Unusual effect of molecular weight and concentration on thermoresponsive behaviors of well-defined water-soluble semirigid polymers

Article abstract of DOI:10.1002/pola.26158

A series of water-soluble semirigid thermoresponsive polymers with well-defined molecular weights based on mesogen-jacketed liquid crystal polymers (MJLCPs), poly[bis(N-hydroxyisopropyl pyrrolidone) 2-vinylterephthalate] (PHIPPVTA) have been synthesized via reversible addition fragmentation chain transfer (RAFT) polymerization. Dynamic light scattering (DLS) revealed that the novel monomer and polymers have thermoresponsive properties with cloud point in the range between 10 and 90 °C. The cloud point was increased by 56.2 °C when the polymer molecular weight increased from 0.47 × 10 ⁴ g mol^-1 to 3.69 × 10⁴ g mol ^-1. In addition, the cloud point of PHIPPVTA was decreased by 18.8 °C with the increase of polymer concentration from 5 to 10 mg mL ^-1. A slight increase (0.1-3.5 °C) of cloud point has been observed after knocking off the end-groups of PHIPPVTA. Moreover, the cloud point of polymer increased with increasing of its molecular weight with or without the trithiocarbonate end-groups, which showed the opposite trend comparing with other thermoresponsive polymers with flexible backbones. These polymers show a dramatic solvent isotopic effect that the cloud point in D ₂O was lower than in H₂O. Copyright

Full text of DOI:10.1002/pola.26158

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Unusual Effect of Molecular Weight and Concentration on
Thermoresponsive Behaviors of Well-Defined Water-Soluble
Semirigid Polymers
Peng Liu, Helou Xie, Haoyu Tang, Guanqun Zhong, Hailiang Zhang
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
Correspondence to: H. Zhang (E-mail: hailiangzhang@xtu.edu.cn)
Received 3 April 2012; accepted 3 May 2012; published online
DOI: 10.1002/pola.26158
ABSTRACT: A series of water-soluble semirigid thermoresponsive
polymers with well-defined molecular weights based on meso-
gen-jacketed liquid crystal polymers (MJLCPs), poly[bis(N-
hydroxyisopropyl pyrrolidone) 2-vinylterephthalate] (PHIPPVTA)
have been synthesized via reversible addition fragmentation
chain transfer (RAFT) polymerization. Dynamic light scattering
(DLS) revealed that the novel monomer and polymers have ther-
moresponsive properties with cloud point in the range between
10 and 90 ꢀC. The cloud point was increased by 56.2 ꢀC when the
polymer molecular weight increased from 0.47 ꢁ 104 g molꢂ1 to
3.69 ꢁ 104 g molꢂ1. In addition, the cloud point of PHIPPVTA was
decreased by 18.8 ꢀC with the increase of polymer concentration
from 5 to 10 mg mLꢂ1. A slight increase (0.1–3.5 ꢀC) of cloud point
has been observed after knocking off the end-groups of
PHIPPVTA. Moreover, the cloud point of polymer increased with
increasing of its molecular weight with or without the trithiocar-
bonate end-groups, which showed the opposite trend comparing
with other thermoresponsive polymers with flexible backbones.
These polymers show a dramatic solvent isotopic effect that the
C
V
cloud point in D2O was lower than in H2O. 2012 Wiley Periodicals,
Inc. J Polym Sci Part A: Polym Chem 000: 000–000, 2012
KEYWORDS: cloud point; dynamic light scattering; liquid-crystal-
line polymers (LCP); mesogen-jacketed liquid crystal polymers
(MJLCPs); reversible addition fragmentation chain transfer
(RAFT); semirigid polymers; thermoresponsive polymers
INTRODUCTION Stimuli-responsive materials, also known as
smart materials, undergo conformational transitions induced
by environmental changes, such as temperature,1–3 salts,4–6
pH,2,7,8 or light.9–12 Thermoresponsive polymers with lower
critical solution temperature (LCST) have gained increasing
attention for their potential application in controlled drug
delivery systems,13–15 tissue engineering,16,17 biotechnol-
ogy,18,19 nanotechnology,20,21 sensors and separation proc-
esses,22,23 and so on. Poly(N-isopropylacrylamide) (PNIPAm)
is one of the most popular temperature-responsive poly-
mers.24–26 Their phase transition in aqueous solutions has
been explained according to the theory of coil-to-globule
transition.27–29
to tune their properties by changing their molecular weights,
which limited their applications. Thus, the development of a
new type of semirigid thermoresponsive polymers will be of
great value for both academic research and practical
applications.
Recently, our group has developed a novel semirigid water-
soluble thermoresponsive polymers based on mesogen-jack-
eted liquid crystal polymers (MJLCPs).39 These polymers
exhibited both liquid crystalline properties and thermores-
ponsive properties. However, these polymers were synthe-
sized by conventional radical polymerizations which resulted
in high polydispersities. It is well known that the LCST of
thermoresponsive polymer is related to polymer molecular
weight and distribution.24,26,30,40,41 The LCST decreased with
the increasing of polymer molecular weight.26,42–45 In addi-
tion, polymer with high polydispersity has a broad LCST
transition. Thus, polymers with well-defined molecular
weight and narrow distribution are essential for precisely
evaluating their thermoresponsive behaviors.12,35,36 This con-
cept has been demonstrated in recent reports on the prepa-
ration of the polymers by living polymerization (e.g.,
While a number of thermoresponsive polymers with flexible
backbones10,30–35 have been synthesized and studied, they
are not able to satisfy the demand of material science. Ther-
moresponsive polymers with semirigid backbones are
another type of attractive smart polymers that have been
less investigated. For example, polypeptide and alkyl-substi-
tuted celluloses36–38 exhibit thermoresponsive phase transi-
tion behaviors in aqueous solution. However, it is impossible
Additional Supporting Information may be found in the online version of this article.
C
V
2012 Wiley Periodicals, Inc.
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reversible addition fragmentation chain transfer [RAFT])46–51 MIXED-D columns (7.5 ꢁ 300 mm, effective molecular weight
Furthermore, the terminal groups of thermoresponsive range of 0.2–400.0 kg molꢂ1) using DMF that contained 0.ꢂ011
polymer also play an important role in their LCST.45,46,52 For M LiBr as the eluent at 80 C at a flow rate of 1.0 mL min
.
ꢀ
example, Jiang et al.52 demonstrated that theꢀhydrophobic Narrowly distributed polystyrene standards in the molecular
end groups decreased the cloud point by 1–20 C depending weight range of 0.5–7.5 ꢁ 107 g molꢂ1 (PSS, Mainz, Germany)
on the molecular weight and the largest depression was were used for calibration.
observed at the lowest molar mass. Xia et al.45 observed that
the LCSTs can be tuned from 32 to 70 ꢀC by adjusting
DSC
DSC examination was carried out on a TA DSC Q100 calorime-
ter with a programmed heating procedure in nitrogen. The sam-
polymer end groups and molecular weights.
In this article, we synthesized a series of water-soluble semi-
rigid rod polymers poly[bis(N-hydroxyisopropyl pyrrolidone)
2-vinylterephthalate] (PHIPPVTA) with well-defined molecu-
lar weights by RAFT. The effects of polymer molecular
weight, concentration and terminal groups on their thermor-
esponsive behaviors were carefully investigated by dynamic
light scattering (DLS). The solvent isotopic effect on this tem-
perature-induced phase transition and dehydration were
ple size was about 5 mg and encapsulated in hermetically
sealed aluminum pans, whose weights were kept constant. The
temperature and heat flow were calibrated using standard
materials (indium and zinc) at the cooling rate (10 ꢀC minꢂ1
)
and the heating rate (20 C minꢂ1). The temperature and heat
ꢀ
flow scale at different cooling and heating rates were calibrated,
using standard materials such as indium and benzoic acid.
1
PLM
tested by DLS and H NMR at various temperatures. In addi-
LC texture of the polymers was observed under PLM (Leica
tion, the liquid-crystalline (LC) behavior of PHIPPVTA has
been studied using a combination of characterization techni-
ques, such as polarized light microscope (PLM), differential
scanning calorimetry (DSC), and one-dimensional wide-angle
X-ray diffraction (1D WAXD).
DM-LM-P) coupled with
a
Mettler-Toledo hot stage
(FP82HT). The films with thickness of ꢃ10 lm were casted
from CH2Cl2 solution and slowly dried at room temperature.
1D WAXD
The supermolecular structures of PHIPPVTA polymers were
examined by 1D WAXD thermal experiments. 1D WAXD pow-
der experiments were performed on a Philips X0 Pert Pro dif-
fractometer with a 3 kW ceramic tube as the X-ray source
(Cu KR) and an X0 celerator detector. The reflection peak
positions were calibrated with silicon powder (2y > 15ꢀ)
and silver behenate (2y < 10ꢀ). The sample stage was set
horizontally, and a temperature control unit (Paar Physica
TCU 100) in conjunction with the diffractometer was used to
study the structure evolutions as a function of temperature.
The heating and cooling rates in the 1D WAXD experiments
were 5 ꢀC minꢂ1. The sample temperature was controlled
within 61 ꢀC. The background scattering was recorded and
subtracted from the sample patterns.
EXPERIMENTAL
Materials
N,N-dimethylformamide (DMF) and trichloromethane were
purchased from Shanghai Chemical Reagents (Shanghai,
China), A.R. grade and purified by drying over anhydrous
magnesium sulfate and distilled. Acetone (Beijing Chemical,
China) was refluxed over potassium permanganate and dis-
tilled before use. 4-Dimethylaminopyridine (DMAP) was pur-
chased from Haili Chemical Reagents (ChangSha, China) and
purified by recrystallization twice from benzene. 1-Amino-2-
propanol and c-butyrolactone were purchased from Shanghai
Chemical Reagents (Shanghai, China), A.R. grade. N, N0-Azobi-
sis-obutyronitrile (AIBN) was purchased from Shanghai
Chemical Reagents (Shanghai, China) and purified by recrys-
tallization twice from ethanol. Tri-n-butyltin hydride (97%)
was purchased from Aladdin. Deionized (DI) water was
obtained from a reagent water system (Aquapro) with a spe-
cific resistivity of 18.25 MX cmꢂ1. All the other chemicals
were used as received. 2-(Ethylsulfanylthiocarbonylsulfonyl)-
2-methylpropionic acid (EMP), 2-Vinyl p-biphthalic acids
(VTA) and 2-vinylterephthaloyl chloride were synthesized
according to reported methods.39,53–55
DLS
DLS measurements were conducted on a commercial spec-
trometer (BI-200 SM) equipped with a multitau digital time
correlator (BI-9000) and an adjustable solid-state laser emit-
ting (output power of ꢃ40 mW at a wavelength of k ¼ 532
nm) as the light source. A temperature controller (BI-TCD)
was used for precisely adjusting the solution temperature.
The solutions were heated in steps and stabilized for 10 min
before the data were recorded. All the DLS measurements
were done at an angle of 90ꢀ, The CONTIN Laplace inversion
algorithm in the correlator was used, and the intensities of
light scattering were recorded.
Instrumentation
1H NMR
1H NMR spectra were recorded on a BRUKER ARX400 MHz
spectrometer with tetramethylsilane (TMS) as the internal
standard at room temperature in CDCl3. Chemical shifts (d)
were given in ppm relevant to TMS.
Synthesis of N-Hydroxyisopropyl Pyrrolidone
1-Amino-2-Propanol (37.5 g, 0.50 mol) and c-butyrolactone
(43.0 g, 0.50 mol) were charged in a 300 mL reaction cal-
ꢀ
Gel Permeation Chromatography
dron. The solution was heated at 250 C for 8 h. The mixture
Gel permeation chromatography (GPC) measurements were was extracted using subsequently distilled water. The water
performed on a Polymer Laboratories (PL)-GPC120 setup layer was concentrated by rotary evaporation and dried with
equipped with a column set consisting of two PL gel 5 lm anhydrous magnesium sulfate. The crude product was
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General Procedure for the RAFT Polymerization
of HIPPVTA
ꢀ
Polymerizations were conducted at 80 C, using AIBN as the
primary radical source and 2-ethylsulfanylthiocarbonylsul-
fonyl-2-methylpropionic acid (EMP) as the RAFT chain trans-
fer agent (CTA). Polymerizations were performed with an ini-
tial monomer concentration ([HIPPVTA]0) of 1.0
M in
individual. The synthetic route is shown in Scheme 2. A typi-
cal polymerization procedure is as follows: HIPPVTA (0.8 g,
1.81 ꢁ 10ꢂ3 mol), EMP (2.03 ꢁ 10ꢂ2 g, 9.07 ꢁ 10ꢂ5 mol),
AIBN (1.19 ꢁ 10ꢂ3 g, 7.23 ꢁ 10ꢂ6 mol) were added to a
reaction tube, followed by the addition of 1.81 mL of DMF.
After three freeze-pump-thaw cycles, the tube was sealed
SCHEME 1 Synthetic route of HIPP and HIPPVTA.
subsequently purified by flash column chromatography using
acetone as eluent. The resulting purified product was color-
less and the yield was 68.3% (Weight: 50.2 g). The synthetic
route is shown in Scheme 1. 1H NMR(CDCl3) of synthesis of
N-hydroxyisopropyl pyrrolidone (HIPP), d (ppm): 4.0–4.03
(m, 1H, ACHA), 3.47–3.51 (t, 2H, ACH2A), 3.20–3.32 (d, 2H,
ACH2A), 2.44 (m, 1H, AOH), 2.40–2.44 (m, 2H, ACH2A),
2.04–2.08 (t, 2H, ACH2A).
ꢀ
under vacuum. The polymerization was carried out at 80 C
for 4.5 h and stopped by exposing to air. The polymerization
solution was diluted with dichloromethane, followed by
slowly precipitating from large amount of ether. The result-
ing polymer was further purified by redissolving in dichloro-
methane and precipitated from ether three times. A straw
yellow solid product was obtained after drying under vac-
ꢀ
uum at 50 C. (0.72 g, yield ¼ 87.8%). These polymers were
Synthesis of Bis(N-hydroxyisopropyl pyrrolidone) 2-
Vinylterephthalate [HIPPVTA]
named as P1–P8 (Table 1).
End Group Removal34–37
VTA (2.0 g, 0.01 mol) and thionyl chloride (40 mL) were
charged in a 250 mL dried round-bottom flask. The flask
was immersed in a thermostatic oil bath at 50 ꢀC for 4.5 h.
Afterward, The reaction solution was concentrated by rotary
evaporation and was then dripped into petroleum ether and
the mixture was further concentrated by rotary evaporation
and repeated three times. 2-vinylterephthaloyl chloride was
successfully synthesized. DMAP (1.35 g, 0.011 mol), triethyl-
amine (TEA) (2.02 g, 0.02 mol), HIPP (3.14 g, 0.022 mol)
and tetrahydrofuran (THF) (50.0 mL) were added into a 250
mL dried round-bottom flask. Tꢀhe flask was immersed in a
thermostatic ice-salt bath at ꢂ5 C, 2-vinylterephthaloyl chlo-
ride was dissolved in THF (100.0 mL). The solution was
added dropwise in this flask for 5.5 h, and then the reaction
mixture was stirred overnight under nitrogen. The solution
was extracted using subsequently distilled water. The organic
layer was dried with anhydrous magnesium sulfate and the
solvent was removed. The product was further purified by
column chromatography (basic alumina, eluent 5% petro-
leum ether in acetone) to get the targeted substance. Weight:
3.9 g; yield: 89.2%. The synthetic route is shown in Scheme
1. 1H NMR (CDCl3), d (ppm): 8.23 (s, 1H, ArAH), 7.88–7.95
(m, 2H, ArAH), 7.37–7.42 (m, 1H, ¼¼CHA), 5.77–5.81 (d, 1H,
¼¼CH2), 5.43–5.46 (d, 1H, ¼¼CH2),5.37–5.39 (m, 2H, ACHA),
3.76–3.81 (m, 2H, ACH2A), 3.55–3.57 (m, 2H, ACH2A), 3.35–
3.41 (m, 4H, ACH2A), 2.33–2.37 (m, 4H, ACH2A), 1.98–2.02
(m, 4H, ACH2A), 1.37–1.39 (m, 6H, ACH3). 13C NMR (CDCl3), d
(ppm): 175.18 (2C, O¼¼CACH2), 166.53–165.32 (2C, O¼¼CAAr),
139.54 (1C, ArAC), 134.75 (1C, CH2¼¼CHAAr), 133.20–132.43
(2C, ArAC), 130.38 (2C, ArAC), 128.06 (1C, ArAC), 117.70 (1C,
CH2¼¼CHA), 70.15–69.38 (2C, OACH(CH3)CH2A), 47.93 (2C,
CHACH2AN), 47.19 (2C, NACH2ACH2A), 30.49 (2C,
ACH2ACH2ACO), 18.99 (2C, ACHACH3), 17.81 (2C,
ACH2ACH2ACH2). Mass Spectrometry (MS) (m/z) [M þ K]þ
Calcd for C24H30N2O6K, 481.610.; found, 481.562.
A typical procedure is as follows: P1 (0.35 g, Mn ¼ 0.47 ꢁ
104 g molꢂ1, Mw/Mn ¼ 1.35, yellow solid), tributylstannane
(4.58 ꢁ 10ꢂ2 g, 1.58 ꢁ 10ꢂ4 mol) and AIBN (1.30 ꢁ 10ꢂ2 g,
6.15 ꢁ 10ꢂ5 mol), and DMF (1 mL) were added to a reaction
tube. The tube was degassed three freeze-thaw cycles and
well sealed under vacuum. The polymerization was per-
formed at 75 ꢀC for 4 h. The reaction solution was diluted
with dichloromethane and the polymer was precipitated
from ether. The polymer precipitate was further purified by
repeatedly dissolving in dichloromethane and precipitated
from ether (three times). A white solid product was obtained
after drying under vacuum at 50 ꢀC. (0.3 g, yield ¼ 85.7%).
The synthetic route is shown in Scheme 3. These polymers
were named P1-H–P8-H (Table 1).
Determination of Phase Transitions in Aqueous
Solutions
PHIPPVTA was fully dissolved in water at solution tempera-
ture below cloud point. The solutions were heated (0.5 ꢀC
minꢂ1) in steps and stabilized for 10 min before recording
the data. All the DLS measurements were done at an angle
of 90ꢀ, The CONTIN Laplace inversion algorithm in the
SCHEME 2 Synthetic route of polymer (P1–P8).
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TABLE 1 Molecular Characteristics and Properties of
PHIPPVTAs
Mn
(ꢁ104)a
Tg
(ꢀC)b
Td
(ꢀC)c
Cloud
Point (ꢀC)d
Sample
PDIa
LCe
P1
0.47
1.18
1.71
2.12
2.58
3.07
3.69
4.42
0.43
1.12
1.68
2.06
2.52
3.01
3.59
4.36
1.35
1.36
1.37
1.38
1.39
1.38
1.37
1.42
1.34
1.35
1.35
1.36
1.37
1.36
1.38
1.40
61.5
82.4
82.8
85.8
86.2
88.0
89.7
90.5
62.9
82.6
86.0
85.6
86.5
86.8
88.7
90.8
326.5
327.8
332.5
334.8
338.8
339.4
340.6
340.3
328.9
328.2
331.5
334.3
338.5
339.6
339.9
340.5
24.0
57.0
62.7
69.9
76.0
79.2
80.2
80.9
27.5
60.0
64.5
71.2
76.8
79.6
80.4
81.0
No
P2
No
P3
No
P4
Yes
Yes
Yes
Yes
Yes
No
P5
P6
P7
P8
P1-H
P2-H
P3-H
P4-H
P5-H
P6-H
P7-H
P8-H
a
No
No
FIGURE 1 1H NMR spectra of HIPPVTA and PHIPPVTA in
Yes
Yes
Yes
Yes
Yes
CDCl3.
formation) and MS (Fig. S2, Supporting Information)
spectrometry.
It is interesting to observe that HIPPVTA exhibited a ther-
moresponsive behavior in aqueous solution, which was
investigated by DLS (Fig. 2). HIPPVTA (5 mg mLꢂ1) wꢀas fully
dissolved in water at solution temperature below 20 C. The
light scatteringꢀ intensity dramatically increased when heat-
ing up to 26.5 C, suggesting the occurrence of phase transi-
tion. This critical temperature is defined as cloud point. In
addition, the cloud point of aqueous HIPPVTA solution was
The Mn and PDI of polymers were measured by GPC, using PS stand-
ards and DMF as the eluent.
b
The glass transꢂit1ion temperatures were obtained by DSC at a heating
rate of 20 ꢀC min under N2 during the second heating process.
c
The decomposition temperaturesꢂa1t 5% weight loss were measured by
TGA at a heating rate of 20 ꢀC min under N2.
d
The polymers were dissolved in the highly pure DI water and then the
aqueous solutions of polymers (10.0 mg mLꢂ1) were measured by DLS.
e
Liquid crystalline phase was observed by PLM.
correlator was used, the intensities of light scattered were
recorded.
RESULTS AND DISCUAAION
Synthesis and Characterization of HIPPVTA
The novel monomer (HIPPVTA) was synthesized via an
esterification reaction between 2-vinylterephthaloyl chloride
and N-hydroxypropyl pyrrolidone with a high yield (yield ¼
89.2%). The molecular structure of HIPPVTA has been veri-
1
fied by H NMR [Fig. 1(a)], 13C NMR (Fig. S1, Supporting In-
SCHEME 3 Synthetic route of polymer end group removal (P1-
FIGURE 2 Phase transitions of HIPPVTA solution were meas-
ured using DLS. (-l-) 10 mg/mL, (-l-) 5 mg/mL.
H–P8-H).
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bulk. The LC behaviors of PHIPPVTA with molecular weight
ranging from 0.47 ꢁ 104 g molꢂ1 to 4.42 ꢁ 104 g molꢂ1
were studied using PLM (Fig. 4) and 1D WAXD (Table 1 and
Fig. 5). The liquid crystal textures of PHIPPVTA were
observed when the polymer molecular weight is higher than
2.06 ꢁ 104 g molꢂ1 and the temperature is above 120 ꢀC,
which was similar as other MJLCPs.39,56,57 In addition, the
birefringence remained before the decomposition tempera-
ture and the textures maintained during the cooling process.
It is believed that a critical aspect ratio of the cylindrical
MJLCPs must be required to stabilize their LC phases.57 The
structural evolution of PHIPPVTA (P6) during the first heat-
ing and cooling was investigated by one-dimensional wide
angle X-ray diffraction (1D WAXD; Fig. 5). When the temper-
ature reached 130 ꢀC, a sharp and intense diffraction peak
appeared at 5.31ꢀ (d ¼ 1.66 nm) [Fig. 5(a)], indicating a co-
lumnar nematic phase of P6.32,42,43 During the cooling pro-
cess, the diffraction peak retained to room temperature [Fig.
5(b)], indicating that the ordered structure of P6 has been
maintained, which is consistent with the PLM results. In
FIGURE 3 GPC results of PHIPPVTAs (P1–P8) with variety of
molecular weights.
10 ꢀC at concentration of 10 mg mLꢂ1. It decreased with the
increasing of concentration, and the phase transition became
sharp at higher concentration. As the monomers assemble more
easily at the higher concentration upon heating, which result in
a lower cloud point and a faster rate of the phase transition.52
Synthesis and Bulk Property of PHIPPVTA
A series of well-defined polymer PHIPPVTA with various mo-
lecular weights (0.47 ꢁ 104 to 4.42 ꢁ 104 g molꢂ1) and nar-
row polydispersities (1.35–1.42) have been synthesized by
ꢀ
RAFT at 80 C in DMF using AIBN as the initiators and EMP
as the CTA. The polymerization results were summarized in
Table 1 and Figure 3. The molecular structures of PHIPPVTA
1
have been verified by H NMR [Fig. 1(b)] and Fourier Trans-
form infrared spectroscopy (FTIR) (Fig. S3, Supporting Infor-
mation). FTIR analysis showed the characteristic absorption
of ester-carbonyl and pyrrolidone-carbonyl at 1725 cmꢂ1
and 1683 cmꢂ1, respectively. The chemical shifts at d ¼ 5.79
and 5.43 ppm (CH2¼CHAr) of HIPPVTA monomer disap-
peared, indicating the formation of polymer backbone and
the high purity of the resulting polymer.
Thermogravimetric Analysis (TGA) and DSC were used to
measure their thermal properties in bulk. All the resulting
ꢀ
polymers have a high thermostability (Td > 320 C) and one
glass transition temperature in the range of 61.5–92.5 ꢀC
depending on their molecular weights (Table 1).
PHIPPVTA is a type of MJLCPs. We reasoned that they would
show similar liquid crystalline behaviors as other MJLCPs in
FIGURE 4 Representative PLM images of the textures of P6 (a)
and P6-H (b) at 120 ꢀC.
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FIGURE 5 Sets of 1D WAXD powder patterns of P6 (a) obtained during the first heating of the as-cast films. The corresponding
first cooling WAXD powder patterns are shown in (b).
addition, PLM and DSC were used to investigate the thermal
and LC behaviors of PHIPPVTA without trithiocarbonate end
groups (P1-H–P8-H), which showed similar results as their
precursors (P1–P8). The samples with molecular weight
above 2.06 ꢁ 104 g molꢂ1 are able to form a LC phase at
properties of the polymers will increase with the increasing
of molecular weight due to congestion effect, which resulted
in an increase of cloud point. The cloud points barely
increased at molecular weights range, which is probably due
to dilution effect and pyrrolidone-shell enrichment of
ꢀ
elevated termperatures (>120 C).
Solution Property of PHIPPVTA with Different
Molecular Weights
The cloud points of PHIPPVTA (0.47 ꢁ 104 g molꢂ1 to 4.42
ꢁ 104 g molꢂ1) in aqueous solution were studied using DLS
(Fig. 6). PHIPPVTAs (10.0 mg mLꢂ1) are highly soluble in
water at the temperature below cloud point and start to ag-
gregate (became opaque) at cloud point, which leads to a
significant transition in the light scattering intensity. We also
observed that the PHIPPVTA solutions became clear when
the temperature decreased below cloud point, indicating a
reversible thermoresponsive behavior (Fig. 7). The cloud
point exhibited remarkable increase (by 56.2 ꢀC) when the
polymer molecular weight increased from 0.47 ꢁ 104
g
molꢂ1 to 3.69 ꢁ 104 g molꢂ1, and became independence on
the molecular weight when it is above 3.69 ꢁ 104 g molꢂ1
.
This behavior may be related to a dilution effect and pyrroli-
done-shell enrichment. At the same mass concentrations (10
mg mLꢂ1), polymer with higher molecular weight also has a
lower molar concentration, which leads to the increase of
cloud point with the increasing of molecular weight. PHIPPV-
TAs with high molecular weight are expected to show a
denser side groups and more rigid molecular structures than
the ones with low molecular weight, which leads to the for-
mation of a more compact pyrrolidone-shell to mask the
hydrophobic inner core of the polymers. The hydrophilic
FIGURE 6 Light scattering intensities of PHIPPVTA polymer
(10.0 mg mlꢂ1) aqueous solutions as a function of solution
temperature on heating.
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FIGURE 7 Temperature dependence of light scattering inten-
sity of aqueous P3 (10.0 mg mlꢂ1) solution during a heating
and cooling cycle.
FIGURE 8 Light scattering intensity of P2 (10 mg mLꢂ1) in D2O
or H2O as a function of solution temperature.
uct P2-H (Mn ¼ 1.12 ꢁ 104 g molꢂ1, Mw/Mn ¼ 1.35) has a
similar molecular weight and polydispersity comparing with
its precursor P2 (Mn ¼ 1.18 ꢁ 104 g molꢂ1, Mw/Mn ¼ 1.36).
After the removal of the trithiocarbonate end-groups, the
molecular weight effects on cloud point of reaction product
(P1-H to P8-H) were similar to their precursors (P1–P8; Fig.
10). Namely, the cloud point of reaction product (P1-H to
P8-H) also increased with increasing of its molecular weight.
These semirigid polymers showed the opposite trends com-
paring with other thermoresponsive polymers with flexible
polymer solutions slightly increased at high molecular weights
range.58 So far, this is the highest cloud point increase by
increasing polymer molecular weight that has ever been
reported. For instance, Jiang et al.52 reported that the LCST ꢀof
aqueous Ph-PTEGSt-ON solution elevated from 6 to 15
C
when the molecular weight increased from 3.9 ꢁ 103 g molꢂ1
to 2.64 ꢁ 104 g molꢂ1. Tong et al.59 also demonstrated that the
ꢀ
LCST increased by 0.5 C with the increasing of PNIPAm molec-
ular weight from 4.94 ꢁ 104 g molꢂ1 to 1.01 ꢁ 105 g molꢂ1
.
To elucidate this solvent isotopic effect, we tested the poly-
mer in D2O and H2O by DLS. These polymers showed a dra-
matic solvent isotopic effect that is similar to poly[N-(2-
methacryloyloxyethyl)pyrrolidone] (PNMEP).40 For instance,
4
ꢀ
the cloud point of P2 (Mn ¼ 1.18 ꢁ 10 ) is 15.5 C lower in
D2O than in H2O (Fig. 8). The cloud point of PHIPPVTA was
further investigated by 1H NMR spectroscopy in D2O as a
function of temperature. Take P2 for instance (Fig. 9; Fig. S4,
Supporting Information), at 25 ꢀC, the polymer chains are
fully solvated and the 1H NMR signals are strong. Above 41
1
ꢀC, the H NMR signals intensity decreased due to a diminu-
tion of chain mobility.17,58
Given that the end-groups of thermoresponsive water soluble
polymers have some effect on LCSTs,45,60–62 we compared
the cloud points before and after the removal of the trithio-
carbonate end-groups. The end groups were knocked off in
the presence of tributylstannane at 75 ꢀC.52 The resulting
polymers (P1-H–P8-H) without trithiocarbonate end-groups
were characterized by GPC (Table 1). The polymer (P1-H–
P8-H) molecular weights and the molecular weight distribu-
tions were about the same as P1–P8. For instance, the prod-
FIGURE 9 1H NMR spectrum evolution of polymer (P2) in D2O
on elevating the solution temperature.
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cloud point decreased with the increasing of molecular
weight after the removal of the hydrophobic end groups.
However, the cloud point values of P1-H to P8-H higher than
their corresponding precursors (P1–P8), because that the tri-
thiocarbonate group is a larger hydrophobic group than
hydrogen. Such effect was more pronounced in the samples
with low molecular weights. For example, the cloud point of
P1 (Mn ¼ 0.47 ꢁ 104 g molꢂ1, Mw/Mn ¼ 1.35) was lower
than P1-H (Mn ¼ 0.43 ꢁ 104 g molꢂ1, Mw/Mn ¼ 1.34) by 3.5
ꢀC at 10 mg mLꢂ1 while the cloud point of P6 (Mn ¼ 3.07 ꢁ
104 g molꢂ1, Mw/Mn ¼ 1.38) was slightly lower than P6-H
(Mn ¼ 3.01 ꢁ 104 g molꢂ1, Mw/Mn ¼ 1.36) by 0.4 ꢀC at 10
mg mLꢂ1 (Fig. 11). This trend is probably ascribed to the
end group effect that the hydrophobic end groups became
negligible when the molecular weight is higher enough.52
The concentration effect on the cloud point has also been
considered. The cloud point of PHIPPVTA solution was dra-
matically decreased with the increasing of concentration
(Fig. 12). For example, P3 (Mn ¼ 1.71 ꢁ 104 g molꢂ1
)
showed a cloud point of 81.5 ꢀC at 5 mg mLꢂ1 and 55.6 ꢀC
at 20 mg mLꢂ1. The influence of the polymer concentration
on the cloud point can be explained as follows. When the
polymer concentration increase, more polymer chains exist
in the solution and the polymers are easier to aggregate
upon increasing the temperature, which will increase the
turbidity of the solution and lower the cloud point.45
FIGURE 10 Cloud point of PHIPPVTA polymers as a function of
their corresponding Mn.
backbones,52,60 which suggested that the backbone rigidity
associated with their end groups plays an important role in
the cloud point. For example, Jiang et al.52 reported that the
cloud point of the polymers with hydrophobic end groups
increased with the increasing of molecular weight. But, the
We also noted that the decreasing rate of the cloud point
varied in different polymer concentration regions. The cloud
point decreased exponentially other than linearly with the
increasing of polymer concentration. Namely, the cloud point
FIGURE 11 Light scattering intensities of P1 and P1-H (a), P6 and P6-H (b) in DI-water (10.0 mg mLꢂ1) as a function of
temperature.
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able molecular weight-dependence and concentration-
dependence on their cloud point when comparing with con-
ventional thermosensitive polymers with flexible backbones
(e.g., PNIPAM). The cloud point increased by 56.2 ꢀC when
the molecular weight of PHIPPVTA increased from 0.47 ꢁ
104 g molꢂ1 to 3.69 ꢁ 105 g molꢂ1. The cloud point
decreased dramatically (upto 18.8 ꢀC) as the concentration
increased from 5 to 10 mg mLꢂ1. PHIPPVTAs showed a dra-
matic solvent isotopic effect which is similar to PNMEP. Fur-
thermore, the cloud points of PHIPPVTA slightly increased
after knocking off their end-groups. Our results will shed
light on the molecular design of new thermoresponsive poly-
mers with semirigid backbones and provide new opportuni-
ties in the field of thermoresponsive materials.
ACKNOWLEDGMENTS
This work was supported by the Innovation Platform Open
Fund Projects of University of Hunan Province (10k067) and
the National Nature Science Foundation of China (51073131,
21104062).
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