SSIMS studies of hydrophobic recovery: Oxygen plasma treated PS

235

Applied Surface Science 47 (1991) 235-242 North-Holland

SSIMS studies of hydrophobic E. Occhiello,

M. Morra,

Istiiuto Guido Donegani S.p.A.,

F. Garbassi Via G. Fauser 4, 28100 Novara, Iiab

D. Johnson

and P. Humphrey

CSMA,

Department

VMIST,

Received 5 September

recovery: oxygen plasma treated PS

of Chemistry, P.O. Box 88, Manchester

1990; accepted for publication

14 November

M60 IQD, UK 1990

SSIMS (static secondary ion mass spectroscopy) has been used to aid in the interpretation of hydrophobic recovery of oxygen plasma treated PS (polystyrene), together with XPS (X-ray photoelectron spectroscopy) and water contact angle measurements. The heterogeneity in sputter yields of different ions did not allow complete qualitative and quantitative information to be obtained. Yet negative ion spectra of surfaces treated with plasmas of isotopically enriched oxygen allowed us to follow closely the disappearance of polar groups during hydrophobic recovery. Furthermore, using isotopically enriched PS samples, it was possible to obtain unambiguous information about molecular weight and temperature-induced changes in the hydrophobic recovery mechanism, which could not have been provided by XPS.

proposed in refs. [ll-131, can be very helpful in providing unambiguous information.

1. Introduction Static secondary ion mass spectroscopy [l] has long been used in the analysis of polymer surfaces, as shown by the work of several groups, namely those of Vickerman [2,3], Briggs [4,5], Hercules [6] and Benninghoven [7]. Our group investigated the use of SSIMS in the characterization of flame [8] and plasma treatment [9-131 of polymeric materials, the latter involving SSIMS studies of hydrophobic recovery of PDMS [ll] and PP [12,13]. Very recently we found that the mechanism of hydrophobic recovery of PS is molecular weight and temperature dependent [14]. This prompted us to use this phenomenon to critically assess which contributions SSIMS is able to offer in this area. The issues we will address include surface specificity, relationship with other techniques, such as XPS and contact angle measurement, quantitation and the influence of sputter yields. We will also show that the use of isotopic substitution, 0169-4332/91/.$03.50

2. Experimental PS standards (M, = 2700 and 50 000; M,/M, < 1.1) were purchased from Aldrich, henceforth they will be referred to a’s P&,, and PS,,. Fully deuterated styrene was also provided by Aldrich and anionically polymerized, the resulting M, was 42 300, M,/M, = 1.1, the polymer will be henceforth referred to as d-PS. Polystyrene was dissolved in dichloromethane (10% w/v solutions) (in the case of blends both components were dissolved at the same time in the appropriate weight percentages) and spin-coated on clean glass slides. Solvent removal was completed by annealing the samples under vacuum for 72 h. Plasma treatments were performed with the apparatus described elsewhere [ 11,131. Plasma parameters were the following: excitation frequency 13.56 MHz, RF power 100 W, pressure 2 Pa,

0 1991 - Elsevier Science Publishers B.V. (North-Holland)

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E. Occhiello et al. / SLUMS studies

gas flow 8 cc(STP)/min, treatment time 20 s. Oxygen from lecture bottles supplied by Carlo Erba was used. For SSIMS studies some samples were treated with ‘*02 plasma. The enriched gas was provided by MSD (Merck, Sharp and Dohme) with 99% isotopic purity. Plasma treated samples were aged at 293, 353, 393, 413 and 433 K both in atmosphere and at reduced pressure (500 Pa). Contact angle measurements were performed after allowing the sample to reach room temperature in air. We always measured, besides the treated sample, untreated PS subjected to the same thermal treatment, checking in this way that the sample surface had reached room temperature. To assess the importance of thermal oxidation events we aged at the same temperature untreated and treated PS samples and performed aging at normal and reduced pressure. No alteration was observed on untreated samples and no difference was found between contact angles measured after aging at different pressure, thereby excluding thermal oxidation effects [15]. Water contact angles were measured by the sessile drop technique [16], using carefully deionized and doubly distilled water (surface tension = 72.8 mJ/m2, checked using a Calm DCA 322 dynamic contact angle analyzer) and averaging over 10 different measurements. Advancing and receding angles were obtained using a Rame’-Hart contact angle goniometer by increasing or decreasing the drop volume until moving the three-phase boundary over the surface. We kept the capillary pipette of the microsyringe immersed in the drop during the entire measurement [16]. XPS spectra were obtained using a PHI model 548 XPS spectrometer. Details on the experimental procedures can be found described elsewhere

P71. The equipment used to obtain static SIMS spectra is a VG SIMSLAB instrument [8]. SSIMS (FABMS) spectra were acquired using 2 keV Ar atoms to excite the formation of secondary ions, which were energy analyzed and then mass analyzed using a quadrupole mass spectrometer (VG MM12-12, O-1200 daltons). A VG LEG 31 electron flood gun (500 eV energy), has been used to prevent sample charging.

of hydrophobic recovery

3. Results 3.1. XPS and contact angle measurements PS samples were oxygen plasma treated and aged at various temperatures until advancing and receding angles were stable with aging time. These angles will be henceforth referred to as limiting contact angles. In tables 1 and 2 the relevant contact angle and XPS information is presented for P%700 and PS,,, respectively. In the case of PS,, aging at any temperature induces only a partial recovery, as shown by limiting advancing and receding angles, which are always quite lower than in untreated PS. XPS parallels this finding showing a surface composition unaltered by aging (table 2). On the other hand, PZ&x,, at temperatures above 373 K, returns to advancing angles identical Table 1 Water advancing (a.a.) and receding @.a.) angles, both in degrees, of untreated, just treated and aged P.!&, and PSsessr, Sample

Untreated Just treated After 16 h Aged, 293 K Aged, 353 K Aged, 373 K Aged, 393 K Aged, 413 K Aged, 433 K

P%MNI

P&Q a.a.

r.a.

a.a.

r.a.

90 15 30 45 59 83 88 90 90

78 9 9 11 12 31 33 70 73

90 13 30 45 58 62 65 66 67

79 8 10 11 11 11 14 15 23

Table 2 XPS surface compositions (atomic percentages) of untreated, just treated and aged P!$,r,s and P& Sample

P%@J

P%WO

0

C

0

C

Untreated After 16 h Aged, 293 K Aged, 353 K

2.5 20.2 20.5 19.7

97.5 79.8 79.5 80.3

3.0 20.5 21.0 -

97.0 79.5 79.0 _

Aged, 373 K Aged, 393 K

15.2 14.0

84.8 86.0

20.0 _

80.0

Aged, 413 K Aged, 433 K

6.2 6.4

93.8 93.6

19.5 18.9

80.5 81.1

E. Occhiello et al. / SSIMS

237

studies of hydrophobic recooery

Table 3 Limiting water advancing (a.a.) and receding (r.a.) angles, both in degrees, of P.!&,/P!Zsoooo blends Aging temp.

P!$,,

(K)

5%

293 353 373 393 413 433

weight percentage

a.a.

r.a.

a.a.

r.a.

45 60 65 66 88 90

11 12 13 13 41 73

45 58 66 67 90 90

12 10 13 13 55 70

to untreated PS, while receding angles reach a value only slightly lower than in untreated samples. Correspondingly XPS allows remarkable decreases of the oxygen concentration to be detected at the surface. These behaviors have been rationalized assuming that hydrophobic recovery of PS,, involves macromolecular motions within the plasma mod-. ified layer, burying polar groups away from the surface, while in the case of PE!&-,, above 373 K, outdiffusion of untreated PS occurs [14]. Further studies have been performed on plasma treatment of blends of Pf&,m in PS,, matrix, to verify the effect of dilution on the mechanism of hydrophobic recovery [18]. In table 3 the corresponding limiting contact angles are reported. With respect to the data in table 1, it can be observed that, after aging at 373 and 393 K, limiting contact angles higher than those observed for PS,, are reached for PS,, weight percentages above 20%. At higher temperatures advancing angles similar to those reached by PS,,, have been observed. In summary, at high P&,e concentrations diffusion is an important mechanism for hydrophobic recovery. At low concentrations it is again important, but it acts at higher temperatures, typically 40 K more. 3.2. Negative

20%

10%

ion spectra

Negative ion SSIMS has been suggested for quantitative purposes, in particular Rriggs et al. suggested the use of the ratio of SSIMS intensities of O- and CH- ions (masses 16 and 13), present

50%

a.a.

r.a.

a.a.

r.a.

46 59 74 78 90 90

12 10 13 13 51 70

45 59 85 89 90 90

11 10 41 45 52 70

in most polymers [19]. In the case of treated polymers, to be completely sure that the observed oxygen derived only from the plasma treatment and not from contaminants, we recently suggested the use of plasmas of isotopically substituted gases, namely “0, [ll-131. In fig. 1 we present SSIMS negative ion spectra of P&7oo untreated (fig. la), shortly (16 h) after ‘*02 plasma treatment (fig. lb) and after reaching limiting contact angles at 413 K (fig. lc). The introduction of 180 is clearly shown in fig. lb by the peaks at 18 and 19 amu (180- and ‘*OH-, respectively). 160 is present as well, as shown by the peaks at 16 and 17 amu (“Oand 160H-, respectively), probably due to contaminants, some 1602 leaked into the reactor, etc. The spectra of corresponding PS,, samples look very much similar to those just described. In fig. lc 180 (and 160 as well) has nearly completely disappeared, in very good agreement with contact angle data (table l), which points to a completely recovered PS. XPS on the same sample detects a rather significant amount of oxygen, the discrepancy is most probably related to the different thickness of the analyzed layer. Receding angles are known to reflect the most wettable portion of polymer surfaces [19]. In the case of plasma oxidized PS surfaces they are closely related to the amount of oxygen containing functions at the sample surface. In fig. 2 receding angles of PS,, and P!$7oo samples aged at different temperatures are plotted versus SSIMS CH-/180intensity ratio, used to convey more efficiently the obtainment of an increasingly apolar surface. As already observed in the case of plasma

238

E. Occhielio et al. / SSIMS

studies of hydrophobic recovery 80

a A

AA.

M

Fig. 2. Water limiting receding contact angles of P$,, PZ&,,,,, (A), as a function of SSIMS CH-/‘*Oratio.

100

Mass

(amu)

Fig. 1. Negative ion SSIMS spectra of: (a) untreated PqTw; (b) P&,c,-,, 16 h after treatment with ‘“0, plasma; (c) P&,00, treated with ‘sOz plasma, after reaching limiting contact angles at 413 K.

treated PP [12], a remarkably good correlation exists. This reflects the fact that contact angles and SSIMS are sensitive to a layer of similar thickness [20,21], while XPS is a more penetrant probe [22,23]. 3.3. Positive ion spectra SSIMS positive ion spectra of untreated, ‘*02 and 1602 plasma treated PS,are presented in

(A) and intensity

fig. 3. Again PS,, and PF&,, did not show any difference. The spectrum relative to untreated PS is in good agreement with that reported in the literature [24]. The O-100 amu range is dominated by peaks at mass 51 (C,Hc), 63 (C,Hl), 77 (C,H;) and 91 (C,H;, tropylium cation). Some sodium contamination is visible at mass 23 (Na+). Higher mass peaks, namely those at 103, 115, 128, 152,165 and 178 amu, are also in good agreement with published spectra [24-261. Shortly after plasma treatment the intensity of the peaks in the spectrum is lowered and the appearance altered, high mass PS “fingerprint” peaks tend to disappear, in agreement with the absence of shake-up features in XPS spectra [14]. It is also interesting to observe that the spectra in figs. 3b and 3c look essentially identical. Being due to samples treated with ‘so2 and 160, plasmas, this implies that no oxygen-related peaks are present in meaningful amount in positive ion spectra. The same feature was previously shown in the case of 1802 and 1602 plasma treated PP [12,13] and PDMS [ll]. In figs. 4a and 4b positive ion spectra of plasma treated PS,, and PSzYoohaving reached limiting contact angles are presented. Interestingly, the two spectra look very much similar between themselves and to that of untreated PS (fig. 3a). All “fingerprint” peaks, including those at mass above 100 amu, as per ref. [24], are present with similar intensity.

E. Occhiello et al. / SSIiUS studies of hydrophobic recovery

Yet, as shown in tables 1 and 2, the two samples are quite different between themselves and even more so from untreated PS. In the case of PS*,oo an incomplete outdiffusion occurs, while in PS5oooOnothing similar happens. A possible explanation lies probably in the very different sputter yields between “untreated” and “treated” PS. Hydrophobic recovery, even by macromolecular motions, drives back to the surface “untreated” oligomers and speculatively one may assume that the appearance of the spectrum is mainly related to these units, while the crosslinked and oxidized

239

a 4

Fig. 4. Positive ion SSIMS spectra of: (a) PSsoooo, treated with oxygen plasma, after reaching limiting contact angles at 413 K; (b) P&,,, treated with oxygen plasma, after reaching limiting contact angles at 413 K.

portions of the surface give smaller contributions, due to much lower stability of the formed ions ~1,241. 3.4. Positive ion spectra of isotopic bled

Fig. 3. Positive ion SSIMS spectra of: (a) untreated Pb; (b) P.&,, shortly after treatment with ‘“0, plasma; (c) P&,,,,,, shortly after treatment with “02 plasma.

SSIMS was used to confirm the occurrence of different hydrophobic mechanisms when PS,,/ P&,,, blends are aged below 413 K (table 3). XPS of course is not able to discriminate between PS%OOOand PS2700, while SSIMS can, if isotopitally substituted polymers are used. First of all we used SSIMS to establish the real surface composition of PSsoooo/PS2,,,a blends. It is known that surface tension decreases with molecular weight [25], so a surface excess of the low molecular weight fraction is expected. We used to verify this event, the molecular d-PS/P%,a

240

E. Occhiello et al. / SSIMS

weight difference between d-PS ( M,,, = 42 000) and is not sufficient to induce meaningful difPS,, ferences in surface tension [27], also comparatively unimportant is the surface tension difference between deuterated and non-deuterated PS [28]. In fig. 5 positive ion spectra of 50 : 50 d-PS/ blends aged at 393 K for PS*,oo and d-PS/PS,, 72 h, a period of time in excess of that needed to reach limiting contact angles after plasma treatment, are displayed. The difference is evident, in the case of the d-PS/P&, blend a strong excess is present, as shown by the higher intenof PS,,, sity of the 91 amu peak (C,Hq , tropylium cation) with respect to the 98 amu peak (C,DT, deuterated tropylium cation). On the other hand, as expected, the two peaks have identical intensity in the case of the d-PS/PS,, blend. Actually, the intensities of 91 and 98 amu peaks provide a rather good way of evaluating the rela-

studies of hydrophobic recovery

OYO..'... 20

40

60

00

100

d-PS bulk percent

Fig. 6. d-PS surface versus bulk concentrations for d-PS/PSs, (A) and d-PS/P&,, (A) blends.

tive percentage of d-PS at the surface, following equation:

as per the

d-PS surface percentage = int.(98)/[int.(91)

h.

b

hkss

(amul

Fig. 5. Positive ion SSIMS spectra of: (a) 50: 50 d-PS/PE& blend; both annealed at 393 K blend; (b) 50 : 50 d-PS/PSs, for 72 h.

+ int.(98)]

X

100.

In fig. 6 d-PS surface percentages are plotted versus d-PS bulk percentages for d-PS/PS,, and blends. A good agreement between d-PS/P%m bulk and surface composition is observed for dblends, while a progressively higher PS/PS,, decrease of surface percentage of d-PS (and excess of PS,,,) was noted with decreasing d-PS bulk percentage for d-PS/PS,,, blends. Positive ion spectra of plasma treated and ansamples having reached limitnealed d-PS/P&, ing advancing angles were then obtained. As shown before, the spectra included features typical of untreated PS, either deuterated or protonated. But, when plotting d-PS surface versus bulk percentages (fig. 7) a huge decrease of d-PS (and was observed for annealing increase of PS,,,) above 373 K. Furthermore, the decrease of d-PS, for compositions with more than 20% P!&,,, is much higher than that due to molecular weight induced surface tension difference. The agreement with contact angle data (table 3) is also very good. At concentrations below 20% PS,,, no strong excess of PS2700, apart from that due to surface tension effects, and no increase in limiting advancing and receding angles with respect to PS,, (table 1) are observed. Above 20% P!&,,,, parallel

E. Occhiello et al. / SSIUS

looI

?

Ii 60. fi

60

c

40

5

20 OA0

20

40

60

d-PS

bulk

percent

80

100

Fig. 7. d-PS surface versus bulk concentrations for plasma treated and annealed (at 393 K for 72 h) d-PS/PSa,m blends.

increases in PS,,, excess and advancing angle are verified. So SSIMS, in combination with contact angle data, allows one to prove unambiguously that, at temperatures between 373 and 413 K, there is indeed a “ transition” between different hydrophobic recovery mechanisms at PS2700 bulk concentrations around 20% weight percentage. Below 20% macromolecular reorientation, burying polar groups away from the surface, operates. Above 20%, recovery is dominated by diffusion to the . . surface of non-modified PS,,,.

studies of hydrophobic recovery

241

As also shown by other groups [29,30], SSIMS, mainly when aided by isotopic substitution, can be rather helpful from a quantitative point of view. Negative ion SSIMS CH-/‘*Oproved very well related to the amount of polar groups remaining at the surface during hydrophobic recovery and as such stricly related to advancing and receding angles. With respect to techniques such as XPS, apart from the higher surface specificity, an advantage is the possibility of using labelled species for treatment, thereby limiting significantly uncertainties related to contamination effects. Furthermore, XPS is less successful whenever chemical species do not differ in atomic composition, for instance when the only diffence is molecular weight. In the case of SSIMS, again using isotopic substitution, such information becomes available. By a proper choice of peaks used for quantitation, to avoid sputter yield effects and yet retain chemical specificity, valuable information can be obtained. In our case the role of diffusion in hydrophobic recovery of plasma treated molecular weight blends has been unambiguously clarified by comparing deuterated and non-deuterated tropylium ion peaks. The above-mentioned advantages can be particularly important when dealing with surface treatments and/or presence of additives to alter surface properties.

4. Discussion and conclusion As compared with other surface spectroscopies, for instance XPS or FTIR-ATR, SSiMS has as a definite advantage its surface specificity [19], which makes it an ideal chemical counterpart of contact angle measurements [20,21]. With respect to XPS it suffers from rather unpredictable sputter yields

111.

In terms of qualitative information, SSIMS has been shown to provide valuable information on hydrophobic recovery of plasma treated polymers, particularly when using isotopically labelled treatment gases [ll]. In this particular case no particularly meaningful structural information was gathered, since no high mass oxygen-containing peak can be found in positive ion spectra.

References 111A. Benninghoven, F.G. Ruedenauer and H.W. Werner,

Secondary Ion Mass Spectroscopy (Wiley, New York, 1987). I21 A. Brown and J.C. Vickerman, Surf. Interf. Anal 6 (1984) 1. [31 A. Brown, J.A. Van den Berg and J.C. Vickennan, Spectrochim. Acta 40B (1985) 871. 141 D. Brigs, Polymer 25 (1984) 1379. [A D. B&s, Surf. Interf. Anal. 9 (1986) 391. WI J.A. Gardella, Jr. and D.M. Hercules, Anal. Chem. 52 (1980) 226. 171 I.V. Bletsos, D.M. Hercules, D. Greifendorf and A. Benninghoven, Anal Chem. 57 (1985) 2384. PI F. Garbassi, E. Gcchiello, F. Polato and A. Brown, J. Mater. Sci. 22 (1987) 1450.

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E. Occhiello et al. / SIMS

[9] E. Gcchiello, F. Garbassi and M. Morra, Surf. Sci. 211/212 (1989) 218. [lo] E. Ckchiello, M. Morra and F. Garbassi, Angew. Makromol. Chem. 173 (1989) 183. [ll] M. Morra, E. Gcchiello, R. Marola, F. Garbassi, D. Johnson and P. Humphrey, J. Colloid Interf. Sci. 137 (1990) 11. [12] E. Occhiello, M. Morra, F. Garbassi, P. Humphrey and J.P. Vickerman, in: Proc. SIMS VII Conf., Monterey, CA, September l-5, 1989. [13] E. Gcchiello, M. Morra, G. Morini, F. Garbassi and P. Humphrey, J. Appl. Polym. Sci., accepted. [14] E. Gcchiello, M. Morra, P. Cinquina and F. Garbassi, Polym. Prepr. 31 (1990) 308. [15] M. Morra, E. Gcchiello and F. Garbassi, J. Colloid Interf. Sci. 132 (1989) 504. [16] S. Wu, Polymer Interface and Adhesion (Dekker, New York, 1982) ch. 8. [17] F. Garbassi, E. Occhiello and F. Polato, J. Mater. Sci. 22 (1987) 207. [18] M. Morra, E. Occhiello and F. Garbassi, in preparation. [19] M.J. Heam, D. Briggs, SC. Yoon and B.D. Ratner, Surf. Interf. Anal. 10 (1987) 384.

studies of hydrophobic recovery [20] M. Morra, E. Gcchiello and F. Garbassi, Adv. Colloid Interf. Sci. 32 (1990) 79, and references therein. [21] W.A. Zisman, Adv. Chem. Ser. 43 (1964) 1. [22] M.P. Seah and W.A. Dench, Surf. Interf. Anal. 1 (1979) 2. [23] D. Brings and M.P. Seah, Eds., Practical Surface Analysis (Wiley, New York, 1983). [24] D. Briggs, A. Brown and J.C. Vickerman, Handbook of Static Secondary Ion Mass spectrometry (SIMS) (Wiley, Chichester, 1989) pp. 42-43. 125) J. Lub, F.C.B.M. van Vroonhoven, D. van Leyen and A. Benninghoven, Polymer 29 (1988) 998. [26] J. Lub, F.C.B.M. van Vroonhoven, E. Bruninx and A. Benninghoven, Polymer 30 (1989) 40. [27] S. Wu, Polymer Interface and Adhesion (Dekker, New York, 1982) p. 72. [28] R.A.L. Jones, E.J. Kramer, M.H. Rafailovich, J. Sokoiov and S.A. Schwartz, Phys. Rev. Lett. 62 (1989) 280. [29] P.N.T. van Velzen, J. J. Ponjee and A. Benninghoven, Appl. Surf. Sci. 37 (1989) 147. (301 J. Lub, F.C.B.M. van Vroonhoven, D. van Leyen and A. Benninghoven, J. Polym. Sci. Phys. Ed. 27 (1989) 2071.