Amphiphilic Maleic Acid-Containing Alternating
Copolymers—2. Dilute Solution Characterization by Light
Scattering, Intrinsic Viscosity, and PGSE NMR Spectroscopy
E. SAUVAGE,1 N. PLUCKTAVEESAK,2 R.H. COLBY,2 D.A. AMOS,1 B. ANTALEK,1 K.M. SCHROEDER,1 J.S. TAN1
1
Research & Development Laboratories, Eastman Kodak Company, Rochester, New York 14650-2116
2
Department of Materials Science and Engineering, The Pennsylvania State University,
University Park, Pennsylvania 16802
Received 13 November 2003; revised 7 April 2004; accepted 5 May 2004
DOI: 10.1002/polb.20201
Published online in Wiley InterScience (www.interscience.wiley.com).
ABSTRACT: The dilute solution behavior of several alternating copolymers of maleic
acid has been characterized by static and dynamic light scattering, intrinsic viscosity,
and pulsed-gradient spin-echo NMR spectroscopy. The copolymer of maleic acid–sodium salt and isobutylene (IBMA-Na, Mw ⬃350 kg/mol) dissolves readily in concentrated aqueous salt solutions. Changes in chain dimensions with ionic strength and pH
are similar to those of the lesser salt solution-soluble poly(acrylic acid-sodium salt). The
hydrophobically modified (with n-butyl, n-hexyl, n-octyl, and phenethyl amines) copolymers of maleic acid–sodium salts and isobutylene (IBMA-NHR-Na) show no sign of
large intermolecular aggregation in 0.1 N sodium acetate (NaAc). However, the sizes of
the copolymers are relatively small compared to that of the ionized parent copolymer
(IBMA-Na, Mw ⬃350 kg/mol), suggesting intramolecular aggregation of the alkyl sidechain groups along the polymer backbone. The copolymer modified with the longer
chain n-decyl, on the other hand, forms stable large intermolecular aggregates containing 33 chains/aggregate. The copolymers of maleic acid–sodium salt and styrene (SMANa) appear to have no signs of aggregation, despite being a hydrophobic polyelectrolyte.
The copolymer of maleic acid–sodium salt and di-isobutylene (DIBMA-Na) has a similar
salting-out concentration as SMA-Na. The radius of gyration measurements by static
light scattering suggest that at least some fraction of the DIBMA-Na chains form large
intermolecular aggregates. The copolymers of maleic acid–sodium salt with n-alkenes
(n-CmMA-Na) in 0.1 N NaAc form small intermolecular aggregates (three to five
chains/aggregate). In contrast to these static light scattering results, PGSE NMR
diffusion measurements for the above aggregated systems indicate only one diffusion
coefficient consistent with the motion of single isolated chains. A plausible explanation
for this discrepancy is that the population of the aggregates is too small to be sufficiently detected in the PGSE NMR experiment. Furthermore, it is likely that the
aggregate has a larger relaxation rate than the nonaggregate, and therefore has a
comparatively reduced signal in the PGSE NMR experiment. © 2004 Wiley Periodicals,
Inc. J Polym Sci Part B: Polym Phys 42: 3584 –3597, 2004
Correspondence to: R.H. Colby (E-mail: rhc@plmsc.psu.edu)
Journal of Polymer Science: Part B: Polymer Physics, Vol. 42, 3584 –3597 (2004)
© 2004 Wiley Periodicals, Inc.
3584
AMPHIPHILIC MALEIC ACID-CONTAINING ALTERNATING COPOLYMERS—2
3585
Keywords: maleic acid alternating copolymers; hydrophobically modified polyelectrolytes; static and dynamic light-scattering; intrinsic viscosity; pulsed-gradient spinecho NMR spectroscopy; intramolecular and intermolecular aggregation
INTRODUCTION
Maleic acid-containing copolymers are a class of
polyelectrolytes with unique properties and many
industrial applications.1–7 The alternating sequence distribution of the hydrophilic maleic acid
and the hydrophobic comonomers offers the possibility of forming a comb-like structure. This
structural feature can facilitate the segregation of
the two comonomers with opposing polarity, leading to either intramolecular or intermolecular aggregation for a preferred chain conformation in
aqueous solutions. Therefore, the choice of chemical compositions of the copolymers has profound
influence on their solution behavior. The parent
anhydride copolymers can be made amphiphilic
and water-soluble, water-dispersible, or water-insoluble, depending on the selection of the hydrophobic comonomers. In addition, hydrophobic side
chains can also be incorporated through ringopening reaction of the maleic anhydride with
alkylamines.
The low molecular weight materials are highly
surface active and useful as dispersing agents for
water-insoluble compounds.1,2 Only a few studies
have been reported on the aggregation behavior
and interfacial activities of some maleic acid copolymers.8 –10 It is not well understood how molecular weight and the length of the side chains of
the comonomers affect the interfacial activity.
The high molecular weight hydrophobically
modified-polyelectrolytes in general are useful as
associative thickeners.11 Examples of the associative thickeners that bear randomly charged carboxylates have been studied.12,13 In these systems, only a few mol % (1–3%) of the hydrophobic
comonomers containing n-C12H25 to n-C18H37 alkyl side chains was found necessary to induce
solution viscosity enhancement. However, the viscosity behavior of the present alternating amphiphilic maleic acid copolymers is less understood. It is not known to what extent the n-alkyl
chain length of the comonomer and the overall
molecular weight affect the solution viscosity.
To optimize the interfacial activity and rheological behavior of the present alternating maleic
acid copolymer, a study of the correlation between
the conformational behavior of the copolymers in
dilute solution and bulk properties is essential.
The early work by Strauss and coworkers14 –16
on conformational behavior of several copolymers
of n-alkyl vinyl ether and maleic acid has led to
many later studies on the characterization of intramolecular microdomain or micelle formation of
the same copolymers.17–21 The copolymer of noctadecene and maleic acid–potassium salt was
studied by Chu and Thomas.22 They found that
there is about one polymer chain (molecular
weight ⬃10 kg/mol) in each micelle consisting of
25 pairs of repeating monomer units. Detailed
structural analysis of a series of copolymers of
n-alkene and maleic acid–lithium salt (molecular
weight up to 20 kg/mol) was reported using smallangle neutron scattering by Chen and coworkers.23,24 The results indicated that the copolymer
derived from the high molecular weight n-octadecene forms cylindrical-shaped micelles in water
with approximately 10 chains per micelle. The
shape of the micelles becomes elliptical when the
copolymers were derived from the lower molecular weight n-alkenes. The number of chains per
micelle was found to decrease to only one or two
for those derived from 1-octene and 1-decene.
Most of the scattering experiments were conducted in dilute aqueous solution (⬍1%), although several measurements for the 1-octadecene copolymer were also made at high concentration (⬃20%).
Previously, we reported the characterization of
dissociation behavior and the determination of
chemical compositions of several amphiphilic maleic acid-containing copolymers.25 In this article,
we describe our results on the dilute solution
characterization of the same materials. The alternating copolymers include the maleic acid with
isobutylene, di-isobutylene, styrene, the n-alkenes ranging from n-hexene to n-octadecene, and
the hydrophobically modified-copolymers of maleic anhydride and isobutylene. We have employed static and dynamic light scattering, intrinsic viscosity, and pulsed-gradient spin-echo NMR
techniques to characterize these copolymers.
Salting-out experiments were conducted as qualitative measures for solubility behavior in aqueous salt solutions. The objective is to utilize the
information on intramolecular and intermolecular interactions gained from these studies to bet-
3586
SAUVAGE ET AL.
Table I. Molecular Weight Data for Parent Anhydride Copolymers
Polymer
IBMAa
DIBMAb
n-CmMAc
m⫽6
8
10
12
14
18
SMAd
SMA-lmwe
SMA-hmwf
Meq (kg/mol) (PEO eq)
(Organic Solvent)
Mw/Mn
330 (350)
2.4
Meq (kg/mol)(PSSNa eq)
(CH3OH/0.1MNa2SO4)
15
57
56
20
23
28
36
4.3
280 (340)g
4.2
3.7
2.0
2.1
2.5
3.0
2.7
4.7
5.5
500
a
Kuraray; DMF solvent. The nominal molecular weight listed by the manufacturer is in
parentheses.
b
Rohm & Haas, water-soluble hydrolyzed form.
c
S.C. Johnson & Sons (m ⫽ 6,8), THF solvent. Eastman Kodak (m ⫽ 10, 12, 14, 18), THF
solvent.
d
Elf Atochem, Inc. (low molecular weight). Scientific Polymer Products, Inc. (high molecular
weight).
e
lmw ⫽ low molecular weight.
f
hmw ⫽ high molecular weight.
g
The molecular weight in parentheses was calculated from the measured intrinsic viscosity of
0.79 dL/g for SMA-hmw in THF using the Mark-Houwink equation for SMA in THF.27 The SEC
equivalent molecular weight for SMA-hmw is most likely erroneous due to the known difficulties
with SEC of SMA in THF.28
ter understand future experiments on interfacial
and rheological behavior26 of the same systems.
EXPERIMENTAL
Materials
The parent copolymers (anhydride form) used in
this study are listed in Table I. The Na salt form of
each copolymer was obtained by hydrolyzing in
strong NaOH, followed by exhaustive dialysis and
freeze drying. The chemical structures of the Na
salt forms are given in Figure 1. The acid form of
each copolymer was obtained by ion exchange using
a mixed-bed ion exchange resin, followed by freeze
drying. Detailed syntheses and molecular weight
characterization were described earlier.25 Only one
of the copoly(isobutylene-alt-maleic anhydride)
(IBMA) series with molecular weight ⬃350 kg/mol
was selected for the present study. The two members, copoly (n-C6H13-alt-maleic anhydride) and copoly(n-C8H17-alt-maleic anhydride) obtained from
S.C. Johnson & Sons, have larger polydispersity
indices (Mw/Mn ⬃3.7– 4.6) than those of copoly (nC10H21, n-C12H25, n-C14H29, n-C18H37-alt-maleic
anhydrides) (Mw/Mn ⬃2.0 –3.0) that were synthesized at Eastman Kodak. These polymers are designated generally as copoly(n-alkene-alt-maleic acid-sodium salt) or n-CmMA, where m is the number
of carbons in the comonomer, making m ⫺ 2 the
number of carbons in the alkyl side chain. The hydrolyzed salt form is designated as n-CmMA-Na.
The hydrophobically modified IBMA copolymers (designated as IBMA-NHR) were obtained
by ring-opening reaction of the anhydride copolymer with n-alkylamines (i.e., n-butyl, n-hexyl,
n-octyl, and n-decyl) and n-phenethylamine in
DMSO. Detailed preparations of the sodium
salt form of these copolymers were described
earlier.25
The hydrolyzed copolymer of maleic acid and
di-isobutylene (copoly(di-isobutylene-alt-maleic acid–sodium salt or DIBMA-Na) was obtained in
aqueous solution from Rohm & Haas (supplied as
the hydrolyzed polymer solid containing excess
NaOH with a trade name Acusol 460N) with a
nominal molecular weight of 12 kg/mol. This polymer was dissolved in water and exhaustively dialyzed to remove the excess NaOH and freeze
dried to a white fluffy solid.
AMPHIPHILIC MALEIC ACID-CONTAINING ALTERNATING COPOLYMERS—2
3587
Also included in Table I are the samples of low
and high molecular weight copoly (styrene-altmaleic anhydride) (SMA) obtained from Elf Atochem and Scientific Polymer Products, Inc., respectively. Detailed preparations of the sodium
salt form of these copolymers, SMA-Na, were described earlier.25
Poly(acrylic acid sodium salt) (PAA-Na, Mw
⬃330 kg/mol) was purchased from Polysciences,
Inc. in an aqueous solution. Excess NaOH was
added to the polymer solution and the ionized
polymer was extensively diafiltered and freeze
dried. Poly (styrene sulfonic acid–sodium salt)
(PSS-Na) molecular weight standards were purchased from American Polymer Standards Corp.
Mentor, OH. They are supplied as dialyzed solid
samples with polydispersity index Mw/Mn ⬍ 1.1.
Light Scattering
Figure 1. Chemical structures of the maleic anhydride-containing copolymers.
Light-scattering measurements were made at
room temperature with a Brookhaven Laser
Light-Scattering system. It was equipped with a
BI 200SM goniometer, an Excel 3000 argon ion
laser tuned at 514.5 nm, and a BI 9000AT digital
autocorrelator with 256 channels operating over
10 decades in delay time. A Wyatt optilab 903
interferometric refractometer was used to measure the refractive increment at the same wavelength. The Zimm plot and the Berry plot routines
were employed to analyze the static light-scattering (SLS) data over the range of scattering angles
between 20° and 150°. The dynamic light-scattering (DLS) measurements were performed at various angles at a delay time ranging from 1 ␮s to
100 ms. The concentration of the polymer was
kept at a minimum (ⱕ0.1% w/w) to ensure that
only one single mode of decay function was observed. The autocorrelation function was analyzed by the cumulant or the CONTIN program,
the former for the soluble polyelectrolyte solutions and the latter for the aggregating systems.
Because of the unreliable and irreproducible
data collection in the decay functions of the aggregated sample solutions, the size measurements by DLS are reported here only for the
wholly soluble polymers PAA and IBMA. Sample
preparation is the most important part of the
light-scattering experiments, particularly for
aqueous polyelectrolyte solutions. The stock solution of sodium acetate (NaAc) was prepared once
every week and stored in the refrigerator. The pH
of the solvent at a given ionic strength was adjusted with NaOH or acetic acid of the same nor-
3588
SAUVAGE ET AL.
mality and filtered with 0.45 ␮m, 0.2 ␮m (Sterile
Acrodisc), and finally with 0.02 ␮m (Whatman
Anotop inorganic membrane) filters several times
into a clean light scattering cell before use. The
cleanliness of the solvent is assured by observing
almost no scattering intensity when the cuvette
containing the solvent is placed in the cell holder
of the laser light instrument. The cleaned solvent
was then used to prepare the polymer stock solutions (polymer concentration, cp, ranged from 0.1
to 1%, w/w) followed by filtration several times
with 0.8- and 0.45-␮m filters. Filters with pore
size smaller than 0.45 ␮m are not suitable for the
polymer stock solutions because the high molecular weight soluble polyelectrolytes or aggregates
may be removed.
For the static light-scattering experiments,
1 mL of the cleaned solvent was first introduced
into a dust-free cleaned cuvette and scattering
intensity was measured from 20° to 150°. The
laser intensity was adjusted so that the count rate
at each angle was maintained at above 10 Kcps
but below a few Mcps. Incremental additions into
the solvent of a few microliters of the filtered
polymer stock solution were made before scattering measurements. All light scattering measurements were made in dilute solution, with typical
polymer concentration 0.05% ⬍ cp ⬍ 0.3%. The
experimental accuracies determined from the
Zimm plot are ⫾10% for the weight-average molar mass Mw, and ⫾15% for the z-average radius
of gyration Rg and the second viral coefficient A2.
For DLS, care must be exercised similarly to
obtain clean polymer solutions (cp ⬍ 0.1%, filtered
with 0.8-␮m and then 0.45-␮m filters). The laser
intensity was adjusted with each sample solution
so that the count rate was above 10 Kcps.
Some specially made funnels fitted with sintered glass filters with porosity MF (medium
fine), VF (very fine), and UF (ultra fine) were used
to clean the organic solvents (methanol or tetrahydrofuran) and polymer solutions. Scattering intensity for the filtered sample solution with the
highest concentration was first measured, and
sequential additions of filtered solvent were made
for the dilution. The dilution procedure was
achieved by adding the filtered solvent from the
filtration funnel directly into the polymer sample
cuvette, followed by weighing.
Intrinsic Viscosity
Intrinsic viscosity, [␩], was used as a means to
estimate polymer size in solution. Measurements
were performed at 25 (⫾0.05)°C with a CannonUbbelohde capillary viscometer. The initial polymer stock solution was prepared at ⬃0.2% in various CNaAc at pH ⬃8.0 and filtered several times
with 0.8- and 0.45-␮m filters. The solvent was
also prepared at the same CNaAc at pH ⬃8.0 and
filtered similarly. The flow time for each solution
was measured three times and averaged. The intrinsic viscosity was determined from the common intercept of the plots of reduced and inherent
viscosities versus cp (in %). The accuracy of each
intrinsic viscosity determination is ⫾1%. The
product of intrinsic viscosity and molar mass is a
measure of the molar volume of the polymer in
solution. Hence, the cube-root of [␩]Mw/NAv is a
measure of the polymer’s size in solution, where
NAv is Avogadro’s number.
Pulsed-Gradient Spin-Echo NMR Spectroscopy
Polymer chain translational diffusion coefficients
were measured using the pulsed-gradient spinecho (PGSE) NMR technique.29 –33 Measurements
were made at 23 (⫾0.5)°C on a Varian Inova
400 MHz spectrometer equipped with a 20-Amp
Highland gradient amplifier and a standard
5-mm, indirect detection, pulsed field-gradient
probe. The combination provides a z-gradient
strength, g, of up to 33 Gauss/cm.
We employed a stimulated echo-pulse sequence
with the following features. (1) A self-compensating bipolar gradient pair for reduced eddy current
ringdown and better lock recovery; (2) a slice selection step to excite only the sample region experiencing a constant z-gradient (4 mm along the
sample length); and (3) a spin lock purge pulse for
better suppression of phase anomalies. The pulse
sequence and various features are described in
detail elsewhere.32,33 The experiments were performed by varying g and keeping all other timing
parameters constant. The relationship between
the echo attenuation and the self-diffusion coefficient (D) is given by
E ⫽ 0.5E 0exp关⫺D␥2 g2 ␦2 共⌬ ⫺ ␦/3兲 ⫺ R兴 ,
(1)
where E is the measured signal intensity, E0 is
the signal intensity for t ⫽ 0, ␥ is the gyromagnetic ratio for the 1H nucleus, ␦ is the gradient
pulse time length, ⌬ is the time between the two
gradients in the pulse sequence (and hence defines the diffusion time), and R is a constant that
takes into account nuclear relaxation. Typically,
AMPHIPHILIC MALEIC ACID-CONTAINING ALTERNATING COPOLYMERS—2
we use a ⌬ value of 200 to 400 ms, a ␦ value of 5
to 10 ms, and vary g from 3 to 33 Gauss/cm in 15
steps. The value ⌬ was chosen to be large enough
so that the root-mean-square (rms) displacement
of the polymer is much larger than the radius of
gyration of the polymer coil, Rg. For the diffusion
measured, we estimate that the polymer rms displacement is greater than 1 ␮m. The combination
of g, ⌬, and ␦ were chosen to obtain 90 –95% total
signal attenuation throughout the experiment.
We used a Hermitian 90-pulse shape of length
800 ␮s and power level of 30 for the slice selection
step. The spin lock purge pulse was 1.5 ms in
length. Other parameters include the following
with typical values: sweep width ⫽ 3600 Hz,
points for Fourier transform ⫽ 32,768, number of
transients ⫽ 64, and acquisition time ⫹ delay
⫽ 4.5 s. We used the integral of the region between 0.5 and 2 ppm to obtain the diffusion coefficient. The same polymer solutions as those used
in the light-scattering experiments were used in
the NMR measurements. Ten percent w/w D2O
was added to the aqueous polymer solution (i.e.,
100 ␮L/1 mL polymer solution) and 10% deuterated THF was added to the THF solutions for
deuterium locking.
Rh, an average hydrodynamic radius, was determined from the measured diffusion coefficient
using the Stokes-Einstein relation
R h ⫽ kT/6␲␩D ,
(2)
where ␩ is the solvent viscosity and k is the
Boltzmann constant. At T ⫽ 23 °C, ␩ values of
0.469 and 0.980 mPa s were used for the THF and
water solvents, respectively. It was found that the
water viscosity increased above 0.3 N NaAc. The
water diffusion coefficient, Dw, was then used to
estimate the bulk solution viscosity by keeping
constant the viscosity-diffusion product, ␩Dw. The
estimated accuracy in Rh is ⬃⫾10%.
Salting-Out Measurement
The salting-out experiment was conducted by
simple visual observation of the onset of light
scattering of a polymer solution upon incremental
additions of concentrated salt solution. A 3-mL
polymer solution (cp ⬃0.5%) containing a small
stirring bar was placed on a magnetic stirring
plate. A focused microscope beam was passed
through the middle of the polymer solution. Aliquots of a concentrated NaAc solution in microli-
3589
ters were added to the polymer solution under
stirring. The onset of turbidity through the scattering of the light beam was recorded for each
polymer solution. The concentration of salt at the
onset of turbidity was used as the salting-out
concentration, that is, the concentration of salt
required to initiate polymer precipitation. The visual observation of the onset of turbidity increase
is accurate to within 1 ␮L of the added salt solution.
RESULTS AND DISCUSSION
IBMA-Na and IBMA-NHR-Na
The static light-scattering results for IBMA-Na in
aqueous solutions as a function of salt concentrations and pH are listed in Table II. The weightaverage molecular weight, Mw, is approximately
constant (⬃330 K ⫾10%) in various aqueous media and is similar to the PEO-equivalent molecular weight (MPEO eq) of the parent anhydride copolymer previously determined by size exclusion
chromatography in DMF.25
The IBMA-Na polymer stock solutions in various salt concentrations show a pH of 8.5. This is
consistent with the partial dissociation (with an
extent of neutralization of ␣ ⫽ 0.72) of this polymer found previously.25 It was difficult to control
the pH of the sample solutions at low salt concentration (0.01 to 0.05 N NaAc) because of lack of
buffering capacity. Several static light-scattering
experiments were performed in this region; however, and the averages were used for Mw, the
radius of gyration, Rg, and the second virial coefficient, A2. Good Zimm plots or Berry plots with
double-linear extrapolations were obtained for
the 0.05 N NaAc solutions, but some curvature
was more apparent at lower salt concentrations
(0.01 and 0.02 N NaAc). More reproducible runs
were obtained, however, in 0.1 N NaAc where the
pH of the solutions was varied extensively as
shown in Table II. This is attributed to the effective screening of the charges on the polyelectrolytes by the added salt. The Acrodisc filters are
damaged by high pH, leading to unusually high
scattering intensity of the solution. Therefore,
data above pH ⫽ 8.7 were discarded. Higher Mw
and Rg values were obtained for experiments in
solutions below pH ⫽ 4.5, indicating aggregation
of the less soluble acidic polymer chains. Good
Zimm plots were also obtained for the 0.5 and
1.0 N NaAc solutions. The Mw determined for the
3590
SAUVAGE ET AL.
Table II. Static Light-Scattering Data in Aqueous Solution at 25°C
Polymer
CNaAc(N)
pH
0.01
0.02
0.05
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.5
1.0
3.0
8.5–8.7
8.1–8.3
8.5–8.7
8.4–8.6
7.0
6.2
6.0
5.0
4.5
4.2
8.2–8.4
8.6
8.5
0.1
0.1
0.1
0.01
0.1
0.1
8.0
8.0
8.0
8.0
8.0
7.0
Mw (kg/mol)
Rg (nm)
A2(10⫺4 cm3 mol g⫺2)
IBMA-Naa
IBMA-NHR-Nab
R ⫽ n-butyl
n-hexyl
n-octyl
n-decyl
phenethyl
PAA-Nac
325
335
344
350
303
330
316
330
331
457
357
340
250
383
506
666
25300
709
333
74
64
60
54
42
61
49
42
43
82
49
42
33
45
26
12.5
81
37.6
41
100
50
43
33
12
35
2
2
—
2
10
2
4
7.4
2.8
0.5
0
4.4
11.6
a
IBMA-Na cannot be salted out in saturated NaAc solutions at pH-7. The average value for dn/dc ⫽ 0.18 was used for all ionic
strengths.
b
The value for dn/dc is 0.18 for all samples. The n-decyl polymer salts out below 0.1 N; therefore, 0.01N was used.
c
PAA-Na is salted out in 1.5 N NaAc at pH-7. The value for dn/dc ⫽ 0.175 was used.
3.0 N NaAc solution is somewhat smaller, possibly due to an inaccurate dn/dc value.
Although the radius of gyration, Rg, decreases
with increasing CNaAc (from 74 to 33 nm), it does
not change markedly with pH at 0.1 N NaAc
(from 54 to 43 nm). The latter is attributed to the
effectiveness of the charge screening in 0.1 N salt
solution where polyelectrolyte coil extension is
greatly hindered. The second virial coefficient, A2,
decreases more sensitively with increasing CNaAc
and decreasing pH, as expected for anionic polyelectrolytes based on carboxylic acid. The magnitudes of Rg and A2 are also similar to those measured for PAA-Na with comparable molecular
weight (see also Table II). The two polymers,
IBMA-Na and PAA-Na, have identical ratios of
carboxylate ions to the number of backbone
carbons.
The plot for Rh vs. CNaAc at pH ⫽ 8.5 for IBMANa measured by PGSE NMR is shown in
Figure 2(a) along with Rg and ([␩] Mw/NAv)1/3 determined by SLS and intrinsic viscosity measurements, respectively. The trends show a definitive
electrostatic screening effect. The ratio, Rh/Rg remains steady, near 0.7, similar to the ratio ob-
served for uncharged flexible polymers in good
solvent.
The three measures of size, Rh, Rg, and ([␩]
Mw/NAV)1/3 as a function of pH are shown in
Figure 2(b). The insensitivity to pH is attributed
to the partial screening of the 0.1 N NaAc solution.
The Rh (by PGSE NMR) dependence on salt
and pH of IBMA-Na is almost identical to that of
PAA-Na of similar molecular weight, as shown in
Figure 3. Only the DLS technique was used to
acquire the PAA-Na data. It is difficult to obtain
PGSE NMR data on PAA-Na because the 1H relaxation times are very short (T2 ⬍ 10 ms) and
very little signal can be refocused in the experiment.
The static light-scattering data for the IBMANHR-Na polymers in 0.1 N NaAc at pH ⫽ 8 are
also included in Table II. The Zimm plots for all
modified copolymers appear normal, and the data
in Table II were averages of at least three repeated experiments. The increase in Mw in this
series is consistent with the increase in the alkyl
length of the side chain from n-butyl to n-octyl or
phenethyl. The much larger Mw (2.53 ⫻ 107) mea-
AMPHIPHILIC MALEIC ACID-CONTAINING ALTERNATING COPOLYMERS—2
Figure 2. (a) Salt concentration dependence of
IBMA-Na size at pH ⫽ 8.5. (b) pH dependence of IBMANa size at 0.1 N NaAc.
sured for the n-decylamine derivative (which is
only soluble below 0.05 N NaAc) suggests the
presence of stable large intermolecular aggregates, with larger Rg (81 nm) and negligible A2.
Nevertheless, double-linear extrapolations were
obtained in the Zimm plot for the latter polymer.
The aggregates are stable with time and the number of chains per aggregate, N, is approximately
33.
The dependence on the number of n-alkyl chain
carbons in the amide group for Rg and Rh (by
PGSE and DLS) is shown in Figure 4. The data at
zero carbon atoms corresponds to the unmodified
IBMA-Na. This result clearly indicates that the
coil size decreases with increasing chain length,
resulting from intramolecular aggregation of the
n-alkyl side chains. This aggregation behavior is
further supported in the 1H NMR spectra shown
in Figure 5. The alkyl resonances severely
broaden as the n-alkyl side chain length increases, indicating a sharp reduction in molecular
mobility. The decrease in A2 with increasing nalkyl side chain length (see Table II) is also consistent with the coil collapsing caused by intramolecular aggregation. This result will be correlated
3591
with the salting-out values for the same series of
copolymers later. The Rg value for the soluble
phenethyl derivative is greater than that for the
n-octyl derivative with equal number of carbon
atoms. This is a result of the effect of the bulkier
phenethyl side chain in the former polymer. The
hydrodynamic radii, Rh, determined by PGSE
NMR track with the Rg values and are lower in
magnitude by approximately 50% up to n-hexyl
and by 70% for n-octyl. Rh obtained by DLS appears to be somewhat less sensitive to chain
length than that obtained by PGSE NMR, attributed partly to the experimental accuracy in Rh
determination and the polydispersity of the polymer.
A dramatic difference between the Rg and Rh
values is seen in the n-decyl modified material.
The 1H NMR spectrum is not shown in Figure 5
but is similar to that of the n-octyl modified polymer, indicating a reduction in molecular mobility
due to chain collapse. A rigorous determination of
the actual dissolved concentration was not performed but based on the signal-to-noise ratio,
clearly much less was truly solvated compared to
the other modified polymers. It has been suggested that the collapse of polyelectrolyte chains
occurs in two stages: first to a “pearl necklace”
structure, followed by precipitation.34,35 Loss of
signal from the backbone hydrogen nuclei is attributed to the formation of the “pearl necklace.”34 We observe the loss of signal from the
backbone maleic acid methines but retain the signal from the more flexible long chain alkyls. The
SLS data suggests that some charge-stabilized
particulates (intermolecular aggregates) exist
that are unobservable in the NMR.
In summary, we have obtained only one single
exponential decay function for all the water-soluble polyelectrolytes in aqueous solutions at cp
⬍ 0.1% by PGSE NMR, indicating uniform size.
We found that in 0.1 N NaAc, IBMA-Na is very
similar to PAA-Na with comparable molecular
weight and charge density. The chain dimensions
decrease with increasing salt concentration but
remain unchanged with pH. Although PAA-Na
can be precipitated with a theta solvent salt concentration of 1.5 N NaCl,36,37 IBMA-Na is extremely soluble in aqueous salt solutions up to
saturated NaAc concentration (⬎5 N) at room
temperature.
The modification of IBMA with n-alkylamines
(n-butyl to n-octyl) does not cause intermolecular
aggregation in aqueous 0.1 N NaAc. The overall
chain dimensions of these hydrophobically modi-
3592
SAUVAGE ET AL.
Figure 3. (a) Comparison of the salt concentration
dependence of PAA-Na hydrodynamic size with that of
IBMA-Na at pH ⫽ 8.5. (b) Comparison of the pH dependence of PAA-Na hydrodynamic size with that of
IBMA-Na at 0.1 N NaAc. The Rh data obtained from
PGSE NMR is the same as that shown in Figure 2.
fied polyelectrolytes decrease with increasing nalkyl side-chain length. This finding is consistent
with the formation of a very tightly coiled conformation within each polymer chain resulting from
side-chain hydrophobic, intramolecular interaction. This result has a direct bearing on the conformational transition found for these copolymers
upon dissociation.25 Intermolecular aggregation
was observed, however, when the side chain increases to n-decyl (aggregation number⬃33),
leading to larger but stable suspended particles.
(1/1 v/v) (see Table I for molecular weight information).
Excellent Zimm and Berry plots were obtained
for DIBMA-Na in 0.1 N NaAc at pH ⫽ 8.0, resulting in Mw ⬃150 kg/mol. Based on the ratio Mw/Meq,
we estimate an aggregation number of ⬃10
chains/aggregate for DIBMA-Na in 0.1 N NaAc at
pH ⫽ 8.0. The Rg value of 48 nm for DIBMA-Na is
similar to those for IBMA-Na and PAA-Na with
higher Mw (⬃300 kg/mol). DIBMA-Na therefore
appears to form intermolecular aggregates in
aqueous 0.1 N NaAc.
The SLS data for SMA-Na in 0.1 N NaAc exhibit ordinary double-linear extrapolations in the
Zimm plot. The value obtained for Mw ⫽ 350 kg/
mol, is somewhat larger than the MPEOeq
⫽ 280 kg/mol determined by SEC (PEO equivalent molecular weight of the parent anhydride
polymer listed in Table I). However, considering
the fact that SEC in THF is known to give erroneous results for SMA28 and our measured intrinsic viscosity of SMA in THF (0.79 dL/g) suggests a
molecular weight of 340 kg/mol for the parent
anhydride polymer based on the Mark-Houwink
equation for SMA in THF,27 we would conclude
that SMA-Na does not aggregate in 0.1 N NaAc.
In contrast, the Rh data for DIBMA-Na, obtained by PGSE NMR and shown in Figure 6,
indicate no aggregation. A hydrodynamic radius
of 3.6 nm is consistent with a single chain molecular weight of 15 kg/mol. Spectral broadening,
consistent with intramolecular interactions, is
found in the 1H NMR spectra of this polymer (not
DIBMA-Na and SMA-Na
The static light-scattering results for the two copolymers, DIBMA-Na and SMA-Na, are listed in
Table III. The molecular weight of 12–15 kg/mol
was given for DIBMA-Na by the manufacturer.
Similar PSS-Na equivalent molecular weight
(Meq) was also obtained earlier25 by aqueous sizeexclusion chromatography with a fluid phase
composed of CH3OH/H2O with 0.1 M Na2SO4
Figure 4. Radius of gyration and hydrodynamic radius versus the number of n-alkyl side chain carbon
atoms for IBMA-NHR-Na copolymers in 0.1 N NaAc at
pH ⫽ 8. Due to its low solubility in the high salt regime,
0.01 N NaAc was used for the n-decyl version.
AMPHIPHILIC MALEIC ACID-CONTAINING ALTERNATING COPOLYMERS—2
3593
Figure 5. 1H NMR spectra of three IBMA-NHR-Na copolymers in 0.1 N NaAc at pH
⫽ 8. The resonance of the acetate ion is at 1.9 ppm. (a) R ⫽ n-butyl, (b) R ⫽ n-hexyl, and
(c) R ⫽ n-octyl. The resonances from the backbone hydrogen nuclei and the methylene
adjacent to the amine are above 2 ppm. The resonances from the long chain alkyls are
between 0.5 and 1.5 ppm.
shown) at high ionic strength, ⬎1 N, where Rh is
considerably smaller [see Fig. 6(a)].
This discrepancy between Rh and Rg is similar
to what was found for the n-decyl modified IBMA
polymer. It appears on the basis of SLS that
charge stabilized intermolecular aggregates, unobservable in NMR, exist in suspended form in
the DIBMA-Na solutions in 0.1 N NaAc. It is
unclear what total fraction of polymer is contained in these suspended particulates. SLS determines a z-average Rg, and hence, is significantly biased to the larger particulates. It is plausible to explain this discrepancy if the population
of aggregates is low with respect to the nonaggregates. NMR is sensitive to the number-average
molecular weight. Furthermore, it is reasonable
to assume, based on the spectral broadening seen
with intramolecular interactions, that the relaxation rates associated with the aggregate are
much higher than that of the nonaggregate. This
may greatly reduce the signal obtained for the
aggregate in the PGSE NMR experiment.
n-CmMA-Na
Although the SLS results for IBMA-Na are normal
for a soluble polyelectrolyte in aqueous solution, the
data for the higher members of the copolymers of
n-CmMA-Na begin to show deviations attributed to
aggregation. The copolymer n-C6MA-Na exhibits
obvious curvature in the Zimm and Berry plots, but
the approximate linear extrapolation scheme remains acceptable at the lowest concentrations and
smallest angles. The values for Mw, Rg, and A2 are
listed in Table III. An aggregation number of 3 is
obtained for n-C6MA-Na, based on the Mw data by
SEC and SLS, in good agreement with the previous
result reported by small angle neutron scattering24
(SANS).
3594
SAUVAGE ET AL.
Table III. Static Light-Scattering Data in 0.1N NaAc/pH 8.5 at 25°C
Polymer
DIBMA-Na
SMA-Na
n-CmMA-Na
m⫽6
8
10
12
14
18
Meq
(kg/mol, SEC)
Mw
(kg/mol,SLS)a
Rg
(nm)
A2
(10⫺4 cm3 mol g⫺2)
N
(Chains/Aggr)
(PSSNa eq)
15b
(PEO eq) 280
(340)c
(PEO eq)
57
56
20
23
28
36
150
48.4
2.08
10
350
41.4
10.0
1d
190
—
—
114
104
186
30
—
—
8
26
6.6
3.84
—
—
0.94
3.58
10.0
3
—
—
5
4
5
a
The value for dn/dc was difficult to measure, hence, an approximate value of 0.18 was used throughout.
The intrinsic viscosity of this polymer was measured to be 0.74 dL/g in the SEC solvent (1:1 methanol:aqueous 0.1N Na2SO4),
possibly suggesting a higher effective molecular weight for DIBMA-Na.
c
The molecular weight in parentheses was calculated from the measured intrinsic viscosity of 0.79 dL/g for SMA in THF using
the Mark-Houwink equation for SMA in THF.27 The SEC equivalent molecular weight for SMA-hmw is most likely erroneous due
to the known difficulties with SEC of SMA in THF.28
d
The molecular weight of 340 kg/mol from intrinsic viscosity in THF is nearly identical to the light scattering molecular weight,
suggesting no aggregation of SMA-Na in 0.1 N NaAc.
b
Very pronounced curvature was observed in
the Zimm and Berry plots, however, for the next
two members of the series, n-C8MA (see Fig. 7)
and n-C10MA, rendering the extrapolation
scheme unreliable. To verify that the curvature is
a result of aggregation in aqueous media for the
more hydrophobic polyelectrolytes, we have performed light-scattering measurements for the corresponding acid form of the same n-C8MA in THF.
A typical Zimm plot is shown in Figure 8, yielding
Mw ⫽ 33 kg/mol and A2 ⫽ 2.9 ⫻ 10⫺3 cm3 mol/g2.
This Mw of 33 kg/mol is of the same order of
magnitude as that obtained by SEC (56 kg/mol,
the PEO-equivalent molecular weight).
Interestingly, normal Zimm and Berry plots
are obtained for the next higher members
n-C12MA and n-C14MA. The estimated values for
Mw, Rg, A2, and the aggregation numbers are
listed in Table III. These polymers form intermolecular aggregates in 0.1 N NaAc, with three to
five chains per aggregate.
The highest member of the series studied here,
n-C18MA, with a PEO-equivalent molecular
weight of 36 kg/mol dissolves readily in aqueous
0.1 N NaAc solution, yielding a normal Zimm Plot
with Mw ⫽ 186 kg/mol as shown in Table III.
Based on the PEO-equivalent molecular weight
determined by SEC in THF and Mw by SLS in
aqueous 0.1 N NaAc, the aggregation number for
this n-C18MA copolymer is five chains/aggregate
(Table III).
Figure 6. (a) Salt concentration dependence of
DIBMA-Na hydrodynamic size at pH ⫽ 8.5. (b) pH
dependence of DIBMA-Na hydrodynamic size at 0.1 N
NaAc.
AMPHIPHILIC MALEIC ACID-CONTAINING ALTERNATING COPOLYMERS—2
3595
Table IV. Rh (nm) data in 0.1N NaAc/pH ⫽ 8.5 and
THF at 23°C by PGSE NMR
Figure 7. Light-scattering Berry plot for the hydrolyzed n-C8MA-Na copolymer in 0.1 N NaAc at pH ⫽ 8.
The rapid decrease at lower angles is a result of high
scattering intensity from large aggregates.
PGSE NMR experiments were performed for
0.1 N NaAc aqueous (sodium maleate form) and
THF (maleic acid form) solutions with cp ⬃0.1%
for all the n-alkyl copolymer samples and Rh is
compiled in Table IV. IBMA and DIBMA results
are also shown for comparison. Again, in contrast
to the SLS results, Rh from PGSE NMR only
shows a single exponential decay with a relaxation time consistent with isolated chains for the
n-CmMA-Na copolymers. DIBMA-Na and the
higher chain-length n-CmMA-Na polymers exhibit a small reduction in size in aqueous solution
compared with THF solution, indicating poorer
solvent conditions in water with 0.1 N NaAc than
in THF. In contrast, the IBMA-Na, shows a clear
increase in size for the aqueous solution that is
attributed to electrostatic repulsion coupled with
the small hydrophobic effect of its isobutylene
comonomer. Figure 9 is a graphical representa-
Figure 8. Light-scattering Zimm plot for hydrolyzed
n-C8MA acid copolymer in THF.
Polymera
Aqueous
THF
IBMA
DIBMA
n-CmMA
m⫽6
8
10
12
14
18
35
3.6
23
5.3
hmw
6.1
4.9
4.5
4.1
4.3
4.6
hmw
6.2
3.1
2.7
3.2
3.6
—
lmw
3.1
1.6
—
3.0
3.9
7.6
a
The acid form was used in the THF solutions and the
sodium salt form in the aqueous solutions.
tion of Table IV. It is interesting to note that a
minimum in size exists for C8–C10 above which
the size increases. This suggests that below C8–
C10 coil collapse dominates and above C8–C10 aggregation dominates the apparent size.
The SLS experiments in aqueous media appear
to be affected by the presence of large aggregates
for the hydrophobic series of n-CmMA-Na. The
aggregation results are consistent with those reported by SANS24 in aqueous media for the same
copolymer series in D2O. It should be noted, however, that the copolymers synthesized in our laboratories are similar in molecular weight to those
Figure 9. Hydrodynamic radius as a function of the
number of carbons in the alkene comonomer (data of
Table IV) Data shown are for the high molecular weight
copolymers in THF (filled squares) and in aqueous 0.1
N NaAc (open squares) and for the low molecular
weight copolymers in aqueous 0.1 N NaAc (open circles). Compared to the THF data, the size reaches a
minimum near C8–C10 in the 0.1 N aqueous medium.
3596
SAUVAGE ET AL.
Table V. The Salting-Out NaAc Concentrations for
Dilute Aqueous Copolymer Solutions (⬃0.5%) at 25°C
[NaAc]50
(mol/L)
Polymer
M (kg/mol)a
hmw
IBMA-Na
IBMA-NHR-Na
R ⫽ n-butyl
n-hexyl
n-octyl
n-decyl
phenethyl
DIBMA-Na
SMA-Na
330
none
383
506
666
25,300
709
15b
280 (340)c
4.3
2.17
0.96
0.12
0.05
1.0
3.0
2.9
n-CmMA-Na
m⫽6
6
8
8
10
12
12
14
14
18
57
11
56
3.5
20
23
12
28
7.8
36
lmw
none
3.05
4.5
0.54
1.9
0.25
0.57
1.3
0.52
0.6
0.30
a
Molecular weights of IBMA-Na and its modified series
were obtained in aqueous media by SLS: DIBMA-Na in mixed
aqueous/CH3OH by SEC; SMA-Na and the n-CmMA-Na copolymers in THF by SEC
b
The intrinsic viscosity of this polymer was measured to be
0.74 dL/g in the SEC solvent (1:1 methanol:aqueous 0.1 N
Na2SO4), possibly suggesting a higher effective molecular
weight for DIBMA-Na.
c
The molecular weight in parentheses was calculated from
the measured intrinsic viscosity of 0.79 dL/g for SMA in THF
using the Mark-Houwink equation for SMA in THF.27 The
SEC equivalent molecular weight for SMA-hmw is most likely
erroneous due to the known difficulties with SEC of SMA in
THF.28
of the previously reported materials24 but with
lower polydispersity indices. Similar to the
DIBMA-Na case, the SLS and SANS data apparently contradict the PGSE NMR diffusion data.
The two sets of data together suggest that there
are a small number of large aggregates that dominate the scattering results, in equilibrium with
many isolated single chains.
Salting Out
The concentrations of NaAc required for saltingout all the polymers studied in this work are
compiled in Table V. The unmodified IBMA-Na is
soluble even in saturated NaAc. Modification in
the maleic anhydride monomer with alkylamines
changes the salt tolerance of this copolymer dramatically. The longer the alkyl side-chain group,
the less tolerance it has to salt. Unlike the ndecyl-derivative whose salting-out concentration
is only 0.05 N, the phenethyl derivative with the
same number of carbon atoms on the side chain,
remains soluble even up to 1.0 N NaAc. Apparently, the polar aromatic substituent prefers intramolecular aggregation, possibly resulting from
the tendency of the planar aromatic rings to
stack. This makes the phenethyl-modified copolymer more soluble in the presence of a high salt
concentration than the linear n-alkyl-modified copolymers.
The salting-out concentration is found to be
⬃3 N for the two copolymers, DIBMA-Na and
SMA-Na (hmw). They have similar neutralization
extents and their titration behaviors are almost
identical.25
Two molecular weights were investigated for
some of the n-CmMA-Na series. n-C6MA-Na is
very soluble in aqueous salt solution. The salt
tolerance decreases with increasing n-alkene side
chain length and is the smallest for n-C18MA-Na.
The lower molecular weight sample for each copolymer shows higher salt tolerance than the
higher molecular weight counterpart. This behavior is generally expected for the effect of molecular weight on solubility.
CONCLUSIONS
IBMA-Na is highly soluble in aqueous solution
and cannot be salted out with saturated NaAc.
IBMA-Na behaves similarly to PAA-Na of comparable molecular weight with respect to the dependence of coil size on ionic strength and pH. The
low molecular weight SMA-Na (4.3 kg/mol) is also
very soluble in aqueous salt and cannot be precipitated in saturated NaAc. The high molecular
weight SMA-Na salts out at 2.9 mol/L NaAc, suggesting this polymer is considerably more hydrophobic than IBMA-Na. However, SMA-Na shows
no signs of intermolecular aggregation in 0.1 N
NaAc. The n-alkylamine modified isobutylenemaleic acid-sodium salt copolymers (IBMA-NHRNa) exhibit intramolecular interactions in aqueous solution causing chain collapse. The saltingout concentration for this series also decreases
dramatically with increasing n-alkyl side-chain
length. A large intermolecular aggregate with an
AMPHIPHILIC MALEIC ACID-CONTAINING ALTERNATING COPOLYMERS—2
aggregation number of ⬃33 was obtained for the
n-decyl derivative by light scattering. Similarly,
DIBMA-Na appears to form a small fraction of
stable intermolecular aggregates in aqueous salt
solutions, with aggregation numbers of order
⬃10. PGSE NMR diffusion measurements on solutions of IBMA-Na and DIBMA-Na only see a
single exponential decay, with a diffusion coefficient suggestive of isolated individual chains. Because scattering is dominated by large sizes, the
NMR and light-scattering results on DIBMA-Na
together suggest a small population of large aggregates in equilibrium with a large population of
isolated (nonaggregated) DIBMA-Na chains.
The copolymer series of maleic acid with nalkenes shows aggregation behavior in aqueous
media. Normal Zimm plots were obtained for the
lowest (n-C6MA) and the higher (n-C12MA,
n-C14MA, and n-C18MA) members of the series,
yielding stable intermolecular aggregates with
three to five chains/aggregate. Large curvatures
in the Zimm and Berry plots were observed, however, for the intermediate members (n-C8MA and
n-C10MA), suggesting the presence of aggregates,
the number or size of which depends on concentration.
The authors are grateful to C.J. Verbrugge for the gift
of the n-alkene-alt-maleic anhydride copolymers, I.
Ponticello for synthesizing some of the same materials,
and Kim Le for providing the SEC data.
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