Langmuir 1995,11, 2576-2584
2576
Quantitative Analysis of Polymer Surface Restructuring Ronald C. Chatelier," Ximing Xie, Thomas R. Gengenbach, and Hans J. Griesser Division of Chemicals and Polymers, CSIRO, Private Bag 10,Rosebank MDC, Clayton 3169, Australia Received December 2, 1994. In Final Form: April 10, 1995@ Polymer surfaces can adapt to surrounding media by reorientation of surface segments; such motions can produce changes in contact angles (CAS). Building on two existing formalisms for time-independent CAS of heterogeneous surfaces, treatments are derived for the analysis of time-dependent CAS, enabling quantitative evaluation of the restructuring of polymer surfaces. In particular, we treat the case of air1 water CAS on polymers which contain some polar surface groups that are immobile and others that are mobile and can migrate away from the surface by rotational and/or translational motions of polymer chains. A first-order process is assumed for the rate-limiting reorientation motion. The resulting equations have three parameters, uiz., the fraction 6f the surface area covered by mobile polar groups, the fraction of the surface area covered by immobile polar groups, and the characteristic time constant (lifetime) of the reorientation process. The theoretical descriptions were applied to sets of contact angles measured on various plasma-modified polymer surfacesover extended periods of time, enabling quantitative comparison of the effects of identical plasma treatments on the surface restructuring of different polymers.
Introduction The mobility of polymer chains enables polymer surfaces to respond to interfacial forces and thus to adapt their surface chemical structure to their environment. By rotational and translational motions of chains and chain segments, the surface composition can change in order to minimize the interfacial free energy between the polymer Specifically, in a nonpolar and its environment such as air, polymers minimize the density of polar groups at the ~ u r f a c e whereas ,~ in an aqueous environment it is energetically more favorable for the polymer surface to become enriched in polar groups and reduce the density of nonpolar group^.^ An equilibrated interface between a polymer surface and its environment may be disturbed by a change in the environment. Alternatively, the insertion into polymer surfaces of extraneous chemical groups by a surface modification process typically creates surfaces that are not a t equilibriuma2Freshly modified polymer surfaces thus are unstable and lose, within hours or days, partially or completely the properties conferred by surface t r e a t m e n t ~ . ~ ,For ~ - linstance ~ when hydrophobic polymers have been modified to create a hydrophilic surface and
* Author for correspondence. Abstract published in Advance A C S Abstracts, J u n e 1, 1995. (1)Andrade, J . D., Ed. Pol.ymer Surface Dynamics; Plenum Press: New York, 1988. (2) Morra. M.: Occhiello. E.: Garbassi. F. Proceedines o f the First International Conference o n Polymer Solid Interfaces; I6P Publishing: Philadelphia, PA, 1992; p 407. (3) Garbassi, F.; Morra, M.; Occhiello, E. Polymer Surface-From Physics to Technology; Wiley: Chichester, 1994; Chapter 2. (4) Baszkin, A,; Ter-Minassian-Saraga, L. Polymer 1974,15, 759. (5)Holly, F. J.; Refojo, M. J. Biomed. Mater. Res. 1976,9,315. (6) Yasuda, H.; Sharma, A. K.; Yasuda, T. J . Polym. Sci., Polym. Phys. Ed. 1981,19, 1285. (7) Holmes-Farley, S. R.; Reamey, R. H.; Nuzzo, R.; McCarthy, T. J.; Whitesides, G. M. Langmuir 1987,3,799. (8) Garbassi, F.; Morra, M.; Occhiello, E.; Barino, L.; Scordamaglia, R.Surf. Interface Anal. 1989,14, 585. (9) Youxian, D.; Griesser, H. J.; Mau, A. W.-H.; Schmidt, R.; Liesegang, J . Polymer 1991,32,1127. (10)Strobel, J. M.; Strobel, M.; Lyons, C. S.; Dunatov, C.; Perron, S. J. J. Adhesion Sci. TechnoE. 1991,5, 119. (11)Griesser, H. J.;Youxian, D.; Hughes, A. E.; Gengenbach, T. R.; Mau, A. W.-H. Langmuir 1991,7 , 2484. (12) van der Mei, H. C.; Stokroos, I.; Schakenraad, J. M.; Busscher, H. J. J. Adhesion Scz. Technol. 1991,5, 757. (13)Xie, X.; Gengenbach, T. R.; Griesser, H. J. J. Adhesion Sci. Technol. 1992,6, 1411. (14) Yasuda, T.; Miyama, M.; Yasuda, H. Langmuir 1992,8,1425. @
are stored in contact with air, polar surface groups are transported into the polymer and untreated, nonpolar chain segments emerge to the surface. This so-called "aging" of surface-modifiedpolymers has been intepreted as arising from the action of interfacial forces which act to re-establish a more suitable surface c o m p ~ s i t i o n . ~ , ~ Often termed surface restructuring or surface rearrangement, the response of monolithic polymers to changes in surrounding media5J6J7 and the aging of surface-modified polymers6-15are observable in changes of aidwater contact angles (CAS) and by surface spectroscopic methods. A number of studies have reported data which document the occurrence of polymer surface restructuring, and its mechanism has been described qualitatively in terms of diffusion of energetically unfavorable groups away from the interface. There is, however, a lack of quantitative analysis of the evolution of polymer surface compositions in response to changes in the environment or a surface modification process. We are interested in the quantitative analysis of surface restructuring to improve understanding of the time dependence of polymer surface properties and to establish quantitative comparisons of the effects of surface treatments applied to a range of polymers. The rate of loss of polar groups upon aging in air, and the percentage of polar groups attached that are lost, are properties of both fundamental and applied interest. Moreover, we are interested in determining how and why changes in the experimental parameters of the surface modification process affect the density and rate of loss of polar groups. Contact angles provide an attractive basis for a quantitative description ofpolymer surface dynamics since they probe the surface layers to a depth comparable to the range of the interfacial forces involved in surface restructuring. CAS relate to the fractions of nonpolar and polar structural elements on a surface.lsJg We have therefore based our treatments of polymer surface dynamics on the time dependence of advancing and receding CAS. Our formal(15) Gengenbach, T. R.; Xie, X.; Chatelier, R. C.; Griesser, H. J. J. Adhesion Sci. Technol. 1994,8, 305. (16) Van Damme, H. S.; Hogt, A. H.; Feijen, J. J. Colloid Interface Sci. 1986,114,167. (17) Laoharojanaphand, P.; Wang, L.; Stoffer, J. 0.ACSPolym.Prepr. 1990,31 (2), 77. (18)Cassie, A. B. D. Discuss. Faraday SOC.1948,3,11. (19) Johnson, R. E.; Dettre, R. H In Surface and Colloid Science; Matijevic, E., Ed.; Wiley Interscience: New York, 1969; Vol. 2, p 85.
0743-746319512411-2576$09.00/0 0 1995 American Chemical Society
Analysis of Surface Restructuring isms enable calculation of the time-dependence of the fractional densities of nonpolar and polar structural elements at the surface. In this communication we derive theoretical treatments for time-dependent CAS and apply them to the quantitative, comparative analysis of the surface restructuring behavior of fluorocarbon and hydrocarbon polymers modified by ammonia and water plasmas. The treatments reveal substantial differences in restructuring rates and the initial and final surface fractions of polar and nonpolar structural elements. In a companionreport,20we use these treatments of surface restructuring to analyze quantitatively the effects of varying surface modification conditions upon the surface dynamics of plasma-modified Teflon FEP.
Theory When a polymer surface is composed of a mixture of polar and nonpolar groups, the surface densities of these groups can change in response to interfacial disturbance~.’-~Structural elements (chains and chain segments) can migrate between polymer surfaces and deeper layers (“bulk”) by rotational and translational motions. With time, chemical groups that are energetically less favorable at the interface become replaced with energetically more favorable groups; i.e., there occurs simultaneous inward migration of interfacially unfavorable groups and emergence of the opposite type of groups to the interface. In this report we treat quantitatively the dynamics of polymer surfaces in contact with air, characterizing their surface restructuring in terms of experimentally observable increases in airlwater CAS, increases which are indicative of the migration of polar groups away from the surface layers. Restructuring in other media can be formulated analogously. For the ensuing discussion we will use the operational definitions that the “surface” is the polymer region, thought to be of the order of 5 1nm thick based on the work of Whitesides et al.,7,21that interacts with the environment across the interface and is probed by CAS, while the “subsurface” is the region of the polymer material which is outside (deeper than) the range of interfacial forces. Thus, CA increases are interpreted as resulting from the migration of polar groups away from the interface to depths L 1nm into the polymer. Our model starts from existing, time-independent formalisms for CAS of two-dimensional surfaces that contain polar and nonpolar groups. We incorporate a kinetic scheme which introduces time dependence into the formalisms. Changes in the depth distribution ofpolar and nonpolar groups are reflected in the time dependence of the CAS. Consider a surface which is composed of a mixture of two types of chemical species. One is a nonpolar polymer segment such as -CFz- or -CH2-. The other type is a polar species such as an amine or a hydroxyl group. In principle, more than two types of groups could be required for an accurate description, but for polymers such as polypropylene and fluorinated ethylene-propylene copolymer (FEP) the differences between -CX~CXZ-and -CXCX3- (X = H or F) segments can be neglected as a first approximation, particularly in the light of experimental uncertainties in CAS. According to Cassie,18 when there are two types of surface patches the equilibrium contact angle can be expressed as (20) Chatelier, R. C.; Xie, X.; Gengenbach, T. R.; Griesser, H. J. Langmuir 199S,ll,2585. (21)Bain, C. D.; Whitesides, G. M . J.Am. Chem. SOC.1988,110, 5897.
Langmuir, Vol. 11, No. 7, 1995 2577
where f, is the fraction of the surface area covered by polar regions, @Ep is the equilibrium airlwater CA of a surface consisting only of polar regions, and fnp and 6Enp analogously apply to a pure nonpolar surface, with fnp = 1 - f,. The Cassie approach assumed distinct, macroscopic patches of groups on the surface. A more recent approach by Israelachvili and Geez2assumed heterogeneity of the surface on a molecular scale and led to the relationship ~
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We extend these time-independent descriptions and derive time-dependentequations for surface restructuring by assuming that, when polymers age in contact with air, the interfacial density of polar species diminishes over time. As the polar groups diffuse into the polymer, they are replaced by nonpolar groups which diffuse toward the surface from the subsurface region. The surface dynamics of a polymer can then be characterized in terms of the time dependence of the fractions of nonpolar and polar chemical groups on the surface. However, CAS are not governed exclusively by the outermost monolayer of chemical groups; contributions by groups below the immediate surface (but within -1 nm) are smaller but not negligible. On restructuring, therefore, the migration of a polar group into the polymer does not cause an instantaneous loss in its interfacial contribution;rather, a gradual reduction of its contribution occurs as it migrates across the top nanometer. The existing descriptions contain heterogeneity in the surface plane but do not include a depth dimension. As interfacial forces decrease with increasing depth by various distance laws, the summation of interfacial forces for a realistic polymer system is prohibitively complex. In addition, compositional profiles over the top nanometer are difficult to obtain with sufficient accuracy. Instead of considering interfacial interactions threedimensionally, in the surface plane and across varying distances (depths) from the interface, in our model the contributions from groups at various depths are projected onto the surface, to regenerate a two-dimensionalmodel. The magnitude and effect of each projection are determined by the nature of the chemical group, its distance from the interface, and the nature of the interfacial force involved. Thus, we picture the surface as being heterogeneous effectively on a submolecular scale, with many tiny surface patches. The nature and area of each patch are related to the type and magnitude of the interfacial force that the particular group exerts. In this model, we represent the two-dimensionalsurface as consisting of numerous tiny polar and nonpolar patches, whose summation then definesf, and fnp. Thus, as a polar group migrates inward, the magnitude of its interfacial contribution diminishes, and this can be represented, by using the approach of projection onto the surface, as a decrease in the surface area occupied by this group. The average contribution of all the effective surface areas of polar groups is related to the fractional surface density, f,. Thus,f, contains informationon the depth distribution. If the absolute number of polar groups does not change over time, then changes in f, can be interpreted in terms of changes to the depth distribution of the polar groups. One needs to be careful, however, in interpreting f p ; it does not describe the absolute density at the outermost (22) Israelachvili, J. N.; Gee, M . L. Langmuir 1989,5, 288.
Chatelier et al.
2578 Langmuir, Vol. 11, No. 7, 1995 surface (a density which is ofrelevance for covalent surface attachment reactions). To summarize the model thus far, contributions to the properties of dynamic polymer surfaces can be described by a two-dimensional model; the migration of a particular component into the polymer is represented by a reduction, or disappearance, of the effective area of its contribution to interfacial properties. The surfaces of alkyl acrylate polymers do not become fully hydrophobic or hydrophilic in nonpolar or polar environments, r e ~ p e c t i v e l y . ~ JTheir ~ J ~ surface restructuring is incompletepresumably because rotation around the central backbone will not remove the interfacially “undesirable”group entirely from interfacial interactions; polar and nonpolar groups attached to the same polymer backbone segment cannot migrate far enough apart. The surface properties of amphiphilic polymers are thus determined by a combination of effects from polar and nonpolar groups, with the relative contributions varying with the environment. Likewise, surface-modified polymers usually do not revert fully to their original (unmodified) properties on aging;9J1-13J5the aged surface is a heterogeneous mixture of polar and nonpolar groups. We allow for the observed noncompletion of restructuring by defining two populations of polar surface groups, attached onto “mobile” and “immobile” surface chain segments, respectively. We use the operational definitions that “immobile” polar groups are those that, for instance due to near by cross-links, cannot participate in polymer chain motions that would transport them beyond the range of interfacial potentials, and “mobile”groups are those that can be transported to deeper than 1 nm over the time scale of the experiment. Contact angle andXPS data13J5suggest an exponential time course for surface restructuring. Assuming an exponential decay of the surface populationof mobile polar groups (fraction f m ) , f, can be expressed as
(3) where t is the characteristic time constant for surface restructuring and fim is the fraction of the surface area covered by immobile polar groups. Insertion of eq 3 and using fnp = (1 - f,) leads to the following time-dependentrelationships for the equilibrium contact angle of a restructuring surface
when extending the Cassie equation, and in the case of the Israelachvili-Gee description
How do the above descriptions relate to experimental CA data? The literature is not unanimous on how the theoretical equilibrium CA, I%, of a surface can be derived experimentally. The information content and problems in CA studies have been reviewed recently.23 We assume that the experimentally observable advancing (ACA)and receding (RCA) contact angles are indicators of surface restructuring, since they are thought to probe for hydrophobic and hydrophilic components of the surface, respectively, and would thus be affected by redistribution of polar and nonpolar groups. Accordingly, we shall (23) Morra, M.; Occhiello,E.; Garbassi, F.Adu. Colloidlnterface Sci. 1990,32,79.
approach the study of restructuring using an experimentally obtained mean airlwater CA, OM, defined by
and assume that OM = @E. We will examine this assumption in the companion reportz0 using a larger set of experimental data. Surface topography also has an effect on contact angles,%the above descriptions are only valid if the surface topography does not become altered on aging.
Experimental Section Plasma Surface Treatments. Perfluorinated ethylenepropylene copolymer(FEP)and poly(tetrafluoroethy1ene)(PTFE) were obtained in the form of tape of 12.7 mm width. The PTFE tape was kindly provided by Dr. C. Lyons, 3M Co., while the FEP tape was of commercial origin (Du Pont FEP 100 Type A). Low density polyethylene (LDPE), from Cadillac Plastics, was used in sheet form. These polymer substrates were surface modified using a gas plasma established in ammonia gas or water vapor. The cleanliness of the substrates prior to plasma modification was checked by XPS analysis as described previously.ll In addition, a thin plasma polymer coating from n-hexane vapor (n-hex pp) was deposited on to FEP tape as described previ0usly.2~The n-hex pp can be considered as a highly crosslinked analogue of polyolefins;it was thus of interest to compare the surface restructuring of surface-modifiedn-hex pp with that of LDPE. Ammonia (Matheson) was supplied from a cylinder via a stainless steel line and a mass flow controller (MKS). Triply distilled water was placed in a round-bottom flask, and the vapor fromevaporation at room temperature was suppliedto the reactor chamber. The general procedure we use for plasma surface treatments has been reported earlier,13J5 and the custom-built plasma apparatus has also been described previously.26 Briefly, the reactor chamber (volume = 7600 cm3)contained two vertically oriented copper electrodes of dimensions 90 x 18 mm placed in a vertical glass cylinder. The electrodes were spaced apart by 16 mm. Plastic and glass fittings defined a controlled path for the flow of incoming processvapor, extendingfrom the gas inlet to the electrodes. The pressure in the reactor was adjusted via the throttle valve at the reactor outlet. The plasma discharge was powered by a commercialgenerator (EN1ACG-3)operating at 13.56 MHz and equipped with a matching network. The plasma treatments were performed using the experimental conditionsof 0.30 Torr pressure, 30 W power,and 60 s treatment time. The reactor also incorporated a tape transport system for semicontinuous treatment of 12.7mm wide tape movingthrough the plasma at a controlled speed. Experiments were performed with sheets of polymer substrates, 75 mm long,attached by thin, double-sided adhesive tape to the face of the electrodes, and also (except for LDPE) with extended lengths of tape moving at a constant speed through the plasma zone. The treatment of extended lengths of tape moving at a constant speed enabled the fabrication of a large number of nominally identical specimens (XPSanalyses of randomly selected samples along the length of tape showed no significant variations in composition),which, afbr plasma treatment,were stored in tissue-culture polystyrene (TCP) dishes at ambient temperature (21 & 1 “C) and assessed periodically,using a fresh piece of tape for each CA measurement. The storage of samples in TCP dishes, which are explicitly designed to avoid contamination by polymer additives, evidently avoids problems with adventitious contamination; we have previously discussed our evidence, obtained by XPS on a range of samples of different surface energies, against the significant buildup of hydrocarbon and other contamination on storage.11J3 (24)Wenzel, R.N.Ind. Eng. Chem. 1936,28,988. (25)Gengenbach,T.R.;Vasic, Z. R.; Chatelier,R. C.; Griesser, H. J. J.Polym. Sci., Part A: Polym. Chem. 1994,32,1399. (26)Griesser, H.J. Vacuum 1989,39,485.
Langmuir, Vol. 11, No. 7, 1995 2579
Analysis of Surface Restructuring 1201
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The surface modification of n-hex pp was performed as a twostep process without breaking vacuum; immediately following deposition of the n-hex pp on to FEP, the remaining n-hexane vapor was pumped off and then ammonia gas supplied for the surface treatment. Contact Angle Measurements. The apparatus and methods used for the determination of advancing and receding airlwater CASwere the same as those detailed elsewhere.13 Measurements were performed in quadruplicate on specimensstored for various periods oftime; standard deviations typically were 1to 2". Clean reference FEP surfaces (SCA = 107", RCA = 98", and ACA = 117") were used to ascertain reproducibility of measurements and purity of the triply distilled water. ScanningTunnelingMicrosocopy. Acustom-builtair STM unit incorporating an inverted piezo-tube scanner design was used to assess surface topography. The sensitivity of the STM head was calibrated to within f10% using highly oriented pyrolytic graphite (JC,y sensitivity) and a nickel compact-disk stamperwith a pit height of 140nm (z sensitivity). The insulating polymer specimens were mounted on stubs using conducting cement, with some of the cement lapped around the specimen edge, and coated by rfsputtering with a layer of platinum 3 nm thick, as assessed by a thickness monitor (Inficon). This Pt overlayer,with a mean grain diameter of between 5 and 10 nm, limits the resolution of STM to topographical features larger than -10 nm. This is not a serious concern for the present study since it is generally assumedthat wettability measurements are not affected by surface roughness on such a small scale.
Results 1. Comparison of Surface Theories. Before the time-dependent CA treatments for analysis of surface restructuring of experimental samples are used, the timeindependent CA formulations for heterogeneous surfaces by Cassie (eq 1)and Israelachvili and Gee (eq 2) will be compared in simulations using values of @Epand @EnPwhich are typical of the common practical situation where hydrophilic groups have been attached onto the surface of hydrophobic polymers. Figure 1shows a plot of & versus fp using eqs 1and 2. The value of @Ep used in this simulation is O", the mean contact angle expected when interfacial interactions are determined exclusively by amine or other similarly polar groups. The value of @Enp used equals the experimental value of @M measured on a n untreated, pure FEP surface. The @M value for PTFE is similar. Thus, Figure 1relates generally to the surface modification of perfluorocarbon polymers with treatments which introduce polar groups onto these materials. Both theories predict a quasi-linear
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relationship between @E and fpfor values of fpbetween 0.2 and 0.8. However, the Cassie equation consistently predicts a higher value of @E for a given value of f,. As a corollary, when the Cassie equation is applied to experimentalvalues of contact angles, it will predict higher values off, than the Israelachvili-Gee theory. The discrepancy between the Cassie and the Israelachvili-Gee equations is further explored in Figure 2 in which the ratios of the values of @E predicted by the two equations are plotted against f,. The calculations show that the discrepancy increases sharply as the value off, deviates from either zero or unity. The effect is also more pronounced as the difference between @Ep and @EnP increases. The discrepancy may be as high as 20% for experimentally realizable values off,, @Ep, and @Enp. 2. Application to the Analysis of Experimental Time-Dependent CA Data. The premise behind the present work is that detailed characterization and quantitative evaluation of the time dependence of CAS of equilibrating surfaces can provide insights into the molecular phenomena involved in polymer surface dynamics and the molecular consequences of surface modification processes. As a first step, we shall in this report apply our analytical treatments to investigate how the two plasma modifications (H20 vapor and NH3) affected the surface restructuring of the various polymers used. We compare the density of polar groups initially attached by the modification, and the rates and extents of burial of the polar groups upon aging of the surfaces in air.
Chatelier et al.
2580 Langmuir, Vol. 11, No. 7, 1995
It appears reasonable to assume that the introduced groups are located randomly on plasma-modified surfaces, i.e., there is no correlation in the distribution of polar and 100 nonpolar molecular structures, on account of the nature of the plasma treatment process, in which reactive 80 molecules from the plasma phase are thought to impinge on the polymer surface independently of each other. Upon 60 plasma treatment with an inorganic gas such as ammonia, oxygen, or water vapor, the resultant molecular structures 40 on the polymer surface are likely to be individual chemical groups derived from incorporation of molecular fragments 20 from the treatment gas into the polymer chain. When organic vapors are used, on the other hand, larger 0 molecular structures can be formed by oligomerization 0 50 100 150 200 250 reactions occurring in the plasma vapor phase prior to the oligomer entity impinging on the polymer substrate Storage time (days) surface. Hence, at low surface coverages such plasmas can give polar surface patches larger than individual 120 I chemical groups. However, when treatingfor sufficiently long times the patches will "coalesce" and eventually form a continuous, amorphous surface. The Israelachvili-Gee approach, which was designed l o80 o specificallyfor surfaces with heterogeneity on a molecular scale, might seem preferable but in the absence of 60 independent information on the distribution of polar groups over the surface, that is, whether the groups are 40 organized in distinct macroscopic patches or whether they are molecularly admixed, we will use both approaches to 20 study the restructuring of plasma-modified surfaces. 2.1. Ammonia-Plasma-Modified Surfaces. We 01 have previously reported that NH3 plasma modification 0 50 100 150 200 250 of FEP and PTFE resulted in very hydrophilic surfaces, but when the treated specimens were stored in air, the S t o r a g e time (days) CAS increased substantially in the course of a few d a y ~ , ~ ~ J ~ Figure 3. Advancing ( 0 )and receding (0)a i d w a t e r contact suggesting that some of the polar groups introduced by angles of an ammonia-plasma-treated FTFE surface as a the treatment2' became buried in the polymer by rotational function of storage time after treatment a n d (B) the m e a n and/or translational motions. In this study we have contact angle, OM, as a function of storage time. The dashed plasma-modifiedfluorocarbon and hydrocarbon polymer line in panel B represents the best-fit based on eq 4, a n d the surfaces using identical NH3 plasma conditions and solid line represents the best-fit based on eq 5. quantitatively analyzed their aging by our newly develTable 1. Surface Restructuring Parameters of oped formalisms, to investigate whether the fluoropolyPlasma-Modified Surfaces Calculated Using Equations 4 mers might behave in a distinctly different way due to the and 5' electronegativity of fluorine which might affect the polymer plasmagas eq fm fim f,pb t(days) properties and reactivity of attached amine groups. Figure 3A reproduces experimental values of aidwater PTFE NH3 4 0.40 0.58 0.02 4.6 5 0.54 0.46 0.00 2.9 ACA and RCA measured on NH3-plasma-modified PTFE FEP NH3 4 0.30 0.70 0.00 3.2 specimens as a function of storage time (aging in air). 5 0.39 0.60 0.01 2.9 Figure 3B shows the values of OM calculated via eq 6 and LDPE NH3 4 0.47 0.38 0.15 2.3 the best-fit curves based on eqs 4 and 5. The values used 5 0.50 0.33 0.17 2.1 for 6EP and OEnp in these fits were the same as those used n-hex plasma NH3 4 0.41 0.55 0.04 46 for the above simulations. The resultant best-fit values polymer 5 0.47 0.48 0.05 43 5.4 PTFE HzO 4 0.09 0.44 0.47 offim,fm,and t a r e tabulated, for both formalisms,in Table 5 0.09 0.29 0.62 5.2 1. The two fits are of comparable quality; it is not possible FEP H2O 4 0.00" 0.60 0.40 c to decide which of the CA theories is superior based on 5 0.00" 0.49 0.51 c statistical considerations alone. The best-fit values offim, LDPE HzO 4 0.33 0.39 0.28 1.2 fm, and z differ somewhat, due to the differences, discussed 5 0.34 0.34 0.32 1.2 I
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(27) The chemical composition ofthe modified surfaces and the nature and densities of the polar groups introduced by the plasma treatment are not well understood (ref 15). Pentafluoroethylamine and its hydrochloride salt have been reported to eliminate HF within minutes, producing nitrile groups (Kumar, R. C . ; Shreeve, J. M. J.Am. Chem. SOC.1980,102,4958; Leidinger, W.; Sundermeyer, W. Chem. Ber. 1982, 115, 2892). However, we detected amine groups on freshly modified surfaces by a 2-h reaction with fluorescein isothiocyanate (ref 15). XPS also revealed post-treatment incorporationof oxygen (ref 15). We neglect possible complications arising from multifunctionality and assume identical values of @Ep for the various polar groups present. Thus, eqs 4 and 5 can still be used to describe the reorientation of the modified polar surfaces since not only aminegroups but also other nonfluorocarbon structural elements, such as nitrile and amide groups possibly resulting from post-treatment reactions, are subject to the restructuring forces driving toward removal of polar groups from the immediate surface.
The plasma conditions were pressure = 0.30 Torr, power = 30 W, frequency = 13.56 MHz, and treatment time = 60 s. fnp is not a fit parameter. It is calculated as 1- f m - fim and represents the fraction of the surface area which is nonpolar immediately after plasma treatment. CAchanges on aging were within experimental uncertainty of the CA determinations.
above, in the quantitative relationships between @E and f , in the two CA formalisms. In Figure 4 the analogous data and evaluations are presented for NHS-plasma-treatedFEP. In this case the best-fit curve based on eq 4 is very close to that based on eq 5. The values of fim and fm (Table 11, however, again differ significantly.
Analysis of Surface Restructuring
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Langmuir, Vol. 11, No. 7, 1995 2581
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Storage t i m e (days) Figure 4. (A)Advancing ( 0 )and receding(0) aidwater contact anglesof an ammonia-plasma-treatedFEP surfaceas a function of storage time after treatment and (B)the mean contact angle, OM, as a function of storage time. The dashed line in panel B represents the best-fit based on eq 4, and the solid line represents the best-fit based on eq 5. For both fluoropolymers and both descriptions, the values of the surface fractions indicated that the freshly prepared surfaces contained virtually no nonpolar structural elements. After extended aging, however, 30-54% ofthe polar surface fractions had disappeared. The "final", aged surfaces thus possessed a distinctly heterogeneous character. In contrast, the NH3 plasma modification of LDPE produced fresh surfaces which still contained a considerable fraction of hydrophobic structural elements. The CA data, the values of OM, and the best-fit curves, which overlap completely, are shown in Figure 5. Avalue of 85" was used for &*p in the fits of the t?M data obtained with modified hydrocarbon surfaces; this value was derived from experimental CASmeasured on a n untreated, clean LDPE surface. The best-fit parameters are listed in Table 1. After aging, the remaining polar fraction of the surface was substantially smaller than for the fluorocarbon polymers. Interestingly, the fraction of attached polar groups that were capable of undergoing inward diffusion was similar to that of PTFE/NHS. Ammonia plasma modification of the n-hexane plasma polymer was performed to study a system which was expected to differ markedly in its surface mobility. Plasma polymers are generally thought to possess a n extensively cross-linked, random, and infinite network structure (although the cross-link density may depend on the process parameters used in the plasma deposition). Accordingly, the mobility of the structural segments of a plasma
Storage t f m e (days) Figure 5. (A)Advancing ( 0 )and receding(0) aidwater contact angles of an ammonia-plasma-treated LDPE surface as a function of storage time after treatment and (B) the mean contact angle, OM, as a function of storage time. The line in panel B represents both the best-fits based on eqs 4 and 5. polymer is likely to be considerably less than the chain segmental mobility of a comparable conventional polymer. In Figure 6 CA and OM values are shown for n-hex/NH~. The best-fit curves based on eqs 4 and 5 overlap. The best-fit values for the surface restructuring parameters are listed in Table 1. Comparison of the values obtained with n-hex/NHs with those of LDPE/NH3 reveals that the plasma polymer restructured much more slowly, as expected. Interestingly, however, the fraction of polar groups that disappeared from the interface with air was not much smaller. We expected that the higher density of cross-links within the plasma polymer structure would not only markedly reduce the rate of surface restructuring but also lead to a much higher fraction of the polar groups being incapable of in-diffusion into the polymer, due to their being in the vicinity of a cross-link. The combination of a substantial loss of polar groups and a very slow rate of restructuring provides an intriguing glimpse of complexities that are not understood. Further work with the n-hex/NHs system is in progress. In summary, all four freshly prepared NH3-plasmamodified surfaces were dominated by polar surface fractions. The aging process then invariably led to a marked extent of disappearance of polar surface groups, a t widely differing rates. However, while there were quantitative differences in the surface restructuring parameters, the qualitative behaviors of the surfaces upon restructuring did not appear to differ fundamentally. Accordingly, we conclude that the CA changes on aging do not reveal fundamental differences in the evolution of modified PTFE
Chatelier et al.
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Storage t i m e (days) Figure6. (A)Advancing (0)and receding (0) aidwater contact angles of an ammonia-plasma-treatedn-hexaneplasma polymer surface as a function of storage time after treatment and (B) the mean contact angle, &, as a function of storage time. The line in panel B represents both the best-fits based on eqs 4 and 5.
and FEP surfaces compared with NH3-modified hydrocarbon surfaces. 2.2. Water-Vapor-Plasma-ModifiedSurfaces. Although the same experimental conditions as for NH3 plasma modifications were used, exposure of FEP and PTFE to a water vapor plasma caused distinct differences in the surfaces produced and their restructuringon aging. While the freshly produced surfaces had CAS higher than those of the fluoropolymersurfaces exposed to NH3 plasma, they demonstrated much lower mobility. In the case of FEP/H20,the CAS were constant on aging, and for PTFEI H20, the CAS increased only marginally. Figure 7 reproduces CA values determined on H2O-plasma-modified FEP specimens as they aged in air, the calcdlated values of OM, and the fits to eqs 4 and 5. The fits of the OM values to eqs 4 and 5 overlapped. Again, due to the different relationship between OM and f p in the two descriptions, the same CA data yielded different values for the value off, (Table 1). For PTFE (CA data and fits not shown) a similar behavior was found except that the small increase in the CAS on aging translated into a small but non-negligible value for the mobile polar fraction, f m (Table 1). With the small extent and relatively slow rate of restructuring of PTFE/H20, the calculated values of f m and t are subject to considerable uncertainty because the uncertainties in the CA measurements are much more critical in this case than for more mobile surfaces, such as those obtained with NH3 plasma treatment, for which the CA changes on
Storage t i m e (days) Figure 7. (A)Advancing ( 0 )and receding (0) aidwater contact angles of a water-plasma-treatedFEP surface as a function of storage time after treatment and (B) the mean contact angle, OM,as a function of storagetime. The line in panel B represents both the best-fits based on eqs 4 and 5. aging exceeded the uncertainties in the CAvalues by more than an order of magnitude. The values offnpshow that the H2O plasma modification was relatively inefficient; the freshly treated surfaces consisted of comparable fractions of polar and nonpolar patches. In other words, while the N H 3 plasma modification led to virtually complete coverage of the fluoropolymer surfaces with polar groups, the water-plasma-modified surfaces were not saturated with polar groups. In contrast to the situation with fluoropolymers, H 2 0 plasma modification of LDPE (under the same conditions) did not produce a surface resistant to aging. The usual pattern of CA increases (raw data not shown) indicative of surface restructuring was observed, similar to that of LDPEYNHS. The best-fit surface restructuring parameters (Table 1)were similar to those obtained for the LDPE/ NH3 case, except that the freshly prepared LDPE/H20 surface was somewhat less polar than the LDPENH3 surface. Interestingly, however, the final equilibrated surfaces produced upon storage in air were identical within experimental error, with 33-39% (depending on the equation used to fit the data) ofthe surface area consisting of polar patches and 61-67% covered with nonpolar patches. 2.3. Topography Effects? FEP was the smoothest of the substrates used, and thus topography contributions to changes of CAS with time should be easiest to detect and quantify. Several STM images were collected for each specimen in order to ensure reliable assessment of these gently undulating but randomly patterned sample sur-
Analysis of Surface Restructuring
Figure 8. Representative STM images recorded on F E P coated with -3 nm of Pt: ( a , top) “side” view; (b, bottom) “top” view of another surface area. Fine ridges evident in t h e top view a r e obscured in t h e side view.
faces. Figure 8 shows representative STM images of the surface topography of ammonia-plasma-modified FEP. In no case did we observe significant changes to the surface topography with time (data not shown). Thus, CA changes on aging are attributable exclusively to changes of the chemical composition of the surfaces.
Discussion We have developed two formalisms for a quantitative description of polymer surface restructuring in terms of experimentally observable changes of aidwater contact angles as surfaces age upon storage after fabrication. These formalisms were applied to CAS measured on various polymer surfaces which were exposed to two plasma modifications and then stored in air. The results demonstrate the applicability and usefulness of our formalisms and show how the restructuring behavior of different base polymers is affected by surface modifications using two different plasma process vapors under identical experimental conditions. The surface restructuring of polymers has in the past been described by qualitative models involving migration of polar and nonpolar groups. Ultimately, however, one
Langmuir, Vol. 11,No. 7,1995 2583 wishes to establish a quantitative understanding of the time dependence of polymer surface properties and correlate surface restructuring phenomena with molecular surface and bulk properties of polymers, such as Tgand segmental mobilities, determined by independent methods, and with process parameters of surface modification procedures. It is clear that in order to seek such correlations, a formalism for the quantitative analysis of surface restructuring is required. Our descriptions provide such a tool. The surface restructuring parameters obtained via our formalisms also enable compact description of substantial CA data sets. It is also clear that the mathematical fit parameters (Table l), while enabling quantitative description of the time dependence of polar and nonpolar surface fractions, do not yet link to fundamental, molecular properties characteristic of the unmodified and modified base polymers. Elucidation of relationships between surface restructuring parameters and molecular properties will require considerable further work, both in molecular modeling and experimental searches for correlations. As a first step, however, the present formalisms enable analysis of restructuring and provide values for rates and extents of changes to the fractional areas of a surface covered by polar and nonpolar patches. The surface restructuring parameters obtained by our formalisms are useful in being more amenable to interpretation and comparison than “raw” CA data. The parameters can be used to quantitate differences in the restructuringof, e.g., various polymers subjected to a given surface modification, or a given polymer subjected to different modifications, and thereby open up new areas of investigation. For instance, the difference in t of LDPE/ NH3 and LDPE/H20 (Table 1) is significant and worthy of further investigation. Does the H20 plasma modification cause, under the same process parameters, more cleavage of LDPE chains than the NH3 plasma modification does, or does the latter process produce an enhancement of cross-link density in the LDPE surface layers? Also of interest is the unexpectedly high reduction upon aging in the fraction of polar groups on the n-hex ppNH.3 surface. Clearly, a quantitative formalism for analyzing the aging of polymer surfaces is a prerequisite for further, detailed analysis of such phenomena. Table 1 shows that for related polymers and plasma treatments, there can be marked variation in the extent and rate of mobility of the modified surfaces. These differences in restructuring are probably related to differences in both the original cross-link density of the substrate polymer and the extents of cross-linking and bond cleavage reactions conferred by the plasma treatments. Compositional changes to the surface layers arising from such reactions cannot be detected by current surface analytical techniques such as XPS. However, analysis of surface restructuring as a function of process parameters might provide some indication of the possible occurrence of such cross-linking and chain scission reactions. This theme is taken up in the companion paper.20 The treatments of polymer surface dynamics derived in this communication rest on a number of assumptions, and it is not clear at this stage to what extent a single mathematical relationship can be expected to be applicable to mobile polymer surface layers of widely varying compositions and experiencing various types of intermolecular and interfacial forces. One simplification in our treatments is the neglect of structural factors such as crystalline and amorphous regions, which may have different extents and time constants of restructuring. Another simplification is the classification into “mobile” and “immobile”groups; probably a gradation of mobility
2584 Langmuir, Vol. 11, No. 7, 1995
ofpolymer chain segments is a more realistic picture, but, by the operational definition that “mobility”equates with transport to deeper than the range of interfacial forces experienced by a water molecule in the CA determination, we can circumvent the need for gradation of mobility. Another assumption in our treatments is that the changes to the CAS on aging are interpreted only in terms of rotational and translational restructuring motions. This model applies to polymers such as PHEMA5 and poly(alkyl methacrylates),16J7 whose surfaces rearrange, probably by chain rotations, when subjected to a sudden change in environment. The chemical composition of the surface layers of surface-modified polymers and plasmadeposited polymers, on the other hand, can, as shown by XPS analyses, undergo considerable changes after processing.l5 Such changes, which comprise oxidative reactions originating from remaining, trapped radicals and in-diffusing oxygen, can proceed for periods of time exceeding the time frame of surface restructuring. In cases where surface chemical changes occur concurrently with restructuring, more refined models and treatments are required for analysis of the observed CA changes on aging. Changes in contact angles appear to be described by a single exponential process even when the XPS elemental ratios change due to posttreatment oxidation rea~ti0ns.l~ It is therefore important to ascertain by other surface analytical techniques whether an exponential change in CAS can indeed be interpreted solely in terms of restructuring. XPS analyses of the composition of aging modified surfaces revealed a (1 - e+) time law for the posttreatment incorporation of oxygen.15 In principle a term of this form could be added to our formalisms to describe the oxidative production of additional polar groups. The assumption would be required that, as for the groups incorporated into the surface by the modification process itself, these groups are partitioned between immobile and mobile surface patches. Oxygen-containinggroups produced in the latter areas would then likewise be subject to surface restructuringforces. Such a treatment requires, however, too many fit parameters for meaningful analysis of available experimental CA and X P S data. For surfaees that oxidize on aging, the surface restructuring parameters f m , fim, and f n p are defined by the net effect of surface restructuring and postmodification oxidation. The CASmeasure, and our fit parameters describe quantitatively, the final (Le., after surface restructuring and oxidation) surface polarity relative to the surface polarity at t = 0. The intrinsic mobility of the surface layers cannot be unraveled accurately in such cases at present. The above treatments of polymer surface restructuring do not address the origin(s1of interfacial (and other) forces that cause surface restructuring. Analysis of forces involved is the subject of current research aimed a t a n improved model. The impetus is that the migration of polar groups into a hydrophobic polymer on storage in air has traditionally been assigned to repulsive surface interactions between polar groups and the nonpolar air environment, but this interpretation does not appear to account fully for experimental observations of the surface restructuring of various modified polymers. l3 Particularly the marked differences in the mobility of HzO-plasmamodified and NHs-plasma-modified fluoropolymer sur-
Chatelier et al. faces (Table 1)present a challenge for theories of polymer surface dynamics and the understanding of plasma modification effects. We suggest that additional considerations need to be incorporated to arrive at a unified interpretation of the surface restructuring of polymers. In particular, we surmize that the emphasis on attractive or repulsive interfacial forces driving the surface restructuring needs to be re-examined and that entropic factors require consideration. When any class of chemical groups is concentrated near the surface, they will tend to diffise into the bulk of the polymer in order to increase the translational entropy of the system; reduction of the concentration gradient provides a driving force. This effect may be opposed by attractive interfacial interactions between the surface groups and the medium in contact with the surface, since such interactions will decrease the enthalpy of the system. When the surface groups are polar and in contact with air, attractive interactions are negligible, and therefore entropic effects can dominate. The disorder ofthe system increases and the surface density of the groups is reduced by dilution into the subsurface layers. However, the ability of surface groups to move below the surface may be reduced by cross-linking of polymer chains. Surface treatments can form cross-links andor cause chain scissions, as well as attach new groups. The relative importance of these processes is a function of the polymer substrate chemistry, the process gas, and process parameters. The relative importance to surface restructuring of factors such as repulsive interactions, entropy, cross-link density, hydrogen bonding, and possibly others, may depend strongly on the system (polymer modification) under study. We expect that analysis of polymer surfaces by the descriptions we have provided will reveal information on the fundamental factors affecting the dynamic properties of polymer surfaces and improve our understanding and interpretation of restructuring in terms of molecular structures and interactions.
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Conclusions Two time-dependent descriptions are proposed for contact angles of heterogeneous, mobile polymer surfaces in order to enable quantitative analysis of surface restructuring. These have been derived from the Cassie and the Israelachvili-Gee descriptions of time-independent contact angles of heterogeneous surfaces. An exponential time law is assumed for the surface restructuring process since this is consistent with a model where the polar groups can exist in two states, i.e., surface or subsurface regions of the polymer. Both treatments fit the observed changes of aidwater contact angles on aging after plasma modification of a number of polymer surfaces. The fraction of the surface which is polar after the treatment and the extent and rate of restructuring vary greatly between polymers and treatments. Acknowledgment. X.X. gratefully acknowledges support under the CSIRO/Academia Sinica Exchange Scheme and the Commonwealth Government’s Cooperative Research Centres Scheme (CRC for Eye Research and Technology). LA940956L
