Article pubs.acs.org/JPCC
Ca Carboxylate Formation at the Calcium/Poly(methyl methacrylate) Interface Huanxin Ju,† Xuefei Feng,† Yifan Ye,† Liang Zhang,† Haibin Pan,† Charles T. Campbell,‡ and Junfa Zhu*,† †
National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei, Anhui 230029, People's Republic of China ‡ Department of Chemistry, University of Washington, Seattle, Washington 98195-1700, United States ABSTRACT: Chemical reactions occurring at the interface during the growth of Ca films on poly(methyl methacrylate) (PMMA) by vapor deposition at room temperature have been investigated in detail by Xray photoemission spectroscopy (XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The changes of the O 1s, C 1s, and Ca 2p core level XP spectra upon Ca deposition directly indicate that Ca atoms preferentially react with the ester groups of PMMA at very low coverages, leading to the formation of polymeric Ca carboxylate and loss of methyl groups. The O K-edge NEXAFS spectra further confirm that the ester groups of the PMMA are the primary active sites for reaction with Ca atoms. The results of XPS and NEXAFS measurements are fully consistent with those from adsorption microcalorimetry studies reported in our previous work. Therefore, the reaction mechanism between Ca and PMMA has confirmed that Ca atoms react with the ester groups in PMMA to form polymeric Ca carboxylate in the early stages of Ca adsorption.
1. INTRODUCTION The interfaces between metals and polymers are of considerable importance in wide-ranging applications, especially in optoelectronic devices,1−4 packing,5,6 and coatings,7,8 because metal layers are usually deposited on polymer films as key functional components (e.g., electrodes and coatings). In general, metal deposition onto polymers may result in simple physical interactions, intradiffusion processes, or chemical reactions, all of which can directly influence the adhesion strength of the metal film to the polymer substrate.9−15 Therefore, a thorough understanding of the nature at the interfaces between metals and polymers can provide important guidance for practical manufacturing. Poly(methyl methacrylate) (PMMA) as an important thermoplastic material has been utilized in a variety of engineering areas ranging from aeronautical applications to electronics industries, due to its attractive physical and optical properties such as remarkable mechanical and superior insulation properties as well as excellent transparency.16−19 Interfaces between metals and PMMA have been extensively studied by various surface analysis techniques including X-ray photoelectron spectroscopy (XPS),20−26 infrared spectroscopy (IR),27−30 and low-energy ion scattering spectroscopy (ISS).31,32 It has been demonstrated that the ester groups in PMMA generally act as the preferred reaction sites during metal deposition, resulting in the formation of the polymer−metal complexes at the interface. For instance, strongly reactive metals such as Al,26,33 Cr,27 or Ni22,25 deposition on PMMA result in strong chemical reactions to form some stable metal− PMMA complexes at the interface. In contrast, there are no © 2012 American Chemical Society
obvious interfacial reactions between the polymers and the weakly reactive metals such as Cu12,26 and Pb.34 Similarly, spin coating PMMA onto the metal or metal oxide surfaces such as Co,35 Al2O3,23,28,36−38 and Co2O3,29,39 ester cleavage has been observed, leading to carboxylate ions bonded to the surface. The adsorption energy released as heat upon metal deposition onto polymer surfaces is directly related to the adhesion strength between the two materials when they form an abrupt interface. In our previous works, we used a unique adsorption microcalorimetry technique40 in combination with sticking probability measurements to study the interfacial interactions between the PMMA and the two metals Pb and Ca,34,41 which have very different reactivities. The very low measured heat (13 kJ/mol) of Pb on the pristine PMMA surface indicates a fairly weak interaction between Pb and PMMA.34 In contrast, the heat of Ca adsorption on PMMA at low Ca coverages (CO). Likewise, the C 1s spectrum can be resolved into four distinct components as shown: the C1 component at 289.0 eV is assigned to the carbonyl carbon (>CO), the C2 component at 286.8 eV is ascribed to the methoxyl carbon (O− CH3), the C3 component at 285.7 eV corresponds to the quaternary carbon (>CCO) (532.2 eV), similar to the observation in the case of PMMA/Al2O3.36 The O 1s intensity ratio of C−O:CO decreases from 1:1 in the pristine PMMA to only 1:2 after 1 ML Ca deposition. This supports our previously proposed model of the Ca/PMMA interfacial reaction,41 wherein this change can be attributed to the dissociation of the C−O bond in CH3−OC(O)R, leading to the loss of methyl groups in the polymer. That model is further supported by the fact that the bond dissociation energy of C− O in CH3−OC(O)CH3 (380 kJ/mol) is 44 kJ/mol lower than that of CH3O−C(O)CH3 (424 kJ/mol)55 and by a study of PMMA decomposition by soft X-ray excitation, which showed the C−O bond breaking in the methoxyl group with the formation of CH3+ ions.56 Therefore, it is likely that the C−O bond in the CH3−OC(O)R breaks, resulting in the change of the local chemical environment of the original methoxyl oxygen. This is further supported by the C 1s spectra below. Figure 3a shows the XP spectra of the C 1s region for various coverages of Ca on PMMA. After Ca deposition, noticeable changes of the C 1s spectra can be observed in Figure 3a: the overall intensity of C 1s signal decreases, especially for the intensity of C 1s from the ester group. Similar to the analysis of the O 1s spectra, the difference spectrum (Figure 3b) between the spectra of the clean and 1.0 ML Ca-deposited PMMA shows that besides the decrease in the intensities of the ester groups, two new features develop at 287.4 and 283.4 eV. We fitted the C 1s spectra for different Ca coverages, and the results are shown in Figure 3c−f. The peak at 287.4 eV (labeled as C6) can be ascribed to the carboxylate.27,39,50 It further confirms that the deposition of Ca on PMMA forms the corresponding Ca carboxylate at the interface by selectively interacting with the ester groups initially. At higher Ca coverages (more than 1 ML), another new but weak C 1s component appears at the low binding energy of 283.4 eV (labeled as C7), which can be attributed to Ca−C species. In the same spectral region, Ni carbide has also been observed as a result of Ni deposition on PMMA.22 The formation of this minor carbide species here is possibly also induced by X-ray beam damage,53,54 as with the aforementioned oxide species. The ratios of the peak intensities for different C 1s components are summarized in Table 1. The relative distribution of the carbon components has changed considerably as compared to that of clean PMMA. After 2 ML Ca deposition, the relative ratios of carbonylic carbon (>CO) and methoxyl carbon (O−CH3) decrease dramatically from 37.9 to 10.3%; however, the relative area ratios of the alkyl carbon increase gradually from 62.1 to 78.8%. The dramatic decrease in the intensity of the methoxyl carbon (O−CH3) is further evidence for the cleavage of the C−O bond in the CH3−OC(O)R with the methyl group removed from the molecule. Figure 4 displays the Ca 2p XP spectra at different Ca coverages. As seen, below 1 ML Ca on PMMA, the Ca 2p3/2 peak remains almost unchanged at 347.3 eV, which is ∼0.6 eV higher than the binding energy of bulk Ca 2p3/2 (346.7 eV), indicating that the deposited Ca forms ionic species on PMMA. After 1 ML Ca deposition, the Ca 2p doublet spectrum can be deconvoluted into two components whose 2p3/2 peaks locate at 347.3 and 346.7 eV, attributed to the ionic Ca and neutral Ca,
Figure 4. XP spectra of the Ca 2p peak at different Ca coverages on PMMA. Peak fittings for the spectra of 1 and 2 ML Ca deposition on PMMA with two components are also shown. Here, the high binding energy plasmon contribution to the neutral Ca components is neglected since the small nanoparticles of neutral Ca grown here have a relatively weaker plasmon intensity.
respectively. The intensity ratio of the two components is about 4:1, indicating that 80% of deposited Ca (i.e., 0.8 ML) has reacted with PMMA and only 20% of Ca (i.e., 0.2 ML) is neutral. As the Ca coverage increases, the relative intensity of the neutral Ca enhances greatly. After 2 ML Ca deposition, the intensity ratio of these two components decreases to 5:3, showing that more neutral Ca appears on PMMA. These results are nicely consistent with those obtained from microcalorimetry.41 In our previous study,41 we showed (based on the heats of Ca adsorption) that (1) all of the adsorbed Ca atoms below 0.5 ML diffuse into the subsurface to bond with the ester groups, (2) the population of Ca bonded to subsurface ester groups starts to saturate above 1 ML, and (3) by 2.5 ML, it fully saturates with a total coverage of 1.23 ML of Ca that reacted with esters. Because one Ca reacts with two ester groups to form Ca carboxylate, this is equivalent to reacting with five layers of monomers in PMMA. The ratio of the integrated XPS intensities of the ionic Ca 2p peak to the O3 1s peak (shown in Figure 2) after 1 ML Ca deposition is 1.42, and it is 1.48 after 2 ML Ca. Multiplying this ratio by the XPS sensitivity factor ratio for O(1s)/Ca(2p) of 0.4257 gives an atomic ratio of reacted Ca: O3 of 0.59 at 1 ML and 0.62 after 2 ML. Both ratios are close to the proposed stoichiometric ratio of 1:2. Therefore, the combination of O 1s, C 1s, and Ca 2p observations gives clear evidence that Ca preferentially reacts with the ester groups in PMMA to form Ca carboxylate at low Ca coverages. We excluded the possibility of any influence from the reactions of Ca with the residual gas in the vacuum such as H2O, CO, or CO2 to the above O 1s, C 1s, and Ca 2p spectral changes as follows. We performed a control experiment under the same experimental conditions by depositing a bulklike Ca film on an Au-coated Si wafer and then leaving it in the UHV chamber. The result shows that almost no oxygen can be detected in XPS at the same time period as that used for Ca on 20469
dx.doi.org/10.1021/jp307010x | J. Phys. Chem. C 2012, 116, 20465−20471
The Journal of Physical Chemistry C
Article
observations are fully consistent with the results from XPS for Ca on PMMA and further confirm the reaction mechanism between Ca and PMMA, as proposed previously.41
PMMA, suggesting that the changes in the O 1s, C 1s, and Ca 2p spectra after Ca deposition on PMMA are truly caused by the Ca−PMMA interaction. As mentioned before, because NEXAFS is very sensitive to πbonded species, using this technique to investigate the Ca/ PMMA interface can be particularly helpful to understand the reaction mechanism. Figure 5 shows the evolution of the O K-
4. CONCLUSION The spectroscopic XPS and NEXAFS techniques have been applied to study the interfacial interaction between Ca and PMMA at room temperature. The results suggest that at low coverages (
