3.1 Preparation of iPP/PVDF blend through melt-mixing
Table 1
Crystallite sizes, heat of fusion values, and crystallinity of PVDF-doped iPP samples.
Sample | PVDF content (phr) | Crystallite size obtained from XRD (nm) | Heat of fusion (J/g) | Crystallinity obtained from DSC (%) |
iPP | 0 | 8.82 | 58.2 | 27.8 |
PVDF 0.001 | 0.001 | 8.88 | 58.6 | 28.1 |
PVDF 0.01 | 0.01 | 8.96 | 55.8 | 26.7 |
PVDF 0.1 | 0.1 | 8.83 | 56.1 | 26.8 |
PVDF 1.0 | 1.0 | 8.72 | 55.1 | 26.4 |
PVDF 3.0 | 3.0 | 8.70 | 56.3 | 26.9 |
*phr: parts per hundred resin |
The blend of iPP and PVDF was prepared by melt-mixing their pellets above their melting temperatures (Tm), followed by hot-pressing the as-mixed mixture. The compatibility of the two polymers was confirmed by evaluating the transition in the Tm peak obtained from differential scanning calorimetry (DSC) as a function of the PVDF content, as shown in Fig. 1. For pristine iPP and PVDF, the single Tm peak values were approximately 169 and 165°C, respectively (Fig. S1). When the polymers were mixed with each other, the Tm peaks of iPP and PVDF were distinguishable, indicating that the two polymers exist independently because of their immiscible characteristics [30]. The two peak temperatures in the iPP/PVDF blend exhibited different transition behaviors as the PVDF content increased. The Tm of iPP was not affected by the presence of PVDF. Meanwhile, the Tm of PDVF increased depending on its content from approximately 157.5°C for 0.001 phr to 160.8°C for 3.0 phr of PVDF. This indicates that PVDF had negligible impact on the crystallinity of iPP. In addition, there was no difference in the degree of crystallinity (Xc), which will be described in detail later (Table 1).
The change in morphology of the iPP/PVDF blend was confirmed by scanning electron microscopy (SEM), as shown in Fig. 2. In the first row, the SEM images after etching the PVDF phase using dimethylformamide (DMF) are shown. The second and third rows show the SEM and the corresponding energy-dispersive spectroscopy (EDS) images before etching for identification of the PVDF phase, respectively. When very small amounts, below 0.01 phr of PVDF, were mixed, the presence of PVDF phases was not detected in either the SEM or EDS images (Fig. 2a and b). When 0.1 phr was mixed, the PVDF phase was observed as a separated domain of several hundred nanometers, in which only one domain was found in the image after etching (center) and before etching (upper side) but was rarely observed (Fig. 2c). As the content of the PVDF was increased above 1.0 phr, the formation of separate PVDF domains with sizes ranging from 1.0 to 5.0 µm was observed due to the immiscible characteristic of PVDF and iPP (Fig. 2d). At a content of 3.0 phr, the PVDF domain size increased by several micrometers to approximately 10 µm, which was confirmed by the EDS image (Fig. 2e).
3.2 Crystal formation behaviors of PVDF-doped iPP
The effect of PVDF on iPP crystal formation and growth behaviors was analyzed using X-ray diffraction (XRD) patterns, differential scanning calorimetry (DSC) traces during cooling, and polarized optical microscopic (POM) images as a function of PVDF content, as shown in Fig. 3. With the addition of PVDF, the intensity of the α peaks in the (110) plane decreased linearly, while other α peaks in the (040), (130), and (131) planes at 16.7°, 18.4°, and 21.7° increased, respectively (Fig. 3a). Meanwhile, the β peak at 15.9°, which existed in the pristine iPP, gradually decreased and entirely disappeared above 3 phr of PVDF. We also calculated the crystallite size in spherulites from the full width at half maximum (FWHM) of the (110) peaks using the Scherrer equation (Table 1) [31]. The crystallite size of iPP lamellar with the (110) plane was slightly increased from 8.82 nm to 8.96 nm as the PVDF content increased to 0.01 phr, and again decreased to 8.70 nm at 3.0 phr. These results indicate that the chain folding was gradually disrupted by the increasing amount of discrete PVDF domains.
We found that the PVDF additives affected the formation of iPP crystals; hence, we further studied the crystallization behavior of the PVDF-doped iPP samples during the cooling sequence in the DSC traces dependent on the PVDF content (Fig. 3b). The crystallization temperature (Tc) of iPP was approximately 115°C and shifted to higher temperatures, when the PVDF content increased, indicating that the PVDF additives acted as nucleating agents for the facile growth of the PP molecules (Table 1). Notably, when Tc increased, the total crystallinity (Xc), which was calculated using the heat of fusion, was nearly changed. Thus, the presence of PVDF affects the crystallization kinetics of iPP but has no effect on the fraction of crystals that are finally formed.
The crystal formation of PVDF-doped iPP was directly observed using POM (Fig. 3c). The iPP was composed of large spherulites, with a lateral size above approximately 20 µm. As 0.001 phr of PVDF was incorporated, the spherulite size decreased, and large crystals were split into smaller pieces of spherulites as the PVDF content increased to 0.1 phr. Above 1.0 phr of PVDF, the presence of discrete micrometer-sized PVDF domains with a spherical shape was observed, which is consistent with the SEM results shown in Fig. 2. However, the size and shape of the spherulites nearly changed, indicating that a smaller PVDF that can act as a nucleating agent still exists and helps crystallize the PP molecules. In addition, it is assumed that the partial decrease in Xc at a high fraction of PVDF is due to the formation of large PVDF domains.
3.3 Dielectric breakdown behaviors of PVDF-doped iPP
We performed direct current breakdown (DC BD) tests on the PVDF-doped iPP samples, as shown in Fig. 4. At 20°C, the pristine iPP and PVDF exhibited a DC BDS of approximately 410 kV/mm and 290 kV/mm, respectively, in which the values of the PVDF were widely distributed due to inhomogeneity (Fig. 4a). Below 1.0 phr of PVDF, DC BDS values of the PVDF-doped iPP samples were higher than those of pristine iPP and PVDF, although PVDF has a lower DC BDS value than the pristine iPP, indicating that small amounts of PVDF synergistically affect the increase in the dielectric BD resistance (Fig. 4c). The BDS value at a 0.01 phr PVDF was approximately 465 kV/mm, whereas a BDS value above 3.0 phr strongly decreased to 380 kV/mm. This is ascribed to the increase in PVDF content, causing the formation of discrete micrometer-sized PVDF domains, thereby increasing the loose interfaces between the PP matrix and PVDF domains that act as unfavorable shallow traps [32]. We also measured the direct current breakdown strength (DC BDS) of the samples at 110°C, the normal operating temperature of the high-voltage power cables, as shown in Fig. 4b [33]. The BDS values of iPP and PVDF decreased to approximately 160 and 30 kV/mm, respectively, owing to the loss in elasticity depending on temperature [34]. Similar to the BDS behavior at 20°C, the DC BDS values at 110°C were also improved when 3 phr of PVDF was incorporated (approximately 270 kV/mm). The highest value was at 1.0 phr, which is 169% higher than that of the pristine iPP. In addition, the volume resistivities of the PVDF-doped iPP samples were comparable to that of pristine iPP, and ranged from 1017 to 1018 Ω cm in spite of the presence of PVDF with few orders of magnitude lower (Fig. S2).
Such efficient improvement of the BDS with low PVDF content was interpreted by relating the results to changes in morphology caused by PVDF dopants. Few studies have previously reported that the BDS of polymers is strongly affected by spherulite size and crystallinity. Because there were no significant differences in crystallinity in our PVDF-doped iPP samples, the crystallinity effect on the BDS can be neglected. Meanwhile, the spherulite size largely decreased due to the presence of PVDF, which served as a nucleating agent for the crystallization of PP molecules (Fig. 3c). As the spherulite size decreases, the free volume at the spherulite boundaries that act as defect sites decreases, and the polymer density increases [35]. It is well known that the initiation and propagation of the electrical tree dominantly occur at the spherulite boundaries [36]. Therefore, the boundaries between smaller spherulites are probably strengthened by the incorporation of the PVDF additive, allowing the suppression of the pathway along the unstable spherulite boundaries, where electrical breakdown can occur, as shown in Fig. 5a. Furthermore, the as-grown PP spherulites were constant in size and distribution, indicating that smaller PVDF additives were capable of acting as nucleating agents and were homogeneously distributed in the PP matrix (Fig. 3c and 5a). Therefore, we hypothesize that the PVDF carrying fluorine is located at the spherulite center. At low PVDF content, fluorine moieties with high electronegativity act as specific sites to trap the injection carriers, contributing to the increase in BDS (Fig. 5b) [37]. In summary, the addition of PVDF synergistically facilitated a decrease in spherulite size and provided fluorine sites with high electronegativity, enabling a reduction in the pathway, where breakdown occurs and charge carriers are trapped.
3.4 Tensile properties of PVDF-doped iPP
The presence of heterogeneous additives can cause mechanical property degradation of the matrix owing to incompatible interfaces between two immiscible phases [38]. Thus, we also measured the tensile properties of the PVDF-doped iPP samples using the universal testing machine (UTM) (Fig. 6). The samples were fabricated through melt-molding followed by quenching, similar to the common extruded cable processing conditions. Pristine iPP exhibited a tensile strength of approximately 40 MPa, tensile modulus of 1100 MPa, and 1000% of elongation at break. For the PVDF-doped iPP samples, as the PVDF content increased from 0.001 to 1.0 phr, the tensile strength values decreased slightly from 40.4 to 38.5 MPa, respectively. In addition, the elongation at break values showed a similar decreasing trends from 1000 to 950%. However, the rates of decrease in tensile strength and elongation at break below 1.0 phr of PVDF can be considered insignificant. Meanwhile, the values of the two properties strongly decreased to 32.8 MPa and 630%, when 3 phr of PVDF was incorporated, which is probably due to the rise in the number of discrete micrometer-sized PVDF domains with incompatible interfaces to PP. In addition, the tensile modulus increased linearly as the PVDF content increased according to the mixture rule, in which the modulus of PVDF was higher than that of PP. Consequently, the use of the PVDF TVS was advantageous not only for introducing fluorine elements into the PP matrix, but also for the facile crystallization of PP molecules that enable the improvement of dielectric BD resistance without significant degradation of tensile properties at a low content of PVDF.