A new technique for foaming submicron size poly(methyl methacrylate) particles

A New Technique for Foaming Submicron SizePoly(methyl methacrylate) Particles

Hiroyuki Ogawa, Akihiro Ito, Kentaro Taki, Masahiro Ohshima

Department of Chemical Engineering, Kyoto University, Kyoto 615-8510, Japan

Received 27 November 2006; accepted 15 May 2007DOI 10.1002/app.26944Published online 2 August 2007 in Wiley InterScience (www.interscience.wiley.com).

ABSTRACT: About 0.7–2 lm diameter poly (methylmethacrylate) (PMMA) foamed particles were prepared viathermally induced phase separation (TIPS) from aPMMA/ethanol mixture and vacuum dried. It was foundthat ethanol, known to be a poor solvent to PMMA, coulddissolve PMMA when the temperature was over 608C. Thesolubility of PMMA (Mw 5 15,000 and Mw 5 120,000) inethanol was measured and was found to increase as thetemperature increased. PMMA particles on the scale ofsubmicron and single micron diameter could be precipi-tated from the PMMA/ethanol solution by temperaturequenching. Then, since the precipitated particles containeda certain amount of ethanol, the precipitated particles

could be foamed using the ethanol as a foaming agent in avacuum drying process. Vacuum drying at temperaturesslightly below the glass transition temperature of the poly-mer could make the particles foam. The effects of foamingtemperature and the molecular weight of the polymer onthe size of foamed particles were investigated. The experi-mental results showed that the vapor pressure and the mo-lecular weight of the polymer are key factors determiningthe expandability of the micro particles. � 2007 Wiley Peri-odicals, Inc. J Appl Polym Sci 106: 2825–2830, 2007

Key words: foam; poly(methyl methacylate); thermallyinduced phase separation; polymeric particles

INTRODUCTION

Polymeric particles are used for many purposes suchas the stationary phase in chromatography, adsorb-ents, catalyst supports, drug delivery vehicles, andpolymeric foams. Thermally expandable micro-spheres are one class of polymeric particles that areused for polymeric foam production as blowingagents or as lightweight fillers. For example, theseparticles are used in printing inks to provide the pro-duction surface textures, and they are also used toachieve weight-reduction of various matrices andimprove the physical properties of the resulting prod-ucts.1,2 Dow Chemical originally invented the conceptof a thermally expandable particle which encapsu-lates a low boiling point hydrocarbon within a ther-moplastic polymer. These particles have beenimproved, and several kinds of particles have beenproduced. The expandable polystyrene beads (EPS)are known as one of the most widely used expanda-ble polymeric particles. They are prepared by suspen-sion polymerization of atactic polystyrene containingpentane as a blowing agent. Because of environmen-tal concerns, Crevercoeru et al. modified the EPS pro-cess and prepared water expandable polystyrene par-ticles by incorporating water in the early stage of

polymerization, stabilizing the particle with the addi-tion of a suitable surfactant, and using water as ablowing agent.3–5 Kawaguchi et al. used acrylonitrile,methacrylonitrile, and vinyl acetate to create a shellfor a thermoplastic expandable microsphere.6,7 In mostof the previously reported processes, polymeric par-ticles are prepared by suspension polymerizationwhere a monomer phase is suspended in a waterphase containing a stabilizing agent, and polymeriza-tion is conducted in the monomer droplets. Further-more, most of the prepared expandable particles are5–50 lm-sized particles before expansion, and no ex-pandable particles in the submicron and single micronsize range have been reported.

Thermally induced phase separation (TIPS) is oneof the techniques that can produce smaller polymericparticles. In the TIPS process, polymer is dissolved insolvents at high temperature. Upon the removal ofthermal energy by cooling, a phase separation isinduced, i.e., nucleation. Schaaf et al. prepared poly-meric particles from the crystallization of semi-dilutesolutions of polyethylene and poor solvents in a TIPSprocess8 where the particle size was in the range of 1–10 lm diameter. Hou and Lloyd reported the prepa-ration of fairly uniform nylon polymer particles usinga TIPS process,9 and they produced 5–13 lm sizedparticles. Polyethylene particles were prepared usingphase separation between solvent (decane) and non-solvent mixture (tetraglyme),10 and submicron sizedpolypropylene particles were made by Matsuyamaet al. from a mixture of polypropylene and diphenyl

Correspondence to: M. Ohshima ([email protected]).

Journal of Applied Polymer Science, Vol. 106, 2825–2830 (2007)VVC 2007 Wiley Periodicals, Inc.

ether (DPE).11 Nano/micro size PMMA particleswere prepared by cooling PMMA and 1-propanolmixtures,12 and it was shown that the particle sizewas in the range of 0.1–10 lm. The particles producedby TIPS are normally smaller than those produced bysuspension polymerization. However, there havebeen no reports on submicron and single micron sizeexpandable polymeric particles prepared via a TIPSprocess.

In this study, since polymeric particles prepared bya TIPS process show submicron size particles andalso contain solvent upon precipitation, both submi-cron size and single micron size expandable poly-meric particles were prepared. Single micron or sub-micron sized PMMA particles were precipitated by aTIPS process from polymer/ethanol solutions. Then,using the ethanol in PMMA particles as a foamingagent, the particles were foamed by a pressure differ-ence and vacuum pressure. The effects of the foamingtemperature and vacuum pressure on particle sizeand morphology were investigated.

EXPERIMENTAL

Materials

Two different molecular weights of poly (methylmethacrylate), (PMMA 15k and PMMA 120k) werepurchased from Aldrich Chemical Co. Their weightaverage molecular weights (Mw) and polydispersityindexes (PI) were measured using a gel permeationchromatography (Shimadzu RID-10A, eluent: chloro-form, column: Shim-pack GPC-802C and 804C, cali-bration curve: polystyrene standards). The Mw and PIwere 14,600 and 1.53 for PMMA 15k and those were123,000 and 1.96 for PMMA 120k, respectively. Theywere powder-like materials and were used withoutfurther purification. The glass transition temperatures(Tg) of the PMMA materials were measured using adifferential scanning calorimeter (Pyris 1, Perkin-Elmer Japan) at 108C/min in a nitrogen purgedatmosphere. Ethanol (99.9% in purity, Wako PureChemical, Japan) was used as received.

Solubility of PMMA in ethanol

To conduct the temperature quenched TIPS process,the solubility of PMMA in ethanol was essential toknow. The solubility of PMMA in ethanol was meas-ured from the weight loss of the PMMA sample afterimmersing the polymer in ethanol. The disk-shapedthin film samples were prepared from PMMA by acompression-molding machine (Big press., Imoto,Japan). The sample was dried at room temperature ina vacuum chamber for more than one day, andweight wi was measured. Then, the sample wasimmersed for 5 h into we gram ethanol in a cylindrical

glass cell with a plug sealing screw cap at a specifiedtemperature under well-mixing conditions. Afterimmersion of the sample in ethanol, the solid samplewas retrieved and dried at room temperature in thevacuum chamber. The completely dried PMMA sam-ple was weighed by an electric balance (AT261 DeltaRange, Mettler Toledo, Germany) to obtain the weightwf. The weight of the sample decreased because someamount of the polymer dissolved into ethanol. Fromthe weight measurements, the solubility of PMMA inethanol was calculated by eq. (1)

S ¼ wi � wf

we(1)

where S is solubility of the polymer in ethanol [g-polymer/g-ethanol], wi is the initial weight of thesample, wf is the weight of the sample after soaking,and we is weight of ethanol.

Preparation of PMMA particles

PMMA particles were prepared by a TIPS processwith ethanol in a 150 mL glass cell. The cell wassealed with a screw cap and immersed in an oil bathwhere the temperature was controlled within 60.18Caround a given setpoint. 5 g of ethanol and 0.05 g ofpowder-PMMA were put into the cell and heated upto the setpoint. By stirring the solution with a mag-netic stirrer at 100 rpm for 30 min, a homogeneousand transparent solution was obtained. Then, the so-lution was quickly cooled down to a lower tempera-ture, 258C. When it was cooled down, the mixtureshowed a slightly opaque color characteristic ofcolloids. The colloidal suspension was filtered byvacuum filtration with a membrane filter of 0.1 lmpore size.

Foaming PMMA particles by vacuum drying

The ethanol remained in the precipitated PMMA par-ticles because the particles were prepared by phaseseparation using the TIPS process. The ethanol inPMMA could be used as a physical blowing agent tofoam the particles. The filtered PMMA particles wereput into a pre-heated vacuum dry oven immediatelyafter filtration. The temperature of the vacuum ovenwas controlled within a precision of 618C around aset-point in the range of 25–1108C. After putting theparticles into the vacuum oven, the pressure in theoven was reduced down to 0.1 kPa within 5 min, andthe vacuum pressure condition was kept for 10 min atthe specified temperature. The vapor pressure ofethanol is a function of temperature. When the par-ticles were heated, the vapor pressure was establishedinside the particles by the ethanol, while the particleswere soft. The vacuuming operation at a certain

2826 OGAWA ET AL.

Journal of Applied Polymer Science DOI 10.1002/app

temperature created a pressure difference betweenthe inside and the outside of the particles. The pres-sure difference made the particles foam. In this stage,heating alone could not make the particles foam, sinceunder atmospheric pressure, a higher temperaturewould be needed to establish a sufficient pressure dif-ference to expand the particles, but this higher tem-perature would reduce the viscoelasticity of polymertoo much and would often lead to particle agglomera-tion and melting.

Particle size measurements

The foamed particles were dried in the vacuumchamber at room temperature and coated with a thinPd-Au film for FE-SEM measurement. The particlesize was measured from micrographs taken by a fieldemission scanning electron microscope (FE-SEM)(JSM-6340FS, JOEL). The particle diameter was deter-mined by averaging the diameter of about 300 par-ticles observed in the FE-SEM micrographs, and thecoefficient of variation (CV) of the sample diameterwas also calculated from the data to evaluate variabil-ity. The number average diameter and CV were calcu-lated by eqs. (2) and (3), respectively

davg ¼PNi¼1

di

N; (2)

CV½%� ¼

0:5�PNi¼1

ðdi�davgÞ2

N�1

0B@

1CA

davg� 100; (3)

where, davg is the average diameter of particle (lm),N is the total number of particles to be measured,and di is the diameter of the ith particle (lm). CVstands for the coefficient of variation.

RESULTS AND DISCUSSION

Glass transition temperature of PMMA

The measured glass transition temperatures, Tg, ofboth PMMA grades are listed in Table I. The Tg ofPMMA 15k was 888C, which was lower than that ofPMMA 120k.

Solubility of PMMA in ethanol

The solubility measurement was conducted by vary-ing solution temperature from 60–908C. The resultingsolubility data are given in Figure 1. In Figure 1,squares represent the solubility of PMMA 15k and tri-angles represent that of PMMA 120k. The solubilityof PMMA 15k in ethanol increases at temperaturesover 608C, and the solubility of PMMA 120k increasesat temperatures over 758C. Ethanol is known to be apoor solvent to PMMA, but it can dissolve PMMA asthe solution temperature increases. The solubilitydata clearly showed that the TIPS method could beperformed to precipitate PMMA particles from thePMMA/ethanol mixture.

PMMA particles prepared by TIPS

PMMA 15k particles were prepared by the TIPS pro-cess at various initial PMMA concentrations in etha-nol. Figure 2 shows an SEM micrograph of PMMA

TABLE IGlass Transition Temperature of PMMA Samples

PMMA 15k PMMA 120k

Tg (8C) 88 108

Figure 1 Solubility of PMMA in ethanol at different solu-tion temperatures.

Figure 2 FE-SEM micrograph of PMMA 15k particles pre-cipitated from 0.5 wt % PMMA/ethanol mixture and driedat 258C under atmospheric pressure. Scale bar is 1 lm.

FOAMING SUBMICRON SIZE PMMA PARTICLES 2827


particles precipitated from a mixture. The precipi-tated PMMA particles are spherical with a fairly uni-form particle size and a smooth surface. Figure 3shows the effect of the initial polymer concentrationon the average particle size. As the concentrationincreases, the diameter of the precipitated particlesincreases. Table II shows the CV value of the particlesobtained at each initial polymer concentration.Because of the lowest variability, the PMMA particlesprecipitated from 0.5 wt % PMMA 15k and 99.5 wt %ethanol solution were further used for the foamingexperiments.

PMMA particle foaming using thevacuum drying process

To confirm whether the precipitated particles con-tained ethanol, simple drying experiments wereconducted on precipitated particles, and the driedparticle size was measured. Without vacuuming, theparticles, which were precipitated from a mixture of0.5 wt % PMMA 15k and ethanol, were dried in anoven at 258C by varying drying time from 10 min to24 h, and the average particle diameter was meas-ured. Figure 4 illustrates the change in the diameterof the dried particles versus drying time. As shown inFigure 4, the diameter of the particles decreased withdrying time, which indicates that the particles containethanol, which swells the particle initially and thengradually diffuses out from the particles.

The precipitated particle, which was prepared from0.5 wt % PMMA/ethanol mixture, was foamed in avacuum oven at 1108C under 20.1 kPa-gauge pres-sure conditions. Figure 5 illustrates an FE-SEM micro-graph of foamed PMMA 15k particles. The foamedparticles were spherical, and their diameter expandedto be 1.3 times as large as that of the nonfoamed par-ticles in Figure 2.

To see the inside of the foamed particle, the foamedparticles were ground in a mortar. Figure 6 shows anFE-SEM micrograph of a ground foamed PMMA 15kparticle, and it clearly shows a hollow structure. Themechanism of forming the hollow structure could beas follows. The precipitated particles contained etha-nol, and when they were placed in a vacuum oven,where the temperature and pressure were controlled,a certain amount of ethanol diffused from the particle

Figure 3 Effect of initial polymer concentration on the di-ameter of precipitated PMMA 15k particles.

TABLE IIAverage Diameter of PMMA 15k Particles Prepared by

Temperature Quenching from 80–258C

Initial PMMA conc. in ethanol [wt %]

0.1 0.5 1.0 2.0 3.0

Diameter (lm) 0.72 1.51 2.89 3.72 3.97CV (%) 16.9 14.7 15.2 16.3 17.8

Figure 4 Change in the average diameter of PMMA 15kparticles against drying time. The particles were dried at258C immediately after precipitation.

Figure 5 FE-SEM micrograph of PMMA 15k particlesfoamed at 1108C under 0.1 kPa vacuum. The particleswere precipitated from a 0.5 wt % PMMA/ethanol mix-ture. Scale bar is 1 lm.

2828 OGAWA ET AL.


surface and established a concentration profile insidethe particle. Then, the pressure difference, which wascreated by the ethanol vapor pressure and by vac-uum, induced bubble nucleation in the particles andexpanded the particles. Since the ethanol had diffusedfrom the surface of the particles, the ethanol concen-tration was low at the surface. Thus, the degree of sol-vent swelling was reduced at the surface of each par-ticle, i.e., the surface of the particles became stiffercompared with the particle interior. Because the sur-face of the particles was stiff and the inside was soft,bubble nucleation occurred dominantly in the particlecore, and the nucleated bubbles were easily coalescedinside the particle. This mechanism would lead to theformation of a hollow structure.

The particle was foamed under vacuum for 10 minby varying the temperature in the range of 25–1108C.

To see the effect of vacuum, the particles were driedunder atmospheric pressure at the same temperature.Figure 7 shows the average diameter of the foamedparticles at each foaming temperature. Solid circlesand triangles, respectively, represent the diameters ofparticles dried under 20.1 kPa vacuum gauge-pres-sure and those foamed under atmospheric pressurewithout vacuuming. Under vacuum drying condi-tions, the diameter increased at temperatures over708C, which was below the glass transition tempera-ture of the original PMMA, 888C. Under atmosphericconditions, the diameter of the particles was slightlyincreased at 808C, but suppressed to around 1.25 lmover this temperature. This temperature correspondsto the boiling point of ethanol, which is 788C. It canbe considered that the glass transition temperatureof polymer, Tg, was reduced by the dissolved etha-nol, and the particles were foamed at a temperaturebelow the original Tg due to the pressure differencebetween the vapor pressure of ethanol and thevacuum pressure.

Effect of molecular weight on foaming

The effect of the molecular weight of the polymer onparticle foaming was investigated using PMMA 120k.The PMMA particles were prepared from 0.5 wt %PMMA 120k/ethanol solutions under the same tem-perature conditions in which the PMMA 15k particleswere prepared. The foaming experiment was con-ducted under 20.1 kPa-vacuum gauge-pressure witha 10-min drying time by varying the drying tempera-ture in the range of 25–1308C. The average diametersof the resulting particles are shown in Figure 8, witherror bars indicating the max and min diameters. Thediameter of the initial precipitated particles was about

Figure 6 FE-SEM micrograph of a ground foamed parti-cle (PMMA 15k). Scale bar is 1 lm.

Figure 7 Average diameters of PMMA 15k particlesfoamed at different foaming temperatures under 0.1 kPavacuum. Error bars indicate the maximum and minimumdiameters observed on the micrograph.

Figure 8 Average diameter of foamed PMMA120k par-ticles at different drying temperatures under 0.1 kPa vac-uum. Error bars indicate the maximum and minimumdiameters observed on the micrograph.

FOAMING SUBMICRON SIZE PMMA PARTICLES 2829


0.7 lm. The diameter actually became larger by foam-ing at temperatures above 1008C. The Tg of PMMA120k is 1088C, as shown in Table I. Because of thehigher Tg of PMMA 120k, the higher viscoelasticity ofthe PMMA matrix suppressed bubble nucleation andgrowth in the polymer below 1008C. The foamingtemperature where the particle diameter increase ofPMMA 120k occurs is at higher temperatures thanthose for PMMA 15k due to the difference in the Tg ofneat polymers. The diameter of particles foamed at1308C was reduced. This is due to the higher diffusiv-ity of ethanol out from the particles.

CONCLUSION

Submicron and single micron sized PMMA expanda-ble particles were prepared from PMMA and ethanolmixtures by the TIPS method. A new technique forpreparing expandable particles was developed by theTIPS scheme with ethanol as the theta solvent. ThisTIPS method could be used to prepare the submicronand micron sized particles containing volatile solvent,ethanol in this study. Owing to the presence of etha-nol in the particles, the particles were foamed undervacuum conditions using ethanol as a physical foam-ing agent. The foaming temperature is a key variable

to manipulate in order to make the particle foam. Thetemperature should be appropriately chosen consid-ering the glass transition temperature of the polymerand the vapor pressure of ethanol. Vacuum wasneeded to create a sufficient pressure difference betweenthe inside and outside of the particles to expand thesoftened particles.

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A new technique for foaming submicron size poly(methyl methacrylate) particles