Nanoindentation and thermal characterization of poly (vinylidenefluoride)/MWCNT nanocomposites

PVDF is a semi-crystalline thermoplastic polymer which has four crystalline phases, among which more attention has been focused to the β-phase because of its piezoelectric and ferroelectric properties. This β-phase in PVDF can be obtained by mechanical deformation while applying the strong electric field on solid state α-phase of PVDF.^1,2 Due to its promising piezoelectric and ferroelectric properties, as well as outstanding mechanical, electrical properties, it is widely used for various device level applications, including sensors and actuators.^3–5 However, the potential applications of PVDF is still limited because of their low coupling between electrical and mechanical properties, also it has relatively low generated voltage and force.⁶ 

The dispersion of nanomaterial into PVDF will have an important effect on conformation, morphology, thermal and crystallization behavior of the composites. But it has much effort to solve the problems such as uniform dispersion of nano-size particles in matrix and compatibility between them. MWCNT is one of the best candidates to improve the mechanical, thermal, electrical and ferroelectric properties of PVDF.^7,8 The PVDF/MWCNT nanocomposites are multiphase systems comprised of amorphous and crystalline phases. Thus, the presence of MWCNT directly affects the microstructures and properties of PVDF. These composites can make high strength and extension due to its high tensile and mechanical properties.⁹ In general, three methods may be used to incorporate MWCNTs into polymer matrix, i.e., i) dispersion of MWCNT in PVDF followed by a film casting, ii) polymerization of MWCNT-monomer mixtures and iii) melt mixing of MWCNTs with polymer matrix. In this work the polymer nanocomposite was prepared by solvent casting method other than melt processing, because the dispersion and distribution of MWCNT by melt mixing process is very poor and there are chances of polymer degradation.

In the present study, the first method has been adopted to make the polymer nanocomposites with PVDF and MWCNT. These composites were undergone with different characterization techniques. Mechanical behavior of these films has been investigated by nanoindentation with controlled constant-load experiments. X-ray diffraction experiments have been employed to study the phase characteristics. Morphological and thermal studies also been reported in this article.

PVDF and MWCNT are obtained from Sigma-Aldrich India Pvt. Ltd., Bangalore and used as received. The average molecular weight (Mw) and density of PVDF pellets are 180000 g/mol; and 0.78 g/cm³ respectively.¹⁰ Diameter of MWCNT was 10–30 nm with a length of 0.1–10 μm and its density was 1.5 g/cm³ having a purity of >90%.¹¹ Dimethylformamide (DMF) was obtained from Merck chemicals and was used as it is.

The PVDF pellets and MWCNTs were mixed separately with DMF solvent. The polymer solution was prepared by heating a mixture of PVDF and DMF in 1 : 4 weight ratios at 70 °C for 2 hrs, while the MWCNT/DMF solution was sonicated using a probe sonicator with 6 mm probe at a frequency of 20 KHz (SONICS Vibra Cell, 750 W) for 3 hrs at room temperature. The final mixture was prepared by adding the contents of the MWCNT/DMF premix to the PVDF solution at 70 °C with vigorous stirring and continued stirring for another 3 hrs. The solution was casted on a glass mold and then transferred to a vacuum oven for drying at 50 °C for 11 hours. The nanocomposites have been prepared with different compositions of MWCNT (1, 2, 5, 7 and 10 w/w %). The thickness of composites was found to be 100–200 μm.

The films were underwent various characterization techniques to determine the physical properties of the nanocomposite. Park XE-100 atomic force microscope (AFM) was employed to study the surface topography of the films in non-contact mode with a silicon cantilever at a scanning rate of 1 Hz. The radius and force constants of the tip are 50 nm and 0.032 N/m respectively. Field emission scanning electron microscope (Hitachi, SU-6600) has been employed for morphological studies of composite films at 20 KV accelerating voltage. The samples where stick on to a conductive carbon tape and coated with thin films of gold using a Hitachi 1010 ion sputter instrument before imaging to reduce electron charging.

The films were analyzed with X-ray diffraction (XRD) studies for their crystalline behavior. X-ray diffraction (XRD) data were collected at room temperature with a Copper Kα radiation source (λ = 0.154 nm) operated at 30 KV. The X-ray diffractograms were collected in the scan range (2θ) of 1–60° at the scan speed of 1° min⁻¹ using a step size of 0.04°.

Hardness and Young's modulus of composite films were measured by nanoindentation technique using AFM. For this study a Berkovich diamond indenter with total included angle of 142.3° was used. The maximum indentation depth was found to be 250 nm at the hold time of 10 seconds at maximum load. The data set was processed using the proprietary software to measure the mechanical properties and constant contact force which gives the loading unloading curves. Young's modulus for the composite films was obtained from the slope of the unloading curve.

Differential scanning calorimetry (DSC) studies of PVDF and its composite films were performed using a TA Instruments Q20 instrument under dry N₂ environment. During the DSC analysis 5 mg of samples were heated and cooled at a rate of 10 °C/min. and it is repeated for three times. Samples were ramped from −50 to 250 °C and cooled from 250 to −50 °C, maintained under isothermal conditions for 5 min. Melting temperature (Tm) and crystallization temperature (Tc) were obtained from the DSC thermograms. The nominal melting temperature was defined as the peak of the melting endotherm during first heating from −50 to 250 °C, and the crystallization temperature was defined as the peak of the crystallization exotherm upon cooling from 250 to −50 °C.

The XRD spectrums of pure PVDF and its nanocomposites are depicted in Figure 1. From the integration of the area under the crystalline peaks, it can be seen that the pure PVDF membrane contains predominantly major crystalline peaks at 2θ of 18.6°, 20.3°, and 27°. These peaks are attributed to the crystal planes associated with the α-phase of PVDF.¹² Pure MWCNT exhibits sharp XRD peaks at 28.5°, 39.5°, and 40.6°. From the XRD analysis it is clear that the addition of MWCNT to the matrix leads to exhibit a sharp peak at 20.6° which is attributed specially to the β-phase. In addition to this peak, it has been observed that the α-peak area is reducing upon loading the MWCNT to the matrix at 2θ of 18.6° and 27°. From that it can be concluded that loading of MWCNT to the matrix continue to promote the crystallization of PVDF in the β-phase, indicating that a restricting effect occurs in α-crystals. This restricting effect results in producing tensile stress in α-crystals.

The α and β-phases of PVDF matrix consist of TGTG and TT conformation chains respectively The total energies of TT and TGTG conformation chains are –23.95 and –25.21 kJ/mol respectively, which indicate that the TGTG conformation is more stable than the TT conformation.¹³ The MWCNT surface consists of zigzag carbons, which match with the TT conformation chain of PVDF. The interaction between PVDF and MWCNT has been confirmed elsewhere.^14–16 Therefore the MWCNTs act as a nucleating agent in PVDF matrix and the polymer chains with TT conformation are adsorbed on the surfaces of the MWCNTs, which results in the formation of β-phase. These polymer chains with TT conformation cannot transform into low energy α-phase during crystallization due to the restricting effect of MWCNT on the TT conformation chains.

DSC thermograms and data of pure PVDF and its composites are shown in Figure 2 and in Table I respectively. DSC thermogram reveals that the melting temperature for composites shifted to higher temperature range when compared to pure PVDF. Moreover the cooling curves show that the crystallization temperature has shifted to a lower temperature in all composites. The addition of MWCNT has decreased the crystallization due to super cooling. Our results of enhancement in β-phase of PVDF by the introduction MWCNT loading are in accordance with the existing reports.¹⁷ 

As shown in Table I, the incorporation of the MWCNTs leads to increase the melting temperature and decrease the degree of crystallinity (ΔHm). The ΔHm decreased sharply at 2% loading of MWCNT by about 25%. Although the nucleating effect of MWCNTs can lead to a higher crystallinity in matrix, the presence of MWCNTs can restrict the motion of its molecular chains. The nucleus generated on the MWCNT surfaces grew under the restricting condition, which will lead to the formation of imperfect crystals and crystal defects.

Surface roughness and mechanical properties of the fabricated films has been characterized at nanoscale and is depicted in Figure 3. Addition of MWCNT to the matrix leads to increase the surface roughness of the matrix. Composites with lower composition of MWCNT show fairly uniform topography. Surface roughness, Young's modulus, and hardness values for pure PVDF and its composites calculated from the AFM images are shown in Table II.

The addition of MWCNT to polymer matrix leads to change in the surface topography of the matrix, as a result of stress, grain growth, and phase transformations. The surface roughness increases significantly due to the growth of MWCNT crystalline phase when the sample is annealed in the air.

Young's modules were determined from the slope in the initial unloading segments of the load displacement curves. MWCNT loading to the polymer matrix leads to improve its hardness, indicating that the MWCNTs provide more resistance to the deformation of PVDF. As shown in Table II and Figure 4(a) and 4(b), the hardness of PVDF increases from 2.14 to 9.34 KPa and the yield strength increases from 40.14 to 83.91 MPa, indicating a significant improvement in the hardness and modulus at 10% of MWCNT loading. The significant improvements in modulus and strength of PVDF clearly confirms that the reinforcing effect of the MWCNTs in the PVDF/MWCNT composites. In general, the modulus and strength of crystalline polymers increase with the degree of crystallinity. So these improvements in modulus and strength are not attributed to the change in crystallinity. In order to reveal the improvements in mechanical properties of matrix it is necessary to relate the properties to their morphological structure.

High resolution electron microscopic images of the fractured surface of nano composites are shown in Figure 5. The micrographs show that the MWCNTs are uniformly dispersed in the PVDF matrix. Most MWCNTs are dispersed individually in the polymer matrix. Some MWCNTs are pulled out of the matrix in the breaking process, but their ends are embedded in the polymer matrix, indicating that there is an interfacial adhesion between PVDF and MWCNT. Therefore, the improved mechanical properties should be attributed to the good dispersion of the MWCNTs and the interfacial adhesion between PVDF and MWCNTs. Observations show that the results are comparable or superior to referred literature.^18,19

The PVDF/MWCNT nanocomposites have been prepared and investigated for the β-phase enhancement. Incorporation of MWCNT into PVDF causes heterogeneous nucleation induced by forming β-phase. The restricting effect of MWCNTs on polymer chains endows the β-phase with a higher thermal stability. Loading of MWCNT to the matrix has shown synergetic enhancement in thermal properties of composites and decreasing its degree of crystallinity. From the nanoindentation studies it has been found that the hardness and Young's modulus of nanocomposites has improved significantly by the addition of the MWCNT to the PVDF matrix.