X. Liu et al. / Polymer 87 (2016) 260e267
265
microphase-segregated model in the supramolecular column of Col
temperatures.
phase can be inferred, where the polyphenylacetylene main-chain
locates in the core of the column and the alkyl chains are disordered
outside [16,29]. The diameter of the main-chain can be estimated to
be 1.08 nm in the case of n ¼ 0.
From the view of chemical structure, Pn, PDAVT and P-OCm have
a similarity, i.e., the Ire phase appears only when the length of alkyl
tails achieves a critical value. As mentioned before, the long alkyl
chains bound to the semi-rigid main-chains behave more likely as
“solvent”. Different volume fractions of the “solvent” determine the
distinct phase behaviors of Pns with short and long alkyl chains.
According to Onsager's theory on the isotropic to nematic phase
transition [35], the volume fraction of the rigid component should
be high enough to form ordered LC phase. For Pns with short alkyl
tails, the volume fractions of the “solvent” are relatively small, more
favorable for the Col phase. While the volume fraction of alkyl tails
Second, d-spacing of the Lam phase of Pn (n ꢁ 8), which are of
2
.00, 2.60, 3.27, and 3.71 nm, increases with increased n. Linear
fitness (Fig. 5a) shows that the increment of layer thickness is of
2.7 Å for each increment of n, comparable of twice of the theoretic
~
value of 1.27 Å for one methylene unit [30,31]. It means that Lam
phase may take a double layer structure, where the stretched side
chains are parallel to each other without interdigitation.
(“solvent”) is increased, the main-chains become “diluted” in the
3
.2. Multiple effects of alkyl tails on phase behavior of Pn
system. We consider that the “dilution” effect shall be tightly
related to the formation of Ire phase in Pns (n ꢁ 8).
It is interesting to find that, compared to the prototype polymer
As is well known, long alkyl tails tend to take stretched con-
formations to form ordered packing with each other, which usually
occurs at temperatures close to the crystallization temperatures of
alkyl groups. Due to this crystallization tendency, Lam phase for-
mation can be expected for Pn with long alkyl tails at low tem-
peratures, which is an enthalpy driven process. As determined by
XRD results of the Lam phase, the stretched side-chains form a
double layer structure, similar to that reported for hairy-rod poly-
mers with high graft density of side-chains [36]. According to DSC
results of Pns of n ꢁ 8, the transition temperatures for Lam phases
of polyphenylacetylene, Pns with alkyl tails attached to 2,5-
position of the phenyl group have thoroughly changed phase be-
haviors. Considering the di(alkyl) terephthalate moiety as a whole,
the bulky pendant groups can impose the “jacketing effect” on the
polyacetylene backbone. As a result, the cis-rich backbone of Pn
adopts a stretched cis-transoidal conformation [32]. For such a
semi-rigid main-chain, the alkyl tails can be regarded as bound
solvent, similar to that in hairy-rod polymers [33,34]. The cova-
lently bonded alkyl tails not only act as plasticizers to greatly
ꢂ
reduce the T
g
, but also contribute to the conformational degrees of
are 10e15 C higher than the crystallization temperatures of al-
freedom to improve the mobility of the semi-rigid macromolecules.
We propose that the packing behavior and motion of side chains,
which vary with the tail length and the temperature, play a sig-
nificant role on the phase evolution of Pns.
n
kanes (C H2nþ2) bearing the same numbers of carbon atoms [19].
This may be ascribed to the microphase separation between main-
and side-chains of Pns, which facilitates the side-chain ordering.
The alkyl chain length determines not only the transition temper-
ature of the Lam phase, but also the ordered degree of this phase.
For example, the onset temperature of the Lam phase for P14 is the
With the pronounced “jacketing effect” of side chains, all the
Pns studied can form Col phases, which take the supramolecular
columns as building blocks. It is worth to note that at higher
temperatures the Col phase is better developed. As shown in Fig. 4,
the diffraction intensity of P6 increases with increased tempera-
ture. This outcome suggests that the electron density contrast be-
tween the core (main-chain) and the shell (alkyl tails) in the
microphase-segregated supramolecular column is improved. For
the samples with n ꢁ 8, the Col phase can only be observed once the
temperature becomes sufficiently high. The enhanced microphase
separation within the supramolecular column and/or the appear-
ance of Col phase at high temperatures clearly indicates that the Col
phase formation of Pn relies on the mobility of alkyl tails. As the
liquid-like alkyl tails bonded to the semi-rigid main-chain seek
more conformations in space and maximize their conformational
entropy with increased temperature, the Col phase of Pn becomes
an efficient packing scheme. In other words, the stronger “jacketing
effect” of side-chains on main-chains at higher temperatures,
which can result in the Col phase, originates from entropy gain of
the alkyl tails.
ꢂ
highest among the samples (~30 C) and the diffraction peak be-
ꢂ
comes well-defined at 20 C upon cooling (Fig. S6).
Based on the above arguments, we consider that the “jacketing
effect” and the crystallization tendency of the side-chains, which are
entropic and enthalpic in nature, respectively, dominate the phase
structures at high and low temperatures. The Ire phase at interme-
diate temperatures should be a result of the balance between the
conformational entropy of side-chains and the main-chain/side-
chain interaction. For Pn of n ꢁ 8, the temperature window of the
I
re phase can be quite narrow, especially for P12 and P14 during the
ꢂ
heating process. As shown in P12 upon heating from 20 to 40
C
(Fig. 6c), there is coexistence of two phases at specific temperatures:
ꢂ
Lam-Ire at 20e35 C, and Ire-Col at higher temperatures, suggesting a
competitive transformation among the three phases.
3.3. Componential dynamics determined by solid state NMR
In the above discussion, we have mentioned that the confor-
mational entropy gain of the side-chains is responsible for the
formation of the Col phase. In the following, we employ in-situ
variable-temperature solid state NMR techniques to study the dy-
namics of the heterogeneous components (main-chain and side-
Interestingly, the Col phase of Pns with n ꢀ 6 is stable during the
ꢂ
entire temperature range (ꢃ50e80 C), while for PDAVT the Col
phase of n ¼ 6 disappeared at room temperature [17]. This differ-
ence is related to the rigidity of backbones. For the semi-rigid
polyacetylene main-chain of Pns with the cis-transoidal confor-
mation, its conformation takes a little change during the cooling
process. As the movement of short alkyl chains slows down below
1
13
chain) in different phases. 2D He C wideline separation (WISE)
NMR experiment is a powerful tool for the determination of het-
erogeneous dynamics in solid polymers, which is useful for the
characterization of polymers including both hard and soft compo-
g
T , the Col phase becomes frozen in the glassy state and the
13
diffraction intensity reduced during cooling (Fig. 4). One can
anticipate that the Col phase should collapse eventually when the
glass transition occurred at a sufficiently low temperature. Such a
transition is observed in Pns with longer alkyl chains (n ꢁ 8) and
other MJLCPs, such as PDAVT (n ꢁ 6) [17] and P-OCm (m ¼ 4e18, m
is the number of carbon atoms of the alkoxy group) [19]. Indeed, the
nents [37,38]. The observed C chemical shifts indicate segmental
compositions of the components. On the other hand, it is well-
1
known that the line width of the H line reflects the nature of the
dipolar interaction between the protons and thus can be exploited
to monitor the dynamic behavior of polymer chains. Therefore,
1
different componential dynamics can be distinguished by the
wide-line spectrum, broad if rigid and narrow if mobile [39].
H
g
T s of these polymers are much lower than the Col-Ire transition