C60-containing polymer complexes

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Macromol. Chem. Phys. 201, 1037–1047 (2000)

Full Paper: Multifunctional 1-(4-methyl)piperazinylfullerene (MPF) and N-[(2-piperidyl)ethyl]aminofullerene (PEAF) were prepared by nucleophilic addition of C60 with amines. MPF and PEAF formed complexes with poly(styrenesulfonic acid) (PSSA), poly(vinylphosphonic acid) (PVPA), poly(acrylic acid) (PAA) or poly(methacrylic acid) (PMA), resulting in fullerene-containing polymer complexes. All the complexes showed no distinct glass transitions up to the degradation temperatures. The PSSA complexes showed unusual thermal behavior. The interactions between the C60 derivatives and the polymers were studied by X-ray photoelectron spectroscopy (XPS). XPS shows that the nitrogen atoms in both MPF and PEAF are protonated by the polymers. The strong ionic interactions between the C60 derivatives and the protondonating polymers led to the formation of complexes. 1

H NMR (90 MHz) spectrum of MPF in CDCl3

C60-containing polymer complexes: Complexation between multifunctional 1-(4-methyl)piperazinylfullerene or N-[(2-piperidyl)ethyl]aminofullerene and proton-donating polymers S. H. Goh* 1, S. Y. Lee1, Z. H. Lu1, C. H. A. Huan2 1 2

Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543 Department of Physics, National University of Singapore, 3 Science Drive 3, Singapore 117543

(Received: August 9, 1999; revised: November 18, 1999)

Introduction The possibility of combining the attractive properties of [60]fullerene (C60) and those of polymers has received considerable attention1). Various methods have been developed to covalently link C60 onto polymers2). C60 may be part of the polymer backbone, part of the polymer pendent group, or can be capped at the polymer chain end. Recently, the use of specific intermolecular interactions such as hydrogen-bonding and ionic interactions to prepare self-organized materials such as liquid crystalline polymers has attracted considerable interest3–8). Similarly, interpolymer complexes obtained through strong intermolecular interactions have also been extensively studied9–16). We have recently reported the preparation of C60-containing polymer complexes through interactions between a multifunctional C60 derivative and suitable Macromol. Chem. Phys. 201, No. 10

polymers17, 18). Multifunctional 2-(2-pyridyl)ethylaminofullerene (PYEAF) forms complexes with poly(styrenesulfonic acid) (PSSA), poly(vinylphosphonic acid) (PVPA), poly(acrylic acid) (PAA) and poly(methacrylic acid) (PMAA) through ionic interactions17). Hydrogensulfated fullerenol also forms complexes with poly(4-vinylpyridine) through ionic interactions18). Unlike the conventional C60-containing polymers in which fullerene is chemically attached to the polymer chains, the fullerene derivative in the complex is physically attracted to the polymer chains. This method provides an alternative route to prepare C60-containing polymers. In this paper, we report the complexation between multifunctional 1-(4-methyl)piperazinylfullerene (MPF) or N-[(2-piperidyl)ethyl]aminofullerene (PEAF) with PSSA, PVPA, PAA, and PMAA. Our recent studies have shown that piperidine- and piper-

i WILEY-VCH Verlag GmbH, D-69451 Weinheim 2000

1022-1352/2000/1006–1037$17.50+.50/0

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S. H. Goh, S. Y. Lee, Z. H. Lu, C. H. A. Huan

azine-containing polymers can form complexes with proton-donating polymers19–21).

Experimental part Materials —

PSSA (Mn = 22 500) in a form of 30 wt.-% aqueous solution was purchased from Polysciences. PAA (viscosity-average — molecular weight Mv = 450 000, Tg = 106 8C) was bought — from Aldrich. PMA (Mw = 400 000, Tg = 156 8C) was purchased from Polysciences. PVPA was also purchased from Polysciences (no molecular weight information available). C60 (99.9%) was obtained from Peking University and used without further purification. 1-(2-Aminoethyl)piperidine (98%) and 1-methylpiperazine (98%), both obtained from Aldrich, were distilled under reduced pressure immediately before use while other reagents were used as received. Measurements FTIR measurements were performed on a Perkin-Elmer 1725X FTIR spectrophotometer; 16 scans were signal-averaged at a resolution of 2 cm–1. Samples were prepared by dispersing the samples in KBr and compressing the mixture to form discs. All the discs were dried in vacuo at 60 8C for 12 h before the spectra were run. UV-vis spectra were recorded on a Hewlett Packard 8452A Diode array spectrophotometer with a Hewlett Packard Vectra QS/165 computer system. The 90 MHz 1H NMR spectra were obtained using a Jeol FX90Q in CDCl3 with TMS as internal standard. The glass transition temperatures (Tg s) of various samples were measured with a TA Instruments 2920 differential scanning calorimeter under nitrogen at a heating rate of 20C min–1. The initial onset of the change of slope in the differential scanning calorimetry curve was taken as the Tg . Each sample was scanned several times to check the repeatability of the results. Thermogravimetric measurements were made with a TA Instruments 2960 simultaneous DTA-TGA under nitrogen at a heating rate of 20 8C min–1. XPS measurements were performed on a VG Scientific ESCALAB MKII spectrometer with a Mg Ka X-ray source (1253.6 eV photons). Samples in the form of fine powder were mounted on a standard sample stud by means of double-sided adhesive tape. The X-ray source was run at 12 kV and 10 mA. A take-off angle of 75 was taken in all spectra and all core-level spectra were referenced by the C1s neutral carbon peak at the binding energy (BE) of 284.6 eV. Synthesis of MPF 15 ml of 1-methylpiperazine was added to a solution of 500 mg of C60 in 150 ml chlorobenzene in a 3-neck round-bottomed flask equipped with a nitrogen inlet and outlet. The mixture was stirred continuously under nitrogen for 72 h to allow C60 dissolve completely. After the removal of 1methylpiperazine and chlorobenzene by means of rotary evaporation at 75 8C, the product was washed with diethyl ether three times. The brown powder product was dried in vacuo at 30 8C for three days to give 850 mg of MPF.

Fig. 1.

1

H NMR (90 MHz) spectrum of MPF in CDCl3

IR (KBr, cm–1): 2 934 (s), 2 791 (s), 1 666 (m), 1 443 (s), 1 288 (w), 1146 (s), 1 006 (m), 790 (w), 523 (w). The 1 H NMR spectrum of MPF is shown in Fig. 1. UV (CHCl3): kmax = 270 nm. Elemental analysis (wt.-%): Found C 79.1 H 5.51 N 12.6

Note that elemental analysis of nitrogen in C60 derivatives gives a lower value due to incomplete combustion22). XPS (atm.-%):

Found C 85.2 N 14.7

The XPS result is consistent with nine 1-methylpiperazine groups per C60, leading to the average stoichiometry of [C60H9(NHC2H4NCH3)9]. Synthesis of PEAF 400 mg of C60 was added to a solution of 2.0 ml of 1-(2-aminoethyl)piperidine, 40 ml of DMSO, and 40 ml of THF in a 3-neck round-bottomed flask equipped with a water condenser and nitrogen inlet and outlet. The mixture was continuously stirred under nitrogen for 48 h to let C60 dissolve completely. After the removal of most of the THF by means of rotary evaporation at 50 8C, the mixture was poured into 100 ml of methanol to precipitate the derivative. The crude product was separated by centrifugation and then dissolved in THF. The THF solution was then poured into an excess of methanol to precipitate the product. The dissolution-precipitation process was repeated three times to purify the fullerene derivative. The brown powder product was dried in vacuo for three days to give 750 mg of PEAF in brown powder. IR (KBr, cm–1): 3 451 (s), 1 653 (m), 1 507 (s), 1 461 (s), 1116 (w), 1 024 (m), 672 (w); The 1H NMR spectrum of PEAF is shown in Fig. 2. UV (in CHCl3): kmax = 270 nm. Elemental analysis (wt.-%): Found C 80.2 H 5.41 N 10.11 XPS (atm.-%): C 87.2 N 12.8

The XPS result is consistent with eight 1-(2-aminoethyl)piperidine groups per C60 , leading to the average stoichiometry of [C60H8(NH2CH2CH2NC5H10)8].

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C60-containing polymer complexes: Complexation between multifunctional ...

remain at the corresponding region as observed in the spectrum of 1-methylpiperazine. The results are consistent with the addition of the secondary amino group to C60 .

Synthesis of PEAF

Fig. 2.

1

H NMR (90 MHz) spectrum of PEAF in CDCl3

Preparation of complexes In a typical procedure, 50 mg of PEAF was dissolved in 50 ml of THF/methanol (v/v, 2/1). PSSA, PVPA, PAA or PMA was dissolved separately in the same solvent at a concentration of 5 g N L–1. The appropriate amount of the PEAF solution and the acidic polymer solution were mixed and stirred for 15 min to give a brown precipitate which was separated from the solution by centrifugation. After washing with the same solvent three times, the precipitate was dried in air for several hours and then further dried in vacuo at 60 8C for one week. The ratios of the amount of the dried complex to the total amount of the PEAF and the acid polymer used gives the yield of the complex. Complexes of MPF were similarly prepared in THF/methanol (v/v, 1/2).

Results and discussion

The reaction between C60 and 1-(2-aminoethyl)piperidine proceeds smoothly in DMSO/THF (1/1) and the product dissolves in THF readily although C60 itself does not. No insoluble products were formed. Based on the XPS results, an average of nine 1-(2-aminoethyl)piperidine molecules have been added to the C60 framework. Fig. 2 shows the 1H NMR spectrum of PEAF. A new multiple peak at 3.3 ppm arises from the protons transferred from amino groups to the C60 framework. All other proton signals remain as observed in the spectrum of 1-(2-aminoethyl)piperidine but the proton signal of the amino group becomes weaker, which is in agreement with the transfer of a proton to C60 .

Complexation of MPF or PEAF with PSSA, PVPA, PAA or PMAA The complexation of MPF or PEAF with the acidic polymers occurred rapidly, leading to the formation of polymer complexes. The characteristics of various complexes are summarized in Tab. 1 – 8. As compared to the previously reported PYEAF/polymer complexes17), the most striking observation is the Tab. 1.

Characteristics of MPF/PSSA complexes

Synthesis of MPF

Complex system

There is one secondary amino group and one tertiary amino group in 1-methylpiperazine. The tertiary amino group cannot add to C60 through nucleophilic multi-addition due to the absence of a transferable hydrogen22–24). Hence only the secondary nitrogen in 1-methylpiperazine can react with C60 to give MPF. Usually, the nucleophilic addition of amines with C60 proceeds smoothly in amines themselves or in the presence of additional strong polar solvents such as DMF and DMSO. However, the reaction between C60 and 1-methylpiperazine can proceed in chlorobenzene. The product dissolves in THF readily although C60 itself does not. No insoluble products were formed. Based on the XPS result, an average of nine 1methylpiperazine molecules have been added to the C60 framework. Fig. 1 shows the 1H NMR spectrum of MPF in CDCl3 . After the addition, the peak at 1.3 ppm of proton on the nitrogen in 1-methylpiperazine disappeared to give a new multiple peak at 3.3 ppm. The proton signals on the piperazine ring at 2.2 – 3.2 ppm overlap with those of the methyl groups at the higher field. All these peaks

Mole ratio of SO3H to N in feed Yield % of complex Mole ratio of SO3H to N in complex Protonation fraction of N in complex Tab. 2.

2

0.37 90 0.47 0.26

3

0.74 1.11 82 51 0.84 1.41 0.56 1.00

Characteristics of MPF/PVPA complexes

Complex system Mole ratio of PO3H2 to N in feed Yield % Mole ratio of PO3H2 to N in complex Protonation fraction of N in complex Tab. 3.

1

1

2

0.72 93 0.42 0.17

3

1.15 1.73 87 78 0.52 0.75 0.34 0.53

Characteristics of MPF/PAA complexes

Complex system

1

2

3

Mole ratio of COOH to N in feed 1.00 1.50 2.00 Yield % of complex 96 98 97 Mole ratio of COOH to N in complex 1.58 1.69 2.14 Protonation fraction of N in complex (%) 0.15 0.31 0.47

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Tab. 4.

Characteristics of MPF/PMAA complexes

Complex system

1

2

3

Mole ratio of COOH to N in feed 1.26 1.67 2.51 Yield % 93 95 94 Mole ratio of COOH to N in complex 1.49 1.82 2.63 Protonation fraction of N in complex (%) 0.13 0.24 0.31 Tab. 5.

Characteristics of PEAF/PSSA complexes

Complex system Mole ratio of SO3H to N in feed Yield % of complex Mole ratio of SO3H to N in complex Protonation fraction of N in complex Tab. 6.

Mole ratio of PO3H2 to N in feed Yield % Mole ratio of PO3H2 to N in complex Protonation fraction of N in complex

3

0.90 1.13 67 69 1.36 1.62 1.00 1.00

1 0.87 62 0.79 0.52

2

3

1.35 1.54 86 83 1.03 1.26 0.57 0.60

Characteristics of PEAF/PAA complexes

Complex system Mole ratio of COOH to N in feed Yield % of complex Mole ratio of COOH to N in complex Protonation fraction of N in complex Tab. 8.

0.63 65 0.57 0.42

2

Characteristics of PEAF/PVPA complexes

Complex system

Tab. 7.

1

1 0.72 86 1.66 0.25

2

3

1.15 1.73 89 86 1.71 2.17 0.35 0.49

Characteristics of PEAF/PAA complexes

Complex system Mole ratio of COOH to N in feed Yield % of complex Mole ratio of COOH to N in complex Protonation fraction of N in complex

1 0.97 81 1.51 0.20

2

3

1.45 1.93 88 90 1.91 2.08 0.27 0.44

high yields of MPF/polymer and PEAF/polymer complexes, especially for complexes involving PAA and PMAA. The complexation between PYEAF and PSSA or PVPA gives 80 – 90% yield, while that between PYEAF and PAA or PMAA gives about 10 – 20% yield. In contrast the complexation between MPF or PEAF and PAA or PMAA gives about 90% yield. The yield of interpolymer complex depends on several factors such as the molecular weights of the polymers, the feed composition, and the number and strength of interpolymer interactions. The formation of interpolymer complexes requires the chain lengths of interacting polymers to exceed a certain critical value25, 26). A long polymer chain allows more cooperative interaction, facilitating complex formation. For example, we have recently

reported that poly(p-vinylphenol) forms complexes with a high-molecular-weight poly(N-acryloyl-N9-methylpiperazine) (PAMP) in ethanol but not with a low molecular weight PAMP21). Since the same polymers were used in both the previous and the present studies, the high yields of complexes involving MPF or PEAF do not arise from the molecular weight effect. The formation of interpolymer complexes requires polymer-polymer interactions to be stronger than polymer-solvent interactions. Therefore, any factors that enhance interpolymer interactions favor the formation of interpolymer complexes. Similarly, one would expect the formation of C60 /polymer complexes to be favored by the enhancement of interaction between the C60-derivatives and the polymers. Since piperidine (pKb = 2.88) and piperazine (pKb = 4.17) are stronger bases than pyridine (pKb = 8.75), it is reasonable to attribute the high yields of MPF/polymer and PEAF/polymer complexes to the stronger basicity of the piperidine and piperazine groups. This reasoning is supported by the extent of protonation of the basic nitrogens in the piperidine and piperazine groups. As will be discussed in a later section, the extent of protonation of nitrogen can be evaluated by XPS measurements. The fractions of protonated nitrogens in PYEAF/PAA complexes and PYEAF/PMAA complexes are around 0.26 and 0.10, respectively17). In comparison, the fractions of protonated nitrogens in MPF/PAA, MPF/ PMAA, PEAF/PAA, and PEAF/PMAA complexes are significantly larger than those of the corresponding PYEAF/polymer complexes. Furthermore, for MPF/ PSSA complexes, complete protonation of nitrogen is achieved. As a result of increasing the extent of protonation of nitrogen, there is an increase in the number of ionic interactions between the C60-derivatives and the polymers, leading to higher yields of complexes. The extent of protonation of nitrogens in MPF and PEAF also depends on the acid strength of the polymer. The pH values of 0.1 M ethanol/water (1 : 1) solutions of PSSA, PVPA, PAA, and PMAA are 1.36, 2.33, 3.04, and 3.57, respectively20). Thus, the stronger the acid, the larger is the fraction of protonated nitrogens. It is noted that the yields of complexes involving PAA or PMAA are higher than those of complexes involving PSSA even though their fractions of protonated nitrogens are lower. The high yields of PAA or PMAA complexes could be the result of the higher molecular weights of PAA and PMAA as compared with PSSA. The larger number of interacting sites associated with a longer polymer chain and the chain entanglement effect could lead to a higher yield of complex.

Thermal characteristics of complexes It is commonly observed that the Tg of an interpolymer complex is higher than that calculated from the linear


Fig. 3. Thermogravimetric curves of (a) MPF/PSSA complex 2, (b) PSSA, and (c) MPF

Fig. 5. Thermogravimetric curves of (a) MPF/PVPA complex 2, (b) PVPA, and (c) MPF

Fig. 4. Thermogravimetric curves of (a) PEAF/PSSA complex 3, (b) PSSA, and (c) PEAF

Fig. 6. Thermogravimetric curves of (a) PEAF/PVPA complex 1, (b) PVPA, and (c) PEAF

additivity rule. The positive deviation in Tg values of complexes is taken to indicate the presence of strong intermolecular interactions that restrict chain motion. The Tg values of PMAA and PAA are 165 8C and 106 8C, respectively, while the Tg s of PSSA and PVPA are difficult to detect. However, distinct glass transitions for all the complexes could not be observed up to the starting degradation temperature of about 150 – 230 8C. It is conceivable that the intermolecular interactions in the complexes are so strong that the Tg values are even higher than the degradation temperatures. Such a behavior has been observed in interpolymer complexes with high ionic crosslinking density27). Similarly, the glass transitions of all PYEAF/polymer complexes could not be observed17).

The thermal stability of all the complexes was evaluated by thermogravimetry (TG). The PSSA complexes showed peculiar TG curves as shown in Fig. 3 and 4. Firstly, the initial degradation temperatures of the complexes are about 100 8C higher than those of PSSA. Secondly, the amount of residues remained at high temperatures are lower than those of PSSA and the C60-derivatives. The strong interactions between PSSA and MPF or PEAF could have delayed the onset of thermal degradation. However, it is difficult to rationalize the second observation. Nonetheless, the TG curves demonstrate that the degradation behavior of the PSSA complexes is different from that of PSSA and the C60-derivatives. We have observed previously that PYEAF/PSSA complexes

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also showed an unusually good thermal stability in the low-temperature region17). Fig. 5 and 6 show the TG curves of two PVPA complexes. The complexes also show a better thermal stability than PVPA in the low-temperature region. Unlike the PSSA complexes, the PVPA complexes do not show unusual behavior in the high-temperature region. Moreover, the thermal behavior of the MPF/PVPA and PEAF/PVPA is similar to that of the PYEAF/PVPA complexes17). In contrast, the TG curves of MPF/PAA, MPF/PMAA, PEAF/PAA, and PEAF/PMAA complexes do not show unusual thermal behavior. The overall shape of the TG

Fig. 7. N1s spectra of (a) MPF, (b) MPF/PSSA complex 1, (c) MPF/PSSA complex 3, (d) MPF/PVPA complex 1, and (e) MPF/PVPA complex 2

curve is similar to the combination of the polymer and the C60-derivative. For brevity, their TG curves are not shown. Once again, the thermal behavior of the complexes is similar to that of the corresponding complexes of PYEAF17).

XPS characterization Fig. 7 shows the N1s core-level spectra of MPF and several complexes. MPF shows a single N1s peak that could be deconvoluted into two component peaks representing the two different types of nitrogens in MPF. The nitrogen attached to the C60 is electron-poor as compared to the nitrogen attached to a methyl group. Therefore nitrogen (a) (underlined) has a higher binding energy (399.2 eV) than nitrogen (b) (bold) (398.8 eV). [C60(NHC4H8NCH3)9]


In Fig. 7 b, MPF/PSSA complex 1 shows the appearance of a high-BE N1s peak which is deconvoluted into two component peaks at 401.2 and 401.6 eV, arising from the protonated nitrogen atoms (b) and (a), respectively. Previous studies have shown that the BE value of the positively charged N1s electron is 2.0 – 2.5 eV higher than that of the neutral N1s electron16, 19, 28, 29). The peak synthesis of the N1s peak was based on the principle that the sum of the areas of protonated and unprotonated nitrogen (a) equals the sum of the areas of protonated and unprotonated nitrogen (b). Furthermore, the widths of all component peaks are maintained at 1.7 eV. As shown in the spectra, the sum of all the peaks is in excellent agreement with the experimental signal. It is noted that the two types of nitrogen atoms of MPF have only been protonated partially and the extent of protonation of nitrogen atom (b) is much higher than that of nitrogen atom (a). The results indicate that both nitrogens (a) and (b) can be protonated by PSSA but nitrogen (b) is the preferred site of protonation. However, the N1s spectrum of MPF/ PSSA complex 3, as shown in Fig. 7 c, reveals only a single N1s peak at the high-BE region. Nevertheless, this peak can also be deconvoluted into two nitrogen environments at 401.2 and 401.6 eV. The absence of the low-BE N1s peak for MPF/PSSA complex 3 clearly shows that all the nitrogen atoms of MPF have been completely protonated. In other words, both the piperazine nitrogen atoms can be protonated entirely by PSSA. Similarly, the N1s spectra of the two MPF/PVPA complexes (Fig. 7 d and 7 e for complexes 1 and 2) also show the appearance of a high-BE peak that is deconvoluted into two nitrogen environments at 401.1 and 401.5 eV. However, the extent of protonation of nitrogen atoms of MPF is much lower in the MPF/PVPA complexes than in the MPF/PSSA complexes, and the nitrogen atoms of MPF are not completely protonated by PVPA. The extent of protonation can be calculated from the area of the deconvoluted peaks and the preferred protonation site is also the nitrogen atoms (b) of MPF. The total protonation fraction of all complexes increases with increasing polyacid content in the feed and the results are shown in Tab. 1 and 2. Since sulfonic acid is a stronger acid than phosphonic acid, it is reasonable to suggest that PSSA is the more effective protonating agent than PVPA in complexation with MPF. XPS measurements could not be performed on PSSA due to its extremely hygroscopic nature. However, we have earlier reported the S2p spectrum of sulfonated polystyrene with a degree of sulfonation of 27 mol-% (SPS27)28). The S2p peak of SPS27 consists of a main spin-orbit split doublet S2p3/2 and S2p1/2 with BE values of 168.4 and 169.6 eV respectively, arising from the free sulfonic acid groups. There is a minor doublet with BE values of 170.2 and 171.4 eV, attributed to hydrogen bonded sulfonic acid groups. When SPS forms complexes with poly(styrene-co-4-vinylpyridine), the

Fig. 8. S2p spectra of (a) MPF/PSSA complex 1, and (b) MPF/ PSSA complex 3

S2p peak shifts to a lower BE region. Each S2p peak can be deconvoluted into two environments, one arising from the sulfonic acid with BE values of 168.4 and 169.6 eV and the other from the sulfonate ion with BE values of 167.3 and 168.5 eV. As shown in Fig. 8, all the S2p peaks of the MPF/PSSA complexes can also be deconvoluted into two environments, one due to the sulfonic acid groups at 168.4 and 169.6 eV (dashed curve) and the other due to the sulfonate ion at 167.3 and 168.5 eV (full curve). Therefore, the S2p spectra of the MPF/PSSA complexes clearly show that some of the sulfonic acid groups have ionized to protonate MPF. The P2p spectrum of PVPA is not available due to the extremely hygroscopic nature of PVPA. However, it has been reported that the P2p peak of poly(phenoxyphosphazine) consists of two spin-orbit split doublet P2p3/2 peak with a BE of 133.85 eV and P2p1/2 peak with a BE of 134. 72 eV and the intensity ratio is 1.9429). We have also reported previously that the P2p peak of poly(2-vinylpyr-

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Fig. 9. P2p spectra of (a) MPF/PVPA complex 2, and (b) MPF/ PVPA complex 3

idine) or poly(4-vinylpyridine)/PVPA complexes can be deconvoluted into two environments, one arising from the neutral phosphonic acid with BE of 133.2 eV for P2p3/2 and 134.1 eV for P2p1/2, and the other from the phosphonate with BE of 132.4 eV for P2p3/2 and 133.3 eV for P2p1/216). As shown in Fig. 9, all the P2p peaks of the MPF/PVPA complexes can also be deconvoluted into two environments, one due to the phosphonic acid groups at 133.2 eV for P2p3/2 and 134.1 eV for P2p1/2 (dashed curve) and the other due to the phosphonate ion at 132.4 eV for P2p3/2 and 133.3 eV for P2p1/2 (full curve). Therefore, the P2p spectra of the MPF/PVPA complexes clearly show that some of the phosphonic acid groups have ionized to protonate MPF. The N1s core-level spectra of various MPF/PAA and MPF/PMAA complexes are similar to those of the MPF/ PVPA complexes. Each N1s peak shows the appearance of a high-BE peak which can also be deconvoluted into two component peaks at 401.1 and 401.5 eV. The relevant information obtained from XPS measurements are given in Tab. 3 and 4. The extent of protonation of nitrogen

Fig. 10. N1s spectra of (a) PEAF, (b) PEAF/PSSA complex 1, and (c) PEAF/PSSA complex 2

atoms of MPF is much lower than that of MPF/PSSA or MPF/PVPA complexes under similar conditions and the nitrogen atoms of MPF are not completely protonated by PAA. The nitrogen atom (b) of MPF is also the preferred protonated site in the complexation with PAA. The total protonation fraction of all complexes increases with


Fig. 11. S2p spectra of (a) PEAF/PSSA complex 1, and (b) PEAF/PSSA complex 2

increasing PAA content in the feed and the results are shown in Tab. 3. The XPS results show that PAA is a less effective protonating agent of MPF than PSSA and PVPA. Similarly, the protonation extent in the MPF/ PMAA complexes is much lower than that in the MPF/ PSSA and MPF/PVPA complexes even with higher acid contents in the complexes. The extent of protonation of the MPF/PMAA complexes is also lower than that of the MPF/PAA complexes at similar compositions of complexes. Therefore, PMAA is the weakest protonating agent among the four acidic polymers. Fig. 10 shows the N1s core-level spectra of PEAF/ PSSA complexes. Unlike MPF, the N1s peak of PEAF at 398.9 eV could not be deconvoluted. The BE values of the two types of nitrogen are too close to each other to allow deconvolution of the N1s peak. For PEAF/PSSA complex 1, the N1s spectrum shows the appearance of a high-BE peak at 401.1 eV (Fig. 10 b), indicating that some of the nitrogen atoms of PEAF have been protonated. However, the N1s spectrum of PEAF/PSSA complex 2 shows only a single peak at 401.1 eV (Fig. 10 c), showing that all the nitrogen atoms of PEAF have been completely protonated. In other words, both the piperidine nitrogen and the amine nitrogen atoms are proto-

Fig. 12. N1s spectra of (a) PEAF/PVPA complex 1, and (b) PEAF/PVPA complex 3

nated by PSSA. Similarly, the nitrogen atoms of PEAF in PEAF/PSSA complex 3 are also completely protonated and the N1s spectrum is not shown. As shown in Fig. 11, the S2p peaks of two PEAF/PSSA complexes can also be deconvoluted into two environments, one due to the sulfonic acid groups at 168.4 and 169.6 eV (dashed curve) and the other due to the sulfonate ion at 167.3 and 168.5 eV (full curve). Once again, the S2p spectra of the PEAF/PSSA complexes provide evidence of protonation by PSSA. Fig. 12 shows the N1s core-level spectra of two PEAF/ PVPA complexes. All spectra show the appearance of a high-BE peak at 401.1 eV, indicating that some of the nitrogen atoms of PEAF have been protonated by PVPA. Unlike the PEAF/PSSA complexes, the appearance of the peak at 398.9 eV in all the PEAF/PVPA complexes clearly shows that the nitrogen atoms of PEAF have not been completely protonated by PVPA. In other words, PVPA cannot protonate all the piperidine nitrogen and the amine nitrogen atoms as PSSA does. The extent of proto-

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The N1s core-level spectra of various PEAF/PAA and PEAF/PMAA complexes are essentially the same as those of the PEAF/PVPA complexes, showing the development of high-BE N1s peaks in all the spectra. For brevity, these spectra are not shown. It suffices to mention that the nitrogens in the PEAF/PAA and PEAF/PMAA complexes are not as highly protonated as those in the PEAF/PVPA complexes.

Conclusion Two C60 derivatives, 1-(4-methyl)piperazinylfullerene and N-[(2-piperidyl)ethyl]aminofullerene, form C60-containing polymer complexes with PSSA, PVPA, PAA or PMA through strong intermolecular interaction. All the complexes show no distinct glass transitions up to the degradation temperatures. The strong interactions between multifunctional C60 derivatives and polymer are shown by XPS. The fractions of protonation of nitrogen in complexes depend on the acidic strength and the content of polyacids. The nitrogens of MPF and PEAF can be completely protonated by PSSA. It is noted that the fullerene derivatives used in the present work are multifunctional and some of the special properties of fullerene may be lost due to multiple functionalization. Future studies will focus on the complexation between polymers and fullerene derivatives with low degree of functionalization.

Fig. 13. P2p spectra of (a) PEAF/PVPA complex 1, and (b) PEAF/PVPA complex 3

nation can be calculated from the area of the deconvoluted peaks and the results are shown in Tab. 6. The extent of protonation for the PEAF/PVPA complexes is much lower than that in the PEAF/PSSA complexes. It is of interest to note that the fractions of protonation of nitrogen atoms in all these complexes are more than 50%, showing that both types of nitrogen atoms can also be protonated by PVPA. However, the extent of protonation for each type of nitrogen atom cannot be calculated. In view of the high basicity of piperidine, the piperidine nitrogen atoms of PEAF are also likely to be the preferred protonation sites by PVPA. Undoubtedly, the phosphonic acid group is not effective as a sulfonic acid group as the protonating agent of PEAF. As shown in Fig. 13, the P2p peaks of the two PEAF/ PVPA complexes can also be deconvoluted into two environments, one due to the phosphonic acid groups at 133.2 eV for P2p3/2 and 134.1 eV for P2p1/2 (dashed curve) and the other due to the phosphonate ion at 132.4 eV for P2p3/2 and 133.3 eV for P2p1/2 (full curve).

Acknowledgement: Financial support of this work by the National University of Singapore is gratefully acknowledged.

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