EUROPEANPOLYMER

European Polymer Journal 40 (2004) 1609–1613

www.elsevier.com/locate/europolj

JOURNAL

Effect of porosity on sulfonation of macroporousstyrene-divinylbenzene beads

Munir Ahmed, Muhammad Arif Malik *, Shahid Pervez, Muhammad Raffiq

Applied Chemistry Laboratories, PINSTECH, PO Nilore, Islamabad, Pakistan

Received 16 March 2004; received in revised form 9 April 2004; accepted 19 April 2004

Available online 9 June 2004

Abstract

Macroporous styrene-divinylbenzene beads having pore volume in the range of 0.1–2.2 ml/g were synthesized by o/w

suspension polymerization using petroleum ether and cyclohexanone as porogen. As the pore volume was increased the

pore size distribution shifted towards large pore diameter and the mechanical strength of the beads in the dry state

decreased. The copolymers were converted into cation-exchange resins by sulfonation under controlled experimental

conditions. The derived resins had the highest capacity when the base copolymers had pore volume in the range of 0.3–

0.4 ml/g. The results are explained on the basis of the effect of the porogen on the spaces between chains and cross-links

in the copolymer phase and the permanent pores in the beads.

� 2004 Elsevier Ltd. All rights reserved.

Keywords: Chemical modification; Styrene-divinylbenzene copolymer; Macro-porous polymer beads; Ion exchange resin; Sulfonation

reaction

1. Introduction

Porous styrene-divinylbenzene (St-DVB) copolymer

beads are produced by oil-in-water (o/w) suspension

polymerization [1–3]. The monomers are diluted with

an inert organic liquid called porogen that introduces

pores during the copolymerization [4]. The porosity

is controlled by the amount and type of porogen

and cross-linkage, i.e., divinylbenzene (DVB) [5–7]. A

number of reviews have been published on this topic [8–

13] and new findings are being reported on a regular

basis [14–23].

Porous St-DVB beads are converted to or used as

adsorbents [21], support for catalysts [24,25], anion-

exchangers [26,27], cation-exchangers [28,29], etc. Cat-

* Corresponding author. Address: Center for Bioelectrics,

ECE, ODU, 830 Southampton Avenue, Suite 510, Norfolk

23510, USA. Tel.: +1-757-683-2231; fax: +1-757-314-2397.

E-mail address: m_arif_malik@yahoo.com (M.A. Malik).

0014-3057/$ - see front matter � 2004 Elsevier Ltd. All rights reserv

doi:10.1016/j.eurpolymj.2004.04.013

ion-exchangers are among the most important com-

mercial products derived from St-DVB for use in

chromatographic separations [30] and as acid catalysts

[31]. The cation-exchangers are obtained by sulfonation,

i.e., by the introduction of –SO�3 H

þ groups with

exchangeable protons onto benzene rings of monomer

units in St-DVB [28,29]. The degree of sulfonation,

which determines the capacity of the derived cation-

exchanger, can be controlled by time and temperature of

the sulfonation reaction [30]. The effect of porosity,

especially the effect of permanent pores (also called

macroporosity or macroreticular porosity), on the sul-

fonation reaction in St-DVB might have been studied by

industries for production of commercial products. To

the best of our knowledge, the research has not been

published.

The following research discusses the effect of porosity

on the degree of sulfonation of St-DVB. Knowledge of

these sulfonation reaction controlling parameters will be

useful for production of tailored products from the

porous St-DVB.

ed.

1610 M. Ahmed et al. / European Polymer Journal 40 (2004) 1609–1613

2. Experimental

The St-DVB copolymer beads were synthesized by

o/w suspension polymerization reported earlier [32]. The

beads were sieved and the fraction having diameter 150–

300 lm were dried at 110 �C till constant weight. Den-sity ðdÞ in g/ml of the dried beads was determined bymeasuring cylinder method [16]. Pore volume (PV) in

ml/g, pore area (PA) in m2/g and pore size distribution

of the dried beads were analyzed by mercury porosi-

meter Autopore II 29220 from Micromeritics [4,33,16].

Pore volume, pore area and pore size distribution can

also be analyzed by nitrogen adsorption/desorption [33].

The pore volume in the dry state, which represents the

macroporosity, can also be calculated from the density

of the dried copolymer beads [16]. Pore Volume can also

be estimated from the composition of monomers and

porogen mixture [32]. Crush load (CL) in Newton (N) of

a sample of 300 lm diameter beads, with no cracks orother abnormalities on the surface, was measured by a

method developed in our laboratory [34]. A selected

bead was placed on the pan of a top loading balance and

it was tarred for zero. Pressure was gradually increased

by rotating the knob of a load stem until the bead

underneath it collapsed, which was indicated by sudden

fluctuations in the load reading. The average of the 15

readings is reported as crush load. The crush load

indicates the mechanical strength of the dried beads.

Table 1

Experimental conditions for synthesis of styrene-divinylbenzene cop

exchangers

Exp.

No.

DVB % Porogen %

(composition)

PV (ml/g) PA (m

1 20 30 (PE) 0.0900 53.0

2 20 40 (PE) 0.1305 77.8

3 20 50 (PE) 0.4210 147.9

4 20 60 (PE) 0.7015 172.9

5 20 65 (PE) 1.6472 218.0

6 20 70 (PE) 1.9566 281.1

7 20 30 (CHN) 0.0714 41.3

8 20 50 (CHN) 0.0795 46.1

9 20 60 (CHN) 0.1047 54.3

10 20 70 (CHN) 0.0701 42.1

11 20 70 (PE:CHN, 25:75) 0.0670 37.2

12 20 70 (PE:CHN, 50:50) 0.2502 107.4

13 20 70 (PE:CHN, 75:25) 0.3121 175.3

14 20 70 (PE:CHN, 85:15) 0.9759 198.2

15 20 70 (PE:CHN, 90:10) 1.4674 254.3

16 15 70 (PE:CHN, 85:15) 0.3730 141.0

17 33 70 (PE:CHN, 85:15) 1.5327 240.8

18 40 70 (PE:CHN, 85:15) 2.2055 290.5

19 60 70 (PE:CHN, 85:15) 2.262 291.2

‘DVB %’ is percentage of pure divinylbenzene isomers in the mon

polymerization mixture, ‘PV’ is pore volume, ‘PA’ is pore area, ‘d’ is‘PE’ is petroleum ether (boiling range 140–180 �C) and ‘CHN’ is cyc

The dried beads were stirred in 98% H2SO4 (1:5

weight to volume ratio) with a glass stirrer at 100 �C for2 h for sulfonation. The resin (sulfonated beads) was

separated by succession filtration and washed with

demineralized water. A 5 ml of wet resin in H-form was

air dried and then dried at 110 �C until constant weightto determine the dry weight of the resin. Another 20 ml

of wet resin in H-form was packed in a column of 1 cm

diameter and excess of NaOH solution (0.36 M, 200 ml)

was passed through it, followed by 50 ml demineralized

water to remove residual NaOH. The collected effluent

(250 ml) was titrated. The capacity of the resin was

calculated in milliequivalents per milliliter (meq/ml) of

wet resin and in milliequivalents per gram (meq/g) of dry

resin from weight and volume of resin used and number

of milliequivalents of NaOH retained by the resin.

3. Results and discussion

Pore volume of the St-DVB beads varied from

around 0.1–2.2 ml/g by using the techniques described in

literature [4–22,32], i.e., by increasing the amount of

porogen, the ratio of petroleum ether (non-solvent) to

cyclohexanone (good solvent) in porogen, or the cross-

linker (DVB). The results are presented in Table 1 and

illustrated in Fig. 1. With increased pore volume the

crush load values of the beads were decreased and the

olymer, properties of the copolymers and the derived cation

2/g) d (g/ml) CL (N) Capacity

meq/g meq/ml

0.61 11 0.31 0.20

0.59 3.9 1.9 0.79

0.40 2.2 4.4 1.50

0.33 0.96 4.0 1.30

0.23 0.69 3.5 0.82

0.19 0.16 2.3 0.43

0.64 11 0.05 0.30

0.64 8.2 0.21 0.14

0.63 4.6 0.45 0.30

0.61 2.7 1.30 0.58

0.61 2.0 1.6 0.65

0.53 1.8 4.1 1.40

0.41 1.5 4.4 1.50

0.33 0.98 3.8 1.20

0.26 0.44 3.5 0.83

0.42 1.2 4.5 1.5

0.23 0.49 3.5 0.83

0.20 0.26 2.9 0.65

0.16 0.26 1.3 0.34

omers, ‘Porogen %’ is volume percentage of porogen in the

density of the copolymer beads in dry state, ‘CL’ is crush load,

lohexanone.

Fig. 2. Pore size distribution of copolymers from exp. # 2–4

(porogen was petroleum ether).

Fig. 1. Variation of pore volume with % petroleum ether–the

non solvent in porogen (the rest of the porogen was cyclohex-

anone–the good solvent) and with % porogen in the polymeri-

zation mixture (the rest of the mixture was monomers) of

styrene-divinylbenzene copolymers (Data from Table 1).

M. Ahmed et al. / European Polymer Journal 40 (2004) 1609–1613 1611

pore size distribution shifted towards larger pore diam-

eter as illustrated in Fig. 2.

The St-DVB beads were sulfonated under controlled

experimental conditions so that variation in the

capacity of the derived resin would be related to the

porosity of the base copolymers. In exp. # 1–15 (Table

1) DVB was 20% and the remaining monomers, i.e.,

styrene, ethylvinyl benzene, etc. were 80%. In this set of

experiments, the highest capacity of 4.4 meq/g was

achieved when the pore volume of the base copolymer

was in the range of 0.3–0.4 ml/g. The capacity gradually

decreased to around 0.3 meq/g when the pore volume

was either increased or decreased from the range of

0.3–0.4 ml/g. Calculations show that 4.4 meq/g capacity

corresponds to sulfonation of around 80% benzene

rings in the copolymer. This observation indicates that

nearly all the benzene rings of monomers (excluding

DVB) were sulfonated under the optimum pore volume

condition.

In exp. # 16–19 (Table 1) DVB was increased from

15% to 60% in the monomers and the capacity of the

corresponding resins was found to decrease with in-

crease in DVB. This observation further supports the

assumption that benzene rings of DVB units were not

sulfonated. The DVB units lie at the points of cross-

linkage where restricted mobility coupled with steric

hindrance posed by two copolymer chains across the

reaction site may limit its accessibility to the sulfonating

agent.

In exp. # 7–10 (Table 1) cyclohexanone was em-

ployed as a single porogen. When amount of cyclohexa-

none was increased from 30% to 70% a significant

reduction in crush load values and a slight increase in

the capacity of the derived resins were observed.

Although, the pore volume remained low at around

0.1 ml/g.

The above results verify the effect of porogen on the

distances between chains and cross-links in the copoly-

mer, i.e., microreticular porosity, and their effect on

the permanent pores, i.e., macroreticular porosity or

macroporosity in the copolymer beads [4–24,32,35]. The

polymerization reaction takes place in a suspended

droplet during o/w suspension polymerization. As the

reaction progresses the copolymer precipitates within

the droplet and form spherical shapes called nuclei.

These droplets form due to the difference in the solu-

bility parameter (d) of the copolymer and the ambientliquid. The nuclei grow into microspheres (also called

microgel) and the microspheres agglomerate with each

other resulting in the primary network. Upon further

polymerization and cross-linkage, the primary network

becomes the cross-linked porous network as illustrated

in Fig. 3. When the porogen is removed, the void space

between microsphere agglomerates, i.e., macroporosity,

is left behind, while the spaces between chains and cross-

links within the copolymer phase shrink. A larger dif-

ference between the d values of the copolymer and theambient liquid, results in a stronger interaction and

earlier phase separation. Consequently, lower crush load

values, wider pores, higher pore volume, and denser

copolymer phase within the microspheres in the beads is

observed. At phase separation, the monomers left be-

hind in the ambient liquid copolymerize with time and

fill-up the smaller pores in the matrix [22]. This explains

the shift of the pore size distribution towards larger pore

diameter as illustrated in Fig. 2.

Increased sulfonation and, consequently, increased

capacity of the resins with increase in pore volume up to

0

0.5

1

1.5

0 0.5 1 1.5 2 2.5

Pore volume (ml/g)

Cap

acity

(meq

/ml)

Fig. 4. Volume capacity versus pore volume for all experiments

from Table 1.

Fig. 3. Mechanism of porous structure formation during suspension copolymerization of styrene-divinylbenzene.

1612 M. Ahmed et al. / European Polymer Journal 40 (2004) 1609–1613

0.3–0.4 ml/g can be explained by the faster diffusion of

reagents (sulfonating reagent) thru interconnected per-

manent pores in the beads [36]. However, the permanent

pores are formed at the expense of the spaces between

chains and cross-links within the copolymer phase,

which may reduce the diffusion of the sulfonating re-

agent in the copolymer phase. This hypothesis is further

supported by the results of exp. # 7–10 where the

porogen (cyclohexanone) had a d value close to that ofthe d value of the copolymer [32]. In this case theporogen keeps the copolymer swollen during polymeri-

zation and increases only the spaces between chains and

cross-links in the copolymer. Decreasing the porogen

content resulted in the decrease in the capacity of the

derived resins. The diffusion within the copolymer phase

appears to become the rate-limiting step for the sulfo-

nation reaction when the pore volume is increased be-

yond the 0.4 ml/g value. This explains the observed

decrease in the capacity of the resins.

We have observed a similar trend for the case of

diffusion of ferric chloride complex anions in macro-

porous 4-vinylpyridine-divinylbenzene anion exchanger

resins. In this case, the rate of diffusion was the fastest in

the resins having pore volume of 0.5 ml/g and the rate

decreased when the pore volume was either decreased or

increased from the optimum value of 0.5 ml/g [37].

Volume capacity of the resins represents their work-

ing capacity. Fig. 4 shows the highest volume capacity of

the resins in the present study was obtained when the

pore volume of the base copolymer was in the range of

0.3–0.4 ml/g.

In conclusion, the sulfonation reaction of porous St-

DVB beads is strongly dependent on their porosity. The

optimum conditions for sulfonation are achieved with

copolymer pore volume in the range of 0.3–0.4 ml/g.

Acknowledgements

Authors thank Mr. Robert J. Minahan, BioElectrics

Inc., Norfolk, VA, USA, for improving English of the

manuscript.

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