Polyvinylidene fluoride (PVDF for short) is highly non reactiveThermoplasticityFluoropolymers。It is available through1,1-DifluoroethyleneSynthesis by polymerization of.Soluble in strong polar solvents such as dimethyl acetamide.Anti aging, chemical resistance, weather resistance, UV radiation resistance and other excellent performance.It can be used as engineering plastics, sealing ring anti-corrosion equipment, capacitor, coating, insulation material, ion exchange membrane material, etc.[1]
PVDF is mainly used in situations where high purity is required and solvent resistance and acid alkali corrosion are required.Compared with other fluoropolymers, such asteflonThe density of PVDF is low (1.78g/cm).
PVDF can be used to produce insulating skins of pipes, plates, films, substrates and cables.At the same time, it can also be used for injection molding or welding, and is widely used in chemical, semiconductor, pharmaceutical and national defense industries, for example, it can be used for manufacturingLithium ion battery。In addition, it can also be made intoCross linkobturatorfoam, which is increasingly widely used in the aerospace field.
PVDF of fine powder grade, such as KYNAR 500 PVDF and HYLAR 5000 PVDF, can be used to manufacture high-endMetallic coating。This paint has extremely high gloss and color stability.In many famous buildings, such asBimodal tower，Taipei 101You can find this kind of paint.Commercial buildings and residential shopsMetal roofThis coating can also be used.
Because PVDF film has non-specific affinity for amino acids, it canWestern blottingIt is used to fix protein in detection.
In 1969, researchers found that PVDF had strongPiezoelectric effect: The piezoelectric coefficient of the polarized (i.e., the net dipole moment generated by placing a strong electric field) film is 6-7pC/N, which is more than 10 times larger than the corresponding value of the polymer found at that time.
PVDFGlass transition temperature（Tg）It is about − 35 ° C, and the crystallinity is usually 50 – 60%.In order to give piezoelectric properties to materials, materials are usually mechanically stretched along the direction of molecular chains, and then polarized under tension.PVDF has a variety of solid phases: α phase (TGTG '), β phase (TTTT) and γ phase (TTTGTTTG').The difference between these phases lies in whether the molecular chain is cis (T) or trans (G).PVDF will becomeferroelectricPolymer, with good piezoelectric and pyroelectric properties.These properties enable it to be used in the production of sensors and batteries. For example, PVDF films are used in the sensors of some new thermographic cameras.[2]
With some otherPiezoelectric material, such asPZT, different, d of PVDFthirty-threeThe value is negative.From a physical point of view, this means that when other materials expand in an electric field, PVDF will shrink, and vice versa.
Synthesis and treatment
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PVDF can be synthesized by free radical (or controlled free radical) polymerization using gaseous vinylidene fluoride monomer.Subsequent casting or solution treatment (such asSolution casting 、Spin coatingOr film casting).At the same time, prepareLangmuir Blodgett film。Solvents commonly used for solution based treatment includeDimethylformamideAnd butanone.In waterborne emulsion polymerization, fluorinated surfactant, perfluoric acid in the form of anion, is often used as a processing aid for solubilizing monomers.Compared with other fluoropolymers, polyvinylidene fluoride has a lower melting point (about 177 ° C), so it is easier to melt.
The treated material is usually in the α phase without piezoelectricity.In order to convert it into piezoelectric beta phase, the material usually needs to be stretched or annealed.The PVDF film with micron thickness can not be treated in this way, and the residual stress between the film and the substrate is enough to convert it into β phase.
In order to obtain piezoelectric response, materials must be polarized in a strong electric field.The polarization electric field is usually greater than 30MV/m.In order to obtain a larger piezoelectric response, thicker films (thicker than 100 µ m) need to be heated during the polarization process at a temperature of 70-100 ° C.
A quantitative defluorination treatment in mechanochemistry can treat PVDF waste in a more environmentally friendly way.[3]
copolymer
The copolymer of PVDF can also be used to make piezoelectric materials and electrostrictive materials.Among them, the most commonly used copolymer is vinylidene fluoride trifluoroethylene copolymer, with a ratio of about 50:50 wt% or 65:35 wt% (equivalent to 56:44 mol% or 70:30mol%).Another common copolymer is vinylidene fluoride-TetrafluoroethyleneCopolymers.They improve piezoelectric response by improving the crystallinity of materials.
Since the structural unit of the copolymer is less polar than that of pure PVDF, the above copolymer usually has higher crystallinity.This will lead to greater piezoelectric response: d of vinylidene fluoride tetrafluoroethylene copolymerthirty-threeUp to − 38pC/N, while the corresponding value of pure PVDF is − 33pC/N.
Terpolymer
The terpolymer of PVDF can be used to manufacture electromechanical strain materials.The commonly used PVDF based ternary polymers include vinylidene fluoride trifluoroethylene-ChlorotrifluoroethyleneCopolymer and vinylidene fluoride trifluoroethylene chlorofluoroethylene copolymer.This terpolymer based on relaxor ferroelectrics can be manufactured by randomly adding bulky chlorotrifluoroethylene into the vinylidene fluoride trifluoroethylene polymer chain (itself a ferroelectrics).This random doping process will destroy the long-range ordering of ferroelectric polar phases, resulting in nano polar domains.When an electric field is applied, the conformation of the disordered nano polar domain will become an all trans conformation, which will lead to the material having a large electrostriction and a high dielectric constant (~50) at room temperature.
application area
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PVDF is often used to make insulation sheath of wires due to its multiple excellent properties such as elasticity, low weight, low thermal conductivity, high chemical corrosion resistance and heat resistance.No. 30 thin wire commonly used for winding circuit andpcbPVDF insulation is commonly used.Cables with PVDF insulation layer are often referred to as "Kynar line" under the trademark of PVDF.
PVDF is commonly used in production due to its piezoelectric propertiesTactile sensorArrays, inexpensive strain gauges, and lightweight audio transducers.
PVDF or lithium batteryComposite electrodeStandard binder of: dissolve inN-methyl-2-pyrrolidone(NMP) PVDF with specific gravity of 1-2% and graphite, silicon, tin, LiCoOtwo、LiMntwoOfourOr LiFePOfourSuch active lithium storage materials and carbon black or carbonNanofiberSuch conductive additives are mixed, and then the slurry is poured on the metal collector and the NMP is evaporated to form a composite electrode or paste electrode.PVDF can be used on this occasion because it is chemically inert within the potential range of battery charging and discharging and will not react with electrolyte or lithium.
In the field of biomedicine, PVDF films are often used for immunoblotting, and proteins will be electrophoretic on them.Because PVDF is solvent resistant, the film used in the test can be easily stripped and reused to detect other proteins.PVDF film can also be used to make syringe type or wheel type membrane filtration devices.The heat resistance, chemical corrosion resistance, low protein binding and other characteristics of this material enable it to be used as a disinfection filter in the preparation of drugs, and also as a filter in the preparation of samples for HPLC and other analysis, to prevent expensive equipment from being damaged due to the small amount of particulate matter in these samples.
PVDF can also be used to make professional monofilament fishing lines to replace the traditional nylon monofilament.Its surface is hard, more resistant to the abrasion of sharp fish teeth, and its optical density is lower than that of nylon, making the fishing line more difficult to be detected by the sharp fish eyes.At the same time, its density is larger than nylon, so that the fishing line can sink toward the fish faster.
Compared with semiconductor piezoresistive sensors, PVDF sensors are more suitable for dynamic modal testing, and in terms of structural integrationPiezoelectric ceramic transducerIt has certain advantages.Due to lower cost and stronger compatibilityActive sensorFor the futureStructural health monitoringThe development of is very important.
