. In this method, the layered silicate is swollen within the liquid monomer or a monomer solution, so that polymer formation can occur inside the clay galleries.

INTRODUCTION

The dispersion of layered silicate (clay) in polymer matrices provides materials with enhanced properties (e.g. mechanical, thermal, barrier and fire properties), depending in particular on the extent of individual clay sheets exfoliation at nanoscale in the matrix

. Among the different methods for the preparation of polymer/clay nanocomposites, in situ polymerization is recognized as a technique of choice to facilitate the exfoliation

. In this method, the layered silicate is swollen within the liquid monomer or a monomer solution, so that polymer formation can occur inside the clay galleries.

It is generally admitted, especially when hydrophobic monomers are concerned, that the in situ strategy requires a pre-treatment of the clay in order to improve its compatibility with the host monomer/polymer and to allow a good final dispersion

. This can be achieved by exchanging the inorganic cations of natural clay with suitable organic compounds. These organic cations can act as compatibilizers

, or they can bear a functional reactive group, such as a monomer

or an initiator

. For example, Zhu et al.

used a home-made ammonium cation containing one styryl group to modify pristine montmorillonite clay and performed in situ radical polymerization of styrene in bulk. They obtained a completely exfoliated nanocomposite with styrene

but observed a mixed exfoliated-intercalated nanocomposite with methyl methacrylate (MMA)

. Diaconu et al.

used a derivative of MMA, 2-methacryloylethyl-hexadecyldimethylammonium as organomodifier, to enhance the compatibility between the clay platelets and an acrylic matrix in waterborne nanocomposites by mini-emulsion polymerization. However, they reported only partial exfoliation, with a preferential location of clay platelets at the surface of polymer particles. In the aim of synthesizing polymers with well-defined architecture and predictable molecular weights, Böttcher et al.

used atom transfer radical polymerization (ATRP) with a suitable initiator previously exchanged in the silicate layers for the polymerization of MMA in acetone. This polar solvent enables a good dispersion of organomodified clay but, due to high concentration of monomer and insolubility of polymer, the medium soon becomes viscous which limits the degree of conversion.

Recently, supercritical carbon dioxide (scCO

) appeared as an interesting alternative medium

for the in situ preparation of polymer/clay nanocomposites

. Indeed, thanks to the excellent

transport properties of scCO

, it is possible to polymerize a monomer inside clay platelets without any restriction associated with high system viscosity like in bulk or in classical solvents

. Besides, this solvent has very attractive characteristics such as low toxicity, non- flammability, low cost, easy recyclability, accessible critical parameters (31.1°C, 73.8 bar) and tunable solvent strength

. Moreover, since CO

is a gas at ambient temperature, polymer product recovery is straightforward upon depressurization.

The interest in this green solvent for the in situ polymerization in presence of clay started with the work of Zerda et al.

. They took advantage of the low viscosity of scCO

to prepare PMMA nanocomposites with high ammonium-modified clay loading (20-50 wt%), through radical polymerization. Subsequently, aliphatic polyesters (poly( -caprolactone)

and poly(lactide)

)/clay masterbatches with a high clay concentration (up to 66 wt%) and a high degree of intercalation were prepared by Urbanczyk et al. by in situ ring-opening polymerization of the corresponding lactones. The final nanocomposites are then obtained by redispersing the masterbatches by melt blending in the polymer matrix of interest. In contrast, Zhao et al. used low inorganic content for the polymerization of MMA in scCO

. The particularity of their contribution is the use of fluorinated

and poly(dimethylsiloxane)-based ammonium cations

for the modification of montmorillonite clay. They showed that these modified clays provide effective steric stabilization and lead to the formation of exfoliated PMMA nanocomposites.

In the first chapter, we established the use of scCO

as a medium for clay organomodification.

When pristine clay is contacted with an appropriate organic cation in a high pressure vessel under supercritical conditions, ionic exchange occurs and after depressurization a ready-to-use powder is obtained. The process was applied for the preparation of thermally stable organoclays suitable for melt blending at high temperature (see Chapter 2-4).

In the present chapter, we describe the application of our scCO

process for clay

organomodification with four different functional organic compounds, i.e. ammonium cations

containing a monomer unit, a fluorinated chain, a polydimethylsiloxane (PDMS)-segment and

an initiator for ATRP. The aim of the study is to examine the potential of those functional

organoclays for the preparation of exfoliated nanocomposites through in situ polymerization

in scCO

, eventually via a one-pot two step convenient process. Methyl methacrylate was

chosen as a model monomer since experimental conditions for its polymerization are already

well established in scCO

.

EXPERIMENTAL SECTION

1 Materials

Sodium montmorillonite (MMT) labeled as Cloisite

Na

(cationic exchange capacity or CEC of 92.6 meq/100g) was supplied by Southern Clay Products (Rockwood Additives Ltd.).

Methyl methacrylate (MMA, Aldrich, 99 %) was deoxygenated by nitrogen bubbling just before use. Free radical initiator 2,2′-Azobis(isobutyronitrile) (AIBN, Aldrich, 98 %) and carbon dioxide (CO

, Air Liquide, 99,998 %) were used as received.

For ATRP experiments, CuBr (Aldrich, 98 %) was purified by dispersion in glacial acetic acid and stirring for a few hours followed by filtration, washing with acetone and drying under reduced pressure at 80°C. Fluorinated macroligand (M

= 12 000 g/mol, 3 TEDETA units/chain) was synthesized according to a reported method

.

2 Organomodifiers synthesis

Organic salt 1 (S1, Scheme 1) was obtained by lyophilization of [2- (methacryloyloxy)ethyl]trimethylammonium chloride solution (Aldrich, 80 wt% in H

O) and was stored in a dessicator. S2 and S3 were synthesized by quaternization of 2- (dimethylamino)ethyl methacrylate (Aldrich, 98 %) with respectively 1-bromododecane (Aldrich, 97 %) and 1H,1H,2H,2H-perfluorooctyl iodide (Aldrich, 96 %) in acetone (50°C under reflux for 3 days), based on literature description

. S4 is a polydimethylsiloxane mono- terminated with a trimethylammonium group (PDMS-N

(CH

)

, Mn 1400 g/mol) described elsewhere

and was kindly provided by Céline Labuyère (SMPC, UMons). S5 was obtained in two steps from 11-bromoundecan-1-ol (Aldrich, 98 %) as previously reported

.

3 Modification of clay

In a typical example, 1 g of unmodified clay (MMT) and a slight excess of organic cation (1.1

equivalents relative to CEC) were poured in a 40 ml high pressure reactor. 1 ml of ethanol

was added as a co-solvent and the vessel was dipped in an oil bath at 40°C. Following

thermostatization, CO

was injected with an Isco automatic syringe pump and pressure was

adjusted to 200 bar. After stirring during 2 h, the reactor was depressurized and the modified

clay was dried under vacuum at 50°C overnight to remove the co-solvent. No purification was

performed before polymerization assays. Only to evaluate the yield of exchange, as-obtained

organoclays were washed at room temperature firstly with water and secondly with acetone followed by drying under vacuum at 80°C overnight, as described previously.

4 Polymerization

Free radical polymerizations were conducted in scCO

in 20 ml high pressure reactors with 4.0 ml of MMA monomer (3.7 g). AIBN (1 wt% compared to monomer) and clay (3 wt% as inorganics compared to monomer) were first poured in the reactor containing a magnetic stirring bar. The vessel was dipped in an oil bath at 65°C and oxygen was removed by CO

venting for 15 min. MMA was then injected under CO

flow, followed by pressurization at 300 bar.

ATRP experiments were carried out using a similar procedure, with 6.6 ml of MMA (6.2 g), 0.290 g of initiator-modified clay (1.95.10

mol of initiator, 3 wt% inorganics to monomer), 0.028 g of CuBr (1.95.10

mol) and 0.780 g of fluorinated macroligand (6.50 10

mol).

Polymerization reactions proceeded at 300 bar and 65°C for various times (between 4 and 120 h), before cooling in an ice bath and slow release of CO

. Residual monomer was eliminated by drying under vacuum at 80°C overnight.

5 Characterization

As-obtained organoclays were analyzed by X-ray diffraction (XRD) in reflectance mode with a powder diffractometer Siemens D5000 (Cu Kα radiation with λ = 0.15406 nm, 50 kV, 40 mA, Ni filter, step size 0.05° and step time 2s) in order to evaluate their interlayer distance.

The organic content before and after washings was determined by thermogravimetric analysis (TGA, Q500 from TA Instruments) at a heating rate of 20K per min, from room temperature to 600°C under nitrogen flow.

The morphology of composites was observed by electron microscopy. For transmission electron microscopy (TEM, Philips CM100 at 100 kV), ultrathin sections (50-80 nm) of samples were prepared with a Leica EM FC6 ultramicrotome at room temperature and deposited on copper grids. Powdery samples were previously pressed for 30 s at 180°C or embedded and cured in an epoxy resin. Samples for scanning electron microscopy (SEM, JEOL JSM 840-A) were metallized with Pt.

Molecular weights of polymers were determined by gel permeation chromatography (GPC) in

dimethylformamide (DMF) with LiBr (0.025 M) at 55°C (flow rate: 1 ml/min), with a Waters

600 liquid chromatograph equipped with a 410 refractive index detector and styragel HR

columns (HR1, 100-5000; HR3, 500-30000; HR4, 5000-500000, HR5, 2000-4000000) calibrated with poly(methyl methacrylate) standards. Samples were first centrifugated in DMF, followed by two successive filtrations (through 0.45 nm and 0.20 nm filters). ATRP samples were extracted from clay sheets by a preliminary exchange with LiCl ions in a saturated THF solution at room temperature during one night, followed by precipitation in methanol.

RESULTS AND DISCUSSION

1 Preparation of functional organoclays in scCO

Organoclays were prepared in scCO

by ionic exchange between the sodium ions present in the interlayer space of natural montmorillonite and ammonium cations. For the first time, the scCO

process was applied to ammonium cations bearing a vinyl monomer or a polymerization initiator. Organomodifier structures are presented in Scheme 1. Organic salt 1 (S1, commercially available) consists of a methacrylate moiety and a trimethylammonium group. It was suggested that, if polymerization could occur with incorporation of the methacrylate moiety of this clay organomodifier in the growing polymer chains, exfoliation will be favored

. S2 is similar to S1 except for the substitution of a methyl group by a dodecyl chain on the nitrogen cation. This modification was indeed proven to be beneficial to expand clay interlayer distance and hence further facilitate exfoliation

. S3 and S4 respectively contain a fluorinated and a PDMS-chain, both known as CO

-philic segments. These ammonium compounds were tested in reference to the work of Zhao et al.

who demonstrated that fluorinated or PDMS-clay dispersions are effective stabilizers for polymerization of MMA in scCO

. Two differences must be noted: our fluorinated segment is smaller (6 fluorinated carbons compared to 10) and bears a methacrylate end group, while our PDMS-surfactant is also smaller (15 Si(CH

)

-O units compared to ~44) and is mono- terminated by a trimethylammonium group to avoid bonding of two adjacent clay sheets.

Lastly, S5 was chosen in reference to the interesting work of Böttcher et al.

on ATRP. A few examples exist with this initiator but none in scCO

to our best knowledge.

All organic salts except for S4 are in the solid state at room temperature and present a poor

solubility in scCO

. Therefore, to favor the exchange reaction, a little amount of ethanol (2.5

vol%) was added as co-solvent (cfr Chapter 1). No co-solvent was added in case of S4. The

results are presented in Table 1. For the first two samples, the stoechiometry of ammonium cation compared to the cationic exchange capacity of clay (CEC) was varied in order to study its influence on the interlayer distance (d

) and the exchange yield. The latter is determined by comparing the weight loss due to organic content as measured by TGA of washed products (classically between 150 and 550°C) to the theoretical weight loss based on CEC and molecular weight of ammonium cation.

Table 1. Results of organomodification in scCO

Code Function Stoech.

Org. cont.

(wt%)

d

(nm)

Yield

(%) MMT-1a

monomer 1.1 11.1 1.43 71

MMT-1b 2.2 20.8 1.87 89

MMT-2a

monomer +alkyl chain 1.1 24.0 1.70 76

MMT-2b 1.6 30.3 1.74 95

MMT-3 monomer +fluor.chain 1.1 27.2 1.30 35

MMT-4 PDMS 1.1 58.5 2.06 85

MMT-5 ATRP initiator 1.1 31.0 1.80 80

a

stoechiometry of ammonium cation compared to the clay cationic exchange capacity (CEC)

b

determined by TGA on as-obtained dried organoclays

c

determined on washed, dried organoclays

O O

N

+ C l

O O

N⁺ B r

O O

N⁺

F F

F F

F F

F

F F

F F F F

I

S i O

S i

O

S i O

N

+

1 5

B r

N

+

O

O B r B r

Scheme 1. Structure of used organomodifiers (numbers refer to those of Table 1)

S1 S2 S3

S4

S5

Figure 1. XRD patterns of functional organoclays prepared in scCO

Compared to natural clay (interlayer distance d

of 1.1 nm), all as-obtained organoclays present an enlarged gallery height, suggesting intercalation of ammonium cations between the clay layers. MMT-1a (with 10 % excess ammonium) presents a relatively small enlargement of d

(from 1.1 to 1.4 nm), which is directly related to the small size of organomodifier 1.

Interestingly, the gallery height may be increased up to 1.9 nm with the addition of twice as much organomodifier, indicating that excess organic cation can be located between the clay layers as noticed by other authors

. A further increase in organic cation concentration did not lead to further interlayer enlargement. In comparison, S2 with an alkyl chain of 12 carbons leads to a maximum d

of 1.7 nm, reached with 1.1 equivalents. A peak shouldering is nevertheless observed with 1.6 equivalents (MMT-2b), which might suggest at least partial location of excess ammonium ion in the galleries and the breaking up to some extent of the organized structure of clay. In both examples, yield of exchange is improved with increasing organomodifier concentration.

Concerning MMT-3, a distance of 1.3 nm does not seem enough for extensive intercalation of

the corresponding ammonium cation. Indeed, after washing, a large fraction of organic

content is eliminated, leading to a very poor yield. The difficulty to intercalate S3 between

clay layers might be explained by the iodide counterion which is less effective for ionic

exchange in scCO

compared to bromide and chloride ions. Indeed, we showed in Chapter 1

that the formation of the inorganic salt from the combination of sodium cations of the clay

and anions of organomodifier is the driving force of the ionic exchange in scCO

and NaI has

the lowest lattice enthalpy, thus less favored formation from its ions under the same

conditions.

MMT-4 presents a gallery height of 2.1 nm, which at first might seem a low value relative to the high molecular weight (1400 g/mol) of the used ammonium salt. In comparison, Labruyère et al.

did the ionic exchange in water and obtained a d

= 1.7 nm, but they did not exclude that it could correspond to d

, as the peak was not well defined. In fact, if the peak corresponded to d

, the related d

would be of 3.4 nm (4.1 nm in our case) which could still be visible on the spectrum (limit of detection is 2θ = 2° which corresponds to d = 4.4 nm). We rather believe that flexible PDMS-ammonium chains are lying flat on the clay surface. According to the relatively well-defined peak obtained and the high yield, PDMS- ammonium chains are thus successfully intercalated in MMT in scCO

medium, without the need of a co-solvent. Moreover, a direct proof of successful ionic exchange is detected on XRD spectra of as-obtained organoclays (Figure 1), by the observation of NaCl or NaBr salt coming from the recombination of sodium ion from the clay and counter-ion from the ammonium cation (Cl

or Br

depending on the ammonium). In contrast, on MMT-3 diffractogram, very thin peaks observed at 2θ=6 and 2θ=23 may be attributed to the non- exchanged salt.

Finally, initiator-modified clay (MMT-5) was prepared in scCO

. With S5, a much broader diffraction peak is observed for the as-obtained clay, with central distance of 1.8 nm. The same organic cation was used by other groups, with different reported gallery heights:

Böttcher et al.

obtained a relatively sharp diffraction peak with d

= 1.88 nm while Behling et al.

noted a distance of 1.45 nm (no XRD spectra was shown). Larger discrepancy are even found among the same group: Shipp et al.

once reported a distance of 1.46 nm and elsewhere 1.96 nm, the latter distance from a weaker and broader diffraction peak compared to the first. In each case (including our work), the same proportion of montmorillonite clay to initiator was used. Yields of exchange are claimed between 80 and 88

% but these must be compared carefully as different washing methods were performed.

The observation of diverse X-ray diffraction patterns for organoclays containing the same

organomodifier in identical proportion is linked to the strong θ dependence of experimental

parameters, as well as arrangement and organization of constituents

. In fact, the differences

in reported interlayer distances may be explained by a late work of Behling et al.

who

demonstrated a two-step addition of initiator to MMT in acetone (Figure 2). After 48 h, the

interlayer distance measured is 1.45 nm, while 1.85 nm is reached after 96 h. In between (72

h), a broad peak is observed, suggesting a transition between the two intercalation heights.

Our obtained pattern corresponds to that “transition state”. We may conclude that ionic exchange in scCO

is much faster, as the transition state is reached after 2 h compared to 72 h.

Further investigation is however needed to better understand the phenomenon in wet solvent (not explained by the authors) and in scCO

.

Figure 2. X-ray patterns of initiator-clay prepared in acetone, showing a two-stage addition of initiator (S5) to MMT, reproduced with permission from Behling et al.

This part demonstrated that supercritical carbon dioxide is an effective medium for the preparation of three types of functional organoclays. Compared to the more classical wet process, the scCO

technique allows an easy recovery of organoclays in a simple, fast and very convenient way. To emphasize this advantage, no washing or purification was carried out on the organoclays before the polymerization tests.

2 Polymerization in scCO

Polymerization tests in scCO

are presented in three sections. First, methacrylate ammonium- modified clays (MMT-1 and MMT-2) are used for the free radical polymerization of methyl methacrylate (MMA). Secondly, PDMS-modified clay is introduced for pseudo-dispersion polymerization and thirdly dispersion atom transfer radical polymerization from initiator-clay is presented. In each paragraph, the morphology of nanocomposites is studied.

2.1 Free radical polymerization with methacrylate-bearing clays

Clays modified by methacrylate-bearing ammonium (MMT-1b and MMT-2b) were used in

the free radical polymerization of MMA in scCO

. The quality of clay dispersion in the

polymer matrix was then evaluated, in order to establish organomodifier structure/degree of

exfoliation relationships.

Free-radical polymerization in scCO

was carried out at 65°C for 16 h at 300 bar with magnetic stirring. Quantity of monomer was 4 ml in a 20 ml reactor, with 1 wt% AIBN and 3 wt% inorganics (both with respect to monomer).

Table 2. Results of free radical polymerization of MMA in the presence of organomodified

clays in scCO

clay time (h)

conversion (%)

Mn (g/mol)

Mw

(g/mol) Mw/Mn sample morphology

1 MMT-1b 16 80 not determined flakes

2 MMT-2b 16 83 46400 257000 5.5 flakes

Conditions: 65°C, 300 bar, 700 rpm, 1 wt% AIBN, 3 wt% inorganics

The visual observation of samples 1 and 2 reveals an aggregated yellowish powder at the bottom of the reactor covered by white solid flakes to the top. A TGA analysis (not shown) indicates that the aggregated powder contains a large majority of inorganics. We must thus conclude that clay was not mixed with MMA during polymerization. This is also confirmed by XRD analysis (not shown) where no enlargement of initial organoclay interlayer distance is observed. A possible explanation is the fast precipitation of growing PMMA chains in scCO

, forming a heterogeneous system and leaving no time for the soluble monomer to penetrate inside the clay layers.

To help clay swelling with MMA and facilitate further polymerization with incorporation of the methacrylate moiety of this clay organomodifier in the growing polymer chains, we added a previous step of “clay soaking” at a temperature below the decomposition temperature of AIBN, namely 40°C, and a pressure of 180 bar. After 2 h, the temperature was increased to 65°C, which consequently increased the pressure to 350 bar. The samples were collected after 7 h under the polymerization conditions and the conversion obtained was around 40 % with both organoclays. Again, visual observation of these samples shows a large heterogeneity, with a phase-separation between clay and PMMA. Clay agglomerates (yellowish) are eye- detected on pressed disks (Figure 3).

Figure 3. Pressed disks of PMMA synthesized in scCO

, left: without clay, middle: with

MMT-1b, right: with MMT-2b

An intercalated morphology was reported in the literature for polymerization in bulk of MMA with clay modified by cations bearing a vinyl group

. However, to our best knowledge, no example exists in scCO

with monomer-modified clay and further investigation is needed to better understand the phenomenon.

2.2 Free radical polymerization with PDMS-clay

Polydimethylsiloxane (PDMS)-modified clay can find application in the polymerization of MMA in scCO

because of the CO

-philic character of the siloxane chains. If the insoluble clay platelets get attached to the growing PMMA chains, the soluble PDMS-organomodifier can help providing steric stabilization of PMMA in this medium. This technique was referred to as pseudo-dispersion polymerization by Zhao et al.

.

Polymerization in scCO

was performed at conditions close to those described by Zhao, i.e. at 65°C and 300 bar for 4 h with magnetic stirring. Quantity of monomer was 4 ml in a 20 ml reactor, with 1 wt% AIBN and 3 wt% inorganics (both with respect to monomer).

Table 3. Results of free radical polymerization of MMA with PDMS-clay in scCO

sample clay time

(h)

conversion (%)

Mn (g/mol)

Mw

(g/mol) Mw/Mn sample morphology

4 MMT-4 4 30 52400 150000 2.9 powder

Conditions: 65°C, 300 bar, 700 rpm, 1 wt% AIBN, 3 wt% inorganics

After 4 h of reaction, 30 % of conversion is reached and a homogeneous fine white powder is collected from the reactor, suggesting a stabilizing effect of PDMS-clay (containing PDMS- monotelechelic trimethyl ammonium, cfr Scheme 1). This effect is consistent with the observation of Zhao et al. who modified clay with an - bis(aminopropyl)-terminated PDMS quaternized with HCl

. The PDMS content to monomer in our study and in the work of Zhao were calculated and compared in Table 4. An equal amount of PDMS to monomer was used.

Table 4. PDMS content in PMMA nanocomposites compared to that of Zhao et al.

wt% of PDMS-clay

to monomer

wt% of PDMS in PDMS-clay

wt% of PDMS to monomer

this study 7.2* 58.5 4.2

Zhao

6.0** 65.0 3.9

*corresponding to inorganic content of 3 wt%, **lowest % to obtain a fine powder

SEM analysis reveals that the PMMA/PDMS-clay nanocomposites primarily consist of spherical PMMA particles, with an average particle size around 10 µm (Figure 4), same as that obtained by Zhao.

Figure 4. SEM picture of PMMA with PDMS-clay (Table 3)

The proposed mechanism of stabilization is firstly based on description by DeSimone et al.

for dispersion polymerization in scCO

. They state that steric stabilization of a colloidal dispersion is usually imparted by amphiphilic macromolecules that become adsorbed onto the surface of the dispersed phase. These amphiphilic macromolecules contain an anchoring segment, which attaches to the particle usually by physical adsorption, and stabilizing moieties that are soluble in the continuous phase. The stabilizing moieties project in the continuous phase and prevent flocculation by mutual excluded volume repulsion. Secondly, Lagaly et al.

have shown that clay platelets can be used as stabilizers for emulsions and inverse emulsions by encapsulation of droplets through formation of a three-dimensional inorganic network. Thus, combining these two statements, it can be suggested that soluble PDMS chains, linked to the clay platelets, may act as the soluble stabilizing moieties, while the clay platelet itself acts as the anchoring segment, being attached to the C=O group of methyl methacrylate by hydrogen bonding with remaining interlayer water molecules (Figure 5). Hydrogen bond interactions between PMMA and clay were indeed detected previously by FT-IR

.

Figure 5. Schematic illustration of stabilization of PMMA particle (in blue) by clay platelets

(in green) bearing PDMS chains (in orange), adapted from

The morphology of the powdery nanocomposites was then investigated by X-ray diffraction and transmission electron microscopy. XRD pattern of PMMA containing PDMS-clay (MMT-4) is a smooth line with no visible peak below 2θ = 10 (Figure 6). The disappearance of the characteristic peak of the organoclay might suggest an exfoliation in the matrix.

However, as commonly admitted, this must be confirmed by other techniques, such as TEM.

Figure 6. XRD pattern of PDMS-clay and PMMA nanocomposite with PDMS-clay

TEM analysis of pressed sample reveals an extensive destructuration of the clay stacks, with preferential arrangement of clay platelets (individual and small stacks) in ribbons, as shown on Figure 7. This observation confirms the proposed mechanism of stabilization, with the location of clay on the surface of the PMMA particles.

Zhao et al. reported a uniform dispersion of silicate layers in the PMMA matrix, however this statement was based on TEM only and the distribution of the silicate layers depends on the angle of observation.

.

Figure 7. TEM pictures of PMMA with PDMS-clay from pressed sample (30 s at 180°C)

500 nm

As a conclusion on free radical polymerization in presence of clay in scCO

, we observed that MMA-like clays were not homogeneously dispersed in PMMA, probably due to a fast precipitation of growing polymer chains. On the contrary, PDMS-like clay was successfully exfoliated in PMMA, thanks to a stabilizing effect of siloxane chains, leading to pseudo- dispersion polymerization with formation of spherical PMMA microparticles stabilized by PDMS modified clay located at their surface. This process is particularly interesting as no supplementary surfactant is needed. Moreover, as the organoclay itself is prepared in scCO

, the preparation of stabilized PMMA nanocomposites could be processed in one-pot in future applications, after optimization of the degree of conversion. The analysis of thermal, mechanical and fire properties of these nanocomposites still needs to be carry out. Finally, it is noteworthy that PDMS-clay may also find applications in the preparation of silicon polymer nanocomposites.

2.3 Atom transfer radical polymerization with initiator-modified clay

Another interesting route to obtain fully dispersed polymer/layered silicate nanocomposites is the use of initiator-modified clay. Indeed, the anchoring of an initiator inside the clay layers is the best way to ensure polymer chain growth from the silicate with a progressive delamination during reaction. We chose the ATRP-initiator developed by Böttcher et al.

because of its simple synthesis in mild conditions. Moreover, atom transfer radical polymerization is a very convenient process to synthesize polymers with well-defined architecture and molecular weights. Böttcher et al. obtained very interesting results regarding control of MMA polymerization in the presence of initiator-modified clay. However, they worked in acetone with a limited monomer conversion ( 50 %). Detailed morphology of samples was also not provided. Our objective was to test the same initiator for ATRP of MMA in scCO

, for its advantageous low viscosity and high mass transport capacity.

DeSimone et al.

studied the effect of using different ligands in the copper-mediated ATRP

in scCO

. They obtained highest yields and best control when a fluorinated ligand was used

and attributed this to the enhanced catalyst solubility in scCO

. This is why CO

-philic

fluorinated ligands with tetraethyldiethylenetriamine (TEDETA) as complexing group were

synthesized and used in this study, i.e. a fluoroalkyl-substituted ligand (f-L) and a fluorinated

macromolecular ligand (f-ML) sketched in Scheme 2. The latter was chosen because of its

demonstrated dual role, (i) the complexation of the ATRP catalyst (copper salt) and (ii) the

stabilization of the growing polymer particles (PMMA)

. At the end of the polymerization,

127 PMMA microspheres can be recovered in powder form

. For further use, the macroligand may either be eliminated through supercritical fluid extraction or serve to produce a material with hydrophobic properties.

N

N

N O

E t

E t Et

Et O

C8F

1 7

[

]

N

N

N O

O

E t

E t Et

Et O

O O O

C₈F_{1 7} H O₂C

S H

[ ( ) n ( ) m ^] p

[

]

N

N

N O

O

E t

E t Et

Et O

O O O

C₈F_{1 7} H O₂C

S H

[ ( ) n ( ) m ^] p

Scheme 2. General scheme of fluorinated ligand (f-L, left) and fluorinated macroligand (f- ML, right, 12 000 g/mol, 3TEDETA units/chain) used for ATRP

ATRP of MMA was initiated in scCO

by an ammonium-terminated bromoisobutyrate (cfr Scheme 1), alone or anchored to clay layers by electrostatic interaction. The weight percentage of clay to monomer was calculated to be of 3 % as inorganics (i.e. MMT-5 as- obtained organoclay with 31 wt% of initiator). An equivalent proportion of initiator, catalyst (CuBr) and ligand was used. Conditions were a temperature of 65°C and a pressure of 300 bar. Quantity of monomer was 6.6 ml in a 20 ml reactor. After 70 h of magnetic stirring at 700 rpm, the experiment carried out without clay (entry a, Table 5) reached a good conversion while the ones containing clay (entries b and c, Table 5) went to around 50 % conversion.

These were then repeated for longer times (5 days, entries d and e, Table 5). Assays d and e are done in the same conditions to test reproducibility.

Table 5. Results of ATRP polymerization in scCO

clay ligand time (h)

conv.

(%)

Mn

(g/mol)

Mn

(g/mol) Mw/Mn f sample

morphology

a - f-ML 70 94 29900 45000 1.75 0.66 fine powder

b MMT-5 f-L 70 53 16900 35200 1.54 0.48 flake

c MMT-5 f-ML 70 54 17200 39900 2.85 0.43 flake

d MMT-5 f-ML 120 97 30800 146000 1.63 0.21 flake + powder e MMT-5 f-ML 120 95 30200 89800 1.69 0.34 fine powder Conditions: 65°C, 300 bar, 700 rpm, [initiator]/[TEDETA] = 1, [CuBr]/[TEDETA] = 1, Mn

= [MMA]

/[initiator]

x Mw

x conv., f = Mn

/Mn

A first general observation is the slower polymerization rate in scCO

compared to polymerization in solution (conversion of 50 % reached after 4 h in 2 ml acetone with 3 ml MMA and same initiator-clay, Böttcher et al.

), mainly due to the higher dilution used in supercritical medium. Secondly, the lower conversion observed with the presence of clay may be explained by the necessity for the catalyst, complexed by the ligand, to enter the gallery space to meet the initiator. Yet, with its fluorinated chains, the ligand has more affinity for CO

then for silicate. As a result, the initiation step might be delayed.

The relatively large polydispersity without clay is probably linked with the poor solubility of the initiator bearing an ammonium-end group in scCO

, inducing inhomogeneous initiation of reaction. As a reference, polydispersity obtained by Böttcher et al.

in acetone is 1.1.

Comparing assay a to assays d and e, the polydispersity is not influenced by the presence of clay. It is however higher with the macroligand compared to that with the smaller ligand, most likely due to its size which hinders the access to initiator molecules localized inside the clay galleries. The suggested mechanism is a start of polymerization through the externally located initiator molecules with a progressive delamination of clay. As the clay gallery height increases, more internally located initiator molecules can be reached.

Concerning the initiator efficiency (f = Mn

/Mn

), it is limited to 0.65 without clay, which is consistent with previous reports for ATRP of MMA in scCO

, with methyl α- bromophenylacetate initiator

. When anchored to silicate layers, the efficiency is decreased.

The result calculated with fluorinated ligand (0.47) is close to the value calculated from the

results of Böttcher et al. (0.52) with the same ammonium-initiator but in acetone. This could

be attributed to the non accessibility of part of initiator molecules located inside clay galleries,

due to insufficient clay sheet delamination. An interesting study published recently reported

the influence of graft density on kinetics of surface-initiated ATRP of polystyrene from

montmorillonite

. With the same initiator as in present study but in bulk, the efficiency is of

95 %, whatever the graft density. Following above hypothesis of insufficient delamination,

their high efficiency can be explained, at least partially, by the use of ultrasonication to

enhance MMT stacks dispersion. Besides, they eliminated termination effects by the

introduction of excess CuBr

, as was done previously for ATRP from silicon surface

. The

use of ultrasonication and CuBr

should thus be considered to optimize our system in scCO

.

Another interesting perspective of these preliminary tests would be to combine the small ligand, to facilitate penetration between the clay layers, and the macroligand to ensure further stabilization of the system. Indeed, stabilization of PMMA particles was effective with the use of the macroligand, as proved by the collection of a fine powder with free initiator without clay (Table 5, entry a) and silicate-anchored initiator (Table 5, entry e). Careful attention should however be paid to the stirring effectiveness, since insufficient stirring might explained the lack of reproducibility between samples c, d and e. These samples were analyzed by scanning electron microscopy (SEM).

Figure 8. SEM pictures of ATRP samples with macroligand (letters refer to entry in Table 5;

a- left: without clay, e-right: with clay)

SEM analysis of sample without clay (Figure 8, sample a) reveals typical spherical particles with an average size of 5 µm, coexisting with larger spherical particles up to 20 µm, consistently with previous results obtained for MMA dispersion polymerization in scCO

with the same stabilizer and methyl α-bromophenylacetate as ATRP initiator

. In the presence of clay (Figure 8, sample e), spherical particles of large size distribution coexist with particles of various forms.

XRD analysis was carried out on sample powder to evaluate the quality of clay dispersion. No peak is detected in PMMA/clay spectrum (Figure 9), which may be an indication of exfoliation. However, we should be careful about this statement since the clay itself did not show a well-defined peak. Further analysis is thus required to prove that a nanocomposite is obtained. In this optic, ultrathin sections of as-obtained samples were observed by TEM (Figure 10).

e

a

Figure 9. XRD pattern of initiator-clay and PMMA nanocomposite with initiator-clay

Figure 10. TEM pictures of PMMA with ATRP-initiator clay (sample e, in epoxy resin)

The TEM images of PMMA powder embedded in epoxy resin reveal the presence of aggregates and stacks of medium and small size. Individual platelets were also observed, together with large zones free of clay, indicating only partial intercalation and small extent of delamination. It is consistent with the difficult access of the macroligand to the initiator inside the clay platelets. We may assume that when all monomer is consumed, clay aggregates subsist with intercalated initiator molecules that did not take part to the polymerization process.

Finally, the morphology of PMMA particles obtained by ATRP may be compared with that of PMMA particles obtained with PDMS-clay. In both cases, spherical particles of micrometric size are obtained, with at least partial exfoliation of clay layers. In fact, the two methods show distinct advantages: the use of a CO

-philic organomodifier (PDMS-ammonium) enables the clay itself to stabilize the PMMA particles. This method is very simple and easy to implement but the possibility to control the size and architecture of PMMA is quite limited. On the

5000 nm 500 nm

contrary, ATRP will offer the possibility to control the molecular weights but therefore it will require a careful choice of initiator and ligand, as well as adjustment of other processing parameters (e.g. stirring, temperature, pressure, quantity of catalyst). Once the ATRP product being optimized, the thermal and mechanical properties of PMMA nanocomposites should be analyzed and compared with those of PMMA nanocomposites obtained by free radical polymerization, to provide supporting information on the interest of each method.

CONCLUSIONS

Three types of functional organoclays were successfully prepared in scCO

. Intercalation of

organomodifier in montmorillonite clay was demonstrated by X-ray diffraction and

polymerization of MMA in presence of these organoclays in scCO

is reported. While

methacrylate-bearing clays were not homogeneously dispersed in PMMA synthesized by free

radical polymerization, exfoliated PDMS-clay platelets served as effective stabilizer for the

synthesis of this polymer in scCO

, leading to the formation of spherical microparticles with

clay located at their surface. Thirdly, ATRP-initiator modified clay in presence of catalyst and

fluorinated macroligand also led to the formation of PMMA microspheres in scCO

, thanks to

the stabilizing properties of the macroligand. The process is convenient since the polymer is

recovered as a fine powder after synthesis and further optimization of the ATRP process

would allow preparing polymers with well-defined molecular weight. Moreover, functional

cations of various structures should be tested to enhance clay exfoliation in this or another

matrix. Finally, as both clay organomodification and polymerization occur in scCO

, a one-

pot process can be easily envisaged.

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