Novel semirigid water-soluble thermoresponsive polymers based on mesogen-jacketed liquid crystal polymers

Article abstract of DOI:10.1039/b923107b

Two novel semirigid smart polymers based on mesogen-jacketed liquid crystal polymers were successfully synthesized via free radical polymerization, which showed both characteristic liquid crystal properties of mesogen-jacketed liquid crystal polymers and

Full text of DOI:10.1039/b923107b

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Novel semirigid water-soluble thermoresponsive polymers based
on mesogen-jacketed liquid crystal polymersw
Chunhua Wang,a Fangkai Du,a Helou Xie,a Hailiang Zhang,*a Erqiang Chenb and
Qifeng Zhoub
Received (in Cambridge, UK) 9th November 2009, Accepted 13th February 2010
First published as an Advance Article on the web 12th March 2010
DOI: 10.1039/b923107b
Two novel semirigid smart polymers based on mesogen-jacketed
liquid crystal polymers were successfully synthesized via free
radical polymerization, which showed both characteristic liquid
crystal properties of mesogen-jacketed liquid crystal polymers
and remarkably reversible thermoresponsive phase transition
behaviors.
ethyl(hydroxyethyl)cellulose (EHEC),8 have LCSTs at 42 1C,
50 1C and 65 1C, respectively. But it is impossible to control
the molecular weight of these natural polymer derivatives and
copolymerize with other special functional comonomers.
Therefore, it is a challenge to see if we can design and
synthesize a water-soluble thermo-responsive polymer possessed
semirigid main chain. Clearly, the different topological
architectures of polymers will have an immense impact on
their properties. So, developing this new type of thermo-
responsive polymer will be meaningful in both academic
research and practical applications.
Stimuli-responsive polymers, often referred to as ‘‘smart or
intelligent’’ polymer systems, which exhibit reversible property
changes in response to changes in environmental factors such
as temperature or pH, have attracted considerable research
interest due to their promising potential.1 As
a good
It is well-known that mesogen-jacketed liquid crystal poly-
mers (MJLCPs) have a typical semirigid chain structure.13
This is because MJLCPs are constructed by laterally attaching
the bulky side groups to the main chain through a short
linkage, forming a dense ‘‘jacket’’ around each chain back-
bone due to their high population.14 The main chains of
MJLCPs are forced to take the extended conformation by
the surrounding jacket.15 So MJLCPs exhibit semirigid
properties similar to those of main-chain liquid crystal polymers
(MCLCPs). So far, the synthesized mesogen-jacketed liquid
crystal polymers are hardly water-soluble and much less
thermo-responsive. Here we design a novel polymer in the
light of the idea of combining mesogen-jacketed liquid crystal
polymer with thermosensitive polymer. The chemical structure
model is shown in Fig. 1. Like other MJLCPs, this polymer
has a semirigid main chain due to the presence of bulky side
groups. What is more, it has not only hydrophobic groups but
also hydrophilic groups in side chains. Accordingly, it is
reasonable to expect that adjusting the hydrophobic/
hydrophilic balance of the macromolecule might trigger a
thermoresponsive behavior. The novelty of our work lies in
successfully designing and synthesizing new thermoresponsive
polymers having both liquid crystal properties and thermo-
responsive properties, which will enrich their species and
broaden their applications.
candidate, thermoresponsive polymers have been widely
investigated because of various applications such as rheological
control additives, thermal affinity separation,2 controlled drug
release3 and gene therapy.4 In general, this kind of polymer is
soluble in aqueous solution at low temperature but can be
rapidly phase-separated from solution when the temperature is
raised above the lower critical solution temperature (LCST).
One of the most widely studied synthetic thermo-responsive
polymers is poly(N-isopropylacrylamide) (PNIPAM). The
repeat unit of PNIPAM consists of hydrophilic (amide) and
hydrophobic (isopropyl) groups, and the LCST is about 32 1C.5
Besides PNIPAM (a typical example of N-alkyl-substituted
polyacrylamides), there are many non-acrylamide-containing
thermoresponsive polymers, which have also attracted
considerable interest, such as poly[(2-dimethylamino)ethyl
methacrylate],6 poly[N-(2-methacryloyloxyethyl) pyrrolidone],7
poly(vinyl methyl ether),8 poly(N-vinylcaprolactam),9 poly-
(proprylene oxide),10 and poly(2-isopropyl-2-oxazoline),11 etc.
However, the remarkable character of these thermo-responsive
polymers is that their main chains are mostly flexible. Of
course, there is another class of interesting and important
thermo-responsive polymer, polypeptide12 and alkyl-substituted
celluloses, which have semirigid main chains. For example,
hydroxypropylcellulose (HPC),5 methylcellulose (MC),8 and
Two new mesogen-jacketed liquid crystal monomers bis-
(N-hydroxyethyl pyrrolidone) 2-vinylterephthalate (M1) and
bis(N-hydroxypropyl pyrrolidone) 2-vinylterephthalate (M2)
were synthesized via direct esterification reactions between
2-vinylterephthaloyl chloride and N-hydroxyethyl pyrrolidone
and N-hydroxypropyl pyrrolidone, respectively. Corresponding
polymers named P1 and P2 were successfully synthesized via
free radical polymerization. The synthetic route is shown in
Scheme 1.
a Key Laboratory of Polymeric Materials & Application Technology
of Hunan Province, Key Laboratory of Advanced Functional
Polymer Materials of Colleges of Hunan Province, College of
Chemistry, Xiangtan University, Xiangtan, Hunan, 411105, China.
E-mail: hailiangzhang@xtu.edu.cn; Fax: +86-731-58293264;
Tel: +86-731-58293377
b Beijing National Laboratory for Molecular Sciences,
Key Laboratory of Polymer Chemistry and Physics of Ministry of
Education, College of Chemistry and Molecular Engineering,
Peking University, Beijing, 100871, China
GPC, TGA, and DSC results of two polymers are listed in
Table 1. From P1 to P2, the glass transition temperatures (Tg)
w Electronic supplementary information (ESI) available: Experimental
section. See DOI: 10.1039/b923107b
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Fig. 1 The molecular model of novel semirigid thermoresponsive
polymers based on MJLCPs.
Fig. 2 2D WAXD patterns of P2 obtained with the X-ray beam
perpendicular (a) and parallel (b) to the fiber direction.
phenomenon had also been reported in poly[N-(2-hydroxy-
isopropyl)acrylamide], the LCST of which exceeded the
temperatures that could be measured.18 Subsequently, in order
to decrease the cloud point, we designed and synthesized
another new polymer P2 by introducing a more hydrophobic
alkyl group. Just as expected, P2 polymer aqueous solution at
the concentration of 2.0 mg mlÀ1 exhibited a sharp and rapid
phase transition around 65.0 1C. When the concentration of
P2 was 20.0 mg mlÀ1, the LCSTs of P2 were 52.0 1C in H2O
and 47.5 1C in D2O. Therefore, the concentration and the
deuterium isotope have a great effect on the cloud point. The
cloud point decreased with the increasing concentration of the
polymer solution. At the same concentration, the cloud point
in D2O was lower than that in H2O. This cloud point
difference was attributed to the polar difference between
D2O and H2O. Apparently, the more hydrophobic propyl
group decreased the cloud point. To the best of our
knowledge, this is the first report on synthetic thermo-responsive
polymers which have both thermoresponsive properties and
liquid crystal properties. The LCSTs of P1 and P2 in D2O were
measured by visual method. Fig. 3 illustrates the appearance
changes of 20.0 mg mlÀ1 P1 and P2 in D2O during phase
separation. The LCST of P2 in H2O was obtained by Laser
Light Scattering. Fig. 4f shows the temperature dependence of
light scattering intensity of 2.0 mg mlÀ1 P2 polymer aqueous
solution during a heating and cooling cycle. Clearly, polymer
Scheme 1 Synthetic route of the monomers and polymers.
decreased with the increasing number of flexible alkyl groups.
Birefringences of these two polymers were observed by POM
and the images are provided in the ESI.w Similar to previous
MJLCPs, the liquid crystal textures did not appear until the
temperature reached 180 1C and still retained before the
decomposition temperature. Furthermore, the textures could
be maintained during the cooling process. To study their
supermolecular structures, we performed the wide angle
X-ray diffraction. Firstly, we used the one-dimensional wide
angle X-ray diffraction (1D WAXD) under variational
temperatures to investigate the structural evolution, as shown
in the ESI.w When the temperature reached 170 1C, the
samples exhibited sharp and intense diffractions at low 2y
angles, indicating the existence of ordered structures.
And then, two-dimensional wide angle X-ray diffraction
(2D WAXD) experiments on the oriented samples were
carried out by aligning the X-ray incident beam along different
directions. Fig. 2 shows 2D WAXD patterns of P2 obtained
with the X-ray beam perpendicular (a) and parallel (b) to the
fiber direction. 2D WAXD patterns of P1 are shown in the
ESI.w Like other MJLCPs, P1 exhibited a columnar nematic
(FN) phase and P2 exhibited a hexatic columnar nematic
(FHN) phase.16 In addition, we took P2 as a typical example
and confirmed the polymers we synthesized were semirigid by
solid-state NMR experiments.17 Details are provided in the
ESI.w
P2 exhibited
a remarkably reversible thermoresponsive
phase transition behavior in aqueous solution. Moreover, no
hysteresis phenomenon was observed in P2 aqueous solution
during a heating and cooling cycle. Fig. S10w in the ESI shows
a light scattering intensity of 20.0 mg mlÀ1 in P2 polymer
aqueous solution as a function of solution temperature on
heating.
In conclusion, we have successfully designed and
synthesized a new family of semirigid smart polymers based
on mesogen-jacketed liquid crystal polymers, which show both
liquid crystal properties and thermoresponsive behaviors. This
new thermosensitive polymeric material undoubtedly provides
new opportunities in the field of thermoresponsive systems.
Further studies on these new smart polymers are in progress,
such as determining the thermoresponsive mechanism,
solution properties and so on.
Interestingly, when P1 was dissolved in pure deionized
water, it was always transparent even when the temperature
rose to 100 1C, indicating that P1 was completely water-
soluble and non-thermoresponsive in aqueous media;
however, when P1 was dissolved in D2O, it was turbid at
88.0 1C, meaning the occurrence of phase separation. Accordingly,
we thought the lower critical solution temperature (LCST)
of P1 in water was too high to measure at 1 atm. This
This research was financially supported by the National
Nature Science Foundation of China (20874082), the Scientific
Research Fund of Hunan Provincial Education Department
(06A068), the Key Project of Chinese Ministry of Education
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Table 1 Molecular characteristics and properties of two polymers
Sample
Mn(Â104)a
PDIa
Tg/1Cb
Td/1Cc
LCST1/1Cd
LCST2/1Ce
LCST3/1Cf
Liquid crystallinityg
P1
P2
8.40
7.75
2.26
2.63
53.8
23.8
388.3
386.9
—
65.0
—
52.0
88.0
47.5
Yes
Yes
a
b
The Mn and PDI of polymers were measured by GPC using PS standards and DMF as solvent. The glass transition temperatures were
obtained by DSC at a heating rate of 20 1C minÀ1 under N2 during the second heating process. The decomposition temperatures at 5% weight
c
loss of the samples measured by TGA heating experiments at a rate of 20 1C minÀ1 under N2. The polymers were dissolved in the highly pure
d
deionized water and then the aqueous solutions of polymers (2.0 mg mlÀ1) were measured by Laser Light Scattering. The aqueous solutions of
e
polymers (20.0 mg mlÀ1) were measured by Laser Light Scattering. The polymers P1 and P2 were dissolved in D2O, respectively, and then the
f
polymer solutions (20.0 mg mlÀ1) were measured by visual method. Liquid crystallinity was observed by polarized optical microscopy.
g
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Chem. Commun., 2010, 46, 3155–3157 | 3157