoluminescence of the TiO2/PS-b-PMMA nanocomposites depends on the location of the TiO2 nanoparticles.
2-5 References
[1] Henglein, A. Chem. Rev. 1989, 89, 1861.
[2] Wang, Y.; Herron, N. J. Phys. Chem. 1991, 95,525.
[3] Murry, C. B.; Kagan, C. R.; Bawendi, M. G. Science, 270, 1335, 1995.
[4] Bates, F. S. Science 1991, 251, 898.
[5] Thomas, E. L. Science 1999, 286, 1307.
[6] Sankaran, V.; Cummins, C. C.; Schrock, R. R.; Cohen, R. E.; Silbey, R. J. J. Am.
[12] Moffitt, M.; Eisenberg, A. Chem. Mater. 1995, 7, 1178
[13] Moffitt, M.; McMahon, L.; Pessel, V.; Eisenberg, A. Chem. Mater. 1995, 7, 1185 [14] Moffitt, M.; Vali, H.; Eisenberg, A. Chem. Mater. 1998, 10, 1021.
[15] Zhao, H.; Douglas, E. P.; Harrison, B. S.; Schanze, K. S. Langmuir 2001, 17, 8428 [16] Zhao, H.; Douglas, E. P. Chem. Mater. 2002, 14, 1418.
[17] Fink, Y.; Urbas, A. M.; Bawendi, M. G.; Joannopoulos, J. D.; Thomas, E. L. J.
LightwaveTechnol. 1999, 17, 1963.
[18] Edrington, A. C.; Urbas, A. M.; DeRege, P.; Chen, C. X.; Swager, T. M.;
Hadjichristidis, N.; Xenidou, M.; Fetters, L. J.; Joannopoulos, J. D.; Fink, Y.; Thomas, E. L. Adv. Mater. 2001, 13, 421.
[19] Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. J. Phys. Chem. 1988, 92, 5196.
[20] Joselevich, E.; Willner, I. J. Phys. Chem. 1994, 98, 7628.
[21] Serpone, N.; Lawless, D.;Khairutdinov, R. J. Phys. Chem. 1995, 99, 16646.
[22] Brus, L. J. Phys. Chem. 1986, 90, 2555
[23] Liu, Y.; Claus, O. J. Am. Chem. Soc. 1997, 119, 5273 [24] Isdoa, K.; Kuroda, K. Chem. Mater. 2000,12, 1702.
[25] Delattre, L.; Babonneau, F. Chem. Mater. 1997, 9, 2385.
[26] Leu, C. M.; Wu, Z. W.; Wei, K. H. Chem. Mater. 2002, 14, 3016.
[27] Guegan, P., Cernohous, J. J., Khandpur, A. K., Hoye, T. R., and Macosko, C. W., Macromolecules, 1996, 29, 4605.
Scheme 2-1. Synthesis of TiO2 nanoparticles by ionic or non-ionic surfactants.
(reverse micelle ) (TiO2-TMAC)
N
(TiO2) (Covalently-bonded
TMS on TiO2)
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Table 2-1. Compositions of TiO2 colloidal solutions.
TMA C (g)
TMS (g)
H2O (g)
HCl(g ) (36%)
TTIP/IPA THF
(ml) (ml)
TiO2-TMAC
in THF 5 0.085 - 0.05 0.026 0.5
TiO2-TMS in
THF 5 - 0.016 0.05 0.026 0.5
TiO2-H+ in
THF 5 - - 0.05 0.026 0.5
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Table 2-2. Onset of UV-vis absorbance and calculated radii of TiO2 nanoparticles.
Absorbance onset
wavelength (nm) radius (nm)
TiO2-TMAC 359 0.96
TiO2-TMS 364 1.07
TiO2-H+ 376 1.59
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340 360 380 400
λos=364nm
λos=376nm λos=359nm
TiO
2-TMS
TiO
2-TMAC TiO
2-H
+Absorbance
Wavelength (nm)
Figure 2-1. UV-vis absorbance spectra of TiO colloidal solutions. 2
[Chem. Mater. 2003, 15, 2936-2941]
-60 -65 -70 -75
T3 peak
Si-O-Ti Si-O-Si -67.7
-67.0
ppm
Figure 2-2 29SiNMR spectrum of the TiO -TMS colloidal solution. 2
[Chem. Mater. 2003, 15, 2936-2941]
50 nm
Figure 2-3.Transmission electron microscopy image and electron diffraction pattern of TiO2 nanoparticles from TiO -H2 + colloidal solution.
[Chem. Mater. 2003, 15, 2936-2941]
15 20 25 30 35 40 45 50 55 60
1.69 oA 1.89 oA
2.37 oA 3.51 oA
rutile
2θ (degree)
Anatase
TiO2-H+
Intensity (a.u.)
-H+ nanoparticles.
Figure 2-4. X-ray diffraction curve of TiO2
[Chem. Mater. 2003, 15, 2936-2941]
b a
50 nm 50 nm
c d
50 nm
Figure 2-5. Transmission electron microscopy images of (a) PS-b-PMMA, (b)
TiO2-TMAC/PS-b-PMMA and (c) shows an energy-dispersive x-ray diffraction pattern of the dark particles in (b), (d) TiO2-TMAC/PS-b-PMMA stained with RuO . 4
[Chem. Mater. 2003, 15, 2936-2941]
70 80 90 100 110 120 130
Endo
TgPS=95oC TgPS=95oC
TgPS=104oC
TiO2-TMS/PS-b-PMMA
TiO2-TMAC/PS-b-PMMA
PS-b-PMMA
Temperature (oC)
Figure 2-6. Differential scanning calorimetry curves of PS-b-PMMA, TiO -TMS/PS-b-PMMA and TiO -TMAC/PS-b-PMMA 2 2
[Chem. Mater. 2003, 15, 2936-2941]
3200 3100 3000 2900 2800 1800 1700 1600 1500
1726 1741 1714
C=C-H C-H C=O
c
b
a
Absorbance
Wavenumber (cm-1) a PS-b-PMMA
b TiO2-TMAC/PS-b-PMMA c TiO2-TMS/PS-b-PMMA
Figure 2-7. Fourier-transform infrared spectra of PS-b-PMMA and TiO2/PS-b-PMMA nanocomposites.
[Chem. Mater. 2003, 15, 2936-2941]
50 nm
Figure 2-8. Transmission electron microscopy image of TiO -TMS/PS-b-PMMA. 2
[Chem. Mater. 2003, 15, 2936-2941]
PS-b-PMMA
TiO2-TMAC aggregates in PS domain TiO2-TMAC (ionic surfactant)
solvent removal
surfactant) TiO2-TMS disperses well in PMMA domain
Figure 2-9. Schematic drawing of different dispersion modes by ionic-polar and covalent bondings between TiO2 and surfactants in PS-b-PMMA.
[Chem. Mater. 2003, 15, 2936-2941]
300 330 360 390 420 450 480
excited at 260nm
326 nm 323 nm
410 nm 400 nm
323 nm
TiO2-TMAC/PS-b-PMMA TiO2-TMS/PS-b-PMMA
TiO2-TMS PS-b-PMMA
PL Intensity
Wavelength (nm)
Figure 2-10. Photoluminescence of TiO -TMS, PS-b-PMMA and TiO2 2/PS-b-PMMA nanocomposites.
[Chem. Mater. 2003, 15, 2936-2941]