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h2so4-oxidized polyethylene surfaces by means of ESCA and45ca2+ adsorption
Characterization of KMnO4/H2SO4-Oxidized Polyethylene Surfaces by Means of ESCA and 450a2+ Adsorption J. C. ERIKSSON,* C.-G. G t g L A N D E R , t A. BASZKIN,~ AND L. TER-MINASSIAN-SARAGA~ *Department of Physical Chemistry, The Royal Institute of Technology, S-IO0 44 Stockholm, Sweden, "~The Institute for Surface Chemistry, Box 5607, S-114 86 Stockholm, Sweden, and ~.Laboratoire de Physico-Chimie des Surfaces et des Membranes, CNRS, UER Biomddicale, 45 rue des Saints-POres, 75270 Paris Cedex 06, France
Received September 14, 1983; accepted December 31, 1983 The chemical constitution of KMnO4/H2SO4-0xidized(sulfated) polyethylene (PE) surfaces has been exploredby means of ESCA utilizing chemical shifts, peak intensity data, and chemicaltaggingreactions. In addition, the adsorption of Ca2+ ions on sulfated PE was quantified by radiotracer measurements. Comparisons have been made with KC103/H2SO4and K2Cr2OT/H2804 as oxidizing agents and through replacing polyethylene by polystyrene. According to the present study the main chemical groups on a O KMnOa/H2SO4-treatedPE surfaceare -OSO3H, -OH, -O-OH, -C----O,-C-O-C-, andc//_O_R (ester). In the oxidation grooves, dissociable -COOH groups are also present in appreciable amounts. By annealing, a smoother and more homogeneous sulfated surface is produced, the composition of which strongly depends upon the state of the ionic groups at the heat treatment. BACKGROUND Chemical modification of polyolefin surfaces is of critical importance for several m a n ufacturing processes in use today as a means of promoting adhesion or wetting. To serve as guidelines when tackling practical adhesion problems a n u m b e r of different "theories" of adhesion have been advanced in the past as is well described, e.g., in a recent review paper centered on polyethylene and polypropylene surfaces by Brewis and Briggs (1). So far, however, the interrelations between the observed adhesion/wetting properties and the detailed surface chemistry of polymers have not been explored extensively enough to enable the most important general features to be clearly distinguished. The prospects for further progress in this research area have become considerably brighter through the advent of the ESCA technique (2) which is well suited for comprehensive chemical analysis of nonconducting polymer surfaces as has been con-
vincingly demonstrated particularly by Clark (3) and Briggs (4). Several methods are now available to improve the adhesion to polyolefin surfaces, e.g., corona discharge, gas plasma exposure, flame treatment, and acid etching, all of which appear to result in an enhanced degree of surface oxidation (cf. Ref. (4)). Rasmussen et aL (5), Blais et al. (6), and Willis and Zichy (7) have studied the surface oxidation of low density PE due to exposure to chromium/sulfuric acid using IR spectroscopy. Rasmussen et al. found that carbonyl and carboxyl groups are formed at this oxidation treatment. Blais et al. found evidence also for hydroxyl surface groups. In addition, Willis and Zichy (7) could establish the presence of sulfate and alkyl sulfonate groups. Briggs et al. (8, 9) using ESCA identified hydroxyl, carbonyl, carboxyl, and sulfonate groups on chromic acid etched low density PE surfaces. In their adhesion/wetting studies of PE surfaces, Baszkin and Ter-Minassian-Saraga (10381 0021-9797/84 $3.00
Journal of Colloid and Interface Science, Vol. 100, No. 2, August 1984
Copyright © 1984 by Academic Press. Inc. All rights of reproduction in any form reserved.
382
ERIKSSON ET AL.
13) utilized KC103 dissolved in concentrated the oxidizing agent. The surface density of polar groups was quantified with a 45Ca2+ tracer adsorption technique which has been employed also in the present investigation. Eriksson et al. (14) used KMnO4 dissolved in concentrated H2SO4 to modify PE surfaces and it was demonstrated that such a modified PE surface is thromboresistant in the restricted sense of exhibiting a minimal degree of blood platelet adhesion (15). Recently, the surface chemistry of the KMnOa/H2SO4-treated PE surface was the subject of an ESCA study (16) which indicated the presence of hydroxyl, sulfate ester, carbonyl, and carboxyl surface groups. Since these results partwise seemed to disagree with the earlier findings concerning the chemical constitution of acid-etched PE surfaces (in particular sulfonate vs sulfate) we decided to carry through a broader study of the surface functionality in the first place of KMnO4/H2SO4oxidized PE surfaces, including variable oxidation conditions while making simultaneous use of ESCA and the 45Ca2+ tracer adsorption technique referred to above. H 2 S O 4 as
MATERIALS AND SAMPLE PREPARATIONS Films of low density PE (0.93 g/cm 3 at 23°C) were purchased from Noax AB, Sweden, and from Grace SocietY, France. Polystyrene (PS) samples were cut out of conventional polystyrene Petri dishes manufactured by Nuric AS, Denmark. The concentrated sulfuric acid used was of pro analysi grade with density equal to 1.84 g/ml. Potassium permanganate (KMnO4), potassium dichromate (K2Cr207), potassium chlorate (KC103), potassium hydrogen phosphate (K2HPO4), potassium hydrogen phthalate (KHC6H4(COO)2), and calcium chloride (CaCI2) were all of Merck's reagent grade. 45Calabeled CaCl2 was obtained from Institute Nationale des Radioelements, Belgium, with specific activity 221.5 mCi/g. Triply distilled water was used throughout the experiments. The ESCA chemical tagging reagents were all Journal of Colloid and Interface Science, Vol. 100, No. 2, August 1984
of Merck's reagent quality except SO2 that was purchased from Alfax, Maim6, Sweden. For the 45Ca 2+ adsorption measurements we utilized a 50-#m-thick low-density PE (Noax) film which after sonication in 70% ethanol/water was practically free of oxygen as checked by means of ESCA. Thicker polymer samples of the size 14 × 10 × 1 mm were used in the ESCA studies. In order to reduce the amount of oxidation products initially present on the surface some PE samples were gently scraped with a knife and subsequently pressed against a microscope cover glass over night at 60°C to produce a smooth surface. Prior to further use all samples were sonicated in 70% ethanol/water, rinsed with water, and stored in a desiccator. The various oxidation treatments were carlied out as appears from Table I using KMnO4, KC103, or K2Cr207 dissolved in concentrated sulfuric acid and were followed by several rinsings with water. METHODS The ESCA spectra were recorded with a Leybold-Heraeus spectrometer (LH 2000) equipped with a AI K~ X-ray source (1486.6 eV), the operating conditions of which were always set to 13 kV and 14 mA. The sample orientation was chosen normal to the direction through the entrance of the hemispherical electron energy analyzer of the spectrometer. The electrons analyzed emanate from a rectangular sample surface area of the approximate size 10 × 1 mm. Complete spectral scans as well as detailed recordings of the main peaks were made for each sample, usually with a sweep time of 5 or 10 min. The ESCA electron binding energy scale was fixed by assigning EB = 285.0 eV to the -CH2- carbon 1s peak. According to previous studies (17) the carbon chemical shifts of different kinds of oxygen containing groups are: (i) hydroxyl, hydroperoxide, ether, alkyl, or sulfate ester, etc, AEB = 1.5 eV, (ii) carbonyl, AEB = 3.0 eV~ and
KMnO4/H2SO4-OXIDIZED
PE
TABLE
383
SURFACES
I Number of different atoms per -CHz- group in each surface
Sample
Preparation
1
PE, Noax
2
PE, Noax
3
PE, Noax
4
PE, Noax
5
PE, Noax
6
PE, Noax
7
PE, Noax
8
PE, Noax
9
PE, Noax
10
PE, Noax
11
PE, Noax
12
PE, Noax
13
PE, Grace
14
PS
70% ethanol/water sonication 2 g/liter KMnO4/H2SO4 2 min 2 g/liter KMnO4/HzSO4 2 sec 2 g/liter KMnO4/H2SO4 10 min 0.2 g/liter KMnO4/H2SO4 2 min 10 g/liter KMnO4/H2SO4 2 rain 2 g/liter K M n O 4 / H 2 S O 4 2 min, 1% HzO 2 g/liter KMnO4]HaSO4 2 min, 5% H20 2 g/liter KC103/H2SO4 2 mm 10 g/liter KC 1O3/H2SO4 2 mln 55 g/liter KC103/HzSO4 2 mm 2 g/liter K 2 C r 2 0 7 / H 2 S O 4 2 mm 2 g/liter KC103/H2SO4 2 mln 2 g/liter KMnO4/H2SO4
Number
-c-o-
/~c=o
-c/?o-
o
-OSO3H
0.46
0.11
0.05
0.72
0.40
0.11
0.07
0.04
0.59
0.12
0.09
0.14
0.45
0.09
0.05
0.64
0.11
0.38
0.10
0.06
0.64
0.07
0.39
0.07
0.06
0.08
0.15
0.05
0.05
0.05
0.05
0.01
0.02
0.20
0.04
0.11
0.01
0.03
0.28
0.03
0.02
0.11
0.05
0.03
0.47
0.11
0.08
0.10
0.03
0.02
0.61
0.17
0.08
0.87
0.03
0.52
0.38
0.40
1.88
0.17
-C-Cl
0.11
0.01 0.02
2 mln
(iii) carboxyl or the c o r r e s p o n d i n g ester, AEB = 4.2 eV. T h e c o r r e s p o n d i n g p e a k intensity c o n t r i b u tions were graphically e s t i m a t e d f r o m the C 1s spectra. The relative cross-sections used in this w o r k were o b t a i n e d f r o m a p a p e r b y Scofield (18): a ( C l s ) = 1.00, a(01s) = 2.93, a(S2p) = 1.68, a(C12p) = 2.29, a(Ca2p3/2) = 3.35, a(Ti2p3/2) = 5.22, a ( F l s ) = 4.43, a ( N l s ) = 1.80, a(Br3d) = 2.84. These cross-section values for $2p3/2 a n d Ca2p3/2 were also c h e c k e d t h r o u g h rec o r d i n g spectra o f s o d i u m d o d e c y l sulfate a n d c a l c i u m stearate a n d satisfactory a g r e e m e n t was found. F o r the 45Ca2+ a d s o r p t i o n m e a s u r e m e n t s circular P E film samples were p u n c h e d having
a d i a m e t e r o f 3 cm. After o x i d a t i o n t r e a t m e n t the s a m p l e s were p l a c e d on t o p o f a n a q u e o u s s o l u t i o n c o n t a i n i n g 45CAC12. T h e excess o f Ca 2+ at t h e o x i d i z e d s a m p l e / s o l u t i o n interface was d e t e r m i n e d f r o m the difference in c o u n t ing rate relative to a n oxygen-free P E film s a m p l e using a gas flow c o u n t e r m o u n t e d above the PE film. T h e set-up used is described in detail in Ref. (10). 2oCa 45 is a t - - e m i t t e r with an average electron energy e q u a l to 0.258 MeV. T h u s the 5 0 - g m PE film causes n o significant r e d u c t i o n o f the r a d i a t i o n intensity. T h e i n f o r m a t i o n d e p t h is a b o u t ~ 0 . 1 m m d o w n into the 45CAC12 s o l u t i o n i m p l y i n g t h a t the b u l k s o l u t i o n c o n t r i b u t i o n to the c o u n t i n g rate b e c o m e s sizable at 45Ca c o n c e n t r a t i o n s in excess o f ~ 10 -4 M. Journal of Colloid and Interface Science, Vol. 100, No. 2, August 1984
384
ERIKSSON ET AL. RESULTS
+11~
5g/I
Surface-Chemical Effects of Different Oxidizing Agents Figure 1 shows some ESCA survey spectra of untreated PE (sample # 1 (a)) and PE oxidized with KMnO4 (sample #2 (b)), KC103 (sample #9 (c)), and KzCr207 (sample # 12 (d)) all in concentrated H z S O 4 . The pure PE sample # 1 exhibits only a minor trace of oxygen, its spectrum is dominated by the large C ls peak at ER = 285.0 eV, and the associated carbon Auger K L L signal at Ekin ~, 275 eV. Besides carbon, all the oxidized surfaces contain substantial amounts of oxygen and in addition also sulfur and nitrogen. From circumstantial evidence we may anticipate that the presence of nitrogen (EB = 402.0 eV) is due to a spurious cationic contaminant that is adsorbed after the oxidation treatment. The KC103 sample (Fig. lc) shows a C12p peak at EB = 200.0 eV indicating the formation of C-C1 bonds (19). It is noteworthy that the ESCA spectra in Figs. lb, d do not show the presence of any traces of Mn or Cr. The sulfur and oxygen peaks are comparatively large for the KMnO4/H2SO4-treated PE surface. When the concentration of KC103 is raised, however, to 55 g/liter corresponding Cls
01s
°
Cls
i
i
i
1486 1000 500
i
0
i
r
i
1486 1000 500
,___..1
0
FIG. 1. ESCA survey spectra. (a) Polyethylene (PE, # 1), (b) PE: KMnO4/H2SO4, 2 g/liter (#2), (c) PE: KC103/ H2SO4, 2 g/liter (#9), (d) PE: K2Cr2OT/H2SO4, g/liter (#12). Journal of Colloid and Interface Science, Vol. 100, No. 2, August I984
=•12• - ~
AE a
eV
i i i 630
K2Cr207
-~9~
2 g/I
i i i 630
i i I 630
KMn04
KCIOa
FIG. 2. C 1s spectra of PE surfaces treated with oxidant/ H2SO4 for 2 min. The vertical lines denote the peak positions o f C - O (AEB = 1.5 eV), C : O (AEB = 3.0 eV), and COOH (AEB = 4.2 eV).
to 2.9% by weight, one observes a considerable increase of the S peak intensity to about the same level as for 2 g]liter ofKMnO4 (cf. Table I) whereas an increase of the KMnO4 concentration leads to a substantial decrease of the S content in the surface (cf. below). It appears already from the O/C ratio that KMnO4 in H2804 is a more powerful oxidant than KC103/H2SO4 which in turn is somewhat more powerful than K 2 C 2 0 7 / H 2 8 0 4 . The corresponding detailed C 1s spectra (of samples #2, 6, 9, 10, 11, 12) are displayed in Fig. 2 from which it is clear that the KMnO4/ H 2 S O 4 agent is also much more efficient than KC103/H2SO4 and K 2 C r 2 0 7 / H 2 8 0 4 in promoting the formation o f - C - O - carbon atoms shifted 1.5 eV away from the - C H 2 - carbon. It has been proposed earlier that in the case ofKMnO4/HzSO 4 oxidation, - O H groups and -OSO3H groups are attached to these carbon atoms (16). Further ESCA evidence will be presented below indicating that perhydroxy groups also contribute to the - C - O - peak. In the KCIO3/H2SO4 and K2CEO7/HESO4 mixtures the oxidation reactions apparently proceed rather differently and probably with a slower rate of formation of O H groups re-
KMnO4/H2SO4-OXIDIZED PE SURFACES suiting in a lower rate of esterification with H2SO 4. Deconvolution of these C ls spectra yields the separate contributions to the resultant peak intensity of the different chemically shifted carbon peaks (Table I). It is noteworthy that the KC103/H2SO4-treated PE surface shows essentially the same C ls spectrum at 55 g/liter as at 10 g/liter of KC103 and that the - C - O - / - C H 2 - a t o m i c ratio is as high as ~ 0 . 4 6 for the KMnO4/H2SO4 treated surface in spite of the fact that the information depth of ESCA for organic surfaces is estimated to be 5-10 nm. In Fig. 3 the detailed S2p spectra are compared for samples 12, 2, and 9: The $2p3/2 peak positions for all samples center around EB = 169.0 eV with a spread of +0.2 eV indicating sulfur in sulfate or possibly sulfonate form. For ESCA samples prepared on PE plates from acidified (pH = 1.0) sodium dodecyl sulfate and sodium decylsulfonate solutions we have obtained EB (sulfate) = 169.0 eV as measured for the $2p3/2 peak relative to the - C H 2 - carbon ls peak at 285.0 eV and EB (sulfonate) = 168.2 eV. Earlier it has been established that the $2p3/2 peak due to sulfate ester groups in heparin and in dextran sulfate are positioned at EB -- 168.8 and 169.1 eV, respectively (20). Furthermore, for the polystyrene sample #14, EB ($2p3/2) equals 168.3 eV indicating sulfonate formation in this case. Thus the more complete ESCA evidence now available gives further support for our preliminary conclusion that mainly ester sulfate groups are introduced in a PE surface as a result of the action of a KMnO4/H2SO4 mix-
x4
eV
6 PE
K2Cr207
Grace
Polystyren
ture. So seems to be the case likewise for the KC103/H2SO4 and K2Cr207/H2SO 4 mixtures but in these cases the rates of formation of r-OSO3H groups is reduced, presumably because of a lower rate of formation of the O H intermediate. In Fig. 4 the C ls peaks obtained after KMnOa/H2SO4 oxidation of PE (Noax), PE (Grace), and PS are shown. A clear tendency for a more extensive reaction on PE (Grace) than on PE (Noax) is noted. In fact with 10 g/liter KC103/H2SO4 as the oxidizing agent a C 1s spectrum is generated for PE (Grace) that is very similar to the KMnOa/H2SO4 2 g/liter standard case (cf. Fig. 2). Apparently, PS reacts much more readily than PE leading to a comparatively high surface density also of carboxyl groups as is seen from Fig. 4c (cf. Table I). In Figs. 5 and 6 the effects of the C l s spectra
0.2g/I
/~EB I
[ 167
PE
FiG, 4. C 1s spectra of different polymer surfacestreated with KMnO4/H2SO4for 2 min.
eV i J 17116g
Noax
x9
~2~ AE 8 ~, eV
385
~ i 17116g
i 167
KMnO 4
L i 17116g
i 167
KCIO 3
FIG. 3. S2p spectra of PE surfacestreated with oxidant/ H2SO4 for 2 rain.
]
630 2 sec
I
I
I
J
630 2 min
I
I
L
630 lomin
FIG. 5. C 1s spectra of PE samples treated with KMnO4/
H2SO4. The vertical lines denote the peak positions of C-O (AEB = 1.5 eV), C--~O(AEB = 3.0 eV), and COOH (AEB = 4.2 eV). Journal of Colloid and Interface Science, Vol. 100, No. 2, August 1984
386
ERIKSSON ET AL.
of varying exposure time, concentration, and water content at the KMnOJHaSO4 treatment are shown. When it comes to favoring the development of the - C - O - shoulder at AEB = 1.5 eV it is obviously important to use a long exposure time and a comparatively low KMnO4 concentration. Furthermore, the water content should be kept at a rather low (less than ~ 1%) level. To conclude this part we wish to draw the attention to the SEM pictures of a pure PE surface and of a sulfated PE surface (21). These SEM pictures reveal a granular structure with grain size ~ 0.1 #m in diameter that is particularly pronounced for the sulfated PE surface. This observation points in the direction that the amorphous regions are more susceptible to oxidative attack than the crystalline spherulites. In view of this clearly visible surface heterogeneity we have to presume that
the actually exposed surface area is considerably larger than the planar surface area. Furthermore, due to the water formation at the esterification and the inhibitive action of water on this same reaction it is not surprising to find as we have done previously (cf. Ref. (16)) using angular dependent ESCA that by large the O surface atoms are situated below the S surface atoms.
Chemical Tagging Experiments In order to obtain further verification of the results described above as to the chemical constitution of the oxidized PE surfaces some chemical tagging experiments have also been carried out. The following derivatization reactions, previously explored by Briggs (22) and by Briggs and Kendall (23) as to the most selective reaction conditions, have been utilized.
H --C--+
--NH--NH2--* C = N - - ~ N - - @
+ H20
[1]
II pentafluorophenylhydrazine (PFPH) in ethanol
O
(Schiff's base)
--CH2--C--+ Br2/H20 ---~--CBr2--C--(Br2 reacts also with II II double bonds) O O
[2]
- - C H = C - - + C1--C--CH2--C1 -~ CH=C--O--C--CH2C1
[3]
I
If
II
OH
O
O
Chloroacetylchloride (reacts predominantly with (CAC) in heptane enolic OH groups)
--CH2--C--
I
+ (acac)2Ti(OPr)2
OH Diisopropoxide-titanium bis-acetylacetonate (TAA) in 2-propanol --CH2--O--OH
+ SO2(g) ~
Journal of Colloid and Interface Science, Vol. 100, No. 2, August 1984
--CH2--C--O-- Ti(acac)2 / OPr
[4]
(reacts predominantly with alcoholic OH groups) --CH2~" O--SO3H
[5]
KMnO4/H2SO4-OXIDIZED PE SURFACES
387 TABLE II
Evaluation of Chemical Tagging Reactions
s%
Tagging reagent PFPH Brz CAC TAA SOz
Functionalgroup
Number of groups per -CH2- in original #2 surface
ESCA peak quantified
-C=O -CH2C=O, - C = C - C - O H (enol) - C - O H (alcohol) -C-O-OH
0.04 0.05 0.06 0.10 0.05
Fls, Nls Br3d Cl2p Ti2p3/2 S2p
ber of accessible - C - - O groups, ~ 0 . 0 5 per - C H 2 - in the standard KMnO4/H2SO4 surface. From the C 1s spectrum (Fig. 2, #2) it is found that the corresponding figure is significantly higher, ~,0.11, indicating that about half of the -C----O groups do not react with the tagging reagents under the conditions choo% sen. Comparing with Table I, one observes that the alcoholic O H surface density as measured by means of reaction [4] is considerably less in magnitude than the - C - O peak intensity of the C I s spectrum. T h e ESCA S2p spectra of PE surfaces first AE B ,< i I I treated with K_MnO4/H2SO4 and then with 6 3 0 SO2(g) (reaction [5]) are displayed in Fig. 7. KMnO 4 It is seen that an extensive SO2 addition occurs FIG. 6. CIs spectra of PE surfacestreated with 2 g/liter on the KMnOa/H2SO4 surface indicating the KMnO4[H2SO4,containing added H20. The verticallines presence of perhydroxy groups. No evidence denote the peak positions ofC-O (AEB = 1.5 eV), C=O for such groups on the KC103/H2SO4 and (AEB = 3.0 eV), and COOH (AEB = 4.2 eV). K2Cr2Ov/H2SOa-treated surfaces was found. Furthermore, it is worth noticing that the sulfur added as a result of reaction [5] gives rise The reactions [1] and [2] are claimed to to an ESCA peak exactly at the sulfate position quantify the density of (aldehyde and keto) EB = 169.0 eV as no peak broadening is obcarbonyl groups on the surface, whereas re- served. This strongly supports that the original actions [3] and [4] m a y be used to distinguish alkyl sulfate assignment for the KMnO4/ between enolic and alcoholic O H groups. Reaction [5] involving gaseous SO2 provides a very convenient way of analyzing perhydroxy groups (24). Some of the ESCA results obtained after carrying through reactions [1][4] with the standard sample #2 are shown in Ea i ~ i I I I Table II, where cross-section-corrected peak ~ev lrl 169167 17116g t67 intensities are given for F, N, Br, el, and Ti KMnO 4 K M n 0 4 + S02 relative to the original - C H 2 - peak intensity. It is evident that the F, N, Br, and C1 probe s FIG. 7. S2p spectra of a KMnO4/H2SO4-treated PE used yield rather similar results as to the n u m - sample before and after SO2 exposure.
/k
Journal of Colloid and Interface Science, Vol. 100, No. 2, August 1984
388
ERIKSSON ET AL.
atively charged group on the surface and the H+/Ca 2+ ion exchange process proceeds to completion. The fact that the observed Ca/S ratio is somewhat below 0.5 can readily be accounted for by referring to (i) some substitution of noncondensed Ca 2+ ions with H + occurs at the rinsing step and (ii) all -OSO3H groups which contribute to the S peak intensity Ionic Groups on KMnO4/H2SO4-Oxidized are not accessible for ion exchange. On this PE Surfaces basis we may conclude that the plateau level The experimental room temperature iso- in the 45Ca 2+ adsorption isotherm most likely therm of 45Ca 2+ adsorption onto a standard refers to the number o f - O S O ~ sites per (forKMnO4/H2SO4 PE surface (sample #2) from mal) surface area which are accessible for ion CaC12 solutions of different concentrations and exchange with Ca 2+. It appears from Fig. 8 pH ~ 6 is plotted in Fig. 8 where the cor- that this number is 14 × 1018 m -2 correspondresponding Ca2p3/2 ESCA peak intensity data ing to ~ 0 . 0 7 nm2/-OSO3. Such a very high are also displayed. The ESCA samples ana- (formal) surface density o f - O S O 3 groups is lyzed were prepared through removing excess definitely not in accord with the S/C and bulk solution with a filter paper followed by S/O ratios derived from the ESCA spectral rinsing with isopropanol and drying. There is data and must hence largely be attributed to a pronounced initial rise in both isotherms. the heterogeneous character of the oxidized The ESCA isotherm reaches a saturation level PE surface. At concentrations above ~ 10 -4 M CaC12 corresponding to a Ca/S atomic ratio of ~ 0 . 4 3 at CaC12 concentrations ~ 2 × 10 -5 there is a marked rise of the 45Ca2+ isotherm M. It is noteworthy that even exposure to a which, however, is not reproduced in the l0 -2 M CaC12 solution did not result in any ESCA isotherm. A closer analysis indicates that this feature hardly can be due to systemobservable C12p peak. The expected Ca/S ratio would be 0.5 for atic experimental errors connected with the the case when -OSO3 is the only kind of neg- growing background correction of the counting rate. A more reasonable explanation may be obtained by noting the electrostatic deC.__~a shielding effect at increasing the electrolyte s concentration coupled with the assumption 0.8 that carboxylic groups (in unesterified form) 2O preferentially are present in depth of the sur0.7 E face grooves below the ESCA sampling depth. 15 0.6 O Accordingly, as the CaC12 concentration is raised, the dissociation of these - C O O H 10 0.5 groups takes place more readily and in this +~ way successively more Ca 2+ adsorption sites (10-2M 0A ~ 5 become available. To within experimental er0 rors the observed Ca/S atomic ratio stays conI I I I I stant ---0.43 in the concentration interval l0 -55 10 15 20 25 l0 -2 M CaC12, i.e., there is no evidence for Ccacl2Xl05 mol I -~ an increasing number of Ca 2+ adsorption sites FIG. 8. Ca2+adsorption on KIV[nO4/H2SO4 treated PE in the ESCA isotherm. Thus we have to consurfaces from radiotraeermeasurementsand from ESCA clude that the carboxylic group contribution to the Cls peak intensity (Table I) predomipeak intensities. H2SO4-treated standard PE sample is correct. As appears from Tables I and II, the relative surface densities o f - C - O H , - C - O - O H , and -C-OSO3H groups are 0.10, 0.05, and 0.11, respectively, for the standard KMnO4/HzSO4 sample #2.
f
o
Journal of Colloid and Interface Science, VoL 100, No. 2, August 1984
KMnO4/H2SOa-OX1DIZED PE SURFACES nantly is due to surface carboxyls in some O
389
Cls
S2p
//
reacted form, e.g., - C - O - R . Figure 9 shows the effects of varying p H of the solution at fixed K + and Ca 2+ concentrations, 8.44 × 10 -3 and 10 -4 M, respectively, as measured by means of 45Ca2+ adsorption. At the concentrations chosen, there is still competition between Ca + and K + for the anionic sites on the surface which is the main reason for the lower 45Ca2+ surface concentrations at low pH as compared with the data of Fig. 8. These 4SCa2+ adsorption data indicate that a gradual dissociation o f - C O O H groups starts at about pH ~ 6.5 at the actual ionic strength. This kind of behavior is in general agreement with the properties of carboxylic polyelectrolyte systems (25) which imply that the ionic strength dependent electrostatic repulsion between - C O O - surface charges has a strong influence on the surface charge density due to a fixed number of carboxyl groups. As the 45Ca2+ adsorption can be increased by about 50% through raising the p H to -~7, the number o f - C O O H groups present may be estimated to roughly ,~ 7 × 10z8 m-2. As already pointed out above these COOH groups are presumably positioned below the ESCA sampling depth, otherwise one would have expected a significant rise of the ESCA isotherm in Fig. 8 at higher CaC12 concentrations.
Annealing Experiments Heat treatment of KMnO4/H2SO4 oxidized PE samples at t ~ 60°C gives rise to substantial
6 '7 E 51 == o 4
d
COO -
3 o
so4-
2 0
1 [ 5
I 7
J 9
r 111 p H
FIG. 9. Influence of pH on the 45Ca2+ adsorption at c~+ = 8.44 × 10-3 Mand Cc~2+= 10 -4 M.
b
) ~
~
-10
/2 AEB
~eV
r
I
i
6 3 0
.5 I
17'1
169
I
~87
FIG. 10. Cls and S2p spectraafterannealingof KMnO4/ H2SO4-treatedPE samples. (a) No further treatment, (b) annealing at 80°C in air, (c) annealing at 80°C in H20, and (d) annealing at 80°C in air after Ca:+ adsorption. changes of the ESCA spectrum (Fig. 10). Depending upon the exact conditions during annealing the end result can be rather variable. As regards the topography, SEM pictures reveal that the granular structure of the sulfated PE surface largely disappears as a result of the heat treatment. Thus a redistribution of matter takes place involving a decrease of the total surface area, i.e., to increased planarity which per se is expected to improve the ESCA spectral resolution. As in all likelihood the surface concentrations produced of polar groups due to the oxidation treatment do not correspond to thermodynamic equilibrium, diffusion processes between the surface and the bulk also take place upon heating. When the dissociable - C O O H and -OSO3H groups are present in the acid form, migration from the air-exposed surface of all kinds of polar groups occurs as may be inferred from the changes of the Cls and S2p ESCA peaks (Fig. 10b). When the surface is contacted with water at annealing there is a tendency for the (dissociated) OSO~H + groups to remain in water contact (Fig. 10c). It is obvious from Fig. 10d, where the - C O - Cls peak (AEB = 1.5 eV) is comparatively Journal of Colloid and Interface Science, Vol. 100, N o . 2, A u g u s t 1 9 8 4
390
ERIKSSON ET AL.
large and well-resolved that the binding of Ca 2+ counterions is an efficient means of blocking the migration of these groups (and also of other polar groups) from the surface to the bulk. Since the surface area diminishes at annealing and one actually observes a small increase of the S2p peak intensity, the sulfate groups appear to accumulate in the residual surface. Thus a surface homogenization process takes place at heat treatment which, in addition, is likely to lead to a more homogeneous lateral distribution of the various polar groups in the surface as had been investigated previously
for KC103/H2SO 4 treated PE surfaces by means of ESR utilizing Mn 2+ as the counterions bonded (cf. Ref. (26)). DISCUSSION
It is well known that the oxidation of hydrocarbons usually follows quite complex reaction pathways. The oxidation ofa PE surface by an oxidant like KMnO4 dissolved in concentrated H2804 is certainly no exception to this rule. Still, on the basis of the present investigation we may devise a simplified, overall scheme of chemical events which by large can account for our experimental results.
--CH2-- + R . ---, - - C H - - + RH
(hydrogen abstraction)
[6]
- - C H - - + RH + O2 ---, - - C H - - + R .
(hydroperoxide formation)
[7]
I
O--OH
--C--HI OH
--C--HI
[8]
~O
O--OH "-~ _ C / / ° \OH - - C H - - + H2SO4 ~ - - C - - + HzO
I OH
[9]
OSO3H
- - C H 2 + 02 "-~ - - C / ' ~ O
I
(esterification)
I + H20
[10]
\OH
OH In this scheme hydroperoxy groups and hydroxyls are of central importance as comparatively long-lived intermediates. Our ESCA data indicate that KMnO4 is particularly efficient in generating both of these kinds of surface groups presumably due to reactions [7] and [8] and a rich supply of 02 through the decomposition reaction Mn207 ~ 2MnO2 + 3/2 02 which presumably occurs in the Journal of Colloid and Interface Science,
Vol. 100,No. 2, August1984
K M n O 4 / H 2 S O 4 solution (27). As a consequence, the esterification with H2SO 4 also
proceeds to a relatively large extent for KMnO4 as compared with KC103 and KzCr207 (cf. Table I). The presence of water naturally inhibits the esterification reaction. Thus the limited rate of water transport away from the surface zone
KMnO4/H2SO4-OXIDIZED PE SURFACES and a rather rapid hydroxyl formation step may provide an explanation for the lower S/C atomic ratios found at high KMnO4 concentrations. Due to essentially the same rea-
391
son, the competitive reaction [10] leading to carboxyl formation may also be favored in the depth of the surface oxidation pits. An alternative to reaction [7] is
- - C H - - + RH + SO3 ---* - - C H - -
J
(sulfonate formation).
[ 11 ]
SO3H + R . However, with the oxidants studied here, this reaction seems to play a minor role for PE, at least within the surface zone monitored by ESCA, whereas for PS the corresponding sup phonation reaction seems to dominate the picture. With KC103 as the oxidant, H abstraction can also be followed by C1 addition as C12 forms during the oxidation reaction: - - C H - - + C12 ~ - - C H - - + CI.. ] C1
[12]
This reaction is of similar nature as reaction [2] above and can account for the hydrocarbon chlorination detected in this case. The chemical constitution of the standard (#2) KMnO4/H2SO4 PE surface as probed by ESCA and the tagging reactions [l]-[5] and, in addition, by means of 45Ca2+ adsorption is represented schematically in Fig. 1 1. As compared with our previously proposed model (16) the main differences are (i) perhydroxy groups contribute to the - C - O - Cls peak, (ii) a significant amount of keto groups are positioned beneath the surface but within the ESCA sam-
piing depth, (iii) the carboxyls are present in esterified form on the external surface and in acid form in the surface oxidation grooves, and (iv) ether groups also contribute to the - C - O Cls and O l s peak intensities. According to the model proposed here, the O/S atomic ratio should amount to 8.5 whereas the O/S ratio actually determined in the present investigation is significantly lower, 6.5. However, as the S2p photoelectrons have higher kinetic energy (1317 eV) than the O 1s photoelectrons (951eV) the corresponding electron escape depths, ke, are also different resulting in a relative weaker O 1s signal by a factor of(951/1317) 0.75 - 0.78. Since 6.5/0.78 8.3 it would thus seem that a reasonably good agreement is within reach between the O/S data recorded and our model of the chemical constitution of a KMnO4/H2SO4 oxidized PE surface. CONCLUSIONS The ESCA and tracer adsorption studies presented in this paper show that the main chemical groups on a KMnO4/H2SO4 treated PE surface are
// O --OSO3H, --OH, - - O - - O H , - - C = O , - - C - - O - - C - - , and - - C - - O - - R
In the oxidation grooves - C O O H groups are also present in appreciable amounts. By annealing a smoother and more homogeneous polymer surface can be formed with a com-
(ester).
position that strongly depends on the chemical state of the ionic groups at the heat treatment. Comparative experiments with KC103/H2SO4 and K2Cr207/H2SO4 support the view that Journal of Colloid and Interface Science. Vol. I00, No. 2, August 1984
392
OH i
ERIKSSON ET AL.
OSO3H - C = 0 [
i
OH i
O-OH i
o~
0~
0
i
ii
FIG. 11. Model showing schematically the chemical constitution of the standard K ] V [ n O 4 / H 2 S O 4 PE surface.
these mixtures are weaker as oxidizing agents than K M n O 4 / H 2 S O 4 . ACKNOWLEDGMENTS The authors wish to heartily thank BjiSrn Akermark, Bernt Lindberg, Rein Maripuu, and Per Stenius for helpful discussions. REFERENCES 1. Brewis, D. M., and Briggs, D., Polymer 22, 7 (1981). 2. Seighahn, K. et al., "ESCA, Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy." Almqvist och Wiksell, Uppsala, 1967. 3. Clark, D. R., PureAppl. Chem. 54, 415 (1982). 4. Briggs, D., "Surface Analysis and Pretreatment of Plastics and Metals" (D. M. Brewis, Ed.), p. 199. Applied Science, London, 1982. 5. Rasmussen, J. R., Stedronsky, E. R., and Whitesides, G. M., J. Amer. Chem. Soc. 99, 4736 (1977). 6. Blais, P., Carlsson, D. J., Csullog, G. W., and Wiles, D. M., J. Colloid Interface Sci. 47, 636 (1974). 7. Willis, H. A., and Zichy, V. J. I., in "Polymer Surfaces" (D. T. Clark and W. J. Feast, Eds.), Chapt. 15. Wiley, New York, 1978. 8. Briggs, D., Brewis, D. M., and Konieczko, M. B., aT. Mater. Sci. 11, 1270 (1976). 9. Briggs, D., Zichy, V. J. I., Brewis, D. M., Comyn, J.,
Journal of Colloid and Interface Science, Vol. 100, No. 2, August 1984
Dahm, R. H,, Green, M. A., and Konieczko, M. B., Surface Interface Anal, 2, 107 (1980). 10. Baszkin, A., and Ter-Minassian-Saraga, L., J. Polym. Sci. C34, 243 (1971). 11. Baszkin, A., and Ter-Minassian-Saraga, L., J. Colloid Interface Sci. 43, 190 (1973). 12. Baszkin, A., and Ter-Minassian-Saraga, L., Polymer 15, 759 (1974). 13. Baszkin, A., Nishino, M., and Ter-Minassian-Saraga, L., J. Colloid Interface Sci. 54, 317 (1976). 14. Larsson, R., Eriksson, J. C., and Olsson, P., Thromb. Res. 14, 941 (1979). 15. Larsson, R., Eriksson, J. C., Lagergren, H., and Olsson, P., Thromb. Res. 15, 157 (1979). 16. Larsson, N., Stenius, P., Eriksson, J. C., Maripuu, R., and Lindberg, B., J. Colloidlnterface Sci. 90, 127 (1982). 17. Gelius, U., Hed6n, P. F., Hedman, J., Lindberg, B., Manne, R., Nordberg, R., Nordling, C., and Siegbahn, K., Phys. Scr. 2, 70 (1970). 18. Seofield, J. H., J. Electron Spectrosc. 8, 129 (1976). 19. Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. F., and Muilenberg, G. E. (Eds.) "Handbook of X-Ray Photoelectron Spectroscopy," p. 58. PerkinElmer, 1979. 20. Lindberg, B., Maripuu, R., Siegbahn, K., Larsson, R., G61ander, C.-G., and Eriksson, J. C., J. Colloid Interface Sci. 95, 308 (1983). 21. G61ander, C.-G., Arwin, H., Eriksson, J. C., LundstriSm, I., and Larsson, R., Colloids Surfaces 5, 1 (1982). 22. Briggs, D., J. Adhes. 13, 287 (1982). 23. Briggs,D., and Kendall, C. R., Int. J. Adhes. Adhesives 13 (1982). 24. Clark, D, T., Pure. Appl. Chem. 54, 415 (1982). 25. Gunnarson, G., Thesis, Lund, Sweden, 1981. 26. Catoire, B., Bouriot, P., Baszkin, A., Ter-MinassianSaraga, L., and Boissonnade, M. M., Y. Colloid Interface Sci. 79, 143 (1981). 27. "Gmelins Handbuch der anorg. Chemic," 8. Auflage, Teil C1, p. 365. Verlag Chemic, GMBH, Weinheim/Bergstrasse, 1973.
Received September 14, 1983; accepted December 31, 1983 The chemical constitution of KMnO4/H2SO4-0xidized(sulfated) polyethylene (PE) surfaces has been exploredby means of ESCA utilizing chemical shifts, peak intensity data, and chemicaltaggingreactions. In addition, the adsorption of Ca2+ ions on sulfated PE was quantified by radiotracer measurements. Comparisons have been made with KC103/H2SO4and K2Cr2OT/H2804 as oxidizing agents and through replacing polyethylene by polystyrene. According to the present study the main chemical groups on a O KMnOa/H2SO4-treatedPE surfaceare -OSO3H, -OH, -O-OH, -C----O,-C-O-C-, andc//_O_R (ester). In the oxidation grooves, dissociable -COOH groups are also present in appreciable amounts. By annealing, a smoother and more homogeneous sulfated surface is produced, the composition of which strongly depends upon the state of the ionic groups at the heat treatment. BACKGROUND Chemical modification of polyolefin surfaces is of critical importance for several m a n ufacturing processes in use today as a means of promoting adhesion or wetting. To serve as guidelines when tackling practical adhesion problems a n u m b e r of different "theories" of adhesion have been advanced in the past as is well described, e.g., in a recent review paper centered on polyethylene and polypropylene surfaces by Brewis and Briggs (1). So far, however, the interrelations between the observed adhesion/wetting properties and the detailed surface chemistry of polymers have not been explored extensively enough to enable the most important general features to be clearly distinguished. The prospects for further progress in this research area have become considerably brighter through the advent of the ESCA technique (2) which is well suited for comprehensive chemical analysis of nonconducting polymer surfaces as has been con-
vincingly demonstrated particularly by Clark (3) and Briggs (4). Several methods are now available to improve the adhesion to polyolefin surfaces, e.g., corona discharge, gas plasma exposure, flame treatment, and acid etching, all of which appear to result in an enhanced degree of surface oxidation (cf. Ref. (4)). Rasmussen et aL (5), Blais et al. (6), and Willis and Zichy (7) have studied the surface oxidation of low density PE due to exposure to chromium/sulfuric acid using IR spectroscopy. Rasmussen et al. found that carbonyl and carboxyl groups are formed at this oxidation treatment. Blais et al. found evidence also for hydroxyl surface groups. In addition, Willis and Zichy (7) could establish the presence of sulfate and alkyl sulfonate groups. Briggs et al. (8, 9) using ESCA identified hydroxyl, carbonyl, carboxyl, and sulfonate groups on chromic acid etched low density PE surfaces. In their adhesion/wetting studies of PE surfaces, Baszkin and Ter-Minassian-Saraga (10381 0021-9797/84 $3.00
Journal of Colloid and Interface Science, Vol. 100, No. 2, August 1984
Copyright © 1984 by Academic Press. Inc. All rights of reproduction in any form reserved.
382
ERIKSSON ET AL.
13) utilized KC103 dissolved in concentrated the oxidizing agent. The surface density of polar groups was quantified with a 45Ca2+ tracer adsorption technique which has been employed also in the present investigation. Eriksson et al. (14) used KMnO4 dissolved in concentrated H2SO4 to modify PE surfaces and it was demonstrated that such a modified PE surface is thromboresistant in the restricted sense of exhibiting a minimal degree of blood platelet adhesion (15). Recently, the surface chemistry of the KMnOa/H2SO4-treated PE surface was the subject of an ESCA study (16) which indicated the presence of hydroxyl, sulfate ester, carbonyl, and carboxyl surface groups. Since these results partwise seemed to disagree with the earlier findings concerning the chemical constitution of acid-etched PE surfaces (in particular sulfonate vs sulfate) we decided to carry through a broader study of the surface functionality in the first place of KMnO4/H2SO4oxidized PE surfaces, including variable oxidation conditions while making simultaneous use of ESCA and the 45Ca2+ tracer adsorption technique referred to above. H 2 S O 4 as
MATERIALS AND SAMPLE PREPARATIONS Films of low density PE (0.93 g/cm 3 at 23°C) were purchased from Noax AB, Sweden, and from Grace SocietY, France. Polystyrene (PS) samples were cut out of conventional polystyrene Petri dishes manufactured by Nuric AS, Denmark. The concentrated sulfuric acid used was of pro analysi grade with density equal to 1.84 g/ml. Potassium permanganate (KMnO4), potassium dichromate (K2Cr207), potassium chlorate (KC103), potassium hydrogen phosphate (K2HPO4), potassium hydrogen phthalate (KHC6H4(COO)2), and calcium chloride (CaCI2) were all of Merck's reagent grade. 45Calabeled CaCl2 was obtained from Institute Nationale des Radioelements, Belgium, with specific activity 221.5 mCi/g. Triply distilled water was used throughout the experiments. The ESCA chemical tagging reagents were all Journal of Colloid and Interface Science, Vol. 100, No. 2, August 1984
of Merck's reagent quality except SO2 that was purchased from Alfax, Maim6, Sweden. For the 45Ca 2+ adsorption measurements we utilized a 50-#m-thick low-density PE (Noax) film which after sonication in 70% ethanol/water was practically free of oxygen as checked by means of ESCA. Thicker polymer samples of the size 14 × 10 × 1 mm were used in the ESCA studies. In order to reduce the amount of oxidation products initially present on the surface some PE samples were gently scraped with a knife and subsequently pressed against a microscope cover glass over night at 60°C to produce a smooth surface. Prior to further use all samples were sonicated in 70% ethanol/water, rinsed with water, and stored in a desiccator. The various oxidation treatments were carlied out as appears from Table I using KMnO4, KC103, or K2Cr207 dissolved in concentrated sulfuric acid and were followed by several rinsings with water. METHODS The ESCA spectra were recorded with a Leybold-Heraeus spectrometer (LH 2000) equipped with a AI K~ X-ray source (1486.6 eV), the operating conditions of which were always set to 13 kV and 14 mA. The sample orientation was chosen normal to the direction through the entrance of the hemispherical electron energy analyzer of the spectrometer. The electrons analyzed emanate from a rectangular sample surface area of the approximate size 10 × 1 mm. Complete spectral scans as well as detailed recordings of the main peaks were made for each sample, usually with a sweep time of 5 or 10 min. The ESCA electron binding energy scale was fixed by assigning EB = 285.0 eV to the -CH2- carbon 1s peak. According to previous studies (17) the carbon chemical shifts of different kinds of oxygen containing groups are: (i) hydroxyl, hydroperoxide, ether, alkyl, or sulfate ester, etc, AEB = 1.5 eV, (ii) carbonyl, AEB = 3.0 eV~ and
KMnO4/H2SO4-OXIDIZED
PE
TABLE
383
SURFACES
I Number of different atoms per -CHz- group in each surface
Sample
Preparation
1
PE, Noax
2
PE, Noax
3
PE, Noax
4
PE, Noax
5
PE, Noax
6
PE, Noax
7
PE, Noax
8
PE, Noax
9
PE, Noax
10
PE, Noax
11
PE, Noax
12
PE, Noax
13
PE, Grace
14
PS
70% ethanol/water sonication 2 g/liter KMnO4/H2SO4 2 min 2 g/liter KMnO4/HzSO4 2 sec 2 g/liter KMnO4/H2SO4 10 min 0.2 g/liter KMnO4/H2SO4 2 min 10 g/liter KMnO4/H2SO4 2 rain 2 g/liter K M n O 4 / H 2 S O 4 2 min, 1% HzO 2 g/liter KMnO4]HaSO4 2 min, 5% H20 2 g/liter KC103/H2SO4 2 mm 10 g/liter KC 1O3/H2SO4 2 mln 55 g/liter KC103/HzSO4 2 mm 2 g/liter K 2 C r 2 0 7 / H 2 S O 4 2 mm 2 g/liter KC103/H2SO4 2 mln 2 g/liter KMnO4/H2SO4
Number
-c-o-
/~c=o
-c/?o-
o
-OSO3H
0.46
0.11
0.05
0.72
0.40
0.11
0.07
0.04
0.59
0.12
0.09
0.14
0.45
0.09
0.05
0.64
0.11
0.38
0.10
0.06
0.64
0.07
0.39
0.07
0.06
0.08
0.15
0.05
0.05
0.05
0.05
0.01
0.02
0.20
0.04
0.11
0.01
0.03
0.28
0.03
0.02
0.11
0.05
0.03
0.47
0.11
0.08
0.10
0.03
0.02
0.61
0.17
0.08
0.87
0.03
0.52
0.38
0.40
1.88
0.17
-C-Cl
0.11
0.01 0.02
2 mln
(iii) carboxyl or the c o r r e s p o n d i n g ester, AEB = 4.2 eV. T h e c o r r e s p o n d i n g p e a k intensity c o n t r i b u tions were graphically e s t i m a t e d f r o m the C 1s spectra. The relative cross-sections used in this w o r k were o b t a i n e d f r o m a p a p e r b y Scofield (18): a ( C l s ) = 1.00, a(01s) = 2.93, a(S2p) = 1.68, a(C12p) = 2.29, a(Ca2p3/2) = 3.35, a(Ti2p3/2) = 5.22, a ( F l s ) = 4.43, a ( N l s ) = 1.80, a(Br3d) = 2.84. These cross-section values for $2p3/2 a n d Ca2p3/2 were also c h e c k e d t h r o u g h rec o r d i n g spectra o f s o d i u m d o d e c y l sulfate a n d c a l c i u m stearate a n d satisfactory a g r e e m e n t was found. F o r the 45Ca2+ a d s o r p t i o n m e a s u r e m e n t s circular P E film samples were p u n c h e d having
a d i a m e t e r o f 3 cm. After o x i d a t i o n t r e a t m e n t the s a m p l e s were p l a c e d on t o p o f a n a q u e o u s s o l u t i o n c o n t a i n i n g 45CAC12. T h e excess o f Ca 2+ at t h e o x i d i z e d s a m p l e / s o l u t i o n interface was d e t e r m i n e d f r o m the difference in c o u n t ing rate relative to a n oxygen-free P E film s a m p l e using a gas flow c o u n t e r m o u n t e d above the PE film. T h e set-up used is described in detail in Ref. (10). 2oCa 45 is a t - - e m i t t e r with an average electron energy e q u a l to 0.258 MeV. T h u s the 5 0 - g m PE film causes n o significant r e d u c t i o n o f the r a d i a t i o n intensity. T h e i n f o r m a t i o n d e p t h is a b o u t ~ 0 . 1 m m d o w n into the 45CAC12 s o l u t i o n i m p l y i n g t h a t the b u l k s o l u t i o n c o n t r i b u t i o n to the c o u n t i n g rate b e c o m e s sizable at 45Ca c o n c e n t r a t i o n s in excess o f ~ 10 -4 M. Journal of Colloid and Interface Science, Vol. 100, No. 2, August 1984
384
ERIKSSON ET AL. RESULTS
+11~
5g/I
Surface-Chemical Effects of Different Oxidizing Agents Figure 1 shows some ESCA survey spectra of untreated PE (sample # 1 (a)) and PE oxidized with KMnO4 (sample #2 (b)), KC103 (sample #9 (c)), and KzCr207 (sample # 12 (d)) all in concentrated H z S O 4 . The pure PE sample # 1 exhibits only a minor trace of oxygen, its spectrum is dominated by the large C ls peak at ER = 285.0 eV, and the associated carbon Auger K L L signal at Ekin ~, 275 eV. Besides carbon, all the oxidized surfaces contain substantial amounts of oxygen and in addition also sulfur and nitrogen. From circumstantial evidence we may anticipate that the presence of nitrogen (EB = 402.0 eV) is due to a spurious cationic contaminant that is adsorbed after the oxidation treatment. The KC103 sample (Fig. lc) shows a C12p peak at EB = 200.0 eV indicating the formation of C-C1 bonds (19). It is noteworthy that the ESCA spectra in Figs. lb, d do not show the presence of any traces of Mn or Cr. The sulfur and oxygen peaks are comparatively large for the KMnO4/H2SO4-treated PE surface. When the concentration of KC103 is raised, however, to 55 g/liter corresponding Cls
01s
°
Cls
i
i
i
1486 1000 500
i
0
i
r
i
1486 1000 500
,___..1
0
FIG. 1. ESCA survey spectra. (a) Polyethylene (PE, # 1), (b) PE: KMnO4/H2SO4, 2 g/liter (#2), (c) PE: KC103/ H2SO4, 2 g/liter (#9), (d) PE: K2Cr2OT/H2SO4, g/liter (#12). Journal of Colloid and Interface Science, Vol. 100, No. 2, August I984
=•12• - ~
AE a
eV
i i i 630
K2Cr207
-~9~
2 g/I
i i i 630
i i I 630
KMn04
KCIOa
FIG. 2. C 1s spectra of PE surfaces treated with oxidant/ H2SO4 for 2 min. The vertical lines denote the peak positions o f C - O (AEB = 1.5 eV), C : O (AEB = 3.0 eV), and COOH (AEB = 4.2 eV).
to 2.9% by weight, one observes a considerable increase of the S peak intensity to about the same level as for 2 g]liter ofKMnO4 (cf. Table I) whereas an increase of the KMnO4 concentration leads to a substantial decrease of the S content in the surface (cf. below). It appears already from the O/C ratio that KMnO4 in H2804 is a more powerful oxidant than KC103/H2SO4 which in turn is somewhat more powerful than K 2 C 2 0 7 / H 2 8 0 4 . The corresponding detailed C 1s spectra (of samples #2, 6, 9, 10, 11, 12) are displayed in Fig. 2 from which it is clear that the KMnO4/ H 2 S O 4 agent is also much more efficient than KC103/H2SO4 and K 2 C r 2 0 7 / H 2 8 0 4 in promoting the formation o f - C - O - carbon atoms shifted 1.5 eV away from the - C H 2 - carbon. It has been proposed earlier that in the case ofKMnO4/HzSO 4 oxidation, - O H groups and -OSO3H groups are attached to these carbon atoms (16). Further ESCA evidence will be presented below indicating that perhydroxy groups also contribute to the - C - O - peak. In the KCIO3/H2SO4 and K2CEO7/HESO4 mixtures the oxidation reactions apparently proceed rather differently and probably with a slower rate of formation of O H groups re-
KMnO4/H2SO4-OXIDIZED PE SURFACES suiting in a lower rate of esterification with H2SO 4. Deconvolution of these C ls spectra yields the separate contributions to the resultant peak intensity of the different chemically shifted carbon peaks (Table I). It is noteworthy that the KC103/H2SO4-treated PE surface shows essentially the same C ls spectrum at 55 g/liter as at 10 g/liter of KC103 and that the - C - O - / - C H 2 - a t o m i c ratio is as high as ~ 0 . 4 6 for the KMnO4/H2SO4 treated surface in spite of the fact that the information depth of ESCA for organic surfaces is estimated to be 5-10 nm. In Fig. 3 the detailed S2p spectra are compared for samples 12, 2, and 9: The $2p3/2 peak positions for all samples center around EB = 169.0 eV with a spread of +0.2 eV indicating sulfur in sulfate or possibly sulfonate form. For ESCA samples prepared on PE plates from acidified (pH = 1.0) sodium dodecyl sulfate and sodium decylsulfonate solutions we have obtained EB (sulfate) = 169.0 eV as measured for the $2p3/2 peak relative to the - C H 2 - carbon ls peak at 285.0 eV and EB (sulfonate) = 168.2 eV. Earlier it has been established that the $2p3/2 peak due to sulfate ester groups in heparin and in dextran sulfate are positioned at EB -- 168.8 and 169.1 eV, respectively (20). Furthermore, for the polystyrene sample #14, EB ($2p3/2) equals 168.3 eV indicating sulfonate formation in this case. Thus the more complete ESCA evidence now available gives further support for our preliminary conclusion that mainly ester sulfate groups are introduced in a PE surface as a result of the action of a KMnO4/H2SO4 mix-
x4
eV
6 PE
K2Cr207
Grace
Polystyren
ture. So seems to be the case likewise for the KC103/H2SO4 and K2Cr207/H2SO 4 mixtures but in these cases the rates of formation of r-OSO3H groups is reduced, presumably because of a lower rate of formation of the O H intermediate. In Fig. 4 the C ls peaks obtained after KMnOa/H2SO4 oxidation of PE (Noax), PE (Grace), and PS are shown. A clear tendency for a more extensive reaction on PE (Grace) than on PE (Noax) is noted. In fact with 10 g/liter KC103/H2SO4 as the oxidizing agent a C 1s spectrum is generated for PE (Grace) that is very similar to the KMnOa/H2SO4 2 g/liter standard case (cf. Fig. 2). Apparently, PS reacts much more readily than PE leading to a comparatively high surface density also of carboxyl groups as is seen from Fig. 4c (cf. Table I). In Figs. 5 and 6 the effects of the C l s spectra
0.2g/I
/~EB I
[ 167
PE
FiG, 4. C 1s spectra of different polymer surfacestreated with KMnO4/H2SO4for 2 min.
eV i J 17116g
Noax
x9
~2~ AE 8 ~, eV
385
~ i 17116g
i 167
KMnO 4
L i 17116g
i 167
KCIO 3
FIG. 3. S2p spectra of PE surfacestreated with oxidant/ H2SO4 for 2 rain.
]
630 2 sec
I
I
I
J
630 2 min
I
I
L
630 lomin
FIG. 5. C 1s spectra of PE samples treated with KMnO4/
H2SO4. The vertical lines denote the peak positions of C-O (AEB = 1.5 eV), C--~O(AEB = 3.0 eV), and COOH (AEB = 4.2 eV). Journal of Colloid and Interface Science, Vol. 100, No. 2, August 1984
386
ERIKSSON ET AL.
of varying exposure time, concentration, and water content at the KMnOJHaSO4 treatment are shown. When it comes to favoring the development of the - C - O - shoulder at AEB = 1.5 eV it is obviously important to use a long exposure time and a comparatively low KMnO4 concentration. Furthermore, the water content should be kept at a rather low (less than ~ 1%) level. To conclude this part we wish to draw the attention to the SEM pictures of a pure PE surface and of a sulfated PE surface (21). These SEM pictures reveal a granular structure with grain size ~ 0.1 #m in diameter that is particularly pronounced for the sulfated PE surface. This observation points in the direction that the amorphous regions are more susceptible to oxidative attack than the crystalline spherulites. In view of this clearly visible surface heterogeneity we have to presume that
the actually exposed surface area is considerably larger than the planar surface area. Furthermore, due to the water formation at the esterification and the inhibitive action of water on this same reaction it is not surprising to find as we have done previously (cf. Ref. (16)) using angular dependent ESCA that by large the O surface atoms are situated below the S surface atoms.
Chemical Tagging Experiments In order to obtain further verification of the results described above as to the chemical constitution of the oxidized PE surfaces some chemical tagging experiments have also been carried out. The following derivatization reactions, previously explored by Briggs (22) and by Briggs and Kendall (23) as to the most selective reaction conditions, have been utilized.
H --C--+
--NH--NH2--* C = N - - ~ N - - @
+ H20
[1]
II pentafluorophenylhydrazine (PFPH) in ethanol
O
(Schiff's base)
--CH2--C--+ Br2/H20 ---~--CBr2--C--(Br2 reacts also with II II double bonds) O O
[2]
- - C H = C - - + C1--C--CH2--C1 -~ CH=C--O--C--CH2C1
[3]
I
If
II
OH
O
O
Chloroacetylchloride (reacts predominantly with (CAC) in heptane enolic OH groups)
--CH2--C--
I
+ (acac)2Ti(OPr)2
OH Diisopropoxide-titanium bis-acetylacetonate (TAA) in 2-propanol --CH2--O--OH
+ SO2(g) ~
Journal of Colloid and Interface Science, Vol. 100, No. 2, August 1984
--CH2--C--O-- Ti(acac)2 / OPr
[4]
(reacts predominantly with alcoholic OH groups) --CH2~" O--SO3H
[5]
KMnO4/H2SO4-OXIDIZED PE SURFACES
387 TABLE II
Evaluation of Chemical Tagging Reactions
s%
Tagging reagent PFPH Brz CAC TAA SOz
Functionalgroup
Number of groups per -CH2- in original #2 surface
ESCA peak quantified
-C=O -CH2C=O, - C = C - C - O H (enol) - C - O H (alcohol) -C-O-OH
0.04 0.05 0.06 0.10 0.05
Fls, Nls Br3d Cl2p Ti2p3/2 S2p
ber of accessible - C - - O groups, ~ 0 . 0 5 per - C H 2 - in the standard KMnO4/H2SO4 surface. From the C 1s spectrum (Fig. 2, #2) it is found that the corresponding figure is significantly higher, ~,0.11, indicating that about half of the -C----O groups do not react with the tagging reagents under the conditions choo% sen. Comparing with Table I, one observes that the alcoholic O H surface density as measured by means of reaction [4] is considerably less in magnitude than the - C - O peak intensity of the C I s spectrum. T h e ESCA S2p spectra of PE surfaces first AE B ,< i I I treated with K_MnO4/H2SO4 and then with 6 3 0 SO2(g) (reaction [5]) are displayed in Fig. 7. KMnO 4 It is seen that an extensive SO2 addition occurs FIG. 6. CIs spectra of PE surfacestreated with 2 g/liter on the KMnOa/H2SO4 surface indicating the KMnO4[H2SO4,containing added H20. The verticallines presence of perhydroxy groups. No evidence denote the peak positions ofC-O (AEB = 1.5 eV), C=O for such groups on the KC103/H2SO4 and (AEB = 3.0 eV), and COOH (AEB = 4.2 eV). K2Cr2Ov/H2SOa-treated surfaces was found. Furthermore, it is worth noticing that the sulfur added as a result of reaction [5] gives rise The reactions [1] and [2] are claimed to to an ESCA peak exactly at the sulfate position quantify the density of (aldehyde and keto) EB = 169.0 eV as no peak broadening is obcarbonyl groups on the surface, whereas re- served. This strongly supports that the original actions [3] and [4] m a y be used to distinguish alkyl sulfate assignment for the KMnO4/ between enolic and alcoholic O H groups. Reaction [5] involving gaseous SO2 provides a very convenient way of analyzing perhydroxy groups (24). Some of the ESCA results obtained after carrying through reactions [1][4] with the standard sample #2 are shown in Ea i ~ i I I I Table II, where cross-section-corrected peak ~ev lrl 169167 17116g t67 intensities are given for F, N, Br, el, and Ti KMnO 4 K M n 0 4 + S02 relative to the original - C H 2 - peak intensity. It is evident that the F, N, Br, and C1 probe s FIG. 7. S2p spectra of a KMnO4/H2SO4-treated PE used yield rather similar results as to the n u m - sample before and after SO2 exposure.
/k
Journal of Colloid and Interface Science, Vol. 100, No. 2, August 1984
388
ERIKSSON ET AL.
atively charged group on the surface and the H+/Ca 2+ ion exchange process proceeds to completion. The fact that the observed Ca/S ratio is somewhat below 0.5 can readily be accounted for by referring to (i) some substitution of noncondensed Ca 2+ ions with H + occurs at the rinsing step and (ii) all -OSO3H groups which contribute to the S peak intensity Ionic Groups on KMnO4/H2SO4-Oxidized are not accessible for ion exchange. On this PE Surfaces basis we may conclude that the plateau level The experimental room temperature iso- in the 45Ca 2+ adsorption isotherm most likely therm of 45Ca 2+ adsorption onto a standard refers to the number o f - O S O ~ sites per (forKMnO4/H2SO4 PE surface (sample #2) from mal) surface area which are accessible for ion CaC12 solutions of different concentrations and exchange with Ca 2+. It appears from Fig. 8 pH ~ 6 is plotted in Fig. 8 where the cor- that this number is 14 × 1018 m -2 correspondresponding Ca2p3/2 ESCA peak intensity data ing to ~ 0 . 0 7 nm2/-OSO3. Such a very high are also displayed. The ESCA samples ana- (formal) surface density o f - O S O 3 groups is lyzed were prepared through removing excess definitely not in accord with the S/C and bulk solution with a filter paper followed by S/O ratios derived from the ESCA spectral rinsing with isopropanol and drying. There is data and must hence largely be attributed to a pronounced initial rise in both isotherms. the heterogeneous character of the oxidized The ESCA isotherm reaches a saturation level PE surface. At concentrations above ~ 10 -4 M CaC12 corresponding to a Ca/S atomic ratio of ~ 0 . 4 3 at CaC12 concentrations ~ 2 × 10 -5 there is a marked rise of the 45Ca2+ isotherm M. It is noteworthy that even exposure to a which, however, is not reproduced in the l0 -2 M CaC12 solution did not result in any ESCA isotherm. A closer analysis indicates that this feature hardly can be due to systemobservable C12p peak. The expected Ca/S ratio would be 0.5 for atic experimental errors connected with the the case when -OSO3 is the only kind of neg- growing background correction of the counting rate. A more reasonable explanation may be obtained by noting the electrostatic deC.__~a shielding effect at increasing the electrolyte s concentration coupled with the assumption 0.8 that carboxylic groups (in unesterified form) 2O preferentially are present in depth of the sur0.7 E face grooves below the ESCA sampling depth. 15 0.6 O Accordingly, as the CaC12 concentration is raised, the dissociation of these - C O O H 10 0.5 groups takes place more readily and in this +~ way successively more Ca 2+ adsorption sites (10-2M 0A ~ 5 become available. To within experimental er0 rors the observed Ca/S atomic ratio stays conI I I I I stant ---0.43 in the concentration interval l0 -55 10 15 20 25 l0 -2 M CaC12, i.e., there is no evidence for Ccacl2Xl05 mol I -~ an increasing number of Ca 2+ adsorption sites FIG. 8. Ca2+adsorption on KIV[nO4/H2SO4 treated PE in the ESCA isotherm. Thus we have to consurfaces from radiotraeermeasurementsand from ESCA clude that the carboxylic group contribution to the Cls peak intensity (Table I) predomipeak intensities. H2SO4-treated standard PE sample is correct. As appears from Tables I and II, the relative surface densities o f - C - O H , - C - O - O H , and -C-OSO3H groups are 0.10, 0.05, and 0.11, respectively, for the standard KMnO4/HzSO4 sample #2.
f
o
Journal of Colloid and Interface Science, VoL 100, No. 2, August 1984
KMnO4/H2SOa-OX1DIZED PE SURFACES nantly is due to surface carboxyls in some O
389
Cls
S2p
//
reacted form, e.g., - C - O - R . Figure 9 shows the effects of varying p H of the solution at fixed K + and Ca 2+ concentrations, 8.44 × 10 -3 and 10 -4 M, respectively, as measured by means of 45Ca2+ adsorption. At the concentrations chosen, there is still competition between Ca + and K + for the anionic sites on the surface which is the main reason for the lower 45Ca2+ surface concentrations at low pH as compared with the data of Fig. 8. These 4SCa2+ adsorption data indicate that a gradual dissociation o f - C O O H groups starts at about pH ~ 6.5 at the actual ionic strength. This kind of behavior is in general agreement with the properties of carboxylic polyelectrolyte systems (25) which imply that the ionic strength dependent electrostatic repulsion between - C O O - surface charges has a strong influence on the surface charge density due to a fixed number of carboxyl groups. As the 45Ca2+ adsorption can be increased by about 50% through raising the p H to -~7, the number o f - C O O H groups present may be estimated to roughly ,~ 7 × 10z8 m-2. As already pointed out above these COOH groups are presumably positioned below the ESCA sampling depth, otherwise one would have expected a significant rise of the ESCA isotherm in Fig. 8 at higher CaC12 concentrations.
Annealing Experiments Heat treatment of KMnO4/H2SO4 oxidized PE samples at t ~ 60°C gives rise to substantial
6 '7 E 51 == o 4
d
COO -
3 o
so4-
2 0
1 [ 5
I 7
J 9
r 111 p H
FIG. 9. Influence of pH on the 45Ca2+ adsorption at c~+ = 8.44 × 10-3 Mand Cc~2+= 10 -4 M.
b
) ~
~
-10
/2 AEB
~eV
r
I
i
6 3 0
.5 I
17'1
169
I
~87
FIG. 10. Cls and S2p spectraafterannealingof KMnO4/ H2SO4-treatedPE samples. (a) No further treatment, (b) annealing at 80°C in air, (c) annealing at 80°C in H20, and (d) annealing at 80°C in air after Ca:+ adsorption. changes of the ESCA spectrum (Fig. 10). Depending upon the exact conditions during annealing the end result can be rather variable. As regards the topography, SEM pictures reveal that the granular structure of the sulfated PE surface largely disappears as a result of the heat treatment. Thus a redistribution of matter takes place involving a decrease of the total surface area, i.e., to increased planarity which per se is expected to improve the ESCA spectral resolution. As in all likelihood the surface concentrations produced of polar groups due to the oxidation treatment do not correspond to thermodynamic equilibrium, diffusion processes between the surface and the bulk also take place upon heating. When the dissociable - C O O H and -OSO3H groups are present in the acid form, migration from the air-exposed surface of all kinds of polar groups occurs as may be inferred from the changes of the Cls and S2p ESCA peaks (Fig. 10b). When the surface is contacted with water at annealing there is a tendency for the (dissociated) OSO~H + groups to remain in water contact (Fig. 10c). It is obvious from Fig. 10d, where the - C O - Cls peak (AEB = 1.5 eV) is comparatively Journal of Colloid and Interface Science, Vol. 100, N o . 2, A u g u s t 1 9 8 4
390
ERIKSSON ET AL.
large and well-resolved that the binding of Ca 2+ counterions is an efficient means of blocking the migration of these groups (and also of other polar groups) from the surface to the bulk. Since the surface area diminishes at annealing and one actually observes a small increase of the S2p peak intensity, the sulfate groups appear to accumulate in the residual surface. Thus a surface homogenization process takes place at heat treatment which, in addition, is likely to lead to a more homogeneous lateral distribution of the various polar groups in the surface as had been investigated previously
for KC103/H2SO 4 treated PE surfaces by means of ESR utilizing Mn 2+ as the counterions bonded (cf. Ref. (26)). DISCUSSION
It is well known that the oxidation of hydrocarbons usually follows quite complex reaction pathways. The oxidation ofa PE surface by an oxidant like KMnO4 dissolved in concentrated H2804 is certainly no exception to this rule. Still, on the basis of the present investigation we may devise a simplified, overall scheme of chemical events which by large can account for our experimental results.
--CH2-- + R . ---, - - C H - - + RH
(hydrogen abstraction)
[6]
- - C H - - + RH + O2 ---, - - C H - - + R .
(hydroperoxide formation)
[7]
I
O--OH
--C--HI OH
--C--HI
[8]
~O
O--OH "-~ _ C / / ° \OH - - C H - - + H2SO4 ~ - - C - - + HzO
I OH
[9]
OSO3H
- - C H 2 + 02 "-~ - - C / ' ~ O
I
(esterification)
I + H20
[10]
\OH
OH In this scheme hydroperoxy groups and hydroxyls are of central importance as comparatively long-lived intermediates. Our ESCA data indicate that KMnO4 is particularly efficient in generating both of these kinds of surface groups presumably due to reactions [7] and [8] and a rich supply of 02 through the decomposition reaction Mn207 ~ 2MnO2 + 3/2 02 which presumably occurs in the Journal of Colloid and Interface Science,
Vol. 100,No. 2, August1984
K M n O 4 / H 2 S O 4 solution (27). As a consequence, the esterification with H2SO 4 also
proceeds to a relatively large extent for KMnO4 as compared with KC103 and KzCr207 (cf. Table I). The presence of water naturally inhibits the esterification reaction. Thus the limited rate of water transport away from the surface zone
KMnO4/H2SO4-OXIDIZED PE SURFACES and a rather rapid hydroxyl formation step may provide an explanation for the lower S/C atomic ratios found at high KMnO4 concentrations. Due to essentially the same rea-
391
son, the competitive reaction [10] leading to carboxyl formation may also be favored in the depth of the surface oxidation pits. An alternative to reaction [7] is
- - C H - - + RH + SO3 ---* - - C H - -
J
(sulfonate formation).
[ 11 ]
SO3H + R . However, with the oxidants studied here, this reaction seems to play a minor role for PE, at least within the surface zone monitored by ESCA, whereas for PS the corresponding sup phonation reaction seems to dominate the picture. With KC103 as the oxidant, H abstraction can also be followed by C1 addition as C12 forms during the oxidation reaction: - - C H - - + C12 ~ - - C H - - + CI.. ] C1
[12]
This reaction is of similar nature as reaction [2] above and can account for the hydrocarbon chlorination detected in this case. The chemical constitution of the standard (#2) KMnO4/H2SO4 PE surface as probed by ESCA and the tagging reactions [l]-[5] and, in addition, by means of 45Ca2+ adsorption is represented schematically in Fig. 1 1. As compared with our previously proposed model (16) the main differences are (i) perhydroxy groups contribute to the - C - O - Cls peak, (ii) a significant amount of keto groups are positioned beneath the surface but within the ESCA sam-
piing depth, (iii) the carboxyls are present in esterified form on the external surface and in acid form in the surface oxidation grooves, and (iv) ether groups also contribute to the - C - O Cls and O l s peak intensities. According to the model proposed here, the O/S atomic ratio should amount to 8.5 whereas the O/S ratio actually determined in the present investigation is significantly lower, 6.5. However, as the S2p photoelectrons have higher kinetic energy (1317 eV) than the O 1s photoelectrons (951eV) the corresponding electron escape depths, ke, are also different resulting in a relative weaker O 1s signal by a factor of(951/1317) 0.75 - 0.78. Since 6.5/0.78 8.3 it would thus seem that a reasonably good agreement is within reach between the O/S data recorded and our model of the chemical constitution of a KMnO4/H2SO4 oxidized PE surface. CONCLUSIONS The ESCA and tracer adsorption studies presented in this paper show that the main chemical groups on a KMnO4/H2SO4 treated PE surface are
// O --OSO3H, --OH, - - O - - O H , - - C = O , - - C - - O - - C - - , and - - C - - O - - R
In the oxidation grooves - C O O H groups are also present in appreciable amounts. By annealing a smoother and more homogeneous polymer surface can be formed with a com-
(ester).
position that strongly depends on the chemical state of the ionic groups at the heat treatment. Comparative experiments with KC103/H2SO4 and K2Cr207/H2SO4 support the view that Journal of Colloid and Interface Science. Vol. I00, No. 2, August 1984
392
OH i
ERIKSSON ET AL.
OSO3H - C = 0 [
i
OH i
O-OH i
o~
0~
0
i
ii
FIG. 11. Model showing schematically the chemical constitution of the standard K ] V [ n O 4 / H 2 S O 4 PE surface.
these mixtures are weaker as oxidizing agents than K M n O 4 / H 2 S O 4 . ACKNOWLEDGMENTS The authors wish to heartily thank BjiSrn Akermark, Bernt Lindberg, Rein Maripuu, and Per Stenius for helpful discussions. REFERENCES 1. Brewis, D. M., and Briggs, D., Polymer 22, 7 (1981). 2. Seighahn, K. et al., "ESCA, Atomic, Molecular and Solid State Structure Studied by Means of Electron Spectroscopy." Almqvist och Wiksell, Uppsala, 1967. 3. Clark, D. R., PureAppl. Chem. 54, 415 (1982). 4. Briggs, D., "Surface Analysis and Pretreatment of Plastics and Metals" (D. M. Brewis, Ed.), p. 199. Applied Science, London, 1982. 5. Rasmussen, J. R., Stedronsky, E. R., and Whitesides, G. M., J. Amer. Chem. Soc. 99, 4736 (1977). 6. Blais, P., Carlsson, D. J., Csullog, G. W., and Wiles, D. M., J. Colloid Interface Sci. 47, 636 (1974). 7. Willis, H. A., and Zichy, V. J. I., in "Polymer Surfaces" (D. T. Clark and W. J. Feast, Eds.), Chapt. 15. Wiley, New York, 1978. 8. Briggs, D., Brewis, D. M., and Konieczko, M. B., aT. Mater. Sci. 11, 1270 (1976). 9. Briggs, D., Zichy, V. J. I., Brewis, D. M., Comyn, J.,
Journal of Colloid and Interface Science, Vol. 100, No. 2, August 1984
Dahm, R. H,, Green, M. A., and Konieczko, M. B., Surface Interface Anal, 2, 107 (1980). 10. Baszkin, A., and Ter-Minassian-Saraga, L., J. Polym. Sci. C34, 243 (1971). 11. Baszkin, A., and Ter-Minassian-Saraga, L., J. Colloid Interface Sci. 43, 190 (1973). 12. Baszkin, A., and Ter-Minassian-Saraga, L., Polymer 15, 759 (1974). 13. Baszkin, A., Nishino, M., and Ter-Minassian-Saraga, L., J. Colloid Interface Sci. 54, 317 (1976). 14. Larsson, R., Eriksson, J. C., and Olsson, P., Thromb. Res. 14, 941 (1979). 15. Larsson, R., Eriksson, J. C., Lagergren, H., and Olsson, P., Thromb. Res. 15, 157 (1979). 16. Larsson, N., Stenius, P., Eriksson, J. C., Maripuu, R., and Lindberg, B., J. Colloidlnterface Sci. 90, 127 (1982). 17. Gelius, U., Hed6n, P. F., Hedman, J., Lindberg, B., Manne, R., Nordberg, R., Nordling, C., and Siegbahn, K., Phys. Scr. 2, 70 (1970). 18. Seofield, J. H., J. Electron Spectrosc. 8, 129 (1976). 19. Wagner, C. D., Riggs, W. M., Davis, L. E., Moulder, J. F., and Muilenberg, G. E. (Eds.) "Handbook of X-Ray Photoelectron Spectroscopy," p. 58. PerkinElmer, 1979. 20. Lindberg, B., Maripuu, R., Siegbahn, K., Larsson, R., G61ander, C.-G., and Eriksson, J. C., J. Colloid Interface Sci. 95, 308 (1983). 21. G61ander, C.-G., Arwin, H., Eriksson, J. C., LundstriSm, I., and Larsson, R., Colloids Surfaces 5, 1 (1982). 22. Briggs, D., J. Adhes. 13, 287 (1982). 23. Briggs,D., and Kendall, C. R., Int. J. Adhes. Adhesives 13 (1982). 24. Clark, D, T., Pure. Appl. Chem. 54, 415 (1982). 25. Gunnarson, G., Thesis, Lund, Sweden, 1981. 26. Catoire, B., Bouriot, P., Baszkin, A., Ter-MinassianSaraga, L., and Boissonnade, M. M., Y. Colloid Interface Sci. 79, 143 (1981). 27. "Gmelins Handbuch der anorg. Chemic," 8. Auflage, Teil C1, p. 365. Verlag Chemic, GMBH, Weinheim/Bergstrasse, 1973.