the segregation of surfactant upon film formation of latex dispersions: an investigation by energy...

Polymer International 43 (1997) 187È196

The Segregation of Surfactant upon FilmFormation of Latex Dispersions: anInvestigation by Energy Filtering

Transmission Electron Microscopy*

Alexander Du Chesne,a¤ Bettina Gerharzb & Gu� nter Liesera

a Max-Planck-Institut fu� r Polymerforschung, PF 3148, D-55021 Mainz, Germanyb Hoechst-AG, PF 800320, D-65926 Frankfurt/M., Germany

(Received 4 November 1996 ; accepted 26 November 1996)

Abstract : We investigated the local distribution of emulsiÐer in dried Ðlms castfrom latex dispersions. Energy Ðltering transmission electron microscopy(EFTEM) allowed the detection of heteroatom-containing domains in unstainedsamples by electron spectroscopic imaging and elemental mapping. If thesamples were prepared at temperatures above the minimum Ðlm-forming tem-perature (MFT), separated domains of surfactant were observed in detachmentreplicas and ultrathin sections. If the preparation was carried out below MFTthe emulsiÐer was retained at the particle surface and similar observations couldnot be made.

It is suggested that the mobility of the emulsiÐer at the surface of the particlesis increased upon deformation and coalescence in the process of Ðlm formation,and that this mobility is a prerequisite for its segregation. Both processes do nottake place upon drying below MFT, owing to lower mobility. The correlationbetween the process of Ðlm formation and the segregation of surfactant is dis-cussed for both a dispersion of poly(vinylacetate) and one of polyacrylate.

Key words : Ðlm formation of latices, emulsifying agents in latex Ðlms, EFTEM.

INTRODUCTION

Ease of handling and, for environmental reasons, thelack of emission of organic solvents are today expectedof polymer dispersions for a multitude of applications.For paints in particular, a good Ðnish is another desiredproperty. Regarding the Ðrst two requirements, disper-sions of latex for water-borne coatings are excellent ;they show low viscosity at high polymer content andoften shear thinning, which make their application easy.However, in many cases, the Ðnish and stability of coat-ings made from polymer solution are superior. In order

* Presented at “The Cambridge Polymer Conference : Partner-ship in PolymersÏ, Cambridge, UK, 30 SeptemberÈ2 October1996.¤ To whom all correspondence should be addressed.

to reduce the emission of volatile organic solvents a lotof e†ort is being put into the improvement of water-borne coatings (latex Ðlms).

The formation of a continuous, stable latex Ðlm isbased upon the processes of latex particle packing,deformation and coalescence with interparticle di†usionof polymer chains. This happens if the coating is driedbeyond the so-called minimum Ðlm-forming tem-perature (MFT).

To prevent coagulation prior to their application, dis-persions contain stabilisers, forming a hydrophilic layerat the surface of the particles. Stabilisation may be elec-trostatic or steric, or a combination of both. Surfactantsare present right from the beginning in most dispersionsas they are essential for the process of emulsion polym-erisation. The additives are retained in the latex Ðlms(pigments are not considered here). Hence all latex Ðlms

187Polymer International 0959-8103/97/$17.50 1997 SCI. Printed in Great Britain(

188 A. Du Chesne, B. Gerharz, G. L ieser

represent multicomponent systems, contrary to coatingsfrom solution which, after solvent evaporation, exhibitthe properties of the polymers only.

Additives can either be homogeneously distributed(solubilised) over the whole Ðlm or form separatedomains,1,2 and may alter the properties of a polymericmaterial depending on their distribution. A homoge-neous distribution will usually give a material withaveraged properties which, owing to the usually lowadditive content in latices (a few per cent), would beacceptable except for special applications. Di†erent pol-arities and surface e†ects, however, result in a strongtendency towards phase separation, which may producea multiphase system where each phase retains its ownproperties. Usually, being water-soluble, segregated sur-factants will inÑuence the Ðlm properties, because mois-ture may disrupt the ÐlmÏs continuity. Enrichment ofadditives at the surface of the Ðlm causes deteriorationof adhesion at the substrate or of Ðlm surface proper-ties, such as stability, tack, reÑectivity and wettability.The properties of water-borne coatings thereforedepend more or less on type, amount and distributionof additives.

Scheme 1 is based on the early works of Brown3 andBradford and Vanderho†4 and represents the process ofÐlm formation, which is divided, somewhat arbitrarily,into distinct steps. The localisation of surfactant shownschematically was also proposed by Chevalier et al.5 Inthe dispersion (Scheme 1A), the emulsiÐer covers theparticle surface forming a swollen hydrophilic layer.Upon drying (for details on drying see Ref. 6), the dis-persion concentrates and particles become closelypacked (Scheme 1B). Though broadly accepted, themodel is partially misleading, as some experiments haverecently shown that particle deformation may takeplace prior to close packing.6h8 In the next stage, wateris expelled from the interstitial voids (Scheme 1C) andpolyhedral cells are formed, divided by hydrophiliclayers (membranes). The circumstances in which thesemembranes collapse depend on the type of surfactant.Membranes built up from low molecular weight ionicsurfactant in general collapse quickly, while highermolecular weight or non-ionic membranes need alonger time. If such surfactant is in large excess, or isgrafted to the polymer, membranes will be maintainedat the boundary between the particles and may preventtheir coalescence.5,9 Prerequisite for coalescence tooccur is the collapse of the membranes, enabling theinterdi†usion of polymeric chains (coalescence, Scheme1D).

As long as water is still present in interstitial voids,the collapse of the particle-dividing surfactant mem-branes can proceed through a micelle inversion, whichhas also been proposed by other authors5 and isdepicted in Scheme 2. Micelles of emulsiÐer can formseparate phases depending on concentration. Moreover,emulsiÐer may migrate to the Ðlm surface through the

Scheme 1. Schematic representation of the process of Ðlm for-mation from latex dispersion arbitrarily divided into foursteps : (A) dispersion ; (B) latex particles form a close packing ;(C) latex particles deform in order to Ðll the volume and expelwater ; (D) the surfactant membrane disrupts and coalescenceoccurs. The surfactant is drawn as a dash with a black dotsymbolising the hydrophilic end-group. The polymer is shown

by grey shading.

channels Ðlled with residual water.9 With time, micro-phases can migrate within the Ðnal Ðlm to form macro-phases.10

In order to gain a better insight into the process ofÐlm formation, and because the properties of a coatingare a function of the presence and distribution of addi-tives, many investigations have focused on their proper-ties and interactions. ReÑectance spectroscopy,9,11nuclear magnetic resonance (NMR) spectroscopy12 andscattering methods5,13 have been used for this purpose.For an interpretation of such measurements it is helpfulto have a method at hand to visualise, for instance,domain sizes and their localisation by direct techniques.

The above consideration is based on the assumptionthat continuous Ðlms are formed. Otherwise a powder

POLYMER INTERNATIONAL VOL. 43, NO. 2, 1997

Segregation of surfactant in latex Ðlms 189

Scheme 2. Schematic representation of the processes impor-tant for the segregation of surfactant. (A) Particles are incontact but surfactant stays adsorbed on their surface. (B)Deformation towards Ðlling of the interstitial voids hasstarted. This is the moment when the mobility at the particlesurface is large enough to allow for the desorption of sur-factant and for micelle inversion from “oil in waterÏ to “waterin oilÏ. (C) The Ðnal state, with inverse micelles and separated

phases of surfactant.

of more or less undeformed particles would be obtained.This phenomenon is used to determine the MFT byobserving the visible changes in a Ðlm dried on a tem-perature gradient bar.14

If electron microscopy is used to localise the emulsi-Ðer in the Ðnal Ðlm directly, mass thickness contrast isnecessary in order to distinguish between phasesenriched with an additive and that of the pure polymer.The detection of surfactant is difficult because of its lowcontent and often poor contrast. Staining was some-times used in order to create contrast, e.g. by hydrazinehydrate and osmium tetroxide for ester groups, or byuranylacetate if acrylic acid groups werepresent.5,13,15,16 If staining is not applicable and con-trast is low, energy Ðltering transmission electronmicroscopy (EFTEM) can be a promising alternative. Incases where the detergent contains at least one species

of heteroatoms, which does not occur in the polymeritself, the method allows the detection of emulsiÐerphases in unstained samples by electron spectroscopicimaging (ESI) and elemental mapping. The investigationof unstained samples also has the advantage that itavoids possible artefacts due to staining, which showsthe tendency to enlarge structures and, if applied in aliquid medium, can wash out soluble substances.

In the present study we investigated the distributionof emulsiÐer in coatings prepared at temperatures aboveand below the MFT. As yet, knowledge of the distribu-tion of surfactant in water-borne coatings dried belowthe MFT is limited.

EXPERIMENTAL

Synthesis

Latices were prepared in a 2 l reactor by semi-continuous emulsion polymerisation of vinylacetatemonomer (VAc) in the case of poly(vinylacetate) (samplePVAc) and methylmethacrylate (MMA) andn-butylacrylate (BuA) in the case of polyacrylate(sample PRA). Amounts of the constituents of the reac-tion mixtures are given in weight percentage based onthe Ðnal monomer content. Only p.a. chemicals wereused for the polymerisation ; water was deionized.

PV Ac. The whole amount of the emulsiÐers and co-monomers (see Table 1) and 91% of the total amount ofwater were stirred continuously and 10% of the VAcwas added. Upon heating, at 45¡C ammonium persul-phate (APS, 0É075%) was added as a solution in 10 g

By heating over a period of 15 min the polymeris-H2O.ation temperature of 70¡C was reached, allowing for theformation of a seed latex. After 30 min, the remaining90% VAc and 0É075% APS in 40 g were added inH2Oparallel over a period of approx. 3 h. The monomer andinitiator feed was adjusted in accordance with theexothermicity of the process. Subsequent to the additionof all the monomer, 0É05% APS in 20 g was addedH2Oand after 30 min, when the exothermic reaction was Ðn-ished, the reactor was cooled down to room tem-perature (RT). The dispersion was bu†ered to pH 4É8 bysodium acetate.

PRA. The homogeneous latex was synthesised accord-ing to Ref. 17 ; the main constituent monomers were50% MMA and 50% BuA (both Merck). Water (54É7%)and 0É75% sodium dodecylsulphate (SDS, Merck) werestirred and heated to 80¡C. Then, a portion of 1/40 ofthe monomer emulsion and 0É05% APS as a solution in1É3% was added. The monomer emulsion consist-H2Oed of the whole amount of monomer, 0É75% SDS, 0É3%APS, 2% methacrylic acid (Merck) and 60% AfterH2O.a prepolymerisation period of 15 min, the remaining



TABLE 1. Composition and characterisation of the latices. Compositions are given

in weight percentage of the monomer basis

PVAc PRA

Monomer basis vinylacetate methylmethacrylate 50%n-butylacrylate 50%

Functional comonomers Na-ethenesulphonate 0·5% methacrylic acid 2%Emulsifier Na-dodecylsulphate 1% Na-dodecylsulphate 1·5%

non-ionic surfactanta 2%Initiator, ammonium persulphate 0·41% 0·35%T

gb 38·6¡C 19·5¡C

Average particle diameter, dw 200 nm 93 nm

Particle size distribution, dw /dn c 1·1 1·1

MFT 20¡C 9¡CSolids content 50·1% 46.9%pH, buffer 4·8, Na-acetate Ã3 H

2O 8, ammonium hydroxide

287 (Hoechst) : oligoethyleneglycol-n-alkyl-ether, x ¼number ofa }Emulsogen-EPN CxE

y;

carbon atoms with a distribution of x ¼9–15; more than 90% are x ¼11; y ¼number of ethyl-

eneglycol monomer units, y B 28.

b Measured by differential scanning calorimetry in a Perkin Elmer DSC 7 at a scanning rate of

20 K minÉ1 in a range from É80¡C to 150¡C.

c Determined by photon correlation spectroscopy, for details see Ref. 18.

main portion of the monomer emulsion was added overa period of 3É5 h. Afterwards, the reaction mixture waskept at 80¡C for another 1 h and then cooled down toRT. The pH of the dispersion was adjusted to 8 byaddition of ammonia solution (12É5%).

The Ðnal composition and characterisation of the latexdispersions are given in Table 1. More data on similarPVAc dispersions can be found in the literature.18

Specimen

Dispersions were cast on glass slides to obtain Ðlms ofapprox. 200 km thickness, or into small vessels for bulksamples with a thickness of approx. 2 mm. Films weredried for 2 days, bulk samples for 2 weeks at RT (25¡C,T [ MFT) and 50% relative humidity, or at 5¡C(T \ MFT) in a dry atmosphere. Replicas were pre-pared in a Balzers BAE 250 evaporation apparatus byshadowing with Pt/C under an angle of 20¡ with sub-sequent evaporation of a carbon supporting Ðlm. Thepolymer was removed from the replicas by dissolutionin tetrahydrofuran (THF). Bulk samples were sectionedin a Reichert Ultracut-E ultramicrotome at [45¡C(FC 4E cryo-attachment) set to 35 mm section thickness.The ultrathin sections were Ñoated o† the diamondknife onto a mixture (60 : 40)dimethylsulphoxide/H2Oand transferred to copper grids. Other sections wereprepared by dry sectioning at room temperature.

Energy filtering transmission electron microscopy(EFTEM)

Using a transmission electron microscope equippedwith an energy Ðlter, one can select imaging electrons

according to their energy, which is lower for thosehaving undergone inelastic interaction with the speci-men.19,20 In particular, if atoms of the sample becomeionised in this process, primary electrons lose aquantum of energy equal to the ionisation energy forthe respective shell.21 These and other processes arereÑected in the electron energy loss (EEL) spectrum.Inner-shell excitations lead to absorption edges charac-teristic of each element. If one particular phase of thespecimen contains heteroatoms of type “AÏ, the intensityof electrons scattered in this phase will be enhanced atenergy losses (*E) shortly behind the correspondingabsorption edge. Hence images of such specimenscontain element-speciÐc contrast to phases not contain-ing “AÏ when recorded with electrons of *E in an energywindow shortly behind the absorption edge. For anunambiguous interpretation, the contribution ofelement-speciÐc contrast has to be separated fromunderlying mass thickness contrast. It is calculated onthe basis of a number of images recorded at *E beforethe absorption edge and subtracted in an elementalmapping procedure.19,20 This procedure is demon-strated in Fig. 1 using a real EEL spectrum from anultrathin section of a PVAc sample containingsodium ethenesulphonate and the emulsiÐer SDS. Thespectrum clearly shows the absorption edge atS-L2, 3165 eV, which is smeared out to higher energy losses(delayed edge22,23). Sometimes, element-speciÐc con-trast between phases can be directly visualised if thescattering due to mass thickness yields low intensity.For most organic compounds this is the case at energylosses 200 eV\ *E\ 283 eV, close to, but not reaching,the carbon K-absorption edge where the scattering due



Fig. 1. EEL spectrum showing the absorption edge, recorded from an ultrathin section of PVAc. On the basis of ESI 1 andS-L2, 3ESI 2, which are not inÑuenced by the edge, the background image is extrapolated assuming the intensity decay as potential(dotted line). This image is subtracted from ESI 3, thus separating the element-speciÐc information from mass thickness back-

ground (elemental mapping, three-window method).

to carbon atoms is at a minimum (low background).Even if an absorption edge is located at lower energyloss, where the intensity due to scattering at carbon maystill be signiÐcant, the tail of the edge and the resultingintensity o†set may reach into the region of low back-ground. Here, the intensity o†set, although small inabsolute value, is relatively high. In the case that ele-ments with such absorption edges are contained in onephase exclusively, so-called structure-sensitive contrastcan be observed, accentuating that particular phasebright. For details and typical examples see Refs 19, 20and 24È26.

For this investigation, EFTEM was performed in aLeo 912 operated at 120 kV using an objective apertureof 8É5 mrad. The width of the energy window dE, con-trolled by the opening of the energy selector slit, was15 eV. Energy losses of the imaging electrons extendfrom the nominal value of *E up to *E] dE. IlfordPan F 35 mm Ðlm was used for photographic recording.Images for elemental mapping were recorded using aslow scan CCD camera (SSCCD, lateral resolution1024 ] 1024 pixel, grey values digitised to 14 bit) andprocessed by means of an SIS-Analysis image pro-cessing system. Parallel EEL spectra were recorded at120 kV using the SSCCD camera. The diameter of theilluminated area (tungsten Ðlament, emission 2 kA, illu-mination angle 0É1 mrad) was 1 km (M \ 20 000).

RESULTS AND DISCUSSION

Samples cast and dried at RT (25¡C), i.e. above theMFT, were optically clear and, in the case of PVAc, of

high mechanical strength. Samples of PRA were softowing to the lower glass transition temperature see(Tg ,Table 1).

The proÐle shown in elastically Ðltered images of rep-licas from the surface of PVAc Ðlms (Fig. 2(a)) originatesfrom the uppermost layer of latex particles which have,to some extent, retained their shape in the directionnormal to the surface. Such a proÐle is typicallyobserved if the preparation temperature is lower than

In the plane of the Ðlm surface the particles areTg .densely packed and deformed, thus Ðlling up thevolume between them. On the other hand, the surface ofthe PRA Ðlm appears Ñat with no trace of the originallatex particles (Fig. 2(b)), which is characteristic of Ðlmsprepared at T [ Tg .

Changing from the elastic to the inelastic imagingmode, a contrast inversion is observed (inelastic “dark-ÐeldÏ). At energy losses *E between 150 and 180 eV, anadditional bright structure becomes visible in both rep-licas, the contrast reaching a maximum at *E\ 250 eV.The same regions as above, but imaged under theseconditions, are shown in Figs 2(c) and (d). In the case ofthe PVAc sample, the image exhibits bright spotsbetween the particles in some of the interstitial valleys(Fig. 2(c)).

The comparison of the sulphur map of an identicalspecimen with the image at *E\ 250 eV (Fig. 3)demonstrates that at the given energy loss, structure-sensitive contrast (SSC) is observed, which originatesfrom the intensity o†set due to the sulphur absorp-L2, 3tion edge at *E\ 165 eV. Hence the domains appear-ing bright at *E\ 250 eV are enriched with sulphur.

The same imaging conditions applied to a replica of



Fig. 2. ESI images of identical regions of replicas from Ðlms dried at T [ MFT: (a) and (c) PVAc; (b) and (d) PRA. (a) and (b) areelastically Ðltered images (*E\ 0 eV) ; (c) and (d) are recorded with electrons of *E\ 250 eV and show structure-sensitive contrast.



Fig. 3. ESI image and elemental map of the same region of areplica of a PVAc Ðlm dried at T [ MFT: (a) ESI at*E\ 250 eV (structure-sensitive contrast) ; (b) elemental mapof sulphur which was evaluated using the absorptionS-L2, 3

edge (see Fig. 1).

the PRA Ðlm show a “netÏ structure of bright contourslabelling the original latex particles at the Ñat surface ofthe Ðlm.

From the above it follows that some sulphur-containing material from the original Ðlm is retained atthe replicas. Also, twofold washing in THF did notchange their appearance, which leads to the conclusionthat the phases observed are insoluble in THF and stemfrom the surface of the Ðlms. Hence the replicas are infact detachment replicas.

The observation of the sulphur-containing domainsat the Ðlm surface leads to the questions : what do they

consist of ? and where do they originate from? Sulphur-containing compounds in the samples are SDS, traces ofunreacted initiator, the initiator bound to the chainends and, in the case of PVAc, ethenesulphonatecopolymerised with VAc.

For the dispersions investigated it can be expectedthat most of the surfactant is located at the surface ofthe particles, but from the information available so far itis not clear if some of it is contained “freeÏ in the serumas well. For PVAc in particular, a signiÐcant amount ofVAcÈethenesulphonate copolymer (VAc-co-ES, approx.15% ES) was found in the serum after ultracentrifu-gation.18

Of the compounds under consideration, only SDSand, in the case of PVAc, VAc-co-ES, possess enoughmobility and are present in sufficient amounts to segre-gate and form phases as shown above. Also, SDS and,depending on the ES content (15%), VAc-co-ES, arepractically insoluble in THF. Since the method usedcannot discriminate between these compounds, in thecase of PVAc they will be referred to as emulsiÐer, whilein PRA samples only SDS has to be considered.

There are di†erences in the appearance of the emulsi-Ðer at the surface of Ðlms from the two samples. In thecase of PVAc, larger domains are observed and forPRA the location of the emulsiÐer at the surface of theparticles is retained to some extent. A possible reason isthe presence of non-ionic emulsiÐer and VAc-co-ES inthe case of PVAc forming larger clusters with SDS. Thedi†erence can also be due to the higher mobility of thepolymer PRA, which enables faster coalescence of theparticles at the Ðlm surface,6,8 thus immediately forminga continuous skin (note the di†erence between drying ofthe whole Ðlm and skin formation at its surface). Thiswould leave less time for surfactant exudation, leadingto smaller phases which are more evenly distributed. Infavour of this argument is the fact that samples of PRAdry much more slowly compared with PVAc. On theother hand, in the case of PVAc a skin forms as well.The faster drying of these samples is therefore explainedby the presence of channels through which serum canÑow to the Ðlm surface, where the water evaporates.This makes it probable that the large segregated phasesobserved at the surface of PVAc Ðlms have been formedupon concentration of the serum at the outlet of thechannels. However, this explanation presupposes thatthe serum contains “freeÏ emulsiÐer.

Ultrathin sections from bulk samples of the laticeswere prepared and investigated in order to obtain animpression of the situation in the interior of the Ðlms.The elastically Ðltered images of the sections (Fig. 4(a))appear continuous, proving that Ðlm formation hasoccurred. Applying the SSC mode of imaging, micro-heterogeneity becomes visible (Fig. 4(b)). Again, elemen-tal mapping detected sulphur at the bright sites. Hencesegregation of emulsiÐer is not restricted to the surfaceof the Ðlms. In contrast to the situation at the surface,



Fig. 4. ESI images of the same region in an ultrathin section of a bulk sample of PVAc dried at T [ MFT: (a) elastically Ðlteredimage (*E\ 0 eV) ; (b) ESI image recorded with electrons of *E\ 250 eV (structure-sensitive contrast). Sections of PRA show

identical features.

the di†erence between PVAc and PRA is not signiÐcantin the bulk, where the size of the segregated domains israther small. That larger domains occur only at thesurface of PVAc (Fig. 2(c)) supports the above explana-tion of shorter drying time through the presence ofchannels.

The bright domains in Fig. 4(b) are sometimes sur-rounding dark regions, which are holes and appear asbright spots in the elastically Ðltered image (Fig. 4(a)).Some of the holes form by irradiation due to beamdamage. (In the replicas this e†ect was not observed,because the specimen is coated by the evaporated, elec-trically conductive carbonÈplatinum Ðlm.) Other holesare visible from the beginning. It is assumed that theyhave been Ðlled with “boundÏ water, which has evapo-rated from the sections in the vacuum of the transmis-sion electron microscope. Domains of “boundÏ waterhave been detected by NMR measurements12 and are inagreement with the hypothesis of micelle inversion.5

To sum up, in water-borne Ðlms prepared at tem-peratures where Ðlm formation occurs (T [ MFT), seg-regated emulsiÐer can be observed. This observationdoes not answer the question of where this emulsiÐer

comes from, i.e. whether it is present in the serum of theoriginal dispersion (“freeÏ) or if it desorbs from thesurface of the particles during Ðlm formation (seeSchemes 1 and 2). If the emulsiÐer was originally con-tained in the serum (“freeÏ emulsiÐer), its segregationshould not depend on whether samples are preparedabove or below MFT.

Therefore, samples of PVAc were investigated inwhich Ðlm formation was prevented by drying at 5¡C,i.e. at T \ MFT. They were white and brittle. This ischaracteristic of samples in which the particles did notdeform, leaving interstitial voids which scatter light, andwhere interparticle di†usion of the polymer did notoccur.

Replicas of the surface of these specimens were pre-pared under conditions identical with those applied tothe Ðlms dried at temperatures above MFT (Fig. 2). Thesurface, as revealed by elastically Ðltered images(*E\ 0 eV), looks the same as Fig. 2A and is thereforenot shown. Imaging the specimen under the conditionsfor SSC (Fig. 5) yields just a contrast inversion of theelastically Ðltered image ; no sulphur-containingdomains were detected at the surface.



Fig. 5. ESI image of a replica of a PVAc Ðlm dried atT \ MFT, imaged at *E\ 250 eV under the condition forstructure-sensitive contrast. The corresponding elastically Ðl-

tered image looks the same as Fig. 2(a).

This indicates that the emulsiÐer, which at T [ MFTforms separate domains, does not stem from the serumbut originally adheres at the surface of the particles.However, the situation in the bulk has to be checked aswell.

Ultrathin sections of the bulk samples prepared atT \ MFT are not continuous owing to the interstitialvoids, which lead to mass thickness Ñuctuations on alength scale of the same order of magnitude as thediameter of the latex particles (Fig. 6(a)). Images record-ed under the conditions of SSC show just an inversionof contrast (Fig. 6(b)). As in the case at the surface, thereare no domains of emulsiÐer. The identical appearanceof dry sectioned samples (without a Ñoating liquid)excludes the preparation artefact that domains of emul-siÐer have been washed out by the Ñoating liquid.

In samples prepared at T \ MFT, the emulsiÐer doesnot appear in segregated domains. However, sulphur-containing emulsiÐer is present in all samples, whichcan be demonstrated by EEL spectroscopy (Fig. 1).Hence the emulsiÐer is retained at the surface of thelatex particles as a thin, possibly monomolecular layerwhich cannot be imaged.

It is broadly accepted that Ðlm formation can be pre-vented by the hydrophilic membrane around the par-ticles, and the disruption of the membrane in theprocess of Ðlm formation upon annealing of drysamples has been followed.5 So far not much attentionhas been paid to the question of whether segregation ofsurfactant can occur upon drying, while there is no Ðlmformation.

Summarising the results, the dense packing alone,which brings the particles into contact, and drying atT \ MFT do not e†ect the segregation of emulsiÐerfrom the latex. For this process to take place, a certainlimit of mobility of the emulsiÐer at the particle surfaceis assumed to be necessary. This mobility can be provid-ed by particle deformation and coalescence. Hence, thesegregation of emulsiÐer takes place only if the samplesare dried at T [ MFT, i.e. when Ðlm formation occurs.During Ðlm formation, perhaps in the process of expel-ling the serum from the interstitial voids by particledeformation, the serum is enriched with surfactant. Themechanism which can be proposed may include the for-mation of inverse micelles, as depicted in Scheme 2. Ifchannels are present, the serum Ñows to the Ðlm surfacewhere it concentrates and dries, leaving large domainsof segregated emulsiÐer (Fig. 2(c)).

Further investigations on the observed interdepend-ence of Ðlm formation and the segregation of emulsiÐerare in progress.

CONCLUSIONS

EFTEM is a useful microscopic method for localisingsurfactant in solid Ðlms of latex, without the necessity toÐnd an appropriate stain for the samples. Domains ofemulsiÐer were observed in structure-sensitive contrastat the Ðlm surface and in the bulk, using detachmentreplicas and ultrathin sections, respectively. Segregateddomains of emulsiÐer were only observed in sampleswhich were dried at T [ MFT and formed continuousÐlms. E†orts to detect such domains in samples inwhich Ðlm formation did not take place were unsuc-cessful. The emulsiÐer in such samples seems to beretained as a very thin layer at the surface of the latexparticles. Its desorption from the particle surface is, onthe one hand, a necessary prerequisite for Ðlm forma-tion, and requires, on the other hand, a certain extent ofmobility. Mobility of the emulsiÐer at the particlesurface seems to play a key role in the combined pro-cesses of Ðlm formation, desorption and segregation ofemulsiÐer.

ACKNOWLEDGEMENTS

The authors gratefully acknowledge support by theGerman Ministry of Education and Research (BMBF),contract no. O3N3O185.



Fig. 6. ESI images of the same region in an ultrathin section of a bulk sample of PVAc dried at T \ MFT: (a) elastically Ðlteredimage (*E\ 0 eV) ; (b) ESI image recorded at *E\ 250 eV under the condition for structure-sensitive contrast.

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the segregation of surfactant upon film formation of latex dispersions: an investigation by energy...

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