polyvinyl chloride blend by loading single walled carbon nanotubes

Progress in Natural Science: Materials International xxx (xxxx) xxx–xxx

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Original Research

Enhancement of the thermal and mechanical properties of polyurethane/ polyvinyl chloride blend by loading single walled carbon nanotubes☆ ⁎

A.M. Hezmaa, I.S. Elashmawia,b, E.M. Abdelrazekb,c, A. Rajehc,d, , Mustafa Kamalc a

Spectroscopy department, Physics Research division, National Research Centre, Cairo, Egypt Physics department, Faculty of Science, Taibah University, Al-Ula, Kingdom of Saudia Arabia c Physics department, Faculty of science, Mansoura University, Mansoura, Egypt d Physics department, Faculty of Science and education, Amran University, Yemen b

A R T I C L E I N F O

A BS T RAC T

Keywords: SWCNTs Nanocomposites X-ray TEM Mechanical properties

Structural, thermal, and mechanical properties of pure blend and nanocomposites based on polyurethane (PU) and polyvinyl chloride (PVC) doped with low different content of single walled-carbon nanotubes (SWCNTs) were studied. The nanocomposites at different concentration were prepared via casting technique. The interaction between PU/PVC and CNTs were examined via FT-IR studies. The changes in the structures of the nanocomposites were examined using X- Ray Diffraction (XRD), and the results indicated that the amorphous domains of nanocomposites increased with increasing SWCNTs content. Transmission electron microscope (TEM) observation indicated that SWCNTs surface was wrapped with the polymer with the thermal properties of nanocomposites improved. The mechanical behavior of the nanocomposites was evaluated as a function of SWCNTs content. The main enhancement in tensile properties was observed, e.g., the tensile strength and elastic modulus increased compared with the pure blend, which may be attributed to the interaction and adhesion between CNTs and the polymer matrices due to the hydrogen bonding between carbonyl groups (C=O) of polymer blend chains and carboxylic acid (COOH) groups of CNTs.

1. Introduction

Moreover, the modification of the polymer and/or polymer blend by the nanofillers can improve their properties and reduce the cost of their nanocomposites [8]. In traditional polymer nanocomposites, inorganic nanofillers have been used to improve the pure blend properties. Also, the nanofillers used in the nanocomposites are prepared in different parameters, such as different shapes, volumes and sizes [9,10]. The new class of polymer nanocomposites is a type of material that is considered to be an alternative to the classical filled polymers. The good dispersion of inorganic nanofillers in the polymer matrices improves the performance properties of the pure polymer. SWCNTs as a nanofiller improves the thermal, electronic and mechanical properties of the nanocomposites [11,12]. One of the most popular polymers used in industrial applications is polyurethane because of its high performance properties. Generally, polyurethane is composed of a high molecular weight polyether macrodiol, and a hard segment which is composed of diisocyanate and low molecular weight diamine or diol. The addition of inorganic nanofillers into the PU matrix is the most effective method to improve its thermal and mechanical properties. PU/CNTs nanocomposites have the advantages of PU and CNTs and have some special functions that pristine polyurethane does not

Carbon nanotubes (CNTs) have exceptional chemical, structural and physical properties, and have been utilized generally as good fillers to strengthen authentic polymer matrices and enhance its thermal, electrical and mechanical properties [1]. Nanocomposites (CNTs/ polymer) have attracted an extensive consideration in the scholarly community due to their excellent characteristics including, thermal properties, strength and conductivity even at low dopant contents [2] which make them suitable for diverse applications such as electronic devices, polymeric composites, hydrogen storage and field emission display. Furthermore, CNTs aggregate easily and form bundles due to their interactions arising from the strong Vander Waals force and large smooth surface areas. These properties made them uniformly dispersed in polymer blends, as well as in either different organic liquids or in aqueous solutions. Various techniques are applied to enhance the dispersion quality of CNTs in polymer matrices. These CNTs functionalization can be accomplished by covalent/non-covalent methods [3], surfactant, in-situ polymerization [3], melt mixing [4,5], and organic solvent dispersion [6,7].

☆ ⁎

Peer review under responsibility of Chinese Materials Research Society. Corresponding author. E-mail address: [email protected] (A. Rajeh).

http://dx.doi.org/10.1016/j.pnsc.2017.06.001 Received 13 January 2017; Received in revised form 29 May 2017; Accepted 2 June 2017 1002-0071/ © 2017 Published by Elsevier B.V. on behalf of Chinese Materials Research Society This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).

Please cite this article as: Hezma, A.M., Progress in Natural Science: Materials International (2017), http://dx.doi.org/10.1016/j.pnsc.2017.06.001

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possess. PU/ MWCNTs nanocomposites were prepared via direct solution mixing as described by Koerner et al. [12] One of the most important polymers with many application fields is Poly vinyl chloride (PVC), which is widely used in construction industry, for floor coverings, window profiles, pipes, and wallpapers. Recently in our group, we have enhanced the electrical, mechanical, and thermal properties of poly vinyl chloride using MWCNTs [13–15]. In another extensive study of the better PU/PVC blend ratio, Hezma et. al. have also found that for the PU/PVC blend, the ratio (75/25 wt%) PU/PVC was the best to get better compatibility and physical properties [16]. This paper studies the influence of SWCNTs doping on the structural, thermal and mechanical properties of PU/PVC with its potential to stand strongly to work in the field of wind turbines blades for electrical power generation.

SWCNTs (Wt%)

Absorbance (a. u.)

0.06

0.04 0.02

0.01

pure blend

2. Experimental section

4000

3500

3000

2500

2000

1500

1000

500

-1

Wavenumber cm

2.1. Materials

Fig. 1. FTIR spectra of pure PU/PVC and nanocomposites with different concentrations of SWCNT.

PU (MW 330,600) was supplied by (Cargill-Dow, South Korea). PVC with high molecular weight was supplied by (Fluka, Romania). Tetra Hydro Furan (THF) was supplied by (Duksan, South Korea). Functionalized Single Walled Carbon Nanotubes (SWCNTs- HCOO) (NTX10) were supplied by (Nanothinx, Greece) with average length ≈22 µm, diameter between 5 and 15 nm and purity 95%.

3. Results and discussion 3.1. Fourier transform infrared (FT-IR) The FT-IR spectroscopy examines the structure and reactions between different chemicals. For polymeric material, the results of FT-IR spectra are utilized to investigate the types of chemical bonds and functional groups in polymeric structures. The FT-IR spectra of pure blend and blend doped with low concentrations of SWCNTs in the range of 4000–500 cm−1 at room temperature is shown in Fig. 1. The FTIR spectrum of pure PU/PVC reveal the presence of the stretching mode of N‒H group at 3342 cm−1 in PU. The two peaks at 2955 cm−1 and 2860 cm−1 are attributed to asymmetric and symmetric stretching of the –CH band, respectively [13,17]. The absorption band at 1730 cm−1 for free carbonyl group and at 1703 cm−1 are assigned for hydrogen bonded carbonyl group. The split of C=O peak observed in Fig. 1 is due to the existence of interhydrogen bonding in polymer matrices [16]. The peaks at 1620 cm−1 and at 1540 cm−1 are attributed to –NH bending vibration and C–C stretching respectively. The absorption bands at 1460 cm−1, 1425 cm−1 and 1225 cm−1 ascribed to the alkane C-H bending, CH3 stretching and C-N stretching vibration modes in the PU/PVC blend. The IR bands at 837 cm−1is due to C–Cl stretching mode while at 1166 cm−1 is due to C–O stretching modes of urethane groups [18]. The FT-IR spectra for the nanocomposites (Fig. 1) show that intensities of the peaks 3341 cm−1, 1621 cm−1 and 821 cm−1 decrease with increasing the ratio of CNTs. The peak at 1540 cm−1 shifts towards lower wavenumber which is attributed to chemical interaction between SWCNTs and PU/ PVC in nanocomposites. These changes in the spectra confirmed interaction and complexation between SWCNTs and PU/PVC due to excellent dispersion of SWCNTs in polymeric matrix. Fig. 2 indicates the possible mechanism of interaction between PU/PVC and SWCNTs.

2.2. Nanocomposites preparation The Polyurethane and polyvinyl chloride(75/25) wt% were dissolved in THF as a solvent at room temperature with continuous stirring for one 24 h to get a homogeneous solution. Also the SWCNTs concentrations were 0.01, 0.02, 0.04, and 0.06 wt% of the total PU/ PVC weight dissolved in THF with sonication for 1 h. The CNTs solution was added drop wise separately to polymer solutions with continuous stirring and occasional shaking in an ultrasonic until to give us a well dispersion of carbon nanotubes achieved. Solution was poured into clean Petri dishes and left to dry at room temperature for 3 days to ensure the removal of solvent traces. After drying, films were peeled of the Petri dishes and kept in vacuum desiccators until use. The thickness of the samples was in rang of ≈ (0.1–0.3) mm. 2.3. Structural and morphological analysis FT-IR spectra of the samples prepared were studied by using single beam light, spectrometer (Nicolet iS10, USA). The spectra of the films were obtained in the spectral range of 4000–400 cm−1. XRD examination was performed using PANanalytical X′Pert PROXRD system (Holland) in reflection or transmission region, using Cu-Ka target radiation (where l=1.540 Å, and tube operating at 45 kV-40 mA) where, the Bragg's angle (2θ) in range of 10–80°. High-resolution transmission electron microscopy from (JEOLJEM-2100) attached to a CCD camera at an accelerating voltage of 200 kV was used to investigate the morphology of the SWCNT in polymer solution. The thermal degradation has been studied over a temperature range from 35 to 650 °C by using Shimadzu Thermogravimetric-45H under air environment at heating rates of 10 °C/min.

3.2. X-Ray Diffraction Analysis (XRD) Fig. 3 shows XRD for PU/PVC and PU/PVC doped with different concentrations of SWCNTs. It is observed that XRD pattern of PU/PVC blend indicate a sharp peak at 21.7° and a small peak at 22.4° due to semicrystalline nature of pure blend [19]. The XRD data show a decrease in the intensity and increase in the broadening of the nanocomposites diffraction peaks with increasing the SWCNTs content. This could be due to the disruption of the PU/PVC crystallinity and the changing in the crosslink density of PU/PVC with increasing of SWCNTs content which decrease the crystallinity of

2.4. Mechanical properties The properties of mechanical of pure blend and filled nanocomposites were scanned utilizing the tensile strength method on a universal testing machine (L.loyd Instruments Ltd, UK). Each sample was scanned at a crosshead speed of two mm / min at 25 °C. 2

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Fig. 2. Possible mechanism interaction between PU/PVC and SWCNTs nanocomposites.

diffraction peak. Table 1 shows a decrease in the degree of crystallinity for pure blend with increasing SWCNTs concentrations. The data in Table 1 have a good agreement with Fig. 2 and confirms the interaction between PU/PVC and functionalized SWCNTs.

nanocomposites [20]. A previous study reported that the intensity of XRD pattern decreases with increasing the amorphous nature by the addition of dopant [21]. No new peak pertaining to SWCNTs appeared in nanocomposites, which indicates a good dispersion of CNTs in nanocomposites and the addition of CNTs low content [22]. The crystallinity degree (D) is calculated from [15]:

D=

A ×100 A−

3.3. Transmission electron microscopy (TEM) (1)

The TEM images for pure SWCNTs and PU/PVC doped with different concentrations of SWCNTs are shown in Fig. 4. The pure SWNCTs figure show tubular structure with an external average

Where A is the total area of the peaks (area of crystalline and amorphous peaks) and A-is the total area under the crystalline 3

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more uniform dispersion in nanocomposites may contribute to its higher thermal stability compared with pure blend.

SWCNT (Wt%)

3.4.1. Determination of the activation energy There are two different methods to calculate thermodynamic activation parameters for the fundamental degradation process from the second decomposition region in TGA curves.

Intinsity (a. u.)

0.06 0.04 0.02

3.4.1.1. Coats - Redfern method. Which is an exemplary integral manner and can be performed as following [28]:

0.01

⎡ − log(1 − α ) ⎤ R ⎡ 2RT ⎤ Ea log ⎢ ⎢1 − ⎥ − 0.434 ⎥ = log 2 ⎣ ⎦ ⎣ T ΔEa Ea ⎦ RT

pure blend

10

20

30

40

50

60

70

(2)

Where, R is the universal gas constant (8.3136 J/mol K), T is the absolute temperature (Kelvin), Eais the activation energy (J/mol)and α is the weight loss of conversion fraction.

80

2θ (degree) Fig. 3. The XRD spectra pure PU/PVC and nanocomposites.

⎡ − log(1 − α ) ⎤ 1000 By plotting the dependence − log ⎢ ⎥ Versus T for each T2 ⎣ ⎦ sample, we obtained a straight lines and from the slopes we can calculated the activation energy as:

Table 1 The degree of crystallinity and activation energy of pure blend and nanocomposites by using Coats – Redfern and Broido methods. (PU/PVC) SWCNTs (wt%)

Coats – Redfern method (KJ/ mol)

Broido method (KJ/ mol)

Crystallinity (%)

0.00 0.01 0.02 0.04 0.06

250.09 240.45 235.64 230.32 227.45

237.125 233.19 229.25 225.81 220.81

55 52 48 42 39

Ea = slope × 2.303R

(3)

3.4.1.2. Broido method. Broido introduce a model to calculate the activation energy associated with second decomposition stage using the equation:

⎡ ⎛ 1 ⎞⎤ ⎛ E ⎞ 1 ln ⎢ln ⎜ ⎟ ⎥ = ⎜ a ⎟ + constant ⎣ ⎝Y ⎠⎦ ⎝ R ⎠ T

diameter nearly 14 nm. After the addition of SWCNTs to the blend (SWCNTs-PU/PVC) a core-shell structure was observed with SWCNTs as a hard core and PU/PVC as a soft shell. This indicates that the PU/ PVC was grafted successfully to the SWCNTs surface. The diameters of the pure SWCNTs and SWCNTs-PU/PVC nanocomposites were measured and the external diameter of the SWCNTs-PU/PVC was not uniform, which is probably dependent on the thickness of the polymer layers on the SWCNTs. The change of the diameters is more dramatic at the end of SWCNTs (Fig. 4), which suggest the greater oxidation occurred at end of nanotubes [23]. This leads to more polymer grafted to this oxidized sites in the end of the nanotubes. With FT-IR spectroscopy, the C=O stretching bands at 1735 cm−1 and (-NHC=O) at 1570 cm−1 were observed in the SWCNTs- PU/PVC indicating that SWCNTs were successfully wrapped by PU/PVC [19].

(4)

Where Y is the fraction of the number of initial molecules not yet decomposed and calculated as;

Y is given by Y =

Wt − W∞ Wi − W∞

(5)

Wt is the weight at any time t; W∞ is the weight equal zero at infinite time and the initial weight is Wi. A plot of ln [ln (1/Y)] vs. 1/T gives good approximation to a straight line. we calculated activation energy from slope for theses straight lines [29]. The values energy of activation (Ea) of the films by Coats – Redfern and Broido methods are listed in Table 1, these values decrease with increasing addition of CNTs this indicates that the CNTs intensively affect the polymer blend.

3.5. Mechanical properties 3.4. Thermogravimetric analysis (TGA) The stress–strain curves of pure blend and nanocomposites as shown in Fig. 6. The tensile strength and elastic modulus of all samples obtained from the stress–strain curve are shown in Fig. 7. It can be seen that the tensile strength and elastic modulus of pure blend enhanced after addition of SWCNTs. The addition of SWCNTs increase the strength to about 46 MPa at 0.06 wt% of SWCNTs, while the elastic modulus increased from about 1.1–4.9 GPa. Compared with tensile strength and elastic modulus the increasing of elongation at break and toughness are more obviously. The values of toughness increased from 1.1 MJ/m3 for pure PU/PVC to 4.01 MJ/m3 at the higher concentration of CNTs. In all cases, tensile strength, elastic modulus, toughness and elongation are improved after addition of SWCNTs low concentration. This can be ascribed to homogeneous dispersion of SCWNTs and a strong interfacial adhesion between SWCNTs and PU/PVC in nanocomposites that improve the material mechanical properties. So, the functional groups on the SWCNTs surface played significant role in accelerating of the interfacial adhesion and SWCNTs dispersion in the nanocomposites [30].

TGA is a good technique used to examine the mass change, thermal stability and thermal decomposition of nanocomposite materials [24]. Fig. 5 demonstrates the thermal stability of PU/PVC and its nanocomposites. A large weight loss appear between 120 °C and 255 °C, due to degradation of large polymer chains into small parts [25]. The sharp loss of weight at temperature higher than 260 °C is due to the thermal decomposition of polymer blend. A plateau at the temperature range 550–650 °C was observed in most of the samples, which is attributed to the carbonization and degradation of the polymer blend [26]. The increase in thermal stability also appears by shifting the curves toward the higher temperature by increasing the SWCNTs content. The thermal stability improvement may result from the excellent thermal properties of CNTs, which could promote heat dissipation in the polymeric matrices [27]. The interfacial bonding between CNTs and polymer blend may further promote the dissipation of the heat, thus delaying the decomposition of the nanocomposites more effectively. The stronger interfacial interaction between CNTs and PU/PVC with 4

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Pure SWCNTs

0.02 Wt.% SWCNTs

0.04 Wt.% SWCNTs

0.06 Wt.% SW CNTs

Fig. 4. TEM micrograph of pure SWCNTs and nanocomposites.

45

110

0.06

SWCNT (Wt%)

100

40

90

Tensile Stress [MPa]

35

80

Weight (Wt%)

0.04

70 60

0.06 0.04 0.02 0.01 pure blend

50 40 30

0.02 30

0.01 25

blend

20 15 10

20

5

10 0 0

100

200

300

400

500

0

600

0

o

Temperature ( C)

2

4

6

8

10

12

14

16

18

Strain %

Fig. 5. TGA thermograms of pure PU/PVC and nanocomposites.

Fig. 6. Stress -Strain curves of pure PU/PVC and nanocomposites.

5

20

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50

The crystallinity of the PU/PVC-SWCNTs nanocomposites decreases with increasing SWCNTs content, due to the change in the crosslink density of PU/PVC. The PU/PVC was grafted successfully to the SWCNTs surface as indicated by the formation of core-shell structure with the SWCNTs as a hard core and PU/PVC as a soft shell. The thermal stability of the prepared nanocomposites increase with increasing SWCNTs content. The main weight loss was observed in a temperature range of about 250–450 °C which is ascribe to the thermal degradation of polymeric chains. The decomposition of nanocomposites is shifted towards higher temperature due to the thermal stability enhancement after addition of the SWCNTs.

(a)

Tensile Strenght (MPa)

40

30

20

References

10

[1] [2] [3] [4]

0

blend

0.01

0.02

0.04

0.06

SWCNTs (Wt.%) (b)

elastic modulus (GPa) Toughness (MJ/m3)

5

[5] [6]

4.9

[7]

4.01

3.8

4

[8] [9] [10] [11] [12]

3.12 2.85

3

2.56

2

1.65

1.9

[13]

2.01

[14]

1.1

[15]

1 [16] [17] [18] [19] [20] [21] [22] [23]

0

blend

0.01

0.02

0.04

0.06

SWCNTs (Wt.%) Fig. 7. (a) Tensile strength and (b) elastic modulus and Toughness of different samples.

[24] [25]

4. Conclusion

[26]

Nanocomposites of PU/PVC-SWCNTs were prepared and examined by different techniques. SWCNTs were well dispersed in the PU/PVC matrix. The mechanical properties of the nanocomposites were improved by the addition of SWCNTs due to the functional group on the SWCNTs surface, which plays a significant role in accelerating both the interfacial adhesion and SWCNTs dispersion in the nanocomposites.

[27] [28] [29] [30]

6

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