Preparation of monodisperse microspheres using the Shirasu porous glass emulsification technique

:

ELSEVIER

Preparation

A:Physicochemical

Colloids and Surfaces and Engineering Aspects

COLLOIDS AND SURFACES

A

109 (1996) 97-107

of monodisperse microspheres using the Shirasu porous glass emulsification technique Shinzo Omi

Graduate School of Bio-Applications

and Systems Engineering, Tokyo University of Agriculture and Technology, Nakamachi, Koganei, Tokyo 184, Japan

Received 31 July 1995; accepted 16 August 1995

Abstract Relatively uniform polymeric microspheres, with diameters ranging from 2.5 to 60 pm, and relative standard deviations of diameters close to lo%, were prepared by the ordinary suspension polymerization of either styrene(hydrophobic) or methyl methacrylate (MMA)-(more hydrophilic) based monomers. Unlike the conventional stirredtank system, a particular microporous glass membrane (Shirasu porous glass; SPG) provided uniform monomer droplets continuously when monomer was allowed to permeate through the micropores under a carefully controlled nitrogen atmosphere. The monomer droplets, a mixture of monomers, diluents, oil-soluble initiator as well as waterinsoluble reagent, were then suspended in the aqueous solution containing stabilizing agents, transferred to a stirred vessel, and polymerized. In the case of hydrophilic MMA spheres, the size distribution tends to become broader because hydrophilic substances easily wet the surface of SPG, leading to a permeation process which is uncontrollable. This difficulty was overcome by adopting the droplet swelling technique, in which the secondary emulsion droplets containing hydrophilic MMA were absorbed in the primary emulsion droplets consisting of hydrophobic components on the principle of the degradative diffusion process. Uniformity of the initial droplets is preserved during the swelling step, and subsequent polymerization. Applications of crosslinked microporous spheres were promising as packing beads for gel permeation chromatography and as carriers for enzyme immobilizations. Keywords:

Membrane emulsification; glass; Suspension polymerization

Monosized

particles; Poly methylmethacrylate;

1. Introduction Polymeric microspheres with a uniform size distribution, in particular those with diameters in the range of 10 pm, have attracted scientific and engineering attention as one of the most sophisticated of materials. Microporous spheres are favored as the packing materials for column chromatography techniques such as gel permeation chromatography and high performance liquid chromatography, and 0927-7757/96/$15.00 0 1996 Elsevier Science B.V. All rights reserved SSDI 0927-7757(95)03477-3

Polystyrene;

Shirasu porous

for affinity separation. Incorporation of magnetite powders improves the efficiency of the separation procedure when the specific ligand is immobilized on the surface of particles to collect a particular cell [ 1,2]. Singularly monodisperse spheres with a relative standard deviation (RSD) of diameter of less than 5% are sold at a price of more than one million yen kg-l as liquid crystal spacers. Manipulation of charged spheres under the influence of an electric field is often demonstrated for

98

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Surfaces A: Physicochem.

the modeling of physical phenomena, and the principle is extended to electronic photocopying and further hybridization with other micromeritic materials. Synthesis of uniform spheres having a diameter of 10 nm or more is not easy. One-step synthesis has not been achieved with the exception of Ober and Hair [3] who reported that 15 pm monodisperse polystyrene (PS) spheres were obtained by non-aqueous phase dispersion polymerization. PS spheres of almost 10 nrn diameter were obtained on a commercial scale in the mixed medium of a short-chain alcohol and 2-methoxy ethanol. Turning to aqueous phase polymerization, Bradford, Vanderhoff and co-workers [4,5] of Dow Chemical Co. introduced standard PS latex, although the diameters were 557 nm at most. They introduced multi-stage seed emulsion polymerization, and established the theoretical background of why the particle size distribution becomes narrower on adopting the seed process. Vanderhoff continued his work on particle growth after he transferred to Lehigh University and, cooperating with the NASA research group, a series of extremely monosized PS spheres of up to 30 nm diameter were obtained from successive seeded emulsion polymerizations carried out on the Space Shuttle in a zero-gravity field [6,7]. The RSDs of the diameters of these spheres were, in most cases, less than 2%. After the flight of the Shuttle was terminated temporarily, they overcame the gravity problem on the ground, and extended the diameter to 50 urn, and eventually up to 100 pm. Ugelstad et al. [S] developed the two-stage swelling technique, which comprised first an absorption of water-insoluble oligomer by the seed particles, and then a swelling of monomers into seeds up to a thousand times greater in volume. The incorporation of water-insoluble oligomers into the seed particles requires an extension of the Morton equation [9] to the three-component system, and a seemingly unbelievable amount of swelling of monomers is justified by the thermodynamic theory. Ugelstad’s group has been producing a variety of uniform spheres as large as 100 nrn in commercialscale plants [l]. Okubo et al. [lo] developed the dynamic swelling technique, in which water is continuously added to the emulsion of seeds and

Eng. Aspects 109 (1996) 97-107

monomer droplets, creating unfavorable conditions for monomer to be present in droplets. Yoshimatsu et al. [ 1l] prepared a very fine monomer emulsion before swelling, and promoted the diffusion process of the monomer by increasing the surface area of monomer droplets. Okubo and Yoshimatsu obtained spheres of 6-8 pm using their techniques. Physicochemical or mechanical processes are also capable of preparing uniform spheres. Panagioutou and Levendis [12] obtained 20-30 nm uniform PS and poly methylmethacrylate (PMMA) spheres by designing a particular spray tower in which the polymer solution was sprayed from a vibrating orifice plate with a very narrow slit. Uniformity of size was guaranteed only when the slit diameter and vibration frequency satisfied a certain relationship. The atomized droplets may contain monomers and an initiator so that further polymerization takes place during the drying. Hou and Lloyd [ 131 obtained various monosized nylon spheres by controlled precipitation from a theta solvent, a mixture of water and formic acid, under the condition of no mechanical agitation. A careful cooling of the solution at a rate of 1°C s-l induces the phase separation, in other words the nucleation of longer chains, and further cooling promotes the growth of these nuclei by the mutual coagulation and/or absorption of precipitated shorter chains. The average diameters of these nylon spheres are around 10 pm. Kaneka Co., Japan [ 141 possesses several patents in relation to a particular plant for continuous suspension polymerization, in which uniform droplets of PS-styrene syrup (partially prepolymerized), generated from a bundle of jet nozzles, undergo further polymerization while floating up through a towerlike reactor against the counter flow of the stabilizer solution. The size of the particles may be well over 1000 pm. As this brief introduction shows, seed emulsion polymerization seems to be a well-established technique; however, tedious step-by-step operations as well as a great deal of expertise are required to perform successful preparations. The fairly simple process of nonaqueous phase dispersion polymerization is offset by the inevitable use of organic solvents and the lack of flexibility. Membrane emulsification with a subsequent

S. OmilColloids

Surfaces A: Physicochem.

polymerization process is, in principle, similar to those processes using a vibrating orifice plate or a liquid nozzle, but offers far more flexibility to obtain a variety of polymeric microspheres with considerable monodispersity. A particular microporous glass membrane (Shirasu porous glass, SPG) is fabricated by a microphase separation of a mixture of CaO-A1,03-B,03-Si02 [ 151. Spinodal decomposition creates a bicontinuous mixture of CaO-B,O, and A1,OJ-Si02. Acid washing leaves a fairly microporous structure of Al@-SiO, phase. Monomer droplets containing other ingredients, such as solvents, crosslinkers and an initiator, as well as a water-insoluble oligomer, are formed by permeation through these micropores, suspended in the aqueous solution of stabilizing agents, transferred to the reactor, and polymerized. We have used the SPG membrane with different pore sizes ranging from 0.5 to 5.25 pm, have produced a variety of monosized spheres, and have demonstrated several successful applications of these spheres [ 16-191. In this paper, the wide capacity of the SPG membrane will be presented.

Dispersion phase storage tank (pressure bottle)

Stainless

yeI

99

T al This outlet IS closed during emulaficarion.

module

t Monomer

mlel

b)

Fig. 1. Schematic diagram of SPG emulsification: (a) total system; (b) MPG module.

Table 1 Applied pressure for SPG emulsification as a function of SPG pore size

2. Experimental

SPG pore size (pm)

2.1. Methods 2.1 .I. Preparation

Eng. Aspects 109 (1996) 97-107

AP (MPa)

of emulsions

A diagram of the particular emulsification process used is given in Fig. 1. Microporous glass membrane (MPG; NA-I, Ise Chemical Co.), of average pore sizes 0.5, 0.9, 1.4, 1.8 and 5.25 pm, was used for the preparation of monomer emulsion. The glass membrane was a cylindrical annulus (o.d. = 10 mm, L = 150 mm, surface area = 50 cm’), installed in a stainless-steel cylinder as shown in a detailed sketch in Fig. l(b). The dispersion phase, a mixture of monomer, solvent, initiator and water-insoluble substance, was stored in a reinforced glass storage tank, and permeated through the membrane into the recirculating flow under an appropriate pressure. The required pressure depends on the pore size of the SPG and the formulation of emulsion, and the approximate values are shown in Table 1. The droplets were suspended in the continuous phase, and a

0.5 0.11-0.16

0.9 0.08

1.36 0.05

1.70 0.04

5.25 0.01

“Approximate values, dependent on the composition of the dispersion phase, and the stabilizer concentration in the continuous phase.

part of the suspension was stored in the emulsion storage tank with a gradual increase of droplet holdup. Gentle stirring of this tank was necessary to prevent creaming of the emulsion only when the droplet size approached 30 pm. Otherwise, the emulsion has excellent stability. After the desired amount of the dispersion phase was emulsified, the emulsion was withdrawn from the storage tank, and transferred to the reactor. The selected recipes of SPG emulsification for the preparation of styrene (ST) and methyl methacrylate (MMA) spheres are shown in Table 2. Notice that these are not exact recipes for polymerization when the swelling step follows, in particular for the case of MMA.

S. OmijColloids Surfaces A: Physicochem. Eng. Aspects 109 (1996) 97-107

100

Table 2 Formulations for SPG emulsification (1) For PS spheres

(2) For PMMA spheres

Run no. SPG pore size (pm) Continuous phase Water (g) PVA (g) Serial No. SLS (g) Sodium sulfate (g) Hydroquinone (g) Sodium nitrite (g)

425 0.5

407 0.9

347 325 356 409 1.36 5.25 1.36 0.9

416 0.9

359 362 380 1.36 1.36 0.5

450 450 450 450 450 450 450 2.0 5-6 4.5 3.0 3.0 3.0 3.0 - 420 -420 -420 -420 -420 - 120 -420 0.5-0.7 03.5 0.20 0.20 0.05 0.30 0.30 0.1 0.1 0.1 0.1 0.1 0.2 0.2 0.15 0.15 0.15 0.15 0.15 0.15 0.15

Dispersion phase Styrene (g) DVB (g) Acrylic acid (g)

0.1 50

50

50

Solvent Toluene (g) Hexane (g) Heptane (g) Water-insoluble oligomer Lauryl alcohol (g) Hexadecane (g) BP0 (g)

450 450 450 3.0 3.0 3.0 -217 -217 -217 0.2 0.2 0.5

12.5 12.5

12.5 12.5

12.5 12.5 12.5 12.5 0.025 0.05

2.5 1.0

2.5 1.0

2.5 1.0

2.5 1.0

2.1.2. Swelling of seed emulsions

The two-stage emulsification process was developed for the preparation of water-soluble monomer emulsion. A diagram of the process is given elsewhere [ 181. The primary (seed) emulsion consisting of the hydrophobic components was prepared by the SPG emulsification method described above. The secondary emulsion, composed of hydrophilic substances such as MMA, ethyleneglycol dimethacrylate (EGDMA) and hexanol, was obtained using a conventional homogenizer with a very small amount of emulsifier. When the secondary emulsion is added to the seed emulsion, absorption of the hydrophilic substances on the hydrophobic seeds takes place rather rapidly due to the degradative diffusion mechanism [20]. While the droplets of the secondary emulsion were polydisperse, the final droplets retained the uniformity of the seed droplets. During the swelling period, nitrogen was bubbled into the emulsion. Typical recipes for the secondary emulsion, and the amount of the primary (seed) emulsion to

25

25

50

25

2.5 1.0

2.5 2.5 1.0

0.15

25

DVB (g)

Benzene (g) 25

0.1

45 30

Hexadecane (g)

2.5

5.0

5.0

1.0

1.0

0.75

1.0

which the secondary emulsion is added, are shown in Table 3. 2.2. Swelling ratio A theoretical value of the swelling ratio can be defined as follows: Sr = (wt. of monomers and solvents in (wt. of organic phase in the secondary emulsion) the primary emulsion)

+l

(1)

2.3. Addition of extra stabilizer solution after the swelling In the runs where a large amount of organic substances was absorbed in the primary droplets, an extra stabilizer solution was added to the emulsion to prevent coalescence or break-up of the swollen droplets [ 191.

S. Omi/Colloids Surfaces A: Physicochem. Eng. Aspects IO9 (1996) 97-107

Table 3 Formulations of secondary emulsion

2.6. Analytical techniques

Run No.

359

Continuous phase Water (g) SLS (g)

100 100 0.05 0.05

Dispersion phase MMA (g) EGDMA (g) Acrylic acid (g)

10

362

7.0

Solvent Heptane (g) Hexanol (g) Octanol (g) Added to the primary emulsion Weight of primary emulsion (g)

101

380 180 0.05 9 9 0.036 10.4

200

210

160

2.4. Polymerization As the initiator is already present during the emulsification and swelling procedures, efficient procedures are essential to eliminate idle time. An ordinary four-neck glass separator flask was employed as a reactor and also served for the swelling process. A semicircular anchor-type blade of Teflon was installed for agitation. A nitrogen inlet nozzle and condenser were connected to the flask. Nitrogen was purged from the top of the condenser. After the formulation of the emulsion was finished, a gentle bubbling of nitrogen into the emulsion was continued. The bubbling nozzle was removed from the emulsion after 1 h, the ingredients were heated to the reaction temperature, normally 343 or 348 K, and polymerized for 24 h under a nitrogen atmosphere. 2.5. Treatment of polymer particles After the polymerization, the general features of the polymer particles were observed with an optical microscope. Then, polymer particles were removed from the serum by centrifugation, washed with methyl alcohol or ethyl alcohol, and dried under vacuum.

Percent conversion of monomers was determined gravimetrically. Polymer was precipitated by methyl alcohol from the reaction mixture, separated by centrifugation, dried in vacuum, and the dried weight was measured. An optical microscope equipped with a camera was used to observe monomer droplets and polymer particles. Diameters of several hundred droplets were measured from the photographs in order to calculate average diameters. General features of polymer particles were observed with scanning electron microscopy (SEM) (JEOL, JSM-35CFII), and the average diameter was determined from SEM images. Average molecular weight and molecular weight distribution were determined with gel permeation chromatography (HLC-801, Toso Co. Ltd.) employing tetrahydrofuran as an elution solvent. Specific surface area of the porous particles was measured with a mercury porosimeter (Micromeritics Poresizer 9310, Shimadzu), and by the Brunauer-Emmett-Teller (BET) nitrogen adsorption technique (Quantasorb Jr., Yuasa Ionics Co.). Average pore diameter, dg, was calculated as d,=4v/A, where V is the volume of the sample, and A is the measured surface area. 2.7. Materials ST, MMA, divinyl benzene (DVB; a mixture of 55% isomeric DVB, 40% ethyl vinylbenzene, and 5% saturated compounds), and glycidyl methacrylate (GMA) were commercial grade and were distilled under vacuum to remove inhibitors. Acrylic acid (AA) was reagent grade, stored in a refrigerator, and was used after the precipitated polymer was removed by filtration. All of these monomers were from Kishida Chemical Co. EGDMA (Tokyo Chemical Industry Co.) was commercial grade, and was used after the removal of the inhibitor, either by distillation under vacuum or by alkaline washing and drying. Benzene (BZ) was used as the seed emulsion droplets for the preparation of hydrophilic PMMA particles. Toluene (TOL), hexane (HX), and heptane (HP) were used as the diluent for PS-DVB

102

S. OmijColloids Surfaces A: Physicochem. Eng. Aspects 109 (1996) 97-107

porous spheres. Hexanol (HA), octanol (OA), and dodecanol (lauryl alcohol, LA) were used as the diluent for PMMA-EGDMA porous spheres. LA was also used as a water-insoluble oligomer for ST emulsion. Methanol and ethanol were used to precipitate polymer particles and for the extraction of the diluents and unreacted monomers. All the solvents were reagent grade, except for toluene and methanol, which were commercial grade, and were obtained from Kishida Chemical Co. Toluene was distilled under vacuum prior to use. Data of solubility and solubility parameters of the principal monomers and solvents, which provide vital information to determine the formulations of the polymerization and to comprehend the morphologies of the resulting polymeric spheres, are shown in Table 4. Sodium lauryl sulfate (SLS; Merck) was biochemical grade. Poly(viny1 alcohol) samples with different degrees of hydrolysis and degrees of polymerization (DP) were used as stabilizers: PVA-120 (98.5599.4% hydrolyzed, DP = 2000, Kishida Chemical Co.); PVA-420 (80% hydrolyzed, DP= 2000, Kuraray); and PVA-217 (88.5% hydrolyzed, DP = 1700, Kuraray). Hexadecane (HD; Tokyo Chemical Industry Co.) was reagent grade and was used as a water-insoluble oligomer to stabilize droplets of the primary emulsion. Benzoyl peroxide (BPO, Kishida Chemical Co.) with a 25% moisture

Table 4 Solubility

of organic

Reagent

reagent

in water and solubility

3. Results and discussion 3.1. Droplet size of primary emulsion Average size of the droplets in the primary emulsion is plotted in Fig. 2 as a function of the SPG pore size. As can be seen from the Figure, the droplet size increases linearly with the pore size, although the data points scatter around the predicted value because of the different formulation of the dispersed phase. For the case of PS spheres, the slope was 6.62 [ 161, whereas for PMMA, in which case benzene was emulsified, it was 6.00

parameter

(“C)

Solubility (wt.%)

Styrene MMA

25 25

0.03 1.6

9.3 8.8

Benzene Toluene Hexane Heptane Hexadecane

20 16 20 20 25

0.171 0.05 9.47 (10-4) 2.66 ( 10-4) 9 (10-s)

9.2 8.9 1.3 1.45 1.9

Butanol Hexanol Octanol Dodecanol (Lauryl alcohol) Hexadecanol

Temperature

content was reagent grade and was used as an initiator. Hydroquinone (HQ) and sodium nitrite (NaNO,) (Kishida Chemical Co.) were reagent grade and were used as inhibitors to prevent the nucleation of polymer particles in the aqueous phase. Sodium sulfate (anhydride, Na$O,; Kishida Chemical Co.) was reagent grade and was used as an electrolyte. All these chemicals were used as received.

Solubility parameter

25 ? ? 16

6.86 0.59 0.054 1.7 (10-4)

11.4 10.7 10.3 8.1

?

4.1 (10-T)

?

0

1

2

3

4

5

6

SPG pore size (j/m) Emulsions

for

0

PS spheres[l6]

??

PS spheres[l9] PMMA spheres[l8]

0

Fig. 2. Average droplet size of primary emulsion as a function of SPG pore size. Slope of the straight hne=6.62 for PS spheres, 6.00 for PMMA.

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[ 181. Nakashima et al. [ 211 reported a value of 3.25, regardless of the formulation of the emulsion. The difference between the coefficients can be attributed to a horn-like wide opening of the micropores of the glass membranes produced commercially. Mass production of the membrane cannot match the sophisticated expertise of Nakashima et al. Also, each membrane was composed of a delicately different fabric even though the nominal pore size was the same. The RSDs of the droplet sizes were normally between 10 and 15%, and < 10% in several cases. 3.2. Particle size after the swelling and polymerization The average size of the PMMA spheres is plotted as a reduced form, dp/del, against the swelling ratio, Sr, in Fig. 3. Here, dp and d,, denote average sizes of the polymer particles and the droplets of the primary emulsion respectively, and if the swelling proceeds ideally, and 100% conversion is attained, the relationship can be written as d,/d,,

= Sr’j3

(2)

Size reduction after polymerization because of the increasing density of the polymer is not considered, for the sake of approximation. Eq. (2) is shown in Fig. 3 as a solid line. While the majority of the

SPG Pore size

0.5

I

I

2

4

Swelling Fig. 3. d,/d,, as a function line indicates a theoretical indicate porous spheres.

I 6

volume

A

1.36

I 8

10

ratio, S,

of the swelling volume ratio. Solid curve, Eq. (2). Smeared symbols

data fell close to the line, crosslinked and porous spheres yielded higher dp values than those predicted. Coarse and grainy structures were observed with these spheres from SEM images [ 171. As the PMMA-EGDMA network and hexanol are fairly hydrophilic, water molecules may have participated in the development of the pore structure, resulting in a coarse structure and an increase in the diameter of the spheres [IS].

3.3. SEM images of uncrosslinked particles

polymer

SEM images of PS polymer particles obtained from SPGs of different pore sizes are shown in Fig. 4. Part (d), Run 356, shows crosslinked porous spheres. The RSD of the particle size was 18.8, 14.2, 11.3, and 10.8% for 0.5, 0.9, 1.36, and 5.25 urn pore sizes respectively, indicating that the polydispersity became slightly larger with the finer pore size. Despite this tendency, all runs retained the narrow size distribution of the emulsion droplets. A control run of microsuspension polymerization was carried out to compare the polydispersity of polymer particles with those obtained by the SPG emulsification. An identical formulation with runs 425, 407, and 325 was selected, and a conventional homogenizer (Ace Homogenizer, Nissei Co. Ltd.) was employed with an agitation rate of 5000 rev mini. 53.2 and 75.1% of RSD were obtained for the emulsion droplets and polymer particles respectively, a demonstration that the SPG emulsification technique is far superior for obtaining a narrower size distribution [ 161. In Run 325, secondary nucleation of smaller particles was observed, despite the addition of hydroquinone in the aqueous phase. These smaller spheres can be seen on the surface of larger spheres in Fig. 4(c), and an enlarged image clearly indicated that the layers of smaller spheres deposited between the larger particles and acted as a binding agent [ 161. Chains of PVA-120 probably prefer to be dissolved in the aqueous phase rather than adsorbed on the surface of polymer particles, due to the loss of lipophilic sites (99.4% hydrolysis), and played the role of a stabilizer for nucleated chains in the aqueous phase. A switch to less

104

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Surfaces A: Physicochem.

Eng. Aspects 109 (1996) 97-107

b)

Fig. 4. SEM images of uniform PS spheres: (a) Run 425, SPG pore size = 0.5 urn, d, = 2.38 pm, RSD = 18.8%; (b) Run 407, SPG pore size=0.9 urn, d,=4.88 urn, RSD=14.2%; (c) SPG pore size=1.36 urn, d,=8.53 urn, RSD=11.3%, secondary polymer particles can be seen attaching to the surface; (d) Run 356, SPG pore size= 5.25 urn, d,=26.5 urn, RSD= 10.8%, crosslinked porous spheres. Specific surface area = 111.3 mZ g-l.

hydrolyzed PVA-420 (80%) and later to PVA-217 (88%) significantly suppressed the nucleation in the aqueous phase. In the case of PMMA spheres, Run 359 with a formulation without a crosslinker yielded peculiar one-eyed spheres, each sphere possessing a uniform hole created by the phase separation of hexadecane [IS]. Unlike hydrophobic PS spheres, the formation of hydrophilic PMMA chains enhances the phase separation of HD, eventually resulting in the complete isolation of the HD domain. Lauryl alcohol is compatible with PMMA; however, its increasing solubility in water, as shown in Table 4, was not favored because the original uniformity of the primary emulsion was lost during the swelling period. 3.4. Crosslinked nonporous spheres In general, regardless of the type of spheres, the addition of good solvents to the particular polymer

yielded spheres of nonporous structure. However, due to the formation of a network, no holes were observed in PMMA spheres when either DVB or EGDMA was added as a crosslinker. HD was probably distributed in microdomains supported by the fine network. 3.5. Crosslinked porous spheres In principle, a porous structure can be obtained if poor solvents (diluent) for the base polymer constituting a network are present in the recipe, and undergo a micro-phase separation during the polymerization. Delicate balancing between monomers, a mixture of good and poor solvents, and water-insoluble substance (HD) is required to obtain uncoagulating uniform porous spheres. Consultation with solubility parameters in Table 4 provides at least an initial clue to determine the formulation. PS-DVB spheres have more flexibility than their PMMA counterparts because normally

S. OmijColloids Surfaces A: Physicochem. Eng. Aspects 109 (1996) 97-107

b) Fig. 5 (above). SEM images of two failed spheres: (a) Run 409, PS-DVB-(AA)-HP-HD spheres. Porous structure was covered with a skin layer; (b) Run 362, PMMA-DVB-BZ-HP-HD Early separation further spheres. phase prevented polymerization.

Fig. 6 (right). SEM images of crosslinked porous spheres: (a) Run 347, PS-DVB-HP-LA spheres, d,=7.71 urn, RSD= 9.99%, specific surface area = 140 mz g-‘, applied for GPC packing beads; (b) Run 416, PS-DVB-(AA)-HP-LA spheres, d,= 5.20 urn, RSD = 10.9%, specific surface area= 142 mz g-r, applied for enzyme carrier; (c) Run 380, PMMA-EGDMA(AAtBZ-HA-HD spheres, dp =4.99 urn, RSD = 10.6%, specific surface area= 138 mz gg’, applied for enzyme carrier.

b)

105

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Surfaces A: Physicochem.

hydrophobic alkanes such as hexane and heptane are acceptable as effective diluents. Failure occurs only when an overdose of such diluents was employed together with the most water-insoluble HD. Fig. 5(a) demonstrates such an example. As shown in Table 2, Run 409 used HD instead of lauryl alcohol with 50% of heptane in the organic phase. A thin skin layer was unexpectedly formed; a porous structure developed only inside the spheres. A hollow space was also created in the spheres, leading to the collapse of the skin layer. The presence of a small amount of acrylic acid may have enhanced the formation of the hydrophilic skin layer. Without a hydroxyl group such as lauryl alcohol, hexadecane was buried inside the particles. Fig. 5(b) shows failed PMMA spheres. In this case, a phase separation between PMMA-DVB and HD-heptane started from the early stage of the polymerization. A more hydrophilic PMMADVB shell developed in the interface, leaving a mixture of the alkanes and unreacted monomers encapsulated inside the spherical shells. Monomer conversion was only 38.2%, and the liquid phase was easily extracted by washing with methanol, leaving a collapsed shell structure. Learning from this failure, we eventually discovered more acceptable diluents, hexanol and octanol, for PMMA-EGDMA porous spheres [IS]. Examples of successful porous spheres are shown in Fig. 6. The average pore size of Run 347(a) was 0.080 urn, and 140 m2 g- ’ of specific surface area was obtained. A maximum value of 373 m2 g-i was attained for PDVB spheres using heptane as a diluent [ 171. These spheres were applied for the packing material of a GPC column, and the resolution of Run 347 spheres was comparable to those of the commercial products (TX gel, Tosoh). PMMA spheres of Run 380(c) also yielded a fine porous structure. A coarse structure developed as the ratio of heptane to hexanol in the diluent increased [17,18]. Runs 416 and 380 were used as carriers for enzyme immobilization, and a good performance of immobilized glucoamylase for glucose production from oligosaccharides was reported elsewhere [ 171.

Eng. Aspects 109 (1996) 97-107

3.6. Monomer conversion after the polymerization Water-soluble inhibitor, either hydroquinone or sodium nitrite, was added to each run as shown in Table 2, in order to prevent the nucleation of submicron spheres in the aqueous phase. In general, the final monomer conversions reached satisfactorily high levels for the styrene polymerizations. No significant effect of the water-soluble inhibitor (hydroquinone) was observed. In the case of MMA polymerizations, hydroquinone is replaced with sodium nitrite due to the coloring reaction with MMA. According to Fitch [22], sodium nitrite undergoes hydrolysis and forms nitrous acid, which eventually dissociates into nitric acid and nitric oxide. Uneven numbers of electrons of nitric oxides promote coupling with polymeric radicals. However, as nitrous acid is fairly soluble in the organic phase, the possibility arises that the same coupling between radicals may indeed take place in polymer particles, leading to incomplete monomer conversions. In fact, monomer conversions around 70% or less were obtained fairly often. Another possibility for the incomplete conversions may be that, due to the sophisticated formulations of MMA polymersolvents, ization, several kinds of untreated which may contain some inhibiting substances, were present in the polymerization system [ 181.

4. Conclusion A new technique for the preparation of uniform monomer emulsion was briefly reviewed together with the subsequent swelling with hydrophilic ingredients, and the final polymerization. Hydrophobic PS and hydrophilic PMMA spheres were obtained with a variety of sizes and morphologies. The investigation was further oriented to the preparation of 60 urn porous spheres using the swelling technique [ 191. The preparation of more hydrophilic spheres containing water-soluble hydroxyethyl methacrylate (HEMA) and dimethylaminoethyl methacrylate (DMAEMA) is now under investigation.

S. OmiJColloids Surfaces A: Physicochem. Eng. Aspects 109 (1996) 97-107

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