Multi-material stretchable optical, electronic and optoelectronic fibers and ribbons composites via thermal drawing

Abstract

The present invention concerns a thermal drawing method for forming fibers, wherein said fibers are made at least from a stretchable polymer. The present invention also concerns drawn fibers made by the process.

Claims

1. A thermal drawing method for forming a fiber comprising the steps of: providing a preform of a material for the fiber; heating the material such that the preform necks down under its own weight and produces a lower end; and continuously drawing a fiber from the lower end of the preform, wherein the material includes an elastomer.

2. The method as defined in claim 1, wherein the step of continuously drawing includes co-drawing the fiber with another material.

3. The method as defined in claim 1, further comprising the step of: providing an additional element to the preform before the step of continuously drawing, the additional element including at least one of a metallic electrode made of a conductive medium, a semiconducting material, an insulating material, and optical material, and a functional material.

4. The method as defined in claim 1, further comprising the steps of: inserting a thin metallic wire in a channel of the fiber to form an embedded electrode; and encapsulating a connection with the embedded electrode by an adhesive to improve mechanical resistance of the connection.

5. The method as defined in claim 1, wherein the material of the preform further includes nanoscale objects to bring functionality to the material.

6. The method as defined in claim 5, wherein the nanoscale objects include at least one of nanoparticles and nanotubes.

7. The method as defined in claim 1, wherein in the step of heating the material, a heating furnace provides a heating temperature to decrease the viscosity of the elastomer for deformation such that the material reaches an elastomeric phase before the preform is subjected to the step of continuously drawing.

8. The method as defined in claim 1, wherein the material further includes a thermoplastic thereby forming a thermoplastic elastomer (TPE) that has a thermoplastic domain that physically cross-links an elastomeric phase.

9. The method as defined in claim 8, wherein in the step of heating the material, a heating furnace provides a heating temperature to reach a softening temperature of the TPE before the preform is subjected to the step of continuously drawing.

10. The method as defined in claim 1, further comprising the step of: attaching the lower end of the preform to a pulling system after the step of heating the material.

Description

DETAILED DESCRIPTION OF THE INVENTION

(1) The present invention will be better understood from the following description of non-limiting examples and embodiments, some illustrated in the attached drawings which show

(2) FIG. 1 represents a bloc-diagram of an example of a thermal drawing process from a preform according to the prior art;

(3) FIGS. 2A to 2F illustrate an example of poly(styrene-block-butadiene-block-styrene) (SEBS) preform 2 and SEBS fiber 1 after thermal drawing, FIGS. 2A and 2B. It also shows a schematic illustration of a thermoplastic elastomer, FIG. 2C. A Transmission Electron Microscope (TEM) micrograph shows the microstructure of the preform (FIG. 2D) and the drawn polymer (FIGS. 2E and 2F). The structures are similar highlighting the fact that the thermal drawing process does not alter the elastic properties of the polymer.

(4) FIGS. 3A and 3B illustrate an example of a fiber 1 made of a copolymer of polydimethylsiloxane and urea (Geniomer) preform 2.

(5) FIGS. 4A to 4F illustrate an example of SEBS fiber 1 with liquid Gallium 3 inside. FIGS. 4A and 4B show the side view and cross-section view respectively of a SEBS fiber integrating 8 liquid Ga electrodes. In FIG. 4C, the preform 2 is schematically shown with the fiber 1 being drawn according to the present invention (FIG. 4D). FIG. 4E shows the cross-section and longitudinal view of a single Ga electrode embedded in the SEBS cladding. The picture below shows that the electrode is continuous over tens of meters, and can be used to light an LED. The fiber 1 obtained can act as a strain sensor since the electrical current through the wire depends on the fiber strain as shown in the graph, see FIG. 4F. This makes a very robust and stable sensor, as an example of application.

(6) FIGS. 5A and 5B illustrate an example of SEBS fiber 1 with solid metal eutectic of Bismuth and Tin inside, that is a rigid material within a soft matrix.

(7) FIG. 6 represents a fiber 1 with a stretchable polymer matrix and exposed metallic electrodes 4 on the fiber surface, in contrast with the embedded electrodes previously shown.

(8) FIGS. 7A to 7B illustrate an example of SEBS fiber 1 with a polymer nanocomposite with Carbon black loaded polyethylene (CPE) inside. In the bottom (FIG. 7C) is a SEBS-Carbon Nanotube composite (in black) thermally drawn within a SEBS matrix 5.

(9) FIGS. 8A to 8C illustrate an example of SEBS fiber 1 with Polycarbonate (PC) thermoplastic 6 inside. FIG. 8C is an example of a microstructured PC fiber with hollow channels 7, within a stretchable cladding.

(10) FIGS. 9A to 9D illustrate an example of SEBS fiber 1 with wavy PC 8 rod inside. In the bottom left (FIG. 9C), the fiber 1 of FIGS. 8A-8C was strained, inducing the microstructured PC 8 to deform plastically. As the stress is released the PC 8 deforms into a helicoidal shape to comply with the initial length recovered by the elastic cladding 1. Launching light as shown in bottom right FIG. 9D creates a strain dependent loss optical fiber 1. At each bend, light can escape but as the fiber 1 is stretched, the bend radius reduces and less light couples out, leading to lower losses.

(11) FIGS. 10A and 10B illustrate two examples of SEBS fiber 1, a rectangular and circular shaped, with a hollow channel 9 inside.

(12) FIGS. 11A to 11D illustrate two examples of optical fibers 1 with a soft cladding 10. These fibers can be rigid in the z direction, but soft along their side for, for example, avoid tissue irritation for fiber probes. One fiber has a hollow channel 9.

(13) FIGS. 12A to 12C illustrate examples of multilayer Bragg mirror fiber 1, a stack of alternative layers made of SEBS and Geniomer. FIG. 12C shows a similar bragg mirror fabricated around a circular SEBS fiber.

(14) FIG. 13 illustrates examples of liquid Gallium 11 in the channel in SEBS fiber 1 surface.

(15) FIGS. 14A to 14C shows SEM (top view, FIG. 14B) and optical microscope (cross-section, FIG. 14A) pictures of a textured fiber 1 made out of a stretchable polymer. On the right (FIG. 14C), a diffraction pattern is shown to shift as the fiber is being stretched.

(16) FIGS. 15A and 15B show the optical micrograph of a stretchable multi-material fiber 1 with a complex architecture that can sense pressure and its direction. Depending on the direction for the pressure, the top CPE electrode can touch a different CPE electrode in the bottom connected to different circuits, hence revealing the pressure direction. A metallic layer 12 right under the surface can sense pressure as shown in the graph where a change of current is shown as a pressure is applied.

(17) FIG. 16 shows the storage (G′) and loss modulus (G″) of SEBS, highlighting a new criteria used to evaluate compatibility with the drawing process by requiring a situation where G″>G′, so that a cross over between the two must happen in some temperature range.

(18) FIG. 17 illustrates an example of a touch sensing fiber and the processing steps to fabricate the same. The preform is fabricated layer by layer as illustrated in FIGS. 17A and 17B and in the steps (a) to (e) listed in FIG. 17B and the whole assembly is then thermally drawn to form the fiber in accordance with the principles of the present invention.

DETAILED DESCRIPTION

(19) Thermal plastic elastomer (TPE) is a kind of copolymers or of physically mixed polymers (usually a plastic and a rubber) which comprises materials with both thermoplastic and elastomeric properties. For example, poly(styrene-block-butadiene-block-styrene or SEBS) is a very typical kind of TPE. The glass transition (Tg) of the polystyrene (PS) part is ca. 120° C., higher than room temperature (RT), so it serves as a thermoplastic part at RT. The Tg of poly(butadiene-block-styrene) (EB block) is −50 to −60° C., lower than RT and hence it forms an elastic part. At RT, the PS will provide a physical cross link (i.e. aggregate), that will ensure that the elastomer will return to its original shape when the stress is removed. When the TPE is heated above the Tg of PS, the physically crosslinked thermoplastic part is “uncrosslinked”, which enable deformation and thermal drawing.

(20) The main difference between TPEs and thermal set rubbers is that thermoset rubbers are chemically cross linked with covalent bonds between the chains. These bonds cannot be destroyed before thermal degradation, so thermal set rubbers cannot be remanufactured, and could not be thermally drawn. The physically cross linked PS domains in the preform are distributed in the ES block matrix and prevent the further slip of EB block after the EB block reaches its maximum stretch ratio under a mechanical constraint. For SEBS to be compatible with the thermal drawing process, the PS domain must be able to deform or be broken down into smaller parts above the T.sub.g of PS. In a large temperature range SEBS is hence compatible to be codrawn with many different kinds of materials, such as high-drawn-temperature polycarbonate (PC), low-drawn temperature poly(lactic-co-glycolic acid) (PLGA), and different metals or semiconductors with various melting points.

(21) The microstructure of TPE in the final fiber and thus the mechanical property is highly temperature dependent. At lower drawing temperatures, for example, 140° C., PS block will maintain a strong phase separation with EB block (similar to the preform state), while PS domain is able to be deformed and drawn. In such low temperature, the PS domain is large and will be reoriented along the thermal drawing direction as demonstrated in the small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) results. In the case of high drawing temperature, such as 220° C., the PS block tend to form small domains, and do not aggregate as much. Thus the PS domain in the low-drawing-temperature fiber is larger than for the high-drawing temperature, in the drawing direction. The larger PS domain size could bring a higher modulus as could be found in the strain-stress test.

(22) To make a TPE drawable, the TPE should have the following properties:

(23) 1. Good thermal resistance. The polymer should not thermally degrade during the drawing process. The thermal resistance property of the elastic block is the key point as commonly the elastic block will degrade thermally before the plastic one due to its lower Tg. One of the good candidate of TPE is SEBS as its EB block is hydrogenated to get rid of the double bond and its thermal degradation temperature could be as high as ca. 280° C. This is high enough to cover a large part of materials for the multimaterial fiber drawing.
2. Proper viscosity and (melt) strength to be compatible with the drawing process and other co-drawn materials. This could be one main challenge in finding a proper TPE to be drawn as interchain interaction between the elastic block is rather week, especially at high temperature, due to the low Tg. The intrachain interaction and the plastic component domain are two aspects we could think about to improve. A high molecular weight, i.e., long polymer chain will increase the polymer entanglement and thus is good for increasing the viscosity and strength.
3. The plastic component domain property (such as molecular weight and its ratio to the elastic block) is another important part to determine the viscosity and strength which is highly dependent on temperature in terms of phase separation and plastic domain size. It should be properly tailored when making the preform and drawing a fiber. A low drawing temperature will be important to maintain a high viscosity and (melt) strength.
4. The TPE, especially the plastic part, should resist crystallization as crystal melt could bring a sudden drop in viscosity during the thermal process.

EXAMPLES

(24) 1. Making SEBS Preform or Plate

(25) A typical process to make a SEBS preform 2, for example, a preform having dimensions 24 mm in width, 170 mm in length and 10 mm in thickness, is the following. The SEBS granule can be acquired from different companies, for example G1657, a product of Kraton Performance Polymers Inc. Its weight-average molecular weight is ca. 70 000 g/mol and the soft/hard ratio is 87/13 (weight). The granule are preferably hot pressed under a pressure of 0.25 bar for 15 min at a temperature at 180° C. The hot press temperature could be varied from 130° C. to 190° C.

(26) 2. Thermal Drawing of a SEBS Based Preform.

(27) The preform 2 was put in a furnace. In a typical process, (1) the temperature could be 90° C. in the top zone, 130° C. to 260° C. in the middle zone temperature, and 80° C. in the bottom zone; (2) The drawing speed could be from 0.05 m/min to 2.4 m/min or higher and the feeding speed could be 1 mm/min, for example. Other temperature ranges depending on the materials, and higher drawing speed can be achieved as well. This is illustrated in FIGS. 2A to 2F, which disclose a SEBS fiber 1 after thermal drawing in FIGS. 2A and 2B.

(28) FIG. 2C illustrates schematically a thermoplastic elastomer.

(29) FIG. 2D to 2F illustrate a Transmission Electron Microscope (TEM) micrograph showing the microstructure of the preform 2 (FIG. 2D) and the drawn polymer 1 (FIGS. 2E and 2F). The structures are similar highlighting the fact that the thermal drawing process does not alter elastic properties of the polymer.

(30) 3. Making SEBS Preform with Hollow Channel Inside.

(31) Channels may be fabricated via conventional milling or thermal embossing methods along the length direction in the surface of a preform 2. A solid rod such as steel or Teflon may then be inserted in the channel 3 and covered by another SEBS plate before consolidation under vacuum in an oven or inside a hot press. The rod is subsequently removed from the preform leaving a channel 3 of prescribed dimensions in the preform 2. Examples are illustrated in FIGS. 4A to 4D and 10A, 10B.

(32) 4. SEBS Preform with Integrated Liquid Metal Electrodes.

(33) Liquid Gallium may be injected inside the hollow channels 3 previously fabricated in a SEBS preform 2. Alternatively, a solid Gallium rod may be inserted into the hollow channel(s) 3. Examples are illustrated in FIGS. 4A to 4F, and 13.

(34) 5. Bragg Mirror (Multi Layers) Fiber

(35) Make different kind of TPE thin films via methods such as solution casting or hot press. Film thickness may be from several micron to hundreds of micron depending on requirement on the final thickness of different layers.

(36) Stack different layers alternatively or stack different layers on an additional thick TPE plate, which may be used to support the thin films. Consolidate under heat and pressure.

(37) Alternatively, different layers may be rolled around a cylinder (Teflon.) that is subsequently removed after consolidation. The multilayer structure is then thermally drawn into a fiber having a Bragg mirror structure with a size that depends on the draw-down ratio.

(38) Example are given in FIGS. 12A to 12C.

(39) 6. Making Wavy Structure in TPE Fiber

(40) A polycarbonate (PC) rod 8 is assembled into a SEBS preform 2, and the assembly in thermally drawn into a fiber 1 with a PC core 8. The fiber 1 is then stretched: the SEBS deforms elastically but the PC 8 quickly deforms plastically. When the pulling force is released, the SBES layer returns to its original length, while the PC rod 8 cannot and is forced to coil in order to comply with the original length. A PC helical structure 8 is obtained and may be deformed elastically, making the whole fiber 1 stretchable. The final fiber 1 may serve as stretchable light guide for example as a possible application. The fiber shows that the light intensity increases when the strain increases, see FIG. 9D. An example is illustrated in FIGS. 9A to 9D.

(41) 7. 4 Galinstan Channels in SEBS Fiber Surface

(42) An SEBS plate, a CPE plate and a second SEBS plate with four cylindrical metal rods at its surface were pressed together under 0.02 bar for 15 min at 145° C. Then the metal rods were removed to obtain hollow channels 11 and filled with Galinstan. After thermal drawing, the CPE layer and the top SEBS layer were peeled off from the fiber. An SEBS fiber 1 with four Galinstan channels 11 at its surface was finally obtained.

(43) An example is given in FIGS. 13, 15A and 15B.

(44) 8. Touch Sensing Fiber

(45) Different component plates as described above were consolidated under 0.02 bar for 20 min at 145° C. The top two layers of SEBS and CPE were peeled off from the fiber after thermal drawing. The Geniomer was pulled out from the fiber end. Alternatively, tiny cuts (2 mm long) can also be made in the SEBS walls to pull out the Geniomer from the side.

(46) An example is illustrated in FIGS. 15A, 15B and 17. As illustrated in FIG. 17, the preform is made layer by layer (see FIG. 17A which indicates the properties of each layer and FIG. 17B), the steps followed being indicated as (a) to (e) in FIG. 17B and the drawn fiber 1 is shown in FIG. 17C and FIG. 15A (in cut-view).

(47) 9. SEBS (Sub)Micro-Channel-Patterned Optical Gratings Fiber

(48) Micron-scale patterns with a period of for example 100 micron or 10 micron were first fabricated on an Si mask by photolithography. The initial patterns in the perform were made with a Heidelberg DWL200 laser writer on Cr-blank masks, then transferred to Si masks with a Suss MA6 mask aligner. The developed Si masks were then etched using a plasma etcher Alcatel AMS 200 SE to obtain the desired pattern depth. The etching depth was the same as the width of the structure, or half of the period, to obtain square shaped patterns, as an example of achievable structure. The Si masks were then molded onto a PDMS precursor via casting (PDMS 84 Dow-Coring) and curing at 80° C. to transfer the pattern onto a soft PDMS substrate. A PMMA plate was subsequently patterned by pressing it on the patterned PDMS at 1 bar for 10 min at 150° C. using a Thermal NanoImprinter EHN-3250. This PMMA plate was assembled with an SEBS plate and another PMMA plate (patterned or non-patterned) and hot-pressed (at pressure of 1 bar for 10 min at 150° C.) to get a preform. After thermal drawing the preform to create a fiber, the two PMMA layers were peeled off and the SEBS patterned optical grating fiber was obtained.

(49) An example is illustrated in FIGS. 14A to 14C.

(50) The embodiments of the invention described in the present application are only illustrative examples and should not be construed in any limiting manner. The present invention may also use equivalent means, materials and method steps to the ones described therein in the embodiments and examples with corresponding results. Also many different applications of the present invention may be envisaged as suggested hereabove, all within the scope of the present invention. It is also possible to combine different embodiments of the present invention according to circumstances and they are not exclusive.

(51) Accordingly, the present description is neither intended nor should it be construed as being representative of the full extent and scope of the present invention. The present invention has been set forth in various levels of detail in the detailed description of the invention and no limitation as to the scope of the present invention is intended by either the inclusion or non inclusion of elements, components, etc. in the present description. Additional aspects of the present invention have become readily apparent from the detailed description, particularly when taken together with the drawings illustrating examples of the invention.

RELATED PATENTS AND SCIENTIFIC PUBLICATIONS

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