Langmuir 1995,11, 2585-2591
2585
Effects of Plasma Modification Conditions on 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 April 10, 1995@ Fluorinated ethylene-propylene (FEP) surfaces modified by ammonia or water vapor plasmas under a range of conditions were stored in contact with air and their surface restructuring on agingwas monitored by periodic determination of airlwater contact angles (CAS). By use of formalisms for the analysis of time-dependentCAS,the fraction of the surface area initially 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 were calculated. For ammonia-plasma-modified FEP surfaces, the fraction of mobile polar groups increased with increasing plasma treatment time up to an optimal treatment time of 45 s,probably due to plasma-inducedchain cleavage. This indication of increasing mobility upon extended plasma treatment was, however, not accompanied by a decrease in the lifetime ofthe reorientation process, implying that the rate-limiting step for surface reorientation may not be associated with the inward motions of plasma-modified surface chains, but instead may depend on the slower outward migration of FEP chains to the surface.
Introduction The ability of polymer surfaces to restructure their composition in response to the environment1s2 is of considerable interest both for fundamental surface science and for a number of technological applications. Surfacemodified polymers are used, or intended, for diverse applications.2 Often, however, the restructuring of polymer surfaces leads to the partial, or even complete, disappearance of the surface modification effects with time.2-11 This aging of surface-modified polymers can be detrimental to the performance of a device, and improved understanding of the aging of modified surfaces on storage is required to gain the ability to process polymer surfaces under conditions that are optimized and lead to interfacial properties that are controlled and predictable at the time of use. Qualitatively, polymer surface restructuring is described in terms of rotational and translational motions of chains and chain segments. Interfacial forces act to cause the burial “inside” the polymer of groups that are energetically unfavorable a t the interface, with the simultaneous emergence to the surface of groups that lower the interfacial e n e r g ~ . l - ~ Quantitatively, J~ however, interpretation of polymer surface restructuring is less clear and the details are insufficiently understood. At present it is not possible to predict rates and extents
* Author for correspondence.
Abstract published in Advance A C S Abstracts, June 1, 1995. (1)Andrade, J. D., Ed. Polymer Surface Dynamics; Plenum Press: New York, 1988. (2) Garbassi, F.; Morra, M.; Occhiello, E. Polymer Surfaces-From Physics to Technology; Wiley: Chichester, 1994. (3) Baszkin, A.; Ter-Minassian-Saraga,L. Polymer 1974,15, 759. (4)Yasuda, H.; Sharma, A. IC; Yasuda, T. J . Polym. Sci., Polym. Phys. Ed. 1981,19,1285. (5) Holmes-Farley,S . R.; Reamey, R. H.; Nuzzo, R.; McCarthy, T. J.; Whitesides, G.M. Langmuir 1987,3,799. (6) Garbassi, F.;Morra, M.; Occhiello,E.; Barino, L.; Scordamaglia, R.Surf. Interface Anal. 1988,14, 585. (7) Strobel, J. M.; Strobel, M.; Lyons, C. S.; Dunatov, C.; Perron, S. J. J . Adhesion Sci. Technol. 1991,5,119. (8) Yasuda, H.; Charlson,E. J.; Charlson,E. M.;Yasuda,T.;Miyama, M.; Okuno, T. Langmuir 1991,7, 2394. (9) Griesser, H. J.; Youxian, D.; Hughes, A. E.; Gengenbach,T. R.; Mau, A. W.-H. Langmuir 1991,7, 2484. (10) Xie, X;Gengenbach, T. R.; Griesser, H. J. J . Adhesion Sci. Technol. 1992,6, 1Ll1. (11) Gengenbach, T. R.; Xie, X.; Chatelier, R. C.; Griesser, H. J. J. Adhesion Sci. Technol. 1994,8, 305. (12) Holly, F. J.;Refojo, M. J . Biomed. Mater. Res. 1975,9,315. @
of surface restructuring, nor is it clear why a specific polymer surface subjected to two related surface modifications exhibits fundamentally different restructuring behaviors.1° To enable quantitative analysis of polymer surface restructuring, we developed theoretical formalisms for the time dependence of airlwater contact angles (CAS) of mobile polymer surfaces13 based on previous timeindependent descriptions for CAS of heterogeneous surfaces.14J6 These formalisms were applied to CA data to determine the restructuring rates and the extent of changes on aging of the populations of nonpolar and polar structural elements on the surface of a number of gasplasma-modified polymers. l3 In this communication we apply the formalisms for surface restructuring to study how changes to plasma process parameters affect rates and extents of surface restructuring of one specific polymer, viz., perfluorinated ethylene-propylene copolymer (Teflon FEP). By correlating restructuring parameters (the characteristictime constant and the percentage of mobile groups)with plasma parameters, we aim to elucidate optimal surface modification conditions (avoiding overtreatment) and to improve understanding of processes occurring during and after plasma treatments at a molecular level. Plasma modification conditions can have marked effects on the initial values of the CAS of modified surfaces and their subsequent aging,ll but molecular interpretations of why the effects of plasma conditions differ are not available. Analysis of surface dynamics may provide information which is not accessible by other methods: XPS surface analysis is not capable of documenting plasma-induced cleavage and/or cross-linking of polymer chains, but such reactions would be expected to alter the restructuring behavior. Our data show that a higher percentage of the attached polar groups become mobile when longer ammonia-plasma treatment times are used, indicating that extended plasma exposure enhances cleavage reactions rather than cross-linking. It is also suggested that the mobility of unmodified subsurface polymer chains determines the rate of restructuring in the FEP/NH3 system. (13) Chatelier, R. C.; Xie, X.; Gengenbach, T. R.; Griesser, H. J. Langmuir 1995,11, 2576. (14) Cassie, A. B. D. Discuss. Faraday SOC.1948,3,11. (15) Israelachvili, J. N.; Gee, M. L. Langmuir 1989,5,288.
0743-746319512411-2585$09.0010 0 1995 American Chemical Society
Chatelier et al.
2586 Langmuir, Vol. 11, No. 7, 1995
In contrast, changes to the plasma treatment time had no affect on FEP surfaces modified in water vapor plasmas. Experimental Section Plasma Surface Treatments. FEP tape of 12.7 mm width was sourced commercially (Du Pont FEP 100 Type A). Its cleanliness was checked, prior to plasma modification, by XPS analysis.9 FEP samples were modified by plasmas established in a low pressure atmosphere of ammonia of water vapor, using procedureslOJ1 and a custom-built plasma reactor reported previously.16Ammonia(Matheson)was supplied from a cylinder via a stainless steel line and amass flow controller (MKS). Triply distilled water was placed in a round-bottomflask and the vapor from evaporationat room temperature was suppliedto the reactor chamber. The plasma treatments were performed using the experimental conditions of 0.4 mbar pressure, 13.56 MHz rf frequency, 20-120 W power, 10-120 s treatment time. Experiments were performed with strips of FEP, 75 mm long, attached by thin, double-sidedadhesive tape to the face of the electrodes, and also with extended lengths of tape moving at a constant speed through the plasma zone, the speed being adjusted t o give a selected residence time in the plasma region. The semicontinuous treatment of extended lengths of FEP tape in a stable plasma enabled the fabrication of a large number of nominally identical specimenswhich, after plasma modification,were stored in clean tissue-culture polystyrene (TCP) dishes at ambient temperature (21 1"C)and asssessed periodically,using a fresh piece of tape for each CA and XPS analysis. Surface Analyses. The apparatus and methods used for the determination of advancing and receding aidwater CAS were the same as those detailed elsewhere.10 Measurements were performed in quadruplicate on specimens stored for various periods of time; 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. XPS spectra were collected as described previouslyll using a VG Escalab V spectrometer with non-monochromatic Al Ka radiation at a power of 200 W. Angle-dependent XPS (ADXPS) analysis was performed using a number of emission angles between 0" and 75" (measured from the surface normal). Raw ADXPS data were converted into depth profiles using an algorithm by Tyler et a1.l' A custom-built air STM unit incorporating an inverted piezotube scanner design was used to assess surface topography. The calibration of the STM head and the preparation of insulating polymer specimens, including coating with a Pt overlayer, have been detailed e1~ewhere.l~
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Results 1. Ammonia-Plasma-ModifiedFEP. We have previously reported that the surface modification of FEP in ammonia plasmas resulted in very hydrophilic surfaces, but when the treated specimens were stored in air, the CASincreased substantially in the course of a few days,ll suggesting that some of the polar groups introduced by the treatment? did not remain at the surface. XPS data indicated burial inside the polymer (at shallow depths) rather than volatilization.ll The initial CAS (measured immediately aRer treatment) and the subsequent changes in the CAS on aging depended on the plasma conditions. Our premise is that quantitative evaluation of the time dependence of properties of equilibrating surfaces can provide insights into the molecular phenomena involved in polymer surface dynamics and the molecular consequences of surface modification processes. We are particularly interested in how the rates and extentsof surface restructuring of polymers vary with changes to surface modification procedures. (16) Griesser, H.J. Vacuum l989,39, 485. (17) Tyler, B.J.;Castner, D.G.; Ratner, B. D. Su$. Interface Anal. R I-) ._14. 44.1. -l 9_ (18) The chemical composition of the FEPNHa surfaces, and the nature and densities of the polar groups introduced by the plasma modification, are not well understood (see refs 11 and 13). I - - I
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Storage t l m e (days) Figure 1. (A)Advancing ( 0 )and receding (0)aidwater contact angles of ammonia-plasma-treated FEP as a function of storage time after plasma treatment and (B)the mean contact angle, OM, as a function of storatge time. The plasma treatment time was 10 s. The best-fit curves based on eqs 2 and 3 overlay one another and are represented as a single solid line. In the following we report experiments involving the systematic variation of one experimental plasma parameter (treatment duration or power) while keeping the other plasma parameters constant. Specimens stored in contact with air were periodically assessed to monitor their surface restructuring via changes to the aidwater CAS. Figure 1A shows experimental values of advancing (ACA) and receding (RCA) aidwater contact angles measured on specimens of FEP modified by an NH3 plasma of 10 s duration and an input power of 30 W, as a function of time elapsed after surface modification (storage time in air). A mean aidwater CA, 9 ~defined , by cos(e,) =
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(1) 2 was obtained for each time point from the ACA and RCA values. Figure 1B shows the values of OM thus obtained from the data of Figure 1A. We have recently derived relationships for-the time dependence of the theoretical equilibrium CAS, &, of restructuring, heterogeneous polymer surfaces.13 Our formalisms were based on two earlier, time-independent descriptions14J5for the CAS of surfaces comprising polar and nonpolar surface fractions. The time-dependent relationship for the equilibrium CA of a restructuring surface is13 cos eE(t) = (fim f,e-t'r) cos 8,' (1 - f i , - fme-") cos e E n p (2)
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Plasma Effectson Surface Restructuring
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