J. Phys. Chem. B 2001, 105, 4565-4571
4565
Heterogeneous Polymerization of Hydrogels on Hydrophobic Substrate Akishige Kii, Jian Xu,† Jian Ping Gong, Yoshihito Osada,* and Xianmin Zhang‡ DiVision of Biological Sciences, Graduate School of Science, Hokkaido UniVersity, Sapporo 060-0810, Japan ReceiVed: September 13, 2000; In Final Form: February 26, 2001
The substrate effect on the inhomogeneous gelation is in situ studied using various monomers: 2-acrylamide2-methyl-1-propanesulfonic acid (AMPS), acrylic acid, and N,N′-dimethylacrylamide on various hydrophobic substrates with different surface tensions: poly(tetrafluoroethylene), polypropylene, polyethylene, polystyrene, and poly(vinyl chloride) in aqueous solution as well as in organic solvent by means of electronic speckle pattern interferometry. When the polymerization is carried out in water, a clear interface appears in the vicinity of the hydrophobic substrates at a critical time when the auto-acceleration of the polymerization starts. The polymerization on the hydrophobic substrate is suppressed after the appearance of the interface which gives rise to a heterogeneous gel structure. The thickness of the substrate-induced inhomogeneous layer increases with the square root of the polymerization time, showing the feature of monomer diffusion. An enhanced heterogeneous polymerization occurs on the hydrophobic substrate with a lower surface tension. These effects are greatly suppressed when the polymerization is carried out in ethanol and in the presence of hydrophobic monomer, giving rise to a homogeneous gelation. The correlation between the surface tension of the substrates and the interface strength in water suggests that the substrate-induced interface formation might be associated with the high interfacial tension between the substrate and the polymerizing solution.
I. Introduction We have previously reported that when a cross-linked hydrogel is synthesized from hydrophilic vinyl monomer by radical polymerization in water, the surface properties of the hydrogel are strongly dependent on the substrate on which the gel was synthesized.1 For example, a piece of poly(2-acrylamide2-methyl-1-propanesulfonic acid) (PAMPS) gel synthesized on a polystyrene plate has an eel-like slimy surface with an extremely low surface frictional coefficient on the order of 10-4-10-5, which is 1-2 orders of magnitude lower than that of a gel prepared on a glass plate.1 The interfacial adhesion and the interaction with biological cells of the gel are also strongly dependent on the substrate on which the gel is formed.2 To study the gel formation process, a novel method, electronic speckle pattern interferometry (ESPI), was first established to spatially monitor the entire polymerization and gelation between a glass substrate and a poly(tetrafluoroethylene) (Teflon) substrate in real time.3 It was found that the polymerization proceeded linearly and homogeneously at the initial stage of reaction, but almost simultaneously with the beginning of autoacceleration (gel-effect) of the polymerization, the polymerization rate on the Teflon surface became lower than that of the bulk region. As a consequence, the monomer diffused to the adjacent region where polymerization was accelerated, resulted in the inhomogeneous network formation. The origin of the suppression of the polymerization on the Teflon substrate is not clear. In this paper, we have systematically investigated the correlation between the polymerization rate and the heterogeneity of the PAMPS gel to clarify the effect of monomer diffusion. To elucidate whether this substrate effect † On leave from Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, China. ‡ On leave from Department of Information and Electronic Engineering, Zhejiang University, Hangzhou 310027, China.
generally occurs for the hydrophilic vinyl monomers when polymerized on hydrophobic surfaces, apart from 2-acrylamide2-methyl-1-propanesulfonic acid (AMSP), experiments of various hydrophilic monomers: acrylic acid (AA), and dimethylacrylamide (DMAAm) on the substrates with various surface tensions have been made using Teflon, polypropylene (PP), polyethylene (PE), polystyrene (PS), and poly(vinyl chloride) (PVC) as the hydrophobic substrates, which have surface tensions in the range of 23.9 mN/m - 41.9 mN/m. The correlation between the surface tension of the substrate and the interface, as well as the heterogeneity of the gel is studied. The effect of solvent surface tension was also investigated by carry out the polymerization in ethanol, which has a much lower surface tension than that of water, and the result was compared with that in water. II. Experimental Section The monomer, 2-acrylamide-2-methyl-1-propanesulfonic acid (AMPS) (99% in purity, Tokyo Kasei Co., Ltd.) was used as received. N,N′-Methylenebisacrylamide (MBAA) (Wako Co., Ltd.), used as a cross-linking agent, was recrystallized from ethanol. 2-Oxoglutaric acid (Wako Co., Ltd.), used as an ultraviolet (UV) initiator, was used as received. Acrylic acid (AA) (Tokyo Kasei Co., Ltd.), dimethylacrylamide (DMAAm) (Kojin Co., Ltd.), styrene (ST) (Junsei Co., Ltd.), and 2,2,2trifluoroethyl acrylate (TFEA) (Daikin Co., Ltd.) were purified by distillation under reduced pressure before usage.4 AMPS, MBAA, and 2-oxoglutaric acid were dissolved in deionized water and poured in the cell shown in Figure 1. The sample cell containing monomer solution was purged with 99.99% nitrogen gas with a flow rate of 2.6 mL/min. for nearly 1 h prior to the polymerization to reduce the inhibitory effects of oxygen. The polymerization was carried out at an ambient temperature of 20 °C under irradiation of a UV lamp (model ENF-260C/J, Spectronics Co.) with a wavelength around 365
10.1021/jp003242u CCC: $20.00 © 2001 American Chemical Society Published on Web 04/26/2001
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Figure 2. Position dependence of the relative light intensity near the Teflon surface during the polymerization. The inserted figure shows the time dependence of the peak position of the light intensity. The data at 20 min is determined visually from the images of the reaction cell.
Figure 1. Schematic illustration of reaction cell (left) and the images of the cell during the polymerization of AMPS from its aqueous solution between a Teflon substrate (left, x ) 0 mm) and a glass substrate (right, x ) 16 mm). Polymerization conditions: 1.0 M AMPS, 5.0 mol % MBAA, 0.3 mol % 2-oxoglutaric acid.
nm. The distance of the UV lamp to the sample cell is 15 cm, and the illumination intensity on the cell surface is about 0.35 mW/cm2. The sample cell used was a cuvette made of Pyrex glass (Yokota Co. Ltd.) with 200 × 20 mm in its cross-section area and 5 mm in the light path. A prescribed hydrophobic plate of 200 mm × 5 mm in size was inserted into the cell as one of the two internal surfaces, and the Pyrex glass wall of the cuvette was used as a hydrophilic substrate. The distance between the substrate surfaces was 16 mm. The cell was long enough to eliminate the edge effects (the top and the bottom of the cell). Teflon (Furon Industry Co. Ltd), PP (Sunplatec Co. Ltd.), PE (Sunplatec Co. Ltd.), PS (Sunplatec Co. Ltd.), and PVC (Homac Co. Ltd.) were used as hydrophobic substrates. The cell and the substrates were carefully washed with 0.5 M hydrochloric acid, and then with detergent (Extran MA02, Merck, Inc.), which was followed by careful rinsing with a large amount of deionized water. The rate of the polymerization was assayed by the relative change in the specific refractive-index of the solution using the ESPI method. The principle and experimental setup of the ESPI was described in detail in the previous paper as was its theory.3 It was composed of a He-Ne laser light source (λ ) 633 nm model 127, Spectra-Physics Lasers, Inc.), an optical system, a cooled CCD camera (model C4742-95, Hamamatsu Co., Japan), and a personal computer. According to our previous results for the long sample cell,3 the intensity variation is the same along the vertical direction (y axis) in the detecting region (16 mm × 16 mm) and can be considered as a one-dimensional system. The normalized intensities related to the x-axis and the time factor can be obtained through normalization of the I by an average I
Inor(x,y,t) ) 1 + γ cos[θ(x,y) + φ(x,t)]
(1)
where θ(x,y) is the initial phase difference between these two beams, γ is the visibility, and φ(x,t) is the change in phase difference during the polymerization. The refractive-index change related to the phase change can be written as
∆n(x,t) )
λφ(x,t) 2πl
(2)
l is the optical path of the cell, and λ is the laser wavelength in a vacuum. Through eqs 1 and 2, the refractive-index variation, ∆n(x,t), in the polymerization process can be evaluated. In our research, the images containing 1280 × 1024 pixels were sequentially taken by a CCD camera and stored on the hard disk. The CCD detector sampled the image for 1000 times with a sampling interval of 15 or 30 s, and 250 or 500 min of the reaction was monitored. The background noise was subtracted from all the images taken in the image acquisition step. The experimental for the in situ observation of the polymerizations of AA, DMAAm, and of the copolymerization of AA and ST or TFEA in the presence of a cross-linking agent were the same with that of the PAMPS gel described above. III. Results and Discussions III.1. In Situ Analysis on Reaction Kinetics. Images of the reaction cell are followed in the course of the polymerization of PAMPS gel. A strong light-line in the region near Teflon suddenly appears at 20-30 min after the beginning of the reaction, as shown in Figure 1, while no light-line appears in the region close to the glass during the entire reaction time. By shaking the reaction cell, it is recognized that this strong lightline is originated from an interface which is the border between a low viscous region facing the Teflon and a high viscous region facing the bulk. The position and the brightness of the light line changes with the time. Figure 2 shows the relative intensity-position curves at various reaction times, where the Teflon surface is defined as x ) 0. Though we have recognized a clear but weak light line at the reaction time of 20-30 min from the images in Figure 1, a clear light intensity peak is observed only at t > 40 min in Figure 2. This difference might be due to the high sensitivity of human eyes. The time dependence of the peak position is shown as the inserted figure of Figure 2. The light line moves toward the bulk to x ) 0.55 mm increasing its strength until 40 min and then, gradually back toward the Teflon surface to x ) 0.3 mm at 90 min, decreasing its strength and finally disappears at a position of x ) 0.25 mm at 150 min. The appearance of the light line indicates that there is a sharp change in the refractive index around this position. The light at the position of minimum intensity is deflected to the position of maximum intensity. Though the change in the refractive index at this interface cannot be accurately determined only from the
Polymerization of Hydrogels
Figure 3. Phase changes at different positions of the reaction cell during the polymerization. Numbers on curves are the distances (mm) from the Teflon surface.
intensity change in Figure 2, we can plausibly explain that there is a sharp change in the refractive index at the position of light line and the refractive index on the side neighboring to the Teflon is much lower than that neighboring to the bulk. Figure 3 shows the time courses of phase changes ∆φ(x,t) during the polymerization at different positions in the sample cell by ESPI method. Numbers in the figure indicate the distance in millimeters from the Teflon surface. As shown in Figure 3, at the very beginning of the polymerization, the refractive index increases monotonically without any induction time, and the increase in the refractive index is originated from the normal polymerization. Details of the kinetic profile at the beginning of the reaction are given in Figure 3 (see inset), which demonstrates clearly a homogeneous polymerization with no position-dependence. The results coincide well with those in Figure 1 and 2, which shows a homogeneous image and no intensity peak at the initial stage of the reaction. After 20 min of reacting, an abrupt increase in the phase change is observed both in bulk and near the Teflon surface. As described in the previous paper,3 the abrupt increase in the change of the phase is due to the so-called “autoacceleration effect” of radical polymerization that occurs due to the increase in the viscosity of the polymerized system (so-called “geleffect”), which in turn suppresses the termination probability between two propagating chains and thus increases the overall rate of polymerization.5 The polymerization process after the autoacceleration effect occurs is very sensitive to position. In the region of 4 < x < 16 mm, the polymerization rates are almost the same to give the same values of the phase changes. However, in the region of 0.8 < x < 4 mm, the phase changes show strong position dependence and the values are much larger than those of the region 4 < x < 16 mm. In the region of x < 0.8 mm, the phase change could not be assayed due to the presence of the bright light line. The lower refractive index in this region shown in Figure 2 suggests that the autoacceleration is suppressed in this region. The suppression of the polymerization near the Teflon where high monomer concentration is maintained should result in monomer diffusion to the neighboring region (x > 0.8 mm) where monomers were consumed rapidly due to auto-acceleration. Thus, an additional phase change is subsequently observed in the region of 0.8 mm < x < 4 mm. The auto-acceleration process appears again at 7085 min, at positions of x ) 1.6 to 2.0 mm, as shown in Figure 3. Thus, the inhomogeneous gelation is accompanied with monomer diffusion.
J. Phys. Chem. B, Vol. 105, No. 20, 2001 4567 By comparing the time profiles of the phase change in bulk (x ) 8.0 mm) with the position of the interface as determined from the intensity peak in Figure 2, we found the light line of the interface appears almost simultaneously with the acceleration, which suggests that the interface appears by the extensive increase in the viscosity of the solution. As the phaseacceleration proceeds, the light intensity peak moves quickly toward the bulk and then gradually moves back toward the Teflon surface. The homogeneous and linear polymerization kinetics at the initial stage of reaction excludes possibilities of any undesired effect of impurity that might be brought about by the Teflon substrate. Therefore, the appearance of the interface in the vicinity of the Teflon might be attributed to the specific surface property of the Teflon. As the interface appears simultaneously with the autoacceleration at a polymer conversion of about 15% that can be calculated from the phase change, or a polymer concentration of 0.1 M, the substrate effect might be related with the formation of concentrated polymer or the network structure. An important fact is that the abrupt increases and their positional dependence of the phase change start simultaneously with the appearance of the interface, at the reaction time of about 25 min. This suggests that the substrate effect appear at the polymer entanglement and/or the network formation. From our previous paper3 as well as the above results, the phase change (refractive index change) shown in Figure 3 is associated with the heterogeneous polymerization and diffusion. Assuming the conversion from monomer to polymer is 100%, change in the refractive index at the end of reaction (∆np) in the bulk region as well as that close to the glass substrate (4 < x < 16 mm) is simply due to the polymerization of monomer with concentration C0. In the region near the Teflon substrate (0.8 < x < 4 mm), the extra refractive index change relative to ∆np results from the monomer diffusing in addition to the polymerization of the monomer in that region. When the entire polymerization is finished, the polymer distribution in the reaction cell is determined by the following relation3
C(x) ) C0 +
∆n(x) - ∆np dnm/dC + dnp/dC
Here dnp/dC ≈ ∆np/C0 and is the refractive-index variation of the polymerization, which is found to be 3.80 × 10-3 per mol, and dnm/dC is the refractive-index variation of the monomer concentration, which is measured by a differential refractometer to be 3.80 × 10-2 M-1.3 Figure 4a shows the concentration distribution of polymer at the polymerization time of 160 min. The inhomogeneous density of the gel exists within the range of x < 4 mm and the highest density of the polymer is at 2 mm from the Teflon substrate. The extra amount of polymer localized in the region from x ) 0.8-4 mm is originated from the monomer diffusion from the region within 0.8 mm of Teflon plate where polymerization takes place very slowly. If the gel is sliced into one millimeter slices and allowed to swell in water, the strip sliced closest to the Teflon plate shows the highest swelling while the one next to it has the lowest value (Figure 4b). The highest swelling of the gel formed near the Teflon indicates a low cross-linking density. In addition, after the equilibrated swelling in water, the strip sliced closest to the Teflon shows an extensive curvature, as schematically shown in Figure 4b, suggesting the gradient network structure. Therefore, if the entire gel synthesized between a hydrophobic and a hydrophilic substrate is allowed to swell in water, it exhibits a significant curvature as shown in Figure 5, where the gel surface
4568 J. Phys. Chem. B, Vol. 105, No. 20, 2001
Figure 4. Final polymer density distribution in the reaction cell (a) and the position dependence of the swelling degree of the gel in water (b). The latter was obtained by slicing the gel into 1 mm strips, which were allowed to swell in water. The shapes of the gel strips swelling in water are schematically shown in the figure.
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Figure 6. Time profiles of the phase change during the polymerization of AMPS using various amount of UV initiator (2-oxoglutaric acid) in (a) the bulk region (x ) 12.0 mm) and in (b) the surface region of Teflon (x ) 2.4 mm). Amount of initiator: (0) 0.1 mol % (upper time axis), (O) 0.3 mol %, and (]) 1.0 mol % (lower time axis). Polymerization conditions: 1.0 M AMPS, 5.0 mol % MBAA.
TABLE 1a initiator concentration (M)
t0 (min)
0.1 0.3 1.0
150 29 20
surface region (x ) 2.4 mm) te (min) τ (min) 391 100 48
241 71 28
bulk region (x ) 12.0 mm) te(min) τ (min) 200 70 46
50 41 26
a t : Reaction time for the starting of the auto-acceleration. t : 0 e Reaction time for the saturation of the auto-acceleration. τ: Duration of the auto-acceleration (τ ) t0 - te). Polymerization conditions: 1.0 M AMPS, 5.0 mol % MBAA.
Figure 5. Photograph of the PAMPS gel prepared between a Teflon plate and a glass plate after swelling in water. The gel was dyed after preparation using the neutral red for the clarity of image.
formed on the Teflon is always the outside of the curvature and that on the glass is the inside. The strips sliced more than 6 mm from the Teflon surface show almost the same swelling ability with a straight shape regardless of the position, which is in agreement with the polymer density profile of Figure 4a. The gradient network structure might occur by the following reason. The suppression of the gelation on the Teflon surface brings about an extensive diffusion of monomers in the surface region to the adjacent region where the monomer has already been consumed, which leads to an enhanced cross-linking by the physical entanglement in this region. On the contrary, the substantial decrease in the monomer concentration at the topmost surface of the hydrophobic substrate consequently results in a gel with a decreased cross-linking density containing extensive branches with free chain ends. III.2. Effect of Reaction Rate. According to the above discussion, the heterogeneous polymer density distribution is associated with the competitive kinetics of polymerization and monomer diffusion. Therefore, if the rate of polymerization is too high, the monomer diffusion could not be competitive with the monomer consumption and the layer thickness of the heterogeneous structure might become thinner. Since the initial rate of polymerization, Rp, increases proportionally with the square root of the initiator concentration, Rp ∝ I1/2,5 we further
investigate the effect of the polymerization rate on the formation of the heterogeneous structure by changing the concentration of UV initiator. Figure 6a,b show the time profiles of the phase change during the polymerization of AMPS containing various amounts of UV initiator in the bulk region (x ) 12 mm, Figure 6a) and in the Teflon surface region (x ) 2.4 mm, Figure 6b), respectively. Since there is no monomer diffusion in the bulk region, the phase changes in this region occur only due to the polymerization, which permits us to calculate the polymerization conversion and the result is shown in the right axis of Figure 6a. Although the time at which the autoacceleration starts strongly depends on the concentration of UV initiator, the critical conversion at which the auto-acceleration starts is almost the same, around 10-20%. The time to start the auto-acceleration, t0, and the time at which the phase change saturates, te, are listed in Table 1 for the surface region and the bulk region, respectively. The duration of the auto-acceleration, τ ) te - t0, is also listed in Table 1. When the concentration of UV initiator is decreased, the critical time for the starting of the autoacceleration as well as the time for the saturation of the autoacceleration increase. The duration for the autoacceleration τ in the surface region of Teflon is much longer than that of the bulk due to the appearance of the interface that suppresses the surface polymerization. Figure 7 shows the final polymer concentration distribution of the gel prepared with various amounts of UV initiator. As expected, the lower the UV initiator concentration (the lower the rate of polymerization), the larger the heterogeneity of the gel, indicating that the heterogeneous structure is formed due
Polymerization of Hydrogels
Figure 7. Final polymer density distributions in the reaction cell for the polymerization of AMS between a Teflon substrate (left side, x ) 0 mm) and a glass substrate (right side, x ) 16 mm) with different amounts of UV initiator. Amount of initiator: (0) 0.1 mol %, (O) 0.3 mol %, (]) 1.0 mol %. Polymerization conditions: 1.0 M AMPS, 5.0 mol % MBAA. The inserted figure is the log-log plot of the position of the maximum polymer concentration, dmax, and the duration of the auto-acceleration, τ, in the surface region of Teflon.
Figure 8. Time profiles of the interface position appeared during the polymerization of various kinds of monomers between Teflon and glass substrates. Polymerization conditions: (O) 1.0 M AMPS, 5.0 mol % MBAA, 0.3 mol % 2-oxoglutaric acid; (0) DMAAm, 1.0 M monomer, 1.0 mol % MBAA, 0.3 mol % 2-oxoglutaric acid; (]) AA, 1.0 M AA, 1.0 mol % MBAA, 0.3 mol % 2-oxoglutaric acid.
to the competitive rates of polymerization and monomer diffusion. Since the heterogeneous structure of the gel is formed after the appearance of the interface, which almost occurs simultaneously with the starting of the auto-acceleration, we attempt to plot the position of the maximum polymer concentration, dmax, in Figure 7 against the duration of the autoacceleration τ in the surface region in the log-log scale. As shown in Figure 7 (inserted), a linear line with a slope of 0.50 (dmax ∝ τ0.50) is obtained. This relationship suggests that the thickness of heterogeneity is closely associated with the rate of monomer diffusion. Since the thickness of the heterogeneous layer is approximately two times of dmax, this gives D ≈ 4d2max/τ. The diffusion constant thus obtained is D ≈ 1.9 × 10-5 cm2/s, which is quite reasonable. These results clearly demonstrate that the formation of a heterogeneous structure could be associated with the diffusion of monomer from the surface region where the polymerization is strongly suppressed to the adjacent region where the polymerization takes place rapidly after the appearance of the interface. III.3. Substrate Effect on Other Hydrogels. The heterogeneous polymerization near the Teflon surface is also observed in ionic monomer systems such as AA, as well as neutral monomers such as DMAAm in water. As shown in Figure 8, similar to AMPS, the interface appears suddenly once autoacceleration occurs, but the position of the interface strongly
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Figure 9. The maximal distance of the interface from the hydrophobic surface, dmax, as a function of surface tension of substrates, γs, for the polymerization of (O) AMPS and (0) DMAAm between a hydrophobic substrate and a glass. Hydrophobic substrates used are indicated in the figure. Polymerization conditions: (O) 1.0 M AMPS, 5.0 mol % MBAA, and 0.3 mol % 2-oxoglutaric acid; (0) 1.0 M DMAAm, 1.0 mol % MBAA, 0.3 mol % 2-oxoglutaric acid. Data are the averages values over 3-5 experiments, and the error bars are the standard deviations from the averages.
depends on the nature of the chemicals. AA and DMAAm show a stronger interface on the Teflon surfaces than that of AMPS, and the maximum interface distance is as high as 3.25 mm in the case of DMAAm. The increased interface distance for AA and DMAAm suggests the pronounced effect of the Teflon substrate on the polymerization of these monomers. III.4. Effect of Other Hydrophobic Substrates. This substrate effect was also observed on other hydrophobic substrate, such as PP, PE, PS, and PVC. Figure 9 shows the relationship between the surface tension of the substrate, γs,6 and the maximum position of the interface from the hydrophobic surface, dmax, which appeared during the polymerization of AMPS. Though the experimental results show a large error range, we can observe a tendency that dmax decreases with the increase in γs. As has been discussed in the previous paper,3 the error for the determination of phase change is mainly from the temperature variation and is less than 5%. The deviation between different experimental runs is considered mainly due to the amount of residual oxygen that could not be removed completely by nitrogen purging, since the reaction rate has a substantial influence on the substrate effect, as has been clarified in section III.2. The polymerization of DMAAm on various hydrophobic substrates was also investigated. As shown in Figure 9, dmax also decreases with the increase in γs for DMAAm. The increased distance of dmax for DMAAm here again suggests the pronounced effect of the substrate on DMAAm gelation. With the increase in γs, the difference between their dmax values becomes smaller, and dmax is the same on the PS substrate. The final polymer concentration distributions of the PAMPS gel prepared between various substrates are shown in Figure 10. In corresponding to the interface results, Teflon shows the most prominent heterogeneity, which decreases with the increase in the surface tension of the hydrophobic substrate used. To illustrate the general feature of the heterogeneity of the gel, the maximum polymer concentration at the final stage of the polymerization gel, Cmax, is plotted against the surface tension of the substrate γs and is shown in the inserted figure of Figure 10. A correlation between Cmax and the substrate surface tension was observed. Thus, one can conclude that, the lower the γs, the more extensive in the heterogeneity of the polymerization.
4570 J. Phys. Chem. B, Vol. 105, No. 20, 2001
Figure 10. Spatial polymer concentration distribution in the reaction cell at the final stage of polymerization. Polymerization conditions: 1.0 M AMPS, 5.0 mol % MBAA, and 0.3 mol % 2-oxoglutaric acid. Substrate: (O) Teflon, (0) PP, (]) PE, (4) PS. The inserted figure shows the relationship between surface tension of hydrophobic substrates, γs, and the maximal polymer concentration in the gel, Cmax. Temperature: 20 °C.
As reported before, the polymerization proceeded almost homogeneously at the initial stage of the reaction, before the appearance of the interface. This result excludes any impurity effect that might inhibit the polymerization. Any convection effects caused by the poor thermal conductivity of the plastic substrates might also be excluded since among the hydrophobic substrates tested in this work, Teflon has the highest thermal conductivity. The above experimental results suggest that the origin of the substrate effect on the formation of the interface and the heterogeneous gelation might be associated with the high interfacial tension between the hydrophobic substrate and the polymerizing aqueous solution. III.5. Effect of Solvent. The surface tension of a solution is approximated as the concentration average of the individual surface tensions of the solute and the solvent.8 Since the surface tension of water is 73 mN/m at 20 °C, the aqueous solution of the hydrophilic polymers and/or gels should have a higher surface tension than that of the hydrophobic substrates used, which changes from 23.9 mN/m for Teflon to 41.9 mN/m for PVC. For example, the surface tensions of 0.2 M aqueous solutions of PAMPS and PAA are 76 and 63 mN/m, respectively, and those of 0.2 M hydrogels of PAMPS and PAA are 67 mN/m and 64 mN/m, respectively, at 20 °C.7 Accordingly, if the polymerization is carried out in an organic solvent that has a surface tension as low as that of the hydrophobic substrate, the interface formation and the inhomogeneous gelation might be suppressed. Therefore the substrate effect on the gelation in ethanol, which has a surface tension of 22 mN/m at 20 °C, was further investigated on the Teflon surface. Figure 11 shows the images of the polymerizing solution near the Teflon surface and the time profiles of the interface position during the polymerization of AA between a Teflon substrate and a glass substrate in water and in ethanol, respectively. Since the appearance of the interface and the heterogeneity of the gel strongly depend on the rate of polymerization, the polymerization with a similar reaction rate was shown in Figure 11. Comparing with the aqueous system, the interface in ethanol is hardly visible, as shown by the inserted images in Figure 11, and is a shorter distance from the Teflon surface. The less substantial interface observed in the ethanol system might be associated with the decrease in the interfacial tension between Teflon and the ethanol solution. It should be noted that when the polymerization is carried out in nonaqueous environments, usually a stronger hydrogel
Kii et al.
Figure 11. Time profiles of the interface position, d, during the polymerization of AA in (b) water and in (.) ethanol between a Teflon substrate and a glass substrate. Inset: Images of the solutions near the Teflon surface at a time that the light-line (interface) has the strongest intensity. Polymerization conditions: 1.0 M AA, 1.0 mol % MBAA, 0.3 mol % 2-oxoglutaric acid. Temperature: 20 °C.
Figure 12. Spatial phase change distribution at the final stage of polymerization in the reaction cell for the polymerization of AA between a Teflon substrate and a glass substrate in ethanol with the presence and absence of hydrophobic monomers. (O) AA, (0) AA:ST ) 95:5, (]) AA:TFEA ) 95:5, and (b) AA:TFEA ) 75:25 in molar ratio. Polymerization conditions: 1.0 M monomer, 5.0 mol % MBAA. The amount of 2-oxoglutaric acid was changed in a range of 0.3-1.5 mol % to adjust the reaction rate closer to each other.
is formed with a higher effective cross-linking density. However, the cross-linking density should not have a substantially effect on monomer diffusion since the radius of a monomer is much smaller than the network size. III.6. Effect of Hydrophobic Monomer. The copolymerization of AA with hydrophobic monomers, ST or TFEA, to form copolymer gels in ethanol was investigated. These hydrophobic monomers may preferentially adsorb on the hydrophobic substrate and eliminate the observed substrate effect. The polymerization rate was adjusted to the comparable reaction rate by changing the initiator concentration. The interface is clearly observed for AA, but it becomes weaker in the presence of 5 mol % ST or TFEA, and disappears when TFEA is increased to 25 mol % (data not shown). Figure 12 shows the corresponding spatial distribution in the final phase changes in the reaction cell. Comparing with the homopolymerization of AA, the heterogeneity substantially decreases in the case of copolymerization containing 5 mol % hydrophobic monomer. In the case of 25 mol % TFEA, no interface formation appears, and polymerization takes place homogeneously. These results again suggest that the substrate-induced interface formation and the heterogeneous polymerization on the hydrophobic surface might be associated with the high interfacial tension between the substrate and the polymerizing solution. The substantial decrease in the interface distance could be related
Polymerization of Hydrogels to the preferential adsorption of hydrophobic monomer onto the Teflon surface. When a hydrogel is polymerized from vinyl monomers in water between a pair of glass substrates, homogeneous polymerization occurs. On the other hand, when the polymerization is carried out between a pair of Teflon substrates, inhomogeneous polymerization occurs at both surface of the Teflon substrate. We have found that no interface appears for the cross-linking reaction from aqueous polymer solutions. For example, when the poly(vinyl alcohol) (PVA) gel was formed by cross-linking reaction from PVA aqueous solution, the gelation occurred homogeneously on the Teflon surface. Thus, the substrate effect on the hydrophobic surface is a specific feature of the polymerization from the hydrophilic vinyl-monomers.
J. Phys. Chem. B, Vol. 105, No. 20, 2001 4571 effect on the inhomogeneous polymerization of a hydrogel might be caused by the high interfacial tension between the hydrophilic polymerizing solution(or gel) and the hydrophobic substrate. Such a macroscopic substrate effect observed in these hydrogels has never been observed in other systems and should be studied further. We propose a possible mechanism of this substrate effect on gelation in the following paper.7 Acknowledgment. This research was supported by Grantin-Aid for the Specially Promoted Research Project “Construction of Biomimetic Moving System Using Polymer Gels” from the Ministry of Education, Science, Culture and Sports, Japan. The authors sincerely thank Mr. T. Ohta of the glassware shop and Mr. Y. Hirata of the machine shop, of the Electronic Research Institute, Hokkaido University for their help in making the special reaction cells for us.
IV. Conclusions When a hydrogel was polymerized from its vinyl monomer in water, an interface appeared during the polymerization to form heterogeneous structures in the vicinity of the hydrophobic substrates. The thickness of the heterogeneous layer increases with the increase of the square root of the polymerization time. This substrate effect was found to have a correlation with the surface tension of the hydrophobic surface. The lower the surface tension, the stronger the substrate effect. On the other hand, the substrate effect was substentially suppressed when the polymerization was carried out in ethanol, which has a much lower surface tension than that of water. Moreover, the substrate effect was completely eliminated when the hydrophilic gel was prepared in ethanol in the presence of a few percent of hydrophobic monomer. These results suggest that the substrate
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