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Langmuir 1997, 13, 3982-3989
Conformational Change Restricted Selectivity in the Surface Sulfonation of Polypropylene with Sulfuric Acid Hiroaki Tada*,† and Seishino Ito‡ Environmental Science Research Institute, Kinki University, 3-4-1, Kowakae, Higashi-Osaka 577, Japan, and Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashi-Osaka 577, Japan Received September 11, 1996. In Final Form: February 1, 1997X Surface sulfonation of solid polypropylene (PP) with hot concentrated H2SO4 has been studied by diffuse reflectance Fourier-transformed infrared (DRIFT) and X-ray photoelectron (XP) spectroscopies. Both measurements indicated that the sulfonation of PP followed by slight oxidation and the formation of CdC bonds is enhanced with both increasing reaction temperature and time. The time dependence of H2O penetration into sulfonated PP mesh sheets was analyzed using a rearranged Washburn’s equation, providing a quantitative indicator of surface hydrophilicity. A positive linear correlation was obtained between the hydrophilicity and the degree of sulfonation. The charge distribution calculated for models of SO3 and PP molecules by the PM3 molecular orbital (MO) method suggested that the sulfonation of PP proceeds via electrophilic addition of SO3. It was further predicted from the MO calculations for models of sulfonated PP molecules that side CH3 groups are more susceptible to sulfonation than C atoms of the main chain due to a drastic conformational change of the main chain in the latter case. The heat of reaction in each pathway (∆E) was estimated to be ∆E(side CH3 groups) ) -63.5 kJ (unit mol)-1, ∆E(C(CH3)SO3H) ) -56.8 kJ (unit mol)-1, and ∆E(CH(SO3H)) ) -55.8 kJ (unit mol)-1, respectively. The selectivity in the sulfonation of PP termed “the conformational change restricted selectivity (CCRS)” was shown to increase with a decrease in the reaction temperature by analyzing the DRIFT signals around 600 cm-1 assigned to the deformation of S-O bonds and the stretching vibration of S-C bonds.
I. Introduction Basic research on the surface modification of polymers is of great importance due to its relevance to polymer degradation1 and the surface design for higher functionalization of polymers.2 In the field of electrochemistry, polyolefin mesh sheets have attracted much attention from the view point of their application to a separator of highcapacitance-cells.3 If the surface of the polyolefin can be changed from a hydrophobic to a hydrophilic state, the high insulating character and low density should make it a desirable material for the separator. Although the surface wetting property is generally evaluated by measuring contact angles for flat plate samples, the method is not applicable to the sample absorbing liquids, e.g., hydrophilic polyolefin mesh sheets. With a view to endowing polyolefins with a hydrophilic character and/or a blood-anticoagulating property, much effort has been devoted to sulfonation of the surface.4 Sulfonation of polyethylene was accomplished by several methods including treatments with fuming H2SO45 or gaseous SO3.6 The technology has been developed to a high degree; however, there is still great room for * To whom correspondence should be addressed: TEL, 06-7212332; FAX, 06-721-3384; e-mail,
[email protected]. † Environmental Science Research Institute. ‡ Department of Applied Chemistry. X Abstract published in Advance ACS Abstracts, March 15, 1997. (1) Schnabel, W. Polymer Degradation Principles and Practical Applications; Macmillan Publishing Co., Inc.: New York, 1981. (2) Suzuki, M.; Tamada, Y.; Iwata, H.; Ikada, Y. Physicochemical Aspects of Polymer Surfaces; Mittal, K. L., Ed.; Plenum: New York, 1983; Vol. 2. (3) Kaiya, H. Hyomen Gijyutsu 1994, 45, 593. (4) (a) Chen, W. Y.; Xu, B. Z.; Feng, X. D. J. Polym. Sci., Polym. Chem. Ed. 1982, 20, 547. (b) Nishizawa, E. E.; Wynalda, D. J.; Lednicer, D. Trans. Am. Soc. Artif. Intern. Organs 1973, 19, 13. (c) Kamide, K.; Okajima, K.; Matsui, T.; Ohnishi, M.; Kobayashi, H. Polym. J. 1983, 15, 309. (5) (a) Olsen, D. A.; Osteraas, A. J. Polym. Sci., Part A: Polym. Chem. 1969, 7, 1913. (b) Bergbreiter, D. E.; Kabza, K. J. Am. Chem. Soc. 1991, 113, 1447. (c) Bergbreiter, D. E.; Kabza, K. Ind. Eng. Chem. Res. 1995, 34, 2733. (6) Gibson, H. W.; Bailey, F. C. Macromolecules 1980, 13, 34.
S0743-7463(96)00885-2 CCC: $14.00
investigating the mechanism on the sulfonation of polyolefins. Ihata revealed by use of IR, UV, and Raman spectroscopies that sulfonation of polyethylene (PE) is accompanied by the formation of long polyene sequences.7 More recently, detail surface characterization of sulfonated PE has been performed by Idage et al. using X-ray photoelectron spectroscopy (XPS).8 Both of them presented a mechanism on sulfonation of PE involving SO3H radicals as an intermediate. As to the sulfonation of polypropylene (PP), there is no report concerning the characterization and the reaction mechanism to the best of our knowledge. This paper presents a method for assessing quantitatively the hydrophilicity of polypropylene mesh sheets (PPMS) sulfonated on different levels. Also, the sulfonation of polypropylene (PP) was studied experimentally and theoretically in order to elucidate its mechanism. Particular emphasis was placed on clarification of the feature in the surface sulfonation of solid PP. II. Experimental Section Procedures for Sulfonation of PP. PPMSs (ca. 150 µm thick, porosity ∼0.44, average mesh size ∼10 µm) obtained from Nippon Glass Fiber Co. were used for sulfonation without any pretreatment.9 The samples were immersed in liquid H2SO4 (Futaba Pure Chemicals, >95%) kept at a given temperature (90-120 °C) for various times (15-90 min). They were rinsed in four steps by H2SO4 aqueous solutions with volume percents of 70, 50, and 37.5% in this order for 10 min each and finally pure water and dried at 50 °C in a vacuum oven. Diffuse Reflectance Fourier-Transformed Infrared (DRIFT) Spectroscopy. DRIFT spectra of the samples (12 × 12 mm2) were obtained with a FT-IR spectrophotometer (JEOL Model JIR-5500) equipped with a diffuse reflectance attachment. Prior to the measurements, the sample compartment was purged (7) Ihata, J. J. Polym. Sci., Part A: Polym. Chem. 1988, 26, 167. (8) Idage, S. B.; Badrimayanan, S.; Verekar, S. P.; Sivaram, S. Langmuir 1996, 12, 1018. (9) The crystallinity of PP (R-isotactic) was determined by X-ray diffraction measurements to be approximately 51.0% (2θ ) 13.85, 16.65, 18.31, 21.39, 25.28, and 28.24°).
© 1997 American Chemical Society
Selectivity in Surface Sulfination with a dry air for 5 min. KBr powder (Kishida Chemicals, spectroscopic grade, > 99%) was used as a reference. The spectra were recorded in the range of 4000-400 cm-1 at a resolution of 4 cm-1 with 200 coadded scans and computed with triangular apodization. X-ray Photoelectron Spectroscopy (XPS). XPS spectra were measured with a PHI 5600 Ci Physical Electronics X-ray photoelectron spectrometer. Photoelectrons were generated by Al (KR1, 2) X-ray radiation (hν ) 1486.6 eV) at 150 W of power. The unpolarized X-ray photons struck the sample surface with their k-vector (kx) at an angle of 45° with respect to the surface. The electrons were always collected at an angle of 90° relative to kx. The residual gas pressure in the spectrometer chamber during data acquisition was less than 10-9 Torr. The binding energy (BE) scales were referenced by setting the hydrocarbon peak maxima in the C1s spectra to 284.6 eV. The precision of BE with respect to this standard value was within (0.1 eV. Evaluation of Surface Wetting Properties. The rate of liquid H2O penetration was measured using an automated apparatus (Hosokawamicron Penetoanalyzer). PPMS’s (90 × 60 mm2) were hung vertically from one arm of a RH electromicrobalance. A pan containing liquid H2O is raised until the end of PPMS is just immersed in the liquid. The weight gain of PPMS with penetration of liquid H2O was recorded as a function of immersion time. The weight change was measured with an accuracy of (0.1 mg. The kinetics of the H2O penetration was analyzed by a rearranged Washburn’s equation10 described in detail later. Molecular Orbital (MO) Calculations. The present authors used a newer MO model, PM3, which is a modification of that of Dewar et al.’s11 improved version of MNDO termed AM1 to calculate models of polyolefins and sulfonated ones (Figures 5 and 6). The PM3 model employs Stewart's reparameterization12 designed especially to yield greater accuracy in calculating heats of formation of molecules without losing accuracy in molecular geometry and dipole moments. All geometries were optimized within MOPAC ver. 5.013 with the PM3 method. Convergence of the self-consistent field was extended by using the keyword Eigenvalue Following (EF). The force constants between all neighboring atoms were obtained by the PM3 method, being converted to a vibrational spectrum using a keyword FORCE, normal mode analysis, calculation with MOPAC. For details see ref 12.
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Figure 1. XPS core-level photoemission lines of virgin PPMS (a) and PPMS treated with concentrated H2SO4 at 105 °C for 15 min (b).
Surface Characterization. Figure 1 shows XPS corelevel photoemission lines of virgin PPMS (a) and PPMS treated with liquid H2SO4 at 105 °C for 15 min (b). In spectrum a, an intense C1s and two very small peaks of O (1s, 2s) can be observed, indicating that the surface of the virgin sample had slightly been oxidized during the preparation process. In spectrum b, new peaks due to both S (2s, 2p) and N (1s, 2s) appear. Also, the O1s peak markedly intensifies. The BEs of S2p and O1s were determined to be 169.0 and 531.7 eV, respectively, which are in complete agreement with the reported values of SO3H groups attached to PE.8 The S/C ratio of 0.0356 is smaller than that of PE at the same treatment time (15 min) by a factor of 0.37.8 These facts may reflect the tendency of PP to more readily undergo oxidative etching, since the more substituted alkane is generally more reactive.5c Taking the reorganization of SO3H groups into consideration with annealing at 50 °C in vacuum, the surface concentration of S would be greater to some extent just after the treatment.5b The narrow scan XPS analysis of N1s determined the BE to be 403.2 eV. This value is close to a literature one for N2 in the gas phase (402.8 eV) and far part from ones for nitrogen oxide groups including
NO (404.3 eV) and NO2 (406.1 eV).14 In the XPS experiments, the sample box was purged with N2 gas after inserting samples. The N1s signal in Figure 1b can be attributed to N2 molecules physisorbed during this process and/or while they had been allowed to stand for a long time in contact with air. The absence of the N1s signal in Figure 1a suggests an increase in the adsorption ability for N2 with sulfonation probably due to generation of the strong acid groups on the surface. This interesting fact offers a further research subject but is outside the scope of the present study. Figure 2A shows 4000-400 cm-1 region DRIFT spectra of PPMSs before (R) and after treatment with concentrated H2SO4 at 105 °C for various times (a, 15 min; b, 40 min; c, 90 min). After treatment (a-c), there are two perceivable changes: a new peak appears at 582 cm-1 and the background in the 1300-800 cm-1 region is increased. The change is more clarified by 2000-400 cm-1 region difference DRIFT spectra before and after treatment shown in Figure 2B. A number of negative peaks due to the CH3 groups of PP are observed. On the other hand, several positive absorption bands characteristic of SO3H groups appear in the 1250-840 cm-1 region and at 582 cm-1. The band regions are in agreement with those of sulfonated polyethylene (PE) reported by Ihata 7 and Fonseca et al.15 Further, from comparison with the IR bands for a model compound, sodium diocty lsulfosuccinate (SDOSS, see Table 3),16 the absorption peak at 582 cm-1 can be assigned to the deformation of the S-O bond (δ(S-O)). Two small and broad bands centered at 1693 and 1590 cm-1 are due to the stretching vibrations of CdO and CdC bonds, respectively. The sulfonation of PE was revealed to be followed by a significant degree of oxidation and/or the formation of some conjugated CdC double bonds.7 The intensities of all the positive and negative peaks tend to increase with increasing treatment time. PP changed from white to pale yellow upon treating at 90
(10) (a) Washburn, E. W. Phys. Rev. 1921, 17, 374. (b) Szekely, J.; Neumann, A. W.; Chuang, Y. K. J. Colloid. Interface Sci. 1971, 35, 273. (11) Dewar, M. J. S.; Zoebisch, E. G.; Healy, E. F.; Stewart, J. J. P. J. Am. Chem. Soc. 1985, 107, 3902. (12) Stewart, J. J. P. J. Comput. Chem. 1990, 11, 543. (13) MOPAC Version 5.00 (QCPE No. 445), Stewart, J. J. P. QCPE Bull. 1989, 9, 10. Hirano, T. JCPE Newsl. 1989, 1 (2), 36; Revised as Version 5.01 by Toyoda, J. for Apple Macintosh.
(14) (a) Someno, M; Yasumori, I. Hyomen Bunseki; Kodansya: Tokyo, 1976. (b) Handbook of X-ray Photoelectron Spectroscopy, JEOL: Tokyo, 1991. (15) Foeseca, C.; Perena, J. M.; Fatou, J. G.; Bello, A. J. Mater. Sci. 1985, 20, 3283. (16) Urban, M. W. Vibrational Spectroscopy of Molecules and Macromolecules on Surfaces; Wiley-Interface Publication: New York, 1993.
III. Results and Discussion
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Figure 2. (A) DRIFT spectra of PPMSs before (R) and after treatment with concentrated H2SO4 at 105 °C for various times (a, 15 min; b, 40 min; c, 90 min). (B) Difference DRIFT spectra before and after treatment.
°C for 90 min, while PE changed from white to dark brown even by more mild treatment with fuming H2SO4 at room temperature for 10 min. This suggests that conjugated CdC bonds responsible for the coloration are quite difficult to yield in sulfonation of PP. It is clear from these results that the sulfonation of PP accompanied by only slight oxidation and the formation of CdC bonds takes place with hot concentrated H2SO4 treatment. Wetting Properties. Figure 3A shows time-courses of liquid H2O penetration into PPMSs treated with H2SO4 at 90 °C for various times (R, virgin; b, 40 min; c, 60 min; d, 90 min). The PPMS without sulfonation only absorbs a small amount of H2O because of large hydrophobicity resulting from the surface CH3 groups. The rate of H2O penetration is remarkably increased with an increase in the extent of sulfonation. As previously reported, useful information on the surface wetting property can be obtained by analyzing the kinetic data on the H2O penetration into PPMS.17 Rearranging Washburn’s equation,10 one can write
2 ln WL ) ln t + {ln cos θ + ln(S)2r + ln(FL2γL/2ηL)} (1) where WL is the uptake weight of H2O; θ is the H2O contact angle of the PP flat plate; S, , and r are the sectional area (∼0.09 cm2), porosity (∼0.44), and pore radius (∼10 µm) of PPMS; FL, γL, and ηL are the density (1 g cm-3), surface tension (72.77 dyn cm-1), and viscosity (1.02 dyn s cm-2) of H2O at 25 °C. Figure 3B shows plots of log WL vs log t for PPMSs treated with H2SO4 at 90 °C for different time (a, 15 min; b, 40 min; c, 60 min; d, 90 min). Each plot provides a straight line having a slope of 1/2 in the time range below (17) Tada, H.; Shimoda, K. J. Electrochem. Soc. 1997, 144, 273.
ca. 2 min. In a more strict model, the diffusion of H2O molecules into PP could be taken into account together with the penetration of liquid H2O into the pores of PPMS. However, it is thought that the contribution of the former to the uptake weight can be neglected due to the hydrophobicity of the interior of the polymer. The intercepts for samples R, a, b, c, and d are determined by extrapolating the straight lines until log t ) 0 to be -3.50, -3.31, -2.88, -2.69, and -2.45, respectively. As the sulfonation of PPMS proceeds, the intercept becomes greater. Since scanning electron microscopy indicated no change in the topology of PPMS before and after treatment, the second and third terms of the intercept can be assumed to be constant in eq 1. Consequently, the increase in the intercept is ascribable to the change of the first term, which increases when θ is decreased (0 < θ < 90 °). The contact angles for samples R, a, b, c, and d can be estimated from eq 1 using the intercepts and the other constants included in the second and third terms to be 81, 76, 58, 39, and 0°, respectively. A fair lower value for sample R compared to the literature one (94°) seems to be due to slight surface oxidation at the initial state (Figures 1a and 2A-R). Similarly, Bergbreiter et al. observed lower contact angles of H2O with increasing treatment time in the sulfonation of PE films.5b These results draw a conclusion that the intercept is a quantitative indicator for the hydrophilicity of the PPMS surface. Figure 3C shows time-dependences of the intercepts for the samples prepared by different reaction temperatures (a, 90 °C; b, 100 °C; c, 105 °C). The intercepts are strongly dependent on the treatment temperature as well as on the time of sulfonation. Irrespective of the sulfonation temperature, the intercept monotonically increases with increasing treatment time, reaching a plateau of ca. -2.4 above 90 min. Also, when the reaction
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Figure 4. Intercepts of log WL vs log t plots and the absorbances at 582 cm-1 (δ(S-O)) + ν(S-C)) in the DRIFT spectra as a function of treatment time. The inset shows the relation between the intercept and the absorbance. Table 1. Net Atomic Charges SO3 molecule S O1 O2 O3
2.4002 -0.8001 -0.8001 -0.8001
model of PP (model B) C1 C2 C3 C4 C5 C6 C7 C8 C9 C10 C11 C12 C13
-0.1132 -0.0737 -0.1096 -0.0755 -0.1135 -0.0761 -0.1100 -0.0738 -0.1130 -0.1241 -0.1345 -0.1341 -0.1242
classificationa T M M M M M M M T S S S S
model of PE (model E) C1 C2 C3 C4 C5 C6 C7 C8 C9
-0.1093 -0.0979 -0.1031 -0.1018 -0.1019 -0.1019 -0.1031 -0.0977 -0.1095
a Symbols of T, M, and S denote the terminal, main chain, and side chain carbon atoms.
Figure 3. (A) Weight gains (WL) with liquid H2O penetration into PPMSs treated by H2SO4 at 90 °C for various times (R, virgin; b, 40 min; c, 60 min; d, 90 min). (B) Plots of log WL vs log t for PPMSs treated with H2SO4 at 90 °C for different times (a, 15 min; b, 40 min; c, 60 min; d, 90 min). (C) Time dependences of the intercepts for the samples prepared by different temperatures (a, 90 °C; b, 100 °C; c, 105 °C).
temperature is raised from 90 to 105 °C, the increasing rate of hydrophilicity significantly augments. Figure 4 shows intercepts of log WL vs log t plots and absorbances at 582 cm-1 (δ(S-O)) in the DRIFT spectra as a function of reaction time. Apparently, as the reaction temperature increases, the intercept increases with a quite similar profile of the absorbance. The inset in Figure 4
also demonstrates a linear relationship between the intercept and the absorbance. This finding suggests that the introduction of very polar SO3H groups attaching onto the surface of PP is mainly responsible for the increase in the hydrophilicity, leading to the increase of the intercept.18 Mechanism on the Sulfonation of PP. The structure of SO3 is known to be planar trigonal having equal S-O bond length of 0.143 nm as shown in Figure 5A.19 The degree of crystallinity of PP can be estimated from the intensity ratio of the IR band at 998 cm-1 to that at 971 cm-1.20 The invariant ratio (0.96) determined from the data in Figure 2A may indicate that the sulfonation occurs mainly in the interlamella amorphous region of PP with a larger free volume,5c because the sulfonation should be accompanied by a large conformational change in PP chains as described below. The same conclusion was drawn in the sulfonation of PE with fuming H2SO4 from constancy of the melting point and the heat of fusion before and after the treatment.15 We assumed that models for PP molecules in the gas phase approximate the molecules on the surface of the amorphous region where the intermolecular interaction is relatively small. (18) It is known that immobilization of SO3H groups and salts onto surfaces of PE produces surfaces that do not thermally reorganize to a more hydrophobic state unless they have been modified so as to contain long-chain hydrocarbon groups (>C6). For detail see ref 5b. (19) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry, 2nd ed.; John Wiley & Sons, Inc.: New York, 1972. (20) (a) Luongo, J. P. J. Appl. Polym. Sci. 1960, 3, 302. (b) Danusso, F.; Moraglio, G.; Natta, G. Ind. Plast. Mod. 1958, 40.
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Figure 5. (A) The structure of SO3 cited from ref 16. The PM3 optimized structures of models for PP (B, model B) and sulfonated PP (C, model C1; D, model C2; E, model C3).
Parts B and C of Figure 5 exhibit the PM3-optimized structures of the models for PP (C13H28, model B) and sulfonated PP (C13H28SO3). Three possible sites for sulfonation in PP, i.e., the side CH3 group (model C1) and the two kinds of C atom in the main chain (-CH3(CH3)-,
model C2; -CH2-, model C3) are taken into account. Since Burkhardt showed that n-hexane is sulfonated by fuming H2SO4, the reactivity of PP can be thought to be predicted by employing a model with a short chain.21 Further, a low concentration of the surface SO3H groups (∼4%) gives
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Table 2. Energy Comparison for PP and PE Models Heat of Formation/kJ (unit mol)-1 SO3
model Ba
model C1a
model C2a
model C3a
-53.7
-79.3
-196.5
-189.8
-188.8
Heat of Reaction/kJ (unit
mol)-1
path 1
path 2
path 3
-63.5
-56.8
-55.8
a
The value calculated for the model was multiplied by 1/4 in order to transform the value per unit mol.
validity of neglecting the influence of the SO3H groups already attached to PP molecules on the reactivity. For comparison, the calculations were also undergone for models of PE (C9H20, model D) and sulfonated PE (C9H20SO3, model E). The PM3-optimized structures are shown in Figure 6A (model D) and Figure 6B (model E), respectively. The net atomic charges calculated by the PM3-MO method for SO3 and model B are listed together with the data of model D in Table 1. A large positive charge of 2.4 on the S atom compensated by negative charge on the O atoms (-0.8 × 3) indicates that the S atom acts as an electrophilic center in the sulfonation of PP. On the other hand, all the C atoms have negative charges ranging from -0.067 to -0.153. It has been well established that the sulfonation of aromatic compounds with hot concentrated H2SO4 occurs via electrophilic addition of SO3.22 Then the sulfonation of PP can be presumed to proceed via electrophilic addition of SO3 in a similar manner as aromatic compounds. A radical mechanism involving SO3H was presented for the sulfonation of PE with gaseous SO3 to account for the significant formation of conjugated CdC bonds.7,8,23 However, since CdC bonds are hardly generated in the present PP system, the possibility of the radical mechanism seems to be small, even if it acts as a concurrent minor one. A probable reaction scheme is expressed by eqs 2-5, where the minor pathways of oxidation and the formation of CdC bonds are omitted. Noticeably, the average charge on the C atoms of the side CH3 groups (-0.124) is greater than that on the dCH2 unit (-0.111) and the dCH(CH3) unit (-0.075), which suggests that the side CH3 groups are more subject to sulfonation than the C atoms of the main chain. In the case of PE, an intermediate negative charge (-0.104 ( 0.006) is uniformly distributed over the C atoms (model D).
Table 2 summarizes heats of formation calculated for SO3 and models B, C1, and C2. The smaller heat of formation in model C1 than of model C2 by 6.7 kJ (unit (21) Sperling, R. J. Chem. Soc. 1949, 1932. (22) Morrison, R. T.; Boyd, R. N. Organic Chemistry, 2nd ed.; Allyn and Bacon, Inc.: Boston, MA, 1966. (23) A referee kindly suggested another possible reaction scheme involving formation of an intermediate alkene followed by SO3 electrophilic substitution and inter- or intramolecular hydride transfer to the carbocation.
Figure 6. PM3-optimized structures of models for PE (A, model D) and sulfonated PE (B, model E).
mol)-1 and model C3 by 7.7 kJ (unit mol)-1 indicates that C1 is much more stable than C2 and C3. A detailed energy analysis attributes this to the greater core-core repulsion energy of model C2 by 688.3 eV and model C3 by 721.07 eV, respectively, related to model C1. This also greatly affects the two-centered bond energies of C-S, which are -19.08 eV for model C1 (C13-S10 bond), -15.93 eV for model C2 (C6-S14 bond), and -17.02 eV for model C3 (C5S14 bond), respectively. As the former value is near to that of model E (-17.58 eV), the great core-core repulsion in model C2 is ascribable to the interaction between the SO3H group and the adjacent side CH3 groups. The heat of reaction (∆E) can be calculated from the difference in the sum of heats of formation between the reactant system and the product system (∆E ) E(model C) - E(SO3) E(model B)). A linear free energy relation, which is known to provide an empirical relationship between the standard free energy change and the activation free energy change in many reaction systems, also suggests the preference of path 1 (∆E ) -63.5 kJ (unit mol)-1) over both path 2 (∆E ) -56.8 kJ (unit mol)-1 and path 3 (∆E ) -55.8 kJ (unit mol)-1). The discussion above is confirmed visually in Figure 5 that the main chain of model C2 is highly bent by the great repulsion between the SO3H and adjacent CH3 groups, while the conformational change of model C1 from model B is considerably smaller, which is also true for model E (Figure 6). The conformational change of polymers in a solid state is generally more difficult than in a solution due to significant intermolecular interaction. It was observed in the sulfonation of solid PE that the reaction is more pronounced with a decrease in its molecular weight, i.e., as the intermolecular interaction weakens.15 Consequently, paths 2 and 3 accompanied by the remarkable conformational change should have a much greater activation energy compared to path 1, which results in the selectivity of the reaction site in the sulfonation of PP. The selectivity can be termed “the conformational change restricted selectivity (CCRS) in the solid polymer surface reaction”. This CCRS reasonably explains why a significant amount of conjugated CdC bonds, detected in the sulfonation of PE, are not obtained in the present PP system. Temperature Dependence of the Selectivity. Figure 7A shows the theoretical IR spectrum obtained from the PM3-MO calculations for models D and E. In the
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Table 3. Analyses of Vibration Spectra of Sulfonated PP and PE exptl vibrational frequency/cm-1 this study
sulfonatedTPE
theor vibrational frequency/cm-1 SDOSSc
model C1
model C2
1216 1050 652
3917 3054 2916 2852 882 + 920 662 200 + 373
3900 3048 2924 2845 900 667 484
3540 2967 2847 1250-1160 1160-840 613 + 582 a
1215a 1050a 700-600b
model E
assign.
898 710 448
ν (SO-H) νas (CH3) νas (CH2) νs (CH) νas (S-O) νs (S-O) δ (S-O) + ν(S-C)
Data were cited from ref 7. b Data were cited from ref 14. c Data were cited ref 15.
Figure 7. Theoretical IR transmission spectra of models for PE (A, model D) sulfonated PE (A, model E) and models for sulfonated PP (B, model C1 and C2).
spectrum of model E, there appear three strong absorption peaks due to the vibrations of the bonds including the S atom at 898 cm-1 (νas(S-O)), 710 cm-1 (νs(S-O)), and 448 cm-1 (δ(S-O)). The spectrum pattern and the relative positions of the three bands are in agreement with those of the experimental spectra for the sulfonated PE and SDOSS; however, the peak positions are ca. 300 cm-1 shifted toward lower wavenumber. Clearly, the PM3MO calculation is effective in the spectral analyses of the sulfonated polymers. There are three primary vibrational bands in Figure 2B, which situate at 1250-1160 cm-1 (B1), 1160-840 cm-1 (B2), and 582 cm-1(B3). In order to analyze the experimental difference DRIFT spectra in Figure 2B, it is important to take care of several seeming peaks due to superposition of the CH3- and CH-negative peaks. Figure 7B shows theoretical transmission IR spectra for models C1 and C2, which are expected to be the most probable ones from the linear energy relationship consideration. Although the ca. 200 cm-1 shifts of all the peak positions toward lower wavenumbers with respect to the corresponding experimental ones are observed, the experimental spectrum pattern is well reproduced. The absorption bands of B1, B2 and B3 in Figure 2B can be assigned to νas(S-O), νs(S-O) and δ(S-O) + ν(S-C) on the basis of the results of the theoretical analyses, respectively. It is noteworthy that the presence of nitrogen on the surface, indicated by the XPS spectrum (Figure 1b), does not make these assignments ambiguous, because it exists as N2 physisorbed (ν ∼ 2360 cm-1).24 In Table 3, it should be noted that there is an as much as 111 cm-1 difference in the δ(S-O) + ν(S-C) peak positions between models C1 and C2, whereas the other peaks due to the vibration modes of the bonds involving S atom are in agreement in position within the range of 20 cm-1. The steric crowd in model C2 may raise up the curvature in (24) Nakamoto, K. Infrared and Raman Spectra of Inorganic and Coordination Compounds, 4th ed.; John Wiley & Sons, Inc.: New York, 1986.
Figure 8. DRIFT spectra of PPMSs treated with H2SO4 at 90 °C (a), 105 °C (b), and 120 °C (c) for 15 min.
its potential energy curve as compared to that of model C1, leading to a shift of the δ(S-O) + ν(S-C) peak toward higher wavenumber. Figure 8 shows DRIFT spectra of PPMSs treated with H2SO4 at 90 °C (a), 105 °C (b), and 120 °C (c) for 15 min. In spectrum a, a broad singlet peak (P1) assigned to δ(SO) + ν(S-C) is observed at 582 cm-1. When the treatment temperature is raised up above 105 °C, a shoulder (P2) appears at 613 cm-1, intensifying with an increase in the reaction temperature as shown in spectra b and c. From comparison with the theoretical results, the two peaks of P1 and P2 are considered as corresponding to the δ(S-O) + ν(S-C) signals for models C1 and C2, respectively. The lower wavenumber and the broader half-width of the δ(S-O) signal for model C1 further support the assignment. Then the selectivity of the reaction site is determined from the intensity ratio of P1/(P1 + P2). Finally, one can obtain a conclusion that the side CH3 groups is predominantly sulfonated at 90 °C, while the selectivity is gradually decreased with increasing reaction temperature. Above 105 °C, a large activation energy resulting from drastic conformational change in path 2 would be overcome by thermal energy. This selectivity may be associated with the mechanical strength of sulfonated PPMSs. The twocentered bond energies (Eb) of C-C in the main chain closest to the SO3H group for models C1, C2, and C3 are -13.41 ({Eb(C5-C6) + Eb(C6-C7)}/2), -13.08 eV ({Eb(C5C6) + Eb(C6-C7)}/2), and -13.03 eV ({Eb(C4-C5) + Eb(C5C6)}/2), respectively. It can be expected from this that the mechanical strength is intrinsically improved when the side CH3 groups are selectively sulfonated. The tensile strength of the PPMS (30 × 60 mm2) sulfonated at 120 °C was remarkably decreased with increasing treatment time from 0.028 kg (t ) 0) to 0.015 kg (t ) 90 min), while it is almost constant for treatments at both 90 and 105 °C (data not shown). This fact provides evidence for the correlation between the mechanical strength of sulfonated PPMS and the selectivity of sulfonation.
Selectivity in Surface Sulfination
Conclusions A quantitative method for evaluating the hydrophilicity of sulfonated PPMS using a rearranged Washburn’s equation was presented to provide a positive linear correlation between the hydrophilicity and the degree of the sulfonation. The PM3-MO calculations for models of sulfonated PP suggest that the sulfonation proceeds via electrophilic addition of SO3 to the side CH3 group (path 1) rather than the main chain of PP (paths 2 and 3). This selectivity due to a remarkable conformational change in paths 2 and 3 was termed “the conformational change restricted selectivity (CCRS)”. By comparison with the theoretical and experimental IR spectra of sulfonated PP, an absorption peak around 600 cm-1 could be assigned to the δ(S-O) + ν(S-C). It was also revealed that the peak is split into a doublet with an increase in the reaction
Langmuir, Vol. 13, No. 15, 1997 3989
temperature, which is direct evidence for the decrease in the CCRS of the PP sulfonation. Since great intermolecular interaction resists the conformational change of the polymer in a solid state, the CCRS would be general in the reaction of solid polymer surfaces. Practically, the CCRS in the sulfonation of PP is of great significance with respect to its mechanical strength and durability. Also, the analysis of the kinetics of H2O penetration presents a general method for evaluating quantitatively the hydrophilicity of liquid absorbers. Investigation of a novel method for sulfonation capable of increasing CCRS is currently in progress in our laboratory. Acknowledgment. The authors express their sincere gratitude to A. Hattori (Nippon Sheet Glass TechnoResearch Co. Ltd.) for XPS measurements. LA960885L
