Universidade de São Paulo
Biblioteca Digital da Produção Intelectual - BDPI
Departamento de Bioquímica - IQ/QBQ
Artigos e Materiais de Revistas Científicas - IQ/QBQ
2008
Poly(aniline)/poly(methylmethacrylate) films
obtained from waterborne latex nanoparticles
E-POLYMERS, 2008
http://producao.usp.br/handle/BDPI/16747
Downloaded from: Biblioteca Digital da Produção Intelectual - BDPI, Universidade de São Paulo
http://www.e-polymers.org
e-Polymers 2008, no. 149
ISSN 1618-7229
Poly(aniline)/poly(methylmethacrylate) films obtained from
waterborne latex nanoparticles
Luciana Oliveira Melo,1 Eduardo A. Ponzio,1,2 Ana M. Carmona-Ribeiro,1 Roberto M.
Torresi1,*
1*
Instituto de Química, Universidade de São Paulo, CP 26077, 05513-970 São Paulo,
Brazil; fax: +55 11 38155579; e-mail: [email protected]
2
Departamento de Físico-Química – Universidade Federal Fluminense, Campus
Valonguinho, 24.020-150 – Niterói (RJ) - Brazil
(Received: 8 May, 2008; published: 10 November, 2008)
Abstract: This paper presents the characterization of poly(aniline) (PANI) and
poly(methyl methacrylate) (PMMA) coatings obtained by mixing PANI with PMMA
aqueous dispersions (latex particles). These dispersions were characterized by
using dynamic light scattering for sizing, zeta-potential analysis and thermal
analysis. PMMA and PANI/PMMA dispersions show negative charged particles
with zeta potential greater than |40| mV, a zeta-average diameter of 64 nm for pure
PMMA and a bi-modal particle-size distribution centered at 45 and 120 nm for a
mixture with 25% w/w of PANI. Films obtained by casting were characterized by
using scanning electron microscopy and they show a conductivity increase upon
PANI content reaching a value of 1 mS cm-1 for a film with 25% w/w of PANI. In
addition, Raman spectroscopy have shown the presence of the conducting form of
PANI in the films and cyclic voltammetry experiments corroborated that they are
electroactive in both acid and neutral solutions.
Introduction
Intrinsic conducting polymers (ICP) have potential applications, like corrosionprevention coatings, light-emitting devices, antistatic materials, etc. These
applications are related to their chemical and electrochemical properties [1-5]. The
processability of ICP like polyaniline (PANI) or polypyrrole (PPy) is related to their
solubility in organic solvents such as m-cresol, chloroform, or xylene [6-9]. These
solvents are pollutants and, considering the increasing environmental concern, the
search for friendly systems regarding the environment becomes inevitable [10, 11].
Therefore, much effort has been dedicated to obtaining water-soluble ICPs. In the
case of PANI, different approaches were used such as the incorporation of
hydrophilic units to aromatic rings or on the nitrogen sites [12-17]. Also, other
strategies, for example, the formation of PANI-blends with water-soluble polymeric
acids [18], doping PANI with protonic acids carrying long hydrophilic tail [19],
controlling the pH of the solution [20], by using β-cyclodextrin [21] or modifying
polyaniline with orthosilicates [22] were also used. However, when coating
applications were considered, some properties in some cases were not suitable, i.e.,
films became disrupted when soaked in water displaying reduced conductivity. To
solve this problem, new strategies such as the confinement of the conducting chains
into networks of organic polymers were proposed. As a matter of fact, the
improvement towards water stability, mechanical properties and good electrical
conductivity need to be pursuit.
1
Therefore, the direct use of PANI dispersions as one of the component of blends and
composite materials is one of the most attractive alternatives to overcome the
problems mentioned before. The synthesis of PANI nanoparticles in colloidal form by
using water and water-soluble polymers such as poly(vinyl alcohol) (PVA), poly(Nvinylpyrrolidone), poly(vinyl methylether), poly(ethylene oxide) (PEO) or cellulose
ethers has been reported [23-25]. However, it is possible to obtain a macroscopic
precipitation during polymerization due to particles agglomeration; so that, it is not
easy to synthesize polymer nanoparticles by using these methods. A more efficient
way of synthesizing PANI as waterborne coatings is to use micro-emulsion
polymerization containing a large amount of nanoscopic micelles. They can change
the local properties acting as nanoreactors giving a specific environment to the
monomer and yielding polymer nanospheres with improved properties. The use of
waterborne latex with PANI dispersions allows obtaining films with good mechanical
and electrical properties and also improves its processing capability as well as
reduces environmental impact [26-30]. Moreover; the particle size can be controlled;
so that, the properties of the coating can be tuned over the specifications of the
applications [31]. Another way to improve film properties is to synthesize blends by
using waterborne PANI dispersions together with organic polymers dispersions as
polyurethane, poly(methylmethacrylate) (PMMA) or polystyrene [32-34].
Waterborne latexes of PANI doped with dodecylbenzenesulfonic acid (DBSA),
poly(acrylic acid) (PAAC), poly(methylmethacrylate) (PMMA) and dispersions formed
by the two latexes were prepared by microemulsion polymerization, where DBSA and
PAAC were used as both surfactants and dopants. The size and shape of PANI,
PMMA and mixtures of latex of PANI and PMMA were studied in detail, showing
particle sizes of few nanometers and negative zeta potential. Waterborne latex
nanoparticles were directly cast onto metallic substrates obtaining homogeneous
films by solvent evaporation. The electrochemical properties and electrical
conductivity of the films were characterized and possible applications of these films
are discussed, mainly considering their use as corrosion protection coatings.
The original contribution of this paper is to point out the formation of stable dispersion
of acrylic PANI in the presence of corrosion inhibitors such as PAAC and DBSA,
leading by casting a homogenous coating with potential applications in corrosion
protection. It is also important to remark that the presence of inhibitors is a crucial
task considering that it is a water-dispersion; so, a mechanism of corrosion inhibition
must act during water evaporation to form the coating.
Results and discussion
Aqueous dispersions
A PANI–DBSA-PAAC aqueous dispersion was obtained after the polymerization
process. The aniline:DBSA molar ratio into the reactor was 1:1, and as it is very well
known that the doping degree of PANI is approximately 33% [15, 35, 36]; at the end
of the polymerization process there is an excess of DBSA in the reactor. Therefore
the formation of micellar structures could take place and this fact leads to the
formation of nano or microstructures. The hydrophobic tails of free or bonded DBSA
molecules are arranged in a way that they all turn to each other or towards PANI
structures, while the hydrophilic groups of DBSA turn to the aqueous phase. In this
way, PANI structures can be stabilized in same degree due to electrostatic repulsion
between negative charges of the DBSA hydrophilic heads [30, 37]. It is also
2
important to mention that all carboxylic groups along the PAAC chain turn to the
aqueous medium forming hydrogen bonds with water which can also contribute to
stabilize the dispersion [38, 39]. However, in this paper it was not possible to
measure Zeta potentials and size of PANI–DBSA-PAAC structures by using light
scattering experiments due to the fact that almost the total precipitation of the PANI
colloid occurs after 30 minutes showing that repulsion forces are not enough to give
long-term stability to these dispersions.
In order to obtain information about morphology of the structures formed in these
dispersions, some FEG-SEM micrographs were obtained. Figure 1 shows the
micrograph of PANI structures obtained by casting the dilute dispersion. It is possible
to see fibres which could be formed by a template guided mechanism associated
with the presence of PAAC in the dispersion [40] showing a diameter of about 100
nm and several micrometers in length.
Fig. 1. FEG-SEM (Field emission gun scanning electron microscope) micrograph of
PANI–DBSA-PAAC fibres.
An important result of the present study is that upon mixing PANI–DBSA-PAAC and
PMMA dispersions, PANI fibres disintegrate; so, it was possible to perform zeta
potential and size distribution measurements. Figure 2 presents data of size
distributions for dispersions of pure PMMA and of different mass percentage of
PANI–DBSA-PAAC. It is clear that, in the mixture of both dispersions, a bi-modal size
distribution appears and there is an increase in the particle size with the increase of
PANI–DBSA-PAAC proportion from 64 nm for pure PMMA dispersions to 100 nm for
25% w/w PANI–DBSA-PAAC ones. Figure 3 shows the values of zeta potential
obtained for both PMMA particles and mixtures with different mass percentages of
PANI–DBSA-PAAC. All zeta potential values reported here are negative which
means that the particle surfaces are negatively charged. Since these values are
more negative than -25 mV, they indicate that the dispersions are stable due to
electrostatic repulsion between particles of the same charge. As mentioned before,
3
DBSA hydrophilic heads are responsible for this stability and, due to this fact, when
PANI–DBSA-PAAC proportion is higher zeta potential values are more negative. It
can be also observed that for PMMA dispersion (0% PANI) ξ was also negative due
to the presence of the sulphate groups formed during the MMA polymerization [41].
Fig. 2. Particle size distributions for PMMA latex particles and various dispersions
with different mass percentages of PANI-DBSA-PAAC as indicated in the figure.
An important conclusion from these experiments is that, when both dispersions are
mixed, PANI–DBSA-PAAC fibres are broken which could be explained by the
presence of the non-ionic surfactant Triton-X. The particles of PMMA are mainly
stabilized by the negative charge of sulphates and the concentration of Triton-X is
several times above the cmc; so, the latex particles are formed inside the micelles of
4
this surfactant and it is possible to conclude that Triton-X is present at the surface of
PMMA latex. So, as the repulsion forces of non-ionic surfactant are weak, these latex
particles can enter in PANI fibres agglomerates and destroy them leading to smaller
particles. As a result, a homogeneous PANI-PMMA dispersion is formed.
Fig. 3. Zeta Potential of PMMA particles and dispersions with different mass
percentages of PANI–DBSA-PAAC.
Fig. 4. FEG-SEM micrographs of films obtained from different mass percentages
dispersions of PANI–DBSA-PAAC: (a) 3%, (b) 8%, (c) 16%, (d) 25%, and (e)
micrograph of PMMA film obtained from a pure PMMA dispersion.
5
Polymeric films obtained from the aqueous dispersions
Figure 4 (a-d) shows FEG-SEM micrographs of various films obtained by casting of
the dispersions. They showed different morphology when compared with PANI–
DBSA-PAAC and PMMA films obtained from the pure dispersions. PANI structures
have presented long fibres conformation (Fig. 1) because the influence of PAAC that,
as it was already mentioned, determines the morphology [40]. On the other hand,
PMMA particles in Figure 4e appear spherical because they were formed inside
surfactant micelles. PMMA particles show a diameter of approximately ~170nm
which is higher than 64 nm obtained by dynamic light scattering experiments (Fig. 2).
The coalescence of PMMA particles obtained after casting and drying at 60 oC for
scanning electronic microscopy experiment can explain these higher diameter
values.
The micrographs of different PANI-DBSA-PAAC proportions films (Figs. 4a-4d)
clearly show that there is no segregation between both polymers and a globular
morphology is obtained with diameters between 50-100 nm.
Fig. 5. Photographs of free-standing films obtained from different mass percentages
dispersions of PANI–DBSA-PAAC: (a) 3%, (b) 8%, and (c) 16%.
Figure 5 shows free-standing films obtained with three different dispersions, 3, 8 and
16 % in mass of PANI-DBSA-PAAC. These films were obtained by casting the
dispersions on a Teflon® slide and, after solvent evaporation; films were removed by
heating the slide. It is clearly observed that the films are green colored and that the
color intensity depends on the proportion of PANI-DBSA-PAAC. Moreover, color
intensity is uniform showing that there is no phase segregation between PANI and
PMMA at macro-scale. In order to demonstrate if there is the formation of a blend,
thermogravimetric experiments were carried out.
Figure 6 shows the thermal behaviour for different percentage of PANI-DBSA-PAAC
films. The degradation of doping agents is represented as an endothermic peak at
200 oC in the DSC curves, clearly seen in Fig. 6d. Another endothermic peak at 370
o
C is related to PMMA particles degradation. This peak is accompanied by practically
100% of mass loss in the TG curves which indicates film degradation meaning that,
at this temperature, both PANI and PMMA suffer entire degradation. Although no
6
separate events are observed for PMMA and PANI, it is hard to say that during film
formation a blend is obtained because of two reasons: first, as it can be seen in Fig.
6, there is no clear change in the baseline for the DSC curves, so that; it was not
possible to accurately determine the Tg of PANI/PMMA films; second, the Tg of PANI
and PMMA were 59 and 64 oC, respectively, showing a ΔT of 5 oC, too small to
conclude if it formed a miscible, partially miscible or immiscible blend [42]. Both Tg
are in agreement with data already published [43-45]. This discussion about blend
formation will be considered later together with conductivity results.
Fig. 6 - DSC (full line) and TG (open circle) curves for the films with different mass
percentages of PANI-DBSA-PAAC. (a) 3, (b) 8, (c) 16 and (d) 25.
The temperature used for drying PANI–DBSA-PAAC /PMMA films was 100 oC
because there are no events at this temperature in the DSC/TG diagrams showed in
Fig. 6; so that, it is assumed that this thermal treatment do not cause any problem to
the film’s chemical properties. Assuming that the Tg for the PANI/PMMA films are
between 59 and 64 oC, the temperature of 100 oC allowed the film formation for the
blends because the drying temperature is higher than the Tg. Actually, PMMA is a
7
matrix for PANI–DBSA-PAAC film formation, which gives good mechanical properties
to the films [46].
Conductivity values of various films are shown in Figure 7. Conductivity increases
with PANI–DBSA-PAAC content, without a clear percolative limit until 25 % w/w of
PANI–DBSA-PAAC. These values are similar to semi-conducting materials, reaching
a maximum at around 2 mS cm-1 for the film with 25% w/w of PANI–DBSA-PAAC.
Films obtained from water dispersions have shown lower conductivity than blends
obtained in organic media. As an example, PANI blends doped with
camphorsulphonic acid in m-cresol presented conductivities around 200 S cm-1 [47].
These blends usually show higher conductivities for low concentrations of the
conducting polymer. This fact is related to their unusual conformation formed by a
interpenetrate fibril network, where PANI forms a conducting path [48]. In the present
case, it is clear that the formation of conducting paths achieved after the
incorporation of a low amount of PANI to the blend does not take place and a certain
degree of segregation could explain the fact that conductivity increases with the PANI
content not showing the typical shape associated with the percolation theory [49]. So,
considering the DSC results (Fig. 6) and the fact that conductivity increases with
PANI content not showing a percolation limit, it is clear that a fully miscible blend is
not obtained, forming a partially miscible or immiscible one.
Fig. 7. Conductivity vs. PANI–DBSA-PAAC content of the various blend films (full line
is only a guide for the eyes).
Figure 8 shows Raman spectra for pure PANI–DBSA-PAAC and for various blend
films with different percentage of PANI. All spectra are similar because they show
predominantly PANI bands because of the resonant Raman effect [8, 9]. The band at
1494 cm-1 corresponds to C=N stretching mode and the band at 1173 cm-1 is
ascribed to C-H deformations. The ratio between the intensities of the bands
corresponding to C-N●+ (1345 cm-1) and to the aromatic –C-C– (1600 cm-1) stretching
is around 1. It indicates that the bands shown in the Raman spectra are ascribed to
the emeraldine salt, the conducting form of PANI. These results show that
emeraldine is present in the films and they are in good agreement with conductivity
results shown before.
8
Figure 9 shows the potentiodynamic j/E profiles obtained with films of 10% w/w
PANI–DBSA-PAAC on Au electrodes; both experiment were run with the same
electrode in order to assure the same amount of mass. The results indicate that the
films presented redox activity in either acid (1.0 mol L-1 H2SO4) or neutral (0.1 mol L-1
NaCl) electrolytic solutions. The potentiodynamic profiles in both media were similar
and the characteristic current peak of redox process leucoemeraldine (the completely
reduced form of PANI) / emeraldine, can be seen at around +0.35V. However, the
redox activity in acid media is higher than in neutral solution as it can be inferred by
the lower current densities observed in the latter case. Even though, these results
are important for technological applications.
Fig. 8 - Raman spectra of pure (a) and various mass percentages of PANI–DBSAPAAC, (b) 3, (c) 8, (d) 16 and (e) 25. λ = 1064 nm.
As an example, corrosion protective materials containing conducting polymers must
be electro-active at more positive potential values than the metallic substrates. It
indicates that the metal can be naturally oxidized by the conducting polymer and this
fact is related to PANI corrosion protection mechanism [5, 50] showing that these
kinds of films could be used as “green” paint coating for corrosion protection.
9
Fig. 9 - j vs. E potentiodynamic profiles for Au/ PANI-DBSA-PAAC/ PMMA (PANI
10% in mass) in electrolytic solution of (a) H2SO4 1.0 mol L-1 and (b) NaCl 0.1 mol
L-1. v =10 mV s-1.
Conclusions
This paper shows new water-friendly PANI blends that do not threaten the
environment and with potential technological applications. The mixing of a dispersion
of doped PANI and PMMA latex particles provides a way to increase PANI
processability and allowed achievement of homogeneous and stable films. The
presence of PMMA was extremely important because it was able to improve the
mechanical properties of the conducting polymer while maintaining a good
conductivity. The film formation is suitable for technological applications like
corrosion protection, since they presented redox activity in both acid and neutral
solutions.
Experimental part
PANI polymerization [28, 30]
Aniline (Aldrich) (5 mmol) and poly(acrylic acid) (PAAC) (Acros) (20 µmol) were
dissolved in 100 mL of purified water (ELGA systems) and stirred for two hours at 25
o
C. After that, 5 mmol of DBSA were added and the mixture was stirred during two
hours till a white turbid dispersion of insoluble tiny particles was obtained. The
dispersion was cooled at 0 oC, and 6 mmol of potassium persulfate (Merck) were
slowly added. The polymerization process was carried out and a colour change from
10
white to dark green was observed after 12 h. The pH of the final dispersion was
equal to 3. These dispersions were purified by dialysis with a semi-permeable
membrane (Sigma D - 9652, molar mass cut-off of 12,400 g mol-1) in purified water,
which was replaced by fresh water frequently for a period of 7 days.
PMMA polymerization [30]
Methylmethacrylate monomer (MMA) (Aldrich) (0.11 mol) and potassium persulphate
(0.1 mmol) were dissolved in 80 mL of purified water. Also, 0.4 mmol of sodium
dodecyl sulphate (SDS) (Mallinckrodt chemical), triton® X-100 (Sigma) (2 mmol) and
potassium persulphate (0.3 mmol) were dissolved in 20 mL of purified water. After
that, both solutions were mixed and stirred by six hours at 70 oC under N2
atmosphere. A white turbid dispersion of insoluble tiny particles was obtained at a pH
= 3.4. After that, dimethylphtalate (Aldrich) was added in a proportion of 33% in mass
and heated at 60 oC for 10 minutes. This plasticizer was added in order to obtain
homogeneous films and all chemicals used were of analytical reagent quality.
Film preparation
PANI/PMMA mixtures were obtained by simple mixing of the aqueous dispersions in
different proportions in order to obtain the desired weight percentage of PANI. The
films were obtained by casting and drying at 80 oC for 15 minutes and repeating this
path until a desired thickness was obtained. Finally, thermal treatment at 100 oC for
one hour and a half was carried out. Typically, film thicknesses were about 10 µm.
Characterization
Determinations of mean diameter (D), size distribution, and zeta potential (ζ) were
performed by using the ZetaPlus Zeta Potential Analyzer (Brookhaven Instruments
Corp., Holtsville, NY), which was equipped with a 677nm laser and dynamic light
scattering at 90° for particle sizing. Zeta potential, ζ, was determined from the
electrophoretic mobility, µ, and the Smoluchowski equation (ξ = µη/ε), where η and ε
are the viscosity and the dielectric constant of the medium. For these experiments,
samples were purified in the same way as it was described for PANI dispersions.
After that, 30 µL of the dispersion were added to 10 mL of a 1 mmol L-1 KCl solution.
Field emission gun scanning electron microscope (FEG-SEM) images were obtained
with a FEG-SEM JSM 6330F at the LME/LNLS, Campinas - Brazil. Raman
spectroscopy was used to follow the oxidation state of PANI in the system by the
utilization of a FT-Raman Bruker FRS100/S with a InGaAs detector and a Nd:YAG
laser (λo = 1064 nm, Coherent Compass 1064-500N) as the exciting radiation. The
conductivity of the films was measured by four-point method by using a Jandel
Universal Probe. Differential scanning calorimetry (DSC) and thermal gravimetric
analysis (TGA) were performed on Shimadzu DSC-50 and TGA-50 instruments,
respectively. Temperature range from room temperature to 800 oC (TGA) and to 500
o
C (DSC) at a heating rate of 10 or 20 oC/min were used.
All electrochemical experiments were carried out by using a potentiostat/galvanostat
Autolab PGSTAT30 (Ecochemie, Netherlands). These experiments were performed
in a conventional three-electrode Pyrex cell using Au as substrate for working
electrodes. A platinum sheet was used as counter electrode and all potentials were
referred to the saturated calomel electrode (SCE). All experiments were performed at
room temperature.
11
Acknowledgements
Authors thank FAPESP (Proc. 03/10015-3), CNPq and CAPES for financial support.
LME/LNLS (Campinas, Brazil) and Laboratório de Espectroscopia Molecular (LEMIQ-USP) are gratefully acknowledged for HRTEM and Raman facilities respectively.
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