J.-J. Yan et al. / Polymer 55 (2014) 6504e6512
6507
All the monomers have vinyl substituent, so they can be poly-
merized via radical polymerization method as illustrated in Scheme
. Each of the resulted polymers had demonstrated good solubility
Table 1
Molecular characterization data of P4-mꢀBF
4
.
4
ꢁ
2
sample
Mn ( ꢂ 10 g/mol)
PDI
Tg ( C)
in variety of organic solvents, such as chloroform, DMF, dimethyl
P
4-2ꢀBF
4
4
4
7.81
11.62
9.47
1.76
2.46
2.09
1.54
113.29
57.37
48.72
38.92
sulfoxide, methyl alcohol and dichloromethane. Fig. 2 depicts the
P4-4ꢀBF
1
H NMR spectra of P4e6ꢀBF
4
. The representative resonance peaks of
was
P
P
4-6ꢀBF
4-10ꢀBF
4
9.81
the vinyl substituent of corresponding monomer M4e6ꢀBF
4
observed to disappear completely after polymerization at chemical
shifts of 7.41e7.30, 5.77e5.73, and 5.44e5.40 ppm (denoted as h, g,
and k, respectively). Moreover, the resonance peaks of P4e6ꢀBF
4
The phase transitions of the polymers were examined by DSC
experiments. To eliminate the thermal history of the samples, all
become quite broad compared to the monomer M4e6ꢀBF
4
due to
ꢁ
ꢁ
the slower motion of the protons, which is common phenomenon
observed for polymer systems. Likewise, FTIR spectroscopic anal-
the samples were first heated to 250 C at a rate of 20 C/min and
kept isothermal for 5 min. Then, the DSC cooling trace and the
second DSC heating trace employed a slower temperature scan of
ysis experiments were carried out to further confirm the results of
1
ꢁ
H NMR. Fig. 2S shows the infrared spectra of M4e6ꢀBF
4
and
10 C/min to determine the Tg values. The DSC thermograms of P4-
P
4e6ꢀBF
4
. It is unambiguous that the characteristic peaks at 954
m
ꢀBF (m ¼ 2, 4, 6, 10) are depicted in Fig. 3. As expected, all the
4
ꢀ
1
and 914 cm are assigned to the CeH out-of-plane bending vi-
bration of the vinyl group which disappeared after polymerization.
On the other hand, the intensity of some peaks after polymerization
obviously decrease compared with the corresponding monomers
due to the restraining of the intramolecular motion. The FTIR
DSC curves exhibited a single glass transition, and no other tran-
sitions were observed during the process of heating or cooling.
However, a strong decrease of the glass transition temperature (Tg)
in this set of polymers with increasing of alkyl spacer length was
observed, as given in Table 1. In the series of P4-mꢀBF
4
(m ¼ 2, 4, 6,
1
experiment results were consistent with the results from H NMR
10), the Tg illustrated a decreasing tendency when the alkyl spacer
spectroscopy. The chemical structures of other monomers and
polymers are shown in supporting information. Finally, GPC was
used to characterize the molecular weights of the prepare ILCPs.
Table 1 provides the MWs and polymer polydispersity index (PDI)
length
increased
< P4e6ꢀBF
(i.e.,
< P4e4ꢀBF
the
Tg
ranking was
4
). This observation
P4e10ꢀBF
4
4
4
< P4e2ꢀBF
was ascribed to a constant increase in a higher molecular flexibility
with increasing alkyl spacer length, which facilitated higher mo-
lecular mobility of ILCPs.
of P4-mꢀBF
4
.
Based on the PLM experiments, we found that all the monomers
M
4-mꢀBF
only for the monomer M4e10ꢀBF
bright birefringence was observed at room temperature due to the
4
(m ¼ 2, 4, 6, 10) did not form the mesophase. Meanwhile,
3.2. Phase transitions and LC ordered structures
4
with more long flexible spacer, a
crystallization. Upon melting, the monomer sample M4e10ꢀBF
4
The thermal stability is an important property for potential
applications and mechanical processing of the ILCPs. Fig. 16S de-
picts the thermal stabilities of ILCPs under nitrogen (N ). The TGA
curves revealed that the thermal stability increased slightly when
increasing the alkyl spacer length. But, more importantly the TGA
flowed freely and the birefringence disappeared. Therefore, we only
focused on the LC structures of the polymers. To evaluate the in-
fluence of the number of carbons in the aliphatic chain, we used
2
PLM to study the LC behavior of P4-mꢀBF
4
(m ¼ 2, 4, 6, 10) with the
films cast from CHCl solutions and slowly dried at ambient tem-
3
data demonstrated that all the ILCPs were stable up to at least
ꢁ
perature. Fig. 4 provides the texture of P ꢀBF (m ¼ 2, 4, 6, 10)
3
20 C (weight loss of 5% or less) indicating that these ILCPs had
4-m
4
ꢁ
under PLM taken at 140 C. Distinct birefringence was observed
when the samples were heated above the Tg and kept at an
isothermal setting before decomposition could take place. During
the cooling process, the birefringence could be maintained, indi-
cating a stable mesophase.
excellent thermal stabilities.
The LC ordered structures of P4-mꢀBF (m ¼ 2, 4, 6, 10) were
4
further investigated using X-ray scattering. Fig. 5 shows the X-ray
diffractions profiles for the series of ILCPs prepared. It was found
ꢀ
1
that there were amorphous halos around 15 nm in the high-angle
region corresponded to the lack of long-range order in the sub-
nanometer length scale. Additionally, there were some distinct
diffractions observed in the low-angle region. As shown in Fig. 5,
ꢀ
1
there was a first-order sharp peak at q
higher-order diffraction peak at q
¼ 4.06 nm for P4e2ꢀBF
ratio of the scattering vectors of the two peaks q : q is approxi-
mately 1:2, which indicated the formation of a lamellar structure
smectic) with a periodicity of 3.08 nm (d10 ¼ 2 /q ) for P4e2ꢀBF
have three diffraction peaks at low-
1
¼ 2.03 nm along with a
ꢀ1
2
4
. The
1
2
(
p
1
4
.
Both P4e4ꢀBF
4
and P4e6ꢀBF
4
angle region and the scattering vector ratio of the three diffrac-
tions was approximately 1:2:3, which was characteristic of smectic
structure, and the d-spacing of (10) for P4e4ꢀBF
4
and P4e6ꢀBF
4
were 3.59 nm and 3.70 nm, respectively. In the cases of P4e10ꢀBF
4
,
,
ꢀ
1
there were also three diffractions peaks, locating at 2.30 nm
ꢀ
1
ꢀ1
3
.98 nm and 4.58 nm , but the ratio of the scattering vectors of
1
/2
1 2 3
the three peaks q : q : q is 1:3 :2, which were attributed to (10),
(
11) and (20) plane, respectively. This result featured an ordered
F
H
Fig. 2. 1H NMR spectra of M4e6ꢀBF
and the corresponding polymer P4e6ꢀBF
4
4
.
structure where the d -spacing value of (10) was 2.73 nm, so the