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Sulfonation of solid polystyrene using gaseous sulfur trioxide.

INTRODUCTION

There is an increasing number of applications for sulfonated polystyrene (SPS) in separation membranes, fuel cells and sensors described in literature (1-7). These high-tech applications require well defined uniformly sulfonated product, hence, homogeneous phase sulfonation reactions are generally used. On the other, there is a large volume of waste solid PS available from both food packaging and construction insulation. Although, only a limited extent of literature has been published dealing with the preparation of SPS via sulfonation in a heterogeneous phase, this method is assumed advantageous for industrial application over the homogeneous process, because it avoids problems with solvents and the separation of sulfonated product from the reaction mixture. The heterogeneous sulfonation reaction, i.e., solid-liquid (s-1) such as PS-[H.sub.2][SO.sub.4] or solid-gas (s-g) such as PS-[SO.sub.3], could be used in commercial process to produce cheap sulfonated ionomers out of waste PS using low cost gaseous [SO.sub.3]. Unlike many methods published so far, heterogeneous sulfonation of waste PS can result in a wide range of application of such as low density concrete, macro defect free cement, cheap PS ionomers, etc. (8-10).

Kim et al. (11) described a heterogeneous s-g sulfonation method based on reaction of gaseous [SO.sub.3] above fuming sulfuric acid with a monodisperse cross-linked PS beads. In this work, PS beads 450 nm in diameter were sulfonated in the gaseous phase for 3 days at room temperature to obtain yellowish sulfonated product. The sulfonated PS beads were used for preparation of a conductive polymer by adding poly(xylidene tetrathiophenium chloride), [AsF.sub.5] and poly(tetrafluoroethylene).

Regas (12) sulfonated polystyrene-divinylbenzene networks ([M.sub.w] from 1000 to 50 000) using an s-1 reaction of cross-linked polystyrene-divinylbenzene beads swollen in dichloroethane with concentrated [H.sub.2][SO.sub.4] at 80[degrees]C for about 10 min. Increased reactivity was obtained, when the [ClSO.sub.3]H sulfonating agent was used instead of pure [H.sub.2][SO.sub.4]. Prepared products were used for ion-exchange capacity measurements.

Carrol et al. (13] developed an s-l heterogeneous sulfonation of a finely divided powder of PS with [M.sub.w] = 240 000. The PS powder was prepared by precipitation of a diluted PS solution in methyl-ethyl ketone with methanol. Sulfonation of this PS powder was carried out using 100% [H.sub.2][SO.sub.4] as the sulfonating agent in the presence of [Ag.sub.2][SO.sub.4] as a catalyst at room temperature for 10 min. The [Ag.sub.2][SO.sub.4] prevents the formation of side products in this reaction. The 100% yield of the sulfonation reaction was achieved within 5-15 min. A gelatinous colorless mass was obtained and no change after 24 h or further reaction was observed. Aggregation of the polymer chains was caused by an electrostatic interaction between the sulfonated chains and, in addition, a chemical crosslinking was observed due to side reaction accompanying the sulfonation process (14).

In this work, kinetic study of the s-g heterogeneous sulfonation process is presented using solid PS beads and gaseous [SO.sub.3]. Effect of the type and size of the solid PS particles on the reaction kinetics and sulfonation yield has also been investigated. The rate of diffusion of [SO.sub.3] through a barrier of sulfonated product formed on the particle surface was studied using scanning electron microscopy (SEM) and electron diffraction scattering spectroscopy (EDS). The experimental data were fitted using Johanson-Mehl-Avrami-Jerofyeev-Kolgomorov's equation to obtain an overall rate constant of heterogeneous sulfonation on solid PS surface.

EXPERIMENTAL

Materials

Spherical reactor PS beads ~0.5 mm in diameter, produced under the trade name Krasten 127/9001 (Kaucuk. a.s., Czech Republic) with an viscosity average molecular weight [M.sub.[eta]] = 180,000 g [mol.sup.-1], were used as the model substrate. Further. PS pellets were produced by extrusion of spherical solid reactor PS beads formed in suspension polymerization of styrene used in industrial synthesis of PS. The cylindrical pellets were ~1 mm in diameter and 4 mm in length. These pellets were freeze-milled to produce powder of required size distribution.

Oleum (Synthesia Semtin, CZ) of 60% concentration was use to produce the [SO.sub.3], which was employed in a gaseous state diluted with nitrogen as inert gas. The gas mixture of [SO.sub.3] and inert gas was prepared by blowing the dried inert gas through the solution of oleum in a bottom glass flask.

Heterogeneous Sulfonation Reaction

The source of gaseous mixture of [SO.sub.3] with nitrogen was 60% oleum. Nitrogen from steel container was dried with [P.sub.4][O.sub.10] in a glass tube and blown using flow rate ~170 [cm.sup.3] [min.sup.-1] through the oleum in a bottom glass flask. The gaseous mixture of ([SO.sub.3] + [N.sub.2]) was transported into sulfonation reactor utilizing PTFE hose. The sulfonation reactor was constructed from the glass bottom flask (2 [dm.sup.3]) equipped with PTFE-glass stirrer and with input/output valves for sulfonating agent. Sampling of SPS during the reaction was accomplished through the sulfonating agent outlet using long scoop. As a result of sulfonation. the color of SPS material changed to yellow or lightly brown from originally white color. The nonchemically bonded [SO.sub.3] was removed by washing samples with distilled water. Significant water swelling of samples sulfonated to a greater extent was observed. Finally, washed reaction products were filtered under reduced pressure and vacuum oven dried at temperature below 50[degrees]C.

Scanning Electron Microscopy, Electron Diffraction Scattering

Particles of the heterogeneously SPS were coated in the vacuum-fussing box (Polaron 5000, UK) with a thin gold layer for about 180 s. The coated samples were inserted into matrix block from Resin 3 (Struers, NL) with boreholes of 3 mm diameter and embedded in two-component epoxy resin. After curing the epoxy resin, the surface of matrix block with embedded SPS particles was polished with sand papers until No. 250 and than wet polished using water suspension of black diamond powder with particle size of 1[micro]m. Finally, the glossy surface of matrix with SPS beads was vacuum-covered in JEE-4B (produced by Noel) with very thin conductive carbon layer.

The structure and chemical composition of sulfonated layers on the solid PS particles were studied using XL 30 SEM (FE1. Czech Republic) with EDS detector from EDAX. Photographs of sulfonated layers were taken in the back scattered electrons mode (BSE) at acceleration voltage of 20 kV. The same accelerating voltage was used for elemental analysis of sulfonated layers on the PS surface. The layers were analyzed across the layer thickness with maximum 10 of measuring point with a distance between sampling points from 1 to 3 [micro]m. Spectra of elemental analysis were acquired for 50 s in each of the sampling points.

RESULTS AND DISCUSSION

Elemental analyses of sulfonated layers on the PS particles were obtained using the EDS measurements of samples sulfonated at three different temperatures. The depth profile of the content of sulfur, oxygen and counter ion (sodium) was determined using EDAX detector. The results of elemental analysis were expressed as stoichiometric coefficients related to one aromatic ring of oneconstitutional unit of PS chain. General stoichiometric formula of SPS layer can be written as--[CH.sub.2]CH([C.sub.6][H.sub.4][S.sub.i][O.sub.j][M.sub.k])--, where i, j, k are the experimentally determined stoichiometric coefficients and M is sulfonic group counter ion, [Na.sup.+] in this case. The structure of the attached sulfonic group was analyzed using elemental analysis result and composition of SPS was proposed as--[CH.sub.2]CH([C.sub.6][H.sub.4][S.sub.1][O.sub.x][M.sub.y]), where x, y are the recalculated stoichiometric coefficients j and k for normalized value of i = 1. The respective values of the coefficients x, y were found for all the layers and all temperatures investigated: for oxygen x = <1.5; 5.1> and for sodium y = <0.6; 1.4> Determined stoichiometric coefficients values are varying around theoretical values x = 3, y = 1 for the case when one sulfonic group was attached to one benzene ring of PS and confirming the hypothesis, that it was mainly one sulfonic group ~[SO.sub.3]M attached to one aromatic ring along the polymer backbone.

The experimentally determined values of stoichiometric coefficients of sulfur related to one aromatic ring of polymeric backbone ([S.sub.x]), which is equal to the sulfonation degree expressed in mole %, obtained by means of elemental analysis of the sulfonated layers were summarized in Table 1. The yield of sulfonation increase in the initial stage of reaction and after about 6 h varying around its maximal value. Apparent decrease of sulfonation yield is probably caused by growing of standard deviation of EDS elemental analysis with increasing time and temperature, which is affected by complex morphology of samples with high sulfonated layers. The results of this measurement have clearly shown that maximum one ~[SO.sub.3]H group was attached to one aromatic ring of SPS.
TABLE 1. The average sulfonation degree [[bar.S].sub.x] in mole
% of ~[SO.sub.3]H and standard deviation related to one aromatic ring
obtained by means of elemental analysis of sulfonated PS pellets
prepared at temperature 5[degrees]C. 22[degrees]C, and 50[degrees]C.

                                Reaction time (min)

                    5         10           30          60        90

[[bar.S].sub.x]                                       0.14      0.14
                                                    [+ or -]  [+ or -]
                                                      0.01      0.01

[mole %]           0.20      0.30    0.43 [+ or -]    0.36      0.41
                 [+ or -]  [+ or -]      0.03       [+ or -]  [+ or -]
                   0.01      0.01                     0.08      0.12

                   0.26      0.46        0.65         0.57      0.58
                 [+ or -]  [+ or -]  [+ or -]0.12   [+ or -]  [+ or -]
                   0.04      0.17                     0.08      0.13

                                Reaction time (min)

                    180       360       720      1440    Temperature
                                                         ([degrees]C)

[[bar.S].sub.x]    0.24      0.36      0.45      0.57         -5
                 [+ or -]  [+ or -]  [+ or -]  [+ or -]
                   0.03      0.01      0.01      0.07

[mole %]           0.43      0.50      0.57      0.48         22
                 [+ or -]  [+ or -]  [+ or -]  [+ or -]
                   0.05      0.08      0.06      0.06

                   0.72      1.01      0.74      1.24         50
                 [+ or -]  [+ or -]  [+ or -]  [+ or -]
                   0.11      0.20      0.20      0.25


As shown previously (15), the mechanism of sulfonation reaction in heterogeneous phase did not differ significantly from that in the homogeneous phase. As in many heterogeneous reactions involving a solid substrate, the diffusion of gaseous [SO.sub.3] into the solid PS substrate was most probably the main factor affecting the kinetics and yield of s-g heterogeneous sulfonation reaction. The rate of diffusion can be influenced by several parameters such as [SO.sub.3] concentration and pressure, temperature, specific surface area and density of PS particles and molecular weight of the PS.

Dependence of sulfonation yield on the specific surface area of the particles used was investigated for several types of commercial grades of PS. The formation of partially or completely water-soluble SPS was expected in the case of PS particles with the largest surface area (smallest particles size). The amount of [SO.sub.3] absorbed in PS particles was dependent on the specific surface area of PS particles and on the reaction time. The influences of concentration of the [SO.sub.3] in the mixture with [N.sub.2] seemed to be less important for the degree of sulfonation. On the other hand, an increase in concentration of [SO.sub.3] over a certain level caused a cross-linking reaction which resulted in formation of insoluble sulfones rather than sulfonated PS. Increasing reaction temperature also resulted in an increasing yield of cross-linked insoluble sulfones.

The complete series of SEM photographs of SPS particles sulfonated at -5[degrees]C, 22[degrees]C, and 50[degrees]C were taken in order to investigate depth of the s-g sulfonation under the conditions used. Visual observations of the changes of PS morphology using SEM micrographs and quantitative EDS microanalysis of the sulfonated PS pellets were performed. The morphological investigations of the SPS particles showed that PS changes its molecules structure and, thus, the contrast upon sulfonation resulting in a formation of a distinct layer of sulfonated PS visible on the SEM of the cross section of sulfonated particles (see Fig. 1).

[FIGURE 1 OMITTED]

The data on thickness of the sulfonated layer obtained from SEM are summarized in Table 2. It seemed that for reaction temperature above 20[degrees]C. the sulfonation rate was almost independent of reaction temperature. On the other hand, low temperature sulfonations were sensitive to reaction temperature providing measurable yield after more than 60 min, whereas at 20[degrees]C, measurable yield was observed after less than 5 min. Analysis of the effect of reaction conditions on the average layer thickness and chemical composition of sulfonated layers on the PS pellets surface were used to study reaction kinetics and mechanism of heterogeneous sulfonation.
TABLE 2. The thickness of sulfonated layer on PS pellets determined
from SEM measurement.

                      Average thickness of sulfonated layer ([mu]m) at
                               given reaction temperature

Reaction time (min)  -5[degrees]C  22[degrees]C  50[drgrees]C

5                        --           -0.50          0.66
10                       --           -1.88          2.28
30                       --           -3.25          5.99
60                      0.59          10.00          9.42
90                      0.75          11.22         14.90
180                     0.85          15.80         18.52
360                     1.30          22.80         25.98
540                      --          -25.28         32.67
720                     5.09          30.38         32.30
1440                    7.51          43.60         49.35


Reaction rate, reaction order and activation energy of sulfonation, diffusion coefficient and activation energy of diffusion were investigated. The relationship between the average sulfonated layer thickness and the square of time at different reaction temperatures was plotted in Fig. 2. Diffusion coefficients and the activation energy of diffusion of [SO.sub.3] into partially sulfonated PS were computed using the basic of Arrhenius type equation, assuming simplified Fick type of diffusion. This simplification did not take into consideration of the fact that the diffused PS continuum was undergoing a sulfonation reaction with the diffusing [SO.sub.3] changing its polarity and structure (SPS). Attempts were made to account for the diffusion with chemical reaction, however, models of diffusion with chemical reaction available in literature do not provide data reduction scheme yielding substantially better description of the observed phenomena compared to the simple model used. Moreover more rigorous kinetic analysis of the data will be outside the scope of this journal.

[FIGURE 2 OMITTED]

The increase of sulfonated layer thickness on the solid PS surface was described using Eq. 1:

D = [(D*T).sup.[1/2]], (1)

where D is the diffusion coefficient in [[mu]m [min.sup.1] and d is the sulfonated layer thickness in [[mu]m].

The calculated diffusion coefficients of [SO.sub.3] into partially sulfonated PS at -5[degrees]C, 22[degrees]C, and 50[degrees]C were summarized in Table 3. The values of the diffusion coefficients of [SO.sub.3] into the PS through the sulfonated layer were substantially lower than the diffusion coefficient of gaseous [N.sub.2] into PS (16) or with diffusion coefficient of water into PS (17). The larger size of the [SO.sub.3] molecule causes most probably the lower diffusion coefficients for system [SO.sub.3]/PS in comparison to [N.sub.2]/PS or [H.sub.2]O/PS. In addition, due to the sulfonation reaction, the polarity of PS changed with time and, thus, the repulsion interaction between sulfonated aromatic ring of SPS and molecule of [SO.sub.3] could also contribute to the reduced diffusion rate of [SO.sub.3] into SPS. In contrary to the temperature dependence of diffusion described generally by Clausius-Clapeyron equation (16), the diffusion coefficients for [SO.sub.3] diffusing into SPS grew with increasing temperature mostly because of the further chemical reaction of [SO.sub.3] with PS.
TABLE 3. The diffusion coefficients D of gaseous [SO.sub.3] into the
solid PS particles determined for three reaction temperatures.

T ([degrees]C)  D (X - [10.sup.10] [cm.sup.2] [S.sup.-1])

     -5                             0.07
     22                             2.18
     50                             2.78


The activation energy of diffusion was evaluated by means of Arrhenius' Eq. 2 (18), (19):

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII], (2)

where A is the pre-exponential factor in [[cm.sup.2] [s.sup.-1]], [E.sub.aD] is the activation energy of diffusion in [kJ [mol.sup.-1]]. For graphic processing of the kinetic data, Eq. 2 was converted into its logarithmic form showed in Eq. 3.

In D = In A - [[E.sub.aD]/[RT]], (3)

The activation energy of diffusion was estimated from experimental data plotted in Fig. 3. The value of activation energy of diffusion, [E.sub.aD] = 49.13 kJ [mol.sup.-1], and the pre-exponential factor. A = 4.12 X [10.sup.-6] [cm.sup.2] [S.sup.-1] were calculated using Eq. 3.

[FIGURE 3 OMITTED]

In the processing of experimental data and analysis of the kinetics of the heterogeneous sulfonation of PS, the general Johanson-Mehl-Avrami-Jerofyeev-Kolgomorov's (JMAJK) Equation was used (20), (21):

-In (1 - [alpha]) = k * [t.sup.m], (4)

where [alpha] is the conversion of PS in heterogeneous reaction defined as the ratio of the molar concentration of SPS to the sum of molar concentration of sulfonated and unreacted polystyrene, [kappa] is the overall rate constant of the heterogeneous sulfonation reaction in [S.sup.-1] and m is the exponent factor. For the cylindrical PS pellets the conversion to SPS was expressed as volume ratio of sulfonated and unreacted PS part using Eq. 5:

[alpha] = [1 - [([r.sub.0] - d).sup.2] * ([[upsilon].sub.0] - 2d)]/[[r.sub.0.sup.2] * [[upsilon].sub.0]] (5)

where [r.sub.0] is the average radius of starting cylindrical particles in [mm], [v.sub.0] is the average height of starting cylindrical particles in [mm]. d is the average sulfonated layer thickness in [mm].

The logarithmic form of Eq. 4 was used for the kinetics data processing in the following form:

ln (-ln(1 - [alpha])) = ln k + m * ln t (6)

The linear relationship described by Eq. 6 was plotted in Fig. 4. The resulting overall rate constants and the exponent factors of the heterogeneous sulfonation reaction at -5[degrees]C, 22[degrees]C, 50[degrees]C, calculated from the slopes of the curves, were presented in Table 4.

[FIGURE 4 OMITTED]
TABLE 4. The overall rate constants and the exponent factors of
heterogeneous sulfonation reaction calculated from JMAJK equation at
temperature 5[degrees]C, 22[degrees]C, 50[degrees]C.

T ([degrees]C)      k [[S.sup.-1]]       m

     -5           1.97 X [10.sup.-6]    0.84
     22           5.50 X [10.sup.-5]    0.74
     50           9.04 x [10.sup.-5]    0.70


The values of exponent m ranging from 0.70 to 0.84 were obtained and compared to the theoretical values 0.53-0.58 for diffusion controlling process (20). We believe that the diffusion of [SO.sub.3] across a polar barrier of SPS to the reaction interface on the surface of bulk PS, at which the sulfonation reaction was proceeding relatively fast, was the controlling step of the heterogeneous s--g sulfonation under experimental conditions used in this investigation. The slightly greater exponent m compared to the theory was most probably caused by the incomplete sulfonation yield in the sulfonated layer. The overall rate constant [kappa] of heterogeneous sulfonation on solid PS surface was slightly increased with increasing temperature. The activation energy of sulfonation of PS was evaluated by means of Arrhenius type Eq. 7:

ln k = ln A - [[E.sub.a]/[RT]], (7)

where A is the pre-exponential factor in [s.sup.-1], [E.sub.a] is the activation energy of sulfonation reaction in [kJ [mol.sup.-1]]. In Fig. 5, the linear relationship between the logarithm of rate constant, calculated from Eq. 7, and the inversion value of temperature was plotted to estimate activation energy of sulfonation reaction. The activation energy of heterogeneous sulfonation reaction, computed from Eq. 2, was equal to [E.sub.a] = 50.9 kJ [mol.sup.-1] and the pre-exponential factor A = 2.45 x [10.sup.4] [s.sup.-1] . From the results shown above, the activation energy of heterogeneous sulfonation using [SO.sub.3] as the sulfonating agent, was practically equal to the to the activation energy of [SO.sub.3] diffusion. One can conclude that the diffusion was the main process controlling the yield of heterogeneous sulfonation. The calculated value of activation energy of heterogeneous sulfonation was also compared to the activation energy of the g-l heterogeneous sulfonation of benzene (dissolved in nitrobenzene) using [SO.sub.3] as sulfonating agent, which was about 23.1 kJ [mol.sup.-1] (22). The higher activation energy of PS sulfonation compared to sulfonation of benzene can be ascribed to the steric hindrances occurring in the case of s-g sulfonation of high molecular weight polymer.

[FIGURE 5 OMITTED]

CONCLUSIONS

Kinetics of heterogeneous sulfonation of solid PS was investigated in this work. The formation of distinct sulfonated layer on PS surface was observed using SEM. Composition of this layer was analyzed using EDS technique. The diffusion of [SO.sub.3] through a barrier of sulfonated product was the primary parameter determining the rate and yield of heterogeneous sulfonation. The measurement of the time and temperature dependence of sulfonated layer thickness formed on the solid PS surface under different reaction conditions provided experimental data to determine diffusion coefficients for [SO.sub.3] in SPS. Diffusion coefficients ranged from 0.07 X [10.sup.-10] [cm.sup.2] [s.sup.-1] at -50[degrees]C to 2.78 X [10.sup.-10] [cm.sup.2] [s.sup.-1] at 50[degrees] C and were substantially lower than the diffusion coefficients for non reacting gas diffusion through PS. The experimentally determined activation energy of diffusion was 49.13 kJ [mol.sup.-1] which is about two times greater than in the case of low molecular benzene l-g sulfonation with [SO.sub.3]. The elemental analysis of sulfonated layer proved, that maximum one ~[SO.sub.3]H group was attached to one aromatic ring. The experimental data were filled using Johanson-Mehl-Avrami-Jerofyeev-Kolgomorov's equation to obtain the overall rate constant of heterogeneous sulfonation on solid PS surface. The rate constant ranged from 1.97 X [10.sup.-6] [s.sup.-1] at -5[degrees]C to 9.04 X [10.sup.-6] [s.sup.-1] at 50[degrees]C. The overall exponent factor calculated using JMAJK equation ranged from 0.70 to 0.84 and indicated that the s-g heterogeneous sulfonation was the diffusion controlled process. The activation energy of reaction calculated using JMAJK equation was 50.94 kJ [mol.sup.-1], which agreed approximately with the activation energy of diffusion.

ACKNOWLEDGMENTS

The authors are indebted to Mrs. D. Janova for her patience with the SEM analyses.

Funding under MS 0021630501 from The Czech Ministry of Education. Youth and Sports is greatly appreciated.
NOMENCLATURE

[alpha]          Conversion of polystyrene to sulfonated polystyrene

A                Pre-exponential factor

BSE              Buck scattered electron

d                Thickness of sulfonated layer

D                Diffusion coefficient

[E.sub.a]        Activation energy of sulfonation reaction

[E.sub.a]        Activation energy of diffusion

[E.sub.aD]       Activation energy of diffusion

EDS              Electron diffraction scattering

JMAJK            Johanson-Mehl-Avrami-Jerofycev-Kolgornorov's equation

[kappa]          Overall rate constant of the sulfonation reaction

m                Exponent factor of JMAJK equation

[M.sub.w]        Weight average molecular weight

[M.sub.[eta]]    Molecular weight determined by viscosimetric
                 measurement

[v.sub.0]        Average height of starting cylindrical PS particles

PS               Polystyrene

PTFE             Polytetrafluorethylene

[r.sub.0]        Average radius of starting cylindrical particles

s-g              Solid-gaseous

s-l              Solid-liquid

SEM              Scanning electron microscopy

SPS              Sulfonated polystyrene

[S.sub.x]        Sulfonation degree at an one point of sulfonated
                 layer which were calculated as a ratio of amount
                 of sulfonic groups in SPS to amount of PS units of
                 sulfur related to one aromatic ring and expressed
                 in mole %

[[bar.S].sup.x]  Average sulfonation degree determined inside the
                 separated sulfonated layer of sample, calculated
                 from the series of measurements accomplished along
                 the perpendicular to the contour of sulfonated
                 layer

t                Time

T                Temperature


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F. Kucera, J. Jancar

Institute of Material Chemistry, Brno University of Technology, Brno 612 00, Czech Republic

Correspondence to: J. Jancar; e-mail: jancar@fch.vutbr.cz

Contract grant sponsor: Czech Ministry of Education, Youth and Sports; contract grant number: MSM 0021630501.

DOI 10.1002/pen.21398
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Date:Sep 1, 2009
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