Self assembly of an organicinorganic block copolymer for nano by mikesanye



Self-assembly of an organic–inorganic
block copolymer for nano-ordered
GE Global Research Center, 1 Research Circle, Niskayuna, New York 12309, USA
*These authors contributed equally to this work

Published online: 3 January 2007; doi:10.1038/nnano.2006.168

Self-assembly is a promising approach for achieving controlled
nanoscale architectures in ceramics. The addition of ceramic-
forming precursors to templating agents such as self-assembled
surfactants or organic block copolymers (BCPs) has thus far
been the primary route to forming ordered nanoporous
oxides1–5 and nanostructured non-oxide ceramics6–9. In spite of
its viability, however, this approach has several intrinsic
shortcomings, including: (1) stringent requirements for                                 Hybrid           CHCl3
amphiphilicity between template and precursor, lack of which                             BCP
may lead to macro-phase separation and loss of nano-scale
order; (2) morphologies that can change uncontrollably with
varying amounts of added ceramic precursor. Here we report a                                     Self-assembly       Ordered BCP                    Ordered ceramic
novel single-source ceramic precursor, based on a hybrid
organic– inorganic BCP of polynorbornene –decaborane, that
enables the formation of ordered ceramic nanostructures with                         Figure 1 The self-assembly and pyrolysis of a block copolymer
tunable morphology and composition. In particular, we                                (PNB-b-PDB30) comprising a polynorbornene (PNB) segment and a
describe the synthesis of nanostructured boron carbonitride                          decaborane-based one (PDB). When cast from solutions of tetrahydrofuran (THF)
and mesoporous boron nitride, the latter of which exhibits the                       a cylindrical morphology is obtained, where the organic PNB blocks (orange) form
highest reported surface area for this material to date.                             cylinders in a surrounding matrix made from the organic–inorganic PDB blocks
     Although infiltration methods have been used to synthesize                       (green). When this material is pyrolysed in an ammonia atmosphere at 1,000 8 C,
ordered ceramic structures at larger length scales10, the intrinsic                  the exclusively organic PNB blocks decompose and are removed from the system,
BCP precursor route provides a direct path towards ordered                           and the PDB matrix converts into boron nitride, which retains the ordering that was
nanostructured ceramics11. The simplicity of the process is                          created by the initial self-assembly of the BCP. In contrast, when PNB-b-PDB30 is
illustrated in Fig. 1, where a diblock copolymer, having two                         cast from a chloroform (CHCl3) solution, a lamellar structure is formed. When this
dissimilar polymer chains (blocks) linked via a single covalent                      material is pyrolysed in a nitrogen atmosphere at 1,000 8 C, a layered boron
bond, is first self-assembled into the desired ordered morphology                     carbonitride/carbon ceramic composite is obtained.
and is then pyrolysed to yield a highly ordered ceramic.
Non-oxide ceramics are generally used in high-temperature and
harsh chemical environments and the ability to manipulate these                      is, Mw/Mn ¼ PDI. A polymer sample in which all of the chains
materials on the nanoscale is expected to further expand their use                   have the same degree of polymerization results in a PDI of 1, and
in current applications as well as enable new ones.                                  a value of less than 1.2 is typically required for a BCP to form
     The formation of ordered nanoscale domains in BCPs using                        ordered assemblies12. Most high-temperature polymeric ceramic
self-assembly is affected by a number of parameters including the                    precursors are ill-defined molecular species13,14 characterized by
Flory –Huggins interaction parameter x , which reflects the                           low molecular weights and broad molecular weight distributions,
incompatibility between blocks, the degree of polymerization and                     precluding their incorporation into polymeric architectures (such
the volume fraction of the blocks12. Generally, self-assembly into                   as BCPs) that are capable of forming ordered nanoscale structures
highly ordered morphologies requires a fairly narrow molecular                       via self-assembly.
weight distribution for the BCP; that is, there should be very little                    Recently, Wei and Sneddon demonstrated the synthesis of
variation in the degree of polymerization of the polymer chains.                     boron carbide via poly(norbornene-decaborane) homopolymers
A measure of this is known as the polydispersity index (PDI),                        derived from ring-opening metathesis polymerization (ROMP)15–17.
which is defined as the ratio of the weight-average molecular                         Advances in ruthenium-based ROMP catalysts by Grubbs have
weight (Mw) to the number-average molecular weight (Mn), that                        enabled the synthesis of various norbornene based block

nature nanotechnology | VOL 2 | JANUARY 2007 |                                                                                    43

                                    Ru catalyst                         Ru

                                                                                                                                        H      H
                                                                                                                                       H        H

                                                         H   H                                Mes        N       N Mes
                                                     H           H                                               Cl Ph           H3C         CH3
                                                                                                     N Ru
                                 Decaborane (DB) =                       Ru catalyst =                                   Mes =
                                                                                                    Cl       N
                                                                                         Br                                            CH3


Figure 2 The chemical synthesis of the hybrid block copolymer containing PNB and PDB segments. Norbornene is a highly reactive monomer that undergoes a
ring-opening metathesis polymerization (ROMP) when reacted with the third-generation ruthenium catalyst developed by Grubbs. Once all of the monomer has been
consumed and polymeric chains of norbornene have been formed, the reactive Ru moiety remains at the chain ends. When the second monomer is introduced a
decaborane compound with a norbornene group attached to it this compound polymerizes from the ends of the PNB chains to form a second block. For experimental
details and procedures see the Methods section.

copolymers with high molecular weights and narrow molecular                      in the cross-sectional transmission electron microscopy (TEM)
weight distributions18. The exploitation of these recent                         micrograph of Fig. 3a, a hexagonally packed cylindrical
developments provides a direct path to well-defined organic –                     morphology results when THF is used as the solvent. However,
inorganic hybrid block copolymers for nanostructured, boron-                     when chloroform was used as the casting solvent, a lamellar
based ceramic synthesis.                                                         morphology was obtained (Fig. 3b).
    In this work, the ROMP method is used to synthesize                              For PNB-b-PDB30, which has a symmetric composition (the
well-defined      polynorbornene-block-polynorbornenedecaborane                   volume ratio for the PNB block and the PDB block is close to
(PNB-b-PDB) copolymers in which the ceramic-forming                              1:1), a lamellar morphology would have been predicted by
polynorbornenedecaborane (PDB) block contains pendant                            classical BCP phase diagrams. This is indeed the case for a non-
decaborane moieties on a polynorbornene (PNB) backbone                           selective solvent such as chloroform, but a cylindrical morphology
(Fig. 2). Norbornyl decaborane was synthesized according to a                    was obtained from THF solutions. In block copolymer
procedure described previously15. Block copolymers were                          microphase-separated structures prepared by solvent casting, it is
synthesized by introducing a dichloromethane solution of                         well known that the block selectivity of the solvent can greatly
norbornene to a solution of Grubbs generation-three ruthenium                    influence the morphology19,20. The formation of a cylindrical
catalyst16 at 240 8C in a nitrogen atmosphere. Reactions are                     structure in this system indicates that THF preferentially solvates
typically complete within 30 min, at which point the norbornyl                   the PDB block. Elemental mapping by electron energy loss
decaborane monomer is added as a concentrated                                    spectroscopy establishes that the PNB block forms the cylindrical
dichloromethane solution. The reaction is allowed to warm up                     phase and the boron-containing PDB block forms the matrix (see
and maintained between 220 and 210 8C, and the decaborane-                       Supplementary Information, Fig. S1).
functionalized monomer is quantitatively consumed within                             Following solvent evaporation, the resulting film cast from
30 min. After quenching the polymerization with ethyl vinyl                      THF was thermally annealed at 100 8C for 24 h in a nitrogen
ether, the catalyst is removed using standard flash                               atmosphere, and then transferred to a tube furnace where it was
chromatography techniques. Polymer solutions are concentrated                    heated in an ammonia atmosphere to form boron nitride.
prior to precipitation into pentane and the polymer is typically                 Thermal gravimetric analysis in argon indicates that these block
isolated by centrifugation with yields .90%.                                     copolymers begin to decompose with an onset temperature of
    Analysis of the polymers by proton nuclear magnetic                          242 8C, which represents the crosslinking and decomposition of
resonance spectroscopy confirms that the relative ratio of the two                the ceramic precursor moieties, whereas the polynorbornene
blocks matches very closely the loading of the respective                        backbone initiates decomposition at about 400 8C. The pyrolysis
monomers in the feed, attesting to the near-quantitative                         process involved heating the sample from room temperature to
conversion of the polymerization. Molecular weights were                         400 8C (at 5 8C min21) with a dwell time of 1 h, followed by a
determined by gel permeation chromatography with multi-angle                     ramp cycle to heat the sample to 1,000 8C (at 1 8C min21), where
laser light scattering and refractive index detection in                         it was held for 4 h. Figure 4a shows an SEM micrograph of the
tetrahydrofuran (THF). The analysis confirms the formation of                     pyrolysed material cast from THF, showing a mesoporous
high-molecular-weight polymers in the range of 50–100 kDa with                   structure in which the highly ordered cylindrical morphology
PDI , 1.2. Results presented hereafter have been obtained with a                 is retained. As expected, the pyrolysed material cast from
30 mol% norbornene– decaborane containing block copolymer                        chloroform provided a layered ceramic (Fig. 4b). In this case,
(PNB-b-PDB30) in which Mn ¼ 61,000 and PDI ¼ 1.12.                               pyrolysis was carried out in a nitrogen atmosphere, leading
    Self-assembly of PNB-b-PDB30 was accomplished by first                        to a layered boron carbonitride/carbon ceramic composite
dissolving the polymer in solvent and allowing it to evaporate                   as assessed by X-ray energy dispersion spectroscopy (see
slowly under an inert atmosphere at room temperature. As shown                   Supplementary Information, Fig. S1).

44                                                                                nature nanotechnology | VOL 2 | JANUARY 2007 |
 a                                         b                                         a

                                                                                                           20 nm

                              100 nm                                     100 nm

Figure 3 TEM bright-field images of the self-assembled microstructures
of PNB-b-PDB30 after solvent evaporation. a, TEM showing the end view                                                                                       200 nm
of a hexagonally packed cylindrical structure obtained using THF as the
solvent. The PNB phase forms the cylinders, and boron-containing PDB forms
the matrix, as revealed by the elemental boron map (see Supplementary
Information Fig. S1 for the map). b, TEM bright-field image of a self-
assembled microstructure of PNB-b-PDB30 after chloroform evaporation,
showing a lamellar morphology. The PNB phase forms the darker layers,
and boron-containing PDB forms the lighter layers, as revealed by the
elemental boron map.

                                                                                                           50 nm
    In samples cast from THF, the ceramization of the PDB block
forms the boron nitride matrix, with cylindrical voids left behind
by decomposition of the PNB. Pyrolysis of the PDB in ammonia
further involves the substitution of carbon with nitrogen,
eventually leading to a ceramic composed primarily of boron
nitride21,22, as revealed by X-ray energy dispersion spectroscopy
analysis, electron energy loss spectroscopy analysis and X-ray                                                                                           100 nm
diffraction. High-resolution TEM imaging reveals a local structure
consistent with turbostratic boron nitride (see Supplementary
Information, Fig. S1). Pyrolysis in ammonia up to 1,000 8C leads
to a 59.7% yield of ceramic, where a theoretical yield of 60% is                     Figure 4 SEM images of a pyrolysed hybrid BCP. a, SEM of boron nitride
expected. Elemental analysis reveals that the ceramic contains 7%                    originating from self-assembled PNB-b-PDB30 cast from THF after pyrolysis to
oxygen and 3% carbon when heated to 1,000 8C. In this case, the                      1,000 8 C in ammonia; the inset is a higher magnification image of the end view
boron nitride ceramic is amorphous, but when this material                           of the hexagonally packed cylindrical mesoporous structure. b, SEM image of
is further heated to 1,400 8C for 2 h, hexagonal boron nitride is                    boron carbonitride originating from self-assembled PNB-b-PDB30 cast from
obtained, as illustrated by the X-ray diffraction results (see                       chloroform after pyrolysis in nitrogen at 1,000 8 C; the inset is a higher
Supplementary Information, Fig. S1). Small-angle X-ray scattering                    magnification image, which clearly reveals the lamellar nature of
analysis supports the cylindrical morphology observed by SEM,                        the microstructure.
and further indicates that the cylindrical structure is maintained
after pyrolysis at temperatures as high as 1,400 8C (see
Supplementary Information, Fig. S5).                                                 walls. This hierarchical pore structure has the potential to be
    For samples pyrolysed at 1,000 8C, the surface area was                          exploited in various applications, particularly in catalysis28,29.
analysed by measuring nitrogen adsorption/desorption isotherms                       Furthermore, the process described in the present paper may
at –196 8C. Brunauer– Emmett– Teller (BET) analysis (see                             produce not only powders, but also continuous structures, such
Supplementary Information, Fig. S6) of these isotherms confirms                       as free-standing films, fibres and coatings, as is typical for
the presence of a highly mesoporous structure with a nitrogen                        polymeric ceramic precursors.
adsorption value close to 700 cm3 g21, a surface area as high as                        In our system, the ROMP method enables the synthesis of
950 m2 g21 and a bimodal pore-size distribution. To our                              well-defined organic –inorganic hybrid BCPs with relative ease
knowledge, this is the first demonstration of using a template-free,                  (that is, without the need for stringent atmospheric
BCP self-assembly approach to create mesoporous boron nitride,                       conditions). Varying the casting solvent type affords direct
and a surface area of 950 m2 g21 is the highest reported to date                    control of BCP morphology, and pyrolysis atmosphere
for this material23–26. Decomposition of the PNB block leaves                        determines the specific chemical composition. Chemical doping
behind mesopores in which the pore size is narrowly distributed                      is certainly tenable, offering the prospect of functional ceramics
and centred at 20 nm diameter. The mesopores account for                            of ternary or even quaternary compositions. Indeed, the self-
more than 70% of the pore volume, yet the majority of the                            assembly approach described herein provides a versatile
surface area comes from micropores having a pore diameter                            framework for exploring the realization of nano-ordered
,2 nm. The existence of micropores is common in boron nitride                        ceramic composites in which the morphology and domain size
produced from polymeric precursors27. As a result, boron nitride                     can be tailored by controlling the interplay between BCP
derived from PNB-b-PDB30 can be described as a hexagonally                           molecular weight, relative block lengths and selective solvent
packed, cylindrical, mesoporous structure, with microporous                          interactions. This technology provides a platform from which

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beginning the temperature ramp.                                                          Acknowledgements
                                                                                         The authors would like to thank M. Blohm for financial support as well as M. Latorre and D. Vissani
                                                                                         (thermal analysis), G. Goddard (MALLS), L. Denault (SEM), P. Donahue (NMR), W. Heward (X-ray),
NUCLEAR MAGNETIC RESONANCE (NMR) SPECTROSCOPY                                            J. Mckiever (BET) and J. Leist (elemental analysis) for technical support. We also wish to thank S.T.
All NMR spectra were obtained on a Bruker Avance 500 equipped with a                     Dhanasekaran at UMASS Amherst for assistance with SAXS data collection and Xiaolan Wei for
                                                                                         valuable discussions.
10-mm multinuclear dual probe. All spectra were obtained using standard                  Correspondence and requests for materials should be addressed to J.W.
parameters supplied with Bruker’s XWINNMR software. These included a 308                 Supplementary information accompanies this paper on
flip angle, 1 s pulse delay, 10 kHz spectral width for proton and 32.1 kHz
spectral width for 11B and 13C at 160.46 MHz.                                            Author contributions
                                                                                         P.M. and J.W. conceived, designed and performed the experiments. S.T. performed the TEM analysis.
                                                                                         All authors discussed the results and commented on the manuscript.
Received 6 September 2006; accepted 20 November 2006; published
3 January 2007.                                                                          Reprints and permission information is available online at

46                                                                                        nature nanotechnology | VOL 2 | JANUARY 2007 |

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