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The morphology of submicronsized core–shell latex particles: An electron microscopy study
Micron 38 (2007) 522–535 www.elsevier.com/locate/micron
The morphology of submicronsized core–shell latex particles: An electron microscopy study Ce´dric Gaillard a,*, Gilbert Fuchs b, Christopher J.G. Plummer c, Pierre A. Stadelmann a b
a Centre Interdisciplinaire de Microscopie Electronique (CIME), EPFL, CH-1015 Lausanne, Switzerland Arkema, Centre de Recherche Rhoˆne-Alpes (CRRA), Rue H. Moissan, BP 63, 69 493 Pierre-Be´nite, France c Laboratoire de Technologie des Composites et Polyme`res (LTC), EPFL, CH-1015 Lausanne, Switzerland
Received 31 March 2006; accepted 19 July 2006
Abstract The core–shell structure of a range of acrylic–acrylic latexes has been investigated by combining different specimen preparation methods with transmission electron microscopy (TEM), dark-field scanning transmission electron microscopy (DSTEM) and low-voltage scanning electron microscopy (LV-SEM), including the first reported use of LV-SEM to observe composite latex particles at ambient and subambient temperatures. Spin-coating of liquid latex dispersions directly onto TEM grids or SEM stubs is shown to be a relatively straightforward mean of avoiding film formation during specimen preparation. In conjunction with double staining techniques, it has been found to be particularly convenient for characterizing the fine structure of particles with diameters down to below 100 nm. # 2006 Elsevier Ltd. All rights reserved. Keywords: Rubber toughened PMMA; Core–shell latex; Morphological characterization; Transmission electron microscopy (TEM); Low-voltage scanning electron microscopy (LV-SEM); Spin-coating; Multiple chemical staining
1. Introduction Synthetic latexes play an important role in the polymer industry, both for coatings and adhesives. Their inclusion in a polymer matrix leads to significantly improved combinations of physical properties, and in particular, enhanced impact performance. Acrylic core–shell latex systems are of particular interest as impact modifiers for industrial thermoplastics such as polyvinylchloride (PVC) (Dompas et al., 1995), polymethylmethacrylate (PMMA) (Lovell et al., 1993) and styreneacrylonitrile copolymer (SAN) (Steenbrink et al., 1998). The basic core–shell latex particles typically consist of a rubbery polybutylacrylate (Steenbrink et al., 1998) or poly(styrenebutadiene) core (Dompas and Groeninckx, 1994) and a glassy polymethylmethacrylate shell. Efficient impact modification generally requires a strong particle–matrix interface (Plummer et al., 1999), and hence a well-formed, integral shell and the
* Corresponding author. Present address: Plate-forme RIO BIBS, Laboratoire de Microscopie, U.R. BIA, INRA, Rue de la Ge´raudie`re, BP 71 627, 44 316 Nantes Cedex 3, France. Tel.: +33 240 67 51 68; fax: +33 240 67 50 05. E-mail address: [email protected] (C. Gaillard). 0968-4328/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.micron.2006.07.023
particle size is also an important factor (Dompas and Groeninckx, 1994). With increasingly versatile synthetic routes, control of both the morphology and particle size has advanced considerably and multiple or irregularly shaped shells or inverted core–shell structures are now widely available (Stubbs et al., 2003; Sundberg and Durant, 2003). Methods of characterization of the particle morphology must progress with these additional levels of complexity if the development of new synthetic techniques and production of high quality latex particles are to be maintained (Sundberg and Durant, 2003). Relevant characterization techniques used for particle morphology control include atomic force microscopy (AFM) (Sommer et al., 1995; Portigliatti et al., 2000; Kirsch et al., 2001), fluorescence non radiative energy transfer (NRET) (Huijs and Lang, 2000; Pe´rez and Lang, 2000), dynamic light scattering (Manzur, 1994), X-ray photoelectron spectroscopy (XPS) (Barthet et al., 1998; Khan et al., 2000), photon correlation spectroscopy (Hatto et al., 2000), small-angle X-ray scattering (SAXS) (Dingenouts et al., 1999), small-angle neutron scattering (SANS) (De Bruyn et al., 2003; Chevalier et al., 1997), dynamic mechanical spectroscopy (Cavaille´ et al., 1991), differential scanning calorimetry (DSC) (Song and Liao,
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2004), solid-state nuclear magnetic resonance (NMR) (Nelliappan et al., 1995; Kirsch et al., 1999), electrophoresis (Makino et al., 1994), surface tension-based methods (the Maron adsorption method) (Landier et al., 2004) and positron annihilation lifetime spectroscopy (PALS) (Lizama et al., 2001). The most widely used direct characterization technique, however, is transmission electron microscopy (TEM), which is applied to both whole latex particles and sections through resinembedded particles (Sundberg and Sundberg, 1993; Chen et al., 1992; Shi et al., 2002). It has been used, for example, to quantify core–shell morphologies by the shape factor method (Li et al., 2003) and to investigate microdeformation behavior in core–shell particle reinforced PMMA, for example (Plummer et al., 2004). Low-voltage scanning electron microscopy (LV-SEM), on the other hand, remains relatively unexplored as a tool for the direct investigation of composite latex morphologies, although it is considered to be highly promising for polymeric materials in general (Vezie et al., 1995). It has been used successfully for the high-magnification imaging of MMA-embedded core–shell particles (Gaillard et al., 2004), and the observation of the external morphology of whole latex particles (Olah et al., 2004; Wang et al., 2004) and epoxy-embedded latex particles (Ashida et al., 1999). However, to our knowledge, the only use of cryogenic LV-SEM has been for size distribution measurements on PS latex particles (Haridas and Bellarc, 1998). The main difficulties with studying core–shell latexes by electron microscopy are the weak contrast between the different phases and/or between the particles and the background, and their poor electron beam resistance (Joy and Joy, 1996; Egerton et al., 2004). The low resolution often obtained in observation of radiation-sensitive specimens is directly attributable to the weak signal imposed by the need to limit radiation damage and the consequent loss of fine structure (Baudet and Kubin, 1982). Acrylic polymers, which are important components in synthetic latex particles, are particularly prone to radiation damage. In principle, these difficulties may be overcome by using low temperature (Winnik et al., 1993; Subramaniam et al., 2004), low-dose (Williams and Fisher, 1970) and/or low-voltage (Lednicky et al., 2002) techniques and specimen preparation methods that avoid film formation and stabilize and enhance the contrast between the different polymer phases (Sawyer and Grubb, 1996). Selective staining with heavy elements (ruthenium, osmium, tungsten and uranium) is a frequently used method for increasing high-angle Rutherford scattering and generating contrast between the stained and the unstained regions of a polymer specimen (Trent et al., 1983; Ferguson et al., 2002). ‘‘Positive’’ staining occurs when the chemical stain interacts directly with the polymer specimen by diffusion or reaction, whereas ‘‘negative’’ staining is usually applied to the specimen substrate (a hydrophilic carbon film, for example) or embedding medium. Phosphotungstenic acid (PTA) and its sodium or potassium salts are widely used for negative staining of biological specimens (Harris, 2002) and, to some extent, polymer latexes (Li et al., 2002). Other conventional chemical stains that have been successfully used for polymer latexes
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(Sawyer and Grubb, 1996) include osmium tetraoxide (OsO4) for unsaturated polymers such as natural rubber-based latexes (Schneider et al., 1996) and polystyrene/polybutylacrylate composite particles (Kanig and Neff, 1964), ruthenium tetraoxide (RuO4) for saturated and unsaturated polymers (Trent et al., 1983) and both OsO4 and RuO4 used for PS/ styrene-butadiene–rubber/polyolefin blends (Luziniv et al., 2000). Combined use has also been made of positive and negative stains for the characterization of core–shell structures, for example, RuO4 and PTA (Chen et al., 1991; Monteiro and de Barbeyrac, 2001), OsO4 and PTA (He et al., 1997) or RuO4 and uranyl acetate (Ferguson et al., 2002). In some cases, use of only one stain may suffice to give both positive and negative contrast, as has been described for PTA (Harris et al., 1999) and RuO4 (Lei et al., 2004). Chemical staining generally also has the advantage of hardening many types of polymer, which may facilitate specimen preparation and observation, although it may also result in nanoscale artifacts (Chou et al., 2002). Alternative electron microscopy-based methods have therefore been proposed to improve contrast in polymers, including transmission electron holography (Chou et al., 2003), electron energy loss spectroscopy (EELS) and energy-filtering imaging (Egerton, 2003). However, these techniques typically require specialized equipment (Stadelmann et al., 1995) and are most appropriate to specimens in which contrast is associated with characteristic chemical bonds or species (for example, use of the p-plasmon energy loss peak of PS), as in PS/polybutadiene (PB) (Simon et al., 2002), PMMA/PS (Correa and Hage, 1999), polyethylene (PE)/PS (Hunt et al., 1995) and PS/PB/PMMA (Varlot et al., 2000). The present communication explores practical aspects of the characterization of a range of nanosized (particle sizes below 100 nm) and nanotextured multiphase latexes using electron microscopy. Starting with core–shell latexes, synthesized under different experimental conditions and exhibiting different morphologies, the effectiveness of different specimen preparation methods and electron microscopy-based techniques is compared, including TEM, dark-field scanning TEM and LVSEM. 2. Material and methods 2.1. Materials The core–shell latex specimens, provided by Arkema (France), comprised organic core–shell particles with different configurations. The characteristics of the core–shell latexes are given in Table 1. PMMA/3L refers to impact modified PMMA containing relatively large composite particles made up of an inner MMA-rich glassy copolymer core, surrounded by an intermediate styrene-rich rubbery layer. The remaining specimens, designated S1, S2, S3, S4, S5 and S6 consist of aqueous suspensions of composite particles with a rubbery styrene/ acrylic core and an MMA-rich glassy copolymer shell. The proportion of each monomer has been varied in the synthesis step, so that more or less complete shells with different morphologies are expected (see Table 1). OsO4 and PTA
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Table 1 Details of the core–shell latex specimens Designation
PMMA/3L
c
Particle characteristicsa Core
Shell
Glassy MMA-rich copolymer
Rubbery styrene-rich copolymer
Expected morphology/schematicb
Overall diameter (nm)
Large core with
200
integral shell S1d
Rubbery styrene-rich copolymer
Glassy MMA-rich copolymer
Large core with
80
integral shell S2d
Rubbery styrene/acrylic copolymer
Glassy MMA/acrylic copolymer
Large core with very
80
discontinuous shell S3d
Rubbery styrene/acrylic copolymer
Glassy styrene-rich copolymer
Large core with
60
discontinuous shell S4
d
S5
d
Rubbery styrene/acrylic copolymer
Glassy MMA/acrylic copolymer
Large core with
60
discontinuous shell Rubbery styrene/acrylic copolymer
Glassy MMA/acrylic copolymer
Large core containing
100
second phase inclusions S6d
Rubbery styrene/acrylic copolymer
Glassy MMA/acrylic copolymer
Large core with partially
160
discontinuous shell a b c d
MMA, methyl methacrylate. The black and grey regions stand, respectively, for core and shell parts. PMMA-embedded latex. Aqueous dispersion.
(H3O40PW12) are purchased from Aldrich. RuO4 is prepared by mixing 100 mg of ruthenium trichloride (Acros) with 5 ml of 10 wt% aqueous sodium hypochlorite solution (Acros) (Montezinos et al., 1985). Styrene monomer and azo-bisisobutyronitrile (AIBN), used for embedding certain latexes, are provided by Sigma. 2.2. Specimen preparation TEM specimens are prepared from the latexes by either: (i) embedding the particles S1, in PS and preparing thin sections by ultramicrotomy, following method A or (ii) dispersing particles S1, S2, S3, S4, S5 or S6 on a carbon covered TEM grid using a specially developed spin-coating technique, which will henceforth be referred to as ‘‘spin-coating electron microscopy supports’’ (SCEMS, method B). SEM specimens are prepared either by ultramicrotomy of embedded latex particles PMMA/ 3L and PS-embedded S1 as described elsewhere (method C) (Gaillard et al., 2004) or from unembedded particles S1 using SCEMS. Each method is combined with chemical staining using one or more of PTA, OsO4 and RuO4. 2.2.1. Method A Styrene monomer is pre-polymerized by mixing 10 ml of freshly distilled styrene with 0.5 wt% of AIBN and heating at
60 8C for 3 h. Two milliliters of the resulting viscous liquid is then mixed with three drops of the undiluted latex suspension and maintained at 70 8C for 12 h. Ultrathin sections (50 nm thick) are taken from the resulting solid blocks using a diamond knife (Diatome, 458) and an Ultracut E ultramicrotome (Reichert-Jung), and picked up from distilled water onto TEM copper grids (200 mesh) covered with a discontinuous carbon film with square perforations (‘‘Quantifoil S7/1’’ grids from Plano). The sections are then observed either without staining or after staining with RuO4 vapor or a combination of RuO4 vapor and PTA. In this latter case, a drop of aqueous PTA (3 wt%) is placed on the thin film for 1 h, after which the specimen was rinsed with distilled water and exposed to RuO4 vapor for about 15 min. This method is preferred to melamine/ formaldehyde-based embedding, often used for aqueous latex dispersion, because of the excellent stability of PS in the electron beam and because the PS could be selectively stained to provide higher contrast. 2.2.2. Method B (SCEMS) A dedicated spin-coating apparatus has been developed for TEM and SEM specimen preparation (Fig. 1). A SEM specimen holder (a metallic, polytetrafluoroethylene (PTFE) or carbon-coated stub), or a holder adapted for commercial TEM copper grids is mounted on a revolving support
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Fig. 1. Overview of the device used for the spin-coating of latex samples onto electron microscopy supports. (a) Schematic of the spin-coating apparatus, (b) photograph of (i) the apparatus and (ii) the TEM and SEM supports (the arrows indicate where the TEM grids are fixed) and (c and d) TEM and SEM images showing an example of an aqueous PMMA latex spin-coated onto (c) a TEM copper grid covered with a carbon film or (d) an aluminum SEM stub. Scale bars are 2 mm.
(1460 revolutions/min). A drop of latex suspension is introduced to the specimen holder and left to rotate for 3 min. The specimen is then removed from the spin coater and dried under vacuum for approximately 30 min. The latex particles are generally found to be well-dispersed, without the need for dilution of the as-received suspensions, as long as these latter were not excessively viscous. Where required, the SEM or TEM specimens are exposed to RuO4 vapor for 15 min. In the case of the SEM specimens, copper stubs are systematically preferred to aluminum stubs because of their improved resistance to RuO4. PTA staining is carried out by mixing equal volumes of the latex dispersion and aqueous PTA (3 wt%), sonication for 2 h, spin-coating onto the chosen support and, if required, exposure to RuO4 vapor for 15 min.
2.2.3. Method C Fifty nanometers thick microtomed sections are prepared from unstained or RuO4-stained specimens and picked up from distilled water onto a copper SEM stub. Excess water is carefully removed in order to eliminate wrinkles. 2.3. Microscopy TEM investigations are carried out using a HF-2000 Hitachi TEM operated at 200 kV, and equipped with a field emission gun and a dark-field scanning stage. SEM investigations are carried out on uncoated specimens using a XL30 SFEG Philips SEM equipped with a field emission gun and a through-the-lens detector (TLD), operated at 3 kV (working distance, 3 mm; spot size, 3 nm). Some of the LV-SEM images are recorded at
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liquid nitrogen temperature (cryo LV-SEM) using a Gatan Alto 2500 cryo-system. 2.4. Monte-Carlo simulations Monte-Carlo simulations of the secondary electrons intensity in SEM and of electron trajectories are performed using the Plano GmbH MOCASIM software. The geometries used for the simulations schematise: (i) a PMMA/3L polymer
film placed on a copper support according to the method C and (ii) S1 and S6 RuO4-stained particles deposited onto a copper stub following the SCEMS method. Each simulation assumes an acceleration voltage of 3 kVand 600 electron trajectories. To take into account mass loss induced by the electron beam, the thickness of the model film is reduced from 50 to 25 nm in the e-beam-sensitive PMMA-rich regions while a 50 nm in thickness is taken for the e-beam resistant PS-rich parts.
Fig. 2. TEM (a–e) and SEM (f) investigations of PMMA/3L PMMA-embedded latex particles. (a) Bright-field TEM image of unstained embedded latex particles, (b) bright-field TEM of RuO4 vapor-stained embedded particles, (c) dark-field TEM image of unstained embedded latex particles, (d) dark-field TEM image of RuO4 vapor-stained embedded particles, (e) dark-field scanning TEM image of unstained latex particles and (f) LV-SEM image of embedded RuO4-stained particles. Scale bars are 200 nm (a + b + c + d + e) and 500 nm (f).
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3. Results and discussion 3.1. Bright-field and dark-field scanning TEM of embedded core–shell latexes Embedding in a suitable resin has been used to disperse latex particles and facilitate ultra-thin sectioning for TEM examination. Moreover, in the case of impact reinforced plastics, the latex particles are already dispersed in a resin. To enhance the contrast between the different phases, chemical staining is applied either to the embedded particles before sectioning or by exposing a thin section mounted on a TEM grid to the vapor of a volatile stain such as RuO4 (cf. Section 2.2.1). Fig. 2 shows bright-field (Fig. 2a and b) and dark-field (Fig. 2c and d) TEM images of RuO4 vapor stained and unstained ultra-thin films of PMMA/3L, along with a dark-field scanning TEM image of an unstained film (Fig. 2e). The core– shell morphology of the latex particles in ultra-thin films prepared from PMMA/3L is apparent in both bright-field TEM images of unstained sections (Fig. 2a) and RuO4 vapor-stained sections (Fig. 2b). Similar observations are made from the darkfield TEM images (Fig. 2c and d). The contrast in the stained sections may be attributed to selective incorporation of the RuO4 in the styrene-rich rubbery
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shell, resulting in stronger electron scattering and hence a dark appearance in bright-field images (Vitali and Montani, 1980; Lee, 2000). Close examination of the vitreous core (Fig. 2b) also reveals the presence of small heterogeneities, which are attributed to diffusion of styrene monomer into the core during synthesis. Core and shell densities being similar, the strong contrast obtained in the absence of staining (Fig. 2a and c) is assumed to be due to preferential beam-induced mass loss in the MMA-rich copolymer core, PMMA homopolymers generally being significantly more beam sensitive than styrenic polymers. So, depending on the nature of the embedding resin and the components of the latex particle, the staining step may sometimes be dispensed with, the contrast being generated from preferential degradation of one of the polymer phases. Similar contrasts in TEM images of unstained epoxy-embedded latex particles with a PMMA core and a PS shell have been already observed previously, for example (Jonsson et al., 2001). Comparable beam-induced mass thickness contrast between the core and shell is also observed even when the observations are made at liquid nitrogen temperature using a cryogenic specimen holder, consistent with previous cryo-TEM investigations of similar materials (Talmon et al., 1986; Talmon, 1999). DSTEM images obtained with a nanosized electron probe scanned across the specimen and a high angle annular
Fig. 3. TEM (a–c) and LV-SEM (d) investigations of PS-embedded S1 latex particles. (a) TEM of unstained PS-embedded S1 particle, (b) TEM of RuO4 vaporstained PS-embedded S1 particle, (c) TEM of PTA and RuO4-stained PS-embedded S1 particles and (d) LV-SEM image of unstained PS-embedded S1 particles. Scale bars are 50 nm (a), 20 nm (b), 100 nm (c) and 200 nm (d).
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dark-field detector (HAADF) reveal also a core–shell morphology (Fig. 2e). This technique is particularly sensitive to thickness variations, such as those that resulted from beam damage in thin sections of PMMA/3L. Hence, DSTEM/ HAADF images of unstained PMMA/3L (Fig. 2e), show
significantly greater contrast than the corresponding dark-field TEM (Fig. 2) and bright-field TEM images (Fig. 2a). In the case of smaller particles (S1 embedded in PS), various chemical staining techniques are applied (Fig. 3). The S1 particles are more precisely embedded in PS (method A) and
Fig. 4. (a–d) Monte-Carlo simulations of the secondary electrons (SE) emitted when scanning the following samples: (a) RuO4-stained PMMA/3L, (b) PS-embedded unstained S1, (c) unembedded RuO4-stained S1, (d) unembedded RuO4-stained S6. The schematics show the models on which the simulations were based and the black curves give the SE intensity (arbitrary units) as function of position (abscises values in nm). Right-hand insets show an experimental LV-SEM image of the simulated specimen. (e–h) Projections of the simulated electron trajectories corresponding to: (e) RuO4-stained PMMA/3L, (f) PS-embedded unstained S1, (g) unembedded RuO4-stained S1 and (h) unembedded RuO4-stained S6.
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stained using either: (i) RuO4 for a positive staining of the core (Fig. 3b) or (ii) PTA/RuO4 for both negative and positive staining (Fig. 3c). Some residual contrast between the core and shell is again observed in the absence of staining (Fig. 3a), but the contrast is considerably enhanced by use of a ‘positive’ stain or a combination of ‘positive’ and ‘negative’ stains. As expected from the composition (Table 1), RuO4 results in preferential (positive) staining of the core (see Fig. 3b). To highlight the core and shell simultaneously, PTA was therefore used to stain the embedding resin prior to exposure to RuO4. As shown in Fig. 3c (PTA and RuO4), PTA staining leads to clear and unambiguous imaging of the shells, apparently further enhanced by beam-induced mass loss. 3.2. LV-SEM imaging of embedded latexes and contrast analysis An example of LV-SEM images obtained by applying method C to RuO4-stained PMMA/3L is shown in Fig. 2f. The core–shell structure of the embedded latex particles is clearly
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visible, owing to relatively strong electron scattering by the RuO4, resulting in the brighter appearance of the preferentially stained rubbery shell. Application of the same method to unstained S1 latex particles embedded in PS (Fig. 3d) also reveals the core–shell structure. In this case, the PMMA-rich S1 shells become thinner with occurring of mass loss damage. Consecutively, electrons from the metallic support are more easily backscattered through the shells and contribute significantly to image formation, so that the bright contrast associated with the unstained shells reflects enhanced transmission rather than increased scattering. Monte-Carlo simulations are used to account for the observed SEM contrast in embedded specimens, assuming: (i) the shell to be preferentially stained, and the unstained core and surrounding matrix to undergo beam-induced mass loss (as in PMMA-embedded RuO4-stained PMMA/3L; Fig. 4a) or (ii) the shell to undergo beam-induced mass loss, and the core and surrounding matrix to remain undamaged (as in PSembedded unstained S1; Fig. 4b). In the first case, the simulated electron emission intensity, represented as a function of the distance across the specimen by the black
Fig. 5. Spin-coating of a filmable latex (S1) onto: (a) a TEM or (b) a SEM support and (c and d) comparison with the classical diluted drop method. (a) TEM of an undiluted S1 suspension spin-coated onto a copper grid covered with a continuous carbon film, (b) SEM of an undiluted S1 suspension spin-coated onto an aluminum SEM stub, (c) TEM of a drop of an insufficiently diluted (1%) S1 suspension placed on a TEM copper grid covered with a holey carbon film and dried and (d) TEM of a drop of a highly diluted (0.1%) S1 suspension placed on a TEM copper grid covered with a holey carbon film and dried. Scale bars are 500 nm.
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curves (Fig. 4a) is found to be significantly higher in the shell, and particularly in regions of the shell adjoining the thinner parts of the specimen, i.e. those that had undergone mass loss. This accounts for the brighter contrast in the corresponding regions in the SEM image (see the right-hand inset in Fig. 4a), an effect that may be attributed to electrons scattered from both the ruthenium and the copper support, as confirmed by the simulated electron trajectories shown in Fig. 4e. In the case of unstained PS-embedded S1 (Fig. 4a and f), the simulated SE emission was also enhanced at the phase boundaries and there was again a significant contribution from electrons scattered by the metallic support, leading to brighter contrast in the shells, i.e. the thinner parts of the specimen, consistent with the experimental LV-SEM images (right-hand inset in Fig. 4b).
observation. Precise control of the dilution of the latex suspension is necessary to avoid aggregation, and the ionic or steric balance of the suspension has also to be adjusted, particularly for filmable latexes. The SCEMS preparation method introduced here relaxes these constraints on dilution. Thus, good dispersions are obtained on both TEM grids (Fig. 5a) and SEM stubs (Fig. 5b) directly from as-received suspensions of the filmable S1 latex. Fig. 5c shows film formation on drying a droplet of the same latex suspension placed on a holey carbon film. To achieve comparable dispersion by this latter method to that obtained by SCEMS, it was necessary to dilute the suspension by a factor of about 1000 (Fig. 5d).
3.3. Application of spin-coating to prepare TEM grids of unembedded latexes
SCEMS is applied systematically to S1 with and without staining. Fig. 6 shows TEM images of the unstained latex (Fig. 6a), the latex after exposure to RuO4 vapor (Fig. 6b), after combined PTA and RuO4 staining (Fig. 6c) and after staining with PTA, OsO4 and RuO4 (right-hand inset in Fig. 6c).
Whole latex particles must be well-dispersed if the particle morphology is to be clearly identified by direct
3.4. TEM of unembedded core–shell latex particles
Fig. 6. Bright-field TEM (a–c) and LV-SEM (d) investigation of unembedded S1 prepared by spin-coating onto (a–c) a TEM grid covered by a continuous carbon film or (d) a copper SEM stub. (a) Unstained S4 particles, (b) RuO4 vapor-stained S1 particles, (c) PTA and RuO4-stained S1 particles and (right-hand inset) multiple staining of S1 particles with PTA, OsO4 and RuO4 and (d) LV-SEM image of PTA and RuO4-stained S1 particles. Scale bars are 100 nm (a + b), 50 nm (c) and 200 nm (d).
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Fig. 7. TEM investigation of unembedded PMMA latex particles spin-coated onto a copper grid covered with a carbon film. (a) Unstained particles, (b) PTA-stained particles, (c) RuO4 vapor-stained particles and (d) PTA and RuO4-stained particles. Scale bars are 100 nm (a + b + c) and 200 nm (d).
For unstained S1 particles slight contrast is apparent between the core and shell, as well as between the shell and the carbon film (Fig. 6a). As with the embedded PMMA/3L particles, similar contrast is obtained under cryogenic conditions. Exposure to RuO4 vapor enhances the contrast by preferential staining of the core (see Fig. 6b). Use of RuO4 for direct staining of latex specimens in aqueous suspension must be avoided owing to crosslinking reactions between latex particles which result in strong agglomeration. The best results are obtained by combining SCEMS with PTA and RuO4, revealing a tulip-like morphology (Fig. 6c). This not only enhances the contrast between the core and the shell and between the shell and the carbon film, but also greatly improves the stability of the specimen structures in the electron beam. The right-hand inset in Fig. 6c shows the result of the multiple staining of S1 with PTA, aqueous OsO4 and RuO4. Although staining with aqueous OsO4 does not cause aggregation, it makes little difference to the final appearance of the specimen, which is dominated by the PTA and RuO4. As artifacts may arise when staining such dispersions, latex particles are often associated with crystals in the size range 5–
10 nm when exposed to RuO4 (Chou et al., 2002) or with chemical modification of their surfaces, leading to aggregation, with long exposure of the latex particles to aqueous PTA (Harris, 1986). The different staining methods have also been applied on pure PMMA latex particles. Fig. 7 shows TEM images of the unstained PMMA latex (Fig. 7a), and the same latex after PTA staining (Fig. 7b), after RuO4 staining (Fig. 7c) and after combined PTA and RuO4 staining (Fig. 7d). PTA is localized around the particles and greatly enhances the contrast with their shells, which remain unstained (cf. Fig. 7a). Similar effects are obtained by staining pure PMMA latexes with RuO4 (Fig. 7c) and with a combination of PTA and RuO4 (Fig. 7d), again with little evidence of staining of the PMMA. Then, artifacts are avoided in the present experimental conditions, by using short exposure times to the stain, and, if necessary, ultrasound treatment during PTA staining. Fig. 8 shows TEM images of S2 (Fig. 8a), S3 (Fig. 8b), S4 (Fig. 8c) and S5 (Fig. 8d), all prepared following method B (SCEMS with multiple staining). The corresponding structures, shown schematically in Table 1, may be interpreted in terms of the extent of completion of the vitreous shell in
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Fig. 8. Bright-field TEM of latex samples routinely prepared by SCEMS combined with both PTA negative staining and RuO4 positive staining. (a) S2, (b) S3, (c) S4 and (d) S5. Scale bars are 50 nm (a + b + c) and 100 nm (d).
each case. Thus, the very incomplete shells of S2 and S3 gave raspberry-like (Fig. 8a) or quarter-moon-like morphologies (Fig. 8b), respectively, whereas a half-moon-like morphology (Fig. 8c) is associated with the more complete shell of S4 and an occluded structure (Fig. 8d) is obtained in the case of S5. 3.5. LV-SEM and cryo-LV-SEM imaging of unembedded whole core–shell latex particles Method B (SCEMS) has also been used successfully for LVSEM. At ambient temperature, S1 particles show a tulip-like core–shell morphology (Fig. 6d), consistent with the TEM observations (Fig. 6c). Indeed, the SEM contrast is very similar to the TEM contrast owing to the particular specimen configuration imposed by this method. Transmission of the electrons emitted from the metallic support is enhanced in the unstained shells, giving rise to brighter contrast, whereas the electron flux is more strongly attenuated in the stained cores, resulting in a darker contrast. Monte-Carlo simulations of S1 also predict the SE intensity to reach a maximum in the unstained shell (Fig. 4c), and
examination of the electron trajectories, shown in Fig. 4g, again confirms the important contribution of the electrons emitted from the support to the SE intensity. Cryo-LV-SEM images of spin-coated S6 particles, stained with PTA and RuO4 (Fig. 9a), indicate a raspberry-like structure to be associated with a distinct phase in the form of external, globular occlusions. Without staining, there is insufficient contrast between the different phases for this structure to be visible in the LV-SEM (Fig. 9b). Bright-field TEM images of S6 after PTA staining (Fig. 9c) and combined PTA and RuO4 staining (Fig. 9d) confirm this raspberry-like morphology. Clearly significant differences in mean atomic number, such as those induced by preferential staining with RuO4, are required for there to be sufficient contrast in LVSEM. The LV-SEM contrast observed when staining S6 with PTA and RuO4 must be interpreted with regard to both the specimen configuration and the strong contribution from the metallic support inherent in method C. The corresponding Monte-Carlo simulations (Fig. 4d and h) predict the SE intensity to reach a maximum in the unstained PMMA occlusions.
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Fig. 9. Cryo-LV-SEM (a and b) and bright-field TEM (c and d) of S6 investigated using different staining conditions. (a) Cryo-LV-SEM of PTA and RuO4-stained particles, (b) cryo-LV-SEM of unstained particles, (c) bright-field TEM of PTA-stained particles and (d) bright-field TEM of both PTA and RuO4-stained particles. Scale bars are 100 nm (a + b + c) and 50 nm (d).
4. Conclusion A comparative electron microscopy study has been carried out on various latex specimens composed of nanosized core– shell particles with complex shell morphologies. Sections of individual particles are obtained by ultramicrotomy of embedded dispersions. Whole latex particles are also isolated for observation by spin-coating a suspension directly onto carbon covered TEM grids or SEM stubs. This spin-coating preparation gives significantly better dispersions at any given concentration than simply drying a drop of the suspension. When beam damage occurs selectively in one or more of the polymer phases present, sufficient contrast is obtained in TEM for coarse structures to be identifiable without chemical staining. However, chemical staining is necessary in all the latex specimens to ensure adequate stability and reveal fine structure. For this purpose negative staining with PTA and/or positive staining with RuO4 and OsO4 were employed. A combination of PTA and RuO4 leads to considerable detail in the images and excellent beam resistance in both the TEM and the LV-SEM. Thus, morphological features such as raspberry-like structures with lobes in the 10 nm size range, and inverted core–shell structures containing nanometric inclusions are identified. Use of cryo-LV-SEM resulted in useful magnifications comparable to those obtained with
TEM, providing improved imaging of fine structures. The enhanced contrast in the stained specimens observed by SEM are accounted for by both enhanced scattering from heavy metal atoms localized in the rubbery regions of stained latex particles and backscattered electrons emitted from the metallic support, because the mean free path of the primary electrons is always greater than the specimen thickness. Although artifacts cannot be ruled out, staining under relatively mild conditions (short exposure times, low stain concentrations and use of an ultrasonic bath) leads to a high degree of reproducibility. Hence, although a number of promising alternatives exist, such as EELS and holography, chemical staining remains a method of choice for observation of multiphase latexes. Acknowledgments The financial support of the Direction Scientifique—Total and Arkema Research Support are gratefully acknowledged. References Ashida, T., Katoh, A., Handa, K., Ochi, M., 1999. Structure and properties of epoxy resins modified with acrylic particles. J. Appl. Polym. Sci. 74, 2955– 2962.
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The morphology of submicronsized core–shell latex particles: An electron microscopy study Ce´dric Gaillard a,*, Gilbert Fuchs b, Christopher J.G. Plummer c, Pierre A. Stadelmann a b
a Centre Interdisciplinaire de Microscopie Electronique (CIME), EPFL, CH-1015 Lausanne, Switzerland Arkema, Centre de Recherche Rhoˆne-Alpes (CRRA), Rue H. Moissan, BP 63, 69 493 Pierre-Be´nite, France c Laboratoire de Technologie des Composites et Polyme`res (LTC), EPFL, CH-1015 Lausanne, Switzerland
Received 31 March 2006; accepted 19 July 2006
Abstract The core–shell structure of a range of acrylic–acrylic latexes has been investigated by combining different specimen preparation methods with transmission electron microscopy (TEM), dark-field scanning transmission electron microscopy (DSTEM) and low-voltage scanning electron microscopy (LV-SEM), including the first reported use of LV-SEM to observe composite latex particles at ambient and subambient temperatures. Spin-coating of liquid latex dispersions directly onto TEM grids or SEM stubs is shown to be a relatively straightforward mean of avoiding film formation during specimen preparation. In conjunction with double staining techniques, it has been found to be particularly convenient for characterizing the fine structure of particles with diameters down to below 100 nm. # 2006 Elsevier Ltd. All rights reserved. Keywords: Rubber toughened PMMA; Core–shell latex; Morphological characterization; Transmission electron microscopy (TEM); Low-voltage scanning electron microscopy (LV-SEM); Spin-coating; Multiple chemical staining
1. Introduction Synthetic latexes play an important role in the polymer industry, both for coatings and adhesives. Their inclusion in a polymer matrix leads to significantly improved combinations of physical properties, and in particular, enhanced impact performance. Acrylic core–shell latex systems are of particular interest as impact modifiers for industrial thermoplastics such as polyvinylchloride (PVC) (Dompas et al., 1995), polymethylmethacrylate (PMMA) (Lovell et al., 1993) and styreneacrylonitrile copolymer (SAN) (Steenbrink et al., 1998). The basic core–shell latex particles typically consist of a rubbery polybutylacrylate (Steenbrink et al., 1998) or poly(styrenebutadiene) core (Dompas and Groeninckx, 1994) and a glassy polymethylmethacrylate shell. Efficient impact modification generally requires a strong particle–matrix interface (Plummer et al., 1999), and hence a well-formed, integral shell and the
* Corresponding author. Present address: Plate-forme RIO BIBS, Laboratoire de Microscopie, U.R. BIA, INRA, Rue de la Ge´raudie`re, BP 71 627, 44 316 Nantes Cedex 3, France. Tel.: +33 240 67 51 68; fax: +33 240 67 50 05. E-mail address: [email protected] (C. Gaillard). 0968-4328/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.micron.2006.07.023
particle size is also an important factor (Dompas and Groeninckx, 1994). With increasingly versatile synthetic routes, control of both the morphology and particle size has advanced considerably and multiple or irregularly shaped shells or inverted core–shell structures are now widely available (Stubbs et al., 2003; Sundberg and Durant, 2003). Methods of characterization of the particle morphology must progress with these additional levels of complexity if the development of new synthetic techniques and production of high quality latex particles are to be maintained (Sundberg and Durant, 2003). Relevant characterization techniques used for particle morphology control include atomic force microscopy (AFM) (Sommer et al., 1995; Portigliatti et al., 2000; Kirsch et al., 2001), fluorescence non radiative energy transfer (NRET) (Huijs and Lang, 2000; Pe´rez and Lang, 2000), dynamic light scattering (Manzur, 1994), X-ray photoelectron spectroscopy (XPS) (Barthet et al., 1998; Khan et al., 2000), photon correlation spectroscopy (Hatto et al., 2000), small-angle X-ray scattering (SAXS) (Dingenouts et al., 1999), small-angle neutron scattering (SANS) (De Bruyn et al., 2003; Chevalier et al., 1997), dynamic mechanical spectroscopy (Cavaille´ et al., 1991), differential scanning calorimetry (DSC) (Song and Liao,
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2004), solid-state nuclear magnetic resonance (NMR) (Nelliappan et al., 1995; Kirsch et al., 1999), electrophoresis (Makino et al., 1994), surface tension-based methods (the Maron adsorption method) (Landier et al., 2004) and positron annihilation lifetime spectroscopy (PALS) (Lizama et al., 2001). The most widely used direct characterization technique, however, is transmission electron microscopy (TEM), which is applied to both whole latex particles and sections through resinembedded particles (Sundberg and Sundberg, 1993; Chen et al., 1992; Shi et al., 2002). It has been used, for example, to quantify core–shell morphologies by the shape factor method (Li et al., 2003) and to investigate microdeformation behavior in core–shell particle reinforced PMMA, for example (Plummer et al., 2004). Low-voltage scanning electron microscopy (LV-SEM), on the other hand, remains relatively unexplored as a tool for the direct investigation of composite latex morphologies, although it is considered to be highly promising for polymeric materials in general (Vezie et al., 1995). It has been used successfully for the high-magnification imaging of MMA-embedded core–shell particles (Gaillard et al., 2004), and the observation of the external morphology of whole latex particles (Olah et al., 2004; Wang et al., 2004) and epoxy-embedded latex particles (Ashida et al., 1999). However, to our knowledge, the only use of cryogenic LV-SEM has been for size distribution measurements on PS latex particles (Haridas and Bellarc, 1998). The main difficulties with studying core–shell latexes by electron microscopy are the weak contrast between the different phases and/or between the particles and the background, and their poor electron beam resistance (Joy and Joy, 1996; Egerton et al., 2004). The low resolution often obtained in observation of radiation-sensitive specimens is directly attributable to the weak signal imposed by the need to limit radiation damage and the consequent loss of fine structure (Baudet and Kubin, 1982). Acrylic polymers, which are important components in synthetic latex particles, are particularly prone to radiation damage. In principle, these difficulties may be overcome by using low temperature (Winnik et al., 1993; Subramaniam et al., 2004), low-dose (Williams and Fisher, 1970) and/or low-voltage (Lednicky et al., 2002) techniques and specimen preparation methods that avoid film formation and stabilize and enhance the contrast between the different polymer phases (Sawyer and Grubb, 1996). Selective staining with heavy elements (ruthenium, osmium, tungsten and uranium) is a frequently used method for increasing high-angle Rutherford scattering and generating contrast between the stained and the unstained regions of a polymer specimen (Trent et al., 1983; Ferguson et al., 2002). ‘‘Positive’’ staining occurs when the chemical stain interacts directly with the polymer specimen by diffusion or reaction, whereas ‘‘negative’’ staining is usually applied to the specimen substrate (a hydrophilic carbon film, for example) or embedding medium. Phosphotungstenic acid (PTA) and its sodium or potassium salts are widely used for negative staining of biological specimens (Harris, 2002) and, to some extent, polymer latexes (Li et al., 2002). Other conventional chemical stains that have been successfully used for polymer latexes
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(Sawyer and Grubb, 1996) include osmium tetraoxide (OsO4) for unsaturated polymers such as natural rubber-based latexes (Schneider et al., 1996) and polystyrene/polybutylacrylate composite particles (Kanig and Neff, 1964), ruthenium tetraoxide (RuO4) for saturated and unsaturated polymers (Trent et al., 1983) and both OsO4 and RuO4 used for PS/ styrene-butadiene–rubber/polyolefin blends (Luziniv et al., 2000). Combined use has also been made of positive and negative stains for the characterization of core–shell structures, for example, RuO4 and PTA (Chen et al., 1991; Monteiro and de Barbeyrac, 2001), OsO4 and PTA (He et al., 1997) or RuO4 and uranyl acetate (Ferguson et al., 2002). In some cases, use of only one stain may suffice to give both positive and negative contrast, as has been described for PTA (Harris et al., 1999) and RuO4 (Lei et al., 2004). Chemical staining generally also has the advantage of hardening many types of polymer, which may facilitate specimen preparation and observation, although it may also result in nanoscale artifacts (Chou et al., 2002). Alternative electron microscopy-based methods have therefore been proposed to improve contrast in polymers, including transmission electron holography (Chou et al., 2003), electron energy loss spectroscopy (EELS) and energy-filtering imaging (Egerton, 2003). However, these techniques typically require specialized equipment (Stadelmann et al., 1995) and are most appropriate to specimens in which contrast is associated with characteristic chemical bonds or species (for example, use of the p-plasmon energy loss peak of PS), as in PS/polybutadiene (PB) (Simon et al., 2002), PMMA/PS (Correa and Hage, 1999), polyethylene (PE)/PS (Hunt et al., 1995) and PS/PB/PMMA (Varlot et al., 2000). The present communication explores practical aspects of the characterization of a range of nanosized (particle sizes below 100 nm) and nanotextured multiphase latexes using electron microscopy. Starting with core–shell latexes, synthesized under different experimental conditions and exhibiting different morphologies, the effectiveness of different specimen preparation methods and electron microscopy-based techniques is compared, including TEM, dark-field scanning TEM and LVSEM. 2. Material and methods 2.1. Materials The core–shell latex specimens, provided by Arkema (France), comprised organic core–shell particles with different configurations. The characteristics of the core–shell latexes are given in Table 1. PMMA/3L refers to impact modified PMMA containing relatively large composite particles made up of an inner MMA-rich glassy copolymer core, surrounded by an intermediate styrene-rich rubbery layer. The remaining specimens, designated S1, S2, S3, S4, S5 and S6 consist of aqueous suspensions of composite particles with a rubbery styrene/ acrylic core and an MMA-rich glassy copolymer shell. The proportion of each monomer has been varied in the synthesis step, so that more or less complete shells with different morphologies are expected (see Table 1). OsO4 and PTA
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Table 1 Details of the core–shell latex specimens Designation
PMMA/3L
c
Particle characteristicsa Core
Shell
Glassy MMA-rich copolymer
Rubbery styrene-rich copolymer
Expected morphology/schematicb
Overall diameter (nm)
Large core with
200
integral shell S1d
Rubbery styrene-rich copolymer
Glassy MMA-rich copolymer
Large core with
80
integral shell S2d
Rubbery styrene/acrylic copolymer
Glassy MMA/acrylic copolymer
Large core with very
80
discontinuous shell S3d
Rubbery styrene/acrylic copolymer
Glassy styrene-rich copolymer
Large core with
60
discontinuous shell S4
d
S5
d
Rubbery styrene/acrylic copolymer
Glassy MMA/acrylic copolymer
Large core with
60
discontinuous shell Rubbery styrene/acrylic copolymer
Glassy MMA/acrylic copolymer
Large core containing
100
second phase inclusions S6d
Rubbery styrene/acrylic copolymer
Glassy MMA/acrylic copolymer
Large core with partially
160
discontinuous shell a b c d
MMA, methyl methacrylate. The black and grey regions stand, respectively, for core and shell parts. PMMA-embedded latex. Aqueous dispersion.
(H3O40PW12) are purchased from Aldrich. RuO4 is prepared by mixing 100 mg of ruthenium trichloride (Acros) with 5 ml of 10 wt% aqueous sodium hypochlorite solution (Acros) (Montezinos et al., 1985). Styrene monomer and azo-bisisobutyronitrile (AIBN), used for embedding certain latexes, are provided by Sigma. 2.2. Specimen preparation TEM specimens are prepared from the latexes by either: (i) embedding the particles S1, in PS and preparing thin sections by ultramicrotomy, following method A or (ii) dispersing particles S1, S2, S3, S4, S5 or S6 on a carbon covered TEM grid using a specially developed spin-coating technique, which will henceforth be referred to as ‘‘spin-coating electron microscopy supports’’ (SCEMS, method B). SEM specimens are prepared either by ultramicrotomy of embedded latex particles PMMA/ 3L and PS-embedded S1 as described elsewhere (method C) (Gaillard et al., 2004) or from unembedded particles S1 using SCEMS. Each method is combined with chemical staining using one or more of PTA, OsO4 and RuO4. 2.2.1. Method A Styrene monomer is pre-polymerized by mixing 10 ml of freshly distilled styrene with 0.5 wt% of AIBN and heating at
60 8C for 3 h. Two milliliters of the resulting viscous liquid is then mixed with three drops of the undiluted latex suspension and maintained at 70 8C for 12 h. Ultrathin sections (50 nm thick) are taken from the resulting solid blocks using a diamond knife (Diatome, 458) and an Ultracut E ultramicrotome (Reichert-Jung), and picked up from distilled water onto TEM copper grids (200 mesh) covered with a discontinuous carbon film with square perforations (‘‘Quantifoil S7/1’’ grids from Plano). The sections are then observed either without staining or after staining with RuO4 vapor or a combination of RuO4 vapor and PTA. In this latter case, a drop of aqueous PTA (3 wt%) is placed on the thin film for 1 h, after which the specimen was rinsed with distilled water and exposed to RuO4 vapor for about 15 min. This method is preferred to melamine/ formaldehyde-based embedding, often used for aqueous latex dispersion, because of the excellent stability of PS in the electron beam and because the PS could be selectively stained to provide higher contrast. 2.2.2. Method B (SCEMS) A dedicated spin-coating apparatus has been developed for TEM and SEM specimen preparation (Fig. 1). A SEM specimen holder (a metallic, polytetrafluoroethylene (PTFE) or carbon-coated stub), or a holder adapted for commercial TEM copper grids is mounted on a revolving support
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Fig. 1. Overview of the device used for the spin-coating of latex samples onto electron microscopy supports. (a) Schematic of the spin-coating apparatus, (b) photograph of (i) the apparatus and (ii) the TEM and SEM supports (the arrows indicate where the TEM grids are fixed) and (c and d) TEM and SEM images showing an example of an aqueous PMMA latex spin-coated onto (c) a TEM copper grid covered with a carbon film or (d) an aluminum SEM stub. Scale bars are 2 mm.
(1460 revolutions/min). A drop of latex suspension is introduced to the specimen holder and left to rotate for 3 min. The specimen is then removed from the spin coater and dried under vacuum for approximately 30 min. The latex particles are generally found to be well-dispersed, without the need for dilution of the as-received suspensions, as long as these latter were not excessively viscous. Where required, the SEM or TEM specimens are exposed to RuO4 vapor for 15 min. In the case of the SEM specimens, copper stubs are systematically preferred to aluminum stubs because of their improved resistance to RuO4. PTA staining is carried out by mixing equal volumes of the latex dispersion and aqueous PTA (3 wt%), sonication for 2 h, spin-coating onto the chosen support and, if required, exposure to RuO4 vapor for 15 min.
2.2.3. Method C Fifty nanometers thick microtomed sections are prepared from unstained or RuO4-stained specimens and picked up from distilled water onto a copper SEM stub. Excess water is carefully removed in order to eliminate wrinkles. 2.3. Microscopy TEM investigations are carried out using a HF-2000 Hitachi TEM operated at 200 kV, and equipped with a field emission gun and a dark-field scanning stage. SEM investigations are carried out on uncoated specimens using a XL30 SFEG Philips SEM equipped with a field emission gun and a through-the-lens detector (TLD), operated at 3 kV (working distance, 3 mm; spot size, 3 nm). Some of the LV-SEM images are recorded at
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liquid nitrogen temperature (cryo LV-SEM) using a Gatan Alto 2500 cryo-system. 2.4. Monte-Carlo simulations Monte-Carlo simulations of the secondary electrons intensity in SEM and of electron trajectories are performed using the Plano GmbH MOCASIM software. The geometries used for the simulations schematise: (i) a PMMA/3L polymer
film placed on a copper support according to the method C and (ii) S1 and S6 RuO4-stained particles deposited onto a copper stub following the SCEMS method. Each simulation assumes an acceleration voltage of 3 kVand 600 electron trajectories. To take into account mass loss induced by the electron beam, the thickness of the model film is reduced from 50 to 25 nm in the e-beam-sensitive PMMA-rich regions while a 50 nm in thickness is taken for the e-beam resistant PS-rich parts.
Fig. 2. TEM (a–e) and SEM (f) investigations of PMMA/3L PMMA-embedded latex particles. (a) Bright-field TEM image of unstained embedded latex particles, (b) bright-field TEM of RuO4 vapor-stained embedded particles, (c) dark-field TEM image of unstained embedded latex particles, (d) dark-field TEM image of RuO4 vapor-stained embedded particles, (e) dark-field scanning TEM image of unstained latex particles and (f) LV-SEM image of embedded RuO4-stained particles. Scale bars are 200 nm (a + b + c + d + e) and 500 nm (f).
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3. Results and discussion 3.1. Bright-field and dark-field scanning TEM of embedded core–shell latexes Embedding in a suitable resin has been used to disperse latex particles and facilitate ultra-thin sectioning for TEM examination. Moreover, in the case of impact reinforced plastics, the latex particles are already dispersed in a resin. To enhance the contrast between the different phases, chemical staining is applied either to the embedded particles before sectioning or by exposing a thin section mounted on a TEM grid to the vapor of a volatile stain such as RuO4 (cf. Section 2.2.1). Fig. 2 shows bright-field (Fig. 2a and b) and dark-field (Fig. 2c and d) TEM images of RuO4 vapor stained and unstained ultra-thin films of PMMA/3L, along with a dark-field scanning TEM image of an unstained film (Fig. 2e). The core– shell morphology of the latex particles in ultra-thin films prepared from PMMA/3L is apparent in both bright-field TEM images of unstained sections (Fig. 2a) and RuO4 vapor-stained sections (Fig. 2b). Similar observations are made from the darkfield TEM images (Fig. 2c and d). The contrast in the stained sections may be attributed to selective incorporation of the RuO4 in the styrene-rich rubbery
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shell, resulting in stronger electron scattering and hence a dark appearance in bright-field images (Vitali and Montani, 1980; Lee, 2000). Close examination of the vitreous core (Fig. 2b) also reveals the presence of small heterogeneities, which are attributed to diffusion of styrene monomer into the core during synthesis. Core and shell densities being similar, the strong contrast obtained in the absence of staining (Fig. 2a and c) is assumed to be due to preferential beam-induced mass loss in the MMA-rich copolymer core, PMMA homopolymers generally being significantly more beam sensitive than styrenic polymers. So, depending on the nature of the embedding resin and the components of the latex particle, the staining step may sometimes be dispensed with, the contrast being generated from preferential degradation of one of the polymer phases. Similar contrasts in TEM images of unstained epoxy-embedded latex particles with a PMMA core and a PS shell have been already observed previously, for example (Jonsson et al., 2001). Comparable beam-induced mass thickness contrast between the core and shell is also observed even when the observations are made at liquid nitrogen temperature using a cryogenic specimen holder, consistent with previous cryo-TEM investigations of similar materials (Talmon et al., 1986; Talmon, 1999). DSTEM images obtained with a nanosized electron probe scanned across the specimen and a high angle annular
Fig. 3. TEM (a–c) and LV-SEM (d) investigations of PS-embedded S1 latex particles. (a) TEM of unstained PS-embedded S1 particle, (b) TEM of RuO4 vaporstained PS-embedded S1 particle, (c) TEM of PTA and RuO4-stained PS-embedded S1 particles and (d) LV-SEM image of unstained PS-embedded S1 particles. Scale bars are 50 nm (a), 20 nm (b), 100 nm (c) and 200 nm (d).
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dark-field detector (HAADF) reveal also a core–shell morphology (Fig. 2e). This technique is particularly sensitive to thickness variations, such as those that resulted from beam damage in thin sections of PMMA/3L. Hence, DSTEM/ HAADF images of unstained PMMA/3L (Fig. 2e), show
significantly greater contrast than the corresponding dark-field TEM (Fig. 2) and bright-field TEM images (Fig. 2a). In the case of smaller particles (S1 embedded in PS), various chemical staining techniques are applied (Fig. 3). The S1 particles are more precisely embedded in PS (method A) and
Fig. 4. (a–d) Monte-Carlo simulations of the secondary electrons (SE) emitted when scanning the following samples: (a) RuO4-stained PMMA/3L, (b) PS-embedded unstained S1, (c) unembedded RuO4-stained S1, (d) unembedded RuO4-stained S6. The schematics show the models on which the simulations were based and the black curves give the SE intensity (arbitrary units) as function of position (abscises values in nm). Right-hand insets show an experimental LV-SEM image of the simulated specimen. (e–h) Projections of the simulated electron trajectories corresponding to: (e) RuO4-stained PMMA/3L, (f) PS-embedded unstained S1, (g) unembedded RuO4-stained S1 and (h) unembedded RuO4-stained S6.
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stained using either: (i) RuO4 for a positive staining of the core (Fig. 3b) or (ii) PTA/RuO4 for both negative and positive staining (Fig. 3c). Some residual contrast between the core and shell is again observed in the absence of staining (Fig. 3a), but the contrast is considerably enhanced by use of a ‘positive’ stain or a combination of ‘positive’ and ‘negative’ stains. As expected from the composition (Table 1), RuO4 results in preferential (positive) staining of the core (see Fig. 3b). To highlight the core and shell simultaneously, PTA was therefore used to stain the embedding resin prior to exposure to RuO4. As shown in Fig. 3c (PTA and RuO4), PTA staining leads to clear and unambiguous imaging of the shells, apparently further enhanced by beam-induced mass loss. 3.2. LV-SEM imaging of embedded latexes and contrast analysis An example of LV-SEM images obtained by applying method C to RuO4-stained PMMA/3L is shown in Fig. 2f. The core–shell structure of the embedded latex particles is clearly
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visible, owing to relatively strong electron scattering by the RuO4, resulting in the brighter appearance of the preferentially stained rubbery shell. Application of the same method to unstained S1 latex particles embedded in PS (Fig. 3d) also reveals the core–shell structure. In this case, the PMMA-rich S1 shells become thinner with occurring of mass loss damage. Consecutively, electrons from the metallic support are more easily backscattered through the shells and contribute significantly to image formation, so that the bright contrast associated with the unstained shells reflects enhanced transmission rather than increased scattering. Monte-Carlo simulations are used to account for the observed SEM contrast in embedded specimens, assuming: (i) the shell to be preferentially stained, and the unstained core and surrounding matrix to undergo beam-induced mass loss (as in PMMA-embedded RuO4-stained PMMA/3L; Fig. 4a) or (ii) the shell to undergo beam-induced mass loss, and the core and surrounding matrix to remain undamaged (as in PSembedded unstained S1; Fig. 4b). In the first case, the simulated electron emission intensity, represented as a function of the distance across the specimen by the black
Fig. 5. Spin-coating of a filmable latex (S1) onto: (a) a TEM or (b) a SEM support and (c and d) comparison with the classical diluted drop method. (a) TEM of an undiluted S1 suspension spin-coated onto a copper grid covered with a continuous carbon film, (b) SEM of an undiluted S1 suspension spin-coated onto an aluminum SEM stub, (c) TEM of a drop of an insufficiently diluted (1%) S1 suspension placed on a TEM copper grid covered with a holey carbon film and dried and (d) TEM of a drop of a highly diluted (0.1%) S1 suspension placed on a TEM copper grid covered with a holey carbon film and dried. Scale bars are 500 nm.
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curves (Fig. 4a) is found to be significantly higher in the shell, and particularly in regions of the shell adjoining the thinner parts of the specimen, i.e. those that had undergone mass loss. This accounts for the brighter contrast in the corresponding regions in the SEM image (see the right-hand inset in Fig. 4a), an effect that may be attributed to electrons scattered from both the ruthenium and the copper support, as confirmed by the simulated electron trajectories shown in Fig. 4e. In the case of unstained PS-embedded S1 (Fig. 4a and f), the simulated SE emission was also enhanced at the phase boundaries and there was again a significant contribution from electrons scattered by the metallic support, leading to brighter contrast in the shells, i.e. the thinner parts of the specimen, consistent with the experimental LV-SEM images (right-hand inset in Fig. 4b).
observation. Precise control of the dilution of the latex suspension is necessary to avoid aggregation, and the ionic or steric balance of the suspension has also to be adjusted, particularly for filmable latexes. The SCEMS preparation method introduced here relaxes these constraints on dilution. Thus, good dispersions are obtained on both TEM grids (Fig. 5a) and SEM stubs (Fig. 5b) directly from as-received suspensions of the filmable S1 latex. Fig. 5c shows film formation on drying a droplet of the same latex suspension placed on a holey carbon film. To achieve comparable dispersion by this latter method to that obtained by SCEMS, it was necessary to dilute the suspension by a factor of about 1000 (Fig. 5d).
3.3. Application of spin-coating to prepare TEM grids of unembedded latexes
SCEMS is applied systematically to S1 with and without staining. Fig. 6 shows TEM images of the unstained latex (Fig. 6a), the latex after exposure to RuO4 vapor (Fig. 6b), after combined PTA and RuO4 staining (Fig. 6c) and after staining with PTA, OsO4 and RuO4 (right-hand inset in Fig. 6c).
Whole latex particles must be well-dispersed if the particle morphology is to be clearly identified by direct
3.4. TEM of unembedded core–shell latex particles
Fig. 6. Bright-field TEM (a–c) and LV-SEM (d) investigation of unembedded S1 prepared by spin-coating onto (a–c) a TEM grid covered by a continuous carbon film or (d) a copper SEM stub. (a) Unstained S4 particles, (b) RuO4 vapor-stained S1 particles, (c) PTA and RuO4-stained S1 particles and (right-hand inset) multiple staining of S1 particles with PTA, OsO4 and RuO4 and (d) LV-SEM image of PTA and RuO4-stained S1 particles. Scale bars are 100 nm (a + b), 50 nm (c) and 200 nm (d).
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Fig. 7. TEM investigation of unembedded PMMA latex particles spin-coated onto a copper grid covered with a carbon film. (a) Unstained particles, (b) PTA-stained particles, (c) RuO4 vapor-stained particles and (d) PTA and RuO4-stained particles. Scale bars are 100 nm (a + b + c) and 200 nm (d).
For unstained S1 particles slight contrast is apparent between the core and shell, as well as between the shell and the carbon film (Fig. 6a). As with the embedded PMMA/3L particles, similar contrast is obtained under cryogenic conditions. Exposure to RuO4 vapor enhances the contrast by preferential staining of the core (see Fig. 6b). Use of RuO4 for direct staining of latex specimens in aqueous suspension must be avoided owing to crosslinking reactions between latex particles which result in strong agglomeration. The best results are obtained by combining SCEMS with PTA and RuO4, revealing a tulip-like morphology (Fig. 6c). This not only enhances the contrast between the core and the shell and between the shell and the carbon film, but also greatly improves the stability of the specimen structures in the electron beam. The right-hand inset in Fig. 6c shows the result of the multiple staining of S1 with PTA, aqueous OsO4 and RuO4. Although staining with aqueous OsO4 does not cause aggregation, it makes little difference to the final appearance of the specimen, which is dominated by the PTA and RuO4. As artifacts may arise when staining such dispersions, latex particles are often associated with crystals in the size range 5–
10 nm when exposed to RuO4 (Chou et al., 2002) or with chemical modification of their surfaces, leading to aggregation, with long exposure of the latex particles to aqueous PTA (Harris, 1986). The different staining methods have also been applied on pure PMMA latex particles. Fig. 7 shows TEM images of the unstained PMMA latex (Fig. 7a), and the same latex after PTA staining (Fig. 7b), after RuO4 staining (Fig. 7c) and after combined PTA and RuO4 staining (Fig. 7d). PTA is localized around the particles and greatly enhances the contrast with their shells, which remain unstained (cf. Fig. 7a). Similar effects are obtained by staining pure PMMA latexes with RuO4 (Fig. 7c) and with a combination of PTA and RuO4 (Fig. 7d), again with little evidence of staining of the PMMA. Then, artifacts are avoided in the present experimental conditions, by using short exposure times to the stain, and, if necessary, ultrasound treatment during PTA staining. Fig. 8 shows TEM images of S2 (Fig. 8a), S3 (Fig. 8b), S4 (Fig. 8c) and S5 (Fig. 8d), all prepared following method B (SCEMS with multiple staining). The corresponding structures, shown schematically in Table 1, may be interpreted in terms of the extent of completion of the vitreous shell in
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Fig. 8. Bright-field TEM of latex samples routinely prepared by SCEMS combined with both PTA negative staining and RuO4 positive staining. (a) S2, (b) S3, (c) S4 and (d) S5. Scale bars are 50 nm (a + b + c) and 100 nm (d).
each case. Thus, the very incomplete shells of S2 and S3 gave raspberry-like (Fig. 8a) or quarter-moon-like morphologies (Fig. 8b), respectively, whereas a half-moon-like morphology (Fig. 8c) is associated with the more complete shell of S4 and an occluded structure (Fig. 8d) is obtained in the case of S5. 3.5. LV-SEM and cryo-LV-SEM imaging of unembedded whole core–shell latex particles Method B (SCEMS) has also been used successfully for LVSEM. At ambient temperature, S1 particles show a tulip-like core–shell morphology (Fig. 6d), consistent with the TEM observations (Fig. 6c). Indeed, the SEM contrast is very similar to the TEM contrast owing to the particular specimen configuration imposed by this method. Transmission of the electrons emitted from the metallic support is enhanced in the unstained shells, giving rise to brighter contrast, whereas the electron flux is more strongly attenuated in the stained cores, resulting in a darker contrast. Monte-Carlo simulations of S1 also predict the SE intensity to reach a maximum in the unstained shell (Fig. 4c), and
examination of the electron trajectories, shown in Fig. 4g, again confirms the important contribution of the electrons emitted from the support to the SE intensity. Cryo-LV-SEM images of spin-coated S6 particles, stained with PTA and RuO4 (Fig. 9a), indicate a raspberry-like structure to be associated with a distinct phase in the form of external, globular occlusions. Without staining, there is insufficient contrast between the different phases for this structure to be visible in the LV-SEM (Fig. 9b). Bright-field TEM images of S6 after PTA staining (Fig. 9c) and combined PTA and RuO4 staining (Fig. 9d) confirm this raspberry-like morphology. Clearly significant differences in mean atomic number, such as those induced by preferential staining with RuO4, are required for there to be sufficient contrast in LVSEM. The LV-SEM contrast observed when staining S6 with PTA and RuO4 must be interpreted with regard to both the specimen configuration and the strong contribution from the metallic support inherent in method C. The corresponding Monte-Carlo simulations (Fig. 4d and h) predict the SE intensity to reach a maximum in the unstained PMMA occlusions.
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Fig. 9. Cryo-LV-SEM (a and b) and bright-field TEM (c and d) of S6 investigated using different staining conditions. (a) Cryo-LV-SEM of PTA and RuO4-stained particles, (b) cryo-LV-SEM of unstained particles, (c) bright-field TEM of PTA-stained particles and (d) bright-field TEM of both PTA and RuO4-stained particles. Scale bars are 100 nm (a + b + c) and 50 nm (d).
4. Conclusion A comparative electron microscopy study has been carried out on various latex specimens composed of nanosized core– shell particles with complex shell morphologies. Sections of individual particles are obtained by ultramicrotomy of embedded dispersions. Whole latex particles are also isolated for observation by spin-coating a suspension directly onto carbon covered TEM grids or SEM stubs. This spin-coating preparation gives significantly better dispersions at any given concentration than simply drying a drop of the suspension. When beam damage occurs selectively in one or more of the polymer phases present, sufficient contrast is obtained in TEM for coarse structures to be identifiable without chemical staining. However, chemical staining is necessary in all the latex specimens to ensure adequate stability and reveal fine structure. For this purpose negative staining with PTA and/or positive staining with RuO4 and OsO4 were employed. A combination of PTA and RuO4 leads to considerable detail in the images and excellent beam resistance in both the TEM and the LV-SEM. Thus, morphological features such as raspberry-like structures with lobes in the 10 nm size range, and inverted core–shell structures containing nanometric inclusions are identified. Use of cryo-LV-SEM resulted in useful magnifications comparable to those obtained with
TEM, providing improved imaging of fine structures. The enhanced contrast in the stained specimens observed by SEM are accounted for by both enhanced scattering from heavy metal atoms localized in the rubbery regions of stained latex particles and backscattered electrons emitted from the metallic support, because the mean free path of the primary electrons is always greater than the specimen thickness. Although artifacts cannot be ruled out, staining under relatively mild conditions (short exposure times, low stain concentrations and use of an ultrasonic bath) leads to a high degree of reproducibility. Hence, although a number of promising alternatives exist, such as EELS and holography, chemical staining remains a method of choice for observation of multiphase latexes. Acknowledgments The financial support of the Direction Scientifique—Total and Arkema Research Support are gratefully acknowledged. References Ashida, T., Katoh, A., Handa, K., Ochi, M., 1999. Structure and properties of epoxy resins modified with acrylic particles. J. Appl. Polym. Sci. 74, 2955– 2962.
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