- Email: [email protected]
Fabrication of a continuous wettability gradient by radio frequency plasma discharge
Fabrication of a Continuous Wettability Gradient by Radio Frequency Plasma Discharge W I L L I A M G. PITT Department of Chemical Engineering, Brigham Young University, Provo, Utah 84602 Received October 21, 1988;accepted February 21, 1989 Radio frequency(RF) plasma discharge in atmospheres of ammonia, oxygen,and sulfur dioxide was used to create a continuous change in wettability on the surfaces of polyethylene, polystyrene, polydimethylsiloxane, and polytetrafluoroethylene.Thesecontinuouschanges,referredto as wettabilitygradients, were produced by exposing the polymers to the RF plasma in a box with a cover which opened at a constant velocity.Contact angles of water on the derivatized polymersurfaces decreased monotonically as the time of plasma exposure increased. A wide range in wettability was produced by manipulating the RF power, the gas species, and the duration of plasma exposure. The production of such surfaces has promisingapplicationin surfacesciencebecausea continuousspectrum of surface energiesor surface chemistries can be produced on one sample at one time. © 1989AcademicPress,Inc. INTRODUCTION It is often useful to study a range of a particular surface property in order to evaluate observed or predicted interfacial phenomena. Such a study may often be tedious and expensive because of the large number of samples that must be prepared to adequately characterize the complete range of the desired surface property. It would be advantageous to create a continuous change, or spectrum, of the desired surface property on one sample and at one time. This would greatly reduce the number of samples that would otherwise be generated and would ensure that the desired range of surface properties was generated under identical experimental conditions. Elwing et al. (1, 2) have produced a continuous change in wettability on a silicon dioxide surface by allowing C12(CH3)2Si to diffuse through xylene and to react with the surface. This procedure produces a 2-cm region on the sample having a continuous change from wettable (hydroxyl) to nonwettable (methyl) surface groups. These samples were called wettability gradients, which refers to the continuous change in contact angle, O0/Ox,
along the surface. Unfortunately, the production of these gradients is limited to SiO2 substrates. This paper presents a novel technique which produces a wettability gradient on a wide variety of polymeric substrates. The technique employs radio frequency plasma discharge ( R F P D ) to chemically modify the substrate surface. The wettability gradient is produced by exposing the substrate to the R F P D in a time-dependent manner along the length of the sample. In these initial studies, gradients have been created on polystyrene (PS), polyethylene (PE), polydimethylsiloxane (PDMS), and polytetrafluoroethylene (PTFE). MATERIALS AND METHODS Polyethylene and polydimethylsiloxane were obtained in sheet form from Abiomed (Danvers, MA) and Mercor (Berkeley, CA) as National Institutes of Health ( N I H ) reference materials. Polystyrene was cut from bacteria culture dishes (Falcon dish, Becton Dickinson), sonicated for 15 min in 50% ethanol, and dried under vacuum. Polytetra-
223 0021-9797/89 $3.00 Journal of Colloid and Interface Science, Vol. 133, No. 1, November 1989
Copyright © 1989 by Academic Press, Inc. All rights of reproduction in any form reserved.
224
WILLIAM G. PITT
fluoroethylene was obtained as "Teflon pipe thread tape" (Harbor Freight Co.) and was used as received after discarding the first 5 m of tape. Contact angles of water on the underivatized surfaces were 92 °, 100 °, 88 °, and 121 o for PE, PDMS, PS, and PTFE, respectively. Radio frequency plasma discharge treatment of the samples is done in the gradient apparatus shown in Fig. 1. This apparatus is placed inside a Plasma Science radio frequency plasma reactor (Model 0500, Santa Clara, CA) which operates at 13.56 MHz. The operation of this reactor (RF power, duration, pressure, gas flow rate) is completely microprocessorcontrolled and also has an automatic matching network which minimizes the reflected RF power. The gradient apparatus consists of three parts; (1) an aluminum gradient "box" with a translating cover, (2) an electrically grounded aluminum plate (27.8 × 20.6 cm) containing a 3.8 × 11.4 cm rectangular hole, and (3) a 27.8 X 20.6 cm aluminum plate which is connected to the RF generator. The cover on the gradient box is driven by a gear drive which passes through the floor of the
plasma reactor and connects to a stepping motor. The polymer sample is placed inside the gradient box on an adjustable aluminum stand, and the sample is raised to within about 0.5 m m of the cover. The electrically grounded plate is placed on the box so that the hole in the plate is directly above the box. The plate connected to the RF power supply is located 5.1 cm above the grounded plate. Plasma is generated in the volume between these parallel plates when the RF power is turned on. Visual inspection shows a spatially uniform glow between the parallel plates. The gradient is produced by retracting the cover at a constant velocity while generating the RF plasma between the plates. The distance and the velocity of the retraction are controlled by a microprocessor which drives the stepping motor. Retracting the cover during the RF discharge produces a linear increase in the time of plasma exposure along the length of the sample; i.e., a uniform gradient in the amount of plasma exposure. Equilibrium water contact angles were measured on the samples with a R a m e - H a r t goniometer according to the manufacturer's instructions. The samples were placed in a saturated water atmosphere inside a glass box and 2-#1 drops of double distilled water were applied with a syringe. The measurements were made at a temperature of 21°C. The contact angle measurements were completed within 20 min of the RFPD treatment. RESULTS AND DISCUSSION
TO STEPPING MOTOR
FIG. l. Apparatus for producing a gradient with radio frequencyplasma discharge. Journal of Colloid and Interface Science, Vol. 133, No. l, November 1989
The water contact angles on the gradient surfaces are shown in Figs. 2 and 3. Water contact angles in the direction perpendicular to the gradient were uniform, indicating that a one-dimensional gradient was produced. The dotted lines in Figs. 2 and 3 indicate how far the cover was opened. Details of the RFPD processing conditions are given in Table I. These contact angles show that the wettability increases monotonically along the length of the gradient as the exposure time to the plasma increases. The increase in wettability
225
C O N T I N U O U S WETTABILITY G R A D I E N T 130
9o "O
!
•.e.- NH3 on PDMS
Q
70
i o
"¢'- 02 on PDMS
50
f
/
I
~
NH3 on Teflon
"
30 10 0
10
Distance (mm)
20
30
40
50
0
FIG. 2. Water contact angles as a function of the distance that the cover was retracted. The dotted line marks the extent of retraction. (A) NH3 plasma on PTFE; (73) O2 plasma on PDMS; ( 0 ) N H 3 plasma on PDMS. The details of the plasma exposure conditions are given in Table I.
is attributed to incorporation of polar or acidbase functionality into the surface of these apolar polymers. The polar groups may originate directly from the plasma (oxygen, nitrogen, or sulfur species) or may form from free radicals trapped on the polymer surface which react with atmospheric oxygen when the samples are removed from the RFPD reactor (3). ESCA analysis of the same polymers treated with RFPD under standard (nongradient) conditions shows an increase in oxygen and
nitrogen on the surface (data not shown). Other investigators have also observed increases in the surface concentration of O and N following RFPD treatment in NH3, 02, or CO2 atmospheres (4-6). Although these wettability gradients imply the existence of chemical gradients in the surface, the chemical gradients may not correlate directly with the contact angle. For example, a chemical gradient may correlate better with the cosine of the contact angle which is proportional to the
12o t O
I !
loo
I
"
"
.-m- NH3 on PS
< 60 11~~80 1,7. ¢ ¢ ~ ~l~
NH3 on PE 16W
t-
O
o
4-
NH3 on PE 160W SO2 on PDMS
40
20
[]
0
,i 10
i
i
i
Distance (mm) 20
30
40
50
FIG. 3. Water contact angles as a function of the distance that the cover was retracted. The dotted line marks the extent of retraction. ([3) NH3 plasma on PS; (•) SO2 plasma on PDMS; ( 0 ) NH3 plasma at 16 W on PE; ( I ) NH3 plasma at 160 W on PE. The details of the plasma exposure conditions are given in Table I, Journal of Colloid and Interface Science, Vol. 133, No. 1, November 1989
WILLIAM G. PITT
226
TABLE I Processing Parameters for the RFPD Treatment of the SamplesShown in Figs. Z and 3 Substrate polymer
Gas
Flow rate (SLM)a
RF power (W)
Distance (mm)
Time (s)
Pressure (Tolr)
Fig.
PTFE PDMS PDMS PDMS PE PE PS
NH3 NH3 02 SO2 NH3 NH3 NH3
0.26 0.32 0.26 0.22 0.26 0.26 0.26
50 50 50 42 16 160 50
50.8 50.8 50.8 38.1 38.1 38.1 38.1
181 181 12.6 15.0 18.3 18.3 18.3
0.20 0.20 0.20 0.20 0.20 0.20 0.20
2 2 2 3 3 3 3
a Standard liters per minute.
work of adhesion at the water-polymer interface. The shape of contact angle plots (Figs. 2 and 3) all show similar changes in the magnitude of the gradient, O0/Ox; the gradient at short exposure times is relatively large but decreases with increasing exposure times and is relatively small at long exposure times. This shape appears to be characteristic of polymers treated with R F P D (4, 5) and can be attributed to the competition between incorporation of new polar species from the plasma onto the surface and the ablation or removal of these species by the plasma. In the region of short exposure times, incorporation of polar or reactive groups is m u c h faster than ablation and the surface chemistry changes in proportion to the exposure time. As the surface fills with polar groups at long times, ablation of these groups increases until a steady-state competition between ablation and incorporation is achieved. Figure 3 shows the effect of the RF power upon the wettability of PE when exposed to NH3 plasma. Increasing the power from 16 to 160 W decreases the contact angle (at 18 s of exposure) from 77 to 37 ° and appears to increase the time required to reach a steady-state condition. Similar observations have been made in other studies (5). Table I shows that the time required to open the cover in order to produce the gradients can differ by an order of magnitude. For exJournal of Colloid and Interface Science, Vol. 133, No. I, November 1989
ample, on PDMS, a contact angle of about 20 ° is produced after 10 s of 02 plasma or after 102 s of NH3 plasma. This qualitative analysis suggests that for similar RF powers, pressures, and flow rates, 02 is most reactive and SO2 is least reactive with respect to increasing the wettability of this polymer. A similar qualitative analysis can also be made of the reactivity of the polymer toward the plasma. In NH3 plasma, the ability of the polymer to incorporate polar or hydrogen bonding groups increases in the order of PTFE < PDMS < PE < PS. For very reactive polymers in very reactive plasmas, the speed at which the cover can be opened m a y limit the length or magnitude of the gradient. Another interesting observation about polymer reactivity is shown in Fig. 3. Although the cover was retracted to 38 m m , the PE and PS surfaces appear to be derivatized beyond this length. It is possible that some of the plasma diffused into the 0.5-mm gap between the sample and the eovegand then reacted with these polymers. This plasma diffusion and reaction are readily observed on the most reactive PS sample. It is also observed on the PE samples but is not observed on the less reactive PDMS and PTFE samples. The main advantages of using these wettability or chemical gradients in surface studies are the reduction in the n u m b e r of samples which must be produced and the continuous nature of the gradient. Other advantages in-
CONTINUOUS WETTABILITY GRADIENT
clude application of the technique to polymer surfaces, control of the length and magnitude of the gradient, a wide selection of gas atmospheres, and the absence of solvents. There are also some limitations to the RFPD gradient technique which should be mentioned. A limitation which is always present with polymers is the stability of the derivatized surface. The mobility of the polymer surface may allow the chemical groups incorporated into the surface to diffuse away from the surface (into the polymer) if the environment is not thermodynamically favorable (7). For example, the contact angle of water on PS and PDMS treated with NH3 or 02 plasma was observed to increase within 1 h when the sample was exposed to air at room temperature. This surface instability may preclude gradients from being prepared in advance and then stored for extended periods of time in a nonequilibrium environment such as air. Other concerns include roughening of the surface by plasma etching and diffusion of plasma into the gap which may be undesirable if a distinct boundary on the treated region is required. Another is the reproducibility of the surface derivatization. The reproducibility has not been thoroughly studied yet, but preliminary data indicate that contact angles are reproducible to about +5%. SUMMARY
Wettability gradients can be produced on a wide variety of polymeric surfaces by radio frequency plasma discharge by using a box with a retracting cover. A wide range ofwettability can be produced by manipulating the radio frequency power, the gas atmosphere, the velocity of the cover retraction, and the duration of exposure.
227
The production of gradient surfaces has great possibilities in colloid and surface studies. For example, interactions of colloidal particles with surfaces can be studied on one sample having a continuous spectrum of surface energy, surface chemistry, charge distribution, etc. This technique could also be used to study the kinetics of RF plasma interaction with polymer surface. Although the present study was limited to "plasma etching" on polymeric surfaces, it is very possible that this technique may be extended to plasma polymerization on polymers or on nonpolymeric substrates (i.e., ceramics, glasses, metals.). ACKNOWLEDGMENTS This work was supported by a research development grant from Brigham Young University. The author thanks Dr. Carl-Gustaf Golander and Dr. Joseph D. Andrade of the University of Utah for helpful discussions. Part of the cost of the plasma reactor was paid for by Plasma Science. REFERENCES 1. Elwing, H., Welin, S., Askendal, A., Nilsson, U., and Lundstrom, I., J. Colloid Interface Sci. 119, 203 (1987). 2. Elwing, H., Askendal, A., and Lundstrom, I., J. Biomed. Mater. Res. 21, 1023 (1987). 3. Clark, D. T., Dilks, A., and Shuttleworth, D., in "Polymer Surfaces" (D. T. Clark and W. J. Feast, Eds.), pp. 185. Wiley, Chichester, UK, 1978. 4. Kogoma, M., Kasai, H., Takahashi, K., Moriwaki, T., and Okazaki, S., J. Phys. D: Appl. Phys. 20, 147 (1987). 5. Allred, R. E., Merrill, E. W., and Roylance, D. K., in "Molecular Characterization of Composite Interfaces" (H. Ishida and G. Kumar, Eds.), pp. 333. Plenum, New York, 1983. 6. Pitt, W. G., Fabrizius, D., Mosher, D. F., and Cooper, S. L., J. Colloid Interface Sci., 129, 231 (1989). 7. Ruckenstein, E., and Gourisankar, S., J. Colloid Interface Sci. 107, 488 (1985).
Journal of Colloid andlnterface Science, Vol. 133, No. 1, November 1989
along the surface. Unfortunately, the production of these gradients is limited to SiO2 substrates. This paper presents a novel technique which produces a wettability gradient on a wide variety of polymeric substrates. The technique employs radio frequency plasma discharge ( R F P D ) to chemically modify the substrate surface. The wettability gradient is produced by exposing the substrate to the R F P D in a time-dependent manner along the length of the sample. In these initial studies, gradients have been created on polystyrene (PS), polyethylene (PE), polydimethylsiloxane (PDMS), and polytetrafluoroethylene (PTFE). MATERIALS AND METHODS Polyethylene and polydimethylsiloxane were obtained in sheet form from Abiomed (Danvers, MA) and Mercor (Berkeley, CA) as National Institutes of Health ( N I H ) reference materials. Polystyrene was cut from bacteria culture dishes (Falcon dish, Becton Dickinson), sonicated for 15 min in 50% ethanol, and dried under vacuum. Polytetra-
223 0021-9797/89 $3.00 Journal of Colloid and Interface Science, Vol. 133, No. 1, November 1989
Copyright © 1989 by Academic Press, Inc. All rights of reproduction in any form reserved.
224
WILLIAM G. PITT
fluoroethylene was obtained as "Teflon pipe thread tape" (Harbor Freight Co.) and was used as received after discarding the first 5 m of tape. Contact angles of water on the underivatized surfaces were 92 °, 100 °, 88 °, and 121 o for PE, PDMS, PS, and PTFE, respectively. Radio frequency plasma discharge treatment of the samples is done in the gradient apparatus shown in Fig. 1. This apparatus is placed inside a Plasma Science radio frequency plasma reactor (Model 0500, Santa Clara, CA) which operates at 13.56 MHz. The operation of this reactor (RF power, duration, pressure, gas flow rate) is completely microprocessorcontrolled and also has an automatic matching network which minimizes the reflected RF power. The gradient apparatus consists of three parts; (1) an aluminum gradient "box" with a translating cover, (2) an electrically grounded aluminum plate (27.8 × 20.6 cm) containing a 3.8 × 11.4 cm rectangular hole, and (3) a 27.8 X 20.6 cm aluminum plate which is connected to the RF generator. The cover on the gradient box is driven by a gear drive which passes through the floor of the
plasma reactor and connects to a stepping motor. The polymer sample is placed inside the gradient box on an adjustable aluminum stand, and the sample is raised to within about 0.5 m m of the cover. The electrically grounded plate is placed on the box so that the hole in the plate is directly above the box. The plate connected to the RF power supply is located 5.1 cm above the grounded plate. Plasma is generated in the volume between these parallel plates when the RF power is turned on. Visual inspection shows a spatially uniform glow between the parallel plates. The gradient is produced by retracting the cover at a constant velocity while generating the RF plasma between the plates. The distance and the velocity of the retraction are controlled by a microprocessor which drives the stepping motor. Retracting the cover during the RF discharge produces a linear increase in the time of plasma exposure along the length of the sample; i.e., a uniform gradient in the amount of plasma exposure. Equilibrium water contact angles were measured on the samples with a R a m e - H a r t goniometer according to the manufacturer's instructions. The samples were placed in a saturated water atmosphere inside a glass box and 2-#1 drops of double distilled water were applied with a syringe. The measurements were made at a temperature of 21°C. The contact angle measurements were completed within 20 min of the RFPD treatment. RESULTS AND DISCUSSION
TO STEPPING MOTOR
FIG. l. Apparatus for producing a gradient with radio frequencyplasma discharge. Journal of Colloid and Interface Science, Vol. 133, No. l, November 1989
The water contact angles on the gradient surfaces are shown in Figs. 2 and 3. Water contact angles in the direction perpendicular to the gradient were uniform, indicating that a one-dimensional gradient was produced. The dotted lines in Figs. 2 and 3 indicate how far the cover was opened. Details of the RFPD processing conditions are given in Table I. These contact angles show that the wettability increases monotonically along the length of the gradient as the exposure time to the plasma increases. The increase in wettability
225
C O N T I N U O U S WETTABILITY G R A D I E N T 130
9o "O
!
•.e.- NH3 on PDMS
Q
70
i o
"¢'- 02 on PDMS
50
f
/
I
~
NH3 on Teflon
"
30 10 0
10
Distance (mm)
20
30
40
50
0
FIG. 2. Water contact angles as a function of the distance that the cover was retracted. The dotted line marks the extent of retraction. (A) NH3 plasma on PTFE; (73) O2 plasma on PDMS; ( 0 ) N H 3 plasma on PDMS. The details of the plasma exposure conditions are given in Table I.
is attributed to incorporation of polar or acidbase functionality into the surface of these apolar polymers. The polar groups may originate directly from the plasma (oxygen, nitrogen, or sulfur species) or may form from free radicals trapped on the polymer surface which react with atmospheric oxygen when the samples are removed from the RFPD reactor (3). ESCA analysis of the same polymers treated with RFPD under standard (nongradient) conditions shows an increase in oxygen and
nitrogen on the surface (data not shown). Other investigators have also observed increases in the surface concentration of O and N following RFPD treatment in NH3, 02, or CO2 atmospheres (4-6). Although these wettability gradients imply the existence of chemical gradients in the surface, the chemical gradients may not correlate directly with the contact angle. For example, a chemical gradient may correlate better with the cosine of the contact angle which is proportional to the
12o t O
I !
loo
I
"
"
.-m- NH3 on PS
< 60 11~~80 1,7. ¢ ¢ ~ ~l~
NH3 on PE 16W
t-
O
o
4-
NH3 on PE 160W SO2 on PDMS
40
20
[]
0
,i 10
i
i
i
Distance (mm) 20
30
40
50
FIG. 3. Water contact angles as a function of the distance that the cover was retracted. The dotted line marks the extent of retraction. ([3) NH3 plasma on PS; (•) SO2 plasma on PDMS; ( 0 ) NH3 plasma at 16 W on PE; ( I ) NH3 plasma at 160 W on PE. The details of the plasma exposure conditions are given in Table I, Journal of Colloid and Interface Science, Vol. 133, No. 1, November 1989
WILLIAM G. PITT
226
TABLE I Processing Parameters for the RFPD Treatment of the SamplesShown in Figs. Z and 3 Substrate polymer
Gas
Flow rate (SLM)a
RF power (W)
Distance (mm)
Time (s)
Pressure (Tolr)
Fig.
PTFE PDMS PDMS PDMS PE PE PS
NH3 NH3 02 SO2 NH3 NH3 NH3
0.26 0.32 0.26 0.22 0.26 0.26 0.26
50 50 50 42 16 160 50
50.8 50.8 50.8 38.1 38.1 38.1 38.1
181 181 12.6 15.0 18.3 18.3 18.3
0.20 0.20 0.20 0.20 0.20 0.20 0.20
2 2 2 3 3 3 3
a Standard liters per minute.
work of adhesion at the water-polymer interface. The shape of contact angle plots (Figs. 2 and 3) all show similar changes in the magnitude of the gradient, O0/Ox; the gradient at short exposure times is relatively large but decreases with increasing exposure times and is relatively small at long exposure times. This shape appears to be characteristic of polymers treated with R F P D (4, 5) and can be attributed to the competition between incorporation of new polar species from the plasma onto the surface and the ablation or removal of these species by the plasma. In the region of short exposure times, incorporation of polar or reactive groups is m u c h faster than ablation and the surface chemistry changes in proportion to the exposure time. As the surface fills with polar groups at long times, ablation of these groups increases until a steady-state competition between ablation and incorporation is achieved. Figure 3 shows the effect of the RF power upon the wettability of PE when exposed to NH3 plasma. Increasing the power from 16 to 160 W decreases the contact angle (at 18 s of exposure) from 77 to 37 ° and appears to increase the time required to reach a steady-state condition. Similar observations have been made in other studies (5). Table I shows that the time required to open the cover in order to produce the gradients can differ by an order of magnitude. For exJournal of Colloid and Interface Science, Vol. 133, No. I, November 1989
ample, on PDMS, a contact angle of about 20 ° is produced after 10 s of 02 plasma or after 102 s of NH3 plasma. This qualitative analysis suggests that for similar RF powers, pressures, and flow rates, 02 is most reactive and SO2 is least reactive with respect to increasing the wettability of this polymer. A similar qualitative analysis can also be made of the reactivity of the polymer toward the plasma. In NH3 plasma, the ability of the polymer to incorporate polar or hydrogen bonding groups increases in the order of PTFE < PDMS < PE < PS. For very reactive polymers in very reactive plasmas, the speed at which the cover can be opened m a y limit the length or magnitude of the gradient. Another interesting observation about polymer reactivity is shown in Fig. 3. Although the cover was retracted to 38 m m , the PE and PS surfaces appear to be derivatized beyond this length. It is possible that some of the plasma diffused into the 0.5-mm gap between the sample and the eovegand then reacted with these polymers. This plasma diffusion and reaction are readily observed on the most reactive PS sample. It is also observed on the PE samples but is not observed on the less reactive PDMS and PTFE samples. The main advantages of using these wettability or chemical gradients in surface studies are the reduction in the n u m b e r of samples which must be produced and the continuous nature of the gradient. Other advantages in-
CONTINUOUS WETTABILITY GRADIENT
clude application of the technique to polymer surfaces, control of the length and magnitude of the gradient, a wide selection of gas atmospheres, and the absence of solvents. There are also some limitations to the RFPD gradient technique which should be mentioned. A limitation which is always present with polymers is the stability of the derivatized surface. The mobility of the polymer surface may allow the chemical groups incorporated into the surface to diffuse away from the surface (into the polymer) if the environment is not thermodynamically favorable (7). For example, the contact angle of water on PS and PDMS treated with NH3 or 02 plasma was observed to increase within 1 h when the sample was exposed to air at room temperature. This surface instability may preclude gradients from being prepared in advance and then stored for extended periods of time in a nonequilibrium environment such as air. Other concerns include roughening of the surface by plasma etching and diffusion of plasma into the gap which may be undesirable if a distinct boundary on the treated region is required. Another is the reproducibility of the surface derivatization. The reproducibility has not been thoroughly studied yet, but preliminary data indicate that contact angles are reproducible to about +5%. SUMMARY
Wettability gradients can be produced on a wide variety of polymeric surfaces by radio frequency plasma discharge by using a box with a retracting cover. A wide range ofwettability can be produced by manipulating the radio frequency power, the gas atmosphere, the velocity of the cover retraction, and the duration of exposure.
227
The production of gradient surfaces has great possibilities in colloid and surface studies. For example, interactions of colloidal particles with surfaces can be studied on one sample having a continuous spectrum of surface energy, surface chemistry, charge distribution, etc. This technique could also be used to study the kinetics of RF plasma interaction with polymer surface. Although the present study was limited to "plasma etching" on polymeric surfaces, it is very possible that this technique may be extended to plasma polymerization on polymers or on nonpolymeric substrates (i.e., ceramics, glasses, metals.). ACKNOWLEDGMENTS This work was supported by a research development grant from Brigham Young University. The author thanks Dr. Carl-Gustaf Golander and Dr. Joseph D. Andrade of the University of Utah for helpful discussions. Part of the cost of the plasma reactor was paid for by Plasma Science. REFERENCES 1. Elwing, H., Welin, S., Askendal, A., Nilsson, U., and Lundstrom, I., J. Colloid Interface Sci. 119, 203 (1987). 2. Elwing, H., Askendal, A., and Lundstrom, I., J. Biomed. Mater. Res. 21, 1023 (1987). 3. Clark, D. T., Dilks, A., and Shuttleworth, D., in "Polymer Surfaces" (D. T. Clark and W. J. Feast, Eds.), pp. 185. Wiley, Chichester, UK, 1978. 4. Kogoma, M., Kasai, H., Takahashi, K., Moriwaki, T., and Okazaki, S., J. Phys. D: Appl. Phys. 20, 147 (1987). 5. Allred, R. E., Merrill, E. W., and Roylance, D. K., in "Molecular Characterization of Composite Interfaces" (H. Ishida and G. Kumar, Eds.), pp. 333. Plenum, New York, 1983. 6. Pitt, W. G., Fabrizius, D., Mosher, D. F., and Cooper, S. L., J. Colloid Interface Sci., 129, 231 (1989). 7. Ruckenstein, E., and Gourisankar, S., J. Colloid Interface Sci. 107, 488 (1985).
Journal of Colloid andlnterface Science, Vol. 133, No. 1, November 1989