J. Biol. Macromol., 23(2), 37-45 (2023)

ReviewSynthesis of polymer nanoparticles using methylcellulose gel as reactor
Tetsuya Yamamoto
Department of Chemical Systems Engineering, Nagoya University, 464-8603 Furo-cho, Chikusa-ku,
Nagoya-Shi, Aichi 464-8603, Japan

Synthesizing polymer nanoparticles have required large amounts of surfactants, which
increase environmental pollution. It is indispensable to develop a synthesis method for polymer
nanoparticles that does not require surfactants. In the present study, a methylcellulose (MC) gel
was used as a polymerization field to prepare polymer nanoparticles with a particle diameter <100 nm. MC gel was a biocompatible hydrophilic polymer and low environmental load. Thiswas used as a reactor to carry out the soap-free emulsion polymerization of styrene. The meshwidth of the gel decreased with increasing MC concentrations. The three-dimensional network
structure of the MC gel prevented the coagulation growth of the particles due to Brownian andthermal motions to decrease the size of the polystyrene particles. Polystyrene particles with theaverage size of 31 nm were obtained at an MC concentration of 1.0 wt%. In addition, curdlan
and melamine foam, which had also three-dimensional network structures, were used as reaction
fields of soap-free emulsion polymerization of styrene to make polystyrene particles nanosized.
Received July 11, 2023; Accepted July 19, 2023
Keywords: Polymer nanoparticle, Thermo-reversible gel, Methylcellulose, Soap-free emulsion
polymerization
Introduction
Polymerabrasion resistance [1-3]. In recent years, byexcellenttaking advantage of the diffusion effect ofadhesion and permeability and are used as

nanoparticles in the body, they have been

additives for paints and coating agents for paper

actively used in the medical field as drug carriers

and leather to improve water resistance andin drug delivery systems and molecular imaging

Corresponding author: Tetsuya Yamamototechnology [4,5].

E-mail:nanoparticles

haveyamamoto.tetsuya@material.nagoya-

Emulsion polymerization is the primaryu.ac.jpsynthesis method for polymer nanoparticles
37

Synthesis of polymer nanoparticles

Fig. 1. Particle formation mechanism of soap-free emulsion polymerization of styrene.

[6,7]. However, this method requires the use ofeffect of the MC concentration on the particle size

large quantities of surfactants. This results inby changing the mesh width of the gel.

issues such as surfactant contamination ofgroundwater, adverse impacts on the aquatic

Effect of MC concentration on gel network

ecosystems, and removal costs due to activatedstructure and particle size

carbon and membrane treatment, necessitating a

Styrene (St, Tokyo Chemical Industry Co.,

reduction in the usage of surfactants [8].

Ltd.) was used as a monomer, and 2,2'-azobis[2-(2-To address these issues, research has been

imidazolin-2-yl)propane] dihydrochloride (VA-

conducted on polymer particle synthesis methods

044, Fuji Film Wako Pure Chemical Industries,that reduce surfactant use. For example, emulsion

Ltd.) [13] was used as a polymerization initiator,

polymerization

can

occur

at

surfactant

along with potassium peroxodisulfate (KPS, Fuji

concentrations

below

the

criticalmicelleFilm Wako Pure Chemical Industries, Ltd.). An

concentration. Ishii et al. proposed a method for

MC1500 (Shin-Etsu Chemical Co., Ltd.) was usedsynthesizing nanoparticles by copolymerizing

as the MC. Distilled water was prepared using astyrene and methyl methacrylate, in which thedistilled water manufacturing apparatus (WG250,

amount of surfactant added was reduced to

Yamato Scientific) 15 mL of distilled water wasapproximately 1/10 compared to conventional

degassed by nitrogen bubbling for 20 min in a 30-

methods [9].

mL screw tube. Next, 0.1 g of styrene was added,

This study aims to synthesize nanoparticles

and ultrasonic waves were applied at 38 kHz for 30

smaller than 100 nm without using surfactants. The

min using an ultrasonic cleaner (US-5KS, SND).

mechanism shown in Figure 1 was proposed for theAfter irradiation, a styrene emulsion (withparticle formation of monomers, such as styrene.

submicron-sized oil droplets) was prepared. After

Nanoparticles are formed during the initial

the screw tube was placed in water at 80 °C for 5

polymerization stage, and the particles aggregate

min, MC was added and stirred with a shaker for 5

and grow owing to Brownian and thermal motions

min to disperse the MC. A polymerization initiator[10-12]. In this study, we used methylcellulose

was added after allowing it to stand in water at 5 °C(MC), which gels when heated, as a reactor andand cooling it. The mixture was stirred on a shaker

investigated a method for synthesizing polymer

for 5 min to disperse the initiator into the solution.

nanoparticles of 100 nm or less by suppressing the

The screw tube was heated at 70 °C for aaggregation growth of particles due to Brownianpredetermined time using a temperature controller

and thermal motions. We also investigated the(RCH-20L, EYELA), and polymerization was38

T. Yamamoto

sample was coated with a thin osmium film via
(a)

(b)

vapor deposition (OPC60A, Filgen). The particle
size was calculated by averaging over 200 particle
sizes identified from the SEM images using image
analysis software, Azo-kun. Zeta potentials of thesynthesized

Fig. 2. Photos of the samples at MC concentrationof 1.0 wt% and the following temperatures (a)25 °C and (b) 70 °C.

particles

weremeasuredusing

Zetasizer (Malvern Panalytical).
To observe the effect of gel mesh width on

carried out under stationary conditions without

particle morphology, the gel mesh width wasstirring or other mixing operations. This heating

measured with the MC concentrations. Because thecaused the MC to gel and become cloudy, as shown

MC gel is thermoreversible, it undergoes a sol-gel

in Figure 2. After polymerization, the screw tube

transition at room temperature and returns to thewas cooled and returned to the sol, the particle

sol state. The MC was lyophilized (FD-1000,

dispersion (10 mL) was sampled, and the

EYELA) in gel form and observed under an

nanoparticles were recovered using a centrifuge

electron microscope. Distilled water, which was(3700, Kubota) at a rotation speed of 15,000 rpm.

degassed by bubbling nitrogen for 20 min, was

The recovered particles were redispersed in 10 mL

placed in a plastic container. After that, the MC wasof distilled water via ultrasonic irradiation for 30

dissolved by the same method as in the fine particlemin.

synthesis described in the previous section andParticle synthesis was considered to haveheated at 70 °C for 1 h to form a gel. The gelproceeded sufficiently in the gel based on thesamples were placed in liquid nitrogen and frozen.

polymerization ratio. The reaction solution during

The gel was allowed to stand for 24 h in a freeze-polymerization was weighed out and placed in adryer to remove the water. The freeze-dried gel wasconstant temperature bath at 80 °C to remove water

fixed on carbon tape and observed under aand unreacted monomers. The polymer mass wasscanning electron microscope (FE-SEM, JEOL)

obtained from the weight difference, and the[14].

polymerization ratio was calculated from the

Figure 3 shows SEM images of the MC gelsamount of monomer added. For the sample with[14]. The mesh grid width of the gel decreased as

added MC, the polymerization ratio was calculated

the MC concentration increased. Because thefrom the polymer mass after subtracting the mass

number of crosslinking points in the gel increasesof MC.

with increasing MC concentration [15], it is

The average particle diameter was calculated

thought that an increase in the number ofbased on the FE-SEM results. The observation

crosslinking points reduces the mesh width.

samples were prepared as follows: A small amountFigure 4 shows SEM images of the particles

of particles redispersed in distilled water was

synthesized in the MC gel. Particles with adropped onto the mica piece. After drying, the39

Synthesis of polymer nanoparticles

Fig. 3. SEM images of MC gels at the following MC concentrations: a) 0.1 wt%; b) 0.5 wt%; c) 1.0 wt%.

Fig. 4. SEM images of polystyrene nano particles with VA-044 of 2.0 mM at the following MC
concentrations: a) 0 wt%; b) 0.1 wt%; c) 0.5 wt%.
diameter of 50 nm or less were observed at an MCwater near the gel mesh [16]. The interface area

concentration of 0.5 wt%. In contrast, when the

between the MC gel and water is believed to

MC concentration was 0.1 wt%, particles with aincrease as the MC concentration increases.

diameter of 100 nm or more were synthesized,

Consequently, the proportion of hydrated andconfirming that the particle diameter was non-intermediate water at the interface between the gel

uniform.

network and water increases, and the fluidity of theBased on the above results, we summarized thewater in the gel decreases. Therefore, the Brownian

effect of the MC concentration on the mesh width

and thermal motions were suppressed by loweringand particle diameter. The smaller the mesh width

the fluidity of water, and the aggregation growth ofof the gel, the smaller the particle size. In particular,fine particles was suppressed, resulting in particles

we confirmed the synthesis of 31-nm particles atof 50 nm or less.

an MC concentration of 1.0 wt%. Presumably, thisOn the other hand, Figures 3a shows that theis because the gel mesh reduces the fluidity of thegel mesh is non-uniform at an MC concentration ofwater in the gel, suppressing the Brownian and

0.1 wt%, and some parts are sparse in the mesh. At

thermal motions of the particles and thean MC concentration of 0.1 wt%, the particle size

aggregation growth of the particles.

increased, and particles with a particle size of 100

Particle displacement due to Brownian and

nm or more were observed shown in Figure 4b.

thermal motions is affected by the particle

One reason for this is that the fluidity of water isdiffusion. Water in the gel is in a state of low

relatively high in the sparsely meshed part, and it

fluidity, such as hydrated water or intermediate

is difficult to suppress the aggregation growth of40

T. YamamotoFig. 5. SEM images of polystyrene nano particles with MC concentration of 0.5 wt% at the following VA044 concentrations: a) 1.0 mM; b) 5.0 mM; c) 10 mM.
particles.

agglomerate in water to form nanoparticles.

As shown in Figure 5, when the initiator

Because the fluidity of water in the gel is low, theconcentration increased, the average particle size

aggregation between nanoparticles is suppressed.

increased, making it possible to control the size ofthe nanoparticles. This is probably because an

Effect of enzymatic degradation reaction of MC

increase in the initiator concentration can generateon dispersion stability of microparticles

the secondary particles which make contributions

When the MC gel was used as reactor, theto particle growth through the coagulations [17].

synthesized particles were covered with MC.

As shown in Figures 3a and 4b, the particle size

Therefore, their dispersion stabilities were unstable.

distribution of the polymerized particles in the gel

Using

enzymatic

degradation

reaction,we

was wide, and the coefficient value, which was

attempted to improve the dispersion stability of thedefined as the standard deviation divided bynanoparticles synthesized in the MC gels.

average size, exceeded 20%. In general, soap-free

The dispersion stability of the nanoparticles

emulsion polymerization produces particles withsynthesized in MC gel is an important. Because

high particle size uniformity because of particle

nanoparticles have large surface energies and are

aggregation was occurred during the particle

unstable, adhesion and agglomeration of particles

growth. On the other hand, the particles

are likely to occur. Therefore, in this study, MC

polymerized in the gel suppressed the aggregation

was enzymatically degraded to improve theof the particles; therefore, it can be assumed that

dispersibility of the nanoparticles in water.

the uniformity of particle coagulation did not

Cellulase (sucrase ™ C, Mitsubishi Chemical Co.)

progress, and the particle size distribution widened.

was used to degrade the MC. Enzymatic

From the above observations, the particle

degradation is a reaction that hydrolyzes the β1-4

generation mechanism in the MC gel was as

bond of cellulose and can decompose water-

followed. Styrene monomers dissolved in watersoluble cellobiose [18].

from styrene oil droplets react with a water-soluble

Enzymatic decomposition experiments were

initiator. As the polymerization progresses, theperformed in two ways. These included enzymatic

polymer aggregates and precipitates to form

decomposition experiments of MC alone and an

particle nuclei. Subsequently, the particle nuclei

enzymatic decomposition experiment of the41

Synthesis of polymer nanoparticles

particles in water was evaluated using dynamic

ζ potential [mV]0light scattering (DLS, Malvern Panalytical) and
Zetasizer.

-5

Figure 6 shows the relationship between the
enzymatic decomposition reaction temperature and-10

the zeta potential of the polystyrene particles-1520

synthesized by KPS. The zeta potential of the
30

40

50

60

70

synthesized particles showed -0.2 mV before theEnzymatic decomposition
reaction temperature [℃]enzymatic degradation reaction, which was not
originated from KPS-derived sulfate ions (SO4-).

Fig. 6. Effect of enzymatic decomposition
reaction temperature on zeta potential of particle
synthesized by KPS with a concentration of 2.0
mM.

However,theirdispersion

stabilities

were

improved after the degradation reaction, as shownin Figure 6. In addition, because the zeta potential

particle dispersion after particle synthesis. First,

after the enzymatic decomposition reaction wasfor the enzymatic degradation experiment of MC

negative, the SO4- covered with MC were exposed

alone, a 0.5 wt% MC sol was prepared, andon the surface because of the decomposition of MC.

cellulase was added. Enzyme decomposition wasAsperformed by heating at 30 – 60 °C for 24 h. Aftertheenzymatic

temperaturecompletion of the decomposition reaction, thedecomposition

increased,thereactionenzymatic

decomposition activity increased; however, theenzymatic reaction was stopped by cooling at 5 °C.

dispersion stability of the particles did not improve.

After that, it was heated to 70 °C, and gelation was

This was probably because the methoxy group of

confirmed visually. For enzymatic decomposition

MC was thermally aggregated, the enzyme couldof the particle dispersion, 0.02 g of cellulase was

not be efficiently adsorbed onto the particle surface,

added to the particle dispersion prepared using

and part of the enzyme was heat-inactivated [19].

KPS. After heating for 24 h at 30 – 60 °C for

The optimized reaction temperature was found to

enzymatic decomposition, 10 mL of the particle

be 50 oC. The SEM images of the particles at eachdispersion was sampled and centrifuged to collectdegradation reaction temperature were shown in

the fine particles. The recovered particles were

Figure 7. MC, which covered with particles, wereredispersed in 10 mL of distilled water using

degraded to keep some distances between theultrasonic waves, and the dispersion stability of theFig. 7. SEM images of polystyrene nano particles with KPS and MC of 0.5 wt% at the following
degradation reaction temperatures: a) 30 °C; b) 40 °C; c) 50 °C; d) 60 °C.

42

T. Yamamoto

particles, which showed enhancement of their

the polymerization ratio of soap-free emulsion

dispersion stabilities.

polymerization in the MC gel was much higher.

The average size of particles not subjected toThis is attributed to the low fluidity of water in the

enzymatic degradation measured by DLS ranged

MC gel, which prevents the styrene monomer

between 700 ~ 800 nm, which is larger than thedroplets from coalescing owing to Ostwald

average particle size obtained by SEM. In addition,

ripening and phase separation in the upper part ofthe zeta potential of the particles was almost zero,

the reactor, allowing the monomers to besuggesting that the MC molecules adhered to theefficiently used for polymerization.

particle

surfaceandtheparticles

formedaggregates. The average particle size measured byThree-dimensional network reactor

DLS after enzymatic degradation, 100 ~ 200 nm

Using curdlan to make the three-dimensional

was smaller than that of the particles not subjected

network reactor, polystyrene nano particles wereto enzymatic degradation. This is thought to beobtained by soap-free emulsion polymerization of

because of the decrease in the amount of MC

styrene, as shown in Figure 8.

adhering

toredispersion

theofparticlethesurfaceparticles

due

and

the

Applying the present method to melamine

to

the

foam (Figure 9c) as reactor enabled us todecomposition of MC.

synthesize polystyrene nano particles [21, 22]
shown in Figure 9, because the fluidity of theEffect of MC gel on polymerization ratio

solvent was decreased due to the hydrogen bonds

The polymerization ratios at reaction times ofbetween water molecules and triazine rings of the

6 and 24 h were 32%, and 41%, respectively [20].

melamine resin [23].

In a previous study, the polymerization ratio in thesoap-free emulsion polymerization of styrene wasapproximately 30% for the same reaction time, 6 h,and it was assumed that a sufficient amount of
nanoparticles could be synthesized in 6 h using thismethod. As styrene forms a submicron-sized
emulsion, the interfacial area between water andstyrene increases, promoting monomer diffusion,

Fig. 8. SEM image of polystyrene nanoparticles synthesized by reactor using curdlan
to
carry
out
soap-freeemulsionpolymerization of styrene.

and the polymerization ratio becomes comparable
to that of conventional soap-free emulsion
polymerization.
In addition, the polymerization ratio of soap-

Conclusions

free emulsion polymerization, in which styrene

Polystyrene nanoparticles were synthesized

was emulsified and synthesized in water without

using an MC gel as the polymerization field andagitation, was 11.5% during the same period, andinvestigated in terms of particle size and degree of43

Synthesis of polymer nanoparticles

polymerization. The MC gel was enzymaticallyof the nanoparticles, however, they were improved

decomposed to improve the dispersion stability ofby the enzymatic degradation reaction for MC.

the particles. The following conclusions were

(3) A sufficient amount of polymer nanoparticles

drawn:

was synthesized after 6 h of polymerization using(1) The particle size decreased as the mesh width

the MC gel as reactor for soap-free emulsionof the MC gel decreased. We succeeded inpolymerization of styrene.

synthesizing 31-nm nanoparticles at an MCconcentration of 1.0 wt%. This is probably because

Acknowledgments

the smaller the mesh width, the lower the fluidity

This research was supported in part byof the water in the gel and the suppression of

Grants-in-Aid

for

Scientific

Research(No.

aggregation growth due to the Brownian and

21H01702). Prof. Nitta in Ochanomizu University

thermal motions of the particles.

gave us helpful advice and fruitful discussion for(2) The dispersion stabilities of the nanoparticles

achievements to develop the novel reactor using

were poor because MC was attached to the surfaces

biopolymers.

Fig. 9. SEM images of polystyrene particles with KPS using the following reactors: a) conventional
reactor; b) melamine foam reactor shown in (c).

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