Colloids and Surfaces A Physicochem. Eng. Aspects 317

Available online at Colloids and Surfaces A:Physicochem.Eng.Aspects317 (2008) 551–556An environmental scanning electron microscopy examination of thefilm formation mechanism of novel acrylic latexKalin I.Dragnevski∗,Athene M.DonaldSector of Biological&Soft Systems,Department of Physics,Cavendish Laboratory,University of Cambridge,J J Thomson Avenue,Cambridge CB30HE,UK Received27July2007;received in revised form20November2007;accepted23November2007Available online 4 December 2007AbstractWe have employed environmental scanning electron microscopy(ESEM)to study thefilm formation mechanisms of two acrylic latex compo-sitions,here defined as standard(carboxymethyl cellulose stabilised)and novel(stabilised with a novel polysaccharide derived from agricultural waste).The ESEM analysis revealed that the microstructure of the standard system consists of individual particles and upon evaporation a contin-uousfilm is formed,which is consistent with the current models.However,in the case of the novel system the microstructure consists of individual particles and clusters and during evaporation a discontinuousfilm is formed with voids present within its structure.Based on the experimental evidence we propose a modification to thefilm formation mechanism for the novel latex system.© 2007 Elsevier B.V. All rights reserved.Keywords:Polymer latex;Film formation;ESEM1.IntroductionPolymer lattices,with their wide range of applications,have been the subject of many theoretical and experimental stud-ies.When used for its traditional applications,i.e.as paint or adhesive,the latex is applied in its wet state to a surface and allowed to dry andfilm form under ambient conditions.There-fore,conventional electron microscopy,with its extreme drying and sample preparation requirements,will not be suitable for the examination of lattices in their natural wet state.On the other hand,environmental scanning electron microscopy(ESEM)[1], which offers the possibility to image‘wet’and insulating spec-imens,has been successfully used in the study of a number of systems and dynamic processes including lattices andfilm formation[2–7].ESEM is based on the use of a multiple aperture graduated vacuum system,which allows specimens to be imaged under water vapour or other auxiliary gases,such as nitrogen or nitrous oxide[4].In this way,the chamber can be held at pressures usu-ally in the range of1–10Torr[8],while the gun and column remain at pressures of∼7.5×10−7Torr.Moreover,by using a ∗Corresponding author.E-mail address:kd281@(K.I.Dragnevski).correct pumpdown procedure[9]and by controlling the temper-ature of the specimen,which in the ESEM is usually done by using a Peltier stage,dehydration can be inhibited and hence samples can be imaged in their‘natural state’.Furthermore,by taking into consideration the saturated vapour pressure(SVP) curve for water as a function of temperature[9]and by increas-ing the temperature of the specimen or reducing the chamber pressure,it is possible to produce evaporation conditions within the specimen chamber,which allows examination of the process offilm formation.As mentioned above,polymer lattices are important indus-trial products and the subject of many research tex, which is an example of a wet insulating material,can be defined as a colloidal suspension of spherical polymer particles with varying diameters.When water is allowed to evaporate from the system,the aqueous suspension undergoes a series of trans-formations,which result in the formation of a continuous dry polymerfilm.This process,known asfilm formation,contains four main stages that can be described as follows[10–18]—stage I:dispersed suspension of polymer particles;stage II:con-centrated suspension of particles in contact with each other, surrounded by solvent-filled interstices;stage III:ordered array of deformed particles;stage IV:a molecularly continuous and homogeneousfilm formed as a result of polymer interdiffu-sion.0927-7757/$–see front matter© 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.colsurfa.2007.11.042552K.I.Dragnevski,A.M.Donald/Colloids and Surfaces A:Physicochem.Eng.Aspects317 (2008) 551–556Fig.1.Schematic representation of an idealizedfilm formation process.Adapted from Keddie et al.to include the intermediate stage II*.Keddie et al.[5]used environmental scanning elec-tron microscopy and multiple-angle-of-incidence ellipsometry (MAIE)in the study of latexfilm formation.They concluded that an intermediate stage,between II and III,has been omitted in the conventional descriptions[10–18].The stage,defined as II*,is characterized by a randomly packed array of deformed particles which still contain water-filled interstices.A schematic representation of the process is shown in Fig.1.More recently,Keddie and co-workers[19,20]investigated the possibility of creating heterogeneousfilms,by mixing car-bon nanotubes(CNTs)with waterborne polymer particles.It was found that the mechanical properties of the nanocomposite coatings can be greatly improved,while maintaining their opti-cal clarity.However,it is important to note that all of the above studies were carried out using continuous polymerfilms.In recent years,environmental considerations have provided another strong motivation for developing coatings from renew-able resources and eliminating coalescing aids[21].These volatile organic components(VOCs)temporarily lower the glass transition temperature(T g)of the latex,which provides mobility to the polymer chains,thus allowing easier particle deformation and interdiffusion.Possible ways of eliminating VOCs,and pro-ducing lattices with lower T g,include blending hard and soft particles[22–24]so that the soft particles deform around the harder ones.However,in this case it is important to avoid seg-regation within the dispersion,ensuring an even distribution of particles throughout thefilm.Alternatively,core–shell particles, with a soft deforming shell surrounding a hard core,can be used for avoiding the issues with phase separation[25,26].In this paper we present the results from an ESEM investi-gation into thefilm formation mechanisms of a novel acrylic latex that has been stabilised by using a new polysaccharide, derived from agricultural waste and a standard polymer sys-tem,where the conventional carboxymethyl cellulose(CMC) has been used as a stabiliser.The novel polysaccharide con-sists of a number of monosaccharides(including arabinose and xylose)formed fromfive-and six-membered rings and has a low molecular weight,only a few thousand a.m.u.s rather than the hundreds of thousands found in cellulose for example.The polysaccharide also contains a significant amount of interfacially active protein∼15%.It is suggested that the initial latex particle stabilization comes from the protein component and ultimately the polysaccharide component stabilises the latex particles by adsorbing on their surface,rather than by chemically grafting on the growing polymer particles,which is the case for the con-ventionally used CMC.Initial examinations[27]have indicated that the novel latex canfilm form without the addition of coa-lescing solvents,which as suggested above,on one hand would provide an alternative method for the production of VOC-free architectural coatings and on the other would comply with the stringent EU and DEFRA regulations[28].2.Materials and methodsTwo aqueous latex compositions,supplied by ICI Plc,based on copolymers of methyl methacrylate(MMA)and2-ethylhexyl acrylate(2-EHA)were studied.In this paper the latex stabilised with the conventionally used by ICI Plc carboxymethyl cellulose will be referred to as‘standard’and the other stabilised with the new polysaccharide as‘novel’.The two lattices were initially about55wt.%polymer.The glass transition temperatures of the two lattices were determined by differential scanning calorime-try(DSC),carried out on dry specimens,using a PerkinElmer Pyris1instrument.The measured temperatures were279.8K for the standard and280.1K for the novel latex.The minimum film formation temperatures(MFFT)of the two lattices were measured by using a MFFT-Bar and were found to be278K and 279K,respectively.The microstructural analysis was carried out on an FEI XL-30environmental scanning electron microscope equipped with a Peltier stage.Wet samples from the above formulations were placed onto the cooling stage in the microscope chamber at a temperature of ca.274K.An evaporation-inhibiting pumpdown sequence was then performed,with the ambient air progres-sively replaced by water vapour.Once the purging cycle was completed,water vapour pressures and working distances of 3.5–4.5Torr and9.5–11.5mm were set,which provided suitable imaging environments.Imaging of the specimens was carried out at an accelerating voltage of10kV.Previous studies[2–5,29] have shown that the use of moderate beam voltages in combina-tion with fairly high pressures results in minimal beam damage, which proved to be the case in this study.Increasing the temper-ature of the specimens by1◦or2◦above the starting temperature of274K,as explained above,resulted in further dehydration of the lattices,which allowed examination of the process offilm formation.3.Results and discussionThe results of the microstructural observations are presented here.Firstly,thefilm formation mechanism of the standard latex will be considered.This will be followed by a presentation of the results for the novel polymer system.However,prior to considering the above,it is important to note that when we refer to lattices as being‘wet’,some water has in actual fact been removed from the surface of the specimens in order to obtain better quality images.Keddie et al.[2,5]used a simi-lar approach in the study of latexfilm formation by means of ESEM.It was found that despite the fact that some of the sur-K.I.Dragnevski,A.M.Donald /Colloids and Surfaces A:Physicochem.Eng.Aspects 317 (2008) 551–556553Fig.2.ESEM micrograph showing the surface of a standard latex specimen in stage II/II*.Imaging conditions:T =274K;p =3.9Torr.face water had been removed,the bulk of the samples remained ‘wet’.3.1.Standard latexFrom the ESEM image of the standard latex (Fig.2),it can be seen that under ‘wet’conditions the microstructure of the speci-men consists mainly of randomly distributed individual particles with an average size of ca.300nm.This was also confirmed by particle size measurements (Fig.3)carried out on a Coulter LS230Light Scattering Apparatus.Due to the fact that some of the water has already been removed,as explained above,some of the polymer particles are in contact.Despite that,they are still physically distinct,i.e.no significant deformation has occurred,and therefore it can be concluded that the latex is in stage II/II*.Fig.4reveals the surface microstructure of a standard latex specimen at a temperature of 276K.It is clearly seen that,at this slightly increased temperature,water evaporation has taken place,which results in the formation of a continuous polymer film.However,due to the fact that not all particles have lost their identity and some boundaries are still clearly visible,itcanFig.3.Particle size distribution of the standard latex,confirming the fact that the microstructure of the specimens in the ‘wet’state consists predominantly of individual polymer particles with sizes in the range of 300nm.Fig.4.ESEM micrograph of a standard latex specimen in stage III/IV .Imaging conditions:T =276K;p =3.9Torr.be concluded that under these conditions the latex is in stage III/IV .Due to the fact that imaging of the latex specimens was carried out below their T g of 279.8K,it is somewhat surpris-ing to observe the latter stages of film formation.However,it is suggested that as the microstructural analysis was carried out at temperatures very close to the minimum film formation tem-perature,partial particle deformation and coalescence,would naturally be expected to take place.It was also found that further increases in temperature did not lead to the observation of other surface morphologies.Based on the above results,which are comparable to those obtained in previous studies [1–5],it can be said that the film formation mechanism of the standard acrylic latex is in a good agreement with the conventional descriptions.3.2.Novel latexFig.5depicts the surface morphology of a novel latex spec-imen under ‘wet’conditions.The microstructure appears to be similar to one observed for the standard latex.The individual polymer particles,although in contact,are physicallydistinctFig.5.ESEM micrograph of a novel latex specimen in stage II/II*.Imaging conditions:T =274K;p =3.9Torr.554K.I.Dragnevski,A.M.Donald /Colloids and Surfaces A:Physicochem.Eng.Aspects 317 (2008) 551–556and therefore it can be concluded that the latex is in stage II/II*of the film formation process.However,another interesting feature that can be seen at slightly higher magnifications within the microstructure of the novel latex specimens (Fig.5inset )is the presence of a large number of clusters with sizes in the range 2–5␮m.The presence of these clusters was also confirmed by AFM [27]and particle size measurements (Fig.6a and b)carried out by ICI Plc using a Coulter LS230Light Scattering Apparatus.From the above ESEM images it is also evident that the clus-ters seen within the structure of the novel latex appear to have been formed by aggregation,followed by partial coalescence of individual particles,rather than complete coalescence.Here,partial coalescence is defined as the process of formation of agglomerates of spherical particles that are physically distinct,whereas complete coalescence is termed to be the formation of a featureless polymer structure.The factors leading to the formation of these clusters are currently being investigatedandFig.6.Particle size distribution (a)and AFM image (b)of the novel latex system,confirming the presence of a large number of clusters with sizes in the range of 2–5␮m.are believed to be related to the low molecular weight of the polysaccharide and its branched molecular structure [27].Further dehydration of the specimens resulted in the for-mation of a discontinuous film,with voids present within its structure (Fig.7).Similar to the standard latex,under these conditions not all particles and/or clusters appear to have com-pletely lost their identity and therefore it can be concluded that the latex is in stage III/IV of the film formation process.At this point,it is important to note that just as with the standard latex,the examination was carried out at temperatures close enough to the minimum film formation temperature of the latex to allow observation of the latter stages of the film formation process.The discontinuity of the film can be explained by taking into consideration the shape and size of the clusters.As seen from the ESEM results,the aggregates of spherical particles have differ-ent sizes and shapes.Therefore,it can be expected that during water evaporation,i.e.when clusters and individual particles come in contact,voids within the polymer film would easily form.It is believed that the presence of the clusters,which are formed during the latex synthesis,reduces the need for coalesc-ing solvent as part of the film formation has been achieved before film lay down and evaporation of the water.Thus,the creation of this partially coalesced polymer network during evaporation results in the formation of a film,which is in the middle of the two extremes,i.e.full coalescence and no coalescence.In summary,it can be said that despite the fact that the mechanism of cluster formation in the novel acrylic latex is yet to be fully revealed,the film formation process that the system undergoes as a result is clearly seen to be different from the standard one.Although,the overall process appears to be similar,some modifications of the individual stages of the conventional model need to be made,in the case of the novel latex,due to the presence of clusters in the starting material.The stages that the system undergoes during water evaporation can be described as follows—stage I:dispersed suspension of polymer particles and clusters,formed by aggre-gation of individual particles;stage II:concentratedsuspensionFig.7.ESEM image of a novel latex specimen in stage III/IV .Imaging condi-tions:T =276K;p =3.9Torr.K.I.Dragnevski,A.M.Donald/Colloids and Surfaces A:Physicochem.Eng.Aspects 317 (2008) 551–556555Fig.8.Schematic representation of thefilm formation mechanism for the novel latex system.of particles and clusters in contact with each other,surrounded by solvent-filled interstices;stage II*:randomly packed array of deformed particles and clusters that still contain water-filled interstices;stage III:ordered array of deformed particles and clusters with voids present in the structure;stage VI:a dis continuousfilm formed as a result of polymer interdiffusion.A schematic diagram of the process described above is shown in Fig.8.4.ConclusionsEnvironmental scanning electron microscopy has proven to be a successful method for studying the process of evolution of an aqueous polymer dispersion into a polymerfilm.The ESEM results,which are consistent with AFM data obtained by ICI[27],revealed that there are differences in both the microstructures and the drying behaviour of the studied latex systems.In the case of the standard latex,the microstruc-ture mainly consists of individual particles,whereas in the case of the novel latex,the microstructure appears to consist of individual particles and clusters with sizes in the range of 2–5␮m.Furthermore,during water evaporation,in the case of the standard system a continuousfilm is formed,which is con-sistent with the classical descriptions,whereas in the novel one thefilm formed,is discontinuous,with voids present within its structure.Based on the experimental results obtained in this study we have proposed a modification to the currently accepted mechanism for 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