WO2008060675A2

WO2008060675A2 - Coaxial polycarbonate/polyurethane composite nanofibers

Info

Publication number: WO2008060675A2
Application number: PCT/US2007/068007
Authority: WO
Inventors: Xiao-Jian Han; Zheng-Ming Huang; Peter C. Qian
Original assignee: Invista Technologies S.A R.L.
Priority date: 2006-06-01
Filing date: 2007-05-02
Publication date: 2008-05-22
Also published as: WO2008060675A3

Abstract

A submicron-sized fiber has a polycarbonate shell and a polyurethane core, and is made by electrospinning liquid solutions of the materials through co-axial capillaries. The composite fiber may be laid onto a collector to form a nonwoven membrane, or may be laid or adhered onto a fabric substrate. A nonwoven membrane or fabric with the composite fiber combines the filtration efficiency of the polycarbonate with the mechanical characteristics, such as elasticity, of polyurethane. The membrane or fabric is useful in exposure suits, filters and aviation dresses/clothing.

Description

COAXIAL POLYCARBONATE/POLYURETHANE COMPOSITE NANOFIBERS

Field Of The Invention The present invention relates to coaxially electrospinning solutions comprising polycarbonate (PC) and polyurethane (PLJ) to form PC(she!l)/PU(core) composite nanofibers for textile application,

Background Of The Invention Electrospinning is a useful way to produce continuous polymer fibers with diameters in a sub-micron range, The fibers may be formed by imposing an external electric field onto a polymer solution or melt According to a typical electrospinning process, a suspended droplet of polymer fluid is charged to a high voltage to produce submicrometer size fibers. Fibrous materials obtained by this technique have been proposed for filter media with high filtration efficiency (FE) and low air resistance or for chemical protective clothing, depending on the specific polymer being used Conventional eiectrospinning apparatuses and the methods of their use have been disclosed, for example, in U.S. Patent 4,044,404 and U S. Patent 6,991 ,702. Currently, nanofibers and nanostructures comprising single or blended polymer materials are generally known, Y.Z Zhang, Z M. Huang, X J Xu, C T. Um, S, Ramakrishna, Chem. Mater, 16 (3406) 2004 (referred to as "Zhang et a!.") have created biodegradable nanofibers for use in drug release bioactive tissue scaffolds and highly sensitive biochemical sensors, As the bi~component material must be biodegradable, it may not be able to provide enough mechanical strength for textile applications. Furthermore, no surface functionalization procedure was suggested for the bi-component biodegradable material. P W Gibson, H L. Schreuder-Gibson, D, Riven, Colloids and Surfaces A Physicochem Eng. Aspects , 187, 469 (2001) (referred to as "Gibson et al. ") have deposited eiectrospun nylon nanofiber membranes onto polyurethane foam containing activated carbons for use in chemical/biological protective applications and filters with greater efficiency than previously known

Another method of forming bicomponent nanofibers is taught by P Gupta, G. L Wilkes, Polymer, 44, 6353 (2003) (referred to as "Gupta et al ") who have simultaneously eiectrospun polyvinyl chlohde)/segmented polyurethane (PVC/Estane®) and polyvinyl chloride)/ poly(vinyiidiene fluoride) (PVC/PVDF) to form a side-by-side construction. However, the method suffers from the shortcoming that the polymers either do not mix continuously, which occurred when the side-by- side eiectrospinning jets were placed too far apart, or two separate single component fibers were formed, which occurred when the jets were too dose together.

Another manner of combining materials to form a composite is through coaxial eiectrospinning, see G Larsen, R Spretz, R Vaierde-Ortiz, Adv. Mater , 16 166 (2004) (referred to as "Larsen, et a!.); Gupta, et al. , G. Lαscertaies, A Barrero, I Guerrero, R Cortijo, M Marques, A M. Ganan-Calvo, Science, 295 (1695) 2002 (referred to as "Loscertales, et al "); D. Li, Y. Xia, Adv. Mater, 16 ( 1 151) 2004 (referred to as "Li, et al."); JM Lopez-Herrera, A. Barrero, A Lopez, I G Loscertales, M Marquez, Aerosol Science, 34 (535) 2003 (referred ti as "Lopez-Herrera, et al "), and G. Larsen, R. Velarde-Ortiz, K Minchow, A. Barrero, ! G Loscertaies, J Am. Chem. Soc , 125 (1 154) 2003 (referred to as "Larsen, et a! ")

For exampie, Loscertales, et al have formed rnonodisperse compound capsules of water coated by oiive oil by electrifying coaxial jets of the two materials, see Both the amount of water and the thickness of the coating oil layer could be weli controlled by means of a coaxial eiectrospinning setup, Lopez-Herrera, et a!.. teaches a similar method of forming compound capsules, including forming a capsule with a Somos exterior around an ethylene-glycol core. However, these techniques were only used to form capsules and were not apparently investigated for use in forming coaxial fibers Furthermore, the techniques were not considered for use in a textile application

Li et a!., discloses that coaxia! eiectrospinning capillaries have been used to fabricate fibers with a core sheath structure. Larsen et ai teaches a method of combining sol-gel techniques with electrohydrodynamic techniques, such as electrospraying, to form sub-micrometer inorganic fibers and core/shell shapes. However, these references fail to teach the creation of a continuous composite coaxia! fiber and a fabric made therefrom that combines the filtering characteristics of one material with the elasticity characteristics of another material

Filtration materials are of great interest particularly for application in areas such as chemical and biological protection. Polycarbonate is one such material that is generally desirable for its high filtration efficiency, see Gibson, et al,; Larsen, et al. , Gupta, et al., Loscertales, et ai,, and Z M Huang, Y Z Zhang, M Kotaki, S Ramakrishna, Composites Science and Technology, 63 2223 (2003) (referred to as "Huang, et al."). P-W Gibson, H.L Schreuder-Gibson, D. Riven, AIChE J , 45, 190 (1999) (referred to as "P.W.Gibson, et al "), D Smith, D H. Reneker PCT/US00/27737(2001) (referred to as "Smith, et al "); and P W. Gibson, H.L

Schreuder-Gibson, C. Pentheny, J of Coated Fabrics, 28, 63 (1998) (referred to as "Schreuder-Gibson, et al "). Charcoal absorbents have also found application in protective clothing (Huang et al.), but these materials are limited in terms of water permeability and impose extra weight to the article of clothing.

Materials for use in fabric applications generafly need to have good mechanical qualities. Polyurethane is generally desirable in textile applications, due in part to its mechanical characteristics, the most notable of which may be its elasticity

Preliminary investigations have indicated that, compared with conventional textiles, electrospun nanofibers could present both minimal impedance to moisture vapor diffusion and good efficiency in trapping aerosol particles (P,W Gibson, et a! and Smith, et al ). Increased attention has also been paid to electrospinning of fibers containing solely PC or solely PU to form nanofibrous mats for textile applications, see M.S Khii, E.! Cha, H Y Kim, N Bhattarai, J Biomed Mater Res Part B . Appt Biomater , 67B 675 (2003) (referred to as "Khil, et al "), P Peter, Heidi Hl Schreuder-Gibson, P W Gibson, Journal of Electrostatics , 54, 333 (2002) (referred to as "Peter, et ai "); J Doshi, D H Reneker, J. Electrostat , 35_r 151 (1995) (referred to as "Doshi, et al "), and P P Tsai, L.C Wadsworth, TAPPI J,, 81 , 274 (1998) (referred to as "Tsai, et al.")

The background art generally fails to disclose combining poSycarboπate (PC) and polyurethane (PU) into a single co-axial nanofiber that retains certain advantages and characteristics of each of the components The art continues to seek ways to enhance the performance of a substrate fabric by forming the fabric of co-axial composite nanofibers comprising a core formed of one constituent material that has certain desirable characteristics and a shell that is coaxial with the core and that is formed of a different constituent material that has other desirable characteristics

Summary Qf The Invention

This invention provides in one aspect a submicron-sized fiber comprising a polycarbonate (PC) shell and a polyurethane (PU) core, the fiber being obtained by electrospinning the materials through co-axial capillaries By this technique, the performance characteristics of each of the component polymers may be combined A nonwoven fabric incorporating such fibers or a membrane of such fibers combines the filtration efficiency of the PC with the mechanical characteristics, such as elasticity, of PU. The fabric is useful in exposure suits, filters and aviation dresses/clothing

According to another aspect of the invention, a coaxial electrospinning technique was developed to electrospin two different polymer solutions into core- she!! structured nanofibers in which polyurethane and polycarbonate were used as core and shell materials, respectively..

According to a further aspect of the invention, a non-woven nanofibrous fabric of PC(sheSI)/PU(core), can be formed with desired fabric properties, such as high strength, low weight and optimal porosity

In yet a further aspect, a nanofibrous membrane according to the present invention may be used to enhance a substrate fabric as chemical/biological protective clothing, boot and glove liners, and flexible gas mask hoods.

Brief Description Of The Drawings

Novel features and advantages of the present invention in addition to those noted above will be become apparent to persons of ordinary skilS in the art from a reading of the following detailed description in conjunction with the accompanying drawings wherein similar reference characters refer to similar parts and in which: Fig, 1 is a side elevationa! view of the coaxial eletrospinning machine used for the present invention,

Fig 2 is a side elevational view of a fiber according to one aspect of the present invention,

Fig. 3 is a schematic top plan view of a portion of a membrane of poiyurethane only fibers;

Fig 4 is a cross-sectional view of a composite fabric formed using a membrane of coaxial fibers according to one aspect of the present invention;

Fig. 5 is a schematic top plan view of a portion of a membrane of coaxial fibers according to one aspect of the present invention; and Fig 6 is a graph comparing the tensile strength and strain of fibers made according to the present invention.

Detailed Description

Embodiments of the present invention will now be described with reference to the drawings.

To form composite fibers with a polycarbonate (PC) shell and a polyurethane (PU) core, the polymers must first be separately dissolved in solvents, such as tetrahydrofuran (THF) and/or dimethylformamide (DMF), so that the polymers can be introduced in liquid form to the electrospinning apparatus The liquid PC and PU may then be introduced to a conventional coaxial electrospinning apparatus, see

Loscertales, et a!., Li, et al , Lopez-Herrera, et al , and G Larsen, R, Velarde-Ortiz, K Minchow, A. Barrero, LG Loscertales, J, Am.. Chβm Sac, 125 (1 154) 2003 (referred to as "G Larsen, et al "), Such an electrospinning apparatus 10 is schematically shown in Fig 1

An eiectrospinniπg apparatus 10 for forming the coaxial PC(she!l)/PU(core) composite nanofibers may include a DC voltage generator 16, a first syringe pump 18 for controlling the flow of materia! through tube 12 for forming the shell of the nanofiber, a second syringe pump 20 for controlling the flow of material through tube 14 for forming the core of the nanofiber, a coaxial nozzle 30 comprising a first capillary 32 in fluid communication with tube 12, a second capillary 34 in fluid communication with tube 14 and coaxially aligned with the first capillary 32, and a collector 22 The collector 22 may be provided with a collector surface 24 that rotates about an axle 26, and it may be electrically grounded to attract the coaxial fibers 36 The collector 22 may be moved back and forth (e g , translated) relative to the electrospinning apparatus, as shown by arrows 38, and may be rotated clockwise or counter-clockwise, as shown by arrow 39, as desired, without substantially altering the finished fabric

The PC liquid (PC dissolved in solvent) enters chamber 28, so that it can be introduced in droplet form at capillary 32 using syringe pump 18 to control the flow rate The PU liquid (PU dissolved in solvent) is introduced in droplet form at capillary 34 using syringe pump 20 to control its flow rate Capillary 34 is coaxiaSly aligned within capillary 32 to provide the coaxial nozzle 30

As the solutions are formed at the coaxial nozzle 30 a high voltage charge is applied using voltage generator 16, as is conventional in the art of electrostatic spinning The high voltage may be in the range of 1-30 kV When the voltage is applied, the solutions at the coaxial nozzle 30 become highly electrified and are subjected to electrostatic forces Such force(s) cause a droplet to eject from the nozzle in the form of a jet The electrified liquid jet undergoes a stretching and whipping, or winding, process, which leads to the formation of long, thin fibers 36 These charged fibers 36 are attracted to the collector surface 24 of collector 22 and orient randomly as shown, for example, in Fig 5 It is possible to operate the electrospinning apparatus with either liquid PU or liquid PC only to form a membrane of single component fibers 46 such as shown in Fig. 3 Fig 3 schematically shows an image from a scanning electron microscope at 5 μm magnification of a membrane of solely PU fibers at 8wt%

However, composite fibers 36 are more advantageous in this invention Through the eSectrospinning process, composite fibers 36 are formed with a polycarbonate shell 42 surrounding a polyurethane core 44 The composite fibers 36 may have an outer diameter in the range of approximately 10 to 2000 nanometers Preferably, the PU core has an outer diameter of less than about 170 nrn and the PC shell has an outer diameter of iess than about 320 nm

After being electrospun and collected on collector 22, fibers 36 are randomly oriented and can form a membrane 56 as shown in Fig. 5 Fig 5 schematically shows an image from a scanning electron microscope at 5μm magnification of a membrane of composite fibers with PC(shell)/PU(core) at 20wt%/8wt% One feature of this core-shell composite nanofibrous membrane is that the core material can provide sufficient strength, whereas the shell material can be functionalized with some proper agents which can deliver desired characteristics when they are attached on a substrate fabric For example, PC(shell)-PU(core) composite nanofiber membranes according to the present invention have shown better tensile properties than a PC membrane alone Additionally or alternatively, nanoparticles of TIO2, Au, and/or Ag can be incorporated into the PC shell material to increase bacteria- resistance As shown in Fig 4, membrane 56 may be attached to fabric substrate 48 using an adhesive 50, Fabric substrate 48 may be comprised of cotton, wool, or other suitable fabric. Adhesive 50 may be a thermally activated glue, such as double-faced retinitis glue

The PC(shell)/PU(core) fibers, the membrane comprised of the fibers, and the composite fabric comprising the membrane, may be useful in a variety of applications including, but not limited to chemical and biological protective clothing (exposure suits), boot and glove liners, filters, aviation dresses/clothing, and flexible gas mask hoods

EXAMPLE

An embodiment of the invention will now be described in more detail through an example, which is not intended to limit the scope of the invention

According to one exemplary method, polycarbonate (PC-KP30) and polyurethane (PU - Tecoflex), were obtained from JINGJIAN Plastic Ltd (Shanghai, China) The solvents, THF (purity≥90%) and DMF (purity≥99 5%), were obtained from Chemical Reagent Co Ltd (Shanghai, China)

The PC and PU starting materials were both dissolved in a mixture of THF and DMF (1 :1 in volume) The PC solution was made in a weight ratio of 1 (polymer): 4 (solvent), i.e , 20wt%, by stirring the polymer in the solvent at 1 1O°C for 8 hours The PU solutions were made at 80°C for 10 hours and were prepared in four different concentrations: 4 wt%, 6 wt%, 8 wt%, and 10 wt% The prepared solutions were stored at room temperature before usage, and the eSectrαspinning process was carried out at room temperature and pressure

The DC voltage generator (such as 16 in Fig. 1) was obtained from Beijing

Machinery & Electricity Institute, The first and second syringe pumps (such as 18, 20 in Fig. 1) were each a model WZ-50C2, available from Zhejiang University Medical

Instrument Co., Ltd The spinneret system (such as 12, 14, 16, 18, 20, 30, 32, 34 in

Fig, 1) was self-made, according to conventional methods.

PC(shell)-PU(core) composite nanofibers were electrospun at 21 KV with a tip-to-collector distance, i.e the distance from capillary tip (such as 32 in Fig, 1) to the nearest surface of collector (such as 22 in Fig. 1), of 12-13cm The PC solution

(20 wt%) was electrospun with each of the PU solutions (4 wt%, 6 wt%, 8 wt%, and

10 wt%) The flow rates of the outer and the inner solutions were 2 00rn!/h and

1 50ml/h, respectively

The nanofiber according to this example had a polyurethane core with a diameter, designated by A in Fig, 2, of about 168nm The outer diameter of the polycarbonate shell, designated by B in Fig 2, was approximately 316nm The core fiber component showed a sharp interface with the shell fiber component, and a relatively smooth core-shell interface was demonstrated.

Fig 5 shows an example of an electrospun PC(shell)/PU(core) composite fiber as it exists in a membrane 56 according to the present invention Fig 5 schematically represents an image from a scanning electron microscope at 5μm magnification, wherein the PC(shell) is 20wt% and the PU(core) is 8wt%.

Fabric 48 with an affixed layer of glue 50 was passed along collector 22, so that the nanofibers could be electrospun directly onto the layer of glue 50 to form the composite fabric shown in Fig 4. The composite fabric was then heated at 120°C for approximately 5-10 seconds

Mechanical behavior of the composite fabric was examined, the results of which are shown in Fig 6 and Table 1 , The fiber membranes containing

PC(shell)/PU(core) in a concentration ratio of 20 wt%/4 wt% and 20 wt%/6 wt% exhibited so called "beads," which are associated with the PC The beads are disadvantageous because they tend to decrease mechanical performance.. The tensile behavior of the composite PU core and PC shell nanofibers was enhanced with the increase of the polymer concentration in the core solutions. No beads formed for PC(shell)/PU(core) at concentration ratios of 20 wt%/8 wt% and 20 wt%/10 wt%. The PC(she)l)/PU(core) (20 wt%/8 wt%) membrane exhibited the highest strain to fracture characteristics and ultimate strength of the coaxial nanofiber membranes created. Tabie 1

Tensile Properties of Electrospun PC(sheil)/PU (Core Composite Fibers

20 wt% 20 wt% 20 wt% 20 Wt %

PC, PC, PC, PC, PU, PC,

Concentration 4 wt% 6 wt% 8 wt% W wt% 8 wt% 20 wt% PU PU PU PU

0 156 0 176 0 209 0 240 0 101 0 270 opscimen

{0 044)^' (0 047) (0 031) (0 020) (0 021 } (0 025) thickness (mm)

6 00 11 14 13 00 12 25 4 52 1 04

IVIOQUlUS

(0 002) (0 005) (0 005) (0 003) {0 002) (0 012)

(MPa)

0 33 0 52 0 82 0 33 2 88 0 04

Ultimate

(0 10) (0 16) (0 12) (0 07) (0 08) (0 01) strength (MPa)

2] U', - x)² In , where A- is an averaged value

/=1

Water vapor transmission of the nanofibrous membranes was measured through a dish method following a Chinese National Standard GB/T 12704-91 The specimen thickness was 0 12 ± 0 03mm, which was obtained by controlling the electrospinning time for 1h The constant temperature and relative humidity used were 39°C and 94%, respectively, in humidistat HWS- 150 (Jinghong Laboratory Instrument Co , Ltd, Shanghai, China)

Pliability of the fabric, which is an important characteristic for textile applications because it reflects sculpting capability and conformabiϋty for dressing, was tested according to the Chinese National Standard GB/ 7689 4-2001 , According to this well-known method, the fabric sample is placed between two wood blocks The front of the fabric was located at the intersection of the planar and inclined surface of the first block A second block was then impelled to drive the fabric to slide in such a way that there is no sliding between the second block and the fabric According to this example, and because of gravity, the fabric droops towards the inclined surface of the first block As soon as there was a touch between the fabric and the inclined surface, the impelling force was stopped The moving distance of the fabric on the planar surface determines the fabric pliability In this experiment, the sample thickness is a key factor that would affect the experimental result The membrane thickness was controlled to 0 12 ± 0 03mm, and 6 repeated tests were performed to gain a reliable data.

The different water vapor transmission rates (WVT) and pliabilities between the cotton fabrics enhanced with PC(sheli)/ PU(core) composite membranes and the pure cotton fabric are listed in Table 2

Table 2

Water Vapor Transmission Rate (WVT) and Pliability of Pure Cotton Fabric and the Fabrics Treated with Different Nanoftber Membranes n Concen „*tr„at«ⁱ„on„ ² _B ⁰ _yή W_O/t_o ⁰/⁰ _P ^P _U ^C- ² ₁ ⁰ _O ^Mwt ^%% ^PP^CU> ^p Ol^υ I- P⁸ M^wrftO%/ ^Pur _f ^e _ab ^C _r ^O _ic ^ttOn

WVT

3258 92±38 55 3417 70±47..7 3240 05±58 04 5570 58±9 56 (g/m²-24h)

Pliability(cm) 8 6±0 6 7 3±1 10 2±0.5 4 8±0 2

TiO₂, Au, or Ag may be incorporated into the PC sheli to enhance bacteria resistance by adding a certain amount of liquid butyi titanate, copper nitrate, or silver nitrate into the PC solution For example, to incorporate TiO₂ into the solution, liquid butyl titanate can be added in amounts of just above about 0 wt% to about 40 wt% To incorporate copper into the solution, copper nitrate can be added in amounts of just above about 0 wt% to about 10 wt% Similarly, silver nitrate in an amount of just above about 0 wt% to about 10 wt% can be added to incorporate silver into the solution. Preferably, about 5 wt% of liquid butyl titanate, about 2wt% of silver nitrate, and/or about 2wt% of copper nitrate could be added to the PC solution to provide an improved antimicrobial effect

According to one exemplary method in which TiO₂, Au, and Ag were incorporated into the PC solution, polycarbonate was dissolved in a solvent made of a mixture of DMF and THF₁ as described above 5 wt% of liquid butyl titanate was added to the PC solution and the components were mixed together by means of an ultrasonic stirrer for three hours. Then, 2 wt% of particulate copper nitrate and 2 wt% of silver nitrate were added and stirred into the PC solution for an additional three hours until all the particles were completely dissolved in the mixture. The solution made by this method was then electrospun according to the methods described above to form a nanofibrous membrane containing nanofibers with a PU core and a PC sheli comprising components of TiO₂, Cu₂ + , and Ag + The membrane was tested by exposing it to 426 pseudomonads where it was determined to exhibit good bacteria-resistant behavior by virtue of the fact that, after the exposure, only 256 pseudomonads retained active.

Thus, according to the invention, the filtering characteristic of pure polycarbonate or polycarbonate with components of TiO₂, Cu₂+ , and Ag + can be combined with the mechanical characteristics of polyurethane through the formation of the composite nanofiber described herein

It should be understood that the above detailed description while indicating preferred embodiments of the invention are given by way of illustration only since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from the detailed description

Claims

What is claimed is.

1 A method of forming a coaxial nanofiber, comprising: flowing a solution comprising polycarbonate through a first capillary of an electrospinning apparatus, flowing a solution comprising polyurethane through a second capillary of the electrospinning apparatus, the second capillary being coaxially aligned within the first capillary; and applying a voltage to the solution comprising polycarbonate and the solution containing polyurethane as the solutions pass through the first and second capillaries, respectively, to form a coaxial nanofiber comprising a polycarbonate shell and a polyurethane core

2 The method according to claim 1 , wherein the solution comprising polycarbonate also includes a solvent comprising a mixture of tetrahydrofuran and dimethylformamide

3 The method according to claim 1 , wherein the solution comprising polyurethane also includes a solvent comprising a mixture of tetrahydrofuran and dimethylformamide

4 The method according to claim 1, where the amount of polycarbonate in the solution comprising polycarbonate is at least about 20 wt%

5 The method according to claim 4, wherein the solution comprising polycarbonate further comprises liquid butyl titanate in an amount between about 0wt% and about 40wt%, copper nitrate in an amount between about 0wt% and about 10wt%, and silver nitrate in an amount of about 0wt% and about 10wt%

6 The method according to claim 5, wherein the solution comprising polycarbonate further comprises liquid butyl titanate in an amount of about 5 wt%, copper nitrate in an amount of about 2 wt%, and silver nitrate in an amount of about 2 wt%

7 The method according to claim 1 , where the amount of polyurethane in the solution comprising polyurethane is between about 4 wt% and about 10 wt%

8. The method according to claim 4, where the amount of polyurethane in the solution comprising polyurethane is about 8 wt%

9 The method according to claim 1 , wherein the voltage is in a range of about 1-3O kV

10 The method according to claim 1 , wherein the coaxial nanofiber has an outer diameter in the range of about 1 to about 1000 nm The method according to claim 1 , further comprising collecting the nanofiber onto a collector The method according to claim 1 1 , wherein the second capillary has a tip at its exit and distance from the second capillary tip to the collector is in the range of about 1 cm to 30 cm The method according to claim 12, wherein the distance from the second capillary tip to the collector is in the range of about 12 cm to 13crn The method according to claim 1 , further comprising collecting the nanofiber onto a fabric substrate The method according to claim 13, wherein the fabric substrate is provided with an adhesive layer prior to collecting the nanofiber thereon A coaxial nanofiber fiber, comprising: a polyurethane core and polycarbonate shell, wherein the fiber has an outer diameter in the range of about 1 nm to about 1000 nm The coaxial nanofiber of claim 16, wherein the polyurethane core has an outer diameter of less than about 170 nm The coaxial nanofiber of claim 16, wherein the polycarbonate shell has an outer diameter of about 320 nm or less The coaxial nanofiber of claim 16, wherein the polycarbonate shell further comprises at least one of the following TiO2, Au₅ or Ag A membrane comprising the coaxial nanofiber of claim 16 A membrane comprising the coaxial nanofiber of claim 19 The membrane of claim 20 having a thickness in the range of about 0 1 mm to 0 2 mm The membrane of claim 21 having a thickness in the range of about 0 1 mm to 0 2 mm A composite fabric comprising: a fabric substrate, the membrane according to claim 20; and an adhesive layer in contact with the fabric substrate and the membrane A composite fabric comprising. a fabric substrate; the membrane according to claim 21 , and an adhesive layer in contact with the fabric substrate and the membrane