A versatile solvent-free "one-pot" route to polymer nanocomposites and the in situ formation of calcium phosphate/layered silicate hybrid nanoparticles

Article abstract of DOI:10.1002/adfm.201000041

Poly(methyl methacrylate) (PMMA), polystyrene (PS), and polyurethane (PU) nanocomposites containing well-dispersed calcium phosphate/layered silicate hybrid nanoparticles were prepared in a versatile solvent-free "one-pot" process without requiring separate steps, such as organophilic modification, purification, drying, dispersing, and compounding, typical for many conventional organoday nanocomposites. In this "one-pot" process, alkyl ammonium phosphates were added as swelling agents to a suspension of calcium/layered silicate in styrene, methyl methacrylate, or polyols prior to polymerization. Alkyl ammonium phosphates were prepared in situ by reacting phosphoric acid with an equivalent amount of alkyl amines such as stearyl amine (SA) or the corresponding ester- and methacrylate-functionalized tertiary alkyl amines, obtained via Michael Addition of SA with methyl acrylate or ethylene 2-methacryloxyethyl acrylate. Upon contact with the calcium bentonite suspension, the cation exchange of Ca ²⁴⁺ in the silicate interlayers for alkyl ammonium cations rendered the bentonite organophilic and enabled effective swelling in the monomer accompanied by intercalation and in situ precipitation of calcium phosphates. According to energy dispersive X-ray analysis, the calcium phosphate precipitated exclusively onto the surfaces of the bentonite nanoplatelets, thus forming easy-to-disperse calcium phosphate/layered silicate hybrid nanoparticles. Incorporation of 5-15wt% of such hybrid nanoparticles into PMMA, PS, and PU afforded improved stiffness/toughness balances of the polymer nanocomposites. Functionalized alkyl ammonium phosphate addition enabled polymer attachment to the nanoparticle surfaces. Transmission electron microscopy (TEM) analyses of PU and PU-foam nanocomposites, prepared by dispersing hybrid nanoparticles in the polyols prior to isocyanate cure, revealed the formation of folly exfoliated hybrid nanoparticles.

Full text of DOI:10.1002/adfm.201000041

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A Versatile Solvent-Free ‘‘One-Pot’’ Route to Polymer
Nanocomposites and the in situ Formation of Calcium
Phosphate/Layered Silicate Hybrid Nanoparticles
By Hans Weickmann, Matthias Gurr, Olaf Meincke, Ralf Thomann, and
Rolf M u¨ lhaupt*
1
. Introduction
Poly(methyl methacrylate) (PMMA), polystyrene (PS), and polyurethane (PU)
nanocomposites containing well-dispersed calcium phosphate/layered silicate
hybrid nanoparticles were prepared in a versatile solvent-free ‘‘one-pot’’
process without requiring separate steps, such as organophilic modification,
purification, drying, dispersing, and compounding, typical for many
Since the pioneering advances by Okada
[
1]
et al. at Toyota, organophilic layered
silicates have been used extensively in
academia and industry to prepare nano-
[
2–5]
composites.
Lowsilicatecontentofafew
conventional organoclay nanocomposites. In this ‘‘one-pot’’ process, alkyl
ammonium phosphates were added as swelling agents to a suspension of
calcium/layered silicate in styrene, methyl methacrylate, or polyols prior to
polymerization. Alkyl ammonium phosphates were prepared in situ by reacting
phosphoric acid with an equivalent amount of alkyl amines such as stearyl
amine (SA) or the corresponding ester- and methacrylate-functionalized tertiary
alkyl amines, obtained via Michael Addition of SA with methyl acrylate or
ethylene 2-methacryloxyethyl acrylate. Upon contact with the calcium bentonite
weight percent can be sufficient to simulta-
neously improve stiffness, strength, and
dimensional stability without sacrificing
toughness, to enhance barrier performance
and to improve thermal stability as well as
[
2–7]
halogen-free fire protection.
The most common layered silicates used
for polymer nanocomposites are montmor-
illonites, to which calcium bentonites
belong. Their layered structure consists of
silicate nanoplatelets with dimensions of
approximately 300 nm in diameter and
2
R
suspension, the cation exchange of Ca in the silicate interlayers for alkyl
ammonium cations rendered the bentonite organophilic and enabled effective
swelling in the monomer accompanied by intercalation and in situ precipitation
of calcium phosphates. According to energy dispersive X-ray analysis, the
calcium phosphate precipitated exclusively onto the surfaces of the bentonite
nanoplatelets, thus forming easy-to-disperse calcium phosphate/layered
silicate hybrid nanoparticles. Incorporation of 5–15 wt% of such hybrid
nanoparticles into PMMA, PS, and PU afforded improved stiffness/toughness
balances of the polymer nanocomposites. Functionalized alkyl ammonium
phosphate addition enabled polymer attachment to the nanoparticle surfaces.
Transmission electron microscopy (TEM) analyses of PU and PU-foam
nanocomposites, prepared by dispersing hybrid nanoparticles in the polyols
prior to isocyanate cure, revealed the formation of fully exfoliated hybrid
nanoparticles.
1
of Al
nm in thickness. Isomorphic substitution
3
þ
2þ
with Mg within the layers
generates negative charges that are counter-
þ
2þ
balanced by cations, i.e., Na and Ca
,
residing in the interlayer spacings. Since
the silicates are hydrophilic and lack affinity
with hydrophobic organic solvents and
water-insoluble polymers, cation ion
exchange reactions with organic cations,
i.e., alkyl ammonium cations, are necessary
in order to render the silicate surface
organophilic. The resulting organophilic
layered silicate is often referred to as
organoclay. In most industrial processes,
calcium-containing bentonites are con-
verted into sodium bentonites by means
of cation exchange. Resulting from their improved water swelling
with respect to calcium bentonites, sodium bentonites promote
[
*] Dr. H. Weickmann, M. Gurr, Dr. O. Meincke, Dr. R. Thomann,
Prof. R. M u¨ lhaupt
Institut f u¨ r Makromolekulare Chemie und Freiburger Material-
forschungszentrum der Albert-Ludwigs-Universit ¨a t
Stefan-Maier-Str. 21/31, D-79104 Freiburg i. Br. (Germany)
E-mail: rolf.muelhaupt@makro.uni-freiburg.de
[
8]
effective cation exchange with organic ammonium cations.
Much less is known on how to exploit calcium bentonites without
requiring this intermediate purification step. In fact, most
conventional processes for making organoclays represent multi-
step processes including sodium bentonite purification, organo-
clay formation, washing, drying, dispersing, and compounding.
An important challenge in nanocomposite development is to
simplify organoclay and nanocomposite fabrication.
Prof. R. M u¨ lhaupt
Freiburg Institute for Advanced Studies, School of Soft Matter
Research
Albertstraße 19, 79104 Freiburg i. Br. (Germany)
DOI: 10.1002/adfm.201000041
1
778
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[
37]
Many attempts have been reported to incorporate organoclay in
poly(methyl methacrylate) (PMMA), polystyrene (PS), or poly-
urethanes(PU),aimingatobtainingimprovedpropertiesresulting
from intercalation and exfoliation of organoclays. Organoclay
ambient temperature. The interlayer cation exchange and in-
situ organophilic bentonite modification is accompanied with
massive swelling in monomers and the precipitation of calcium
phosphate, thus forming novel calcium phosphate/layered silicate
hybrid nanoparticles in monomers such as styrene, methacrylate,
and polyol prior to polymerization. This simple ‘‘one-pot’’ process
does not require conversion of calcium bentonite into sodium
bentonite and eliminates the need for extensive washing, drying,
milling, and dispersion technology typical for organoclay
preparation in aqueous medium.
[
9–15]
dispersions in monomer were polymerized in bulk,
solu-
Alternatively,
extrusion of polymer melts was employed successfully to prepare
[
16–21]
[22–33]
tion,
and also in aqueous emulsions.
[
34–36]
thermoplastic organoclay nanocomposites.
A large number
of publications have addressed the influence of the organoclay
preparation processes and of the swelling agent on morphology
and material properties, including investigations of enhancing
exfoliation by polymerization in intergallery spacings. However,
almost all approaches require syntheses of at least two steps.
Usually, the organoclay is produced by exchanging the intergallery
cation in aqueous solution in the presence of the organic swelling
agent. The consecutive washing, drying, milling, and dispersing of
the resulting organoclay represent time-consuming and relatively
costly steps required prior to the polymerization. One of the few
attempts to bypass two-step procedures has been conducted by
2
. Results and Discussion
2
.1. PMMA and PS Nanocomposites Containing in situ
Formed Calcium Phosphate/Layered Silicate Hybrid
Nanoparticles
[
21]
Huski c´ etal., reportingthesolutionpolymerizationofMMAina
dispersion of pristine montmorillonite in a mixture of ethanol,
water, and varying quaternary ammonium ions. Resulting
nanocomposites were determined to exhibit intercalated morphol-
ogies and enhanced thermostability. However, this one-step
approach required the use of solvents and was restricted to matrix
polymers soluble in the polar solvent mixture.
Natural Ca-bentonite was rendered organophilic by means of ion
exchange using protonated stearyl amine, stearyl di(methoxycar-
bonylethyl) amine (SAE), or stearyl di(2-methacryloxyethyl
carbonylethyl) amine (SAMA). The ester-functionalized tertiary
amine (SAE) and the methacrylate-functionalized tertiary amine
(SAMA) were synthesized via a Michael Addition of stearyl amine
with methyl acrylate or 2-methacryloxyethyl acrylate (EGAMA)
with exclusive reaction of the acrylate group at ambient
temperature. This Michael Addition chemistry represents a very
versatile tool to match the polarity and reactivity of bentonite
organophilic modifiers with those of the monomers. The alkyl
amineswereprotonatedwiththeequivalentamountofphosphoric
acid in MMA or styrene solution in order to produce swelling
agents for calcium bentonite, respectively. Amine and acid content
were equimolar to the Ca-bentonite’s cation exchange capacity of
Here we report on a novel solvent-free ‘‘one-pot’’ process,
illustrated in Scheme 1, for producing PU, PS, and PMMA
nanocomposites based upon Ca-bentonites. Organophilic mod-
2þ
ification was achieved in situ by means of cation exchange of Ca
in the bentonite interlayer spacings for alkyl ammonium
phosphates, prepared by neutralizing alkyl amines with the
equivalent amount of phosphoric acid. Ester and methacrylate-
functionalized stearyl amines were obtained by means of Michael
Addition of stearyl amine with methyl acrylate or 2-methacryloxy-
ethyl acrylate with exclusive reaction of the acrylate groups at
ꢀ1
0.8 meq g . Then Ca-bentonite was added and the cation ion
exchange of the bentonite’s intergallery Ca
2þ
afforded organophilic bentonite modification
accompanied by swelling and the simultaneous
precipitation of calcium phosphate. Stable
dispersions were formed under high shearing
conditions (Ultrathurax) and polymerization
was initiated at 60 8C. In order to examine the
influence of calcium phosphate/layered silicate
hybrid particles onmorphologyandmechanical
properties of the composite system, the content
of organoclay was varied from 5 to 15 wt%. The
composites’ stiffness, measured as Young’s
modulus, and fracture toughness, determined
as critical stress intensity factor K1c (mode 1) are
summarized in Table 1. As reference, non-
modified PMMA and PS, prepared under
identical conditions, were included. Samples
are labeled FAx/polymer with FA representing
the organophilic functionalization agent used
in hybrid nanoparticle preparation, x represent-
ing the filler content in wt%, and polymer
Scheme 1. Solvent-free ‘‘one-pot’’ process for the formation of nanocomposites containing in-
situ-formed calcium phosphate/layered silicate hybrid nanoparticles. a) Protonation of alkyl
2
þ
amines with equivalent amounts of phosphoric acid, b) cation exchange of intergallery Ca by
quaternary alkyl ammonium cations, accompanied by the simultaneous precipitation of calcium
phosphate, c) swelling of organophilic calcium phosphate/layered silicate hybrid nanoparticles
with monomer, and d) polymerization and in situ formation of intercalated and fully exfoliated relating to the polymeric matrix of the nano-
nanocomposites.
composite material.
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Table 1. Mechanical properties of PMMA and PS nanocomposites.
ꢀ3/2
Sample
Organoclay
Young’s
K1c [MN m
]
content [wt%]
modulus [MPa]
PMMA
0.0
5.0
2650 ꢁ 20
2800 ꢁ 90
3180 ꢁ 55
3250 ꢁ 25
2990 ꢁ 60
3220 ꢁ 90
3280 ꢁ 60
2920 ꢁ 20
2980 ꢁ 55
3150 ꢁ 90
3440 ꢁ 60
3510 ꢁ 95
3620 ꢁ 95
3820 ꢁ 90
3590 ꢁ 50
3740 ꢁ 100
4060 ꢁ 120
3650 ꢁ 100
3780 ꢁ 100
4070 ꢁ 160
1.52 ꢁ 0.08
0.84 ꢁ 0.06
0.78 ꢁ 0.05
0.88 ꢁ 0.07
0.79 ꢁ 0.12
0.65 ꢁ 0.11
0.69 ꢁ 0.06
0.75 ꢁ 0.06
0.54 ꢁ 0.02
0.57 ꢁ 0.06
1.31 ꢁ 0.10
2.15 ꢁ 0.13
1.04 ꢁ 0.04
1.16 ꢁ 0.10
0.80 ꢁ 0.05
0.70 ꢁ 0.05
0.70 ꢁ 0.05
0.60 ꢁ 0.10
0.70 ꢁ 0.05
0.50 ꢁ 0.05
SA5/PMMA
SA10/PMMA
SA15/PMMA
SAE5/PMMA
SAE10/PMMA
SAE15/PMMA
SAMA5/PMMA
SAMA10/PMMA
SAMA15/PMMA
PS
10.0
15.0
5.0
10.0
15.0
5.0
10.0
15.0
0.0
SA5/PS
5.0
SA10/PS
10.0
15.0
5.0
SA15/PS
SAE5/PS
SAE10/PS
10.0
15.0
5.0
SAE15/PS
SAMA5/PS
SAMA10/PS
SAMA15/PS
10.0
15.0
It is well known that the organoclay content has a major
38]
influence on the mechanical properties of nanocomposites.
[
From Figure 1a it is apparent that in the case of PMMA–layered
silicate nanocomposites, the Young’s Modulus significantly
increases with increasing filler content for all swelling agents
used, while the amount of stiffening is dependent on the type of
swelling agent. At a filler content of 15 wt%, the modulus is
increased by about 23% in the case of SA, 24% in the case of SAE,
and 19% in the case of SAMA with respect to pure PMMA.
A similar tendency can be observed for the PS–layered silicate
composites. As shown in Figure 1b, the stiffness of the PS
composites also increases with increasing organoclay content. At
an organoclay content of 15 wt%, the stiffness was enhanced by
about 11% for SA15/PS and 18% in case of both SAE15/PS and
SAMA15/PS with respect to pure PS.
Figure 1. Young’s Modulus of a) PMMA–layered silicate composites and
b) PS–layered silicate composites, prepared with protonated SA (&), SAE
(
ꢂ), and SAMA (~) as swelling agents.
10 wt% of calcium phosphate/layered silicate hybrid
nanoparticles.
The structure of montmorillonite consists of 2D layers formed
by fusing two silica tetrahedral sheets to an edge-shared octahedral
sheet of aluminium hydroxide. Stacking of the layers of clay
In Figure 2 the composites’ critical stress intensity factor K1c is
plotted as a function of organoclay content. For the PMMA
composites (Fig. 2a), no significant decrease of the stress intensity
factor was obtained with increasing content of hybrid nanopar-
ticles. All nanocomposites show a significantly lower stress factor
[
39]
particlesisheldbyweakdipolarorvanderWaalsforces. Figure3
shows the X-ray diffraction patterns of the unmodified clay
compared with that of PMMA or PS composites with a filler
content of 15 wt%. The Ca-bentonite pattern shows a d100 peak at
2Q ¼ 5.98, which corresponds to an interlayer distance of
d ¼ 1.5 nm. In all PMMA composites the 2Q value is shifted
towards smaller angles, reflecting the increase of the interlayer
distancecausedbyintercalationofthevariousswellingagents. The
basal spacing of SA15/PMMA was determined to be 3.5 nm, and
that of SAE15/PMMA to be 3.7 nm. In the case of SAMA15/
PMMA, a shoulder at about 5.1 nm was observed. The degree of
intercalation or exfoliation is not only influenced by the type of the
swelling agent, but also by the filler content. With decreasing
bentonite content, an increasing interlayer spacing was observed.
In Table 2, results of all wide-angle X-ray scattering (WAXS)
investigations are listed. The highest interlayer distances were
found for the samples SAMA/PMMA, in which the swelling agent
ꢀ
3/2
with respect to the value of pure PMMA (1.52 MN m
). The
fracture toughness of respective PS nanocomposites are summar-
ized in Figure 2b. In case of SAE/PS and SAMA/PS, only a very
small influence of the filler content on the K1c was detected. A
different behavior can be observed for the SA/PS samples. The
intensity stress factor of SA/PS runs through a maximum of
ꢀ
3/2
2
.15 MN m
increase of 65% with respect to pure PS (1.31 MN m
higher organoclay contents, the stress intensity factor decreases to
for SA5/PS, representing a fracture toughness
ꢀ3/2
). For
ꢀ3/2
ꢀ3/2
1
.04 MN m
for SA10/PS and 1.16 MN m for SA15/PS. SA/
PS nanocomposites offer the opportunity to obtain materials that
exhibiteithersignificantly increasedtoughnesscombinedwiththe
stiffness of pure PS at low filler contents, or increased stiffness
combinedwiththetoughnessofPSwhenincorporatingmorethan
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Figure 2. Critical stress intensity factor of a) PMMA–layered silicate com-
posites and b) PS–layered silicate composites, prepared with SA (&), SAE
(
ꢂ) and SAMA (~) as swelling agents.
Figure 3. WAXS traces of nanocomposites containing 15.0 wt% hybrid
nanoparticles modified with SA, SAE, and SAMA in comparison to
a) natural Ca-bentonite and PMMA nanocomposites and b) natural Ca-
bentonite and PS nanocomposites.
exhibits polymerizable methacrylate moieties. In this case,
intergallery spacings of up to 5.2 nm were achieved. The WAXS
spectra of the PS–calcium phosphate/layered silicate composites
withfillercontents of15 wt%aredisplayedinFigure3b. Nosignals
were observed in the range between 1 and 5.98 (8.8 and 1.5 nm).
The WAXS spectra suggest that the silicate layers are either
exfoliated in the polymer matrix or exhibit interlayer spacings
higher than 8.8 nm. Samples with lower organoclay contents show
similar behavior in WAXS investigations, regardless of the
swelling agent used (Table 2).
Table 2. Interlayer spacings of PMMA– and PS–calcium phosphate/layered
silicate composites calculated from WAXS measurements.
Swelling agent
Organoclay
d001 in
d001 in
content [wt%]
PMMA [nm]
PS [nm]
The nanocomposite morphologies were examined by means of
transmission electron microscopy (TEM). As shown in WAXS
measurements, the best results of intercalation and of exfoliation
were obtained in the case of nanocomposites with low content of
bentonite using SAE or SAMA as swelling agent. Figure 4 shows
nanocomposites containing 5 wt% SA-swollen bentonite in a
PMMA (Fig. 4a) and a PS matrix (Fig. 4b). Fairly large silicate
particles are visible. In both systems, mostly intercalated
structures were obtained, although no clear reflexes can be
observed in WAXS measurements of SA5/PS. In the case of SA5/
PS, the composite morphology appears to show an ideal
combination of matrix ligaments, single silicate layers, and
SA
5.0
10.0
15.0
5.0
4.3[a]
4.1[a]
3.5
> 8.8[b]
> 8.8[b]
> 8.8[b]
> 8.8[b]
> 8.8[b]
> 8.8[b]
> 8.8[b]
> 8.8[b]
> 8.8[b]
SA
SA
SAE
SAE
SAE
SAMA
SAMA
SAMA
4.9[a]
4.3[a]
3.7
10.0
15.0
5.0
5.2[a]
5.1
10.0
15.0
5.1[a]
[a] Reflexes cannot be assigned exactly and represent approximate values.
[b] Interlayer spacings exceeding 8.8 nm cannot be detected due to overlap
with the primary X-ray beam.
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responsible for the signal at 3.7 nm in WAXS
measurements (Fig. 5a). In SAE5/PS (Fig. 5b),
individual layers as well as stacks of two to four
layers are dispersed within the matrix. Similar
results were obtained for the composite
SAMA5/PMMA (Fig. 5c). While WAXS traces
of the sample SAMA5/PMMA indicate an
intergallery spacing of about 5.1 nm, measure-
ments showed no signals for SAE5/PS. In the
case of SAMA5/PS, parallel arranged indivi-
dual silicate layers are observed (Fig. 5d). With
an average intergallery spacing of more than
5
nm, the 2Q angle in WAXS investigation is
too close to the primary X-ray beam to detect
clear signals.
The exchange of Ca ions by organophilic
ammonium phosphates was accompanied by
precipitation of calcium phosphates, such as
2þ
Figure 4. TEM images of composites swollen in situ with protonated stearylamine: a) SA5/
PMMA and b) SA5/PS.
aggregates of intercalated layers, accounting for additional energy
dissipation during fracture, which leads to the increased fracture
toughness with respect to pure PS.
From Figure 5 it is obvious that composites with swelling
agents, protonated SAE and SAMA, lead to more homogeneously
dispersed hybrid particles. SAE swollen clay in a PMMA matrix
shows large areas with anisotropically distributed fully exfoliated
silicate nanoplatelets. Other silicates form an intercalated system
4 3 4 2
CaHPO orCa (PO ) . Tolocalizethesalts, energydispersiveX-ray
(EDX) line-scan measurements were conducted (Fig. 6). The
silicon signal clearly shows the distribution of silicate as function
of the distance from the origin. Peaks of calcium and phosphorus
are located exclusively in the same positions as silicon. The results
confirm the formation of calcium phosphate/layered silicate
hybrid nanoparticles by precipitation of CaHPO
the interfaces of the silicate nanoplatelets.
4 3 4
and Ca (PO ) at
2
.2. PU and PU-Foam Nanocomposites
Containing Calcium Phosphate/Layered
Silicate Hybrid Nanoparticles via in situ
Swelling
Polyurethane composites were prepared using
a trihydroxy-terminated oligo(propylene oxide)
(
Desmophen PU-IK-01) as monomer, in which
stearyl amine was protonated by phosphoric
acid. Amine andacidcontent wereequimolar to
the Ca-bentonites cation exchange capacity of
ꢀ1
0
.8 meq g . Addition of 4 wt% of Ca-bentonite
resulted in the exchange of stearyl ammonium
2þ
ions with the bentonite’s intergallery Ca ions
and simultaneous precipitation of calcium
phosphate. Consecutive high shear treatment
leads to a uniform dispersion of calcium
phosphate/layered silicate nanoparticles in
the polyol (PU-SA-bent). This dispersion was
crosslinked by the addition of the methylene
diphenyl diisocyanate (Baymidur-KL-3-5002)
without any further purification. Morphology
investigations revealed a homogeneous distri-
bution of swollen inorganic hybrid particles in
the PU matrix. Particle sizes were determined
to be in the range of 20–1500 nm in the case of
PU-SA-bent, while the particle diameters of
nonswollen calcium bentonite in sample PU-
bent were determined to be more than 10 mm
(Fig.7a).Intercalatedstructureswithanaverage
Figure 5. TEM images of nanocomposites swollen in situ with protonated tertiary amines:
a–b) intercalated and exfoliated silicate layers in SAE5/PMMA (a) and in SAE5/PS (b); c–d) higher
intergallery spacings and exfoliated layers in SAMA5/PMMA (c) and SAMA5/PS (d).
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Figure 6. EDX line-scan of SAE10/PMMA: a) scanning vector and b) element distribution as a function of position, showing Ca-phosphates to be located in
close proximity of silicate layers.
interlayer distance of 2.8 nm were observed in
the case of PU-SA-bent (Fig. 7b). One can
assume thattheincorporation ofthequaternary
ammonium ions in the silicate layers led to
massive swelling and intercalation of the
primary particles of the Ca-bentonite. In the
case of PU-bent, the major part of the inorganic
filler settled at the bottom of the mold due to the
ineffective dispersion of the large primary
particles.
Tensile tests were carried out in order to
investigate the mechanical properties of the
PU-composites. The results are displayed in
Table 3. In comparison to the unfilled poly-
urethane (PU-pure), the modulus of the filled
samples increased by about 210% in the case of
PU-bent and 160% in the case of PU-SA-bent.
Polymer composites filled with inorganic
materials often show increased stiffness com-
pared to the corresponding matrix materials.
This stiffening effect is commonly aligned with
[
7]
a decrease in toughness. For elastomeric
materials, toughness can be described with the
elongation at break. While the elongation at
break in PU-bent was in the order of magnitude
of the unfilled sample, it was significantly
enhanced by about 160% for PU-SA-bent. The
extraordinary effect of the swelling procedure
on composite materials’ toughness may be
explained with the different morphologies
observed. Within PU-composites with natural
Ca-bentonite, few large silicates agglomerates
are divided by broad matrix ligaments. The
fracture behaviorof theunfilled matrixmaterial
dominatesthecompositesoverallperformance,
which is reflected in a comparable elongation at
break. Better dispersion of the filler within the
PU matrix, along with enhanced interaction
Figure 7. TEM images of PU composite materials: a) untreated primary bentonite particles
dark) in bulk PU matrix (bright) in PU-bent, b) homogenously distributed hybrid particle
(
fragments showing intercalated silicate layers after swelling in situ with stearyl amine and
phosphoric acid (PU-SA-bent), and c–d) PU foam containing exfoliated and intercalated silicate
layers compatibilized by swelling in situ with polyols featuring protonated amine moieties (PU-f-
3 4
H PO -bent). Ca-bentonite content is 4 wt% in all composite materials.
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Table 3. Mechanical properties of PU-pure, PU-bent, and PU-SA-bent.
functionalized bentonites are highly reactive and can copolymer-
ize in free radical polymerization. Such organoclays are difficult to
handle in conventional multistep nanofiller preparations with
drying temperatures at which polymerization of the methacrylate
groups and crosslinking of the nanoparticles can occur. Moreover,
methacrylate-functional bentonites can copolymerize and account
for the effective attachment of styrene and acrylic polymers to the
nanoparticle surfaces. Such attachment was also achieved for
polyurethanes when protonated amine-functionalized polyether
polyols were used as monomer components. During the cation
exchange of calcium cations for alkyl ammonium ions, the in situ
organophilic modification of bentonite was accompanied by
swelling and precipitation of calcium phosphates exclusively on
the bentonite nanoplatelet interfaces. The resulting novel calcium
phosphate/layered silicate nanoparticles were effectively dis-
persed in the monomer and afforded good adhesion between
nanoparticles and polymer matrix. Moreover, this ‘‘one-pot’’
process enabled effective intergallery polymerization and attach-
ment of polymer chains, which accounted for excellent nano-
particle dispersions, nanoparticle adhesion, and energy dissipa-
tionatthecracktip. Inconclusion, theone-potprocessrepresentsa
very versatile method for the preparation of nanocomposites and
for upgrading and diversifying thermoplastics, engineering
plastics, thermosets, elastomers, adhesives, and coatings.
Toughness/stiffness balances of polymers such as styrenics,
acrylics, and polyurethanes can be improved significantly.
Synergisms appear feasible when hybrid nanoparticles are
dispersed in multiphase polymers such as rubber-toughened
styrenics and fiber-reinforced polymers. Alkyl ammonium
phosphate-functionalized polyols together with calcium bento-
nites represent attractive intermediates for the preparation of
polyurethane nanocomposite and nanoparticle-armored polyur-
ethane foams.
Sample
Young’s modulus [MPa]
Elongation at break [%]
PU-pure
0.7 ꢁ 0.2
1.5 ꢁ 0.2
1.1 ꢁ 0.2
140 ꢁ 25
145 ꢁ 10
220 ꢁ 20
PU-bent[a]
PU-SA-bent
[a] Bentonite precipitated during polymerization process.
strength due to compatibilization, intercalation, and exfoliation
allows additional mechanisms of energy dissipation to become
efficient in PU-SA-bent nanocomposites. The specific changes in
the fracture mechanism that lead to increased toughness were not
further investigated in this work, but it is well known from the
literature that nanocomposites, especially when featuring inter-
calated silicate layers, facilitate energy dissipation in the form of
[
40]
local plastic deformation, crack pinning, and crack deflection.
PU-foam composites containing calcium phosphate/layered
silicate hybrid nanoparticles were prepared using an amine-
functionalized polyether polyol derived from ethylene diamine
and propylene oxide (Lupranol 3402). The tetrafunctional polyol
contains two tertiary amine groups. The polyol was dissolved in
waterandaminegroupswerepartiallyprotonateduponadditionof
phosphoric acid. The cation exchange process took place in the
reactivesolutionbyaddingCa-bentonite. Afterwaterwasremoved,
a PU-foam was prepared upon crosslinking with methylene
3 4
diphenyl diisocyanate (PU-f-H PO -bent). Morphological investi-
gations by scanning electron microscopy (SEM) and TEM revealed
pore sizes in the range of 100 mm. Single silicate layers as well as
particles with diameters up to 300 nm were homogeneously
distributed in the PU matrix, indicating that the untreated
bentonite particles (with diameters of several micrometers) were
exfoliated during the swelling process. The accumulation of the
hydrophilic inorganic filler at the interface air/polymer, typical for
some conventional PU nanocomposite foams, was not observed,
most likely due to the very high wettability of the polyol. Exfoliated
as well as intercalated structures with an average interlayer
distance of 1.9 nm were found (Fig. 7c,d).
This solvent-free ‘‘one-pot’’ process with its ‘‘functionalized
nanoparticle tool box’’ can be exploited for many formulated
systems based upon bulk polymerization of monomers such as
styrene, acrylics, isocyanates, and also other thermoset resins.
4. Experimental
Materials: Ca-bentonite EXM 918, which represents a natural mon-
tmorillonite, was supplied by S ¨u d-Chemie, Moosburg, Germany. The
3
. Conclusions
2þ
negative charge of the silicate layers is compensated by Ca ions in the
ꢀ1
Aversatile ‘‘one-pot’’ process for the formation of nanocomposites
containing calcium phosphate/bentonite hybrid nanoparticles
was developed. Amine functionalization, protonation with
phosphoric acid, cation exchange with calcium bentonite,
intercalation, exfoliation, and polymerization were performed in
bulk monomers such as styrene, methyl methacrylate, and polyols
without requiring solvents. Moreover, extensive purification
associated with conventional organoclay preparation and special
milling and dispersing processes were eliminated. Various
primary and tertiary amines proved to be very effective swelling
agents for calcium bentonite when the amines were protonated
with phosphoric acid in monomer solution. Methacrylate-
functionalized amines prepared in situ via Michael Addition of
stearyl amine with 2-methacryloxyethyl acrylate and protonated
with phosphoric acid produced methacrylate-functionalized
organoclay by means of cation exchange. Such methacrylate-
interlayer galleries. The cation exchange capacity (CEC) was 0.8 meq g
The interlayer spacing calculated from WAXS was 1.47 nm. Methyl
.
methacrylate and styrene (Fluka, Germany) were distilled under reduced
˚
pressure and stored over a 3 A molecular sieve. The following commercial
products were used as received: Ethoquad C12 (Akzo-Nobel, Germany);
Lupranol 3402, Lupranat M50, and Accelerator NL64-10P (BASF,
Germany); Desmophen PU21-IK-01 and Baymidur KL-3-5002 (Bayer,
Germany); stearyl amine (SA), triethyl amine, and 2-hydroxyethyl
0
methacrylate (Aldrich, Germany); 2,2 -azoisobutyronitrile (AIBN), methyl
acrylate and acryloyl chloride (Fluka, Germany); phosphoric acid 85%
(Merck, Germany); [2-(methacryloy-l-oxy)-ethyl]trimethyl ammoniumchlor-
0
ide (MOETACl; R o¨ hm, Germany); and 2,2 -azobis-(isobutyl amidine
hydrochloride) (AIBA; WAKO, Germany).
Synthesis of Swelling Agent, Stearyl Di(methoxycarbonylethyl) Amine
SAE): Methyl acrylate (29.20 g, 0.340 mol) was dissolved in a toluene/
(
methanol mixture (150 mL, 16/10). To the resulting solution, stearyl amine
(45.75 g, 0.170 mol), dissolved in a toluene/methanol mixture (500 mL, 16/
10), was added at 0–5 8C under stirring within 3 h. After completion of the
1784
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Preparation of PMMA– and Polystyrene–Calcium
Phosphate/Layered Silicate Nanocomposites: PMMA–
and PS–calcium phosphate/layered silicate nanocompo-
sites with various swelling agents (SA, SAE, SAMA) and
various organoclay contents (5, 10, 15 wt%) were
prepared according to Table 4. Phosphoric acid
(equimolar to amine content) was dissolved in MMA
or styrene. The respective swelling agent (SA, SAE or
SAMA) and Ca-bentonite were added. The mixture was
stirred for 10 min using a magnetic stirrer and then
dispersed using an Ultrathurax (10 min, 18 000 rpm).
AIBN, dissolved in MMA or styrene, was added. The
mixture was degassed and polymerized between two glass
plates for 48 h at 60 8C in a water bath.
Scheme 2. Structure of SAE.
Preparation of Polyurethane–Calcium Phosphate/
Layered Silicate Nanocomposites: A mixture of trihydroxy-terminated
oligo(propylene oxide) with a number average molar mass of 3800 g mol
addition, the mixture was allowed to react for a further 24 h at room
temperature. Then the solvent was evaporated and the product (see
Scheme 2) was dried under vacuum. Yields of up to 93% were obtained.
ꢀ1
(192 g, Desmophen PU-IK-01), stearyl amine (1.36 g), phosphoric acid
(0.58 g), and Ca-bentonite (6.36 g) was dispersed using an Ultrathurax
(IKA) at 18 000 rpm for 10 min. Amine and acid content were equimolar to
1
H-NMR (300 MHz, CDCl
18), 1.50–1.26 (2-17), 0.88 (1); C-NMR (75.4 MHz, CDCl
21), 55.2 (18), 52.9 (22), 51.1 (19), 35.8 (20), 24.0–33.9 (2–17), 15.4 (1).
Synthesis of Swelling Agent, Stearyl Di(2-methacryloxyethyl carbonylethyl)
3
): d ¼ 3.69 (22), 2.88 (19), 2.60 (20), 2.53
13
3
(
(
): d ¼ 174.6
ꢀ1
the Ca-bentonites cation exchange capacity of 0.8 meq g . Traces of water
were removed at 100 8C and 3 mbar using a Molteni Planimax high shear
mixer. After cooling to room temperature, the mixture was blended
together with diisocyanato diphenyl methane isomers (17.6 g, 55 wt% 2,4-
Amine (SAMA): 2-Methacryloxyethyl acrylate (EGAMA) was prepared
according to a synthesis reported by Luchtenberg and Ritter [41]. In short,
acryloyl chloride was slowly added to a solution of 2-hydroxyethyl
methacrylate and triethyl amine in toluene at 0–5 8C. After completion
of the reaction, precipitated triethyl ammonium chloride was removed by
filtration and the solvent was evaporated. The resulting product was
isolated by fractional vacuum distillation. Yields of up to 83% were
obtained.
0
isomer and 45 wt% 4,4 -isomer, isocyanate functionality: 2.0, Baymidur KL-
3-5002). After stirring for 3 min at room temperature and 3 mbar to remove
air bubbles, the mixture was poured into a mold (200 mm ꢃ 200 mm ꢃ
4 mm). Curing was performed at 80 8C for 24 h in a vented oven (PU-SA-
bent). For comparison, the pure matrix material (PU-pure) and a sample
without organophilic compatibilization (PU-bent) were prepared.
For preparing polyurethane foams, a tetrafunctional polyether polyol
derived from ethylene diamine and propylene oxide with an average molar
EGAMA (50.40 g, 0.273 mol) was dissolved in a toluene/methanol
mixture (180 mL, 16/10). To the resulting solution, stearyl amine (36.77 g,
ꢀ1
0.137 mol), dissolved in a toluene/methanol mixture (600 mL, 16/10), were
mass of 480 g mol (18.8 g, Lupranol 3402) were diluted in distilled water
(50 mL) and H PO (85%, 1.01 g, 0.8 meq per g Ca-bentonite). Ca-
added at 0–5 8C under stirring within 2 h. After completing the addition, the
mixture was allowed to react for a further 24 h at room temperature. The
solvent was evaporated, and the product (see Scheme 3) was dried under
vacuum. Yields of up to 94% were obtained.
1
3
4
bentonite EXM 918 (2.44 g) was added to the resulting mixture. After
dispersion with an Ultrathurax (IKA) for 10 min at 18 000 rpm, the water
was removed at 110 8C and 7 mbar using a Molteni Planimax high shear
mixer. Tegostab B 8443 (0.4 g), N,N-dimethyl benzylamine (0.4 g, NL64-
H-NMR (300 MHz, CDCl
3
): d ¼ 6.07 (27b), 5.53 (27a), 4.27 (22, 23),
13
0
2
(
(
.71 (19), 2.39 (18, 20), 1.89 (26), 1.20–1.24 (2–17), 0.82 (1); C-NMR
75.4 MHz, CDCl
): d ¼ 174.0 (21), 169.2 (24), 137.8 (25), 127.7 (27), 64.2
23), 63.8 (22), 55.6 (18), 51.0 (19), 34.0 (20), 24.5–31.5 (2–17), 20.1 (26),
5.9 (1).
10P), 4,4 -diisocyanato diphenylmethane (30 g, functionality: 2.9, Lupranat
M50) and distilled water (0.5 g) were added. The obtained PU-foam (Pu-f-
3
H
3
4
PO -bent) was stored at room temperature. For comparison, a sample
1
without phosphoric acid was prepared (Pu-f-bent).
Scheme 3. Preparation of SAMA via Michael Addition.
Adv. Funct. Mater. 2010, 20, 1778–1786
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1785

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Table 4. Recipes for the preparation of nanocomposites of PMMA and PS containing calcium phosphate/layered silicate hybrid nanoparticles.
Chemicals
Sample, X ¼ filler content [wt%]
SAE X/PMMA or PS
SA X/PMMA or PS
SAMA X/PMMA or PS
PMMA or PS
5
10
15
5
10
15
5
10
15
MMA or Styrene [g]
285
1.14
2.66
–
270
2.27
5.30
–
255
3.41
7.98
–
285
1.02
–
270
2.04
–
255
3.07
–
285
0.92
–
270
1.83
–
255
2.75
–
270
3.41
–
H
3
PO
SA [g]
SAE [g]
4
[g] [a]
3.92
–
7.83
–
11.75
–
–
–
–
–
SAMA [g]
–
–
–
5.07
9.93
0.57
2.28
10.15
19.85
0.54
2.16
15.22
29.87
0.55
2.04
–
Ca-bentonite [g] [b]
AIBNMMA [g] [c]
AIBNPS [g] [d]
12.34
0.57
2.28
24.68
0.54
2.16
37.02
0.51
2.04
11.08
0.57
2.28
22.17
0.54
2.16
33.25
0.51
2.04
–
0.54
2.16
a] nacid:namine ¼ 1. [b] CEC: 0.8 meq gꢀ . [c] 0.2 wt% based on MMA. [d] 0.8 wt% based on styrene.
1
[
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13
3
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1786
ß 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
Adv. Funct. Mater. 2010, 20, 1778–1786