surface modification of temperature-responsive polymer particles by an electrically conducting...

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Research ArticleReceived: 18 April 2013 Revised: 14 May 2013 Accepted article published: 28 May 2013 Published online in Wiley Online Library: 1 July 2013

(wileyonlinelibrary.com) DOI 10.1002/pi.4565

Surface modification of temperature-responsive polymer particles by an electricallyconducting polyaniline shell layerHasan Ahmad,a∗ Mehnaz Rashid,a Mohammad Mahbubor Rahman,a

Mohammad Abdul Jalil Miah,a Klaus Tauerb and Mohammad Abdul Gafurc

Abstract

In this research an attempt was made to prepare biocompatible electrically conductive composite polymer particles inview of their wide applications in biotechnology. Temperature-sensitive polymer particles have applications as drugcarriers, bioseparators, bioreactor cell activators and diagnostic reagents. So a combination of diverse properties in asingle polymer composite is expected to increase its application potential. Here temperature-responsive poly(N-isopropylacrylamide-methyl methacrylate-N,N′-methylene-bis-acrylamide) (P(NIPAM-MMA-MBAAm)) core particles were prepared byemulsion copolymerization without using any stabilizer. In a second step seeded chemical oxidative polymerization ofdifferent amounts of aniline was carried out in the presence of submicron-sized core particles to obtain P(NIPAM-MMA-MBAAm)/polyaniline composite particles. For a comparative study, reference polyaniline particles were prepared by chemicaloxidative polymerization. Fourier transform IR spectroscopy, UV−visible spectroscopy, thermal and X-ray diffraction analysesshowed that composite particles prepared with higher aniline content (0.8 g) per unit mass (g) of core particles had high surfacecoverage compared with lower aniline content (0.1 g).c© 2013 Society of Chemical Industry

Keywords: temperature-responsive; chemical oxidative polymerization; polyaniline; conductivity

INTRODUCTIONComposite polymer particles exhibiting reversible phase transitiondue to small variations in physical or chemical stimuli

such as temperature,1–3 pH,4–7 electric current,8 ions orchemical species9,10 are most commonly known as stimuli-responsive composite microspheres. These composite particlesfind applications in drug delivery systems, separation operationsin biotechnology, sensors and diagnostic reagents. Both poly(N-isopropyl acrylamide) (PNIPAM) and poly(2-(dimethylamino) ethylmethacrylate) (PDMAEMA) have been extensively studied withregard to the temperature-responsive phase transition, definedby the phenomenon of the lower critical solution temperature

(LCST).11–22 These polymers show a nearly continuous volumephase transition and associated phase transition from a lowtemperature, highly swollen network to a collapsed, hightemperature phase near the LCST. The LCST depends on themicrostructure of the molecule and lies mostly between 30 ◦Cand 35 ◦C. The reason for this phase transition is a goodbalance between hydrophilic and hydrophobic interactions inthe polymer. With an increase in temperature, the polymer chaindehydrates, promoting attractive segmental interactions amongthe hydrophobic groups of the polymer chains, which induces aconformation change from a coil to a globular state.

The objective of this research was to design biocompatibleconducting latex particles. These types of electrical conductingparticles have been found to have particular applicationsin, for example, tissue engineering, regenerative medicineand biosensors. Responsiveness of some tissues to electrical

stimuli makes the biocompatible conducting particles particularlyattractive for several biomedical applications. The availabilityof such materials may provide solutions to many problems inneural biology/medicine.23 They have been shown to modulateactivities of nerve, cardiac, skeletal muscle and bone cells.They stimulate cell growth, migration and adhesion, andenhance DNA synthesis and protein secretion. They mightalso be useful for time predetermined drug release anddelivery. A few reports are available on the preparationof such microgel particles. Lin and Chiu reported thepreparation of temperature-sensitive polypyrrole/P(NIPAM-acrylicacid) composite microgels by chemical oxidation of pyrrole inthe presence of dodecylbenzene sulfonic acid as the primarydopant and P(NIPAM-acrylic acid) microgels as the polymericco-dopant and template.24 In this case the polymerization ofpyrrole occurred directly inside the microgel networks leadingto the formation of composite microgels. In a similar workMolina et al. described the in situ polymerization of anilineand N-methylaniline inside thermosensitive PNIPAM hydrogels.25

∗ Correspondence to: Hasan Ahmad, Department of Chemistry, RajshahiUniversity, Rajshahi 6205, Bangladesh. E-mail: [email protected]

a Department of Chemistry, Rajshahi University, Rajshahi 6205, Bangladesh

b Max Planck Institute of Colloid and Interfaces, Am Muhlenberg, 14476 Golm,Germany

c Pilot Plant and Process Development Centre, BCSIR, Dhaka 1205, Bangladesh

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www.soci.org H Ahmad et al.

CL-

NIPAMMMA

Cross-linked hydrophobic core particleMonomers

PANI

PANI composite

70°C, N2,100 rpm, 12 h

APS, H2O

Ambient temp, N2, 250 rpm, 24 h

Aniline, APS, PVA, H2O

+ -

H2C C C

O

CH3

O H2C CH C

O

N

H

C CH3

CH3

H

MBAAm

H

H2C CH C

O

N

H

C CH2

H

CHC

O

N

H

NHCL

NHCL+ -NHCL

+ -

CL-

NIPAMMMA

Cross-linked hydrophobic core particleMonomers

PANI

PANI composite

70°C, N2,100 rpm, 12 h

APS, H2O

Ambient temp, N2, 250 rpm, 24 h

Aniline, APS, PVA, H2O

+ -

H2C C C

O

CH3CH3

CH3

O H2C CH C

O

N

H

C CH3

CH3

H

MBAAm

H

H2C CH C

O

N

H

C CH2

H

CHC

O

N

H

MBAAm

H

H2C CH C

O

N

H

C CH2

H

CHC

O

N

H

NHCL

NHCL+ -NHCL

+ -

NHCL

NHCL+ -NHCL

+ -

Scheme 1. Preparation of P(NIPAM-MMA-MBAAm)/PANI composite polymer particles.

Cruz-Silva et al. reported the preparation of polyaniline (PANI)colloids by enzymatic polymerization in the presence of chitosanand PNIPAM as steric stabilizer.26

PANI is a hydrophobic conducting polymer that has beenintensively studied for its applications as substrates for biosensorsand matrices for immobilizing proteins. In the present investigationwe attempted to prepare conducting composite polymer particleshaving a core − shell morphology comprising a temperature-sensitive core and a PANI shell. The formation of structuredsemiconducting polymer composites was demonstrated byMezzenga et al. via a self-assembly approach in which continuouspercolation of doped PANI was achieved in an insulatingpolymer matrix.27 This process offers a theoretical advantageconsidering that a low volume fraction of continuous andconducting PANI improves the overall conductivity. However,the use of organic solvents and adverse processing conditionsmay reduce their potential particularly from the viewpointof a clean environment. Here temperature-sensitive P(NIPAM-methyl methacrylate-N,N′-methylene-bis-acrylamide) (P(NIPAM-MMA-MBAAm)) core particles were prepared by soap-freeemulsion copolymerization followed by seeded chemical oxidativepolymerization of aniline. The incorporation of hydrophobiccomonomer (MMA) and crosslinking agent (MBAAm) impartsrigidity to the core particles and hence seeded polymerizationof aniline is expected to occur at or near the surface of the particlesrather than inside them. In order to control the magnitude ofthe conductivity, seeded oxidative polymerization was carriedout with 0.8 and 0.1 g of aniline respectively. The preparation ofbiocompatible P(NIPAM-MMA-MBAAm)/PANI composite particlesis illustrated in Scheme 1.

EXPERIMENTALMaterials and instrumentsAniline (99% pure) of reagent grade purchased from (ThomasBaker, Mumbai, India) was used without purification. NIPAMobtained from Across Organics, USA, was recrystallized from

a mixture of 90% hexane and 10% acetone and dried undervacuum at low temperature before preserving in a refrigerator.MMA of monomer grade from Fluka, Chemika, Switzerland,was passed through basic alumina to remove any inhibitorand preserved in a refrigerator. MBAAm, a crosslinking agentpurchased from Sigma Chemical Co., USA, was used withoutpurification. Ammonium persulfate (APS) from (Thomas Baker,Mumbai, India) was recrystallized from water before use. Poly(vinylalcohol) (Thomas Baker) of molecular weight 1.4 × 104 g mol−1

was used as a polymeric stabilizer. Other chemicals used were ofreagent grade. Deionized water was distilled using a glass (Pyrex)distillation apparatus.

Transmission electron microscopy (TEM) (Zeiss EM 912 Omega)was used to investigate the morphology and particle size distri-bution. An IR spectrophotometer (Perkin Elmer, FTIR-100, UK),double beam UV − visible spectrophotometer (Shimadzu, UV-1650pc), X-ray diffractometer (Bruker D8 Advance, Germany),precision impedance analyzer (PIA-4294A, Agilent, USA) andpH meter (Mettler Toledo MP 220, Switzerland) were used inthis study. Thermal analyses were carried out using a thermo-gravimetric analyzer from Seiko Instruments Inc. (EXSTAR-6000TGA, Japan).

Preparation of PANI particlesAniline (6 g) was dissolved in HCl (1 mol L−1) solution and the pHwas adjusted to 1.0. The aniline content was maintained at 4.1%(w/w). The aniline solution was transferred into a three neckedround flask. APS (18.3775 g) was added immediately under anitrogen atmosphere and the polymerization was continued atambient temperature for 12 h while the reaction medium wasmagnetically stirred at 200 rpm.

P(NIPAM-MMA-MBAAm) core particles by soap-free emulsioncopolymerizationP(NIPAM-MMA-MBAAm) core particles were prepared by soap-freeemulsion copolymerization of NIPAM (3.6 g), MMA (0.28 g) and

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Conducting temperature-responsive polymer particles www.soci.org

Table 1. Preparation of P(NIPAM-MMA-MBAAm)/PANI compositepolymer particles by seeded oxidative polymerizationa

Ingredients

Core − shell

ratio 1/0.8

Core − shell

ratio 1/0.1

P(NIPAM-MMA-MBAAm ) (g) 1.0 1.0

Aniline (g) 0.8 0.1

APS (g) 1.6 0.2

PVA (g) 0.08 0.08

H2O (g) 100.0 100.0

PVA, poly(vinyl alcohol).a Ambient temperature, N2, 24 h, 250 rpm.

MBAAm (0.12 g) using APS (0.08 g) as the water-soluble initiator.The copolymerization was carried out in a 250 mL three neckedround bottomed flask under a nitrogen atmosphere at 70 ◦Cmaintained by a thermostat water bath. The reaction mixturewas mechanically stirred (100 rpm) and the polymerization wascontinued for 12 h.

P(NIPAM-MMA-MBAAm)/PANI composite polymer particlesby seeded chemical oxidative polymerizationP(NIPAM-MMA-MBAAm)/PANI composite polymer particles wereprepared by seeded chemical oxidative polymerization of anilinein the presence of submicron-sized P(NIPAM-MMA-MBAAm) coreparticles using APS as an oxidant. Aniline was first dissolved in HCl(1 mol L−1) solution and the pH was adjusted to below 2.5. ThenP(NIPAM-MMA-MBAAm) core emulsion and the remaining waterwere added to aniline hydrochloride salt solution. Finally, thepH of the reactant solution was adjusted to 2.5 by the furtheraddition of a few drops of HCl solution. The polymerizationwas immediately started by adding APS solution at ambienttemperature under a nitrogen atmosphere. The reaction mixturewas magnetically stirred at ambient temperature for 24 h tocomplete the polymerization. Two different composites wereprepared with different core − shell ratios. The preparationconditions are shown in Table 1.

Thermogravimetric analysisThermal properties of the dried powder were measured by heatingsamples under a flowing nitrogen atmosphere from 30 ◦C to1000 ◦C at a heating rate of 20 ◦C min−1 and the weight loss wasrecorded.

X-ray diffractionThe XRD patterns of the powder samples were taken with ascanning X-ray diffractometer using Cu Kα radiation (λ ≈ 1.54 A), atube voltage of 33 kV and a tube current of 45 mA. The intensitieswere measured at 2θ values from 4.5◦ to 50◦ at a continuous scanrate of 10◦ min−1 with a position-sensitive detector aperture at 3◦

(equivalent to 0.5◦ min−1 with a scintillation counter).

Measurement of electrical conductivityThe conductivities of the reference PANI particles, P(NIPAM-MMA-MBAAm) core particles and P(NIPAM-MMA-MBAAm)//PANIcomposite particles prepared with two different core − shell ratioswere obtained by measuring conductances using an impedanceanalyzer. For this measurement a circular disk of ∼10 mm diameter

0

200

400

600

800

1000

1200

10 20 30 40

Hyd

rod

ynam

ic d

iam

eter

(n

m)

2000

2500

3000

3500

4000

4500

5000

5500

Temperature (°C)

Figure 1. Temperature-dependent variation of the average hydrodynamicdiameter of P(NIPAM-MMA-MBAAm) core particles and P(NIPAM-MMA-MBAAm)/PANI composite polymer particles prepared with a differentamount of aniline. The arrows indicate the relevant y axis.

and ∼ 0.1 mm thickness was prepared from each dried polymersample. Then conductance values were measured from threedifferent points for each sample disk at ambient temperature.The conductivity (S cm−1) was calculated from the averageconductance.

RESULTS AND DISCUSSIONFigure 1 shows the temperature-dependent variation ofhydrodynamic diameter for washed P(NIPAM-MMA-MBAAm)core particles and P(NIPAM-MMA-MBAAm)/PANI compositepolymer particles prepared with different aniline contents. Thehydrodynamic diameter of P(NIPAM-MMA-MBAAm) core particlesdecreases gradually over the temperature range 20 − 37 ◦C withthe mid-point of the transition region about 28 ◦C, which isclose to the average LCST of PNIPAM aqueous solution (32 ◦C).The incorporation of hydrophobic MMA comonomer possiblydecreases the LCST to lower temperature.28 Also in the early workwe observed that incorporation of hydrophilic methacrylic acid(MAA) in temperature- and pH-responsive polystyrene/P(NIPAM-MAA-MBAAm) composite particles broadened the phase transitionregion from 32 to 50 ◦C at higher pH value.5 In other work Alamet al. reported the same broadening effect on the phase transitiondue to the incorporation of hydrophilic DMAEMA comonomer.29

The gradual decrease in hydrodynamic diameter for P(NIPAM-MMA-MBAAm) core particles suggests that at temperatures belowthe LCST the particles swell with water and with increasingtemperature the particles dehydrate and collapse. Compared withthis P(NIPAM-MMA-MBAAm)/PANI composite polymer particlesprepared with 0.8 g aniline do not show any change inhydrodynamic diameter whereas composite particles preparedwith 0.1 g aniline show only a weak phase transition. In the lattercase, the poor surface coverage of P(NIPAM-MMA-MBAAm) coreparticles by the PANI shell layer is probably responsible for thecontribution to the temperature-responsive phase transition. Thisresult suggests that by lowering the surface coverage of P(NIPAM-MMA-MBAAm) core particles by a PANI shell layer it is possibleto induce a temperature-responsive phase transition in the

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composite particles to a certain extent. After centrifugal washingthe colloidal stability of PANI-modified composite particles becamepoor and hence the hydrodynamic diameters became relativelylarger compared with the electron micrograph images.

Figure 2 shows the TEM photographs of P(NIPAM-MMA-MBAAm) core particles and PANI particles, as well as P(NIPAM-MMA-MBAAm)/PANI composite polymer particles prepared withdifferent amounts of aniline. P(NIPAM-MMA-MBAAm) coreparticles are spherical and monodispersed. The average diameterand coefficient of variation of the core particles are 340 nmand 14.47% respectively. Compared with this PANI particlesare rod shaped; a similar morphology has also been reportedin the literature.30,31 Irrespective of core − shell ratio, P(NIPAM-MMA-MBAAm)/PANI composite polymer particles prepared byseeded oxidative polymerization are rather spherical. The averagediameters and coefficients of variation are 604.8 nm and 9.68%for composite particles prepared with 0.8 g aniline and 374.5 nmand 6.75% for composite particles prepared with 0.1 g aniline. Itis reasonable to expect that composite particles prepared withhigher aniline content would be larger than those preparedwith lower aniline content. However, the diameters of thecomposite particles are much larger than the theoretical diametersparticularly at higher aniline content. The theoretical diametersare calculated to be 413.6 and 351 nm respectively for compositesprepared with 0.8 g and 0.1 g aniline. The reason is not clear. But it isreasonable to assume that P(NIPAM-MMA-MBAAm) core particlesare highly hydrophilic and swelled with water in the dispersionstate; when dried during sample preparation the particles shrankand deswelled. In comparison, the encapsulation of hydrophilicP(NIPAM-MMA-MBAAm) core particles by the hydrophobic PANIshell layer possibly prevented the dehydration even when driedat ambient temperature during sample preparation. In bothcomposite particles darker pockets indicating the PANI networkare visible both at the surface and inside the particles and suchpockets quantitatively decrease from an average of nine perparticle prepared with higher aniline content to an average oftwo to three per particle prepared with lower aniline content.The use of hydrophobic comonomer and crosslinking agent inthe core particles increases the localization of PANI at the surfacecompared with the previously reported PNIPAM hydrogel25 andproduces mostly a core − shell type morphology particularly athigher aniline content. In similar work Okubo et al. reportedthe formation of a core − shell morphology in polystyrene/PANIcomposite particles prepared by seeded chemical oxidativepolymerization in the presence of hydrophobic polystyrenecore particles.32 Rod shaped particles similar to PANI are notvisible in TEM images of the two composite particles. All theseresults indicate that seeded oxidative polymerization occurredsuccessfully without secondary nucleation.

Fourier transform IR (FTIR) spectra were used to characterize thecomposite structure. Figure 3 shows the FTIR spectra of P(NIPAM-MMA-MBAAm) core particles, reference PANI particles and thecorresponding composite particles prepared with different anilinecontents. These spectra were taken in KBr pellets. In P(NIPAM-MMA-MBAAm) core particles the characteristic absorption banddue to symmetric stretching of the C=O (associated) groupappears at 1657 cm−1. The peaks appearing at 3430 cm−1 and2974 cm−1 are due to secondary N−H (symmetric) stretching andC−H stretching vibrations. The absorption band at 1548 cm−1

represents the secondary N−H bending of the substituted amidegroup. Additionally the peaks at 1172 cm−1 and 1131 cm−1

correspond to C−C (or C−N) stretching modes. The bands

Figure 2. TEM photographs of (a) P(NIPAM-MMA-MBAAm) core particles,(b) PANI particles and (c), (d) P(NIPAM-MMA-MBAAm)/PANI compositepolymer particles prepared with different aniline contents: (c) 0.8 g; (d)0.1 g.

at 672 cm−1 can be attributed to out-of-plane N−H bendingmodes. In the spectrum of PANI the characteristic continuousabsorption appears from 1800 to 4000 cm−1 with a shoulder bandat 3377 cm−1 representing the N−H stretching vibration of an

aromatic amine as well as of its salt.33–35 The absorption signalsappearing in the 1440 − 1600 cm−1 region are related to the C = Nstretching vibration of benzoid and quinoid rings respectively andthat at around 1300 cm−1 is due to the C−N stretching vibration.Comparing the spectrum of composite particles prepared by using0.8 g aniline with the spectra of P(NIPAM-MMA-MBAAm) coreparticles and PANI, the vibration bands for the composite particlescan be reasonably assigned to PANI except for a slight deviation.This result indicates that a composite of P(NIPAM-MMA-MBAAm)decorated with PANI has been formed. In composite particlesprepared with lower aniline content (0.1 g) the vibration bandsare similar to those for P(NIPAM-MMA-MBAAm) core particlesindicating a low or no inclusion of PANI. It should be mentionedthat the FTIR technique is an acceptable tool for confirming theformation of a surface layer even on nanoparticles.36

The UV − visible absorption spectra of a dilute dispersionof P(NIPAM-MMA-MBAAm) core particles, PANI particles andP(NIPAM-MMA-MBAAm)/PANI composite particles prepared withdifferent aniline content are illustrated in Fig. 4. In PANI twocharacteristic absorption peaks appear near 345 and 450 nm andone absorption peak appears above the 600 nm region. The peakat 345 nm arises from π−π* electron transitions within benzenerings. The peak at 450 nm represents the transition betweenpolaron and π* bands normally observed for emeraldine salts. Thebroad band above 600 nm is due to the presence of polarons

resulting from the doping process.37–39 In P(NIPAM-MMA-MBAAm) core particles no such absorption signal is observed.Compared with this, the spectrum of the composite particlesprepared with 0.8 g aniline shows a strong resemblance to thereference PANI particles, whereas composite particles preparedwith 0.1 g aniline show no such absorption band. These results


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4001300220031004000

Wavenumber (cm-1)

T (

%)

a

b

c

d

Figure 3. FTIR spectra of P(NIPAM-MMA-MBAAm) core particles (curvea), PANI particles (curve b) and P(NIPAM-MMA-MBAAm)/PANI compositepolymer particles prepared with different aniline contents: curve c, 0.8 g;curve d, 0.1 g.

0.0

0.5

1.0

1.5

2.0

250 450 650 850

Wavelength (nm)

Ab

sorb

ance

a

d

b

c

Figure 4. UV−visible spectra of P(NIPAM-MMA-MBAAm) core particles(curve a), PANI particles (curve b) and P(NIPAM-MMA-MBAAm)/PANIcomposite polymer particles prepared with different aniline contents:curve c, 0.8 g; curve d, 0.1 g.

are in agreement with the FTIR spectra and it is expected thatcomposite particles prepared with higher aniline content are richin PANI while those prepared with lower aniline content do notcontain enough PANI to be identified. The disappearance of thePANI band in composite particles prepared with 0.1 g aniline mayalso result from a change in electron conjugation, i.e. a changein chemical structure of PANI, although such probability is aminimum as both particles were prepared under the sameconditions with the same doping degree.

Figure 5 shows the thermograms of P(NIPAM-MMA-MBAAm)core particles, PANI particles and P(NIPAM-MMA-MBAAm)/PANIcomposite particles. The reference PANI material showsmonotonic mass loss over a wide temperature range, exhibitingthree distinct mass-loss regions. The initial mass loss between 60

0

20

40

60

80

100

0 200 400 600 800 1000

Temperature (°C)

Wei

gh

t lo

ss (

%)

d

b

c

a

Figure 5. TGA thermograms of P(NIPAM-MMA-MBAAm) core particles(curve a), PANI particles (curve b) and P(NIPAM-MMA-MBAAm)/PANIcomposite polymer particles prepared with different aniline contents:curve c, 0.8 g; curve d, 0.1 g.

and 130 ◦C is associated with the loss of moisture.40,41 Further massloss between 200 and 280 ◦C represents the evolution of dopantHCl.42 A sharp mass loss starting at around 470 ◦C correspondsto large scale thermal degradation of PANI main chains39 andnearly 49% residual mass is left at 1000 ◦C. The thermogramof composite particles prepared with 8% aniline presumablyexhibits three minor mass-loss regions related to the referencePANI and one major mass-loss region related to P(NIPAM-MMA-MBAAm) core particles. The residual mass left at 1000 ◦C is around32% indicating successful quantitative incorporation (>25%) ofaniline in P(NIPAM-MMA-MBAAm) core particles. In comparison,P(NIPAM-MMA-MBAAm)/PANI composite particles prepared with0.1 g aniline do not show a mass-loss region like the reference PANIbut the thermal stability improves considerably as the residualmass left at 1000 ◦C is higher (16%) than that (7%) of the coreP(NIPAM-MMA-MBAAm) particles. So this behavior supports theincorporation of aniline (>9%) in composite particles even atlower aniline content (0.1 g) although the amount may not beenough for complete encapsulation of core particles and may notbe measurable by FTIR and UV − visible spectroscopy.

The XRD pattern of reference PANI particles shows peaks at26.11◦ (110 face) and 20.5◦ (100 face), which are similar tothose of highly doped emeraldine salt.40 Such a scattering profile(Fig. 6) points to the unstructured and amorphous characterof the polymer. Compared with this P(NIPAM-MMA-MBAAm)core particles show one broad characteristic amorphous peakcentered at 21.9◦, whereas composite particles irrespective ofaniline content show a characteristic broad peak of both coreparticles and PANI particles. The doping degree and incorporationof core particles possibly affect the peak as well as the peakshape.43

The electrical conductivities of P(NIPAM-MMA-MBAAm) coreparticles, reference PANI particles and composites prepared withdifferent amounts of aniline are presented in Table 2. Theconductivity of PANI is 4.44 × 10−4 S cm−1. The conductivity ofthe electrically insulating P(NIPAM-MMA-MBAAm) core particlesis much lower. Relative to this the conductivity of P(NIPAM-MMA-MBAAm)/PANI composite particles prepared with 0.8 g aniline is


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0 20 40 60 80

Two theta

Lin

(co

un

ts)

a

bc

d

Figure 6. XRD patterns of P(NIPAM-MMA-MBAAm) core particles (curvea), PANI particles (curve b) and P(NIPAM-MMA-MBAAm)/PANI compositepolymer particles prepared with different aniline contents: curve c, 0.8 g;curve d, 0.1 g.

Table 2. Electrical conductance and conductivity of P(NIPAM-MMA-MBAAm) core particles, PANI particles and corresponding compositeparticles

Sample name Conductivity (S cm−1)

PANI 4.43 × 10−4

P(NIPAM-MMA-MBAAm) 4.0 × 10−7

P(NIPAM-MMA-MBAAm)/PANI,core − shell ratio 1/0.8 (w/w)

1.6 × 10−6

P(NIPAM-MMA-MBAAm)/PANI,core − shell ratio 1/0.1 (w/w)

4.0 × 10−7

1.6 × 10−6. However, for better comparison with the conductivityof reference PANI the overall conductivity of two-phase P(NIPAM-MMA-MBAAm)/PANI composite particles is normalized by takinginto account the volume fraction of the conductive PANI shell.The volume fraction of the conductive PANI shell in P(NIPAM-MMA-MBAAm)/PANI composite particles prepared with 0.8 ganiline is calculated to be ca 22.8% and hence the conductivityis approximated as 3.6 × 10−5 S cm−1,27,44 which is only oneorder of magnitude lower than the reference PANI. The ratherhigh conductivity suggests that the continuous phase in thedisk consists of the green protonated emeraldine salt form of

PANI which is known to be electrically conductive.45–47 Thegreen protonated emeraldine salt has the following structure:

In comparison the composite particles prepared with loweraniline content (0.1 g) exhibit very low electrical conductivityindicating only negligible surface coverage of the core particlesby PANI. This result gives the important information that bycontrolling the aniline content during seeded chemical oxidativepolymerization it is possible to control the electrical conductivity.It should be added that the electrical conductivity of our preparedcomposite particles cannot be compared with already publishedwork on conducting PNIPAM nanocomposites prepared by in

situ polymerization of aniline inside thermosensitive hydrogelsbecause of the non-availability of the relevant data.25 However,it is reasonable to believe that the presence of PANI pocketsinside the PNIPAM hydrogel microspheres reduces the electricalconductivity by a larger margin from the reference PANI material.

CONCLUSIONSThe surface of temperature-responsive P(NIPAM-MMA-MBAAm)core particles has been successfully modified by conductivePANI particularly at higher aniline content. The incorporationof hydrophobic comonomer (MMA) and crosslinker (MBAAm)limits the polymerization of hydrophobic aniline inside the coreparticles and it occurs mostly at the surface. The electricalconductivity of the modified core − shell composite polymerparticles can be controlled by varying the amount of anilinecontent. Biocompatible PANI composites are expected to becomea novel material in biotechnology because of their core − shellmorphology, size monodispersity and ability to control particlecharacteristics such as PANI content and hence the electricalconductivity.

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