Introduction

Additive manufacturing (AM) has brought a revolution in engineering of various products (Ref 1). Out of commercially available thermoplastics, many researchers have highlighted use of PVDF for sensors and actuators (because of the existence of four different phases in PVDF) (Ref 2). It has been reported that composite of carbon fiber-reinforced PVDF has good tribological properties in terms of less wear rate and friction. FDM of such reinforced thermoplastic material has been reported by some researchers in which good mechanical properties were observed (Ref 3, 4). The perfect two dimensional lattices and presence of sp2 hybridized carbon atoms in Gr gave excellent conducting and thermal properties to it. Some researchers have reported PVDF polymer matrix based composite for super-capacitance applications by using cellulose plastics for reinforcing Gr in it (Ref 5, 6). Cement, PVDF matrix composite and barium titanate-glass based composites have been reported as electrically conductive material for electromagnetic induction, sensing and bone regeneration applications. It has been reported that recycled plastic waste can be used in road construction, which may help in reduction of plastic solid waste (PSW) (Ref 7,8,9). Also, PVDF has been reported as acceptable material for battery applications due to efficient electro-active and dielectric properties, whereas titanate has been found responsible for imparting piezoelectric property in base polymer matrix (Ref 10,11,12,13). The concept of smart materials with one or two way shape memory programming is of great interest in AM these days. Poly-caprolactum, polycarbonate, fiber-reinforced polymer composites and concrete based smart polymers have been investigated by some researchers to ascertain 4D or smart properties in composites for exploring such materials in construction of civil structures (Ref 14,15,16,17). The data shown in Table 1 represent the research carried in past 30 years (1990-2020) on PVDF and Gr materials for different engineering applications as per web of science input using VOS viewer open source software.

Table 1 Relevance of research of PVDF and graphene
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Based on the above data, Fig. 1(a) was obtained for clear understanding of relevant research carried in the proposed field. Investigations on PVDF and Gr have been reported for 3D printing of sensors, actuators, superconductors and many other robotic applications. Figure 1(b) shows that less work is reported on 4D capabilities of PVDF-Gr composite for maintenance and repair of cultural heritage structures.

Fig. 1
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(a) Web of research carried on PVDF and Gr. (b) Highlighted research on PVDF matrix and Gr

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It has been reported that composite of PVDF, polyamide (PA6), acrylonitrile butadiene styrene (ABS), polylactic acid (PLA), etc. having reinforcement of Gr, BaTiO3, TiO2, can be used successfully for 3D printing on open source FDM setup with acceptable mechanical properties (Ref 18,19,20,21,22,23,24,25,26,27). The literature review reveals that the previous studies on PVDF were focused on increasing mechanical and electrical properties of PVDF for sensor and actuators. But hitherto little has been reported on PVDF-Gr composite for 4D applications such as self-assembly in maintenance and repair of heritage structures. This study reports the investigations on mechanical, thermal, morphological, magnetic and piezoelectric properties of PVDF-Gr composite as customized self-assembly based solution for filling of surface cracks of heritage structures.

Experimentation

Mechanical blending of PVDF and Gr nano-particles was performed to investigate rheological, thermal, mechanical, piezoelectric, morphological and 4D properties of the composite. Figure 2 shows the work methodology of the present research work.

Fig. 2
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Process outflow of the proposed work

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MFI and Viscosity Testing

To investigate the rheological properties of PVDF and its composite, nano sized Gr particles were reinforced in PVDF in four different weight proportions i.e., 2.5, 5.0, 7.5, and 10% by mechanical blending. MFI of each composition/proportion was tested as per ASTM D1238 standard. Table 2 shows the MFI of prepared samples. The wire sample obtained from MFI tester was weighed and volume of each sample was recorded for viscosity calculations as per literature reference (Ref 18). Shear stress and shear rate for each case was recorded as per standard equations. Table 3 shows density and viscosity of each composition/proportion. The results obtained for rheological and viscosity test were discussed.

Table 2 MFI results of PVDF-Gr composites
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Table 3 Viscosity of PVDF-Gr composites
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Thermal and Mechanical Characterization

Thermal analysis of each composition/proportion was performed on Mettler Toledo DSC setup. The heat capacity observed for each sample is shown in Table 4. All the proportions were processed on TSE to prepare 3D printer feedstock filament wire. Wires were extruded at screw temperature of 195 °C with 0.3 Nm torque by applying 10 kg load. Mechanical properties of each filament wire were tested on UTM. 3D printing of disk-shaped pallet was performed on FDM based open source printer.

Table 4 Thermal (heat) capacity of PVDF-Gr composite
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4D Characterization

The DC poling of 3D printed disk was performed on 5 kV poling unit and dielectric constant (D33) for each composition was measured. After ascertaining piezoelectric properties in PVDF-Gr composite, 4D characteristics of compositions were investigated by performing VSM test. The FTIR of PVDF and its composites was performed to analyze bonding characteristics and effect of Gr reinforcement on mechanical properties. Morphology of each composition/proportion was studied using Tool Maker’s and metallurgical microscope.

Results and Discussion

Rheological and Viscosity Analysis

Based upon Tables 2 and 3, MFI and viscosity of PVDF-Gr composite matrix were plotted with common X-axis (Fig. 3). As observed from Fig. 3, PVDF-5%Gr composition/proportion is representing intersection point for MFI and viscosity observations. One of the major outcomes is that beyond this point usually mechanical and thermal properties are improved (which may be counter verified from mechanical and thermal testing). Hence this proportion may be considered as a limiting/ bare minimum proportion for preparing PVDF-Gr composites.

Fig. 3
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Correlation graph between MFI and viscosity of PVDF-Gr composite

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Thermo-Mechanical Analysis

Based upon Table 4 it has been ascertained that heat capacity of PVDF composite matrix increased with increase in weight percentage of Gr (Fig. 4).

Fig. 4
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DSC observations of PVDF-Gr composite

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The results obtained for UTM test of filament wire is shown in Table 5. It was observed that mechanical properties like peak strength and Young’s modulus were best obtained for PVDF + 10%Gr. The stress-strain curve obtained during tensile test is shown in Fig. 5. As compared to virgin PVDF wire (sample no.1), PVDF-10%Gr has shown better strain at break (SB) due to which this may be considered as better solution for 3D printing.

Table 5 UTM results of PVDF and PVDF-Gr composites
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Fig. 5
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Stress-strain curves for PVDF filament and PVDF-Gr composites

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Piezoelectric Analysis

3D printing of nanoGr-reinforced PVDF composites was found successful as 0.3 mm thick disk-shaped pallets were printed for performing DC poling and measuring dielectric constant D33. Figure 6(a) shows 0.3 mm thick disks 3D printed for direct current (DC) poling and measurement of dielectric constant (D33). Figure 6(b) and (c) shows 5 kV DC poling setup and D33 equipment for measuring piezoelectric characteristics in PVDF-Gr composites. After DC poling for 3 hours on disk-shaped samples, PVDF-10%Gr attained the highest electric charge of 45 pC.

Fig. 6
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3D printed disks (a), DC poling setup (b), D33measuring equipment (c)

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VSM and FTIR Analysis

The results obtained for VSM of PVDF-Gr samples showed that sample 3, 4 and 5 possess acceptable value of magnetization (emu/g), coercivity (Oe) and retentivity (G) for good magnetic properties. Figure 7(a), (b) and (c) shows the hysteresis plots obtained for VSM testing of sample 3, 4 and 5, respectively. Table 6 shows the magnetic characteristics of each tested composite. VSM analysis outlined that composition/proportion of PVDF with 10%Gr can be magnetized up to maximum content with average magnetization of 0.00131 × 10−3 emu/g. This proportion has higher average retentivity and coercivity of 6.16 × 10−3 G and 1660.34 Oe, respectively.

Fig. 7
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Hysteresis curve obtained from VSM test for PVDF-Gr specimen

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Table 6 Magnetic properties of PVDF-Gr composites
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The FTIR analysis of sample 1 and sample 5 (as per Table 2) was conducted to investigate the effect of infra-red (IR) rays intensity on nature of bonding in PVDF due to reinforcement of nanoGr particles. The FTIR of samples shown in Figure 8(a) and (b), respectively, highlighted the FTIR absorption spectrum of virgin PVDF in comparison to PVDF-10%Gr composite. The sharp peaks in FTIR of PVDF-10%Gr show the increase in bond strength of composite that possibly increased the mechanical properties of the composite. The stretch of C-F bond (observed in the range of 830-890 WN) was also confirmed by FTIR. The sharp peaks showing C-O stretch at 1160-1180 WN frequency range show the strong chemical bonding in atoms of sample 5 in comparison to sample 1. The incremental change in bond strength can be seen clearly in sample 5 as the peaks got sharper in comparison to sample 1 at 865 WN for C-F stretch and at 1178 WN for C-O bond stretch. It shows that reinforcement of 10%Gr in PVDF matrix is responsible for bond stretching and increase in bond strength as the absorption of composite increased in sample 5 as compared to PVDF absorption spectra. Further at high WN the absorbance is approaching zero value both for virgin PVDF and PVDF-10%Gr matrix; hence the prepared composite may be used as sensor in remote sensing applications.

Fig. 8
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(a) FTIR absorption spectra of PVDF wire specimen. (b) FTIR absorption spectra of PVDF + 10%Gr

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Morphological Analysis

Tool Maker’s microscopic images obtained at ×30 magnification (Fig. 9) show uniform distribution of nano Gr particles in PVDF matrix due to which acceptable mechanical and 3D/4D properties were obtained. The images show that uniform cross section of feedstock filament wire can be prepared using TSE at proposed processing parameters.

Fig. 9
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Tool Maker’s microscope images of PVDF-Gr composites (at ×30)

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Morphological properties of PVDF-Gr composites were investigated using metallurgical microscope (at ×100 magnification) is shown in Fig. 10(a). Images for surface of filament wire along the longitudinal axis and radial axis were captured and processed on metallurgical image analysis software (MIAS) 4.0 to measure the porosity% in the filament wire along its length and cross section. Figure (10b) shows that PVDF-10%Gr possesses lesser porosity% (i.e., 2.93% along longitudinal axis and 2.21% along the radial axis). As regards to better morphological and mechanical properties, this composition/proportion may be considered as the better for required 3D/4D applications.

Fig. 10
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(a) Optical photo micrographic images (at x100 magnification). (b) Porosity% images (at x100 magnification)

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Conclusions

Following conclusions may be drawn from the present study:

  • The rheological analysis outlined that blending of Gr in PVDF decreases the MFI and increases the viscosity of the composite matrix. The 5% Gr reinforcement in PVDF resulted into a turning point beyond which better mechanical, thermal and morphological properties were noticed. The thermal testing showed that PVDF-10%Gr composite have highest heat capacity of − 64 J/g (during heating cycle).

  • The maximum peak strength of 43.01 N/mm2 was observed, while processing at screw temperature 195 °C with 0.3 Nm torque for filament wire preparation of PVDF-10% Gr matrix. Finally the 3D printing of selected composition/ proportion was also successful.

  • The piezoelectric properties of composite (PVDF-10% Gr) were observed (after DC poling) by dielectric constant measurement (D33 = 45 pC/N), which is sufficient for self expansion/ contraction properties.

  • The feed stock samples prepared were counter checked for morphological testing, which outlined that porosity for PVDF-10%Gr was observed as minimum (2.93 % for surface along longitudinal axis and 2.21 % for surface along radial axis).

  • For self-assembly applications, vibration sample magnetometry (VSM) was performed on PVDF-Gr composite. Finally based upon observed magnetic properties (magnetization 0.00131 ×10−3 emu/g, retentivity 6.16 ×10−3 G and coercivity 1660.34 Oe) and piezoelectric property (45pC/N) of PVDF-10%Gr composite, it may be used as 4D material for self-assembly applications by preparing 3D printed smart fillers for repairing cracks in heritage structures.