characterization of advanced morphologies in polymer dispersions by auc and hdc

ORIGINAL CONTRIBUTION

Characterization of advanced morphologies in polymerdispersions by AUC and HDC

Vikas Mittal

Received: 16 August 2009 /Revised: 14 September 2009 /Accepted: 16 September 2009 /Published online: 6 October 2009# Springer-Verlag 2009

Abstract Polymer latex particles are used in a widespectrum of applications that are directly influenced bythe surface characteristics as well as particle size and sizedistribution of these polymer particles. Accurate analysis ofsuch characteristics is required to efficiently control thebehavior of such particles. Analytical ultracentrifugation(AUC) and hydrodynamic chromatography (HDC) are theparticle characterization methods of high relevance owingto their statistical ability, and the combination of these twotechniques also allows to generate information about thesurface morphology of the particles. Such a comparison isfacilitated owing to different principles of particle charac-terization in these two methods. AUC relies on the densitydifference between the particles and the suspendingmedium to correlate to the particle size, whereas HDC isbased on the measurement of hydrodynamic diameter byUV absorption. When particles functionalized by a thinlayer of hydrophilic polymer are analyzed, these twomethods by the virtues of their characterization principlesallow to detect the presence of such a functionalizingpolymer layer on the surface. Subsequently, these methodsalso provide accurate estimation of the thickness of such alayer. The comparisons can also be carried out as a functionof time or amount of surface functionalization to tune theproperties according to the requirement. In the case where athick polymer layer is present in the surface of the particles,the comparisons are more qualitative in nature owing to thebridging and aggregation of the particles especially noticedin HDC. However, even in such a case, the combination of

these methods is the only way to characterize the specialmorphology of the particles.

Keywords Latex . Density . Diameter . Adsorption .

Grafting . AUC . HDC . Aggregation

Introduction

Optimization of the physical properties as well as surfacemorphology of the colloidal polymer particles is of utmostimportance as it directly impacts the applications of theseparticles. Polymer dispersions with particles of varioussizes, chemical compositions, and surface characteristicsare used in the commercial products, and these particularproperties of the polymer particles are the drivers of theiruse in particular product formulations [1]. The smallchanges in these characteristics can bring altogetherdifferent behaviors of these particles; therefore, control ofthese parameters is very important. This ability to controlalso helps subsequently to tune the properties of thepolymer particle formulations according to need.

Particle size and its distribution is the most commonparameter in the analysis of polymer particles, and as aresult, there are numerous ways to achieve this property.Colloid fractionating techniques like analytical ultracentri-fugation (AUC) [2–7] and hydrodynamic chromatography(HDC) [8, 9] are one of the most commonly usedtechniques for characterization of particle size and itsdistribution owing to their very high statistical analysiscapability. In one experimental run, particles in the range of1012 are analyzed to generate the information on the size[1]. For the determination of particle size and its distribu-tion of compact particles, use of any of these twotechniques would accurately provide the information on

V. Mittal (*)BASF SE, Polymer Research,67056 Ludwigshafen, Germanye-mail: [email protected]

Colloid Polym Sci (2010) 288:25–35DOI 10.1007/s00396-009-2119-8

the size. However, for the non-compact particles, i.e.,particles with morphologies like compact core and softshell, or swollen particles, etc., the use of any singletechnique to identify and characterize the particles withthese morphologies may not be sufficient. Polymer chainsare also chemically grafted or physically adsorbed on thesurface of polymer particles to enhance their surfacecharacteristics important for their stability, rheology, pack-ing, interactions, etc. The amount or thickness of the shellof such polymer chains or brushes present on the surface ofthe particles also affects the behavior or performance ofthese particles. Therefore, the characterization of thethickness of the surface layer is also important to tune theproperties of the polymer particles. For such analysis too,not a single method can provide the full information [5, 10–12]. In this paper, we intend to highlight the synergismbetween the two techniques of AUC and HDC to firstidentify and subsequently quantify the morphologies ofpolymer particles which are not compact in nature. Apartfrom that, those systems are also reported where these twotechniques are not complementary to each other howeverstill provide valuable insights into the nature and advancedmorphologies of the polymer particles.

Both AUC and HDC are based on different principles ofsize analysis, and owing to these different principles, thecombination of the two techniques can yield informationregarding the surface morphology of the particles as well ascorresponding particle sizes, which otherwise is notpossible from single analytical method alone. In AUC, thesedimentation of particles in the ultracentrifugation cell inthe rotor depends on the density differences between thesolute and the solvent. This density difference is correlatedwith the sedimentation coefficient (which is related to thespeed of sedimentation of these particles in the centrifugalfield) to generate distributions of size of particles [1, 13].HDC is, on the other hand, density insensitive and is basedon the size exclusion fractionation according to thehydrodynamic diameter of the particles [14]. The UVabsorption by the particles at 254 nm wavelength iscorrelated to the particle size distributions. Because of itsdensity-insensitivity, HDC is complementary to AUC togain further insights into the advanced morphologies ofcolloidal particles.

Materials and experimental methods

Polymer dispersions

The polymer dispersions used in the study are polystyreneand acrylic copolymer-based compact particles. In order toachieve different extents of hairy layer or corona on thesurface of the polymer particles, poly(acrylic acid) and

polyacrylamide with molar masses in the range of 500,000–1,000,000 g/mol were either physically adsorbed orchemically grafted on the surface. Apart from that, in orderto ascertain the effect of glass transition temperature on thebehavior of the particles in AUC and HDC, acryliccopolymer dispersions with two different glass transitiontemperatures were synthesized by varying the chemicalcomposition.

Turbidity and interference AUC

The particle size distributions of the polymer dispersionswere analyzed in OPTIMA XL ultracentrifuge fromBeckman Coulter [1]. Either the self-made turbidity opticsor interference optics were employed for the analysis [1].In the turbidity analysis, green detection light with awavelength of 546 nm was generated using a stabilizedincandescent lamp as light source and a monochromaticlight filter. The light was focused to 0.5 mm at mid-cell.The intensity of the light after reduction owing to thescattering by the latex particles, according to Mie’s lightscattering theory, is recorded by the photomultiplier as afunction of time. The experiment is run till the intensityreaches the same value as the incident amount. On theother hand, in the interference optics, which is based onthe principle of Rayleigh interferometer, refractive indexdifference between the solution and the solvent wasmonitored. This difference is proportional to thecorresponding concentration at this radial position. A laserdiode with a wavelength of 675 nm was used as a lightsource.

Static density gradients

Self-modified OPTIMA XL model of the AUC fromBeckman Coulter was used for the static density gradientanalysis [15, 16]. The machine is equipped with eight-holerotors and self-made Schlieren optics with a multiplexer.Schlieren optics setup was similar to the setup of Rayleighinterferometer, but it had a phase plate in the focus of thecondenser lens as an additional element. The radial scanswere collected at a fixed interval of time in order toascertain the time when the equilibrium has reached or toknow about the possible movement of the particles out ofthe used density range owing to their higher density, etc.Mono sector cells containing −2° horizontal wedge win-dows to compensate the steep radial optical refractive indexgradient were used. Water Nycodenz density gradientsystem with a range of density 1.05–1.17 g/cm3 was used.A small amount of emulsifier was also added to water inorder to inhibit any aggregation of the particles. The rotorspeed was chosen to be 30,000 rotations per minute, andthe cells were nearly completely filled in order to cover the

26 Colloid Polym Sci (2010) 288:25–35

maximum density range. The concentration of particlesranged from 0.065 to 0.076 g/l. The samples were run for22 h.

Hydrodynamic chromatography

HDC was performed on the particle size distributionanalyzer supplied by Polymer Labs. The equipment wasfitted with an integrated autosampler. The equipment alsoknown as gel permeation chromatography for particlescontains a packed column of polystyrene beads of definedsurface functionalization and pore size distribution. Theeluent used is a mixture of anionic and non-ionicsurfactants in order to reduce any aggregation of theparticles or to avoid any possible interaction between theparticles and the column. The HDC elution time and elutionshape are calibrated with standard latexes with sizedistributions covering the whole measurement range possi-ble by using a particular column. The latex particles afterdilution were injected into the column and owing to thepore size distribution in the column, the particles getfractionated, and the larger particles start to elute out first

followed by the smaller particles, which get delayed as theyoccupy the interstitial sites in the column. On elution, theparticles were detected by UV absorbance at 254 nm tomeasure the hydrodynamic size of the particles.

Results and discussion

In many applications, the morphology or surface character-istics of the polymer particles are modified in order toachieve functional behaviors from them. These modifica-tions may include physical adsorption or chemical graftingof polymer or oligomeric chains on the surface to form aless compact thin layer or corona loosely held on thesurface. This generates roughly a core–shell morphology ofthe particles with a compact core and soft shell. In thefollowing paragraphs, two general cases are discussed; the

Fig. 1 Representation of the thin layer of hydrophilic polymerphysically adsorbed on the surface of polymer latex particles

Fig. 2 H2O-D2O plots of sedimentation profile of particles before(dotted lines) and after (solid lines) adsorption of the polymer on thesurface. From left to right: analysis in H2O, H2O:D2O 1:1 mixture,and D2O. I is the transmitted intensity of light through latex at time t,whereas I0 represents intensity of light through solvent

Fig. 3 a Cumulative and (b) differential size distributions of theparticles before (dotted lines) and after (solid lines) the surfacemodification, when measured by turbidity optics analysis in analyticalultracentrifugation

Colloid Polym Sci (2010) 288:25–35 27

one when there is a thin layer of corona present around theparticles and the other case where the grafted or adsorbedlayer around the particles is thick and the synergistic use ofAUC and HDC to achieve more information on theparticles characteristics would be highlighted.

Figure 1 represents the immobilization (by physicaladsorption) of a thin layer of polymer chains on the surfaceof the compact particles to achieve certain surface func-tionality. As the thickness of the layer also affects theproperties and colloidal stability of the particles, therefore,it is of immense importance to first know if such a layerexists on the surface and what is its thickness if its presenceis confirmed. By using other characterization methods, e.g.,microscopy, it is difficult to detect such a thin layer, and thesample preparation requires considerable effort. Both theabove questions can be answered accurately by combina-

tion of the AUC and HDC techniques because of theirdifferent principles of particles size determination. As thesize determination by the use of AUC requires theknowledge of accurate density of the polymer particles,therefore, a sophisticated technique for density determina-tion called H2O-D2O method was used [2, 17]. In thistechnique, sedimentation analysis of the particles in H2Oand D2O (and even in mixtures of H2O and D2O) is carriedout using the turbidity optics. By evaluating these sedi-mentation profiles, simultaneous evaluation of both theparameters of density as well as particle size can be carriedout owing to the presence of the two sets of sedimentationdata for the particles. One has to assume that the particlesize and density are identical in all the liquid mediums usedfor the analysis. Figure 2 shows such an analysis for theparticles before and after the adsorption of thin polymerlayer (poly(acrylic acid)) on the surface, where the changein intensity of the incident light is plotted as a function oftime achieved by turbidity detector. Particles in H2O, D2O,and 1:1 H2O/D2O mixture were analyzed. A density of1.091 g/cm3 was observed for the particles before thepolymer adsorption, whereas density value of 1.088 g/cm3

was measured for the particles with adsorbed polymer layerindicating a minor change in density owing to polymeradsorption. By using this density information from theH2O-D2O method, the sedimentation profiles of thesamples in water were further analyzed for particle size

Fig. 4 a Cumulative and (b) differential particle size distributionanalysis of particles before (dotted lines) and after (solid lines) thephysical adsorption of the polymer on the surface, when measured byhydrodynamic chromatography

Fig. 5 Hydrodynamic chromatography differential size distributionplots of the particles for polyacrylamide adsorption system: (1)squares, particles before adsorption; (2) circles, particles withadsorption of polyacrylamide in an amount of 2 wt.% correspondingto the latex solid content and measured after 5 days; (3) triangles,particles with adsorption of polyacrylamide in an amount of 5 wt.%corresponding to the latex solid content and measured after 5 days, (4)inverted triangles, particles with adsorption of polyacrylamide in anamount of 10 wt.% corresponding to the latex solid content andmeasured after 5 days; and (5) diamonds, particles with adsorption ofpolyacrylamide in an amount of 10 wt.% corresponding to the latexsolid content and measured after 40 days


determination. The cumulative as well as differentialdistributions of particle size for both the samples have beenplotted in Fig. 3. The particles have bimodal sizedistributions, the distribution representing higher particlesize range accounting for roughly 20% of the particles. Thepeak particle size of the first distribution in the particlesbefore adsorption was observed at 134 nm. The peakdiameter value for the particles reduced to 125 nm in thesample after polymer adsorption. This is not expected as thesize of the particles should increase owing to polymeradsorption on the surface. Firstly, such an effect on particlesize analysis in AUC may result from the effectivedifference in density distribution inside the particles owingto polymer adsorption and resulting change in morphology(because of resulting core–shell morphology with compactcore and soft shell). Although the density of the particlesafter physical adsorption was measured and was applied forthe size evaluation, the inhomogeneous density inside theparticles may change their sedimentation behavior unpro-portional to the density change. Secondly, the adsorption ofhydrophilic polymer chains on the surface would signifi-cantly affect the dynamics of particle sedimentation owingto the insertion of the water in the adsorbed corona. Theparticles after polymer adsorption may be more swollen inwater phase than the compact particles, and it effectivelyslows down their sedimentation in the centrifugal field,which leads to their evaluation as small particles owing tosmaller sedimentation coefficient or lower sedimentationvelocity. This behavior of apparent reduction in diameter

already signifies changes of possible surface characteristicsof particles, but owing to a small change in the apparentdiameter, the presence of corona needs to be furtherconfirmed. Moreover, in real cases, the standard samples,i.e., particles before adsorption, are not always available forcomparison.

Figure 4 shows the cumulative and differential sizedistributions of the particles before and after adsorption,when measured by using HDC. The similar bimodalbehavior was also observed as depicted by AUC. The peakdiameter of the particles in the first distribution for thesample without polymer adsorption was observed at157 nm, and the peak diameter increased to 171 nm afterthe polymer adsorption. As the particle fractionation inHDC is independent of density or density changes in theparticles and is purely affected by the hydrodynamic size ofthe particles, the polymer adsorption on the surface of theparticles correctly reflects an increase in the size of theparticles. It can be argued that the HDC results shown inFig. 4 provide direct confirmation of the presence ofpolymer layer on the surface of the particles; however, asmentioned above, the reference samples are not always

Fig. 6 Analytical ultracentrifugation cumulative size distributionplots of the particles for polyacrylamide adsorption system: (1)squares, particles before adsorption; (2) circles, particles withadsorption of polyacrylamide in an amount of 2 wt.% correspondingto the latex solid content and measured after 5 days; (3) triangles,particles with adsorption of polyacrylamide in an amount of 5 wt.%corresponding to the latex solid content and measured after 5 days; (4)inverted triangles, particles with adsorption of polyacrylamide in anamount of 10 wt.% corresponding to the latex solid content andmeasured after 5 days

Fig. 7 Representation of grafting of a thick layer of polymer on thesurface of the polymer particles

Fig. 8 Static density gradient analysis of (a) ungrafted and (b) graftedparticles


available to be compared with the polymer-adsorbedparticles. Therefore, it is of interest if the information aboutadsorption can be generated from the polymer-adsorbedparticles only by using different characterization methods.

To quantify the amount of corona adsorbed on thepolymer particles by combining AUC and HDC, a simpleequation was suggested [18]:

dCorona ¼ DHDC

21� DAUC

DHDC

� �2=3 !

where DHDC and DAUC correspond to particle diametersfrom HDC and AUC, respectively. The equation takes intoaccount the swelling of the periphery of the particles afterthe polymer adsorption. By applying this equation, a coronaof 16 nm shell thickness was observed for the particlesshown in Figs. 3 and 4 after adsorption of the polymer. Itshould also be noted that a corona value of 8 nm was alsoobserved for the reference particles without polymeradsorption. This amount, however, falls into the experi-mental error range of the techniques used and should not bequantified. Presence of long chain emulsifier molecules onthe surface of particles may also lead to this effect. However,with reasonable accuracy, it is still correct to say that thestarting latex particles were compact in nature. Degree ofswelling of the shell polymer has also been studied inliterature by comparison of swollen and un-swollenparticles [19, 20].

The above-described synergism between the AUC andHDC methods can also be used to study the polymeradsorption on the surface of the polymer particles asfunction of adsorbent loading as well as time. By relativecomparisons between the peak diameters as well as sizedistributions, one can optimize the conditions which wouldresult into the required morphology generation on thesurface. Figure 5 shows the HDC results on the adsorptiontrials of polyacrylamide on the surface of compactpolystyrene particles. The amount of polyacrylamide wasvaried in the weight percent of 2%, 5%, and 10%,

Fig. 9 Cumulative and differen-tial size distribution of ungrafted(dotted lines) and polymer-grafted particles (solid lines),when measured by analyticalultracentrifugation

Fig. 10 a Differential and (b) cumulative size distribution ofungrafted (dotted lines) and polymer-grafted particles (solid lines),when measured by hydrodynamic chromatography


corresponding to the solid content in the latex. The sizedetermination was performed after 5 days, and the 10%sample was also measured after 40 days. The peak diametersteadily increased from 176 for the latex particles withoutadsorption to 179, 184, and 187 nm for the particlesadsorbed with polyacrylamide in the weight ratio of 2%,5%, and 10%, as compared to the solid fraction of the latex.Increasing the amount of polymer in the system also led tobroadening of the size distributions as well as steadyincrease in the potential aggregation of the particles asobserved by the generation of tailing in the size distribu-tions. The high shearing forces in the HDC column mayalso aggravate the aggregation formation owing to the softshell on the particles. The AUC results, as shown in Fig. 6,present the similar behavior as explained above. Withincreasing polymer loading as well as adsorption time, theapparent size distributions shifted to lower diameter valueseven though no significant change in the density of theparticles could be recorded after adsorption of polymer inany amount. Comparisons of these behaviors with HDC canfirst, help to confirm the presence of corona and secondly,help to tune the surface of the particles as requirement. Noaggregates were observed in the AUC measurements,indicating that either the interactions holding the particles

Fig. 11 Cumulative anddifferential size distributions ofparticles. Curve I representsungrafted particles andmeasured at a pH of 5.6. CurvesII–IV represent polymer-graftedparticles and measured at a pHof 3.9, 5.6, and 9, respectively

Fig. 13 a Cumulative and (b) differential size distributions ofparticles. Squares represent ungrafted particles and measured at a pHof 5.6. Curves with circles, triangles, and inverted triangles representpolymer-grafted particles and measured at a pH of 3.9, 5.6, and 9,respectively

Fig. 12 Representation of the lower diameter evaluation of thegrafted particles in analytical ultracentrifugation after swelling athigher pH


together in the aggregate are not strong or the particles hadshear-induced aggregation in the HDC column.

There are many applications of the particles whichrequire a thick layer of polymer to be immobilized on thesurface of the particles, mostly by using chemical graftingmethods. In many instances, the polymer particles aredirectly synthesized in the presence of shell-forming layerin order to achieve its chemical incorporation on thesurface. Figure 7 shows such a representation of thegrafting process. Such layers are of immense importanceto control rheological, wetting, and colloidal stabilityproperties of the particles. Figure 8 shows the densityprofiles of the grafted and ungrafted compact particlesmeasured by using static density gradients. It should benoted that the ungrafted sample is not completely similar tothe core in the grafted sample, as the grafted particles aregenerated in situ in the presence of shell-forming polymer.Therefore, as mentioned above, in real cases, it is always ofinterest to generate information regarding surface character-istics and particle size only from the grafted particles byusing different characterization methods. The density of theparticles after grafting was measured at 1.11 g/cm3 ascompared to 1.09 g/cm3 in the case of ungrafted particles,indicating no significant change in the density owing tografting. Figure 9 shows the AUC analysis of such asystem. A considerable shift of the particle size distributionto the left was observed in grafted particles as compared tothe ungrafted particles, owing to significant amount of lesscompact soft corona chemically bounded to the surface ofthe particles. It is again important to stress upon thecomplex phenomena associated with sedimentation of thegrafted particles. Though the measured density does notseem to be affected by grafting, however, change ofmorphology as well as dynamic interactions of particlesurface with the liquid phase significantly affect thesedimentation behavior which unrealistically results into

Fig. 14 Field flow fractionationprofile of the ungrafted (dottedline) and grafted (solid line)particles using a UV detectormeasuring at a wavelength of202 nm. The particles weremeasured at a pH of 3.9. Thefirst peak in the profile is astandard material for calibration

Fig. 15 a Cumulative and (b) differential distributions of particleswith low Tg (solid line) and high Tg (dotted line), when measured byhydrodynamic chromatography


low particle size. Figure 10 reports the differential andcumulative size distributions of the grafted and ungraftedparticles, when measured by HDC. As is evident from theseplots, the apparent size of the grafted particles was veryhigh, which is highly unlikely owing only to the chemicalgrafting of the polymer chains on the surface. Two possiblereasons for such a phenomenon can be hypothesized:interaction of chains on the particles with each other toform physical networks and shear-induced aggregation ofthe particle in the HDC column. If interactions between thechains are the main reason for such a behavior, then itsabsence in AUC would mean that such interactions may bedisturbed during fractionation in the centrifugal field. Mostprobably, a combination of both the factors may beconsidered to better explain such a behavior. Thus, in thecases of high extent of polymer grafting on the surface ofparticles, it is possible that none of the AUC or HDCmethods provides an absolute value of particle size;however, the combination of the two methods is still theonly way to characterize such morphologies qualitatively.

The extent of swelling of the polymer-grafted particleswas also studied as a function of pH. Figure 11 shows theAUC analysis of the grafted particles at pH values of 3.9,5.6, and 9.0. Apart from that, the size distribution ofungrafted particles measured at a pH of 5.6 is also plotted.As pH increased more, more shifting of the size distributionto the lower apparent particle sizes was observed. Byincreasing pH, the compact core of the particles wasswollen accordingly leading to incorporation of water deepinside the particle cores. This leads to more inhomogeneitiesin the density distribution in the particles as well as increased

extent of interactions with the aqueous phase leading to theirevaluation as small particles owing to their slow rate ofsedimentation. The phenomenon is also represented inFig. 12, where pH increase leads to reduction in the effectiveparticle size deduced from AUC measurements; however, thehydrodynamic diameter in the HDC should increase if anyother additional factor associated with swelling does notcome into the picture. The HDC results of the system arepresented in Fig. 13. The size distributions are observed toshift to much higher diameter ranges than expected as afunction of pH. It is not completely unexpected as theincreased extent of swelling further accelerates the bridgingof particles with each other as well as increases the extent ofshear aggregation owing to the softer core than the ungraftedparticles.

To further confirm which of the reasons of inter-particleinteractions or shearing in the HDC column or a possiblecombination of both are responsible for the much highersize distributions observed in the HDC results, two differentexperiments were carried out. Figure 14 shows the fieldflow fractionation profile of the grafted and ungraftedparticles measured at a pH of 3.9. As the channel used inthis equipment has no packing material, therefore, theextent of shearing is expected to be less than that observedin the HDC column. The grafted particles had the apparentparticle size distributions in higher particle size range,which indicates that probably the shearing in the HDCcolumn was less responsible than the inter-particle bridgingfor the behavior of the particles observed in HDC, andpossibly such bridging was disturbed in the AUC duringfractionation leading to no detection of such aggregation in

Fig. 16 Cumulative anddifferential distributions ofparticles with low Tg (solid line)and high Tg (dotted line), whenmeasured by analyticalultracentrifugation


AUC. However, the second experiment pointed contrary tothe above notion. Figure 15 shows the cumulative anddifferential particles size distributions of particles with low(below room temperature) and high (above room temper-ature) Tg, when measured with HDC. The correspondingAUC measurements are also presented in Fig. 16. In low Tgmaterial, a large fraction of material had much higherparticle size indicating bridging or aggregation, whereas nosuch behavior was observed in the AUC measurements. Onthe other hand, for the high Tg material, most of the higherparticle size component was absent, and the AUC mea-surement was also similar to the HDC findings. Thisindicates clearly towards the shear-induced aggregation ofthe soft or swollen particles in the HDC column. As thehigh Tg particles were compact in nature, therefore, theirbehavior in HDC and AUC was comparable. However, asthe studied system in this case is not completely similar tothe grafted polymer particles and as the grafted polymerchains from the surface of particles are more mobile tointeract with the other particles in the vicinity, therefore, itcan be concluded that the HDC findings of much higherparticle sizes are a combined result of inter-particleinteractions and shear aggregation in the column, both ofwhich are absent in the AUC. As a result, combination ofboth AUC and HDC helps to identify and either quantita-tively or qualitatively analyze such morphologies owing totheir different principles of characterization.

Conclusions

Synergism between the AUC and HDC can be used tocharacterize advanced morphologies of the polymer latexparticles apart from particle size and its distribution. Bycombinations of particle size determined from AUC andHDC, it is possible to first identify the presence ofmorphologies like core–shell morphology (with compactcore and soft shell of grafted or adsorbed polymer), swollenparticles, etc. Subsequently, it is also possible to calculatethe thickness of the soft shell on the surface of the particlesby using a simple relation. It should be noted that in AUC,the change in morphology of the particles, because of theadsorption or grafting of polymer on the surface, leads tochanges in the density distribution in the particles. Theseinhomogeneities in density as well as changed interactionsof the polymer-adsorbed/grafted particles as compared tounadsorbed or ungrafted particles lead to significantlychanged sedimentation behavior of the particles whichresults into the apparent particles diameters which aresmaller than the parent particles. HDC, on the other hand, isinsensitive to such changes in density and interaction withthe aqueous phase and correctly measures the hydrody-namic size of the particles. Therefore, any combination of

the results of these two techniques to quantify the amountof polymer on the surface of the particles should includethese considerations.

Comparisons of AUC and HDC can also be performedas a function of extent of polymer adsorption or adsorptiontime. These comparisons are helpful to tune the surfaceproperties of the particles according to the requirement. Inother cases, especially where a thick shell of polymer isgrafted on the surface of particles, only qualitativecomparisons are possible. A large extent of particlebridging or aggregation is observed in HDC which is alsoenhanced as a function of pH owing to the swelling of thecore of the particles at higher pH. The AUC results on theother hand show reduced apparent size as a function ofextent of grafting owing to changed morphology, non-uniform density distribution in the particles, and theslowed sedimentation of the particles because of thepresence of hydrophilic polymer chains on the surface ofthe particles. Although in such cases, neither HDC norAUC can provide an absolute value of particles size;however, their comparison is the only way to identifyand control such morphologies.

Acknowledgements The author would like to thank M. Kaiser, K.Werle, S. Machauer, and K. Vilsmeier for the excellent experimentalsupport.

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characterization of advanced morphologies in polymer dispersions by auc and hdc

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