Temperature controlled dispersion of carbon nanotubes in water with pyrene-functionalized poly(N-cyclopropylacrylamide)

Article abstract of DOI:10.1021/ja905803f

(Figure Presented) Despite their immense potential, the ability to control the dispersion and microstructure of carbon nanotubes remains a hurdle for their widespread use. Poly(N-cyclopropylacrylamide), containing 5 mol % pyrene-bearing repeat units (p-PN

Full text of DOI:10.1021/ja905803f

Published on Web 09/08/2009
Temperature Controlled Dispersion of Carbon Nanotubes in Water with
Pyrene-Functionalized Poly(N-cyclopropylacrylamide)
Krishna C. Etika,† Florian D. Jochum,‡ Patrick Theato,*,‡ and Jaime C. Grunlan*,†
Materials Science and Engineering Program, Texas A&M UniVersity, College Station, Texas 77843, Institute of
Organic Chemistry, UniVersity Mainz, Germany
Received July 13, 2009; E-mail: theato@uni-mainz.de; jgrunlan@tamu.edu
Suspensions of single-walled carbon nanotubes (SWNTs) are
being studied for variety of applications that include biocompatible
transportation,1 drug delivery systems,2 and optical sensors.3 As
produced SWNTs are aggregated because of the van der Waals
force of attraction and hydrophobic interactions in aqueous environ-
ments which severely limit their dispersion, especially in water.4,5
For this reason, both covalent6 and noncovalent7 techniques are
used to stabilize SWNTs in water. Recently, the use of pH
responsive polymers to stabilize SWNTs in water was examined
where changes in conformation and degree of ionization of
poly(acrylic acid) (PAA) as a function of pH were utilized for
exfoliation and bundling of SWNTs.8,9 The ability to use temper-
ature to control the level of nanotube exfoliation would be a much
more convenient extension of this powerful concept. In this
communication, we report the use of a thermo-responsive poly(N-
cyclopropylacrylamide) (PNCPA) copolymer with 5 mol % pyrene
side groups (p-PNCPA), to stabilize aqueous SWNT suspensions.
Figure 1. Cryo-TEM of p-PNCPA/SWNT suspensions as function of
temperatue (a) and a schematic representing the stabilization mechanism
The changes in conformation (i.e., coil to globule transformation)
as a function of temperature are believed to control the dispersion
(b).
state of SWNTs (i.e., stabilized or bundled) in water.
Few attempts have been made to control the dispersion of
SWNTs using stimuli-responsive polymers, especially as a function
of temperature. Lack of suitable thermo-responsive polymers that
have affinity for SWNTs may be one of the reasons for this. Most
of the studies reported were based on poly(N-isopropylacrylamide)
(PNIPAM) due to its transition temperature being near human body
temperature (i.e., 32 °C).10 However, the physical interaction
between SWNTs and pure PNIPAM is modest and not sufficient
for effective stabilization and/or control of the microstructure of
aqueous SWNT suspensions as a function of temperature.11
Accordingly, there is a need to develop new temperature responsive
polymeric systems with better physical interaction with SWNTs.
Pyrene-functionalized PNCPA (p-PNCPA) was synthesized to
disperse SWNTs in water (see Supporting Information). PNCPA
shows temperature responsive dissolution in water and has a lower
critical solution temperature (LCST) of 53 °C.12,13 Addition of
pyrene side groups, however, lowers the LCST of PNCPA to 30
°C for 5 mol % pyrene, due to the hydrophobic effect of the unpolar
aromatic system (see Figure 3 and Supporting Information). The
pyrene moiety has a strong tendency to adsorb on SWNTs due to
π-π stacking along the sidewalls, but the hydrophilic PNCPA
backbone tends to dissolve in water below the LCST.14 At
temperatures above LCST, the otherwise hydrophilic backbone turns
hydrophobic and becomes immiscible in water.15 This transition
of the backbone can influence the dispersion state of SWNTs as
shown in Figure 1b. The stabilization of SWNTs in water by
addition of p-PNCPA is believed due to the presence of a steric
layer of polymer on SWNT surfaces. The thickness of the steric
layer is greater below the LCST as the backbone exists in an
expanded form due to the formation of hydrogen bonds with
water.15 This increased steric layer thickness counteracts the
intertube van der Waals force of attraction and restricts the SWNTs
from bundling. At temperatures above the LCST, the backbone
collapses and assumes the form of a globule, which in turn reduces
the steric layer thickness and the obstacle to SWNT aggregation.
Similar steric layer thickness controlled dispersion of colloidal
suspensions was observed for polystyrene latex stablilized with
acidic polysaccharides.16 The observation of SWNTs in an “exfo-
liated” state at temperatures below the LCST and a more bundled
state at temperatures above the LCST of p-PNCPA (see Figure 1a)
supports this assertion.
Suspensions with well-dispersed SWNTs are known to have
lower viscosities as compared to those that have agglomerated or
bundled SWNTs, due to lack of entanglements.8 To investigate this
behavior, viscosity measurements were made on dilute suspensions
containing 0.011 wt % SWNT and 0.1 wt % p-PNCPA at
temperatures below and above the LCST of the polymer (i.e., 50
and 10 °C), as shown in Figure 2. It can be seen that, at 10 °C, the
suspension shows a nearly Newtonian behavior with little change
in viscosity as a function of shear rate. However, suspension
viscosity at 50 °C exhibits a shear-thinning behavior (i.e., decrease
of viscosity with increasing shear). Concentrated solutions of
SWNTs show shear- thinning behavior because of the high degree
of entanglements that look like polymeric solutions.17
Also, cryo-TEM micrographs of the suspensions (see Figure 1a)
show SWNT microstructure to be bundled at 50 °C and more
exfoliated at 10 °C. The results obtained from viscosity measure-
† Texas A&M University.
‡ University Mainz.
9
13598 J. AM. CHEM. SOC. 2009, 131, 13598–13599
10.1021/ja905803f CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Table 1. Electrical Conductivity of p-PNCPA/SWNT Composite
wt %
SWNT
drying temp
(°C)
electrical conductivity
(S/m)
10
20
20
50
20
50
0.04
0.26
0.91
2.29
control the microstructure and behavior in the solid state will have
potential applications in polymer nanocomposite preparation.
To conclude, we have synthesized a novel thermo-responsive
polymer with 5 mol % pyrene side groups (p-PNCPA) to disperse
SWNTs in water. Cryo-TEM micrographs obtained at temperatures
above and below the LCST of this thermo-responsive polymer
revealed the bundled and exfoliated microstructure of SWNTs,
respectively. Viscosity measurements show shear-thinning and
nearly Newtonian behavior of the suspensions above and below
LCST. Turbidity measurements suggest that p-PNCPA retains its
intrinsic thermo-responsive properties in the presence of SWNTs,
with no significant change in the transition temperature. Evaluation
of dried composite films suggests that the microstructure in the
liquid state is preserved in the solid state to a large extent.
Temperature controlled dispersion of SWNTs in water provides a
new technique for controlling properties in the liquid and solid states
that could impact nanotube-based sensing and composites respectively.
Figure 2. Viscosity of p-PNCPA/SWNT suspensions as a function of shear
rate at 10 and 50 °C.
ments are complementary to cryo-TEM, and thus it is reasonable
to deduce that p-PNCPA/SWNT suspensions are well dispersed at
10 °C and relatively bundled at 50 °C.
To understand the temperature response of p-PNCPA in the
presence of SWNTs, turbidity (actually decadic absorbance) at 400
nm was examined in the absence and presence of SWNTs, as shown
in Figure 3. It can be seen that there is a significant increase in
turbidity of the p-PNCPA solution at 30 °C, and this is because of
the coil-globule transition of p-PNCPA. The p-PNCPA/SWNT
suspensions also show a small but significant increase at ∼30 °C,
which corresponds to the transition temperature of p-PNCPA. Thus,
the thermo-responsive characteristics of p-PNCPA are retained in
the presence of SWNTs.
Acknowledgment. The authors would like to acknowledge
financial support for this work from the Texas Engineering
Experiment Station (TEES) and the National Science Foundation
(CMMI 0644055). Dr. Christos Savva is thanked for his assistance
with cryo-TEM (part of the Microscopy and Imaging Center [MIC]
at Texas A&M). The acquisition of the FEI Quanta 600 FE-SEM
was supported by the National Science Foundation (DBI 0116835),
the VP for Research Office, and TEES. The University of Mainz
is gratefully acknowledged for financial support.
Supporting Information Available: Synthesis scheme of p-PNCPA,
experimental details, 1H NMR spectra of p-PNCPA, UV/vis absorption
and fluorescence emission spectra of p-PNCPA, SEM of dried
composite films, and hydrodynamic radius of p-PNCPA as a function
of temperature. This material is available free of charge via the Internet
at http://pubs.acs.org.
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