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Pulsed Radio Frequency Plasma Polymerization of Allyl Alcohol: Controlled Deposition of Surface Hydroxyl Groups Christopher L. Rinsch,† Xiaolan Chen,‡ V. Panchalingam,‡ Robert C. Eberhart,† Jenn-Hann Wang,‡ and Richard B. Timmons*,‡ Department of Chemistry and Biochemistry, Box 19065, The University of Texas at Arlington, Arlington, Texas 76019-0065, and Biomedical Engineering Program, University of Texas at Arlington and University of Texas Southwestern Medical Center at Dallas, Dallas, Texas 75235-9031 Received August 15, 1995. In Final Form: April 12, 1996X The utility of employing a variable duty cycle pulsed plasma polymerization technique to control film chemistry during plasma depositions is examined using allyl alcohol as monomer gas. Large scale progressive variations in film composition are observed with sequential changes in the plasma duty cycles employed, all other plasma variables being held constant. In particular, the -OH functionality of the monomer is increasingly retained in the plasma generated thin films as the radio frequency duty cycle is lowered. Fourier transform infrared and X-ray photoelectron spectroscopic analyses of the films obtained reveal that excellent film chemistry control is achieved during plasma polymerization of this monomer. The surface density controllability of functional groups, coupled with a gradient layering technique described herein to improve film adhesion to substrate surfaces, provides ideal opportunities for molecular tailoring of surfaces via subsequent derivatization reactions.
Introduction The use of plasma polymerization techniques is becoming an increasingly popular approach to surface modifications. Certainly, the number and diversity of presentations at recent symposia attest to the rapid growth in this field.1-4 Advantages of this technique include the fact that pinhole-free, conformal thin films can be deposited on most substrates using a relatively simple one-step coating procedure. Additionally, a wide range of compounds, including even saturated hydrocarbons, can be employed as “monomers” in providing interesting surface modifications. Yasuda5 has provided a thorough discussion of plasma polymerization techniques and the important variables involved in film deposition studies. The present paper focuses on a further advance in the use of plasma polymerizations to provide surface modifications. Specifically, we address the issue of controlling film chemistry during plasma polymerization of a given monomer. Controlling film chemistry is a desirable but, at the same time, somewhat elusive goal in many plasma polymerization studies. Nevertheless, in recent years, significant advances in film chemistry controllability have been reported in a variety of plasma polymerization studies.6-16 For example, film chemistry control during plasma operation under continuous-wave conditions (CW) † Biomedical Engineering Program, The University of Texas at Arlington and University of Texas Southwestern Medical Center at Dallas. ‡ Department of Chemistry and Biochemistry, The University of Texas at Arlington. X Abstract published in Advance ACS Abstracts, June 1, 1996.
(1) Symposium “Plasma polymerization and plasma interactions with polymeric materials”, ACS National Meeting, Boston, MA, 1990; Polym. Mater.: Sci. Eng. 1990, 62. (2) Symposium “Plasma deposition of polymeric thin films: Chemistry, characterization and applications, ACS National Meeting, Denver, CO, 1993; Polym. Prepr. 1993, 34. (3) International Symposium “Plasma Polymerization/Deposition: Fundamental and Applied Aspects”, Las Vegas, NV, 1993. (4) Symposium “Polymer surface modification for biomedical applications, ACS National Meeting, Anaheim, CA, 1995; Polym. Prepr. 1995, 36. (5) Yasuda, H. Plasma Polymerization; Academic Press: Orlando, FL, 1985.
S0743-7463(95)00685-8 CCC: $12.00
can frequently be achieved to a fairly high degree by variations in the ratio of power (W) to monomer flow rate (F). In the case of functionalized monomers, it is generally observed that decreases in the value of W/F during plasma polymerization of a given monomer leads to increases in the relative retention of monomer functionality in the plasma generated films. In fact, recent detailed studies of the plasma polymerization of monomers containing -OH groups clearly reveal increased incorporation of the hydroxyl groups as the value of W/F is reduced.6,9,11,16 In addition, both substrate temperature17,18 variations and changes in the location of the substrate relative to the plasma discharge zone have been shown to provide some film chemistry variations.5,11,19 Despite the recent advances in film chemistry control during plasma polymerizations, problems and limitations are still frequently encountered when attempting to utilize the plasma approach for synthesis of specific polymeric films. In particular, it is often observed that experimental difficulties such as good film quality (e.g., avoiding powdery or oily deposits) or good film adhesion to underlying substrates are encountered when attempting to vary a (6) Gombotz, W. R.; Hoffman, A. S. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1988, 42, 285. (7) Lopez, G. P.; Chilkoti, A.; Briggs, D.; Ratner, B. D. J. Polym. Sci., PART A: Polym. Chem. 1992, 30, 2427. (8) Ward, A. J.; Short, R. D. Polymer 1993, 34, 4179. (9) Ameen, A. P.; Short, R. D.; Ward, R. J. Polymer 1994, 35, 4382. (10) Ward, A. J.; Short, R. D. Surf. Interface Anal. 1994, 22, 477. (11) Fally, F.; Virlet, I.; Riga, J.; Verbist, J. J. J. Appl. Polym. Sci., Appl. Polym. Symp. 1994, 54, 41. (12) Ameen, A. P.; Beck, A. J.; O’Toole, L.; Short, R. D.; Jones, F. R. J. Chem. Soc., Chem. Commun. 1995, 1053. (13) O’Toole, L.; Short, R. D.; Ameen, A. P.; Jones, F. R. J. Chem. Soc., Faraday Trans. 1995, 91, 1363. (14) O’Toole, L.; Beck, A. J.; Ameen, A. P.; Jones, F. R.; Short, R. D. J. Chem. Soc., Faraday Trans. 1995, 91, 3907. (15) Ward, A. J.; Short, R. D. Polymer 1995, 36, 3439. (16) Fally, F.; Virlet, I.; Riga, J.; Verbist, J. J. J. Appl. Polym. Sci. 1996, 59, 1569. (17) Lopez, G. P.; Ratner, B. D. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1990, 46, 493. (18) Lopez, G. P.; Ratner, R. D. Langmuir 1991, 7, 766. (19) O’Kane, D. F.; Rice, D. W. J. Macromol. Sci. Chem. 1976, A10, 567.
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parameter such as W/F over too wide a range under CW deposition conditions. Additionally, only relatively modest film chemistry variations have been reported in studies involving the substrate temperature variation approach. The present study examines an alternate route to film chemistry control during plasma depositions. Specifically, a pulsed radio frequency (rf) plasma is employed in lieu of the traditional CW approach. There are two earlier reports of pulsed plasma polymerizations which are noteworthy. As shown by Yasuda and Hsu,20 large scale differences in residual free radical content are observed when comparing CW and pulsed plasma generated films. The differences observed are strongly monomer dependent. In the studies of Nakajima et al.,21 it was observed that the film deposition rates and film compositions obtained from CW depositions were virtually identical to those observed under pulsing conditions, when the comparison was made on an equivalent average rf power input basis for studies involving C2F4 monomer. In the present study, emphasis was placed on examination of variations of the rf duty cycle (i.e., the ratio of plasma-on to plasma-off times) during plasma polymerization of allyl alcohol, all other plasma variables being held constant. As in our previous studies with pulsed plasmas,22-27 we observe large scale progressive changes in film composition with sequential variation of the rf duty cycle. The results obtained provide conclusive evidence that excellent film chemistry control is available through use of this variable duty cycle pulsed technique as shown spectroscopically by FT-IR and XPS analyses, as well as surface energy measurements, of films obtained from allyl alcohol. In addition, we also describe the use of the pulsed plasma technique to improve the adhesion of plasma-deposited films to the underlying solid substrates. In this application, the pulsed plasma is initiated, for a brief period, at a relatively high duty cycle which is then progressively reduced to a final value which provides the desired surface density of functional groups. We reason that the brief initial high duty cycle provides a sublayer film strongly grafted to the substrate. The subsequent depositions, at progressively lower duty cycles, provides a gradient layered structure with each layer bound relatively tightly to adjacent layers. This gradient layering technique is one of general applicability using monomers other than allyl alcohol. Experimental Section Reactions were carried out in an all-glass cylindrical reactor. The rf power was delivered to the reactor through two concentric ring electrodes located around the exterior of the reactor. A detailed description of the apparatus employed has been provided elsewhere.24,25 A major difference between this work and typical CW experiments is incorporation of a pulse generator to provide the intermittent plasma operation. Although the pulse generator (20) Yasuda, H.; Hsu, T. J. Polym. Sci., Polym. Chem. Ed. 1977, 15, 81. (21) Nakajima, K.; Bell, A. T.; Shen, M.; Millard, M. M. J. Appl. Polym. Sci. 1977, 23, 2627. (22) Savage, C. R.; Timmons, R. B. Polym. Mater.: Sci. Eng. 1991, 64, 95. (23) Savage, C. R.; Timmons, R. B.; Lin, J. W. Chem. Mater. 1991, 3, 575. (24) Panchalingam, V.; Chen, X.; Savage, C. R.; Timmons, R. B.; Eberhart, R. C. J. Appl. Polym. Sci.: Appl. Polym. Symp. 1994, 54, 123. (25) Panchalingam, V.; Poon, B.; Huo, H.-H.; Savage, C. R.; Timmons, R. B.; Eberhart, R. C. J. Biomater. Sci. Polym. Ed. 1993, 5, 131. (26) Rinsch, C. L. Molecular Engineering of Biomaterial Surfaces Via Pulsed RF Plasma Polymerization. M.Sc. Thesis, The University of Texas Southwestern Medical Center at Dallas, Dallas, TX, 1994. (27) Chen, X. Molecular Tailoring of Surfaces Via Pulsed RF Plasma Deposition. Ph.D. Thesis The University of of Texas at Arlington, Arlington, TX, 1995.
Rinsch et al. employed is capable of providing a wide range of duty cycles, we have found that plasma on and off times in the one to several hundred milliseconds range are effective in providing controlled film chemistry changes. A second distinguishing feature of the present work is use of relatively high (usually 200-300 W) peak rf power during the plasma on periods. This can be contrasted with values of 50 W or less typically employed in CW plasma polymerizations. However, since the pulsed plasma studies include significant plasma off times, the total equivalent power consumed during the pulsed experiments is, in fact, comparable to those employed in CW experiments. For example, a pulsed run carried out at 300 W peak power and a duty cycle of 1 ms on and 10 ms off corresponds to an equivalent CW power of only 27 W [i.e., (1/11) × 300]. All work reported here was carried out using an rf frequency of 13.56 MHz. The monomer flow rate was typically 4 cm3 (STP)/min and reactor pressure was maintained at ∼0.11 Torr. The plasma-generated films were deposited on silicon, KCl and poly(ethylene terphthalate) (PET) substrates. Substrates were treated initially with an Ar plasma for surface cleaning prior to the plasma deposition. No differences in film compositions were detected on the different substrates for the 10003000 Å films typically involved in this work. The film compositions were determined spectroscopically using primarily X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared methods. The XPS spectra were obtained using a Perkin-Elmer PSI 5000 series instrument equipped with an X-ray source monochromator. The X-ray source employed is an Al KR at 1486.8 eV. A pass energy of 17.90 eV giving a resolution of 0.60 eV with Ag(3d5/2) was used. Spectra were recorded using a 70° take-off angle relative to the sample surface. An electron flood gun was employed to neutralize charge build-up on the nonconductive films examined in this work. The electron flood gun was operated under conditions which provided optimum resolution of the C(1s) peaks. The C(1s) binding energies were established by centering the lowest binding energy peak of the C(1s) multiplets at 284.6 eV.28 FT-IR spectra were recorded using a Bio-Rad Model FTS40 spectrometer operated at 8 cm-1 resolution. Data were recorded as transmission spectra of films deposited on KCl disks. Film thickness measurements were obtained using a Tencor Alpha Step 200 profilometer. A metal-tipped pen was employed to scribe a thin line in films deposited on silicon substrates, after which film thickness was determined using the profilometer. The thickness of a given film was measured at three different positions for each sample. The hydrophilic character of the films was established using a Rame´-Hart goniometer and the liquid water sessile drop method. Quantitation of the surface hydroxyls was obtained using the trifluoroacetic anhydride (TFAA) derivatization technique. This well-documented technique can be used to reveal the presence of hydroxyl groups that are masked in high-resolution C(1s) XPS spectra by other bonds.6,28,30 For example, one cannot distinguish between alcohol and ether linkages by C(1s) XPS. However, selective derivatization of the C-O-H groups with TFAA introduces easily discernible CF3 and OC(O)CF3 carbons via large C(1s) binding energy shifts created by the presence of the highly electronegative fluorine atoms.28 In the present case, the derivatizations were carried using using vapor phase TFAA. Pyridine vapor was also present to neutralize the acid byproduct created during the reaction. On the basis of trial experiments, a standard 5 min reaction time was employed. At longer reaction times the integrity of the plasma films seemed to be affected by excess contact with TFAA molecules. At reaction times less than 5 min, incomplete reaction of surface hydroxyls was noted. After derivatization, the samples were pumped overnight to remove any volatile materials. The overall efficiency of this procedure was checked using poly(vinyl alcohol) (PVA) films spin-coated onto silicon substrates.6 The high-resolution C(1s) peak assignments after TFAA derivatization are the same as those employed by previous workers employing this derivatization technique. The allyl alcohol (+99%) was obtained from Aldrich Chemical (28) Cf.: Beamson, G.; Briggs, D. High Resolution XPS of Organic Polymers, The Scienta ESCA 300 Database; John Wiley and Sons, Inc.: New York, 1992. (29) Everhart, D. S.; Reilly, C. N. Anal. Chem. 1981, 53, 665. (30) Ameen, A. P.; Ward, R. J.; Short, R. P.; Beamson, G.; Briggs, D. Polymer 1993, 34, 1795.
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Figure 1. FT-IR transmission spectra of pulsed plasma polymerized allyl alcohol films deposited at 300 W and rf duty cycles ranging from 1/2 to 1/30 (ms/ms) as shown. Co. The TFAA (99%) was obtained from Sigma Chemical Co. These chemicals were outgassed before use, but they were not subjected to any additional purification. FT-IR analyses of the allyl alcohol monomer did reveal the presence of a slight CdO containing impurity.
Results FT-IR spectra of the pulsed plasma deposited films, although only qualitative in nature, reveal progressive changes in film composition with variations in the rf duty cycles employed. This film chemistry controllability is illustrated in Figure 1 via a stacked plot fo FT-IR transmission spectra of films deposited at rf duty cycles (i.e., plasma on/plasma off times, in milliseconds) ranging from 1/2 to 1/30 as shown. Each run was carried out using a peak power of 300 W. As these spectra clearly reveal there is a progressive increase in the retention of the monomer’s hydroxyl content with decreasing rf duty cycle employed during the deposition. This increase is easily discerned by comparison of the relative intensities of the O-H (∼3400 cm-1) and C-H (∼2900 cm-1) stretching vibrations as the duty cycle changes. Additionally, there is a progressive increase in the intensity of the C-O (∼1100 cm-1) stretching vibration with decreasing rf duty cycle. It is of interest to note the continual slow shift in the O-H stretching frequency to lower wavenumbers with increasing O-H concentrations. This variation is particularly notable between films deposited at 1/10 and 1/30 duty cycles in which the O-H absorption band maxima change from 3462 cm-1 at 1/10 to 3358 cm-1 at 1/20. We believe this abrupt change in frequency signals the presence of sufficiently high OH concentrations to form a H-bonded network among adjacent OH groups. The observed 100 cm-1 frequency shift to the 3358 cm-1 maximum is in agreement with well-documented examples of hydrogenbonded OH frequencies at ∼3350 cm-1.31 The exact controlllability of the OH concentration under pulsed deposition conditions was further demonstrated in runs carried out at intermediate rf duty cycles between 1/10 and 1/20. The OH absorption maxima were observed to vary progressively from 3462 cm-1 to 3432 cm-1 to 3404 cm-1 to 3358 cm-1 as the duty cycle was changed from 1/10 to 1/12 to 1/15 and 1/20, respectively.26 Another notable aspect of these films is the presence of the CdO groups as shown by the absorptions at 1700 cm-1. Although, as noted earlier, there is a small CdO containing impurity in the starting monomer, additional CdO is created under the high-energy plasma conditions employed for polymerization. It is clear that the extent of (31) Cf.: McIntyre, G. B. Practifal Infrared Spectroscopy; John Wiley and Sons, Inc.: New York, 1987.
Figure 2. C(1s) high-resolution XPS spectra of allyl alcohol films obtained at pulsed plasma rf duty ranging from 1/2 to 1/30 (ms/ms) top to bottom as shown.
CdO formation relative to OH incorporation of the film decreases rapidly with decreasing rf duty cycles. On the other hand, the relative intensities of the CdO and C-H absorptions appear to change only slightly with rf duty cycle variations. XPS analyses of the films shown in Figure 1 provide quantitative values for the increasing oxygen atom incorporation in the plasma-generated polymers with decreasing rf duty cycle. The high resolution C(1s) XPS results are shown in Figure 2, with the peak assignments indicated in the lowest duty cycle (1/30) run. Clearly, there is a progressive increase in the number of carbon atoms bonded to oxygen relative to carbon bonded to other carbons as the duty cycle is decreased. Table 1 provides a quantitative measure of the ratio of surface carbon to oxygen atoms obtained from the XPS analyses of the C(1s) and O(1s) peaks. As these data show, there is a steady decrease in the C/O atom ratio as the rf duty cycle employed
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Table 1. Surface Atom Concentrations of Plasma Films from Allyl Alcohol before and after Derivatization with TFAAa atomic concentrations (%) before derivatization after derivatization rf duty cycle on (ms)/off (ms) carbon oxygen carbon oxygen fluorine 1/2 1/5 1/10 1/12 1/15 1/20 1/30
300 W Peak Power 93.0 7.0 86.7 89.4 10.6 79.6 85.5 14.5 70.8 82.0 18.0 63.3 80.3 19.7 59.8 77.4 22.6 56.8 75.0 25.0 54.2
9.3 13.1 15.1 17.0 18.0 18.3 20.2
4.0 7.3 14.1 19.7 22.2 24.9 25.6
10/20 10/50 10/200 10/300 10/600
200 W Peak Power 84.8 15.2 75.9 84.1 15.9 70.3 77.6 22.4 56.2 74.0 26.0 50.6 73.2 26.8 52.1
16.4 15.9 19.7 24.3 20.4
7.7 13.8 24.1 25.0 27.5
a Allyl alcohol flow rate was 4 cm3 (STP)/min and reactor pressure was 0.11 Torr in all runs.
in film generation is decreased. This ratio approaches the limiting stoichiometric value of ∼3:1 present in the CH2dCHCH2OH starting monomer at the lowest duty cycle. As noted earlier, XPS spectra cannot differentiate between C-O-H and C-O-C carbons in view of the similarity of the C(1s) electron binding energy in these two functional groups. However, quantitation of surface hydroxyls is readily achieved using the TFAA derivatization reaction. In this procedure, hydroxyl groups are selectively converted to OC(O)CF3 groups via the reaction shown in eq 1. The newly introduced surface carbon atoms
(underlined in eq 1) are easily distinguished from other surface carbons. This is shown in Figure 3 for XPA spectra of TFAA derivatized surfaces for the same plasmagenerated samples shown previously in Figure 2. It is clear that the extent of fluorine atom incorporation after TFAA derivatization increases rapidly as the rf duty cycle employed in generating the film is reduced (top to bottom). It should be noted explicitly that TFAA also reacts with epoxide groups. The presence of any plasma-generated epoxides would thus lead to an overestimate of the film -OH content using the TFAA titration reaction method. However, the absence of the characteristic epoxide absorption between 1280 and 1260 cm-1 in the plasmadeposited films suggests that epoxide formation during the plasma polymerization must be of relatively little importance. The success of the TFAA derivatization process in quantitating surface hydroxyls was checked using a PVA standard. The results obtained are shown in Table 2 for TFAA vapor phase derivatizations carried out at room temperature for 25 min. As these data reveal, there is good agreement between the measured surface hydroxyls and the theoretical value predicted by the PVA repeat structure of H (
CH2
C OH
)n
Figure 3. C(1s) high-resolution XPS spectra of pulsed plasma polymerized allyl alcohol films after derivatization with TFAA vapor. The rf duty cycle employed in obtaining the original films is as shown and corresponds to the same duty cycles shown in Figure 2. Table 2. Comparison of Experimental and Theoretical Atom Concentrations Obtained with Conventionally Synthesized Poly(vinyl alohol) Film before and after Derivatization with TFAA atom concentrations as synthesized experimental (XPS) theoretical
after derivatization
C
O
C
O
F
65.0 67.7
35.0 33.3
46.9 44.4
23.9 22.2
29.2 33.3
As shown in Figure 3, equal areas are obtained for -OC(O)CF3 and -CF3 peaks as anticipated on the basis of the reaction stoichiometry (reaction 1). Comparison of Figures 2 and 3 reveals that the C-O group is reduced but not eliminated by the TFAA derivatization procedure. Thus, we conclude that some C-O-C (i.e., ether) type
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Table 3. Normalized Integrated Areas of High-Resolution XPS C(1s) Peaks after Derivatization of the Plasma Polymerized Allyl Alcohol Films with TFAA for Runs at 300 and 200 W Peak Power C(1s) peak relative areas rf duty cycles on/off (ms/ms)
C-C 284.6 eV
C-O-C 286.0 eV
CdO 287.6 eV
COC(O)CF3 286.9 eV
OC(O)CF3 289.7 eV
CF3 293.0 eV
1/2 1/5 1/10 1/12 1/15 1/20 1/30
83.2 82.0 72.6 64.3 59.4 54.5 53.2
6.7 8.5 10.0 8.9 11.6 11.6 7.4
300 W 1.5 1.2 1.2 1.5 1.9 1.3 1.6
3.9 4.2 6.7 9.5 9.4 11.0 13.6
2.0 1.8 4.3 7.5 8.4 10.9 11.5
2.8 2.3 5.3 8.4 9.3 10.8 12.7
10/20 10/50 10/200 10/600
76.6 72.5 53.6 48.7
11.1 10.3 12.0 12.5
200 W 2.4 1.7 2.0 2.3
5.2 6.8 10.9 12.3
2.3 3.7 10.1 11.9
2.4 4.9 11.4 12.3
linkages are generated in the films during the plasma deposition process. Alternately, the residual peak at this binding energy may reflect the presence of some unreacted hydroxyl groups. If this is so, the hydroxyl concentrations calculated via the TFAA derivatization procedure represent a lower limit. Also as shown in Table 3, and as noted previously in the FT-IR spectra of the films, the CdO group is also present in these films. However, the prominence of this group remains relatively small under all plasma deposition conditions employed in this work. The last three columns in Table 3 are all associated with the original presence of surface -OHs. As such, we expect the relative intensity of all three C(1s) atoms to be equal. Considering the inherent precision of XPS measurements, there is excellent agreement between CF3 and OC(O)CF3 peak areas. However, the COC(O)CF3 peak area is generally slightly higher than the other two. Possibly, this may indicate a small contribution from some unidentified oxidized carbon species. For example, the more detailed analysis of TFAA derivatized hydroxyl films by Ameen et al.30 included additional peaks for β-shifted CH2 which might account for this difference. In the present case, only the CF3 and OC(O)CF3 peaks are utilized in computing the -OH content of the films. Power Input Variation. Several runs were carried out using significantly lower rf peak power (i.e., 200 instead of the previous 300 W) and longer plasma on times (i.e., 10 ms instead of the previous 1 ms). Again film chemistry was monitored as a function of plasma-off times, all other plasma variables being held constant. Data from these runs have been included in Tables 1 and 3. The results obtained parallel, to a very high degree, those obtained at higher power and shorter plasma on time. As before, a progressive change in film chemistry is observed with decreasing rf duty cycle corresponding to increased retention of oxygen, speficically the -OH group, in the plasma-generated films. This increase is clearly evident in FT-IR transmission spectra of films obtained at different rf duty cycles as shown in Figure 4. As this stacked plot reveals, there is a very large increase in the O-H absorption band intensity relative to that of C-H as the rf duty cycle is decreased. As in the higher power runs, the O-H stretching frequency maximum shifts to progressively lower wavenumbers as the relative concentration of hydroxyl groups in the film is increased. Contact Angle Measurements. The hydrophilic character of the plasma-generated films was determined using the Rame´-Hart contact angle goniometer with water as the sessile drop fluid. The surfaces exhibited a systematic increase in hydrophilicity for films deposited at decreasing rf duty cycles. This increase is shown in Figure 5 as a progressively decreased contact angle with
Figure 4. FT-IR transmission spectra of pulse-plasmapolymerized allyl alcohol films deposited at 200 W and rf duty cycles ranging from 10/50 to 10/600 (ms/ms), as shown.
Figure 5. Advancing contact angle of sessile water dropet on plasma-polymerized allyl alcohol films deposited at rf duty cycles ranging from 1/2 to 1/30 (ms/ms). All films were deposited using a peak rf power of 300 W and a constant plasma on time of 1 ms.
increasing plasma-off times. The contact angles shown represent the average of multiple measurements (at least four per sample) of advancing contact angles taken at different points on the coated substrates. The decrease in the contact angle with increasing plasma-off times employed during film deposition correlates with increasing
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Figure 6. Film deposition rate per complete pulse for allyl alcohol films deposited at a constant plasma-on time of 1 ms and plasma-off times ranging from 2 to 30 ms. All depositions were done at 300 W peak power.
Figure 7. Film deposition rates per complete pulse cycle for allyl alcohol films deposited at a constant plasma-on time of 10 ms and plasma-off times ranging from 20 to 600 ms. All depositions were done at 200 W peak power.
film oxygen content, particularly hydroxyl group incorporation, and thus increasingly polar surfaces. It is of interest to note that a break in the relatively smooth decreasing contact angle with increasing -OH film content is observed at plasma-off times longer than 15 ms. This break occurs roughly at the point of essentially complete hydrogen bonding of the surface hydroxyls. Film Thickness vs rf Duty Cycles. The deposition rates were determined by measurement of film thicknesses at the end of a run. In view of the varying duty cycle pulsed discharges employed, the usual practice of plotting deposition rates per unit time is of relatively little value. Instead, these deposition rates are plotted with respect to the rate per complete pulse cycle (i.e., rate per unit energy input). The results obtained are shown in Figure 6, as a function of the plasma-off times for runs carried out at a constant plasma on time of 1 ms and peak power of 300 W. In general, the film thickness observed for a given sample was relatively uniform corresponding to variations of
