pubs.acs.org/Langmuir © 2009 American Chemical Society
Unusual Swelling of HPC in Toluene Forming a Microspherical Domain Structure that Causes Christiansen Scattering Coloration Susumu Edo,† Kento Okoshi,*,† Sungmin Kang,† Masatoshi Tokita,† Tatsuo Kaneko,‡ and Junji Watanabe*,† † Department of Polymeric and Organic Material, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8552, Japan, and ‡School of Materials Science, Japan Advanced Institute of Science and Technology, 1-1 Asahidai, Nomi, Ishikawa 923-1292, Japan
Received July 22, 2009. Revised Manuscript Received August 19, 2009 The unusual swelling behavior of hydroxypropyl cellulose (HPC) by toluene is described. At temperatures as high as 100 °C, toluene molecules can enter the HPC film up to the weight fraction of 55%; however, they are segregated from the HPC matrix and form microspherical domains. The size of the spherical domain is ∼4.5 μm in diameter on average. Such an unusual swelling behavior is due to the amphiphilic nature of the HPC; HPC polymers rearrange to contact their hydrophobic group with toluene and confine the toluene molecules in spherical domains. Because of the similarity in refractive indices of the toluene microspherical phase and the HPC continuum phase, the swollen film shows a beautiful scattering color that is called the Christiansen filter effect.
Introduction One of the most widely recognized and intensely studied colored colloidal dispersion systems is the Christiansen filter, which historically consists of quartz dispersed in an organic liquid whose refractive index is very close to that of quartz. The coloration phenomena are the consequence of an exact matching of the refractive index of the dispersed material and that of the continuous phase over a narrow range of light wavelengths.1-4 Under the refractive index matching condition, the light passes through the dispersion as if it is a transparent homogeneous medium although the lights of other wavelengths significantly scatter. Such coloration is based on the difference in the Abbe number of the dispersed material and continuous phase. Because the refractive index changes with temperature, in many cases, the passband shifts with changes in the temperature. Described is our finding that hydroxypropyl cellulose (HPC) films immersed in aromatic solvents, such as toluene, show colors that change from blue to red with the increasing temperature (see Figure 2). A possible explanation is the selective reflection of light due to the formation of lyotropic cholesteric liquid crystals.5,6 However, the color is not a circularly polarized light but a simple light. Thus, we need to find another structural formation attributable to the coloration. In this study, we found that the coloration is due to the scattering of light by a microsegregation domain structure. Toluene molecules used as the solvent enter the HPC film but are segregated into microspherical domains with 3-5 μm diameters because of the amphiphilic nature of the HPC. The coloration is attributed to the light scattering due to the *To whom correspondence should be addressed. E-mail: kokoshi@ polymer.titech.ac.jp (K.O.),
[email protected] (J.W). (1) (a) Christiansen, C. Ann. Phys. Chem. 1884, 23, 298. (b) Christiansen, C. Ann. Phys. Chem. 1885, 24, 439. (2) (a) Auerbach, L. Am. J. Phys. 1957, 25, 440. (b) Auerbach, L. Am. J. Phys. 1960, 28, 743. (3) Okoshi, K.; Sano, N.; Okumura, T.; Tagaya, A.; Magoshi, J.; Koike, Y.; Fujiki, M.; Watanabe, J. J. Colloid Interface Sci. 2003, 263, 473. (4) Wormuth, K.; Bruggemann, O.; Strey, R. Langmuir 2002, 18, 5989. (5) Werbowyi, R. S.; Gray, D. G. Macromolecules 1980, 13, 69. (6) Kosho, H.; Hiramatsu, S.; Nishi, T.; Tanaka, Y.; Watanabe, J. High Perfom. Polym. 1999, 11, 41.
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Christiansen filter effect of the microspherical domains in the HPC matrix.
Materials and Methods Materials. HPC with a viscosity of 3-6 cps in a 2 wt % aqueous solution at 20 °C, Mw = 77 000, Mw/Mn = 1.81, MS (molar substitution of hydroxypropyl group) = 4.0, and DS (degree of substitution of hydroxypropoxyl group) = 2.6, was used as purchased from Tokyo Kasei Kogyo.7 Anhydrous toluene (water content < 30 ppm) and methanol (water content < 50 ppm) were purchased from Wako (Osaka, Japan) and used without further purification. The HPC film with a thickness of 0.1 mm was cast from a 20 wt % methanol solution and thoroughly dried in a vacuum to remove any residual methanol and water before the measurements. Instruments. The absorption spectra were obtained in a 2.0 mm quartz cell using a Jasco V570 spectrophotometer (Jasco, Tokyo, Japan). The temperature was controlled by a Jasco ETC-505 instrument for the absorption measurements. The SEM observations were performed using a JEOL JSM-7500F scanning electron microscope (JEOL, Tokyo, Japan). The refractive index measurements were carried out using a thermostatic multiwavelength Abbe refractometer NAR-1T (ATAGO, Tokyo, Japan).
Results and Discussion The HPC film can be swollen by toluene when immersed in a rich amount of anhydrous toluene because the thickness and width of the film steadily increase. However, the shape of the film is maintained even at the highest temperature of 110 °C, which is the boiling point of toluene. This means that there is a limited degree of swelling. Such limited swelling is not due to the crosslinking because HPC can be completely dissolved in various solvents, such as water, methanol, THF, and chloroform. Figure 1 shows the degree of swelling (Ds) of HPC in toluene as a function (7) The solution viscosity, MS, and DS data for the HPC sample are nominal values of the commercial products.
Published on Web 09/04/2009
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Figure 1. Degree of swelling (Ds) of HPC in toluene as a function of temperature. The irreversibility of the swelling is shown by swelling upon heating (open circles) and deswelling upon cooling (closed circles).
of temperature. Here, the Ds is designated as (the weight of toluene/the weight of HPC) 100. The Ds increases from 20 to 55 wt % with the increasing temperature from 40 to 100 °C. Deswelling takes place with the decreasing temperature from 100 to 80 °C; however, upon further cooling to a temperature below 80 °C, the Ds is constant as long as the sample is immersed in the toluene.8 This irreversibility is due to the formation of the microsegregated structure shown below. As mentioned in the Introduction, such swollen films display various colors (see Figure 2A). The color systematically changes from muddy brown, orange, blue, to violet with decreasing temperature from 90 to 20 °C. A reversal trend in the color change is observed upon heating. Because HPC can form the cholesteric LC in organic solvents, we first speculated that the colors result from the selective reflection of light due to the cholesteric helical structure with the pitches comparable to the wavelength of visible light.6 However, no expected circular polarization was detected in the reflected light. By careful observation, the recognized colors were found to be due to the light scattering because the transmitted light possessed a complementary color. To clarify this point, the absorption spectra of swollen films were measured by a UV-visible spectrometer equipped with a thermostat using quartz cells filled with toluene, and their temperature dependency was examined. The HPC polymer has an absorption band only in the UV region below 300 nm so that it should be transparent to visible light with wavelengths ranging from 350 to 800 nm. As shown in Figure 2B, however, the absorption spectra are not simple; the transmission takes place in the limited wavelength that travels from red to blue with the increasing temperature. In other words, there is a significant scattering of the light in the other wavelengths of the spectra. The complementary scattering color is shifted from blue to red, which qualitatively coincides with the color observed by the naked eye (refer to Figure 2A). The significant light scattering indicates some heterogeneity of the structure in the swollen HPC. It was actually detected by optical microscopy. As shown in Figure S1 (Supporting Information), many droplets (or microspherical domains) are dispersed in the swollen film, but not in the original one. The size of the droplets ranged from 3 to 5 μm in diameter at 100 °C. Furthermore, a scanning electron microscope (SEM) image for (8) The film was kept in toluene for about 3 h at the designated temperature before measuring the Ds because it takes about 3 h to reach complete swelling equilibrium for the film, based on our observations.
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Figure 2. (A) Photographs of the bright scattering colors of the HPC film swollen by toluene at 20, 40, and 60 °C, illuminated with white light from the top. In (B) are shown the light absorbance spectra of colored HPC films measured at various temperatures from 20 to 70 °C.
the freeze-fracture surface of the swollen film was analyzed. Here, the swollen film was immediately frozen by placing it in liquid nitrogen and fractured, and then the fracture surface was coated with a platinum-palladium layer by vacuum sputtering. Figure 3A shows a typical SEM photograph of the swollen HPC film. One can observe very clear spherical domains dispersed in the HPC matrix. The sizes of the spherical domains are widely dispersed with an average value of 4.5 μm in diameter, which corresponds to that observed by the optical microscope. Figure 3B is a SEM image after the partial evaporation of toluene. Obviously, the microdomains disappear near the film surface. These results indicate that toluene molecules have been confined in spherical domains; that is, the toluene molecules have been microsegregated from the HPC continuum. Such a microsegregation is also supported by the fact that the relative ratio in area of the spherical domain to the HPC continuum corresponds qualitatively to that expected from the Ds. A question now arises as to why the toluene molecules, which once entered the HPC matrix, have to be segregated from the HPC matrix. This may be attributed to the amphiphilic nature of the HPC,3 which is theorized from the following two facts. At first, HPC can be dissolved in both water and organic solvents, such as tetrahydrofuran (THF), dioxane, chloroform, etc. Second, the colloidal suspension can be prepared in the mixture of two solutions, an aqueous solution of a salt (sodium thiosulfate) and a THF solution of HPC.3 In this suspension, the aqueous phase is confined in droplets dispersed in a matrix of the THF solution of HPC and the amphiphilic HPC polymers create a hydrophobic layer on the surface of aqueous droplets. There are Langmuir 2010, 26(3), 1743–1746
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Figure 4. Dispersion of the refractive index of toluene and HPC at 20 and 40 °C. The refractive indices for toluene (closed circles and dotted lines) are from a reference.15 Curves for HPC (solid lines) are regression functions, which are obtained by fitting the threeterm Sellmeier dispersion relation with experimental refractive indices (open circles).
Figure 3. SEM images observed on freeze-fracture surfaces of (A) the swollen film at 100 °C and (B) the film in which the toluene is partially evaporated from its top surface.
several other examples of water-soluble polymers with an amphiphilic nature.9,10 Such an amphiphilic nature may be the source that induces the interesting segregation in the present system. When the toluene molecules enter the HPC film, the HPC polymer rearranges to contact their hydrophobic (oleophilic) group with toluene and then form the hydrophobic layer to confine the toluene molecules inside the spherical domain. The present results show that the size of the spherical domain is almost constant with temperature.11 It should be noted here that similar swelling phenomena have been observed in other aromatic solvents, such as benzene, xylene, and styrene. Considering such a segregation structure, the scattering coloration may be attributed to the Christiansen filter effect, as mentioned above. The Christiansen filter usually consists of small particles dispersed in some continuum whose refractive index is very close to that of the particles.1,2 In the present case, the particles and continuum correspond to the toluene (or toluenerich) microspherical phase and HPC (or HPC-rich) matrix, respectively. Although the exact compositions of the microspherical and continuum phases are unknown and difficult to measure, the absorption spectra were calculated using the refractive indices of pure toluene and HPC. Here, the refractive indices of HPC were measured using a thermostatic multiwavelength Abbe refractometer at the wavelengths of 470, 589, and 660 nm and then fitted on the three-term Sellmeier dispersion relation.12-14 The (9) Cheng, J. T. J. Macromol. Sci., Part B: Phys. 1981, B20, 365. (10) Hayakawa, K.; Kawaguchi, M.; Kato, T. Langmuir 1997, 13, 6069. (11) The spherical domain of toluene in the film was not observed in the SEM and POM images upon first heating up to 80 °C, and the film showed no color. The spherical domain and the color of the film abruptly appeared at 80 °C at the same time, and the size of the spherical domain stayed almost the same upon further heating and subsequent cooling. Therefore, the spherical domain is necessary for the coloration; however, its size does not play any role in the observed change in color of the HPC film. (12) Fleming, J. W. Appl. Opt. 1984, 23, 4486. (13) Ghosh, G.; Endo, M.; Iwasaki, T. J. Lightwave Technol. 1994, 12, 1338. (14) Ghosh, G. Appl. Phys. Lett. 1994, 65, 3311.
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Figure 5. Absorption spectra calculated with Mie scattering theory of a sufficiently large nonabsorbing sphere with the regression functions of the refractive index dispersion described in the text and the droplet size distribution in Figure S2 (Supporting Information).
resulting dispersion curve of the refractive index of the HPC is given in Figure 4. The refractive indices of toluene collected from a reference15 are also given in the same figure. One can see that the difference in the refractive indices between the HPC and toluene is subtle over the measured wavelength range and that the refractive indices cross at a certain wavelength that decreases with the increasing temperature. Mie scattering theory16 was taken into consideration to reproduce the spectroscopic data over the entire wavelength range in Figure 5. According to the theory, the scattering efficiency of a nonabsorbing sphere, which is sufficiently large compared with the wavelength, in the continuum with a slight different refractive index can be described as follows Qsca ¼ 2 -
4 4 sin Fþ 2 ð1 -cos FÞ F F
F ¼
4πr n1 -1 λ n2
ð1Þ ð2Þ
where Qsca is the scattering efficiency of a single sphere, r is the radius of the sphere, λ is the wavelength, and n1 and n2 are the (15) Rubio, J. E. F.; Arsuga, J. M.; Tarovillo, M.; Baconza, V. G.; Caceres, M. Exp. Therm. Fluid Sci. 2004, 28, 887. (16) Mie, G. Ann. Phys. 1908, 25, 377.
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refractive indices of HPC and toluene, respectively.17 The scattering efficiency corresponds to the extinction efficiency because the system includes no absorbing compounds in the considered wavelength region. To calculate the absorption spectra, the regression function for the refractive index dispersion of both HPC and toluene was put into eq 2. The calculated Qsca was then multiplied by the normalized frequency for the corresponding radius (r) of the sphere according to the size distribution of the microspherical domain evaluated by the SEM observation in Figure S2 (Supporting Information) and summed to take into account the droplet size distribution. The resulting absorption spectra can be seen in Figure 5. It qualitatively reproduces the observed spectra, showing that the observed colorlation of the colloidal dispersion is due to the Christiansen effect. The color change, that is, the shift in the matching wavelength, with the increasing temperature is also explainable because the refractive index of the toluene phase in a microspherical domain may more strongly decrease than that of the HPC matrix (refer to Figure 4).
Conclusion In conclusion, HPC can be swollen by toluene, but its swelling behavior is distinct. At temperatures as high as 100 °C, toluene (17) Hulst, H. C. In Light Scattering by Small Particles; Mayer, M. G., Ed.; Wiley: New York, 1957.
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molecules can enter the HPC film up to the weight fraction of 55%; however, they are segregated from the HPC matrix and form microspherical domains. The size of the spherical domains is around 3-5 μm in diameter. Such an unusual swelling behavior is due to the amphiphilic nature of HPC; HPC polymers rearrange to contact their hydrophobic group with toluene and confine the toluene molecules inside the spherical domain. Because of the similarity in the refractive indices of the microspherical phase and continuum phase, the swollen HPC film shows a beautiful scattering color due to the Christiansen filter effect. Acknowledgment. This research was supported by a Grant-inAid for Creative Scientific Research from the Ministry of Education, Science, Sports and Culture in Japan. Supporting Information Available: Figure S1 shows optical microphotographs and polarized optical micrographs of as-cast film for (A) and (B), respectively, and of the film swollen by toluene at 100 °C for (C) and (D), respectively. Figure S2 shows the distribution of the diameter of the microspherical domain evaluated by SEM observations. This material is available free of charge via the Internet at http://pubs.acs.org.
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