Polyelectrolyte-surfactant complexes as thermoreversible organogelators

Article abstract of DOI:10.1021/ma201023v

Poly(N,N-dimethyl-n-octadecylammonium p-styrenesulfonate) (PSS-DMODA) polymers were prepared and investigated as organogelators for low-polarity aromatic solvents. Gels were prepared by heating polymer solutions (2.5-20% w/v polymer) at elevated temperatu

Full text of DOI:10.1021/ma201023v

ARTICLE
pubs.acs.org/Macromolecules
PolyelectrolyteꢀSurfactant Complexes as Thermoreversible
Organogelators
Yuqing Liu,† Ashley Lloyd,‡ Gustavo Guzman,§ and Kevin A. Cavicchi*,†
†Department of Polymer Engineering, The University of Akron, Akron, Ohio, United States
‡Franklin W. Olin College of Engineering, Needham, Massachusetts, United States
§Department of Chemical Engineering, Universidad Nacional de Colombia, Bogota, Colombia
S Supporting Information
b
ABSTRACT: Poly(N,N-dimethyl-n-octadecylammonium p-styrene-
sulfonate) (PSS-DMODA) polymers were prepared and investi-
gated as organogelators for low-polarity aromatic solvents. Gels
were prepared by heating polymer solutions (2.5ꢀ20% w/v
polymer) at elevated temperature and then cooling in an ice bath.
Gelation was confirmed by the formation of self-supporting sam-
ples that did not flow when inverted 180°. Measurement of the gel
transition temperature by inversion testing showed a dependence
on the concentration of the polymer, the molecular weight of the
polymer, and the gelled solvent. Cavitation rheology measurements
on a subset of the gels demonstrated that they were viscoelastic
solids. Scanning electron microscopy measurements of freeze-dried
xerogels and polarized optical microscopy measurements showed
the formation of network structures and birefringent samples, respectively. Aging studies showed syneresis of the gels especially at
low concentration and temperature. Gelation was interpreted using a model for reversibly associating polymers. The gelation was
attributed to the clustering of the ionic groups to form a physically cross-linked network that restricts the motion of the chains. These
polyelectrolyteꢀsurfactant complexes should be a useful class of organogelators as a number of characteristics of the polymer
(molecular weight, ionic groups, side-chain length) can be independently varied to tune the properties of the resultant organogels.
’ INTRODUCTION
driven by the microphase separation between the ionic polymer
and the surfactant side-chains.13 The details of the morphological
behavior are dependent on various factors including the electro-
static and hydrophobic interactions and the spontaneous curva-
ture between the ionic and nonionic domains similar to the
organization of surfactants and block copolymers.14,15
In addition to the neat phase behavior, a number of interesting
properties are obtained in solutions of PE-SURFs in organic
solvents.12,16ꢀ20 For poly(N-ethyl-4-vinylpyridinium dodecyl
sulfate) and poly(sodium styrenesulfonate) complexed with dif-
ferent quaternary ammonium surfactants in sufficiently nonpolar
solvents the dissociation of the surfactant from the polyelectrolyte
is not observed.16,18 This can lead to a reduction in the intrinsic
viscosity due to a contraction of the chain driven by attractive elec-
trostatic interactions. Interchain aggregation has also been inferred
from a reduction in the diffusion coefficient as observed by dynamic
light scattering measurements.18 Comparison of water swollen
sodium bis(2-ethylhexyl) sulfosuccinate (AOT) reverse micelles in
hexane with the PE-SURF of poly(N-ethyl-4-vinylpyridinium AOT)
Gels are a complex state of soft matter that are phenomeno-
logically defined as systems containing large quantities of fluid
that display solid-like viscoelastic properties over a broad range of
time-scales.1 Organogels are formed when a nonaqueous fluid is
gelled, such as an organic solvent. A number of different com-
pounds have been found to act as organogelators including
semicrystalline polymers, block copolymers, and low molecular
mass organogelators.2ꢀ4 Physical gelation occurs when the
organogelator is able to form a stable, three-dimensional perco-
lating network that traps the surrounding fluid. Organogelators
have found application in a number of areas from viscosity
modification in foodstuffs and organic photovoltaics to templat-
ing nanostructured materials.5ꢀ7 In this paper it is demonstrated
that stoichiometric polyelectrolyteꢀsurfactant complexes (PE-
SURFs) of poly(N,N-dimethyl-n-octadecyl ammonium p-styrene-
sulfonate) act as organogelators for low-polarity aromatic solvents.
Stoichiometric polyelectrolyteꢀsurfactant complexes (PE-SURFs)
are formed by neutralizing each backbone ionic group of a
polyelectrolyte with an oppositely charged surfactant. They have
attracted interest for their ability to self-assemble into periodic
ordered structures.8ꢀ12 The neat polymers have been observed
to form cylindrical, lamellar and frustrated lamellar morphologies
Received: May 4, 2011
Revised:
August 18, 2011
Published: October 05, 2011
r
2011 American Chemical Society
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in hexane found that this PE-SURF formed a similar structure to
the reverse micelles where a comb-like conformation is obtained
with the alkyl tails of the AOT solubilized by the solvent.20
Similarly, lyotropic behavior was found for poly(2-vinylpyridine)
(P2VP) or poly(4-vinylpyridine) (P4VP) complexed with
p-dodecylbenzesulfonic acid (DBSA) in xylene.19 Here the xylene
was found to selectively swell the hydrophobic surfactant side-
chains. However, large polymer concentrations (>50% P4VP or
>70% P2VP) wererequired to observe birefringent morphologies in
these systems.
chloroform, and 5.55 mL (12.1M, 1:1 molar ratio) concentrated
aqueous hydrochloric acid was added dropwise to the solution. The
chloroform layer was collected, and concentrated on a rotary evaporator.
DMODA-HCl was recovered after drying in a vacuum oven (22.1 g,
white powder). Yield: 98.4%. 1H NMR (300 MHz, CDCl3, 7.27, ppm):
2.92 (2H, t, NCH2CH2ꢀ), 2.74 (6H, s, N-(CH3)2,), 1.73 (2H, m,
NCH2CH2CH2ꢀ), 1.14 (30H, m, NCH2CH2(CH2)15CH3), 0.80
(3H, t, NCH2CH2(CH2)15CH3). This salt was then used to synthesize
the SS-DMODA monomer. 200 mL of a 0.1 g/mL chloroform solution
of the DMODA-HCl was combined with 100 mL of a 0.17 g/mL
aqueous solution of sodium p-styrenesulfonate and stirred for 30 min.
The mixture was poured into a separation funnel and allowed to phase
separate for 1 day. The chloroform layer was collected, and dried with
anhydrous sodium sulfate and filtered to obtain a clear solution. The
solution was concentrated by a rotary evaporator, and the SS-DMODA
monomer was recovered by drying in a vacuum oven at room tempera-
ture. (31.3 g, white powder). Yield: 97.5%. 1H NMR (300 MHz, CDCl3,
7.27, ppm): 7.86 and 7.43 (4H, d, C6H4), 6.72 (1H, q, C6H4ꢀCHCH2,),
5.81 and 5.31 (2H, d, C6H4ꢀCHCH2), 3.03 (2H, t, NCH2CH2ꢀ), 2.87
(6H, s, N(CH3)2), 1.78 (2H, m, NCH2CH2CH2ꢀ), 1.26 (30H, m,
NCH2CH2(CH2)15CH3), 0.88 (3H, t, NCH2CH2(CH2)15CH3). 13C
NMR (75 MHz, CDCl3, 77.00, ppm): 144.22, 138.95, 135.75, and
125.93 (6C, s, C6H4), 57.82 (1C, s, NCH2CH2ꢀ), 42.83 (2C, s,
N(CH3)2), 31.73 (1C, s, NCH2CH2ꢀ), 29.52ꢀ22.49 (15C, m,
NCH2CH2(CH2)15CH3), 14.07 (1C, s, NCH2CH2(CH2)15CH3).
Didodecyl-1,2-phenylene Bis(methylene)bistrithiocarbonate
RAFT Agent Synthesis. 1-Dodecanthiol (9.05 g, 45 mmol), Aliquat
336 (0.72 g, 1.8 mmol), and 60 mL of toluene were added into a three
neck round-bottom flask. The solution was stirred under nitrogen gas in
an ice bath for 15 min. Then 50% sodium hydroxide aqueous solution
(3.63 g, 46 mmol) was added to the flask and stirred for 15 min. Carbon
disulfide (3.44 g 45 mmol) dissolved in 20 mL of toluene was added to
the solution, and the color of the solution quickly changed to yellow.
After stirring for 15 min, a solution of 1,2-bis(bromomethyl)benzene
(5.94 g, 23 mmol) dissolved in 30 mL of toluene was added to the flask.
The solution was stirred at room temperature under nitrogen for 12 h.
The reaction was terminated by adding 100 mL of deionized water and
stirring for 30 min. The mixture was poured into a separation funnel, and
a yellow toluene layer was collected and washed with deionized water
3 times. The product was recovered by rotary evaporation (14 g, yellow
solid), and further purified by recrystallization in hexane (9.5 g, yellow
powder). Yield: 64%. 1H NMR (300 MHz, CDCl3, 7.27, ppm): 7.38 and
7.28 (4H, m, CH2C6H4CH2), 4.70 (4H, s, SCH2C6H4CH2S); 3.39 (4H,
t, SCH2CH2(CH2)9CH3), 1.72 (4H, m, SCH2CH2(CH2)9CH3), 1.28
(36H, s, SCH2CH2(CH2)9CH3), 0.90 (6H, t, SCH2CH2(CH2)9CH3).
13C NMR (75 MHz, CDCl3, 77.00, ppm): 223.10 (2C, s, 2CS3), 133.90,
130.99, and 128.41 (6C, s, CH2C6H4CH2), 38.89 (2C, S, CH2C6H4CH2),
37.13 (2C, s, 2CS3CH2CH2(CH2)9CH3), 29.62ꢀ27.98 (18C, m, 2CS3-
CH2 (CH2)9 CH2CH3), 22.66 (2C, s, 2CS3CH2 (CH2)9 CH2CH3),
14.09 (2C, s, 2CS3CH2 (CH2)9 CH2CH3).
Interchain ionic aggregation and/or micelle formation of PE-
SURFs in solution can potentially lead to macroscopic gelation if
three-dimensional network structures are formed. For example,
stoichiometric hydrogen-bonded complexes of a 36 kDa poly-
(N-vinylpyrrolidone) with p-dodecylphenol have been shown to
gel toluene solutions above 10 wt % loading of the polymer.21
Small angle X-ray scattering measurements of these systems were
found to be consistent with the formation of worm-like micelles
of the polymer complexes. In addition the solꢀgel transition
temperature was found to increase with increasing concentration
of the polymer in solution. Gelation of ionomers in nonpolar
solution was predicted theoretically where the interchain aggre-
gation of ionic dipoles results in the formation of a physically
cross-linked ionomer network, which gels the solution.22 In these
systems, the solꢀgel transition temperature increases with
increasing concentration of polymer in solution. Also, macro-
phase separation occurs at lower concentration due to the
unfavorable interactions between the ionomer and the solvent
and the attractive interactions between the ionic groups. Quali-
tatively similar behavior was observed for the acid form of
sulfonated polystyrene solutions in Decalin.23 For example,
gelation was observed on cooling solutions of a high molecular
weight polymer (MW = 355 kDa, 0.8 mol % sulfonation) at
concentrations above 10 wt % polymer where the solꢀgel
transition temperature increased with increasing polymer con-
centration. Macrophase separation of the polymer and solvent
was observed at lower temperatures in the gels and at lower
concentrations.
This paper will demonstrate that organogelation is possible
using ionically complexed PE-SURFs. Here well-defined PE-SURFs
have been synthesized by reversible additionꢀfragmentation
chain transfer (RAFT) polymerization of a surfactant neutralized
monomer. The gelation behavior has been examined as a
function of the molecular weight of the polymer, its concentra-
tion in solution, and the choice of solvent. It will be shown that in
the room temperature gels the PE-SURFs form aggregated
network morphologies. Given that the gelation behavior of
PE-SURFs is dependent on the molecular structure and the wide
number of polyelectrolytes and surfactants that can be purchased
or prepared, PE-SURFs are potentially a useful system to
molecularly engineer organogels with tunable properties, such
as the gel transition temperature.
RAFT Polymerization of PSS-DMODA. RAFT polymerizations
to synthesize poly(N,N-dimethyl-n-octadecylammonium p-styrene-
sulfonate) (PSS-DMODA) polyelectrolyteꢀsurfactant complexes were
conducted using conditions developed previously for the polymerization
of tri-n-octylammonium p-styrenesulfonate.24 For each polymerization
10 g of SS-DMODA, RAFT agent, AIBN (1:5 molar ratio to RAFT
agent) and 21 mL chlorobenzene were added to make a 1 M monomer
solution. The solution was sparged with nitrogen gas for 20 min and then
heated to 80 °C for 8 h. The reaction was terminated by quenching in a
water bath, and an aliquot was collected to characterize the conversion.
The polymer was precipitated from hexane. To remove unreacted
monomer from the polymer, the polymer was redissolved in 50 mL of
tetrahydrofuran, and 100 mL of deionized water was slowly added to the
solution to precipitate the polymer. Three polymerizations were run
’ EXPERIMENTAL METHODS
Materials. 2,20-Azobis(2-methylpropionitrile) (AIBN) was purified
by recrystallization in methanol. All other reagents were used as received.
N,N-Dimethyl-n-octadecylammonium p-Styrenesulfonate
(SS-DMODA) Monomer Synthesis. Synthesis of SS-DMODA
was based on a previously reported method.24 First, N,N-dimethyl-n-
octadecylammonium hydrochloride (DMODA-HCl) was synthesized.
20 g N,N-dimethyl-n- octadecylamine was dissolved in 100 mL
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with RAFT agent concentrations of 1 mmol/mL, 0.5 mmol/mL, and
0.2 mmol/mL (mmol of RAFT agent/mL of chlorobenzene) to target
PSS-DMODA molecular weight of 10 kDa, 20 kDa and 50 kDa.
1H NMR (300 MHz, CDCl3, 7.27, ppm): 7.62 and 6.43 (4H, m,
C6H4), 2.99 (2H, t, NCH2CH2ꢀ), 2.67 (6H, s, Nꢀ(CH3)2), 2.09
(1H, s, C6H4ꢀCHCH2), 1.69 (4H, m, C6H4ꢀCHCH2 and NCH2CH2-
(CH2)15CH3), 1.26 (30H, m, NCH2CH2(CH2)15CH3), 0.90 (3H, t,
NCH2CH2(CH2)15CH3). 13C NMR (75 MHz, CDCl3, 77.00, ppm):
127.69 and 125.92 (6C, s, C6H4), 57.87 (1C, s, NCH2CH2ꢀ), 42.81
(2C, s, N(CH3)2), 31.84 (1C, s, NCH2CH2ꢀ), 29.66ꢀ22.48 (15C, m,
NCH2CH2(CH2)15CH3), 14.01 (1C, s, NCH2CH2(CH2)15CH3).
Polymer Characterization. 1H NMR spectra were measured on a
Varian Mercury-300 MHz spectrometer. Samples were dissolved in
deuterated chloroform (99.8%D, Cambridge Isotope laboratories)
at concentrations of 10 mg/mL and relaxation time of 5s was used.
13C NMR spectra were measured on a Varian Mercury-300 MHz
spectrometer. Samples were dissolved in deuterated chloroform (99.8%D,
Cambridge Isotope laboratories) at concentrations of 150 mg/mL and
relaxation time of 5s was used. Polymer molecular weights were deter-
hot-plate using a lab-jack. The pressure was zeroed, data collection was
started and the syringe pump was turned on and run at a flow-rate of
6.7 μL sꢀ1. The critical pressure, Pc, was determined from the peak in the
pressure vs time curve. A representative pressure vs time curve is shown
in the Supporting Information (Figure S2). Each gel was prepared by
heating at elevated temperature and quenching for 5ꢀ15 min in an ice
bath and placed in the heating block. Three measurements of the critical
pressure were taken at each temperature. Then, 9ꢀ12 measurements
could be taken before the gel was reannealed at elevated temperature,
quenched and then placed back in the heating block for additional
measurements. The temperature was adjusted in 4 °C increments and
tests were conducted 2ꢀ5 min after the temperature reading had
stabilized.
This critical pressure was used to determine the modulus of the gel,
E, from
2γ
5
6
Pc ¼
þ
E
ð1Þ
r
where γ is the surface tension of the gel and r is the inner radius of the
syringe needle. This equation was derived from the pressure-growth
relationship of the cavity using a neo-Hookean strain-energy function.
The surface tension of the gel was assumed to be the same as o-xylene
with the following temperature dependence27
1
mined based on the conversion determined from H NMR and the
expected molecular weight of the polymer at 100% conversion.
The molecular weight distribution of the polymers was characterized
by size exclusion chromatography (SEC) using a Waters breeze system
with three Styragel columns at 35 °C and a refractive index detector with
a mixture of THF and 0.02 g/mL tri-n-octylamine. The molecular weight
vs elution time was calculated using narrow polydispersity polystyrene
standards.
Organogel Preparation and Characterization. First 0.025ꢀ
0.2 g of PSS-DMODA polymer was dissolved in 1 mL of organic solvent
at elevated temperature in a 4 mL screw cap vial followed by placing the
vial in an ice bath for 5 min. Gelation was confirmed if the solution did
not flow when the vial was inverted 180°. Benzene and styrene solutions
were heated at 80 °C, but toluene and o-xylene solutions were heated to
110 and 140 °C, respectively.
After placing the vial in the ice bath, the bath was allowed to warm to
room temperature under ambient conditions from 0 °C. The vials were
periodically removed and inverted. If any flow of the gel was observed
this temperature was recorded as the gel transition temperature, Tgel. For
the gels that were stable at room temperature Tgel was first determined
by slowly heating the vials in a thermostated reaction block with step-
wise temperature changes of 2 °C/15 min. Tgel was measured as the
temperature at which the gels first broke-up when inverted after
annealing for 15 min. Two sets of gels were prepared and the Tgel
measurements were made four times on the first set and three times on
the second set. Four additional measurements were made with the
lowest molecular weight polymers in o-xylene (PSS-DMODA-20).
Heating rates of 2, 1, 0.5, and 0.33 °C/m were used for these gels.
Aging studies of the gels were conducted by leaving the gels on the lab
bench at room temperature or on a hot plate (IKA Works RCT basic) in
an aluminum heating block (Chemglass Pie-Block) at 60 °C.
Cavitation rheology was used to measure the modulus of the 5 and
10% w/v concentration solutions of the lowest molecular weight
polymer (PSS-DMODA-20) in o-xylene. The experiment was con-
ducted as described in previous reports.25,26 The setup consisted of a
syringe pump (Braintree Scientific BS-8000), a differential pressure
sensor (Omega PX26), a 5 mL gastight syringe (Hamilton) with a
22 gauge needle (BD) and a hot plate (IKA Works RCT Basic) with an
aluminum heating block (Chemglass Pie-Block). The pressure was
recorded using a 1/8 DIN Process Meter (Omega DP25BꢀS-A), a
USB Data Acquisition Kit (DATAQ DI-145) and a personal computer.
A schematic of the experimental setup is shown in the Supporting
Information (Figure S1). Vials were sealed with a rubber septum during
the cavitation rheology measurements. For each experiment the syringe
was filled with air and the needle was lowered into the gel by raising the
γ ¼ 32:513 ꢀ 0:1101T ð°CÞ
ð2Þ
The birefringence of the gels was examined using a cross-polarized
optical microscope (OLYMPUS BX51) with a digital camera in trans-
mission mode. The optical path consisted of the light source, a polarizer,
a condenser, the sample, and an analyzer. A hot solution of the PSS-
DMODA/solvent gel was taken up into a glass capillary, which was then
flame-sealed and allowed to cool to room temperature. The gels were
compared to a capillary filled with pure o-xylene to confirm the
birefringence of the gel samples. Annealing experiments were conducted
by placing the capillary in a hot state (Linkham) to control the
temperature. The temperature was incrementally increased by 2 °C
and an image was captured after annealing for 5 min. Cooling ramps
were run by allowing the stage to cool under ambient conditions. Images
were captured at 5 °C intervals during cooling. The birefringence
intensity was measured using ImageJ software by measuring the average
intensity of a region of the capillary and subtracting the average back-
ground intensity obtained in the middle of the image.28
The bulk morphology of xerogels of the PSS-DMODA/benzene gels
was characterized by scanning electron microscopy (SEM, JEOL-JSM-
7401F). The PSS-DMODA benzene gel was frozen in liquid nitrogen,
and freeze-dried to obtain a white powder cake. A portion of the powder
was placed on conductive tape and sputter coated with silver.
’ RESULTS AND DISCUSSION
1. Polymer Synthesis. A series of poly(N,N-dimethyl-n-
octadecylammonium p-styrenesulfonate) (PSS-DMODA) poly-
mers were prepared by RAFT polymerization of the correspond-
ing N,N-dimethyl-n-octadecylammonium p-styrenesulfonate as
shown in Scheme 1. The synthesis of the monomer and poly-
merization scheme followed a previously reported method for
the direct polymerization of ammonium sulfonate monomers.24
Three polymers were synthesized with target molecular
weights of 10, 20, and 50 kDa at 100% conversion by varying
the concentration of RAFT agent in the polymerization. Figure 1
shows size exclusion chromatography traces of these polymers
run in tetrahydrofuran +2 wt % tri-n-octylamine as the mobile
phase. The tri-n-octylamine was added to mitigate interactions
between the polymer and the columns.24 Fairly narrow poly-
dispersity (PDI) polymers were obtained where the PDI
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Scheme 1. RAFT Polymerization of SS-DMODA Monomer to Produce PSS-DMODA
Table 1. Characteristics of PSS-DMODA Polymers
polymer id
target M (kDa)
M (kDa)a
N
PDIb
PSS-DMODA-20
PSS-DMODA-38
10
20
50
9.4
18.3
43.5
20
38
90
1.18
1.20
1.26
PSS-DMODA-90
1
a Calculated by H NMR. b Characterized by GPC with THF+2 wt %
trioctylamine as the mobile phase.
Figure 1. SEC traces of (a) 10 kDa, (b) 20 kDa, and (c) 50 kDa target
molecular weight PSS-DMODA polymers. The curves have been shifted
vertically for clarity.
increased with the target molecular weight. The conversion and
molecular weight (M) of the polymers were determined by
1H NMR. As the SS-DMODA monomer is nonvolatile, for each
polymerization an aliquot was removed from the reactor, dried
under vacuum to remove the solvent and then directly char-
acterized to determine the conversion by measuring the amount
of residual monomer. The molecular weight, the degree of
polymerization (N), and PDI for the three polymers are listed
in Table 1.
2. Gelation Behavior. The PSS-DMODA polymers were
soluble in chloroform and tetrahydrofuran, but were all insoluble
in benzene, toluene, styrene, and o-xylene at room temperature.
Upon heating at elevated temperature in these less polar solvents,
transparent, homogeneous solutions were formed. Assuming
that ion dissociation is negligible in these nonpolar solvents, at
elevated temperature, the thermal energy is apparently enough to
prevent extensive ionic clustering and allow dissolution. These
solutions were found to form optically transparent gels after
cooling in an ice bath. Figure 2 shows pictures of self-supporting
gels formed using PSS-DMODA-38 in benzene over a range of
concentrations (Figure 2a) and in benzene, styrene, toluene, and
o-xylene at 10% w/v polymer. These gels were thermoreversible
as the vial could be reheated to form a homogeneous solution,
which would regel upon cooling.
The PSS-DMODA/benzene samples that formed gels at room
temperature could be freeze-dried to produce PSS-DMODA
xerogels. These were imaged by scanning electron microscopy
(SEM) and are shown in Figure 3. These SEM micrographs show
primarily sheet-like structures, which are reminiscent of the lamellar
morphologies observed in neat PE-SURFs, PE-SURF/organic
solvent solutions, and bilayer morphologies in surfactant reverse
micelles.8,19 No clear dependence was observed in these images on
Figure 2. PSS-DMODA-38/aromatic solvent organogels. Key: (a) benzene
organogels at (1) 2.5, (2) 5, (3) 10, and (4) 20% w/v PSS-DMODA-38;
(b) 10% w/v PSS-DMODA-38 in (5) benzene, (6) styrene, (7) toluene, and
(8) o-xylene.
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Figure 3. SEM micrographs of PSS-DMODA/benzene gels: (a) N = 90, c = 2.5%, (b) N = 90, c = 5%, (c) N = 90, c = 10%, (d) N = 90, c = 20%, (e) N =
38, c = 10%, (f) N = 38, c = 20%, (g) N = 20, c = 10%, and (h) N = 20, c = 20%. The scale bars are 2 μm.
Figure 4. POM micrographs of the PSS-DMODA/o-xylene gels. The concentration of the gels increases from top to bottom, while the degree of
polymerization of the polymer increases from left to right. The scale bar is 500 μm.
the molecular weight or the concentration of the PE-SURF.
However, it is not known to what extent the morphology is
perturbed by the formation of the xerogel. For example, capillary
forces during drying are known to result in shrinkage and further
aggregation in gels.29 Detailed identification of the morphology of
the gels would require other characterization techniques, such as
X-ray scattering.
The gels were also examined by polarized optical microscopy
(POM). Figure 4 shows POM micrographs of 12 PSS-DMODA/
o-xylene gels sealed in glass capillaries. Each gel was birefrigent
exhibiting a nonzero intensity when imaged through crossed
polarizers. On the basis of the SEM data in Figure 3, the
birefringence should arise from a combination of both the
intrinsic birefringence of the PE-SURF due to differential
polarizability of the stretched chains and form birefringence
arising from the index of refraction difference between the
solvent and the polymer network.30 The transmitted intensity
will then depend on the orientation of the polymer and network
with respect to the polarizers. In general, in the lower concentra-
tion samples specific regions of bright intensity were observed
and more uniform intensity was observed at higher polymer
concentrations.
for 216 h and at 60 °C for 16 h in Figure 5. In general, syneresis
was observed at lower concentrations and aging tempera-
tures, while clouding of the samples was observed at higher
concentration.
This syneresis implies that a macrophase separated system is
the equilibrium state for many of these gels. Therefore, gelation
appears to be a competing process with phase separation result-
ing in kinetically trapped gels. This is similar to the results seen in
ionomers where ionic clustering results in gel formation, which
can further undergo phase separation.22,23 This points to the
main gelation mechanism being the interchain ionic clustering
of the PE-SURF. The larger self-assembled structures of the
PE-SURF in the network, as inferred from the SEM and polarized
optical microscopy measurements, could still contribute to the
bulk gel properties. Macrophase separation after gelation would
then occur by breaking and reforming of the ionic clusters
allowing the chains to reorganize and expel solvent. In general
the amount of solvent expelled decreases with increasing poly-
mer concentration. This is consistent with the restricted mobility
with increasing ionic association. However, this result would
also be expected thermodynamically as the volume of the
solvent-rich phase would decrease as the concentration of
polymer is increased. A more detailed analysis and separation
of the thermodynamics and kinetics of these systems requires the
measurement of both the miscibility gap and the gel transitions of
these systems as a function of concentration and temperature.
3. Temporal Stability of Gels. While the initial gels formed
after cooling were always transparent and homogeneous, syneresis
and clouding were observed as the gels were aged. A series of
images is shown for the o-xylene gels aged at room temperature
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Figure 6. Images of a 5% w/v PSS-DMODA-20/o-xylene solution
taken on heating.
Figure 5. Images of o-xylene/PSS-DMODA organogels aged at
(a) room temperature, 216 h, and (b) 60 °C, 16 h. The numbers on
the left-hand-side indicate the degree of polymerization of the polymer
and the numbers across the top of the images indicate the polymer
concentration.
Significantly less solvent expulsion is seen at the shorter anneal-
ing time at higher temperature, especially at higher concentra-
tion. This allows for the determination of the gel transition
temperature, which is the subject of the next section.
4. Thermal Transitions of Gels. The transition temperature
from a gel to a solution (Tgel) was measured by annealing the
organogels with stepwise temperature increments and determin-
ing the temperature at which they breakup upon inversion. This
temperature was assigned to Tgel. For the 2.5% w/v concentra-
tion gels, Tgel was difficult to accurately measure due to the
simultaneous expulsion of solvent during heating, making the
determination of when the gel broke-up very subjective. For the
higher concentration gels, as they were heated the gels would
become noticeably more fragile and would fracture at higher
Figure 7. Tgel vs composition for (9) PSS-DMODA-N20, (b) PSS-
DMODA-N38 (2) PSS-DMODA-N90 in (a) benzene, (b) toluene, and
(c) o-xylene.
concentration are shown in Figure 7. The error bars indicate
one positive and negative standard deviation from the mean.
Each point is the average of seven measurements, except for the
PSS-DMODA-20/o-xylene gels, where 13 (5%), 11 (10%), and
11 (20%) measurements were run.
As the inversion test for Tgel has some measure of subjectivity,
cavitation rheology was used as a second measurement to
confirm Tgel. This is a new technique where the modulus of a
soft gel is measured by initiating a cavity in the system using a
syringe and monitoring the critical pressure for mechanical
instability.25,26 Figure 8 shows the modulus of the PSS-DMO-
DA-20/o-xylene gels at 5 and 10% w/v concentration as a
temperatures if inverted too quickly or shaken. Above Tgel
a
heterogeneous solution was obtained that would transition to a
homogeneous solution upon further increase of the temperature.
Figure 6 displays images of a 5% w/v PSS-DMODA-20 in
o-xylene during an inversion test. At lower temperatures the
system is a gel. At 87 °C, just below the gel transition, some
sliding of the entire gel is observed. The gel transition is observed
at 89 °C and pieces of the gel are observed in solution until 97 °C.
Tgel vs composition for the three PSS-DMODA polymers
in benzene, toluene, and o-xylene for 5, 10, and 20% w/v
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Figure 9. Measured intensity of polarized optical microscopy images
for 10% w/v PSS-DMODA-20/o-xylene samples as a function of
temperature: (squares) sample 1, (circles) sample 2, (open symbols)
heating, (closed symbols) cooling.
through attractive interactions to form a percolating network.
While the example of ionomer solutions has already been
discussed, this same qualitative behavior is seen experimentally
and theoretically in a range of associating polymer systems.31ꢀ36
In nonpolar solution the dissociation of the ionic groups is
minimal, so the association of the PE-SURF occurs through
attractive dipoleꢀdipole interactions.18 The free energy gain
from this association will increase with decreasing temperature
driving gelation upon cooling.22 Further details of the gelation
mechanism depend on the strength of the attractive interactions.
For strong interactions where the associations are essentially
irreversible gelation will occur when the percolation of the
physically cross-linked network occurs.2 For weak interactions
the associations are reversible. This results in a different model
for gelation and is summarized as follows.36 At the geometrical
percolation limit gelation does not occur because the chains can
still relax due to the reversibility of the bonds. As the temperature
is decreased the size of the ionic clusters grows resulting in a
clustering transition where the ionic groups become localized
over long time scales. This results in gelation of the solution due
to the immobilization of the chains in between clusters. This
dynamic behavior was also found to be similar to the vitrification
of glasses. In terms of the phase behavior, this model results in a
similar phase diagram to the strongly interacting systems, but
with lower gel transition temperatures compared to that pre-
dicted by percolation.
Figure 8. Modulus vs temperature for PSS-DMODA-20/o-xylene at
(a) 5% w/v and (b) 10% w/v concentration. The error bars indicate one
positive and negative standard deviation from the mean. The solid lines
in each plot indicate the lowest and highest Tgel value measure by
inversion and the dotted line indicated the average Tgel measured by
inversion.
function of temperature. This measurement demonstrates that
the gels behave as viscoelastic solids. The modulus shows a sharp
drop at a temperature near the highest Tgel measured by
inversion testing at both concentrations. This point at the highest
temperature was the highest temperature where a nonzero
modulus could be measured in the material. Therefore, while
inversion testing for Tgel tends to slightly underestimate Tgel it
should give the proper scaling.
A third measurement was to monitor the birefringence of a
10% w/v PSS-DMODA/o-xylene gel as a function of tempera-
ture. Figure 9 shows the intensity measured by polarized optical
microscopy on heating followed by cooling for two capillary
samples. The raw images at a subset of temperatures are shown in
the Supporting Information (Figure S3). One sample showed
little intensity on heating, while the second showed a large
increase in intensity at 50 °C, which peaked at 60 °C and rapidly
decayed back to zero intensity by 70 °C. This intensity rise is
likely some restructuring of the sample as the chain mobility
increases at elevated temperature. On cooling, both samples
showed an increase in intensity starting at ca. 65 °C. It is
important to note that the Tgel was determined to be 87 °C by
inversion and 90 °C by cavitation rheology for this gel. As the
birefringence is attributed to the self-assembly of the polymer
into larger mesophase structures in solution, this provides
evidence that it is not required for gelation. However, smaller,
more isotropic interchain ionic clusters must still exist in the
system for the solutions to remain gels.
For PE-SURFs solutions this weak association model appears
applicable. Macrophase separation is seen after gelation indicat-
ing that the ionic associations are reversible and the system can
equilibrate during the time scale of the measurement. In the PE-
SURF solution, ionic aggregation should be similar to micelliza-
tion of small molecule surfactants in organic media.14 In this case,
low critical micelle concentrations have been observed for
reverse micelles. This has led to a description of micellization
by a step-growth mechanism
2S T S2 T S3 T
T Sn
ð3Þ
3 3 3
5. Gelation Mechanism. The preceding experiments demon-
strate that the gelation behavior of PE-SURF solutions has a
strong dependence on the polymer concentration, molecular
weight and choice of solvent. Furthermore, the aging studies
demonstrate that the gels are not always the thermodynamic
equilibrium state of the system, especially at lower temperature
and solution concentration. This behavior is consistent with
gelation driven by the physical association of the polymer
where S represents an isolated surfactant molecule, S2 is a dimer,
S3 is a trimer, and Sn is an n-mer. Decreasing the temperature of
the system would favor aggregation and shift the equilibrium to
the right in eq 3. When the system is quenched from higher
temperatures, this ionic clustering should quickly occur resulting
in gelation. At low temperatures, the large extent of clustering
could result in the formation of larger mesophase structures as
observed by the SEM and polarized optical microscopy
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measurements. However, these mesophase structures do not
appear necessary for gelation as their birefringent signal disap-
pears at temperatures below the gel transition. This gelation
process also occurs faster than macrophase separation. Macro-
phase separation in these systems should also be driven by the
attractive interactions between the ionic groups, therefore the
initial clustering of the ionic groups appears to kinetically hinder
larger scale macrophase separation. Upon heating, the cluster size
should begin to decrease, and the gel transition will occur when
the clusters are no longer able to restrict the chain dynamics on
the time scale of the measurement.
Two other items to consider are the molecular weight and
solvent dependence of the gelation behavior. First, the gels
persist to higher temperature with increasing molecular weight
of the PE-SURF. In terms of the weak association model this is
attributed to the slower dynamics of the higher molecular weight
chains. This could arise from the additional number of clusters
per chain more effectively restricting chain motion. An analogy is
the reduction of the glass transition of polymer with decreasing
molecular weight.37 This could be tested by examining gelation
over a larger molecular weight range to see if Tgel asymptotes to a
higher molecular weight limit. The gel stability also persists to
higher temperature going from benzene to toluene to o-xylene.
The normalized solvent polarity parameter (ET(30)N) of these
solvents decreases from benzene (0.111) to toluene (0.099) to
o-xylene (0.074 for p-xylene). ET(30)N is a dimensionless,
empirical parameter, which quantifies the solvent polarity or
the solvating power of a solvent.38 Therefore, as the ionic groups
become less soluble in the organic solvent their interactions
becomes more favorable and persist to higher temperatures.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: kac58@uakron.edu. Telephone: 330-972-8368.
’ ACKNOWLEDGMENT
Acknowledgment is made to the Donors of the Petroleum
Research Fund, administered by the American Chemical Society,
for support of this research and the National Science Foundation
under Grant 1004747, the Goodyear Tire and Rubber Company
and the Lubrizol Corporation for support of G.G. and A.L. as
summer REU students. The authors thank Dr. Bojie Wang for
assistance with the SEM measurements and Prof. Alfred Crosby
for helpful discussions about cavitation rheology.
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It has been shown that a polyelectrolyteꢀsurfactant complex
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S
Supporting Information. Additional figures showing a
b
schematic of the cavitation rheology experiment, pressure vs time
for 10% w/v PSS-DMODA-20/o-xylene gel, and polarized
optical microscopy images. This material is available free of
charge via the Internet at http://pubs.acs.org.
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