Organic Molecules and Polymers

The molecular probe technique in combination with XPS has seldom been used since the early 1970s and has mainly been applied to zeolites [162-164]. These studies were aimed at identifying and quantifying Lewis acidic and basic sites at catalyst surfaces by monitoring the BE shifts of N1s from adsorbed pyridine [162,163] and pyrrole [164], respectively. We shall discuss the application of this approach to molecular and polymeric species.

a. Molecules: Relationship with Gutmann’s and Drago’s Acid-Base Constants. Burger and Fluck [165] established, for quickly frozen solutions of SbCl5-Lewis base complexes in 1,2-dichloroethane, a linear relationship between Sb3d5/2 BE and DN, the donor number of the complexing Lewis bases. On this basis, Chehimi [166] showed that the Sb3d5/2 BE was linearly correlated with the ДИАВ of (base-SbCl5) adduct formation calculated using Drago’s equation. Figure 16 depicts a linear correlation of Sb3d5/2 BE versus ДИАВ (base-SbCl5). Therefore, XPS is a potential tool for estimating Drago’s parameters for polymer surfaces [166], which have actually been confirmed for PPO [167] and plasma-treated polypropylene [46] (see Table 3).

b. Polymers: Sorption of Specific Probes. Chehimi and co-workers

[154,166,168-171] established protocols to quantitatively estimate the acid-base properties of polymer surfaces using (ad)sorbed molecular probes. In practice, a polymer is exposed to liquid vapors (solutes) of known acid-base properties for a few minutes. The polymer is then allowed to outgas the excess of solute and is transferred into the XPS equipment for surface characterization. If the polymer-solute interaction is strong enough (e. g., via acid-base forces) then a residual amount of solute is detected and quantified.

(i) Choice of Molecular Probes. Several molecular probes can be used to charac­terize the acid-base properties of solid surfaces by XPS. For example, chlorinated and

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323.2 322.9

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-ЛНАВ of base-SbCI5 pair, kj/mol

 

Figure 16 Plot of (Sb3d5/2-Cl2p) BE energy difference versus AHab for Lewis base: SbCl5 adducts in quickly frozen solutions of 1,2-dichloroethane. AN, acetonitrile; DEE, diethylether; DMF, dimethylformamide; DMSO, dimethylsulfoxide; HPMA, hexamethylphosphoramide. The data point corresponding to a zero value of AHab (corresponding to ‘‘No donor’’) is obtained for a quickly frozen solution of SbCl5 in the absence of any basic solute. (Reproduced from Ref. [166] by kind permission of Kluwer Academic Publishers.)

 

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Table 12 Molecular Probes Used for XPS Determination of Acid-Base Properties of Materials

Probe

Core Line

Material Property Investigated

CHCl3

Cl2p

basicity

CH2Cl2

Cl2p

basicity

HFIP

F1s

basicity

CF3COOH

F1s

basicity

I2

I3d5/2

basicity

Pyrrole

N1s

basicity

Pyridine

N1s

acidity

DMSO

S2p

acidity

fluorinated acidic species probe Lewis basicity, whereas pyridine [168,154,163] or DMSO [83] are suitable to characterize surface acidity (Table 12).

Figure 17 depicts a survey scan of a basic aromatic moisture-cured urethane resin (ArMCU) before and after exposure to the vapors of hexafluoroisopropanol (HFIP) (CF3CH(OH)CF3), a reference Lewis acid. The F1s from HFIP is easily detected indicat­ing that it was retained by the ArMCU. This retention, despite the high vacuum, is believed to be governed by acid-base interactions between the OH group from HFIP and the carbamate (HN-C = O) group from the resin. The molar ratio of solute per polymer repeat unit (%S, where S stands for solute) was evaluated and used as a measure of the uptake (or retention) of solute by the host polymer. Figure 18 shows plots of

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Figure 17 X-ray photoelectron survey spectra of ArMCU (a) before and (b) after exposure to HFIP. Uptake of the Lewis acid HFIP is indicated by the presence of the F1s feature.

 

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Figure 18 Uptake (%S) of Lewis acids by PMMA and ArMCU versus the acidic character of the solutes. The %S is the solute per repeat unit molar ratio in the case of PMMA and the solute per nitrogen atom in the case of ArMCU. The solutes were characterized by AN, the Gutmann acceptor number: CCl4, 2.3; 1-2,dichloroethane (DCE), 6.4; dichloromethane (DCM) (CH2Cl2), 13.5; trichloromethane (TCM), (chloroform) 18.7; hexafluoroisopropanol (HFIP), 66.3; and tri — fluoroacetic acid (TFAA), 111.

 

%S versus AN (Gutmann’s acceptor number) of the solute, for the host polymers PMMA and ArMCU. The plots are S shaped, showing an increasing uptake of solute with AN, which denotes the basic character of both polymers. This is in agreement with Fourier Transform Infrared (FTIR) studies of the Lewis basicity of PMMA [13] and ArMCU [170]. In contrast, XPS did not detect any retained chloroform at the polyethylene surface following exposure to the vapors because the polymer-solute interactions reduce to London dispersive forces only.

(ii) Chemical Shifts of the Molecular Probes. The binding energies (BEs) of core electrons from the solutes’ elemental markers were also investigated for various poly­mers and resins (Table 13). Chloroform has been the most extensively used Lewis acid to characterize polymer basicity. Table 13 shows that acidic (CHCl3 and CF3Ca) and basic (pyridine) probes undergo negative and positive chemical shifts (lower and higher BEs), respectively, when they interact with host surfaces via acid-base forces. Indeed, a Lewis acid is an electron acceptor and upon interaction with a base via acid-base forces, electron density is transfered to the Lewis acid thus yielding a lower binding energy of its electrophilic site [43,168,169]. The opposite reasoning holds for basic probes [154,168].

Table 13 Binding Energy (eV) for Molecular Probes Adsorbed onto Polymers and Resins

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168.81

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166.5-168[4]

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Figure 19 Cl2p3/2 BE versus ДНАБ for CHCl3 sorbed in ArMCU, poly(vinyl acetate) (PVAc), poly(methyl methacrylate) (PMMA), poly(ethylene oxide) (PEO), poly(butyl methacrylate) (PBMA), and poly(cyclohexyl methacrylate) (PCHMA).

in the high vacuum, there was a continuous desorption of the probe (the purple color was vanishing) leading to a very weak I3d5/2 peak intensity. Nevertheless, the I3d5/2 peak recorded during desorption shifted from 621 to 619 eV, thus towards a lower binding energy. This indicates that the strongly adsorbed iodine molecules were subject to electron density transfer from PS thus leading to low BE. Perhaps PS was not a strong enough Lewis base to retain adsorbed iodine in the high vacuum. Nevertheless, the ESCA group led by J. J. Pireaux in Namur was more successful in characterizing the basicity of plasma — treated polypropylene by iodine vapor [46,172].

(vi) Pyridine. Pyridine is a molecular probe for the acidic sites of catalysts [173]. When adsorbed on polymer surfaces, the N1s core electron undergoes a +1 eV chemical shift (in comparison to the N1s BE for pure pyridine) in the case of the host PMMA owing to the donation of electron density from pyridine to the carbonyl carbon of the metha­crylate repeat unit (acidic site) [168]. The N1s chemical shift is even larger (+1.7 eV) when pyridine is sorbed in PVB since it is predominantly acidic due to its OH pendent groups. The N1s BE positions for pyridine-polymer complexes are higher than those of the pure pyridine because the pyridine-pyridine interaction occurs via dispersive forces only, for pyridine is a monofunctional species [7,54].

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