Complementary multiple hydrogen bonding interactions induce the self-assembly of supramolecular structures from heteronucleobase-functionalized benzoxazine and polyhedral oligomeric silsesquioxane nanoparticles

Article abstract of DOI:10.1021/ma302077x

We prepared octuply adenine (A)-functionalized polyhedral oligomeric silsesquioxane [(octakis(vinylbenzyladenine-siloxy)silsesquioxane, OBA-POSS] nanoparticles through the reaction of A with octakis(benzyl chloride) POSS (OVBC-POSS), itself prepared through hydrosilylation of octakis(dimethylsiloxy) silsesquioxane (Q₈M₈^H) with vinyl benzyl chloride. We observed the self-assembly of lamellar structures from the complexation of a thymine (T)-functionalized polybenzoxazine (PA-T) with OBA-POSS, stabilized through complementary multiple hydrogen bonding interactions between the T groups of PA-T and the A groups of OBA-POSS. In addition, incorporating POSS presenting multiple, strong, complementary hydrogen bonding A units into the PA-T matrix significantly enhanced the thermal stability of this polymer, as evidenced using differential scanning calorimetry and thermogravimetric analysis.

Full text of DOI:10.1021/ma302077x

Article
pubs.acs.org/Macromolecules
Complementary Multiple Hydrogen Bonding Interactions
Induce the Self-Assembly of Supramolecular Structures from
Heteronucleobase-Functionalized Benzoxazine and Polyhedral
Oligomeric Silsesquioxane Nanoparticles
Wei-Hsun Hu, Kai-Wei Huang, Chin-Wei Chiou, and Shiao-Wei Kuo*
Department of Materials and Optoelectronic Science, Center for Nanoscience and Nanotechnology, National Sun Yat-Sen University,
Kaohsiung 804, Taiwan
S
* Supporting Information
ABSTRACT: We prepared octuply adenine (A)-functionalized
polyhedral oligomeric silsesquioxane [(octakis(vinylbenzyladenine-
siloxy)silsesquioxane, OBA-POSS] nanoparticles through the reac-
tion of A with octakis(benzyl chloride) POSS (OVBC-POSS),
itself prepared through hydrosilylation of octakis(dimethylsiloxy)-
silsesquioxane (Q8M8H) with vinyl benzyl chloride. We observed
the self-assembly of lamellar structures from the complexation of
a thymine (T)-functionalized polybenzoxazine (PA-T) with OBA-
POSS, stabilized through complementary multiple hydrogen
bonding interactions between the T groups of PA-T and the A groups of OBA-POSS. In addition, incorporating POSS pre-
senting multiple, strong, complementary hydrogen bonding A units into the PA-T matrix significantly enhanced the thermal
stability of this polymer, as evidenced using differential scanning calorimetry and thermogravimetric analysis.
(DNA bases) allows the formation of complexes stabilized
through multiple hydrogen bonding interactions with comple-
mentary bases. Multiple hydrogen bonding arrays play a fun-
damental role in complex biological systems (e.g., DNA com-
plexation); they are influential structures in polymer science,
where they are often presented as defined macromolecules
possessing near-perfect molecular structures.9 As a result, the
preparation of synthetic polymers that mimic DNA remains an
important challenge in polymer science.10 Chang et al. prepared
a benzoxazine derivative with supramolecular functionality (i.e.,
a uracil unit) through the reaction of a uracil-functionalized
amine with phenol and formaldehyde.11 In addition, we have
also synthesized a thymine (T)-functionalized benzoxazine (PA-T)
through Michael addition of T to an acryloyl-functionalized
benzoxazine (Pa-Ac).12
INTRODUCTION
■
Benzoxazine monomers are heterocyclic compounds featuring
an oxazine ring; they are synthesized from a primary amine, phenol,
and formaldehyde. Benzoxazines can be polymerized through ring-
opening polymerization in the absence of a catalyst, releasing
no byproducts.1 The advantageous properties of benzoxazine
resins include near-zero shrinkage upon polymerization, low
water absorption, high char yield, excellent dimensional stability,
flame retardance, stable dielectric constants, and low surface free
energy.2
Procedures for the modification of benzoxazine resins are grow-
ing rapidly, with many useful products having been developed
in recent years.3 To improve the performance of polybenzoxazines
(PBZs), polymerizable alkynyl and allyl side groups have been
introduced into benzoxazine monomers.4 Benzoxazine resins
have also been modified through blending with other polymers
[e.g., epoxy, polyurethane, poly(N-vinyl-2-pyrrolidone)],5 inorganic
species (e.g., clays),6 polyhedral oligomeric silsesquioxane (POSS),7
and carbon nanotubes.8 Unlike clay or conventional fillers, POSS
has a monodisperse molecular weight, a well-defined structure, low
density, high temperature stability, an absence of trace metals, and
sizable interfacial interactions between its composite particle and
polymer segments.7l As a result, highly functionalized benzoxazine
or epoxy POSS monomers have been employed widely to prepare
covalently bonded PBZ/POSS nanocomposites.7 In general, these
kinds of PBZ/POSS nanocomposites exhibit featureless morphol-
ogies, with no discernible phase separation, suggesting that the
POSS nanoparticles are dispersed homogeneously throughout
the matrix. Functionalizing polymers with nucleic acid bases
In this study, we synthesized a new octuply adenine (A)-
functionalized POSS-based material and then blended it with a
thymine (T)-functionalized PBZ (PA-T) to form PBZ/POSS
nanocomposites. First, we produced octakis(benzyl chloride)
POSS (OVBC-POSS) through hydrosilylation (Scheme 1a)
of vinylbenzyl chloride (VBC) with octakis(dimethylsiloxy)-
H
silsesquioxane (Q8M8 ); we then synthesized the star-shaped
octuply A-functionalized POSS [octakis(vinylbenzyladenine-
siloxy)silsesquioxane, OBA-POSS] from the reaction of OVBC-
POSS with A (Scheme 1b). We used Fourier transform infrared
Received: October 3, 2012
Revised: November 4, 2012
Published: November 14, 2012
© 2012 American Chemical Society
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Scheme 1. Synthesis of (a) PA−OH, (b) PA-ac, and (c) PA-T
and K2CO3 (0.49 g, 3.9 mmol) in DMF (50 mL) was stirred at ambient
temperature for 4 h, then OVBC-POSS (1.0 g, 0.45 mmol) was added,
and the mixture was then stirred at 80 °C for 24 h. After removing the
solvents under reduced pressure, the residue was dissolved in THF and
the salts were removed using a neutral alumina column. A yellowish
viscous liquid was obtained (55%) after drying in a vacuum oven
overnight at room temperature. The chemical reaction is summarized
(FTIR) spectroscopy and transmission electron microscopy
(TEM) to characterize the supramolecular structures self-
assembled through multiple hydrogen bonding interactions
between PA-T and OBA-POSS. In addition, we found that the
octuply A-functionalized POSS imparted PA-T with surpris-
ing aggregation stability as a result of the formation of multiple
hydrogen bonded pairs. The resulting self-assembly of supra-
molecular lamellae based on molecular recognition between
complementary components led to a morphology similar to
that for the microphase separation of block copolymers in the
bulk state. This approach should allow the synthesis of a wide
range of self-assembled supramolecular structures with specified
degrees of internal organization.
1
in Scheme 2. MALDI-TOF mass (Figure S1, calcd 2947). H NMR
(DMSO, ppm): δ 5.31 (s, 2H), 8.16 (s, 2H, NH2), 8.22 (s, 1H), 3.54
(s, 2H). 13C NMR (DMSO, ppm): δ 45.9, 118.7, 144.1, 149.5, 152.6,
156.0. FTIR (KBr, cm−1): 3317, 3157 (NH str), 2959, 2844 (aliphatic
CH str), 1062 (SiOSi), 1652 (CN str), 1601 (NH def). Elemental
analysis: Anal. Calcd for OBA-POSS C, 44.18; H, 3.94; N, 18.41.
Found: C, 56.82; H, 6.49, N, 19.40.
PA-T/OBA-POSS Nanocomposites. Solutions of desired
amounts of PA-T and OBA-POSS in THF were mixed and stirred
for 2 h at room temperature, poured onto an Al plate, dried for 6 h in
the open air, and then placed in an oven at 100 °C under vacuum for
2 h. The cast film was polymerized in a stepwise manner: at 140 and
160 °C for 3 h each and then at 200 °C for 4 h. Each cured sample was
transparent and had a dark red color.
EXPERIMENTAL SECTION
■
Materials. Octakis(dimethylsiloxy)silsesquioxane (Q8M8H) containing
eight hydrosilane groups was obtained from Hybrid Plastics. Paraformalde-
hyde, acryloyl chloride, and 4-hydroxybenzyl alcohol were purchased from
Aldrich. Potassium tert-butoxide, T, and A were obtained from Showa
(Japan). Vinylbenzyl chloride, dimethylformamide (DMF), toluene,
and dimethyl sulfoxide (DMSO) were purchased from Acros Organics
(Germany) and distilled from CaH2 under vacuum prior to use. Sodium
hydride in oil (57−63%) was purchased from Alfa. All other chemicals
were of reagent grade and used as received without further purification.
The benzoxazine monomer (3-phenyl-3,4-dihydro-2H-benzo[e][1,3]-
oxazin-6-yl)methylthymine (PA-T) was prepared as previously reported,
and the chemical reaction is summarized in Scheme 1.12
Characterization. 1H and 13C nuclear magnetic resonance (NMR)
spectra were recorded using an INOVA 500 instrument with CDCl3 as
solvent and TMS as the external standard; chemical shifts are reported
in parts per million (ppm). FTIR spectra of the polymer blend films
were recorded using the conventional KBr disk method with a Bruker
Tensor 27 FTIR spectrophotometer (32 scans; spectral resolution,
1 cm−1); the films were sufficiently thin to obey the Beer−Lambert
law. DSC was performed using a TA-Q20 instrument operated at a
scan rate of 20 °C/min over a temperature range from 0 to 250 °C
under a N2 atmosphere. The thermal stability of the samples was
characterized using a TA Q-50 thermogravimetric analyzer operated
under a N2 atmosphere. The cured sample (ca. 7 mg) was placed in a
Pt cell and heated at a rate of 20 °C/min from 30 to 800 °C at a N2
flow rate of 60 mL/min. TEM was conducted using a JEOL 2100
microscope (Japan) operated at 200 kV. Ultrathin sections of the
samples were prepared using a Leica Ultracut S microtome equipped
with a diamond knife; slices (thickness: ca. 700 Å) were cut at room
Octakis(vinylbenzyladenine-siloxy)silsesquioxane (OBA-POSS).
H
A solution of Q8M8 (1.00 g, 0.980 mmol) and vinyl benzyl chloride
(1.20 g, 7.86 mmol) in toluene (50 mL) was heated at 60 °C under Ar,
and then Pt(dvs) (0.07 mL, 0.13 mmol) was added via syringe. After
stirring for 4 h, the Pt(dvs) catalyst was removed by passing the mix-
ture through activated charcoal.7i,10i,13 The solvent was evaporated in a
rotary evaporator to give OVBC-POSS as a viscous liquid {29Si NMR
(ppm): δ 12.16 [(CH3)2SiCH2CH2], 10.28 [(CH3)2Si(CHCH3)], −109.2
[SiOSi(CH3)2H]}. Subsequently, a solution of A (0.53 g, 3.9 mmol)
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Scheme 2. Synthesis of OBA-POSS: (a) Q8M8H Hydrosilylation with VBC To Form (b) OVBC-POSS and Subsequent Reaction
with A To Form OBA-POSS
Figure 1. H NMR spectra of (a) Q8M8H, (b) OVBC-POSS in CDCl3, and (c) OBA-POSS in d6-DMSO.
1
́
temperature. Nanoindentation tests were performed using an XP Nano
Indenter (MTS, Nano Instruments Innovation Center, Oak Ridge, TN),
a three-sided pyramid (Berkovich) diamond indenter, and a continuous
stiffness measurement (CSM) technique.
signals for the protons marked b (2H, β-side groups) and b
(1H, α-side groups).7i,10i,13 The complete substitution of chloride
atoms by A groups was confirmed in the H NMR spectrum of
1
OBA-POSS (Figure 1c); the signal for the benzylic CH2Cl
groups (4.50 ppm) shifted to 4.31 ppm for the corresponding
T-substituted benzylic moieties. The absence of any remnant
resonance at 4.50 ppm suggested that the substitution had reached
completion under the reaction conditions.
RESULTS AND DISCUSSION
■
1
Synthesis of OBA-POSS. Figure 1 displays H NMR spec-
H
tra of Q8M8 , OVBC-POSS, and OBA-POSS. The signal for
the Si−H units (4.7 ppm) of Q8M8H (Figure 1a) was absent in
the spectrum of OVBC-POSS (Figure 1b), confirming com-
plete hydrosilylation. The latter spectrum reveals that the vinyl
groups of VBC underwent hydrosilylation of the Si−H bonds
Figure 2 displays the 13C NMR spectra of Q8M8H, OVBC-POSS,
and OBA-POSS. The signal for the benzylic carbon nucleus (C-h)
of OVBC-POSS at 47.2 ppm (Figure 2b) shifted to 45.8 ppm
after substitution with A (Figure 2c). The methyl and methylene
signals of OVBC-POSS remained in the spectrum of OBA-POSS
(0.2, 15.2, 20.7, 29.9, and 31.8 ppm).
H
of Q8M8 in both α and β configurations (i.e., a mixture of
these two orientations exists). We determined the ratio of β to
α linkages (1.64:1) for OVBC-POSS through integration of the
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Figure 2. 13C NMR spectra of (a) Q8M8H, (b) OVBC-POSS in CDCl3, and (c) OBA-POSS in d6-DMSO.
H
Figure 3 presents FTIR spectra of Q8M8 , OVBC-POSS, and
OBA-POSS, recorded at room temperature. The strong absorption
PBZ/POSS composites. The composites of the PA-T monomer
with both Q8M8 and OBA-POSS were soluble in THF. The
H
mixtures were poured onto an Al plate; after evaporation of
the solvent, the samples were cured in air in an oven. The
PA-T/Q8M8H composites (which lack complementary multiple
hydrogen bonding interactions) were cloudy and friable both
before (Figure S2a) and after (Figure S2c) thermal curing,
suggesting that the Q8M8H units aggregated considerably during
the thermal curing process. In contrast, the mixtures of PA-T
and OBA-POSS (i.e., with complementary multiple hydrogen
bonding interactions) were all homogeneous and transparent,
suggesting that the two components were completely miscible in
the composition range investigated (Figure S2b); all of their cured
samples were transparent and were colored dark red (Figure S2d).
FTIR spectroscopy is a highly effective tool for investigating
hydrogen bonding interactions in supramolecular arrays. The
presence of multiple hydrogen bonds between the PA-T mono-
mer and OBA-POSS was evident in the FTIR spectra in Figure S3.
Figure 4 presents expanded FTIR spectra (1800−1500 cm−1) for
various PA-T/OBA-POSS blends prior to thermal polymerization.
Analyzing these spectra using the second-derivative technique,10i,k
we observed five major peaks for pure PA-T: ring vibrations at
1598 cm−1; self-complementary multiple T−T hydrogen bond-
ing at 1641 cm−1; free CO groups of T units at 1671 cm−1;
acetyl CO groups hydrogen-bonded with the T moieties at
1710 cm−1; and free acetyl CO groups at 1730 cm−1. The A
groups of OBA-POSS provided characteristic signals at 1657 cm−1
(bonded NH2 scissor plus ring stretching) and 1600 cm−1 (ring
stretching plus bonded NH2 scissor). The intensity of signal of the
free CO groups of T at 1671 cm−1 decreased upon increasing
the OBA-POSS content in the blend, consistent with the existence
of multiple hydrogen bonding interactions between the A and T
groups.
Figure 3. FTIR spectra of (a) Q8M8H, (b) OVBC-POSS, and (c)
OBA-POSS.
peaks near 1100 cm−1 for all compounds represented the vibra-
tions of the siloxane Si−O−Si groupsa general feature of POSS
derivatives. The characteristic stretching vibrations of the Si−H
groups appeared as a signal at 2200 cm−1 in Figure 3a. In the
spectrum of OVBC-POSS (Figure 3b), this peak was completely
absent, indicating that the reaction had reached completion. After
the substitution reaction, the spectrum of OBA-POSS (Figure 3c)
featured signals at 3317, 3157, 1652, and 1601 cm−1, correspond-
ing to N−H stretching and bending of the A groups. Thus, the 1H
and 13C NMR and FTIR spectroscopic analyses were all indicative
of the successful synthesis of OBA-POSS.
We used DSC to examine the glass transition behavior of the
PA-T/OBA-POSS nanocomposites after thermal curing (Figure 5).
For pure PA-T and pure OBA-POSS, the glass transition tem-
peratures (Tg) were 115 and 50 °C, respectively. Pure PA-T
features strong self-complementary multiple hydrogen bonding
Thermal Properties of PA-T/OBA-POSS Nanocompo-
sites. Figure S2 displays the macroscopic morphologies for our
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OBA-POSS complexes, it reached its maximum value at an
OBA-POSS content of 5 wt %. In previous studies, Lee found7d−g
that multifunctionalized benzoxazine POSS derivatives at a content
of up to 10 wt % in benzoxazine hybrids led to aggregation of the
POSS units and, thereby, macro-phase separation, which decreased
their overall effectiveness in hindering polymer movement. Thus,
multifunctionalized benzoxazine POSS, which features an in-
organic silsesquioxane core, appears to undergo considerable
aggregation during the thermal curing process. In this present
study, we suspected that strong, complementary, multiple hy-
drogen bonding interactions would exist between the T groups
of PA-T and the A groups of OBA-POSS and, therefore, an
increase in the glass transition temperature upon increasing the
OBA-POSS content in the blend. Alternatively, taking into
account the effects of the chain mobility and viscosity of PA-T
and OBA-POSS, we might also have expected cross-linking and
self-complementary multiple hydrogen bonding to decrease the
chain mobility and possibly decrease the degree of interasso-
ciation through complementary multiple hydrogen bonding
of PA-T with OBA-POSS. Figure 5 reveals, however, that essen-
tially each of our PA-T/OBA-POSS nanocomposites exhibited
an exothermal peak in its DSC trace at a temperature above its
value of Tg at higher OBA-POSS contents. We assign this exo-
thermal peak to the enthalpy of complementary hydrogen bond
formation.15 The temperature of this exothermic peak and its
enthalpy depended on the DSC heating rate or the number of
heating scans, as revealed in Figure 6 for PA-T/OBA-POSS =
Figure 4. FTIR spectra of the PA-T monomer blended with various
OBA-POSS contents at room temperature.
Figure 5. DSC thermograms (second heating runs) of various PA-T/
OBA-POSS nanocomposites.
interactions among its T groups, leading to a relatively high
value of Tg.12 The glass transition temperature of pure OBA-
POSS, which also features strong, self-complementary, multiple
hydrogen bonding interactions, is higher than that of a similar
octakis-functionalized POSS lacking multiple hydrogen bond-
ing interactions.7l In addition, the exothermic peak at 100−
140 °C and the endothermic peak near 150 °C for pure OBA-
POSS correspond to crystallization and melting, respectively, of
the POSS moieties.14 This melting temperature was confirmed
through small-angle X-ray scattering (Figure S4), where the
scattering peak from aggregation of OBA-POSS disappeared
completely at 180 °C.
Figure 6. DSC analyses revealing the reversibility of multiple hydrogen
bonding in each heating cycles.
80/20. Notably, this exothermic peak did not correspond to
thermal polymerization of the PA-T monomer because it exhib-
ited reversible behavioreven after nine heating scans. Never-
theless, the enthalpy of hydrogen bonding formation decreased
upon increasing the number of heating scans (Figure 6). Further-
more, when we thermally annealed the sample at 250 °C for
Although the glass transition temperature did not change ap-
preciably upon increasing the OBA-POSS content in the PA-T/
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2 weeks and then quickly quenched it to room temperature,
the enthalpy of hydrogen bond formation disappeared almost
totally during the subsequent heating scan, indicating that the
formation of complementary multiple hydrogen bonds was al-
ready complete during the cooling process. As a result, the
value of Tg increased by ∼35 °C after thermal annealing of the
PA-T/OBA-POSS complexes (Figure 6).
We speculate that the chain mobility increased at temper-
atures above the values of Tg of PA-T and OBA-POSS, with
rearrangement of the chains’ conformations allowing more inti-
mate mixing and providing more appropriate orientations at
the interassociation sites (i.e., the T groups of PA-T and the A
groups of OBA-POSS). This result is similar to that observed
for the high glass transition temperatures of polymer blends
of poly(vinylphenol) (PVPh) and poly(methyl methacrylate)
(PMMA),16 where it is concluded that even though hydrogen
bonding between PVPh and PMMA favors their miscibility,
phase separation still occurs, resulting in a nonequilibrium multi-
phase system that is effectively frozen; nevertheless, thermal treat-
ment influences the formation of hydrogen bonds again, thereby
enhancing the miscibility of PVPh/PMMA blends.
Figure 8. Glass transition behavior of PA-T/OBA-POSS nano-
composites, determined using the Kwei equation.
complexes, we obtained values of k and q of 1 and 275, respectively.
A positive value of q usually suggests that strong specific inter-
molecular interactions exist between the two polymers.18 In
Figure 8, the value of Tg of pure PA-T is 115 °C and those of
its blends with OBA-POSS at contents of 5, 10, 20, 30, 40, and
50 wt % are 127, 132, 149, 151, 120, and 110 °C, respectively.
Thus, the value of Tg of the complex increased dramatically
upon increasing the OBA-POSS content to 30 wt % (<30 wt %
provided a good fit with the Kwei equation) but decreased
thereafter (>40 wt % led to a large negative deviation with the
Kwei equation). This behavior presumably resulted from two
opposing effects of the POSS cages in the PBZ matrix.7l One,
the nanoreinforcement effect of OBA-POSS on the PA-T PBZ
matrix, tending to increase the value of Tg of the blend through
complementary multiple hydrogen bonding interactions at
relatively low OBA-POSS contents. The other, the inclusion of
OBA-POSS, which features an inorganic silsesquioxane core, in
the system leading to serious aggregation during the thermal
curing process, thereby decreasing the cross-linking density in
the network structure and decreasing the values of Tg at re-
latively high OBA-POSS contents.
Figure 7 displays DSC thermograms of blends at various
PA-T/OBA-POSS ratios after thermal annealing at 250 °C for
Figure 9 reveals the thermal stabilities of the PA-T/OBA-POSS
nanocomposites under N2. To compare the thermal stabilities,
Figure 7. DSC thermograms of blends at various PA-T/OBA-POSS
ratios after thermal annealing at 250 °C for 2 weeks.
2 weeks and then quickly quenching to room temperature. The
exothermic peaks disappeared completely during the subsequent
heating scan, with the values of Tg having increased for all PA-T/
OBA-POSS ratios, compared with those prior to thermal annealing.
Figure 8 summarizes the corresponding glass transition tem-
peratures. The Kwei equation17 is usually employed to charac-
terize systems displaying specific interactions:
WT + kW T
1 g1
2
2
g2
T =
+ qWW
g
1 2
W + kW
(1)
1
where W1 and W 2 are the weight fractions of the components,
Tg1 and Tg2 are the corresponding glass transition temperatures,
and k and q are fitting constants. The parameter q represents
the strength of the specific interactions in the system; it reflects
a balance between the breaking of self-association interactions
and the forming of interassociation interactions. From non-
linear least-squares “best fits” of the plots for the PA-T/OBA-POSS
Figure 9. TGA analyses of PA-T/OBA-POSS blends incorporating
different amounts of OBA-POSS.
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Figure 10. TEM images and SAED patterns of PA-T/OBA-POSS blends incorporating different amounts of OBA-POSS.
here we use the 10 wt % weight loss temperature as a standard.
The decomposition temperature (Td) of the PA-T/OBA-POSS
nanocomposites gradually increased upon increasing the OBA-
POSS content up to 20 wt %; further increases in the OBA-
POSS content caused the decomposition temperature to de-
crease, due to a phenomenon similar to that for the trend in the
values of Tg. The variation in thermal stability, as measured
in terms of the decomposition temperature, may be construed
as an effect of creating nanocomposites. In a nanocomposite
material, thermal motion of the tether units is restricted, there-
by decreasing the number of organic decomposition pathways
accessible to the tether.19 The char yield, another indicator of
thermal stability, also increased upon increasing the OBA-POSS
content to 20 wt %, but decreased thereafteragain, similar to
the trend in the values of Td. For these hybrids, increasing the
POSS content appeared to improve the composite’s thermal
properties through the formation of network structures be-
tween the PBZ and the incorporated inorganic silsesquioxane.
Self-Assembled Structures of PA-T/OBA-POSS Nano-
composites. As in a previous study,12 we found that pure
PA-T self-assembled into an alternating packed structure, with a
long period of packing of approximately 4.0−4.5 Å, as revealed
in the TEM image in Figure 10a. The orientation of the packing
(111) is consistent with the presence of T units interacting
with the side chains of the PBZ through π−π stacking between
T layers.12 Because of the T units in the PBZ matrix, multiple
hydrogen bonds formed favorably with A or diaminopyridine
units to construct supramolecular structures through simple
blending. In addition, OBA-POSS also formed a lamellar struc-
ture through the A groups on the corner sites of POSS (Figure 10h
and Figure S5). Therefore, we suspected that self-assembly of
PA-T/OBA-POSS hybrid materials would lead to interesting
structures. Figure 10 displays TEM images PA-T/OBA-POSS
nanocomposites incorporating various OBA-POSS contents. At
5 wt % of OBA-POSS, the morphology is similar to that of pure
PA-T; we attribute the dark regions to POSS-rich layers because
the Si atoms of POSS provide a high mass contrast relative to the
organic domain. At 10 and 20 wt % of OBA-POSS, we observe
self-assembled lamellar nanostructures (period of ca. 4.5 nm,20
estimated from Figure S5) in the local region; therefore, we used
selected area electron diffraction (SAED) to determine the micro-
structure. The SAED pattern in Figure S6 (10 wt % of OBA-
POSS) reveals a spot pattern, indicating a highly ordered packing
structure in the PBZ matrix. Moreover, the four most intense
reflections could be unambiguously identified as (210), (210),
(111), and (012) crystal planes, consistent with the POSS elec-
̅
̅
̅
tron diffraction pattern,21 suggesting that lamellar structures
were formed by POSS packing in the PBZ matrix with a pre-
ferred orientation along the (012) plane through A−T inter-
actions between PA-T and OBA-POSS. The TEM images
revealed that the nanolamellar structure disintegrated after
incorporation of more than 40 wt % of OBA-POSS; the cor-
responding SAED patterns transformed into ring patterns
(inset of Figure 10f), presumably because the self-aggregation
of POSS was stronger than the complementary A−T multiple
hydrogen bonding interactions and because the ordered struc-
ture of POSS packing was disrupted. These observations are
consistent with the best fits of the Kwei equationquite pre-
dictable at OBA-POSS contents of less than 40 wt %, but with a
divergence of the experimental values of Tg at OBA-POSS
contents greater than 40 wt %, reflecting the disruption of
specific interassociation interactions through self-aggregation of
OBA-POSS. We suspect that the self-assembled lamellar struc-
tures resulted from POSS separating from the matrix and aggre-
gating along a specific orientation; presumably, complementary
multiple hydrogen bonded pairs in PA-T/OBA-POSS and the
linear-like structure of PA-T also played import roles in deter-
mining the structure formed.
Scheme 3 summarizes these possible processes. Initially,
PA-T and OBA-POSS were miscible in the uncured benzoxazine;
OBA-POSS separated from PA-T during subsequent thermal
curing as a result of reaction-induced phase separation.22 Finally,
the self-aggregation of OBA-POSS was restricted through A−T
multiple hydrogen bonding interactions with the linear structure
of PA-T; it would later occur in the nanosize domains, with
growth along the (012) plane. This morphology is similar to
that the microphase separation of block copolymers in the bulk
state23 and different polymer/POSS nanocomposites.24
Thin Film Properties of OA-T/OBA-POSS Nanocompo-
sites. Surface properties are important factors influencing the
applications of PBZs in, for example, nanoimprint lithography,
microelectromechanical systems (MEMS), and superhydrophobic
surfaces.2 Supramolecules have been reported to exhibit good film
forming characteristics though noncovalent bonds,10i,20b similar to
the behavior of our PA-T/OBA-POSS = 80/20 nanocomposite
(see Figure S2). Although dynamic mechanical analysis (DMA) is
typically used to measure the mechanical properties of PBZs in
the bulk state,7j,k the corresponding thin film properties are very
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homodistribution in our thin film. Therefore, the nanodomains
of OBA-POSS were dispersed very well in the PBZ, allowing
the preparation of thin films (or coating layers) of PBZ/POSS
nanocomposites through multiple hydrogen bonding inter-
actions. The OBA-POSS content in this system increased the
strength (high modulus) relative to that of pure PBZ, but the
sample could not scratch the surface of a different material
(relatively lower hardness).
Scheme 3. Possible Self-Assembly through Three Steps:
(a) the PA-T Monomer Is Miscible with OBA-POSS in the
Uncured Benzoxazine; (b) OBA-POSS Separates from the
Cured PA-T Benzoxazine through a Reaction-Induced Phase
Separation Mechanism; (c) Self-Aggregation of OBA-POSS
Restricted by Complementary Multiple Hydrogen Bonding
Interactions (A−T) and the Linear Structure of PA-T PBZ,
Resulting in Self-Assembly of POSS Units through
Subsequent Growth along the (012) Plane
CONCLUSIONS
■
Complementary multiple hydrogen bonding between a well-
defined star-shaped A-functionalized POSS and a T-functionalized
benzoxazine has resulted in the formation of new supramolecular
structures. With a surprising aggregation stability, the self-assembly
of alternating lamellar structures resulted from spontaneous as-
sociation of the complementary components through poly-
condensation based on multiple hydrogen bonding interactions.
This approach might lead to the preparation of a wide range of
self-assembled supramolecular structures with properties similar
to those from the microphase separation of block copolymers
in the bulk state.
difficult to measure. Figure 11 presents the loading−displacement
curve of the PA-T and OBA-POSS hybrid incorporating 20 wt %
ASSOCIATED CONTENT
■
S
* Supporting Information
Additional characterization data (Figures S1−S7). This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail kuosw@faculty.nsysu.edu.tw.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This study was supported financially by the National Science
Council, Taiwan, Republic of China, under Contracts NSC
100-2221-E-110-029-MY3 and NSC 100-2628-E-110-001.
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Figure 11. Loading−displacement curves of pure PA-T and a PA-T/
OBA-POSS thin film.
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