PVA blend

Indian J Phys (October 2013) 87(10):983–990 DOI 10.1007/s12648-013-0333-1

ORIGINAL PAPER

Enhancement of structural and thermal properties of PEO/PVA blend embedded with TiO2 nanoparticles F H Abd El-kader1, N A Hakeem2, I S Elashmawi2,3* and A M Ismail2 1

Department of Physics, Faculty of Science, Cairo University, Giza, Egypt

2

Physics Division, Department of Spectroscopy, National Research Center, Giza, Egypt 3

Department of Physics, Faculty of Science, Taibah University, Al-Ula, Saudi Arabia Received: 8 March 2013 / Accepted: 21 May 2013 / Published online: 1 June 2013

Abstract: Blend films based on PEO/PVA (50/50 wt%) undoped and doped with different concentration of TiO2 nanoparticles are prepared by casting technique. Characteristic properties of the blend films are investigated using Fourier transform infrared, X-ray diffraction (XRD), ultraviolet–visible spectra (UV-Vis), scanning electron microscope (SEM), differential scanning calorimetry (DSC) and thermogravimetric analysis. On the basis of results obtained from IR and DSC data, blends appear to be immiscible. XRD, UV-Vis and TG reveal that 1.0 wt% TiO2 is most ordered concentration. SEM reveals that the presence of TiO2 leads to changes in the surface morphology and gives rise to crystalline domains with coarse spherulitic structure. It has been seen that spherulites increases with addition of TiO2 nanoparticles. TG studies conclude that this concentration has low E*, DS* and DH*. So, it is more ordering and has low thermal motion. Kinetic thermodynamic parameters such as activation energy, enthalpy, entropy and Gibb’s free energy are evaluated from TG data using Coat’s–Redfern model. Keywords: Nanocomposites; Fourier transform infrared (FT-IR); X-ray diffraction (XRD); Optical properties; Thermal analysis PACS Nos.: 61.25.hk; 78.30.Jw; 61.05.cp; 78.67.Sc; 81.70.Pg

1. Introduction Polyethylene oxide (PEO) and polyvinyl alcohol (PVA) have important applications and their blends are of significant practical utility [1–4]. PEO is a simple chain polymer with enteric linkages, while PVA is a polymer with a carbon chain backbone with hydroxyl (OH) groups attached to methane carbons. These OH groups are sources of hydrogen bonding (H-bonding) and hence of assistance in formation of polymer blend [5]. It has been found that blends from PEO and PVA system are essentially incompatible, although evidence of weak interpolymer chain interactions have been obtained from IR and NMR studies [6]. With the addition of nanosized inorganic particles into polymer matrices the new composite material exhibits

*Corresponding author, E-mail: [email protected]

unexpected properties, which greatly differ from that of conventional materials [7]. Titanium dioxide (TiO2) powder is currently believed to be one of the most promising known semiconductor materials. Recently nanostructured materials have drawn a great deal of interest due to their unique properties and potential applications [8–13]. The nano size TiO2 particles exhibit many special properties due to the fact that band gap of nanoparticles increases with decrease of their size and small TiO2 particles offer a very large surface area [14–16]. TiO2 nanocrystals have high potential for environmental applications due to their physical and chemical stability, relatively lower cost, non-toxicity, resistance to corrosion and large surface area and stable colloidal suspension [17–20]. The main purpose of this work is to achieve a deeper insight into fundamental characterization and physical properties of PEO/PVA blend films doped with different concentration of TiO2 nanoparticles.

Ó 2013 IACS

984

F H Abd El-kader et al.

2. Experimental details

3. Results and discussion Figure 1(a)–1(i) show X-ray diffraction of pure PEO, pure PVA, pure TiO2 and PEO/PVA blend films undoped and doped with different concentration of TiO2 nanoparticles respectively. Pure PEO has two well defined reflection peaks at 2h values of 19.1° and 23.3° [20, 21] while pure PVA exhibits only a broad and shallow diffraction feature around the 2h value of 20° [22, 23]. The reflection peaks of TiO2 at 2h &25.3° and 48° correspond to anatase phase of TiO2 [24]. Spectrum for pure blend sample shows well defined broad peaks at around 19° and 23°, which indicates a semicrystalline nature of polyblend system. It is noticed that reflection peaks of TiO2 at 2h &25.5° appear at concentration C1.0 wt% and their intensities increase with increasing TiO2 content in blend sample.

(i) (h) (g) (f) (e)

Intensity (a.u.)

Poly ethylene oxide (ACROS, NJ, USA) with MW &900 and Poly vinyl alcohol (MP Biomedicals, Inc, France) with MW &15.000 were used as basic polymeric materials. Nano-powder of TiO2 was taken from Qualikems Laboratory reagent, MW = 79.89, Purity 99 %, New Delhi, India. All chemicals were used as received without further purification. Equal amount of PEO and PVA (50/50 wt%) was dissolved in double distilled water separately and then the polymer blend solution was stirred continuously about 6 h at 70 °C until a homogenous viscous liquid was formed. TiO2 nanoparticles were dissolved in double distilled water in the same condition. Resulting solution of TiO2 nanoparticles was added drop by drop to the polymer solution with 0.5, 1.0, 2.5, 5.0 and 10 %wt. The resulting solution was cast to PET Petri dishes and kept in a dry atmosphere at 70 °C about 48 h. After drying, the films were peeled from Petri dishes and kept in vacuum desiccators until use. Fourier transform infrared measurements were performed using JASCO, FT/IR-6100 in the spectral range of 4,000–400 cm-1. X-ray diffractions were performed using X-ray differactometer Diano Corporation-USA using Cu-Ka radiation (k = 0.1540 nm). Ultraviolet–visible absorption spectra were measured in wave length region of 200–1,000 nm using V-570 UV-Vis-NIR (JASCO, Japan) spectrophotometer. Scanning electron micrograph was performed using scanning electron microscope (JEOL-JSM 6100), operating voltage at 30 kV accelerating voltage. Differential scanning calorimetry of prepared samples were carried out using (DSC-50, Shimadzu, Japan) with heating rate was 10 °C min-1. Thermogravimetric measurements (TGA) were made in a TGA A Perkin-Elmer TGA-7.

(d)

(c)

(b) (a)

10

20

30

40

50

60

2θ (degree) Fig. 1 X-ray diffraction of: (a) TiO2, (b) PVA, (c) PEO, (d) PEO/ PVA blend and blend with: (e) 0.5, (f) 1.0, (g) 2.5, (h) 5.0 and (i) 10 wt% TiO2 nanoparticles

The crystalline fraction of the samples (Xc), is calculated according to the Hermans–Weidinger method [25]. XC ¼

Acrystalline Acrystalline þ KAamorphous

ð1Þ

where Acrystalline and Aamorphous are the areas of crystalline reflections and amorphous halo, respectively and K is constant and can be set to unity for comparative purposes. Owing to small amount crystalline material in samples, these measurements may not accurately reflect absolute degree of crystallinity, but reproducible relative values are consistently obtained. Table 1 shows variation of crystalline fraction of samples as a function of TiO2 concentration. It is apparent from Table 1 that addition of TiO2 to blend increases crystalline fraction. Thus, addition of TiO2 nanoparticles enhances degree of crystallinity for PEO/ PVA blend sample. The degree of crystallinity is highest at concentration 2.5 wt% TiO2 compared to other investigated samples. Figure 2(a)–2(i) depicts FT-IR spectra of PEO and PVA homopolymers and their blend sample of PEO/PVA in the range 4,000–400 cm-1. FT-IR spectra of both PVA and PEO homopolymers are consistent with those reported previously [21, 26]. In the spectra, C=O stretching vibration at 1,730 and 1,567 cm-1, O–H bending vibration at 1,328 cm-1 and C–O–C stretching vibration around 1,100 cm-1 are clearly observed. Vibrational bands appeared at 2,884 and 3,344 cm-1 correspond to t(CH) and t(OH), respectively whereas t(COC) stretching is present only in spectrum of

Enhancement of structural and thermal properties of PEO/PVA

985

Table 1 The values of crystalline fraction, absorption edge and band tail for undoped PEO/PVA and doped with different concentrations of TiO2 nanoparticles TiO2 (wt%)

Crystalline fraction

Absorption edge (eV)

Band tail (eV)

0.0

19.10

4.79

0.37

0.5

22.21

4.87

0.32

1.0

19.82

4.99

0.33

2.5

24.91

5.10

0.23

5.0

23.24

4.59

0.73

10

22.74

4.63

0.59

PEO but t(OH) and t(C=O) are present only in spectrum of PVA. In case of blend sample spectrum, intensities of the most absorption bands such as t(OH), t(CH) and t(COC) stretching vibrations are changed irregularly compared to their values in individual polymers, while their positions remain unaffected. Therefore intermolecular hydrogen bonding between PEO and PVA may not be significant at all. Bands observed (for doped samples) in the region 450–800 cm-1 correspond to Ti–O stretching vibration which is consistent with that reported earlier [27]. Absorbance values of all vibration bands in IR spectra for PEO/ PVA/TiO2 nanoparticle samples are higher than that of undoped blend sample. Among these samples the intensity of most stretching bands for blend sample doped with 2.5 wt% TiO2 are highest. These results indicate that there is an interaction between TiO2 nanoparticles and PEO/PVA blend which is optimum at 2.5 wt% TiO2 concentration.

Ultraviolet–visible spectra of all films, recorded at room temperature in the range of 200–800 nm are as shown in Fig. 3(a)–3(f). Spectrum of undoped blend sample exhibited three absorption bands, an intense band at 210 nm and humps at 280 and 340 nm, which are related to high energy absorption as shown in Fig. 3(a). The first band was associated with presence of some residual acetate groups of PVA and/or chromophoric groups of PEO, while humps at 280 and 340 nm are assigned to existence of carbonyl groups associated with ethylene unsaturation [28]. The bands at 280 and 340 nm may be due to p ? p* (K-band) and n ? p* (R-band) electronic transitions respectively. In addition, there is no absorption band on visible region for all samples under investigation since films are transparent. The hump at 385 nm corresponds to chromophoric group of TiO2. It is observed that band at 385 nm increases with increasing concentration of TiO2 while bands at 280 and 340 nm show opposite behaviour. The absorption coefficient (a) is calculated from the absorbance (A) [28]. I ¼ Io expðadÞ

ð2Þ

2:303 Io 2:303 log A Hence; a ¼ ¼ d d I

where Io and I are the intensities of incident and transmitted radiation respectively, d is thickness of sample. Figure 4(a)–4(e) show plot of absorption coefficient with photon energy for PEO/PVA blend samples undoped and doped with different concentration of TiO2. Extrapolation of linear portion of the curves has been used to find values of

(i)

TiO2

(f)

Absorbance (a.u.)

(h)

Absorbance (a.u.)

ð3Þ

(g) (f) (e) (d) (c)

(e) (d) (c) (b)

(b) (a)

(a) 4000

3500

3000

2500

2000

1500 -1

1000

500

Wavenumber (cm )

Fig. 2 FT-IR absorption spectra of: (a) TiO2 (b) pure PVA, (c) pure PEO and (d) PEO/PVA blend and blend with: (e) 0.5, (f) 1.0, (g) 2.5, (h) 5.0 and (i) 10 wt% of TiO2 nanoparticles

200

300

400

500

600

700

800

Wavelength (nm) Fig. 3 UV-Vis spectra of: (a) PEO/PVA undoped blend and blend sample doped with: (b) 0.5, (c) 1.0, (d) 5.0, (e) 5.0 and (f) 10 wt% TiO2 nanoparticles

986


where ao is a constant and Ee is width of tail of localized states in band gap that associated with amorphous nature of materials. In general largest value of Ee, related to greater in structural disorder. Figure 5(a)–5(f) show relation between ln a and ht for all investigated samples. Straight lines obtained suggest that absorption follows quadratic relation for inter-band transitions and Urbach rule is obeyed. Correlation factor of R2 close to unity is chosen. Values of band tails, Ee are calculated from slope of straight lines and are listed in Table 1. It is clear that values of band tail lie between 0.23 and 0.73 eV for all samples. Values of Ee vary slightly with composition but in an irregular trend. Among these samples, value of Ee for blend sample of 2.5 wt% TiO2 content is lower one, reflecting that it is most ordered sample. This result is in agreement with that found in X-ray data.

1000

800

α (cm-1)

600

400

(e) 200

(d) (c) (b) (a)

0 4.5

5.0

5.5

6.0

(eV) Fig. 4 Relation between absorbance coefficient (a) versus ht for (a) PEO/PVA undoped blend sample and blend sample doped with; (b) 1.0, (c) 2.5, (d) 5.0 and (e) 10.0 wt% TiO2 nanoparticles

6.4

6.0

(f) 5.6

(e)

ln(α)

absorption edge which are listed in Table 1. It is clear that values of the absorption edge for doped PEO/PVA with TiO2 content up to 2.5 wt% are higher than that for undoped blend sample. While with further addition of TiO2 concentration beyond 2.5 wt% the value of absorption edge decreases and becomes less than undoped one. The blend sample of 2.5 wt% TiO2 content has highest value of absorption edge. The transition occurs between extended states of band and localized states of tail of other band and absorption coefficient (a) is given by Urbach relation [29]: hm aðmÞ ¼ a0 exp ð4Þ Ee

5.2

(d)

4.8

(c) 4.4

4.0

(b)

(a) 5.1

5.2

5.3

5.4

5.5

(eV) Fig. 5 Relation between ln(a) versus ht for (a) PEO/PVA undoped blend sample and blend sample doped with; (b) 0.5, (c) 1.0, (d) 2.5, (e) 5.0 and (f) 10.0 wt% TiO2 nanoparticles

Figure 6(a)–6(f) show typical SEM images of the PEO/ PVA blend without and with different concentrations of TiO2 nanoparticles content. For undoped polymer blend as given in Fig. 6(a) it is found to be softer, homogenous and coherence. It is apparent that presence of TiO2 in PEO/ PVA polyblend leads to changes in surface morphology of such system. These figures give rise to crystalline domains with coarse spherulitic structure. This is due to the segregation of TiO2 into interlamellar regions of blend. From these micrographs it can be seen that spherulites increase with increase of addition of TiO2 nanoparticles. The DSC thermograms of PEO/PVA polymer blend and PEO/PVA/TiO2 nanocomposites films are shown in Fig. 7(a)–7(f). The values of glass transition temperature (Tg), melting temperature (Tm1 and Tm2), decomposition temperature (Td) and heat of fusion of PVA DHf (PVA) are recorded in Table 2. Differential scanning calorimetry thermograms of all samples show four endothermic peaks. In case of undoped blend sample first endothermic peak at about -34 ± 1 °C was assigned to Tg1 of PEO. The second endothermic peak at about 65 ± 1 °C is attributed to overlapping of Tm1 of PEO and Tg2 of PVA. The third endothermic peak at about 190 ± 2 °C is assigned to Tm2 of PVA. The fourth endothermic peak Td is observed in range between 310 up to 315 °C. It is reported previously [30] that glass transition temperature of PVA is expected to be close to 71 °C. Coincidently, it is near that melting point of PEO, 66 °C. Therefore, glass transition peak of PVA might overlap with melting peak of PEO in DSC thermogram and it could be difficult to observe glass transition temperature separately. It is clear from Table 2 that the position of Tm1 of PEO, Tg2


987

Fig. 6 SEM images of: (a) PEO/PVA undoped blend sample and blend sample doped with: (b) 0.5, (c) 1.0, (d) 2.5, (e) 5 and (f) 10 wt% of TiO2 nanoparticles

Exo.

(f) (e)

Heat flow

(d) (c) (b)

Endo.

(a) Tm2( PVA)

Tg1(PEO)

Td

Tm1(PEO) & Tg2 ( PVA) -100

0

100

200

of PVA and Tm2 peaks remain unaltered with increasing TiO2 content in blend sample. While position of Td peak change irregularly with increasing TiO2 content in blend sample but still higher than undoped blend sample; so thermal stability slightly increase for samples doping with TiO2. It is generally accepted that presence of two separate Tg’s in polymer blends provides a strong signature of immiscibility. Immiscible blends may be further described as compatible or incompatible. In present case, blend sample of PEO/PVA undoped and doped with TiO2 nanoparticles are immiscible but still compatible. To understand change in structural characteristic induced by adding TiO2 nanoparticles, crystallinity percentage (DC) for PVA is measured from heat of fusion at melting for PVA by following equation [31].

300

Temperature (°C) Fig. 7 DSC of: (a) PEO/PVA undoped blend sample and blend sample doped with: (b) 0.5, (c) 1.0, (d) 2.5, (e) 5 and (f) 10 wt% TiO2 nanoparticles

DC ¼

DHf 100 DHf ð100Þ

ð5Þ

where DHf is heat of fusion of PVA; DHf(100) is heat of fusion of 100 % crystallinity of pure PVA (DHf(100) =

988


Table 2 The values of (Tm1(PEO) and Tg2(PVA)), Tm2, Td, DHf (PVA) and DC (PVA) for undoped PEO/PVA and doped with different concentration of TiO2 nanoparticles TiO2 (wt%)

Tm1(PEO) and Tg2(PVA) (°C)

Tm2 (°C)

Td (°C)

DHf (PVA) (Jg-1)

DC (%)

0.0

64

188

310

15.52

9.70

0.5

65

189

315

16.08

10.05

1.0

66

192

313

18.95

11.84

2.5

65

191

314

20.64

12.9

5.0

66

189

312

18.32

11.45

10.0

65

189

311

17.04

10.65

160 Jg-1) [32]. Since Tg of PVA is overlapped with Tm of PEO, so it is difficult to determine exactly enthalpy associated with melting point of PEO. Thus, calculation of

change in crystallinity percentage for PEO with increasing TiO2 concentrations is not preferable. Estimated values of the degree of crystallinity for the PVA after adding TiO2 to blend film are tabulated in Table 2. It is clear from Table 2 that there is an increase in percentage of crystallinity after addition of different concentration of TiO2 for all samples compared to undoped one. Figure 8(a)–8(f) shows TGA thermograms as a function of temperature for PEO/PVA blend undoped and doped with different concentrations of TiO2 nanoparticles. From Fig. 8, it is seen that there are three steps of decomposition in all samples. Table 3 represents decomposition steps and percentage weight loss for PEO/PVA blend undoped and doped with different concentration of TiO2 nanoparticles. The lower values of percentage weight loss, in first decomposition step which includes melting point of PEO (1.14–3.00 %) may be due to splitting or

0.0000

0.0000

90

90

-0.0008 -0.0015 60

60

-0.0016

-0.0030

-0.0024

30

30

(d)

(a)

-0.0032

-0.0045 0

0 0

100

200

300

0

400

100

200

300

400

100

40 -0.00150

(b)

20 0

-0.00225 0

100

200

300

400

Weight loss %

Weight loss %

-0.00075

-0.00075 60 -0.00150 30

(e)

Dr TGA (mg/min)

60

Dr TGA (mg/min.)

80

0.00000

90

0.00000

-0.00225

0 0

100

200

300

400 0.0005

100

0.000

100 0.0000 80

80 -0.001 60 40

-0.002

20

60

-0.0005

40

-0.0010

20

(c)

(f)

-0.0015

-0.003 0

0 0

100

200

300

Temperature (°C)

400

0

100

200

300

400

Temperature (°C)

Fig. 8 TGA and DrTGA of: (a) undoped PEO/PVA blend sample and blend sample doped with: (b) 0.5, (c) 1.0, (d) 2.5, (e) 5.0, (f) 10 wt% TiO2 nanoparticles


989

Table 3 Parameters obtained from TG and Dr TG data for undoped PEO/PVA and doped with different concentration of TiO2 nanoparticles TiO2 (wt%)

Region of decomposition

0.0

0.5

1.0

2.5

5.0

10.0

Temperature(°C) Start

Weight loss (%) End

Tp

Partial

Total 92.00

1st

38

115

69

3.00

2nd

162

346

309

40.00

3rd

348

450

402

49.00

1st

31

87

67

1.14

2nd

181

340

288

38.00

3rd

341

435

386

50.69

1st 2nd

37 191

95 342

66 302

1.20 36.00

3rd

342

436

390

49.50

1st

31

91

61

1.65

2nd

185

356

313

38.40

3rd

356

439

385

51.00

1st

30

140

74

2.01

2nd

198

353

304

38.00

3rd

353

449

378

40.38

1st

30.1

194

66

2nd

194

349.6

301

39.10

3rd

350

439

382

43.60

89.83

86.70

91.05

80.39

2.4 0

85.10

Tp Peak temperature of DrTG

Table 4 Thermodynamic parameters for undoped PEO/PVA blend and doped with different concentrations of TiO2 nanoparticles TiO2 (wt%)

E* (kJ mol-1)

DS* (J K mol-1)

DH* (kJ mol-1)

2nd Region

2nd Region

2nd Region

3rd Region

3rd Region

0.0

101

159

-87

-32

0.5

106

200

-70

50

1.0

99

254

-87

134

2.5

84

169

-117

-3

5.0

108

228

-72

10.0

106

308

-77

DG* (kJ mol-1) 3rd Region

2nd Region

3rd Region

96

153

147

175

101

195

141

162

94

248

144

159

79

163

148

165

96

103

223

145

160

218

101

303

145

160

volatilization of small molecule and/or the evaporation of moisture. The second decomposition region in TG curves which cover a wider temperature range including melting point of PVA have a percentage weight loss (36.0– 40.0 %). The latter process in TG curves includes degradation process, having percentage weight loss (40.38–51.00 %). The difference in thermal decomposition behaviour of investigated samples can be seen more clearly from derivative thermogravimetric (DrTG) curves shown in Fig. 8(a)–8(f). DrTG curves show three temperature broad peaks Tp’s corresponding to three decomposition regions (see Table 3). It is noted that peak temperature Tp of DrTG curves blend sample doped with various TiO2 content are irregularly changed in all decomposition steps.

Thermodynamics activation parameters of decomposition process are evaluated by making use of well known Coats– Redfern equation for first order reaction in form [33]: lnð1 dÞ E AR þ ln ln ð6Þ ¼ 2 T bE RT where A is constant, b is heating rate, R is the universal gas constant, d is fraction of decomposition and E* is the h i activation energy. Therefore plotting ln lnð1dÞ against T2 1/T according to Eq. (6) should give a straight line whose slope is directly proportional to the activation energy E R . The activation entropy DS*, the activation enthalpy DH* and the free energy (Gibbs function DG*) are calculating using the following equations [34]:

990


Ah DS ¼ 2:303 log R KT

ð7Þ

DH ¼ E RT

ð8Þ

DG ¼ DH TDS

ð9Þ

where k and h are Boltzmann and Planck constants respectively, T is temperature involved in calculation selected as peak temperature of DrTG. Entropy DS* gives information about degree of order of system, enthalpy DH* gives information about total thermal motion and Gibbs or free energy DG* gives information about stability of the system. According Coats–Redfern method calculated thermodynamic parameters values are given in Table 4. It is clear that the variation of DG* values is not significant (within the experiment error) in 2nd and 3rd regions. The values of E*, DH* and DS* are changed irregularly in second and third decomposition steps. In second decomposition step it can be noted that polyblend sample of 2.5 wt% TiO2 content has lower E*, DS* and DH* compared to undoped one. Thus it is more ordering and has low thermal motion. In addition, by increasing temperature, random scission of macromolecule chain predominates and activation energy has a greater value. Thus, it can be concluded that second decomposition region have relatively high order and less thermal motion than third decomposition region.

4. Conclusions Blend films formed by both undoped and doped with TiO2 are homogenous, coherent and show neither separation into bilayers nor any precipitation, IR and DSC data reveal that they are immiscible. XRD patterns and band tail in UV– Visible range reveal that 2.5 wt% TiO2 additive gives higher crystalline fraction and more order for polyblend system. SEM images reveals that spherulites increase with increasing TiO2 concentration in blend sample. TG data and thermodynamic parameters indicate that 2.5 wt% TiO2 content blend sample has low thermal motion and more ordering in second decomposition region. Finally, it can be concluded that additive concentration of 2.5 wt% TiO2 to blend sample is the best one at which most physical properties are significantly enhanced.

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