Hybrid Aluminum Colored Pigments Based on Gradient Copolymers Design

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Hybrid Aluminum Colored Pigments Based onGradient Copolymers Designa

Mathieu Joubert, Abdel Khoukh, Jean-Francois Tranchant, Fabrice Morvan,Laurent Billon*

A colored polymer/aluminium hybrid pigment was synthesized by nitroxide mediatedpolymerization initiated from an inorganic surface. This approach requires the preparationof a vinyl dye monomer able to copolymerize with n-butyl acrylate (n-BuA) and styrene (S)from the surface of aluminium flakes. The linearity of the ln([M]0/[M]t) and of the Mn as afunction of time and conversion constitute the criteria of control/‘‘living’’ polymerization, i.e.linearity of respectively ln([M]0/[M]t)¼ f(t) and theMn ¼ f(conversion) plots. Kinetic measurements revealan upward deviation from the linearity for n-BuApolymerization for very high conversion. The introduc-tion of Smonomer restores the control of the polymeri-zation. Both the length of the grafted chains and thedye/styrene molar ratio influence the color of thehybrid material.

Introduction

The main prerequisite in academic and industrial labora-

tories remains the development of novel materials with

improved properties and performance. In this scope, the

great potential of polymer-based inorganic/organic mate-

M. Joubert, A. Khoukh, L. BillonInstitut Pluridisciplinaire de Recherche sur l’Environnement et lesMateriaux, Equipe de Physique et Chimie des Polymeres, IPREM/EPCP, UMR 5254,Universite de Pau et Pays de l’Adour, Helioparc, 2avenue Angot, 64053 PAU Cedex 9, FranceE-mail: [email protected]. TranchantLouis Vuitton Moet Hennessy, Parfums et Cosmetiques, 185,avenue de Verdun, 45 804 Saint Jean de Braye, FranceF. MorvanTOYAL Europe, Route de Lescun, 64 490 ACCOUS, France

a : Supporting information for this article is available at the bottomof the article’s abstract page, which can be accessed from thejournal’s homepage at http://www.mcp-journal.de, or from theauthor.

Macromol. Chem. Phys. 2009, 210, 1544–1555

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

rials is highlighted by the possibility to design specific

properties. In this researchfield, a largenumberof synthetic

strategies based on in situ heterophase polymerization,[1–4]

controlled assembling of preformed latex on seedminerals

(heterocoagulation)[5–7] or grafting of polymer chains onto

the inorganicfillerhavebeendevotedto this technology.[8,9]

Although each of these synthetic routes has been applied to

synthesize sub-micron hybrid materials, only the grafting

way is not limited by the dimension and the geometry of

the inorganic particles and thus, could be applied to

micrometric platelets and planar surface.[9]

Polymer chains tethering onto an inorganic surface can

be performed following three main routes: chemisorption

of a reactive polymer end group to the surface (‘‘grafting

to’’),[10–13] copolymerizationwithgraftedco-monomeronto

the inorganic surface (‘‘grafting through’’) [14–16] and the

growth of polymer chains from chemically linked initiator

onto the inorganic surface (‘‘grafting from’’).[17–19] With

respect to the recent development in controlled/‘‘living’’

radical polymerization (CRP), the ‘‘grafting from’’ way

becomes the most powerful strategy to finely tune the

dimension, functionality, composition and graft density of

the polymer brushes on inorganic surfaces.[20–26]

DOI: 10.1002/macp.200900254

Hybrid Aluminum Colored Pigments Based on . . .

Atomtransfer radicalpolymerization (ATRP)was thefirst

CRP technique applied in order to synthesize well defined

hybrid materials.[20,21,27–29] Reversible addition-fragmen-

tationchain transfer (RAFT)[26,30–33] andnitroxidemediated

polymerization (NMP)[22–25,34,35] were also extrapolated to

the ‘‘grafting from’’ techniques. In many respects, all of the

living free radical polymerization (NMP, ATRP, RAFT) are

similar in their overall scope. Nevertheless, subtle differ-

ences exist which confer to each of them limited suitability

for specific applications. ATRP requires metal complex

which reduces the range of potential functionality and

makespurificationdifficult.AlthoughRAFT is compatible to

a wide variety of monomer families, a universal transfer

agent for all of themdoesnot exist. It implies a restriction of

the range of monomer that could be copolymerized.

Furthermore, another drawback of thesemethods is related

to the inherent complex multi-step synthesis of a bi-

functional initiator. Moreover, due to the presence of the

terminal sulfur group, colored polymers are formed.

Consequently, NMP appears to be the friendliest candidate

for the direct synthesis of polymers with controlled colors.

In a previous paper, we reported an original method to

synthesize well defined poly(butyl acrylate) grafted on

silica particles based on the so-called ‘‘grafting from’’

approach.[35] In this scope, the reactivity difference

between a tertiary and a secondary alkoxyamine provides

the synthesis inone-stepofabi-functional initiator. Indeed,

in a solvent, a tertiary alkoxyamine (MAMA) activates

above 25 8Cwhile acryl secondary alkoxyamine is stable up

to 110 8C. This procedure was based on trapping alkox-

yamine initiator (MAMA) for living free radical polymer-

ization, on the C¼C double bond of the trimethoxysilyl-

propyl acrylate (TPMA).

The objective of the present work is to extrapolate this

approach to metallic pigments in order to obtain colored

hybrid materials by copolymerization with a vinyl dye

monomer. The pigment used was aluminium platelets

coatedwitha thin silica layer. After the chemical graftingof

the initiator through silylation reactions, NMP was

performed to achieve poly(butyl acrylate), poly(butyl

acrylate)/polystyrene and polystyrene copolymers brushes

containing a dye unit on the inorganic particles. First,

kinetics measurements were investigated in order to

establish the influence of both the aluminium pigment

and the vinyl dye unit onto the polymerization. Then, an

optimization of the color intensity was performed follow-

ing twoparameters: the lengthof thegraftedchainsand the

proportion of vinyl dye unit inserted in the polymer chains.

Experimental Part

Materials

Absolute ethanol (99.9%, Aldrich), ammonium hydroxide (28% in

water, Aldrich), trimethoxysilylpropyl acrylate (TPMA, 98%,



Aldrich), 2-methyl-2-[N-tert-butyl-N-(dimethoxyphosphoryl-2,2-

dimethylpropyl)aminoxy] propionic acid (BlockBuilder, 95%, so-

called MAMA) and N-tert-butyl-N-1-diethylphosphono-2,2-

dimethylpropylnitroxyl (SG1, 92%) were supplied by ARKEMA

and used as received. N-butyl acrylate (99%, Aldrich) and styrene

(99%,Aldrich)weredistilledundervacuumprior touse.Aluminium

pigments supplied by TOYAL were washed with ethanol by

centrifugation-redispersion cycles and dried under vacuum at

room temperature prior to use.

Characterization Techniques

1H NMR spectra were recorded at 400MHz on a Bruker Advanced

AM400 spectrometer in CDCl3 and the chemical shifts (d) in ppm

were referenced to internal tetramethylsilane.

TGA experiments were carried out using a system TA 2950

apparatus to determine the amount of bound polymer chains onto

aluminum surface in a temperature range of 40 8C to 550 8C at a

scan rate of 10 8C �min�1 in air.

Themolecularweightsand thepolydispersity indexes of the free

polymerweredeterminedbySEC,usinga2690WatersSystemwith

tetrahydrofuran as the mobile phase. Molecular weights were

calculated relative to polystyrene (PS) standards.

X-ray photoelectron spectroscopy analyses were performed

with a surface science instrument (SSI) spectrometer at room

temperature,usingamonochromaticandfocused (spotdiameterof

600mm, 100W) AlKa radiation (1486.6 eV) under a residual

pressure of 5 �10�8 Pa. The hemispherical analyzer worked under

constant pas energy mode, 50 eV for high resolution spectra and

150 eV for quantitative analysis. The binding energy scale was

calibrated from the carbon contamination using the C(1s) line

(284.6 eV) (a mean atomic percentage of 8% was determined).

Synthesis

Chemical Modification of Purple 2 Dye

We used herein a chemical procedure already described by our

group.[36] The purple 2 dye (5 g, 15.2 mmol) was mixed with

triethylamine (1.97 g, 19.5mmol) in 90mL of dichloromethane and

cooled to 0 8C with an ice bath. The mixture was degassed with

nitrogen and the acryloyl chloride (2.7 g, 29.9 mmol) was added

dropwise. Upon complete addition, the mixture was stirred at 0 8Cduring onehour and thenbrought to roomtemperature and stirred

for 24h. The mixture was washed several times with water and

dried over anhydrous Na2SO4 before removal of the solvent under

vacuum. The residuewas elutedwith chloroformover a silica gel to

yield 3 g (60%) of pure vinyl dye monomer (see Supporting

Information for 1H and 13C NMR spectra).

Synthesis of Initiators by ‘‘in situ Thermo-DependantTrapping of Carbon Radicals’’

Multifunctional initiator by TPMA and MAMA coupling: Initiator

synthesis by ‘‘in situ radical trapping’’wasperformedat 80 8C,with

a reaction time varying from 2h to 4h with two MAMA/TPMA

molar ratios equal respectively to 1.0 and 1.2 in absolute ethanol at

www.mcp-journal.de 1545

M. Joubert, A. Khoukh, J.-F. Tranchant, F. Morvan, L. Billon

1546

80 8C. The weight ratio of ethanol/MAMA is 2 and TPMAwere first

dissolved in absolute ethanol. This solution was stirred at room

temperature under nitrogen for 30min and heated at 80 8C.Model initiator by n-BuA and MAMA coupling: The same

procedure as described above was used except that the MAMA/n-

BuA molar ratio was equal to 0.7. To avoid an excess of MAMA as

initiator when the mono-adduct will be used to check the kinetic

behavior ofmodel initiator (see Supporting Information for 1H and31P NMR analysis).

Initiator Grafting

The grafting of the initiator on aluminium platelets was realized

according to theproceduredescribedbyPhilipseandVrij.[37] To45 g

of absolute ethanol, 0.600g of H20 (1.7 mol � L�1) and 0.600g of

ammoniac (0.25 mol � L�1) were added. 3 g of pigments were

dispersed into the abovemixture prior to the addition of 0.800 g of

the initiator solution. The resulting dispersion was stirred at room

temperature for 24 h under nitrogen. To favor condensation, 20mL

of ethanol was slowly distilled off under vacuum at room

temperature. Aluminiumplateletswerepurified fromfree initiator

through a series of centrifugation-redispersion cycles.

Polymerization Process

Without aluminium pigments: Homopolymer of n-BuA as well as

copolymers of S/n-BuA (25/75), were prepared in glass tubes

containing the model alkoxyamine obtained from n-BuA/MAMA

coupling, anexcessofSG1 (5%with respect to thealkoxyamine)and

n-BuA or a mixture of S and n-BuA. In both cases, [monomer]/

[alkoxyamine]¼400. The reactor tubeswere degassed by nitrogen

bubbling for 30min. and immersed into an oil bath at 115 8C for

specific times. Conversion was determined by 1H NMR spectro-

scopy while molecular weights were obtained from SEC.

Scheme 1. Formation of a ‘‘new’’ alkoxyamine by ‘‘in situ thermo-dependant trapping ofa carbon radical’’.

With aluminum pigments: The aforemen-

tioned procedure was used except that the

reactors were round bottomflaks fittedwith a

septum.Amothersolution(M1)containingthe

alkoxyamine obtained from n-BuA/MAMA

coupling, an excess of SG1 (5% with respect

tothealkoxyamine)andn-BuAoramixtureofS

andn-BuA(25/75)wasdispatched into6 round

bottom flasks. The weight ratio of this above

mixture on aluminum pigments is 10. [mono-

mer]/[alkoxyamine] was adjusted by decreas-

ing the amount of alkoxyamine. The round

bottom flasks were degassed by nitrogen

bubbling for 30min and immersed in an oil

bath at 115 8C for specific times. After poly-

merization and centrifugation, the first super-

natant was used to determine the conversion

through 1H NMR spectroscopy and molecular

weights from SEC measurements. Polymer/

aluminium hybrids were purified from free

polymer through a series of centrifugation-

redispersioncycleswithTHFtill theweightloss

measured by TGA was constant. The grafted

particles were dried under vacuum at 40 8Cduring 48h prior their composition was

determined by TGA.



Results and Discussion

Synthesis of the Surface Initiator (TPMAM)

The general procedure to create polymer brushes through

the so-called ‘‘grafting from’’ approach requires a bi-

functional initiator with an anchoring group (chloro or

alkoxysilanes) and a polymerization initiator group. In

regard to this bi-functional character, multi-step synthesis

processes were involved whatever the polymerization

methods. For nitroxide mediated polymerization, a one

step synthesis has been developed in our laboratory based

on ‘‘in situ thermo-dependent trapping of carbon radicals’’.

The main alkoxyamine initiators are MONAMS

(N-tert-butyl-N-1-diethylphosphono2,2-dimethylpropyl-

0,1-methoxycarbonylethylhydroxylamine) and MAMA. In

relation to the degree of substitution of the carbon in a

position of the nitroxide function, their respective decom-

position temperatures are above 100 8C and 25 8C. Thus, thereaction between MAMA and acrylate monomers at low

temperature leads to the insertion of only one monomer

molecule and thus, provides the formation of a new

alkoxyamine by conversion of a tertiary to a secondary

alkoxyamine (Scheme 1).

In a first step, we optimize the reaction condition to

obtain a high yield. Indeed, it is important to minimize the

amount of TPMA in thefinal product since it canparticipate

to the polymerization and disturbs it. Two variables were

selected: the reaction time and the molar ratio MAMA/

TPMA. The yield of the reaction has been quantified from1H NMR analysis.

DOI: 10.1002/macp.200900254


Figure 1. 1H NMR spectrum of the alkoxyamine initiator obtainedfrom TPMA and MAMA coupling.

Figure 2. Yields in % of the ‘‘in situ trapping carbon radical’’between MAMA and TPMA with MAMA/TPMA molar ratio of 1

Figure 1 exhibits characteristics peaks of the organosi-

lane propyl chains (�CH2�Si, �CH2�CH2�Si

and CH2�CH2�CH2�Si at respectively 0.7, 1.9 and

3.8 ppm). Residual C¼C are observed at 5.8, 6.2 and

6.4 ppm. The conversion degree of the TPMA-MAMA

coupling reaction was estimated from the ratio of the

integral of these signals to the methylene groups of the

propyl chains linked to the silicon. The results are depicted

in Figure 2.

The maximum yield is 93% with a MAMA/TPMA¼ 1.2

and for a reaction time of 4 h.

(a) and 1.2 (b) at 80 8C in absolute ethanol.

Immobilization of the Surface Initiator

The initiator grafting was realized according to the

procedure described by Philipse and Vrij[37] in ethanolic

media containing a know amount of water and in basic

condition. In regard to the poor specific surface of inorganic

pigment, most of the usual techniques involved to

characterize qualitatively or quantitatively the grafting

such as solid stateNMRor thermogravimetric analysis TGA

arenot efficient. Only, XPS analysis provides someevidence

of the grafting (Figure 3).

With respect to thepigmentsused,nocleardifferencecan

be observed by comparing the XPS spectra of pure to the

treated aluminium pigment. We must focus on the C(1s)

signal to confirm the grafting of the superficial initiator. It

was evidenced by the apparition of a shoulder at 288.5 eV

on the C(1s) signal related to C¼O function of the initiator

(Figure 3). Another confirmation is provided by the

quantitative XPS analysis which exhibits an increase of

the atomic ratio C/Si after the grafting (Table 1). The

presence of phosphor atomsproper to the initiator supports

also the presence of TPMAM on the inorganic platelet.

Therefore, no changeswere recorded on the Si(2p) signal. As



aluminium atoms are detected, it implies an inhomoge-

neous coating of the pigments by silica or a thin layer with

less than 5nm of thickness.

Synthesis of Polymer Brushes

The low concentration of initiating sites on the inorganic

platelet surface related to its poor specific surface requires

the addition of free initiators to increase the initial initiator

concentration and the concentration of persistent radical

that generated upon activation of the initiator and thus, to

control the growth of tethered polymer chains from the

surface.[9,21] As a matter of fact, the amount of grafted

initiators was neglected in regard to the free initiators.

For nitroxidemediated polymerization ofnBuAornBuA/

S, kinetic studies in the presence and in the absence of

aluminium pigment would give all the criteria of ‘‘living-

ness’’. For that aim,we prefer to use a free initiatorwith the

most identical structure than the TPMAM in order to avoid

difference in the rate of decomposition or initiation. In this

scope, the initiatorwas prepared by trappingMAMAon the



Figure 3. XPS spectra of pure aluminium pigments (a) and aluminium pigments grafted TPMA (b).

Table 1. Atomic percent of pure aluminum platelet (ALT) andinitiator–grafted aluminium platelet (ALT þTPMA) and C/Siatomic ratio before and after the chemical modificationextracted from XPS analysis.

Sample Surface composition C/Si

atomic %

Si (2p) C(1s) P(2p) Al(2p)

ALT 28.67 11.63 / 0.86 0.407

ALTRTPMAM 26.53 18.52 0.24 0.56 0.698

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C¼C double bond of the n-butyl acrylate as a model

initiator. Moreover, according to previous studies,[38–40] we

chose a [SG1]/[alkoxyamine]¼ 0.05 molar ratio.

A polymerization can be ascertained of quasi-living

with respect of two conditions: the number of active

species and of polymer chains remains almost constant

throughout the reaction. In accordance to the well-known

relation (Equation (1)) in presence of an nitroxide excess

and where the time dependence of [P–X] and [X8] can be

neglected, the first criterion can be demonstrated by the

linearity of the ln([M]0/[M]t) plot as a function of time,



with [M]0 and [M]t equal to the initial and the current

monomer concentration.

Lnð½M�0=½M�tÞ ¼ kp½P��t (1)

The second condition is ratified by the linear dependence

of Mn with conversion.

Poly(butyl acrylate) Brushes

Whereas in the absence of aluminium particles the plot of

ln([M]0/[M]t)¼ f(t) is a straight line over the range of

selected time, in the presence of aluminium particles, a

positive drift from the linearity at high conversions is

observed. This upwarddeviation ismore pronouncedwhen

the dye is present and appears earlier when the DPthincreases. At the same time, the rate of polymerization

increases which is in contraction with the features of the

NMP initiated with alkoxyamine.[38] Our results indicate

side reactions occurring during the polymerization

(Figure 4).

This loss of control might be ascribed to an unusual

consumption of the transfer agent SG1. An upward

deviation has already been observed for n-BuA bulk

DOI: 10.1002/macp.200900254


Figure 4. a) Kinetics of bulk polymerization of n-butyl acrylate at115 8C in presence of SG1 at a initial ratio [SG1]/[initiator]¼0.05;* and * in presence of aluminium pigments with respectively aDPn;th ¼400 and 800; ^ with aluminium pigment and vinyl dyeunit (DPn;th ¼800) and V2/free alkoxyamine molar ratio¼ 1; &

polymerization for high conversion resulting from slow

degradation of SG1.[38] Transfer reaction to the methylene

protonof then-BuAmonomerunits couldalso contribute to

the disruption of the polymerization by reducing the

number of SG1 moieties available to trap the propagating

transient radicals.[41] However, such purposes cannot only

explain the deviation from the straight line since no

positive drift is observed when the polymerization occurs

without aluminium particles. Another concomitant phe-

nomena ascribed to aluminium particles must prevail to

explain the discrepancies from controlled/‘‘living’’ poly-

merization. Their presence emphasizes the viscosity of the

media and thus, affects the physical situation of the

polymerization system. Evidences have been highlighted

that the rate constants involved in radical polymerization

present a diffusion-controlled dependence.[42–44] Some

authors interpreted this tendency by radical trapping till

the chain length is above the critical entanglement

length[42,43] or by a decrease in the translational and

rotational degrees of freedom as the chain grows.[45] In

controlled/‘‘living’’ stable free radical polymerization,

changes in viscosity could disturb the equilibrium constant

between active and dormant species in favor in accordance

to the Figure 4.a to active specieswith respect to the abrupt

increase of themonomer conversion.Herein, this argument

is confirmedby thekinetic curveperformedwithaDP¼ 800

since the positive drift appears earlier. Moreover, the

diffusion-controlled dependence of the equilibrium con-

stant might also explain the increase of the rate of

polymerization with the polymerization degree. The

presence of vinyl purple 2 monomer tends to accentuate

this upward deviation. At the above suggestions,

the inherent chemical structure of the dye closed to an

anthraquinone which is a radical inhibitor might act as a

transfer agent and leads to a complete loss of the control/

living character of the polymerization.

As wementioned above, the limitation of the number of

transfer reaction is another prerequisite to ensure the

controlled character of the polymerization. Figure 4b. tends

to underline the quasi-living behavior of the system in the

absence or in the presence of the inorganic pigment.

Nevertheless, in agreement with the ln([M]0/[M]t)¼ f(t)

plots, the polydispersity indexes increases in the course of

the polymerization in the presence of the particles and of

dye in a more relevant manner.

without aluminium pigments(DPn;th ¼400); b) evolution of themean number molecular weight versus conversion (* and &

with and without respectively aluminium pigments, DPn;th ¼ 400in both cases) and of the polymolecularity index versus conver-sion (* and & with and without aluminium pigments); c) inpresence of aluminium pigments and DPn;th ¼800 in both cases,evolution of the mean number molecular weight versus conver-sion (^ and & with and without respectively vinyl purple 2monomer) and of the polymolecularity index versus conversion(^ and &).

Poly[(butyl acrylate)-grad-styrene] Brushes

In contrast to the polymerization of n-butyl acrylate,

aluminiumpigmentsordyedonotaffect the linearityof the

kinetic curveduring theoverall copolymerization (Figure5).

Moreover, the rate of polymerization remains constant

whatever the DPth. From individual conversion of each


� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mcp-journal.de 1549


1550

monomer, the variation of ln([M]0/[M]t)¼ f(t) for each of

themcanbe ascribed as afirst order reactionwith respect to

themonomer. As expected, the linear dependence of theMn

with conversion and the low polymolecularity indexes

confirm the controlled character of the polymerization.

For controlled free-radical copolymerizationoperatingvia

an activation-deactivation mechanism, the average activa-

tion-deactivationequilibriumconstant hKidependsnotonlyon the respective monomers K, but also on their respective

propagation constant (kp) and reactivity ratio.[46,47] In the

present case,kpbutyl acrylate (88000L �mol�1 � s�1, at120 8C)[48]

is higher than kpstyrene (2000 L �mol�1 � s�1, at 120 8C).[49] Therespective value of K and r for S and n-BuA are

Kstyrene¼ 1.9.10�8 mol � L�1 at 125 8C,[39] Kbutyl acrylate¼1.2 � 10�10 mol � L�1 at 125 8C[50] and rstyrene¼ 0.74, rbutyl

acrylate¼ 0.29.[51]Thehighkp,butyl acrylatewascounterbalanced

by an increase of the hKi related to the high value ofKstyrene,

the difference in the monomer reactivity ratio and the low

kp,styrene.Comparedton-butylacrylatehomopolymerization,

only 50% of n-butyl acrylate is consumed against 90% of

styrene during the first 8h. Madruga et al. [51] obtained

theoretical values of the fraction of propagating radicals

with an S terminal unit for the n-BuA/S copolymerization

through SG1/AIBN system. For two composition feeds (0.2

and 0.8), these values were close to 1 which attests that the

styrene monomer is more rapidly consumed than n-butyl

acrylate monomer. This behavior related to the intrinsic

features of S monomer tends to compensate the increasing

viscosity, the eventual unusual consumption of the SG1

moiety, the diffusion-controlled dependence and decreases

transfer reactions related to the dye. It implies also no

changes in the kinetic curves between n-BuA homopoly-

merization and n-BuA/S copolymerization.

With respect to theco-monomer feed in thepresentwork,

the tendency exhibited in Figure 6 underlines the slight

gradient-like composition of the macromolecular chains

between 0.45 and 0.3 of styrene. Such behavior has been

previously described by Karaky et al.,[52,53] when using

batch polymerization, and could be overcome by semi-

batch polymerization to increase the gradient profile.[53–55]

Figure 5. a) Global kinetics of bulk copolymerization of n-butylacrylate and styrene (fs¼0.25) at 115 8C in presence of SG1 at ainitial ratio [SG1]/[initiator]¼0.05 V2/free alkoxyamine molarratio¼ 1; * and * respectively with and without aluminiumpigments and DPn;th ¼400 and 800; ^ with aluminuim pigmentsand vinyl purple 2 monomer with DPn;th ¼800; b) evolution of themean number molecular weight (^, * respectively with andwithout vinyl purple 2 monomer) versus conversion and of theirrespective polymolecularity index (^, *) versus conversion withaluminum pigments; c) kinetics of each monomer respectivelywith and without aluminium pigments (styrene: *, & and n-butyl acrylate: &, *).

Hybrid Composition

The evolution of the hybrid composition was followed by

TGA measurements. For PBuA-aluminum hybrids, as

expected, the higher the targeted DP the higher the weight

loss is (Figure 7a). The organic part of the hybrid gets richer

and richer with the conversion in the both cases and

increases in a linear manner. Whereas the linear depen-

dence between the organic mass loading and the conver-

sion is in agreement with the kinetic curves displayed

above for BuA/St copolymerization, this observation for


� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/macp.200900254


Figure 7. Organic mass loading in the hybrids versus conversion a)for PBuA-aluminium hybrid (* and & with initial DPn;th ¼ 400and 800 respectively) and b) for PBuA-g-St-aluminium hybrid(DPn;th ¼ 800).

Figure 6. Plots of styrene content as a function of conversion & inabsence and * in presence of aluminium pigments.

BuA homopolymerization supports also a controlled

growth of the grafted chains although the kinetic curves

does not.



Although these results highlight limitations of our

strategy, some of them deal with the loss of the control

of the polymerization. However, the main drawback

remains the absence of color in the metallic pigment even

if the free polymers are colored.

Improvement of the Coloration

In view of these results, we have modified our method. The

conditions investigated neglected the control of the poly-

merization infavorofthecolor.Firstly, styrenewaschosenas

monomer instead ofn-butyl acrylate according to the better

solubility of thevinyldye instyrene than inn-butyl acrylate.

In this sense, dioxane was tested as solvent in order to

improve the amount of vinyl dye in the polymerization

media. The two variables involved are theDPn;th and the dye

concentration. In the latest case, no free initiator was added

and chains were grown only from the particles surface.

The color dependence on DPn;th is not so obvious. Indeed,

from runs 1 to 3 in Table 2, only theDPn;th changes. A slightly

color appears only for the pigment issued from run 2. This

observation seems to underline a relationship between both

the polymer chain length and the proportion of dye unit

incorporatedintothegratedpolymerchainsandtheresulting

color. If the dye units are in close vicinity of the pigment

surface,no color isobserved. This tendency ismorerelevant if

we compared the two runs 1 and 4 (Table 3). The hybrid

obtained in run 1 is richer in dye unit than run 4 but not

colored. This observationmust be correlated to the difference

of the anchored chain length. In the first case, with the

hypothesis that the anchored and free chains displayed

the same characteristics, theMn was about 40000 g �mol�1.

For the run 4, where no free initiator was added, the

corresponding Mn is twice higher. Thus, the chain length

appears to be the first factor playing a role in the color of

the hybrid. The second factor is related to the amount of dye

present at the surface andmore precisely, to the styrene/dye

ratio.Asthisratioincreases, thecolorof thehybrid isswitched

off (run 5 and 6). It must be a consequence of the inherent

properties of the aluminiumplateletswhich strongly reflects

light and so switches off the color if the amount of dye is too

weakor locates intooclosedvicinityofthesurface. Itwasalso

the case for run 3 where the polystyrene dilutes the amount

of dye present at the pigment surface. Moreover, the use of

free initiator does not seem to be a prerequisite for the

synthesis of coloredmaterials. To confirm that and to obtain

hybrids with more intense color, two experiments were

performed with and without free initiator in an initial

reaction media saturated in dye (Table 4).

Although it is obvious that in our condition, the

criteria of controlled/‘‘living’’ polymerization were not

ascertained, the 2 experiments lead also in colored hybrid

(Figure 8).



Table 3. Influence of the dye concentration on the color of the hybrid.

Runsa) [dye] mol � L�1 Weight lossb) Conver-

sionc)

Hybrid compositiond) Color

% % %

St dye St dye pigment

4 0.13 8.9 31 20.0 7.8 1.1 91.4 slightly colored

5 0.07 10.9 40 49.0 9.6 1.3 89.1 slightly colored

6 0.04 10.4 36 52.0 9.5 0.8 89.7 No

a)Metallic pigment/styrene mass ratio¼0.17, dioxane/styrene mass ratio¼3.5, duration¼ 6.5h, temperature¼ 115 8C; b)Determined by

TGA and measured between 50 8C and 550 8C; c)Determined from 1H NMR; d)Determined from the following equations (%St¼weight

loss � [St] � %conversionSt/([St] � %conversionStþ � [dye] � %conversiondye), %dye¼weight loss � [dye] � %conversiondye/([St] �%conversionStþ � [dye] � %conversiondye), % pigment¼ 100�weight loss)

Table 2. Influence of the DPn;th on the color of the hybrid.

Runsa) [dye] DPnthb) Weight

lossc)Conver-

siond)

Hybrid

compositione)

Color

mol � L�1 % % %


1 0.09 840 9.2 51 48.0 7.7 1.5 91.8 no

2 0.06 1200 10.9 49 62.0 9.5 1.5 89.0 slightly colored

3 0.04 1500 12.9 48 59.0 11.6 1.3 87.1 no

a)Metallic pigment/styrene mass ratio¼0.18, dioxane/styrene mass ratio¼ 4.6, duration¼6.5 h, temperature¼115 8C, dye/free initiatormolar ratio¼ 42; b)DPnth¼ [St]0/[free initiator]; c)Determined by TGA and measured between 50 8C and 550 8C; d)Determined from1H NMR; e)Determined from the following equations (%St¼weight loss � [St] � %conversionSt/([St] � %conversionStþ � [dye] �%conversiondye), %dye¼weight loss � [dye] � %conversiondye/([St] � %conversionStþ � [dye] � %conversiondye), % pigment¼ 100�weight loss

Table 4. Influence of the free initiator on the color of the hybrid.

Runsa) [dye] DPnthb) Weight lossc) Conver-

sion d)

Hybrid compositione) Color

mol � L�1 % % %


7 0.10 2000 16.7 40 62.0 14.8 1.9 83.3 colored (Figure 8)

8 0.10 / 21.6 14 24.0 18.9 2.7 78.4 colored (Figure 8)

a)Metallic pigment/styrene mass ratio¼0.1, dioxane/styrene mass ratio¼ 1, duration¼6.5 h, temperature¼ 115 8C; b)DPnth¼ [St]0/[free

initiator]; c)Determined by TGA and measured between 50 8C and 550 8C; Determined from 1H NMR; e)Determined from the following

equations (%St¼weight loss � [St] � %conversionSt/([St] � %conversionStþ � [dye] � %conversiondye), %dye¼weight loss � [dye] �%conversiondye/([St] � %conversionStþ � [dye] � %conversiondye), % pigment¼100�weight loss)

1552Macromol. Chem. Phys. 2009, 210, 1544–1555

� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim DOI: 10.1002/macp.200900254


Figure 8. Photographs of dye-grafted copolymer, colored hybridpigment (run 7) and PS-grafted hybrid pigment (from left toright).

Moreover, the intensity of the color was more pro-

nounced without free initiator in agreement with the

determined composition. It is difficult to bring explanation

of this behavior butwemight suppose that it dealswith the

average activation-deactivation equilibrium constant hKiof the 2 monomer and their respective reactivity ratio.

Figure 9. SEM images of raw aluminium platelets (a,b), PBuAaluminium hybrids (c,d,e) and gradient copolymer aluminiumhybrids (f,g,h).

MEB Analysis

Figure 9 represents MEB pictures of raw aluminium

platelets and hybrids materials corresponding to PBuA

grafted aluminium platelets and PBuA-g-S grafted alumi-

nium platelets. Although the polymer layer cannot be

distinguished, it induces an arrangement of the platelets

(Figure 9b–f). Indeed, the raw aluminium particles are

disposed randomly over the substrate whereas in the case

of hybridmaterials, each of the aluminium platelets seems

to be collapsed or packed. Two phenomena can provide the

transition from non-sticky to sticky particles. First, if the

particles are in close vicinity, the space between platelets is

filled by grafted polymer chains. In such case, PBuA can be

considered as a theta solvent. The grafted polymer chains

stretch and entangle with chains from the neighboring

platelets. However, from TGAmeasurements, SEC analysis

andwith a low specific surface of aluminiumplatelet equal

to 2 m2 � g�1, we can approximate the grafting density of

grafted PBuA and PBuA-g-S. The calculated values corre-

spond respectively to 0.88 and 0.7 molecule �nm�2. In the

case of random coil, the cross section area deduced from,

Rg¼ a �N1/2 (45 A)with a designating the length of c-c bond

(2.5 A) andN the number of C-C bonds is about 63 nm2. This

result is inconsistent with the calculated grafting densities

and supports fully extended chains due to steric crowding.

Thus the chains entanglement is not obvious and themore

relevant explanation must involve the inherent adhesive

properties of PBuA which acts as cement between

the platelets. Then, these hybrid materials can be con-

sidered as adhesive building blocks leading to a strong


� 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim www.mcp-journal.de 1553


1554

particles alignment. This efficient surface cover makes

aluminiumparticlesagoodcandidate forpaintsorcoatings.

Indeed,we canobserve on the SEMimages the alignment of

the hybrid platelets horizontally on the glass substrates

(Figure 9f to h).

Conclusion

The introduction of a colored polymer brushes onto

micrometric aluminium platelets was reached through

surface-initiated nitroxide-mediated polymerization. With

respect to the reactivity of a tertiary and secondary

alkoxyamine, a bi-functional initiator was obtained in a

one-step synthesis according to ‘‘in situ thermo-dependant

trapping of carbon radicals’’ and used after its surface

anchoring to create a polymeric layer on the inorganic

platelet. This approach enabled the formation of hybrid

material with controlled polymer contents up to 15% by

mass depending on the [monomer]/[initiator] molar ratio

and the conversion. However, in controlled/‘‘living’’ con-

dition, no colorwas observed.Moreover, kinetic curves ofn-

butyl acrylate polymerization in presence of aluminium

platelets reveal an abrupt upward drift of the variation of

ln([M]0/[M]t]¼ f(t) for conversion above 60%which ismore

pronounced in presence of vinyl dyemonomer. Such result

suggests that during the course of the polymerization of n-

butyl acrylate, the quasi living character of the NMP was

diffusion-controlled dependent especially with this mono-

mer having a high kp and that the dye induces transfer

reaction.When n-butyl acrylate and styrene copolymeriza-

tion was performed with fs¼ 0.25, a high degree of

livingness was highlighted throughout the polymerization

even if in presence of the vinyl dye monomer. This feature

was correlated to the influence of the rate constants (K and

kp) and also the reactivity ratios of the monomers. The

addition of styrene induces an increase of the average

activation-deactivation equilibrium constant but a slower

consumption of n-butyl acrylatemonomer compared to its

homopolymerization in regard to the low kp,styrene and the

reactivity ratios. In this way, the diffusion-controlled

dependence was overcome.

To achieve colored hybrid materials, two factors have

been highlighted: the anchored chain length and the

styrene/dye ratio. It must be a consequence of the inherent

properties of the aluminium platelets which reflects light

and so switchof the color if theamountof dye is tooweakor

locates in too closedvicinityof thesurface. Toovercomethis

drawback, it is necessary to work in no controlled/‘‘living’’

conditions.

Acknowledgements: The authors want to thank V. Pellerin andC. Guimon for SEM images and XPS analysis, respectively. The



initiator Block Builder, so-called MAMA, has been provided byARKEMA.

Received: May 29, 2009; Published online: August 24, 2009;DOI: 10.1002/macp.200900254

Keywords: copolymerization; dyes/pigments; grafting from;hybrid material; nitroxide mediated polymerization

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Hybrid Aluminum Colored Pigments Based on Gradient Copolymers Design

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