<<

Polymer Journal. Vol. 31, No.5, pp 447-451 (1999)

Effects of and Molecular on the Porous Structure Formation of Polymer Chemical Gels

Toshiaki MIURA, Ryoichi KISHI, and Hisao lcHuo

National Institute of' Materials and Chemical Research, Twkuha. Jharaki 305 8565. Japan

(Received November 5, 1998)

ABSTRACT: The e!Tects of temperature and molecular weight on the formation of inhomogeneous polymer gels, in which chemical gelation and phase separation take place simultaneously, were investigated. Slow chemical gelation by gamma ray irradiation was suitable for systematic analysis. In starting from ordinary polymer , the characteristic wavenumber of the porous structure decreases very sharply with depth of temperature jump. However, with appropriate amount of pre-dose, the effects of temperature are almost taken over by those of the pre-dose. The shape of the phase boundary also contributes to this temperature insensitive region. Hence, pre-dose of polymer solutions. which induces the molecular weight build up, is an important factor that facilitates controlled preparations of inhomogeneous porous structures of chemical gel networks. KEYWORDS Poly(vinylmethylether) I Chemical Gels I Porous Structure I Pre-Dose I

Polymer gels continue to attract considerable interest tempts.l. 2 bulk PVME gels of porous structures were from experimental and theoretical points of view. Some obtained without temperature control. In this case. the polymer gels show sharp transition in by temperature of the sample cells increased gradually from changing temperature or solvent. However, the slow the room temperature to around 500C. due to the heat dynamics of gels sometimes restricts applications. One transmitted from the radiation source as shown in Figure effective method to improve this slow dynamics is to use I. However. when we tried to prepare the same porous various higher order structures instead of homogeneous PVME gels with controlled stepwise temperature jump gels. The inhomogeneous gels that have interconnected above the phase separation point. we were not able to porous structure are examples. The volume relaxation obtain such porous gels. We only found that the dense times of inhomogeneous gels of appropriate porous homogeneous gels precipitated to the bottom of sample structures are several hundred times faster than homo­ cells in most cases. This seems strange and one question geneous gels. 1 - 4 For the preparation of gel networks arises. Is the gradual temperature increase the essential with large inhomogeneity in the order of hundred mi­ factor for the preparation of inhomogeneous polymer crometers. it is very effective to use phase separations. chemical gels? In order to clarify this problem and find The phase separation produces polymer rich and solvent suitable preparation conditions, quantitative analysis rich region. The competition of the phase separation and that systematically changes the experimental conditions gelation leads to inhomogeneous gels. We reported on in the porous structure formation of polymer chemical the rapid responsive porous poly(vinylmethylether) gels is necessary. This study investigates the effects of (PVME) gels that are prepared by }'-ray irradiation 1 . preparation temperature and molecular weight of the This corresponds to the simultaneous phase separation polymer solutions in detail and finds out the essential and chemical gelation. Preparations of porous gels have factors that determine the inhomogeneity of chemical gel also been reported on other gels as poly(N-isopropyl­ networks. acrylamide) gels 5 or some physical gels. 6 - 9 some of which were prepared by freezing and toughening.

Polymer solutions from which chemical gels are produced 50 have strong or molecular weight de­ pendency in viscoelastic properties, which may add additional features that are different from polymer u 40 blends. Although the studies on competitive phase ....Cl> separation have been extensive for polymer blends, 1 0 - 14 ::I there is little detailed quantitative analysis on the -nl.... Cl> formation of porous chemical gels. This is partly because 0. E 30 the control of the gelation is usually difficult for

the polymer chemical gels. However. by adopting the chemical gelation by }'-ray irradiation, the speed of gelation is directly related to the exposure. 20 2 4 6 8 10 Hence, it becomes easy to control this experimental 0 Time (h) parameter. The of porous structure depends on the balance Figure I. Without temperature control, the temperature of the irradiation cell increases gradually due to heat transmitted from the of the phase separation speed and gelation speed. radiation source. In our previous attempts, opaque gels with the However. the actual preparation of inhomogeneous inhomogeneous porous structures were obtained only at this condition. chemical gels has been very difficult. In previous at- Dashed line indicates the point where phase separation begins. 447 T. MIURA, R. KISHI, and H. !CHIJO

Temperature r -ray Cross-linking Temperature u Jump __T_.' +-C------1------J 35 e::I 41 Time c.. E 30 1-phase region {! Gelation Time Molecular Weight 25 0 20 40 60 80 100 (%) Figure 3. A phase diagram of PVME solutions. Filled circles indicate the boundary of one-phase region and two-phase region. Above the open circles, phase separation takes place very rapidly.

Inhomogeneous gels thus obtained were cooled and immersed in water at 25oC for observation. Porous A structures of thin gel films were observed using a phase contrast optical microscope. Microscopic images were Time recorded on videotape. Characteristic length of these porous gels was calculated by image analysis using a personal computer. Two-dimensional Fourier transfor­ Gelation Time mation of a real space image gives us the wave space information, equivalent to the results that can be Gelation Threshold obtained by light scattering. Since the microscopic images are not periodic, the procedure to reduce the edge effect Figure 2. Experimental scheme to change chemical gelation speed. is necessary. We cut the original images whose are 640 x 480 pixels to those of 400 x 400 pixels. These images EXPERIMENTAL were extrapolated to 512 x 512 by zero padding. The fast Fourier transformation (FFT) was applied using the Aqueous of PVME (Tokyo Kasei Co., Ltd.) hamming window function. Since these gels are isotropic, was used without further purification. Weight-averaged we calculated the radial distribution of FFT intensity by molecular weight was 92000 (M,) Mn = 3.3) measured averaging the two-dimensional FFT results for all from GPC. We prepared narrowly distributed PVME directions. The scattering functions were averaged over samples by GPC fractionation. Fractionated PVME many snapshots of gels. was weight-averaged molecular weight of 83000 with polydispersity of 1.24. The phase diagram of PVME RESULTS AND DISCUSSION solutions was determined by phase contrast microscope (Olympus BH-2) and differential scanning calorimeter Since the phase separation temperature is the baseline (Seiko Instruments SSC5200). to determine the depth of the temperature jump, we first PVME solution of 30wt% becomes gel with irradia­ determined the phase diagram of our PVME samples tion of 21.4 kGy (21.4 kJ kg -l) of the y-ray from the with narrow molecular weight distribution. As shown 6 °Co source (IIOTBq). The dose rate was 8.56kGyh- 1 . in Figure 3, the PVME solution becomes two-phase in the The gelation threshold of PVME solution depends on high temperature region. Although the phase boundary the concentrations of polymers as reported elsewhere. 15 between the mixed region and two-phase region has Temperature jump was carried out by circulating the unique dependency on the polymer concentration, phase temperature-controlled water around the sample cells. separation becomes very fast over 35°C, from which Experimental conditions of the temperature jump and endothermic also showed a sharp increase. Opaque irradiation are illustrated in Figure 2. In addition to the inhomogeneous gels were not obtained below 35°C. The temperature factor. we changed the molecular weight of shape of the phase diagram of PVME does not come polymer solution by pre-dose. By inducing the molecular from the polydispersity effect of the polymer samples, weight build up of polymer sample, the gelation time since we used fractionated samples. It may be ascribed decreases. Specifically, when the original polymer to the specific hydrophobic interaction of PVME. There solution becomes gel with the irradiation of time tgei• we was no significant difference in the transition points prepare samples that are irradiated for 0.2tgei• 0.4tgei• between fractionated samples and original ones. Hence, 0.6! gel· and 0.8/ gel at room temperature before simul­ we used the original PVME samples hereafter. taneous phase separation and chemical gelation. As Figure 4 shows typical results of image analysis for shown in Figure 2, pre-dosed polymer solutions form the inhomogeneous gel samples. The temperature of gels with irradiation of 0.8/gei• 0.6tgei• 0.4tgeb 0.2/gel re­ the preparation was 39oC and pre-dose amount was spectively, which are faster than the original polymer 17.1 kGy. As with ordinary phase separation of binary solution. solutions, the scattering function of inhomogeneous gels

448 Polym. J., Vol. 31, No. 5, 1999 Porous Structure Formation of Polymer Chemical Gels

0.2

48

46 Ill VI 544 c: 0.1 Ill 'Iii c: lii 42 - 0. E40

38 0 104 105 10' 107 Wavenumber (m.') 5 10 15 Pre-dose (kGy) Figure 4. Typical example of the scattering functions of the in­ homogeneous chemical gels. Figure 5. Structure mapping of inhomogeneous PVME gels obtained from 30 wt% solutions. The darkness represents the characteristic showed a broad peak. Hereafter, we use this peak porous size of gels. This chart is painted with seven ranges in the darkness, in which the brighter part indicates the larger porous wavenumber as the convenient characteristic length of structure. The length range is from 25 .urn to 210 .urn. Boundaries of the porous structure. ranges are 51 .urn, 78 !liD, I 04 .urn, 131 .urn, 157 .urn, and 184 !liD. Characteristic wavenumbers of inhomogeneous gels obtained under various conditions in the quench temperature and gelation speed are summarized in Figure 49 5. The concentration of original PVME solution is 30 wt%. We obtain only the homogeneous transparent u 47 gels if the final quench temperature is below 35°C. In t!! 45 this figure, the upper direction of vertical axis means a :::s deep temperature quench. The depths of the temperature ..I'll... 43 Gl jump that we used were I, 2, 3, 4, 5, 10, and 15oC. The 0. (S) horizontal axis indicates pre-dose time of polymer E 41 solutions. The darkness of the paint represents the 39 characteristic wavenumber of a porous structure. The bright part means small wavenumber (i.e., large porous 37 0 5 10 15 20 structure) and dark part, large wavenumber (i.e., small Pre-dose (kGy) porous structure). The data range is from 3 x 104 m - 1 to 2.5 x 10 5 m - 1 in wavenumber, which corresponds to Figure 6. Structure mapping of inhomogeneous PVME gels obtained porous size, 210 J.lm to 25 J.lm in real space units. from 50 wt% solutions. The mapping procedure and classification We carried out the same experiments at different ranges of the porous size are the same as in Figure 5. polymer concentrations, since the different concentra­ tion means different phase separation or gelation . even a slight temperature fluctuation from the chemical Figure 6 shows characteristic wavenumbers of the porous reactions easily leads to unfavorable structure, which is structures of gels obtained from 50 wt% PVME solutions. negligible for thin films but cannot be ignored for bulk Although absolute values of the vertical or horizontal samples. Fortunately, for PVME gels prepared without axis were different from the results of 30 wt% samples, temperature control as in our first report, gradual the shape of contour is the same. Specifically, the char­ temperature increase transmitted from the irradiation acteristic wavenumber drops very sharply with depth instrument is ideal. Irradiation before the system reaches of temperature quench from the phase separation point the phase separation temperature in previous attempts when the system experienced no pre-dose. The pre-dosed corresponds to pre-dose time, which induces molecular samples have less quench temperature dependency in the weight build-up and also shortens the gelation time. porous structure. After an appropriate pre-dose, porous size becomes very Let us compare these results of systematic investigation insensitive to the quench depth of temperature jump. with the experimental conditions of our first report of Thus, we were able to prepare fine porous gels with high preparation of rapid responsive inhomogeneous PVME reproducibility in previous attempts by gradual tempera­ gels. These rapid responsive PVME gels have porous ture increase. structures around a hundred micrometers. This corre­ There is a factor that may cause collapse of porous sponds to the dark region, marked (P) in Figure 5 and structures in the preparation of the bulk gels. When we 6. This explains difficulties in preparing rapid responsive want to make very large gel samples several centimeters inhomogeneous gels before. When we start from ordinary in height, difference in the between the polymer polymer solutions with constant temperature, the final and solvent can invoke unfavorable effect at certain structures of inhomogeneous gels are largely affected conditions. When the speed of the phase separation is by slight differences of quench temperature depth from too fast in such samples, the samples become macro­ the phase transition temperature. We cannot obtain scopically phase separated structures instead of large inhomogeneous gels of fine porous size, unless we con­ porous structure, in which the upper side is the solvent trol temperature within an accuracy of0.5°C. In this case, and the lower side is the densely precipitated polymer.

Polym. J., Vol. 31, No.5, 1999 449 T. MHJRA, R. KISHI, and H. ICHIJO

This is because largely porous structures cannot sustain (a) heavy loads. We used thin samples of less than 400 fill thickness and this effect of condensation was mini­ -=! 4 mized. Hence, although our experimental results in ca Figure 5 and 6 show that the very large porous films are .5 obtained in the region (S), we cannot obtain bulk samples - e! 3 of extremely large porous structures. :I To verify the universality of experimental results, we - Cll analyzed our results with the time-dependent Ginzburg c. E 2 Landau (TDGL) model. The TDGL model gives good description for universal properties of phase separation processes of polymer systems, 16 - 18 and this model has been the basis for simulation of various phase separation 600 400 200 0 phenomena. We included the molecular weight effect and Gelation threshold differences of mobility in polymer concentration in our model to describe the chemical gelation of polymer solutions. The free energy can be described by polymer (b) concentration 200000

f(c)=fdr( _c lnc+(l-c)ln(l-c) Mw i 150000 '- <1l 2 .0 +xc(l-c)++K(Vc) ) (l) E 100000 :::J c: <1l The last term comes from the surface energy of phase > 50000 boundaries. Here, we introduced the molecular weight M,. that increases as the of -1.8 to crosslink 0 probability. The conservation law requires, 0 5 10 Temperature ("C) (1_·:_.· = V. (Mv. bf {c(r)J) (2) 8t (k(r) (c) M where is mobility. Chemical gelation can be realized 300000 by modification of the mobility of polymers. The pre­ cise form of the mobility depends on the features of competitive phase separation. In the case of competitive g 200000 chemical gelation, we introduced the power Jaw of the '-

450 Polym. J., Vol. 31, No.5, 1999 Porous Structure Formation of Polymer Chemical Gels these situations, very precise temperature control is phase separation take place. Systematic investigation of necessary for preparation of inhomogeneous chemical these processes has not been explored, because of the gels. Figure 7(c) shows experimental and numerical re­ difficulty in the control of the chemical gelation speed. sults less sensitive to temperature. Thus, our exper­ However, chemical gelation by the irradiation is suitable imental results can be described by the TDGL model for this study, since the reaction rate is proportional to with appropriate time and molecular weight dependent exposure. Experimental results showed that the char­ viscosity terms of polymer solutions and first order ap­ acteristic wavenumber of gels decreases very sharply as proximation on temperature dependency terms. The one the depth of temperature jump increases in the case of thing that is slightly different from the experiments starting from ordinary PVME solutions. However, by is that the temperature insensitive region in simulation increasing the pre-dose amount, thus inducing the data covers only limited narrow in the parameter molecular weight build up, the characteristic size of space, while it covers a wide area in Figure 6. This can porous structure is insensitive to the depth of temperature be ascribed to the approximation in the free energy form, jump. This primarily comes from the combined effects in which the temperature dependence of x seems weak­ of the high viscosity and shortened gelation time of the er for hydrophobic interactions of PVME solutions. pre-dosed samples, but also comes from characteristic As shown above, the temperature dependence of phase diagram of thermally responsive polymer solutions porous structures of chemical gels diminishes as pre-dose due to hydrophobicity. In the temperature above the time increases, and finally the effects of the depth of the unstable boundary, the system sharply separates into two temperature jump are almost taken over by that of the phases as the very dense polymer domain and solvent pre-dose time. The experimental and numerical results domain, which shows sharp contrast to ordinary binary showed that the control of porous structures only by the polymer mixtures that have gradual changes in the temperature factor alone is not always effective. This coexistence concentrations in the two-phase region. method is possible in principle, but requires very precise The experimental results were analyzed with the TDGL temperature control, which may be sometimes impossible model. For the preparation of the rapid responsive gels for bulk samples because of the reaction heat of gelation. with the inhomogeneous porous structure, the appropri­ Our systematic investigation on the entire parameter ate amount of the pre-dose is essential, since it brings range of both the temperature jump and pre-dose time the system less influenced by the precise depth of revealed that the pre-dose processes are essential factors temperature jump or temperature fluctuation during for the preparation of inhomogeneous chemical gels. chemical gelation. Although the speed of phase separation in polymer systems is extremely slower than ordinary binary mix­ REFERENCES tures of low molecules, it is still very fast compared with chemical gelation by irradiation. When we add I. X. Huang, H. Unno, T. Akehara, and 0. Hirasa, J. Chem. Eng. the pre-dose condition, it has the combined effect of the Japan, 20, 123 (1987). slow phase separation by increased viscosity and the 2. R. Kishi, H. Ichijo, and 0. Hirasa, J. Intelligent Mater. Syst. Struct., 4, 533 ( 1993). accelerated gelation by shortened gelation time. These 3. R. Kishi, 0. Hirasa, and H. Ichijo, Polymer Gels a/Ill Net11·orks effects are what we have predicted qualitatively before, 5, 145(1997). but quantitative analysis has not been carried out. Our 4. T. Miura, E. Ono, and H. Ichijo, Jpn. J. Appl. Phys., 34, Ll603 systematic studies reveal these details on how the pre-dose (1995). processes suppress the temperature quench factor. This 5. B. G. Kabra and S. H. Gehrke, Polym. Commun., 32, 322 (1990). 6. F. Sciortino, R. Bansil. H. E. Stanley, and P. Alstrom, Ph_\'S. Rev. would give us useful information for the preparation of £, 47, 4615 (1993). inhomogeneous chemical gels with simultaneous phase 7. L. M. Jelich, S. P. Nunes, E. Paul, and B. A. Wolf, separation. Our experiments were carried out for Macromolecules, 20, 1943 ( 1987). chemical polymer gels. We think that this can be also 8. R. Bansil, J. La], and B. L. Carvalho, Polymer, 33, 2961 (1992). 9. R. H. Tromp and R. A. L. Jones, Macromolecules, 29. 8109 applied to the controlled preparation of the bulk samples (1996). of porous physical polymer gels. In this case, the pre- 10. N. lnaba. K. Sato. S. Suzuki. and T. Hashimoto. Macromolecules. dose in the chemical gelation corresponds to the initial 19, 1690(1986). molecular weight of polymer solute for the physical gels. II. H. K. Lee, A. S. Myerson. and K. Lcvon. Macromolecules. 25. 4002 ( 1992). However, in the case of many physical gels with little 12. S. Song and J. M. Torkelson, Macromoll'ctdes, 27, (1994). hydrophobic interaction, strong concentration de­ 13. T. Hashimoto. M. Takenaka, and H. Jinnai, Polpn. Commun., pendence of the spinodal line in phase diagrams may 30. 177 ( 1989). become obstacles, which reduces this temperature in­ 14. Q. Tran-Cong. T. Nagaki, 0. Yano, and T. Socn. Macro­ molecules. 24. 1505 (1991 ). sensitive region. 15. T. Miura. H. Okumoto. and H. lchijo. Phrs. Rev. E. 54, 6596 (1996). SUMMARY 16. R. Petschck and H. Mctiu. J. Chem. Phrs .. 79. 3443 (1983). 17. A. Chakrabarti, R. Toral. J. D. Gunton, and M. Muthukumar, We studied the effects of temperature and molecular Ph1·s. Ret•. Lett .. 63, 2072 ( 1989). 18. G .. Brown and A. Chakraharti, J. Chem. Phrs .. 98.2451 (1993). weight on the formation of rapid responsive inhomogen­ eous gels in which simultaneous chemical gelation and

Polym. J., Vol. 31, No.5, 1999 451