Abstract
In this study microwave plasma initiated polymerization in aim of synthesize of
desired polymer for the first time was experienced.
Ultrahigh molecular weight Sulfonated polyacrylamides (SPAM) were obtained by
plasma-induced polymerization in water solutions. The influence of monomer
concentration, initiation time and post polymerization period on polymer yield and
intrinsic viscosity were investigated. Also optimum of intrinsic viscosity at time
glow of 60s by altering power is evaluated at 150 w and reciprocally another
optimum is calculated as 30s for glow discharge time at fixed power of 100 w as
power of plasma.
The TG result shows two stages of the weight loss. The first one occurred between
270oC and 330oC, with a weight loss about 45%, and the second one happened
between 350oC and 460oC, with a weight loss about 25%. Fourier Transform
Infrared spectroscopy (FTIR) and Hydrogen Nuclear Magnetic Resonance (HNMR) spectroscopy results alongside all mentioned above indicate good properties
which satisfy efficient viscosity modifiers in displacement of fluids and especially
for our purpose, Enhanced Oil Recovery (EOR).
Introduction:
Recent Scientific researches show that the use of plasma initiated polymerization
(PIP) has established as an advantageable method to produce free radicals in
polymerization process. There are some systematic methods for routine
polymerization such as: chemical, thermal, UV radiation, etc, while the positive
point of this method compared with others is that it can obtain ultra high molecular
weight polymers [1-4]. These polymers have a large number of applications in
special industries, so the PIP method can address a vast variety of needs in
industry.
Initially In 1979, Osada’s research group applied plasma initiated polymerization
for some special vinyl monomer and applied a radio-frequency generator for
production of plasma [5-6]. However, this method could not achieve to huge
success in industry, mainly because of a wide range of problems encountered in
industry using RF plasma [7]. By way of illustration, in the former PIP method,
which plasma was generated by radio frequency (RF) method, a low pressure about
10-3-10-4 Torr is needed. This vacuum must be maintained for few days as post
polymerization. In order to suppress the drawbacks of this method regard to
sustainable development and economical concerns, we should use plasma
generator system that could be applied in industry.
In general, by employing the low pressure in the process of monomers degassing
and the stage of glowing the plasma is necessary. However, the process of
degassing can be carried out in pressure about 1 Torr. Moreover, Microwave
(MW) generated plasma, glows easily at pressure about 1 Torr. Therefore, using
the Microwave (MW) generated plasma in the PIP method in pressure about 1
Torr, which can be achieved by the use of a Rotary pump, is more practical and
economical. Also, the Microwave generator rather than RF generator has a lower
cost and working with it, is easier. Consequently, in the present work as an
innovation, it was tried to use the Microwave to generate the plasma for initiating
the polymerization process.
Polyacrylamide is one of the practical polymers in industry which can be produced
by PIP method. One of the most, important applications of this polymer is in the oil
industry, especially in Enhanced Oil Recovery (EOR) projects [8]. The chosen
monomers for this study were acrylamide (AM, M1) and 2-acrylamido-2-methylpropanesulfonic acid (AMPS, M2). The AMPS monomer is well-known for its
hydrolytic stability, high solubility in water and good thermal behavior [9-12]. As
a consequence, MW generated plasma was used for plasma initiated
polymerization and the effects of microwave discharge power and plasma exposure
time on intrinsic viscosity of synthesized copolymers were investigated.
Experimental setup:
Synthesis of the copolymers. The monomers of acrylamide(AM) and 2acrylamido-2- methylpropane sulfonic acid (AMPS) were purchased from Merck
Co. Then the purification of monomers was performed by methanol. Different
ratios of monomers were mixed which in this step the concentration of 5% wt
comonomer in deionized water was provided. 20cc of the comonomers solution
was poured in a Pyrex container tube (figure 1). After connecting the tube to the
vacuum pump, it was placed within the liquid nitrogen to turn the comonomers
frozen. Then, we vacuum the system to 10-2-10-3 Torr and allow the monomer to
melt. This process is known to the degassing of the system. Repeating the
degassing process -3 to 4 times- ensures the removal of oxygen from the solution.
Finally, we froze the solution and evacuated the system again (with pressure about
10 -3- 10-2 which was provided by rotary pump). After that, the vacuum valve was
closed. The monomers turn to melt slowly which in turn set the stage for raising
the pressure of the monomer tube. On the verge of observing the pressure about
1torr, the monomer tube was placed in the field applicator of Microwave wave
guide (Figure.1), and plasma was ignited. Other applied conditions are presented in
table I.
After turning off the plasma, the container tube must be put in a dark place and
maintains its vacuum for several days which is so-called post polymerization step.
In all the tests represented in this report, the post polymerization process lasted for
3 days. The obtained polymer gel was purified and dried by distilled water and
pure methanol. In this stage, polymers were separated from remained monomers in
the container by the help of methanol. Meanwhile the percentage of polymer
conversion was measured.
Physical Tests. The FTIR and 1H-NMR spectra of the copolymers were obtained
as the physical test. In order to obtain FTIR spectrum, purified and dried powder of
the polymer was mixed with potassium bromide (KBr ) and prepared pills of this
mixture were put in the spectrometer device, Tensor27 model (Bruker Co). The 1HNMR spectra are obtained in deuterated water (D2O) solutions at 60oC on a JEOLC60HL spectrometer with an electromagnet at 300 MHz.
Measurements of intrinsic viscosity ([η]). Intrinsic viscosity of different samples
of synthesized copolymers (AM-co-AMPS) was measured in different conditions
in a solvent of distilled water at a temperature of 25oC by Lauda Proline PV15
viscometry device.
Results and discussions
Sulfonated polyacrylamide has especially synthesized which is very practical in
Enhanced Oil Recovery industry. In order to identify produced polymer, FTIR and
H-NMR spectrum was obtained. In addition, the conversion percentage in
comparison with pervious PIP methods was satisfying. As a significant parameter,
the viscosity of polymer was measured which in turn results in insurance of its
high molecular weight. Consequently, with the investigation of polymer thermal
behavior, we sure that the appropriate polymer for EOR applications is produced.
Copolymer Characterization
Fourier Transform Infrared Spectroscopy (FTIR): The transmission FTIR
spectrum for copolymer (AM-co-AMPS) was presented in Figure 2. The spectrum
show typical absorption bands as follows: N-H stretching at 3442 cm-1, C=O
stretching at 1655 cm-1, CH, CH2 and CH3 stretching interactions at 1547 cm-1 and
1454 cm-1, CH, CH2 and CH3 bending interactions at 2928 cm-1, So  group
3
stretching at 1204,1117 and 1040 cm-1.
1
Proton Nuclear Magnetic Resonance Spectroscopy( H-NMR)
In the H-NMR spectrum, the peak of 1.6 ppm region is atributted to the hydrogens
of the CH3 groups of C  (CH 3 ) 2 , and the CH2 groups of  CH 2  So3 . Additionally,
the peak of 2.31 ppm region is related to the hydrogen of the CH group of the main
polymer chain, and the peak of the 3.5 ppm region is related to the hydrogen
groups of CH2 of the main polymer chain.
Copolymerization Data: Copolymerization data obtained from 1H-NMR spectra
(Figure 3) were summarized in Table 2.
In the above table, changing results of 3 parameters, included plasma exposure
time, plasma power and monomers ratio in various tests are presented. These testes
were implemented for 3 arbitrary points, and for every one of them these 3
parameters were measured. By and large, the conversion percentage which is
obtained by Microwave Plasma Initiated Polymerization, in comparison with
pervious PIP methods, shows considerably a higher conversion [13].
Plasma parameters and its effects on intrinsic viscosity: How plasma
parameters affect on viscosity? In order to answer this question, plasma variation
which has correlation with molecular weight was identified by several equations.
Then the reaction of intrinsic viscosity in response to change of these variables was
investigated.
Radicals are created by plasma emission on the interface of frozen monomers.
Most of these radicals will terminate with each other and lose the ability of
initiating polymerization except a very few radicals reach the surface of the frozen
monomers which initiate the polymerization quickly and form a long chain
polymer. The energetic electrons of plasma impact with the monomer interface by
collision number N which causes free radical generation that can be calculated by
the following equation:
N  vt  S ,
(1)
where ν is known as the molecular collision rate, t as the exposure time and S as
the surface of frozen monomers interface under glow (irradiation). Then the
molecular collision rate ν can be calculated by:
v  [n1 (8kT
1
2
.
m) ] 4
(2)
ν depend on the absolute temperature (T), mass of gaseous molecules plasma (m)
and molecular density of the gas (n1). On the other hand the whole number due to
the collision is as follows:
Ne  N  ,
(3)
where α, is ionization factor, is equal to:

plasma density .
n1
(4)
So, according to equations (1), (3) and (4) it can be concluded that the main
parameters in the plasma controling the radical generation in the monomers surface
are the plasma density and also the exposure time [14]. Needless to say, working
pressure (gas pressure) is also important that here was kept constant at 1 Torr.
Effect of plasma density can be investigated by changing plasma power. In
general, plasma power is directly related to plasma density. Figure 4 shows that
increasing the discharge power at first increases the intrinsic viscosity of
copolymers.
But there is an optimum point which by raising the power beyond it, the intrinsic
viscosity begins to decrease.
At the beginning, growing the power results in increasing of generated radicals.
Therefore, because of direct relation between numbers of collisions with
polymerization rate, numbers of these free radicals at optimum point are enough
that during three days of post polymerization, polymer chain could be reached to
maximum in regard to its rate and growth condition which in this optimum
condition molecular weight and subsequently viscosity are desired.
After that by more increasing of the plasma power, number of free radicals surplus
optimum one, and collision probability of these free radicals at the head of these
polymer chains which cause to end polymerization, could be increased. In other
words increasing power will cause decreasing desired viscosity.
Another important parameter related to plasma generation is the exposure time, by
increasing this parameter, number of collisions and accordingly free radical
generation increase. This phenomenon has similar effect of plasma power on the
viscosity, Figure 5 shows the effect of exposure time on the viscosity.
The Thermal Behavior of copolymer: The thermogravimetric analysis (TGA)
was carried out by Mettler device, Sdta 851 e model, to investigate the thermal
behavior of copolymer. The TG curve (Figure. 6) shows two stages of the weight
loss. The first one occurred between 270oC and 330oC, with a weight loss about
45%, and the second one happened between 350oC and 460oC, with a weight loss
about 25%.
Conclusion
From our experimentally data on the synthesis of SPAM by PIP, for the firstime it
is illustrated that Microwave can be used as utile as other plasma generators and
also by correlating several operational parameters and polymer properties, it is
possible now to select the optimum reaction conditions for the process. Also both
the physical test data and ThermoGravimetric results motivate a possible use of the
obtained SPAM polymers in EOR which satisfy the most important demands for
those applications where it is desirable for the polymer solutions such as:
• They are completely soluble in water;
• Their molecular weights are ultrahigh, achieving ultrahigh viscosity in dilute
solutions;
• They are satisfactorily indicate a good thermal stability with respect to TG data
Reference
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Fig. 1.Microwave reactor.
Fig. 2. FTIR spectrum of AM-co-AMPS.
Fig 3. 1H-NMR spectrum of AM-co-AMPS.
30
discharge time: 60s
feed composition: 3
(M1/M2)
intrinsic viscosity (dl/g)
25
20
15
10
5
0
0
50
100
150
200
250
300
discharge power (W)
Fig. 4. Discharge power effect on intrinsic viscosity.
350
35
discharge power: 100W
feed composition: 3
(M1/M2)
intrinsic viscosity (dl/g)
30
25
20
15
10
5
0
0
30
60
90
120
150
discharge time (s)
Fig 5. The glow discharge duration effect on intrinsic viscosity.
Fig 6. TG and DTG curves of AM-co-AMPS: sample size 2.6860 mg, heating rate 10oC/min, nitrogen purge.
Table 1. Various parameters for copolymer.
plasma exposure time (s)
15, 30, 60, 120
discharge power: 100w
(s)
feed composition (M1/M2):3/1
Discharge power (w)
50, 100, 150, 200, 300
plasma exposure time: 60 s
(w)
Feed composition (M1/M2): 3/1
Feed composition (M1/M2)
1/1, 3/1, 7/1
Discharge power:100w
Plasma exposure time:60 s
Table 2. copolymeization* data, percent of conversion and intrinsic viscosity for AM(M1)/AMPS(M2) copolymers
obtained by PIP.
Conversion (%)
Copolymer
d(M1)/d(M2)***
M1/M2: 3/1
Various time(s)
Power:
100W
Time: 60s
Time: 60s
Various
M1/M2**
Power:
100W
Various
power(W)
M1/M2: 3/1
Intrinsic
viscosity(dl/g)
30(s)
93.18
5.48
26.03
60(s)
96.58
1.75
20.16
120(s)
79
2.01
19.19
50(w)
97.39
2.22
17.53
100(w)
96.58
1.75
20.16
150(w)
72.35
4.55
27.12
7/1
79.94
9.78
21.42
3/1
96.58
1.75
20.16
1/1
87.36
1.58
21.1
*from 1H-NMR spectra. **for 5% wt monomer in water.***monomer ratio in the copolymer