Carbon nanotubes (CNTs) attract great attention due to the unique combination of physical, chemical, electrical and mechanical properties which have high potential for design of nanocomposite materials with customized properties (electromagnetic shielding, high strength and electroconductivity materials), actua-tors (e.g. artificial muscles and tissues), sensors, components of microfluidic devices, etc. Usage of CNT in practical applications is limited by their lower solubility in most solvents. Remaining primary agglomer-ates of NT not only increase the amount of filler needed for percolation, but also act as imperfections under mechanical stress. Therefore, in this investigation, the influence of surfactant type and its concentration, along with sonication treating time on sedimentation behavior of multi-walled carbon nanotubes was in-vestigated. To a greater extent, the influence of monomer composition, surface-active substances, and car-bon nanotubes on gelation and physicochemical properties of thermo- and pH-sensitive hydrogels and hy-drogel nanocomposites based on N-isopropylacrylamide with carbon nanotubes were investigated too.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3559

Goncharuk, A.I.

Korotych, O.I.

Samchenko, Yu.M.

Sliusar, Yu.S.

Starchenko, V.Yu.

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  
NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 2 No 3, 03NCNN38(4pp) (2013) 
 
 
2304-1862/2013/2(3)03NCNN38(4) 03NCNN38-1  2013 Sumy State University 
Thermo- and pH-sensitive Hydrogels with Incorporated Multi-Walled Carbon Nanotubes 
 
O.I. Korotych1,*, A.I. Goncharuk1, Yu.S. Sliusar2, Yu.M Samchenko1,†, V.Yu. Starchenko1 
 
1 Ovcharenko Institute of Biocolloid Chemistry, National Academy of Sciences of Ukraine, Vernadskogo Blvd. 42, 
03680 Kyiv, Ukraine 
2 National University of Kyiv-Mohyla Academy, Hryhoriya Skovorody St. 2, 
 04070 Kyiv, Ukraine 
 
(Received 10 June 2013; published online 03 September 2013) 
 
Carbon nanotubes (CNTs) attract great attention due to the unique combination of physical, chemical, 
electrical and mechanical properties which have high potential for design of nanocomposite materials with 
customized properties (electromagnetic shielding, high strength and electroconductivity materials), actua-
tors (e.g. artificial muscles and tissues), sensors, components of microfluidic devices, etc. Usage of CNT in 
practical applications is limited by their lower solubility in most solvents. Remaining primary agglomer-
ates of NT not only increase the amount of filler needed for percolation, but also act as imperfections under 
mechanical stress. Therefore, in this investigation, the influence of surfactant type and its concentration, 
along with sonication treating time on sedimentation behavior of multi-walled carbon nanotubes was in-
vestigated. To a greater extent, the influence of monomer composition, surface-active substances, and car-
bon nanotubes on gelation and physicochemical properties of thermo- and pH-sensitive hydrogels and hy-
drogel nanocomposites based on N-isopropylacrylamide with carbon nanotubes were investigated too. 
 
Keywords: Thermosensitive, pH-sensitive, Hydrogels, Carbon Nanotubes, Nanocomposites. 
 
  PACS numbers: 07.10.Cm, 62.23.Pq 
 
 
                                                          
* korotych.elena@gmail.com  
† yu1sam@yahoo.com 
1. INTRODUCTION 
 
Carbon nanotubes (CNTs) attract great attention in 
the last decades due to the unique combination of phys-
ical, chemical, electrical and mechanical properties 
which have high potential for design of composite ma-
terials with customized properties (construction mate-
rials, electromagnetic shielding, high strength and 
electroconductivity materials), actuators (e.g. artificial 
muscles and tissues), sensors, components of microflu-
idic devices, etc. Usage of CNT in practical applications 
is limited by their lower solubility in most solvents. 
However, for reinforcement of different polymeric ma-
trices, CNTs must possess uniform dispersion. In this 
context, an excellent distribution and dispersion of 
CNTs is an essential precondition for the effective use 
of the potential of the nanofiller. Remaining primary 
agglomerates of carbon NTs not only increase the 
amount of filler needed for percolation but also act as 
imperfections under mechanical stress. 
Carbon nanotubes can be incorporated into thermo-
sensitive hydrogels based on N-isopropylacrylamide 
(NIPAAm) with promise to yield significant improve-
ments and new functionalities. Hydrogel system based 
on NIPAAm has emerged as one of the most extensive-
ly studied ones for thermoresponsive applications. A 
key feature of such system is a lower critical solution 
temperature (LCST) that occurs in the temperature 
range of 32-34°C, and it is comparable to the tempera-
ture range which can be triggered by both physiological 
and environmental conditions. Upon heating above the 
LCST, the phase transition in hydrogel occurs resulting 
in a drastic change in the hydrogel volume by water 
expulsion, and it is connected with the transition be-
tween two conformation structures of the polymer side 
groups [1]. Taking into account the fact that the phase 
transition temperature or LCST of thermosensitive 
systems depends on hydrophilic-lipophilic balance, it 
can be moved to any side during copolymerization of 
NIPAAm with hydrophilic or hydrophobic monomers. 
Thus, NIPAAm's copolymerization with hydrophilic 
monomers leads to the increase of phase transition 
temperature, while its copolymerization with hydro-
phobic monomers gives the opposite effect [2]. Fur-
thermore hydrogels systems sensitive to a several 
stimuli can be obtained copolymerizing of NIPAAm 
with different stimuli responsive monomers. A rare 
instance of such copolymeric systems is hydrogels 
based on NIPAAm and acrylic acid (AA), which possess 
thermo- and pH-sensitivity. 
It have been shown [3] that hydrogel nanocomposites 
based on NIPAAm with single-walled carbon nanotubes 
exhibit up to 5 times enhancement in the thermal re-
sponse time compared to pure NIPAAm hydrogels. 
Therefore, in this investigation, the influence of surfac-
tant type and its concentration, along with sonication 
time on sedimentation behavior of multi-walled carbon 
nanotubes was investigated. To a greater extent, the 
influence of monomer composition, surface active sub-
stances, and carbon nanotubes on gelation and physico-
chemical properties of thermo- and pH-sensitive hydro-
gels and hydrogel nanocomposites based on N-
isopropylacrylamide with CNTs were investigated too. 
 
2. MATERIALS AND METHODS 
 
2.1 Materials 
 
N-isopropylacrylamide (NIPAAm) (Sigma-Aldrich, 
97%) was recrystallized from n-hexane and dried under 
vacuum. Acrylic acid (AA) (Merck) was distilled and 
then subjected to fractional crystallization. Acrylamide 
 O.I. KOROTYCH, A.I. GONCHARUK, YU.S. SLIUSAR PROC. NAP 2, 03NCNN38 (2013) 
 
 
03NCNN38-2 
(AAm) (Merck, 99.9%), N,N′-methylenebisacrylamide 
(MBA) (Merck, 98%), ammonium persulfate (PSA) 
(Sigma, 98%), N,N,N′,N′-tetramethylendiamine 
(TMED) (Merck, 99%), multi-walled carbon nanotubes 
(CNT), Triton X-100 (TX-100) (AppliChem Gmbh, 
M=646.85Da), Tween 80 (T-80), sodium dodecyl sulfate 
(SDS), and sodium dodecylbenzenesulfonate (SDBS) 
were used as received without preliminary purification. 
 
2.2 Synthesis of hydrogels 
 
(Co)polymeric hydrogels based on NIPAAm, AAm 
and AA were synthesized by means of radical polymer-
ization in water medium at 15◦C. Covalent cross-
linking of copolymeric chains was accomplished using 
bifunctional monomer MBA. Initiation of the polymeri-
zation was performed with the help of the redox system 
composed of ammonium persulfate (PSA) and 
N,N,N',N'-tetramethylethylenediamine TMED [2]. 
(Co)polymeric gel composition was determined by 
the composition of the reaction mixture and was found 
to be as follows: weight fraction of monomers constitut-
ed 10%, 15% or 20%; MBA – 0.20%, 0.10% or 0.05%; 
concentration of the equimolar initiating mixture was 
0.04mol/L for hydrogels based on NIPAAm (NIPAAm 
with AAm) and 0.08mol/L for hydrogels based on 
NIPAAm with AA. 
 
2.3 Synthesis of nanocomposites 
 
In order to obtain nanocomposites an aqueous dis-
persion of NT stabilized by surface-active substances 
(SAS) were added during the synthesis of hydrogels. 
(Co)polymeric gel composition with carbon CNTs 
was determined by the composition of the reaction mix-
ture and was found to be as follows: weight fraction of 
monomers constituted 20%; MBA – 0.10%; concentra-
tion of NTs – 0, 0.05, 0.10 and 0.25%; concentration of 
the equimolar initiating mixture was 0.04mol/L for 
hydrogels based on NIPAAm (NIPAAm with AAm) and 
0.08mol/L for hydrogels based on NIPAAm with AA. 
 
2.4 Dispergation of CNT 
 
The sedimentation behavior of CNTs dispersed in 
water and aqueous surfactant solutions was investigat-
ed. CNTs were added into an aqueous solution of SAS 
with different concentration, concentration of CNTs 
was 0.01%, the ratio between SAS and NT was 0:1, 1:1, 
2.5:1, 5:1, 7.5:1, 10:1, 15:1, 20:1, 30:1, 40:1, and 50:1. 
Carbon nanotubes were dispersed at room temperature 
using ultrasound bath (Transsonic T 460/H, frequency 
36 kHz, Elma, Germany) for 5 min and ultrasonic dis-
perser (UZDN-2T, frequency 44 kHz, and the output 
power of 150 W, Ukrrospribor, Sumy, Ukraine) for 
1min. Control pictures were taken in 7 days after dis-
pergation. 
 
3. RESULTS AND DISCUSSIONS 
 
The sedimentation stability of CNTs suspensions in 
water and water solutions of SAS may serve as charac-
teristic of their dispersion ability. The dispersability of 
carbon nanotubes is mainly influenced by the entan-
glement structure of the tubes within the as-produced 
primary CNT agglomerates determining also the ag-
glomerate strength. In general, due to strong van der 
Waals forces CNTs exhibit a tendency to formation of 
agglomerates and bundles during synthesis (see Fig. 1), 
and also during their dispergation and incorporation 
into the polymeric matrixes. 
 
 
 
Fig. 1 – Microphotography (TEM) of multi-walled carbon 
nanotubes. 
 
Pristine CNTs dispersed in water start to precipi-
tate immediately after sonication. As can be seen on 
Fig. 2 the sedimentation rate increases with increasing 
of NTs concentration due to so-called swarm sedimen-
tation. 
 
 
 
Fig. 2 – Time dependence of optical density at 490nm of CNT 
suspensions in water with different concentration of nano-
tubes: 1 – 0.0050%; 2 – 0.0025%. 
 
Dispergation of nanotubes with formation of stable 
suspensions is of crucial importance for obtaining of 
hydrogel nanocomposites of high quality. For this 
purpose CNTs were sonicated in water solutions of 
four types of SAS (see Fig. 3) at different concentra-
tions. Nonionic (TX-100 and T-80) and anionic (SDBS 
and SDS) SAS have been used for stabilization of wa-
ter suspensions of carbon NTs. 
The influence of all types of SAS on the stability of 
aqueous dispersions of CNTs was analyzed using SAS 
 Thermo- and pH-sensitive hydrogels… PROC. NAP 2, 03NCNN38 (2013) 
 
 
03NCNN38-3 
series (see Fig. 4). Stable dispersions were obtained 
with certain ratio of SAS to CNTs for all surfactants, 
e.g. for TX-100 and T-80 it is 2.5:1, for SDBS –2:1, and 
for SDS – 3:1. Comparing the results it can be con-
cluded that TX-100 and SDBS stabilized water sus-
pensions of CNTs is the best. High dispersive efficien-
cy of these substances can be explained by the pres-
ence of benzene ring combined with the small head 
group. 
 
 
 
Fig. 3 – Structures of surface-active substances: TX-100, T-80, 
SDBS, and SDS. 
 
 
 
Fig. 4 – Photo of CNTs suspensions stabilized by TX-100 with 
different ratio between SAS and NTs. Concentration of CNTs 
is 0.01%. 
 
Addition of monomers to CNTs suspensions influ-
ences on its stability. Thus, AAm improves disperga-
tion of nanotubes and does not change the ratio be-
tween SAS and carbon NTs, while NIPAAm and AA 
have opposite effect. This have to be taken into ac-
count for obtaining nanocomposites with CNTs based 
on NIPAAm and AA. 
Study of the dependence of hydrogel equilibrium 
swelling degree on composition allowed choosing ma-
trix, which possess a sufficient strength and have 
optimal swelling degree (see Fig. 5). Thereby copol-
ymeric hydrogels and nanocomposites with CNTs 
were synthesized based on matrix with 20%-content 
of monomers. 
Since hydrogels and nanocomposites are promis-
ing in terms of their use as actuators, components of 
microfluidic devices, etc. they have quickly respond 
to external changes. Studying of hydrogel collapse 
have allowed choosing the suitable matrixes which 
will be used for obtaining nanocomposites with in-
corporated NTs based on them. As can be seen on 
Fig. 6 homopolyisopropylacrylamide matrix in the 
initial stage collapses linearly. Furthermore collapse 
rate decreases with increasing content of MBA and 
comonomer content. 
 
 
 
Fig. 5 – Equllibrium swelling degree of hydrogels based on 
NIPAAm with different concentration of monomer and at 
different concentration of MBA: 1 – 0.05%; 2 – 0.10%; 3 – 0.20%. 
 
 
 
Fig. 6 – Time dependence of normalized hydrogel swelling 
degree of matrixes with 20%-content of NIPAAm and different 
concentration of MBA when the temperature changes from 
25°С to 50°С: 1 – 0.20%; 2 – 0.10%; 3 – 0.05%. 
 
To a greater extent, the influence of monomer 
composition, SAS and carbon NTs on gelation and 
physicochemical properties of thermo- and pH-
sensitive hydrogels and hydrogel nanocomposites 
based on N-isopropylacrylamide with CNTs were 
investigated. It has been shown that different addi-
tives to gel composition based on NIPAAm have dif-
ferent effect on the polymerization (gelation). Thus, 
addition of AAm and CNTs accelerate the polymeri-
zation (see Fig. 7.1, Table 1), while AA and SAS have 
opposite effect (see Fig. 7.2, Table 1). 
Hydrogel nanocomposites with incorporated 
CNTs improve such properties as collapse rate and 
strength, and increase the electroconductivity of 
thermo- and pH-sensitive systems. 
 O.I. KOROTYCH, A.I. GONCHARUK, YU.S. SLIUSAR PROC. NAP 2, 03NCNN38 (2013) 
 
 
03NCNN38-4 
 
 
a 
 
 
 
b 
 
Fig. 7 – Temperature (T) during the polymerization of 
(co)polymeric hydrogels based on NIPAAm with AAm (a), and 
AA (b): 1 – 100% NIPAAm; 2 – 95% NIPAAm; 3 – 90% 
NIPAAm; and 4 – 0% NIPAAm.  
 
 
Table 2 – Polymerisation time (t) for copolymeric hydrogels 
based on NIPAAm with AAm and AA. 
 
w(NIPAAm),% 
NIPAAm+AAm NIPAAm+AA 
t, sec t, sec 
100 200 
95 175 461 
90 145 1355 
0 110 4717 
 
4. CONCLUSIONS 
 
Our study has shown that investigation of the sed-
imentation behavior of carbon nanotubes dispersed in 
aqueous surfactant solutions are suitable in order to 
determine type and ratio between SAS and NTs. It has 
been shown that TX-100 and SDBS have a high disper-
sive efficiency with the ratio ca. 2-2.5. Compared to oth-
er surfactants, we found SDBS to be more stable in 
NIPAAm and AA monomer solutions. 
Investigating the equilibrium swelling degree and 
collapse we determined the most suitable composition 
of polymeric matrixes which can be used as actuators 
and components of microfluidic devices. It has total 
content of monomers equal to 20% and 0.10%-content 
of MBA. Copolymer hydrogel based on NIPAAm with 
5%-content of AAm and 10%-content of AA has optimal 
properties (strength, collapse rate, the value of swelling 
degree and LCST, etc.) among other copolymeric ma-
trixes. 
Hydrogel nanocomposites with incorporated CNTs 
improve such properties as collapse rate and strength, 
and increase the electroconductivity of thermo- and pH-
sensitive systems. 
 
AKNOWLEDGEMENTS 
 
Financial support from The State Fund for Funda-
mental Researches (project №GP/F36/106) is gratefully 
acknowledged. 
REFERENCES 
 
1. E.S. Ron, L.E. Bromberg, Adv. Drug Deliv. Rev. 31, 197 
(1998). 
2. Yu. Samchenko, Z. Ulberg, O. Korotych, Adv. Colloid In-
terface Sci. 168, 247 (2011). 
3. X. Zhang, C.L. Pint, M.H. Lee, et al., Nano Lett. 11, 3239 
(2011). 
 


English

Thermo- and pH-sensitive Hydrogels with Incorporated Multi-Walled Carbon Nanotubes

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 2 No 3, 03NCNN38(4pp) (2013)   2304-1862/2013/2(3)03NCNN38(4) 03NCNN38-1  2013 Sumy State University Thermo- and pH-sensitive Hydrogels with Incorporated Multi-Walled Carbon Nanotubes  O.I. Korotych1,*, A.I. Goncharuk1, Yu.S. Sliusar2, Yu.M Samchenko1,†, V.Yu. Starchenko1  1 Ovcharenko Institute of Biocolloid Chemistry, National Academy of Sciences of Ukraine, Vernadskogo Blvd. 42, 03680 Kyiv, Ukraine 2 National University of Kyiv-Mohyla Academy, Hryhoriya Skovorody St. 2,  04070 Kyiv, Ukraine  (Received 10 June 2013; published online 03 September 2013)  Carbon nanotubes (CNTs) attract great attention due to the unique combination of physical, chemical, electrical and mechanical properties which have high potential for design of nanocomposite materials with customized properties (electromagnetic shielding, high strength and electroconductivity materials), actua-tors (e.g. artificial muscles and tissues), sensors, components of microfluidic devices, etc. Usage of CNT in practical applications is limited by their lower solubility in most solvents. Remaining primary agglomer-ates of NT not only increase the amount of filler needed for percolation, but also act as imperfections under mechanical stress. Therefore, in this investigation, the influence of surfactant type and its concentration, along with sonication treating time on sedimentation behavior of multi-walled carbon nanotubes was in-vestigated. To a greater extent, the influence of monomer composition, surface-active substances, and car-bon nanotubes on gelation and physicochemical properties of thermo- and pH-sensitive hydrogels and hy-drogel nanocomposites based on N-isopropylacrylamide with carbon nanotubes were investigated too.  Keywords: Thermosensitive, pH-sensitive, Hydrogels, Carbon Nanotubes, Nanocomposites.    PACS numbers: 07.10.Cm, 62.23.Pq                                                             * korotych.elena@gmail.com  † yu1sam@yahoo.com 1. INTRODUCTION  Carbon nanotubes (CNTs) attract great attention in the last decades due to the unique combination of phys-ical, chemical, electrical and mechanical properties which have high potential for design of composite ma-terials with customized properties (construction mate-rials, electromagnetic shielding, high strength and electroconductivity materials), actuators (e.g. artificial muscles and tissues), sensors, components of microflu-idic devices, etc. Usage of CNT in practical applications is limited by their lower solubility in most solvents. However, for reinforcement of different polymeric ma-trices, CNTs must possess uniform dispersion. In this context, an excellent distribution and dispersion of CNTs is an essential precondition for the effective use of the potential of the nanofiller. Remaining primary agglomerates of carbon NTs not only increase the amount of filler needed for percolation but also act as imperfections under mechanical stress. Carbon nanotubes can be incorporated into thermo-sensitive hydrogels based on N-isopropylacrylamide (NIPAAm) with promise to yield significant improve-ments and new functionalities. Hydrogel system based on NIPAAm has emerged as one of the most extensive-ly studied ones for thermoresponsive applications. A key feature of such system is a lower critical solution temperature (LCST) that occurs in the temperature range of 32-34°C, and it is comparable to the tempera-ture range which can be triggered by both physiological and environmental conditions. Upon heating above the LCST, the phase transition in hydrogel occurs resulting in a drastic change in the hydrogel volume by water expulsion, and it is connected with the transition be-tween two conformation structures of the polymer side groups [1]. Taking into account the fact that the phase transition temperature or LCST of thermosensitive systems depends on hydrophilic-lipophilic balance, it can be moved to any side during copolymerization of NIPAAm with hydrophilic or hydrophobic monomers. Thus, NIPAAm's copolymerization with hydrophilic monomers leads to the increase of phase transition temperature, while its copolymerization with hydro-phobic monomers gives the opposite effect [2]. Fur-thermore hydrogels systems sensitive to a several stimuli can be obtained copolymerizing of NIPAAm with different stimuli responsive monomers. A rare instance of such copolymeric systems is hydrogels based on NIPAAm and acrylic acid (AA), which possess thermo- and pH-sensitivity. It have been shown [3] that hydrogel nanocomposites based on NIPAAm with single-walled carbon nanotubes exhibit up to 5 times enhancement in the thermal re-sponse time compared to pure NIPAAm hydrogels. Therefore, in this investigation, the influence of surfac-tant type and its concentration, along with sonication time on sedimentation behavior of multi-walled carbon nanotubes was investigated. To a greater extent, the influence of monomer composition, surface active sub-stances, and carbon nanotubes on gelation and physico-chemical properties of thermo- and pH-sensitive hydro-gels and hydrogel nanocomposites based on N-isopropylacrylamide with CNTs were investigated too.  2. MATERIALS AND METHODS  2.1 Materials  N-isopropylacrylamide (NIPAAm) (Sigma-Aldrich, 97%) was recrystallized from n-hexane and dried under vacuum. Acrylic acid (AA) (Merck) was distilled and then subjected to fractional crystallization. Acrylamide  O.I. KOROTYCH, A.I. GONCHARUK, YU.S. SLIUSAR PROC. NAP 2, 03NCNN38 (2013)   03NCNN38-2 (AAm) (Merck, 99.9%), N,N′-methylenebisacrylamide (MBA) (Merck, 98%), ammonium persulfate (PSA) (Sigma, 98%), N,N,N′,N′-tetramethylendiamine (TMED) (Merck, 99%), multi-walled carbon nanotubes (CNT), Triton X-100 (TX-100) (AppliChem Gmbh, M=646.85Da), Tween 80 (T-80), sodium dodecyl sulfate (SDS), and sodium dodecylbenzenesulfonate (SDBS) were used as received without preliminary purification.  2.2 Synthesis of hydrogels  (Co)polymeric hydrogels based on NIPAAm, AAm and AA were synthesized by means of radical polymer-ization in water medium at 15◦C. Covalent cross-linking of copolymeric chains was accomplished using bifunctional monomer MBA. Initiation of the polymeri-zation was performed with the help of the redox system composed of ammonium persulfate (PSA) and N,N,N',N'-tetramethylethylenediamine TMED [2]. (Co)polymeric gel composition was determined by the composition of the reaction mixture and was found to be as follows: weight fraction of monomers constitut-ed 10%, 15% or 20%; MBA – 0.20%, 0.10% or 0.05%; concentration of the equimolar initiating mixture was 0.04mol/L for hydrogels based on NIPAAm (NIPAAm with AAm) and 0.08mol/L for hydrogels based on NIPAAm with AA.  2.3 Synthesis of nanocomposites  In order to obtain nanocomposites an aqueous dis-persion of NT stabilized by surface-active substances (SAS) were added during the synthesis of hydrogels. (Co)polymeric gel composition with carbon CNTs was determined by the composition of the reaction mix-ture and was found to be as follows: weight fraction of monomers constituted 20%; MBA – 0.10%; concentra-tion of NTs – 0, 0.05, 0.10 and 0.25%; concentration of the equimolar initiating mixture was 0.04mol/L for hydrogels based on NIPAAm (NIPAAm with AAm) and 0.08mol/L for hydrogels based on NIPAAm with AA.  2.4 Dispergation of CNT  The sedimentation behavior of CNTs dispersed in water and aqueous surfactant solutions was investigat-ed. CNTs were added into an aqueous solution of SAS with different concentration, concentration of CNTs was 0.01%, the ratio between SAS and NT was 0:1, 1:1, 2.5:1, 5:1, 7.5:1, 10:1, 15:1, 20:1, 30:1, 40:1, and 50:1. Carbon nanotubes were dispersed at room temperature using ultrasound bath (Transsonic T 460/H, frequency 36 kHz, Elma, Germany) for 5 min and ultrasonic dis-perser (UZDN-2T, frequency 44 kHz, and the output power of 150 W, Ukrrospribor, Sumy, Ukraine) for 1min. Control pictures were taken in 7 days after dis-pergation.  3. RESULTS AND DISCUSSIONS  The sedimentation stability of CNTs suspensions in water and water solutions of SAS may serve as charac-teristic of their dispersion ability. The dispersability of carbon nanotubes is mainly influenced by the entan-glement structure of the tubes within the as-produced primary CNT agglomerates determining also the ag-glomerate strength. In general, due to strong van der Waals forces CNTs exhibit a tendency to formation of agglomerates and bundles during synthesis (see Fig. 1), and also during their dispergation and incorporation into the polymeric matrixes.    Fig. 1 – Microphotography (TEM) of multi-walled carbon nanotubes.  Pristine CNTs dispersed in water start to precipi-tate immediately after sonication. As can be seen on Fig. 2 the sedimentation rate increases with increasing of NTs concentration due to so-called swarm sedimen-tation.    Fig. 2 – Time dependence of optical density at 490nm of CNT suspensions in water with different concentration of nano-tubes: 1 – 0.0050%; 2 – 0.0025%.  Dispergation of nanotubes with formation of stable suspensions is of crucial importance for obtaining of hydrogel nanocomposites of high quality. For this purpose CNTs were sonicated in water solutions of four types of SAS (see Fig. 3) at different concentra-tions. Nonionic (TX-100 and T-80) and anionic (SDBS and SDS) SAS have been used for stabilization of wa-ter suspensions of carbon NTs. The influence of all types of SAS on the stability of aqueous dispersions of CNTs was analyzed using SAS  Thermo- and pH-sensitive hydrogels… PROC. NAP 2, 03NCNN38 (2013)   03NCNN38-3 series (see Fig. 4). Stable dispersions were obtained with certain ratio of SAS to CNTs for all surfactants, e.g. for TX-100 and T-80 it is 2.5:1, for SDBS –2:1, and for SDS – 3:1. Comparing the results it can be con-cluded that TX-100 and SDBS stabilized water sus-pensions of CNTs is the best. High dispersive efficien-cy of these substances can be explained by the pres-ence of benzene ring combined with the small head group.    Fig. 3 – Structures of surface-active substances: TX-100, T-80, SDBS, and SDS.    Fig. 4 – Photo of CNTs suspensions stabilized by TX-100 with different ratio between SAS and NTs. Concentration of CNTs is 0.01%.  Addition of monomers to CNTs suspensions influ-ences on its stability. Thus, AAm improves disperga-tion of nanotubes and does not change the ratio be-tween SAS and carbon NTs, while NIPAAm and AA have opposite effect. This have to be taken into ac-count for obtaining nanocomposites with CNTs based on NIPAAm and AA. Study of the dependence of hydrogel equilibrium swelling degree on composition allowed choosing ma-trix, which possess a sufficient strength and have optimal swelling degree (see Fig. 5). Thereby copol-ymeric hydrogels and nanocomposites with CNTs were synthesized based on matrix with 20%-content of monomers. Since hydrogels and nanocomposites are promis-ing in terms of their use as actuators, components of microfluidic devices, etc. they have quickly respond to external changes. Studying of hydrogel collapse have allowed choosing the suitable matrixes which will be used for obtaining nanocomposites with in-corporated NTs based on them. As can be seen on Fig. 6 homopolyisopropylacrylamide matrix in the initial stage collapses linearly. Furthermore collapse rate decreases with increasing content of MBA and comonomer content.    Fig. 5 – Equllibrium swelling degree of hydrogels based on NIPAAm with different concentration of monomer and at different concentration of MBA: 1 – 0.05%; 2 – 0.10%; 3 – 0.20%.    Fig. 6 – Time dependence of normalized hydrogel swelling degree of matrixes with 20%-content of NIPAAm and different concentration of MBA when the temperature changes from 25°С to 50°С: 1 – 0.20%; 2 – 0.10%; 3 – 0.05%.  To a greater extent, the influence of monomer composition, SAS and carbon NTs on gelation and physicochemical properties of thermo- and pH-sensitive hydrogels and hydrogel nanocomposites based on N-isopropylacrylamide with CNTs were investigated. It has been shown that different addi-tives to gel composition based on NIPAAm have dif-ferent effect on the polymerization (gelation). Thus, addition of AAm and CNTs accelerate the polymeri-zation (see Fig. 7.1, Table 1), while AA and SAS have opposite effect (see Fig. 7.2, Table 1). Hydrogel nanocomposites with incorporated CNTs improve such properties as collapse rate and strength, and increase the electroconductivity of thermo- and pH-sensitive systems.  O.I. KOROTYCH, A.I. GONCHARUK, YU.S. SLIUSAR PROC. NAP 2, 03NCNN38 (2013)   03NCNN38-4   a    b  Fig. 7 – Temperature (T) during the polymerization of (co)polymeric hydrogels based on NIPAAm with AAm (a), and AA (b): 1 – 100% NIPAAm; 2 – 95% NIPAAm; 3 – 90% NIPAAm; and 4 – 0% NIPAAm.    Table 2 – Polymerisation time (t) for copolymeric hydrogels based on NIPAAm with AAm and AA.  w(NIPAAm),% NIPAAm+AAm NIPAAm+AA t, sec t, sec 100 200 95 175 461 90 145 1355 0 110 4717  4. CONCLUSIONS  Our study has shown that investigation of the sed-imentation behavior of carbon nanotubes dispersed in aqueous surfactant solutions are suitable in order to determine type and ratio between SAS and NTs. It has been shown that TX-100 and SDBS have a high disper-sive efficiency with the ratio ca. 2-2.5. Compared to oth-er surfactants, we found SDBS to be more stable in NIPAAm and AA monomer solutions. Investigating the equilibrium swelling degree and collapse we determined the most suitable composition of polymeric matrixes which can be used as actuators and components of microfluidic devices. It has total content of monomers equal to 20% and 0.10%-content of MBA. Copolymer hydrogel based on NIPAAm with 5%-content of AAm and 10%-content of AA has optimal properties (strength, collapse rate, the value of swelling degree and LCST, etc.) among other copolymeric ma-trixes. Hydrogel nanocomposites with incorporated CNTs improve such properties as collapse rate and strength, and increase the electroconductivity of thermo- and pH-sensitive systems.  AKNOWLEDGEMENTS  Financial support from The State Fund for Funda-mental Researches (project №GP/F36/106) is gratefully acknowledged. REFERENCES  1. E.S. Ron, L.E. Bromberg, Adv. Drug Deliv. Rev. 31, 197 (1998). 2. Yu. Samchenko, Z. Ulberg, O. Korotych, Adv. Colloid In-terface Sci. 168, 247 (2011). 3. X. Zhang, C.L. Pint, M.H. Lee, et al., Nano Lett. 11, 3239 (2011).  

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Thermo- and pH-sensitive Hydrogels with Incorporated Multi-Walled Carbon Nanotubes

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