Macromol. Chem. Phys. 2001, 202, 3065–3071 3065

Studies on Polymer–Metal Interfaces, 4a

Effect of Substituent of Polymethacrylate on theInterfacial Characteristics between Polymethacrylatesand Copper

Dong Ha Kim, Won Ho Jo*

School of Materials Science and Engineering and Hyperstructured Organic Materials Research Center,Seoul National University, Seoul 151-742, KoreaFax: +82-2-885-1748; E-mail: whjpoly@plaza.snu.ac.kr

IntroductionThe properties of polymer molecules adsorbed onto metalsurfaces are of major importance in applications such asadhesion,[1] wetting,[2] corrosion protection,[3] electro-chemistry,[4] biocompatibility,[5] colloid stabilization,[6]

microelectronics,[7] and many other areas. Accordingly,the understanding of the interfacial characteristicsbetween polymer and metal is crucial in order to controlits chemical and physical properties.

In recent years, the role of oxygen-containing function-ality in polymers for polymer–metal adhesion has been ofan important subject. Burkstrand[8–13] reported that theinterfacial adhesion strength between polymer and metalwas significantly enhanced through complex formation ofmetal–oxygen-polymers when metal atoms were vapor-deposited on oxygen-containing polymers by X-ray photo-

electron spectroscopy (XPS), and the presence of interac-tion between the carbonyl group in PMMA and metal isverified by the changes in the XPS peak shapes of the oxy-gen atoms of PMMA.[9] The interaction between PMMAand oxidized aluminum surfaces was also studied usingFourier transform infra-red multiple specular reflectancespectroscopy and inelastic electron tunneling spectroscopyby Sondag and Raas.[14] Grohens et al.[15] investigated theeffect of chain conformation of PMMA stereoisomers onthe interaction between the carbonyl group and the alumi-num surface. In this report, they found that the fraction ofcarbonyl groups bonded to the aluminum surface is higherfor isotactic PMMA than for atactic and syndiotacticPMMA. Although the carbonyl group in PMMA has beenwell recognized as an adhesion site, the effect of the bulki-ness of alkyl groups in polymethacrylates on interfacialcharacteristics between polymethacrylates and metal hasnot been systematically analyzed.

Full Paper: The interfacial characteristics between poly-methacrylates and copper were investigated. The adhesionstrength between poly(methyl methacrylate) (PMMA)/copper was the highest, while that between poly(ethylmethacrylate) (PEMA)/copper was the lowest among thefour polymethacrylates/copper systems, i. e., PMMA/cop-per, PEMA/copper, poly(n-butyl methacrylate) (PnBMA)/copper, and poly(i-butyl methacrylate) (PiBMA)/copper.Analysis of X-ray photoelectron spectroscopy (XPS)shows that the carbonyl functional groups in polymeth-acrylate interact specifically with copper and the degreeof specific interaction decreases in the order of PMMA/copper A PnBMA/copper F PiBMA/copper A PEMA/cop-per. Results from reflection–absorption infrared (RA-IR)spectroscopy indicate that the adsorption behavior of

polymethacrylate is closely related to the orientation ofthe carbonyl group upon the copper surface and that thecarbonyl group is more perpendicularly oriented near theinterface due to the specific interaction with the coppersurface.

Macromol. Chem. Phys. 2001, 202, No. 15 i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2001 1022-1352/2001/1510–3065$17.50+.50/0

a For the previous paper the series see ref.[26]

3066 D. H. Kim, W. H. Jo

In order to examine such an effect, PMMA, poly(ethylmethacrylate) (PEMA), poly(n-butyl methacrylate)(PnBMA), and poly(i-butyl methacrylate) (PiBMA) areused as model polymers containing carbonyl group withdifferent steric environments, and the interfacial charac-teristics between the polymers and metal are comparedwith one to another in terms of adhesion strength, specificinteraction, and orientation behavior of carbonyl groupsat the interface.

Experimental Part

Materials

PMMA, PEMA, PnBMA, and PiBMA were obtained fromScientific Science Incorporation. The glass transition tem-perature (Tg) of PMMA, PEMA, PnBMA, and PiBMA meas-ured by differential scanning calorimetry was about 100, 68,18, and 548C, respectively and each of their intrinsic viscos-ities was about 0.38, 0.52, 0.47 and 0.15 dl N g–1, respec-tively. Copper metal used in this study is a commercial pro-duct of 99.9% purity.

Preparation of Metal Substrates

Copper substrates for RA-IR spectroscopy and XPS wereprepared as follows.[16, 17] Commercial copper plates with athickness of 1.5 mm were mechanically polished with a ser-ies of dry silicon carbide abrasive papers ranging from 320to 2000 grit. After the surface was ground, wet polishingwas performed using 0.05-lm alumina powder in Micro-cloths (Buehler Inc.) with deionized water as the lubricant.The resulting mirrors were ultrasonically rinsed with deio-nized water and dried by blowing with nitrogen gas.

As substrates for adhesion test, copper plates of size100 mm625 mm61 mm were mechanically polished asdescribed above. The ground metal plates were etched by20% hydrochloric acid solution for 1 min to remove theoxide layer on the surface. The resulting plates were ultraso-nically washed with ethanol, followed by rinsing the plateswith deionized water, and then dried by blowing with nitro-gen gas.

Film Formation onto Metal Substrates

Copper substrates prepared as described above wereimmersed into the polymer solution of 0.05–4% concentra-tion in chloroform for 1–24 h, and after removal of the sam-ple from the solution, the solvent was slowly evaporated.Samples obtained from 0.5–4% solution were used for thereflection absorption infrared (RA-IR) spectroscopy andthose from 0.05% solution were used for X-ray photoelec-tron spectroscopy (XPS).

Film Thickness Measurement

Film thickness upon metal substrates used for the RA-IRspectroscopy and XPS was determined by ellipsometry(Rudolph Auto EL-II). The refractive index of the copolymerwas assumed to be 1.50. The film thickness obtained from

0.5, 1, and 4% solutions for RA-IR spectroscopy was about95, 160, and 550 �, respectively. The thickness of films forXPS was about 15 �.

Adhesion Strength Measurement

Test specimens were prepared by applying a film of 8%polymer solution to the end of one metal plate and then byoverlapping another plate onto the film so that the lap areawas 25 mm615 mm. The adhesion strength was measuredby the lap shear test (ASTM D1092-72) at a pull rate of5 mm N min–1, using a universal tensile machine (LLOYD,LR 10K). The measurements were performed at –108Cwhich is sufficiently below the glass transition temperatures(Tg) of the polymers. The lap shear strength was calculatedby dividing the strength by the lap area. Five sets of speci-mens for each polymer were tested, and the average isreported as the lap shear strength.

X-Ray Photoelectron Spectroscopy

The XPS experiments were performed on a PHI Instrument’sspectrometer (PHI 5700) equipped with a monochromatic AlKa X-ray source with a power of 350 W. The pass energywas 23.5 eV with 0.05 eV steps and the pressure in the cham-ber was kept below 5610–10 torr. All the XPS spectra werecorrected to eliminate charging effect by referencing the C1s peak of hydrocarbons to 285.0 eV. High-resolution spec-tra were analyzed in order to determine the various chemicalspecies present. Each spectrum was curve-fitted using theXPSPEAK3.1 software. The XPS spectra of polymer bulk oncopper were obtained first, and then the polymer/copperinterface spectra were recorded after the polymer overlayerwas sputtered until characteristic peaks of copper appearstrongly. In sputtering experiments, the samples were irra-diated with Ar+ ion at 3 keV and the current was kept below2 lA. It has been reported that the chemical structures andcompositions of polymers may be significantly changedwhen the polymers are irradiated under strong energy condi-tions, e.g., 20 keV l 2 MeV, and therefore the structures andcompositions observed at the interface by XPS are far fromactual structures and compositions arising from the polymer-metal interactions.[18] In our experimental condition, the lowenergy condition (3 keV) is used to minimize the effect ofradiation on the polymer structures and compositions at theinterface.

Infrared Spectroscopy

The bulk spectra of polymers were recorded on a Fouriertransform infrared spectrometer (FT-IR, Perkin-Elmer1760X) at a resolution of 4 cm–1 with 32 scans. The RA-IRspectra of polymer/metal interface were obtained using aBomem MB-100 spectrometer at a resolution of 4 cm–1 with32–200 scans. A Graseby Spec P/N 19650 monolayer/graz-ing angle accessory was used. The angle of incidence was788, and freshly polished copper substrates were used for thereference spectrum.

Studies on Polymer–Metal Interfaces, 4 3067

Results and Discussion

Adhesion Strength

The adhesion strengths between polymethacrylates andcopper are listed and compared with one to another inTable 1. The lap shear strength between PMMA/copper isthe highest, while the strength between the PEMA/copperis the lowest among four polymethacrylates/copper sys-tems. It is also observed that the adhesion strengthbetween PnBMA/copper is slightly higher than thatbetween PiBMA/copper. It is interesting to find that theadhesion strength between PEMA/copper is lower thanthat between PnBMA/copper and that between PiBMA/copper. This result will be discussed in terms of theadsorption behavior of the carbonyl group in later sec-tions.

X-Ray Photoelectron Spectroscopy

The XPS is used to analyze the adsorption behavior ofpolymethacrylates upon the copper surface. Figure 1a and1b show the high-resolution C 1s spectra obtained fromthe PMMA bulk and the PMMA/copper interface, respec-tively. The XPS spectrum of the PMMA bulk consists ofthree types of carbon atoms which can be assigned toC1C or C1H carbon at 285 eV, C1O carbon at 286.5eV, and O2C1O carbon at 289 eV.[19–21] The C 1s spec-trum from the PMMA/copper interface is decomposedinto two peaks at 285 eV and 286.3 eV. When this inter-face spectrum (Figure 1b) is compared with the bulkspectrum (Figure 1a), it is realized that most of theO2C1O peak at 289 eV in the bulk spectrum shifts to alower binding-energy region. This indicates that there is astrong specific interaction between the carbonyl group inPMMA and copper.

The specific interaction between PMMA and copper isalso supported by the analysis of O 1s spectra. As shownin Figure 1c, the O 1s spectrum of the PMMA bulk isdecomposed into two components at 532.2 eV and 533.7eV which can be assigned to C2O and C1O oxygen,respectively.[19–21] Since each type of oxygen atom is ofequal amount, the area of the two O 1s peaks shouldalmost be equal (Figure 1c). When the O 1s spectrum ofthe PMMA/copper interface is also decomposed into twocomponents at 532.3 eV and 533.8 eV, as shown in Fig-

ure 1d, some noticeable change in the O 1s spectrum canbe observed when the PMMA is adsorbed onto copper.The ratio of the C2O peak intensity to C1O peak inten-sity at the polymer/metal interface spectrum (Figure 1d)is higher than at the polymer bulk spectrum (Figure 1c).This result suggests that the amount of C2O oxygenincreases near the interface due to the specific interactionbetween C2O oxygen and copper. The absence ofexpected shift of the O 1s peaks in Figure 1d can beexplained by charge delocalization from oxygen to car-bon concurrent with interaction.[22, 23] The PEMA bulkand PEMA/copper interface are also examined by XPS.Similar to the high-resolution C 1s spectrum of PMMAbulk (Figure 1a), the high-resolution C 1s spectrum ofPEMA bulk (Figure 2a) consists of three characteristicpeaks at 285 eV, 286.4 eV, and 289 eV. Unlike the caseof the PMMA/copper interface, the shift of the carbonylC 1s peak to a lower binding-energy region cannot beclearly observed in the high-resolution C 1s spectrum ofPEMA/copper interface, as shown in Figure 2b, indicat-ing that the carbonyl group in PEMA may not interactsignificantly with copper. Figure 2c and 2d show the O1s spectra of the PEMA bulk and PEMA/copper inter-face, respectively. The two spectra are similar in shape,indicating that no significant interaction exists betweenthe carbonyl group of PEMA and the copper surface.

The high-resolution C 1s spectra obtained from thePnBMA bulk and the PnBMA/copper interface (notshown here) were also analyzed in the same manner.

Table 1. Comparison of adhesion strength between four poly-methacrylate/copper systems.

Sample Adhesion strengthMPa

PMMA/copper 1.9 l 0.2PEMA/copper 1.4 l 0.4

PnBMA/copper 1.7 l 0.3PiBMA/copper 1.6 l 0.2

Figure 1. (a) XPS high-resolution C 1s spectra of PMMAbulk; (b) XPS high-resolution C 1s spectra of PMMA/copperinterface; (c) XPS high-resolution O 1s spectra of PMMA bulk;(d) XPS high-resolution O 1s spectra of PMMA/copper inter-face.

3068 D. H. Kim, W. H. Jo

When the interface spectrum of PnBMA/copper is com-pared with its bulk spectrum, a part of O2C1O peak at288.9 eV of the bulk spectrum shifts to a lower bindingenergy region with significant reduction of the peak areaat the interface spectrum while some amount of O2C1Ocarbon peak still remains at 288.7 eV at the interfacespectrum. These results indicate that the carbonyl groupof PnBMA can interact with the copper surface moreeffectively than that of PEMA. The overall behavior ofthe PiBMA bulk and PiBMA/copper interface is similarto the case of the PnBMA/copper system. The ratio of theintensity of the highest binding energy component(O2C1O carbon peak) to that of the neutral carbon peakat 285 eV are calculated in order to compare the adsorp-tion behavior of PnBMA/copper with that of the PiBMA/copper system in more detail. The ratio of the PnBMA/copper interface is 0.71 times the value of PnBMA bulk,whereas the value of PiBMA/copper interface is 0.86times the value of the PiBMA bulk. This indicates thatthe carbonyl group of PnBMA interacts more effectivelywith copper that that of PiBMA. In summary, a simpleinspection of the high-resolution C 1s spectra of fourpolymethacrylates/copper systems leads to a conclusionthat the degree of specific interaction between the carbon-yl group and copper decreases in the following order:PMMA/copper A PnBMA/copper F PiBMA/copper >PEMA/copper. The selective increase of the C2O peakintensity was not clearly identified in O 1s spectra fromthe PnBMA/copper and PiBMA/copper interface (notshown here).

It is expected that the specific interaction between thecarbonyl group and the metal surface would be weak forpolymethacrylate with bulkier side group because the car-bonyl group has less chance to gain access to the coppersurface due to unfavorable steric hindrance. However,our experimental results show that PBMA interacts morestrongly with the copper surface than PEMA, althoughPBMA has a bulkier side group than PEMA. This sug-gests that the effective number of carbonyl groups inter-acting with the copper surface is larger for the PBMA/copper interface than for the PEMA/copper interface.When polymer molecules are adsorbed from solutiononto the metal surface, a more flexible chain would itselfadjust its conformation at the interface so that the func-tional group can interact more readily with the metal sur-face. Considering that the glass transition temperature ofthe polymers is related to their chain flexibility, it is real-ized that PnBMA is more flexible than PEMA (comparethe Tg’s of PnBMA and PEMA: 188C versus 688C). Con-sequently, it is probable that the adhesion strengthbetween PnBMA and copper becomes stronger than thatbetween PEMA and copper. Therefore, the adsorptionbehavior of four polymethacrylates/copper can be sum-marized as follows. When the substituent is smaller thanthe ethyl group, the dominant factor is steric hindrance,but the chain flexibility becomes the dominant factor forinteraction between carbonyl group and metal when thesubstituent is larger than the ethyl group. This explana-tion agrees well with the above experimental observation:the adhesion strength between PMMA/copper is higherthan that between PEMA/copper, whereas the adhesionstrength between PEMA/copper is lower than thatbetween PnBMA/copper. The higher adhesion strengthbetween PnBMA/copper compared to between PiBMA/copper can also be explained by the fact that the Tg

(188C) of PnBMA is lower than that (548C) of PiBMA.

IR Spectroscopy

The transmission IR and RA-IR spectroscopy is used tointerpret the results of adhesion strength measurementand XPS in terms of the orientation behavior of thecarbonyl group upon the copper substrate. RA-IR spectraof polymethacrylates/copper are obtained from very thinfilms. It is known that in RA-IR spectroscopy the inter-face region contributes more to the overall spectrum asthe film becomes thinner. Since the RA-IR spectroscopyis very sensitive to preferential orientation of functionalgroups at the surface of a metal, i.e., vibrational modeswith transition moments perpendicular to the surface ofmetal appear with much stronger intensity than do vibra-tions with transition moments parallel to the surface, thedifference between the transmission and the RA-IR spec-trum may come from the orientation difference of func-tional groups at the interface.[24–33]

Figure 2. (a) XPS high-resolution C 1s spectra of PEMA bulk;(b) XPS high-resolution C 1s spectra of PEMA/copper interface;(c) XPS high-resolution O 1s spectra of PEMA bulk; (d) XPShigh-resolution O 1s spectra of PEMA/copper interface.

Studies on Polymer–Metal Interfaces, 4 3069

Figure 3a shows the transmission IR spectrum ofPMMA bulk, and Figure 3b–d show the RA-IR spectraobtained from PMMA adsorbed onto a copper substratefrom 4–0.5% solution in chloroform, respectively. In Fig-ure 3a, the PMMA bulk shows a characteristic absorptionband at 1730 cm–1 due to the C2O stretching mode. Therelative intensity of the carbonyl increases with decreas-ing film thickness, accompanied by a change in peakshape, as shown in Figure 3b–d. The change in peakshape may arise from specific interaction between thecarbonyl group and copper, and the increase of relativeintensity indicates that the carbonyl group is oriented in amore perpendicular direction near the interface due to thespecific interaction between the carbonyl group and cop-per.

For quantitative analysis of RA-IR spectra of sampleswith different film thickness, the quantitative absorption-reflection thickness IR (QUART-IR) method is used.[34, 35]

A possible alternative measure of film thickness is to usethe spectral average absorbance, since RA-IR band inten-sities are expected to vary monotonically if not linearlywith film thickness.[34] Absorbances of the carbonyl bandare normalized with respect to the average absorbance of

major bands. Table 2 summarizes the results obtainedfrom the spectra of PMMA bulk and the PMMA/copperinterface. For the case of a single transition moment ofthe ith mode whose direction with respect to the surfacenormal is given by the angle b, it is known that the frac-tional change in intensity di, is related to b by

di = 3cos2 b – 1 (1)

where the di is obtained by dividing the normalized absor-bance of C2O band of the projected interface by that ofthe bulk. From Equation (1) with the measured value ofthe fractional change in the carbonyl band intensity, the

Figure 3. (a) Transmission IR spectrum of PMMA; (b) RA-IRspectrum of PMMA adsorbed on copper from 4% solution inchloroform; (c) RA-IR spectrum of PMMA adsorbed on copperfrom 1% solution in chloroform; (d) RA-IR spectrum of PMMAadsorbed on copper from 0.5% solution in chloroform.

Table 2. Variation of the thickness-normalized C2O bandabsorbance with film thickness for the PMMA/copper system.

Average absorbance Normalized absorbanceof C2O band

bulk 2.52820.3617 2.85290.0996 3.74500.0290 4.2759

projected interface 4.2848

Figure 4. (a) Transmission IR spectrum of PEMA; (b) RA-IRspectrum of PEMA adsorbed on copper from 4% solution inchloroform; (c) RA-IR spectrum of PEMA adsorbed on copperfrom 1% solution in chloroform; (d) RA-IR spectrum of PEMAadsorbed on copper from 0.5% solution in chloroform.

3070 D. H. Kim, W. H. Jo

averaged angle bC2O of PMMA onto the copper surface isestimated to be ca. 198.

When the transmission IR spectrum of the PEMA bulkis compared with the RA-IR spectra of PEMA adsorbedonto the copper substrate of different thickness, as shownin Figure 4a–d, it reveals that the selective increase incarbonyl band intensity can hardly be observed unlike thecase of PMMA/copper. This result is consistent with theobservation of XPS that the carbonyl groups of PEMA donot interact significantly with copper. The spectra ofPnBMA/copper and PiBMA/copper systems (not shownhere) are analyzed in the same manner. The thickness-normalized C2O band absorbances for the RA-IR spectraobtained from the 0.5% solution are calculated in order tocompare the interfacial activity of carbonyl groups in thefour polymethacrylates/copper systems. The values are4.28, 2.85, 3.31, and 3.14 for PMMA/copper, PEMA/cop-per, PnBMA/copper, and PiBMA/copper systems, respec-tively. This result indicates that the carbonyl group ofPMMA is more perpendicularly oriented near the inter-face compared to that of the other polymers, and the trendis exactly consistent with the results of adhesion strengthand XPS.

ConclusionsThe interfacial characteristics between polymethacrylatesand copper were investigated using adhesion strengthmeasurement, XPS, and RA-IR spectroscopy. The adhe-sion strength between PMMA/copper was the highestamong four polymethacrylate/copper systems studied,while that between PEMA/copper was the lowest. Whenthe adhesion strength between PnBMA/copper was com-pared with that of PiBMA/copper, the former was higherthan the latter. The specific interaction between poly-methacrylates and copper was identified by XPS. Thecarbonyl C 1s peak of the polymer bulk spectra shifts tolower binding energy in the polymer/copper interfacespectra and the degree of peak shift decreases in the orderof PMMA/copper A PnBMA/copper F PiBMA/copper APEMA/copper. For the PMMA/copper system, the specif-ic interaction between the carbonyl group and coppercould also be identified by the selective increase of theC2O peak intensity in the O 1s spectrum of the PMMA/copper interface. The RA-IR spectroscopy indicates thatthe adsorption behavior of polymethacrylates is closelyrelated to the orientation of the carbonyl group upon thecopper surface. The use of quantitative absorption-reflec-tion thickness IR (QUART-IR) spectroscopy reveals thatthe relative intensity of the carbonyl band increases withdecreasing film thickness, indicating that the carbonylgroup is more perpendicularly oriented near the interfacedue to the specific interaction with copper. The results ofXPS and RA-IR spectroscopy were consistent with theresult of adhesion strength measurement.

Acknowledgement: The authors thanks the Korea Science andEngineering Foundation for financial support through theHyperstructured Organic Materials Research Center.

Received: November 13, 2000Revised: May 1, 2001

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