Thermally-drawn fibers with spatially-selective porous domains

Abstract

The control of mass transport using porous fibers is ubiquitous, with applications ranging from filtration to catalysis. Yet, to date, porous fibers have been made of single materials in simple geometries, with limited function. Here we report the fabrication and characterization of thermally drawn multimaterial fibers encompassing internal porous domains alongside non-porous insulating and conductive materials, in highly controlled device geometries. Our approach utilizes phase separation of a polymer solution during the preform-to-fiber drawing process, generating porosity as the fiber is drawn. Engineering the preform structure grants control over the geometry and materials architecture of the final porous fibers. Electrical conductivity of the selectrolyte-filled porous domains is substantiated through ionic conductivity measurements using electrodes thermally drawn in the cross-section. Pore size tunability between 500 nm-10 µm is established by regulating the phase separation kinetics. We further demonstrate capillary breakup of cylindrical porous structures porous microspheres within the fiber core.Porous polymer fibers show great potential for a range of applications, but their simple structures typically limit their functionality. Here, the authors combine a thermal drawing process with polymer solution phase separation to fabricate porous multimaterial fibers with complex internal architectures.

Fig. 1

Fiber fabrication method and cross-sections. (a) General illustration of thermal drawing process, with the associated temperature profile. The dashed line denotes the phase separation temperature for the polymer solution in the core. (b) Schematic phase diagram for a generic polymer solution. State 1 is the homogeneous state in the furnace and state 2 is the phase separated state at room temperature. (c) Illustration of a section of drawn fiber with the dense cladding surrounding the porous core. (d, e) Cross-sectional SEM images of porous fibers made of (d) polyvinylidene fluoride and (e) polycaprolactone (PCL) obtained after cladding dissolution

Fig. 2

Control of external geometry and architecture. (a) Schematic illustration of a preform with a cross-shaped reservoir for PVdF solution and (b) SEM micrograph of the cross-shaped porous PVdF fiber after drawing and cladding dissolution. (c) Schematic illustration of a preform with a triangle-shaped reservoir for PVdF solution and (d) SEM micrograph of the triangle-shaped porous PVdF fiber after drawing and cladding dissolution. (e) Schematic illustration of a cyclic olefin copolymer preform with a cylindrical reservoir for polycaprolactone (PCL) solution lined with a thin LDPE wall and (f, g) SEM micrographs of the final porous PCL core/dense LDPE shell fiber after drawing and cladding removal

Fig. 3

Influence of quenching temperature on microstructure. (a) Average pore size for different quenching temperatures in ethylene–glycol–water bath. The error bar corresponds to the SD over the mean pore size in the six images analysed per quenching conditions. The inset displays an illustration of the possible phase diagram with a dashed line at the working concentration, highlighting both L–L and S–L demixing. (b–d) Associated cross-sectional SEM images for fibers quenched at −20, 20 and 60 °C

Fig. 4

Transverse ionic transport measurement through impedance spectroscopy of ionic liquid-filled porous core fibers. (a) Optical micrograph of fiber sample displaying a porous core filled with a 10⁻³ m PYR13TFSI in propylene carbonate solution, adjacent CPEs and contiguous Bi-In metal buses. (b) Simple equivalent circuit expected from fiber samples. CPA refer to Constant Phase Angle elements. (c) Photograph of a connected fiber sample. (d) Impedance spectra for a fiber between 12 and 1 cm, sequentially cut by 10 mm decrements. Solid lines are fittings results with equivalent circuit. The inset shows the dependence of R _Gel as a function of the inverse-length of the fiber and shows a linear relationship, as expected from geometric considerations

Fig. 5

Porous microsphere production with controlled capillary breakup of porous core fibers. (a) General illustration of the capillary breakup process. Fibers are reheated above the phase transition temperatures of the solution and glass transition of the cladding. The core then evolves into a row of spheres under the effect of surface tension. Once breakup is completed samples are rapidly quenched. (b, c) SEM image and close-up of a polycaprolactone (PCL) porous microsphere and of (d, e) a polyvinylidene fluoride porous microsphere