INTRODUCTION - bictel.ulg.ac.bebictel.ulg.ac.be/.../unrestricted/Naveau_Chapter5.pdf · Chapter 5:...
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Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
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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 matrix1,2
. Among the
different methods for the preparation of polymer/clay nanocomposites, in situ polymerization
is recognized as a technique of choice to facilitate the exfoliation3-6
. 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 dispersion6. This can be achieved by
exchanging the inorganic cations of natural clay with suitable organic compounds. These
organic cations can act as compatibilizers7-9
, or they can bear a functional reactive group, such
as a monomer4,10-14
or an initiator9,11,15-18
. For example, Zhu et al.13,14
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 styrene13
but observed a mixed exfoliated-intercalated
nanocomposite with methyl methacrylate (MMA)14
. Diaconu et al.11
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.15
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 (scCO2) appeared as an interesting alternative medium
for the in situ preparation of polymer/clay nanocomposites19-25
. Indeed, thanks to the excellent
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
114
transport properties of scCO2, it is possible to polymerize a monomer inside clay platelets
without any restriction associated with high system viscosity like in bulk or in classical
solvents24
. 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 strength26,27
. Moreover, since CO2 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.19
. They took advantage of the low viscosity of scCO2 to prepare
PMMA nanocomposites with high ammonium-modified clay loading (20-50 wt%), through
radical polymerization. Subsequently, aliphatic polyesters (poly( -caprolactone)24
and
poly(lactide)28
)/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 scCO2. The
particularity of their contribution is the use of fluorinated20
and poly(dimethylsiloxane)-based
ammonium cations21
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 scCO2 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 scCO2 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 scCO2, 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 scCO226,27,29
.
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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 (CO2, 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 (Mn = 12 000 g/mol, 3 TEDETA
units/chain) was synthesized according to a reported method30
.
2 Organomodifiers synthesis
Organic salt 1 (S1, Scheme 1) was obtained by lyophilization of [2-
(methacryloyloxy)ethyl]trimethylammonium chloride solution (Aldrich, 80 wt% in H2O) 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 description31
. S4 is a polydimethylsiloxane mono-
terminated with a trimethylammonium group (PDMS-N+(CH3)3, Mn 1400 g/mol) described
elsewhere32
and was kindly provided by Céline Labuyère (SMPC, UMons). S5 was obtained
in two steps from 11-bromoundecan-1-ol (Aldrich, 98 %) as previously reported15
.
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, CO2 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
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
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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 scCO2 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 CO2
venting for 15 min. MMA was then injected under CO2 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 (1.95.10-4
mol of initiator, 3 wt% inorganics to monomer),
0.028 g of CuBr (1.95.10-4
mol) and 0.780 g of fluorinated macroligand (6.50 10-5
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 CO2. 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
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
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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 scCO2
Organoclays were prepared in scCO2 by ionic exchange between the sodium ions present in
the interlayer space of natural montmorillonite and ammonium cations. For the first time, the
scCO2 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 favored4. 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 exfoliation33
. S3 and S4 respectively
contain a fluorinated and a PDMS-chain, both known as CO2-philic segments. These
ammonium compounds were tested in reference to the work of Zhao et al.20,21
who
demonstrated that fluorinated or PDMS-clay dispersions are effective stabilizers for
polymerization of MMA in scCO2. 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(CH3)2-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.15
on ATRP. A
few examples exist with this initiator but none in scCO2 to our best knowledge.
All organic salts except for S4 are in the solid state at room temperature and present a poor
solubility in scCO2. 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
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
118
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 (d001) 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 scCO2
Code Function Stoech.a
Org. cont.b
(wt%)
d001b
(nm)
Yieldc
(%)
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)
bdetermined by TGA on as-obtained dried organoclays
cdetermined on washed, dried organoclays
O
O
N+
C l
O
O
N+
B r
O
O
N+
F
F
F
F
F
F
F
FF
F
FF
F
I
Si
O
Si
O
Si O
N+
15
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
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
119
Figure 1. XRD patterns of functional organoclays prepared in scCO2
Compared to natural clay (interlayer distance d001 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 d001 (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 authors34
. 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 d001 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 scCO2 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 scCO2 and NaI has
the lowest lattice enthalpy, thus less favored formation from its ions under the same
conditions.
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
120
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.32
did the ionic exchange in water and obtained a d001 = 1.7 nm, but they did
not exclude that it could correspond to d002, as the peak was not well defined. In fact, if the
peak corresponded to d002, the related d001 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 scCO2 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 scCO2. 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.15
obtained a relatively sharp diffraction peak with d001 = 1.88 nm while
Behling et al.35
noted a distance of 1.45 nm (no XRD spectra was shown). Larger discrepancy
are even found among the same group: Shipp et al.36,37
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 constituents38
. In fact, the differences
in reported interlayer distances may be explained by a late work of Behling et al.39
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.
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
121
Our obtained pattern corresponds to that “transition state”. We may conclude that ionic
exchange in scCO2 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 scCO2.
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.39
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 scCO2 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 scCO2
Polymerization tests in scCO2 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 scCO2. The quality of clay dispersion in the
polymer matrix was then evaluated, in order to establish organomodifier structure/degree of
exfoliation relationships.
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
122
Free-radical polymerization in scCO2 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 scCO2
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
scCO2, 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 scCO2, left: without clay, middle: with
MMT-1b, right: with MMT-2b
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
123
An intercalated morphology was reported in the literature for polymerization in bulk of MMA
with clay modified by cations bearing a vinyl group4. However, to our best knowledge, no
example exists in scCO2 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 scCO2 because of the CO2-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.21
.
Polymerization in scCO2 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 scCO2
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 HCl21
. 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.21
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
Zhao21
6.0** 65.0 3.9
*corresponding to inorganic content of 3 wt%, **lowest % to obtain a fine powder
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
124
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.27
for dispersion polymerization in scCO2. 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.40
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-IR20
.
Figure 5. Schematic illustration of stabilization of PMMA particle (in blue) by clay platelets
(in green) bearing PDMS chains (in orange), adapted from21
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
125
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
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
126
As a conclusion on free radical polymerization in presence of clay in scCO2, 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 scCO2,
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.15
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 scCO2, for its
advantageous low viscosity and high mass transport capacity.
DeSimone et al.41
studied the effect of using different ligands in the copper-mediated ATRP
in scCO2. They obtained highest yields and best control when a fluorinated ligand was used
and attributed this to the enhanced catalyst solubility in scCO2. This is why CO2-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)29,42
. At the end of the polymerization,
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
127
PMMA microspheres can be recovered in powder form30,42
. For further use, the macroligand
may either be eliminated through supercritical fluid extraction or serve to produce a material
with hydrophobic properties.
NN
N
O
Et
Et Et
Et
O
C8F
17
[ ( )n ( )m ]p
NN
N
O
O
Et
Et Et
Et
O
OOO
C8F
17
HO2C
S H
[ ( )n ( )m ]p
[ ( )n ( )m ]p
NN
N
O
O
Et
Et Et
Et
O
OOO
C8F
17
HO2C
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 scCO2 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 scCO2
clay ligand time
(h)
conv.
(%)
Mnth
(g/mol)
MnGPC
(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, Mnth
= [MMA]0/[initiator]0 x MwMMA x conv., f = Mnth/MnGPC
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
128
A first general observation is the slower polymerization rate in scCO2 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.15
), 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
CO2 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 scCO2, inducing inhomogeneous initiation of
reaction. As a reference, polydispersity obtained by Böttcher et al.15
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 = Mnth/MnGPC), it is limited to 0.65 without clay, which
is consistent with previous reports for ATRP of MMA in scCO2, with methyl α-
bromophenylacetate initiator30
. 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
montmorillonite39
. 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 CuBr2, as was done previously for ATRP from silicon surface43
. The
use of ultrasonication and CuBr2 should thus be considered to optimize our system in scCO2.
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
129
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 scCO2 with
the same stabilizer and methyl α-bromophenylacetate as ATRP initiator30,42
. 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
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
130
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 CO2-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
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
131
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 scCO2. Intercalation of
organomodifier in montmorillonite clay was demonstrated by X-ray diffraction and
polymerization of MMA in presence of these organoclays in scCO2 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 scCO2, 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 scCO2, 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 scCO2, a one-
pot process can be easily envisaged.
Chapter 5: Functional organoclays and routes for the in situ preparation of polymer nanocomposites in scCO2
132
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