Influence of the microstructure of acrylic acid-n-butyl acrylate copolymers prepared by pseudoliving radical polymerization by the reversible addition-fragmentation chain transfer mechanism on their surface activity in aqueous solutions

Article abstract of DOI:10.1134/S1070427209040181

The influence of the microstructure of acrylic acid-n-butyl acrylate copolymers prepared by pseudoliving radical polymerization by the reversible addition-fragmentation chain transfer mechanism on their surface activity in aqueous solutions was studied. The mechanisms of adsorption of polymer macromolecules on the phase boundary, accounting for the difference in the behavior of the random and block copolymers, were suggested. The dependence of the surface activity on the chain flexibility was elucidated. This dependence is very significant for the random copolymers and insignificant for the block copolymers.

Full text of DOI:10.1134/S1070427209040181

ISSN 1070-4272, Russian Journal of Applied Chemistry, 2009, Vol. 82, No. 4, pp. 630−638. © Pleiades Publishing, Ltd., 2009.
Original Russian Text © E.V. Sivtsov, E.V. Chernikova, P.S. Terpugova, O.G. Yasnogorodskaya, 2009, published in Zhurnal Prikladnoi Khimii, 2009,
Vol. 82, No. 4, pp. 635−643.
MACROMOLECULAR CHEMISTRY
AND POLYMERIC MATERIALS
Influence of the Microstructure of Acrylic Acid–n-Butyl Acrylate
Copolymers Prepared by Pseudoliving Radical Polymerization
by the Reversible Addition–Fragmentation Chain Transfer
Mechanism on Their Surface Activity in Aqueous Solutions
E. V. Sivtsova, E. V. Chernikovab, P. S. Terpugovab, and O. G. Yasnogorodskayaa
aSt. Petersburg State Institute of Technology (Technical University), St. Petersburg, Russia
bLomonosov Moscow State University, Moscow, Russia
Received November 13, 2008
Abstract—The influence of the microstructure of acrylic acid–n-butyl acrylate copolymers prepared by
pseudoliving radical polymerization by the reversible addition–fragmentation chain transfer mechanism on their
surface activity in aqueous solutions was studied. The mechanisms of adsorption of polymer macromolecules
on the phase boundary, accounting for the difference in the behavior of the random and block copolymers, were
suggested. The dependence of the surface activity on the chain flexibility was elucidated. This dependence is
very significant for the random copolymers and insignificant for the block copolymers.
DOI: 10.1134/S1070427209040181
In the past decades, synthetic polymeric surfactants
have found wide use in various branches of industry,
biology, and medicine [1]. Surfactant properties are
exhibited by polymers having pronounced diphilic
nature of macromolecules, such as polyvinyl alcohol
and its copolymers with vinyl acetate, (co)polymers
of acrylic and methacrylic acids, and polymers based
on oxyethylated acrylates. These polymers are used as
emulsifiers, dispersing agents, stabilizers of various
colloid systems, compatibilizes, foam suppressors,
rheological additives, and superplasticizers for cement
mixes. Despite wide use of macromolecular surfactants in
engineering, the number of papers concerning the nature
and mechanism of the surface activity of these compounds
is relatively few.
therapeutic systems, as artificial organs, and as auxiliary
medical equipment. In this field, surface phenomena are
of particular significance, as they occur on the phase
boundary with living tissues and biological media.
In this study, to examine the effect of the microstructure
on the surface activity, we used polymeric surfactants
based on acrylic monomers. It should be noted that the
method of classical radical polymerization, traditionally
used for preparing polymeric surfactants based on
acrylic monomers, has a number of serious limitations.
In particular, under these conditions it is impossible to
prepare compositionally uniform copolymers (except
azeotropic copolymerization) at high conversions and,
the more so, to prepare block copolymers. Furthermore,
classical radical polymerization does not allow synthesis
of homo- and copolymers of a given molecular weight
with narrow molecular-weight distribution, which is
in some cases of principal importance (e.g., for use in
medicine).
In one of the basic monographs dealing with
adsorption and surface activity of polymers, it was noted
that the surface activity of polymers and even the general
relationships between the surface activity and chemical
structure are studied insufficiently [2]. Along with
engineering, studies of the surface activity of polymers
are also topical for biology and medicine, in view of rapid
expansion of the assortment of polymers and materials
based on them, used in drug forms, in macromolecular
The problems inherent in traditional radical poly-
merization can be solved by using pseudoliving radical
polymerization, which allows preparation of polymers
of the preset structure with controllable molecular-
weight characteristics [3, 4]. In the course of this
630

INFLUENCE OF THE MICROSTRUCTURE OF ACRYLIC ACID–n-BUTYL ACRYLATE COPOLYMERS 631
process, the forming macroradicals can reversibly react
with stable radicals (polymerization by the reversible
inhibition mechanism and polymerization in the
presence of iniferters [5, 6]) or with special additives,
reversible chain-transfer agents (atom transfer radical
polymerization, ATRP; polymerization by the reversible
(II)
addition–fragmentation chain transfer mechanism [3,
7]). The principal difference of the pseudoliving radical
polymerization from the classical polymerization is that
the forming macromolecules constantly “revive” in the
(III)
course of the process, regenerating macroradicals which
can again participate in the chain propagation reaction up
to the next reversible termination or transfer event. It is
important that, in these processes, the role of quadratic
termination of macroradicals, in contrast to classical
radical polymerization, is insignificant. The multiple
The first step of chain-transfer reaction (I) in the RAFT
repetition of the termination (or transfer) and “revival”
process consists in addition of a polymeric (oligomeric)
of chains (i.e., reversibility of the chain termination and
·
propagating radical Pn to the RAFT agent with the
transfer reactions) provides successive propagation of
formation of radical intermediate Int-1 [reaction (Ia)].
macromolecules in the course of polymerization and
This process is reversible, but only partially, because
narrowing of the molecular-weight distribution.
there is also another pathway (Ib) of fragmentation of
Today one of the most promising procedures for
Int-1, with elimination of the leaving group (in the form
controllable synthesis of polymers is pseudoliving
radical polymerization occurring by the reversible
addition–fragmentation chain transfer (RAFT) mechanism,
suggested by Australian researchers [3, 8, 9]. In a typical
RAFT polymerization process, sulfur-containing com-
pounds of the general formula Z–C(=S)–S–R, where Z
is a stabilizing group and R is a leaving group, are used,
in particular, dithiobenzoates (Z = Ph), trithiocarbonates
(Z = SR), dithiocarbamates (Z = NRR'), and xanthates
(Z = OR) [9].
·
of the PhCHn radical) and formation of a new RAFT
agent (polyRAFT-1) containing polymeric substituent
Pn. Process (Ib) is, generally speaking, reversible, but
·
in the real process the equilibrium is shifted to the
right, because the benzyl radical in the presence of
the monomer irreversibly adds to it, initiating a new
chain. The occurrence of this step leads to gradual and
irreversible consumption of Int-1 with its transformation
first into intermediate Int-2 [reaction (II)] and then into
intermediate Int-3 [reaction (III)].
The mechanism of RAFT polymerization, along with
the elementary steps of chain initiation and propagation,
common for radical polymerization, involves reversible
chain-transfer reactions specific for the RAFT process.
Let us consider this process for dibenzyl trithiocarbonate
(BTC) used in this study as example [9]:
Thus, the performance of the initial RAFT agent
(BTC) is determined by the ratio of the rates of two
pathways (Ia, Ib) of Int-1 fragmentation. The more
readily is the leaving group eliminated in the course of
the monomer polymerization, the more effective is this
RAFT agent in the pseudoliving polymerization of the
monomer; the more active the propagating radical, the less
probable is its elimination from Int-1 and the stronger is
the equilibrium shifted to the left, i.e., toward regeneration
of the initial RAFT agent (in the limiting case, instead of
RAFT polymerization we will deal with common radical
polymerization).
Naturally, all the above-said concerns only the
initial RAFT agent and correspondingly the reactions
involving Int-1. With the progress of the polymerization,
the initial RAFT agent is consumed and transforms into
(I)
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SIVTSOV et al.
EXPERIMENTAL
polyRAFT-1. Elimination of the second leaving benzyl
group results in formation of a polymeric RAFT agent
[polyRAFT-2, reaction (II)] containing two polymeric
substituents. The latter reacts with the propagating
Prior to use, nBA and tBA were vacuum-distilled.
Azobis(isobutyronitrile) (AIBN) was recrystallized from
ethanol.
·
radical Pn to form intermediate Int-3 [reaction (III))].
BTC was synthesized by the procedure described
previously [11]. For this purpose, 5.85 g (0.045 mol)
of sodium sulfide was dissolved in 50 ml of water, after
which 0.30 g of a 75% aqueous solution of methyl-
tributylammonium chloride and 3.54 g (0.046 mol) of
carbon disulfide were added. The mixture was stirred for
1 h at 35°C. Then 10.08 g (0.08 mol) of benzyl chloride
was added to the reaction mixture over a period of
15 min, and the mixture was stirred for 3 h at 50°C and
for 30 min at 70°C. For the reaction completion, 0.15 g
of methyltributylammonium chloride was added, and
the mixture was stirred for an additional 15 h at room
temperature. After the reaction completion, the aqueous
phase was separated by decantation, and to the yellow
mass obtained 70 ml of ethanol was added. The crystalline
product was filtered off, washed with ethanol, and dried
to constant weight. The yield of BTC (yellow crystalline
substance) was 83.6%. 13C NMR spectrum (CDCl3), δC,
ppm: 41.37, 127.60, 128.52, 129.08, 134.75, 222.35.
In its structure, all the three substituents are equivalent,
and any reaction of polyRAFT-2 with Pn is efficient.
·
Specifically this fact determines the higher performance
of the polymeric RAFT agent compared to the initial
agent [10]. Repeated occurrence of step (III) in the
system, providing “revival” of macroradicals capable
of further participation in the chain propagation, leads
not only to an increase in the molecular weight of the
polymer with the conversion, but also to narrowing of
the molecular-weight distribution. It is important that
the polymer isolated from the reaction mixture (actually
polyRAFT-2) contains an active group in the chain and,
after being introduced into a new portion of the same or
another monomer, is capable to continue pseudoliving
polymerization. In the process, a homopolymer is
formed in the first case and a triblock copolymer, in the
second case [11].
It was shown previously that BTC is efficient in
homopolymerization of n-butyl acrylate (nBA) and tert-
butyl acrylate (tBA), allowing controllable synthesis of
both homopolymers [10]. Thus, it was natural to expect
that this RAFT agent would also be efficient in synthesis
of random and block copolymers from these monomers.
Acid hydrolysis of such copolymers [12] will yield
amphiphilic copolymers of acrylic acid (AA) and nBA
with preset molecular weight and narrow molecular-
weight distribution, which, at certain composition, can
exhibit surface activity in aqueous solutions. Thus, RAFT
polymerization offers a unique opportunity to study how
the structure of copolymers affects their properties, as it
allows purposeful synthesis of copolymers having similar
molecular weight and overall composition but different
microstructure.
To prepare prepare poly(n-butyl acrylate) trithio-
carbonate (PBA), in the freshly distilled monomer we
dissolved AIBN (10–3 M) and BTC (0.6 M). The mixture
was transferred into an ampule (~10 ml), degassed to
a residual pressure of ~5 × 10–3 mm Hg, and sealed off.
The ampule was placed in a thermostat (80°C), and the
contents were polymerized for 8 h, after which the ampule
was cooled in liquid nitrogen and opened. The mixture
was dissolved in a tenfold excess of benzene and freeze-
dried. The PBA yield was 31.7%; according to GPC,
Mn = 920, Mw/Mn = 1.2.
Samples for RAFT polymerization were prepared
by dissolving calculated amounts of BTC or PBA and
AIBN in the monomer or a mixture of comonomers. The
solutions were transferred into ampules. After degassing
by repeated freezing–pumping–thawing, the ampules
were sealed off. The polymerization was performed at
60 or 80°C. The polymers were isolated from benzene
by freeze drying; the conversion was determined
gravimetrically.
The goal of this study was to examine the behavior
of AA and nBA copolymers of various microstructures
in aqueous solutions, reveal mechanisms responsible
for their surface activity on the aqueous solution–air
phase boundary, and elucidate the effect exerted on the
surface tension of copolymer solutions by the chemical
composition of copolymers (content of nBAhydrophobic
units), distribution of units along the chain, and degree
of ionization.
To synthesize random copolymers of nBAand tBAby
RAFT polymerization, we prepared reaction mixtures
with different content of the comonomers and equal
concentrations of AIBN (10–3 M) and BTC (10–1 M). In
the first case, the mixture contained 10 vol % nBA and
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INFLUENCE OF THE MICROSTRUCTURE OF ACRYLIC ACID–n-BUTYL ACRYLATE COPOLYMERS 633
Table 1. Molecular-weight characteristics and composition
of copolymers of nBA with tBA, prepared by RAFT
copolymerization
90 vol % tBA(sample R1), and in the second case, 6 vol%
nBA and 94 vol % tBA (sample R2). Polymerization
was performed at 80°C to limiting conversions. The
molecular-weight characteristics of the copolymers
obtained are given in Table 1.
polymeriza-
To synthesize block copolymers, we prepared the
following reaction mixtures. To obtain sample B1, we
dissolved in tBA7.5 × 10–2 M PBAand 10–4 MAIBN; to
obtain sample B2, 7.5 × 10–2 M PBAand 10–3 MAIBN; to
obtain sample B3, 1.5 × 10–1 M PBA and 10–3 M AIBN;
and to obtain sample B4, 8.2 × 10–3 M PBA and 10–3 M
AIBN. Samples B1–B3 were prepared by polymerization
to limiting conversion at 60°C, and sample B4, at 80°C.
The molecular-weight characteristics of the samples are
given in Table 1.
where Δρ = ρaq – ρair is the difference between the
solution and air densities at the test temperature; g, free
fall acceleration (9.81 m s–2); r, capillary radius; and h,
height of solution ascent in the capillary.
The molecular-weight characteristics of the polymer
samples were determined by GPC in THF at 35°C on
a Waters liquid chromatograph equipped with a re-
fractometric detector, Ultrastyrogel columns (pore size
103, 105 Å), and a linear column. The chromatograms
were processed on a Data Module-730 integrator with
calibration against polystyrene reference samples.
Surface phenomena in macromolecular systems
are more complex than with low-molecular-weight
substances. For low-molecular-weight substances, the
decisive factors are surfactant type (cationic, anionic,
nonionic) and its hydrophilic–lipophilic balance de-
termined by the ratio of hydrophilic and hydrophobic
groups of atoms. For macromolecular surfactants,
significant (and in some cases decisive) role should be
played by steric arrangement and accessibility of these
groups, which, in turn, is determined by a number of
factors: conformation of macromolecules in solution,
depending on the solvent “quality” and on possible
Acid hydrolysis of random and block nBA–tBA
copolymers was performed according to [13] under the
conditions when only tBA units undergo hydrolysis.
A polymer sample was dissolved in 1,4-dioxane with
stirring at room temperature. To the resulting solution,
concentrated HCl was added, and the mixture was placed
in a thermostat heated to 85°C. The mixture was heated
for 5–6 h, after which it was stirred at room temperature
for 24 h, benzene was added, and the polymer was
isolated by freeze drying. The hydrolysis completeness
was checked by 1H NMR.
To prepare AA–nBA copolymers by classical radical
polymerization, 5 × 10–2 MAIBN was dissolved in a mix-
ture containing DMSO and the comonomers (AA and
nBA). The total concentration of the comonomers was
2 M, and the nBA to AA molar ratio was varied from 0
to 1 : 9. The polymerization was performed in an inert
atmosphere at 70°C for 2 h up to the limiting conversion.
After completion of the precipitation copolymerization,
the copolymers were filtered off, washed with DMSO,
and vacuum-dried to constant weight.
To determined the surface tension, we used the method
of capillary ascent; σ was calculated by the equation
[14]
log c [g l–1]
Fig. 1. Surface tension σ as a function of the logarithm of the
concentration c of aqueous solutions of AA–nBA copolymers
at 18°C. Content of nBA in the copolymer, mol %: (1) 0, (2)
5, (3) 10, and (4) 15.
σ = Δρgrh/2,
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SIVTSOV et al.
interactions inside macromolecular coil (e.g., hydrogen
bond network or hydrophobic interactions); degree
of ionization of ionogenic groups (which in aqueous
solutions depends on pH, presence of foreign electrolytes,
and nature of counterion for polymers in the salt form);
formation of intermolecular associates; molecular weight;
microstructure and flexibility of the polymeric chain [15,
16].
copolymerization (curves 2–4). It should be noted that
these copolymers were prepared at high conversions
and, owing to the difference in the copolymerization
constants (rBA = 1.08, rAA = 0.59 [81]), were nonuniform
in composition. However, their overall composition
corresponded to the composition of the starting monomer
mixture. It can be seen that, with an increase in the
content of hydrophobic nBA units in the copolymer in
the examined composition range (0–15 mol % nBA),
the surface activity of the copolymers increases. The
AA homopolymer also exhibits surface activity
(Fig. 1, curve 1) due to the presence of a hydrophobic
hydrocarbon fragment (–CH2–CH–) and a hydrophilic
carboxy group in each unit of the macromolecule, which
makes each unit diphilic and capable of adsorption on
the phase boundary. Similar concept of the mechanism
of macromolecule adsorption on the water–air boundary
was developed previously for PVA [19].
Common for low- and high-molecular-weight
surfactants is the dependence of the surface activity on
the contrast of the hydrophilic and hydrophobic fragments
of molecules: The more pronounced is their character
and the better is their separation in space, the stronger
the diphilic nature of molecules is manifested. Therefore,
AA–nBAcopolymers having pronounced diphilic nature
are effective surfactants [17].
Figure 1 shows the surface tension isotherms
of aqueous solutions of AA–nBA copolymers of
various compositions, synthesized by classical radical
In going from PAA homopolymer to AA–nBA
copolymers, with an increase in the content of nBA
units, the surface activity appreciably increases (Fig. 1,
curves 2–4). This trend may be due to a change in the
adsorption mechanism. The macromolecule can be
attached to the phase boundary through hydrophobic nBA
units acting as anchors of a sort. Apparently, an increase
in the concentration of nBA units in the macromolecule
should lead to an increase in the number of such contacts.
Adsorption by the mechanism characteristic of PVA is
possible for copolymers also, but its role is insignificant
for them.
(a)
Similar trends were observed for random copolymers
prepared by RAFT copolymerization: With an increase in
the fraction of hydrophobic units in the chain, the surface
tension at the same polymer concentration appreciably
decreased (Fig. 2a). This effect is manifested more
strongly than for the copolymers prepared by classical
radical copolymerization, which is probably due to
compositional uniformity of the RAFT copolymerization
products.
(b)
High performance of AA–nBA copolymers as
surfactants may also be due to changes in the density
of macromolecular coils in the adsorption layer
compared to the bulk of solution, caused by repacking
of macromolecules in the surface layer, characteristic
of adsorption on the phase boundary in dilute solutions
[19]. On introduction into PAA macromolecules of
nBA units linking the polymer to the phase boundary,
macromolecules are unrolled when reaching the surface,
log c [g l–1]
Fig. 2. Surface tension σ as a function of the logarithm of the
concentration c of aqueous solutions of AA–nBA copolymers
at 18°C. (a) Random and (b) block copolymers. (a) (1) R1 and
(2) R2; (b) (1) B1, (2) B2, (3) B3, and (4) B4.
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INFLUENCE OF THE MICROSTRUCTURE OF ACRYLIC ACID–n-BUTYL ACRYLATE COPOLYMERS 635
the coils become looser, and a loose adsorption layer
is formed. In this state, functional groups distributed
along the chain participate in the surface phenomena to
the greatest extent depending on fine structural features
of macromolecules. Beyond the adsorption layers,
macromolecules are in a denser conformation, equilibrium
under these conditions. This phenomenon is noticeably
manifested in the dependence of the surface activity of
AA–nBA copolymers of various microstructures on the
solution concentration.
via anchor groups, hydrophobic nBAunits. Separate nBA
units (under the experimental conditions, the content of
nBAunits does not exceed 10%) in the random copolymer
are surrounded by blocks consisting of several AA units
exhibiting high affinity for water and low tendency to
adsorption on the water–air boundary. Therefore, AA
blocks “entrain” anchor nBA units into the aqueous
phase, preventing their adsorption. In block copolymers,
each macromolecule contains two short hydrophobic
fragments consisting of 2–5 nBA units (Fig. 3) showing
pronounced tendency to adsorption. Therefore, in this
case the whole sequence (nBA)n can participate in the
adsorption. As a result, under identical conditions larger
number of nBAunits are adsorbed in the case of the block
structure of the polymer.
Thus, in adsorption of polymers on the phase boundary,
decisive effect on their surface activity is exerted by the
presence of “anchor” groups with which a macromolecule
can be linked to the surface and by the conformation of
macromolecular coils in the adsorption layer. At dense
conformation of coils (in concentrated solutions or in
a “bad” solvent), their internal structure affects the surface
activity insignificantly or does not affect it at all. In this
case, only a minor fraction of functional groups occurring
on the surface of a macromolecular coil participates in
surface phenomena and polymer adsorption on the phase
boundary. It is interesting that both factors are interrelated,
because the presence of “anchor” groups favors unrolling
of macromolecules in the adsorption layer.
The process under consideration should be influenced
by the rigidity of the polymer chain, because, if two
adjacent nBA units are within a fragment whose length
does not exceed the Kuhn segment length, their motion
is not independent. In adsorption of one of these units
on the phase boundary, the second unit with a high
probability will be located beyond the adsorption layer,
being surrounded by hydrophilicAAblocks (Fig. 3). The
next nBAunit located in another kinetically independent
fragment of the macromolecule will fulfill its anchor
function without hindrance.
To study manifestations of the surface activity of
random and block AA–nBA copolymers, we plotted
surface tension isotherms in a wide range of the polymer
concentrations, 0.001–1 wt % (Fig. 2).
Adsorption of a separate unit is reversible and is
characterized by a certain dynamic equilibrium specific
for the given conditions. Because of the large number
of anchor units, adsorption of the macromolecule as a
whole is irreversible. Such dynamics affects considerably
more strongly the state of random copolymers than the
state of block copolymers, because it is hardly probable
Asnotedabove, fortherandomcopolymerssynthesized
by RAFT copolymerization, the surface activity increases
with an increase in the content of hydrophobic units
(Fig. 2a). In going from random to block copolymers,
this trend is preserved (Fig. 2b). However, there are
also certain differences. Under equal other conditions
(overall composition of the polymer, molecular weight,
polydispersity), block copolymers exhibit higher surface
activity than do random copolymers (Fig. 2a, curves 1,
2; Fig. 2b, curves 1, 3), which is manifested in that the
same decrease in the surface tension is attained at lower
polymer concentrations. In addition, the difference in the
surface activity of random and block copolymers depends
on the content of anchor nBA units.
Air
l > lK
lK
Phase boundary
l < lK
Water
To elucidate how the microstructure of the copolymers
affects their surface activity, we should consider
adsorption of macromolecules on the phase boundary
from the viewpoint of flexibility of the polymer chain.
It was assumed previously that random copolymers are
adsorbed on the phase boundary surface by linking to it
Fig. 3. Adsorption of AA–nBA copolymer [(1) random and (2)
block] on the aqueous solution–air phase boundary. (lK) Kuhn
segment and (l) distance between nBA units. Chain segment
consisting of (1) nBA and (2) AA units.
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SIVTSOV et al.
that desorption of a separate nBA units in a block is
accompanied by removal of the nBA unit from the
effective adsorption layer, as this unit is retained by the
adjacent units. However, at random structure of the chain,
when an nBAunit is surrounded by sequences ofAAunits,
this process is more probable. This remark seems to be
significant, because at equal energy characteristics of the
adsorption–desorption processes they affect the state of
the effective adsorption layer differently for copolymers
of different microstructures.
This is apparently due to the fact that, with an increase in
the content of nBA in the copolymer, the solvent quality
gets worse, and the density of the polymer coil increases.
Therefore, the character of distribution of hydrophobic
units along the chain affects the surface activity of the
polymer to a lesser extent.
The results obtained for the block copolymers allow
one more important conclusion. Under the experimental
conditions, the macromolecules do not undergo
aggregation in which chain segments consisting of nBA
units would form an internal sphere of aggregates and
hydrophilic sequences of AA units would be oriented
toward solution, as it was observed in [21–23]. Otherwise
this phenomenon would lead to a drastic decrease in the
surface activity of the block copolymer relative to the
random copolymer. Actually the relationship is inverse
(Fig. 2). Furthermore, aggregation considerably expands
the limits of solubility ofAA–nBAcopolymers of different
compositions in water.According to [23], diblockAA–nBA
copolymers containing up to 50 mol % nBA units are
soluble in water. This considerably exceeds the solubility
limit of the copolymers studied in this work. The probable
cause of the observed differences in the behavior of
copolymers of the same monomer composition is
difference in their structure. Tendency to aggregation
is exhibited by diblock copolymers consisting of a long
sequence (90–100) of hydrophobic nBAunits and a block
of 100–300 AA units [21, 22].
This concept is in good agreement with data on
relative decrease of the surface tension by samples R1
and B4 having the same overall composition but different
chain structures. Table 2 shows how samples B4 and R1
decrease the surface tension relative to that of distilled
water σaq under the experimental conditions (73 mJ m–2).
The quantities δσ were defined as ratios of the difference
between the surface tensions of the polymer solution of
the given concentration and water to σaq.
At low polymer concentrations, the performance
of B4 is approximately two times that of R1. This is
consistent with the suggested mechanism and with the
apparent length of the Kuhn segment, measured in a good
solvent [20]. At lK ≈ 5 nm and monomeric unit length
of 0.259 nm, only a fragment consisting of 19 units is
kinetically independent, i.e., approximately every second
nBA unit experiences hindrance in fulfilling the anchor
function. With increasing concentration (≥0.05 g l–1),
this effect disappears, and the random and block samples
show virtually equal performance. This phenomenon
can be rationalized when considering the state of
macromolecules in the adsorption layer. When adsorbed on
the phase boundary from dilute solutions, macromolecules
are unrolled, taking a conformation more favorable for
manifestation of the effects of the microstructure. In more
concentrated solutions, macromolecules in the adsorption
layer occur in relatively compact conformation and are
closely packed, so that the chain microstructure cannot
affect to full extent their surface activity.
Figure 4 shows the pH dependences of the surface
tension of 1% solutions of random and block AA–nBA
copolymers. In the region of complete neutralization
of all carboxy groups, σ sharply increases. This may
be due to higher solubility of polyacrylic acid and its
copolymers in the salt form. Therefore, with the progress
of neutralization, macromolecules adsorbed on the phase
boundary leave the surface and pass into the solution,
excess of polymer molecules on the surface disappears,
and the surface tension on the polymer solution–air phase
boundary decreases [24, 25]. In the process, the random
and block copolymers behave differently.
The microstructure of polymer chain, in our case the
character of alternation of elementary units in polymer
macromolecules, affects the surface activity ofAA–nBA
copolymers. It is important that the difference between
the random and block copolymers in the surfactant
performance depends on the content of anchor nBA
groups. At a relatively high content of nBAunits (10 mol
% at the limit of solubility in water of about 15 mol %),
the effect of the microstructure is less pronounced than
at lower (6–8 mol %) content of nBAunits (Figs. 2a, 2b).
With samples of different microstructures, we can
observe certain interesting relationships consistent with
the above-suggested adsorption model for random and
block copolymers. The titration curves of 1% aqueous
solutions of random and block copolymers (Fig. 4) show
that, with the progress of ionization, random copoly-
mers sharply lose the surface activity and rapidly reach
a plateau corresponding to the surface tension of water
under the experimental conditions. An increase in pH
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INFLUENCE OF THE MICROSTRUCTURE OF ACRYLIC ACID–n-BUTYL ACRYLATE COPOLYMERS 637
shifts the equilibrium toward dissociation of carboxy
groups, which in this study exhibit higher affinity for
water and entrain the anchor nBA groups located on the
phase boundary. At pH ~6.8, for both examined random
samples, the macromolecules are fully detached from
the solution surface, and the surface tension is no longer
determined by their adsorption on the phase boundary.
The block copolymers lose the surface activity more
smoothly (Fig. 4, curves 3–6). The boundary between
the two portions of the titration curves is smeared, and
flattening-out at pH > 7 is observed at higher pH values.
In addition, for the block copolymers the curves flatten
out at the surface tension values that are considerably
lower than the surface tension of water. This means that,
in contrast to random copolymers, the block copolymers
do not fully lose the surface activity upon ionization. Even
a strongly ionized block of AA units cannot detach from
the surface the fully linked block of nBA units.
Fig. 4. pH dependence of the surface tensoin σ of 1 g l–1 aqueous
solutions of AA–nBA copolymers: (1) R1, (2) R2, (3) B1, (4)
B2, (5) B3, and (6) B4.
units in adsorption of random copolymers on the phase
boundary.
The difference between random and block copolymers
in the adsorption mechanism is of large applied
significance. The majority of ionic surfactants are pH-
dependent, which is also illustrated by the results of our
study. However, in many cases such dependence may be
extremely undesirable. For example, when surfactants are
used in cement mixes as rheological or foam-controlling
additives, their molecules get into a strongly alkaline
medium. Polycarboxylates (copolymers of acrylic or
methacrylic acid) are frequently used for this purpose. Our
results show that block copolymers can be recommended
for use under the conditions of strong ionization, as their
performance depends on pH more weakly.
(3) Ionization of carboxy groups of acrylic acid–n-
butyl acrylate copolymers leads to a decrease in their
surface activity. At pH > 7, the random copolymers
fully lose the surface activity, in contrast to the block
copolymers preserving the capability for adsorption
on the phase boundary. It is suggested to use the block
copolymers synthesized by the RAFT procedure as
surfactants whose performance weakly depends on pH
of the medium.
ACKNOWLEDGMENTS
The study was financially supported by the Russian
Foundation for Basic Research (project nos. 08-03-
00269 and 08-03-00445, RFBR–NWO project no. 06-
04-89402).
CONCLUSIONS
(1) Random copolymers of acrylic acid and n-butyl
acrylate, synthesized by the RAFT mechanism, exhibit
increased surface activity. The higher the content of
“anchor” n-butyl acrylate units, the greater the extent to
which the copolymers decrease the surface tension of
aqueous solutions.
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