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Y. Li et al. / Polymer 54 (2013) 3017e3025
on the formation of LC phase is investigated. In addition, the LC
phase of fully cured resins is characterized using various experi-
mental techniques. The glass transition temperature, dynamic
mechanical properties, and thermal expansivity of the resins cured
in LC and non-LC state are examined systematically.
hydrochloric acid was added into dimethylformamide to produce
hydrochlorination reagent. Cresol red solution was used as acid-
base indicator and was prepared by dissolving cresol red in a
mixture of acetone and distilled water. A small amount of BP was
dissolved in the hydrochlorination reagent. Then the excess acid
was titrated with a 0.1 N sodium hydroxide solution. The EEW was
found to be 170.6, which is consistent with the value previously
reported by Su [27]. The chemical structures of the epoxy monomer
and the curing agent are illustrated in Fig. 1.
2. Experimental section
2.1. Materials
The thermal properties of BP and the fully cured resins were
studied using a Q2000 DSC (TA Instruments, Inc.). The DSC cell was
purged with helium gas at a flow rate of 25 mL/min. The epoxy
monomer was tested at a heating and cooling rate of 10 ꢀC/min. For
the fully cured resins, the first heating scan was used to erase the
thermal history. While the second heating scan was recorded to
evaluate Tg.
To study the curing behavior, the mixture of BP and SAA was
loaded into a hermetic aluminum DSC pan then sealed with a lid. A
series of isothermal cure studies were carried out using a Q20 DSC
(TA Instruments, Inc.). The DSC cell was purged with nitrogen gas at
a flow rate of 50 mL/min. The samples were cured at 150 ꢀC, 160 ꢀC,
170 ꢀC, 180 ꢀC, 190 ꢀC, 200 ꢀC, and 210 ꢀC for 180 min respectively.
Morphologies of BP were investigated using a polarized optical
microscope (POM) from Olympus (model BX51-TRF equipped with
a Linkam LTS-350 hot stage and TMS-94 temperature controller).
Small amounts of BP (2w3 mg) was pre-melted on a microscope
slide then covered with a piece of cover glass to form a uniform thin
film. The samples were heated and cooled repeatedly from room
temperature to 170 ꢀC at a rate of 1 ꢀC/min to investigate the change
of birefringence. The isothermal cure of BP with SAA was also
monitored using POM. The formation and development of the LC
phase were examined under polarized light.
4,40-dihydroxybiphenyl with 97% purity, benzyltrimethyla
mmonium bromide, and sulfanilamide (SAA) were purchased from
SigmaeAldrich (Milwaukee, WI). Epichlorohydrin with 99% purity
was obtained from Acros Organics (Belgium). Sodium hydroxide,
isopropyl alcohol, chloroform, methanol, hydrochloric acid, and
acetone were supplied by Fisher Scientific (Fair Lawn, NJ). All
chemicals were used as received without further purification.
2.2. Synthesis of 4,40-diglycidyloxybiphenyl (BP)
The epoxy monomer was synthesized according to a procedure
reported in an earlier work by Su and coworkers [27]. A mixture of
4,40-dihydroxybiphenyl (57.26 g), benzyltrimethylammonium bro-
mide (2.09 g) and epichlorohydrin (481 ml) was placed in a three-
neck flask and refluxed for 40 min. NaOH (24.6 g) was dissolved in
139 ml of water to prepare 15% NaOH aqueous solution. Then the
solution was added into the flask dropwise over a period of 3 h
under reflux. The reaction was carried out for an additional hour at
room temperature. The excess epichlorohydrin was removed by
vacuum distillation and the final product was washed with water
and methanol. A white powder was obtained by recrystallization
from isopropyl alcohol and chloroform.
Wide angle X-ray diffraction (WAXD) was used to explore the
crystal structure of BP and the fully cured LCERs. For the epoxy
monomer, a high temperature XRD experiment was carried out
using Rigaku Rint 2000 diffractometer equipped with a high tem-
perature furnace. The diffraction patterns were collected at 30 ꢀC,
100 ꢀC, 140 ꢀC on heating process and 100 ꢀC, 30 ꢀC on cooling
2.3. Sample preparation and curing process
Uncured resin samples were prepared by dissolving BP and SAA
in tetrahydrofuran (THF) in a stoichiometric ratio. Then the solvent
was removed at room temperature and the mixture was dried
under vacuum for 24 h to prevent further reaction. To study the
curing behavior, the mixture was loaded into aluminum differential
scanning calorimeter (DSC) pans and hermetically sealed. A small
hole was made in the center of the lids to prevent pressure buildup.
To study the thermomechanical properties of fully cured resins, the
samples were cured in a convection oven at 170 ꢀC, 180 ꢀC, 190 ꢀC,
and 200 ꢀC for 12 h and post-cured at 230 ꢀC for 2 h.
process respectively with a Zr-filtered MoKa radiation. In the
experiment, a platinum plate was used as a sample holder, and the
scan rate was 0.15ꢀ/min over a scan angle from 0ꢀ to 40ꢀ. For the
fully cured resins, the diffraction patterns were collected using
Scintag XDS2000 powder diffractometer with Kevex Peltier cooled
silicon detector and Ni-filtered CuKa radiation. The scan rate was
2ꢀ/min over a scan angle from 0ꢀ to 40ꢀ.
Dynamic mechanical properties of the fully cured resins were
studied using a model Q800 dynamic mechanical analyzer (DMA,
TA Instruments, Inc.). All the samples were heated from room
2.4. Characterization of BP and fully cured resins
The chemical structure of BP was characterized using fourier
transform infrared spectroscopy (FTIR) and nuclear magnetic
resonance (NMR). The FTIR spectrum was recorded on a Bruker’s
IFS66V FTIR with a resolution of 2 cmꢁ1 from 400 to 4000 cmꢁ1 at
temperature to 280 ꢀC at 3 ꢀC/min, at 1 Hz frequency and 25
amplitude in three-point bending mode.
mm
room temperature. The characteristic peaks at 2927 cmꢁ1
,
O
O
1606 cmꢁ1, 1500 cmꢁ1, 1244 cmꢁ1, 1037 cmꢁ1 and 910 cmꢁ1 can be
assigned to the stretching of (CH2), stretching of (C]C), bending of
(C]C), stretching of (CeO) on aromatic rings, stretching of (CeO)
on aliphatic chain, and epoxy group, respectively. The 1H NMR
spectrum was obtained by means of a Varian VXR-300 NMR in-
strument at room temperature, in the presence of CDCl3 as the
H
C
H
C
H
C
H
C
H
C
H
C
2
O
O
2
2
2
4,4’-diglycidyloxybiphenyl (BP)
O
solvent. 1H NMR (CDCl3):
CH2 of epoxy), 3.38(2H, m, CH of epoxy),
glycidyl), 4.25(2H, dd, CH2 of glycidyl), 6.96(4H, d, biphenyl),
7.45(4H, d, biphenyl).
The epoxy equivalent weight (EEW) of BP was determined by
titration using the hydrohalogenation method. Concentrated
d
2.78(2H, dd, CH2 of epoxy),
d2.93(2H, dd,
H2N
NH2
S
d
d4.01(2H, CH2 dd, of
O
d
d
d
Sulfanilamide (SAA)
Fig. 1. Chemical structures of the epoxy monomer and the curing agent.