Reaction-induced microphase separation in polybenzoxazine thermosets containing poly(N-vinyl pyrrolidone)-block-polystyrene diblock copolymer

Article abstract of DOI:10.1016/j.polymer.2010.10.047

Poly(N-vinyl pyrrolidone)- block-polystyrene diblock copolymer (PVPy- b-PS) was synthesized via sequential reversible radical-fragmentation transfer polymerization with S-1-phenylethyl O-ethylxanthate as a chain transfer agent. The block copolymer was incorporated into polybenzoxazine to access the nanostructures in the thermosets. The nanostructures in the thermosets were investigated by means of transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). It was found that disordered and/or ordered PS nanophases were formed in the PBa thermosets. It is judged that the formation of nanophases followed the mechanism of reaction-induced microphase separation in terms of the miscibility of the subchains of the diblock copolymer (viz. PVPy and PS) with polybenzoxazine after and before curing reaction.

Full text of DOI:10.1016/j.polymer.2010.10.047

Polymer 51 (2010) 6346e6354
Contents lists available at ScienceDirect
Polymer
journal homepage: www.elsevier.com/locate/polymer
Reaction-induced microphase separation in polybenzoxazine thermosets
containing poly(N-vinyl pyrrolidone)-block-polystyrene diblock copolymer
*
Di Hu, Sixun Zheng
Department of Polymer Science and Engineering and State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, Shanghai 200240, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Poly(N-vinyl pyrrolidone)-block-polystyrene diblock copolymer (PVPy-b-PS) was synthesized via
sequential reversible radical-fragmentation transfer polymerization with S-1-phenylethyl O-ethyl-
xanthate as a chain transfer agent. The block copolymer was incorporated into polybenzoxazine to access
the nanostructures in the thermosets. The nanostructures in the thermosets were investigated by means
of transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS). It was found that
disordered and/or ordered PS nanophases were formed in the PBa thermosets. It is judged that the
formation of nanophases followed the mechanism of reaction-induced microphase separation in terms of
the miscibility of the subchains of the diblock copolymer (viz. PVPy and PS) with polybenzoxazine after
and before curing reaction.
Received 23 July 2010
Received in revised form
11 October 2010
Accepted 23 October 2010
Available online 30 October 2010
Keywords:
Polybenzoxazine
Poly(N-vinyl pyrrolidone)-block-polystyrene
diblock copolymer
Ó 2010 Elsevier Ltd. All rights reserved.
Reaction-induced microphase separation
1. Introduction
Recently, RIMPS has been proved to an efficient alternative to the
nanostructures in thermosets [7e15]. In a previous work, Zheng
During the past decades considerable attention has been paid to
the investigations on reaction-induced phase separation (RIPS) in
multi-component thermosets [1,2]. The goal of these studies is to
establish the correlation of morphology with the properties of
materials. The modified thermosets are prepared starting from
a homogenous solution of modifier (e.g., thermoplastics or elasto-
mers) with precursors of thermosets and then the fine morphol-
ogies are formed by modulating the competitive kinetics between
phase separation and polymerization. The driving force of RIPS is
ascribed to the decreased entropic contribution to the mixing free
energy owing to the occurrence of polymerization (viz. cross-
linking). Nonetheless, the miscible thermosetting blends can be
obtained while there were the favorable intermolecular specific
interactions (e.g., hydrogen bonding) between these polymers and
thermosets [3e6].
Generally, RIPS takes place on the macroscopic scale since these
modifiers are some homopolymers or random copolymers. While
the polymeric modifiers are some amphiphilic block copolymers,
which contain both thermoset-philic and thermoset-phobic sub-
chains, the RIPS could be confined to the micrometer scale i.e.,
so-called reaction-induced microphase separation (RIMPS) occurs
and thus the nanostructures in thermosets would be obtained [1,2].
et al. [7] demonstrated that the formation of nanostructures in
epoxy thermosets containing poly(
-caprolactone)-block-poly-
butadiene-block- poly( -caprolactone) triblock copolymer followed
3
3
the RIMPS mechanism and RIMPS has been found in a variety of
thermosetting blends containing amphiphilic block copolymers
[7e16]. It should be pointed out that Bates et al. [17,18] have ever
proposed the strategy of self-assembly to access the nanostructures
of thermosets. In this protocol, the precursors of thermosets act as
the selective solvents of block copolymers and some self-organized
nanostructures are formed in the mixtures before curing reaction.
These preformed nanostructures are fixed through the subsequent
curing reaction. The self-assembly behavior has been extensively
investigated during the past decade vis-à-vis RIMPS of block
copolymers in thermosetting polymers [9,19e32]. Mechanistically,
there is a significant difference between self-assembly and RIMPS
approaches. In self-assembly approach, the formation of nano-
phases is governed by the equilibrium thermodynamics of the
multi-component system comprised of precursors of thermosets
and amphiphilic block copolymer. Nonetheless, the formation of
nanostructures via RIMPS could be greatly influenced by the
competitive kinetics between polymerization and phase separa-
tion. With the reaction proceeding, a series of structural changes
involving chain extension, branching and crosslinking occurred in
succession, i.e., the system undergoes the solegel transition within
a short time.
* Corresponding author. Tel.: þ86 21 54743278; fax: þ86 21 54741297.
E-mail address: szheng@sjtu.edu.cn (S. Zheng).
0032-3861/$ e see front matter Ó 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.polymer.2010.10.047

D. Hu, S. Zheng / Polymer 51 (2010) 6346e6354
6347
Benzoxazines (Ba) are a class of bicyclic heterocycle consisting of
a benzene ring fused to that of oxazine, which can be prepared via
Mannich condensation among phenol, formaldehyde and amine
[33e48]. Polybenzoxazines (PBa) thermosets can be obtained via
thermally activated ring-opening polymerization of benzoxazine
monomers and no hardeners are required involving the curing
process [34e36] (See Scheme 1). PBa thermosetting polymers are
a class of novel thermosets and are becoming the promising
alternatives to some traditional thermosets such as epoxies,
phenolic resins and bismaleimides owing to their excellent
thermal, mechanical and electrical properties [33e48]. The
potential applications of PBa motivate to prepare their thermoset-
ting blends with improved properties [36e38,49e51]. Chang et al.
[49] reported that PBa thermoset was miscible with poly(N-vinyl
pyrrolidone) and the miscibility was ascribed to the formation of
inter-component hydrogen-bonding interactions. This could be the
first report on the fully miscible blends of PBa. The most of PBa
thermosetting blends found hereto are phase-separated. Ishida
et al. [36] investigated that the synergism in PBa blends containing
expected that the nanostructured epoxy thermosets can be
prepared via RIMPS approach. In this work, the PVPy-b-PS diblock
copolymer was firstly synthesized via sequential reversible radical-
fragmentation transfer (RAFT) polymerization approach and then
transmission electron microscopy (TEM) and small-angle X-ray
scattering (SAXS) are used to examine the nanostructures in the
thermosets. The formation mechanism of nanostructures is judged
on the basis of the miscibility of subchain with the thermoset after
and before curing reaction.
2. Experimental
2.1. Materials
4,40-Dihydroxyldiphenylisopropane, aniline, 1-bromoethylben-
zene and carbon disulfide were of analytical pure grade, purchased
from Shanghai Reagent Co., China. Paraformaldehyde was obtained
from Aldrich Co., USA. Styrene was purchased from Shanghai
Reagent Co., Shanghai, China and the inhibitor was eliminated by
washing thrice using aqueous solution of sodium hydroxide (5%) and
the monomer was dried with anhydrous magnesium sulfate before it
was distilled under decreased pressure. N-vinyl-2-pyrrolidone was
obtained from Acros Co., Shanghai, China. and used as received. The
solvents such as chloroform and toluene were obtained from
commercial source and they were purified with standard procedures
prior to use. Benzoxazine was synthesized by following the approach
of literature on the basis of Mannich condensation among 4,40-
dihydroxyldiphenylisopropane, aniline and paraformaldehyde with
toluene as the solvent [33].
0e15 wt% of poly(3-caprolactone) by means of dynamic mechanical
analysis. Fourier transform infrared spectroscopy showed that
within the compositions investigated there were the intermolec-
ular hydrogen-bonding interactions between PBa and poly(3-cap-
rolactone). Zheng et al. have found the occurrence of macroscopic
phase separation-induced by reaction in the thermosetting blends
of PBa with poly(3-caprolactone) [50] and the similar results were
also reported that in the thermosetting blends of PBa with poly
(ethylene oxide) [51]. The reaction-induced phase separation
occurred at the micrometer scale since the polymeric modifiers
used to prepare the blends were some homopolymers. Recently, it
has been recognized that the formation of the nanostructures in the
multi-component thermosets can further optimize the interactions
between thermosetting matrix and modifiers and thus the
mechanical properties of materials were significantly improved,
which has been called “toughening by nanostructures” [52]. To the
best of our knowledge, however, no precedent works were reported
on the formation of nanostructures in PBa thermosets by the use of
an amphiphilic diblock copolymer.
The purpose of this work is to demonstrate that disorder and/or
ordered nanostructures could be accessed via RIMPS approach in
the thermosetting blends of PBa by the use of amphiphilic block
copolymer. Toward this end, a diblock copolymer, poly(N-vinyl
pyrrolidone)-block-polystyrene (PVPy-b-PS) was used by knowing
that PVPy is miscible with PBa thermosets [49] whereas PS will
undergo reaction-induced phase separation in PBa thermosets. It is
2.2. Synthesis of S-1-phenylethyl O-ethylxanthate
To a 150 ml flask anhydrous ethanol (20 ml) and potassium
hydroxide (2.8 g, 0.05 mol) were charged with vigorous stirring
until a transparent solution was obtained. The flask was immersed
into ice-water bath and carbon disulfide (10 ml) was slowly drop-
ped and the reaction was allowed to carry out at room temperature
for 10 h. The excess carbon disulfide was distilled out at 70 ꢀC and
then 1-bromoethylbenzene (5.66 ml, 0.0413 mol) dissolved in 10 ml
ethanol was added into the mixture and the reaction was per-
formed at 60 ꢀC for 5 h. The solids (viz. inorganic salts) in the system
were removed via filtration and then ethyl ether (20 ml) and
deionized water (20 ml) were added. The solution was washed with
deionized water thrice and the organic layer was collected and
dried with anhydrous magnesium sulfate. After filtration the
OH
N
O
C
N
n
Thermal Curing
H3C
CH3
CH3
CH3
N
n
O
N
OH
Scheme 1. Thermal curing of Ba monomer.

6348
D. Hu, S. Zheng / Polymer 51 (2010) 6346e6354
solvents were eliminated via rotary evaporation and yellow oil
liquid (8.61 g) was obtained with the yield of 76.2% after drying in
vacuo at 30 ꢀC for 30 h. 1H NMR (CDCl3, ppm): 1.39 (3H, CH3CH2),
1.72(3H, CH3CH), 4.62 (2H, CH3CH2), 4.91 (1H, CH3CH), 7.27e7.40
(5H, C6H5).
2.3. Synthesis of polystyrene macromolecular chain transfer agent
(PS-CTA)
To a 50 ml flask styrene (15.0 g, 0.14 mol), S-1-phenylethyl
O-ethylxanthate (15.0 g, 0.14 mol), azodiisobutyronitrile (AIBN)
(0.0371 g, 0.22 mmol) and anhydrous toluene (2.4 ml) were
charged. The system was connected to a Schlenk line and the
reactive mixture was degassed via three pump freezeethaw cycles
and then immersed in a thermostated oil bath at 80 ꢀC for 26 h.
The crude product was dissolved in tetrahydrofuran and the
solution was dropped into a great amount of methanol to afford
the precipitates and this procedure was repeated three times. The
precipitates were dried in vacuo at 30 ꢀC for 24 h. By controlling
the conversion of monomer the product (10.18 g) was obtained
with the yield of c.a. 66.2%. The molecular weight of the macro-
molecular chain transfer agent (PS-CTA) was estimated to be
Mn ¼ 8900 in term of the molar ratio of monomer (at the
conversion of 66.2%) to the RAFT agent (i.e., S-1-phenylethyl O-
ethylxanthate). 1H NMR (CDCl3, ppm): 6.19e7.23 (5H, C6H5 of
polystyrene), 1.44 (1H, CH2CHC6H5), 1.85 (2H, CH2CHC6H5). GPC:
Mn ¼ 9100, Mw/Mn ¼ 1.51.
Fig. 1. 1H NMR spectrum of PS-CTA and PVPy-b-PS diblock copolymer.
eliminated in vacuo at 30 ꢀC for 2 h. The mixtures were poured into
Teflon molds and cured at 180 ꢀC for 4 h. The contents of the diblock
copolymer in the PBa thermosets were controlled to be 10, 20, 30
and 40 wt%, respectively.
2.4. Synthesis of poly(N-vinyl pyrrolidone)-block-polystyrene
diblock copolymer
N-vinyl pyrrolidone (3.4600 g, 30 mmol), PS-CTA (2.0005 g,
0.22 mmol), 1,4-dioxane (4 ml) and AIBN (0.0092 g, 0.05 mmol)
were charged to a 50 ml flask. The flask was connected to a Schlenk
line and the reactive mixture was degassed via three pump
freezeethaw cycles and then immersed in a thermostated oil bath
at 70 ꢀC for 26 h. The crude product was dissolved in tetrahydro-
furan and the solution was dropped into a great amount of petro-
leum to afford the precipitates. This procedure was repeated three
times and the precipitates were dried in vacuo at 30 ꢀC for 24 h. By
controlling the conversion of monomer the product (3.2 g) was
obtained with the yield of c.a. 36.7%. The length of the PVPy block
was estimated to be Mn ¼ 5500 in term of the molar ratio of
monomer (at the conversion of 46.7%) to the PS-CTA. 1H NMR
(CDCl3, ppm): 6.19e7.23 (8.3H, C6H5 of polystyrene), 2.15e2.55
[2H, NCH2CH2CH2CO], 3.75 [1H,CHN(CH2)3CO], 3.24 [2H,
NCH2CH2CH2CO], 1.2e2.13 (methylene protons as indicated by the
peaks a, b, h and e in Fig. 1). Gel permeation chromatography (GPC):
Mn ¼ 15,200 with Mw/Mn ¼ 1.57.
2.7. Measurement and characterization
2.7.1. Fourier transform infrared spectroscopy (FTIR)
FTIR measurement was carried out on a PerkineElmer Paragon
1000 Fourier transform spectrometer. For the measurement of the
PVPy-b-PS diblock copolymer, the trace of water in the polymer
was eliminated via azeotropic distillation technique. The block
copolymer (0.1 g) was dissolved in 20 ml anhydrous toluene and
10 ml solvent was distilled out and then the solution of the diblock
copolymer was cast onto KBr windows at 60 ꢀC and the residual
solvent was removed in vacuo at 60 ꢀC for 1 h. For the samples of
thermosets, the powder was mixed with KBr pellets to press into
small flakes. All the specimens were sufficiently thin to be within
a range where the BeereLambert law is obeyed. In all cases 64 scans
at a resolution of 2 cmꢁ1 were used to record the spectra.
2.7.2. Nuclear magnetic resonance spectroscopy (NMR)
2.5. Preparation of Ba and PS blends
The samples were dissolved in deuteronated chloroform and the
NMRspectraweremeasuredonaVarianMercuryPlus 400 MHzNMR
spectrometer with tetramethylsilane (TMS) as the internal reference.
The blends of Ba with the model PS were prepared by solution
casting from tetrahydrofuran at room temperature. The concen-
tration was controlled within 5% (w/v). To remove the residual
solvent, all the blend films were further desiccated in vacuum at
50 ꢀC for 48 h.
2.7.3. Gel permeation chromatography (GPC)
The molecular weights and molecular weight distribution of
polymers were determined on Waters 717 Plus autosampler gel
permeation chromatography apparatus equipped with Waters RH
columns and a Dawn Eos (Wyatt Technology) multi-angle laser light
scattering detector and the measurements were carried out at 25 ꢀC
with tetrahydrofuran (THF) as an eluent at the rate of 1.0 ml/min.
2.6. Preparation of PBa thermosets containing PVPy-b-PS
The desired amount of diblock copolymer PVPy-b-PS was dis-
solved in the smallest amount of tetrahydrofuran and the solution
was added to Ba with continuous stirring until the homogenous
solution was obtained. Before curing reaction, the majority of
solvent was evaporated at 60 ꢀC for 2 h and the residual solvent was
2.7.4. Atomic force microscopy (AFM)
The specimens of thermosets for AFM observation were trim-
med using
a microtome machine and the thickness of the

D. Hu, S. Zheng / Polymer 51 (2010) 6346e6354
6349
specimens is about 70 nm. The morphological observation of the
samples was conducted on a Nanoscope IIIa scanning probe
microscope (Digital Instruments, Santa Barbara, CA) in tapping
nitrogen. The fractured surfaces so obtained were immersed in
tetrahydrofuran at room temperature for 30 min. The PS phases
could be preferentially etched by the solvent while epoxy matrix
phase remains unaffected. The etched specimens were dried to
remove the solvents. The fracture surfaces were coated with thin
layers of gold of c.a. 100 Å. All specimens were examined with
a Hitachi S210 scanning electron microscope (SEM) at an activation
voltage of 20 kV.
mode. A tip fabricated from silicon (125
mm in length with ca.
500 KHz resonant frequency) was used for scan, and the scan rate
was 2.0 Hz.
2.7.5. Transmission electron microscopy (TEM)
Transmission electron microscopy (TEM) was performed on
a JEOL JEM-2010 transmission electron microscope at an accelera-
tion voltage of 200 kV. The samples were trimmed on a microtome
machine with a diamond knife and the sections of the samples
were stained with 5% RuO4 aqueous solution for 20 min to increase
the contrast. The stained specimen sections with the thickness of
70e100 nm in thickness were placed in 200 mesh copper grids for
observations.
3. Results and discussion
3.1. Synthesis of PVPy-b-PS diblock copolymer
It has ever been a challenge to synthesize well-defined PVPy
block copolymer via living radical polymerization techniques
owing to the high reactivity of N-vinyl pyrrolidone [53]. Under the
normal conditions of living radical polymerization, the polymer-
ization of such an unconjugated monomer can give rise to the loss
of controllability in living radical polymerization and the occur-
rence of several side reactions such as disproportionation and
chain transfer to form dead polymers. Therefore, some measures
of deactivation must be taken to perform the living radical poly-
merizations of N-vinyl pyrrolidone [54]. By organostilbine-medi-
ated living radical polymerization [55], Yamago et al. have
reported the synthesis of poly(N-vinyl pyrrolidone)-block-poly-
styrene (PVPy-b-PS). Arsalani et al. [56] synthesized PVPy-b-PS
diblock copolymer by nitroxide-mediated living free radical
polymerization. More recently, Hussain et al. [57] reported the
synthesis of diblock copolymers of polystyrene (PS) and poly(N-
vinyl pyrrolidone) (PVPy) by a combination of the atom transfer
radical polymerization and macromolecular design via inter-
change of xanthates (i.e., ATRP/MADIX). In this work, the RAFT/
MADIX process [57] was used to obtain the PVPy-b-PS diblock
copolymer. The route of synthesis is shown in Scheme 2 and
a two-step sequential reversible radical-fragmentation transfer
(RAFT) polymerization approach was employed. S-1-Phenylethyl-
O-ethylxanthate was used as the chain transfer agent [58,59]. In
the first step, the polystyrene macromolecular chain transfer
agent (PS-CTA) was prepared via the RAFT polymerization with
the desired conversion of styrene. In this work, the conversion of
styrene was controlled to be 66.2% to obtain the molecular weight
of PS-CTA to be Mn ¼ 8900, which is quite close to the value
measured by means of GPC (i.e., Mn ¼ 9100 with Mw/Mn ¼ 1.51).
2.7.6. Small-angle X-ray scattering (SAXS)
The SAXS measurements were taken on a small-angle X-ray
scattering station (BL16B1) with a long-slit collimation system in
the Shanghai Synchrotron Radiation Facility (SSRF), Shanghai,
China, in which the third generation of synchrotron radiation light
sources was employed. Two dimensional diffraction patterns were
recorded using an image intensified CCD detector. The experiments
were carried out with the radiation of X-ray with the wavelength of
l
¼ 1.24 Å at room temperature (25 ꢀC). The intensity profiles were
output as the plot of scattering intensity (I) versus scattering vector,
q ¼ (4/
l
) sin (
q
/2) (
q
¼ scattering angle).
2.7.7. Differential scanning calorimetry (DSC)
The calorimetric measurements were performed on a Per-
kineElmer Pyris 1 differential scanning calorimeter in a dry
nitrogen atmosphere. An indium standard was used for tempera-
ture and enthalpy calibrations, respectively. The samples (about
10.0 mg in weight) were first heated to 140 ꢀC and held at this
temperature for 3 min to remove the thermal history, followed by
quenching to ꢁ40 ꢀC. A heating rate of 20 ꢀC/min was used in all
cases. Glass transition temperature (Tg) was taken as the midpoint
of the heat capacity change.
2.7.8. Scanning electron microscope (SEM)
In order to observe the phase structure of PBa blends, the
samples were fractured under cryogenic condition using liquid
S
O
O
S
n
N
AIB
S
S
N
O
O
S
n
m
IBN
A
N
S
O
Scheme 2. Synthesis of poly(N-vinyl pyrrolidone)-block-polystyrene (PVPy-b-PS) diblock copolymer.

6350
D. Hu, S. Zheng / Polymer 51 (2010) 6346e6354
The PS-CTA was used to polymerize N-vinyl pyrrolidone with AIBN
as the initiator. By controlling the conversion of N-vinyl pyrroli-
done, the PVPy-b-PS diblock copolymer was obtained with the
length of PVPy subchain to be Mn ¼ 5500. Shown in Fig. 1 are the
1H nuclear magnetic resonance spectra of PS-CTA and the PVPy-
b-PS diblock copolymer. The PS block was the characteristic of the
signals at 1.44, 1.85 and 6.0e8.0 ppm, which are assignable to the
resonance of protons of methylene, methane and aromatic ring,
respectively. It is worth noticing that the signals of CTA are
discernible at 1.35, 1.71, 2.36 and 7.30e7.5 ppm, which are
attributed to the protons of methyl, methylene and aromatic rings,
respectively. In the 1H NMR spectrum of PVPy-b-PS diblock
copolymer, the signals from methylene groups of pyrrolidone ring
were detected at 3.2, 2.4 and 2.1 ppm, which are ascribed to the
protons of methylene groups connected to nitrogen atom,
carbonyl groups and those between these two methylene groups,
respectively. In addition, the resonance of protons for the methane
of PVPy main-chain was detected at 3.72 ppm. The 1H NMR
spectroscopy indicates that the PS-CTA and PVPy-b-PS diblock
copolymer with designed structures were successfully obtained.
Both PS-CTA and PVPy-b-PS diblock copolymer were subjected to
gel permeation chromatography (GPC) and the curves of GPC are
presented in Fig. 2. It is seen that the GPC curves of PS-CTA and
PVPy-b-PS diblock copolymer displayed unimodal peaks. The
polymerization of N-vinyl pyrrolidone with the PS-CTA as chain
transfer agent did not yield detectable unreacted PS-CTA in the
resulting diblock copolymer. The molecular weight of the diblock
copolymer was determined to be Mn ¼ 15,200 with the poly-
dispersity of Mw/Mn ¼ 1.57, which was slightly higher than that of
PS-CTA. The polydispersity of the diblock copolymer is quite close
to the results reported by other investigators [57]. The results of
1H NMR and GPC indicate that the PVPy-b-PS diblock copolymer
was successfully obtained.
obtained with the content of PVPy-b-PS diblock copolymers up to
40 wt%. All the thermosets were homogenous and transparent,
implying that macroscopic phase separation did not take place
with the curing reaction. Nonetheless, the reaction-induced
microphase separation (RIMPS) cannot be excluded only in view of
the clarity. The morphology of PBa thermosets containing PVPy-b-
PS diblock copolymer was examined by means of transmission
electron microscopy (TEM) and small-angle X-ray scattering
(SAXS).
The PBa thermosets containing PVPy-b-PS diblock copolymer
were trimmed using an ultra-thin microtome machine and the
specimens were subjected to the morphological observations by
means of TEM. In order to increase the contrast the sections of the
samples were then stained with RuO4, the PBa matrix containing
PVPy can be deeply stained whereas PS microdomains were only
lightly stained. Shown in Fig. 3 are the TEM micrographs of the
thermosets containing 10, 20, 30 and 40 wt % PVPy-b-PS diblock
copolymer. It is noted that all the thermosetting blends possess
microphase-separated morphologies (i.e., nanostructures). The
dark continuous regions are assignable to the PBa matrix, which
was miscible with the PVPy subchains of the block copolymer
whilst the light regions are attributed to PS domains. While the
contents of PVPy-b-PS are 10 and 20 wt%, the spherical PS nano-
phases were imbedded in the continuous PBa matrix at the
average size of c.a. 20e30 nm in diameter (Fig. 3A and B). With
increasing the content of PVPy-b-PS diblock copolymer, the
number of PS nanodomains increased (Fig. 3B). While the content
of PVPy-b-PS diblock copolymer is 30 wt% or higher, the PS
nanodomains became increasingly interconnected and some
cylindered nanophases were exhibited (See Fig. 3C and D). It is of
interest to note that the PS nanophases were to some extent
arranged into ordered structures. The order of the nanostructures
increased with increasing the concentration of PVPy-b-PS diblock
copolymer. The ordered nanostructures were further investigated
by means of SAXS.
3.2. Nanostructures of PBa thermosets containing PVPy-b-PS
The SAXS profiles of the thermosets containing 10, 20, 30, and
40 wt% of PVPy-b-PS diblock copolymer are shown in Fig. 4. It is
seen that the well-defined scattering peaks were observed in all the
cases, indicating that the PBa thermosets containing PVPy-b-PS
were indeed microphase-separated. According to the position of
the primary scattering peaks, the principal domain spacing dm can
be obtained to be 55.6, 51.0, 43.6, and 41.9 nm for the thermosets
containing 10, 20, 30 and 40 wt % PVPy-b-PS diblock copolymer,
respectively. It is noted that except for the thermoset containing 10
and 20 wt% of PVPy-b-PS diblock copolymer the thermosetting
blends displayed the multiple scattering maxima, indicating that
the materials possessed long-range ordered nanostructures. The
scattering peaks of the thermosets situated at q values of 1, 40.5 and
90.5 relative to the first-order scattering peak positions (qm) are
discernable. It is proposed that these are the scattering peaks of
cylindrical nanophases arranged in hexagonal lattices. In addition,
body-centered (or face-centered) cubic lattice is also possible. It
should be pointed out that it is not easy unambiguously to judge
the types of packing lattices only in terms of SAXS profiles for the
thermosetting blends containing 30, 40 wt% PVPy-b-PS since the
scattering peaks are quite broad, i.e., the ordering is apparently not
good enough. Nonetheless, it is noted that in the thermosetting
blends the local rearrangement owing to the increase in the
concentration of PVPy-b-PS diblock copolymer leads to an
enhancement of the long-range order.
The PVPy-b-PS diblock copolymer was incorporated into pol-
ybenzoxazine to access the nanostructures in the thermosets.
Before curing, all the binary mixtures composed of benzoxazine
and PVPy-b-PS diblock copolymer were homogenous and trans-
parent. After cured at 180 ꢀC for 4 h, the PBa thermosets were
PS-CTA
PVPy-b-PS
15
20
25
30
3.3. Formation mechanism of nanostructures
Elution time (min)
It has been recognized that the formation of nanostructures in
thermosets by the use of amphiphilic block copolymers could
Fig. 2. GPC curves of PS-CTA and PVPy-b-PS diblock copolymer.

D. Hu, S. Zheng / Polymer 51 (2010) 6346e6354
6351
Fig. 3. TEM micrograph of PBa thermoset containing: A) 10, B) 20, C) 30 and D) 40 wt% PVPy-b-PS diblock copolymer. The sections of samples were stained with RuO4.
1
follow either self-assembly [17,18] or reaction-induced microphase
separation mechanism [7,8]. For the mechanism of self-assembly,
the precursors of thermosets act as selective solvents of block
copolymers and self-organized nanophases (i.e., micelle) are
formed prior to curing and these nanostructures are further locked
in with subsequent curing reaction. For reaction-induced micro-
phase separation mechanism, it is not required that the amphi-
philic block copolymers are self-organized into the nanophases
before curing reaction, i.e., all the subchains of block copolymers
may be miscible with precursors of thermosets. Upon curing, a part
of subchains of block copolymers are demixed to form the micro-
phases whereas other subchains of the block copolymer remain
mixed with the matrix of thermosets. In the present case, it is
important to know the miscibility and phase behavior in the blends
of the subchains of diblock copolymers with PBa after and before
curing reaction for the judgment of the formation mechanism of
the nanostructures. The miscibility of Ba and PVPy blends has been
previously reported by Chang et al. [49] and it has been demon-
strated that the thermosetting blends of PBa and PVPy were
miscible after and before curing. Before curing reaction, the
miscibility of PVPy with Ba monomer is ascribed to the non-
PVPy-b-PS
41/2
(wt %)
91/2
1
40
30
20
41/2
91/2
10
negligible contribution of mixing entropy (
DSm) to mixing free
0.0
0.1
0.2
0.3
q (nm-1)
0.4
0.5
0.6
0.7
energy ( Sm) since the molecular weight of Ba monomer is quite
D
low. The miscibility of PBa thermosets with PVPy was attributed to
the formation of intermolecular hydrogen-bonding interactions
between PVPy and the PBa networks.
Fig. 4. SAXS curves of the PBa thermosets containing PVPy-b-PS diblock copolymer.

6352
D. Hu, S. Zheng / Polymer 51 (2010) 6346e6354
Thermally activated ring-opening polymerization of benzox-
1667 1650
azine monomers affords the PBa networks which bear a great
amount of phenolic hydroxyl groups (See Scheme 1). These
phenolic hydroxyl groups are potential to form the intermolecular
hydrogen-bonding interactions with the pyrrolidone rings, which
are readily investigated by means of Fourier transform infrared
spectroscopy (FTIR). Shown in Fig. 5 are the FTIR spectra of control
PBa and the nanostructured PBa thermosets in the range of
3000e3800 cmꢁ1. The bands in this region are assignable to the
stretching vibration of phenolic hydroxyl groups. For control PBa,
a broad band centered at 3437 cmꢁ1 is ascribed to the stretching
vibration of hydrogen-bonded phenolic hydroxyl groups [60].
Upon adding PVPy-b-PS diblock copolymer to the thermoset,
a shoulder band at 3373 cmꢁ1 appeared. The intensity of the
shoulder band increased with increasing the content of PVPy-b-PS
diblock copolymer. The new band is assignable to the phenolic
hydroxyl groups which were associated with pyrrolidone ring of
PVPy subchains, i.e., the intermolecular hydrogen-bonding inter-
actions between PBa matrix and PVPy subchains were formed. The
observation that the stretching vibration of phenolic hydroxyl
groups partially shifted to the lower frequencies indicates that
the intermolecular hydrogen-bonding interactions between the
phenolic hydroxyl groups and the pyrrolidone rings are much
stronger than the self-associations of the phenolic hydroxyl groups
[49,61e63]. The intermolecular hydrogen-bonding interactions
can be further evidenced by the change of infrared bands for the
carbonyl groups of pyrrolidone rings. Shown in Fig. 6 are the FTIR
spectra of PVPy-b-PS diblock copolymer and the nanostructured
PBa thermosets in the range of 1600e1760 cmꢁ1. For PVPy-b-PS
diblock copolymer, the infrared band in this region of frequency is
ascribed to the stretching vibration of amide groups in pyrrolidone
ring. The quite broad band reflects the broad distribution of
frequencies assignable to free and dipoleedipole interacted
1687
PBa/PS-b-PVP
90/10
80/20
70/30
60/40
0/100
1760
1720
1680
1640
1600
Wavenumbers (cm-1)
Fig. 6. FTIR spectra of PVPy-b-PS and the PBa thermosets containing PVPy-b-PS
diblock copolymers.
pyrrolidone rings, respectively [63,64]. Upon adding PBa to PVPy-
b-PS diblock copolymer two shoulder bands at 1687 and
1650 cmꢁ1 are increasingly discernible. It is plausible to propose
that the component at the higher frequency is ascribed to free
pyrrolidone rings whereas that at the lower frequency could be
attributed to the pyrrolidone rings which are hydrogen-bonded
with the phenolic hydroxyl groups of PBa networks. Table 1
summarizes the results of curve-fitting [62,63,66,67] for the
PVPy-b-PS diblock copolymer and the nanostructured PBa ther-
mosets containing PVPy-b-PS diblock copolymer. It is seen that the
fractions of the H-bonded and free carbonyl groups increased with
increasing the content of PBa in the thermosets, indicating that the
pyrrolidone rings were H-bonded with the phenolic hydroxyl
groups of PBa networks. The slight increase in the quantity of free
pyrrolidone ring could be ascribed to the restriction of crosslinked
PBa networks on the conformation of PVPy chains. The FTIR
spectroscopy indicates that the PVPy subchains are intimately
mixed with the PBa matrix.
The miscibility and phase behavior of Ba with PS before and
after curing reaction were investigated by means of differential
scanning calorimetry (DSC) and scanning electron microscopy
(SEM), respectively. The model PS having the identical molecular
weight with the length of PS in the PVPy-b-PS diblock copolymer
was used to prepare the PS and Ba blends. The uncured blends
were subjected to thermal analysis and the DSC thermograms of
the blends were shown in Fig. 7. The Ba monomer displayed the
glass transition at 18.3 ꢀC whereas the model PS at 83.8 ꢀC, which
is slightly is lower than the value of normal PS (Tg ¼ 100 ꢀC)
owing to its lower molecular weight (Mn ¼ 8900). It is seen that
each blend exhibited a single Tg and the Tg’s of the blends were
intermediate between those of plain Ba and PS and decreased
with increasing the content of Ba as shown in Fig. 8. The Tg-
composition relationship was accounted for by GordoneTaylor
equation [65]:
Fig. 5. FTIR spectra of PBa and the PBa thermosets containing PVPy-b-PS diblock
copolymers.

D. Hu, S. Zheng / Polymer 51 (2010) 6346e6354
6353
Table 1
Curve-fitting results of the spectral line in the range of 1638e1760 cmꢁ1 for PVPy-b-PS diblock copolymer and the nanostructured PBa thermosets containing PVPy-b-PS
diblock copolymer.
PVPy-b-PS (wt%)
Free carbonyl band
Self-associated carbonyl band
H-bond carbonyl band
n
(cmꢁ1
)
width
area (%)
n
(cmꢁ1
)
width
area (%)
n
(cmꢁ1
)
width
area (%)
100
40
30
20
10
1687
1687
1687
1687
1687
25
25
25
25
25
8.8
9.2
8.9
11.0
12.3
1666
1667
1666
1666
1668
54
54
54
54
54
91.2
64.8
57.2
29.9
10.5
e
e
e
1650
1650
1650
1648
54
54
54
54
26.0
33.9
59.2
78.7
À
Á
spherical PS particles at the size of 0.2e2.0 mm in diameter were
Tg ¼ Tg1 þ kðW2=W1Þ Tg2 ꢁ Tg
(1)
dispersed into the continuous PBa matrix, i.e., a heterogeneous
morphology was exhibited. The result of SEM confirmed that the
reaction-induced phase separation occurred in the thermosetting
blends of PBa with PS.
where Tg is the glass transition temperature of the blends, and the
Tg1 and Tg2 are those of the blend components. Wi is the weight
fraction. The k is an adjusting parameter related to the degree of
curvature of the Tg-composition curve. The application of Gor-
doneTaylor equation to the experimental data yielded a k value of
0.34, fitting the experimental data quite well. The behavior of
single and composition-dependent Tg’s indicates that the blends
of Ba with the model PS are completely miscible. The homogenous
mixtures of Ba with the model PS was cured at elevated
temperature to obtain the binary blend of PBa with PS. It was
observed that with the curing reaction proceeding the transparent
and homogenous mixture gradually became cloudy, implying that
the reaction-induced phase separation occurred. The cured blends
were subjected to the morphological observation by means of
scanning electron microscopy (SEM). Before measurement, the
fractured end of the sample was etched with tetrahydrofuran
to rinse the PS phase whereas the matrix of PBa remained
unaffected. Shown in Fig. 9 is the SEM micrograph of the ther-
mosetting blend containing 10 wt% PS. It is seen that some
According to the miscibility of Ba monomer with PVPy and PS
blocks, it is proposed that the curing reaction will start from the
homogenous solutions composed of Ba and PVPy-b-PS diblock
copolymer. This speculation has been further confirmed by the fact
that no scattering peaks were displayed on the SAXS profiles (not
shown) of the mixtures of Ba monomer with PVPy-b-PS diblock
copolymer. In views of the difference in miscibility and phase
behavior in the blends of the subchains (i.e., PS and PVPy) of the
diblock copolymer with PBa after and before curing, it is judged
that the formation of the nanostructures in PBa thermosets con-
taining PVPy-b-PS diblock copolymer followed the mechanism of
reaction-induced microphase separation, i.e., at the beginning of
the curing reaction, all the subchains (viz. PVPy and PS) of the
diblock copolymer are miscible with Ba monomer. With the curing
reaction proceeding, the PS subchains were separated out at the
nanometer scale whereas the PVPy subchains remained mixed
with PBa networks via the intermolecular hydrogen-bonding
interactions.
400
380
360
Gordon-Taylor equation
with k = 0.34
340
320
300
280
260
240
0
20
40
60
80
100
Ba (wt%)
Fig. 8. Plot of glass transition temperatures (Tg’s) of the binary mixtures of Ba with the
Fig. 7. DSC curves of the blends of Ba monomer with the model PS.
model PS as a function of PS content.

6354
D. Hu, S. Zheng / Polymer 51 (2010) 6346e6354
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Acknowledgment
Financial support from Natural Science Foundation of China (No.
20474038 and 50873059) and National Basic Research Program of
China (No. 2009CB930400) is gratefully acknowledged. The authors
thank the Shanghai Synchrotron Radiation Facility under the
project of 08sr0157 and Shanghai Leading Academic Discipline
Project (Project Number: B202) for partial support.
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