Langmuir 2001, 17, 2965-2972
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Surface Characterization of Hydrosilylated Polypropylene: Contact Angle Measurement and Atomic Force Microscopy J. Long and P. Chen* Department of Chemical Engineering, University of Waterloo, 200 University Avenue, Waterloo, Ontario, Canada N2L 3G1 Received November 6, 2000. In Final Form: February 7, 2001
In this study, axisymmetric drop shape analysis-profile (ADSA-P) and atomic force microscopy (AFM) were employed to study the surface features of polypropylene (PP) and hydrosilylated polypropylene (SPP). Static and dynamic contact angles were measured using ADSA-P. Water permeability was calculated from the results of static contact angle measurements. To our knowledge, this is the first attempt to obtain permeability using ADSA-P. The water permeability and wettability of PP are greater than those of SPP. Surface free energy was calculated by the equation of state theory and other methods, and the influence of surface roughness on surface free energy was taken into consideration and incorporated into the calculation. Topographic images of the sample surfaces were obtained using AFM. Compared to PP, the SPP surface shows larger but smoother peaks. Calculated results show that SPP has a lower surface free energy than PP. The surfaces of both PP and SPP can be modeled as a homogeneous but rough surface.
1. Introduction It is well-known that polypropylene (PP) is a material widely used in many fields of industry. For many applications, its surface properties have a profound influence. The modification of its surface is therefore an area of tremendous scientific and commercial interest. The surface modification of PP can be accomplished by various methods1-8 ranging from wet chemical processes to dry physical processes, such as flame treatment, corona discharge, or low-pressure plasma. These modifications introduce polar functional groups into surfaces to improve their wetting and adhesive properties. However, in some applications such as paper release mechanisms, surfaces of PP need to be modified to reduce the wettability and adhesion. Such a modification can be achieved by hydrosilylation of terminal double bonds in PP through reactive processing.9,10 A better understanding of the effects of such a modification on the surface properties of PP is required before the technique can be applied commercially, but a study for this purpose has not yet been done. Contact angle measurement has been a major experimental approach to many problems concerning solidliquid interfaces. There are three main techniques for measuring the contact angle on flat solid surfaces. These * Corresponding author. E-mail:
[email protected]. Fax: 519-746-4979. (1) Harth, K.; Hibst, H. Surf. Coat. Technol. 1993, 59, 350. (2) Nie, H.-Y.; Walzak, M. J.; Berno, B.; McIntyre, N. S. Appl. Surf. Sci. 1999, 144-145, 627. (3) Scho¨nherr, H.; Hruska, Z.; Vancso, G. J. Macromolecules 1998, 31, 3679. (4) Vallon, S.; Brenot, R.; Hofrichter, A.; Dre´villon, B.; Gheorghiu, A.; She´maud, C.; Klemberg-Sapieha, J. E.; Martinu, L.; Poncin-Epaillard, F. J. Adhes. Sci. Technol. 1996, 10 (12), 1313. (5) Mahlberg, R.; Niemi, H. E.-M.; Denes, F. S.; Rowell, R. M. Langmuir 1999, 15, 2985. (6) Nihlstrand, A.; Hjertberg, T.; Schreiber, H. P.; Klemberg-Sapieha, J. E. J. Adhes. Sci. Technol. 1996, 10 (7), 651. (7) Good, R. J.; Shu, L. K.; Chiu, H.-C.; Yeung, C. K. J. Adhes. 1996, 59, 25. (8) Poncin-Epaillard, F.; Chang, Y.-I. Langmuir 2000, 16, 1450. (9) Shearer, G.; Tzoganakis, C. J. Appl. Polym. Sci. 1997, 65, 439. (10) Malz, H.; Tzoganakis, C. Polym. Eng. Sci. 1998, 38 (12), 1976.
are the sessile drop (or captive bubble in liquid), the Wilhelmy plate, and the inclined plane methods. A detailed discussion about the three methods can be found elsewhere.11 In this study, we chose one of the sessile drop methods, axisymmetric drop shape analysis-profile (ADSAP), which is a novel technique to determine liquid-fluid interfacial tensions and contact angles from the shape of axisymmetric menisci (i.e., from sessile as well as pendant drops).12 Its basic principle is to fit the experimental drop profile to a theoretical one given by the Laplace equation of capillarity, and the surface tension is generated as a fitting parameter. Other parameters, such as contact angle, drop volume, surface area, and three-phase contact radius, can also be obtained. Details of the methodology and experimental setup can be found elsewhere.13,14 A more recent development of the ADSA-P application is also available.15 As discussed above, the ADSA-P technique can obtain the volume of a liquid drop sitting on a solid surface. Thus, the liquid amount penetrating into or through the solid sample and further the permeability coefficient of the sample can be obtained from the volume change of the drop with time. This provides a means to measure the permeability of polymeric materials. Permeability is very important in many applications, such as the control of the rate of delivery of a drug, the rate of release of an active agent in water in agricultural applications, and the decrease in transport of liquid and plasticizer in plasticized poly(vinyl chloride) in contact with various liquids. In the present study, modified polypropylene (SPP) is used to make stickers that are required to release easily and to (11) Erbil, H. Y.; McHale, G.; Rowan, S. W.; Newton, M. I. Langmuir 1999, 15, 7378. (12) Rotenberg, Y.; Boruvka, L.; Neumann, A. W. J. Colloid Interface Sci. 1983, 93, 169. (13) Lahooti, S.; del Rı´o, O. I.; Cheng, P.; Neumann, A. W. In Applied Surface Thermodynamics; Neumann, A. W., Spelt, J. K., Eds; Marcel Dekker: New York, 1966; Vol. 63, p 441. (14) del Rı´o, O. L.; Neumann, A. W. J. Colloid Interface Sci. 1997, 196, 136. (15) Long, J.; Chen, P. 74th Colloid and Surface Science Symposium; Bethlehem: PA, 2000.
10.1021/la001547u CCC: $20.00 © 2001 American Chemical Society Published on Web 04/10/2001
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Table 1. Purity, Density, and Surface Tension of the Liquids Useda liquid
purity (%)
density (g/cm3)
γlv (mJ/m2)
water glycerol DMSO DMFM
ultrafiltered 99.8 97.0 99.9
0.977 1.258 1.101 0.944
72.70 ( 0.09 65.02 ( 0.04 42.68 ( 0.03 36.65 ( 0.03
a The values of surface tensions were measured in this study using ADSA-P. The properties of purity and density are the values provided by the producers of these liquids.
last for a long time. Because the polymers are permeable, water will diffuse into adhesive joints. Reducing the permeability will be necessary. Many methods are available to measure permeability.16-18 Usually, these methods require a closed system and are costly and applicable only to membranes with high permeability. In this study, the samples are solid polypropylene (modified and unmodified) with a thickness of more than 1 mm. The permeability of these samples is rather low. It is not easy to measure the permeability using conventional methods in an open system. We, therefore, made the first attempt to use the ADSA-P technique to measure the permeability of the polymers. The experiments were carried out in an open system (real environment), and thus the results are more significant to applications. In recent years, another powerful tool, atomic force microcopy (AFM), was widely employed to characterize solid surfaces in both micrometer and nanometer size regimes. It has been used to study the surface properties of PP/modified PP by several investigators.2,3,5 A combination of contact angle measurement with atomic force microscopy will certainly help to understand the surface properties much better as macroscopic observations are correlated with microscopic structure. In this paper, we present the first systematic study using both ADSA-P and AFM to quantify the surface characteristics and permeability of SPP. The goal of this study is to obtain new insights into the effects of surface modification on surface characteristics, such as surface wettability, permeability, free energy, roughness, and structure, and then to provide guidelines for the modification process and the final product application. 2. Experimental Section 2.1. Materials. Polypropylenes and modified polypropylenes were provided by Dr. C. Tzoganakis’ lab in the Department of Chemical Engineering, University of Waterloo, Ontario, Canada. The modification was achieved by hydrosilylation of terminal double bonds in PP through reactive processing. Details about the process and method of the modification can be found in refs 9 and 10. The samples were strip-shaped with a width of around 20 mm and a thickness of around 1 mm and were further cut into rectangular/square shapes for use in experiments. The four liquids used were water, glycerol, dimethyl-sulfoxide (DMSO), and N,N-dimethyl-formamide (DMFM). Some physical properties of the four liquids are listed in Table 1. 2.2. Surface Preparation. The sample surfaces for contact angle measurements and AFM imaging needed to be carefully prepared to eliminate contamination. In our experiments, the sample surfaces were first rinsed several times using alcohol and double-distilled water, and then the samples were fixed in a sample stand and immerged into double-distilled water for (16) Comyn, J.; Polymer Permeability, 1st ed.; Elsevier Science Publishing: New York, 1985; Chapter 1. (17) Vergnaud, J. M. Liquid Transport Processes in Polymeric Materials, 1st ed.; Prentice Hall: Englewood Cliffs, NJ, 1991. (18) Vieth, Wolf R. Diffusion In and Through Polymers, 1st ed.; Hanser Publisher: New York, 1991.
ultrasonic cleaning. The cleaned samples were dried and placed into a container with a cover. 2.3. Contact Angle Measurements Using ADSA-P. 2.3.1. Static Contact Angles and Permeability. To obtain the water permeability of a sample, we needed to know the amount of water penetrating through the sample within a certain period of time. Static sessile drop experiments of ADSA-P could be employed. The experimental procedure was the following. A sessile drop of water was manually deposited onto the surface of sample. The drop radius usually was larger than 3 mm. To get rid of the influence of evaporation, the sample was placed into a sealed chamber saturated with water. A sequence of images of the drop was then recorded and analyzed by the ADSA-P program. As the penetration was slow, a static sessile drop experiment usually lasted for more than 10 hours. The penetrating amount and contact area were calculated from the results of drop volume and three-phase contact radius, respectively. All the experiments of this study were carried out at room temperature (21.3 ( 0.5 °C). For static contact angle measurements, each individual experiment was repeated three times and the results in this paper were the average values. 2.3.2. Advancing and Receding Contact Angle. Sessile drop contact angle measurements using ADSA-P could be performed as a function of time. An initial drop with a radius larger than 3 mm was deposited onto the sample surface to ensure that the drop was axisymmetric. By use of a motor-driven syringe to pump liquid steadily into the sessile drop from below the surface,19 a sequence of images of the growing drop were then captured; the advancing contact angles were obtained. Subsequently, by withdrawing liquid from the drop, we obtained the receding contact angles. The advancing/receding rate used in this study was 0.5-5 motor steps/s (1 step is equivalent to 2 micros that the motor piston moves, or 0.083708 mm3 liquid/s). The corresponding velocities of the three-contact lines ranged from 0.5 to 2.5 mm/min. All advancing and receding contact angle measurements were repeated two or three times in this study, and the results were averaged. 2.4. Atomic Force Microscopy. The contact mode of a commercial AFM (Digital Instruments, Santa Barbara, CA) was employed in this study. Triangular silicon nitride cantilevers with silicon nitride tips (Digital Instruments) were used to obtain topographic images of sample surfaces. The cantilevers selected were 200 µm in width and had a spring constant of 0.12 nN/m. The scanning scales carried out in our experiments ranged from 100 nm to 15 µm. The statistics of surface roughness were derived from ASME B46.1 (Surface Texture: Surface Roughness, Waviness and Lay) available from the American Society of Mechanical Engineers. The definition of surface roughness used in this study, Ra, is the arithmetic average of the absolute values of the surface height deviations measured from the mean plane.20
Ra )
1
N
∑|Z | N j
(1)
j)1
where N is the number of points and Zj is the height of the jth point. For each scanning scale, five experiments were carried out at different locations of the sample surface, and the result of mean roughness is the average value of these five experiments.
3. Results and Discussion 3.1. Static Contact Angles. Figures 1 and 2 show the results of static contact angles for PP and SPP, respectively. The results are summarized in Table 2. From Figures 1 and 2, one can find the following features: (1) For both samples, the contact angles and the volumes of sessile drops linearly decrease with time, and the radii of three-phase contact lines remain nearly constant. (2) The initial contact angle of SPP, around 102°, is obviously (19) Kwok, D. Y.; Lin, R.; Neumann, A. W. Colloids Surf., A 1996, 116, 63. (20) NanoScope Command Reference Manual, version 4.42; Digital Instruments.
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Figure 3. Driving force for the calculation of permeability. Table 2. Results of Static Contact Angle Experiments SPP initial static contact angle (deg) decreasing rate of static contact angle (deg/h) decreasing rate of drop volume (cm3/h) permeability (m2/(Pa S))
PP
102 ( 1 0.100
84 ( 2 2.364
5 × 10-5
6 × 10-4
7.330 × 10-15
7.644 × 10-14
3.2. Permeability. To quantify the penetration capability of liquids through the polymers, we employed the permeability coefficient whose definition is16
P) Figure 1. Static contact angles of a water drop deposited on a polypropylene surface measured by ADSA-P. θ, V, and R are the contact angle (deg), drop volume (cm3), and three-phase contact radius (cm), respectively.
(penetrating amount/time/area) (driving force/sample thickness)
(2)
For this study, the penetrating amount per unit time can be directly calculated from the plots of volume versus time as in Figures 1 or 2. From these two figures, one can also find that the three-phase contact radius is nearly constant and the corresponding area can be calculated. The driving force is the pressure difference between the two sides of the sample as shown in Figure 3. Here, we assume that the pressure of the bottom side of the sample is the atmospheric pressure P0. Because of the surface tension, a pressure difference across the liquid surface will exist and can be expressed by the classic Laplace equation of capillarity,21
P2 - P0 )
2γ R
(3)
where γ is the surface tension and R is the radius of the curvature at the apex, which can be obtained from the images via image analysis. The driving force can be obtained by
∆P ) P1 - P0 )
Figure 2. Static contact angles of a water drop deposited on surface-modified polypropylene measured by ADSA-P.
larger than that of PP, which is around 84°. (3) The decreasing rates of contact angle and drop volume for PP (2.364°/h and 6 × 10-4 cm3/h, respectively) are much larger than those for SPP (0.1°/h and 5 × 10-5 cm3/h, respectively). A phenomenon observed in all the static contact angle experiments is that after a long period of time, many small liquid drops appeared on the bottom side of the sample, which indicated that the liquid of the drop penetrated through the sample.
2γ + Fgh R
(4)
where F is the density of the liquid, h is the height of the drop, and g is the acceleration of gravity (9.807 m2/s). The calculated results of permeability are 7.33 × 10-15 m2/(Pa S) for SPP and 7.644 × 10-14 m2/(Pa S) for PP. Clearly, the permeability of PP is much greater than that of SPP. This indicates that SPP will work longer if used in stickers because moisture and water will need a longer time to penetrate through the SPP layer and to reach the joint interface. Why the surface modification to PP induced such a great decrease in permeability and the relationship between permeability and other properties obtained in this study, such as contact angles and surface free energy, will be discussed in section 3.5. (21) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons: New York, 1997.
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Figure 5. Receding contact angles of SPP. The liquid was water, and the motor rate was 1 step/s. Figure 4. Advancing contact angles of SPP. The liquid was water, and the motor rate was 1 step/s.
Table 3. Results of Advancing and Receding Contact Angles PP
3.3. Dynamic Contact Angles. We measured the advancing and receding contact angles of four liquids on both PP and SPP samples. For all four liquids, except DMFM on the surface of PP, stable and reproducible advancing and receding contact angles were obtained. The liquid DMFM did not produce stable advancing and receding contact angles on the surface of PP samples. Typical experimental results of advancing and receding contact angles for water are shown in Figures 4 and 5. The results of advancing and receding contact angles and contact angle hysteresis are also summarized in Table 3. From all the results of advancing/receding contact angle measurement, similar features are observed. Here, we discuss only Figures 4 and 5. From Figure 4, it can be found that the contact angle increases rapidly in the first 100 s and then reaches a plateau, which represents the advancing contact angle. From Figure 5, one finds that initially the contact angle decreases linearly, whereas the three-phase contact radius hardly changes with the decrease in drop volume. After a certain period of time, the three-phase contact radius starts to decrease and a stable plateau on the plot of contact angle versus time can be observed. The plateau represents a receding contact angle. Comparing the advancing contact angles (Table 3) of water with its initial static contact angles (Table 2), one can find that they are approximately identical. The slight difference between the advancing and initial static contact angles might be caused by the experimental procedure. All the results listed in Table 3 have an error of (1% at the 95% confidence level. From Table 3, one can find the following: (1) For all liquids, the advancing and receding contact angles of the modified sample are larger than those of the unmodified sample. (2) With the decrease in surface tension of the liquid used, both advancing and receding contact angles decrease for both samples. (3) The
b
b
OPPa
SPP
liquid
θA
θR
Hb
water glycerol DMSO DMFM
89.3 86.4 63.5
76.2 62.2 35.3
15.1 24.2 28.2
θA
θR
H
θA
θR
H
102.5 91.5 76.6 59.5
82.5 65.1 40.4 39.5
20.0 26.4 36.2 20.0
106 88
86 64
20 24
a Oriented polypropylene (OPP) film (ref 7). b θ , advancing A contact angle; θR, receding contact angle; H ) θA - θR.
difference between the advancing and receding contact angles, or contact angle hysteresis, H, increases with the decrease in surface tension of the liquid. (4) The contact angle hysteresis of the modified sample is larger than that of the unmodified sample for all liquids used. (5) The results of advancing and receding contact angles for modified polypropylene obtained in this study approximate the literature values of oriented polypropylene film.7 3.4. Surface Characteristics with AFM. Figures 6 and 7 show the typical 3D topographic images of PP and SPP samples with scanning areas of 5 µm × 5 µm and 500 nm × 500 nm, respectively. From Figure 6a, one can find that the surface of the PP sample is well organized and comprises parallel peaks. The distance between two peaks is approximately 3 µm. Comparing images a and b in Figure 6, one finds that the surface structure of SPP is quite different from that of PP and that the size of peaks on the surface of SPP seems larger than the scanning size. On the basis of the experiments on larger scanning scales, such as 15 µm, we found that the size of the peaks on surfaces of SPP is around 8 µm. The mean roughnesses obtained from the two images are 11.59 nm for PP and 24.38 nm for SPP. This indicates that the surface of SPP is rougher than the surface of PP on the scale of 5 µm. However, from Figure 7 it can be observed that on the 500 nm scanning scale the surface of PP is rougher than the surface of SPP. The mean roughnesses from the two images are 4.10 nm for PP and 2.72 nm for SPP.
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Figure 6. Topographic images with a scanning size of 5 µm: (a) PP, mean roughness of 11.59 nm; (b) SPP, mean roughness of 24.38 nm.
Figure 8 shows the results of mean roughness obtained in this study. The scanning scales range from 10 nm to 15 µm. From the two curves of mean roughness versus scanning size in Figure 8, one finds that the mean roughness of SPP is lower than the mean roughness of PP when the scanning size is less than 4 µm. Once the scanning size is large than 4 µm, the mean roughness of SPP is larger than that of PP. Such a transition indicates that the surface of PP comprises smaller peaks but with a larger number than the surface of SPP. From the discussion above, one finds that both surfaces of PP and SPP can be qualitatively modeled as “homogeneous but rough surfaces”. Details about the model can be found elsewhere.22 Using this model, the surface free
energy changes with contact angle can be calculated and plotted. The quantitative description of the model for the PP and SPP samples in this study will be carried out in a later paper. Theoretically, similar to the results obtained in ref 22, one can find many thermodynamic stable/ metastable states of contact angles. There is a stable state that corresponds to the experimental advancing contact angle. Each metastable state will result in a receding contact angle, but not all metastable states or receding contact angles from the theoretical calculations can be experimentally observed. Therefore, the phenomenon of contact angle hysteresis well observed in this study can (22) Li, D. Q.; Neumann, A. W. In Applied Surface Thermodynamics; Neumann, A. W., Spelt, J. K., Eds.; Marcel Dekker: New York, 1966.
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Figure 7. Topographic images with a scanning size of 500 nm: (a) PP, mean roughness of 4.10 nm; (b) SPP, mean roughness of 2.72 nm.
be attributed to surface roughness. The difference of contact angle hysteresis between PP and SPP is then attributed to the change in surface features as shown in Figures 6 and 7. 3.5. Surface Free Energy. Because of the absence of surface mobility, a solid phase is very different from a liquid phase; hence, one cannot directly measure the surface tension (surface free energy) of a solid. An approach to calculate the surface tension of a solid polymer is to use the contact angle data. At the center of contact angle research is Young’s equation21
γlv cos θY ) γsv - γsl
(5)
which interrelates the Young contact angle θY with the interfacial tensions of the liquid-vapor (γlv), solid-vapor (γsv), and solid-liquid (γsl). Experimental investigations23,24 have shown that the three interfacial tensions satisfy the functional relation
γsl ) f(γsv, γlv)
(6)
The relationships of the above equation have been in the (23) Kwok, D. Y.; Li, A.; Lam, C. N. C.; Leung, A.; Neumann, A. W. Langmuir 1998, 14, 2221. (24) Kwok, D. Y.; Lam, C. N. C.; Li, A.; Zhu, K.; Wu, R.; Neumann, A. W. Polym. Eng. Sci. 1998, 38 (10), 1675.
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γlv cos θA/δ ) γsv - γsl
(13)
where δ is a roughness factor, which is defined as the ratio of a rough surface area to the geometrically projected area, and θA is the apparent/advancing contact angle. This equation indicates that the surface roughness enhances the hydrophilicity of hydrophilic surfaces and enhances the hydrophobicity of hydrophobic ones because δ is always larger than 1. In this study, the values of δ can be directly obtained by using the AFM software to analyze the topographic images. The average δ is 1.03508 for SPP and 1.02551 for PP. By replacing all occurrences of cos θY in eqs 9, 10, and 12 with cos θA/δ, we obtained the following equations:
literature for a long time. Two examples are Antonow’s rule25
γsl ) |γlv - γsv| and Berthelot’s geometric mean
(8)
1 + cos θY γlv 2
(9)
(1 + cos θY)2 γlv 4
(10)
and Berthelot’s rule
The results of Kwok et al.24 show that surface tensions obtained from the above two rules do not agree well with the real solid-liquid surface tensions. An alternative way is the equation of state approach that is considered as a modified Berthelot’s rule. On phenomenological grounds, the equation of state has been formulated:24
γsl ) γlv + γsv - 2xγlvγsve-β(γlv-γsv)
2
(11)
where β is a constant which was found to be 0.0001247 (m2/mJ)2 for many polymer surfaces. Combining this equation with Young’s equation yields
cos θY ) -1 + 2
x
γsv β(γlv-γsv)2 e γlv
(12)
From the above equations (eqs 9, 10, and 12), the surface free energy can be calculated. However, these equations are applicable only to a flat surface and not to a rough one, and as discussed in the above section, the sample surfaces used in this study are considered to be rough. Thus, these equations need to be modified so that they can be applied in this study. Several models describing the contact angle at a rough solid surface have been proposed. Wenzel proposed a theoretical model that modified Young’s equation as follows:27 (25) Antonow, G. J. Chim. Phys. 1907, 5, 372. (26) Betthelot, D. Comp. Rend. 1898, 126, 1703.
γsv )
(1 + cos θA/δ)2 γlv 4
(15)
x
(16)
cos θA/δ ) -1 + 2
relationship26
By combination with Young’s equation, the surface tension of a solid can be expressed by Antonow’s rule
γsv )
(14)
(7)
γsl ) γlv + γsv - 2xγlvγsv
γsv )
1 + cos θA/δ γlv 2
γsv )
Figure 8. Results of mean roughness.
γsv β(γlv-γsv)2 e γlv
The surface free energy for rough surfaces can be calculated using eqs 14-16. All the calculated results for the three approaches are listed in Table 4. From Table 4, one finds that Antonow’s rule gives the highest surface free energy, and Berthelot’s rule results in the lowest free energy. On the basis of the investigations carried out by Kwok et al.,23,24 the results obtained by using the equation of state approach are most reliable. Thus, we consider that the surface tensions are 27.26 mJ/ m2 for PP and 21.85 mJ/m2 for SPP by averaging results obtained from different liquids. Clearly, the surface tension of PP is larger than that of SPP (a decrease of 20% from PP to SPP). The reason for such a decrease in surface free energy is that the modification changed the surface composition and structure. The existence of silicon in SPP may decrease its surface free energy. From the experimental contact angles and the computed results of surface free energy, one can conclude that SPP has lower wettability compared with PP. 3.6. Work of Adhesion and Its Influence on Permeability. Another important quantity, which more directly indicates the interaction between two phases (A and B), is the work of adhesion wAB given by21
wAB ) γA + γB - γAB
(17)
The wAB represents the work necessary to separate a unit area of the interface AB into liquid-vapor or solid-vapor interfaces A and B. Obviously, for a solid-liquid system, the larger the work of adhesion wsl, the more easily the solid is wetted by the liquid. In this study, the work of adhesion between PP/SPP and water can be calculated by combining eqs 14 and 11. The results are 68.82 mJ/m2 for the PP-water system and 57.74 mJ/m2 for the SPP-water system. These results confirm the conclusion that SPP has low wettability compared with PP. Furthermore, the work of adhesion wsl will also play an important role in permeation. Two important factors influencing the permeability coefficient are the microstructure of the solid and the interaction between the liquid and the solid. The interaction can be represented by the work of adhesion. With the increase in work of adhesion, (27) Wenzel, R. N. J. Phys. Colloid Chem. 1949, 53, 1466.
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4. Conclusions
Table 4. Results of Surface Free Energy surface energy (mJ/m2) Berthelot’s rule
Antonow’s rule
equation of state
liquid
PP
SMPP
PP
SMPP
PP
SMPP
water glycerol DMSO DMFM average
17.45 18.60 19.99
10.44 15.68 17.12 20.85
35.52 34.76 29.79
27.56 31.93 27.57 27.66
28.00 26.80 27.00
21.50 24.10 19.80 22.00 21.85
27.26
the interaction between the solid samples and the liquids will enhance, and thus the liquids (water) will more easily wet and further penetrate the surfaces of the samples. In this study, the work of adhesion of PP is larger than that of SPP. The permeability coefficient of PP, therefore, should be larger. On the other hand, the microstructure of the solid samples, such as the number, size, and tortuosity of capillary pores in the samples, will also have an important influence on the permeability. As one can observe from Figure 7, the topographic image of the PP surface shown in (a) indicates more dark patches compared with that in (b). These dark patches probably represent pores. The more the pores, the larger the permeability. Therefore, the larger permeability of PP should be attributed to both its microstructure and the interaction between the solid and water.
All results show that the modification has a great influence on the surface characteristics of PP. The surface of PP comprises a sequence of smaller parallel peaks. The surface of SPP comprises larger random peaks. The surface of SPP is rougher on a larger scale but smoother on a smaller scale compared with the surface of PP. Both surfaces of PP and SPP can be modeled as a homogeneous but rough surface. All the advancing and receding contact angles and contact angle hystereses of SPP are larger than those of PP. This indicates that the wettability of PP is higher than that of SPP. The modification also greatly changes the water permeability. The water permeability coefficient of PP is nearly 10 times that of SPP. The modification obviously decreases the surface free energy of PP (a decrease of 20%) and the work of adhesion. Acknowledgment. This research was financially supported by the Natural Sciences and Engineering Research Council of Canada (NSERC), the Canadian Foundation for Innovation (CFI), and Ontario Graduate Scholarships in Science and Technology (OGSST). LA001547U
