Macromolecules 2006, 39, 8207-8209
8207
Ruthenium(II) Complex-Induced Dispersion of
Montmorillonite in a Segmented Main-Chain
Liquid-Crystalline Polymer Having Side-Chain
Terpyridine Group
Wenyi Huang and Chang Dae Han*
Department of Polymer Engineering, The UniVersity of
Akron, Akron, Ohio 44325
ReceiVed August 24, 2006
ReVised Manuscript ReceiVed October 13, 2006
Compatibility is one of the most important factors for
achieving a very high degree of exfoliation of clay aggregates
in a polymer matrix. Montmorillonite (MMT), which has widely
been used to prepare polymer nanocomposites, is a type of
smectic natural clay that tends to swell when exposed to water.1
Thus, in its pristine state, MMT is only compatible with
hydrophilic polymers, such as poly(ethylene oxide) (PEO)2 or
poly(vinyl alcohol) (PVA).3 To render the compatibility between
MMT and hydrophobic polymers, in most cases MMT was
treated with a surfactant through ion-exchange reactions to
convert the hydrophilic silicate surface into an organophilic one,
and thus form the so-called organoclay.4-6 A very high degree
of exfoliation has seldom been observed in nanocomposites
when there is little or no attractive interaction between the
polymer matrix and MMT or organoclay. Therefore, one must
design and synthesize a polymer such that the selected orga-
noclay can have strong attractive interactions with the selected
polymer.7,8 The use of a ruthenium(II) complex for the prepara-
tion of a clay-metal complex hybrid film9 or selective absorp-
tion onto clay surfaces has extensively been studied,10-13 usually
in connection with harnessing the specific interactions between
metal-ion center and negatively charged clay surface.
In this study, we first synthesized a segmented main-chain
liquid-crystalline polymer having side-chain terpyridine group
(PTBP) and prepared a ruthenium complex [RuII(PTBP)(6TPy)]-
(PF6)2 (referred to as PTBP-Ru-6TPy) by mixing PTBP with
a monocomplex (6TPy-RuCl3), which was formed between a
terpyridine (6-(2,2′:6′,2′′-terpyridyl-4′-oxy)hexane (6TPy) and
ruthenium chloride (RuCl3‚3H2O). Subsequently, we prepared
a (PTBP-Ru-6TPy)/MMT nanocomposite by solution blend-
ing. The MMT employed in this study has Na+ ion, the ion-
exchange capacity of 92 mequiv/100 g, and the dimensions of
1 nm in thickness, and approximately 100 nm in width and
length. We found the MMT aggregates in the nanocomposite
to be very well dispersed in the matrix of PTBP-Ru-6TPy.
We attribute the very high degree of exfoliation of MMT
aggregates observed to the Coulombic interactions between the
negatively charged surfaces of pristine MMT and the positively
charged ruthenium center in the polymer matrix. We are not
aware of any previous study, which has reported on the
formation of very high degree of exfoliation of pristine MMT
aggregates in a polymer matrix having a metal complex.
Scheme 1 shows the reaction route employed to prepare
ruthenium complex with PTBP, PTBP-Ru-6TPy, the detailed
synthesis procedures and characterizations of which are sum-
marized in the Supporting Information. Briefly stated, 6TPy was
reacted with an equimolar amount of RuCl3‚3H2O in methanol
Figure 1. XRD patterns of (a) MMT, (b) (PTBP-Ru-6TPy)/MMT
nancomposite, and (c) PTBP/MMT nancomposite.
at 60 °C overnight to form 6TPy-RuCl3 monocomplex, which
was further refluxed with PTBP in a cosolvent of tetrahydrofuran
and ethanol (9:1, v/v), using N-ethylmorpholine as a catalyst,
followed by the addition of an excess amount of ammonium
hexafluorophosphate to finally obtain the PTBP-Ru-6TPy
complex. It has been found from differential scanning calorim-
etry that PTBP has a glass transition temperature (Tg) of 49.9
°C and a smectic-to-isotropic transition temperature of 80.7 °C
during heating, while PTBP-Ru-6TPy only exhibits a Tg of
123.4 °C upon heating (see the Supporting Information).
(PTBP-Ru-6TPy)/MMT nanocomposite was prepared by
mixing a predetermined amount of PTBP-Ru-6TPy dissolved
in a cosolvent of N,N-dimethylformamide (DMF) and H2O (10:
1,v/v) and MMT aggregates suspended in a mixture of DMF
and H2O (10:1, v/v) under vigorously stirring. The solvent in
the mixture was evaporated slowly under constant stirring for
2 days at ambient temperature. When we prepared the (PTBP-
Ru-6TPy)/MMT nanocomposite, the exchangeable interlayer
cation Na+ was exchanged by ruthenium complex in PTBP-
Ru-6TPy and formed sodium hexafluororophophate salt, which
was still present in the final product.
For comparison, the PTBP/MMT nancomposite was prepared
using the same procedure, except that slight heating was applied
to ensure the solubility of PTBP. Nanocomposites were dried
completely in a vacuum oven at 100 °C until no weight change
was detected. The amount of MMT (inorganic clay) in each
nanocomposite (PTBP-Ru-6TPy)/MMT and PTBP/MMT was
3 wt %.
To determine the degree of dispersion of MMT aggregates
in the nanocomposites, X-ray diffraction (XRD) experiments
were conducted on a Rigaku Rotaflex rotating anode diffrac-
tometer with slit collimation. Figure 1 shows XRD patterns,
showing that MMT has a gallery distance (d-spacing) of 1.1
nm, while the d-spacing of MMT in PTBP/MMT nanocomposite
has increased only slightly from 1.1 to 1.3 nm, indicating that
there would be little chance to obtain a highly dispersed MMT
aggregates in PTBP. Interestingly, it can be seen in Figure 1
that (PTBP-Ru-6TPy)/MMT nanocomposite has a featureless
XRD pattern, the origin of which can be attributed to the
incorporation of ruthenium complex into PTBP that largely
improves the compatibility between the MMT and the polymer.
Transmission electron microscopy (TEM, JEM1200EX 11,
JEOL) was conducted to verify the observations made from
XRD patterns. It is clearly seen in Figure 2 that (PTBP-Ru-
6TPy)/MMT nanocomposite has a very high degree of disper-
sion of MMT aggregates in the ruthenium complex matrix, while
* To whom correspondence should be addressed. E-mail:
10.1021/ma0619637 CCC: $33.50 © 2006 American Chemical Society
Published on Web 11/03/2006