Microfluidic-based wet spinning of individual solid polymer fibers

The invention relates to a method for producing individual solid polymer fibers from a precursor liquid comprising polymerizable and/or cross-linkable polymer precursors by microfluidic-based wet spinning. Furthermore, the invention is directed to a microfluidic-based wet spinning device for producing an individual solid polymer fiber.

Synthetic fibers are used in the manufacture of materials in many fields of technology ranging e.g. from optics over textiles to mechanical engineering. Usually, synthetic fibers are produced by spinning. Thereby, depending on the material of the synthetic fiber, continuous filaments or fiber mats are produced by different spinning techniques. For example, melt spinning allows to produce synthetic fibers from molten thermoplastic materials. Another approach, called solution spinning uses spinning solutions comprising precursor materials in a solvent whereby the precursor materials of the spinning solutions are solidified to form the target fibers. Solution spinning can be carried out quite differently. Well-known methods are inter alia dry spinning, wet spinning, dry-jet wet spinning, and electrospinning.

WO 2014/143866 A1 (Arsenal Medical, Inc.) discloses for example a method for obtaining multicomponent fibers by coaxial electrospinning. Thereby, fibers are provided, which comprise (a) a polymeric core that comprises a core-forming polymer and (b) a polymeric sheath that comprises a sheath-forming polymer that is different than the core-forming polymer. Examples of core-forming polymers include, for instance, crosslinked polysiloxanes and thermoplastic polymers, among others. Examples of sheath-forming polymers include, for instance, solvent-soluble polymers, degradable polymers and hydrogel-forming polymers, among others. However, this method produces fiber meshes formed by a collection of rather thin fibers interlaced to form a three-dimensional network but is not able to directly produce individual fibers in a targeted manner.

KR 10169598 B1 (Dong-A Univ. Res. Found for Industry-Academy Coop.) relates to a method for manufacturing a polymer fiber using a microfluidic device and, particularly, to a method for manufacturing a polymer fiber manufactured by injecting a precursor solution and a support crosslinking solution to a microfluidic device. The method comprises the steps of: (1) preparing (i) a precursor solution containing a monomer, a crosslinking agent, an alginate, and a photoinitiator, and (ii) an alignate crosslinking solution containing a metal cation; (2) injecting the precursor solution and the alginate crosslinking solution into the microfluidic device to manufacture a metal-alginate support structure; (3) photopolymerizing the monomer by irradiation with a light source in the microfluidic device, in order to manufacture a cross-linked polymer/alginate support composite fiber; and (4) reacting the manufactured cross-linked polymer/alginate support composite fiber with a chelating agent to remove the alginate support from polymer/alginate support composite fiber so as to obtain a cross-linked polymer fiber without support. However, fibers produced in this manner comprise a considerable amount of imperfections, making them for example unsuitable for optical applications. Furthermore, this method is practical only for producing hydrophilic fibers, of which the precursors are miscible with the alginate aqueous solution. It is however not possible to produce hydrophobic fibers from other precursors that are not miscible with alginate aqueous solutions, such as e.g. polydimethylsiloxane (PDMS) fibers.

There is thus a need to develop new and improved methods for producing polymer fibers, which at least partly overcome the aforementioned drawbacks.

It is the object of the present invention to provide new and improved solutions for producing polymer fibers. Especially, the method should allow to produce individual polymer fibers consisting of different materials and having various thicknesses and lengths in a targeted manner. Further preferred, the solution should make it possible to produce polymer fibers as uniform as possible and in particular having a quality suitable for optical applications.

Surprisingly it was found that these objects can be achieved with a method according to claim.

Thus, according to a first aspect, the invention is concerned with a method for producing an individual solid polymer fiber from a precursor liquid comprising aggregatable, polymerizable and/or cross-linkable polymer precursors, especially from a precursor liquid comprising polymerizable and/or cross-linkable polymer precursors, by microfluidic-based wet spinning, the method comprising the steps of:

The inventive method makes use of a removable shell as tubular mold for shaping and trapping the curable polymer precursor in a core channel. This without need of mixing the curable polymer precursor and the shell liquid beforehand. This allows for producing a wider range of individual polymer fibers and by adjusting the dimensions of the capillary tubes, polymer fibers with various thicknesses up to several millimetres and lengths of more than one meter can be produced in a highly targeted manner. Furthermore, the polymer fibers obtainable with the inventive method are highly uniform and, depending on the materials used, even suitable for optical applications.

Furthermore, during the inventive method, a fiber having a liquid core embedded in a cured shell is produced. This allows for decoupling the curing of the liquid curable polymer precursor from the wet spinning process.

In contrast, in existing microfluidic spinning methods, such as for example described in KR 10169598 B1, the solidification of the polymer fibers has to be induced during the spinning process, in particular when the flow of the liquids in the wet spinning apparatus is well maintained. Furthermore, such an approach requires rapid curing of the precursor fluid during the rather short spinning process, thus limiting the materials suitable for fiber production. Also, because specific polymer materials require different solidification methods, e.g. photo-initiated polymerization, chemical crosslinking and/or solvent exchange, the microfluidic wet spinning devices have to be designed and fabricated with specific considerations, thus making polymer fiber production rather complicated for users.

All these drawbacks can be circumvented with the inventive method, whereby curing of the liquid curable polymer precursors can be decoupled in time and place from the wet spinning process and in particular of the curing of the shell. Therefore, for example, curing of the liquid curable polymer precursors can be effected outside the wet spinning apparatus and during any long time, if desired.

Therefore, in a preferred implementation, steps g) and/or h) take place outside the stationary liquid phase and/or in time after steps a) to e).

In particular, unlike processes such as common melt spinning or thermal drawing, the inventive method allows to produce high quality continuous polymer optical fibers (POF) even with sensitive functional molecules, e.g. perovskite nanocrystals.

Furthermore, with the inventive method, polymer opticalfibers (POF) having a low modulus of elasticity of <3 MPa or even <1 MPa are available. Such POF feature an increased sensitivity for small pressures and cannot be produced by common moulding techniques because the fibers would break during demoulding due to the too low modulus of elasticity.

In particular, the polymerizable and/or cross-linkable polymer precursors are selected from polymerizable and/or crosslinkable monomers, oligomers and/or polymers.

For example, the precursor liquid comprises:

According to an especially preferred implementation, the precursor liquid comprises (meth)acrylates and/or crosslinkable polydimethylsiloxane polymers.

Examples of aggregatable polymers are polylactic acid (PLA) and/or polycaprolacton (PCL). Such polymers can be solidified via solvent-exchanged polymer aggregation. Thus, in particular, aggregatable polymers are dissolved in one or more solvent(s) as the precursor liquid.

However, depending on the desired solid polymer fiber, other polymerizable and/or cross-linkable polymer precursors can be used as well.

Further preferred polymerizable precursors are lactic acid or its cyclic di-ester lactide.

Other highly preferred polymerizable precursors are polyols as well as diisocyanates, triisocyanates and/or polyurethane prepolymers, capable of forming polyurethane polymers.

Further preferred polymerizable precursors are mixtures of crosslinkable siloxane polymers or alkene polymers and/or macromers, e.g. dodecyldimethacrylateand, and (metha) acrylate precursors, especially capable of forming amphiphilic polymer co-networks.

Especially, the precursor liquid furthermore comprises for example a solvent, a crosslinking agent, a thermal polymerization initiator, a photopolymerization initiator, a chain transfer agent, a functional molecule and/or a molecular weight regulator. In particular, these substances are chosen depending on the specific polymerizable and/or cross-linkable polymer precursors and are in particular added in order to enable and control the solidifying of the liquid core in step g).

Functional molecules can e.g. be selected from fluorophores, chromophores and/or nanoparticles. Such molecules can for example be incorporated in the fibers to adapt the fibers to specific applications. Especially, functional molecules may comprise nanocrystals, e.g. perovskite nanocrystals.

The shell liquid in particular is a solution of a solvent, especially water, and a solvent-soluble non-crosslinked polymer.

In particular, the shell liquid is a solution of water and a homopolymer and/or a copolymer formed for example from one or more of the following monomers: ethylene oxide, vinyl pyrrolidone, vinyl alcohol, vinyl acetate, vinyl pyridine, methyl vinyl ether, acrylic acid and salts thereof, methacrylic acid and salts thereof, hydroxyethyl methacrylate, acrylamide, N,N-dimethyl acrylamide, N-hydroxymethyl acrylamide, alkyl oxazolines, saccharide monomers, polysaccharides, dextran, alginate, amino acids, hydrophilic polypeptides, proteins and/or gelatin.

Especially preferred, the shell liquid is an aqueous hydrogel precursor solution, in particular an aqueous polysaccharide solution. Particular preferred, the shell liquid is an aqueous alginate solution, in particular an aqueous alkaline metal alginate solution, e.g. a sodium alginate solution. These kind of solutions turned out to produce highly stable shells and are compatible with various precursor liquids. Additionally, these solutions can easily be removed later on. In particular, with respect to the total weight of the shell liquid, a concentration of the homopolymer and/or a copolymer, especially the hydrogel precursor, in particular an alginate, is from 0.05-10 wt %, especially 0.1-5 wt %, in particular 1-3 wt %.

Especially, the precursor liquid is essentially immiscible with the shell liquid. Thereby, in step b) a well-defined phase interface between the core and the shell in the liquid fiber can be obtained. Additionally, diffusion of precursor liquid into the cured shell is reduced.

In step d) the liquid fiber with core-shell structure, optionally having a sheath, as produced in step b) or in step c) preferably is introduced into the stationary liquid phase below the liquid surface of the stationary liquid phase. This reduces turbulences and improves the overall quality of the fibers obtainable.

Thus, preferably, the second capillary tube or the third capillary tube, if the latter is present, is submerged in the stationary liquid phase.

Especially, the third liquid in step c) and/or the stationary liquid phase in step d) comprises or consists of a polar solvent, especially methanol, ethanol and/or water, in particular water.

Particularly, the stationary liquid phase can have a temperature between 0-95° C., in particular between 15-90° C., especially between room temperature and 80° C.

According to a first preferred embodiment, the temperature of the stationary liquid phase is between 15-30° C., especially room temperature.

In a second preferred embodiment, the temperature of the stationary liquid phase is an elevated temperature, especially above room temperature, in particular above 30° C. This allows performing a pre-curing step and/or a partial curing of the liquid core in stationary liquid phase.

Especially, the temperature of the stationary liquid phase is between 45-85° C., especially between 55-75° C., particularly between 60-70° C., e.g. 65° C. These temperatures are especially beneficial when using crosslinkable siloxane polymers, especially polydimethylsiloxane polymers, as precursor liquid and/or when producing PDMS fibers.

In particular, the third liquid in step c) and/or the stationary liquid phase in step d) is a curing agent for the shell liquid and is selected from an aqueous solution of a salt of a divalent metal cation, especially selected from of Ca, Mg, Zn, Fe, Cu, and/or Ba. Especially preferred is an aqueous solution Casalt, e.g. an aqueous solution of CaCl.

In particular, with respect to the total weight of the aqueous solution, a concentration of the salt of the divalent metal cation, especially CaCl, in the aqueous solution is from 0.05-10 wt %, especially 0.1-5 wt %, in particular 0.5-2 wt %.

According to a particularly preferred embodiment, optional step c) is performed and in step c) a curing agent for the shell liquid selected from an aqueous solution of a salt of divalent metal cation is used as the third liquid. In this case, the shell of the liquid fiber with core-shell structure is directly cured through the concentric sheath flow of the third liquid within the third capillary tube. Thereby, the third liquid forms another tubular mold for shaping and trapping the liquid fiber with core-shell structure. This may further improve uniformity of the liquid fiber. In this case, the stationary liquid phase in step d) allows the fibers to be spun-out of the device in a smooth and stable manner. In particular, the stationary liquid phase in step d) is an aqueous solution, e.g. water.

According to another highly preferred embodiment, step c) is not performed and in step d) a curing agent for the shell liquid selected from an aqueous solution of a salt of divalent metal cation is used as the stationary liquid phase. In this case, the shell of the liquid fiber with core-shell structure is cured within the stationary liquid phase, which in addition allows the fibers to be spun-out of the device in a smooth and stable manner.

According to a preferred embodiment, in the liquid fiber with core-shell structure produced in the second capillary tube, the shell liquid is in direct contact with the precursor liquid at least in a downstream section of the second capillary tube.

Thereby, preferably, the downstream end of the first capillary tube is located within the second capillary tube, whereby the downstream end of the first capillary tube, in flow direction of the liquid fiber, in particular is located inside the first half, especially within the first quarter, of the second capillary tube. In this case, if a third capillary tube is present, the downstream end of the second capillary tube in particular is located inside the third capillary tube.

In particular, with respect to an upstream end, at least an outer diameter, especially an inner and the outer diameter, of the first capillary tube tapers, especially step-like, towards the downstream end, in particular to form a first capillary nozzle. Likewise, in a preferred embodiment, with respect to an upstream end, at least an outer diameter, especially an inner and the outer diameter, of the second capillary tube tapers, especially step-like, towards the downstream end, in particular to form a second capillary nozzle.

This allows for focussing the flow of the precursor liquid when injecting it into the second capillary tube and for focussing the liquid fiber with core-shell structure when leaving the second capillary tube. However, other configurations might be suitable as well.

In particular, a length of the tapered section of the first capillary tube is 25-75%, especially 40-60%, of the whole length of the first capillary tube; and, especially, a length of the tapered section of the second capillary tube is 25-75%, especially 40-60%, of the whole length of the second capillary tube.

According to another preferred embodiment, in the liquid fiber with core-shell structure produced in the second capillary tube, the shell liquid and the precursor liquid in the liquid fiber with core-shell structure are separated by the first capillary tube when flowing through the second capillary tube. Put differently, in this case, the shell liquid and the precursor liquid are not in direct contact in the second capillary tube.

Thereby, preferably, the first capillary tube extends completely through the second capillary tube, and, preferably, the downstream end of the first capillary tube, in flow direction of the liquid fiber, is located further downstream the downstream end of the second capillary tube. If a third capillary tube is present, the downstream ends of the first and the second capillary tubes are located inside the third capillary tube, especially within the first half, in particular within the first quarter, of the third capillary tube.

Thereby, especially, the sheath fluid first is contacted with the shell fluid in the third capillary tube in order to pre-cure the shell fluid before the precursor fluid is introduced through the first capillary tube further downstream into the hollow central section of the pre-cured shell fluid.

The pre-cured shell fluid thereby still is fluid but forms a more stable interface with the precursor fluid what further reduces mixing of the fluids at the interface.

Thus, the expression “whereby at least a downstream end of the first capillary tube coaxially protrudes into a second capillary tube”' in particular is to be interpreted as to include the first capillary tube extending completely through the second capillary tube, and, preferably, the downstream end of the first capillary tube, in flow direction of the liquid fiber, being located further downstream the downstream end of the second capillary tube.