High modulus gel-spun PVDF fiber thin films
Mechanically and piezoelectrically anisotropic polymer fibers may be formed by spinning a polymer solution or gel that includes a high molecular weight crystallizable polymer and a liquid solvent. The solvent may be configured to interact with the polymer to facilitate chain alignment and, in some examples, create a higher crystalline content within the spun fibers. The polymer solution may also include a low molecular weight additive. The high and low molecular weight polymers may each be characterized by a bimodal molecular weight distribution where the molecular weight of the additive is less than the molecular weight of the crystallizable polymer. The polymer(s) and the additive(s) may be independently selected from vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, hexafluoropropene, vinyl fluoride, etc. The spun fibers may be oriented, annealed, poled, and woven or laminated to form a polymer thin film having a high elastic modulus and a high electromechanical coupling factor.
1 . A polymer fiber comprising:
a crystalline polymer comprising a material selected from the group consisting of vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, hexafluoropropene, vinyl fluoride, homopolymers thereof, co-polymers thereof, tri-polymers thereof, and derivatives thereof, wherein the crystalline polymer has a molecular weight between 100,000 g/mol and 500,000 g/mol, wherein
the polymer fiber is formed by stretching a gel spun polymer fiber at a stretch ratio between 8 and 40, annealing the stretched polymer fiber under an applied stress between 100 MPa and 250 MPa at a temperature greater than 80° C., and poling the annealed polymer fiber at an applied voltage between 200 V/micrometer and 600 V/micrometer, wherein the polymer fiber has a Young's modulus between 4 GPa and 15 GPa, and an electromechanical coupling factor (k 31 ) between 0.1 and 0.3.
2 . The polymer fiber of claim 1 , wherein the polymer fiber has a piezoelectric coefficient (d 31 ) of at least 5 pC/N.
3 . The polymer fiber of claim 1 , further comprising an additive having a molecular weight of less than 25,000 g/mol.
4 . The polymer fiber of claim 3 , wherein the additive comprises a moiety selected from the group consisting of vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, hexafluoropropene, vinyl fluoride, homopolymers thereof, co-polymers thereof, tri-polymers thereof, and derivatives thereof.
5 . The polymer fiber of claim 1 , wherein a molecular weight distribution of polymers within the polymer fiber is selected from the group consisting of monodisperse, polydisperse, and bimodal.
6 . A polymer thin film comprising the polymer fiber of claim 1 .
7 . A polymer thin film, comprising:
a plurality of polymer fibers each comprising a crystalline polymer comprising a material selected from the group consisting of vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, hexafluoropropene, vinyl fluoride, homopolymers thereof, co-polymers thereof, tri-polymers thereof, and derivatives thereof, wherein the crystalline polymer has a weight average molecular weight between 100,000 g/mol and 500,000 g/mol; and
the plurality of polymer fibers within the polymer thin film are formed by stretching a gel spun plurality of polymer fibers at a stretch ratio between 8 and 10, annealing the stretched plurality of polymer fibers under an applied stress between 100 MPa and 250 MPa at a temperature greater than 80° C., and poling the annealed plurality of polymer fibers at an applied voltage between 200 V/micrometer and 600 V/micrometer, wherein the polymer thin film has a Young's modulus between 4 GPa and 15 GPa, and an electromechanical coupling factor (k 31 ) between 0.1 and 0.3.
8 . The polymer thin film of claim 7 , wherein the crystalline polymer of the each of the plurality of fibers is preferentially oriented along a predetermined axis.
9 . The polymer thin film of claim 7 , further comprising an additive having a molecular weight of less than 25,000 g/mol.
10 . The polymer thin film of claim 7 , wherein the polymer thin film is optically clear and has less than 10% bulk haze.
11 . A method comprising:
forming a polymer solution comprising a crystallizable polymer content selected from the group consisting of vinylidene fluoride, trifluoroethylene, chlorotrifluoroethylene, hexafluoropropene, vinyl fluoride, homopolymers thereof, co-polymers thereof, tri-polymers thereof, and derivatives thereof, and a liquid solvent;
forming a gel from the polymer solution;
spinning the gel to form a polymer fiber;
stretching the polymer fiber to a stretch ratio between 8 and 40 to form an oriented fiber;
annealing the oriented fiber under an applied stress between 100 MPa and 250 MPa at a temperature greater than 80° C.; and
poling the oriented fiber at an applied voltage between 200 V/micrometer and 600 V/micrometer, wherein a Young's modulus of the oriented fiber is between 4 GPa and 15 GPa, and an electromechanical coupling factor (k 31 ) of the oriented fiber is between 0.1 and 0.3.
12 . The method of claim 11 , wherein forming the gel comprises a process selected from the group consisting of cooling the polymer solution, evaporating the liquid solvent, and adding a poor solvent to the polymer solution.
13 . The method of claim 11 , wherein spinning the gel comprises exposing the polymer fiber to a liquid solvent or a heated gas.
14 . The method of claim 11 , wherein spinning the gel comprises electro-spinning.
15 . The method of claim 11 , wherein an electric field is applied during or after the stretching.
16 . The method of claim 11 , wherein an electric field is applied during or after the annealing.
17 . The method of claim 11 , further comprising forming a polymer fiber thin film from the oriented fiber by a process selected from the group consisting of weaving, cross-linking, and laminating the oriented fiber.
18 . The method of claim 17 , further comprising weaving a selected number and type of the oriented fibers along each of two or more in-plane directions.
19 . The method of claim 17 , further comprising embedding the polymer fiber thin film in a polymer matrix.