Structure-property relationship of highly ??-conjugated schiff-base moiety in liquid crystal diepoxide polymerization and mesophases stabilization

Article abstract of DOI:10.1021/jp202998x

A study of the structure-property relationship of the highly ??-conjugated Schiff-base moiety in polymerization of a liquid crystal (LC) diepoxide oligomer (PBMBA) and mesophases stabilization has been investigated. We first proposed two exothermic peaks

Full text of DOI:10.1021/jp202998x

ARTICLE
pubs.acs.org/JPCB
StructureꢀProperty Relationship of Highly π-Conjugated Schiff-Base
Moiety in Liquid Crystal Diepoxide Polymerization and
Mesophases Stabilization
Huan Liu,†,‡ Zien Fu,†,‡ Kai Xu,*,† Hualun Cai,†,‡ Xin Liu,†,‡ and Mingcai Chen†
†Key Laboratory of Polymer Material for Electronics, Guangzhou Institute of Chemistry, Chinese Academy of Sciences, P.O. Box 1122,
Guangzhou 510650, People's Republic of China
‡Graduate School of the Chinese Academy of Sciences, Beijing 100049, People's Republic of China
S Supporting Information
b
ABSTRACT: A study of the structureꢀproperty relationship of the highly
π-conjugated Schiff-base moiety in polymerization of a liquid crystal (LC)
diepoxide oligomer (PBMBA) and mesophases stabilization has been
investigated. We first proposed two exothermic peaks distinctly observed
in nonisothermal polymerization curves for thermal copolymerization of
PBMBA monomer with a diamine comonomer that corresponded to two
different reactions, namely, an epoxy-amine polymerization and anionic
polymerization. For PBMBA, note that an unexpected homopolymerization
accompanying an appearance of enantiotropic mesophase transitions had
taken place in the absence of any initiators, evidencing the possibility of an
anionic mechanism yielding a homopolymer. A novel Schiff-base model compound (SBM) was synthesized and used to induce the
polymerization of different types of epoxies. Based on the structureꢀproperty relationship, we considered a specific role of highly π-
conjugated Schiff-base moieties in the anionic polymerization of PBMBA and hoped the mesophases could be stabilized using this
mechanism, which may provide a key strategy for design of the polymer-dispersed liquid crystal (PDLC) materials via a chemical
process.
’ INTRODUCTION
research areas. Intensive research has been dedicated to the
ability of self-assembly of π-conjugated systems, which has
mainly been approached from both a materials and a supramo-
lecular chemistry point of view.15ꢀ18 Based on the ability of self-
assembly of π-conjugated systems, π-conjugated polymer chains
acting as rigid chains could function as mesogenic units for liquid
crystal properties.19ꢀ21 However, ordered structures formed by
LC mesophases have a well-known drawback, they are difficult to
be stabilized, which limits their wide application. According to
the theory of supramolecular chemistry, two important second-
ary interactions in mesophase formation and stabilization are
πꢀπ22 and hydrogen bond interactions. Generally, πꢀπ inter-
actions often exist in π-conjugated systems and hydrogen bonds
are formed when a donor (D) with an available acidic hydrogen
atom is interacting with an acceptor (A) carrying available
nonbonding electron lone pairs. In comparison with the πꢀπ
interactions, the hydrogen bonds, being highly selective and
directional, are ideal secondary interactions to construct supra-
molecular architectures. Use of intermolecular hydrogen bond-
ing, the controlled morphology of epoxy-based responsive
polymers has been reported.23ꢀ27 Special interests have been
Owing to their superior electrical and mechanical properties,
ease of processing, good adhesion to many substrates, and
excellent chemical resistance, epoxy polymers have been widely
used as structural, coating, and adhesive materials in many
demanding application fields, such as automotive, aeronautics,
electronics, and electrical engineering.1ꢀ4 Recently, epoxy-based
polymers also contribute to the development of advanced
functional materials, such as self-healing polymers,5,6 shape
memory polymers,7,8 hybrid organicꢀinorganic polymers,9,10
and thermoresponsive inorganic/organic polymers.11,12 The
important research in materials consisting of specific organic
structure units (such as mesogenic group, π-conjugated moiety,
etc.) embedded in an epoxy matrix have been developed.13,14
One way to prepare these organicꢀorganic functional materials
is to find the optimum interplay of weak and strong physical
interactions (supramolecular chemistry) with covalent bonds
(crosslinks). This implies the strategy not only of the chemistry,
but also of the rheology and thermodynamics in the same
process.
In the absence of a detailed understanding of the supramole-
cular interactions between the individual π-conjugated mol-
ecules, the introduction of π-conjugated systems in electronic
devices and the dream to arrive at molecular electronics based on
these systems have become one of the most challenging scientific
Received: March 31, 2011
Revised:
May 9, 2011
Published: May 23, 2011
r
2011 American Chemical Society
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Scheme 1. Synthetic Routes of PBMBA (A) and SBM (B); Chemical Structure of Epoxy (C)
focused on the design of the epoxy-based responsive polymers
with controlled dispersion of the low molar mass liquid crystals to
find correlations between fast response to the external field, such
as thermal gradient, electric, or magnetic fields, and the final
morphology generated in these systems. Several representative
low molar mass crystal structures are known for EBBA that was
used to generate polymer-dispersed liquid crystals (PDLCs) by
polymerization-induced phase separation (PIPS) in an epoxy
matrix.28,29 This type of PDLC, consisting of well-dispersed and
stabilized LC-rich domains in a polymer matrix, displays func-
tional properties and is a material with a potential application in
electrooptical devices.30ꢀ32
In this work, the liquid crystal diepoxide oligomer (PBMBA),
alternately having imine groups (CdN) and benzene rings in the
main chain and being π-conjugated through rigid molecular
scaffolds, was prepared in a manner similar to the method
reported in ref 33. This polymer was investigated since early
1998 and was reported to show enantiotropic mesophase
transitions;33ꢀ35 however, to our knowledge, the specific role
of a highly π-conjugated Schiff-base moiety in the polymerization
of PBMBA has not been investigated from the point of view of
the structureꢀproperties relationship. Meanwhile, whether these
ordered structures appearing in the vicinity of the enantiotropic
mesophase transitions temperature region of both the smectic
and nematic phases could be stabilized by using this anionic
homopolymerization of the one-component LC diepoxide that
has not yet been reported. As one aim of this study, we are going
to focus on its anionic polymerization mechanism of the LC
diepoxide based on the structureꢀproperty relationship of a
highly π-conjugated Schiff-base moiety in the polymerization of
PBMBA. For the PBMBA/MDA copolymerization system, two
exothermic peaks were distinctly observed in nonisothermal
polymerization curves, and corresponding various enthalpies
during the two polymerization stages could be derived, which
is considered as two different reaction mechanisms: the epoxy-
amine polymerization reaction and the anionic polymerization
reaction. Note that the anionic initiation mechanism, being
different from the one reported by others,34,35 was first proposed.
This type of anionic initiation mechanism could also be used to
explain the occurrence of the unexpected homopolymerization
accompanying with the appearance of enantiotropic mesophase
transitions in the absence of any initiators for the nonisothermal
polymerization curve of PBMBA. Likewise, we hoped that the
highly π-conjugated Schiff-bases moiety could also initiate the
polymerization of different types of epoxies. Accordingly, a
Schiff-base model compound (SBM) was synthesized and used
to induce the polymerization of epoxy. Owing to the simultaneity
of the mesophase transitions and the homopolymerization, we
wondered whether these mesophases could not be disrupted and
could even be stabilized using this reaction. As a second aim of
this study, differential scanning calorimetry (DSC), polarizing
optical microscopy (POM), and wide-angle X-ray diffraction
(WAXD) were used to explore the possibility of stabilization of
mesophases via a chemical process. Once the LC ordered
structures could be stabilized by the homopolymerization of
the diepoxide, there can be no doubt for a new strategy for the
design of the PDLC materials via a chemical process, which will
be the focus of our further study.
’ EXPERIMENTAL SECTION
Materials. All reagents and solvents were purchased as reagent
grade and were purified or dried by standard methods before use.
The following reagents were purchased from Aladdin-reagent
Co. Shanghai, China: terephthalaldehyde, p-aminophenol, ani-
line, and 4,40-diaminodiphenylmethane (MDA). Analytical grade
zinc chloride obtained from Shanghai Chemical Reagent, Ltd.,
China, was used as catalyst. Epichlorohydrin (ECH) was pur-
chased from Lingfeng Chemical Reagent Co., Shanghai, China.
The epoxies DGEBA (diglycidyl ether of bisphenol A) and
HWEP (high molecular weight diglycidyl ether of bisphenol A;
see Scheme 1C) with an epoxy equivalent weight (EEW) of
185 g/eq and 502 g/eq were purchased from Shell Chemical Co.
BAEP (bisphenol A diglycidyl ether) monomer was purchased
from Alfa Aesar.
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1
Physical Measurements. H NMR spectrum was recorded
heated from 50 to 300 °C at a heating rate of 10 °C/min under
nitrogen.
on a Bruker AM-400S (400 MHz) spectrometer using TMS as
internal standard and chloroform-d (at 7.26 ppm), DMSO-d
(at 2.50 ppm), and acetone-d (at 2.05 ppm) as solvents. FT-IR
spectrum was recorded with an Analect RFX-65A infrared
spectrophotometer using KBr discs. EA was performed on a
PE 2400 Series II CHNS/O Analyzer. Mass spectrum was
determined on a VG-7070E spectrometer (EI, 70 eV).
Synthesis of PBMBA and SBM. The synthetic pathways of
PBMBA and SBM are shown in Scheme 1. The procedures were
carried out in a four-neck round-bottom flask equipped with a
nitrogen inlet and stirrer under controlled temperature and
pressure. Schiff-base-type diphenol (DPBA) was synthesized
from terephthalaldehyde (20.0 g, 0.15 mol) and p-aminophenol
(30.0 g, 0.3 mol) in a solvent mixture (dimethyl sulfoxide
(DMSO) 150 mL, ethanol (EtOH) 300 mL), using zinc chloride
(0.5 g) as the catalyst (see Scheme 1A). The reaction mixture was
stirred at 85 °C for 4 h and then poured into 1000 mL of H2O. A
yellow solid was collected by filtration, recrystallized, and dried.
The liquid crystal (LC) diepoxide oligomer (PBMBA) was
synthesized from 0.1 mol of DPBA, 5 mol of ECH, and 20 mL
of DMSO as a solvent using 22.0 g of a 40 wt % NaOH solution as
the catalyst (see Scheme 1A). SBM was synthesized via the
condensation of terephthalaldehyde with aniline in a similar way
as described in the synthesis of DPBA (see Scheme 1B).
Calculation of Activation Energy Value (Ea). Ea obtained
with respect to the analysis of secondary reactions and the
conversion of epoxy groups at the end of the DSC runs allowed
the following assumptions in the further analysis of DSC data:
(i) negligible influence of secondary reactions; (ii) 100% con-
version of the limiting reactant; and (iii) heat of reaction constant
and independent of conversion. The DSC data were correlated to
the rate of conversion at constant temperature to some function
of the concentration of reactants using the following equation:
dR
dt
¼ kfðRÞ
ð1Þ
where (dR)/(dt) is the rate of cure; R is the fractional conversion
at any time t; k, the Arrhenius rate constant, and f(R), a function
form of R that depends on the reaction mechanism. For
nonisothermal conditions, when the temperature varies with
time with a constant heating rate, β = dT/dt, eq 1 is represented
as follows:
ꢀ
ꢁ
dR
dT
A
Ea
¼
exp
ꢀ
fðRÞ
ð2Þ
β
RT
1
DPBA: Yield, 42.0 g, 90%. mp, 272ꢀ275 °C. H NMR
where A is the pre-exponential factor, Ea is the activation energy,
and R is the gas constant. The straightforward application of eqs 1
and 2 yields a single kinetic triplet for the overall process. This
type of analysis does not allow for possible changes in the rate-
limiting step. The derivative modes by Dole can be used to give
Ea from the plot ln βi against TꢀRi1 (here i is the ordial number of
DSC runs performed at different heating rates, βi).36
UVꢀVis Experiment. Ultraviolet and visible light (UVꢀvis)
absorbance spectrum was measured using a Shimadzu UV-2550
spectrophotometer (Shimadzu Corporation, Kyoto, Japan). The
measurements of the optical absorption were preformed at room
temperature, where a deuterium lamp is a source of ultraviolet
(200ꢀ380 nm) and a tungsten lamps are used for visible
(380ꢀ800 nm) and near-infrared (800ꢀ3300 nm) parts of light.
The samples, PBMBA, MDA and mixture of PBMBA/MDA
(1:1 mol ratio) were well soluble in the nonpolar chloro-
form (CHCl3). The concentration of these solutions has been
on the level 10ꢀ5 mol/L, suitable for the UVꢀvis optical
investigations.
POM Experiment. Optical studies were conducted with a
polarizing optical microscope (POM; Optiphot-Po1, Nikon)
equipped with a hot stage attachment (Mettler FP52/FP5).
The phase and crystalline morphology of different types of
aggregation structures were observed under POM with a heating
rate of 10 °C/min. For the stabilization of the smectic phase,
PBMBA was heated from room temperature to 225 °C
(a temperature of smecticꢀnematic transition) but soon
dropped to 210 °C. Heated at this temperature for 30 and 120
min, respectively, the samples, namely, as S30 and S120, were
obtained. For the stabilization of the nematic phase, PBMBA was
heated from room temperature to 240 °C (a temperature of
nematicꢀisotropic transition) but soon dropped to 210 °C.
Heated at this temperature for 30 and 50 min, respectively, the
samples, namely, as N30 and N50, were obtained. During the
entire observation period, we found the fixed mesophases could
be sustained 120 min for the smectic phase and 50 min for the
nematic phase. The given temperature, 210 °C, was found to be
(DMSO-d) δ (ppm): 9.57 (1H, single, OH), 8.57 (1H, single,
NdCH), 7.80 (2H, single, ArH), 7.24ꢀ7.26 (2H, multiplet,
ArH), 6.80ꢀ6.82 (2H, multiplet, ArH). FT-IR (KBr cmꢀ1):
3384 (OH), 1619 (CN). MS: m/z 316 [Mþ]. Elem. Anal. (%)
Calcd for C20H16N2O2: C, 75.93; H, 5.10; N, 8.86; O, 10.11.
Found: C, 75.77; H, 5.15; N, 8.81; O, 10.27.
PBMBA: mp: 194ꢀ203 °C. 1H NMR (CDCl3) δ (ppm): 8.51
(1H, single, NdCH), 7.97 (2H, single, ArH), 7.24ꢀ7.26 (2H,
multiplet, ArH), 6.94ꢀ6.96 (2H, multiplet, ArH), 4.85 (1H,
single, OH), 4.23ꢀ4.27 (2H, multiplet, CH2), 3.95ꢀ4.00 (2H,
multiplet, CH2), 3.53ꢀ3.38 (1H, multiplet, CH), 2.90ꢀ2.91
(2H, multiplet, CH2), 2.76ꢀ2.78 (2H, multiplet, CH2). FT-IR
(KBr cmꢀ1): 3384 (OH), 1610 (CN), 914 (epoxy ring). EEW:
225 g/eq.
SBM: Yield: 24.5 g, 86%. mp: 160ꢀ162 °C. 1H NMR
(acetone-d) δ (ppm): 8.68 (1H, single, NdCH), 8.10 (2H,
single, ArH), 7.41ꢀ7.45 (2H, multiplet, ArH), 7.28ꢀ7.32 (2H,
multiplet, ArH), 7.24ꢀ7.28 (1H, multiplet, ArH). FT-IR
(KBr cmꢀ1): 1617 (CN). MS: m/z 284 [Mþ]. Elem. Anal.
(%) Calcd for C20H16N2: C, 84.48; H, 5.67; N, 9.85. Found: C,
84.53; H, 5.65; N, 9.82.
DSC Experiment. Differential scanning calorimetry (DSC)
was recorded with a Diamond/Pyris DSC analyzer under a
nitrogen atmosphere. For the mixture of PBMBA/MDA,
PBMBA was dissolved completely in chloroform, and then a
stoichiometric amount of MDA was added. Afterward, the
solvent was removed by rotary evaporator under reduced pres-
sure. Three mixtures of SBM/BAEP, SBM/DGEBA, and SBM/
HWEP with a 1:1 mol ratio of CdN and epoxy were prepared in
the same way as mentioned above. The nonisothermal DSC
copolymerization of PBMBA/MDA was recorded under nitro-
gen from 50 to 300 °C at heating rates of 5, 10, 15, 20 °C/min.
For the homopolymerization of PBMBA, the same type of
experiment has been carried by DSC from 50 to 320 °C. For
the study of SBM, the mixtures of SBM/BAEP, SBM/DGEBA,
and SBM/HWEP and the corresponding neat diepoxides were
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Table 1. Thermogravimetric Data for the Nonisothermal
DSC at Various Heating Rates of the PBMBA/MDA
Copolymerization
heating ratio
Tp1
Tp2
ΔH1
ΔH2
Ea1
Ea2
(β; °C/min) (°C)
(°C)
(J/g)
(J/g)
(kJ/mol) (kJ/mol)
5
137.96 238.64 ꢀ65.73 ꢀ179.72 124.65
143.45 252.21 ꢀ92.51 ꢀ234.88
149.17 264.95 ꢀ84.19 ꢀ219.04
152.90 273.57 ꢀ86.76 ꢀ199.30
82.27
10
15
20
Figure 1. Nonisothermal DSC curves for the PBMBA/MDA copolym-
erization at various heating rates.
the most suitable temperature for the stabilization of the LC
ordered structures.
FT-IR Experiment. The samples prepared for observing a shift
of PBMBA CHdN absorption band using FT-IR spectra were
obtained by DSC calcination at a heating rate of 10 °C/min
under nitrogen. The two samples, obtained at 290 °C for
PBMBA-1 and at 320 °C for PBMBA-2 were rapidly taken out
from the DSC pool and quenched in liquid nitrogen.
WAXD Experiment. Wide angle X-ray diffraction (WAXD)
scanning curves ranging from 0 to 60° were collected at room
temperature with a Rigaku D/max-1200 X-ray diffractometer
using copper filtered Cu KR radiation (40 KV, 30 mA). The
samples, S30, S120, N30, and N50, with a glass slide as substrate,
were used to perform WAXD measurements.
Figure 2. FT-IR spectra of PBMBA (A) and PBMBA/MDA (B).
copolymerization reaction, have attracted considerable attention.
It is generally accepted that the epoxy-amine polymerization
reaction includes three principle reactions: the reaction of the
primary amine hydrogen with the epoxy group, the reaction of
the secondary amine hydrogen with the epoxy group, and the
reaction of the hydroxyl hydrogen with the epoxy group. These
abnormalities in the PBMBA/MDA copolymerization reaction
suggest that it did so through dissimilar mechanisms. For stage 1,
the classical epoxy-amine polymerization reaction, nucleophilic
addition on the epoxy ring proceeds with the preliminary
activation of the epoxy ring by amines, takes place. In contrast
to the results for Ea of the epoxy-amine polymerization reported
by other researchers,38ꢀ41 the Ea value of PBMBA/MDA copo-
lymerization for low conversion (R = 0ꢀ0.4) is considerably
high. This may be due to the specific role of highly π-conjugated
Schiff-base in the copolymerization of PBMBA/MDA. Regarding
stage 2, it can be noted that the onset polymerization of this stage
starts after the end of stage 1. According to the literature,39,41,42
the reactions of stage 2 may become diffusion controlled at a
higher conversion. However, due to the presence of the rigid rod-
like mesogen, the increasing diffusion restriction hinders the
mobility of the functional groups, which was further evidenced by
higher Tps for stage 2. The larger ΔH value implies that the
reaction occurring in this stage can be attributed to the anionic
polymerization reaction of the epoxy,34 different from the epoxy-
amine polymerization. The mechanism of the anionic polymer-
ization being different from other authors’ elucidation will be
discussed further below.
’ RESULTS AND DISCUSSION
PBMBA/MDA Copolymerization. Copolymerization Kinetics.
The thermal copolymerization of PBMBA monomer with MDA
comonomer was studied by DSC. The nonisothermal DSC
polymerization curves at the various heating rates (see Figure 1)
showed two exothermic peaks that were distinguished clearly from
each other. For instance, the polymerization curve obtained at the
heating rate of 10 °C/min exhibited one broad exothermic peak
ranging from 70 to 190 °C and the other broad exothermic peak
ranging from 200 to 300 °C. The peak temperature, Tp, and the
polymerization enthalpy, ΔH, values for each exothermic peak
obtained from Figure 1 are summarized in Table 1. As can be seen,
Tp and the polymerization enthalpy values for stage 2 were
considerably higher than those for stage 1. Tp2s appearing at the
higher temperatures and corresponding ΔHs largely increased.
The observed heat of PBMBA/MDA copolymerization for the
stage 1 was calculated as 38 kJ/mol (based on a molecular weight
of 450 g/mol obtained by titration), which was lower by almost
58 kJ/mol than that reported in the literature for the polymeriza-
tion of epoxy/diamine, 98ꢀ99 kJ/mol.37 This observation sug-
gested 40% epoxy monomer conversion (R) during stage 1.
Moreover, the observed heat for stage 2 was calculated as 95 kJ/mol.
The resulting activation energies for each peak were, respectively,
found to be Ea1 = 124.65 kJ/mol for stage 1 and Ea2 = 82.27 kJ/mol
for stage 2.
FT-IR Spectrum and UVꢀVis Absorbance Spectrum. The
presence of the imine groups was confirmed, respectively, by
FT-IR spectrum and UVꢀvis absorbance spectrum because the
band characteristic of the CdN stretching deformations was
The characteristic phenomena related to the appearance of
two exothermic peaks and the corresponding and prodigiously
different Tp, ΔH, and Ea values, occurring in the PBMBA/MDA
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Figure 3. Chloroform solution UVꢀvis absorbance spectra of PBMBA
Figure 4. Proposed D [Hþ] A assembly model between the
highly π-conjugated imine structure and the amine.
3 3 3
3 3 3
(A), PBMBA/MDA (B), and MDA (C).
detected in the case of both PBMBA and PBMBA/MDA. In the
FT-IR spectrum of PBMBA (see Figure 2), a strong absorption at
1610 cmꢀ1 suggested the presence of the CdN structure, which
shifted to a higher wavenumber in the case of PBMBA/MDA
(1619 cmꢀ1). In addition to the CdN stretching band, a band at
about 1590 cmꢀ1 could be distinguished and ascribed to the
CdC stretching deformations in the aromatic ring. The lower
wavenumber of the imine group absorption indicates the better
conjugation of π-electrons. The higher wavenumber value im-
plies that the bond between carbon and nitrogen atoms in the
imine group is shorter, which means that the π-electrons in the
double bond are less involved in conjugation.
reaction. For stage 1, because the primary amines of MDA are
involved in the protonation process of the Schiff-base moiety
achieved in solution, the mobility of those is decreased. When the
epoxy-amine copolymerization occurs, more energy is needed to
achieve a deprotonation process that makes the primary amines
be released, thus, resulting in the increase of the Ea values. When
the reaction becomes the diffusion controlled at the higher
conversion, the hydroxyl groups, initially present in the epoxy
prepolymer and those generated during the epoxy-amine copo-
lymerization, may move and are involved into the D
3 3 3
[Hþ] A assembly. As shown in Figure 5, the D [Hþ]
A
3 3 3
3 3 3
3 3 3
assembly between the hydroxyl groups and the Schiff-base
moiety leads to the formation of alkoxy anions. The formed
key intermediate alkoxy anions can react with another hydroxyl
groups that are not involved into the protonation process of the
Schiff-base moiety through the ionic transition effect, which
results in the formation of hydroxyl oxygen anions. This step can
be considered to be the initiation process of the anionic
polymerization (see Figure 6A). Based on this type of initiation
mechanism, the hydroxyl oxygen anion can be regarded as
initiation center that reacts with another epoxy ring, placing
the actively site on another alkoxy anion. Thus, a kinetic chain
causing branching and cross-linking is generated (see Figure 6B).
Rozenberg has acknowledged that the presence of a proton donor
or electrophilic agent such as a hydroxyl group forming a complex
between the hydroxyl group, the oxygen in the epoxy ring and the
tertiary amine that is required by initiation of epoxy anionic
polymerization by tertiary amines.46 Many other researchers suppose
that the initiation mechanism involves formation of a zwitter-ion by
interaction between the tertiary amine and the H-bonded oxirane ring
and that the subsequent propagation reaction occurs via the
alkoxide.35,47ꢀ49 Whatever the true underlying initiation mechanism,
they all focused their attention on the important role of tertiary amine
that was beneficial to the formation of alkoxide or alkoxide anion,
both of which could be the propagation species. However, it is
necessary to point out no tertiary amines was used in our work.
Accordingly, from the point of view of structureꢀproperties relation-
ship, it requires us to take into account what is the specific role of the
highly conjugated Schiff-base moiety in polymerization of the epoxies.
PBMBA Homopolymerization. If the anionic polymerization
was initiated by the protonation process of the Schiff-base
moiety, this type of initiation mechanism should not only be
suitable for the PBMBA homopolymerization, but also be
Figure 3 illustrates the UVꢀvis absorbance spectra, within the
optical range from 260 to 450 nm, of PBMBA and PBMBA/
MDA respectively in chloroform solution. The short-wave’s border
of UV spectra of solution was limited by a permeability of solvent, i.e.
240 nm for chloroform. Two absorbance bands at λ1
(280ꢀ320 nm) and at λ2 (320ꢀ440 nm) were distinctly present
in the spectra of PBMBA and PBMBA/MDA, except that of MDA.
PBMBA exhibited one well-defined absorption band in the range of
325ꢀ440 nm, being responsible for πꢀπ* transition in the imine
group.43,44 The introduction of MDA to PBMBA in comparison
with neat PBMBA leaded to a hypsochromic shift of the λmax band
in chloroform solution from 386 to 375 nm. The blue shift of the
absorption maxima observed for the Schiff-base moiety described
here is ascribed to the resonance effect of the protonic electron-
withdrawing group.44,45 These results reveal that due to the presence
of the highly π-conjugated imine structure acting as a proton
acceptor and amino proton acting as a proton donor in chloroform
solution, the assembly behavior of donorꢀprotonꢀacceptor
(D [Hþ] A) shown in Figure 4 may be readily achieved in
3 3 3
3 3 3
solution.44 The protonation process of the Schiff-base moiety causes
“break” of the conjugation system and diminishes the length of
conjugated part of molecule, which leads to the hypsochromic
transition of the long-wavelength absorption band.
Combination of evidence in the FT-IR spectrum and the
UVꢀvis absorbance spectrum, the shift transitions of the char-
acteristic peak for the CdN stretching deformations are con-
sistent with the assembly behavior of D [Hþ] A between
3 3 3
3 3 3
the Schiff-base moiety and the amino group.
Copolymerization Mechanism. The mechanism of the ther-
mal copolymerization of PBMBA/MDA is postulated to attempt
to elucidate our experimental observations that could not be
explained simply by the classical epoxy-amine polymerization
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Figure 5. Proposed D [Hþ] A assembly model between the highly π-conjugated imine structure and the hydroxyl group.
3 3 3
3 3 3
Figure 6. Proposed anionic polymerization for initiation (A) and propagation (B).
Figure 7. Nonisothermal DSC curves for SBM/DGEBA (A), SBM/
HWEP (B), SBM/BAEP (C), DGEBA (D), and HWEP (E) at a heating
rate of 10 °C/min.
Figure 8. Nonisothermal DSC curves for the PBMBA homopolymer-
ization at a heating rate of 10 °C/min.
suitable for SBM/epoxies system. Therefore, the specific role of
the highly conjugated Schiff-base moiety in polymerization of the
epoxies will be discussed in detail in the following chapter. The
additional work on discovering the feasibility of the stabilization
of the mesophases using the anionic homopolymerization also
will be described below.
peaks and two exothermic peaks. Table 2 shows Tp and ΔH
values for each exothermic peak and each endothermic peak
obtained from the dynamic DSC. The resulting activation
energies for each peak were, respectively, found to be Ea1
=
93.90 kJ/mol for stage 1 and Ea2 = 120.73 kJ/mol for the stage 2.
For the three endothermic peaks, the first one appearing in the
broad range 180ꢀ207 °C, with a maximum peak value at 198 °C,
was due to the monomer melting. The other two weak
DSC and FT-IR Investigations. The DSC thermogram re-
corded for PBMBA (see Figure 8) showed three endothermic
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Table 2. Thermogravimetric Data for the Nonisothermal
DSC at the Various Heating Rates of the PBMBA
Homopolymerization
arising from the following three cases: weight loss, change in
heating rate and change in heat capacity, did not appear. To
resolve these doubts, the samples obtained respectively at 290 °C
(PBMBA-1) and at 320 °C (PBMBA-2) were characterized by
FT-IR spectra. Observed from the position of the CdN absorp-
tion band, it was found that the CdN stretching band appeared
at 1623 cmꢀ1 for PBMBA-1 and at 1612 cmꢀ1 for PBMBA-2 (see
Supporting Information). A combination of the calculated heat
value with the enthalpy and the change of the CHdN absorption
band at position show there are some interesting views as follows.
First, the calculated heat value, being much lower than the energy
of the covalent bond breakage, may result from the breaking of
some noncovalent bonds, namely the deprotonation process of
the Schiff-base moiety. It is, however, generally believed that the
energy of the noncovalent bonds breakage is not up to our
calculated heat value. But if we take into account the exothermic
contribution of the homopolymerization, the higher calculated
heat value should attribute to the two proportions’ contribution.
Second, the CdN stretching band arising from a slight shift to
the lower wavenumber may be due to the occurrence of the
deprotonation process of the Schiff-base moiety. Summarizing,
the conjunct contribution of the coordination bond breakage and
the heat release in the homopolymerization process may lead to
the appearance of the sharp exothermic peak.
heating ratio
Tp1
Tp2
ΔH1
ΔH2
Ea
(β; °C/min)
(°C)
(°C)
(J/g)
(J/g)
(kJ/mol)
5
257.91
273.91
284.39
291.37
291.10
304.99
313.35
321.05
ꢀ209.65
ꢀ164.38
ꢀ139.71
ꢀ133.33
ꢀ411.97
ꢀ402.96
ꢀ411.56
ꢀ410.15
93.90
10
15
20
endothermic peaks at 225 and 240 °C might be, respectively,
associated with a smecticꢀnematic transition and a nematicꢀ
isotropic transition. Similar results, the appearance of the en-
antiotropic mesophase transitions, have been obtained by other
authors33ꢀ35 and were monitored by means of POM. For the two
exothermic peaks, the first peak observed in the temperature
range between 200 and 300 °C and the second was a consider-
ably sharp peak at 300ꢀ310 °C. Here it should be pointed out
that the two endothermic peaks related to the enantiotropic
mesophase transitions accompanied the appearance of the first
exothermic peak. Figure 8 illustrates that an exothermic trend
became more distinct after 225 °C, indicating that a slow
homopolymerization occurred before the appearance of the
smectic mesophase and was accelerated after that. When the
nematic mesophase was presented, the exothermic trend became
more and more distinct. Accordingly, we consider that these
LC mesophases may be performed jointly by a homopolymer
and PBMBA, and the heat release may also be composed of
contributions from the polymerization of amorphous domain
and the LC mesophases. However, that may be difficult to
distinguish.
The first exothermic peak can be associated with the PBMBA
anionic homopolymerization. Based on the previous discussions in
the mechanism of anionic polymerization, the anionic polymeri-
zation is related to the hydroxyl, whether the concentration of the
hydroxyl groups, initially present in the epoxy prepolymer and
those generated during the epoxy-amine polymerization, would
affect the anionic polymerization process. A comparison of Ea
values of the two anionic polymerization stages for PBMBA/MDA
and PBMBA reveals that the occurrence of the PBMBA homo-
polymerization needs more energy, which may be ascribed to the
discrepancy of the hydroxyl concentration. Because a substantial
amount of hydroxyl groups is obtained from the PBMBA/MDA
copolymerization, the hydroxyl concentration for PBMBA/MDA
is higher than that for PBMBA. Accordingly, it is speculated that in
the anionic polymerization stage of PBMBA/MDA, a substantial
amount of hydroxyl groups obtained from the epoxy-amine
polymerization stage makes it possible for more hydroxyl groups
being protonated, which results in the increase of initiation of rate
and polymerization reactivity.
Schiff-Base Model Compound (SBM). Now that the anionic
homopolymerization of the one-component LC diepoxide took
place in the absence of tertiary amines, we wondered if the
protonated Schiff-base moiety could also initiate the polymeri-
zation of the different types of epoxies. To this purpose, a Schiff-
base model compound (SBM) was synthesized via the conden-
sation of the aromatic aldehyde with the aromatic amine.
Figure 7 displays the thermogram of the neat diepoxides and
the mixtures of diepoxides and SBM analyzed by nonisothermal
DSC. The endothermic peaks appearing in the broad range
90ꢀ160 °C for the mixtures of diepoxides and SBM were due to
SBM melting. It was noted that the nonisothermal polymeriza-
tion curve for SBM/HWEP did exhibit an exothermic peak that
failed to appear in those for SBM/DGEBA, SBM/BAEP, and the
neat diepoxides. The appearance of this significant exothermic
peak should be a strong hint for the polymerization heat release
and indicated an occurrence of a polyetherification reaction.
Although, the polyetherification reaction for epoxies is well
described in the literature and can be thermally initiated and
does not even require a specific initiator,38,50,51 it did not seem to
support this view with the absence of the exothermic peak for the
neat diepoxide. On the other hand, even if the reaction starts at
about 180 °C, there is still a catalytic effect that should not be
ignored. In our case, based on the initiation mechanism, it must
be pointed out that the interactions between the Schiff-base
moiety and the hydroxyl groups are essential for the initiation.
Regarding the effect of the concentration of the hydroxyl
groups discussed above, HWEP, obviously possessing the higher
hydroxyl concentration than DGEBA, may be easier to form the
hydroxyl oxygen anion and then initiate the anionic polymeriza-
tion. Meanwhile, it should be noted that PBMBA, possessing the
lower hydroxyl concentration than HWEP, could be involved in
the homopolymerization. This contrary situation should be
related with the occurrence of the enantiotropic mesophases,
because the unexpected homopolymerization of PBMBA took
place accompanying the appearance of the enantiotropic meso-
phase transitions. Owing to the formation of these mesophases,
the highly π-conjugated Schiff-base moiety contributes to the
For the second considerably sharp exothermic peak, it was
referred to the thermal degradation reported by Lee.35 However,
we have held different views in this regard, since there is some
doubt based on our DSC results. First, the observed heat of this
stage was calculated as 181 kJ/mol (based on a molecular weight
of 450 g/mol obtained by titration), implying that may not be
simply due to the breaking of the covalent bond. The second, this
peak, appearing in DSC with a considerably narrow temperature
range, was not consistent with the peak of the thermal degrada-
tion. Last, the shift of DSC baseline after the exothermic peak
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Figure 9. Smectic morphologies observed at 225 °C (A) and stabilized at 210 °C for 30 min (B) and for 120 min (C) by POM. Magnification: 200ꢁ .
Figure 10. Nematic morphologies observed at 240 °C (A) and stabilized at 210 °C for 30 min (B) and for 50 min (C) by POM. Magnification: 200ꢁ .
πꢀπ stacking that may enhance the initiation ability of the
mesogenic unit. Accordingly, to explain the specific role of the
highly conjugated Schiff-base moiety in polymerization of the
epoxies, an electronic sink effect should be proposed. Based on
Drude’s free electron theory,52 it is a simplified model of
electrical conduction and can be represented as a process of
accumulation of free electrons inside a battery-positive. Due to
the molecular electronic resonance formed by the D
3 3 3
[Hþ] A assembly, the electron sink effect may be
3 3 3
produced.53ꢀ55 Accordingly, the protonated Schiff-base moiety
resulting from the D [Hþ] A assembly might serve as an
3 3 3
3 3 3
electronic sink for imine/amine or imine/hydroxy chemistry (see
Figures 4 and 5). It is just because of the electronic sink effect that
the mobility of the amino is decreased, which leads to the
increase of the Ea value for the epoxy-amine polymerization. It
is also due to the presence of the electronic sink effect that the
alkoxy anions can be formed in the “protonation-induced”
assembly process, thus, resulting in the occurrence of the
unexpected anionic polymerization.
Stabilization of Mesophases. Depending on annealing tem-
perature, two types of ordered structure, the smectic phase and
the nematic phase, were observed under POM, as shown in
Figures 9A and 10A. The smectic phase and the nematic phase
formed in the temperature range of 218ꢀ232 °C and
232ꢀ245 °C, respectively, and the onset polymerization tem-
perature of PBMBA was about 200 °C, which means that the
ordered structures may be stabilized directly by the polymer
network. Inspection by POM showed that, at 210 °C, the smectic
phase gradually disappeared and became discontinuous with the
increase of the polymerization time (see Figure 9B,C). The
similar results were obtained when the sample possessing the
nematic phase was heated at 210 °C for different time (see
Figure 10B,C). Additionally, the fixed domain of the smectic
phase and the nematic phase could be maintained for at least 3 h
and for no more than 50 min, after then they were transformed
into an isotropic phase. To confirm the ordered structures
presenting in the polymer network, the crystalline morphology
was examined using WAXD. The room temperature WAXD
Figure 11. Room temperature WAXD patterns for PBMBA (A), S30
(B), S120 (C), N30 (D), and N50 (E).
patterns of PBMBA, S30, S120, N30, and N50 are presented in
Figure 11. PBMBA displayed a typical WAXD pattern of a liquid
crystal. After the isothermal treatment, WAXD patterns of S30,
S120, N30, and N50 clearly showed that, although the amor-
phous structure appeared, the diffraction peak of the crystal
structures did not entirely disappear.
During the isothermal treatment, the mesophase was stabi-
lized after that partially disappeared. This may be due to the
partial diepoxide compounds were involved in the PBMBA
homopolymerization that increased the length of the molecule
chain and broke the ordered aggregation of the mesogens. Owing
to the lower polymerization temperature, the diffusion ability of
the rest of the aggregative diepoxide compounds was restricted
by long-chain macromolecules to some degree. Therefore, the
partial PBMBA involved in the homopolymerization that formed
the isotropic phase and the partial PBMBA possessing the
mesophase became immobile and was stabilized. It can also be
used to explain the appearance of the amorphous structure and
the disappearance of the crystal structure confirmed by WAXD.
With the increase of isothermal treatment time, the mobility of
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the long-chain macromolecules is increased, which may release
the restricted LC monomers to react. The formed LC polymer
may be identified as a side-chain LC polymer that can form LC
mesophases.56,57 Therefore, the stabilization mesophases may be
attributed to either the monomer or the polymer, but that may be
difficult to distinguish. Furthermore, the difference in the holding
periods between the stabilization smectic phase and nematic
phase may be related to the structural characteristics of them. It is
well-known that there are two fundamental orders, orientational
order and translational order. For a crystal, 3-dimensional
periodic array of atoms or molecules are orientational and
translational order. For the LC mesophase, with the increase of
temperature, the orientational order will be lost first, but the
translational order will be retained. For the nematic phase,
however, it is the only mesophase without the translational order
at higher temperature.58 Thereby, the stabilized nematic phase
exhibited the shorter holding period than the stabilized smectic
phase. Altogether, we can state that these ordered structures can
be stabilized by the anionic homopolymerization depending on
the polymerization time and temperature. It is conceivable, even,
that this kind of method opens new strategies for the design of
materials with well-dispersed LC mesophases. However, the
work, how to control the dispersion homogeneity, and the size
of the discontinuous phase domain, is important and will be
further studied as our next work.
homopolymerization reaction were well stabilized. This kind of
method, via the chemical process to obtain the LC mesophases
based on the structureꢀproperty relationship, opens new stra-
tegies for the design of the PDLCs materials.
’ ASSOCIATED CONTENT
S
Supporting Information. Figure showing FT-IR spectra
b
of PBMBA-1 and PBMBA-2. This material is available free of
charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*Tel.: þ86 20 85232302. Fax: þ86 20 85231058. E-mail: xk@
gic.ac.cn.
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