Ions in Polymers - American Chemical Society

19 Structure Variation of Sulfonated Polystyrene Surfaces R. W. BIGELOW, F. C. BAILEY, W. R. SALANECK, J. M. POCHAN, D. F. POCHAN, H. R. THOMAS, and H. W. GIBSON 1

Downloaded by UNIV LAVAL on July 12, 2016 | http://pubs.acs.org Publication Date: June 1, 1980 | doi: 10.1021/ba-1980-0187.ch019

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A type of angle-dependent x-ray photoemission spectroscopy was used to investigate the molecular orientation at the surface of sulfonated polystyrene as a function of reaction depth. A model based on these measurements indicates that at a critical sulfonation depth the aliphatic hydrocarbon backbone becomes exposed preferentially at the surface. These results are consistent with surface energy and triboelectric charging measurements, which also reveal the effects of associative interactions in the form of conversion dependencies.

' " T h e chemical modification of polymers is an established technique for the attainment of new and useful surface properties. I n particular, there is considerable interest i n surface modification as a means of controlling dyeability (1,2), electrical charging (3,4), wettability ( 5 , 6 ) , and adhesion (7,8), where presumably the functional group orientation becomes a critical factor i n determining the interfacial characteristics. T o date, however, direct analysis of such modified surfaces has been carried out i n only a few instances (9-14). F o r example, i n a recent study of the surface structure of potassium-exchanged sulfonated polystyrene using angle-dependent x-ray photoemission spectroscopy [ X P S ( 0 ) ] (10), it was concluded that both the potassium and sulfur atoms reside below the surface in accordance with thermodynamic arguments requiring surface free energy minimization (15,16). Also, using the X P S ( 0 ) technique Briggs and coworkers (11) examined the surface structures of chromic acid-etched polyethylene and polypropylene, and correlated the relative 1

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atomic concentrations with surface energy measurements. However, each of these studies examined a limited number of samples. This chapter examines surface properties of a series of polystyrene (PS) films, modified to varying depths by sulfonation, using X P S ( 0 ) , surface energy measure ments, and triboelectric charging. A model based on the results of these measurements indicates that at a critical sulfonation depth the heteroatoms become preferentially oriented into the bulk.


Experimental Films of varying sulfonation depth were obtained by exposing free standing 80-/xm thick films of oriented PS ( D o w Trycite, Type 1000) to 99.7 - » 100.3% sulfuric acid at 25°C under dry conditions (~ 100 p p m water) for selected periods of time up to 60 min. The films were then quenched i n 96% sulfuric acid, washed with deionized water, dried, and stored i n a dry nitrogen atmosphere. Sulfonation was confirmed inde pendently by IR difference spectroscopy (17) and the X P S results. The degree of sulfonation was established by three independent techniques: (1) direct titration of the acidic protons; (2) measurement of visible absorption of companion films ion-exchanged with methylene blue (a cationic dye); and (3) interferometric estimation of reacted depth. These methods are described i n detail elsewhere ( I S ) . In conversion from the number of moles of sulfuric acid per gram of film (Methods 1 and 2) to number of monolayers reacted, two assumptions were made: (1) an abso lutely smooth surface and (2) 100% reaction of each layer in sequence, that is, a diffusion-controlled process (19). Above about 30 ideal mono layers the estimated accuracy is approximately ± 5 % . F r o m zero to 30 monolayers the accuracy is less, on the order of 25 to 100%, being less for the lesser extents of reaction, of course. The XPS(0) characterization was accomplished with an ΑΕΙ 200B photoelectron spectrometer. The instrumental conditions and geometrical constraints imposed i n the present study have been described previously (10). Films were transferred from the storage containers to the instru ment sample probe in a dry nitrogen atmosphere (typical residual oxygen concentration was ^40 p p m and water was removed by exposing the environment to ~ 200 c m of copper tubing held at liquid nitrogen tem perature) and inserted directly into the high vacuum of the instrument. Surface tension measurements were made via the advancing contactangle ( y ) technique (15) with a Rame-Hart rheogoniometer. Values for y are estimated to be accurate to ± 1.0 dyn/cm. Triboelectric charging data were accumulated on vacuum-dried ( > 2 weeks, ~ 1 torr) samples at zero humidity ( ^ 100 p p m water) using the device of Figure 1 as described previously (20). 2

c

c

Results and Discussion X P S ( 0 ) . T o establish a standard for comparison of the X P S ( 0 ) data, Figures 2 and 3 present the relevant photoemission intensities (1(0)) for an untreated sample and one sulfonated to a depth of approximately


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y METALLIC (STEEL, NICKEL, ZINC, ETC; 4 100, 250/x DIAMETER) BEADS

ELECTROMETER Figure 1.

Cascade device for measurement of triboelectric charging (20, 38, 39)

600 monolayers, respectively. T h e details of the application of this tech nique have been discussed previously (10). O f particular importance to the ensuing arguments is the approximately constant ratio of intensities in Figure 2 between the main C ( l $ ) core-level emission and the clearly resolved shake-up satellite structure at approximately 6.6 eV higher bind ing energy i n going from θ ~ 1 0 - » 80°. Since these particular satellite features reflect π* « - π valence orbital excitation processes localized on the benzene chromophore (21,22,23,24), their uniform magnitude rela tive to the primary peak intensities as a function of angle provides compelling evidence for the similarity i n bulk and surface concentration of unperturbed pendent aromatic rings of the untreated polymer. The relatively insensitive angle dependence of the C ( ls)/0( Is) intensity ratio indicates that the residual oxygen i n the untreated film is distributed approximately uniformly throughout the bulk. Since a typical escape depth for detected electrons i n these measurements is between 10 and 30 A (25), the bulk sampling depth extends approximately 20 A below the surface.


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Figure 2. The C(ls) and O(ls) x-ray photoemission signals of a typical untreated PS film taken at θ ~ 10°, bulk mode (upper,) and θ ~ 80°, sur face mode (lower). The intensity (l^ of a displayed signal is calculated by the relationship [(area of displayed signalXsensitivity scale factor)/ (number of scans required to produce the signal)]. The respective scan (A) and scale factors (B) are given below each signal (A/B). ( · · -) KE of —6.6 eV relative to the main C(ls) peak.


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Figure 3. The C(ls), O(ls), and S(2p) emission signals at Θ ~ 10° (upper) ana θ ~ 80° (lower) for the most heavily sulfonated sample considered by XPS (see text). (- - ·) KE of — 6.6 eV relative to the main C(ls) peak. Figure 3 confirms, for example, that i n the more strongly sulfonated samples the heteroatoms lie preferentially below the surface; the emission intensities for the Ο (Is) and S(2p) levels decrease more rapidly than the matrix C ( L s ) signal as the sample is rotated from θ ~ 10 - » 80°. A l though there are some important variations i n the angle-dependent C(ls) heteroatomic intensity ratios as a function of sulfonation (this w i l l be



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addressed in greater detail later), profound increases i n the matrix/heteroatom signal ratios in going from θ ~ 10 -> 80° are observed beginning at approximately 10 monolayers sulfonation depth. In addition to indicating a preferred interfacial structure, Figure 3 exhibits some interesting irregularities i n the satellite structure of the C(ls) signal obtained at θ ~ 80°. To explore this problem in greater detail Figure 4 compares the C(ls) signal (θ ~ 8 0 ° ) of representative samples from this study with the C ( l s ) spectrum of an oxidized film reported by Clark et al. (26). It should be noted that while correspond ing measurements on Films A D of Figure 4 at θ ~ 10° yielded some low-intensity satellite structures not present in the parent film, the shakeup features were clearly resolvable and of a magnitude comparable with that presented in Figure 2. A t least a portion of the C ( L s ) satellite structures of Figure 4 may be attributed to chemical shifts arising from various oxidation states of the surface carbon atoms. However, it should be emphasized that the intensities of the satellite structures in Figure 4 do not increase with the degree of sulfonation. Comparison measure ments conducted on solvent-cast PS films showed a shake-up structure comparable with the untreated free-standing film (θ ~ 10 -> 80° ). H o w ever, relative to the free-standing untreated samples, much less residual oxygen was present. Although sulfonation of the solvent-cast films d i d not seriously perturb the shake-up features evident in the untreated film at θ ~ 10°, satellite maxima were absent at θ ~ 80°. These results indicate that prolonged exposure to the reagent merely alters the state of a limited number of carbon atoms that may initially exist in a lower oxidized form at the surface of the untreated free-standing films (26,27). The notable absence or decrease in the C ( L s ) shake-up intensity characteristic of the unsubstituted aromatic structure at θ ~ 80° is more difficult to resolve. T w o possibilities merit consideration. A n obvious one is that sulfonation leads to a preferential ring opening of the surface phenyl groups, and that the θ ~ 80° measurements are, therefore, sampling a polyethylene-like structure. W i t h i n this context Storp and H o l m (28) attributed the decrease observed in PS shake-up intensity concomitant with ion bombardment as evidence of ring destruction. Such a mechanism i n the present case, however, does not appear well-founded in the literature. A more plausible explanation is that the shake-up intensity is quenched because of strong intra- and/or intermolecular inter actions. In a detailed study of para-substituted PS and appropriate model compounds, for example, Clark and co-workers (29) attribute the shakeup process at approximately 6.6 eV higher binding energy relative to the main peak as primarily attributable to two one-electron ττ* D. These are compared with the highly oxidized PS film reported by Clark et al (26).



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Figure 5. Representation of the π*

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