Polymer modification and Self-Assembly of Layered Silicate Clays
Jiang-Jen Lin
( Department of Chemical Engineering, National Chung Hsing University )
ABSTRACT
We prepare organoclays from intercalation of
layered silicates with polyoxyalkylene-amines. Due
to the incorporation of hydrophobic organic moieties,
the resultant hybrids are amphiphilic and tend to
self-assemble. We observed unique morphologies of
self-aligned arrays including rod-like and star-like
fibrous microstructures, as an example, lengthy rods
in a dimension of 10μm in length and 0.2-0.8μm in
diameter. From the observations of SEM, TEM and
AFM images, we postulate the formation of
micro-fiber structures in microscopic and
biomaterial-like bundles in microscopic were
self-piled from the amine-intercalated silicate stacks
of amphiphilic nature through noncovalent bonding
forces in a synchronized manner.
INTRODUCTION
Tremendous efforts have been focused on the
understanding of self-assembling mechanisms and
the finding of new nanomaterials in recent years.
Research has shown that materials on the nanometer
scale (1 to 100 nm) may behave differently such as
quantum dots in electron movement1 from their
conventional bulk properties.2 Different geometric
shapes of the synthetic nanostructures including
nanowire, nanosphere, nanotube, nanocube, and
nanorod have been reported.3 Among the naturally
abundant minerals, the smectite clays are particularly
interesting because of their unique high-aspect-ratio
characteristics. For example, montmorillonite is
composed of layered silicate platelets which are
bound by ionic bridges in primary units.4 In the
nanocomposites, layered silicates of hydrophilic
nature are organically modified by alkyl quaternary
Table 1. XRD Basal Spacing and Solvent Dispersing Properties
of the POP-amine/MMT Hybrids at Various POP-amine
Equivalents to CEC.
d
Weight
Dispersion c
POP-amine molar
spacing
fraction
to CEC of MMT
H2O
Toluene
(w/w) b
(Å) a
1.0
58
63 / 37
－
＋
0.8
20
46 / 54
＋
－
0.5
20
34 / 66
＋
－
0.2
19
22 / 78
＋
－
a
d spacing by X-ray diffraction. b Weight fraction of intercalated
organics/silicates (measured by TGA). c ＋ dispersible; －
aggregate (0.1 g sample in 1.0 g solvent).
ammonium salts to improve their compatibility with
polymers. Works have shown that a range of 19-30 Å
basal spacing after the organic introduction.5
Previously, we reported a high 92 Å basal spacing
using polymeric oxyalkylene amine quaternary salts.6
Further works showed the organoclays were
amphiphilic
and
self-assembling.7
These
self-assembling properties are mainly controlled by
the high aspect ratio of platelets, including
organo-modified momtmorillonite, mica and random
silicate plates.8
EXPERIMENTAL
Materials
Sodium montmorillonite (Na+-MMT) is a natural
smectic aluminosilicates, obtained from Kunimine
Industries, Inc. and have a cation exchange capacity
of 1.15 mequiv/g. The synthetic fluorine mica
(SOMASIFTM ME-100) has a cation exchange
capacity (CEC) = 1.20 mequiv/g.
Poly(oxyalkylene)diamines,
including
the
poly(oxypropylene)- (POP) and poly(oxyethylene)(POE) backboned diamines, are commercially
available under the trade name Jeffamine ® amines.
The POP-2000 is an α, ω-diamine of
poly(oxypropylene)-diamine (2000 Mw). The
POE-2000 is a diamine with a backbone of
poly(oxyethylene) rich.
Preparation of the Amine Intercalated Clays
The general procedures for the ionic exchange
reaction of Na+-MMT and synthetic mica with
quaternary ammonium salts are reported previously
and exemplified below. Na+-MMT (10g, 1.15
mequiv/g) was placed in a beaker and mechanically
stirred in 1-liter of deionized water at 80 oC until
swollen. POP-2000 (23 g, 11.5 mmol) was acidified
in a separate beaker with the designated amount of
hydrochloric acid (37 % in water; 1.13 g, 11.5
mequiv), and added to Na+-MMT at 80 oC with
vigorously stirring and then filtered, dried.
Preparation of nanofiber and nanorod arrays
The SEM samples were prepared by dispersing
the sample in water and spread on a glass plate
surface, evaporated at 80 oC. The random silicate
platelets were prepared from a direct exfoliation of
Na+-MMT, reported elsewhere.
RESULTS AND DISCUSSION
The intercalation involves the acidification of
polymeric amines into the corresponding quaternary
ammonium salts and ionic exchange reaction with
Na+-MMT at various molar ratios to CEC value of
1.15 meq/g. The organic content is ranged from 22
to 63 wt% (Table 1).
Because of the presence of hydrophobic POP
organics, the hybrids are amphiphilic with respect to
their solvating ability towards organic solvents. Their
self-assembled morphologies are shown in Figure 1C
and 1D, directional and ordered rod-like arrays were
- 5 -
observed. The formation of rod arrays is in high
uniformity with individual rod dimension from 100
to 800 nm for the width and 2~10 μm for the length
(Figure 1).
SEM micrographs of POP2000/H+-MMT (B;
19Å d spacing; 26 wt% organics).
Figure 3.
Figure 1. SEM micrographs of (A) Na+-MMT (B)
POP2000/MMT (58Å d spacing; 63 wt% organics). (C)
POP2000/MMT (0.5 CEC intercalated, 20 Å; 34 wt%)
and (D) POP2000/MMT (0.2 CEC intercalated, 19Å;
22 wt%).
For the rod growing mechanism, the unit rods
must grow in two synchronized directions, vertically
and horizontally (Figure 2). The layered POP-amine
organics align themselves side-by-side through
hydrophobic interaction which directs the horizontal
growth and the increasing rod width. On the other
hand, the vertical growth is mainly controlled by the
ionic attraction on the flat silicate platelets, resulting
in a unit piling and the length increase.
In addition, POP-intercalated micas are
inorganic/organic hybrids, consisted of hydrophilic
silicates and hydrophobic organics. Similar to MMT,
the hybrids can be considered to be a new class of
surfactant structure which could behave like
surfactants and form film at the toluene/water
interface. In bulk, the POP-D2000 intercalated mica
hybrids (41.6 Å d spacing) have shown to have the
ability of self-assembling. When the finely dispersed
POP-D2000/mica in toluene was placed on the top of
water layer, the hybrid self-aggregated at the
toluene/water interface (Figure 4).
SEM micrograph of POP-D2000
intercalated mica film surfaces on water side,
formed at the toluene/water interface.
Figure 4.
Figure 2. Conceptual diagrams of vertical and
horizontal self-aligning mechanism.
100~800 nm
2~40μm
Vertical Self-aligning
Horizontal Self-aligning
10 μm
19~20 Å
The proper balance between the POP hydrophobic
interaction and the silicate ionic attraction is the key
element for controlling the directional rod formation.
A second intercalating method has been adopted to
prepare the organoclays without sodium ion
contamination. The Na+-MMT was first acidified
into H+-MMT by the excess HCl treatment. The
direct POP-amine intercalation afforded the
POP-MMT of similar XRD (19Å d spacing) and
organic fraction (26 wt%). The subsequent rod
formation is observed to be of highly regular and
lengthy (e.g., 40 μm in Figure 3).
Overall, the formation of an unusual length in such
as structure must be involved with an intensive
self-assembling force. By judging from the primary
unit structure, there are at least two noncovalent
bonding forces, hydrophobic interactions of the
embedded organics and ionic charge attraction of the
silicate surfaces. These distinctly different noncovalent
bonding forces are complimentary in directing the
primary unit aligning mechanism. The high
aspect-ratio of the silicates tends to be responsible
for the lengthy formation. During the vertical growth
from platelet aligning, the POP organics are
widening by edge-to-edge aligning. It appears that
the vertical growth into lengthy rod is the dominant
direction in resulting the lengthy rod morphology
(Figure 5).
- 6 -
Figure 5. DFM
observed during the TEM electron beam
bombardment and also deduced from the shadow
surrounding the tube picture. The diameter of the
tube-like array structure is approximately 100 nm,
the space originally occupied by the layered platelets.
The occurrence of this unique transformation is
attributed to the high aspect ratio of the platelets and
their electrostatic surface.
image of the bundle of rod-like
crystals.
500 nm
250 nm
When the lengthy fibers were examined by a TEM
under intense electron beam bombardment, it was
observed that the structures of stacked platelets
underwent a further transformation into another
nanoarray with the dimension of 5.6 μm in length,
120 nm in diameter and 30 nm in wall thickness as
shown in Figure 4b. The array is presumably formed
through silicate platelets scattering under the electron
beam bombardment. In early stage of the TEM
examination, different dimensions of fiber arrays
were observable (Figure 6). Under the high-energy
field exposure by continuing the TEM observation,
the silicate arrays have been shattered and the
individual platelets diffuse away.
Figure 6. TEM micrographs of silicate platelet arrays
under initial observation a), and transformation into b)
after further electron beam bombardment (a conceptually
proposed picture in the inserted).
CONCLUSION
A unique morphology was observed from
organoclay self-assembly. By judging from the
primary unit structure, there are at least two
noncovalent
bonding
forces,
hydrophobic
interactions of the embedded organics and ionic
charge attraction of the silicate surfaces. These
distinctly different noncovalent bonding forces are
complimentary in directing the primary unit aligning
mechanism. The high aspect-ratio of the MMT and
mica silicates tends to be responsible for the lengthy
formation. The stacking mechanism is unprecedented
and apparently different from the formation of clay
in the nature process. The potential applications of
these lengthy fibrous silicates in blending with
organic polymers are being pursued.
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