End Coupling of Block Copolymer Nanotubes to Nanospheres

Article abstract of DOI:10.1021/ja034847h

Triblock copolymer nanotubes bearing end-exposed poly(acrylic acid) or PAA core chains were prepared. The exposed PAA chains were reacted by amidization with a large excess of polystyrene spacer chains possessing amino end groups or amino-containing end b

Full text of DOI:10.1021/ja034847h

Published on Web 10/23/2003
End Coupling of Block Copolymer Nanotubes to Nanospheres
Guojun Liu,*,† Xiaohu Yan,† Zhao Li,† Jiayun Zhou,† and Scott Duncan‡
Contribution from the Department of Chemistry, UniVersity of Calgary,
2500 UniVersity DriVe, NW, Calgary, Alberta, Canada T2N 1N4, and
Defence Research DeVelopment Canada - Suffield, P.O. Box 4000 Station Main,
Medicine Hat, Alberta, Canada T1A 8K6
Received February 24, 2003; Revised Manuscript Received September 18, 2003; E-mail: gliu@ucalgary.ca
Abstract: Triblock copolymer nanotubes bearing end-exposed poly(acrylic acid) or PAA core chains were
prepared. The exposed PAA chains were reacted by amidization with a large excess of polystyrene spacer
chains possessing amino end groups or amino-containing end blocks to graft the spacer chains. The amino
groups at the other end of the spacer chains were then reacted with nanospheres bearing surface carboxyl
groups to connect the nanotubes to nanospheres. The products from such a coupling reaction ranged
from multiarm adduct to surfactant- and dumbbell-like objects. Product control using different strategies
was explored. The products may have interesting properties and applications.
PtBA following procedures reported before,8 where PtBA de-
notes poly(tert-butyl acrylate).
Introduction
There have many reports in the past several years on the
preparation of nanostructures from inorganic and organic
compounds and from block copolymers. For the nanostructures
to function as nanoelectronic devices, controlled coupling
between different components is essential. There have been, to
our knowledge, no reports on the coupling of different block
polymer nanostructures. In this paper, we report on the end
coupling of polystyrene-block-poly(2-cinnamoyloxyethyl meth-
acrylate)-block-poly(acrylic acid) or PS-PCEMA-PAA nano-
tubes to nanospheres bearing surface carboxyl groups.
The preparation of permanent block copolymer nanostructures
normally involves taking advantage of the block segregation
property of the polymers in either the solid state1 or a block-
selective solvent.2 The block-segregated structures are then
chemically processed,3 invoking selective domain cross-linking
or degradation, to yield stable nanostructures including nanofi-
bers,4 nanotubes,5 and thin films containing nanochannels.6,7 The
nanotubes used in this study were prepared from PS-PCEMA-
The preparation of these nanotubes involved casting films
from the triblock containing some PS homopolymer. In such
films, PtBA and PCEMA formed hexagonally packed concen-
tric core-shell cylinders dispersed in the PS matrix. The films
were then irradiated to cross-link the PCEMA shell cylinders.
PS-PCEMA-PtBA nanofibers were obtained after separating
the cross-linked cylinders by solubilizing the PS matrix chains
in THF. They were shortened by ultrasonication to expose the
core PtBA chains at the fiber ends. PS-PCEMA-PAA nano-
tubes were obtained after the hydrolysis of the tert-butyl groups
from the PtBA cores.
† University of Calgary.
‡ Defence Research Development Canada.
(1) For block segregation patterns of di- and tri-block solids, see, for example:
Bates, F. S.; Fredrickson, G. H. Phys. Today 1999, February, issue 32.
(2) For block segregation patterns of diblocks in block-selective solvents, see,
for example: Eisenberg, A. Can. J. Chem. 1999, 77, 1311.
(3) Liu, G. J. Curr. Opin. Colloid Interface Sci. 1998, 3, 200.
(4) See, for example: (a) Liu, G. J.; Qiao, L. J.; Guo, A. Macromolecules
1996, 29, 5508. (b) Liu, G. J.; Ding, J. F.; Qiao, L. J.; Guo, A.; Gleeson,
J. T.; Dymov, B.; Hashimoto, T.; Saijo, K. Chem.-Eur. J. 1999, 5, 2740.
(c) Massey, J.; Power, K. N.; Manners, I.; Winnik, M. A. J. Am. Chem.
Soc. 1998, 120, 9533. (d) Won, Y.-Y.; Davis, H. T.; Bates, F. S. Science
1999, 283, 960. (e) Templin, M.; Franck, A.; DuChesne, A.; Leist, H.;
Zhang, Y. M.; Ulrich, R.; Schadler, V.; Wiesner, U. Science 1997, 278,
5344.
(5) (a) Stewart, S.; Liu, G. J. Angew. Chem., Int. Ed. 2000, 39, 340. (b) Yan,
X. H.; Liu, F. T.; Li, Z.; Liu, G. J. Macrmolecules 2001, 34, 9112.
(6) (a) Liu, G. J.; Ding, J. F.; Guo, A.; Herfort, M.; Bazett-Jones, D.
Macromolecules 1997, 30, 1851. (b) Liu, G. J.; Ding, J. F. AdV. Mater.
1998, 10, 69. (c) Liu, G. J.; Ding, J. F.; Hashimoto, T.; Saijo, K.; Winnik,
F. M.; Nigam, S. Chem. Mater. 1999, 11, 2233. (d) Liu, G.; Ding, J.;
Stewart, S. Angew. Chem. 1999, 38, 835. (e) Hashimoto, T.; Tsutsumi, K.;
Funaki, Y. Langmuir 1997, 13, 6869.
The nanospheres used in this study bear surface carboxyl
groups and were prepared from emulsion polymerization or from
PCEMA-PAA.9 To couple the nanotubes to the nanospheres,
the end-exposed PAA chains of the nanotubes were first reacted
with an excess of a polymer spacer, which can be either poly-
[4-(2-aminoethyl)styrene]-block-polystyrene-block-poly[4-(2-
aminoethyl)styrene], PAES-PS-PAES (Scheme 1), or poly-
(7) (a) Lee, J.-S.; Hirao, A.; Nakahama, S. Macromolecules 1989, 22, 2602.
(b) Zalusky, A. S.; Olayo-Valles, R.; Taylor, C. J.; Hillmyer, M. A. J. Am.
Chem. Soc. 2001, 123, 1519. (c) Thurn-Albrecht, T.; Schotter, J.; Kastle,
G. A.; Emley, N.; Shibauchi, T.; Krusin,-Elbaum, L.; Guarini, K.; Black,
C. T.; Tuominen, M. T.; Russell, T. P. Science 2000, 290, 2126.
(8) (a) Liu, G. J.; Yan, X. H.; Qiu, X. P.; Li, Z. Macromolecules 2002, 35,
7742. (b) Yan, X. H.; Liu, G. J.; Liu, F. T. Angew. Chem., Int. Ed. 2001,
40, 3593.
(9) Henselwood, F.; Liu, G. J. Macromolecules 1997, 30, 488.
9
10.1021/ja034847h CCC: $25.00 © 2003 American Chemical Society
J. AM. CHEM. SOC. 2003, 125, 14039-14045
14039

A R T I C L E S
Liu et al.
Scheme 1. Coupling Nanotubes to Diblock Nanospheres
lithium amide solution in hexane. The solution was cooled to 0 °C
which resulted in the formation of a yellow precipitate. The precipitate
was redissolved with the addition of 30 mL of dry THF. Chloromethyl
methyl ether, 8.8 g or 110 mmol, was then added dropwise from the
addition funnel. This caused the lithium chloride salt to precipitate
shortly after from the reaction mixture. The mixture was left stirring
for 8 h before the salt was filtered and the solvent was removed by
rota-evaporation. The product was purified by vacuum distillation to
give 16 g of clear liquid (80% yield).
Grignard Reagent of 4-(Chloromethyl)styrene.13 To a 500-mL
three-neck round-bottomed flask fitted with an addition funnel and
purged with argon were added 150 mL of dry diethyl ether, 2.6 g or
107 mmol of magnesium, and a small iodine crystal. To the addition
funnel was added 14.5 mL or 100 mmol of distilled 4-chloromethyl
styrene in 50 mL of dry diethyl ether. Approximately 10 mL of the
4-chloromethyl styrene solution was added in one aliquot to the flask
to start the reaction. The reaction mixture was heated to 40 °C, and
stirring commenced. The remainder of the 4-chloromethyl styrene
solution was added dropwise over 2 h. After that, the reaction was
continued for another 30 min. The resultant solution contained a trace
amount of residual magnesium and had a green color.
styrene end-labeled with amino groups, NH2-PS-NH2, to yield
nanotubes containing amino end groups. The amino groups were
then reacted with nanospheres bearing surface carboxyl groups
(Scheme 1) to couple the nanotubes to nanospheres.
4-[2-N,N-Bis(trimethylsilyl)amino ethyl]styrene.14,15 N,N-Bis(tri-
methylsilyl)methoxymethylamine, 20.7 g or 100 mmol, was added
dropwise to the above Grignard reagent solution. The mixture was
stirred at room temperature for 24 h. The salt was then filtered, and
solvent was removed by rota-evaporation. The product was purified
by vacuum distillation twice to give 4-[2-N,N-bis(trimethylsilyl)amino
ethyl]styrene or TMS-AES as a colorless liquid with 50% yield.
Polymerization. Polymerization was done in a 1-L three-neck round-
bottomed flask attached to a vacuum line. The flask was first evacuated
and flamed. To it was distilled approximately 500 mL of THF. The
flask was then cooled in a dry ice acetone bath at -78 °C. The
impurities in THF were titrated with a 0.05-M lithium naphthalenide
initiator solution until a faint green color of the initiator was observed.
At this time, 20 mL of the initiator solution or 1.0 mmol of initiator
was added. This was followed by the injection of 5.0 mL or 44 mmol
of purified styrene. Styrene was polymerized for 1 h before 4.5 mL of
TMS-AES was added and polymerized for 1 h. The polymerization
was terminated with the addition of degassed methanol. After being
warmed to room temperature, the polymerization mixture was concen-
trated by rota-evaporation and was added into methanol to precipitate
out the polymer. The polymer was dried in vacuo.
Experimental Section
Reagents. Tetrahydrofuran (THF) was dried by refluxing with
potassium and a small amount of benzophenone until a deep purple
color developed and was distilled just before use. Dry diethyl ether
was obtained by refluxing and distillation over calcium hydride. Styrene
was first filtered through an alumina column and then stirred with
benzylmagnesium chloride at room temperature for 3 h. It was freshly
distilled under reduced pressure just before polymerization. The anionic
initiator lithium naphthalenide was prepared by the reaction of
naphthalene with an excess of lithium in THF.10 2,2,5,5-Tetramethyl-
1-(3-chloropropyl)-1-aza-2,5-disilacyclopentane was prepared following
a literature method.11
NH2-PS-NH2 with protected amino end groups was prepared
similarly. The polymerization was terminated by 2,2,5,5-tetramethyl-
1-(3-chloropropyl)-1-aza-2,5-disilacyclopentane.
Deprotection of the Amino Groups. To hydrolyze P(TMS-AES)-
PS-P(TMS-AES), 1.0 g of the triblock was dissolved in 15 mL of
THF containing 1.0 mL of 2.0 M HCl. The mixture was stirred for 1
h before solvent was removed by rota-evaporation. The polymer was
then washed with 1.0 M KOH aqueous solution and water and dried
under vacuum at 40 °C overnight. The amino end groups of NH2-
PS-NH2 were deprotected by stirring the sample in methanol overnight.
Polymer Characterization. The polymer PAES-PS-PAES was
characterized in the P(TMS-AES)-PS-P(TMS-AES) form, because
PAES did not dissolve well in organic solvents such as THF. The light
scattering (LS) and size exclusion chromatography (SEC) measurements
and the specific refractive index increment dnr/dc determination of the
polymer spacers were all performed in THF. Proton NMR characteriza-
tion was done in CDCl3. The amino group content in NH2-PS-NH2
was determined by acid-base titration.11
N,N-Bis(trimethylsilyl)methoxymethylamine.12 A 500-mL three-
neck round-bottomed flask was fitted with an addition funnel and purged
with argon. To it was added 100 mL of a 1.0-M N,N-bis(trimethylsilyl)-
The LS instrument used was a Brookhaven BI-200SM model
equipped with 10-mW He-Ne laser operating at 632.8 nm. The
(10) Sorenson, W. R.; Sweeny, F.; Campbell, T. W. PreparatiVe Methods of
Polymer Chemistry, 3rd ed.; John Wiley & Sons: New York, 2001; p 295.
(11) Ueda, K.; Hirao, A.; Nakahama, S. Macromolecules 1990, 23, 939.
(12) Peters, M. A.; Belu, A. M.; Linton, R. W.; Dupra, W.; Meyer, T. J.;
Desimone, J. M. J. Am. Chem. Soc. 1995, 117, 3380.
(13) Gilman, H.; Catlin, W. E. Org. Synth. 1937, Col. Vol. 1, 417.
(14) Morimoto, T.; Takahashi, T.; Sekiya, M. J. J. Chem. Soc., Chem. Commun.
1984, 794.
(15) Suzuki, K.; Hirao, A.; Nakahama, S. Makromol. Chem. 1989, 190, 2893.
9
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Coupling of Block Copolymer Nanotubes to Nanospheres
A R T I C L E S
determination of dnr/dc was achieved with a differential refractometer
from Precision Instruments. SEC was performed on a Waters instrument
using a Waters HT-4 broadband column. The SEC column was
calibrated in THF using poly(methyl methacrylate) standards.
Reaction between Nanotubes and Spacer Chains. An example
run involved first mixing nanotubes (45 mg containing 0.067 mmol of
carboxyl groups), DMF/water (v/v ) 98/2, 3.0 mL), NH2-PS-NH2
(0.60 g containing 0.085 mmol of NH2 groups), and 1-hydroxybenzo-
triazole (HBA, 18 mg, 0.133 mmol) with 1-[3-(dimethylamino)propyl]-
3-ethylcarbodiimide hydrochloride (EDCI, 26 mg, 0.136 mmol). After
the mixture was stirred at room temperature for 12 h, another 26 mg
of EDCI and 18 mg of HBA were added, and the reaction was allowed
to proceed for another 12 h. The nanotubes were then precipitated into
methanol, and the precipitate was redispersed in THF. The THF solution
was dialyzed in a dialysis tube (Spectra/Por, molar mass cutoff )
500 000 g/mol) against THF that was changed some 10 times over a
week under nitrogen atmosphere to remove the coupling agent that
was not chemically attached to the nanotubes. The purified nanotubes
were precipitated into methanol, redispersed in DMF, and dialyzed in
another tube (Spectra/Por, molar mass cutoff ∼14 000 g/mol) against
DMF to remove the residual methanol under nitrogen atmosphere.
Coupling Nanospheres and Nanotubes. To connect the PAES-
PS-PAES-treated nanotubes with the nanospheres, an example initial
amidation mixture consisted of nanotubes (3.0 mg containing 4.5 µmol
of carboxyl groups), the emulsion spheres (3.0 mg containing 7.6 ×
10-3 µmol of carboxyl groups), 1.0 mL of DMF/water (v/v ) 98/2),
triethylamine (4.5 mg, 45 µmol), EDCI (1.7 mg, 9 µmol), and HBA
(1.2 mg, 9 µmol). The mixture was stirred for 12 h before another
batch of EDCI (1.7 mg, 9 µmol) and HBA (1.2 mg, 9 µmol) was added,
and the mixture was left stirring for another 12 h. After that, the mixture
was diluted with more DMF and centrifuged at 1750 × g to settle the
spheres and the coupled products. The precipitate was redispersed in
DMF/THF (v/v ) 80/20) and settled again by centrifugation. After
this rinsing step was repeated three to four times, the sample was
dispersed in DMF for characterization by transmission electron
microscopy (TEM).
The product from coupling the nanotubes with PCEMA-PAA
nanospheres was purified by dialysis against THF. This caused most
of the nanospheres to precipitate due to their low solubility in THF.
The aggregated nanospheres were separated from the supernatant con-
taining the soluble nanotubes and the coupled products by centrifuga-
tion.
Reaction between Rhodamine B and Grafted NH2-PS-NH2
Chains. To label with Rhodamine B the free amino groups of the
NH2-PS-NH2 chains that have been grafted to the nanotube ends,
the initial amidation mixture consisted of 20 mg of nanotubes, 20 mg
or 0.042 mmol of Rhodamine B (Aldrich, dye content ∼90%), 2 mL
of DMF, 17.0 mg or 0.089 mmol of EDCI, and 11.5 mg or 0.085 mmol
of HBA. The mixture was stirred for 12 h before another batch of EDCI
(17.0 mg, 0.089 mmol) and HBA (11.5 mg, 0.085 mmol) was added,
and the mixture was left stirring for another 12 h. After reaction, the
sample was purified by precipitation into methanol and dialysis of the
redispersed sample in THF against replenishing THF/methanol (v/v )
90/10) for 1 week and then THF until no further colored species left
the tube. Absorption of Rhodamine B was analyzed at 543 nm.
PS-PCEMA-PAA Nanotube Synthesis. The first step involved
the preparation of a 15 wt % toluene solution of the triblock (150 mg)
and a polystyrene homopolymer (60 mg, Mn ) 2500 g/mol, Mw/Mn )
1.07). The solution was poured in a ring glued onto a leveled glass
plate. The top of the ring was covered with another glass plate to slow
the evaporation of toluene to 4-5 days. This yielded a film that was
∼50 µm thick. In step 2, the film was annealed at 120 °C under vacuum
for 2 days to achieve more regular packing of the cylinders. Step 3
involved film irradiation with a focused beam, from a 500-W Hg lamp,
that had passed a 302-nm cutoff filter to cross-link the PCEMA shell
cylinders.16 In step 4, the irradiated films were stirred in 500 mL of
THF for 4-5 days to separate the cross-linked cylindrical domains
(nanofibers). The nanofibers were separated from the insoluble gels
by centrifugation at 1350 × g. Methanol, ∼150 mL, was then added
gradually into the supernatant to precipitate the nanofibers, which were
separated from the solubilized PS homopolymer again by centrifugation.
In step 5, the redispersed nanofibers in THF at ∼1.5 mg/mL were
shortened by ultrasonication17 in a Branson model 1200 R-C (voltage
equals 117 V and current equals 1.3 A) ultrasonicator for 8 h to expose
the core chains at the ends. Finally, PS-PCEMA-PAA nanotubes were
obtained after hydrolyzing the tert-butyl groups from the PtBA cores
of the nanofibers at 2 mg/mL in CH2Cl2 containing 25 vol %
trifluoroacetic acid for 2 h. For solvent switching, the nanofibers were
precipitated in methanol first and then dispersed in the second solvent
without fully drying the fibers.
Emulsion Nanospheres. The emulsion spheres are of the core-
shell type.18 To prepare the core particles, 6.0 g of tert-butyl acrylate
(tBA), 3.5 g of methyl methacrylate (MMA), 0.50 g of ethylene glycol
dimethacrylate (EGDMA, cross-linker), 100 mL of water, 0.030 g of
poly(ethylene glycol) monolaurate [CH3(CH2)10CO(OCH2CH2)nOH,
Aldrich, Mn ≈ 600 g/mol, emulsifier], and 0.12 g of potassium persulfate
(Aldrich, initiator) were stirred at 150 rpm at room temperature for 5
min and then immersed into an oil bath preheated to 80 °C. The
polymerization was allowed to proceed for 1.5 h. After that, 4.75 g of
MMA, 0.25 g of EGDMA, and 0.020 g of poly(ethylene glycol)
methacrylate [CH2dC(CH3)CO(OCH2CH2)nOH, PEG-MA, Aldrich,
Mn ≈ 526 g/mol] mixed in one syringe were injected concurrently with
0.12 g of potassium persulfate solubilized in 50 mL of water in another
syringe over a 2 h span to enable shell formation. The polymerization
was continued for another 1 h after reagent addition. The spheres thus
prepared were settled by centrifugation and cleaned by repeated washing
with water, methanol, and THF. To convert the surface hydroxyl groups
of PEG-MA to carboxyl groups, the spheres were reacted with a 100
molar excess of succinic anhydride in dry pyridine at 50 °C overnight.19
PCEMA-PAA Nanospheres. The second type of nanospheres was
prepared from PCEMA-PtBA. Step 1 involved dissolving 200 mg of
PCEMA-PtBA in 20 mL of THF. To the solution was then added 80
mL of 2-propanol, which is a precipitant for PCEMA, to induce
formation of spherical micelles with PCEMA cores and PtBA coronas.
Nanospheres were obtained after photolysis to achieve a CEMA double
bond conversion of 31%. The PtBA coronal chains were hydrolyzed
to PAA chains by stirring the spheres in CH2Cl2 containing 20 vol %
trifluoroacetic acid for 4 h. The nanospheres after hydrolysis were
separated from the supernatant by centrifugation and redispersed in
DMF. The spheres were further purified by dialysis against DMF.
(16) For the CEMA double bond conversion determination, see, for example:
Guo, A.; Tao, J.; Liu, G. J. Macromolecules 1996, 29, 2487.
(17) For length variation with ultrasonication time, see, for example: Liu, G.
J.; Yan, X. H.; Duncan, S. Macromolecules 2002, 35, 9788.
(18) See, for example: (a) Westby, M. J. Colloid Polym. Sci. 1988, 266, 46.
(b) Levesque, G.; Moitie, V.; Bacle, B.; Depraetere, P. Polymer 1988, 29,
2271.
Reaction between NH2-PS-NH2 and Rhodamine B. To react
NH2-PS-NH2 with Rhodamine B, the amount of NH2-PS-NH2,
Rhodamine B, EDCI, and HBA used initially was 40, 40, 32.8, and
22.8 mg, respectively. The amount of DMF used was 5.0 mL. Another
(19) Li, Z.; Liu, G. J.; Law, S.-J.; Sells, T. Biomacromolecules 2002, 3, 984.
9
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A R T I C L E S
Liu et al.
Table 1. Characteristics of the Polymers
LS M × 10-4
(g/mol)
NMR repeat unit
number ratio
w
sample
SEC M /M
dn /dc (mL/g)
r
n
m
l
w
n
PS-PCEMA-PtBA
PCEMA-PtBA
P(AES-TMS)-PS-P(AES-TMS)
NH2-PS-NH2
1.17
1.18
1.17
1.26
0.151
0.121
0.184
0.195
11.5
26
2.0
1.00/0.25/0.28
0.95/1.00
7.2/1.00
560
660
115
140
140
700
16
160
1.42
batch of catalysts equal in amounts to the first was added 12 h after
the first one, and a total of 24 h was allowed for the reaction. After
reaction, the polymer was precipitated into methanol and washed with
methanol several times to remove most of the excess Rhodamine B.
The precipitate was redispersed in distilled THF and dialyzed against
THF/methanol (v/v ) 90/10) (Spectra/Pro, molar mass cutoff ) 1000
g/mol) for 4 days. It was finally dialyzed against distilled THF, and
the resultant solution was used for visible absorption measurement.
Transmission Electron Microscopy. All transmission electron
microscopy (TEM) studies were performed on a Hitachi H-7000
instrument operated at 75 kV. Specimen of the PCEMA-PAA
nanospheres and their coupled products were prepared by aspirating
methanol and THF solutions of the two on carbon-coated copper grids
using a home-built device.20 TEM specimens of the emulsion spheres
and their coupled products were prepared by aspirating DMF solutions
of the samples onto water surface. The films on water surface were
then picked up by carbon-coated copper grids. All samples except the
emulsion sphere sample were stained with OsO4 vapor for 4 h before
being viewed by TEM.
The polymer grew at both ends of the initiating molecule.
Styrene polymerization should essentially complete within
minutes, but we allowed 1 h for the process. The resultant poly-
styrene dianions were then used to initiate TMS-AES polym-
erization,14 which was also given 1 h to complete. The polym-
erization was terminated by the addition of some methanol.
To prepare NH2-PS-NH2, the styrene polymerization was
also initiated with lithium naphthalenide.11 The polymerization
was terminated by 2,2,5,5-tetramethyl-1-(3-chloropropyl)-1-aza-
2,5-disilacyclopentane, and the amino groups were deprotected
by 2,2,5,5-tetramethyl-1-aza-2,5-disilacyclopentane ring opening
in methanol.11 An acid-base titration experiment showed that
the end labeling of the PS chains by amino groups was higher
than 95%.11 The preparation procedure for 2,2,5,5-tetramethyl-
1-(3-chloropropyl)-1-aza-2,5-disilacyclopentane was not de-
scribed in any detail in the Experimental Section, because it
was well documented in ref 11.
Polymer Characterization. All polymers were carefully
characterized by LS, 1H NMR, and SEC. LS allowed the deter-
mination of the weight-average molar masses Mw of the samples.
Results and Discussion
Polymer Synthesis. A total of four polymers were prepared
for this project. The procedures for PS-PCEMA-PtBA8 and
PCEMA-PtBA9 syntheses were not given in the Experimental
Section, as they have been reported before. Briefly, PS-
PCEMA-PtBA was derived from PS-P(HEMA-TMS)-
PtBA, where HEMA-TMS denotes 2-trimethylsiloxyethyl
methacrylate, and PCEMA-PtBA was derived from P(HEMA-
TMS)-PtBA. Both PS-P(HEMA-TMS)-PtBA and P(HEMA-
TMS)-PtBA were prepared by anionic polymerization.
The polymer spacer PAES-PS-PAES was obtained from
hydrolyzing P(TMS-AES)-PS-P(TMS-AES). Monomer
TMS-AES was prepared following a literature method invoking
the following reactions:12-15
1
Analysis of the intensity ratios of the H NMR peaks of the
different blocks yielded the relative lengths of the blocks. The
weight-average number of repeat units for each block was
obtained by combining the LS and NMR results. SEC was used
mainly to obtain Mw/Mn as a measure for the width of the molar
mass distribution of the samples, where Mn denotes the number-
average molar mass of a sample. The polymer characterization
results thus obtained are summarized in Table 1. The n, m, and
l values denote the numbers of repeat units for the appropriate
blocks with their definition given in the formulas for the block
copolymers.
Nanotubes. The nanotubes were prepared by taking advan-
tage of the block-segregation properties of the triblock co-
polymer in the solid state. PS homopolymer was mixed with
PS-PCEMA-PtBA in toluene before film casting to ensure
that the volume fractions of PCEMA and PtBA in the final solid
were ∼20% and ∼10%, respectively. At such ratios, PCEMA
and PtBA formed shell-core cylinders dispersed in the PS
matrix.21 The shell PCEMA cylinders were then cross-linked
with UV light, and the cross-linked shell-core cylinders were
separated from one another by PS chain solubilization in THF.
The cylinders were shortened by ultrasonication to expose the
PtBA core chains. Nanotubes with end-exposed PAA chains
were prepared after PtBA core hydrolysis. Following this
approach, several batches of nanotubes were prepared. The
CEMA conversion of the nanotubes was typically ∼23% unless
otherwise specified.
P(TMS-AES)-PS-P(TMS-AES) was prepared by living
anionic polymerization at -78 °C in THF using lithium naph-
thalenide as the initiator following the procedure of Nakahama
and co-workers.15 The initiation mechanism was:
Figure 1a shows a TEM image of a nanotube sample prepared
from the hydrolysis of the PS-PCEMA-PtBA nanofibers that
(20) Ding, J. F.; Liu, G. J. Macromolecules 1999, 32, 8413.
(21) Breiner, U.; Krappe, U.; Abetz, V.; Stadler, R. Macromol. Chem. Phys.
1997, 198, 1051.
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Coupling of Block Copolymer Nanotubes to Nanospheres
A R T I C L E S
Figure 1. TEM images of (a) the nanotubes, (b) emulsion spheres, and (c) the diblock nanospheres.
Table 2. Effect of Varying [NH2-PS-NH2] and CEMA Double
the quantitative grafting of PEG-MA onto the nanosphere
surface, and the quantitative conversion of the hydroxyl groups
of PEG-MA to carboxyl groups, we estimated an upper bound
of 40 000 carboxyl groups per nanosphere.
Bond Conversion on nc
nanotube
sample
Ln
(nm)
10-8 × M
(g/mol)
CEMA
conversion
[NH −PS−NH ]
2 2
n
(g/mL)
n
c
1
2
410
460
4.0
41%
23%
0.075
0.30
0.075
0.30
38
74
92
PCEMA-PAA Nanospheres. This type of sphere was pre-
pared by taking advantage of PCEMA-PtBA micelle formation
in the block-selective solvent mixture THF/2-propanol. Such
micelles possess cores made of the insoluble PCEMA block
and coronas of the soluble PtBA block. Nanospheres were
prepared after PCEMA core cross-linking by photolysis. PAA
coronal chains were obtained after PtBA hydrolysis. Figure 1c
shows a TEM image of the PCEMA-PAA nanospheres stained
with OsO4. The core diameter averaged over 78 spheres is 28
( 4 nm. Using this core diameter and assuming a density of
1.0 g/mL for the core, we estimated an aggregation number of
∼40 for the nanospheres. Because the number of repeat units
for PAA is 700, the total number of carboxyl groups per
PCEMA-PAA nanosphere is ∼28 000.
4.5
104
had been ultrasonicated for 8 h. Better TEM images and
spectroscopic evidence for nanotube preparation can be found
in a prior publication.8b Statistical analysis of the lengths of
188 tubes for the sample shown in Figure 1a yielded the weight-
and number-average lengths Lw and Ln of 701 and 515 nm,
respectively. Another two batches of samples obtained under
similar conditions possessed Ln values of 410 and 460 nm (Table
2), respectively. Before ultrasonication, the Ln value of the
nanofibers was ∼5 µm. Thus, most of the nanotubes used in
this study had two exposed ends.
Amidation. Four kinds of amidation experiments were
performed. The first type involved reaction between PAA chains
of the nanotubes and the amino groups of the polymer spacers,
and the second type was between the amino groups of the
spacers that have been grafted to the nanotubes and the surface
carboxyl groups of the nanospheres. In the third and fourth types,
amid bonds were formed between the carboxyl group of
Rhodamine B and the amino groups of either free NH2-PS-
NH2 chains or NH2-PS-NH2 chains that have been grafted to
the nanotube ends by one end only. While the receipts used
differed, the general experimental protocol was the same. It
involved first mixing carboxyl- and amino-containing compo-
nents in a solvent with the catalysts EDCI and HBA each at 2
molar equivalents relative to the carboxyl groups.22 When
PAES-PS-PAES was involved, the solvent used was DMF/
water (v/v ) 98/2), where water was used to increase PAES
solubility. Otherwise, the solvent used was DMF. In the second
type of amidation experiment, triethylamine at 10 molar excess
relative to the carboxyl groups was also used to ensure that the
amino groups to be reacted were in the neutral form. The initial
amidation mixture was stirred at room temperature for 12 h
before another batch of EDCI and HBA equal in amount to the
first batch was added. The mixture was then allowed to react
for another 12 h.
The number-average molar masses Mn of the nanotubes were
obtained from multiplying the TEM Ln and the unit length molar
mass Mu of 9.7 × 105 g/(mol‚nm) for nanotubes. Mu was
estimated following procedures detailed in ref 8a. Column 3 of
Table 2 gives the Mn values for two nanotube samples that will
be invoked in a later discussion.
Emulsion Nanospheres. The emulsion spheres used are of
the core-shell type prepared for another project. The core
consisted of a copolymer of MMA and tBA cross-linked with
EGDMA. The shell was made of cross-linked PMMA. PEG-
MA was used as the surfactant in shell formation and got
anchored chemically on the nanosphere surface. The hydroxyl
group at the other end of the immobilized PEG-MA chains
was converted to a carboxyl group via the following reac-
tion:19
The efficiency of this reaction should be high as succinic
anhydride reacted with poly(ethylene glycol) monolaurate under
similar conditions quantitatively.
Reaction between the Spacer Chains and Nanotubes. The
first step in the preparation of the nanotube-nanosphere-coupled
products involved grafting either NH2-PS-NH2 or PAES-
Figure 1b shows a TEM image of the emulsion spheres. The
average diameter is 365 ( 6 nm. As described in the
Experimental Section, the spheres were prepared by reacting
15.0 g of various monomers with 0.020 g or 3.8 × 10-5 mol of
PEG-MA. Assuming a density of 1.0 g/mL for the spheres,
(22) See, for example: Zeng, H.; Miller, R. S.; Flowers, R. A.; Gong, B. J.
Am. Chem. Soc. 2000, 122, 2635.
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J. AM. CHEM. SOC. VOL. 125, NO. 46, 2003 14043

A R T I C L E S
Liu et al.
PS-PAES chains to the ends of PS-PCEMA-PAA nano-
tubes. To estimate the number nc of NH2-PS-NH2 chains
attached to each nanotube end, we made use of the fact that
NH2-PS-NH2 was used in a large excess and assumed that
NH2-PS-NH2 reacted with the nanotubes by only one amino
group or one end. Thus, nc determination was reduced to the
measurement of the amino group content. To determine the
amino group content, the amino groups were reacted with a
large excess of Rhodamine B for their quantitative labeling by
Rhodamine B. The validity of the quantitative labeling assump-
tion was deduced from the quantitative reaction of the amino
groups in a control experiment involving the free NH2-PS-
NH2 chains and a 10 molar excess of Rhodamine B under
identical conditions. After removal of the unreacted Rhodamine
B from the nanotubes by prolonged dialysis, the amount of
Rhodamine B groups that were chemically attached to the
grafted NH2-PS-NH2 chains was analyzed by visible absorp-
tion spectrophotometry. To ensure that the Rhodamine B that
was detected by visible absorption spectrophotometry was
chemically attached to the amino groups of the grafted NH2-
PS-NH2 chains, we have also performed one more control
experiment. This involved stirring Rhodamine B under identical
conditions in the presence of EDCI, HBA, and nanotubes that
were not end-grafted with NH2-PS-NH2. After nanotube
purification by dialysis, the amount of Rhodamine B that was
retained with the nanotubes inside the dialysis tube was
negligible in this case.
ends of the spacer-grafted nanotubes. The PS backbone design
was retained to increase its compatibility with the PS nanotube
coronal chains and to prevent the penetration of the coupling
agent into the PAA-lined channels. Using this agent, we
determined that the efficiency of coupling between the nanotubes
and the PCEMA-PAA nanospheres increased only slightly to
∼40%. The increase in the efficiency of coupling between the
nanotubes and the emulsion spheres was, however, much more
dramatic as demonstrated by the abundance of the coupled
products. We could not estimate the coupling efficiency in this
case, as nanotubes that did not couple with the nanospheres
did not settle with the emulsion spheres during the centrifugation
purification step.
Coupled Products. Figure 2a-c shows the typical products
from coupling the PAES-PS-PAES-treated nanotubes with the
emulsion nanospheres. The product in Figure 2a resulted from
the coupling between one tube and one sphere. Because the
spherical “head” is water-dispersible and the tube “tail” is
hydrophobic, this structure may be viewed as a macroscopic
counterpart of a surfactant molecule or a “supersurfactant”.
There is a light aura around each sphere in this image and also
in images 2b and 2c probably due to the fact that the spheres
extended out of the plane of focus.
Figure 2b shows the attachment of two tubes to one sphere,
which has fused with another sphere probably during TEM
specimen preparation. The attachment of one tube to two spheres
at the opposite ends is seen in Figure 2c. The products depicted
in Figure 2a-c coexisted regardless of how we changed the
tube to microsphere mass ratio from 20/1 to 1/20. At the high
tube to emulsion sphere mass ratio of 20/1, the supersurfactant
and dumbbell-shaped species were the major products. At the
mass ratio of 1/1 and 1/20, the dumbbell-shaped product
dominated. Other than product control by adjusting the stoi-
chiometry, an effective method to eliminate the dumbbell-shaped
product was to use nanotubes labeled with spacer chains at only
one end. These tubes were obtained by breaking up nanotubes
that contained end-grafted polymer spacer chains by ultrasoni-
cation. For example, the ultrasonication of the PAES-PS-
PAES-grafted nanotubes for 8 h reduced Lw of a nanotube
sample from 701 to 252 nm and Ln from 515 to 187 nm. The
reaction between the shortened tubes and the nanospheres at
the tube to sphere mass ratio of 1/20 yielded almost exclusively
the supersurfactant structure with unreacted nanospheres. The
content of the multiarmed structure increased as the mass ratio
increased.
Table 2 shows the variation in nc thus determined as a
function of NH2-PS-NH2 concentration and PCEMA shell
cross-linking density while holding the other parameters of the
amidation protocol constant as described in the Experimental
Section. The data suggest that nc increased with increasing
NH2-PS-NH2 concentration and decreasing nanotube cross-
linking density. A lower PCEMA cross-linking density leads
to a larger nc because the PAA core swells more under this
condition and a larger core cross-section facilitates the grafting
of more NH2-PS-NH2 chains. Increasing bulk concentra-
tion of NH2-PS-NH2 helps to reduce the spacer chain
concentration gradient between the grafted NH2-PS-NH2
layer and the bulk and thus favors a higher grafting density.
Under the typical conditions of [NH2-PS-NH2] ) 0.30 g/mL
and a CEMA conversion of ∼23%, nc is ∼100. We expect the
nc values for PAES-PS-PAES to be similar, because the nc
value is limited mainly by the number of chains that can be
squeezed into the grafted layer and not by the reactivity between
the amino and carboxyl groups. The fact that the grafted layer
is crowded and has a “brush” conformation can be easily
appreciated by performing calculations following procedures
detailed before.23
An effective method to eliminate the multiarmed structure is
to use smaller spheres. Figure 2d shows a TEM image of
products formed from coupling the nanotubes to the much
smaller PCEMA-PAA spheres. Multitube attachment to the
same sphere was not seen after the examination of many TEM
images. It appears that the smaller surface area of these spheres
impeded multitube attachment.
Performance of Different Polymer Spacers. NH2-PS-NH2
was the first polymer spacer that we prepared and used. This
polymer was ineffective in coupling the nanotubes to the
emulsion spheres. When this polymer was used to couple the
nanotubes to the PCEMA-PAA spheres, the fraction of
nanotube ends that were labeled by nanospheres as seen by TEM
was ∼30%. The coupling efficiency improved in the latter case
probably due to the much higher local concentration of carboxyl
groups in the PAA coronas than in the emulsion sphere coronas.
Potential Applications. The supersurfactants shown in Figure
2a may self-assemble like surfactant molecules to form micelles
and help order molecules on the micrometer size scale.
Alternatively, a supersurfactant with a sufficiently long fluo-
rescent tail may be studied by the optical tweezers method to
(23) See, for example: (a) Ding, J. F.; Tao, J.; Guo, A.; Stewart, S.; Hu, N.;
Birss, V. I.; Liu, G. J. Macromolecules 1996, 29, 5398. (b) Tao, J.; Guo,
A.; Liu, G. J. Macromolecules 1996, 29, 1618.
The second polymer spacer PAES-PS-PAES was designed
and prepared to increase the amino group concentrations at the
9
14044 J. AM. CHEM. SOC. VOL. 125, NO. 46, 2003

Coupling of Block Copolymer Nanotubes to Nanospheres
A R T I C L E S
Figure 2. TEM images of nanotube and nanosphere coupling products. The arrows indicate nanosphere attachment.
interrogate the dynamics of the nanotubes.24 Such studies will
help verify if the dynamic theories developed for polymer chains
would still apply to large objects such as nanotubes.
with an average length of ∼5 µm. The fibers were shortened
by ultrasonication to expose the PtBA core chains at the fiber
ends. PS-PCEMA-PAA nanotubes were obtained after hy-
drolyzing the tert-butyl groups from the PtBA cores. Reacting
the end-exposed PAA chains with excess PAES-PS-PAES
or NH2-PS-NH2 yielded nanotubes containing amino end
groups. The number of coupling chains that were attached to
each nanotube end under our typical reaction conditions was
∼100. The amino groups were then reacted with nanospheres
bearing surface carboxyl groups prepared from PCEMA-PAA
or emulsion polymerization to couple the nanotubes to nano-
spheres. The products from such coupling reaction ranged from
multiarm adduct to surfactant- and dumbbell-like objects and
may have interesting properties and applications. The coupling
products can be controlled either by changing the nanotube to
nanosphere mass ratio or by changing the size of the nano-
spheres. The use of nanotubes labeled with the coupling agent
at only one end helped to eliminate the dumbbell-shaped
product.
The structure of Figure 2b will be of particular interest if the
nanotubes are replaced with Fe2O3-impregnated PS-PCEMA-
PAA nanotubes or PS-PCEMA-PAA/Fe2O3 hybrid nanofibers.
The hybrid nanofibers are superparamagnetic as demonstrated
previously.8b They attracted one another in a magnetic field due
to magnetization and demagnetized once the field was off. After
their attachment to a sphere to form the structure in Figure 2b,
the closing motion of the fibers or “fingers” in a magnetic field
may serve as the basis for a magnetic nanohand.
Conclusion
Four polymers PS-PCEMA-PtBA, PAES-PS-PAES,
NH2-PS-NH2, and PCEMA-PtBA were prepared and char-
acterized. The PtBA and PCEMA blocks of PS-PCEMA-
PtBA formed concentric core-shell cylinders dispersed in the
continuous PS matrix in films of the triblock containing PS
homopolymer. Photo-cross-linking the PCEMA shell cylinders
and separating the cross-linked cylinders by solubilizing the PS
matrix chains in THF yielded PS-PCEMA-PtBA nanofibers
Acknowledgment. NSERC of Canada and Defence Research
Development Canada are cordially acknowledged for sponsoring
this research.
(24) See, for example: Perkins, T. T.; Quake, S. R.; Smith, D. E.; Chu, S. Science
1994, 264, 822.
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