Novel hydrophobically modified ethoxylated urethanes end-capped by percec-type alkyl substituted benzyl alcohol dendrons: Synthesis, characterization, and rheological behavior

Article abstract of DOI:10.1021/ma500876d

Novel dendron hydrophobically modified ethoxylated urethanes (DHEUR) with almost the same molecular weights, molecular weight distributions, and identical hydrophilic portion but different terminal hydrophobic group numbers were prepared by using Percec-t

Full text of DOI:10.1021/ma500876d

Article
pubs.acs.org/Macromolecules
Novel Hydrophobically Modified Ethoxylated Urethanes End-Capped
by Percec-Type Alkyl Substituted Benzyl Alcohol Dendrons:
Synthesis, Characterization, and Rheological Behavior
†
†
,†,‡
†
†
Jun Peng, Renfeng Dong, Biye Ren,* Xueyi Chang, and Zhen Tong
†
‡
Research Institute of Materials Science and The Key Laboratory of Polymer Processing Engineering, Ministry of Education, South
China University of Technology, Guangzhou 510641, China
*
S Supporting Information
ABSTRACT: Novel dendron hydrophobically modified
ethoxylated urethanes (DHEUR) with almost the same
molecular weights, molecular weight distributions, and
identical hydrophilic portion but different terminal hydro-
phobic group numbers were prepared by using Percec-type
alkyl substituted benzyl alcohol dendrons as new end-cappers.
These DHEUR polymers in solution possess interesting
associative and rheological behavior. For DHEUR-1 with 4-
mono(decyloxy)benzyl alcohol (2), the solutions are domi-
nantly composed of the isolated and separated micelles and
exhibit Newtonian behavior in a wide shear rate range
accompanied by shear thinning at high shear rate region.
DHEUR-2 with 3,5-di(decyloxy)benzyl alcohol (4) in solutions form a relatively more complete network through dominant
micellar junctions and process a relatively higher solution viscosity and similar solution viscosity behavior to DHEUR-1.
However, shear thinning behavior shifts to a lower shear rate region due to a relatively longer relaxation time. Interestingly, the
solutions of DHEUR-3 with 3,4,5-tri(decyloxy)benzyl alcohol (6) have developed a complete physical network and show
pronounced shear thinning behavior over the whole shear rate range. The oscillatory measurements further confirm that a
gradually developing associative network leads to their different solution rheological behavior, i.e., viscous fluid (DHEUR-1),
viscoelastic fluid (DHEUR-2), and elastic body (DHEUR-3) with increasing the hydrophobic tail number of dendrons.
Furthermore, the rheological activation energy of these DHEUR polymers increases with the increase of terminal hydrophobic
group numbers, indicating that DHEUR polymers with more hydrophobic tail chains need more energy potential barrier for the
disengagement of hydrophobes from micelles due to stronger association strength. In general, the results demonstrate that the
terminal hydrophobic tail number of dendrons plays a key role in determining the associative and rheological behavior of
DHEUR in solutions. This work opens a new perspective for more efficient thickeners and also promises the potential of these
DHEUR polymers in waterborne coating, cosmetics, dyestuff, medicines, and so on for the first time.
INTRODUCTION
noticeable thickening effect even at relatively smaller doses and
lower molecular weights. This has led to the use of HEUR
polymers as effective rheology modifiers for improving the
rheological properties in paints and coating industry and in
some other applications, such as cosmetics, dyestuff, and
medicines.
■
4
Associative polymers which comprise hydrophilic backbone and
hydrophobic end groups have gained considerable attention
over the past decade. When associative polymers dissolve in
aqueous solution, their hydrophobic groups associate with each
other in both intramolecular and intermolecular. This leads to
the formation of network structures. For their amphiphilic
The basic structure of HEUR is composed of poly(ethylene
glycol) (PEG) chains as hydrophilic segment chain-extended by
diisocyanates and two hydrophobe groups such as alkyl
1
property, associative polymers have been widely applied in
many fields, including waterborne coatings, ink, oil production
and cosmetics as rheology control agents, and DNA sequencing
where the network structure acts as a sieving medium. The
5
6−9
10−12
phenyl, fluorocarbons,
or aliphatic alkyl
attached
2
through urethane on both ends. Above a critical aggregation
concentration (CAC), HEUR in aqueous solutions form
rheological properties, such as viscosity increasing and shear
thinning and thickening, are the major uses of associative
1
3,14
flower-like micelles.
The hydrophobic segments associate
3
with each other and form micellar cores and the hydrophilic
polymers. Therefore, these polymers are commonly referred to
as associative thickeners. Hydrophobically modified ethoxylated
urethane (HEUR) is a class of representative associative
thickener. Compared to traditional thickeners such as
polyacrylates and cellulose derivatives, HEUR polymers have
Received: April 28, 2014
Revised: August 3, 2014
©
XXXX American Chemical Society
A
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Scheme 1. Synthetic Route of Mono-, Di-, and Trisubstituted Benzyl Alcohol Dendrons
backbone come into being flower loops. When the concen-
trations are low, the micelles are isolated, separated, and
difficult to associate with each other. However, at higher
concentration these flower-like micelles connect mutually
through the bridge connection of the hydrophilic chains bridge.
As a result, a network structure is formed and the viscosity
the hydrophobic association would be enhanced with increasing
the size of hydrophobic chain. However, nearly all the studies of
HEUR focused on hydrophobes of a single substituted alkyl tail
chain linked to the PEG spacer through urethane.
It is well know that dendrons are one of most interesting
architectural motifs and have been utilized as powerful building
blocks for construction of complex nanostructures and
supramolecular systems due to the abundant variation in the
1
5−19
increases.
Thus, hydrophilic groups, molecular weight,
molecular weight distribution, and parameters of hydrophobic
end-capper agents are important factors in determining the
thickening effect of HEUR associative thickeners.
31−33
composition and structure.
In our previous article, based
on an amphiphilic dendron−coil macromonomer (3,5-di(n-
hexadecyloxy)benzoyloxy polyoxyethylene methacrylate), the
Percec-type alkyl substituted benzoic acid dendron was
successfully introduced into the hydrophobically modified
alkali-soluble emulsion (HASE) polymer by emulsion copoly-
Previous research has been focused on the thickening effect
1
2,20−29
21
of HEUR associative thickeners.
Barmar et al. studied
the influence of the hydrophilic group of HEUR prepolymer on
the viscoplastic property. For HEUR with different lengths of
the hydrophilic segment but same hydrophobic ends, a stronger
association is achieved with a reduced length of the hydrophilic
34
merization. Our results confirmed that the dendron hydro-
phobically modified alkali-soluble emulsion (DHASE) polymer
resulted in a significant increase in terms of DHASE solution
viscosity with increasing the content of the Perce-type dendron,
representing an obvious thickening effect. We attempted to
introduce the Percec-type dendron into the HEUR polymers to
enhance hydrophobic associative networks. However, the
combination of synthetic thickeners with linear polyurethane
block and dendritic hydrophobic moiety are not common data.
Little is known about the rheological behavior of the dendron-
end-capped HEUR polymers as well.
1
2,22−25
section. The Glass group
reported that an HEUR
thickener with a molecular weight of probably 23 000 was the
most effective rheology modifier. For HEUR polymers with the
same molecular weights but different molecular weight
distributions, a broad molecular weight distribution would
result in a defective network, so thickening efficiency might be
reduced. The parameters of hydrophobic end-capper agent
including the hydrophobic chain length, the size, and structure
of the hydrophobic groups exert significant influences on
thickening performance of HEUR polymers. HEUR polymers
associate with each other to form flower-like micelles only if
their end groups are sufficiently hydrophobic because low end
In this study, three kinds of alkyl substituted benzyl alcohol
dendron with one, two, and three hydrophobic alkyl chains, i.e.,
4
-mono(decyloxy)benzyl alcohol (2), 3,5-di(decyloxy)benzyl
30
alcohol (4), and 3,4,5-tri(decyloxy)benzyl alcohol (6), were
used as new end-capper agents. Novel dendron hydrophobically
modified ethoxylated urethanes (DHEUR) were prepared by
the step-growth polymerization of poly(ethylene glycol) (PEG)
with a slight excess of diisocyanates followed by the reaction of
the terminal isocyanate groups with the Percec-type dendrons.
The associative structures and rheological behavior of these
novel DHEUR polymers in aqueous solutions were studied.
The objective of this study is to obtain detailed insights into
rheological behavior of these novel DHEUR polymers and
demonstrate the potential of these Percec-type alkyl substituted
benzyl alcohol dendron as new end-capper agents in HEUR
associative thickeners in the future.
chain lengths has unfavorable hydrophobic interactions. For
26
example, Kim et al. investigated the effect of the end chain
length of the hydrophobic groups on the thicken efficiency.
They found that HEUR polymers with end chain lengths of C12
and C18 had completed transient networks structures, but
HEUR with that of C8 did not. This means long end chains are
beneficial to the formation of both more compact micelles and
transient networks at low concentrations for HEUR. Barmar et
27
al. studied the effect of the structure of the hydrophobic
groups on the thicken efficiency. For HEUR end-capped
respectively by cetyl alcohol, dodecyl alcohol, and cyclohexanol,
only HEUR with cyclic end group did not exhibit
viscoelasticity. Rheology behavior of HEUR polymers with
variable terminal hydrophobic size was reported by Lundberg et
22
al. As the concentration of HEUR increased, the size of
terminal hydrophobic groups affected the magnitude of the
viscosity. A high viscosity was obtained when the linear HEUR
end-capped by the terminal nonylphenol. This indicated that
EXPERIMENTAL SECTION
■
Materials. Methyl 4-hydroxybenzoate, methyl 3,5-dihydroxyben-
zoate, and methyl 3,4,5-trihydroxybenzoate (all from Aldrich, 98%)
were used as received. Other reagents, such as tetrahydrofuran (THF)
B
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Scheme 2. Synthetic Route of Three DHEUR Polymers
and N,N′-dimethylformamide (DMF), were all analysis grade
in dry THF (60 mL) was added by dripping slowly. After addition of 5
within an hour, the ice−water bath was removed and the temperature
was increased to 30 °C. The reagents were stirred at 30 °C for 2 h, and
the reaction end point was determined by thin layer chromatography
chemicals and freshly distilled prior to use. 1-Bromodecane (Aladdin,
9
9
8%) was distilled before use. Lithium aluminum hydride (Aladdin,
9%) was used as received. Poly(ethylene glycol) (PEG) with the
molecular weight of 6000 was purchased from Aldrich. Dibutyltin
dilaurate (DBTDL) (98%) was used as received from Aladdin. 3-
Isocyanatomethyl-3,5,5-trimethylcyclohexyl isocyanate (IPDI) from
BASF was distilled under vacuum and stored under argon before use.
Toluene and petroleum ether were stirred for 24 h over 40 mesh
calcium hydride and distilled under argon. The mono-, di-, and
trisubstituted benzyl alcohol dendrons, i.e., 4-mono(decyloxy)benzyl
alcohol (2), 3,5-di(decyloxy)benzyl alcohol (4), and 3,4,5-tri-
(TLC). Subsequently, a small quantity of H O was added to consume
2
the unreacted LiAlH . 30 mL of hydrochloric acid was used to
4
neutralize the reaction. The mixture was transferred to a separator
funnel and extracted three times by adding dichloromethane. Next, the
liquid ballistic under layers was obtained, dried over MgSO , and
4
filtered to remove the solvent. The product was further purified by
silica gel column chromatography (eluent: petroleum ether/ethyl
acetate = 5:1 by volume) to yield 4.5 g (98.1 wt %) of white solid 6.
1
(
decyloxy)benzyl alcohol (6), were synthesized mainly according to
The H NMR spectrum of 6 is given in Supporting Information Figure
3
5,36
1
the method reported by Percec and co-workers
with some
SI2. H NMR (CDCl , TMS) δ (ppm): 0.88 (t, 9H, −CH ), 1.27 (s,
3
3
modifications (Scheme 1).
36H, CH −(CH ) −), 1.46 (s, 6H, −CH −CH −CH −O−Ar), 1.79
3
2
6
2
2
2
Measurements. 1H NMR spectra were obtained on a Bruker
Avance Digital 400 MHz spectrometer. The carbon, hydrogen, and
nitrogen contents (wt %) were determined with a Vario EL elemental
analyzer. Fourier transform infrared (FTIR) spectra of DHEUR
polymers were recorded on a Thermo Nicolet 6700 spectrometer
using KBr pellets at room temperature (25 °C). Molecular weights and
molecular weight distributions of DHEUR polymers were measured by
gel permeation chromatography (GPC) with a Waters 515 pump/
M717 data module/R410 differential refractometer, using THF as the
mobile phase with a flow rate 1.5 ML/min, distributed polystyrene as
standard, and a column temperature of 40 °C. The rheological
properties of DHEUR polymers in aqueous solution were measured
on an ARG-2 controlled stress rheometer (TA Instruments Inc.) with
a cone−plate geometry (40 mm diameter and 2° cone angle). Silicone
oil was applied to seal the cone−plate in order to protect from water
evaporation. Both steady-state shear and oscillatory measurements
were recorded at 25 °C to obtain the shear rate versus viscosity profiles
and the viscoelastic properties of the DHEUR solutions.
(m, 6H, −CH −CH −O−Ar), 3.97 (m, 6H, −CH −O−Ar), 4.59 (s,
2
2
2
2H, −Ar−CH −OH), 6.55 (s, 2H, −ArH−CH −OH). Anal. Calcd for
2
2
6: C, 76.96; H, 11.79; O, 11.09. Found: C, 77.14; H, 11.85; O, 11.02.
3,5-Di(decyloxy)benzyl alcohol dendron 4 and 4-mono(decyloxy)
benzyl alcohol dendron 2 were synthesized following a similar
1
procedure to 6. The H NMR spectrum of dendron 2 is given in
Supporting Information Figure SI3. 1H NMR (CDCl , TMS) δ
3
(ppm): 0.88 (t, 3H, −CH ), 1.27 (m, 12H, −(CH ) −), 1.45 (q, 2H,
3
2 6
−CH −CH −CH −O−Ar), 1.61 (m, 1H, −Ar−CH −OH), 1.77 (q,
2
2
2
2
2H, −CH −CH −−O−Ar), 3.95 (t, 2H, −CH −O−Ar), 4.61 (m, 2H,
2
2
2
−Ar−CH −OH), 6.89 (d, 2H, −O−HAr−), 7.28 (d, 2H, −ArH−
2
CH −OH). Anal. Calcd for 2: C, 77.16, H, 10.59 O, 12.10. Found: C,
2
77.32, H, 10.67 O, 11.93.
The 1H NMR spectrum of dendron 4 is given in Supporting
1
Information Figure SI4. H NMR (CDCl , TMS) δ (ppm): 0.88 (t,
3
6H, −CH ), 1.27 (m, 24H, −(CH ) −), 1.44 (q, 4H, −CH −CH −
3
2
6
2
2
CH −O−Ar), 1.66 (m, 1H, −Ar−CH −OH), 1.78 (q, 4H, −CH −
2
2
2
CH −O−Ar), 3.93 (t, 4H, −CH −O−Ar), 4.61 (m, 2H, −Ar−CH −
2
2
2
Syntheses of 3,4,5-Tris(decyloxy)benzyl Alcohol (6). First, methyl
OH), 6.37 (t, 2H, −O−HAr−), 6.5 (d, 2H, −ArH−CH −OH). Anal.
2
3
,4,5-tri(decyloxy)benzoate (5) was synthesized (Scheme 1) according
Calcd for 4: C, 77.01, H, 11.41 O, 11.41. Found: C, 77.36, H, 11.73 O,
10.93.
31
to the method reported in our previous paper. The 1H NMR
1
spectrum of 5 is given in Supporting Information Figure SI1. H NMR
Synthesis of DHEUR Polymers. Synthesis of Isocyanate-
Terminated Polyurethane Prepolymers. Before polymerization of
DHEUR, traces of water which can react with diisocyanates easily must
be removed from the reaction vessel to prevent the chain termination.
The processes can be completed through azeotropic ditillation of PEG
with toluene. In a four-necked 250 mL round-bottom flask, equipped
with a magnetic stirrer, a nitrogen inlet tube, a condenser with Dean-
Stark trap, and a thermometer, 5 g (0.83 mmol) of PEG with an
average molecular weight of 6000 dissolved by 80 mL of toluene was
added. The temperature was increased to 112 °C to remove the water
by azeotropic distillation of PEG in toluene with stirring for 2 h. A
total of 30 mL of solvent was removed to obtain dry PEG in toluene
(
CDCl , TMS) δ (ppm): 0.89 (t, 9H, −CH ), 1.28 (m, 36H,
3
3
−
−
−
(CH ) −), 1.48 (m, 6H, −CH −CH −CH −O−Ar), 1.79 (m, 6H,
2
6
2
2
2
CH −CH −O−Ar), 3.89 (s, 3H, −CO −CH ), 4.02 (m, 6H,
2
2
2
3
CH −O−Ar), 7.26 (s, 2H, −ArH−CO −CH ). Anal. Calcd for 5: C,
2
2
3
7
5.38; H, 11.24; O, 13.22. Found: C, 75.64; H, 11.53; O, 12.82.
Second, 6 was synthesized by chemical reduction from 5. LiAlH4
0.45 g, 11.91 mmol) was weighed and placed under an infrared lamp
(
for 0.5 h to remove water and was then was dissolved by dry THF (20
mL) and added to a three-necked 150 mL round-bottom flask which
was oven-dried. The flask was cooled at 0 °C in an ice−water bath and
under an atmosphere of dry argon. A solution of 5 (4.8 g, 8.32 mmol)
C
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solution. Following dewatering, the flask was cooled; the Dean-Stark
trap was remove and replaced by a reflux condenser. The nitrogen was
added continuously into the reaction system, and 0.24 g (1.1 mmol) of
IPDI (4 equiv of NCO to 3 equiv of OH) as raw material along with
0
.011 g of DBTDL (0.2% of the total mass of reactants) as the catalyst
were added into the flask. After 2 h of the reaction at 80 °C under the
protection of nitrogen, the isocyanate-terminated polyurethane
prepolymers were obtained.
Synthesis of DHEUR Polymers End-Capped with 2, 4, and 6. After
preparation of the isocyanate-terminated polyurethane prepolymer, the
end-capping reaction was performed by adding a toluene solution of
0
.17 g of 2, 0.25 g of 4, and 0.34 g of 6 into the four-neck round-
bottom flask. The reaction temperature was further increased to 90 °C,
and the end-capping reaction lasted for 3 h. Afterward, the DHEUR-1,
DHEUR-2, and DHEUR-3 were obtained. The general reaction is
given in Scheme 2.
Purification of DHEUR Polymers. The warm DHEUR toluene
solutions were precipitated into petroleum ether (3 volumes of
petroleum ether to 1 volume of thickener solution) in order to remove
unreacted benzyl alcohol and diisocyanate residues. Later, the solution
was collected on a sintered glass funnel and filtered to remove the
solvent. The purified products were dissolved in hot toluene,
reprecipitated into petroleum ether, and filtered with filter papers.
Finally, DHEUR-1, DHEUR-2, and DHEUR-3 polymers were
obtained by drying under vacuum at 40 °C for 12 h.
Preparation of Samples. Weighted amounts of DHEUR
polymers were dissolved in Millipore Milli-Q water by stirring at
room temperature until they were totally dissolved. Finally, the
aqueous solutions of DHEUR with different concentrations were
obtained.
1
Figure 1. H NMR spectrum of DHEUR-3 polymer.
In the range of chemical shifts δ from 0.7 to 1.8 ppm for
DHEUR-3 (Figure 1), there is a significant overlap of the
proton resonances of the alkyl tail (C H O) of the Percec-
type dendron end-cappers and the isophorone diurethane
IPDU) moieties. In δ range between 0.7 and 1.1 ppm, the
10
21
(
integration I(0.7−1.1) of the NMR signals compose by 12 protons
contributed by IPDU and 3 protons contributed by the alkyl
group C H O, which can be described by eq 1. In the range of
RESULTS AND DISCUSSION
Characterization of DHEUR Polymers. GPC measure-
ments are shown in Table 1 for the three DHEUR polymers. As
■
10
21
1
.1−1.8 ppm, IPDU contributes 3 protons, while each C H O
1
0
21
group contributes 16 protons. The integration I
can be
(
1.1−1.8)
Table 1. Molecular Weights and Molecular Weight
Distributions of Three DHEUR Polymers
described by eq 2. There is also an overlap of the proton
resonances of PEG, C H O, and IPDU moieties in δ range of
10
21
3
.5−4.3 ppm. In δ range between 3.5 and 3.8 ppm, the
polymers
Mn
Mw
PDI
integration I consist of 544 protons contributed by PEG,
(
3.5−3.8)
DHEUR-1
DHEUR-2
DHEUR-3
21 000
21 300
23 500
29 100
31 900
34 100
1.38
1.49
1.45
1
proton contributed by IPDU, and 2 protons contributed by
C H O, which can be described by eq 3. As the average IPDU
10
21
number is always one more than the average PEG number in
DHEUR polymer chain, eq 4 can be obtained.
can be seen, these DHEUR polymers have almost the same
molecular weights and molecular weight distributions. A target
I(
0.7−1.1)
= 3N(C10
H21O)
+ 12N(IPDU)
(1)
(2)
(3)
(4)
M of probably 22 000 which is the scope of the proposal of the
n
I
= 16N(C10H21O) + 3N(IPDU)
= 544N(PEG) + N(IPDU) + 2N(C10H21O)
= N(PEG) + 1
2
2,24,25
(1.1−1.8)
Glass group
has been chosen in our syntheses by
adjusting the molar ratio of NCO/OH. The FTIR spectra are
shown in Figure SI5 for the three DHEUR polymers. The
I(
3.5−4.3)
−
1
strong characteristic absorption band at ca. 2200−2300 cm
N(
IPDU)
corresponding to the stretching vibration of isocyanate groups
of IPDI disappears, and a new absorption band appears at ca.
716−1721 cm corresponding to the stretching vibration of
carbonyl group of carbonate, indicating that isocyanate groups
reacts with hydroxyl groups to form carbonate.
The H NMR spectrum is shown in Figure 1 for the purified
DHEUR-3 polymer. The H NMR spectra of the purified
In eqs 1−4, N is the average number of group per DHEUR
polymer chain. According to the integral area of the above three
δ range from H NMR spectra of DHEUR polymers, N(IPDU),
N(PEG), and N(C10H21O) can be calculated by applying the above
−1
1
1
1
equations.
1
The number-average molecular weight M (NMR) and the
end-capping ratios of hydrophobic alkyl chain can be obtained
in terms of the calculation eqs 5 and 6
n
DHEUR-1 and DHEUR-2 polymers follow similar trends and
are represented in Supporting Information Figures SI6 and SI7.
The number of alkyl groups per HEUR association polymer
1
Mi
chain can be sought from an H NMR analysis for HEUR
M (NMR) = 6000N
+ 222N(IPDU)
+
N
(C10H21O)
28
n
(PEG)
polymers with a single substituted alkyl tail chain. For these
DHEUR polymers, the degree of end-capping reaction can be
defined as end-capping ratio (ECR) of hydrophobic alkyl chain
i
(5)
N
(C10H21O)
28
according to a similar procedure, correlation with the number
of end alkyl group per DHEUR polymer chain
ECR =
× 100%
2i
(6)
D
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where M is the molecular weight of the Percec-type dendron
end-cappers and i is the number of terminal hydrophobic tail
For DHEUR-1, the solution viscosity is very low and shows
essentially Newtonian behavior in a wide γ range for each
concentration, while a highly shear thinning behavior appears in
i
chains of the Percec-type dendron end-cappers.
1
−1
The results derived from H NMR measurements are
a high γ region (approximately 40−100 s ), where the
summarized in Table 2 for these DHEUR polymers. The
viscosity rapidly falls with increasing γ. As aforementioned, the
hydrophobic segments of HEUR polymers in aqueous solution
can associate to form micellar cores and the hydrophilic
backbone come into being flower loops. When the micelles are
isolated and separated, they are difficult to associate with each
other. However, when these flower-like micelles connect
mutually through the bridge connection of the hydrophilic
1
Table 2. H NMR Characterization of Three DHUER
Polymers
polymers
N(PEG)
N(IPDU)
N(C10H21O)
M (NMR)
n
ECR (%)
DHEUR-1
DHEUR-2
DHEUR-3
3.22
3.21
3.33
4.22
4.21
4.33
1.8
20 731
20 848
21 975
90
87
88
3.48
5.28
chains, thus a network structure is formed and the viscosity is
15−19
increased.
As the network is temporal and weak, so the
network can be destroyed by the shear stress. When the shear
rate exceeds the inverse of relaxation time, i.e., the disengage-
ment rate of the hydrophobe from micellar junction surpasses
1
molecular weights derived from H NMR are consistent with
the values from GPC measurements. The end-capping ratios of
the three DHEUR polymers are above 87%, indicating that the
majority of the Percec-type dendron end-cappers are
successfully introduced into the molecular chain of DHEUR
polymers and only little DHEUR have fewer than the two
expected end-cappers per chain, which is consistent with the
15
the rate at which micellar junction can be re-formed, the
39
network is destroyed and the viscosity rapidly drops. Clearly,
the isolated and separated micelles should play a dominant role
in DHEUR-1 solutions, while micellar junction through the
bridged connection of the hydrophilic chains is secondary.
Therefore, DHEUR-1 polymers in solution cannot develop a
complete physical network to give a high viscosity.
Furthermore, a short relaxation time also reduces a rapid
shear-thinning behavior to occurs at high γ, which is almost
independent of the polymer concentration, suggesting that the
polymer concentration do not significantly influence on the
association and relaxation behavior of DHEUR-1 polymer. The
relaxation behavior of HEUR polymer is attributed to the
disengagement rate of the hydrophobe from micellar
2
5,28,37
results reported by other authors.
Steady Shear Behavior of DHEUR Aqueous Solutions.
The steady shear viscosity η profiles as a function of shear rate γ
for DHEUR aqueous solutions at indicated concentrations are
illustrated in Figure 2. The shear rates incremented from 0.01
to 1000 s− in logarithmically spaced steps (5 points per
decade). Prior to measurement, an equilibrium time of 10 s was
given at each experiment to ensure the shear stress reaches a
1
3
8
15
constant value. As shown, the shear viscosity of the three
DHEUR solutions processes interesting behavior.
junction. Thus, the relaxation time is related to the
association degree of transient network and shear-thinning
Figure 2. Plots of steady shear viscosity vs shear rate for (a) DHEUR-1, (b) DHEUR-2, and (c) DHEUR-3 aqueous solutions at indicated
−1
concentrations (wt %). Zero-shear (0.01 s ) viscosity as a function of DHEUR concentration (d).
E
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behavior of HEUR solutions. The more complete the
associative network is, the longer the relaxation time is. It
should be mentioned that shear thickening behavior is also
observed before shear thinning appears at intermediate γ
observed. Note that η of DHEUR-2 solutions increases sharply
when the concentration is over 3 wt %. Analogously, DHEUR-
0
3 solutions show a mutation of η above the concentration of 2
0
wt %. The difference in η should be attributed to the different
0
−
1
(
approximately from 10 to 100 s ) in low polymer
hydrophobic associative structures of these DHEUR polymers,
4
,6,15,48,49
concentrations, which is the same as the behavior of telechelic
which agrees with data reported by other authors.
The
7
,19,29,40−44
association polymer.
The shear thickening of HEUR
more terminal hydrophobic tail chain number the DHEUR
have, the more significant thickening efficiency the DHEUR
aqueous solutions get. Thus, both DHEUR-2 and DHEUR-3
solutions promise significant thickening efficiency due to their
stronger hydrophobic association than DHEUR-1 solutions.
The results further indicate that the number of the hydrophobic
end groups in these DHEUR polymers plays a key role in
determining the viscosity behavior.
solutions has been verified by some recent publica-
5
,40,45−47
tions.
The shear thickening may result from a
rearrangement of the molecular chain of DHEUR stretch
from intramolecular aggregation to intermolecular association
under shear. So the viscosity increase is due to more
intermolecular association. With the increase of the concen-
tration, the degree of shear thickening is debased because of the
establishment of intermolecular association.
Oscillatory Shear Measurement of DHEUR Aqueous
Solutions. The above steady shear measurement has indicated
that the different viscosity behaviors of these DHEUR aqueous
solutions should be due to the difference in their associative
structures. In order to verify the associative network structures,
the oscillatory shear measurement is performed to investigate
viscoelastic behaviors of DHUER aqueous solutions. DHEUR
aqueous solutions show linear viscoelastic behavior at strain
below 100% according to strain sweep measurements. In this
linear viscoelastic region, oscillatory shear measurement of
DHEUR aqueous solutions is conducted in angular frequency
DHEUR-2 solutions have similar steady shear flow behavior
to DHEUR-1 solutions, and shear thickening is also observed
at low polymer concentration, but the solution viscosity is
relatively higher and represents essentially Newtonian only in a
narrow γ range. On the contrary, a highly shear thinning
behavior shifts to low γ region (approximately from 0.05 to 5
−1
s ). This means that DHEUR-2 in solutions process a longer
relaxation time than DHEUR-1. Clearly, the flower-like
micellar junctions through the bridge connection of the
hydrophilic chains should be dominant in DHEUR-2 solutions
besides some isolated and separated micelles. Furthermore, the
shear rate at which shear thinning occurs significantly reduces
as the solution concentration increases, suggesting the increase
of the bridged micellar junctions and thus a relatively longer
relaxation time. This indicates that the polymer concentration
has an important influence on the associative structure and the
relaxation behavior of DHEUR-2 solutions due to the change
of the micellar junctions. The difference in associative
structures and the relaxation behavior for DHEUR-1 and
DHEUR-2 solutions should attribute to the number of
hydrophobic end groups in their molecular structures.
range ω from 10− to 10 rad/s. The storage modulus G′ and
loss modulus G″ of DHEUR aqueous solutions at indicated
concentrations as a function of the angular frequency ω is
shown in Figure 3. As an illustration, DHEUR solutions at
concentrations of 1, 3, and 5 wt % are given.
2
2
Figure 3a illustrates the changes of G′ and G″ dependence on
ω for DHEUR-1. At each concentration, the G″ increases with
a slope of one and G′ with a slope of two in the whole ω range.
G′ and G″ intersected at ω close to the maximum one. G″ is
higher than G′ over the frequency range, which reflects that
there is no rubbery plateau evident. They behave like
Newtonian viscous fluids, indicating that the associative
junction is weak and micellar aggregation play a dominant role.
Figure 3b illustrates the plots of G′ and G″ as a function of ω
for DHEUR-2. At low frequency region (approximately from
0.01 to 1), G″ increases with a slope of one, G′ with a slope of
two, and the value of G″ is less than that of G′, indicating a
similar viscous behavior to DHEUR-1 solutions. Interestingly,
with increasing ω (approximately from 1 to 100), G′
asymptotes to a constant value higher than G″, while G″
exhibits a maximum and a crossover of G′ and G″ is observed.
This plateau modulus is analogous to a classical rubber plateau
The solutions of DHEUR-3 show very high solution
viscosity and especially pronounced shear thinning behavior
over the whole γ range. This indicates that DHEUR-3 polymer
in solution form a complete physical network through the
micellar junctions to greatly improve the thickening effect due
to more hydrophobic alkyl tails without isolated and separated
micelles. So the relaxation time tends to infinity according to
previous discussion. In other words, even though the shear rate
is very low, the disengagement rate of the hydrophobe from
micellar junction would surpass the rate at which micellar
junction can be re-formed. As a result, DHEUR-3 polymer in
solution shows a rapid shear thinning behavior over the tested γ
range. As aforementioned, the DHEUR-3 have almost the same
molecular weights, molecular weight distributions, and nearly
identical hydrophilic portion with both DHEUR-1 and
DHEUR-2 polymers but more terminal hydrophobic group
numbers. Thus, it comes to conclusion that the number of
hydrophobic tail chain has a significant effect on the viscosity
behavior of these novel DHEUR polymers due to their different
hydrophobic associative structures.
modulus (G ). This plateau implies that the solution behaves in
0
this region as an elastic body. This suggests the formation of
50−52
the associative network,
which should be attributed to the
large number of mechanically active micellar junctions. Thus,
DHEUR-2 solutions show a viscoelastic behavior with elastic
regions at high ω and viscous regions at low ω. With the
increase of concentration, G value increases and the point of
0
intersection G′/G″ comes out at lower ω, indicating that the
associative network is more and more complete and the
relaxation time of associative network get longer and longer due
to the increase of micellar junctions. The relaxation time can be
obtained by fitting the G′ and G″ data of DHEUR-2 aqueous
solution from the single Maxwell model, consisting in an elastic
component connected in series with a viscous element. The
single Maxwell model is described by the following
In order to clearly demonstrate the effect of hydrophobic tail
chain number on thickening efficiency of DHEUR, a plot of the
−1
zero-shear (0.01 s ) viscosity η of DHEUR solutions as a
0
function of polymer concentrations is shown in Figure 2d. For
DHEUR-1, η is low and increases slightly with the polymer
0
concentration, and a linear relationship between the value of η0
and polymer concentration in the examined range can be
1
5,21,29,40
equations.
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concentration increases, which is consistent with previous
steady shear measurements. The results indicated that
DHEUR-2 solution form a relatively more complete network
with increasing polymer concentration and exhibit significant
viscoelastic behavior.
A Cole−Cole plot of G′ vs G″ is another way to investigate
6,15
the viscoelastic behavior of telechelic associating polymers.
The Cole−Cole plot can be described by the following eq 9
obtained by rearranging eqs 7 and 8:
2
m
G″(ω) = [G′(ω)G − G′(ω) ]
(9)
0
where the exponent m equal to 0.5. The data of a Maxwell fluid
should be in the form of a semicircle. Figure 5 shows Cole−
Cole plots for DHEUR-2 solutions at three concentrations (1,
3
, and 5 wt %) with the accompanying fitting of the Maxwell
model shown in solid lines using eq 9. Perfect semicircles can
be observed and G determined from the intersection of the
0
semicircle with the horizontal x-axis are consistent with the
values obtained from fitting of the Maxwell model. This further
confirms the fitting of oscillatory measurement data to the
single element Maxwell model.
Figure 3c illustrates the plots of G′ and G″ versus ω for
DHEUR-3. Note that in the whole ω range the value of G′ is
higher than that of G″. Moreover, G′ is independent of the ω
examined in this work and remains a constant value. A clear
rubbery plateau is observed, indicating the DHUER-3 solutions
behave like a highly elastic body due to the formation of a
complete physical network with strong associative cross-linking.
Thus, the system has infinite relaxation time in this tested
range, which is in accord with the steady shear viscosity
measurements.
From above results, one can draw one conclusion that the
number of hydrophobic tail chain is a significant contributing
factor affecting on the associative structures and rheological
behavior of these DHEUR polymers in solutions. The more
hydrophobic tails for dendron end-cappers favor the formation
of a complete physical network with strong micellar junctions,
which achieve high thickening efficiency.
Influence of Temperature on Rheological Behaviorof
DHEUR Aqueous Solutions. The rheological activation
energy is an important property of associative polymers and
represents the potential barrier for disengagement of the
hydrophobic chain from micellar junction. In order to obtain
the rheological activation energy of these DHEUR polymers,
the influence of temperature on the viscoelastic response is
investigated for the three DHEUR aqueous solutions. The
steady state and oscillatory shear measurements were
performed for these DHEUR solutions with fixed concentration
at different temperatures ranging from 15 up to 65 °C. Figure
6a shows the effect of temperature on steady shear viscosity η
for 5 wt % DHEUR-1 aqueous solutions. As can be seen, η
decreases with increasing temperature. Similar results are
observed for DHEUR-2 in Figure 6b and DHEUR-3 in Figure
Figure 3. Storage modulus G′ (closed symbols) and loss modulus G″
(
open symbols) modulus dependence on angular frequency ω for (a)
DHEUR-1, (b) DHEUR-2, and (c) DHEUR-3 aqueous solutions at
indicated concentrations.
2
2
G ω τ
0
G′(ω) = 1
+ ω τ
2
2
(7)
(8)
G ωτ
0
G″(ω) =
2
2
1
+ ω τ
where G is the plateau modulus, ω is the angular frequencies,
0
and τ is the relaxation time. The fitting data according to the
Maxwell eqs 7 and 8 are shown in Figure 4a−c; the solid line
represents the best fit to the single Maxwell model. The fitting
plots of DHEUR-2 solutions possess similarity as long as the
concentration is over 1 wt %. From the fitting data, the plateau
6
c. For DHEUR-1 and DHEUR-2, the onset of shear shinning
shift to the lower γ, and DHEUR-3 remain shear shinning
behavior over the whole γ range as temperature decreases.
These steady shear experiments indicate that the associative
structures of these DHEUR polymers in solutions almost retain
no change, but their relaxation time becomes shorter with
increasing temperature, which will be discussed below through
oscillatory shear measurements.
modulus G and the relaxation time τ can be obtained. The
0
concentration dependence of the relaxation time for DHEUR-2
solutions obtained from fitting of the Maxwell model is plotted
in Figure 4d. As is seen, the relaxation time increases as the
G
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Figure 4. Fitting of single Maxwell model to the oscillatory measured data of DHEUR-2 aqueous solutions at various concentrations: (a) 1, (b) 3,
and (c) 5 wt %. The relaxation time obtained from fitting to single Maxwell model as a function of concentration of DHEUR-2 solutions (d).
8
,54
given polymer concentration.
The values of Em for these
DHEUR polymers increase with the increase of terminal
hydrophobic tail chain number, further confirming that the
number of hydrophobic end groups is a significant contributing
factor affecting the strength of the association junctions.
To verify the influence of temperature on association and
relaxation behavior of DHEUR polymers, G′ and G″ profiles as
a function of ω are shown in Figure 7a−c for 5 wt % DHEUR
solutions at various temperatures. As an illustration, the data
only at temperature of 15, 35, and 55 °C are given for these
DHEUR solutions. It is clearly observed that the association
behavior of the DHEUR solutions maintains no change under
different temperature; i.e., DHEUR-1 has only viscous behavior
with short relaxation time τ, DHEUR-2 behaves as viscoelastic
fluid with a relatively longer τ, and DHUER-3 is highly elastic
with infinite τ. For 5 wt % DHEUR-2 aqueous solution, the
temperature dependence of τ obtained by fitting the G′ and G″
data from the single Maxwell model is shown in Figure 7d in
Arrhenius form, based on the transient network theory
Figure 5. Cole−Cole plots of storage modulus G′ vs loss modulus G″
for DHEUR-2 solutions at indicated concentration with the
accompanying fitting of the Maxwell model shown by solid lines.
The temperature dependence of zero-shear viscosity η0
extracted from the steady shear experiments can be fitted by
53
55
the Arrhenius equation expressed as
proposed by Tanaka and Edward. Just like η , τ also exhibits
0
Em/RT
a similar temperature dependency and a typical Arrhenius
η0(T) = Ae
(10)
relationship. E evaluated from the relaxation date possesses a
m
−1
where Em is the activation energy representing the potential
barrier to disengagement of the hydrophobic chain from
micellar junction, T is the thermodynamic temperature, A is a
pre-exponential constant, and R is the gas constant, equal to
value of 100 kJ mol , which is close to the data determined
from viscosity measurements. As temperature increases, the
network association strength become weaker, and terminal
hydrophobic chains exit from the micellar junctions more
1
−1
45
8
.31 J mol− K . The η profile as a function of temperature in
easily, which result in τ decreasing. Furthermore, as τ
0
Arrhenius form for 5 wt % DHEUR aqueous solutions is shown
decreases with increasing temperature, the onset of shear
thinning shift to higher γ, verifying the previous steady shear
measurements.
in Figure 6d. Clearly, η follows an Arrhenius temperature
0
dependence, with a linear relationship between the natural log
of η and 1/T. The rheological activation energys E obtained
from the slope of linear fittings are 61, 117, and 140 kJ mol
for DHEUR-1, DHEUR-2, and DHEUR-3, respectively. A
Structural Models of DHEUR Polymers. To illustrate the
change of associative networks, an overview of structural
models for these DHEUR polymers in aqueous solution is
detailed in Figure 8. The systems are composed of flower-like
0
m
−1
higher E value suggests stronger association interaction at a
m
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Figure 6. Steady shear viscosity profiles as a function of shear rate for 5 wt % of (a) DHEUR-1, (b) DHEUR-2, and (c) DHEUR-3 aqueous
solutions at different temperatures. Arrhenius plots of zero-shear viscosity η for 5 wt % DHEUR aqueous solutions (d).
0
Figure 7. (a) Storage modulus G′ (closed symbols) and loss modulus G″ (open symbols) modulus dependence on angular frequency ω for (a)
DHEUR-1, (b) DHEUR-2, and (c) DHEUR-3 aqueous solutions at 5 wt % at various temperature. (d) Arrhenius plot of relaxation time τ for 5 wt
%
DHEUR-2 aqueous solutions.
micelles and chain bridged junctions. The end hydrophobic
groups of DHEUR aggregate to form the core of flower-like
micelles, and hydrophilic segments form the loops or bridged
junctions of flower-like micelles. Flower-like micelles can
connect mutually to form micellar junctions. Those active
junctions through the bridged connection of the hydrophilic
chains are temporary. The lifetime of those junctions is very
short. Since the junctions in the network are dynamic, they are
broken and re-formed all the time by thermal energy. So the
capacity to rupture and re-form the existing junctions allows the
I
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behavior to occur at lower shear rate due to a relatively longer
relaxation time, although they show a similar solution viscosity
behavior to DHEUR-1. DHEUR-3 have more hydrophobic tail
chains and thus stronger hydrophobic association interactions
than DHEUR-2, so DHEUR-3 in solutions can form the most
complete transient networks by the micellar junctions through
the bridged connection of the hydrophilic chains, leading to a
significant increase in viscosity values and rapidly shear thinning
behavior over the whole γ range due to a very long relaxation
time.
CONCLUSIONS
■
Figure 8. Schematic diagram of structural models of (a) DHEUR-1,
Novel dendron hydrophobically modified ethoxylated ure-
thanes (DHEUR) were prepared by using Percec-type alkyl
substituted benzyl alcohol dendrons as new end-cappers. These
DHEUR polymers have almost the same molecular weights,
molecular weight distributions, and nearly identical hydrophilic
portion but different terminal hydrophobic group numbers.
The end-capping ratios of these DHEUR polymers are above
(
b) DHEUR-2, and (c) DHEUR-3 polymers in aqueous solution. The
terminal hydrophobic groups of DHEUR aggregate to form the core of
flower-like micelles and hydrophilic segments form the loops and
bridged junctions of flower-like micelles.
networks to relax applied stress of strains. When the number of
terminal hydrophobic tails increases from one to two and to
three, the association is stronger and stronger, and the network
structures tend to reach more and more complete, and thus the
relaxation time is longer and longer.
Figure 9 shows the steady shear viscosity as a factor of shear
rate for DHEUR aqueous solutions and the accompanying
1
87% estimated from their H NMR measurements; thus the
majority of the Percec-type dendron end-cappers are
successfully introduced into DHEUR polymers chain. These
DHEUR polymers in solution exhibit significantly different
rheological behavior due to their different associative structures.
DHEUR-1 solutions are dominantly composed of the isolated
and separated micelles and exhibit Newtonian behavior in a
wide shear rate range accompanied by shear-thinning at high
shear rate region due to a short relaxation time. DHEUR-2
solutions represent a relatively more complete network through
dominant micellar junctions and process a relatively higher
solution viscosity but a similar solution viscosity behavior to
DHEUR-1. However, shear thinning behavior occurs at lower
shear rate due to a relatively longer relaxation time. DHEUR-3
solutions have developed a complete physical network and
show only pronounced shear thinning behavior over the whole
γ range due to a very long relaxation time. The results indicate
that interesting rheological behaviors such as viscous fluid
(
(
DHEUR-1), viscoelastic fluid (DHEUR-2), and elastic body
DHEUR-3) can be triggered by increasing the terminal
Figure 9. Steady shear viscosity vs shear rate of DHEUR aqueous
solutions and the accompanying structure models of micelles at
different shear regions.
hydrophobic tail number of dendrons. Furthermore, the
rheological activation energy of these DHEUR polymers
increases with the increase of terminal hydrophobic group
numbers, indicating that DHEUR polymers with more
hydrophobic tail chains need more energy potential barrier
for the disengagement of hydrophobes from micelles due to
stronger association strength. In general, the results indicate
that the number of terminal hydrophobic tail chain plays a key
role in determining the rheological behavior of the DHEUR
aqueous solutions. This study opens a new perspective for more
efficient thickeners and further confirms the potential of these
novel DHEUR polymers in waterborne coating, cosmetics,
dyestuff, medicines, and so on for the first time.
structural models of micelles at different shear regions. As
shown in Figure 8, DHEUR-1 solutions are dominantly
composed of the isolated and separated micelles, and a
complete physical network of micellar junctions could not be
developed. Hence, DHEUR-1 solutions show Newtonian
behavior in wide shear rate range. However, the relaxation
time is very short. When the shear rate is very high, the
disengagement rate of the hydrophobe from micellar junctions
surpasses the rate at which micellar junction can be re-formed,
so those secondary micellar junctions is also destroyed and the
shear-thinning behavior occurs, while shear-thickening should
be caused by a rearrangement of the molecular chain from
intramolecular aggregation to intermolecular association at
intermediate shear rates at high shear rate region. DHEUR-2
polymers have stronger hydrophobic association interactions
than DHEUR-1, so they can form more bridged micellar
junctions than DHEUR-1. DHEUR-2 solutions represent a
relatively more complete network through dominant micellar
junctions, although some isolated and separated micelles are
present. So, DHEUR-2 solutions process high viscosity.
However, dominant micellar junctions cause shear thinning
ASSOCIATED CONTENT
■
*
S
Supporting Information
1
H NMR spectra of dendron 5, 6, 2, and 4; FTIR spectra of the
1
three DHEUR polymers; H NMR spectra of DHEUR-1 and
DHEUR-2 polymers. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*Tel +86-20-87112708; e-mail mcbyren@scut.edu.cn (B.R.).
J
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Notes
(31) Cheng, Z.; Ren, B.; Shan, H.; Liu, X.; Tong, Z. Macromolecules
2
008, 41, 2656−2662.
32) Cheng, Z.; Ren, B.; Zhao, D.; Liu, X.; Tong, Z. Macromolecules
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The authors declare no competing financial interest.
(
2
(
ACKNOWLEDGMENTS
■
The financial support from the NSFC (21274047) and the
Specialized Research Fund for the Doctoral Program of the
Education Ministry (20120172110005) is gratefully acknowl-
edged.
(
2
(
(
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dx.doi.org/10.1021/ma500876d | Macromolecules XXXX, XXX, XXX−XXX