Structural, Dielectric, Optical and Magnetic Properties of Ti3

Hindawi Publishing Corporation Journal of Polymers Volume 2013, Article ID 295291, 10 pages http://dx.doi.org/10.1155/2013/295291

Research Article Structural, Dielectric, Optical and Magnetic Properties of Ti3+, Cr3+, and Fe3+: PVDF Polymer Films M. Obula Reddy and L. Raja Mohan Reddy Department of Physics, Loyola Degree College (YSRR), Pulivendla 516390, India Correspondence should be addressed to M. Obula Reddy; [email protected] Received 12 July 2013; Accepted 14 August 2013 Academic Editor: Yves Grohens Copyright © 2013 M. O. Reddy and L. R. M. Reddy. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. Highly transparent and very clear nature of PVDF: Ti3+ , PVDF: Cr3+ , and PVDF: Fe3+ polymer films of good quality have been synthesized by employing solution casting method. XRD profiles have confirmed semicrystalline structures in 𝛼-, 𝛽-, and 𝛾-PVDF phases. IR spectra have confirmed these findings and revealed some structural defects such as monofluorinated alkenes. Emission spectra reveal that PVDF: Ti3+ has a blue emission, PVDF: Cr3+ has blue emission, and PVDF: Fe3+ red emission was observed. The conductivity and dielectric measurements have also been carried out as function of frequency and temperature changes. Due to the presence of the transition metal ions in these films, significant improvement in the ionic conductivity has been noticed. The dielectric behaviors of these films have been analyzed using dielectric permittivity (𝜀1 ), dissipation factor (tan 𝛿), and impedance spectra (Z1 and Z11 ). VSM measurements have confirmed that the PVDF: Ti3+ exhibits antiferromagnetic nature, PVDF: Cr3+ film ferromagnetic nature, and PVDF: Fe3+ film strong paramagnetic nature. Thus, the present study has successfully explored the fact that these optical materials are also potential enough in both conductivity and magnetic properties for their use in applications suitably.

1. Introduction Polymer based magnetoelectric materials are promising materials such as conductive adhesives, supported catalysts, sensors, luminescent films, electrooptical devices, integrated optics, memory devices, and optical data processing technologies, and it is possible to achieve impressive enhancements of material properties as compared with the pure polymers, as being metal free and environmentally acceptable, due to the polymers unique characteristics such as flexibility, light weight, versatility, and low cost, and in some cases, biocompatibility can be taken to advantage [1–8]. As a semicrystalline polymer, poly(vinylidene fluoride) (PVDF) is fit for membrane material due to its excellent chemical resistance, physical and thermal stability, high strength, high dielectric constant, and flexibility. Another feature that distinguishes PVDF from other polymers is its polymorphism; that is, it may present at least four crystalline phases of 𝛼, 𝛽, 𝛿, and 𝛾. Moreover, an increasing interest has been devoted to PVDF in the development of electric or magnetic field sensors. For this application PVDF was added with

transition metal (Ti3+ , Cr3+ and Fe3+ ) ions to evaluate their electrical and magnetic properties.

2. Experimental Study 2.1. Sample Preparation and Characterization. The materials used in the present work are all high purity grade. PVDF, DMA, TiCl3 , CrCl3 , and FeCl3 are used for this study. Both the host polymer and dopant chemicals have been dissolved separately in 99.9% pure dimethylacetamide (DMA) in the proportion of 9 : 1 (Wt%) and such mixed solutions were stirred thoroughly using a magnetic stirrer at temperature 323 K for 24 hrs and each of these polymer solutions was cast onto Petri dishes and was kept in a vacuum oven at 333 K for two days in removing solvent traces while evaporation process takes place. The thickness of the films was in the range of ≈100 𝜇m. The structures of the prepared polymers were characterized on XRD 3003 TT Seifert diffractometer with Cuk𝛼 ˚ at 40 KV and 20 mA and the 2𝜃 radiation (𝜆 = 1.5406 A)

2

Journal of Polymers Table 1: X-ray diffraction peaks semi-crystalline PVDF phases. Values of 2 and respective 𝑑 spacing observed in the X-Ray Diffraction ˚ 𝑑 (A) Assignment

2𝜃 (in degrees) 20.25 39.64 42.06 46.21

𝛽 𝛼 𝛼 𝛾

4.30 2.28 2.13 2.06

120 Transmittance (%)

Intensity (a.u.)

40 0 80

(b)

40 0 80

(c)

40 0 80

(b)

40 0 80

(a)

40

40 0

110 002 042 114

80

(c)

80

Plane

10

20

30

40 2𝜃 (deg)

50

60

70

Figure 1: ((a)–(c)) XRD profile of PVDF: Ti3+ , PVDF: Cr3+ and PVDF: Fe3+ polymer films.

range was varied between 10∘ and 70∘ . Perkin-Elmer FT-IR spectrophotometer was used for recording FTIR spectrum of the PVDF film in the region of 4000–400 cm−1 . Both excitation and emission spectra of these polymer films were recorded on a SPEX FLUOROLOG (model-II) attached with a Xe-arc lamp (150 W). The dielectric constant, dielectric loss, and conductivity measurements were carried out on a Phase Sensitive Multimeter (PSM) Model-1700. The magnetic moment profile as function of applied magnetic field was measured on a Vibrating Sample Magnetometer (VSM) (Model-4500 magnet walker (model HF-20H)).

3. Results and Discussion 3.1. XRD Analysis. XRD diffraction reveals that the PVDF polymer films under investigation are characterized by semicrystalline structure and it has three crystal structures including 𝛼-, 𝛽- and 𝛾-phases [9, 10]. From Figures 1(a)–1(c), the spectra of the (a) PVDF: Ti3+ , (b) PVDF: Cr3+ , and (c) PVDF: Fe3+ polymer films present peaks at 2𝜃 ≈ 20.25∘ , 39.64∘ , 42.06∘ and 46.21∘ and labeled to the diffractions in planes of (110), (002), (042) and (114), respectively; those could be due to the characteristic peaks of 𝛼, 𝛽, and 𝛾 crystalline phases, and are confirmed by FTIR analysis as well. The results is shown in Table 1 and in good agreement with the reports made available in the literature [11, 12]. 3.2. FTIR Analysis. Figures 2(a)–2(c) show the plots of IR transmission spectra of the PVDF: Ti3+ , PVDF: Cr3+ , and PVDF: Fe3+ polymer films in the spectral range of 4000– 400 cm−1 and all the spectra are found to be more similar

0 4000

(a) 3500

3000 2500 2000 1500 Wavenumber (cm−1 )

1000

500

Figure 2: ((a)–(c)) FTIR spectra of PVDF: Ti3+ , PVDF: Cr3+ and PVDF: Fe3+ polymer films.

in nature. The main PVDF characterizing frequencies are observed, where 𝛽 phase occurs mostly at 440 cm−1 , 510 cm−1 and 840 cm−1 and, for 𝛼 phase at 430 cm−1 , 610 cm−1 , 765 cm−1 , and 1420 cm−1 . However, the peaks at 880 cm−1 and 950 cm−1 were identified as belonging to the 𝛾-phase. Couple bands that are identified at 1665 cm−1 and 1629 cm−1 have been assigned to the C=C stretching as was reported earlier [13, 14] and this indicates the presence of polarons in the polymeric matrix. The band at 748 cm−1 which refers to the head to head (h-h) and tail to tail (t-t) defects [15]. The change of the intensities of these IR bands indicates a change of the chelation mode of Ti3+ , Cr3+ , and Fe3+ in PVDF matrix. It is interesting to notice that the C=C mode and the h-h defects are suitable sites for polarons and bipolarons formation in the PVDF matrix. 3.3. Photoluminescence Analysis. Figures 3(a)–3(f) show both excitation and emission spectra of PVDF: Ti3+ , PVDF: Cr3+ and PVDF: Fe3+ polymer films. For PVDF: Ti3+ , the emission spectrum has been obtained with 𝜆 exci = 387 nm, the emission spectrum displays emissions at 413 nm (could not be assigned) 435 nm (3 T1 (F) → 3 A2 (F)), and 457 nm (also could not be assigned to a transition). For PVDF: Cr3+ , the emission spectrum has been measured with 𝜆 exci = 385 nm, strong emission bands are observed at 421 nm ( 2 Eg → 4 T2g ) and 435 nm ( 4 T2g → 4 A2g ). For PVDF: Fe3+ emission spectrum has been recorded with 𝜆 exci = 329 nm, strong emission bands are observed at 588 nm and 625 nm (4 A2 →4 T1 (4 P)). All assignments that are made on the measured emission bands are in agreement with the literature reported results [16].

Journal of Polymers

3 ×105 5

×106 9

PVDF: Ti 3+

387 nm Emis.Int. (a.u.)

4 Excit.Int. (a.u.)

3

8

3 2

T1 (F) → 3A 2 (F) 435 nm

7

PVDF: Ti 3+ 457 nm

6

413 nm

5 4 3

1 0 300

2 1 325

350 375 Wavelength (nm)

400

400

420

440 460 Wavelength (nm)

(a)

73000

(b) ×10 18

PVDF: Cr 3+ 372 nm

385 nm

5

4

16

71000

Emis.Int. (a.u.)

Exci.Inten. (a.u.)

72000

70000 69000

14

2

PVDF: Cr 3+

T2g → 4 A 2g 435 nm

460 nm

E g → 4 T2g 421 nm

12

68000 10

67000 66000 365

370

375 380 385 Wavelength (nm)

8

390

410

420

430 440 450 460 Wavelength (nm)

(c)

470

480

(d)

PVDF: Fe3+ 386 nm

1020

15000 Emis.Inten. (a.u.)

980 960 940

PVDF: Fe3+

16000

1000 Exci.Int. (a.u.)

480

4

A 2 → 4 T1 (4 P) 625 nm

14000 13000 588 nm

12000 11000 10000

920

9000

900 340 350 360 370 380 390 Wavelength (nm)

8000 400 410

(e)

570

580

590 600 610 Wavelength (nm)

620

630

(f)

Figure 3: ((a)-(b)) Excitation and emission of PVDF: Ti3+ polymer films. ((c)-(d)) Excitation and emission of PVDF: Cr3+ polymer films. ((e)-(f)) Excitation and emission of PVDF: Fe3+ polymer films.

3.4. Impedance Analysis. Impedance analysis reflects the collective response of microscopic polarization process under an external electric field. Under an alternating field, frequency dispersion or dielectric relaxation is observed due to different polarization mechanisms within a dielectric. Typical spectroscopic plots of dielectric constant (𝜀1 ) versus frequency (𝑓)

for the three polymer (PVDF: Ti3+ , PVDF: Cr3+ and PVDF: Fe3+ ) films are shown in Figures 4(a)–4(c). It is observed that the 𝜀1 decreases rapidly with increasing frequency. The rapid fall of dielectric constant (𝜀1 ) in their frequency range shows that space charge polarization contributes significantly to the total polarization observed in these films. Space

4

Journal of Polymers 1200

500

PVDF: Cr 3+

PVDF: Ti 3+ 1000 Dielectric constant (𝜀)

Dielectric constant (𝜀)

400

300

200

100

800 600 400 200 0

0 0

1

2

3 4 Frequency (Hz)

5

6 ×105

0

70∘ C 80∘ C 90∘ C 100∘ C

RT 40∘ C 50∘ C 60∘ C

1

2

3 4 Frequency (Hz)

5

6 ×104

70∘ C 80∘ C 90∘ C 100∘ C

RT 40∘ C 50∘ C 60∘ C

(a)

(b)

800

PVDF: Fe3+

Dielectric constant (𝜀)

600

400

200

0 0

4

8 12 Frequency (Hz)

16

20 ×104

70∘ C 80∘ C 90∘ C 100∘ C

RT 30∘ C 40∘ C 50∘ C 60∘ C (c)

Figure 4: ((a)–(c)) Dielectric constant as function of frequency for PVDF: Ti3+ , PVDF: Cr3+ and PVDF: Fe3+ polymer films.

charge polarization (interfacial polarization) arises whenever phases of different conductivities are present in the same material. When an electric field is applied, the charges more through a conducting phase but are interrupted as they come across a high resistivity phase. This leads to a buildup of charge at the interface, which is manifested as enhanced polarization. These builtup charges cannot follow the applied field rapidly at high frequencies and hence, the resultant loss. The dielectric loss (tan𝛿) versus frequency (𝑓) for the three polymer (PVDF: Ti3+ , PVDF: Cr3+ , and PVDF: Fe3+ ) films are shown on Figures 5(a)–5(c). A single relaxation peak was

observed. The loss peak maximum shifts to a lower frequency with decreasing temperature, and the peak intensity decreases and the distribution broadens. This may be due to the result of a dominant effect of the polarization by migrating charges at low frequency. The dispersion observed at low frequencies could be attributed to the interfacial polarization mechanism and the dispersion observed at high frequencies could be attributed to the dipolar relaxation [15, 17]. Complex plane plots (𝑍1 versus 𝑍11 ) for a typical polymer films are shown in Figures 6(a)–6(c). The behaviour of films and at different temperatures is similar. The Cole-Cole

Journal of Polymers

5

PVDF: Ti 3+

20

PVDF: Cr 3+

40

Dielectric loss (Tan𝛿)


35 15

10

30 25 20 15 10

5

5 0

0 0

1

2

3 Log(f)

4

5

6

0

70∘ C 80∘ C 90∘ C 100∘ C

RT 40∘ C 50∘ C 60∘ C

1

2

3 Log(f)

4

5

6

70∘ C 80∘ C 90∘ C 100∘ C

RT 30∘ C 40∘ C 50∘ C 60∘ C (b)

(a)

35

PVDF: Fe3+


30 25 20 15 10 5 0 0

1

2

3 Log f (Hz)

4

5

6

70∘ C 80∘ C 90∘ C 100∘ C

RT 30∘ C 40∘ C 50∘ C 60∘ C (c)

Figure 5: ((a)–(c)) Dielectric loss as function of frequency for PVDF: Ti3+ , PVDF: Cr3+ and PVDF: Fe3+ polymer films.

plots showed two well-defined regions. semi circle pattern indicating the involvement of a single conduction mechanism in the composite system. The single mechanism is attributed to grain size or due to the mobility of free charges (polaron or free ions) induced by the increment of temperature. The bulk dc resistance (𝑅𝑏 ) of the electrolyte was determined as

the point where the high frequency semicircle in the plots cuts the 𝑍1 axis. The ionic conductivity (𝜎) is calculated

𝜎dc =

𝑑 , (𝑅𝑏 ⋅ 𝐴)

(1)

6

Journal of Polymers 2000

PVDF: Ti 3+

PVDF: Cr 3+

20000

1500

Z󳰀󳰀 (Ohm)

Z󳰀󳰀 (Ohm)

15000

1000

10000

500

5000

0

0 2000

4000

6000

8000

0

10000

2

󳰀

Z (Ohm)

70∘ C 80∘ C 90∘ C 100∘ C

RT 40∘ C 50∘ C 60∘ C

4 Z󳰀 (Ohm)

6

8 ×104

60∘ C 70∘ C 80∘ C

RT 40∘ C 50∘ C 60∘ C

(a)

(b) ×10 15

3

PVDF: Fe3+

RT 40∘ C

Z󳰀󳰀 (Ohm)

12

9 50∘ C 70∘ C

6

80∘ C 60∘ C 3

90∘ C 100∘ C

0 0

1

2 3 Z󳰀 (Ohm)

4

5 ×104

(c)

Figure 6: ((a)–(c)) Cole-Cole plots for PVDF: Ti3+ , PVDF: Cr3+ and PVDF: Fe3+ polymer films.

where 𝑑 = thickness of the film and 𝐴 = area of the electrodes. It has been observed in polymer films that 𝜎dc values first increased with increased temperature; this can be explained with the help of free volume theory. As the temperature increases, the polymer can expand easily and produce free volume. Thus ions, solvated molecules, or polymer segments can move to the free volume and hence the increase in conductivity. At higher temperatures 𝜎dc values

decrease due to that metal halide cations act as transient crosslinker, resulting in progressive immobilization of the polymer chain segments thereby decreasing the conductivity. Figures 7(a)–7(c) shows the temperature dependence of ionic conductivity for the PVDF: Ti3+ , PVDF: Cr3+ , and PVDF: Fe3+ polymer films. The conductivity values of these polymer films are shown in Table 2. These results suggest that Arrhenius phenomenological relationship can be used to

Journal of Polymers

7 ×10−5

×10−4

PVDF: Ti 3+

PVDF: Cr 3+

2.2

Conductivity 𝜎dc (S/cm)


3.5

3

2.5

2

1.8

1.6

1.4

2

1.2 1.5 300

310

320

330 340 350 Temperature (K)

360

300

370

310

320

(a)

330 340 Temperature (K)

350

360

(b) ×10

−5

PVDF: Fe3+

6


5

4

3

2

1 300

310

320

330 340 350 Temperature (K)

360

370

380

(c)

Figure 7: ((a)–(c)) Conductivity with temperature change for PVDF: Ti3+ , PVDF: Cr3+ and PVDF: Fe3+ polymer films.

Table 2: The 𝜎dc conductivity values of PVDF: Ti3+ /Cr3+ /Fe3+ polymer films. Temperature (K) 303 313 323 333 343 353 363 373

PVDF: Ti3+ (×10−4 S/cm)

PVDF: Cr3+ (×10−4 S/cm)

PVDF: Fe3+ (×10−4 S/cm)

1.667 1.782 1.891 1.983 2.054 2.243 2.592 3.673

0.1183 0.1235 0.1335 0.1747 0.1938 0.2170 — —

0.1625 0.1735 0.2114 0.2684 0.3273 0.3827 0.4672 0.6063

8

Journal of Polymers −3.4 PVDF: Ti 3+

−3.45

−4.7

−3.5 −3.55

log𝜎dc (S/cm)

log𝜎dc (S/cm)

PVDF: Cr 3+

−4.65

−3.6 −3.65

−4.75 −4.8

−3.7

−4.85

−3.75

−4.9

−3.8

−4.95 2.6

2.7

2.8

2.9 3 3.1 1000/T (/K)

3.2

2.8

3.3

2.9

3 3.1 1000/T (/K)

(a)

3.2

3.3

(b)

PVDF: Fe3+

−4.2

log𝜎dc (S/cm)

−4.3 −4.4 −4.5 −4.6 −4.7 −4.8 2.6

2.7

2.8

2.9

3 3.1 1000/T (/K)

3.2

3.3

(c)

Figure 8: ((a)–(c)) Arrhenius plots for PVDF: Ti3+ , PVDF: Cr3+ and PVDF: Fe3+ polymer films.

describe the conductivity behaviour for the aforementioned polymer films. Indicating Arrhenius type thermally activated process given by the relation 𝜎 = 𝜎0 exp(−𝐸𝑎/KT), where 𝜎0 is the preexponential factor, Ea the activation energy, and 𝐾 is the Boltzmann constant. The activation energies are calculated from the slopes of log 𝜎dc versus 1000/𝑇 plots as shown in Figures 8(a)–8(c) and it has been found that the activation energy decreases from region I to region II. The activation energy of PVDF: Ti3+ is 0.452 eV, PVDF: Cr3+ is 0.460 eV, and for PVDF: Fe3+ 1.16 eV. It is noteworthy that the films with low activation energies are desirable for practical applications [18]. 3.5. Magnetic Properties. Figures 9(a)–9(c) show the room temperature magnetization (M-H) hysteresis loops of PVDF:

Ti3+ , PVDF: Cr3+ , and PVDF: Fe3+ polymer films with a maximum field of 20 kOe. Figure 9(a) shows the results of PVDF: Ti3+ polymer film and depicts the reverse-S-type curve, indicating the intrinsic diamagnetism. From the loop the coercive field (𝐻c ), saturation magnetization (𝑀s ), and remanent magnetization (𝑀𝑟 ) were estimated to be 839.57 Oer, 1.01 × 10−3 emu/gm and 75.96 × 10−6 emu, respectively. Figure 9(b) shows the results of PVDF: Cr3+ polymer film, indicating the existence of RT ferromagnetism nature. From the loop coercive field, saturation magnetization (𝑀s ) and remnant magnetization (𝑀𝑟 ) were estimated to be 38.19 Oer, 1.211 × 10−3 emu/gm and 61.156 × 10−6 emu respectively, we conform that the observed ferromagnetism is intrinsic. Therefore it is reasonable to suggest that the observed ferromagnetism is due to oxygen vacancies and/or defects in the polymer

9

Magnetic moment (emu/gm)

×10−4 8

PVDF: Ti 3+

6 4 2 0 −2 −4 −6 −8

−10


Journal of Polymers

PVDF: Cr 3+

0.001 0.0005 0 −0.0005 −0.001

−2

−1.5

−1

0 −0.5 0.5 Applied field (Oer)

1

1.5

−20000 −15000 −10000 −5000 0 5000 10000 15000 20000 Applied field (Oer)

2 ×104

(a)

(b)


0.006

PVDF: Fe3+

0.004 0.002 0 −0.002 −0.004 −0.006 −20000 −15000 −10000 −5000 0 5000 10000 15000 20000 Applied field (Oer) (c)

Figure 9: ((a)–(c)) Hysteresis loops for PVDF: Ti3+ , PVDF: Cr3+ and PVDF: Fe3+ polymer films.

film. Figure 9(c) shows the results of PVDF: Fe3+ polymer film, indicating weak ferromagnetic nature. From the loop coercive field, saturation magnetization (𝑀s ) and remnant magnetization (𝑀𝑟 ) were estimated to be 118.84 Oer, 5.875 × 10−3 emu/gm and 114.04 × 10−6 emu respectively, which reveals the existence of defects and impurities; we believe that the observed weak ferromagnetism is caused both by the defects (such as oxygen vacancies) and with Fe ions incorporation [19, 20]. From the graphs PVDF: Ti3+ shows antiferromagnetism, PVDF: Cr3+ shows ferromagnetism, and PVDF: Fe3+ shows weak ferromagnetic nature.

4. Conclusion In summary, it is concluded that we have successfully synthesized both undoped (reference) and doped PVDF films containing transition metal ions such as Ti3+ , Cr3+ , and Fe3+ , respectively, and analyzed their structural, electrical and magnetic properties from the measurements of their XRD, FTIR spectra, photoluminescent spectra, dielectric constant, dielectric loss, and magnetic properties. XRD results have revealed that these polymer films possess monoclinic, 𝛼-, 𝛽-, and 𝛾-phases. Conductivity 𝜎dc in the place of electrical

conductivity. From magnetic profile measurements, PVDF: Ti3+ film has shown antiferromagnetic nature, PVDF: Cr3+ film has revealed ferromagnetic nature, and PVDF: Fe3+ has displayed strong paramagnetic nature.

Disclosure The authors confirm that the paper has been read and approved by all named authors and that there are no other persons who satisfied the criteria for authorship but are not listed. The authors further confirm that the order of authors listed in the paper has been approved by all of them. The authors confirm that they have given due consideration to the protection of intellectual property associated with this work and that there are no impediments to publication, including the timing of publication, with respect to intellectual property. In so doing they confirm that they have followed the regulations of their institutions concerning intellectual property.

Conflict of Interests The authors wish to confirm that there is no known conflict of interests associated with this paper and there has been no

10 significant financial support for this work that could have influenced its outcome.

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Nanoparticles Hindawi Publishing Corporation http://www.hindawi.com

Volume 2014

Nanomaterials Journal of

Advances in

Materials Science and Engineering Hindawi Publishing Corporation http://www.hindawi.com

Volume 2014

Journal of


Volume 2014

Journal of

Nanoscience Hindawi Publishing Corporation http://www.hindawi.com

Scientifica


Volume 2014

Journal of

Coatings Volume 2014


Crystallography Volume 2014


Volume 2014

The Scientific World Journal Hindawi Publishing Corporation http://www.hindawi.com

Volume 2014


Volume 2014

Journal of

Journal of

Textiles

Ceramics Hindawi Publishing Corporation http://www.hindawi.com


Biomaterials

Volume 2014


Volume 2014