Effect of Surface Roughness of Hydrophobic Substrate on

J. Phys. Chem. B 2002, 106, 3073-3081

3073

Effect of Surface Roughness of Hydrophobic Substrate on Heterogeneous Polymerization of Hydrogels Mao Peng,†,‡ Takayuki Kurokawa,‡ Jian Ping Gong,† Yoshihito Osada,*,‡ and Qiang Zheng‡ DiVision of Biological Science, Graduate School of Science, Hokkaido UniVersity, Sapporo 060-0810, Japan, and Department of Polymer Science & Engineering, Zhejiang UniVersity, Hangzhou 310027, China ReceiVed: July 3, 2001; In Final Form: January 25, 2002

Theoretical analysis predicted that a gas layer is trapped between an aqueous solution and a rough hydrophobic substrate, and that the volume of the gas increases with the hydrophobicity and the surface roughness of the substrate. The heterogeneous structure formation of hydrogel on hydrophobic substrate is explained in terms of the retardation of the radical polymerization by residual oxygen trapped at the hydrophobic surface. The polymerization of 2-acrylamide-2-methyl-1-propanesulfonic acid (AMPS) aqueous solution on Teflon substrate is experimentally studied by using a novel real time laser sheet refraction (RT-LSR) technique, and it is elucidated that residual oxygen in the trapped gas phase obviously retarded the polymerization of AMPS on the Teflon surface and lead to the formation of heterogeneous structure of the gel. The higher the oxygen concentration, the rougher the Teflon surface, the more significant of the substrate effect, which is in agreement with the theoretical prediction.

I. Introduction Hydrogel is a kind of wet, soft, and flexible material consisting of an elastic cross-linked macromolecular network and large amount of water filling in the interstitial space of the network. Intensive studies have revealed the unique advantages of this material for some applications, for example, drug delivery system, soft contact lenses, chemical valves, and biomaterials.1,2 In our preceding papers, it has been reported that the surface of hydrogels prepared by radical polymerization from aqueous vinyl-monomer solution on hydrophobic substrates exhibits quite different properties from that synthesized on hydrophilic surfaces, showing, for example, a much lower surface friction coefficient, a lower elastic modulus,3,4 a larger swelling degree,5 and a weaker interfacial adhesion and interaction with biological cells.6 Another example of the substrate effect is the fact that when making macroporous poly(2-hydroxyethyl methacrylate) hydrogels through polymerization-induced phase separation in NaCl aqueous solution, a dense skin layer with no porous structure is formed if polymerization undergoes in a polystyrene vessel.7 In considering the great importance of the surface properties of hydrogels to various practical applications, especially to biomaterials, it is extremely necessary to reveal the mechanism of this substrate effect and establish the relationship between polymerization conditions and the resultant surface properties. We have developed a noninvasive technique, electronic speckle pattern interferometry (ESPI), to spatially and temporally monitor the total polymerization and gelation process.8 It was found that if the polymerization is carried out between a glass and a hydrophobic plate,8 heterogeneous polymerization occurs on the hydrophobic surface. The substrate effect on the polymerization has been observed in a wide variety of hydrophilic vinyl-monomers, such as 2-acrylamide-2-methyl-1-pro* To whom correspondence should be addressed. † Division of Biological Science. ‡ Department of Polymer Science & Engineering.

panesulfonic acid (AMPS) and its sodium salt, the sodium salt of styrene sulfonate (NaSS), acrylic acid (AA), acrylamide (AAm), and N,N′-dimethyl acrylamide (DMAAm) in water.5 This kind of heterogeneous polymerization has been confirmed on various hydrophobic substrates, such as poly(tetrafluoroethylene) (Teflon), polyethylene (PE), polypropylene (PP), polystyrene (PS), and poly(vinyl chloride) (PVC) but does not occur on hydrophilic substrates, such as glass, sapphire, silicon, and mica.5 A correlation between the substrate effect and the surface tension of the hydrophobic substrate was observed: The lower the surface tension of the substrate, the more substantial of the heterogeneity occurs in the hydrogel.5 Furthermore, when the polymerization is carried out in ethanol, which has a much lower surface tension than that of water, the substrate effect is substantially suppressed. According to these experimental phenomena, a thermodynamic explanation in terms of the high interfacial energy between the hydrophobic substrate and the hydrophilic solution was proposed.9 However, there are some phenomena that contradict with this explanation and the true mechanism of the substrate effect is still obscure. The disadvantage of ESPI technique is that it cannot determine whether the phase change is induced by the increase or decrease of refractive index. This is of no problem for the polymerization in the bulk region, because refractive index increases monotonically with the increase of conversion. But, as to the interface region close to the hydrophobic substrate, the refractive index decreases, as described before, due to migration of monomer from the interface region into the bulk region. This is the reason the evolution of the refractive index in the near-substrate region (0 to 1 mm) was not discussed in our preceding papers.5,8 Due to the experimental difficulty, up to date, we know little about the spatial distribution of concentration in the near-substrate region during polymerization. Recently, a novel real time laser sheet refraction (RT-LSR) technique that can in situ measure the heterogeneity in the refractive index spatially and temporarily, was developed in our laboratory.10 This allowed us to analyze the heterogeneous

10.1021/jp012521u CCC: $22.00 © 2002 American Chemical Society Published on Web 02/26/2002

3074 J. Phys. Chem. B, Vol. 106, No. 12, 2002

Figure 1. Schematic illustration of the wetting behavior of an aqueous solution on a rough hydrophobic substrate with a sinusoid surface morphology y ) Y0[1 + cos(2πx/λ)].

distribution of concentration in the interface region during the whole polymerization process with a spatial resolution of about 50 µm. By using this technique, we first found that the polymerization is substantially suppressed from the beginning of the reaction in the interface region. This phenomenon is obviously in contradiction with our previously proposed thermodynamic explanation, which predicted that the suppression of the polymerization at the interface region occur only after the polymer concentration increased to a critical value. It is known that if a liquid has a large intrinsic contact angle on a solid and the solid surface is sufficiently rough, the liquid may trap air so as to give a composite interface.11,12 Such a composite interface has been observed for water on paraffin wax, fluorocarbon wax, porous polyolefins, and a fluoropolymer.11 Accordingly, in this work, an additional explanation of the substrate effect in terms of retardation of the radical polymerization by the residual oxygen entrapped at the rough hydrophobic surface is proposed. The main content of the explanation is as follows: A very thin gas layer is adsorbed between the ridges of the rough hydrophobic substrate, and residual oxygen in the gas would dissolve into the aqueous solution and obviously influence the polymerization process near on the surface of hydrophobic substrate. In part II, critical conditions for the gas entrapping and the relationships between the volume of the gas and the surface roughness as well as the hydrophobicity of the substrate are theoretically deduced. Formation of heterogeneity in the gel is simulated using the diffusion and polymerization theories. In part III, polymerization of polyAMPS (PAMPS) gels from its aqueous solution on Teflon surface is studied by the RT-LSR technique. The effects of oxygen concentration, the surface roughness, the surface tension of substrate, and the surface tension of monomer solution on the heterogeneous polymerization are investigated and the essential feature of the explanation is discussed. II. Theoretical Consideration 1. Formation of Composite Interface. As is well-known, an aqueous solution does not spread automatically on a hydrophobic surface, but tends to minimize the contact area as far as possible. In addition, if the hydrophobic substrate has surface roughness in an order of micrometer, it shows enhanced water-repellency. Unwettable lotus leaves are explained by this reasoning. Beetles living in the soil can move freely in the soil with low resistance, because their cuticles usually exhibit a great hydrophobicity and a regular roughness pattern, which prevent the adhesion of soil.13 Therefore, rough hydrophobic materials

Peng et al. can potentially be used in those fields requiring good water or soil repellency. Even though the surface of a hydrophobic substrate is completely covered with an aqueous solution, area between ridges on the rough surface may not be filled by the solution, and so a composite interface forms. As a result, there exists a residual gas layer in the solution-substrate interface region. The surface morphology of a solid is of a random geometry in three dimensions of not any special symmetry. Therefore, it is hard to quantitatively calculate the volume of gas trapped on a real rough solid surface. To deduce the condition for the formation of the composite interface, we only consider an idealized wave formed surface. This simple model has been widely used in the study of interface phenomena. It simplifies the mathematics but does not affect the general features of the results.11 In two dimensions, the surface can be represented as a sinusoid with amplitude 2Y0 and wavelength λ:

[

y ) Y0 1 + cos

(2πλx)]

(1)

Figure 1 presents the schematic illustration of the profile of the model surface. The x-axis is defined to be parallel to the apparent solid surface and passes through the bottom of the sinusoid. The y-axis is vertical to the apparent surface and pass through the peak of the sinusoid. The definition of the intrinsic contact angle is the contact angle between the solution and an idea (i.e., rigid, flat, chemically homogeneous, insoluble, and nonreactive) solid surface, which can be predicted by the well-known Young equation:

cosθ )

γGS - γLS γLG

(2)

Here, γGS, γLS, and γLG are the gas-solid, liquid-solid, and liquid-gas interfacial tensions, respectively. As shown in Figure 1, the contact angle between the solution and the solid surface at point A is supposed to be equal to the intrinsic contact angle θ. It has been proved that if the weight of the liquid and the pressure of the gas are neglected, the gas-liquid interface can be treated as planar and the condition to form gas-containing interface is expressed by the relationship between θ and R at point A:11

θ ) 180° - R

(3)

Here, R is defined as the angle between tangential force (γGS and γLS) and the x-axis. Because γGS and γLS are tangential to the surface of substrate at point A, it can be found that

2πY0 2π dy sin xA tan R ) - |A ) dx λ λ

(4)

By definition, θ is constant for a given solid-liquid-gas system. Therefore, from eqs 1-4, it can be found that the position of point A is the function of θ and the geometry of the surface (Y0 and λ). By submitting eq 3 into eq 4, one can obtain the x-coordinate of point A as

xA )

(

λ -1 λ tan θ sin 2π 2πY0

)

(5)

To understand the influence of residual gas (air) to the polymerization, it is necessary to discuss the relationship between the volume of residual gas and the surface hydropho-

Heterogeneous Polymerization of Hydrogels

J. Phys. Chem. B, Vol. 106, No. 12, 2002 3075

Figure 2. Relationships between volume of residual gas, η, intrinsic contact angle, θ, and surface roughness, F, as calculated by supposing a model surface with a sinusoid surface morphology as shown in Figure 1. λ ) 1µm was used in the calculation.

bicity and roughness. Although the volume of gas calculated according to the above model is not qualitatively comparable to that on a real solid surface, the general feature of the dependence of gas volume on the surface hydrophobicity and roughness should be comparable. If η denotes the volume of residual gas on per unit shadow area of the surface, it can be obtained from

x dx ∫xλ/2{yA - Y0[1 + cos(2π λ )]}

2 λ

η)

(6)

A

Here, xA and yA are the coordinates of point A. Submitting eq 1 into eq 6, one can obtain the solution of eq 6 as

[ ( ) ( ) ( )]

η ) 2Y0

1 2π 1 xA 2π sin xA + cos xA 2π λ 2 λ λ

(7)

Let F denotes the surface roughness, which is described by the ratio between the real area of the rough surface and its shadow area, so

F)

∫0λ

1 λ

x [ 1+

( )] dx

2πY0 2π sin x λ λ

2

(8)

From eqd 5 and 7, it can be found that η is the function of θ, Y0, and λ: η ) f(θ,Y0,λ). So η is determined only by two factors: the surface hydrophobicity and the roughness. To better understand the relationship between η and the two factors, let us relate η directly with θ and F, thus we get η as the function of θ and F: η ) f(θ,F). To further simplify the analysis, λ, the wavenumber of the surface roughness is supposed to be 1 µm in the calculation. Therefore, the surface roughness is increased only due to the increase of amplitude of surface fluctuation. By combining eqs 5, 7, and 8, we obtain the relationships between η, θ, and F, as shown in Figure 2. η increases with the increase of θ, which indicates that it is easier to form gas-containing interface on a substrate with a stronger hydrophobicity. On the other hand, the fact that η also increases with the increase of F suggests that the substrate with a stronger roughness forms gas-containing interface more easily. This result proves that both hydrophobicity and surface roughness are necessary to form a gas-containing interface. It should be noted that there exists a critical intrinsic contact angle θc in the plots of η versus θ. If the intrinsic contact angle between the solution and the substrate is smaller than the critical

Figure 3. Relationship between critical contact angle θc and the surface roughness F (a), and between critical surface roughness Fc and contact angle θ (b).

value, the area between ridges would be fully filled by the aqueous solution and gas-containing interface does not form. There may exist a great deviation between the theoretical values of θc and the practical values on a real solid surface, but it at least presents a good qualitative explanation of the experimental results that no substrate effect can be observed on hydrophilic substrates as mentioned in the Introduction section. Furthermore, the critical value strongly depends on the surface roughness. Figure 3a presents the relationship between critical contact angle and the surface roughness F, showing that when F decreases, the θc increases, indicating that rough surface is more beneficial to the formation of gas-containing interface, which is in consistence with the result mentioned above. On the other hand, it can also be found from Figure 2 that, to a given θ, surface roughness should be larger than a critical value (Fc) for the formation of gas-containing interface. As is shown in Figure 3b, when θ decreases, Fc increases. For a less hydrophobic substrate, stronger roughness is necessary for the formation of gas-trapping interface. As a result, we can conclude that strong hydrophobicity and surface roughness are necessary to obtain gas-containing interface. Though the above discussion is based on a model surface with a periodical roughness and the actual solid surface exhibits random roughness, it qualitatively helps us to better understand the conditions that are necessary for the formation of gascontaining interface as well as to establish the relationship between the volume of entrapped gas and the surface features. 2. Influence of Oxygen to Free Radical Polymerization Residual oxygen in the gas entrapped on the hydrophobic rough surface could obviously influence the polymerization and hydrogel formation process. Free radicals are extremely reactive


Peng et al.

toward oxygen and there are many examples for the addition reaction

R + O2 f R-O-O Usually, the peroxidic radical generated in this way is relatively inactive at room temperature to reinitiate the polymerization, so oxygen acts as a retarder. Low molecular weight copolymer containing peroxidic groups can be obtained for some monomers.14,15 Accordingly, if there exists enough oxygen in the hydrophobic substrate-solution interface, the polymerization of hydrogels may be retarded in the surface region, which in turn leads to the monomer diffusion to the neighboring region where polymerization occurs normally. Therefore, the inhomogeneous polymerization kinetics can be described as a diffusion-combined reaction:

∂Cm ) -kmCm + Dm∇2Cm ∂t

(9)

where Cm is the monomer concentration. Here, the kinetics of polymerization follows the first-order reaction with a reaction velocity constant km,10 and the diffusion of monomer follows the Fick’s law with a diffusion coefficient Dm. We suppose that the polymerization is carried out between the hydrophobic substrate on which the polymerization is retarded in a layer of thickness h, and a hydrophilic substrate on which the polymerization occurs normally and homogeneously at the rate the same with the bulk region. The distance of the two surfaces is H. Supposing that h is much larger than the amplitude of surface fluctuation 2Y0sthat is, h . 2Y0swe can neglect the monomer variation along the x direction in Figure 1 and eq 9 reduces to a one-dimensional problem: 2

∂ Cm ∂Cm ) - kmCm + Dm 2 ∂t ∂y

(10)

The polymer concentration Cp(y,t) should satisfy

∂2Cp ∂Cp ) kmCm + Dp 2 ∂t ∂y

(11)

Here Dp is the diffusion coefficient of polymer in the sample solution. For simplicity, we further suppose that the polymerization in the h layer is completely prohibited, then

km ) 0 for y∈ [0, h]

(interface region)

km ) constant for y∈ [h,H]

Figure 4. Simulation result of spatial distribution of refractive index at early and middle stage of polymerization (a), and at late stage of polymerization (b).

by the solution compositions as follows:10

ns ) nw +

dnm dnp Cm + C dCm dCp p

(12)

where dnm/dCm is the refractive index variation with monomer concentration and dnp/dCp is that with polymer concentration. nw is the refractive index of water. It can be seen that at early and intermediate stages of the polymerization, the refractive index in interface region decreases obviously. But the refractive index in the neighboring region increases faster than that in the bulk region. This results from the diffusion of monomer from the interface region into the bulk region during polymerization. At a late stage of polymerization, the spatial heterogeneity of the refractive index obviously decreases because the polymer diffuses into the dilute interface region. Thus, the simulation result indicates that when polymerization near a hydrophobic surface is suppressed, heterogeneity is induced as a result of diffusion-coupled polymerization.

(bulk region).

Because there is no diffusion on the solid substrates, the boundary condition can be expressed as

∂Cm,p | )0 ∂y y)0,H Using the parameters km ) 5 × 10-4 s-1 for the bulk region, Cm(y,0) ) 1 mol/L-1 and Cp(y,0) ) 0 mol/L-1 for 0 < y < H; h ) 0.1 cm; H ) 1 cm; Dm ) 1 × 10-5 cm2/s and Dp )1 × 10-6 cm2/s, eqs 10 and 11 are solved numerically. Figure 4 shows the refractive index distribution ns of the sample solution in the reaction cell at various reaction times. ns is determined

III. Experimental Results and Discussions III.1. Experimental Results. Surface Morphology of Substrates.The surface profiles and roughness of substrates was characterized using a tapping mode AFM (Nano Scope III, Digital Instruments). The as-purchased Teflon surface is used. To compare the influence of different surface roughness of Teflon, one Teflon substrate was ground using grinding disk and so its surface is rougher. Before experiments, both Teflon substrates were washed carefully by using 0.5 M hydrochloric acid and detergent and then rinsed with large amount of deionized water. The scanning range is 20 × 20 µm2. Surface roughness of Teflon, PP, PE, PS, PVC, and glass were also characterized by AFM in a scanning range 100 ×



Figure 6. Surface profiles of untreated Teflon (dash line) and ground Teflon (solid line) as measured by AFM.

Figure 5. Experimental setup for in-situ monitoring of polymerization of hydrogels by the real time laser sheet refraction technique.

100 µm2. Rq, the root-mean-square average of height deviations taken from the mean data plane, and Ra, the arithmetic average of the absolute values of the surface height deviations measured from the mean plane, were estimated. Each value was the average of the independent samples. Rq and Ra are defined by

Rq )

x

N

f(xi)2/N ∑ i)1

and N

Ra )

|f(xi)|/N ∑ i)1

respectively, where f(xi) is the surface height at point xi.16 Polymerization. All chemical reagents and monomers used were purified by the same methods as described in previous works.5 AMPS (1 mol L-1) as the monomer, 5 mol % N, N′methylenebisacrylamide (MBAA) as the cross-linker, and 0.74 mol % 2-oxoglutaric acid as the UV initiator were dissolved in deionized water at room temperature. Before polymerization, the solution was purged by nitrogen with the purity of 99.99% for 30 min in a dark room in order to get rid of the oxygen resolved in the solution. The solution was then poured into the cell in the ambient conditions. Because the sample cell was not purged by nitrogen, the residual gas in the interface region should be air. Some visible small bubbles absorbed on the Teflon surface were driven away before the experiment. The polymerization was then conducted under irradiation of UV light with wavelength around 365 nm. The distance between the UV lamp (ENF-260C/J, Spectronics Co.) and the sample cell is 15 cm and the illumination intensity on the cell surface is about 0.35 mW/cm2. The polymerization is carried out at 20 °C. Before experiments, Teflon in the sample cell is washed in the same way as that used in the AFM experiment. Real Time Laser Sheet Refraction. Figure 5 presents the experimental setup for the in-situ monitoring of the polymerization process by the real time laser sheet refraction technique. A He-Ne laser (Model 127, Spectra-Physics Lasers, Inc.) is used as the light source. The laser beam is first focused through a pinhole spatial filter by a 25 × microscope objective, then

passes through a serial of collimating lenses and is enlarged into a parallel light column with the diameter of 50 mm. The light column is then filtered by a slit with the width of 1 mm and height of 50 mm. The stripe-shaped laser beam transverses the sample cell and is refracted by the solution, then appears on the screen. A triangular sample cell is used in the experiment as is shown in Figure 5. The direction of the laser is deflected when the laser beam passing through because the sample solution in the cell acts as a prism. The refracted light then impinges onto a vertical screen placed at the left side of the cell. The screen is perpendicular to the incident laser beam. During polymerization, the refractive index of the solution in the cell increases, the deflection angle between the refracted beam and the incident beam increases. Therefore, the considerable displacement of the refracted light can be observed on the screen. The refracted laser beam on the screen is recorded with a cooled CCD camera (C4742-95, Hamamatsu Co., Japan), which is electronically interfaced to a personal computer. To investigate the influence of hydrophobic template, the Teflon substrate is placed on the bottom of the cell. The detailed principle and apparatus of this RT-LSR technique is available in the proceeding paper.10 III.2. Results and Discussions. 1. Surface Morphology of Teflon. Figure 6 presents the surface profiles of the as-purchased Teflon and ground Teflon measured by the AFM. It can be found that the surface of the as-purchased Teflon has spatial fluctuation with maximum amplitude of about 200 nm and wavelength of about 8µm. The surface of ground Teflon exhibits larger roughness with maximum amplitude of about 400 nm. As shown in Figure 6, the zero point of the y-axis is at the average height of the profile curves. The Ra values for the aspurchased Teflon and ground Teflon are 32.54 and 66.89 nm, respectively. In considering the low surface tension of Teflon (23.9mN/m), the two Teflon surfaces are rough enough for the formation of composite interface, and the ground Teflon surface is able to trap more residual gas than the untreated one. 2. Polymerization on Air-Containing Teflon. To introduce more oxygen into the interface region, the monomer solution was purged not in the sample cell but in another vessel, and then poured into the sample cell in the ambient conditions. Because the sample cell was not purged by nitrogen, the residual gas in the interface region has the same composition with air, although the air in the monomer solution has been driven away. Figure 7 presents the typical series pattern of refracted light on the screen at various moments during polymerization of AMPS. The x- and y-axes in Figure 7 are the coordinate axis of the screen. The zero point of the y-axis corresponds to the surface of Teflon substrate. It can be easily found that the refracted


Figure 7. Patterns of refracted light on the screen at various moments during the polymerization of AMPS solution on the air-containing surface of untreated Teflon.

light on the screen move leftward during polymerization, which is obviously induced by the increase of refractive index of the solution in the sample cell due to polymerization. From Figure 7, it is also noted that in the interface region adjacent to the Teflon surface, the position and shape of refracted light is quite different from that in the bulk region. This remarkable heterogeneity obviously results from the different refractive index distribution in the interface region in the sample solution. According to the evolution of refracted light pattern on the screen, the distribution of refractive index in the solution during polymerization can be obtained, as is shown in Figure 8. The solution can be divided into three regions. In region I that has a thickness of about 2 mm from the Teflon, the refractive index decreases with the reaction time. On the other hand, the refractive index increases rapidly with the time in region II, and this gives rise to a sharp interface between regions I and II. The refractive index in the bulk region III modestly increases with time and shows no position dependence, indicating that polymerization in this region is homogeneous and not affected by the substrate effect. Figure 9 presents the development of the refractive index during polymerization at various positions from the Teflon surface. One can see that the refractive index in the nearsubstrate region decreases from the beginning of the polymerization, which increases gradually after the polymerization in the bulk region reach to the late stage. These phenomena suggest that the polymerization in the interface region is retarded from the beginning of polymerization in the presence of air.

Peng et al.

Figure 8. Distribution of refractive index at various moments during the polymerization obtained from Figure 7.

As mentioned above, oxygen can react with monomer radicals. Although the concentration of growing radical in the aqueous solution and that of oxygen in the residual air in the interface region is quite small, they can recombine quickly due to their large reactivity to retard the polymerization in the interface region until the oxygen in the interface region is entirely consumed by free radicals in the solution. Therefore, the decrease in the refractive index in this region is mainly due to the migration of monomers toward region II where monomer has been consumed rapidly. The rapid increase in the refractive index in region II should be associated to two effects: one is the continuous migration of monomer and initiator from region I due to low concentration in this region; the other is the progress of polymerization with time. The formation of the sharp interface at the boundary of the two regions might be attributed to the accelerated effect of these two effects. At the later stage of the polymerization, refractive index in the near-substrate region gradually increases. The reason may be partly due to consumption of oxygen that increases the polymerization and partly due to the swelling of the formed gel into the dilute solution region near the interface. In Figure 9b, we can find a decrease in the refractive index in region II at late stage of polymerization, which suggests the gel swelling. The result of theoretical simulation in Figure 4 satisfactorily agrees with the experimental result, which explains the substrate effect in terms of diffusion-induced heterogeneous polymerization in hydrogels.



Figure 9. Time dependence of the refractive index during the polymerization in (a) the interface region and (b) the neighboring and bulk region.

Figure 11. Patterns of refracted light at various moments during the polymerization of AMPS solution on the air-containing surface of ground Teflon.

Figure 10. Time dependence of width of interface region for different concentration of UV initiator.

In the ESPI method, the refractive index is obtained basing on the assumption that the phase change is caused by the increase of the refractive index. From the above discussion, it can be found that this assumption cannot be applied to the near Teflon region (region I) and later stage of the neighboring region (region II). Therefore, the RT-LSR technique used in the present work is more suitable to study the heterogeneous polymerization of hydrogels. To investigate the influence of polymerization kinetics on the substrate effect, different amounts of UV initiator were used in the polymerization. Figure 10 presents the time dependence of the width of the interface region for different concentration of UV initiator for AMPS polymerization on air-containing Teflon. It can be found that the width of interface region increases with the decrease of the concentration of UV initiator. This phenomenon is understandable. Oxygen reacts with free radicals, therefore, when the concentration of initiator is smaller, the spatial range that can be influenced by oxygen is larger. On the other hand, the width of the interface region is also related to the diffusion of oxygen and monomer. When the concentration of UV initiator in the monomer solution is small,

the polymerization rate is slow; therefore, oxygen and monomer have enough time to diffuse into the farther space, bringing about a more obvious substrate effect. 3. Effect of Surface Roughness. To investigate the influence of the surface roughness, the same experiment was carried out using the ground Teflon that has an increased surface roughness. Figure 11 presents the evolution of refracted light. It can be seen that the substrate effect is much enhanced, because the width of the interface region where polymerization is retarded is much larger than the case of untreated Teflon. Furthermore, interestingly, two interfaces appear during polymerization. The first interface appears at 6.10 mm from the Teflon surface, which is much larger than that of the untreated Teflon. After some time, the second interface appears at the 3.50 mm from the Teflon surface. In the region sandwiched by these two interfaces, the polymerization occurs very fast and the refractive index increases rapidly. At the late stage of the polymerization, the first interface disappears entirely. The enhanced substrate effect observed here is in good agreement with our theoretical prediction. The formation of double interfaces might be related to the competitive diffusion and consumption of oxygen. Since the volume of residual oxygen on the ground Teflon is larger, oxygen can diffuse into the region far from Teflon surface at the beginning of polymerization to give an interface wider than in the case of untreated Teflon. However, oxygen is consumed rapidly by reacting with free radicals, and oxygen in the surface


Peng et al. TABLE 1: Surface Tension and Surface Roughness of Various Substrates Teflon PP PE PS* PVC Glass

Figure 12. Patterns of refracted light at various moments during the polymerization of AMPS solution on untreated Teflon purged with high purity nitrogen. Please take notice on the difference in the scale bar from that of Figure 7.

reservoir has no enough time to diffuse into the region far from Teflon surface before the oxygen there is consumed completely. Therefore, polymerization in the region far from Teflon substrate is not retarded by oxygen any longer, and a new interface appears at a position with a closer distance to the Teflon surface. 4. Effect of Highly Purified Nitrogen. To understand how much oxygen can lead to the substrate effect, it is necessary to get rid of oxygen in the interface region as far as possible. In this experiment, AMPS monomer solution is purged by twostep with large amount of high purity nitrogen (99.999%) as follows: The sample solution was first prepared and purged in a vessel, and then poured into the sample cell under protection of nitrogen. The sample cell was also purged before this operation. Then, the sample solution is further purged in situ in the sample cell by using 99.999% nitrogen. After the twostep purge, the residual gas in the interface region should be substituted by 99.999% nitrogen. Figure 12 presents the evolution of refracted light on the screen during the polymerization of sample solution on purged

γs/mN‚m-1

Rq/nm

Ra/nm

23.9 29.4 36.8 40.7 41.9 286.7

144.5 99.6 74.0 24.5 48.2 16.1

101.3 69.9 48.8 14.7 34.3 12.1

Teflon. It can be found that heterogeneity near the Teflon surface is much weaker comparing with the experiment on the surface of Teflon containing air. Therefore, the residual oxygen trapped at the hydrophobic surface plays very important role in the formation of heterogeneous structure in hydrogels. The above fact shows that the oxygen-induced substrate effect could not be completely eliminated even by purge the reaction system with 99.999% nitrogen. Because of the difficulty of further purification of nitrogen and the sample cell, it is hard to conclude whether the presence of oxygen is the sole reason for the substrate effect. However, from above experimental results, it can be concluded that when there is enough oxygen or air on the substrate surface, polymerization in the interface region can be retarded obviously. 5. Effect of Interface Tension. In our proceeding studies on the substrate effect,5,8,9 the monomer solution was placed into the samples cell and then was carefully purged by nitrogen with a purity about 99.99% for a long time, so it is reasonable to believe that the composition of the residual gas on the hydrophobic surface is the same as that of the nitrogen used for purging. Accordingly, the residual oxygen should retard the polymerization in a same mechanism. Many factors that determine the substrate effect have been clarified in the previous studies, and they can be summarized as follows:5,9 (1) The heterogeneity of the gel shows good dependence on the surface tension of the substrates. With the decrease of surface tension of substrates, namely, the increase of hydrophobicity, the heterogeneity becomes more obvious. (2) When salt is added into the sample solution, the heterogeneity becomes more obvious. (3) When organic solvent with a surface tension as low as the hydrophobic substrate, such as ethanol, is used as solvent instead of water, the heterogeneity is suppressed obviously. (4) When a hydrophobic monomer, such as 2,2,2-trifluoroethyl acrylate (TFEA), is added into the ethanol solution of acrylic acid (AA) to form copolymer hydrogels, the substrate effect was obviously suppressed and even completely disappeared. These phenomena can be explained as the results of different amount of residual oxygen in the interface region caused by different interface tension and the surface roughness. As shown in Table 1, except PS, the surface roughness of the substrates increases with the decrease in the surface tension5,17 (hydrophobicity), which may be mainly determined by the production process and the partial crystallization behavior of these plastics. Therefore, the volume of entrapped gas increases with the decrease in the surface tension of the substrates, which brings about more significant substrate effect. Less hydrophobic substrate, low surface tension organic solvent or hydrophobic monomers decreases the volume of residual gas and suppresses the substrate effect. When salt is added into the monomer solution, the surface tension of monomer solution is increased, which consequently increases the volume of residual gas to bring about enhanced substrate effect.

Heterogeneous Polymerization of Hydrogels IV. Conclusion Gas would be entrapped at the interface between an aqueous solution and a rough hydrophobic solid surface. Therefore, the radical polymerization on a hydrophobic substrate to form a hydrogel would be retarded by the residual oxygen in the entrapped gas, which gives rise to a heterogeneous structure of gel. This oxygen-induced substrate effect could not be completely eliminated even by purge the reaction system with 99.999% nitrogen. Accordingly, from a practical viewpoint, this substrate effect always exists for hydrogels synthesized by radical polymerization from an aqueous solution. Since the substrate effect increases with the increase in the amount of oxygen trapped on the hydrophobic surface, this supplies a potential way to control the surface properties of hydrogels by simply change the hydrophobicity or the surface roughness of the substrate. 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. References and Notes (1) Osada, Y.; Okuzaki, H.; Gong, J. P. Trends Polym. Sci. 1994, 2, 2. (2) Gong, J. P.; Osada, Y. In Electrical and Optical Polymer Systems; Wise, D. L., et al., Eds.; Marcel Dekker: New York, 1998.

J. Phys. Chem. B, Vol. 106, No. 12, 2002 3081 (3) Gong, J. P.; Kurokawa, T.; Narita, T.; Kagata, G.; Osada, Y.; Nishimura G.; Kinjo, M. J. Am. Chem. Soc. 2001, 123, 5582. (4) Narita, T.; Knaebel, A.; Munch, J.-P.; Candau, S. J.; Gong, J. P.; Osada, Y. Macromolecules 2001, 34, 5725. (5) Kii, A.; Xu, J.; Gong, J. P.; Osada, Y.; Zhang, X. M. J. Phys. Chem. B 2001, 105, 4565. (6) Narita T.; Hirai, A.; Xu, J.; Gong, J. P.; Osada, Y. Biomacromolecules 2000, 1, 162. (7) Liu, Q.; Hedberg, E. L.; Liu, Z.; Bahulekar, R.; Meszlenyi, R. K.; Mikos, A. G. Biomaterials 2000, 12, 2163. (8) Zhang, X. M.; Xu, J.; Okawa, K.; Katsuyama, Y.; Gong, J. P.; Osada, Y.; Chen, K. S. J. Phys. Chem. B 1999, 103, 2888. (9) Gong, J. P.; Kii, A.; Xu, J.; Hatori Y.; Osada Y. J. Phys. Chem. B 2001, 105, 4572. (10) Peng, M.; Gong, J. P.; Osada, Y.; Zhang, X. M.; Zheng, Q. Macromolecules 2001, 34, 7829. (11) Wu, S. H. Polymer Interface and Adhesion; Marcel Dekker: New York, 1982; pp 16-26. (12) Adamson, A. W. Physical Chemistry of Surfaces, 5th ed.; John Wiley & Sons: New York, 1990; pp 387-389. (13) Jia, X.; Ren, L. Q.; Chen, B. C. Chin. J. Mater. Res. 1996, 10, 556. (14) Flory, P. L. Principle of Polymer Chemistry; Cornell University Press: Ithaca, NY, 1953. (15) Bevington, J. C. Radical Polymerization; Academic Press: London, 1961; p 166. (16) Japan Society for the Promotion of Science, 131st Committee, Ed. Thin Film Handbook; Japan Society for the Promotion of Science: Tokyo, 1983. (17) Brandrup, B.; Immergut, E. H. Polymer Handbook, 3rd ed.; John Wiley & Sons: New York, 1989.

Effect of Surface Roughness of Hydrophobic Substrate on

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