Enhanced physical properties of poly(vinyl alcohol)-based single-walled carbon nanotube nanocomposites through ozone treatment of single-walled carbon nanotubes

Materials and film preparation

SWCNTs were supplied by Chengdu Organic Chemistry Co. Ltd., China (purity >90, diameter 1–2 nm, length 5–30 µm). Functionalization of SWCNTs was performed through UV-irradiation using UVO generator (Model 42-220, Jelight Co.) in ambient laboratory air at a distance of 6 mm from the lamp of power 28,000 W per cm ² and wavelength of 254 nm. The powder was stirred with a spatula at 10 min intervals to provide a uniform exposure up to 180 min. The FT-IR spectra of UVO treated SWCNTs together with as received SWCNTs were measured by us previously. ³⁶ We showed that the functional group which was formed on the surface of SWCNTs after 180 min UVO irradiation was carboxylic acid. Also, we demonstrated the TEM in addition to SEM micrographs for these nanotubes. However, in the present work, a dispersion test was established for both UVO treated and as received SWCNTs in water as shown in Figure 1(a), in order to confirm the effect of ozone treatment in the dispersion of SWCNTs in water. It is clear from this figure that the dispersion of SWCNTs is improved after 180 min UVO exposure and no precipitation of SWCNTs was observed for three days.

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Figure 1.

(a) Dispersion of SWCNTs in water (after three days) and (b) FT-IR spectra of PVA and ozone-treated SWCNTs-PVA composite.

Polymer composites consist of untreated SWCNTs-poly(vinyl alcohol), or ozone-treated SWCNTs-poly(vinyl alcohol) were used to prepare the filmsat 1% SWCNTs by weight using the casting technique. SWCNTs were added to water used as solvent and ultrasonicated for 30 min using UP 200 S ultrasonicator (Hielsher Company-Ultrasonic Technology). Poly(vinyl alcohol) (PVA, Aldrich, Mw = 85,000) was dissolved in water and was stirred at 75℃ by a magnetic stirrer for 24 h. The dispersed SWCNTs in water were then mixed with polymer solution. The mixture was again stirred for 24 h until the CNT powder became highly dispersed in PVA solution. The resultant solution was poured to a glass Petri dish (dia. 10 cm). The solvent was evaporated at room temperature for 24 h. The untreated and ozone-treated SWCNTs-PVA films were kept inside a vacuum dissector. The film thickness (∼0.1 mm) was measured using digital micrometer (MOTOTOYO – China) with 0.001 mm resolution.

To observe the nature of interaction between ozone-treated SWCNTs and PVA, Fourier transformed Infrared (FTIR) spectra were recorded for composite films using a PerkinElmer 100 FTIR Fourier Transformed Infrared Spectrophotometer in the transmission mode at wavenumber range 4000–1000 cm⁻¹, as shown in Figure 1(b). It is obvious that there are some changes in the bandwidths of PVA characteristic peaks and no other significant changes can be detected in the FT-IR bands of ozone-treated SWCNTs-PVA composite. This indicates, however, an absence of any chemical interactions which may occur between PVA and SWCNTs. ³⁷

Rheological tests

The rheological tests were carried out using the dynamic mechanical analysis (DMA Q800, TA Instruments LLC, Delaware, USA) instrument, a film-clamp was used in dry mode. The storage modulus (G′) and loss modulus (G″) were measured at room temperature and oscillating frequency range. 0.1–100 Hz. Rheological test experiments were examined with the film under tension while the frequency was changed. A static pre-load force of 0.01 N was applied to the sample prior the dynamic oscillating force to prevent film buckling. Dynamic mechanical tests were also performed at fixed frequency of 5 Hz and at the temperature range 70–100℃.

Electrical measurements

Electrical measurements were carried out using 4200-SCS Semiconductor Characterization System (KEITHLEY Co.) with AC and DC applied electrical fields. Composite films were shaped into circular discs of area 1.2 cm ² and thickness of 0.1 mm. The DC electrical conductivity (σ_DC) was determined at RT from the relation σ_DC = (I/V) (d/A), where I is the electrical current, V is the applied voltage, d is the sample thickness, and A is the sample area. The relative permittivity (ɛ′) and AC electrical conductivity (σ_AC) were calculated from the measured capacity (C) and loss angle (δ) values at RT in the frequency range 2 KHz–1 MHz using the following relations:

The dielectric loss (ɛ″) can be calculated as follows:

Differential scanning calorimetric (DSC)

The thermal analysis of the samples was studied using Shimadzu DSC 60. The DSC experiments were run at a heating rate of 10℃/min with nitrogen purged at a flow rate of 100 mL/min. Each sample was carefully prepared and measured out to have approximately the same weight (7:9 ± 0:2 mg).

Rheological properties

Figures 2 and 3 show the variation of storage modulus (G′) and loss modulus (G″) with frequency at RT for the films containing 0 and 1% SWCNTs by weight, respectively. The values of storage modulus (G′) at RT and frequency of 5 Hz are listed in Table 1. Figures 4 and 5 show the variation of storage modulus (G′) and loss tangent (tan(δ)) with temperature at 5 Hz. In general, the results show that incorporation of ozone-treated SWCNTs into neat PVA has significantly improved the mechanical properties of PVA as compared to the untreated SWCNTs-PVA nanocomposites and such enhancement is stable up to 100℃. The values of glass transition temperature (T_g) for prepared samples were determined from Figure 5 and are listed in Table 1. As can be seen from Table 1, incorporation of ozone-treated SWCNTs (1% by wt) to neat PVA increases the T_g of PVA from 83.07℃ (PVA) up to 87.22℃ (treated SWCNTs-PVA composite). This indicates that treatment of SWCNTs with UVO has enhanced the interfacial bonding between PVA and SWCNTs.^9,36,37 In other words, UVO treatment of SWCNTs enhances the physical interaction between PVA and SWCNTs leading to the enhancement of the interfacial bonding between them.

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Figure 2.

Storage modulus (G′) versus frequency for PVA and PVA-based SWCNTs composites.

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Figure 3.

Loss modulus (G″) versus frequency for PVA and PVA-based SWCNTs composites.

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Figure 4.

Variation of storage modulus (G′) with temperature at 5 Hz for PVA and PVA-based SWCNTs composites.

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Figure 5.

Variation of tangent loss (tan(δ)) with temperature at 5 Hz for PVA and PVA-based SWCNT composites.

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Table 1.

Storage modulus (G′), glass transition temperature (T_g), DC electrical conductivity and melting temperature (T_m) of PVA and PVA-based SWCNT composites.

Sample	G′ (MPa) at RT and frequency 5 Hz	Tg (℃)	DC electrical conductivity (σDC) (S/cm)	Tm (℃)
PVA	717.52	83.07	1.5E-10	200.75
Untreated SWCNTs-PVA	798.40	83.65	5.0E-08	199.30
Treated SWCNTs-PVA	1430.88	87.22	2.5E-06	206.17

PVA: poly(vinyl alcohol); SWCNTs-PVA: single-walled carbon nanotubes-poly(vinyl alcohol).

Electrical properties

The calculated values of room temperature DC electrical conductivity for prepared PVA and PVA-based SWCNTs nanocomposites are listed in Table 1. Incorporation of ozone-treated SWCNTs (1% by wt.) to the neat PVA increases the DC electrical conductivity up to four orders of magnitude compared with the neat PVA. This indicates that ozone treatment of SWCNTs improves the dispersion of SWCNTs in PVA and enhances the formation of network structure between nanotubes. Generally, electrical conductivity of CNTs-polymer composites depends on the polymer layer in the internanotube connections which presents highly resistive section in the electrical pathway, acting as a barrier to efficient carrier transport between the nanotubes and models fluctuation-induced tunneling for the conductivity.^9,15,38 However, comparing with untreated SWCNTs-PVA, ozone treatment of SWCNTs enhances SWCNTs dispersion in the neat PVA and reduces the resistive layers between CNTs in the CNTs-polymer network; as a result, such a structure network will facilitate the electron transport through tunneling throughout the polymer or by electron hopping along CNTs interconnects.^{9,12,13,38,39}

Figures 6 and 7, show the relative permittivity (ɛ′) and AC electrical conductivity (σ_AC), respectively, for PVA and PVA-based SWCNTs nanocomposites at room temperature and frequency range 2 KHz–1 MHz. It can be seen that the relative permittivity of PVA increases with SWCNTs and such enhancement is improved with the incorporation of ozone-treated SWCNTs into neat PVA. This indicates that ozone treatment of SWCNTs enhances the polar character of PVA-based SWCNTs nanocomposites.^12,13,40 Figure 7 shows the AC electrical conductivity of the prepared composites at RT. At frequency of 2 KHz, incorporation of ozone-treated SWCNTs (1% by wt.) to neat PVA increases the electrical conductivity up to four orders of magnitude with respect to PVA which indicates that ozone treatment of SWCNTs improves their dispersion in PVA matrix and reduces the distance between CNTs in the CNTs-polymer network.

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Figure 6.

The dependence of relative permittivity (ɛ′) on frequency at RT for PVA and PVA-based SWCNTs composites.

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Figure 7.

The dependence of AC electrical conductivity (σ_AC) on frequency at RT for PVA and PVA-based SWCNTs composites.

Thermal analysis

DSC measurements were carried out for all prepared samples to recognize the CNTs-polymer interaction that leads to the observed composite mechanical properties. DSC curves for PVA, untreated SWCNTs-PVA, and ozone-treated SWCNTs-PVA composites are shown in Figure 8. The melting temperature (T_m) values for the prepared samples were estimated from the endothermic peaks shown in Figure 8 and are listed in Table 1. It is clear that the ozone treatment of SWCNTs enhances the interfacial bonding in the PVA-based SWCNTs; as a result, there is a shift in the T_m value of PVA toward the increase within 6℃. Such a result is a good accordance with those results obtained from the rheological properties. Besides, the area under the melt peak increases with the incorporation of ozone-treated SWCNTs into PVA, which may indicate a growing PVA crystallinity as a result of the polymer-CNTs interaction, where CNTs may act as nucleation sites. Thus, the presence of a shell of crystalline polymer around each nanotube or nanotube bundles appear to strongly enhance the stress transfer, and hence, the overall composite mechanical properties.^41–43

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Figure 8.

DSC curves for PVA and PVA-based SWCNT composites.