Fluorescence probe studies on the complexation between poly(methacrylic acid) and poly(N, N-diethylacrylamide)

Article abstract of DOI:10.1016/j.saa.2004.06.018

The complexation between poly(methacrylic acid) (PMAA) and poly(N, N-diethylacrylamide) (PDEAM) in aqueous phase was studied by UV-vis and fluorescence probe techniques. It was demonstrated that the complexation of PMAA with PDEAM occurs within a pH range of 1-6.5 and along with the complexation, the conformation of PMAA changed from a hypercoiled to a loose coiled form. The complex ratio between the two polymers is 1:1 (PMAA:PDEAM, in monomer unit). Salt effect studies showed that the complexation occurred due to formation of hydrogen bonds between the two polymers. Based upon these conclusions and the "compact micelle-like structure" for PMAA at low pH, a "ladder" model was proposed for the structure of PMAA-PDEAM complex formed at low pH.

Full text of DOI:10.1016/j.saa.2004.06.018

Spectrochimica Acta Part A 61 (2005) 887–892
Fluorescence probe studies on the complexation between
poly(methacrylic acid) and poly(N, N-diethylacrylamide)
Shouxin Liua, Yu Fanga,∗, Gailing Gaoa, Mingzhu Liub, Daodao Hua
a
School of Chemistry and Materials Science, Shaanxi Normal University, Xi’an 710062, PRChina
b
Department of Chemistry, Lanzhou University, Lanzhou 730000, PRChina
Received 26 April 2004; accepted 14 June 2004
Abstract
The complexation between poly(methacrylic acid) (PMAA) and poly(N, N-diethylacrylamide) (PDEAM) in aqueous phase was studied
by UV–vis and fluorescence probe techniques. It was demonstrated that the complexation of PMAA with PDEAM occurs within a pH range
of 1–6.5 and along with the complexation, the conformation of PMAA changed from a hypercoiled to a loose coiled form. The complex
ratio between the two polymers is 1:1 (PMAA:PDEAM, in monomer unit). Salt effect studies showed that the complexation occurred due to
formation of hydrogen bonds between the two polymers. Based upon these conclusions and the “compact micelle-like structure” for PMAA
at low pH, a “ladder” model was proposed for the structure of PMAA–PDEAM complex formed at low pH.
© 2004 Elsevier B.V. All rights reserved.
Keywords: Poly(methacrylic acid); Poly(N; N-diethylacrylamide); Complexation; Conformation; Fluorescence probe
1. Introduction
ther mutually or independently in specific environments,
with particular emphasis on temperature and pH responsive
polymers that have been prepared by copolymerizing the
temperature-sensitive N, N-diethylacrylamide (DEAM) with
monomers containing base or carboxylic acid groups, such
as methacrylic acid (MAA), to give a pH-dependent LCST
[2,10].
Poly(methacrylic acid) (PMAA) is a pH sensitive poly-
mer. Unlike other polyacids, it adopts hypercoiled confor-
mation at low pH because of the hydrophobic interactions
introduced by the methyl groups along the polymer back-
bone. However, on addition of base to solution, the carboxyl
groups ionize and acquire negative charges. The increase in
Coulombic repulsive forces results in a non-uniform sudden
conformational transition from the hypercoiled to expanded
form. This conformational change is reversible [11,12]. It is
expected that the smart behavior of PMAA may be introduced
into a temperature sensitive polymer, such as PDEAM, forms
a copolymer, and this copolymer could be rendered double
sensitive properties to external temperature and pH stimulus.
This copolymer may form the basis of new intelligent films
and hydrogels.
The unique and novel properties of intelligent polymer
materials, which exhibit large property changes in response
to small changes in external conditions such as, for exam-
ple, temperature [1], pH [2], electric fields [3], chemicals
[4], offer an unlimited amount of potential applications rele-
vant to industry, environment and the biomedical field. These
applications include drug delivery systems [5], temperature-
sensitive coatings [6], smart catalysts [7], and pervious mem-
branes [8].
Poly(N, N-diethylacrylamide) (PDEAM) is a temperature-
sensitive polymer [9], that exhibits a well-defined lower crit-
ical solution temperature (LCST) in water around 30 ◦C.
It is known that the phase transition and accompanying
polymer conformation changes result from a delicate bal-
ance between the hydrophobic interaction and hydrogen
bonding. Recently, there has been considerable interest
in the use of materials that respond to two stimulus, ei-
∗
Corresponding author. Tel.: +86 2985307534; fax: +86 29854307025.
E-mail address: yfang@snnu.edu.cn (Y. Fang).
1386-1425/$ – see front matter © 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.saa.2004.06.018

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S. Liu et al. / Spectrochimica Acta Part A 61 (2005) 887–892
At present, we have synthesized a series of temperature
2.2.2. Preparation of DEAM
and pH sensitive DEAM–MAA random copolymers by free
radical copolymerization techniques, and determined their
temperature and pH double sensitive properties by trans-
mittance measurements, respectively. In order to investigate
further the natures of copolymer occurring phase transi-
tion and copolymer temperature and pH double sensibili-
ties properties, we decided to investigate the interactions be-
tween PMAA and PDEAM. Fluorescence techniques, such
as non-radiative energy transfer, fluorescence lifetime, flu-
orescence anisotropy measurements and fluorescence probe
studies have been used widely to investigate interpolymer
complexation [13,14], and segmental mobility and confor-
mational behavior of polymers. The advantage of fluores-
cence techniques is that information about the behavior of
the polymers on the molecular level can be obtained, as op-
posed to the bulk properties determined by non-spectroscopic
techniques.
A solution of 46.7 mL acryloyl chloride dissolved in
30 mL dichloromethane was gradually added to another solu-
tion of 208 mL diethylamine previously dissolved in 450 mL
dichloromethane at 0 ◦C under N2 atmosphere. The reaction
mixture was stirred for 4 h at 0 ◦C. The precipitated salt was
removed by filtration and washed with double distilled water
to remove traces of the filtered solution. After drying over
magnesium sulfate, the solvent was removed under reduced
pressure. The crude product was distilled in the presence of
hydroquinone at 85–88 ◦C under vacuum at 68 mmHg, to
1
yield a colorless liquid product. H NMR (CDCl3, δ ppm):
6.5 (1H, CH ), 6.3 (1H, CH2 ), 5.6 (1H, CH2 ), 3.4 (4H,
CH2 ), 1.1 (6H, CH3).
2.2.3. Preparation of PDEAM
PDEAM was prepared by free radical polymerization,
according to literature procedures [9]. A solution of 1.27 g
(0.01 mol) DEAM dissolved in 1.5 mL methanol was stirred
with 8 mg (0.0488 mmol) AIBN under N2 at 62 ◦C. Stirring
was discontinued after 30 min. Heating was continued for
6 h. The polymer was cooled to room temperature and then
dissolved in 5 mL acetone and precipitated from 80 mL hex-
ane. The polymer was purified by multiple dissolutions (×3)
in acetone, followed by precipitation into hexane, and then
dried at room temperature under vacuum.
In this paper, we describe these preliminary investiga-
tion, using UV–vis and fluorescence probe techniques to
study the complexation between PMAA and PDEAM, the
effects of complexation upon the hypercoiled conformation
of PMAA and the nature of the complexation between the two
polymers.
2. Experimental
2.2.4. Preparation of PMAA
2.1. Materials
PMAA was prepared by free radical polymerization using
AIBN as initiator in benzene. Polymerization was terminated
at less than 10% conversion. The polymer was purified by
multiple dissolutions (×5) in methanol, followed by precip-
itation into diethyl ether.
Pyrene (Py, Aldrich-96%) was purified by recrystalliza-
tion from ethanol and then extracted with ethanol in a Soxh-
let’s extractor. Diethylamine, dichloromethane, magnesium
sulfate, sodium hydroxide, acetone, methanol, diethyl ether,
and hexane were used as received (analytical grade). 2,2ꢀ-
Azobisisobutyronitrile (AIBN) was purified by recrystalliza-
tion from ethanol. MAA was distilled under vacuum before
use. Double distilled water was used throughout. The pH of
the solution was adjusted using 0.5 M NaOH solution and/or
0.5 M HCl solution.
2.3. Polymer characterization
Molecular weights were determined by laser light scat-
tering (LLS) techniques (Brookhaven BI-2000SM, USA)
using methanol as solvent. The weight-average molecular
weight [Mw/(g mol−1)] of PMAA and PDEAM are 7.5 × 105
and 1.24 × 105, respectively. 1H NMR measurements were
recorded on a NMR spectrometer (BRUKER AM-400, Bil-
lerica, MA). Transmittances of the solutions were determined
on a UV–vis spectrophotometer (Shimadzu UV-240, Kyoto,
Japan) and pH measurements were conducted on a PHS-1
acidimeter. All fluorescence measurements were conducted
on a Perkin-Elmer LS-50B luminescence spectrometer.
2.2. Synthesis
2.2.1. Preparation of acryloyl chloride
The preparation of acryloyl chloride is according to
literature procedures [15]. A mixture of 70 g (0.97 mol)
acrylic acid, 234 g (1.66 mol) benzoyl chloride, and 0.5 g
(0.0045 mol) hydroquinone was distilled at a fairly rapid rate
through an efficient 25 cm distilling column. The distillate
was collected in a receiver containing 0.5 g (0.0045 mol) hy-
droquinone, immersed in ice. When the temperature at the
top of the column, which remained between 60 and 70 ◦C
for most of the distillation, had reached 85 ◦C, the distil-
lation was discontinued. The crude product was redistilled
through the same column and the fraction boiling at 72–74 ◦C
at 740 mmHg was collected.
2.4. Sample preparation
Polymer solutions were prepared from their stock solu-
tions. The concentrations of the stock solutions for PMAA
and PDEAM are all 0.1 wt.%. It is to be noted that the pH of
the stock solution of the polyacid was about 10, where that
of PDEAM was around 7.

S. Liu et al. / Spectrochimica Acta Part A 61 (2005) 887–892
889
For the experiments involving dissolution of organic probe
molecules into the water-soluble polymer solutions, the
probe, Py, was initially dissolved in diethyl ether to obtain
a stock solution of known concentration (ca. 10−3 mol L−1).
This solution was diluted to 10−5 mol L−1 just before use.
One milliliter of the probe solution (10−5 mol L−1) was in-
jected into a 10 mL volumetric flask. The ether was evapo-
rated at room temperature. Subsequently, a polymer solution
of known pH (10−3 wt.%) was added to the flask. To ensure
solubilization and equilibration, the polymer/probe solution
was sonicated for 20 min, and then left at room temperature
for more than 12 h.
For complexation measurements, all samples of different
PMAA to PDEAM ratios (xMAA = nMAA/(nMAA + nDEAM):
0; 0.1; 0.2; 0.3; 0.4; 0.5; 0.6; 0.7; 0.8; 0.9; 1.0) were prepared
in a similar manner. The method is described by using an
example, that is the preparation of a solution containing 2.8
× 10−6 mol% of PMAA and 2.8 × 10−6 mol% of PDEAM
(xMAA = 0.5, in residue unit, pH 4.0). To make this solu-
tion, 0.1 mL of PMAA stock solution (2.8 × 10−4 mol%)
and 0.1 mL of PDEAM stock solution (2.8 × 10−4 mol%)
were added, respectively, to a 10 mL volumetric flask with
shaking. The mixture was diluted to about 9 mL and its pH
was adjusted to 4.0 using 0.1 mol L−1 HCl solution and/or
NaOH solution. The solution obtained in this way was di-
luted to 10 mL with double distilled water before using.
Fig. 1. Plots of transmittance (%) against pH in PMAA, PDEAM, and
PMAA–PDEAM aqueous solutions (0.1 wt.%, for PMAA, PDEAM, or
PMAA–PDEAM).
to Fig. 1, it reveals that the transmittance of PMAA solution is
almost pH independent. This result can be understood by con-
sidering the fact that the hydrophobic interaction introduced
bythemethylgroupsalongthepolymerbackboneatlowerpH
will dominate PMAA conformational behavior and PMAA
will form many hydrophobic microdomains under this hy-
drophobic interaction at pH lower 6.5. The hydrophobic in-
teraction makes PMAA to adopt hypercoiled conformation at
lower pH and not to form intermolecular association through
hydrogen bonding. So, the transmittances of PMAA solution
are pH independent. For PDEAM solution, its transmittance
is also pH independent, indicating that the polymer may adopt
relatively open coil conformation within the wide pH range
studied. It is easy to understand that the PDEAM is a temper-
ature sensitive polymer and has no base or carboxylic acid
groups along the polymer backbone, so, the intermolecular
association of PDEAM solution through hydrogen bonding at
lower pH does not occur. But, compared with transmittances
of PMAA, PDEAM solutions, with reference to Fig. 1, there
is a transition in transmittances of PMAA–PDEAM complex
system solution at pH lower than 6.5. Clearly, the decrease
in transmittances of PMAA–PDEAM complex system solu-
tion can be attributed to the complexation of PMAA with
PDEAM. Compared the transmittances of PMAA–PDEAM
complex system with the PMAA system or the PDEAM sys-
tem at pH higher 6.5, their transmittances have no much dif-
ferent, indicating that there is no complexation between the
two polymers in this pH range. This result can be understood
by considering the fact that at pH higher 7 [12], PMAA ex-
ists as polyanions, and the negatively charged structure is
unfavorable for the H-bonding formation and for the com-
plexation between the two polymers. At pH lower than 6.5,
the carboxylic acid groups along the PMAA backbone exists
as polyacids partially, and the lower the solution pH is, the
more the content of the non-ionized carboxylic acid groups
along the PMAA backbone is. For PMAA, the non-charged
structure is favorable for the H-bonding formation and
2.5. Analytical methods
Because complexation between PMAA and PDEAM
will change solution transmittance, the determination of the
complexation is focused on measuring transmittances for
PMAA–PDEAM complex system. Otherwise, it is reported
that a significant change in conformational behavior of poly-
mers occurs upon complexation [13,14]. Therefore, fluores-
cence probe measurements should find increasing use in this
aspect.
3. Results and discussion
3.1. UV–vis spectrophotometric studies
Concentration of the polymer solution used for the de-
termination of complexation was 0.1 wt.%. Because of its
sensitivity to the changes in the turbidity of the solution [9],
550 nm was selected as analyzing wavelength. The sample
solution was put in a sample holder and double distilled wa-
ter was adopted as reference for the measurement.
The complexing behavior of the polymer solutions was de-
termined by studying the turbidity of PMAA, PDEAM, and
PDEAM/PMAA complex system solutions over a wide pH
range, from pH 3.5 to 10. Fig. 1 depicts that the transmittances
for PMAA, PDEAM, and PMAA–PDEAM (1:1, in residue
unit) solutions, respectively, at a polymer concentration of
0.1 wt.%, were measured as a function of pH. With reference

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S. Liu et al. / Spectrochimica Acta Part A 61 (2005) 887–892
complexation between PMAA and PDEAM. In order to fur-
ther confirm the complexation between PMAA and PDEAM
described above, some fluorescence probe solubilization ex-
periments were conducted.
3.2. Probe studies
It is to be expected that the tightly coiled conformation of
PMAA might favor solubilization of organic guests into its
hydrophobic microdomains. Therefore, fluorescence probe
studies should be useful in investigating the effects of com-
plexation upon the conformational behavior of PMAA. The
probe used in the current study is Py. Py was used because
the fine structure of its fluorescence emission spectrum, espe-
cially the I3 (383 nm)/I1 (373 nm) value is highly sensitive to
the changes in the polarity of its microenvironment [16,17].
The larger I3/I1 value indicates a more hydrophobic envi-
ronment. This property has been widely used to monitor the
conformational behavior of watersoluble polymers in aque-
ous phase [18].
Considering the effect of complexation upon conforma-
tional behavior of PMAA, the determination of the complex-
ation between PMAA and PDEAM is focused on the effect
of complexation upon the polarity of Py microenvironment,
namely the I3/I1 values for Py. Fig. 2, the fluorescence emis-
sion spectra of Py (10−6 mol L−1) dispersed in PMAA (2.8
× 10−5 mol%, in reside unit) and PMAA–PDEAM (1:1, 2.8
× 10−5 mol%, in reside unit) aqueous solution at pH 4.0 re-
spectively, shows the effect of complexation upon the fine
structures of the fluorescence emission spectra of Py and it
reveals that complexation between PMAA and PDEAM was
accompanied by PMAA conformational change as proved by
the decrease in the hydrophobicity of the environment of the
probe. The I3/I1 values for Py (10−6 mol L−1) solubilized in
PMAA (2.8 × 10−5 mol%, in reside unit), PDEAM (2.8 ×
10−5 mol%, in reside unit), and PMAA–PDEAM (1:1, 2.8
× 10−5 mol%, in reside unit) complex system aqueous solu-
tions, respectively, measured as a function of pH. The results
Fig. 3. Plots of I3/I1 values for Py against pH in PMAA, PDEAM,
and PMAA–PDEAM aqueous solutions (2.8 × 10−5 mol%, for PMAA,
PDEAM, or PMAA–PDEAM, and 10−6 mol L−1, for Py).
are shown in Fig. 3. The Fig. 3 reveals that, for the PMAA/Py
system, there is a transition in its I3/I1 values between pH
5 and 7, which corresponds to the PMAA conformational
transition from hypercoiled to extended coil structure. The
larger I3/I1 values for Py at lower pH are likely to be a con-
sequence of PMAA hypercoiled structure. In contrast, the
I3/I1values for PMAA/Py at higher pH should be smaller.
However, for the PDEAM/Py system, the I3/I1 values for Py
(with reference to Fig. 3) are significantly lower than that
of the corresponding PMAA/Py system at pH lower 6.5 and
is pH almost independent, indicating that the PDEAM may
adopt relatively open coil conformation within the wide pH
range studied. For the PMAA–PDEAM/Py complex system,
the I3/I1 value at pH lower than 6.5 is also much lower than
that of the corresponding PMAA/Py system. Clearly, for the
PMAA–PDEAM/Py complex system, the decrease in I3/I1
value may be attributed to the complexation of PMAA with
PDEAM at pH lower than 6.5. At pH higher than 6.5, there
is no complexation between the two polymers, because at
pH higher than 6.5, PMAA exists as polyanions and the neg-
atively charged structure is unfavorable for the H-bonding
formation between the two polymers and unfavorable for the
complexation between the two polymers.
Fig. 4 presents results for the complex system at differ-
ent PMAA to PDEAM ratios xMAA (xMAA = nMAA/(nMAA
+ nDEAM)) at pH 4.0. With reference to the figure, it reveals
that when the xMAA is lower than 0.5, the I3/I1 value for
Py in the complex system basically maintains a lower con-
stant, ca. 0.7488 (xMAA = 0) and ca. 0.7568 (xMAA = 0.5)
and when the xMAA is higher than 0.5, the I3/I1 values for
Py gradually increases with further increasing xMAA. Obvi-
ously, this is a result of complexation between PMAA and
PDEAM. When xMAA is lower than 0.5, the I3/I1 values for
Py are smaller because the complexation between PMAA
and PDEAM destroyed hydrophobic microdomains within
PMAA. When the xMAA is higher than 0.5, the I3/I1 values
Fig. 2. Fluorescence emission spectra of Py (10−6 mol L−1) dispersed in
PMAA (2.8 × 10−5 mol%, in reside unit) and PMAA–PDEAM (1:1, 2.8 ×
10−5 mol%, in reside unit) aqueous solution at pH 4.0, respectively.

S. Liu et al. / Spectrochimica Acta Part A 61 (2005) 887–892
891
Fig. 4. Plots of I3/I1 values for Py against xMAA in PMAA–PDEAM (1:1,
2.8 × 10−5 mol%, in residue unit, for either PMAA or PDEAM) aqueous
solutions at pH 4.0.
Fig. 5. Plots of I3/I1 values for Py probe against NaCl concentration in
PMAA–PDEAM (1:1, 2.8 × 10−5 mol%, in residue unit, for either PMAA
or PDEAM) aqueous solutions at pH 4.0.
for PMAA–PDEAM/Py complex system begin to increase
progressively from 0.7568 to 1.0345 with increasing xMAA
from 0.5 to1.0, indicating hydrophobic microdomains of
PMAA gradually form because of excessive PMAA. Above
results indicates that the complexation occurs most effec-
tively at xMAA about 0.5, namely at a ratio of about 1:1
(PMAA:PDEAM, in residue unit). This result can be un-
derstood by considering the fact that PDEAM is a typical
H-bond acceptor and PMAA a H-bond donor because of the
amide group in DEAM and carboxyl group in MAA. Each
residue unit in PDEAM or in PMAA only has one functional
group. Because all of the interaction sites are equally ac-
cessible and can participate in the complexation, the ideal
complexation is complete at stoichiometric ratio of about 1:1
(PMAA:PDEAM, in residue unit), namely xMAA 0.5. When
the xMAA exceeds 0.5, because of excessive PMAA, the hy-
drophobic microdomains of complex system gradually form,
and the I3/I1 values for Py also progressively increase with
increasing PMAA content. The higher the PMAA concentra-
tion is in complex system at pH 4.0, the more the hydrophobic
microdomains are, and the greater the I3/I1 values for Py are
also.
Fig. 6. Schematic diagram of the model for the structure of PMAA–
PDEAM.
tion, which was commonly found in other polyelectrolyte
complex systems [14,19], but most likely hydrogen-bonds
formation.
Considering the “compact micelle-like structure” for
PMAA at low pH [20] and the experimental results described
above, it might be reasonable to propose a “ladder model” for
the structure of the PMAA–PDEAM complex (ref. Fig. 6). In
the model, it was suppose that the two polymer chains would
be connected by hydrogen bonding. With this model it should
have no difficult to explain all the results described above.
3.3. Nature of complexation
In order to gain further understanding of the nature of the
cpmplexation between PMAA and PDEAM, an experiment
was undertaken to study the effect of NaCl on the interac-
tion. Fig. 5 depicts the I3/I1 values for PMAA–PDEAM/Py
complex system at pH 4.0 as a function of NaCl concen-
tration. Inspection of the figure reveals that the I3/I1 val-
ues for the complex system do not change very much with
increasing NaCl concentration. Furthermore, the I3/I1 val-
ues for PMAA–PDEAM/Py complex system in NaCl so-
lution at pH 4.0 are also significantly lower than that for
PMAA/Py system. Therefore, the nature of the complexation
between PMAA and PDEAM may be not electrostatic attrac-
4. Conclusion
UV–vis and fluorescence probe studies show that inter-
polymer complexation between PMAA and PDEAM is both
pH and molar ratio dependent and a major conformational
change occurs when PMAA is mixed with PDEAM in aque-
ous phase at pH lower than 6.5. The conformational change
of PMAA from hypercoiled to loose coiled form is evidenced
by the decrease in the hydrophonic microdomain size or do-
main number. At pH 4.0, the complexation occurs most ef-
ficiently at xMAA 0.5, namely at a molar ratio of about 1:1
(PMAA:PDEAM, in residue unit), suggesting that almost all

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S. Liu et al. / Spectrochimica Acta Part A 61 (2005) 887–892
of the segments of PMAA have taken part in the complex-
ation. Introduction of NaCl has little effect upon complexa-
tion between PMAA and PDEAM showing that the nature of
the complexation is hydrogen bonding and non-Coulombic
interaction. Based upon these conclusions and the “com-
pact micelle-like structure” for PMAA at low pH, a “ladder
model” was proposed for the structure of PMAA–PDEAM
complex formed at low pH.
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