Synthesis and montmorillonite-intercalated behavior of dendritic surfactants

Article abstract of DOI:10.1039/b600544f

A series of novel dendrons serving as surfactants for surface modification of montmorillonite (MMT) have been synthesised via a convergent approach. The most distinguishing characteristic of the prepared dendrons is the long alkyl chain at the periphery.

Full text of DOI:10.1039/b600544f

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PAPER
www.rsc.org/materials | Journal of Materials Chemistry
Synthesis and montmorillonite-intercalated behavior of dendritic
surfactants
Cheng-Che Tsai,a Tzong-Yuan Juang,a Shenghong A. Dai,a Tzong-Ming Wu,*a Wen-Chiung Su,b
Ying-Ling Liuc and Ru-Jong Jeng*a
Received 13th January 2006, Accepted 1st March 2006
First published as an Advance Article on the web 20th March 2006
DOI: 10.1039/b600544f
A series of novel dendrons serving as surfactants for surface modification of montmorillonite
(MMT) have been synthesised via a convergent approach. The most distinguishing characteristic
of the prepared dendrons is the long alkyl chain at the periphery. This would certainly bring about
good solubility, high yield and easy purification. By simple acidification of the ‘‘head group’’
upon the dendron and then ion-exchange with Na+-MMT, the interlayer distance could be
enlarged significantly between the layered silicates. The organic/inorganic ratio determined by
thermogravimetric analysis (TGA) is in the range of 60%–91%. This also confirms the occurrence
of dendron ion-exchange with Na+-MMT. Transmission electronic microscopy (TEM) reveals the
intercalated and exfoliated morphology of the dendron-modified MMTs. Modification using
the generation 2 dendritic surfactant (G2) with molecular weight (Mw) = 2887.5 has achieved the
exfoliated morphology in this work.
influence on the intercalated technique. Singh and Balazs24
1 Introduction
had studied the effect of polymer architecture in polymer/clay
Dendritic macromolecules with architectural and functional
nanocomposites by using linear and branching units. At the
superiority have been attracting great interest.1–3 Unlike linear
same molecular weight, a linear structure tends to be inter-
polymers and traditional branched polymers, dendrimers are
calated; increasing the extent of branching units decreases the
monodisperse and highly branched macromolecules that
free energy of interaction between surfactants and MMT, so
consist of multiple functional groups at the periphery, which
the layered silicates tend to be exfoliated. Furthermore, the
are synthesized through covalent bonds with a divergent or
hydrophilicity property at the periphery of the dendron was
convergent strategy.4,5 Because of these unique advantages,
utilized as a surfactant in an organically-modified layered
their molecular size, shape and nature can be precisely con-
silicate system. However, the shape and size of dendrons did
trolled. Dendrimers are nanostructures with specific, precise
and predictable physical properties (lower intrinsic viscosity
and higher solubility)6 that make them especially useful for
various applications including drug delivery,7–9 self-assembly
systems,10–13 electro-optical materials,14–17 catalysts,18–20 etc.
Montmorillonite (MMT) is the most commonly used
layered silicate with high aspect ratio.21 Employing suitable
surfactants with simple ion-exchange reactions could achieve
different natures of morphologies: intercalated, intercalated-
and-flocculated and exfoliated.22 Many studies indicate that
incorporating very small amounts of layered silicate into a
polymer matrix can greatly improve the mechanical and
physical properties.23 In general, most of the traditional
surfactants are linear forms; even though the interlayer
d-spacing can be increased, they still retain the intercalated
features of nanocomposites. Theoretically incorporating a 3-D
structure into a layered silicate should have a significant
not have any discernable influence on the intercalation or
exfoliation behavior.
Unlike other previous studies, Ma˚nson and co-workers25
exploited the external multiple hydroxyl groups of a dendritic
structure as a polar feature, and there were no cation-exchange
reactions with layered silicate. Their results only showed
intercalated silicates. Furthermore, Simanek and co-workers26
identified another dendritic surfactant based on triazines and
diamines since the dendritic part did not fully stretch out
between layered silicates but hung outside. Therefore, they
believed this dendritic structure could not afford expectable
exfoliation, but only increased the d-spacing slightly.
In this report, we suggest a new method to modify the
surface of MMT using polyurea/malonamide dendrons
synthesized previously in our laboratory using [4-isocyanato-
49-(3,3-dimethyl-2,4-dioxo-acetidino)diphenylmethane] (MIA)
and diethyltriamine (DETA) without painstaking protection–
deprotection or activation methodology.27 The same efficient
convergent route was introduced to successfully synthesize a
series of dendrons consisting of decyl chain groups on the
exterior with high yield and easy purification (Scheme 1).
aDepartments of Chemical Engineering and Materials Engineering,
National Chung Hsing University, Taichung 402, Taiwan, ROC
E-mail: rjjeng@dragon.nchu.edu.tw
bChung-Shan Institute of Science and Technology, Lungtan, Taoyuan
325, Taiwan, ROC
Subsequently,
a secondary aliphatic amine of different
generation dendrons (G1–G3) was utilized as the hydrophilic
segment by simple acidification, with a long alkyl chain as
the hydrophobic segment. Therefore, the resultant dendritic
cDepartment of Chemical Engineering and R&D Center for Membrane
Technology, Chung Yuan Christian University, Taoyuan 320, Taiwan,
ROC
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Scheme 1 Synthesis of G1, G2 and G3 dendrons from MIA and 1-decanol.
surfactants were incorporated into MMT to form an
organically modified layered silicate system. Herein we could
effortlessly achieve large d-spacings of intercalation or direct
exfoliation by different 3-D structures and hydrophobicity of
dendritic surfactants. Dendron-containing hybrids have been
explored in various applications, such as nano-containers,
reactive scaffolds and drug deliveries.28–30 Via respective
incorporation of the different generation dendrons onto
layered MMT, the interlayer distance of the intercalated
hybrid can be precisely controlled to form a unique vessel.
Moreover, exfoliated samples can be applied to serve as
surface modifying agents or metal-encapsulated templates.31,32
IR measurements were performed on a Perkin Elmer Spectrum
One Fourier transform infrared (FTIR) spectrometer.
Elemental CHN analysis was performed on a Heraeus CHN-
OS Rapid Analyzer. Thermal analysis was performed in N2 on
a TA Instruments DSC2010 at a heating rate of 10 uC min21
.
Thermogravimetric analysis was performed by a Seiko SSC-
5200 Thermogravimetric Analyzer (TGA) at a heating rate of
10 uC min21 under nitrogen. Fast atom bombardment mass
spectrometry (FAB MS) analysis was performed on a JEOL
JMS SX/SX 102A mass spectrometer equipped with the
standard FAB source, whose upper limit for measuring
molecular weight is 2000. Matrix-assisted laser desorption
ionization with time of fight (MALDI-TOF) mass spectra
were recorded on a Voyager DE-PRO (Applied Biosystems,
Houston, TX) equipped with a nitrogen laser (337 nm)
operating in linear detection mode to generate positive ion
spectra with dithranol as a matrix, dimethyl sulfoxide (DMSO)
as a solvent and sodium trifluoroacetate as an additive agent.
Gel permeation chromatography (GPC) was performed in
THF as an eluent with a Waters Apparatus equipped with
Waters Stygel columns with a refractive index (RI) detector
and polystyrene calibration. The d-spacing of the intercalated
MMT was analyzed by an X-ray powder diffractometer
(XRD, Shimadzu SD-D1 using a Cu target at 35 kV,
30 mA). The d-spacing of the intercalated MMT was
2 Experimental
Methylene di-p-phenyl diisocyanate (MDI), isobutyryl chlor-
ide, triethylamine, 1-decanol and diethyltriamine (DETA) were
purchased from Aldrich and Acros, and used without further
purification. Tetrahydrofuran (THF) was distilled under
nitrogen from sodium/benzophenone before use. Na+-MMT
was supplied by Nanocor Co., is a sodium type with a cationic
exchange capacity (CEC) of 1.20 mequiv g21 and a surface
area of 750 m2 g21
.
1H NMR spectra were taken on a Varian Gemini-200
FT-NMR spectrometer with chloroform-d6 and DMSO-d6.
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analyzed by using Bragg’s equation (nl = 2d sin h). The
value for n = 1 was calculated from the observed values for
n = 2, 3, 4, etc. Transmission electronic microscopy (TEM) was
performed on a Zeiss EM 902A and operated at 80 kV and
samples of approximately 70 nm were microtomed at room
temperature.
C62H89N7O8 requires C, 70.46; H, 7.66; N, 9.13%); nmax/cm21
3300 (NH), 1856 (CLO), 1738 (CLO), 1700 [(NH)CLO(O)];
dH(DMSO; 200 MHz) 0.83 (6H, t, CH3), 1.23 (28H, s, CH2),
1.38 (12H, s, CH3), 1.44 (6H, s, CH3), 1.57 (4H, t, CH2), 3.30
(8H, m, CH2(N)), 3.86 (4H, s, Ar–CH2–Ar), 3.94 (2H, s, Ar–
CH2–Ar), 4.02 (4H, t, CH2), 7.06–7.62 (24H, m, Ar–H); m/z
(FAB) 1380 (M+).
Synthesis of MIA
Synthesis of G2 (Scheme 1)
Synthesis of MIA was performed according to the litera-
ture.27,33 A solution of triethylamine (45 g, 0.445 mol) in
xylene (100 mL) was added to a solution of methylene
di-p-phenyl diisocyanate (MDI) (125 g, 0.5 mol) and isobutyryl
chloride (38.6 g, 0.362 mol) in the same solvent (250 mL). The
reaction mixture was refluxed for 7 h and then cooled to room
temperature. The resulting solution was filtered to remove the
quaternary salt and concentrated to about 50 mL. The product
was then crystallized from cyclohexane to give MIA (14.9 g,
33%) as white powder (Found: C, 71.32; H, 5.17; N, 8.96.
C19H16N2O3 requires C, 71.24; H, 5.03; N, 8.74%); nmax/cm21
2260 (NLCLO), 1852 (CLO), 1731 cm21 (CLO); dH(CDCl3;
200 MHz) 1.40 (6H, s, CH3), 3.87 (2H, s, Ar–CH2–Ar), 6.90–
7.69 (8H, m, Ar–H); m/z (FAB) 320 (M+).
To a solution of G1.5 (5.1 g, 3.69 mmol) in dry THF (50 mL),
DETA (0.187 g, 1.81 mmol) was added. The solution was
stirred at 60 uC under a nitrogen atmosphere for 2 h. The
solvent was evaporated and the product was purified by silica
gel chromatography with acetone to give G2 (4.8 g, 90%)
as yellow powder (Found: C, 69.23; H, 8.18; N, 10.33.
C166H223N21O22 requires C, 69.60; H, 7.85; N, 10.27%);
n
max/cm21 3300 (NH), 1700 [(NH)CLO(O)], 1650 cm21
[CLO(NH)]; dH(DMSO; 200 MHz) 0.83 (12H, t, CH3), 1.23
(56H, s, CH2), 1.38 (36H, s, CH3), 1.57 (8H, t, CH2), 2.55
(4H, t, CH2(NH)), 3.30 (20H, m, CH2(N)), 3.86 (12H, s,
Ar–CH2–Ar), 4.02 (8H, t, CH2), 7.06–7.62 (48H, m, Ar–H);
m/z (MALDI-TOF) 2887.5 (M + Na+).
Synthesis of G0.5 (Scheme 1)
Synthesis of G2.5 (Scheme 1)
Decanol (2.469 g, 15.60 mmol) was added to a solution of
MIA (5 g, 15.63 mmol) in dry THF (100 mL). The solution
was stirred at 80 uC under nitrogen for 3 h. The solvent was
evaporated and purified by recrystallization from cyclohexane
to give G0.5 (7.1 g, 95%) as white powder (Found: C, 72.70;
H, 8.19; N, 6.18. C29H38N2O4 requires C, 72.77; H, 8.00; N,
5.85%); nmax/cm21 3300 (NH), 1856 (CLO), 1738 (CLO),
1700 cm21 [(NH)CLO(O)]; dH(DMSO; 200 MHz) 0.83 (3H, t,
CH3), 1.23 (14H, s, CH2), 1.38 (6H, s, CH3), 1.57 (2H, t, CH2),
3.86 (2H, s, Ar–CH2–Ar), 4.02 (2H, t, CH2), 7.06–7.62 (8H, m,
Ar–H); m/z (FAB) 478 (M+).
To a solution of G2 (5 g, 1.73 mmol) in dry THF (50 mL),
MIA (0.56 g, 1.75 mmol) was added. The solution was stirred
at 60 uC under a nitrogen atmosphere for 2 h. The solvent
was evaporated and the product was purified by silica gel
chromatography with ethyl acetate to give G2.5 (4.7 g, 86%)
as yellow powder (Found: C, 70.09; H, 7.09; N, 9.90.
C185H239N23O25 requires C, 69.76; H, 7.56; N, 10.11%);
n
max/cm21 3300 (NH), 1856 (CLO), 1738(CLO), 1700
[(NH)CLO(O)]; dH(DMSO; 200 MHz) 0.83 (12H, t, CH3),
1.23 (56H, s, CH2), 1.38 (36H, s, CH3), 1.44 (6H, s, methyl),
1.57 (8H, t, CH2), 3.30 (24H, m, CH2(N)), 3.86 (12H, s,
Ar–CH2–Ar), 3.94 (2H, s, Ar–CH2–Ar), 4.02 (8H, t, CH2),
7.06–7.62 (48H, m, Ar–H); m/z (MALDI-TOF) 3208.1
(M + Na+).
Synthesis of G1 (Scheme 1)
To a solution of G0.5 (5.1 g, 10.66 mmol) in dry THF (50 mL),
DETA (0.539 g, 5.22 mmol) was added. The solution was
stirred at 60 uC under a nitrogen atmosphere for 2 h. The
solvent was evaporated and purified by recrystallization from
acetone to give G1 (5.3 g, 95%) as white powder (Found: C,
69.82; H, 8.66; N, 9.67. C62H89N7O8 requires C, 69.82; H, 8.66;
N, 9.67%); nmax/cm21 3300 (NH), 1700 [(NH)CLO(O)], 1650
[CLO(NH)]; dH(DMSO; 200 MHz) 0.83 (6H, t, CH3), 1.23
(28H, s, CH2), 1.38 (12H, s, CH3), 1.57 (4H, t, CH2), 2.55 (4H,
t, CH2(NH)), 3.30 (4H, m, CH2(N)), 3.86 (4H, s, Ar–CH2–Ar),
4.02 (4H, t, CH2), 7.06–7.62 (16H, m, Ar–H); m/z (FAB)
1060 (M+).
Synthesis of G3 (Scheme 1)
To a solution of G2.5 (5.1 g, 1.59 mmol) in dry THF (50 mL),
DETA (0.080 g, 0.78 mmol) was added. The solution was
stirred at 60 uC under a nitrogen atmosphere for 2 h. The
solvent was evaporated and the product was purified by silica
gel chromatography with acetone to give G3 (4.5 g, 88%)
as yellow powder (Found: C, 69.21; H, 8.26; N, 10.16.
C166H223N21O22 requires C, 69.39; H, 7.65; N, 10.60%);
n
max/cm21 3300 (NH), 1700 [(NH)CLO(O)], 1650 cm21
[CLO(NH)]; dH(DMSO; 200MHz) 0.83 (24H, t, CH3), 1.23
(112H, s, CH2), 1.38 (84H, s, CH3), 1.57 (16H, t, CH2), 2.55
(4H, t, CH2(NH)), 3.30 (52H, m, CH2(N)), 3.86 (28H, s,
Ar–CH2–Ar), 4.02 (16H, t, CH2), 7.06–7.62 (96H, m, Ar–H);
m/z (MALDI-TOF) 6496.3 (M + Na+).
Synthesis of G1.5 (Scheme 1)
To a solution of G1 (5 g, 4.72 mmol) in dry THF (50 mL),
MIA (1.52 g, 4.75 mmol) was added. The solution was stirred
at 60 uC under nitrogen for 2 h. The solvent was evaporated
and the product was purified by silica gel chromatography
with 2 : 1 ethyl acetate–hexane to give G1.5 (5.5 g, 86%)
as yellow powder (Found: C, 69.96; H, 7.97; N, 9.19.
Acidification of dendritic surfactant
All of the dendrons (G1–G3) have structural characteristics
such as secondary aliphatic amine located on one end and long
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alkyl chains on the other. For the G1, G2 and G3 dendritic
surfactant solutions, the formulations of each were prepared
by adding 254, 686 and 1553 mg, respectively, of the dendrons
dissolved in 20 mL THF, followed by acidification of the
secondary aliphatic amine by adding hydrochloric acid (37%
in water, 24 mg, 0.24 mmol).
Modification of organoclay
A series of organoclays were prepared from each dendron.
Typical experimental procedures are described below. First,
clay suspensions were prepared by adding 20 mL of distilled
water to 200 mg of MMT at 80 uC and stirring for 24 hours
until fully swollen. Afterward, different generations (G1–G3)
of dendritic surfactant solutions were added. The organic
solvents were completely miscible with water. Finally, the
suspensions were stirred for 24 hours at 80 uC. The resulting
agglomerated precipitate was collected, and washed with water
and with THF twice to thoroughly remove any residual ions
and non-exchanged dendritic surfactants. The dendron–clay
materials (G1/MMT–G3/MMT) were dried in a vacuum oven
at 80 uC for 24 h and characterized by using XRD, TEM, TGA
and DSC.
Fig. 2 Infrared spectrum of G1.
Fig. 2, the peaks at 1738 cm21 and 1856 cm21 (CLO stretching
of azetidine-2,4-dione) disappeared, and a peak at 1675 cm21
(CLO stretching of malonamide) appeared. The 1H NMR
spectrum (DMSO-d6) of G1 exhibited chemical shifts at
3.30 (4H, m, CH2(N)), 3.86 (4H, s, Ar–CH2–Ar) and 4.02
(4H, t, CH2). The ratio of integrated areas conformed to the
number of hydrogens. The syntheses and analysis of G1.5–G3
were also performed in the same manner.
3 Results and discussion
In the synthesis of G0.5, MIA and 1-decanol were mixed at
80 uC with a molar ratio of 1 to 1. The addition reaction was
completed with a high yield of 95%, and monitored by FTIR.
As shown in Fig. 1, the peak at 2260 cm21 (NLCLO absorp-
tion of isocyanate) disappeared, and a peak at 1700 cm21
(CLO stretching of urethane) appeared. The 1H NMR
spectrum (DMSO-d6) of G0.5 exhibited chemical shifts at
3.86 (2H, s, Ar–CH2–Ar) and 4.02 (2H, t, CH2). The ratio of
integrated areas conformed to the number of hydrogens.
Subsequently, G1 was synthesized by ring-opening addition
reaction of G0.5 with DETA with an inward secondary amine
at the focal point and the reaction was completed at 60 uC with
a high yield of 95%, and monitored by FTIR. As shown in
The chemical structures of all fabricated specimens were
1
fully confirmed by H NMR, elemental analysis (CHN), and
MALDI-TOF spectroscopy. In one example, Fig. 3 shows
the mass spectrum of third generation dendron G3. In this
mass spectrum, only one peak is observed at m/z = 6496.3
(M + Na+), which is consistent with the theoretical molecular
weight. Although MALDI-TOF offers much more accurate
molecular weight, GPC provides invaluable information
confirming the purity of these dendrons, and their globular
conformation in solution.34 Molecular weights and thermal
properties of the dendrons are provided in Table 1. The
polydispersity according to GPC still maintains a narrow
range (PDI = 1.02–1.03), which is further evidence of a
monodisperse distribution. For highly branched or globular
materials,34,35 the relatively significant difference in molecular
weights obtained from MALDI-TOF and GPC is due to
the more pronounced change of hydrodynamic volume as
Fig. 1 Infrared spectrum of G0.5.
Fig. 3 MALDI-TOF MS spectrum of G3.
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Table 1 Characteristics, polydispersities and thermal properties for
dendritic compounds
Compound Calcd. Mw Mw Mw Mn PDIb Td Tm
Tg
a
b
b
c
d
G0.5
G1
G1.5
G2
G2.5
G3
478.6
1060.4
1380.8
2864.7
3185.0
6473.2
478 565 547 1.03 260 99 ND
1060 1711 1654 1.03 234 163 ND
1380 2099 2054 1.02 272 NDe 45
2864 3996 3917 1.02 265 ND 65
3185 4396 4283 1.02 254 ND 82
6473 8635 8465 1.02 254 ND 74
b
a
Determined by FAB MS and MALDI-TOF MS. Determined
by GPC analysis in THF (calibration with polystyrene standards).
5% weight loss. Melting point. Not detectable.
c
d
e
compared to linear polymers. The degradation temperatures
(Td) were in the range of 234–272 uC and only compounds
G0.5 and G1 exhibited melting temperatures (Tm) at 99 and
163 uC, respectively. As the molecular weight and branching
units increased, the other compounds (G1.5–G3) only revealed
the presence of a glass-transition temperature (Tg) without any
trace of melting temperature. There are two factors which
influence the Tg value of a dendritic structure. Firstly, the
increase of branching units leads to an increase of Tg, due to
the decreased flexibility of adjacent chain segments. However,
the increasing number of end groups diminishes the Tg because
of the larger conformational flexibility of the end groups
compared to segments of the dendritic core. The former factor
affects the increasing tendency of Tg from 45 to 82 uC (G1.5 to
G2.5). In addition, as a result of increasing number of end
groups, the latter factor has higher priority in competition.
Consequently the Tg of G3 decreases to 74 uC. This
phenomenon was also observed in previous reports.27,36 The
solubility of each dendron was qualitatively analyzed in
various solvents, and the results are summarized in Table 2.
All of them exhibited good solubility in a variety of polar
solvents that can be attributed to the flexible alkyl chains at the
periphery. Even the higher generation dendron still preserves
excellent solubility. All the above evidence indicates that a
series of dendrons with high yield, excellent solubility and easy
purification have been successful synthesized.
Fig. 4 XRD patterns for various dendritic surfactant/MMT blends
and pristine MMT.
The X-ray diffraction data of the pristine and organically-
modified MMTs are shown in Fig. 4. In this spectrum, the
d-spacings of G1, G2 and G3 modified-MMTs based on
Bragg’s law (nl = 2d sinh and the observed values for n =
˚
2, 3, 4, etc.) were 40 A, 67 A and 38 A, respectively. The
˚
˚
˚
addition of G1 into MMT enlarged the d-spacing to 40 A
which was three times higher than that of pristine MMT. This
result indicates that the amphiphilic dendritic surfactants
could easily intercalate layered silicates. When the molecular
weight increases to almost double (G2) or triple (G3) as
compared to that of G1, the d-spacings of G2/MMT and
G3/MMT would be expected to be larger than that of
˚
G1/MMT. The d-spacing of G2/MMT increased to 67 A and
was almost two times larger than that of G1/MMT. As
anticipated, the number of hydrophobic end groups and the
solidly packed volumes of dendritic molecules increased, so the
modified MMT could achieve larger d-spacing effortlessly.
˚
However, the d-spacing of G3/MMT only reached 38 A
against prior anticipation. In order to confirm the previous
assumptions observed by XRD patterns, TEM was utilized to
visualize the spatial distribution among the modified MMTs.
As shown in Fig. 5a, the TEM micrograph of G1/MMT
indicates ordered intercalation between oriented silicate layers
Three generations of dendrons (G1–G3) were acidified by an
equimolar ratio of hydrochloric acid in THF and then formed
amphiphilic dendritic surfactants with a hydrophilic part
(secondary amine salt) on one end and a hydrophobic part
(long alkyl chain) on the other. After mixing these surfactants
and MMT in solution, organically-modified MMTs were
prepared by ion-exchange reactions and then characterized by
using XRD, TEM, TGA and DSC.
˚
with well-defined d-spacing (40 A) corresponding to the
analysis of the XRD pattern. Amazingly, the micrographs of
G2/MMT either on widespread scale (Fig. 5b) or particule
Table 2 Solubility behavior for dendritic compounds
Sample
Toluene
Chloroform
THF
Acetone
Ethyl acetate
DMSO
Ethanol
Methanol
G0.5
G1
G1.5
G2
G2.5
G3
—
—
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
—
—
+–
+–
+–
+–
—
—
+–
+–
+–
+–
+–
+
—
+
—
+
+
+
+
+
+
+
+
+
+
+
+
+
a
Qualitative solubility was determined with 10 mg of sample in 1 mL of solvent. +: soluble at room temperature; +–: soluble after heating to
60 uC; –: insoluble even on heating to 60 uC.
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of G3/MMT (Fig. 5d) revealed both intercalated and
exfoliated morphology, but the d-spacing was even equal to
˚
or larger than 38 A as calculated from the XRD pattern. At
the intercalated region (left arrow of Fig. 5d), only a few
layered silicates were stacked without regular distribution. The
phenomenon might result from increased steric hindrance
arising from the high molecular weight dendritic surfactant.
Consequently, part of G3 surfactants can not fully stretch
and intercalate layered silicates.26 Moreover, random and dis-
ordered single silicate platelets were found in more than half of
the region (right arrow of Fig. 5d). Conceptual diagrams of
MMTs intercalated by different generation dendrons (G1–G3)
are illustrated in Scheme 2. A rough estimation of the length of
31
the extended alkyl chain is 14 A and the length of the
˚
˚
building block is about 16 A. The dimensions of different
˚
˚
˚
generation dendrons (G1–G3) are 30 A, 46 A, and 62 A,
respectively. According to prior X-ray results shown in Fig. 4
and the estimation of the molecular dimensions, we assumed
that the conformation of G1/MMT was ordered intercalation
containing G1 monolayers between layered silicates.37,38 For
the G2/MMT series, as the periphery groups increase, the
interpenetration of dendrons between silicate layers can be
anticipated. Therefore the interlayer distance of G2/MMT is
larger than that of the G2 monolayer. As the periphery groups
continuously increase, a larger interlayer distance is expected.
But the increasing periphery groups also induced significant
steric hindrances of dendritic surfactants and parts of G3
surfactant are exposed to the exterior of layered silicates. Thus,
˚
the interlayer distance for G3/MMT was only 38 A. Based on
the above, this series of dendritic surfactants are effective in
modifying MMT, especially G2/MMT.
As shown in Table 3, the organic/inorganic ratio was
measured by TGA, conforming to the result calculated by
CEC. Hence it is evident that this series of dendritic
surfactants are powerful intercalating agents and could
effortlessly attach to Na+-montmorillonite (yield . 90%).
Furthermore, the modified organoclays could be well dis-
persed into organic solvents like toluene after a long period of
time without precipitation or aggregation. The molecular
mobility of dendritic surfactants attached to layered silicates
was also monitored by DSC (Fig. 6). The DH (enthalpy of
crystallinity at Tm) of pristine G1 was 67.3 (mJ mg21) and
then decreased to 0.3 (mJ mg21
) after attachment to
Na+-montmorillonite. In addition, the DCp (heat capacity
difference at Tg) values of pristine G2 and G3 were
0.438 (mJ uC21 mg21) and 0.471 (mJ uC21 mg21), which
decreased to 0.216 (mJ uC21 mg21) and 0.186 (mJ uC21 mg21),
respectively, after modification. This tendency clearly reveals
that the reduction in both enthalpy of crystallinity at Tm and
heat capacity difference at Tg after modification may be due to
the fact that the dendritic surfactants were restricted by the
layered silicates without sufficient spaces for molecular
relaxation. This conforms to the observations of the earlier
literature.39,40
Fig. 5 TEM micrographs: (a) intercalated morphology of G1/MMT;
(b) exfoliated morphology of G2/MMT (200 nm); (c) exfoliated
morphology of G2/MMT (50 nm); (d) intercalated and exfoliated
morphology of G3/MMT.
4 Conclusion
scale (Fig. 5c) all exhibited exfoliated morphology with
˚
d-spacing larger than 300–500
A
between random and
A new series of dendrons have been synthesized via a
disordered single silicate platelets. However, the micrograph
convergent approach with high yield, excellent solubility and
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Scheme 2 Conceptual representation of conformations for MMTs intercalated by G1, G2 and G3 dendrons.
Table 3 Organic fraction and dispersibility of dendritic compounds
Weight ratio (% w/w) Dispersibilityc
between layered silicates because of the presence of their
geometric architecture and hydrophobic property. The den-
dritic structure even with low molecular weight could
effectively intercalate the layered silicates and enlarge the
Intercalation Based on Based on
CECa
agent
TGAb
H2O EtOH Toluene
˚
d-spacing to 40 A. On the other hand, dendritic structures
G1/MMT
G2/MMT
G3/MMT
a
56/44
77/23
88/12
60/40
75/25
91/9
—
—
—
+–
+–
+–
+–
+
with higher molecular weight would increase the steric
hindrance and hydrophobic effect, which could effortlessly
lead to exfoliated morphology as compared to the traditional
linear surfactants.41 The utilities of these dendrons in other
aspects are under investigation.
+
Values calculated from the following equation: X 6 1.2 6 1023
6
Mw = Y, where X is the pristine weight of clay, Mw is the molecular
weight of the dendron, and Y is the weight of dendron. The
theoretical organic ratio is given by Y/(X + Y). Measured by
thermogravimetric analysis (TGA) in air.
aggregate after 12h; –: aggregate.
b
c
+: dispersible; +–:
Acknowledgements
The authors thank NSC of Taiwan and Chung Shan Institute
of Science and Technology for financial support. Partial
financial support from the Green Chemistry Project (NCHU)
and the Center-of-Excellence Program on Membrane
Technology from the Ministry of Education, Taiwan, is
gratefully acknowledged.
easy purification, which consisted of active amine groups on
one end and flexible alkyl chains on the other. The dendritic
moieties were feasibly converted into amphiphilic structures by
simple acidification of the ‘‘head group’’. The incorporation
of dendritic surfactants can precisely control the d-spacing
Fig. 6 DSC thermograms for pristine dendrons and modified organoclays: (a) G1 and G1/MMT; (b) G2, G2/MMT, G3 and G3/MMT.
2062 | J. Mater. Chem., 2006, 16, 2056–2063
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