Synthesis of polymer/Laponite nanocomposite latex ... · We report the synthesis and...

Progress in Solid State Chemistry 34 (2006) 121e137www.elsevier.com/locate/pssc

Synthesis of polymer/Laponite nanocomposite latexparticles via emulsion polymerization using silylated

and cation-exchanged Laponite clay platelets

Norma Negrete-Herrera a, Jean-Luc Putaux b, Elodie Bourgeat-Lami a,*

a Laboratoire de Chimie et Procedes de Polymerisation e CNRS/CPE, UMR 140, Bat. F 308,43 Bd du 11 Novembre 1918, BP 2177, 69616 Villeurbanne Cedex, France

b Centre de Recherches sur les Macromolecules Vegetales e ICMG/CNRS, BP 53,

F-38041 Grenoble Cedex 9, France

Abstract

We report the synthesis and characterization of polymer/Laponite nanocomposite latex particlesthrough emulsion polymerization using organically modified Laponite clay platelets as seeds. Two ap-proaches were adopted for the organic modification of Laponite. The first one is based on the graftingof either g-methacryloyloxy propyl dimethyl-methoxysilane (g-MPDES) or g-methacryloyloxy propyltriethoxysilane (g-MPTES) on the clay edges. The other strategy consists in exchanging the clay interlayersodium ions by either a free radical initiator, 2,2-azobis(2-methyl propionamidine)hydrochloride (AIBA)or a cationic vinyl monomer, 2-(methacryloyloxy)ethyl trimethyl ammonium chloride (MADQUAT). Thegrafting was characterized both qualitatively using FTIR and quantitatively using elemental analysis orUV analysis. The results show that the degree of functionalization depends on the nature of the organicmodifier. Before performing the emulsion polymerization reaction, the functionalized clay plateletswere successfully dispersed in water. Nanocomposite latexes were then synthesized using a mixture ofstyrene (Styr) and butyl acrylate (BA) and sodium dodecyl sulphate (SDS) as anionic surfactant. An im-portant result of the present work is that clay redispersion in water is a key step of the overall process. Thelarger the size of the clay aggregates, the poorer the stability of the resulting latex suspension. The mor-phology and mechanism of formation of the nanocomposite particles are discussed.� 2005 Elsevier Ltd. All rights reserved.

Keywords: Laponite; Cation exchange; Silylation; Emulsion polymerization; Nanocomposite latexes; Cryo-TEM

* Corresponding author. Tel.: þ33 4 72 43 17 77; fax: 33 4 72 43 17 68.

E-mail address: [email protected] (E. Bourgeat-Lami).

0079-6786/$ - see front matter � 2005 Elsevier Ltd. All rights reserved.

doi:10.1016/j.progsolidstchem.2005.11.040

mailto:[email protected]

http://www.elsevier.com/locate/pssc

122 N. Negrete-Herrera et al. / Progress in Solid State Chemistry 34 (2006) 121e137

1. Introduction

Polymer-layered silicate nanocomposites (PLSN) have recently attracted intense industrialand academic interests due to their remarkable and enhanced properties compared to unfilledresins or to more conventional composite materials. In PLS nanocomposites, the filler is com-posed of sheets, with a thickness of one to a few nanometers and a length of 30 nm to severalmicrometers that belong to the generic family of phyllosilicates. Among these materials, claysdisplay many attractive structural features. Clay minerals contain active sites such as hydroxylgroups, Lewis and Bronsted acidity and are also characterized by the presence of exchangeablemetal cations such as Naþ, Liþ, Ca2þ, etc. located in the spacing between the negativelycharged layers. Consequently, organic cations can be electrostatically attached onto clay surfa-ces by a cation exchange process.

The elaboration of polymer/clay nanocomposite materials was first reported in 1961, whenBlumstein demonstrated the polymerization of vinyl monomers intercalated into Montmorillon-ite (MMT) [1]. But it is only recently that this class of materials has found an increased interestwhen researchers at the Toyota Central Research laboratories reported 10 years ago that the in-corporation of small amounts of MMT into nylon-6 resulted in a remarkable enhancement of thethermal and mechanical properties of the nanocomposite material [2,3]. Since then, a huge num-ber of works have been reported in this field [4e6]. Because the physical properties of PLS nano-composites mainly depend on the ability to produce intercalation or exfoliation of the clay layerswithin the polymer matrix, much efforts have been directed towards the incorporation of suitablemolecules within the clay galleries in order to promote clay dispersion at the nanometer level andreach enhanced properties. The nature of these molecules mainly depends on the polymerizationprocedure. Three methods are currently reported: exfoliation/adsorption, in situ polymerizationand melt intercalation. Typical molecules for melt intercalation processes are quaternary alkylammonium salts which are incorporated on the clay surface through cation exchange to imparthydrophobicity to the clay layers. In situ polymerization mostly involves the incorporation of or-ganic groups capable of reacting with polymers or initiating polymerization of monomers in or-der to create a chemical link between the inorganic phase and the polymeric matrix. Amongst thepioneering works in this field are those of Giesenking [7], Hendricks [8] and Grim and Allaway[9] who showed that the structural inorganic cations of bentonite could be successfully replacedby organic cations. Henry and Dekking [10,11] reported in a series of papers the modification ofbentonite and kaolinite by adsorption of 2,20-azobisisobutyramidine hydrochloride (AIBA) bymeans of an ion exchange reaction and the subsequent polymerization of methyl methacrylate,styrene, styreneebutadiene, vinyl acetate, chloroprene, acrylamide and acrylonitrile. More re-cently, Fu and Qutubuddin [12] intercalated vinylbenzyl dimethyl dodecylammonium chloride(VDAC) into MMT through cation exchange, dispersed the organoclay in styrene and performedpolymerization in bulk. Fan et al. [13] reported the synthesis of mono and bicationic free radicalinitiators and their intercalation into the MMT galleries to initiate the free radical polymerizationof styrene from the clay surface. In a related strategy, Weimer et al. [14] synthesized delaminatedPS/silicate nanocomposites by anchoring a living free radical polymerization initiator into thesilicate layers followed by bulk polymerization. At last, it is worth mentioning the recentwork of Velten et al. [15] on the intercalation of various tert-butylammonium peroxides, and theiruse as polymerization initiators to produce exfoliated nanocomposites.

In addition to exchangeable cations, the presence of OH groups at the broken edges of theclay platelets, also offers an easy route to accommodate entities with specific functionalities onthe clay surface via the use of organosilanes. It is now well recognized that the grafting of silane

123N. Negrete-Herrera et al. / Progress in Solid State Chemistry 34 (2006) 121e137

coupling agents onto mineral surfaces allows a substantial improvement of material properties.In polymer composites, silane compounds are beneficial because they improve not just a singleproperty, but multiple properties which influence the performances of the composite material.These include, but are not limited to, chemical adhesion, wetting and rheology. Although manystudies have been reported on the grafting of silane coupling agents to the surface of mineraloxide particles, comparatively, only a limited number of works have been published on the si-lylation reaction of natural or synthetic clays [16e27]. For instance, Tonle et al. [16] recentlyreported on the chemical modification of natural smectite-type clay samples with amine andthiol groups by the reaction of aminopropyl and mercapto propyl trimethoxy silanes and high-lighted the potential interest of the functional clays for electroanalytical applications. Wasser-mann et al. [17] described the silylation reaction of bentonite with octadecyl trimethoxysilaneand the effect of the chemical modification on the interlayer chemistry of iron. It is worth men-tioning also the recent work of Bourlinos et al. [18] on the chemical modification of MMT withaeu bridging organosiloxanes. Most of these works concern natural clays and only little workhas been performed on synthetic clays although synthetic layered silicates have gained increas-ing interest in numerous fields of application. Among these kinds of materials is Laponite,a synthetic hectorite clay with a thickness of 1 nm and a diameter of about 30 nm. Laponitehas the advantages over natural clays of being chemically pure and free from crystalline silicaimpurities. The shape of the Laponite crystal, combined with its anionic nature, enables Lapon-ite to produce films which can be used in the manufacture of low cost electrically conductive,antistatic, protective coatings and in optical waveguide applications [28].

In this paper, we report on the synthesis and characterization of poly(styrene-co-butyl acry-late)/Laponite nanocomposite latex particles (hereafter referred to as poly(StyreBA)/Laponite)through emulsion polymerization using organically modified Laponite.

The article is divided into two parts. In the first part, we focus on the grafting of organosilanemolecules carrying a polymerizable methacryloyl functionality and either one (g-MPTDES) orthree (g-MPTMS) hydrolyzable alkoxy groups. These compounds can thus potentially reactwith Laponite particles through condensation reaction between the surface hydroxyl groupslocated on the clay edges and the alkoxy groups of the silane molecules, giving rise to the for-mation of SieOeSi covalent bonds. In this section, we bring valuable information on themolecular orientation of the molecules in the interlayer space and on the mechanism of inter-action of the silane compounds with the Laponite surface. The functionalized clay plateletswere then suspended in water and used as seeds to produce nanocomposite latexes by free rad-ical emulsion polymerization.

The second part of this paper is devoted to the synthesis of polymer/Laponite nanocompositelatexes using Laponite clay particles functionalized by ion exchange using either a free radicalinitiator (AIBA) or a cationic vinyl monomer (MADQUAT). These organic cations were ad-sorbed at the Laponite/water interface in order to again promote polymer formation on theclay surface and create colloidal nanocomposite particles by in situ emulsion polymerization.

2. Experimental section

2.1. Materials

The clay used in this study is Laponite RD. Laponite RD is a fully synthetic microcrystallineswelling clay with a cation exchange capacity of 0.75 meq/g and a hydroxyl group concentra-tion of 0.36 meq/g as determined by the titration with triethyl aluminum [29]. Laponite RD was


supplied by Rockwood Additives Ltd. (UK) as a white powder. The trifunctional, g-methacry-loyloxy propyl trimethoxysilane (g-MPTMS, Fig. 1a) and the monofunctional, g-methacryloy-loxy propyl dimethyl-methoxysilane (g-MPDES, Fig. 1b) coupling agents are commerciallyavailable from Gelest Inc., and were used as received. The cationic initiator: 2,2-azobis(2-methylpropionamidine)hydrochloride (AIBA, Acros Organics, Fig. 1c) and the cationic monomer:2-(methacryloyloxy)ethyl trimethyl ammonium chloride (MADQUAT, Aldrich, Fig. 1d) wereused as supplied. The monomers, styrene and butyl acrylate were purchased from Aldrichand were distilled in vacuum before polymerization. The surfactant, sodium dodecyl sulphate(SDS, Acros Organics); the peptizing agent, tetrasodium pyrophosphate (Na4P2O7, Aldrich) andthe free radical initiator, azobis cyanopentanoic acid (ACPA, Wako Chemicals) were used with-out further purification. Toluene (from Aldrich) was of synthetic grade and used as received.Water was purified using a Milli-Q Academic system (Millipore Cooperation).

2.2. Methods

2.2.1. Organic modification of Laponite clay platelets

2.2.1.1. Grafting of silane coupling agents. In a typical run, Laponite was suspended in tolueneat a concentration of 10 g L�1 and the required amount of the g-MPDES or g-MPTMS cou-pling agent ([silane]¼ 5.22 mmol g�1) was introduced into the reaction flask and stirred for21 days at room temperature. The resulting grafted Laponite will be referred to as MPDES-grafted Laponite and MPTMS-grafted Laponite, respectively. The grafted amount (expressedin millimoles of grafted silane per gram of bare Laponite) was determined from the difference,DC (wt%), of carbon content after and before grafting as follows:

Grafted amount ðmmol=gÞ ¼ 103 �DC

ð1200 NC�DCðM� 1ÞÞ ð1Þ

where NC and M (g/mol) designate the number of carbon atoms and the molecular weight of thegrafted silane molecule, respectively. NC¼ 7 and M¼ 206 g/mol for MPTMS while NC¼ 9 andM¼ 202 g/mol for MPDES.

O

H2C O

H3CN

+

CH3

CH3

CH3

Cl-

dcH2N

NH2

N

N

N+

N +

Cl-

Cl-

H

H

HH

CH3

CH3

CH3

CH3

b

O Si (OCH2CH3)

O

CH3

CH3

a

O Si (OCH3)3

O

Fig. 1. Structure of reagents used as modifiers for the Laponite clay platelets.


2.2.1.2. Cation exchange reaction. In a typical experiment, the cation exchange was carried outas follows: 1 g of Na-Laponite was introduced into 100 mL of deionized water and stirred forapproximately 3 h until the suspension was free flowing and transparent. Then, a predeterminedamount of AIBA or MADQUAT was added to the clay suspension under magnetic stirring. Themixture was stirred at room temperature for 3 h. In the following, the resulting intercalationcomplexes will be referred to as AIBAeLaponite and MADQUATeLaponite, respectively.The amounts of adsorbed AIBA or MADQUAT per gram of Laponite, Qe (mmol g�1), weredetermined by titration of the supernatant solutions (after clay decantation by centrifugation)using the depletion method according to:

Qe ¼ðC0 �CeÞV

mð2Þ

where C0 (mmol L�1) is the initial AIBA or MADQUAT concentration, V (L) is the volume ofsolution, m (g) is the mass of Laponite and Ce (mmol L�1) designates the AIBA or MADQUATequilibrium concentration determined by UV analysis using a predetermined calibration curve.

In every case, the organically modified Laponite samples were extensively washed with theappropriate solvent (toluene or water) in order to remove the organic modifier in excess, anddried at 40 �C in a vacuum oven for 24 h before analysis.

2.2.2. Emulsion polymerization using organically modified Laponite particles

2.2.2.1. MPDES or MPTMS-grafted Laponite as seed. The polymerizations were carried out inbatch at 70 �C for up to 24 h under nitrogen atmosphere. The reactor was charged with 100 g ofthe aqueous MPDES or MPTMS-grafted Laponite suspensions (10 g L�1) containing the sur-factant (SDS, 2 g L�1) and the peptizing agent (Na4P2O7, 1 g L�1). After degassing, the mono-mers: a mixture of styrene (3 g) and butyl acrylate (7 g), and the initiator (ACPA, 0.1 g) weresuccessively introduced at 70 �C under stirring.

2.2.2.2. AIBA or MADQUATeLaponite as seed. Three grams of the peptizing agent was intro-duced into 100 g of the aqueous suspension of ion-exchanged Laponite. The mixture was son-icated for 3 min (Branson Unifier, 95% output power) and magnetically stirred for 2 days.Then, the surfactant (SDS, 2 g) and the monomers (butyl acrylate (7 g) and styrene (3 g))were introduced into the clay suspension and the mixture was sonicated for 2 min (BransonUnifier, 95% output power). The polymerization was initiated by increasing the temperaturein the case of the AIBAeLaponite intercalation complex and by introducing 0.05 g of ACPAdissolved into 3 mL of water in the case of MADQUATeLaponite.

2.3. Characterization techniques

FTIR spectra were recorded using a Nicolet Impact 460 Fourier Transform Infrared Spec-trometer coupled to a personal computer for data output and analysis. Specimens for the meas-urements were prepared by mixing 2 mg of the sample powder with 100 mg of KBr andpressing the mixture into pellets. Spectra were acquired in a wave number range between600 and 4000 cm�1 at a resolution of 2 cm�1 and averaged over 100 scans at room temperature.13C solid state nuclear magnetic resonance (NMR) was performed on a Bruker DSX-300 spec-trometer operating at 59.63 MHz by use of cross-polarization from proton. The contact time


was 5 ms, the recycle delay 1 s and the spinning rate 10 kHz. The absorption UV spectra wererecorded on a UVevis V-530 spectrophotometer. Spectra were collected at room temperature atthe wavelength of 360 nm and 210 nm for AIBA and MADQUAT, respectively. Particle sizewas determined by dynamic light scattering using a Malvern autosizer apparatus with a detec-tion angle of 90 �. The measurements were performed on highly diluted samples in order to ruleout interaction and multiple scattering effects. The intensity average diameter was computedfrom the intensity autocorrelation data using the cumulant analysis method. Monomer conver-sions were determined gravimetrically. Typically, 3e7 g of the latex suspension was placed inan aluminum dish and dried to constant weight at 70 �C. The morphology of the nanocompositeparticles was characterized by cryo-transmission electron microscopy (cryo-TEM). The sam-ples were prepared according to the method described elsewhere [27] using an EMCPC Leicafast-freezing workstation and a Gatan 626 cryoholder. Images were recorded on Kodak SO163films using a Philips CM200 ‘Cryo’ microscope operating at 80 kV, under low doseillumination.

3. Results and discussion

3.1. Poly(StyreBA)/Laponite nanocomposite latexes using silylated Laponite

3.1.1. Silylation of LaponiteLaponite RD is a fully synthetic clay similar in structure and composition to natural hectorite

of the smectite group [30,31]. The dimensions of the elementary platelets are the followings:diameter 30 nm and thickness 0.1 nm. In the dry state or in organic solvents, the plateletsare pilled up, forming 2e3 layers thick tacto€ıds held together by long-range attractive forces.Reactive silanols, corresponding to structural defects, are located at the broken edges of thesestacks while MgOH groups are contained into the internal space of the individual clay sheets[32]. If we consider this structural property, it appears that silane coupling agents can be graftedon the Laponite’s surface through condensation with the edge silanol groups. Evidence of graft-ing of g-MPTMS and g-MPDES on the Laponite clay platelets was provided by FTIR and solidstate NMR spectroscopies. We assume that washing the clay powder after grafting enabled toremove all the molecules which were physisorbed on the clay surface. The FTIR spectra of bareLaponite, g-MPDES-grafted Laponite and g-MPTMS-grafted Laponite are shown in Fig. 2aec.

Spectra 2b and 2c show characteristic vibrations of the carbonyl (nC]O, 1736 cm�1) and ofthe aliphatic CH2 and CH3 groups (nCH, 2850, 2920 and 2980 cm�1 and dCH, 1380 cm�1) of themono and trifunctional silane molecules, which indicate that grafting onto the Laponite par-ticles was successful. It should be noticed that the absorption band at 1634 cm�1, correspondingto the stretching vibration of the C]C double bond, overlaps with the dOH deformation band at1635 cm�1 due to physisorbed water. Moreover, FTIR provides additional information on themolecular orientation in the interlayer space and on the mechanism of interaction of the silanemolecules with the silicate surface. In the case of MPTMS-grafted Laponite, a sharp band at-tributed to hydrogen-bonded carbonyl groups [33] was observed at 1700 cm�1 while a shoulder,which can be assigned to free carbonyl groups, appeared at 1720 cm�1. This result suggests thatthe major part of the carbonyl groups of MPTMS interacts with the hydroxyls present on theedge surface of the clay platelets through hydrogen bonding (band at 1700 cm�1). Nevertheless,as the MPTMS molecules can undergo self-condensation in the presence of water, a multilayercoverage can be formed on the clay edges which can account for the presence of free carbonylgroups (band at 1720 cm�1). Contrary to the trifunctional silane, the monofunctional silane


cannot condense in solution. Therefore, the FTIR spectrum of the MPDES-functionalizedLaponite exhibits a unique signal at 1700 cm�1 indicating that the silane molecules form inthis particular case a monolayer coverage lying flat on the border of the clay plates as schemati-cally represented in Fig. 3.

The previous interpretation is also supported by 13C solid state NMR analysis (Fig. 4). Inboth the cases, the spectra show peaks at 132 ppm (CH2]) and 141 ppm (]C(CH2))

5001000150020002500300035004000

Wave number (cm-1)

Tra

nsm

issi

on a

.u.

a

b

c

1500160017001800

Wave number (cm-1)

b

a

c1700 cm-1

1720 cm-1

Fig. 2. Infrared spectra of (a) Laponite RD, (b) MPDES-grafted Laponite, and (c) MPTMS-grafted Laponite. The inset

shows the carbonyl region enlarged for clarity.

Grafting of amonofunctional silane

Disordered laponite claystacks suspensedin toluene

Grafting of atrifunctional silane

(CH2)3 Si-OEt

OCH3

CH3

CH3

(CH2)3O

O

SiO

CH3

CH3

(CH2)3O

O

SiO

CH3

CH3

(CH2)3O

O

SiO

CH3

30 nm

0.9 nm

O

O

O(CH2)3

SiO

O

(CH2)3O

O

SiO

O

Si

OH

O O

(CH

2 )3

Si

O

OOH

OSi

(CH

2 )3 O

O

HO

Si

(CH2)3O

O

OH

SiO

O

O(CH2)3

O

O

«Free»carbonyl

«H-bonded»carbonyl

O(CH2)3Si(OMe)3

O

O

Fig. 3. Schematic representation of the grafting reaction of silane molecules on Laponite clay particles.


corresponding to vinylic carbon atoms. In addition, the 13C NMR spectrum of the MPTMS-grafted Laponite shows two signals at 172 and 175 ppm corresponding to free and H-bondedcarbonyls, respectively, while the NMR spectrum of MPDES-functionalized Laponite exhibitsonly one signal at 175 ppm in agreement with the previous FTIR analysis.

The grafted amount (expressed in millimoles of grafted silane per gram of bare Laponite)was determined from the difference, DC (wt%), of carbon content after and before grafting(Table 1). The results indicate a lower silane content for the MPDES-grafted Laponite thanfor the MPTMS-grafted Laponite in agreement with the previous description. Indeed, themonofunctional silane contains only one alkoxy group and can react solely with the claysurface. Therefore, the number of silane molecules attached to the Laponite clay edges cannot belarger in this case than the surface hydroxyl groups’ concentration. In contrast, the trifunctionalsilane can undergo both reaction with the clay plates and self-condensation in solution givingrise to the formation of oligomers. These oligomers can in turn attach to the silanol groupsof Laponite forming somehow a polysiloxane coating surrounding the clay tacto€ıds. The pos-sibility that siloxane oligomers react with hydroxyl groups of two neighboring clay plates ina stack cannot be also completely ruled out, which issue would result in chemical cross-linkingof the clay tacto€ıds as schematically represented in Fig. 3 (left).

Chemical shift (ppm)

Free carbonyl

IntermolecularH- bonded carbonyl

115120125130135140145150155160165170175180185

H2C=C - C - O-(CH2)3-Si-O

H2C=C C - O-(CH2)3-Si-O

O

CH3

CH3

O

O

O-Si-O

OH

(b)

(c)

O

O

O(a) (b)

(c’)

MPDES-graftedLaponite

MPTMS-graftedLaponite

(c)

(c’)

(a)

(b)

Fig. 4. 13C solid state NMR spectra of MPTMS (bottom) and MPDES (top)-grafted Laponite.

Table 1

Initial silane concentrations, carbon contents and grafted amounts of silane-functionalized Laponites

Silane concentration

(mmol g�1)

DC (%)a Grafted amount

(mmol g�1)b

MPTMS-grafted Laponite 5.22 11.2 1.86

MPDES-grafted Laponite 5.22 5.0 0.51

a Determined by elemental analysis.b Determined using Eq. (1).


3.1.2. Emulsion polymerization

3.1.2.1. g-MPDES-grafted Laponite as seed. The synthesis of the nanocomposite latexes in thepresence of MPDES-grafted Laponite was carried out using 1 g of the organically modifiedLaponite suspended in 100 mL of water in the presence of SDS and of the peptizing agentin order to facilitate clay redispersion. Under such conditions, a satisfactory clay suspensionin water with an average diameter of around 120 nm four times larger than the particle sizeof Laponite, was obtained, indicating that the functionalized clay platelets were slightly aggre-gated within the suspension presumably due to the presence of the organic modifier on theirsurface. Despite the occurrence of clay aggregation, emulsion polymerization allowed the for-mation of a stable polymer latex with an average particle diameter of around 130 nm and a finalmonomer conversion larger than 97% suggesting successful incorporation of the clay plateletswithin the polymer particles. Similar experiments were subsequently performed in the absenceof clay and in the presence of bare ungrafted Laponite. It must be underlined that noticeabledifferences in colloidal stability were observed when bare Laponite was introduced into the po-lymerization reaction. Although the latexes were stable just after polymerization, they showedonly a limited stability with time and coagulation was systematically observed upon storage.This is presumably related to the fact that as the clay platelets were not incorporated withinthe latex particles, they could form a gel in a similar way as they do in pure water solutions,a mechanism which is additionally promoted by the high ionic strength of the suspension me-dium. In contrast, successful incorporation of the clay sheets within the polymer particles al-lowed to achieve a better long term stability of the suspension medium as there were no‘‘free’’ clay plates present in the surrounding aqueous solution.

3.1.2.2. g-MPTMS-grafted Laponite as seed. In a first set of experiments, the emulsion poly-merization reaction was carried out as described above for the MPDES-grafted Laponite. How-ever, contrary to the MPDES-grafted clay, the polymer latexes prepared under these conditionswere unstable and coagulated after around 20 min of reaction. Other experimental conditionslike, for instance, an increase in the temperature, a decrease in Laponite concentration or theuse of different types of surfactants (block copolymers, anionic surfactants, non ionic surfac-tants or mixtures of anionic and non ionic soaps) were explored in order to improve the stabilityof the latex suspension. However, all these polymerizations yielded unstable latexes. This resultis presumably related to the fact that the aqueous dispersion of g-MPTMS-grafted Laponitecontains large agglomerates that can induce destabilization of the growing latex spheres.Clay aggregation is presumed to be, in this case, due to cross-linking of the clay platelets bythe trifunctional coupling agent. It is worth noticing that the aggregates were so large thatthey settled down on standing. Therefore, an attempt to determine their size by DLS was un-successful supporting the hypothesis of formation of chemical linkages between individual clayplatelets in a stack or even between two clay stacks. It is thus clear from these results that theoriginal quality of the organically modified clay suspension is a determinant parameter that hasa drastic influence on the stability of the final nanocomposite latexes.

3.1.2.3. Morphology of the nanocomposite particles. In order to characterize low Tg polymers,cryo-TEM analysis is a powerful technique that has found an increasing interest in recent yearsin the field of polymer nanocomposites [27,34,35]. Nanocomposite particles embedded in a thinfilm of vitreous ice and observed by cryo-TEM are shown in Fig. 5. The micrograph in Fig. 5acorresponds to a sample prepared by emulsion polymerization in the presence of raw Laponite.

Fig. 5. Cryo-TEM images of (a) ‘‘hybrid latexes’’ synthesized by seeded emulsion polymerization of styrene and butyl

acrylate in the presence of raw Laponite (Laponite aggregation is indicated by arrows), (b) nanocomposite particles

obtained by emulsion polymerization in the presence of 10 wt% of MPTMS-functionalized Laponite clay platelets,

and (c) nanocomposite particles obtained by emulsion polymerization in the presence of 10 wt% of MPDES-functionalized

Laponite clay platelets.


As mentioned in a previous work [27], Laponite crystals appear as dark ‘filaments’ and polymerparticles as gray spheres. As expected, no particular interaction could be detected in this casebetween the clay platelets and the latex particles. Moreover, clay aggregates can be clearly seenin between the latex spheres. The specimen shown in Fig. 5b corresponds to the polymerizationperformed in the presence of the MPTMS-grafted clay sample. The Laponite platelets clearlyform flocculating aggregates with again no detectable interaction with the polymer particles inagreement with the macroscopic observation mentioned previously. The clay aggregates are toolarge to allow polymerization taking place on their surface as they offer only a limited surfacearea for capture of the growing radicals and may additionally undergo sedimentation in courseof polymerization. In contrast, when the monofunctional silane molecule was used as couplingagent, the clay platelets were homogeneously distributed within the latex sample and formeda nanocomposite structure (Fig. 5c), the clay constituting somehow the ‘‘shell’’ of the compos-ite latexes. Moreover, it is worthwhile noticing that the particles are slightly polygonal as thenatural rigidity of Laponite crystals generates a faceting of the surface.

3.2. Poly(StyreBA)/Laponite nanocomposite latexes using cation-exchanged Laponite

3.2.1. Organic modification of Laponite clay platelets through cation exchangeSince Laponite is a cationic exchanger, organic cations can electrostatically adsorb onto its sur-

face. Basically, the procedure involves the replacement of the inorganic metal ions saturating thestructural negative charges of the clay. Source of negative charges in Laponite is both from iso-morphic substitution (permanent charges) and broken edges (pH dependent) (Fig. 6). Isomorphicsubstitution is due to substitution within the lattice structure of lithium ions by magnesium ions.The edges are pH dependent due to the presence of amphoteric groups: MgOH, LiOH and SiOH.These surface groups can be protonated or deprotonated depending on the pH value of the sur-rounding solution. While MgOH and LiOH are positively charged below pH¼ 9, SiOH groupsare negatively charged [36,37]. It should be noticed that the edge charge represents less than10% of the total cation exchange capacity of the mineral [38]. Walker [39] has demonstrated intheir study on the exchange reaction of clays in aqueous salt solutions that cation exchange startsat the edges of the particles and proceeds towards the centre in a highly regular fashion.

Qualitative evidence of the presence of either AIBA or MADQUAT in the interlayer space ofLaponite was provided by FTIR spectroscopy. The FTIR spectra shown in Fig. 7aec displaysignals characteristic of the cationic initiator and of the cationic monomer (nNeC]N at

O

Si

O

SiO

O

Si

O

OO

O

O

O

SiSi

Si

O

Si

O

Si

O

Tetrahedralsheet

Tetrahedralsheet

Octahedralsheet

O

O

Negative charges by isomorphicsubstitution in octahedral sheets

Negative charges arising from the silanol groupslocated on the broken edges of the clay sheets

--

-

O -

O -O -

O -

O -

O -O -

O - O -O -

O-

O-

-O- ---

--

-

-

----

-

Fig. 6. Source of negative charges in Laponite clay platelets.


1680 cm�1 and nC]O at 1722 cm�1, respectively) together with signals of Laponite (namely thenSieO and nOH stretching frequencies at 1005 cm�1 and 3450 cm�1). It is worth noting againthat the peak at 1634 cm�1, characteristic of the C]C double bond stretching vibration ofMADQUAT, overlaps with the dOH deformation band of physisorbed water of Laponite. Thepeak assignment is reported in the figures.

The amounts of adsorbed AIBA and MADQUAT molecules, for initial organic cation con-centrations corresponding to two times the cation exchange capacity (CEC) of the clay, are re-ported in Table 2. It must be first pointed out that as AIBA molecules carry two positivecharges, one molecule can potentially exchange two sodium ions. Therefore, in this particularcase, the data of Table 2 must be multiplied by a factor 2 in order to compare them with theCEC of Laponite. Thus, assuming that two sodium cations are replaced by one AIBA molecule,it can be concluded that the exchange reaction is complete as the concentration of adsorbedamidinium cations (e.g., 0.74 meq/g) precisely corresponds to the CEC of Laponite. Similar re-sults have been reported in the literature by Fan et al. [13] or by Weimer et al. [14] for the in-tercalation of monocationic or bicationic quaternary ammonium initiator molecules.

Tra

nsm

issi

on (

a.u.

)

5001000150020002500300035004000

Wave number (cm-1)

OH3450 cm-1

Si-O1005 cm-1

OH1634 cm-1 a

b

c

C=C1634 cm-1

C=O1722 cm-1

N-C=N1680 cm-1

Fig. 7. Infrared spectra of (a) Laponite RD, (b) AIBAeLaponite, and (c) MADQUATeLaponite.

Table 2

Initial AIBA and MADQUAT concentrations, equilibrium concentrations and adsorbed amounts determined by UV

analysis of the supernatant solutions

Samples C0 (mmol L�1) Ce (mmol L�1)a Adsorbed amount

(mmol g�1)b

AIBAeLaponite 7.4 3.7 0.37

MADQUATeLaponite 15.4 8.9 0.65

[Laponite]¼ 10 g L�1.a Determined by UV titration of the supernatant solution.b Determined using Eq. (2).


Concerning the MADQUATeLaponite intercalation complex, the adsorbed amount (e.g. typi-cally 0.65 meq/g) is slightly lower than the CEC. It is known from the literature that many pa-rameters may affect the exchange reaction [40,41]. Restrictions on quantitative exchange canarise when the size of the cation is larger than the equivalent charge on the surface. The acces-sibility to the clay interlayer can also be drastically affected by the nature of the intercalatedcation and by kinetic factors. Moreover, high organic cation concentrations in solution createosmotic forces that may reduce the interlayer separation in the crystallites, thus making diffu-sion more difficult.

3.2.2. Emulsion polymerization

3.2.2.1. AIBAeLaponite as seed. The AIBAeLaponite clay suspensions were used to initiatethe emulsion polymerization of styrene and butyl acrylate. As the initiator was originally pres-ent on the clay surface, no additional initiator was introduced into the suspension. From thispoint of view, this system is quite original and different from traditional emulsion polymeriza-tion recipes. The clay content of the nanocomposite latexes was fixed to 10% by weight (rel-ative to the monomers) which is quite a high value compared to the literature data [42]. Theemulsion polymerization was prepared as reported in Section 2 using fully exchangedAIBAeLaponite and without removing the excess of cation contained in the water suspension.Special care was taken to avoid premature decomposition of the AIBA initiator by adding someice to the sonication water bath. Sonication of the reaction medium composed of the or-ganoclay, the surfactant, the peptizing agent and the monomers resulted in the formation ofa milky suspension that remained stable for several hours. Attempt to determine the droplets’diameter by DLS was unsuccessful suggesting only limited stability of the resulting emulsion.This polymerization procedure can be thus considered as intermediate between a conventionalemulsion polymerization process and a miniemulsion polymerization and will be called here-after a ‘‘miniemulsion-like’’ process. Polymerizing the ‘‘miniemulsion’’ allowed converting themonomer droplets into a milky white dispersion of polymer particles with an average diameterof 153 nm and free of coagulum. It is worth noticing that the reaction was very fast as themonomers were fully converted after only 30 min. The polymerization was performed severaltimes and was shown to be highly reproducible.

3.2.2.2. MADQUATeLaponite as seed. The aim of these experiments was to extrapolate theprocedure described previously for the AIBAeLaponite samples to the MADQUATeLaponiteintercalation complex. Indeed, it seems interesting to investigate the effect of the nature of theintercalative cation on the morphology of the resulting nanocomposite particles. The procedureused for the preparation of the aqueous clay dispersion and the synthesis of the polymer latexeswas identical to that reported in the previous section for AIBA. However, in this case, 0.05 g ofACPA dissolved in 3 mL of water was added to the suspension medium to initiate polymeriza-tion. The polymerization performed under these conditions again produced a stable latex withan average particle size of 138 nm and a high conversion (typically 98%).

3.2.2.3. Morphology of the nanocomposite particles. Fig. 8a and b is cryo-TEM images of com-posite nanoparticles prepared using AIBAe and MADQUATeLaponite, respectively. In bothcases, more or less spherical particles are observed with a diameter ranging from 30 to130 nm. Only a very small fraction of free clay platelets was detected. The spherical shapeof the particles is more regular in the AIBAeLaponite suspension. As seen from the


distribution of dark ‘filaments’ corresponding to the Laponite platelets seen edge-on, Laponiteforms an incomplete shell around the polymer core (Fig. 8a). The covering seems to be higherfor the larger nanospheres while only a small number of platelets are seen on the surface of thesmaller ones. The particles appear to be more aggregated in the MADQUATeLaponite sample(Fig. 8b) than in the AIBAeLaponite one. The covering of the surface by Laponite also seemsto be rather heterogeneous which may explain the more irregular shape of the composite par-ticles. In both AIBAe and MADQUATeLaponite samples, polymer particles appear to‘‘share’’ common Laponite platelets, suggesting that they can simultaneously develop onboth faces of some clay crystallites.

3.3. Mechanism of nanocomposite latex particles formation

It is generally admitted that determining which nucleation mechanism predominates in con-ventional emulsion polymerization is not straightforward [43]. In the present system, one canexpect that the mechanism involved in the synthesis of the nanocomposite particles is far more

Fig. 8. Cryo-TEM images of nanocomposite particles obtained by emulsion polymerization in the presence of 10 wt%

AIBAeLaponite (a) and MADQUATeLaponite (b).


complicated. Therefore, only a few directions will be given here in order to account for nano-composite particle’s formation and morphology.

The reaction mixture initially consists of monomer droplets, monomer swollen surfactantmicelles and organically modified Laponite dispersed in the continuous aqueous phase. Theclay particles can either initiate the polymerization reaction (in case of AIBA-Laponite) orundergo copolymerization with styrene and butyl acrylate (in case of MADQUAT orMPDESeLaponite). From the mechanistic point of view, one must therefore distinguishbetween the polymerizations performed in the presence of the AIBAeLaponite particles andthose performed in the presence of MPDESeLaponite and MADQUATeLaponite since thetwo systems may involve different mechanisms.

Let us first consider the MADQUAT and MPDES-Laponite systems, where polymerizablegroups are present on the clay surface. By analogy with classical emulsion polymerization,nanocomposite particle formation can be described as follows. The diazoic initiator (ACPA)starts to decompose in the water phase giving rise to the formation of free radicals. These rad-icals will propagate with aqueous phase monomers until they undergo one of the followingfates: (i) aqueous phase termination or (ii) entry into a micelle or precipitation (dependingon the surfactant concentration), creating somehow a new particle. Aqueous-phase oligomersof all degree of polymerization can also undergo frequent collision with the surface of theclay particles, and have therefore a high probability to copolymerize with the double bondsof either MADQUATeLaponite or MPDESeLaponite, thus generating chemisorbed polymerchains in the early stages of polymerization. These discrete loci of adsorption are preferredto adsorb further oligomers or radicals and will become discrete loci of polymerization. Asthese newly formed polymer nuclei develop a large surface area (the number of seed clay pla-telets is much larger than the expected number of polymer particles), they ‘‘coagulate’’ with aneighboring clay platelet to decrease the overall surface area and form a ‘‘mature’’ polymer/clay nanocomposite particle. The clay platelets end up on the surface of the latex particles be-cause this presumably corresponds to the most thermodynamically favored morphology giventhe hydrophilic character of the clay.

In the case of the AIBAeLaponite intercalation complex, polymerization is expected to startboth in the water phase, as previously for the MADQUAT or MPDES system, and on the claysurface since free initiator molecules are present in the clay interlayer and also in the aqueoussurrounding solution. The initiator-derived free radicals propagate until the formation of precur-sor particles stabilized by surfactant adsorbed on their surface. The newly formed particles canagain undergo coagulation events with neighboring growing latexes in order to decrease theoverall surface area until the formation of stable mature composite particles. The possibilitythat some monomer droplets stabilized by clay platelets adsorbed on their surface would be nu-cleated like in miniemulsion polymerization cannot be also completely ruled out although it ishard to discriminate between the two mechanisms at this stage given the limited number of ex-perimental data which have been presented in the present article.

4. Conclusions

In this work, two routes were tested in order to ultimately produce poly(StyreBA)/Laponitenanocomposite latexes through emulsion polymerization.

In the first route, monofunctional and trifunctional silane molecules carrying a polymerizablemethacryloyl functionality were grafted on the edges of the clay plates. The grafting was char-acterized both qualitatively and quantitatively using FTIR and elemental analysis, respectively.


We showed that the trifunctional silane coupling agent formed a polysiloxane coating that pen-etrates the clay galleries whereas the monofunctional silane formed a monolayer coverage onthe clay edges. The functionalized clay particles were then suspended in water in the presenceof an anionic surfactant and a peptizing agent. Distinct behaviours were observed depending onthe functionality of the silane derivative. Since the platelets were irreversibly locked together bythe trifunctional silane, the clay stacks could not be redispersed into water. In contrast, the claylayers grafted with the monofunctional coupling agent could be suspended in water solutions.Nanocomposite latexes the surface of which was homogeneously covered with the clay plateswere successfully obtained by this method.

In the second route, the interlayer sodium ions were replaced by either a cationic monomeror a cationic initiator through cation exchange. The intercalation was successfully demonstratedby FTIR and UV analyses. A satisfactory clay dispersion in water was achieved using a combi-nation of tetrasodium pyrophosphate and SDS. Successful formation of nanocomposite par-ticles was confirmed by cryo-TEM, the clay platelets being again located at the surface ofthe polymer particles or sandwiched between two polymer spheres. A tentative explanationof the mechanism of formation of the nanocomposite particles involving the organoclay asthe seed was proposed for each routes.

Acknowledgments

The authors are grateful to the SFERE-CONACYT exchange program for its financial sup-port to this work. The gift of a sample of Laponite RD by Rockwood Additives is also greatlyacknowledged.

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