Increasing PVDF Content Research Articles

Notably enhanced proton conductivity by thermally-induced phase-separation transition of Nafion/ Poly(vinylidene fluoride) blend membranes

Blending perfluorosulfonic acid (e.g. Nafion) with other polymers is an effective approach to reduce the cost and adjust the properties of durable proton conducting membranes. In particular, poly(vinylidene fluoride) (PVDF) composites have attracted much attention because of its outstanding chemical/thermal stability and it can enhance the barrier property, mechanical strength and processibility of the membranes. One of the major issues with this approach, however, is that the proton conductivity of the Nafion/PVDF composite drops rapidly with increasing PVDF content. In this work, we demonstrate for the first time that through a simple and facile annealing process at 180 °C, the proton conductivity of the Nafion/PVDF blend membrane can be boosted by more than 200%, and the annealed membrane with 60 wt% Nafion shows proton conductivity as high as 45.2 mS cm−1. The barrier properties of the blend membranes are significantly superior to the neat Nafion as indicated by the vanadium ion permeability of approximately one third of the Nafion. In the single cell test of vanadium redox flow battery, the energy efficiency of the annealed membrane with 60 wt% Nafion reaches 83.6% at 100 mA cm−2, which is comparable to Nafion membranes. The annealed blend membranes possess a high chemical and dimensional stability, which makes them promising for practical applications in redox flow batteries or fuel cells.

Promotion of poly(vinylidene fluoride) on thermal stability and rheological property of ethylene-tetrafluoroethylene copolymer

Abstract In this work, the thermal stability, rheological properties and mechanical properties of ethylene-tetrafluoroethylene copolymer (ETFE)/poly(vinylidene fluoride) (PVDF) blends were investigated by thermogravimetric analysis, rheometer and the tensile test. Thermal results indicated that blends had better thermal oxidation resistance than pure ETFE. Particularly, the initial thermal decomposition temperature (T d 0) and the temperature at maximum decomposition rate (T d max) of PVDF/ETFE (10/90 wt%) blends were at 374.49°C and 480°C, which were 52.6°C and 34°C higher than pure ETFE. The activation energy of thermal degradation (E d ) of ETFE was 66 kJ/mol, while the PVDF/ETFE (10/90 wt%) blends presented a higher E d , near 187 kJ/mol. Furthermore, rheological measurements demonstrated that the shear-thinning tendency of blends became stronger with increasing PVDF content. PVDF/ETFE (10/90 wt%) blends had somewhat lower mechanical properties than ETFE, which was still high enough for various applications. Blends with PVDF provided an efficient method to extend the application area of ETFE.

Electrospun Nafion/PVDF single-fiber blended membranes for regenerative H2/Br2 fuel cells

Nafion® perfluorosulfonic acid (PFSA) and poly(vinylidene fluoride) (PVDF) were electrospun simultaneously as a polymer solution mixture using a single needle spinneret. The nanofiber morphology was highly unusual, with bundled 2–5nm fibril strands of Nafion and PVDF aligned along the fiber axis. Membranes were made from fiber mats by a simple hot-pressing step, followed by thermal annealing, where the fibril morphology of Nafion and PVDF in the original nanofibers was retained in the membrane. The PVDF component in the final membrane served three roles, as an electrospinning carrier polymer for PFSA, a mechanical reinforcement, and a hydrophobic (uncharged) component to limit PFSA ionomer swelling. A series of single-fiber membranes with Nafion/PVDF contents ranging from 40/60 to 90/10wt%/wt% were prepared, characterized, and evaluated for use in a regenerative hydrogen/bromine fuel cell. As expected, there was a decrease in proton conductivity, water/electrolyte swelling, and Br2/Br3- permeability with increasing PVDF content. Membrane conductivity was lower than expected based on the weight fraction of PVDF, due presumably to an unusually large fraction of highly structured water. Nevertheless, a single-fiber Nafion/PVDF membrane with a thickness of 18µm and an 80/20wt%/wt% Nafion/PVDF composition performed well in a H2/Br2 regenerative fuel cell due to a combination of low area-specific resistance and low Br2/Br3- crossover. Thus, with an electrolyte containing 0.9M Br2 in 1.0M HBr, the maximum power output was 46% higher than that with a Nafion 212 membrane (1.31 vs. 0.90W/cm2).

Binary and ternary nanocomposites based on PVDF, PMMA, and PVDF/PMMA blends: polymorphism, thermal, and rheological properties

Binary and ternary nanocomposites based on poly(vinylidene fluoride) (PVDF), poly(methyl methacrylate) (PMMA), and PVDF/PMMA blends were successfully prepared through a melt-mixing process, using a commercial organoclay (15A) as the nanofiller. The 15A was more finely dispersed (intercalated/partially exfoliated) within the PVDF matrix compared with the PMMA matrix. A higher PVDF content in the ternary composite essentially led to a superior degree of 15A dispersion. The 15A addition induced the polar β-form PVDF crystals, whereas the presence of PMMA in ternary composites reduced the efficiency in promoting β-form formation by 15A. Adding 15A also enhanced the nucleation of PVDF, but the enhancement was inferior while PMMA was present. Conversely, the crystal growth of PVDF was retarded with the existence of 15A, and the PVDF/15A binary composite exhibited the greatest retardation. The equilibrium melting temperature (Tm°) of PVDF in the neat state and in the blends increased after 15A addition. The PVDF/15A binary composite possessed an evidently higher β-form Tm° than the α-form Tm° of neat PVDF (~20.1 °C rise). Similar effects on the individual components, 15A declined the thermal stability of PVDF but increased that of PMMA in the ternary composites. Rheological property measurements revealed that the ternary composites performed in-between that of individual PVDF/15A and PMMA/15A binary composites. A percolation of 15A (pseudo)network structure was developed in the composites, and a more elastic behavior was observed with increasing PVDF content in the composites.

Perfluorocyclobutane and poly(vinylidene fluoride) blend membranes for fuel cells

Polymer electrolytes based on block copolymers are considered promising proton exchange membrane (PEM) materials for fuel cell applications. A new class of block copolymers of perfluorocyclobutane (PFCB) units with pendant side chains containing sulfonic acid groups have recently been developed and studied for fuel cell applications. In order to enhance the membrane mechanical and hydrothermal stability, sulfonated PFCB copolymers are blended with fluoropolymers such as poly(vinylidene fluoride) (PVDF). Membrane fundamental properties (e.g., proton conductivity, gas permeability, water uptake) and fuel cell performance and durability were evaluated as a function of PVDF content. The proton conductivity and fuel cell performance only decrease slightly with PVDF content at PVDF blend levels of up to 30wt.%. However, at the PVDF levels higher than 40wt.%, the proton conductivity and fuel cell performance decrease rapidly with increasing PVDF content. The PFCB/PVDF composite membranes have lower hydrogen, oxygen and nitrogen gas permeability as well as a higher ratio of conductivity to permeability than perfluorosulfonic acid PFSA membranes (e.g., Nafion®). Membrane hydrothermal swelling is significantly reduced by blending sulfonated PFCB with PVDF. This reduced swelling leads to improved membrane mechanical durability as measured by increased lifetimes in RH cycling tests with increasing PVDF content.

Proton exchange membranes based on semi-interpenetrating polymer networks of Nafion ® and poly(vinylidene fluoride) via radiation crosslinking

A new method to prepare proton exchange membranes based on semi-interpenetrating polymer networks (semi-IPN) of Nafion ® and poly(vinylidene fluoride) (PVDF) via radiation crosslinking was proposed. The tensile strength, degree of crosslinking, water uptake, and swelling ratio of the composite membranes were studied. Compared to the recast Nafion ® 212 membrane, the composite membranes show much better mechanical properties and improved dimensional stability. The tensile strength of the composite membranes ranges from 34.3 MPa to 53.4 MPa, which is higher than that of the recast Nafion ® 212 membrane (23.9 MPa). The dimensional stability of the composite membranes also increases with increasing PVDF content in the membranes. The composite membranes show considerable proton conductivity even at 100–120 °C. The membrane containing 40% PVDF shows the highest proton conductivity of 3.37 × 10 −2 S/cm at 115 °C. These properties make them a great potential in polymer electrolyte membrane fuel cells (PEMFC).

Sulfonated PEEK and fluorinated polymer based blends for fuel cell applications: Investigation of the effect of type and molecular weight of the fluorinated polymers on the membrane’s properties

This work clearly demonstrates the effect of the type and molecular weight of the fluorinated polymer of SPEEK/Fluorinated polymer blends for low temperature (<80 °C) Fuel Cell Applications. Comparisons with trademarks (e.g., Nafion ®) suggests that the membranes we have prepared in this study have good compatibility in all application respects. Membranes were prepared by solution casting method from four different fluorinated polymers; poly (vinylidene fluoride) with three different molecular weights (PVDF, M w: 180.000, M w: 275.000, M w: 530.000); Poli(vinylidene fluoride-co-Hexafluoro propylen) (PVDF-HFP M n:130.000) and sulfonated poly(ether ether ketone) (SPEEK) with sulfonation degree (SD) of 70. The sulfonation degree (SD) of SPEEK was determined by FTIR, 1H NMR and ion exchange capacity (IEC) measurements. Thermo-oxidative stability and proton conductivity of the membranes were determined by using thermal gravimetric analysis (TGA) and BT-512 BekkTech membrane test systems, respectively. Chemical degradation of SPEEK membranes was investigated via Fenton test. The morphology of the membranes were examined by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Water uptake and proton conductivity values decreased with the addition of fluorinated polymers (PVDF, PVDF-HFP) as expected, but proton conductivity values were still comparable to that of Nafion 117 ® membrane. Addition of fluorinated polymers improved chemical degradation of the blend membranes in all ratios while addition of PVDF-HFP to the SPEEK70 caused phase separations in all ratios. Methanol permeability value of SPEEK70/PVDF( M w = 275.000) blend membrane (3.13E-07 (cm 2/s)) was much lower than Nafion 117 ® (1.21E-06 (cm 2/s). PVDF addition to the SPEEK polymers caused increase in elongation of the membranes. Increase in the molecular weight of the PVDF did not show any effect on the Young modulus, but resulted in high elongation values. On the other hand, increasing PVDF content of the blend membranes caused lower elongation values at break and didn’t have any effect on the Young modulus. The gas permeability values of SPEEK70/PVDF type blend membranes were lower than that of the Nafion ®. Hydrogen and oxygen permeability values were one-tenth and one-fifth of the Nafion®, respectively.

Crystallization and morphology of poly(vinylidene fluoride)/poly(3‐hydroxybutyrate) blends. I. Spherulitic morphology and growth by polarized microscopy

AbstractThe development of the poly(3‐hydroxybutyrate) (PHB) morphology in the presence of already existent poly(vinylidene fluoride) (PVDF) spherulites was studied by two‐stage solidification with two separate crystallization temperatures. PVDF formed irregular dendrites at lower temperatures and regular, banded spherulites at elevated temperatures. The transition temperature of the spherulitic morphology from dendrites to regular, banded spherulites increased with increasing PVDF content. A remarkable amount of PHB was included in the PVDF dendrites, whereas PHB was rejected into the remaining melt from the banded spherulites. When PVDF crystallized as banded spherulites, PHB could consequently crystallize only around them, if at all. In contrast, PHB crystallized with a common growth front, starting from a defined site in the interfibrillar regions of volume‐filling PVDF dendrites. It formed by itself dendritic spherulites that included a large number of PVDF spherulites. For blends with a PHB content of more than 80 wt %, for which the PVDF dendrites were not volume‐filling, PHB first formed regular spherulites. Their growth started from outside the PVDF dendrites but could later interpenetrate them, and this made their own morphology dendritic. These PHB spherulites melted stepwise because the lamellae inside the PVDF dendrites melted at a lower temperature than those from outside. This reflected the regularity of the two fractions of the lamellae because that of those inside the dendrites of PVDF was controlled by the intraspherulitic order of PVDF, whereas that from outside was only controlled by the temperature and the melt composition. The described morphologies developed without mutual nucleating efficiency of the components. © 2003 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 873–882, 2003

Thermal diffusivity and heat capacity of poly(vinylidene fluoride)/poly(methyl methacrylate) blends by flash radiometry

The thermal diffusivity D and the relative heat capacity HC were measured for poly(vinylidene fluoride)/poly(methyl methacrylate) (PVDF/PMMA) blends by flash radiometry. The effect of the crystal form on D of these polymer blends as well as the dependence of D and HC on temperature were discussed. In the PVDF content region between 70 and 80 wt%, the blend samples preferentially form the β crystal phase when they are prepared by melt quenching. The dependence of D on PVDF content for the melt-quenched sample showed a noticeable change in the composition region in which the β crystal phase is formed. D decreased with increasing PVDF content up to 70 wt% and then increased up to 100 wt%, showing a peak between 75 and 80 wt%. From the X-ray diffraction patterns, the crystallinity of the blends monotonically decreases with decrease of PVDF content. The results imply that the peak in D in the melt-quenched 75 25 and 80 20 PVDF/PMMA blends is not related to the crystallinity but to another structural factor. The temperature profile of D showed inflection points corresponding to the glass transition temperature and the melting temperature of the α crystal phase. The temperature profile of HC exhibited changes of HC due to the melting transition of the β and α crystal phases. These points from the temperature profile of D and HC correspond well to those from thermal analysis.

Dielectric and TSC study of semicompatible PVDF/PMMA blends

Dielectric and TSC studies of charged (by stabilized electrical breakdown) and discharged PVDF/PMMA blends have been performed. The relative changes in e″ are found to decrease with increasing PVDF content except for samples with 50 and 70 weight-% of PVDF. The temperature Tmax of the TSC maximum is found to be shifted linearly towards lower temperatures with increasing PVDF content up to 30 weight-% of PVDF. For higher concentrations Tmax is found to be independent on PVDF content except for samples of 70 weight-% of PVDF. These behaviours indicate that for PVDF concentrations higher than 30 weight-% the system starts to lose its compatibility.