3.1 Effect of Membrane Material on Mechanical Properties:
To assess the physical characteristics of the two types of filter membranes, tensile testing was used to generate stress-strain curves for PTFE and PVDF, both with a pore size of 0.22 µm. The stress-strain curves were constructed by measuring the deformation and the applied force for the tensile cross-sections of the specimens. Figure 2 provides a graphical representation of these curves, illustrating the relationship between the stress applied to the material and the resulting strain when subjected to stretching. This analysis was a critical tool for understanding the mechanical behavior and performance attributes of the filter membranes.
The stiffness of materials, as quantified by Young's modulus, was determined by analyzing the slope of the elastic section displayed on the stress-strain graph. In this study, the Young’s modulus values of the PTFE and PVDF membranes were estimated by selecting two points within the respective elastic phases while maintaining a significant distance between them. The results revealed higher Young’s modulus values for the PTFE membranes than those of the PVDF membranes. Tensile strength, a crucial measure of a material's maximum stretching resistance, was calculated using the appropriate formula. The PTFE membranes (45.605 MPa) exhibited higher tensile strength than the PVDF membranes (42.143 MPa). Elongation at break, which indicates a material's ability to withstand deformation before failure, was calculated for each sample. The PVDF membranes exhibited superior elongation at break (29.297%) compared to PTFE membranes (23.267%). In summary, the PTFE membranes displayed higher Young's modulus and tensile strength, indicating greater stiffness and strength compared to the PVDF membranes. However, the PVDF membranes exhibited superior elongation at break, demonstrating the greater ability of these membranes to deform before reaching the breaking point.
The surfaces and tensile sections of the filter membranes were thoroughly examined using a biomicroscope, as illustrated in Fig. 3. There were significant differences between the PTFE and PVDF membranes. The PTFE membranes displayed uniform and consistent surfaces, but the PVDF membranes exhibited readily distinguishable fibrous compositions [31]. This dissimilarity in surface morphology between the two kinds of membranes suggests that these materials possess distinct characteristics. Figure 4 presents a comprehensive analysis of the tensile cross-sections of the stretched filter membranes. The tensile cross-sections of the PTFE membranes displayed uniform, compact, and even arrangements [32], however, the tensile sections of the PVDF membranes exhibited distinguishable fiber layers or bundles with evidence of fiber breakage or fractures [33]. Two distinct regions existed within the membranes: a semi-stretched region and a curled region. In the semi-stretched region, a slightly curled morphology was observed. This phenomenon was attributed to the incomplete stretching and elongation of the fiber filaments during the stretching process. The PVDF membranes exhibited a distinctive curled region, which may be correlated to their superior elongation at break, with increased retraction and entanglement of fiber filaments during rupture. Furthermore, cavities and empty spaces were detected in the cross-sectional area, as highlighted in the rectangular boxes in Figs. <link rid="fig4">4</link>-a and 4-b.
In summary, the microscopy findings revealed noticeable disparities in the surface and cross-sectional morphology between PTFE and PVDF membranes. These differences provide valuable insights into the different mechanical properties of these two membranes and how they differ in performance for various applications.
Polyvinylidene fluoride (PVDF) has a molecular formula of -[CH2-CF2]n- and exhibits a molecular structure characterized by intertwined carbon-hydrogen (C-H) and carbon-fluorine (C-F) bonds, molecular model is shown in Fig. 5-a. Polytetrafluoroethylene (PTFE) has a molecular formula of -[CF2-CF2]n- and forms a linear polymer composed of polymerized tetrafluoroethylene monomers, with a molecular structure that is securely bonded by carbon-fluorine (C-F) links. The molecular model is shown in Fig. 5-b. PTFE is a modified polyethylene in which each hydrogen atom has been replaced by a fluorine atom, resulting in low polarity. This low polarity of PTFE contributes to its exceptional non-adhesive properties. Conversely, PVDF has a fluorine atom content that ranges from 65 to 70 percent. This difference in the level of fluorine content explains the observed difference in material properties. With an increase in fluorine content, the surface energy of the polymer film decreased and both Young’s modulus and tensile strength increased. Overall, the modification of PTFE results in exceptional mechanical properties, making it an excellent choice for a wide range of applications.
The remarkable tensile strength of PTFE, an amorphous polymer, allows it to withstand greater deformation than PVDF [34]. PTFE's exceptional properties are explained by its molecular structure, as its high concentration of carbon and fluorine atoms result in a densely packed molecular configuration and enhanced crystallinity [35]. This structure reduces the elongation at break. In contrast, PVDF contains fewer carbon atoms in conjunction with fluorine atoms, resulting in reduced crystallinity and increased elongation at break [36]. Crystallinity plays a pivotal role in determining the mechanical properties of these polymers. The high crystallinity in PTFE resulted in lower elongation at the breaking point under tension, primarily due to the orderly alignment of molecular chains within the lattice, causing localized fracture under force application. Conversely, PVDF’s lower crystallinity resulted in a more irregular arrangement of molecular chains, enabling it to absorb more energy and exhibit greater elongation at the breaking point when stretched. PTFE has impressive compressive strength, and its highly organized polymer structure and exceptionally low coefficient of friction made it an outstanding material for filtration, effectively removing small particles and microorganisms [37]. Additionally, there is reduced need for frequent membrane replacement and maintenance, for reduced costs. In summary, the exceptional attributes of PTFE of outstanding tensile strength, minimal elongation at break, and effective filtration capabilities make it a versatile and extensively employed material across a wide spectrum of applications.
The resistance of polymeric materials to external damage primarily relies on the intricate interplay of intramolecular chemical bonding and intermolecular interactions. Atoms with higher electronegativity demonstrated a greater capacity to attract shared electrons [38]. For instance, fluorine, with its elevated electronegativity, can enhance the stability of carbon atoms in covalent bonds more effectively than hydrogen, which has a lower electronegativity. This variance in electronegativity can significantly impact the polarity and stability of covalent bonds, with the carbon-fluorine (C-F) bond playing a pivotal role in polymeric materials such as PTFE. The electron-attracting fluorine resulted in an electron bias within the C-F bond towards the fluorine atom, for exceptional stability. Carbon-fluorine bonds (485 kJ/mol) exhibited higher binding energies, more pronounced dipole moments, and greater charge densities compared to hydrocarbon bonds (413 kJ/mol) [39]. As a result, C-F bonds display significantly greater resilience than C-H bonds. Carbon-hydrogen bonds manifest lower bond energies and reduced chemical inertness, rendering them susceptible to damage and substitution in various chemical reactions. PTFE contains four covalent C-F bonds, while PVDF has two C-F bonds and two C-H bonds. Thus, PTFE molecules possess higher bond energies, enabling them to withstand more substantial external stresses, for exceptional durability and excellent mechanical properties. PTFE and PVDF also differ in intermolecular cross-linking. PTFE molecules have superior symmetry and more robust intermolecular cross-linking compared to PVDF molecules. This can be attributed to the presence of subphases within the molecular structure of PVDF, leading to molecular interpenetration and entanglement that ultimately weakening intermolecular cross-linking. Additionally, the relatively small electronegativity difference between hydrogen and carbon atoms in PVDF molecules results in a less robust C-H bond, further diminishing intermolecular cross-linking in PVDF. In contrast, PTFE molecules exhibited a compact and highly ordered structure with enhanced symmetry. The high electronegativity of fluorine atoms also contributes to the stability of C-F bonds, promoting intermolecular cross-linking [40]. The interactions between covalent bonds, van der Waals forces, and hydrogen bonds within PTFE molecules can further enhance intermolecular cross-linking.
In summary, PTFE has better binding energy and intermolecular cross-linking than PVDF, resulting in PTFE membranes with superior mechanical properties. In particular, the stability of C-F bonds driven by electronegativity and the precise molecular structure of PTFE are critical factors that explain the excellent performance of this material.
3.2 Influence of preparation process on mechanical properties of filter membranes
A primary objective of this study was to investigate potential effects of preparation process on the properties of microfiltration membranes manufactured from identical materials. Materials were tested that were prepared using the stretching method or the phase transformation method. Biomicroscopy was utilized as an analytical tool to assess potential differences in the mechanical properties of these membranes.
3.2.1 Mechanical properties of filter membranes prepared by stretching method
Biomicroscopy was used to examine the surfaces and tensile cross-sections of the filter membranes, as illustrated in Fig. 6. As depicted in Fig. 6-a, a spatial mesh-like fiber structure was observed on the surfaces of the membranes, consisting of both transverse and longitudinal fiber filaments. There were apparent gaps between these filaments, indicating a relatively open structure. As in our above analysis, imaging of the tensile cross-section, as depicted in Fig. 6-b, revealed two distinct regions: a semi-stretched region and a curled region. In the semi-stretched region, the filaments exhibited a subtle curling morphology, suggesting that, despite some stretching during production, they had not completely straightened. Conversely, the curled region contained filaments that had retracted and twisted, assuming a curved morphology. This phenomenon can be attributed to the high elasticity and amorphous nature of the filter membrane material, as during breakage, the fiber filaments can retract and twist.
Analysis of the tensile cross-section revealed a flattened profile. Nevertheless, the stretching process caused certain filaments to elongate and fracture, resulting in the formation of pores and holes within the material (as indicated within the rectangular box in Fig. 6-b).
Filter membranes produced through the stretching process exhibited distinctive characteristics. These membranes, despite possessing high tensile strength, were notably brittle and susceptible to brittle fracture, resulting in the formation of short and thick fiber filaments. This can be attributed to the stretching process itself, during which the membrane material underwent substantial elongation, leading to a directional alignment of the molecular chains within the material. This directional alignment raised the energy of intermolecular bonds and also enhanced the order of the lattice structure, consequently increasing intermolecular interaction forces to increase the tensile strength of the membrane.
However, the stretching process subjected the polymer material to a substantial amount of stress and deformation. This stress led to the breakage of polymer chains in specific regions. When the membrane eventually fractured, the cross-section of the fiber filaments typically exhibited a flattened configuration, accompanied by the prominent formation of pores and holes due to polymer chain breakage and deformation during stretching. Importantly, these structural modifications affected the mechanical properties, filtration performance, and overall service life of the filter membrane. Overall, understanding how structural changes alter function is essential to optimize filter membrane design and application.
3.2.2 Mechanical properties of filter membranes prepared by phase transition method
As depicted in Fig. 7-a, the filter membrane produced via the phase transition method exhibited a distinct, spatially interlaced fiber structure. The membrane was predominantly comprised of closely aligned, interlaced slender fiber filaments, creating a uniform crystalline region. The surface of this filter membrane was exceptionally smooth, which could offer advantages for various applications. The tensile cross-section of the filter membrane is shown in Fig. 7-b. The membrane displayed straight and slender fiber filaments that were moderately spaced. This filter membrane had a high level of transparency, which could be advantageous in specific filtration processes. Again, there were obvious structural alterations in the filter membranes resulting from the stretching process, as indicated in the rectangular box in Fig. 7-b. These changes determined the filter membrane’s properties and would affect overall performance.
In their tensile cross-sections, filter membranes produced through the phase transition method exhibited slender fiber filaments and had low tensile strength, making them resistant to brittle fracture. Comprehensive molecular analysis revealed that the phase transition method induced lattice rearrangement and amorphization within the material. This resulted in a more uniform and orderly molecular bonding that significantly enhanced the material’s ductility. Furthermore, the amorphization process led to a more homogeneous and less densely packed molecular structure, for enhanced ductility and toughness, consequently increasing the membrane’s elongation at break. These attributes make these membranes well-suited for applications that require flexibility and toughness. The phase transition process utilizes an organic solvent, resulting in the formation of fibrous filaments with minimal impurities, high transparency, and a uniform and dense structure, devoid of discernible voids or pores. Overall, filter membranes prepared through the phase transition method exhibited superior mechanical properties, making them an excellent choice for a variety of applications.
3.2.3 Comparative analysis of tensile method and phase transformation method
To directly compare filter membranes prepared using the stretching method and the phase transition method, an identical standardized testing procedure was used, as detailed in section 3.1.
The results of these tests are visually depicted in Fig. 8. Notably, filtration membranes prepared through the phase transformation method exhibited a significantly higher Young's modulus compared to their counterparts produced via the tensile method. The filter membranes prepared using the tensile method (40.603 MPa) had higher tensile strength values than those prepared via the phase transformation method (23.343 MPa). Filter membranes prepared through the phase transformation method had higher elongation at break (23.028%) compared to filter membranes prepared using the tensile method (15.703%). These findings suggest that filter membranes manufactured via the tensile method possess superior tensile strengths but exhibit greater brittleness, rendering them prone to brittle fracture. In contrast, filter membranes prepared through the phase transformation method demonstrate enhanced elongation at break and increased toughness, thereby reducing the likelihood of brittle fracture. These findings are consistent with the observations. Filter membranes manufactured through the stretching method exhibited tendencies towards brittle fractures, while those crafted via the phase transformation method displayed remarkable toughness. Furthermore, filter membranes produced via the phase transformation method showcased more organized fiber filament structures, flatter surfaces, reduced incidence of cracks, and a more uniform pore size and distribution.
Overall, filter membranes generated through the phase transformation method outperformed membranes created using the tensile method in terms of mechanical properties, underscoring the pivotal role of the chosen production method in achieving the desired characteristics of filter membranes. First, the cross-section of filter membranes subjected to tensile testing and produced using the phase transformation method revealed elongated and uniformly distributed fiber filaments. In contrast, the stretching method resulted in shorter, thicker filaments with higher tensile strength. Second, the phase transformation method resulted in more consistent material transformation and orderly filament alignment. In contrast, there was irregularity in the stretching method owing to the stress and deformation involved, resulted in irregular stretching, filament breakage, and twisting. Additionally, the gentler phase transformation method led to a flatter and smoother surface compared to the uneven cross-sections, surface irregularities, and the formation of small bumps in membranes produced by the stretching method. Filter membranes produced via the phase transformation method underwent minimal tensile stress during preparation, resulting in fewer and less pronounced surface cracks. In contrast, the stretching method induced higher stress and deformation, resulting in visible crack morphology with a noticeable direction of extension. The materials also differed in the pore structure, as the material produced by phase transformation resulted in a uniform pore structure with consistent pore size and distribution, but the stretching method resulted in non-uniform pore size and distribution. Stress concentration during stretching led to pore formation and expansion, often accompanied by the creation of rows of agglomerates. Overall, these structural differences have significant implications for the performance and utility of the filter membranes, highlighting the profound impact of selection of an appropriate fabrication method to achieve the desired functional properties.
This comprehensive analysis of microfiltration membranes produced using two distinct preparation methods revealed that the varying stresses and deformation mechanisms applied during the manufacturing process resulted in significant disparities in several aspects of membrane structure. Notably, filter membranes crafted through the phase transformation method exhibited uniform fiber filament structures, flat surfaces, fewer cracks, and minimal pore structures. In stark contrast, those prepared via the stretching method displayed heterogeneity and irregularity in terms of fiber filament structure, surface flatness, crack morphology, stretching direction, and pore structure. Overall, these findings indicate that the phase transformation method is the superior approach to produce filter membranes. This method should yield higher-quality filter membranes, with enhanced mechanical properties for long service life.
3.3 Effect of pore size on the mechanical properties of filter membranes
We next explored the filtration performance of two distinct types of PTFE pore size filter membranes: one with a pore size of 0.45 µm and the other with a pore size of 3.0 µm. These tests were performed according to the same rigorous methodology outlined in section 3.1, maintaining a uniform and standardized approach for the assessment. The outcomes of these tests are depicted in Fig. 9 and provide valuable insights into the filtration capabilities and characteristics of these PTFE membranes.
The comparative analysis of PTFE filter membranes with different pore sizes (0.45 µm and 3.0 µm) revealed notable distinctions in mechanical properties, including Young’s modulus, tensile strength, and elongation at break, as shown in Fig. 9. After progressing through the elastic phase, the specimens were stretched to reach the yield point. The PTFE membrane with a 0.45 µm pore size had a higher tensile strength (11.808 MPa) than the PTFE membrane with a 3.0 µm pore size (10.815 MPa). Additionally, the PTFE membrane with a 0.45 µm pore size had a higher elongation at break (49.306%) than the PTFE membrane with a 3.0 µm pore size (19.452%). Thus, PTFE membranes with smaller pore sizes outperformed their counterparts in terms of tensile strength and elongation at break. Smaller pore size PTFE membranes exhibited a dense and uniform pore structure, which uniformly distributed external forces and mitigated local stress concentration. Consequently, these materials exhibited superior pressure resistance, thereby extending their service life. Furthermore, the reduced pore size of PTFE membranes effectively prevented the infiltration of larger particles and impurities, for improved filtration effectiveness that can further prolong the membrane’s service life. These findings underscore a pivotal role of pore size in determining the mechanical performance and functional durability of PTFE filter membranes.
As the filter membrane specimen surpassed the yield point, it entered the plastic deformation stage, a critical phase characterized by a sharp transition to ductile yielding, ultimately leading to specimen fracture. For PTFE membranes with a 0.45µm pore size, a distinctive phenomenon known as necking became apparent during the yielding stage. This phenomenon, however, gradually diminished and eventually disappeared with increased pore size. From a molecular chain perspective, PTFE material exhibits a highly dense arrangement where each fluorine atom shares an electron pair with two surrounding carbon atoms, resulting in a strongly crystalline nature. If a material has smaller pore size, molecular chains tended to align within tightly packed crystalline regions. Consequently, PTFE membranes with smaller pores displayed a higher degree of crystallinity and more robust interactions between molecular chains, leading to higher Young’s modulus and improved tensile strength. Furthermore, the reduced pore size constrained the movement of molecular chains and intensified their close interactions. In contrast, larger pore sizes in PTFE membranes weakened interactions between molecular chains. Therefore, when subjected to stress, PTFE membranes with smaller pores exhibited better resistance to the slippage and relative movement of molecular chains, resulting in higher tensile strength. In summary, the observed necking phenomenon during plastic deformation, along with subsequent molecular-level explanations, revealed a pivotal role of pore size in influencing the mechanical properties of PTFE membranes.
In conclusion, PTFE membranes with smaller pore sizes exhibited higher Young's modulus, improved tensile strength, superior elongation at break, and excellent compressive strength for improved filtration effectiveness. These exceptional attributes suggest PTFE membranes with smaller pore sizes are the preferred choice for diverse filtration and separation requirements.