Novel Sample Preparation Method of Polymer Emulsionfor SEM ObservationJING XU,1* ZHAOSHENG HOU,2 AND TIANDUO LI11Instrumental Analysis Center, Shandong Institute of Light Industry, Jinan 250353, People’s Republic of China2Department of Chemical Engineering, School of Materials Science and Engineering, Tsinghua University, Beijing 100084,People’s Republic of China

KEY WORDS polymer emulsion; scanning electron microscopy (SEM); novel samplepreparation method; morphology

ABSTRACT The aim of this study was to design a simple and reliable method for obtaining thedetailed information about the average size, size distribution, and the surface morphology ofparticles with variation of the sample preparation of a polymer emulsion. In this work, the charac-teristic features of the particles of rosin size with high viscosity were first described by scanningelectron microscopy (SEM). The morphologies of polymer emulsion of solid lipid nanoparticles andof the microspheres were observed. The advantage of the method is that not only the true size andshape of emulsion particles can be shown, but the problem of high-viscosity emulsion that preventsthere study with SEM is solved. Using this new method, the micromorphology and size distributionof the emulsion particles with different viscosities have been clearly observed. Microsc. Res. Tech.70:847–850, 2007. VVC 2007 Wiley-Liss, Inc.

INTRODUCTION

The characterization of a polymer emulsion hasattracted considerable attention owing to the impor-tance of shape and texture of the emulsion particles indetermining their properties. It is well known that theaverage size, size distribution, and the surface mor-phology of the particles are closely related to theirfunctions. According to the literature reports, the char-acterization of the particles can be achieved using elec-tron spectroscopy for chemical analysis (ESCA) (Wanget al., 2003), photon correlation spectroscopy (PCS)(Dubes et al., 2003; Ricci et al., 2005), X-ray diffraction(Qi et al., 2002), transmission electron microscopy(TEM) (Dubes et al., 2003; Kang et al., 2006), scanningelectron microscopy (SEM) (Dubes et al., 2003; Qiet al., 2002; Quintanar-Guerrero et al., 2005; Ricciet al., 2005; Tuncel et al., 1999; Wang et al., 2003), andvarious other physical methods. Among all the techni-ques, SEM is the easiest visual technique to obtain in-formation about the average size, size distribution, andthe surface morphology of particles.

SEM has been widely applied to observing the mor-phology of the polymer emulsions. However, polymeremulsions with higher viscosity, observed by SEM,have rarely been reported. In this article, a novel newmethod of emulsion sample preparation for SEM hasbeen described and clear three-dimensional images ofsize and surface morphology are presented for polymeremulsions (especially for the high-viscosity emulsion).The advantage of this method is that the true size andshape of the emulsion particles can be observed. Theaim of this work is to develop a new preparationmethod for SEM so that the characteristic features ofthe polymer emulsion samples are revealed visually.

In this work, the characteristic features of the par-ticles of three polymer emulsion systems, including

high-viscosity emulsion and conventional low-viscosityemulsion, have been revealed using the preparationmethod described here.

MATERIALS ANDMETHODS

The preparation of emulsion samples is described asfollows: First, a 1.0-mm-thick glass slide is thoroughlycleaned with organic solvent and then is cut into shortsegments (0.5 cm 3 0.5 cm). Second, the emulsion sam-ple was diluted by anhydrous ethanol with a properproportion using a clean beaker (5.0 mL). The slide seg-ments were laid to the diluted solutions next to eachother and deposited for 2 min, and small moleculeswere removed by evaporation at 258C for 24 h. Finally,the dried samples were coated with gold (*20 nmthickness) using a Sputter Coater SCD-005 (TEC, Eng-land), and were then observed under a Quanta-200SEM (Quanta, Holland).

The polymer emulsion viscosity was measured withNDJ-8S rotary viscosity meter (Shanghai, China) at258C. The viscosity was measured five times to obtainthe average value.

RESULTSRosin Size

In the first system, the surface morphology of theneutral emulsion-type rosin size suspended in benzenesolution was investigated by SEM. The rosin emulsionproducts have been widely used in the papermaking

*Correspondence to: Jing Xu, Instrumental Analysis Center, Shandong Insti-tute of Light Industry, Jinan 250353, People’s Republic of China.E-mail: xujing@sdili.edu.cn

Received 4 January 2007; accepted in revised form 18 March 2007

DOI 10.1002/jemt.20484

Published online 18 June 2007 in Wiley InterScience (www.interscience.wiley.com).

VVC 2007 WILEY-LISS, INC.

MICROSCOPY RESEARCH AND TECHNIQUE 70:847–850 (2007)

industry for paper sizing for more than 190 years(Hedborg and Lindstrom, 1993; Paiste, 1981; Subrah-manyam and Biermann, 1992; Wang and Tanaka,2002; Wang et al., 2003). Compared with a conven-tional acidic rosin sizing, a neutral rosin applicationhas good sizing efficiency under neutral-alkaline condi-tions of paper manufacture because its emulsionparticles are stable to the action of hydroxide.

The rosin sizing has been described by differentialscanning calorimetry (DSC), ESCA, Fourier transforminfrared, and so on (Wang and Tanaka, 2000; Wanget al., 2003). In this communication, we report on thefirst characterization of rosin size by SEM, and thechanges of emulsion particles of rosin size were studiedbefore/after introducing emulsifiers to the rosin sizing.The rosin sizing was dissolved in benzene without add-ing the emulsifiers and only a kind of viscous fluid wasobtained. Figure 1A shows the sizes of the rosin par-ticles range from 0.7 to 4.6 lm. When the aqueous solu-tion of the emulsifiers, ethylcellulose, and magnesiumaluminum hydroxide (MgAl(OH)5), was introduced inthe system, the stable oil-in-water microemulsion ofthe rosin sizing was achieved. Because of high viscosity(4034 mPa s) of the rosin emulsion, the sample prepa-ration for SEM is a challenging task that often limitsthe use of SEM for visualizing a rosin emulsion. Withproper dilution, it is possible to avoid forming a thinfilm with the rosin sample, which prevents the obser-vation of particle morphology. Clear pictures of mor-phology and the size distribution of rosin sizing arepresented (Fig. 1B). The size of the rosin emulsion dis-tribution ranges from 125 to 600 nm. Compared withFigure 1A, the narrow size distribution can be achievedby introducing inorganic emulsifier (MgAl(OH)5). Withthe same preparation method as the sample in Figure1B, except for higher proportion of rosin sizing in thesystem, rosin emulsion does not spread as separateparticles (Fig. 1C).

Solid Lipid Nanoparticles

In the second system, the images of solid lipid nano-particles (SLN) in aqueous solution of surfactants andTween80 (T80) were visualized by SEM. SLN repre-sents a rapidly growing class of colloidal transport sys-tem of considerable interest for pharmaceutical appli-cations (Dubes et al., 2003; Mehnert and Mader, 2001;Muuer et al., 2002; Quintanar-Guerrero et al., 2005;Wissing et al., 2001). Compared with predecessors,drug stability, bioactive agent controlled release, anddrug payload of SLN were increased. The particles sizewould ensure high adhesion to the stratum corneum,thus enhancing the amount of encapsulated agent,which penetrates into skin. And the narrow size distri-bution would contribute to emulsion stability.

SLN has been described by TEM, atomic force mi-croscopy, SEM, PCS, DSC, and so on. A freeze-dryingmethod and other preparation methods were usedwhen the SLN samples were evaluated by SEM (Muueret al., 2002; Quintanar-Guerrero et al., 2005), and thesurface of samples showed collapse and shrinkage,which led to the mean diameter determined by SEMbeing smaller than that determined by PCS and TEM(Dubes et al., 2003). However, the true size and theplump shape of SLN particles were shown when thesample preparation method for SEM described here

was used. Furthermore, the mean diameter deter-mined by SEM was in agreement with the mean parti-cle diameter determined by DLS and TEM.

In this study, clear pictures of morphology and sizedistribution of SLN are presented. Compared with the

Fig. 1. SEM images of rosin size with variation of the sample prep-aration conditions. A: SEM image of rosin size formed in benzene. B:High-magnification SEM image of rosin size in mixed benzene-aque-ous-emulsifiers solution. C: SEM image of instable emulation withhigher proportion of rosin size in the system.

Microscopy Research and Technique DOI 10.1002/jemt

848 J. XU ET AL.

first system, the SLN emulsion has not only high vis-cosity (3215 mPa s) but a high saponification degree,since the amount of surfactants and T80 were intro-duced during the synthesis of SLN. As with the firstsystem, a proper dilution needed to be found. Further-more, the dilution would be shaken by simple son-ication for 3–5 min, which keeps the SLN particlessuspended in solution. A high-magnification SEMimage of the SLN particles (Fig. 2A) shows that trian-gle-, rectangle-, and sphere-like particles were pro-duced. And the sizes of SLN particles range from 80 to130 nm, with an average size of about 116 nm. It iscloser to the result obtained from DLS, with an averagesize of about 115 6 3 nm. For comparison, the SLNemulsion was investigated by TEM, and rectangle- andsphere-like particles were also observed.

SLN consist of pure solid lipids while nanostructuredlipid carriers (NLC) are made of a solid lipid matrixentrapping variable liquid lipid nanocompartments(Ricci et al., 2005; Souto et al., 2004). NLC has beendeveloped to overcome some limitations of SLN, suchas the insufficient total drug payload (Muuer et al.,2002). As SLN emulsion, NLC emulsion also has highviscosity (2804 mPa s), so a proper dilution is needed to

prepare SEM samples. Comparing with Figure 2A, theimage of NLC (Fig. 2B) shows the triangle- and rectan-gle-like particles disappear, and irregular shaped par-ticles are observed with the range from 100 to150 nm(the average size: ca. 132 nm), which is closer to theresult from DLS analyses with an average size of about

Fig. 2. High-magnification SEM images of A: SLN and B: NLCemulsion particles.

Fig. 3. High-magnification SEM images of poly(St-MAA-AA) latexmicrospheres with A: lower, B: higher and C: suitable amount ofSDBS.

Microscopy Research and Technique DOI 10.1002/jemt

849POLYMER EMULSION FOR SEM OBSERVATION

134 6 3 nm. From Figure 2B, some shrinkage can beseen in the particles, which may be caused by theincreased specific surface area of particles and someliquid molecules volatilizing under high vacuum.

The results of the first and second systems indicatethat scanning of properly prepared samples allows usto reveal the morphological features of the polymeremulsion with high viscosity. The sample preparationmethod is also suitable for the polymer emulsion withlow viscosity, such as the third that follows.

Microspheres

In the third system, the microspheres morphologiesof poly(styrene-methacrylic acid-acrylic acid) (poly(St-MAA-AA)) latex particles were exhibited. Recentadvances in polymerization technology have enabledthe preparation of a variety of latex particles, andamong them, monodispersed and functionalized par-ticles have attracted much attention in biomedical andbiochemical fields (Kang et al., 2004; Lee et al., 2001;Tuncel et al., 1999). In this study, low-viscosity (82mPa s) poly(St-MAA-AA) latex microspheres with nar-row size distribution and with surface carboxyl groupswere synthesized by soap-free emulsion polymeriza-tion, and the diameter of particles can be controlled tobe in the range of 600–650 nm.

The surface micromorphology of spheres was investi-gated by changing the amount of emulsifier (sodiumdodecyl benzene sulfonate, SDBS). With the samplepreparation method for SEM described in the article,the surface micromorphology of spheres can beobserved clearly. When the amount of SDBS is lowerduring the polymerization process, the surface of themicrospheres is adhesive (Fig. 3A) and the emulsion isunstable; while the amount of SDBS is higher, the sur-face of the microspheres is smooth (Fig. 3B); only whenthe amount of SDBS is suitable, the microspheres haverough surface (Fig. 3C). It can be estimated that thechange of the surface morphology is caused by theincreasing ratio of surface carboxyl.

DISCUSSION

In this study, we have presented a novel samplepreparation method of polymer emulsion, and the sur-face micromorphology and size distribution of theemulsion particles are observed clearly. The advantageof the method is that not only the true size and shapeof the particles of emulsion can be shown, but the prob-lem of preparing high-viscosity emulsions for SEM hasbeen solved. Using this method, the micromorphologyand size distribution of the high-viscosity emulsionparticles (the first and second systems) have beenobserved clearly for the first time. We only engage in

preliminary research of the high-viscosity emulsionparticles and hope to offer a new tool for the study ofthis kind of emulsion. The novel sample preparationmethod of polymer emulsion for SEM is also suitablefor the conventional polymer emulsion with low viscos-ity (the third system).

ACKNOWLEDGMENTS

We thank Dr. Wenxia Liu and Dr. Liqiang Zheng forproviding the test samples.

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