Exfoliated PP/Clay Nanocomposites Using Ammonium-Terminated PP as the Organic Modification for Montmorillonite

Article abstract of DOI:10.1021/ma035291l

The applications of chain end-functionalized polypropylene (PP) in polyolefin/clay nanocomposites were discussed. Transmission electron microscopy (TEM) was used to examine the degree of exfoliated structure. The side chain-functionalized or block copolym

Full text of DOI:10.1021/ma035291l

Macromolecules 2003, 36, 8919-8922
8919
Exfolia ted P P /Cla y Na n ocom p osites Usin g
Am m on iu m -Ter m in a ted P P a s th e Or ga n ic
Mod ifica tion for Mon tm or illon ite
nanocomposites with a mixed nanomorphology, includ-
ing both intercalated and exfoliated structures coexist-
8
g
ing in the system. Unfortunately, the availability of
functional PP is very limited due to synthetic chemical
difficulties, and most of such approaches are employing
a single, commercially available, maleic anhydride-
grafted PP (PP-g-MAH) polymer, despite its shortcom-
ings (e.g., PP-g-MAH is typically characterized by very
complicated molecular structure due to many side
Z. M. Wa n g, H. Na k a jim a , E. Ma n ia s,* a n d
T. C. Ch u n g*
Department of Materials Science and Engineering,
The Pennsylvania State University,
University Park, Pennsylvania 16802
9
reactions, including severe chain degradation during
Received September 2, 2003
Revised Manuscript Received October 19, 2003
the free radical grafting process as well as many
impurities).
Although the miscibility of silicates, including clay
minerals, with organics and polymers has been known
The hurdle of obtaining appropriately functionalized
PP can be overcome by a general functionalization
approach involving the combination of metallocene
since the early 1950s, an industrial report of a nylon-
1
6
/montmorillonite (mmt) material from Toyota has
1
0
catalysts and reactive comonomers, which can yield a
broad range of side-chain functionalized polyolefins with
relatively well-defined molecular structures. Recently,
we also discovered a facile route using reactive chain
transfer agents to prepare end-functionalized poly-
recently revitalized the field. Those materials were
synthesized by polymerization of the monomer in the
presence of the inorganic filler and showed remarkable
enhancements of several thermal and mechanical prop-
erties with a moderate inorganic loading (2-5 wt %).
1
1-13
2
olefins
containing a terminal functional group (OH,
Later, Giannelis et al. discovered that it is possible to
NH2, COOH, anhydride, etc.), while maintaining a well-
controlled polymer molecular weight and narrow mo-
lecular weight and composition distributions.
melt-mix polymers with clays bearing cationic organic
surfactants, eliminating the need for organic solvents
or in situ polymerization schemes. Such melt-blending
approaches generally yield similar concurrent enhance-
ments of many materials properties by the nanodisper-
sion of inorganic silicate layers, which cannot be realized
by conventional fillers; for example, simultaneously
increasing tensile strength, flex modulus, and impact
toughness, contributing a general flame-retardant char-
The availability of a broad range of well-defined
functionalized polyolefins provides us with a great
advantage in evaluating their applications in polyolefin/
clay nanocomposites. The most desirable ammonium
+
14
group-terminated i-PP (PP-t-NH3 ) polymers (dis-
cussed in the Supporting Information) were prepared
3
2 2 2
by the combination of rac-Me Si[2-Me-4-Ph(Ind)] ZrCl /
MAO catalyst and p-NSi -St/H (p-NSi -St: 4-{2-[N,N-
2 2 2
acter, and affording a dramatic improvement in barrier
properties.4
,5
On the basis of theoretical models, Balazs6a suggested
bis(trimethylsilyl)amino]ethyl}styrene) chain transfer
+
+
agent. Both pristine Na -montmorillonite clay (Na -
mmt), with an ion-exchange capacity of ca. 0.95 mequiv/
g, and a dioctadecylammonium-modified montmorillo-
nite organophilic clay (2C18-mmt) were obtained from
that an increase in the length of the organic molecules
tethered on clays is a promising approach to further
promote dispersion of clay sheets in a polymer matrix.
7
Toward this end, recent reports employing a variety
2
7
a
Southern Clay Products. Static melt intercalation was
of polymers (such as polystyrene, poly(methyl meth-
acrylate),7b and poly(ꢀ-caprolactone)7c ) grafted on mont-
employed to prepare all PP/clay nanocomposites.
+
-
+
+
-
morillonite surfaces observe varied extents of clay
dispersion. Furthermore, subsequent annealing of these
polymer-bearing clays, or mixing them with the respec-
tive homopolymers, can lead to retention or collapse of
Both PP-t-NH3 Cl /Na -mmt and PP-t-NH3 Cl /
2C18-mmt nanocomposites were prepared and evalu-
ated under the same conditions. Figure 1 compares the
X-ray diffraction (XRD) patterns before and after static
annealing of a physical mixture (90/10 weight ratio) of
7
the clay dispersion depending on the polymer. The
+
-
differences in these responses were attributed to the
an ammonium-terminated i-PP (PP-t-NH3 Cl ; Mn )
6
b
specific polymer-clay interactions in each case, indi-
58 900 and Mw ) 135 500 g/mol; Tm ) 158.2 °C) and a
7
+
cating that where there exist strong interactions the
pristine Na -mmt clay. Namely, simple mixing of dried
+
-
+
clay dispersions collapse, whereas in those cases with
weak polymer-clay interactions the dispersions can be
retained.
PP-t-NH3 Cl powder and Na -mmt, ground together
by mortar and pestle at ambient temperature, creates
the XRD pattern in Figure 1a, with a (001) peak at 2θ
8
+
Many research efforts have also focused on dispers-
= 7°, corresponding to the characteristic Na -mmt d
spacing of ca. 1.26 nm. The mixed powder was then
ing montmorillonite in polypropylene (PP), which is a
fast growing thermoplastic that dominates the indus-
trial applications due to its attractive combination of
properties and low cost. As expected, because of the
absence of any strong interactions, it has been a
scientific challenge to disperse silicate clays in the
highly apolar polyolefins.6b The general approach for
improving the compatibility of PP with organically
modified clays has been the addition of polar functional
groups to the PP polymer, typically resulting in PP/clay
heatedsannealed under static conditionssat 190 °C for
+
2 h under vacuum. The resulting PP-t-NH3 /mmt hybrid
shows a featureless XRD pattern in Figure 1b, indicat-
ing the formation of an exfoliated clay structure, which
corresponds to the thermodynamically stable state, as
the ammonium-terminated PP exchange the alkali
+
(Na ) cations at the mmt surfaces.
Similar results were also observed in the PP-t-NH3+-
-
+
Cl /2C H -2CH -N -mmt nanocomposite case, sug-
1
8
37
3
gesting that the dioctadecylammonium cations in orga-
+
nophilic clay may also be ion-exchanged by PP-t-NH3
*
To whom all correspondence should be addressed: e-mail
+
17
chung@ems.psu.edu or manias@psu.edu.
to form a similar PP-t-NH3 /mmt structure. It is clear
1
0.1021/ma035291l CCC: $25.00 © 2003 American Chemical Society
Published on Web 10/31/2003

8
920 Communications to the Editor
Macromolecules, Vol. 36, No. 24, 2003
F igu r e 1. X-ray diffraction patterns of PP-t-NH +
Cl /Na -
-
+
3
mmt (90/10 weight ratio): (a) physical mixture by simple
powder mixing at ambient temperature and (b) the same
+
mixture after static melt-intercalation (PP-t-NH
brid).
3
/mmt hy-
that an organic surfactant is not needed to promote
+
-
+
compatibility between PP-t-NH3 Cl and pristine Na -
mmt clay; beyond any economic benefits, such an
elimination of the mmt’s organic surfactant also offers
some significant materials advantages; for example, it
eliminates two major concerns relating to the thermal
stability of the surfactant during high-temperature melt
processing and to the long-term stability of the small
organic surfactant in the polymer/clay nanocomposite
under various application conditions.
F igu r e 2. X-ray diffraction patterns of the 50/50 mixture by
+
The binary PP-t-NH3 /mmt hybrid was subsequently
further mixed/blended (at a 50/50 weight ratio) with a
neatsunfunctionalizedsi-PP (Mn ) 110 000 and Mw )
+
weight of exfoliated PP-t-NH
3
/mmt structure (90/10 weight
ratio) and neatsunfunctionalizedsi-PP. The XRD traces
+
shown correspond to (a) the physical mixture of PP-t-NH /
3
mmt and i-PP and (b) the same mixture after static melt-
2
50 000 g/mol). Figure 2 shows the XRD patterns of (a)
intercalation. Bright-field TEM image of the melt-mixed
the physical mixture and (b) the melt blending of the
exfoliated PP-t-NH3 /mmt structures with neat i-PP.
+
ternary PP-t-NH
3
/mmt/PP system (b in the XRD) shows the
+
typical exfoliated montmorillonite structure.
The exfoliated structure is maintained after further
mixing with i-PP, which is compatible (cocrystallizable)
+
with the backbone of the largely isotactic PP-t-NH3
polymer, as also directly observed in the TEM inset
(
Figure 2). Apparently, the i-PP polymer chains largely
+
serve as diluents in the ternary PP-t-NH3 /mmt/i-PP
system, with the thermodynamically stable PP-t-
1
5
+
NH3 /mmt exfoliated structure dispersed in the i-PP
matrix.16
For comparison, several functionalized PP polymers,
containing randomly distributed functional groups in
the side chains or lumped together in a block copolymer
microstructure, were also evaluated in PP/montmoril-
lonite nanocomposites. Similar static melt-intercalation
procedures were followed, except for employing alkyl-
ammonium-modified montmorillonites (2C18-mmt for
all random copolymers and C18-mmt for the block
copolymer). Figure 3 shows the XRD patterns of four
nanocomposites made with 6 wt % of 2C18-mmt clay
and 94 wt % of three side-chain-functionalized PPs (Mn
)
%
95 000 and Mw ) 210 000 g/mol) containing (a) 1 mol
p-methylstyrene, (b) 0.5 mol % maleic anhydride, and
(c) 0.5 mol % hydroxy side groups and the original 2C18-
mmt clay All the functionalized PPs were derived from
the same random PP copolymer synthesized by metal-
locene catalysis, which contained 1 mol % p-methylsty-
rene (p-MS) comonomer. Subsequently, the p-MS’s were
selectively functionalized toward hydroxy (OH) and
maleic anhydride without changing the PP backbone.
Moreover, we show (Figure 3) the XRD pattern of PP-
b-PMMA block copolymer/6 wt % C18-mmt which
contains a 5 mol % methyl methacrylate block. These
XRD patterns clearly show that there is a definite
F igu r e 3. X-ray diffraction patterns of 2C18-mmt clay and
four nanocomposites with 6 wt % of alkylammonium-mmt and
9
1
4 wt % of three side-chain-functionalized PPs containing (a)
mol % p-methylstyrene, PP-r-MS; (b) 0.5 mol % maleic
anhydride, PP-r-MA; and (c) 0.5 mol % hydroxy, PP-r-OH. Also
shown, a 6 wt % C18-mmt nanocomposite of a PP-b-PMMA
block copolymer, with 5 mol % of methyl methacrylate.
intercalated structure for all the side-chain-functional-
ized PP cases, manifesting itself through an interlayer
d spacing increase of about 1 nm compared to that of

Macromolecules, Vol. 36, No. 24, 2003
Communications to the Editor 8921
Ack n ow led gm en t. The authors gratefully acknowl-
edge financial support from the Fire Research Division
of the NIST (through Grant 70NANB0H0097). H. Na-
kajima acknowledges support from Sumitomo Chemical,
J apan.
Su p p or tin g In for m a tion Ava ila ble: Experimental pro-
cedures, syntheses details, and characterization data. This
material is available free of charge via the Internet at http://
pubs.acs.org.
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end-functionalized polyolefin and (b) side-chain-functionalized
polyolefin located between clay interlayers.
1
3, 3516.
the parent alkylammonium-mmt. TEM was also em-
ployed to examine the degree of exfoliated (disordered)
structure, and the quantitative image analysis indicates
only between 20 and 40% exfoliated layers in all four
(9) (a) Ruggeri, G.; Aglietto, M.; Petragnani, A.; Ciardelli, F.
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8
g
systems.
(
10) Chung, T. C. Functionalization of Polyolefins Academic
Summarizing, the herein experimental results dem-
Press: London, 2002.
onstrate the advantage of chain-end-functionalized PP
(
(
11) Chung, T. C. Prog. Polym. Sci. 2002, 27, 39.
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34, 8040.
+
(
PP-t-NH3 ) that seems to adopt a unique molecular
structure atop of the clay surfaces and results in an
1
6
exfoliated montmorillonite structure (Figure 4a). The
+
terminal hydrophilic NH3 functional group anchors the
(
13) (a) Chung, T. C.; Dong, J . Y. J . Am. Chem. Soc. 2001, 123,
PP chains (via ion exchange) on the inorganic surfaces,
and the hydrophobic high molecular weight and semi-
crystalline PP “tail” effectively exfoliates the clay plate-
lets, an exfoliated structure which is maintained even
after further mixing with neat PP. In contrast, side-
chain-functionalized or block copolymer PPs form mul-
tiple contacts with each of the clay surfaces, as illus-
trated in Figure 4b, which not only results in aligning
the polymer chains parallel to the clay surfaces but also
can bridge consecutive clay platelets promoting inter-
calated structures, especially for the higher lateral size
montmorillonites.
4
3
871. (b) Dong, J . Y.; Chung, T. C. Macromolecules 2002,
5, 1622.
(14) Dong, J . Y.; Wang, Z. M.; Han, H.; Chung, T. C. Macromol-
ecules 2002, 35, 9352.
(
15) The fact that all polymer/inorganic mixings were carried out
under melt intercalation2 (static annealing in absence of
mechanical shear or solvent) underlines that the dispersions
achieved are thermodynamically favored rather than kineti-
cally trapped by the processing conditions. For more infor-
6
,7b
mation the reader can refer to previous review articles.
16) Krishnamoorti, R.; Giannelis, E. P. Langmuir 2001, 17,
448. Krishnamoorti, R.; Ren, J . X.; Silva, A. S. J . Chem.
(
1
Phys. 2001, 114, 4968. Ren, J . X.; Casanueva, B. F.; Mitchell,
C. A.; Krishnamoorti, R. Macromolecules 2003, 36, 4188.

8
922 Communications to the Editor
Macromolecules, Vol. 36, No. 24, 2003
(
17) There is no sufficient thermodynamic driving force for PP-
PP-t-NH3+ is also doubtful, albeit more probable. The ion-
exchange mechanism for the development of an exfoliated
structure in this case is speculative.
+
t-NH3 to exfoliate, or even intercalate, an alkylammonium-
6
b,8g
modified montmorillonite.
At the same time, the exist-
ence of a thermodynamic driving force for the replacement
of the alkylammonium surfactant to be ion-exchanged by
MA035291L