Fluorescence and Light-Scattering Studies on the Formationof Stable Colloidal Nanoparticles Made of SodiumSulfonated Polystyrene Ionomers

MEI LI,1 MING JIANG,1 CHI WU2

1 Department of Macromolecular Science, Fudan University, Shanghai, 200433, People’s Republic of China

2 Department of Chemistry, The Chinese University of Hong Kong, Hong Kong

Received 15 October 1996; revised 4 February 1997; accepted 10 February 1997

ABSTRACT: The lightly sulfonated polystyrene ionomer is only soluble in some organicsolvents, such as toluene and tetrahydrofurnan (THF). The mixture of its organicsolution with water normally leads to macroscopic phase separation, namely precipita-tion. In this study, using the steady-state fluorescence, the nonradiative energy transferand dynamic laser light scattering, we demonstrate that the sulfonated polystyreneionomers can form stable colloidal nanoparticles if the THF solution of the ionomersis dropwisely added into an excessive amount of water, or vice verse, water is addedin a dropwise fashion into the dilute ionomer THF solution under ultrasonification orfast stirring. The hydrophobic core made of the polystyrene backbone chains is stabi-lized by the ionic groups on the particle surface. Such formed stable nanoparticles havea relatively narrow size distribution with an average diameter in the range of 5–12nm, depending on the degree of sulfonation, the initial concentration of the ionomerTHF solution, and the mixing order. This study shows another way to prepare surfac-tant-free polystyrene nanoparticles. q 1997 John Wiley & Sons, Inc. J Polym Sci B: PolymPhys 35: 1593–1599, 1997Keywords: laser light scattering; fluorescence; surfactant-free colloid; nanoparticle;polystyrene ionomer

INTRODUCTION as solvated ion-pairs, which form electric dipoles.The attraction between the electronic dipoles andthe repulsion between the dipoles and the hydro-The association/dissociation of ionomers in non-carbon backbone chain lead to the aggregation ofpolar and polar solvents have been a focused issuethe dipoles and the consequent association of thefor more than 10 years, and the lightly sulfonatedionomers, which has been experimentally con-polystyrene (SPS) ionomer was usually chosen asfirmed by rheology, light scattering, small-anglea model compound.1–8 In nonpolar solvents, suchneutron scattering, and fluorescence.9–16 Whenas tetrahydrofuran (THF) and xylene, the ionicdissolving in a polar solvent, such as dimethyl-groups are not completely dissociated, but existformamide (DMF), the ionomer chains in solutionbehave like polyelectrolytes. All phenomena re-

Correspondence to: C. Wu lated to the aggregation in nonpolar solvents dis-Contract grant sponsor: Natural Science Foundation of appear because of the dissociation of the ion-China

pairs.17–20 On the other hand, the study of the S-Contract grant sponsor: National Basic Research Project-Macromolecular Condensed State PS in aqueous solutions has attracted much less

Contract grant sponsor: Research Grants Council of the attention because of the poor solubility of its hy-Hong Kong Government Earmarked Grant; contract grantdrophobic backbone in water. Recently, Jiang etnumber: CUHK 458/95P, 2160046

q 1997 John Wiley & Sons, Inc. CCC 0887-6266/97/101593-07 al.21 reported that the carboxylated poly(styrene-

1593

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1594 LI, JIANG, AND WU

block-butadiene-block-styrene) triblock copoly- centrations with deionized water containing 1 v% THF for fluorescence and light-scattering mea-mer ionomer (SEBS) can form stable colloidal

particles in aqueous solution if a dilute THF solu- surements. The dispersion can also be preparedby very slowly (one drop per minute for the firsttion of the ionomer is added in a dropwise fashion

into an excessive amount of water. In the present 10 mL of water) adding 99 mL deionized waterinto 1 mL ionomer THF solution under fast stir-work, fluorescence together with laser light scat-

tering were used to demonstrate that the ran- ring.domly sulfonated polystyrene ionomers can alsoform stable colloidal particles in water. The em- Fluorescence Measurementsphasis of this study will be on how the formation

The steady-state fluorescence was measured us-of the stable nanoparticles depends on the degreeing a Perkin–Elmer Luminescence Spectrometerof sulfonation, the method of preparation, and theLS50 with a right-angle geometry (907 collectinginitial concentration of the ionomer THF solutionoptics) . The excitation and emission slits were 5used in the preparation.and 3 nm, respectively. Pyrene and naphthalene/anthracene were respectively used here as a fluo-rescence probe and a pair of donor/acceptors toEXPERIMENTALstudy the aggregation of the ionomer chains inwater. The pyrene concentration was 6 1 1006M .The naphthalene and anthracene concentrations

Materials were 2 1 1005 and 1 1 1005M , respectively. Thesolutions were ultrasonificated for at least 10 minThe preparation of sodium sulfonated polystyreneto ensure a complete dispersion of the fluorescenceionomers has been done as described previously.9probe molecules. Then, the solutions were allowedThe sulfonation extents were calculated based onto stand at room temperature for at least 12 hsulphur analysis. Three samples were used in thisbefore the fluorescence measurements. The exci-study. Two of them are made by free radical poly-tation wavelength for the pyrene and the nonradi-merization. The samples are soluble in THF toate energy transfer (NRET) studies are 335 andform a stable solution. The apparent weight-aver-275 nm, respectively. The excitation spectra of py-age molar mass of these two ionomers in THFrene were recorded with the emission wavelengthwere 2.8 1 105 and 3.1 1 105 g/mol, respectively,of 390 nm.which are close to the values before the ionomeri-

zation, indicating that the ionomerization did notLaser Light Scattering (LLS)alter the length of polymer chains. The extents of

sulfonation of these two samples are 3.67 and 5.52 A commercial LLS spectrometer (ALV/SP-150,mol %, respectively. The third sample was made Germany) equipped with an ALV-5000 multi-tby anionic polymerization. Its weight-average mo- digital correlator and a solid-state laser (ADLASlar mass and extent of sulfonation are 3.1 1 105

DPY425II, Germany; output power É 400 mw atg/mol and 5.23 mol %, respectively. According to l Å 532 nm) as the light source was used. Thethe extent of sulfonation, these samples are de- incident light beam was vertically polarized withnoted as 3.67 NaSPS, 5.52 NaSPS, and 5.23 respect to the scattering plane. The detail of LLSNaSPS hereafter. instrumentation and theory can be found else-

where.22,23 All LLS measurements were done at25.0 { 0.17C. All samples used in the LLS mea-Preparation of the NaSPS Nanoparticlessurements were clarified by 0.2 mm Anotop filter.

First, each ionomer was dissolved in tetrahydrofu- The measured time correlation functions were an-ran (THF) to form a solution with a concentration alyzed by both the cumulants and Laplace inver-of 2 1 1002 g/mL or less. Using the THF solution, sion (CONTIN) programs equipped with the cor-the stable dispersion of NaSPS in water was made relator.by adding the THF solution in a dropwise fashioninto an excessive amount of water under ultrason-

RESULTS AND DISCUSSIONification or magnetic stirring. The dispersion con-tains 1 v % THF and the final ionomer concentra-

Evidence of the Formation of Nanoparticlestion is 2 1 1004 g/mL or less. Such prepared dis-persion is transparent and stable. Further, the Pyrene, as one of the most sensitive probes, has

been widely used to study the association and mi-dispersion was diluted to a series of desired con-

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STABLE COLLOIDAL NANOPARTICLES 1595

centration is extremely dilute, indicating thatmost of the pyrene molecules are surrounded bywater molecules. When the ionomer concentrationis higher than 1006 g/mL, the ratio starts to de-crease, very similar to the change of I373 . A pla-teau value of Ç 1.1 at the ionomer concentrationhigher than 1 1 1004 g/mL indicates the completetransference of pyrene molecules from water tothe hydrophobic environment of the colloidal par-ticles. The plateau value of I1 /I3 is not as low as0.95 measured in bulk polystyrene, which is rea-sonable because the collapsed polymer chainmight still containing some water molecules.29

Figure 1 shows that the type of sample, the poly-Figure 1. Ionomer concentration dependence of the dispersity of molar mass, and the extent of sulfo-fluorescence intensity (I373) and the intensity ratio of

nation have nearly no effect on the aggregation(I1 /I3) of pyrene in different NaSPS ionomer disper-behavior observed in fluorescence.sions.

Moreover, we found that for all of the threeNaSPS ionomers in aqueous solution the (0,0)band of the S2–S0 absorption of pyrene is shiftedcellization of macromolecules in solution because

of the remark changes of its photophysical and Ç 5 nm to red, indicating the aggregation of theionomer chains because in a micelle system theother properties when it is transferred from a po-

lar environment to a nonpolar one,24–28 such as red shift is 2–3 nm in comparison with that inpure water.24,26 Wilhelm et al.26 reported that thean increase in the quantum yield, a change in the

fine vibrational structure reflected in the inten- intensity ratio I338 /I333 of the pyrene excitationspectra could give additional information on thesity ratio (I1 /I3) of the first and third bands in its

emission spectrum, and a shift of the low energy onset of block copolymer aggregation because py-rene in water has a maximum absorption at 333band of the La (S2–S0) transition from 333 to 338

nm in its excitation spectrum. nm, and its absorption at 339 nm is very weak. Incontrast, the absorption of pyrene in a less polarFigure 1 shows typical ionomer concentration

dependence of the fluorescence intensity (I373) and solvent at 339 nm increases substantially. There-fore, the shift of the maximum absorption peakthe intensity ratio of I1 /I3 in the pyrene emission

spectrum. The enhancement of the fluorescence and the concentration dependence of the I338 /I333

ratio are also very sensitive to the formation of aintensity shows that the quantum yield of pyreneincreases as the ionomer concentration increases. hydrophobic domain.

Figure 2 shows the ionomer concentration de-For a constant pyrene concentration, the increasein the quantum yield indicates the variation of pendence of the intensity ratio I1 /I3 of the emis-

sion spectra and the intensity ratio I338 /I333 of thepolarity around pyrene. On the other hand, theincrease of I373 also reflects a longer life time of excitation spectra of pyrene in the 3.67 NaSPS

dispersion prepared by different methods: (1)the excited state of pyrene, indicating a changein the local environment of pyrene.25 In the low adding the THF solution (Cinitial Å 21 1002 g/mL)

into water; (2) diluting the THF solution to Cinitialconcentration range, the fluorescence intensityI373 of pyrene is nearly a constant, but when the Å 2 1 1003 g/mL and then adding it into water;

(3) adding water to the THF solution (Cinitial Å 2ionomer concentration is higher than 3 1 1006 g/mL, the intensity rapidly increases as the ionomer 1 1003 g/mL). The decrease of I1 /I3 and the in-

crease of I338 /I333 further indicate the formationconcentration increases, indicating the partitionof pyrene between water and the hydrophobic mi- of colloidal particles. The turning point of the I338 /

I333 curve is close to that of I373 and I1 /I3 showncrodomain, i.e., between inside and outside of theaggregates of the polystyrene backbone chains. in Figure 1. At the extremely dilute ionomer con-

centration, the ratio of I338 /I333 is onlyÇ 0.5, char-On the other hand, the intensity ratio I1 /I3 ofpyrene is also sensitive to the polarity of the me- acterizing pyrene in pure water. When the iono-

mer concentration is higher than 3 1 1006 g/mL,dium. The ratio can change from a value of Ç 1.8in water to Ç 1.0 in anionic surfactant micelles the ratio of I338 /I333 starts to increase. The plateau

value of I338 /I333 at the high ionomer concentrationor even 0.62 in cyclohexane.26 Figure 1 shows thatthe intensity ratio isÇ 1.7 when the ionomer con- is typical for pyrene in a hydrophobic environ-

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1596 LI, JIANG, AND WU

Figure 2. Ionomer concentration dependence of theFigure 4. Typical hydrodynamic radius (Rh ) distribu-intensity ratio (I1 /I3) of the emission spectra and thetions of the stable colloidal dispersion made of differentintensity ratio (I338 /I333) of the excitation spectra of py-NaSPS ionomers. The ionomer concentration in the dis-rene in the 3.67 NaSPS dispersion prepared by differentpersion was kept at 2 1 1003 g/mL for all dispersions.methods.

ment, indicating the formation of colloidal parti- the acceptor. The energy transfer efficiency (E ) iscles in aqueous solution. It is clear that the prepa- directly related to the average distance (R ) be-ration method has little effect on the aggregation tween donor and acceptor by E Å R6

0 / (R60 / R6) ,

of the ionomer chains. Further, the nonradiative where R0 is the distance at which 50% of the en-energy-transfer (NRET) technique was also used ergy transfer takes place. For a given donor/ac-to confirm the formation of stable colloidal parti- ceptor pair in a given medium, R0 is a constant.cles in aqueous solution. Experimentally, the efficiency of the NRET is

Figure 3 shows the ionomer concentration de- characterized by the intensity ratio of Ia /Id in thependence of the efficiency of the NRET between emission spectrum. Figure 3 shows that in the lownaphthalene and anthrance (Ia /Id ) . According to ionomer concentration range the NRET efficiencythe NRET theory, if the emission spectrum of a between naphthalene and anthrance is very low.donor molecule overlaps the absorption spectrum When the ionomer concentration is higher than 1of a acceptor molecule, the NRET will happen due 1 1005 g/mL, Ia /Id starts to increase, indicatingto the dipole–dipole interaction between the do- the formation of a hydrophobic microdomains andnor in its excited state and the acceptor in its the partition of naphthalene and anthrance in theground state leads to the emission spectrum of hydrophobic microdomains. It has been noted that

the concentration at which Ia /Id starts to increaseis Ç 10 time higher than that detected by usingpyrene as a probe. This is understandable becausethe solubility of naphthalene in water is muchhigher than that of pyrene and only when naph-thalene concentration in the hydrophobic micro-domain is higher than a certain value can theenergy transfer takes place. Figure 3 also showsthat the samples with a lower sulfonated degreehave a larger NRET efficiency and Ia /Id starts toincrease at a lower concentration, which agreeswell with the fact that the polystyrene ionomerwith a lower degree of sulfonation is more hy-drophobic. Dynamic light scattering as a powerfulanalytical tool can be used to directly determinethe size distribution of the colloidal particles.

Figure 4 shows typical apparent size distribu-Figure 3. Ionomer concentration dependence of thetions of the stable colloidal particles made of dif-NRET efficiency between naphthalene and anthrance

(IA /Id ) in different NaSPS dispersions. ferent sodium sulfonated polystyrene ionomers. It

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STABLE COLLOIDAL NANOPARTICLES 1597

sults leads us to the following picture: it is notdifficult to realize that when the ionomer THFsolution was added into water in a dropwise fash-ion the hydrophobic polystyrene backbone chainsstart to collapse, in which the ionic groups willhave a tendency to stay at the surface of the in-terchain aggregates. Assuming these aggregatesare spherical with an average radius of R , wehave that the particle surface area (S ) is propor-tional to R2 . On the other hand, the number(Nionic ) of the ionic groups should be proportionalto the number of the ionomer chains incorporatedinside the particle, or equivalent, to the particlemass (M ) . Therefore, assuming all the ionic

Figure 5. Typical hydrodynamic radius (Rh ) distribu- groups are on the particle surface, S /Nionic } R2 /tions of the stable colloidal dispersion made of the 3.67 M } R01 because M } R3 , where S /Nionic repre-NaSPS ionomer after a successive dilution. sents the average surface area occupied by each

ionic group. During the aggregation, the size ofthe particles increases, so that S /Nionic decreases.Eventually, the particles will be stabilized by theclearly shows that the particles formed through

the hydrophobic aggregation of the ionomers in ionic groups on the surface because for a givenpolymer/solvent system S /Nionic should has a min-water are very small in the nanometer range and

the size distribution is fairly narrow. The more imum value that corresponds to a fully coveredsurface by the ionic groups. Therefore, dilutingimportant point is that the average particle size

depends on the degree of sulfonation, namely, the the dispersion has no effect on the particle size,but the relative amount of the particles. When thehigher the degree of sulfonation, the smaller the

average particle size, which agrees well with our concentration is lower than 1007 g/mL, the valueof I1 /I3 remains Ç 1.7, which is typical for pyreneprevious study of the carboxylated ionomers.30 In

static light scattering, we found that the weight- in aqueous media. However, we still cannot ex-clude the possibility of the existence of the hy-average molar masses of the 3.67 NaSPS, 5.52

NaSPS, and 5.23 NaSPS nanoparticles are 3.9 drophobic microdomains because the ionomer con-centration is extremely low, even if the ionomer1 105, 2.9 1 105, and 3.3 1 105 g/mol, respec-

tively, indicating these nanoparticles are nearly chains could form the hydrophobic microdomainsrather than existing as individual chains. The rel-made of a single ionomer chain.

Figure 5 shows a successive dilution of the 3.67 ative amount of pyrene in the microdomainsmight not be high enough to make a detectableNaSPS dispersion in the range of 8 1 1006–2

1 1004 g/mL has nearly no effect on the particle change in I1 /I3 . The LLS study in this extremelydilute range is also very difficult, but still under-size distribution. On the one hand, Figure 5 indi-

cates the effect of the ionic interaction on the mea- going, which might be able to provide evidence toanswer this question.sured particle size distribution is not significant.

Otherwise, the increase of the interparticle dis-tance due to the dilution will decreases the ionic

Effect of Dialysis on the NaSPS Nanoparticlesinteraction and affect the measured size distribu-tion. On the other hand, Figure 5 shows that the In order to remove the 1 v % THF introduced in

the preparation, the dispersion was dialyzed fornanoparticles stabilized by the ionic groups on thesurface are very stable and they are very different 1 week. Figures 6 and 7 indicate that the dialysis

has nearly no effect on the stability of the colloidalfrom simple aggregation, namely there is no equi-librium between individual chains and particles. particles in water; namely, there is no change in

the minimum value of I1 /I3 , the concentration atIt should be noted that it is difficult to determinethe particle size distribution for such small nano- which the ratio of I1 /I3 starts to drop, and the

particle size distribution. These results in Figuresparticles when the concentration is lower than1005 g/mL. In contrast, the intensity ratio of I1 / 6 and 7 imply that there is no strong preferential

adsorption of THF inside the colloidal particles.I3 , I338 /I333 and Ia /Id show substantial variationsover the same concentration range. Otherwise, the removal of THF would change the

hydrophobility inside the particle and alter theA combination of the LLS and fluorescence re-

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1598 LI, JIANG, AND WU

Figure 8. Initial ionomer concentration dependenceFigure 6. Concentration dependence of the intensityand the preparation method influence on the hydrody-ratio I1 /I3 of pyrene in the 3.67 NaSPS ionomer disper-namic radius (Rh ) distributions of the stable polysty-sion before and after 1-week dialysis of the dispersion.rene nanoparticles made of the 3.67 NaSPS chains.

particle size distribution. Therefore, we can as- the initial ionomer concentration can greatly af-sume that when the ionomer THF solution is fect the final particle size distribution, namely adropwisely added into a large amount of water 10-time dilution of the 3.67 NaSPS THF solutionthe THF molecules immediately diffuse into water from 2 1 1002 g/mL to 2 1 1003 g/mL leads toresulting in the collapse and aggregation of the much smaller particles. This is understandablehydrophobic polystyrene backbone chains because because when the ionomer THF solution is dilutedTHF is infinitely soluble in water. the number of the ionomer chains in each drop,

i.e., the local ionomer concentration decreases, sothat the collapsed ionomer chains have lessInfluence of the Sample Preparation Methodschance to form larger particles before they are

Figure 8 shows the initial ionomer concentration stabilized by the ionic groups. On the other hand,dependence and the influence of the preparation Figure 8 shows that adding water in a dropwisemethod on the particle size distribution. It clearly fashion into the ionomer THF solution can alsoshows that ultrasonification and stirring essen- form stable colloidal particles as long as the initialtially lead to the same particle size distribution ionomer concentration is lower than 2 1 1003 g/when the 3.67 NaSPS THF solution with an initial mL. It should be stated that at a higher initialconcentration of 2 1 1002 g/mL was added in a ionomer concentration, such as 2 1 1002 g/mL,dropwise fashion into water. It also shows that adding water into the ionomer THF solution re-

sulted in a milky dispersion. The particles formedby adding water in the ionomer THF solution istwice as large as those obtained by adding thesame ionomer THF solution into water. This canalso be explained in terms of the local ionomerconcentration. When the ionomer THF solution isadded into water in a dropwise fashion, the num-ber of the polystyrene chains in each drop is lim-ited. In this case, the local environment of theionomer molecules changes rapidly from THF toalmost pure water, the collapsed and aggregatedpolystyrene chains are rapidly dispersed into thesurrounding water by ultrasonification or stir-ring, and the particles are relatively small. In con-trast, when water is added in a dropwise fashioninto the ionomer THF solution, the polystyreneFigure 7. Typical hydrodynamic radius (Rh ) distribu-chains around the water droplet are collapsed andtion of the 3.67 NaSPS ionomer dispersion before and

after 1-week dialysis of the dispersion. the local ionomer concentration is much higher,

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STABLE COLLOIDAL NANOPARTICLES 1599

5. D. G. Peiffer, M. W. Kim, and D. N. Schulz, J.so that larger particles are formed. In contrast,Polym. Sci., Polym. Phys. Ed., 25, 1615 (1987).Figure 2 shows no difference in the excitation and

6. P. K. Agarwal, R. T. Garner, and W. W. Graessley,emission spectra of pyrene in the solutions pre-J. Polym. Sci., Polym. Phys. Ed., 25, 2095 (1987).pared by different methods, indicating that fluo-

7. M. Hara, A. H. Lee, and J. L. Wu, J. Polym. Sci.,rescence is insensitive to the particle size distribu-Polym. Phys. Ed., 25, 1407 (1987).

tion, but only to the amount of the hydrophobic 8. M. Hara, J. L. Wu, and A. H. Lee, Macromolecules,microdomains (nanoparticles) formed in the dis- 22, 754 (1989).persion because the change of fluorescence signals 9. C. W. Lantman, W. J. Macknight, D. G. Peiffer,is related to the change of the hydrophility around S. K. Sinha, and R. D. Lundberg, Macromolecules,

20, 1096 (1987).the probe molecules.10. M. Sedlak and E. J. Amis, J. Chem. Physiol. 96,

817 (1992).11. B. D. Ermi and E. J. Amis, Macromolecules, 29,CONCLUSIONS

2701 (1996).12. B. Gabrys, J. S. Higgins, C. W. Lantman, W. J.

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Burchard, Macromolecules, 23, 1434 (1990).or vice verse, the ionomer chains can form surfac-14. K. C. Dowling and J. K. Thomas, Macromolecules,tant-free polystyrene nanoparticles stabilized by

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cules, 24, 4578 (1991).tribution is strongly dependent on the extent of16. K. N. Bakeev, I. Teraoka, W. J. Macknight, and

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