magnetically doped multi stimuli-responsive hydrogel microspheres with ipn structure and application...

Accepted Manuscript

Title: Magnetically doped multi stimuli-responsive hydrogelmicrospheres with IPN structure and application in dyeremoval

Author: Hasan Ahmad Mohammad Nurunnabi MohammadMahbubor Rahman Kishor Kumar Klaus Tauer HidetoMinami Mohammad Abdul Gafur

PII: S0927-7757(14)00584-6DOI: http://dx.doi.org/doi:10.1016/j.colsurfa.2014.06.038Reference: COLSUA 19317

To appear in: Colloids and Surfaces A: Physicochem. Eng. Aspects

Received date: 22-1-2014Revised date: 11-6-2014Accepted date: 23-6-2014

Please cite this article as: H. Ahmad, M. Nurunnabi, M.M. Rahman, K.Kumar, K. Tauer, H. Minami, M.A. Gafur, Magnetically doped multi stimuli-responsive hydrogel microspheres with IPN structure and application in dyeremoval, Colloids and Surfaces A: Physicochemical and Engineering Aspects (2014),http://dx.doi.org/10.1016/j.colsurfa.2014.06.038

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http://dx.doi.org/doi:10.1016/j.colsurfa.2014.06.038

http://dx.doi.org/10.1016/j.colsurfa.2014.06.038

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Magnetically doped multi stimuli-responsive hydrogel microspheres

with IPN structure and application in dye removal

Hasan Ahmad,1 Mohammad Nurunnabi,1 Mohammad Mahbubor Rahman,1

Kishor Kumar,1 Klaus Tauer,2 Hideto Minami,3 Mohammad Abdul Gafur4

1Department of Chemistry, Rajshahi University, Rajshahi 6205, Bangladesh 2Max Planck Institute of Colloid and Interfaces, Am Mühlenberg, 14476 Golm, Germany 3Graduate School of Engineering, Kobe University, Kobe 657-8501, Japan 4Pilot Plant and Process Development Centre, BCSIR, Dhaka 1205, Bangladesh

Corresponding author. Tel.: +88 0721 750916 Fax: +88 0721 750064

Email address: [email protected] (H. Ahmad)

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ABSTRACT

Interpenetrating polymer network (IPN) hydrogel microspheres composed of

temperature-sensitive crosslinked poly(N-isopropylacrylamide) (PNIPAM) and pH-

sensitive crosslinked poly(methacrylic acid) (PMAA) are prepared by sequential

polymerization method. The IPN hydrogel microspheres are characterized for their

temperature- and pH-responsive behaviors by measuring the variation of hydrodynamic

diameters. The results showed that these hydrogel microspheres exhibited both

temperature- and pH- sensitive volume phase transitions. The structure and properties are

systematically characterized using FTIR, 1H-NMR, transmission electron microscope

(TEM), scanning electron microscope (SEM), differential scanning calorimetry (DSC)

and thermo-gravimetric analyses (TGA). IPN hydrogel microspheres are then

magnetically doped by in-situ formation of Fe3O4 nanoparticles. The

adsorption/desorption of various dyes and subsequent separation of dye loaded magnetic

hydrogel microspheres from the aqueous medium are studied under the influence of

magnetic field.

Key words:

IPN (interpenetrating polymer network); Stimuli-responsive; Hydrogel microsphere;

Fe3O4; Dye removal.

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1. Introduction

Stimuli-responsive hydrogels are attractive soft materials capable of responding to

small external stimuli such as temperature, pH, ionic strength, light etc [1-15]. These

hydrogels undergo drastic swelling and shrinkage in response to environment stimuli and

have been intensely investigated in a wide range of biomedical and pharmaceutical

applications [16-21]. The hydrogels in general have superior water uptake properties in

the interstitial space of the network and high permeability of nutrients, long chain

molecules, proteins and oxygen. However, poor mechanical strength particularly in the

swollen state often limits their application versatility. Recently numerous interpenetrating

polymer network (IPN) hydrogels with improved mechanical properties have been

reported [22-26]. IPN hydrogels are defined as a mixture of two or more interwinding

crosslinked polymers where one of the network polymers is synthesized in the presence

of the other. Since there is no chemical bonding between the two component networks,

each network can retain its individual property while the interpenetration of the two

networks can lead to a much higher mechanical strength with respect to the respective

homopolymer network hydrogel.

Temperature and pH are the two important triggering signals for phase transitions

among the reported stimuli-responsive polymer hydrogels. Crosslinked poly(N-

isopropylacrylamide) [PNIPAM] and poly(methacrylic acid) [PMAA] are two well-

known examples of temperature- and pH-responsive hydrogels. Crosslinked PNIPAM

hydrogels are known to exhibit a temperature-dependent volume phase transition with the

lower critical solution temperature (LCST) at about 34°C [27, 28]. At temperature below

the LCST the hydrophilic amide groups in the side chains of the PNIPAM hydrogels

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form hydrogen bonds with surrounding water molecules result in a hydration shell around

the hydrophobic groups. However, as the temperature increases above the LCST the

hydrogen bonding interactions either weakened or disrupted and the hydrophobic

interactions among the hydrophobic isopropyl groups become stronger. On the other pH

dependent swelling/deswelling behavior of PMAA is due to the ionization/deionization of

carboxylic acid groups. The incorporation of a pH-sensitive PMAA component into

PNIPAM based temperature-sensitive hydrogels by random copolymerization often result

in an increase of their volume phase transition temperature and a reduction of their

temperature responsiveness [29, 30]. The ionization of pH-responsive components may

even smear out the temperature-induced phase transition in a random copolymer

hydrogel.

Peppas et al. synthesized IPN systems composed of PNIPAM and PMAA and

showed that each polymer network retained its own properties with negligible

interference of each other [31, 32]. IPN microgels based on PNIPAM and poly(acrylic

acid) exhibited both temperature- and pH-responsive volume phase transition with little

interference of each other are also prepared as reported elsewhere [33-35]. In this work

multi stimuli-responsive hydrogel microspheres with IPN structure based on PNIPAM

and PMAA are prepared by sequential polymerization method. The IPN hydrogel

microspheres are then magnetically doped with Fe3O4 nanoparticles and finally applied

for the removal of organic dyes from aqueous solution, to check the usefulness for the

treatment of wastewater. It is to be mentioned that organic dyes widely used in industries

like textile, garments, cosmetics, printing and paper are causing serious pollution to water

bodies resulting in various undesirable consequences [36-38]. A limited number of hybrid

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polymer composites/hydrogels have been fabricated for dye removal considering the

consequences of dye pollution [39-43]. However, those are mostly dealing with the

removal of only one kind of dye, either cationic/anionic. It is expected that our IPN

hydrogel network may improve the dye uptake, stimuli-responsive property may facilitate

partial recovery of the dye from wastewater and the additional magnetic property can be

used to isolate hydrogel microspheres from treated water by external magnetic field for

reuse.

2. Experimental

2.1. Materials and instruments

NIPAM of monomer grade, obtained from Across Organics, USA, was

recrystallized from a mixture of 90% hexane and 10% acetone, dried under vacuum at

low temperature before preserving in the refrigerator. MAA of monomer grade obtained

from Fluka, Chemika, Switzerland, was distilled under reduced pressure to remove any

inhibitor. Ethyleneglycol dimethacrylate (EGDM) and N, N’-methylene-bis-acrylamide

(MBAM) used as a crosslinker were purchased from Fluka, Chemika, Switzerland and

were used without any purification. Potassium persulfate (KPS) from LOBA Chem.

India, was recrystallized from distilled water. The dyes congo red, magenta and

fluorescein sodium purchased from Matheson Coleman and Bell, Ohio, were used

without purification. Ferric chloride hexahydrate (FeCl3.6H2O), ferrous sulfate (FeSO4),

NH4OH, oleic acid and other chemicals were of analytical grade. Deionized water was

distilled using a glass (Pyrex) distillation apparatus.

Scanning electron Microscope (SEM) (LEO Electron Microscopy Ltd., UK),

transmission electron microscope (TEM) (Zeiss EM-912, Omega), FTIR (Perkin Elmer,

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FTIR-100, USA), NMR spectrophotometer (JEOL spectrometer, 400 MHz, JNM-

LA400, Japan), double beam UV-visible spectrophotometer (Shimadzu UV-1650),

centrifuge machine (TG16-WS, China), NICOMP 380 particle sizer (Santa Barbara,

California, USA), thermogravimetry analyzer, TGA (SDT Q600, USA), differential

scanning calorimeter, DSC (DSC6, Perkin Elmer, USA), X-ray diffractometer (XRD)

(D8 Bruker AXS, USA), pH meter (MP220, Mettler Toledo, Switzerland) were used for

the characterization of hydrogel microspheres. Sherwood scientific magnetic

susceptibility balance (MK I, UK) was used for the measurement of magnetic

susceptibility.

2.2. Preparation of P(NIPAM-MBAM) hydrogel microspheres

Emulsion polymerization of NIPAM (4.9 g) was carried out at 70°C in a three

necked round bottomed flask dipped in a thermostat water bath using MBAM (0.1 g) as a

crosslinker and KPS (0.05 g) as water soluble initiator. Distilled water (150 g) was used

as the dispersion medium. Polymerization was carried out for 12 h under a nitrogen

atmosphere while the reaction mixture was mechanically stirred at 100 rpm. The

conversion was nearly 100% as measured gravimetrically.

2.3. Preparation of P(NIPAM-MBAM)/P(MAA-EGDM) IPN hydrogel microspheres

P(NIPAM-MBAM)/P(MAA-EGDM) IPN hydrogel microspheres were prepared

by sequential emulsion copolymerization of MAA (0.576 g) and EGDM (0.024 g) in the

presence of 1.0 g of P(NIPAM-MBAM) hydrogel microspheres. The solid content was

fixed at 3% (w/w). Prior to the copolymerization the P(NIPAM-MBAM) hydrogel

microspheres were swollen with comonomers for about 40 min. The copolymerization

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was carried out at 70°C for 24 h in a three necked round flask dipped in a thermostat

water bath using KPS (0.005 g) as water soluble initiator.

2.4. Measurement of hydrodynamic diameter

Average hydrodynamic diameters of the hydrogel microspheres were measured

by a dynamic laser scattering particle seizer. Before the measurement the dispersion was

diluted to around 0.1% solid using water as diluent. Each measurement was repeated

twice and the average is reported. The deviation in the average size measurement was

less than ±5%.

2.5. Preparation of magnetite (Fe3O4) particles

Nano-sized magnetite (Fe3O4) particles were produced by co-precipitation of Fe2+

(4.005 g) and Fe3+ (8.01 g) from their aqueous solution (molar ratio 2:1) using 45.6 g of

25% NH4OH. The precipitation was carried out in a three necked flask under a nitrogen

atmosphere at 70°C for 2 h. Oleic acid (0.23 g) was slowly added towards the end of the

process to stabilize the Fe3O4 dispersion. The produced Fe3O4 particles were washed by

serum replacement with deionized distilled water, followed by 0.1M HCl aqueous

solution and again by deionized distilled water in order to remove the residual electrolyte

and excess oleic acid.

2.6. Preparation of P(NIPAM-MBAM)/P(MAA-EGDM)/Fe3O4 IPN hydrogel

microspheres

P(NIPAM-MBAM)/P(MAA-EGDM) IPN hydrogel microspheres (0.5 g) were

taken in a round bottomed flask containing 16 g water. The dispersion was cooled (0-

5°C) in an ice-water bath under a nitrogen gas bubbling; 0.534 g of FeCl3.6H2O and

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0.267 g of FeSO4 were dissolved in 18 g water. The iron solution was added slowly to the

flask containing the IPN hydrogel microspheres. A light brown color mixture was

formed. The ice bath was removed, and the flask was immersed in a preheated water bath

at 70°C. Immediately 3.04 g of 25% NH4OH solution was added. The reaction mixture

gradually turned to black. The mixture was kept stirring at 70°C for 2 h. Oleic acid (0.015

g) was slowly added towards the end of the process to stabilize the Fe3O4 doped IPN

hydrogel microspheres. Then the dispersion was cooled to room temperature and the

resultant magnetic IPN hydrogel microspheres were repeatedly washed by applying

magnetic field.

2.7. Measurement of magnetic susceptibility

In each case the microspheres were washed by repeated serum replacement and

dried in oven at 70°C for 2 h. The dried powders were placed in a pre-weighed sample

tube and measured the magnetic susceptibility (χg) using the following equation:

Where, C is calibration constant of the balance, L is length of the sample, R0 and

R are readings of the empty and sample tubes and M is weight of the sample (dry basis) in

C.G.S unit.

2.8. Thermogravimetric analysis (TGA)

Thermal properties of the dry powdered samples of P(NIPAM-MBAM)/P(MAA-

EGDM) and P(NIPAM-MBAM)/P(MAA-EGDM)/Fe3O4 IPN hydrogel microspheres and

reference Fe3O4 particles were measured by heating samples under flowing nitrogen

C x L x (R-R0)

M x 109=χgC x L x (R-R0)

M x 109=χg

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atmosphere from 40° to 800°C at a heating rate of 20°C/min and the weight loss was

recorded.

2.9. XRD measurement

The XRD patterns of the powder samples were taken by scanning X-ray

diffractometer using Cu KR radiation (Î») 1.54 Ã…, tube voltage of 33 kV, and tube

current of 45 mA. The intensities were measured at 2-theta values from 4.5Â° to 100 Â°

at a continuous scan rate of 10Â°/min with a position sensitive detector aperture at 3Â°

(equivalent to 0.5Â°/min with a scintillator counter).

2.10. DSC measurement

The glass transition temperatures (Tg) of the vacuum-dried hydrogels were

determined by DSC. All samples were, firstly, heated from room temperature to 250◦C at

50°C min−1 under a nitrogen atmosphere and then were cooled to room temperature. The

samples were then reheated to 235°C at 50°C min−1. Tg of the dried hydrogel was

determined from the second cycle. The midpoint of the inflection was taken as Tg.

2.11. Absorption of dye molecules

For each measurement, 0.2 g of purified magnetic IPN hydrogel microspheres

were mixed with 50 mL dye (0.7 mg) aqueous solution. The pH value of the mixture was

immediately adjusted at 10 with dil. NaOH aqueous solution. In order to examine the

absorption behavior of each dye molecule in hydrogel microspheres the amount of

absorption was measured at 40° and 20°C respectively after allowing the mixture to stand

at each temperature for 2 h. The Fe3O4 doped hydrogel microspheres were separated by

applying magnetic field. Dye concentration in the medium was measured by UV visible

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spectrophotometer at the respective wavelength maxima. The amount of absorption was

calculated by subtracting the dye concentration in the medium from that of the initial

concentration. A calibration curve was used for the purpose.

To check the reusability of P(NIPAM-MBAM)/P(MAA-EGDM)/Fe3O4 IPN

hydrogel microspheres, absorption-release experiments were carried out by measuring the

amount of absorption alternatively at 40° and 20°C, keeping the mixture under the same

conditions as stated above.

3. Results and discussion

The TEM images of hydrogel microspheres and Fe3O4 nanoparticles are shown in

Fig. 1. The average particle diameters and coefficient of variations (CV) are 476.6 nm

and 8.7% for P(NIPAM-MBAM), 337.8 nm and 1.70% for P(NIPAM-MBAM)/P(MAA-

EGDM) and 469.6 nm and 6.24% for P(NIPAM-MBAM)/P(MAA-EGDM)/Fe3O4

hydrogel microspheres respectively. It is expected that the average size of hydrogels

should be increased after each step. However, relative to P(NIPAM-MBAM) hydrogels

the average size of P(NIPAM-MBAM)/P(MAA-EGDM) IPN hydrogel microspheres

decreased. The exact reason is unknown but the formation of intermolecular hydrogen

bond among the –COOH and –CONH2 groups in IPN hydrogel microspheres may induce

rapid shrinkage during drying of the sample preparation. It is evident that some of the

IPN hydrogel microspheres had rather different morphology possibly hollow, either

resulted from artifacts or due to the sudden collapse of the hydrogels during drying of the

sample. Such morphology may also arise from the partial phase separation of two

different polymer networks while drying of the IPN hydrogels for sample preparation.

Similar morphology was also observed (not shown) in the SEM image of IPN hydrogel

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microspheres. The average size of magnetically doped P(NIPAM-MBAM)/P(MAA-

EGDM)/Fe3O4 IPN hydrogel microspheres appreciably increased followed by in-situ

deposition of Fe3O4 nanoparticles. The surface morphology of magnetically separated

P(NIPAM-MBAM)/P(MAA-EGDM)/Fe3O4 IPN hydrogel microspheres is illustrated in

the SEM image (Fig. 2). The presence of Fe3O4 nanoparticles on the surface of

P(NIPAM-MBAM)/P(MAA-EGDM) IPN hydrogel microspheres is clearly observed.

Some free magnetic nanoparticles are also visible as expected under the preparation

condition. Both TEM and SEM images suggest that the deposition of Fe3O4 nanoparticles

on the surface of P(NIPAM-MBAM)/P(MAA-EGDM) IPN hydrogel microspheres most

likely improved the stability during sample preparation as the microspheres remained

mostly spherical. The measurement of magnetic moment of washed P(NIPAM-

MBAM)/P(MAA-EGDM)/Fe3O4 hydrogel microspheres (>>1.23×10-4 BM) indicates that

the microspheres are strongly paramagnetic. The hydrogel microspheres dispersed in

water also visibly attracted towards the magnetic field leaving almost clear supernatant.

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Fig. 1. TEM images of a) P(NIPAM-MBAM) hydrogel microspheres, b) P(NIPAM-

MBAM)/P(MAA-EGDM) IPN hydrogel microspheres, c) Fe3O4 particles, and d)

P(NIPAM-MBAM)/P(MAA-EGDM)/Fe3O4 IPN hydrogel microspheres.

Fig. 2. SEM image of washed P(NIPAM-MBAM)/P(MAA-EGDM)/Fe3O4 IPN hydrogel

microspheres.

FTIR spectra of both P(NIPAM-MBAM) hydrogel and P(NIPAM-

MBAM)/P(MAA-EGDM) IPN hydrogel microspheres shown in Fig. 3 exhibited

500 nm500 nm

a) b)

c) d)

2 μm 1 μm

50 nm 1 μm

a) b)

c) d)

2 μm 1 μm

50 nm 1 μm

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characteristic C=O stretching vibration due to substituted amide group between 1642-

1648 cm-1. Additionally in IPN hydrogel microspheres a small shoulder signal shown in

the inset of Fig. 3 appeared at around 1715 cm-1 is assigned to C=O stretching of

carboxyl group derived from MAA monomer. In the spectrum of Fe3O4 particles the

characteristic broad stretching vibrations due to Fe-O bonds of Fe3O4 particles appeared

at around 574.68 and 391.24 cm-1 respectively, as reported elsewhere [44-46]. The

magnetically doped IPN hydrogel microspheres exhibited similar but weak signal for Fe-

O bonds at 591.19 and 407.27 cm-1 respectively. The characteristic signal due to C=O

stretching of substituted amide and acid groups derived from NIPAM and MAA

comonomers are also observed at 1710 and 1637.72 cm-1 respectively in P(NIPAM-

MBAM)/P(MAA-EGDM)/Fe3O4 hydrogel microspheres. The above results confirmed

the formation of IPN hydrogel microspheres as well as the corresponding magnetically

doped microspheres.

Fig. 3. FTIR spectra of a) P(NIPAM-MBAM) hydrogel microspheres, b) P(NIPAM-

MBAM)/P(MAA-EGDM) IPN hydrogel microspheres, c) Fe3O4 particles, and d)

P(NIPAM-MBAM)/P(MAA-EGDM)/Fe3O4 IPN hydrogel microspheres taken in KBr

pellets.

250125022503250

Wavenumber (cm-1)

T (%

)

a

b

c

d

15001700

b

250125022503250

Wavenumber (cm-1)

T (%

)

a

b

c

d

15001700

b

15001700

b

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A comparative 1H NMR plots for P(NIPAM-MBAM) hydrogel and P(NIPAM-

MBAM)/P(MAA-EGDM) IPN hydrogel microspheres is shown in Fig. 4. Prior to the

measurement, both microspheres were washed by repeated replacement of continuous

phase. The characteristic NMR signal due to acidic (-COOH) protons normally appeared

in the region 10.0-12.0 ppm. But such peak in P(NIPAM-MBAM)/P(MAA-EGDM) IPN

hydrogel microspheres is assigned at 8.2 ppm. It is known that the chemical shift of this

group is variable, depending not only on the chemical environment but also on

concentration, temperature, and solvent [47]. The characteristic proton signals of –

CONH– group in P(NIPAM-MBAM) hydrogel and P(NIPAM-MBAM)/P(MAA-EGDM)

IPN hydrogel microspheres are appeared at 3.4 and 7.2 ppm respectively. The positive

chemical shift in DMSO is possibly due to cumulative effect of solvent and resonance

interaction between unshared electron pair on nitrogen and carbonyl group [48].

Fig. 4. 1H NMR spectra of A) P(NIPAM-MBAM) hydrogel, and B) P(NIPAM-

MBAM)/P(MAA-EGDM) IPN hydrogel microspheres dissolved in CDCl3 and DMSO.

0246810

ppm

0246810

ppm

CH2 CH CH2 CH

C

HN

CHH3C CH3

C

NH

C

NH

CH2

CH2 CH

O O

O

P(NIPAM-MBAAm)

b c

d

e

a

f

CH2 C C

C C

C

CH2

CH2 C

H2C

O

O

CH3 CH3

CH3

OH

CH2

OO

O

P(MAA-EGDMA)

j

i

lk

a, i{b, c, jDMSOH2O

bcdef

kdl

CDCl3A

B

P(NIPAM-MBAM)

P(MAA-EGDM)

0246810

ppm

0246810

ppm

CH2 CH CH2 CH

C

HN

CHH3C CH3

C

NH

C

NH

CH2

CH2 CH

O O

O

P(NIPAM-MBAAm)

b c

d

e

a

f

CH2 CH CH2 CH

C

HN

CHH3C CH3

C

NH

C

NH

CH2

CH2 CH

O O

O

P(NIPAM-MBAAm)

b c

d

e

a

f

CH2 C C

C C

C

CH2

CH2 C

H2C

O

O

CH3 CH3

CH3

OH

CH2

OO

O

P(MAA-EGDMA)

j

i

lk

CH2 C C

C C

C

CH2

CH2 C

H2C

O

O

CH3 CH3

CH3

OH

CH2

OO

O

P(MAA-EGDMA)

j

i

lk

a, i{b, c, jDMSOH2O

bcdef

kdl

CDCl3A

B

P(NIPAM-MBAM)

P(MAA-EGDM)

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The Tg is a characteristic temperature for polymer blend or IPN and is a very

effective way to determine the miscibility of polymer component in an IPN. The Tg of

P(NIPAM-MBAM) hydrogel shown in Fig. 5 is 154.77°C. The incorporation of

crosslinked PMAA in the copolymer hydrogel increased the Tg to 162.64°C. The

polymer-polymer interaction may have a role in determining the phase boundary and

hence the Tg of the IPN. Usually the phase separation in an IPN often results in

independent thermal properties and hence the Tg [49]. However the single Tg observed in

P(NIPAM-MBAM)/P(MAA-EGDM) IPN hydrogel microspheres supported that

PNIPAM and PMAA components formed miscible pair owing to the strong

intermolecular interaction among the chains due to the formation of hydrogen bond

between amide and carboxylic acid groups. In comparison with P(NIPAM-MBAM)

hydrogel the broad temperature interval of glass transition in P(NIPAM-

MBAM)/P(MAA-EGDM) is a normal characteristic feature of IPN as reported elsewhere

despite having compact miscible structure [50].

Fig. 5. DSC heating scans of a) P(NIPAM-MBAM) hydrogel, and b) P(NIPAM-

MBAM)/P(MAA-EGDM) IPN hydrogel microspheres.

3

6

9

12

15

18

30 80 130 180 230

Temperature (°C)

Hea

t flo

w e

ndo

up (m

W)

Tg: 154.77°C

Tg: 162.64°C

a

b

3

6

9

12

15

18

30 80 130 180 230

Temperature (°C)

Hea

t flo

w e

ndo

up (m

W)

Tg: 154.77°C

Tg: 162.64°C

a

b

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Fig. 6 shows the TGA thermograms of Fe3O4 particles, P(NIPAM-

MBAM)/P(MAA-EGDM) and P(NIPAM-MBAM)/P(MAA-EGDM)/Fe3O4 IPN hydrogel

microspheres respectively. It is expected that as the temperature is raised from ambient

temperature to 800°C the organic part of the composite would be burned off and the

remaining percentage after calcination would represent the iron oxide content. As can be

seen in Fig. 6, there is only negligible weight loss for Fe3O4 particles as the temperature

is increased to 800°C. However, relative to reference Fe3O4 particles in P(NIPAM-

MBAM)/P(MAA-EGDM)/Fe3O4 IPN hydrogel microspheres about 45% of the total

weight burned off which corresponded to the organic polymer content. The inorganic

Fe3O4 content in magnetically doped IPN hydrogel microspheres is calculated to be about

52% (w/w), which included both free and doped. The important point is that compared to

P(NIPAM-MBAM)/P(MAA-EGDM) the weight loss onset temperature i.e. degradation

temperature for magnetically doped hydrogel microspheres shifted to higher value. This

indicates that the thermal stability of hydrogel microspheres improved following the

incorporation of inorganic Fe3O4.

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Fig. 6. TGA thermograms of a) Fe3O4 particles, b) P(NIPAM-MBAM)/P(MAA-EGDM),

and c) P(NIPAM-MBAM)/P(MAA-EGDM)/Fe3O4 IPN hydrogel microspheres.

The XRD spectrum of P(NIPAM-MBAM)/P(MAA-EGDM) hydrogel

microspheres suggests that the microspheres are substantially amorphous in character and

a broad reflection centered at around 14º is appeared (Fig. 7). In Fe3O4 doped IPN

hydrogel microspheres two strong sharp reflections at around 36º and 63º are observed,

which are the characteristic peaks normally observed for Fe3O4 nanoparticles [51]. The

broad reflection band in IPN hydrogel microspheres at around 14° almost disappeared

after doping the hydrogel microsphere surface with Fe3O4 nanoparticles.

0

20

40

60

80

100

0 200 400 600 800

Temperature (°C)

Wei

ght l

oss

(%)

a

b

c

0

20

40

60

80

100

0 200 400 600 800

Temperature (°C)

Wei

ght l

oss

(%)

a

b

c

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Fig. 7. XRD patterns of a) Fe3O4 particles, b) P(NIPAM-MBAM)/P(MAA-EGDM), and

c) P(NIPAM-MBAM)/P(MAA-EGDM)/Fe3O4 IPN hydrogel microspheres.

The variations of hydrodynamic diameters of P(NIPAM-MBAM) hydrogel and

P(NIPAM-MBAM)/P(MAA-EGDM) IPN hydrogel microspheres at different pH values

are illustrated in Fig. 8. The average hydrodynamic diameter of P(NIPAM-MBAM)

hydrogel microspheres decreased sharply at around 30°C. This result indicates that the

hydrogel microspheres are hydrophobic and shrink at temperature above the LCST of

around 30°C and below the LCST the microspheres are hydrophilic and swell. This

property is associated with the formation and rupture of hydrogen bond between amide

groups and surrounding water molecules with changing temperature. Compared to

P(NIPAM-MBAM) hydrogel microspheres the average hydrodynamic diameter of

P(NIPAM-MBAM)/P(MAA-EGDM) IPN hydrogel microspheres decreased sharply at

around 33°C (LCST) with the increase of temperature at pH 10 but at lower pH values

the average hydrodynamic diameter remained constant until 33°C and then increased

0 20 40 60 80 100

Two theta scale

Lin

(cou

nts)

a

b

c

0 20 40 60 80 100

Two theta scale

Lin

(cou

nts)

a

b

c

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slightly showing upper critical solution temperature (UCST). At higher pH value the IPN

hydrogel microspheres are in the swollen state due to the deprotonation of carboxyl group

favoring the formation of hydrogen bond with surrounding water molecules. In contrast

at lower pH values the carboxyl groups are protonated and thereby facilitated the

formation of intermolecular hydrogen bond with amide groups forming compact

hydrophobic structure which ultimately reduced the average hydrodynamic diameter

compared to that at higher pH value. At lower pH value as the temperature is increased

above the UCST, the IPN hydrogel microspheres are swelled due to the dissociation of

compact structure by the breakage of intermolecular hydrogen bond. The incorporation of

polar MAA in the IPN hydrogel network possibly shifted the LCST to higher temperature

[52]. The temperature dependent variation of average hydrodynamic diameter of Fe3O4

doped IPN hydrogel microspheres at pH 10 is shown in the inset of Fig. 8. The doping of

Fe3O4 nanoparticles on IPN hydrogel microspheres induced faster transition and shifted

the LCST to lower temperature (30°C). The change in surface property following

deposition of Fe3O4 nanoparticles possibly somehow favored the rupture of hydrogen

bond with the surrounding water molecules at lower temperature and hence lowered the

LCST [53, 54].

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Fig. 8. Variation of hydrodynamic diameters of P(NIPAM-MBAM) hydrogel (circle) and

P(NIPAM-MBAM)/P(MAA-EGDM) IPN hydrogel microspheres at pH 4 (square), pH 7

(triangle), pH 10 (diamond) with temperature. Figure in the inset represents variation of

hydrodynamic diameter of P(NIPAM-MBAM)/P(MAA-EGDM)/Fe3O4 IPN hydrogel

microspheres at pH 10.

So far various technologies such as sedimentation, filtration, coagulation,

adsorption/absorption, ion-exchange have been utilized to remove dyes from industrial

effluents. Among these absorption technology is an effective way for the removal of dyes

due to high efficiency and simplicity. Fig. 9 shows the temperature dependent absorption

of different dyes in P(NIPAM-MBAM)/P(MMA-EGDM)/Fe3O4 IPN hydrogel

microspheres at pH 10. The magnitude of absorption of congo red and magenta at 40°C is

higher than that at 20°C. The higher magnitude of absorption at 40°C is due to the

increased hydrophobic and/electrostatic interaction between the hydrogel microspheres

and dye molecules. At 20°C the hydrogel microspheres turned hydrophilic and hence the

magnitude of absorption is reduced. In contrast the magnitude of absorption of

250

450

650

850

1050

1250

10 20 30 40 50

Temperature (°C)

Dia

met

er (n

m)

700800900

100011001200

20 30 40

250

450

650

850

1050

1250

10 20 30 40 50

Temperature (°C)

Dia

met

er (n

m)

700800900

100011001200

20 30 40

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fluorescein sodium at both 40° and 20°C remained almost same. Fluorescein sodium is an

anionic dye and the IPN hydrogel microspheres are also negatively charged at pH 10 due

to the deprotonation of carboxyl groups. So the electrostatic repulsion between

fluorescein sodium and hydrogel microspheres neutralized the hydrophobic interaction at

40°C. The difference in magnitude of absorption between congo red and magenta is

attributed to the difference in molecular characteristics. Congo red is a hydrophobic dye

whereas magenta is relatively hydrophilic and cationic in nature. The higher

hydrophobicity of congo red favored lower magnitude of absorption at 20°C as the

hydrogel microspheres swelled and became hydrophilic. Comparatively higher

magnitude of absorption for magenta at both 40° and 20°C is based on the electrostatic

attraction between cationic magenta and anionic hydrogel microspheres as the

measurement was carried out at pH 10. A digital photographic representation showing the

removal of dye molecules and separation of magnetically doped IPN hydrogel

microspheres under the influence of magnetic field is shown in Fig. 10. It is clearly

observed that the color intensity decreased after absorption of dye molecules.

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Fig. 9. Magnitude of absorption of different dyes in P(NIPAM-MBAM)/P(MAA-

EGDM)/Fe3O4 IPN hydrogel microspheres at 40° and 20°C measured under the constant

concentration against the total hydrogel content. Conditions: pH 10, dye immobilized 250

mg g-1 of particles, immobilization time 2 h.

Fig. 10. Digital photographic images of (a,b) congo red and (c,d) magenta before and

after absorption from aqueous solution using P(NIPAM-MBAM)/P(MAA-EGDM)/Fe3O4

IPN hydrogel microspheres at 40°C.

-0.1

0.9

1.9

2.9

3.9

Am

ount

ads

orbe

d (m

g/g)

40°C

20°C

Congo red

magenta FluoresceinNa

-0.1

0.9

1.9

2.9

3.9

Am

ount

ads

orbe

d (m

g/g)

40°C

20°C

Congo red

magenta FluoresceinNa

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The reusability of P(NIPAM-MBAM)/P(MMA-EGDM)/Fe3O4 IPN hydrogel

microspheres was also assessed by carrying out reversible alternative absorption-release

experiments for magenta at 40° and 20°C respectively. As shown in Fig. 11 the

magnitude of absorption is always higher at 40°C than that at 20°C. However, the

efficiency of the reusability of the hydrogel microspheres decreased with the number of

recycle. The stability as well as poor redispersibility of the hydrogel microspheres

probably reduced the efficiency of dye removal during repeated measurements.

3.2

3.3

3.3

3.4

3.4

3.5

Temperature (°C)

Am

ount

ads

orbe

d (m

g/g)

40° 20° 40° 20°20° 40°3.2

3.3

3.3

3.4

3.4

3.5

Temperature (°C)

Am

ount

ads

orbe

d (m

g/g)

40° 20° 40° 20°20° 40°

Fig. 11. Magnitude of absorption of magenta in P(NIPAM-MBAM)/P(MAA-

EGDM)/Fe3O4 IPN hydrogel microspheres measured alternatively at 40° and 20°C under

identical conditions. Conditions: pH 10, magenta 250 mg g-1 of particles, immobilization

time 2 h.

4. Conclusion

P(NIPAM-MBAM)/P(MAA-EGDM) hydrogel microspheres with IPN structure

were prepared by sequential polymerization method. The hydrogel microspheres had

multi stimuli-responsive volume phase transition behavior. The pH-responsive volume

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phase transition occurred due to the protonation and deprotonation of carboxylic acid

groups at lower and higher pH values respectively. Regarding temperature-responsive

volume phase transition, formation of inter/intra molecular hydrogen bond among

carboxyl and amide groups at lower pH values induced UCST behavior and the

breakdown of inter/intra molecular hydrogen bond at higher pH value (pH 10) imparted

LCST behavior. The IPN hydrogel microspheres were magnetically doped by in-situ

precipitation of Fe3O4 nanoparticles. The magnetic IPN hydrogel microspheres having

stimuli-responsive volume phase transition were applied for the removal of organic dyes

from their aqueous solutions at pH 10. The nature of the dye influenced the magnitude of

absorption/release at temperatures above and below the LCST. The hydrophobic nature

of the dye favored both adsorption and recovery of dye from dye aqueous solution as the

temperature was changed above and below the LCST. Relative to this the cationic dye

favored maximum adsorption from dye solution at above the LCST but the recovery was

negligible at temperature below the LCST due to the electrostatic attraction with the

negatively charged microspheres. The electrostatic repulsion between anionic dye and

hydrogel microspheres neither favored absorption nor recovery at temperatures above and

below the LCST.

Acknowledgments

This work has been supported by the financial grant from MOSICT, Dhaka. The author

also acknowledges the support for spectral analyses from Central Science Laboratory,

Rajshahi University.

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Magnetically doped multi stimuli-responsive hydrogel microspheres with IPN structure and application in dye removal

Hasan Ahmad, Mohammad Nurunnabi, Mohammad Mahbubor Rahman, Kishor Kumar, Klaus Tauer, Hideto Minami, Mohammad Abdul Gafur Graphical Abstract

H2C CHNHC ON

H3C CH3

+

H2C CHC ONHCH2

NHC OCHCH2

H2C

HCNHC ON

H3 C CH3

H2C

HCC ONHCH2NHC OCH

CH2

Fe3O4 Particle

MAAEGDM,

,MB A mNIPAM

MA

A, EGD

MM

onomer sw

elling

KPS70° C, 24 h

Pptn.

Fe3O4

KPS, 70° C

12 h

P(NIPAM-MBAM)NIPAM MBAM

Magnetic IPN hydrogelmicrosphere

H2C CHNHC ON

H3C CH3

+

H2C CHC ONHCH2

NHC OCHCH2

H2C CHNHC ON

H3C CH3

+

H2C CHC ONHCH2

NHC OCHCH2

H2C

HCNHC ON

H3 C CH3

H2C

HCC ONHCH2NHC OCH

CH2

Fe3O4 Particle

MAAEGDM,

,MB A mNIPAM

Fe3O4 Particle

MAAEGDM,

,MB A mNIPAM

MA

A, EGD

MM

onomer sw

elling

KPS70° C, 24 h

Pptn.

Fe3O4

KPS, 70° C

12 h

P(NIPAM-MBAM)NIPAM MBAM

Magnetic IPN hydrogelmicrosphere

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Highlights • Magnetically doped IPN hydrogel microspheres were prepared.

• The IPN hydrogel microspheres were both temperature- and pH-

responsive.

• Recovery of dye from water was investigated.

• The nature of dye influenced the adsorption-release behaviors.

• The magnetic property can be used to recover dye from industrial

wastewater.

magnetically doped multi stimuli-responsive hydrogel microspheres with ipn structure and application...

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