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Materials Science and Engineering A 527 (2010) 2363–2369

Contents lists available at ScienceDirect

Materials Science and Engineering A

journa l homepage: www.e lsev ier .com/ locate /msea

olymer chain pinning at interfaces in CaCO3–SBR latex composites

. Touaiti a,∗, P. Alama, M. Toivakkaa, D.W. Bousfieldb

Laboratory of Paper Coating and Converting, and Center for Functional Materials, Åbo Akademi University, Porthansgatan 3, Turku 20500, FinlandPaper Surface Science Program, Department of Chemical Engineering, University of Maine, Orono, ME 04469-5737, USA

r t i c l e i n f o

rticle history:eceived 13 August 2009eceived in revised form0 November 2009ccepted 24 December 2009

eywords:article

a b s t r a c t

The viscoelastic properties of carbonate-based coatings in ambient and water saturated conditions havebeen investigated using dynamic mechanical thermal analysis (DMTA). At low frequencies, viscous flowis suggested to be due to the reorientation of polymer chains at particle interfaces. At higher frequenciesthe composite storage modulus, as normalised to that of the latex, decreases due to the conjoint action ofincreased heat and decreased molecular chain mobility at the interfaces. Water is found to weaken inter-facial pinning of the polymer molecules. Water is able to conduct heat away from the solid componentof the microstructure more effectively than air, and reduces the effect of temperature on the softening oflatex. Pore space allows greater microstructural mobility and consequently polymer chains at interfaces

oatingsMTAolymer pinningorosityatexomposite

can orient more easily to the loading direction. The glass transition temperature, the normalised storagemodulus and the normalised loss modulus were found to depend on both the latex fraction and on theporosity. This paper highlights and explains particular conditions that may arise during processing bywhich carbonate-based coatings soften.

© 2010 Elsevier B.V. All rights reserved.

iscoelasticarbonate

. Introduction

Particle coatings are porous, particulate-based composite mate-ials used mainly to improve the surface properties of papers. Theain constituents of paper coatings typically include mineral par-

icles, polymeric binding material and air (pore space). Duringrinting and calendering, these composites are subjected to high-ate deformations at various temperatures under the impact of aariety of solvents like water (fountain solutions), vegetable andineral oils during converting processes. Mechanical properties

lay an important role in the final quality of the coated paper andn the ease of process runability. The mechanical performance ofhese composites is strongly dependent on their intrinsic propertiesnd the volume fractions of each phase [1–5]. The microstructuresf interest in this work consist of a network of carbonate (CaCO3)articles bound together by styrene–butadiene (SBR) latex, whichesult in an anfractuous pore space network (Fig. 1).

Optimally, latex must ensure good stress transfer during load-

ng. This depends strongly on the strength of adhesion between theolymer chains and the CaCO3 particle surfaces. The establishmentf strong intermolecular bonds across the interface is thereforessential [6,7]. In CaCO3–SBR latex composites, acid–base interac-

∗ Corresponding author. Tel.: +358 2 2154232; fax: +358 2 215 3226.E-mail addresses: ftouaiti@gmail.com, ftouaiti@abo.fi (F. Touaiti).

921-5093/$ – see front matter © 2010 Elsevier B.V. All rights reserved.oi:10.1016/j.msea.2009.12.041

tions have been reported as the dominant intermolecular forces[4,8,9]. Water and other polar solvents negatively affect such inter-actions and weaken these materials [2,10], in a similar mannerto aluminium-epoxy composites, where this weakening effect isa function of the high surface free energy of water as compared toother solvents [11].

Porosity occupies approximately 30% of the total composite vol-ume and increases microstructural heterogeneity [12], influencingboth the mechanical and viscoelastic properties. The glassy storagemodulus by example, is inversely proportional to porosity [10]. Theglass transition temperature (Tg) has a strong dependency on thepore volume fraction and Tg may decrease [13] or indeed increase[14] as an artefact of porosity. Using Scanning Electron Microscopy(SEM) combined with Monte Carlo simulations, Rahmi et al. [15]found that the Tg of poly(ether imide) PEI decreases as porosityincreases. The results were interpreted in terms of polymer chainconfinement in the pore-to-pore region, which introduces a freesurface that enhances chain mobility and thus decreases the Tg

[16,17]. The confinement regions are illustrated in Fig. 2.In fully filled composites, the polymer chains at particle inter-

faces are immobilised due to the presence of strong interactions

between the particulate surfaces and the polymer. Restrictingchain mobility increases the glass transition temperature and thestorage modulus [18–22]. Nuclear Magnetic Resonance (NMR)and Dynamic Mechanical Thermal Analysis (DMTA) studies onCaCO3–SBR latex composites showed that the material relaxation

2364 F. Touaiti et al. / Materials Science and Engineering A 527 (2010) 2363–2369

Fig. 1. Schematic microstructure of CaCO3–SBR latex composites.

F[

t(prZcrb

cttwste

2

Ic

�c, density of the composite samples; �L, latex density (1.03 g/cm3);

TP

ig. 2. Illustration showing confinement regions between pores described in15,16].

ime broadens as a function of segmental mobility restrictionschain pinning) at CaCO3 particles interfaces [23]. However, theolymer chains in the bulk material remain free and can, inesponse to cyclic loading, align in the direction of the load [24].hou and Komvopoulos [25] used nano-DMA to study the interfa-ial viscoelasticity of thin PMMA films and concluded that chainearrangement at low frequencies is the main reason for viscousehaviour.

In this paper the viscoelastic behaviour of pinned polymerhains at the interfaces of CaCO3–SBR latex composites will be scru-inised. DMTA will be used in frequency sweep mode to understandhe interfacial response to different loading rates under dry andater saturated conditions. Temperature sweeps will be used to

tudy the thermal transitions of CaCO3–SBR latex composites andhe effect porosity has on the glass transition temperature will belucidated.

. Materials and methods

Ground calcium carbonate (GCC) CarbitalTM 90 produced bymerys Minerals Ltd. in the form of slurry was used to manufactureoating samples. The slurry had a solids concentration by weight

able 1hysical properties of calcium carbonate.

Particle Cumulative particle size distribution

3 �m 2 �m 1 �m 0.75 �m 0.5 �m

CarbitalTM 90 mass (%) 96.8 90.2 65.0 52.8 38.3

Fig. 3. Pressure filtration rig used to make a particle coating tablets.

of 78%, and a density of 2.1 g/cm3. The particle size distribution,measured by a sedimentation technique, is shown in Table 1.

SBR latex made by Dow Deutschland Anlagengesellschaft mbHwas used as binding material for the GCC particles. DifferentialScanning Calorimetry (DSC) showed that the latex possessed aglass transition temperature (Tg) of 8 ◦C. The latex particle size was132 nm and its solid content was 49% with a pH of 6.3.

The solid contents of both particle and latex suspensions (theweight ratio between the solid residue in the suspension heated to105 ◦C to that of the suspension at room temperature) were mea-sured using a Precisa Junior 310M. Five test series were made withthe following formulations: pure latex, 5 pph (parts per hundredby weight) latex, 10 pph latex, 15 pph latex and 50 pph latex, theratios (pph) are measured for dry latex to dry CaCO3 particles. Thecoating suspensions were mixed at 1000 rpm for 20 min using aHeidolph RZR2020 mixer. Following this, the coating colour wasleft for 10 min to free the slurry of air bubbles. 40 ml of the coat-ing slurry was then injected into a pressure filtration rig designedand constructed at Åbo Akademi University (Fig. 3). A pressure of0.7 MPa was then applied for 3 h after which, the rig was openedand the compacted tablet put in an oven at 70 ◦C for 12 h.

Once dry, the tablets were cut into small samples using ablade. The samples were a suitable size for DMTA, with dimen-sions 30 mm (length) × 5 mm (width) × 1 mm (thickness). Particle,latex and air volume fractions were calculated using the weightpercent fractions, and the densities of each composite phase. Usingmathematical formulae, the volume fractions of each phase werecalculated according to Eqs. (1) and (2).

˚L = �c

�L(1 + 100/˛)(1)

˚GCC = �c

�GCC(1 + 100/˛)(2)

where ˚L, volume fraction of latex; ˚GCC, volume fraction of CaCO3;

�GCC, particle density (2.71 g/cm3); ˛, latex level in pph (5, 10, 15,50).

A Rheometric Scientific DMTA IV was used in tensile mode toperform both frequency and temperature sweeps. The auto-tension

BET surface area, m2 g−1 (N2)

0.25 �m 0.2 �m 0.1 �m

21.3 17.8 9.70 12

nd Engineering A 527 (2010) 2363–2369 2365

rrasltsacrsss(pffw

3

3

fuFfl3

2esase

oepehWaecs

b(thh

3

i

tatat

Fig. 4. The effect of water on (a) the storage modulus, (b) the loss modulus and (c)the loss factor (tan ı) of pure SBR latex as a function of frequency.

Table 2Volume fractions for each test series.

5 pph latex 10 pph latex 15 pph latex 50 pph latex

Composite density(g/cm3)

1.77 1.71 1.54 1.61

Latex volumefraction (%)

8 15 21 52

F. Touaiti et al. / Materials Science a

ate was 0.01 mm/s with a pre-load of 10−3 N. The frequency sweepanged from 1 to 200 Hz and was ramped at 1 Hz/min. The temper-ture scan (−20 to +50 ◦C) was ramped at a rate of 3 ◦C/min. A straincan at room temperature (21 ◦C) was first used to determine theinear viscoelastic properties (elastic region). The tensile strain haso be chosen within an appropriate elastic range. The most brittleample (8% latex volume fraction) was subjected to a strain sweepnd was linearly elastic to a strain of 0.012%. The testing strain wasonsequently taken to be 0.005%, as it is safely within the elasticange for the most brittle sample. The less brittle materials willtrain more before they reach a limit of elastic proportionality. Thistrain was kept constant for both the frequency and temperatureweeps. The storage modulus (E′), loss modulus (E′′) and loss factortan ı) were measured initially in a frequency sweep. Similar sam-les were then subjected to a temperature sweep at 80 Hz. Samplesrom each of the test series were immersed in (de-ionised) wateror 24 h and for 36 h. These samples were tested to observe howater would affect the viscoelastic properties of the coatings.

. Results and discussion

.1. Viscoelastic properties of pure SBR latex

Since the effect of water on composite viscoelasticity and inter-acial pinning is under scrutiny, it is deemed appropriate to firstnderstand the effect water has on the properties of pure latex.ig. 4 shows E′, E′′ and tan ı as a function of frequency, f, for three dif-erent latex samples, this includes dry latex (ambient conditions),atex immersed in water for 24 h and latex immersed in water for6 h.

There is little obvious difference in water saturating latex for4 h or 36 h. A similar observation has been reported by Husbandt al. when studying the effect of water saturation time on the ten-ile strength of pure latex [10]. Wet samples do nevertheless show10–30% lower storage modulus values as compared with dry latexamples (Fig. 4(a)). This difference is a result of the plasticisingffect of water [26] on carboxylated latexes.

Increasing the frequency augments internal friction in the bodyf latex, which generates heat and encourages viscous flow. How-ver, heat in the dry sample also allows for easier reorientation ofolymer chains in contrast to the water saturated samples. Thisncourages intermolecular secondary bonding, which supersedeseat induced viscous flow, and consequently the latex stiffens up.ater saturated samples heat up at a lower rate because water

bsorbs heat and thus the polymer chains do not receive sufficientnergy to allow extensive polymer chain reorientation. This effectan be most easily observed at the higher loading frequencies. Theame trend is evident for the loss modulus chart (Fig. 4(b)).

The water saturated samples show marginally higher dampingehaviour than the dry sample in the whole range of frequenciesFig. 4(c)). Internal friction is lower at higher frequencies due tohe reorientation of polymer chains in dry samples however, waterinders the mechanism of reorientation by reducing the effect ofeat arising through internal friction [25].

.2. CaCO3–SBR latex composites

The porosity, particle and latex volume fractions are providedn Table 2.

Similarly to the pure SBR latex, the effect of water saturation

ime on the viscoelastic properties was studied. Fig. 5 shows asn example, the E′ for 8% latex fraction composites plotted againsthe loading frequency. With respect to material constituents, thesere the most dissimilar composites to pure latex as they containhe highest fraction of GCC and the lowest fraction of latex. The

Particle volumefraction (%)

62 57 50 40

Air volume fraction(%)

30 28 29 8

2366 F. Touaiti et al. / Materials Science and Engineering A 527 (2010) 2363–2369

Fc

wsbts

imiclm

Table 3Distinct frequency ranges where the rate of change differs for dry and water satu-rated composites.

Low frequency Medium frequency High frequency

Table 3 summarises the distinct frequency ranges for wet and dry

ig. 5. The effect of water saturation on the storage modulus of an 8% latex fractionomposite.

et samples have a lower storage modulus as compared to the dryample however, there is no big difference in the storage modulusetween the 24 h and the 36 h water saturated samples. 36 h satura-ion times were therefore chosen as a constant for water saturatingamples.

Attention now turns to understanding the behaviour of latex atnterfaces, which are directly involved in load transfer within the

aterial. At higher composite latex fractions, latex will assemble

nto latex rich regions. This will happen because it is energeti-ally more favourable [27]. These regions behave essentially as pureatex. The viscoelastic properties of the composite have been nor-

alised to that of the pure latex with an aim of highlighting only

Fig. 6. The normalised storage and loss modu

range (Hz) range (Hz) range (Hz)

Dry 1–10 10–50 50–200Wet 1–10 10–100 100–200

viscoelasticity of the latex at the interfaces. The dry and wet stor-age and loss moduli are normalised to those of dry and wet purelatex respectively. Dividing the composite storage and loss mod-uli with the pure latex storage and loss moduli respectively, leavesonly the contribution to loading resistance at the particle–polymerinterfaces. Consequently, the loss factors for dry and wet samplesare also normalised in the same manner.

3.2.1. Distinctive behaviours at different frequenciesThe normalised storage modulus and loss modulus curves for

dry and water saturated composites are shown in Fig. 6 as plot-ted against the loading frequency. In these graphs, three differentand distinct ‘near-linear’ rates of change can be identified. Thefrequency ranges are different for wet and dry samples, thoughthe trends are the same. In order to comprehend the physico-mechanical meaning of these graphs for the normalised storage andloss moduli as a function of frequency, the wet and dry frequencyranges indicating differential rates of change need to be clarified.

composites.The reason why the medium frequency range is extended some-

what for the wet samples is because water conducts heat away fromthe solid material more effectively than air. As a result, heat induced

li for dry (left) and wet (right) samples.

F. Touaiti et al. / Materials Science and Engineering A 527 (2010) 2363–2369 2367

Fl

ebflttluwtiiaFa

fuld

ii‘icu

ril

Fig. 8. Different gradients between E′n and f plotted for each sample. Dry and

ig. 7. General idealisation of E′n and E′′

n behaviours as a function of frequency inow, medium and high frequency ranges.

ffects occur at higher frequencies in water saturated samples. Inoth dry and wet cases nevertheless, the trend is such that at a lowrequency range, the rate of change in the normalised storage andoss moduli is highest, in the medium range this decreases and inhe higher frequency range, the rate of change increases again forhe storage modulus whilst reversing direction in the case of theoss modulus. It could also be noted that the storage modulus val-es are by and large 2-fold higher for the dry samples as comparedith the wet samples and the loss modulus is approximately, one

hird higher in the dry samples. This could be interpreted accord-ng to [11] as a function of the high water preferentially diffusingnto the particle–latex interfaces rather than into the latex itself. Asresult the latex does not swell but rather the interfaces weaken.

ig. 7 is a generic idealisation of the storage modulus curves plottedgainst a changing frequency.

The high-rate of change in the low frequency range observedor E′

n is believed to be due to microstructural changes that occurnder an applied stress. This microstructural change is a result of

atex chain reorientation at the particle interfaces aligning to theirection of loading.

At lower frequencies, polymer chains pinned at the particlenterfaces have more time to reorient to the direction of load-ng. Consequently, extra resistance to load results from both;nearer-to-unidirectional’ polymer chain loading and secondarynteractions that may develop when polymer chains in sufficientlylose proximity align. As the deformation rate increases, heat buildsp as intermolecular friction increases.

In the medium frequency range, the time available for moleculareorientation becomes shorter. Contrarily, intermolecular frictionncreases and heat develops further. The storage modulus dropsess rapidly in the medium frequency range as orientation and heat

water saturated samples are considered within the low, medium and high frequencyranges.

adjust in opposition and essentially balance between one anotherwith respect to stored and released energy respectively.

Within the high frequency range, heat dominates viscoelasticbehaviour. The polymer molecules now soften in tandem with theheat and a larger rate of change for E′

n and E′′n is observed. E′′

n shows adecrease in the lower frequency range. This decrease in viscous flowis also induced by intermolecular reorientation. As reorientationdecreases with frequency, so too does E

′′n. At medium frequen-

cies, where further reorientation is hindered E′′

n begins to plateau.Water having higher heat conduction than air, reduces the effectsof heat on the normalised storage modulus. This can be seen inFig. 8, where the gradient of E′

n is plot for each sample at the threedifferent frequency ranges.

Lower latex samples have higher storage moduli. This is sug-gested to be due to the reinforcing effect at the particle interfaces,where the higher the ratio of molecular pinning there is to bulk latexmolecules, the more effective the resistance to load. This can be eas-ily understood by the composite rules of mixture where the ratioof the particle fraction reflects the magnitude of elastic modulusbecause the particles have high modulus compared to latex.

Another way to understand this is in that the interfacial areaincreases with the particle volume fraction. These interfacial areaspin the latex molecules and improve the load carrying resistance.This can be further evidenced by the normalised loss factor (tan ın)graphs (Fig. 9), which describes the damping of the samples as afunction of frequency. Wet samples show more of the viscous flowcharacteristics than do the equivalent dry samples. Water softensthe composites and they behave as leathery materials. The waternot only plasticises the latex, but also weakens interfacial bondingand decreases the extent of polymer chain pinning at the parti-cle interfaces. The lower latex fraction samples exhibit a distinctstored energy response because they have a high percentage ofpinned interfacial chains relative to free bulk chains. The majorityof polymer chains in these samples are stiffer compared to thosein higher latex fraction composites. Hence there is a reduced plas-ticising effect in lower latex fraction composites. As heat increaseswith frequency, the molecules become softer and a sharp increasein tan ın is observed for dry samples.

Water saturated samples though, are more resistant to heat andthe increase is less profound. The peaks in the loss factor (>150 Hz)are due to a ‘resonance frequency’ effect of the material–instrument

system. The phenomenon is common to DMTA and has beenexplained by Menard [28]. The results above these frequencies can-not be sensibly interpreted.

2368 F. Touaiti et al. / Materials Science and Engineering A 527 (2010) 2363–2369

Fig. 9. Normalised loss factor versus frequency for different samples in dry and water saturated states.

arying

tmelic

3

otwbln

mlIpccra

apbtntr

a function of latex fraction (Fig. 12).As the latex level increases the Tg decreases up to around 20%

latex fraction after which it rises. It is postulated that at the lowestlatex volume fraction, the majority of polymer chains are pinned at

Fig. 10. (a) Storage modulus and (b) loss factor (tan ı) plotted against v

Porosity increases composite heterogeneity as we can see fromhe broadening of loss factor curves, additionally it allows greater

anoeuvrability within the composite. Thereby, molecular reori-ntation is made easier. It is also for this reason that loweratex content composites (comprising greater volumes of poros-ty) exhibit superior E′

n values. As the porosity decreases, polymerhain reorientation becomes more difficult and E′

n lessens.

.2.2. Distinctive behaviours at different temperaturesAnother way of analysing the viscoelastic properties is by means

f temperature, T, variation, which is a means to understandinghermal transitions that occur in the materials. A frequency of 80 Hzas chosen for the temperature sweeps as it lies sufficiently far

elow the resonance frequency [28]. The storage modulus and theoss factor curves are shown in Fig. 10. E′ is as expected, correlatedegatively with porosity in the glassy state [12].

High latex fraction composites have a higher glassy storageodulus, this is due to the stiffness of the glassy latex coupled to the

ow pore fraction (i.e. more glassy latex is available to resist load).n the leathery region however (5–32 ◦C) E′ of the latex rich com-osites drops more dramatically as compared to low latex fractionomposites. The leathery region of a polymer indicates the onset ofhain mobility, which in turn induces toughening whilst the mate-ial still has an elastic response. This is because latex starts to softenllowing for greater chain mobility.

E′ of the lower latex fraction composites decreases more gradu-lly in this region because the proportion of the interfacial (pinned)olymer to bulk polymer is higher. The ratio therefore of immo-

ilised polymer chains to mobile polymer chains is greater inhese lower latex fraction composites. The gradient in E′ dropson-linearly with respect to and increase in porosity (Fig. 11). Inhe rubbery region (high polymer chain mobility that drasticallyeduces the material elastic response), low latex fraction compos-

temperatures for different CaCO3–SBR dry composites and pure latex.

ites have the highest storage moduli due to the reinforcing effectsof the particles.

The loss factor (tan ı) curves (Fig. 10(b)), detail the dampingbehaviour of the materials. The 52% latex fraction composite showsthe highest tan ı values as compared with the other composites.This behaviour is a reflection of the lower levels of polymer chainpinning at the particle interfaces. Lower latex fraction composites,having more interfaces, require a higher activation energy to softenthe pinned chains. The tan ı peaks indicate that the Tg will shift as

Fig. 11. dE′/dT calculated from Fig. 8(a) plotted as a function of porosity.

F. Touaiti et al. / Materials Science and En

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[25] J. Zhou, K. Komvopoulos, Applied Physics Letters 90 (2007) 021910.

ig. 12. Tg plotted as a function of latex volume fraction. The values are inferredrom Fig. 10(b).

nterfacial sites. Therefore, the activation energy for polymer chainotion is highest and consequently Tg is high. As the latex frac-

ion increases to ca. 20%, there exists a higher ratio of free polymerhains to interfacial (immobilised) polymer chains. Consequently,he Tg decreases because the energy required to activate chain

otion is lower. Porosity moreover now acts as a so called ‘free vol-me’ for the free chains which adds to the reduction in Tg. Abovea. 20% latex fraction, the volume of free latex is high enough toecessitate more energy for the activation of free chain movementnd therefore Tg increases once again. Furthermore, the porosity isower and the ‘free volume’ effect of porosity on the free polymerhains reduces as a function of the lowered porosity.

. Conclusions

CaCO3–SBR latex microstructures are composed of latexomains binding particulate particles, which result in porosity. Ahorough DMTA investigation has been conducted, under both drynd water saturated condition, to gain further understanding oniscoelasticity and the effects of polymer molecule pinning at inter-aces. Water is found to have a slight plasticising effect on pureatex. Moreover, water saturation decreases the storage modulus

hilst increasing the damping of both pure latex and CaCO3–SBR

atex composites. At low deformation rates (low frequencies) vis-ous flow occurs in CaCO3–SBR latex composites and this is mostrobably due to the reorientation of polymer chains pinned athe particle interfaces. Increasing the loading frequency decreasesolymer chain reorientation but heat arises from intermolecular

[[[

gineering A 527 (2010) 2363–2369 2369

friction, which softens the samples. Water reduces the effect of heatas it is thermally more conductive than air. Consequently, the pres-ence of water decreases the effect heat has on polymer softening. Intemperature sweeps, composites with the highest interfacial pin-ning also had the highest rubbery storage modulus. This is believedto be because such materials require the highest activation energyto onset molecular motion. Tg, E′

n and E′′n are intimately balanced

between the effects of porosity and the effects of the latex volumefraction.

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