Surface Spectroscopic Studies of Poly( Methyl Met hacry late) (PMM A)

and Modified PMMA Surfaces

LAWRENCE SALVATI, J R .

Perkin-Elmer Physical Electronics Laboratories Edison, New Jersey 08820

THOMAS J. HOOK and JOSEPH A. GARDELLA, JR.

Department of Chemistry State University of New York at Buffalo

Buffalo, New York 14214

and

ROLAND L. CHIN

Allied Chemical Corporation Buffalo Research Laboratory

Buffalo, New York 14210

Multitechnique surface characterization of plasma and chemically modified poly(methy1 methacrylate) (PMMA) is reported using Low Energy Ion Scattering Spectrometry (LEIS or ISS) and angular-dependent X-Ray Photoelectron Spectroscopy (XPS or ESCA). A complete picture of the depth, extent, and mechanism of modifications developed. ISS yields results, which especially complement ESCA be- cause of the sensitivity to functional group arrangement due to shadowing and shielding of atoms in the topmost layer.

INTRODUCTION ow Energy Ion Scattering Spectroscopy L (LEIS or ISS) has proven to be a valuable

complement to other surface analytical meth- ods such as ESCA, SIMS, and Auger Spectros- copy, especially in catalysis research ( 1 , 2). Ap- plication of ISS to problems in polymer surface structure, bonding, and composition has been limited (3), but work in our laboratories (4, 5) has identified several analytical characteristics that should increase the use of ISS in areas such as composition and bonding within the topmost atomic (or molecular) layer. These re- sults can be compared with data from other surface sensitive spectroscopic methods which sample deeper (i.e., ESCA, FT-IR) to obtain “depth profiles” of composition and bonding.

In this investigation, ISS was used to distin- guish the surface structure of a series of poly(methy1 methacrylate) (PMMA) materials.

The ISS results clearly distinguished between isotactic, syndiotactic, and atactic PMMA ma- terials. Angle resolved ESCA measurements were also obtained on the same set of PMMA materials. The results from these two experi- ments will be discussed.

ISS and ESCA studies were also performed on a series of surface modified PMMA materials. Two different methods were employed to pre- pare modified PMMA surfaces. One method co- polymerized PMMA with poly(methacry1ic acid) (PMAA). A second method was a n HzO plasma treatment. Experimental data for both of these systems will be presented.

PMMA thin films were studied by ESCA and ISS to characterize the surface structure of the polymers. Three tactic PMMA polymers (listed in Table 1) were investigated as both unmodi- fied and modified polymer films. The unmodi- fied surfaces of the various PMMA materials were studied via ESCA and ISS to elucidate the

POLYMER ENGINEERING AND SCIENCE, JULY, 7987, Yo/. 27, No. 73 939

L. S . Salvati. Jr., T. J . Hook, J . A. Gardella, Jr., a n d R. L. Chin

Table 1. Poly(Methy1 Methacrylate) (PMMA) Series.

Stereoregularity Manufacturer

lsotactic Polymer Laboratories

Sy ndiotactic Polymer Laboratories

Atactic Scientific Polymer

LTD, Shrewsbury, UK

LTD, Shrewsbury, UK

Products, Inc., Webster, NY

Poly(Methy1 Methacrylate)/Poly(Methacrylic Acid) (PMMA/PMAA) Series

Monomer Units: Random Copolymers

CH3 I

C H3

PMAA: fCH2--6-j I \ x \ x

PMMA: fCHt,C-)-

0 AOH % PMMA/% PMMA SAMPLES 90/10, 80/20, 75/25

Manufacturer: Polysciences, Inc., Warrington, PA

surface morphology and composition of the polymer films before surface modification. The thin films were modified by two techniques. The primary approach involved an Ar/H20 Rf- plasma treatment. A second method of modifi- cation involved copolymerizing the PMMA (atac- tic only) with various concentrations of PMMA. The modified polymer films were also studied by ESCA and ISS to deduce the compositional and structural changes that occur after modi- fication.

EXPERIMENTAL Sample Preparation

The polymers analyzed in this study are listed in Table 1. All polymer samples were analyzed as thin films cast onto silver substrates. A 1% (by weight) solution was used to cast the poly- mer films. The PMMA and PMMA/PMAA series used a mixed solvent of 50% chloroform and 50% methanol (by volume) for the polymer so- lutions. The use of the same solvent ensured that all solubility effects on surface composition were constant. The solution casting procedure involved pipetting 10 ml of the polymer solution into a Petri dish containing the silver coupons. The Petri dish was covered and the solvent allowed to evaporate over a 24-hour period.

Instrumentation ISS spectra were recorded on a Physical Elec-

tronics model 560 ESCA/SAM modified for ISS using a cylindrical mirror analyzer with a n an- gle resolved drum. For ISS, a 2 kV beam of 3He+ ions was used with a beam current density of <6 nA/cm2. The primary ion beam was defo-

cused and scanned over 3 x 3 mm2 except where indicated. A 144" scattering angle and 90" sec- tor of the CMA was used for analysis. Calibra- tion was done using Ag foil. Each ISS spectrum took approximately 2 min to acquire. Eight to ten consecutive spectra were obtained to esti- mate depth profiles and beam damage effects. Standard MACS Version 6 software was used for all data acquisition and data massage. ESCA analyses were performed before and after ac- quiring ISS data to check for any sample dam- age resulting from the sputtering process.

Angle dependent ESCA measurements were done on a Physical Electronics (PHI) 5000 ESCA. The PHI instrument utilizes a 180" hem- ispherical analyzer with a single channel detec- tor. Mg K a radiation (300 watts, 15 kV, 20 mA) from the dual anode source was used for excita- tion. High resolution scans were done at a pass energy of 18 eV. A variable angle sample stage (5"-90" continuous) was used. Data were ac- quired with the Perkin Elmer 7500 professional computer fitted with a standard software pack- age. Data manipulation (peak fitting, integra- tion, background subtraction) was also per- formed using this system.

Figure 1 illustrates the angle dependant ESCA technique. The sampling depth ( d ) is di- rectly proportional to the photoelectron trajec- tory. This angular dependence is defined by (6):

d = Xsin6'

where d is the sampling depth, X is the inelastic mean free path, and 6' is the angle between the plane of the sample surface and the electron spectrometer. This equation defines the basis of the angle dependent ESCA technique. Angle dependent ESCA measurements were per- formed by rotating the sample about the axis of rotation (or varying 6' ) , which ultimately resulted in a change in the effective sampling depth (d ) . For example, changing from 90" to 10" produced a 6-fold change in the effective ESCA sampling depth. The C 1s photoelectron line from mean free path measurements for PMMA materials have been experimentally determined by Rob- erts et al. (7).

Computer curve fitting was employed to iden- tify and quantify the oxidized carbon functional groups. Chemical shift values from the main hydrocarbon line at 285.0 eV were +1.6 eV for alcohol groups (C-0) and +4.0 eV for carbonate (0-60) groups. By constraining the peaks to these chemical shifts (+O. 1 eV) and maintaining the same Gaussian peak shape, the relative amounts of each of these functional groups were determined. For simplicity, the peak areas determined by the curve fits were normalized to the 0-0 peak area and presented in terms of the 0-C=O: C-0: CH, ratio.

RF-plasma modification was carried out in a Harrick Model PDC-23G plasma chamber with a base pressure of 60 millitorr. The operating pressure of the Ar/H20 (saturated) plasma was

940 POLYMER ENGINEERING AND SCIENCE, JULY, 1987, Vol. 27, No. 13

Surface Spectroscopic Studies of PMMA and Modified PMMA Surfaces

A -

GE\\ . . .

* /

MPLE

ANALYZER

d =ASIN 0

B

Fig. 1 . Schematic drawing illustrating the effect of sam- p l e tilt angle on d e p t h of analysis .

150 +. 20 millitorr. The lo level Rf-plasma mod- ification involved a 1 min plasma exposure of 20 watts in power. The hi level Rf-plasma mod- ification involved a 4 min plasma exposure of 100 watts in power.

RESULTS AND DISCUSSION Unmodified PMMAs

The first results to be considered involve the angle dependent ESCA analysis of the unmodi- fied PMMA films. As expected, ESCA analysis showed two core level photoelectron lines; the carbon 1s and oxygen 1s. Along with this ele- mental information, functional information was accessible by computer curve fitting of the carbon 1s peak envelope. The ESCA C/O ratios (uncorrected) for the three PMMAs a t angles of 15", 45", and 90" are listed in Table 2. All C/O ratios are within experimental (listed) and in- strumental (5%) error limits.

The carbon 1s envelope was also curve fit for

all the PMMA polymers a t the various angles (illustrated in Fig. 2) to determine if function- ality differences could be detected. These curve fits gave a 1 : 1 : 3 (O4-=O:c-0:cHx) relation- ship (within error limits) as expected for the various PMMAs. Also a 1 : 1 : 3 relationship (within error limits) was observed for the var- ious take-off angles for all polymers. These ESCA results were expected due to the sampling depth of ESCA for PMMA (7). Since more than one molecular layer is being sampled even for the shallowest measurement, the ESCA tech- nique was not sensitive to tacticity changes on the molecular level.

ISS analyses, performed on these same PMMA materials, were able to differentiate between the tactic PMMA polymers based on the mea- sured atomic C/O peak height ratios (uncor- rected). Table 3 lists this data along with bulk atomic C/O ratios (corrected). These differences

Table 2. Angle Resolved ESCA Surface Characterization of Unmodified PMMA Polymers.

C/O Uncorrected Peak Area Ratios

< = 15O < = 450 c = gooo Sample C/O (22 A) C/O (61 A) C/O (86 A)

lsotactic 1.16f .04 1.10 f .01 1.10 * .03 Syndiotactic 1.10 f .03 1.06 t .03 1.09 f .02 Atactic 1.11 f .02 1.10 f .02 1.10 * .03

Note: Corrected bulk atomic C/O ratios are 2.500 for all samples.

Lo Level RF-Plasma Modified PMMA Polymers C/O Uncorrected Peak Area Ratios

c = 15". < = 4500 c = gooq Sample C/O (22 A) C/O (61 A) C/O (86 A)

lsotactic 1.40 f .03 1.23 f ,051 1.24 f .05 Syndiotactic 1.37 f .05 1.14 f .08 1.15 k .06 Atactic .96 t .07 .96 f .05 .99 f .06

Hi Level RM-Plasma Modified PMMA Polymers C/O Uncorrected Peak Area Ratios

c = 150 c = 450- c = 90: Sample C/O (22 A) C/O (61 A) C/O (86 A)

lsotactic 1.26 k .06 1.19 f .04 1.22 + .04 Syndiotactic 1.02 f .04 1.05 f .01 1.09 f .06 Atactic 1.26 k .03 1.25 t .04 1.29 +. .05

Binding Energy, EV Fig. 2. Representative curve f i t of a carbon is core level ESCA spectra.

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L. S . Salvati, Jr., T. J . Hook, J . A. Gardella, Jr., and R. L. Chin

Table 3. ISS of Unmodified and Modified PMMAs Uncorrected C/O Scattered Ion Intensity Ratios.

Samde Unmodified Lo Level Hi Level

lsotactic ,726 f ,028 ,216 f ,021 ,171 f ,031 Syndiotactic ,340 f .030 ,262 f .036 ,178 f ,029 Atactic ,464 ? ,024 ,185 k ,018 .164 ? ,013

Note: Coneaed bulk atomic C/O ratio for the unmodified PMMAs is 2.500.

were interpreted via structural analysis and consideration of atomic shielding and shadow- ing of the primary ion beam. The surface struc- ture of the tactic PMMAs has been investigated by contact angle measurements of critical sur- face tension (8). Zisman has proposed that the surface is dominated by methyl groups for all tactic PMMAs. Therefore the differences in C/O peak height ratios observed were related to the shielding ability of methyl groups in the various PMMAs.

Isotactic PMMA was determined to have a small degree of carbon shielding (large degree of oxygen shielding). This occurred since all the methacrylate (-COOCH3) functional groups were orientated in the same configuration along the main carbon chain (i.e., every second car- bon has the same chirality). This resulted in a low degree of shielding to the main carbon chain but a high degree of shielding to both oxy- gens in the closely constrained methacrylate (-COOCH3) functional group. Conversely syn- diotactic PMMA had a relatively lower degree of oxygen shielding and an enhanced degree of carbon shielding. This was due to the alternat- ing functionality of the methacrylate group. The main carbon chain was shielded much more effectively due to the alternating chirality of adjacent carbon atoms. This alternation of functionality also resulted in an increased oxy- gen signal compared to the more highly shielded environment of isotactic PMMA. These expla- nations agreed with the results in Table 3. Iso- tactic PMMA produced a high C/O ratio (low carbon shielding, high oxygen shielding), syn- diotactic PMMA a low C/O ratio (enhanced car- bon shielding, reduced oxygen shielding). Atac- tic PMMA is a random distribution of both iso- tactic and syndiotactic PMMA and results in a C/O ratio which is between the values for iso- tactic and syndiotactic PMMA as expected.

Plasma Modified PMMAs The surface characterization of the unmodi-

fied tactic PMMAs provided a reference for the starting material composition before modifica- tion. The primary method of surface modifica- tion involved treatment of the polymer films with an Ar/H20 (saturated) RF-plasma. ISS was performed on the modified tactic PMMAs to elu- cidate the newly generated surface composition. Results are listed in Table 3. The data showed that the near surface region had increased amounts of oxygen for all the tactic PMMA ma- terials. In addition as the plasma exposure was

increased the degree of modification also in- creased as evident by the C 1s curve fits. ESCA analysis of these tactic PMMA films was per- formed to elucidate the functionality of the modified surface as detected by ISS.

The modified PMMA samples were character- ized by angle dependent ESCA at take off angles of 15", 45". and 90". Computer curve fits were performed on each of the high resolution C I s spectra. The results of these measurements are presented in Figs. 3 and 4 for the low and high level treatments. These plots illustrate the nor- malized functional group compositions (nor- malized to 0--0 equal to 1) as a function of sampling depth for each of the tactic polymers.

Based on the functional group composition determined for the unmodified PMMA materials (1 : 1 :3) it was obvious from the data shown (Fig. 3 ) that the lo level plasma treatment had signif- icantly altered the PMMA surface. In terms of the carbon functional group composition, the modified isotactic surface (45") exhibited a com- position of approximately 1 : 1.8:6 (0-C=O:C- O:CH,).The syndiotactic surface (45") was de- termined to have a carbon functional group ratio of 1: 1.5:5.5 and the atactic surface (45") 1: 1.2:3.5. Thus it was apparent that the isotac-

An le Resolved ESCA Surface Characterization of Lo P eve1 Rf-Plasma Modified PMMA Polymers C i s Curve Fits (C=O:C-O:CHxl

0.1 lsotactic - &----€---fCHx Y 0 5 8 4

I I I

1 I I

c.) Atactic

1 1 I I

Estimated Sampling Depth (Angle) 22 A (i5") 6iA(45') 86A(90°)

Fig. 3. Angle resolved ESCA surface characterization of lo level Rf-plasma modified PMMA polymers.

942 POLYMER ENGINEERING AND SCIENCE, JULY, 1987, Vol. 27, No. 13

Surface Spectroscopic Studies of PMMA and Modified PMMA Surfaces

Angle Resolved ESCA Surface Characterization of Hi Level Rf-Plasma Modified PMMA Polymers CisCurveFits(C-O:C-O:CHx)

a) lsotactic \

1 \\

-ICHx Y f- 1 0 7 a“

I I I

b.) Syndiotactic

c.) Atactic

22 A(i50) 61A(457 86&907

Estimated Sampling Depth (Angle) Fig. 4. Angle resolved ESCA surface characterization of hi level Rf-plasma modified PMMA polymers.

tic and syndiotactic materials were modified to a much higher degree than was the atactic PMMA. Considering the angle-dependent data provides additional data concerning the modi- fication process. No significant variation in the relative functional group composition were ob- served for the lo level modified isotactic material as a function of sampling depth. This suggested the modification depth WGS beyond the maxi- mum sampling depth (86 A) of the ESCA anal- ysis. For the syndiotactic material, the relative CH,-concentration droppTd steadily with sam- pling depth. At the 86 A level the ratio was calculated to be 1:1.5:4.3 versus 1:1.5:6 at a sampling depth of 22 A. The modification in the atactic PMMA was slight and was observed to incKease with sampling depth. The near surface (22A) values (1 : 1 :3) were very close to fhe un- modified material while the “bulk (86 A) num- bers were 1 : 1.2:3.8.

The primary process evident from the carbon 1s curve fit results appeared to be reduction of the polymer surface (observed for all tactic PMMA materials). The highest degree of reduc- tion occurred for 0-+O functional groups. A smaller but detectable reduction also occurred

for c-0 functional groups. A secondary pro- cess was evident from the C/O atomic ratios. These results suggested hydrophobicity of the polymer surface had changed especially for the atatic PMMA material. The reduced atactic PMMA surface is postulated to undergo a change in hydrophobicity followed by an ab- sorption of H20 from the Rf-plasma. This ac- counted for lower C/O ratios (Table 2) for the tactic PMMA materials than was observed from the carbon 1s core level curve fit results.

The hi level modification showed similar re- sults. Figure 4 illustrates the data for the tactic PMMA materials. A s previously discussed the two processes also occurred in the hi level mod- ification process. The isotactic PMMA showed a high degree of modification occurring (evident from the high CH, content) on the surface. This reduction occurred to an increased degree com- pared to the lo level modification. The modified layer for the hi level treatment dropped off slightly (to the values of the lo level modified isotactic PMMA materials] into the sample sur- face. As was observed in the lo level modifica- tion, the 0-+O functionality was reduced to the greatest extent. This reduction was either more pronounced, deeper, or both for all sam- ples. The reduction of the c-0 group occurred for all the tactic PMMA materials. Syndiotactic PMMA materials results in Fig. 4 showed the same degree of surface reduction occurred with an increase in the depth of the modification (compared to the lo level). Similar results for the atatic PMMA materials in Fig. 4 were also ob- served. An increase in the degree of surface reduction was observed deep into the polymer film. For both the syndiotactic and atactic PMMA materials, the O-c=O group was more readily reduced than the C-0 functional group.

Based on the curve fit results it was evident that the plasma modification had reduced the PMMA surface. However the C/O ratios (Table 2) determined for the modified polymers did not reflect a reduction in the oxygen concentration relative to the unmodified polymers. This fact suggested that a second process was occurring during the plasma treatment. It was postulated that this “extra” oxygen was present as ad- sorbed H20. Examining the oxygen 1 s core level ESCA spectra supported this hypothesis. The peak shape of the 0 1s photoelectron line dif- fered significantly between the modified and unmodified polymer materials. The FWHM full width at half maximum (FWHM) were deter- mined to be 0.25 to 0.90 less in the modified PMMA samples. The surface charging was also found to be less in the plasma treated materials. These observations support the presence of a hydration layer at the polymer surface.

P M A A Modi f i ed PMMA Another type of modification for atactic

PMMA involved copolymerization with PMAA.

943 POLYMER ENGINEERING AND SCIENCE, JULY, 1987, Yo/. 27, No. 13

L. S. Salvati, Jr., T. J . Hook, J . A. Gardella, Jr., and R. L. Chin

This modification was a bulk process. ESCA and ISS were used to determine the surface effects of copolymerization. Table 1 lists the concentrations of PMAA studied. The ESCA C/O atomic ratios for these polymers are listed in Table 4. The angle dependent study of these modified atactic PMMA films showed C/O ratios within error limits for all the concentrations of PMAA and all the angles (15". 45", 90") inves- tigated (Table 4). This was expected since the modification is a bulk process and the ESCA technique samples deeper than a single molec- ular layer. The C/O ratios are also within error limits of unmodified atactic PMMA. This data shows that ESCA was not sensitive to changes induced by copolymerization with PMAA for atactic PMMA. Further confirmation of this lack of sensitivity was established by curve fit- ting the carbon 1s core level spectra (see Fig. 2). All concentrations of PMAA at all angles studied (15, 45, 90) showed a 1:1:3 ((0-C= O:C-O:CH,) ratio agreement. This data also il- lustrates that angle resolved ESCA does not have the ability to detect compositional changes for this copolymer series.

The ISS technique was used for the copolymer series to detect surface composition. As for the unmodified tactic PMMAs, the ISS technique was sensitive to compositional changes in the copolymer series. Table 4 lists these results. The differences observed in atomic C/O ratios reflected the differences in shielding and shad- owing (of the primary ion beam) of the PMAA

Table 4. ESCA Surface Characterization of Modified Atactic PMMA.

C/O Uncorrected Peak Area Ratios Sample (%PMMA/

'YoPMAA) C/O (< = lS0)C/O (< = 45O)C/O (< = 90')

9011 0 1.10 f .02 1.07 f .04 1.08 f .04 80120 1.11 f .01 1.06 f .05 1.12 f .03 75/25 1.10 2 .03 1.07 f .03 1.10 f .02

ISS Analysis of Modified Atactic PMMA

Uncorrected C/O Scattered 'Yo PMMA/%PMAA Ion Intensity Ratio

9011 0 80120 75/25

,535 f ,019 ,656 f ,030 ,757 f ,026

Note: Corrected bulk atomic C/O ratio for all copolymers is approximately 2.48.

segments of the copolymer. The absence of the -0-CH3 group (replaced by -0-H) in PMAA reflects less total carbon and subsequently less shielding of underlying carbons. This decreased shielding dominates, which produces increased C/O ratio as the PMAA concentration increases.

This data suggests that modification of atac- tic PMMA via copolymerization with PMAA re- sults in changes undetectable by ESCA and monitorable by ISS.

CONCLUSIONS The surface composition and structure has

been elucidated for unmodified and modified tactic PMMA materials. The plasma modifica- tion process altered the surface in two ways: depletion of 0-C=O and C-0 functionalities, and oxygen incorporation in the near surface region. This oxygen incorporation appears to be a water adsorption following polymer modifi- cation. The increase in hydrophobicity of the polymer layer from reduction of the argon con- taining plasma is evident. This accounts for two different processes occurring during modifica- tion: 1) oxygen functionality depletion, and 2) water adsorption from an increase in hydropho- bicity. The modified atactic PMMA polymer films (copolymerized with PMAA) were investi- gated to determine the sensitivity of ESCA and ISS. The data suggests that ESCA is not as sensitive to tacticity and compositional changes for the polymer systems studied, while ISS is very sensitive to these parameters.

REFERENCES 1. R. L. Chin, and D. M. Hercules, J . Phys. Chern., 86,

3079 (1 982). 2. L. Salvati, L. E. Makovsky, J . M. Stencel, F. R. Brown,

and D. M. Hercules, J . Phys. Chern., 85. 3700 (1981). 3. J. A. Gardella J r . and D. M. Hercules, Anal. Chern., 53,

1979 (1981). R. L. Schmitt , J. A. Gardella J r . , R. L. Chin, J. H. Magill, a n d L. Salvati, Macromolecules, 18(12). 2675 (1985).

4. R. L. Schmitt , J . A. Gardella Jr . , R. L. Chin, a n d L. Salvati, Macromolecules, 19. 648 (1986).

5 . J . A. Schmitt J r . , J. S . Chen, J. H. Magill, a n d D. M. Hercules, J. Am. Chern. SOC., 105, 4536 (1983).

6. T. J . Hook, R. L. Schmitt , J. A. Gardella Jr . , L. Salvati Jr . , a n d R. L. Chin, Anal. Chern., 58, 1285 (1986).

7. R. F. Roberts, D. L. Allara, C. A. Pyde, D. N. Buchanan, and N. D. Hobbins, Surf. lnterf. Anal., 2, 5 (1980).

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