In this work, two kinds of PVDF copolymer structures were used (as shown in
Figure 1). PVDF
c1 and PVDF
c2 are the abbreviation of poly(vinylidene fluoride-
co-hexafluoropropylene) (PVDF-HFP) and poly(vinylidene fluoride-
co-tetrafluoroethylene) [P(VDF-TFE)], respectively. Compared with the PVDF homopolymer structure, an asymmetrical unit of fluoropropylene (HFP) was incorporated into the main constituent vinylidene fluoride (VDF) blocks in PVDF
c1, but a symmetrical unit of tetrafluoroethylene (TFE) in PVDF
c2. Thus, the regularity of the copolymer chain is totally different from the homo-PVDF. In the following discussion, the crystallization behaviors of copolymers will be investigated.
Figure 1.
Molecular structures of the PVDF copolymers, poly(vinylidene fluoride-co-hexafluoropropylene) [PVDF-HFP (PVDFc1)] and poly(vinylidene fluoride-co-tetrafluoroethylene) [P(VDF-TFE) (PVDFc2)].
2.1. The Effect of Chain Structures on the Crystallization Behavior of PVDF Copolymers
DSC results, including the crystallization of homo-PVDF and co-PVDF at a cooling rate of 2.5 °C/min when cooling from 200 °C and subsequent melting at a heating rate of 10 °C/min, are summarized in
Table 1.
Table 1.
DSC crystallization results of homer- and co-PVDF crystallizing from the melt at a cooling rate of 2.5 °C/min.
Table 1.
DSC crystallization results of homer- and co-PVDF crystallizing from the melt at a cooling rate of 2.5 °C/min.
Sample | Tcon (°C) | Tcp (°C) | Tcf (°C) | ΔTc (°C) | Tmp (°C) * | ΔHm (J·g−1) | Xc (%) |
---|
PVDF | 141.3 | 138.1 | 135.8 | 3.2 | 174.2 | 49.8 | 47.7 |
PVDFc1 | 133.9 | 129.2 | 126.0 | 4.7 | 162.8 | 29.2 | 27.9 |
PVDFc2 | 117.5 | 116.2 | 113.8 | 1.3 | 141.0 | 37.8 | 37.2 |
As illustrated in
Table 1, the crystallization temperatures of those two copolymers (including the values of
Tcon,
Tcp and
Tcf) are lower than homo-PVDF. This can be attributed to the lower degree of structural regularity for co-PVDF. The crystallinity of each co-PVDF is lower than the homo-PVDF, which indicates that the comonomer unit introduced into the PVDF chain retards the polymer chains crystallizing into lamellae. The difference between the onset and peak crystallization temperature, ∆
Tc, of PVDF
c1 is 4.7 °C, which is higher than PVDF
c2 and also higher than homo-PVDF. The value, ∆
Tc, of PVDF
c2 is 1.3 °C, which is much lower than homo-PVDF. The lower the value, ∆
Tc, is, the faster the crystallization rate is [
14]. Thus, the crystallization rate is in the order of PVDF
c2 > homo-PVDF > PVDF
c1. This can be explained by the different crystallization mechanisms, which will be discussed in the following section.
Figure 2 shows the typical spherulitic texture of homo-PVDF and PVDF
c1 crystallizing from the melt at a cooling rate of 0.5 °C/min. When cooling from the melt, both homo-PVDF and PVDF
c1 reveal well-defined nucleation and growth processes. The spherulites of these two samples impinge each other in about 10 min after crystallization begins. However, as for PVDF
c2, ribbon-like structures that mostly arrange into concentric needles are observed, and the growth of these crystals nearly completes in 6 min. This result indicates that the HFP unit in the PVDF chain did not change the crystallization style, which is three-dimensional crystallites (spherulitic structure). The reduced crystallization temperature, crystallinity and crystallization rate can be assigned to the asymmetry of the HFP unit in the chain. On the contrary, the TFE unit in the PVDF chain changed the crystallization style from three-dimensional crystallites to one-dimensional crystallites (rod-like or needle-like crystals). This can be attributed to the fast crystallization rate of the TEF chains, which is similar to the crystallization of PTFE [
15]. Therefore, the crystallization rate of PVDF
c2 is faster than PVDF
c1 and homo-PVDF, which agrees with the DSC results.
Figure 2.
Polarized optical micrograph showing the crystalline morphology of homer- and co-PVDF crystallizing from the melt at a cooling rate of 0.5 °C/min.
Figure 2.
Polarized optical micrograph showing the crystalline morphology of homer- and co-PVDF crystallizing from the melt at a cooling rate of 0.5 °C/min.
In this study, the crystalline phases of PVDF in the diluted samples are identified by FTIR spectroscopy (
Figure 3). Homo-PVDF and PVDF
c1 have well-defined absorption bands at 1423, 1400, 1383, 1211, 1181, 1069, 975, 872, 794 and 763 cm
−1. As reported, these IR absorption bands represent the characteristic spectrum of the α phase of the PVDF crystal [
16,
17]. This indicates that only the crystallization of α phase PVDF predominates in the crystallization in PVDF
c1. However, with regard to PVDF
c2, except for the absorption bands of the α phase PVDF, an additional strong absorption band at 840 cm
−1 was observed, which is characteristic of the β phase of PVDF. Therefore, the TFE unit in the PVDF chain favors the β-PVDF crystal formation, which confirms the polarized optical micrograph (POM) observation that the needle-like crystals are the β-PVDF.
To confirm the crystalline phase in the co-PVDF samples, X-ray diffraction was carried out (
Figure 4). According to the researchers’ work [
16,
18], the peaks at 2θ = 17.75°, 18.36°, 19.96°, 26.58°, 33.10°, 36.99°, 38.64° and 39.00° in the curve for the homo-PVDF sample represent the diffractions in the planes, (100), (020), (110), (021), (130), (200), (210) and (002), respectively, which are all characteristic of the α phase of PVDF. For the PVDF
c1 sample, the diffraction peaks are the same as the homo-PVDF. However, for the PVDF
c2 sample, only plane (110) has a strong diffraction peak, and a small peak at around 40° is also different from the other two samples, which confirms only α phase crystals in the PVDF
c1, but mainly β phase crystals in the PVDF
c1 [
19].
Figure 3.
FTIR spectra of homer- and co-PVDF samples crystallizing from the melt.
Figure 3.
FTIR spectra of homer- and co-PVDF samples crystallizing from the melt.
Figure 4.
X-ray diffractograms of homer- and co-PVDF samples crystallizing from the melt.
Figure 4.
X-ray diffractograms of homer- and co-PVDF samples crystallizing from the melt.
In the TIPS process, the diluent is crucial in determining the polymer crystallization process and the resulting membrane morphology.
Table 2 shows the solubility parameters for homo-PVDF, co-PVDF and MS. As shown in
Table 2, the difference in the solubility parameters between the PVDF and MS is little, so MS can be selected as the diluent. However, an obvious spherulitic structure was obtained in the membrane, which is due to the strong interaction between the MS molecule and the PVDF chain [
20]. As calculated by molecular dynamics simulations, the solubility parameters for PVDF
c1 and PVDF
c2 are 13.5 and 15.1 MPa
1/2, respectively [
11]. This indicates weak interactions between the polymer chain and MS molecules, which is due to the presence of the copolymerized units.
Table 2.
Solubility parameters. MS, methyl salicylate.
Table 2.
Solubility parameters. MS, methyl salicylate.
Materials | Solubility parameter δ (MPa1/2) [11] |
---|
PVDF | 19.2 |
PVDFc1 | 13.5 * |
PVDFc2 | 15.1 * |
MS | 21.7 |
2.2. The Effect of Chain Structures on the Membrane Formation of PVDF Copolymers
The values of dynamic crystallization temperature and the cloud point are plotted to obtain the phase diagrams of the co-PVDF/MS system, as shown in
Figure 5. The phase diagram of thehomo-PVDF/MS system has been reported in previous work [
10]. Although the L-L region can be obtained by adding PMMA, which lowers the crystallization temperature of PVDF, the crystallization of PVDF in the MS dilutions is still obvious, because of the strong interaction between the PVDF chains and MS molecules.
Figure 5.
Phase diagram for PVDF copolymer/MS systems.
Figure 5.
Phase diagram for PVDF copolymer/MS systems.
Due to the weak polymer-diluent interaction, the phase diagram of the co-PVDF/MS-diluted system shows the upper critical solution temperature (UCST) type L-L phase behavior [
7], as shown in
Figure 5. Compared with a PVDF/MS system [
10], an obvious wide L-L phase separation region is observed, in which the monotectic point, φ
m (the intersection of the crystallization temperature curve with the cloud curve) [
21], of PVDF
c1 and PVDF
c2 is around 50 wt% and 40 wt%, respectively. For the PVDF
c2/MS system, the cloud points are about 20 °C lower than the PVDF
c1/MS system, which is due to a weaker interaction between the PVDF
c1 and MS molecules, indicated by the aforementioned solubility analysis. Similarly, the crystallization temperature of PVDF
c1 is higher than PVDF
c2. This can be attributed to the crystalline form changing in the presence of the TEF unit in the PVDF chains. For both systems, for samples with a lower polymer concentration (<φ
m), the dilution effect induces the L-L phase separation with a horizontal crystallization curve, but in a high polymer concentration (>φ
m) region, the L-L phase separation will be arrested by the crystallization.
As shown in
Figure 6, the morphologies of PVDF copolymer/MS dilutions crystallizing from the melt at a cooling rate of 1 °C/min were observed by polarized light microscopy. This reveals the well-defined large spherulites of PVDF
c1 in MS, which is the same as is observed in
Figure 2. In the MS dilution, the crystal size is smaller than in the pure polymer sample (
Figure 2), which is due to the supercooling in the ice water bath for MS dilution samples. For the PVDF
c1/MS-diluted system, the number of nuclei and the impinged spherulitic crystal size is decreased with an increase of the polymer concentration, because the entangled chains and highly viscous state restrain chain mobility [
22].
Figure 6.
Polarized optical micrographs showing the spherulitic morphology of PVDF copolymer/MS dilutions, crystallized from the melt at a cooling rate of 1 °C/min: (A) PVDFc1/MS dilution; (B) PVDFc2/MS dilution.
Figure 6.
Polarized optical micrographs showing the spherulitic morphology of PVDF copolymer/MS dilutions, crystallized from the melt at a cooling rate of 1 °C/min: (A) PVDFc1/MS dilution; (B) PVDFc2/MS dilution.
With regard to the PVDF
c2/MS-diluted system, the number of crystal nuclei is decreased with an increase of the polymer concentration, which is similar to the PVDF
c1/MS dilution, but the crystal size is independent of the polymer concentration, because of the ribbon-like structures. Similar to the neat PVDF
c2 sample observation (
Figure 2), this ribbon-like crystal cannot grow into a larger size. However, the number of crystals is decreased with an increase of the polymer concentration, which still can be explained by the entangled chains and highly viscous state restraining chain mobility in a high polymer condition.
The membrane structures of PVDF
c1 and PVDF
c2 are shown in
Figure 7 and
Figure 8, respectively. The membranes obtained from these two systems have a micropore structure in the cross-section.
Figure 7.
SEM micrographs of PVDFc1 membranes prepared from MS dilutions quenched in the ice water bath (the polymer content is marked in the upper left corner of each picture).
Figure 7.
SEM micrographs of PVDFc1 membranes prepared from MS dilutions quenched in the ice water bath (the polymer content is marked in the upper left corner of each picture).
Figure 8.
SEM micrographs of PVDFc2 membranes prepared from MS dilutions quenched in the ice water bath (the polymer content is marked in the upper left corner of each picture).
Figure 8.
SEM micrographs of PVDFc2 membranes prepared from MS dilutions quenched in the ice water bath (the polymer content is marked in the upper left corner of each picture).
As for the PVDF
c1 membranes, due to the growth of spherulites, the membrane structure is similar to the homo-PVDF membrane [
10], which is full of spherulites with small pores in it (
Figure 7). For the PVDF
c1/MS dilution, the largest temperature ranging from the cloud point to the crystallization point (∆
T) is 49 °C, which is smaller than the PVDF
c1/MS dilution (∆
T = 53 °C). This indicates that the region of the L-L phase separation is still small (
Figure 5). Therefore, the L-L phase separation will be quickly arrested by the crystallization of PVDF
c1, leading to the rejection of the liquid diluent to inter- and intra-spherulitic regions. When the polymer concentration is lower than 40 wt%, the small pores formed through the L-L phase separation are observed, but a dense and small spherulitic structure is obtained when the polymer concentration is higher than 40 wt%. Thus, the polymer concentration (40 wt%), which shows that the dramatic morphology change is lower than the monotectic point, φ
m. This can be attributed to the high degree supercooling (quenching in the ice water) condition. When the PVDF
c1 concentration is higher than 40 wt%, a smaller pore size and spherulite size are observed, because of the limited mobility of polymer chains.
With regard to the PVDF
c2/MS system, micropore structures derived from an L-L phase separation are more obvious than the PVDF
c1/MS system. As shown in
Figure 8, at a polymer concentration of 35 wt%, the pore morphology observed microscopically changed dramatically, indicating the L-L phase separation before the crystallization and that the monotectic point, φ
m, is around 35 wt%. However, this is lower than that determined by the phase diagram (
Figure 5). This also can be attributed to the high degree supercooling (quenching in the ice water) condition. When the PVDF
c2 concentration is higher than 35 wt%, the crystallization occurs before the L-L phase separation. In this circumstance, the S-L phase separation could be approximately considered. However, the size of the spherulitic structure (the same as the α-crystal morphology) is increased with the polymer concentration, which is different from the PVDF
c1 membrane. This can be explained by the crystal formation mechanism of PVDF
c2. As discussed in the crystallization behavior of PVDF
c2, mainly β-crystals are formed, but still, small α-crystal exists. Therefore, when the PVDF
c2 concentration is higher, the spherulitic structure becomes more obvious.
Figure 9 shows the tensile strength and elongation at break of PVDF
c1 and PVDF
c2 membranes obtained from various polymer concentrations by quenching in the ice water bath. The tensile strength for PVDF
c1 and PVDF
c2 membranes are increased with the polymer concentration increasing. The highest elongation at break of PVDF
c1 and PVDF
c2 membranes with a polymer concentration of 35 wt% is 55% and 350%, respectively. The enhanced performances of elongation at break of PVDF
c2 can be attributed to the small and ribbon-like crystals induced by the TFE unit in chains. For the elongation at break of PVDF
c1, it is nearly the same as the homo-PVDF membrane [
23], which confirms that the spherulitic structure in the membrane can reduce the elongation at break.
Figure 9.
Tensile strength and elongation at break for membranes derived from PVDF copolymer/MS dilutions with various polymer contents that were quenched, from the melt to the ice water bath. (a) PVDFc1/MS diluted system; (b) PVDFc2/MS diluted system.
Figure 9.
Tensile strength and elongation at break for membranes derived from PVDF copolymer/MS dilutions with various polymer contents that were quenched, from the melt to the ice water bath. (a) PVDFc1/MS diluted system; (b) PVDFc2/MS diluted system.