Gogotsi Y. (Ed.) Nanotubes and Nanofibers

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Figure 8.2 shows a commercial nanofiber electrospinning unit known as NEU produced by the

Kato-Tech Co. The NEU system consists of a metering pump, which controls the volume flow rate

of the spinning dope in a syringe. An electrical potential is applied by a power supply between the

steel syringe needle and the fiber collection ground in the form of a metallic screen or a drum. The

temperature and humidity of the spinning chamber can be controlled.

8.3 KEY PROCESSING PARAMETERS

A key objective in electrospinning is to generate fibers of nanometer diameter consistently and

reproducibly. Considerable effort has been devoted to understand the parameters affecting the

spinnability and more specifically the diameter of the fibers resulting from the electrospinning

process. Many processing parameters have been identified, which influence the spinnability and the

physical properties of nanofibers. These parameters include electric field strength, polymer con-

centration, spinning distance, and polymer viscosity.

According to Rutledge et al. [5], the diameter of electrospun fibers is governed by the following

equation:

d ⫽

冤

γ ε

冥

1/3

(8.1)

where d is the fiber diameter,

the surface tension,

the dielectric constant, Q the flow rate, I the

current carried by the fiber, and

the ratio of the initial jet length to the nozzle diameter.

One can control the fiber diameter by adjusting the flow rate, the conductivity of the spinning

line, and the spinneret diameter. Fiber diameter can be minimized by increasing the current carry-

ing capability of the fiber through the introduction of a conductive filler such as carbon black, car-

bon nanotube, metallic atoms, or mixing with an inherently conductive polymer. For example, an

increase of current carrying capability, I, of the fiber by 32 times will bring about 10-fold decrease

in fiber diameter. Alternatively, if the current I is kept constant, one can bring about a 10-fold

decrease in the fiber diameter by reducing the flow rate by 32 times. Reducing the spinneret nozzle

diameter can also bring about a reduction in fiber diameter. When the value of

is increased from

10 to 1000, the diameter of the fiber will decrease by approximately 2 times.

Experimental evidence has shown that the diameter of the electrospun fibers is influenced by

molecular conformation that is related to the molecular weight and the concentration of the poly-

mer in the spinning dope [6]. It was found that the diameter of fibers spun from dilute polymer solu-

tions can be expressed in terms of the Berry number B, a dimensionless parameter and a product of

intrinsic viscosity η, and polymer concentration, C. This relationship has been observed in a large

number of polymers. An example for this relationship is illustrated using polylactic acid (PLA). The

relationship between polymer concentration and fiber diameter of a PLA of different molecular

weight in chloroform are shown in

Figure 8.3. It can be seen that fiber diameter increases as poly-

mer concentration increases. The rate of increase in fiber diameter is greater at higher molecular

weight. Accordingly, one can tailor the fiber diameter by proper selection of polymer molecular

weight and polymer concentrations.

Expressing fiber diameter as a function of B, as shown in

Figure 8.4, a pattern emerges illus-

trating four regions of B vs. diameter relationships. At region (I), B⬍1, characterizing a very dilute

polymer solution with molecular chains barely touching each other. It is almost impossible to form

fibers by electrospinning of such solution, since there is not enough chain entanglement to form a

continuous fiber, and the effect of surface tension will make the extended conformation of a single

molecule unstable. As a result, only polymer droplets are formed. In region (II), 1⬍B⬍3, fiber

diameter increases slowly as B increases within the range ⬃100 to ⬃500 nm. In this region, the

degree of molecular entanglement is just sufficient for fiber formation. The coiled macromolecules

of the dissolved polymer are transformed by the elongational flow of the polymer jet into orientated

ᎏᎏ

(2ln

⫺3)

ᎏ

Nanofiber Technology 235

molecular assemblies with some level of inter- and intramolecular entanglement. These entangled

networks persist as the fiber solidifies. In this region, some bead formations are observed as a result

of polymer relaxation and surface tension effect. In region (III), 3⬍B⬍4, fiber diameter increases

rapidly with B in the range from ⬃1700 to ⬃2800 nm. In this region, the molecular chain entan-

glement becomes more intensive, contributing to an increase in polymer viscosity. Because of the

intense level of molecular entanglement, it requires a higher level of electric field strength for fiber

formation by electrospinning. In region (IV), B⬎4, the fiber diameter is less dependent on B. With

a high degree of inter- and intramolecular chain entanglement, other processing parameters such as

electric field strength and spinning distance become dominant factors affecting fiber diameter. A

schematic illustration of the four Berry regions is shown in

Table 8.1, showing the corresponding

relationship between fiber morphology and Berry numbers.

More recently, we further demonstrated, as shown in Figure 8.5, that the diameter, d, of

electrospun fibers is related to the Berry number B for three amorphous polymers with different

236 Nanotubes and Nanofibers

Average diameter vs. Concentration (Mw=300,000)

= 0.8478

y = 895.29 ×−1648.5

500

1000

1500

2000

2500

3000

3500

4000

0 1 2 3 4 5 6

Concentration (

g /dL)

Average diameter (nm)

Average diameter vs. Concentration (Mw=200,000)

y = 330.3 × −1018.4

= 0.9005

500

1000

1500

2000

2500

3000

3500

0 5 10 15

Concentration (

g/dL)

FIGURE 8.3 Relationship of fiber diameter and concentrations of polymer with different molecular weights.

molecular weights. It was shown that the relationship of fiber diameter to Berry number can be

expressed by d ⫽ a B

, where a and c are experimental coefficients. The molecular weight of the

three polymers polystyrene, polybutadiene, and SBS are 48,000, 60,000, and 100,000 dalton,

respectively. It is of interest to note that the slope of the diameter vs. Berry number curves are

approximately the same, with a value of 5, reflecting the degree of crystallinity of the polymer. The

coefficient a

is thought to be related to the molecular weight, radius of gyration of molecular chains,

and entanglement of the molecular chains. From this relationship, the polymer concentration

necessary to produce nanofiber can be predicted.

Nanofiber Technology 237

TABLE 8.1

Schematic of Polymer Chain Conformation and Fiber Morphology Corresponding to Four

Regions of Berry Number

Region I Region II Region III Region IV

Berry Number B<1 1<B<2.7 2.7B<3.6 B>3.6

Polymer chain conformation in

solution

Fiber morphology

Average fiber diameter (Only droplets ⬃100–500 nm 1700–2800 nm ⬃2500–3000 nm

formed)

Average diameter vs. Berry number

500

1000

1500

2000

2500

3000

0 1 2 3 4 5

Berry number

Average diameter (nm)

I II III

FIGURE 8.4 The relationship between Berry number and fiber diameter.

8.4 NANOFIBER YARNS AND FABRICS FORMATION

In order to translate nanoeffect to the macroscopic structural level, the nanofibers must be organ-

ized into linear, planar, and 3-D assemblies. The formation of nanofiber assemblies provides a

means to connect nanofiber effect to macrostructure performance. Electrospun fiber structures can

be assembled by direct fiber-to-fabric formation to create a nonwoven assembly and by the creation

of a linear assembly or a yarn from which a fabric can be woven, knitted, or braided. Linear fiber

assemblies can be aligned mechanically or by electrostatic field control. Alternatively, a self-assem-

bled continuous yarn can be formed during electrospinning by proper design of the ground elec-

trode. Self-assembled yarn can be produced in continuous length with appropriate control of

electrospinning parameters and conditions. The fibers are allowed to accumulate until a tree-like

structure is formed. Once a sufficient length of yarn is formed, the accumulated fibers attach them-

selves to the branches and continue to build up. A device such as a rotating drum can be used to

spool up the self-assembled yarn in continuous length as shown in

Figure 8.6. This method pro-

duces partially aligned nanofibers yarn bundles of continuous length.

8.5 POTENTIAL APPLICATIONS OF ELECTROSPUN FIBERS

The Donaldson Company was the first to realize the commercial value of electrospinning taking

advantage of the enormous availability of the specific surface of electrospun fibers and the ultrafine

nature of the fibers. They introduced the Ultra-Web

cartridge filter for industrial dust collection in

1981 and more recently the Hollingsworth & Vose Company introduced the Nanoweb

for auto-

motive and truck filter applications. To date, industrial filter remains the primary commercial use of

electrospun fibers. The rediscovery and popularization of the electrospinning process in the 1990s

have changed the dynamics of the field of nanofiber technology. Electrospinning has been trans-

formed from an obscure technology to a word common in academia and one gradually being rec-

ognized by industry and government organizations. This was most evident in the Polymer Nanofiber

Symposium held in New York at the American Chemical Society (ACS) in September 2003,

wherein 75 papers and posters were presented [7]. On the basis of the papers presented in the ACS

symposium and the rapidly growing number of publications, one can categorize the potential

applications of electrospun fibers into (1) biomedical applications — tissue engineering scaffolds,

wound care, superabsorbent media, drug delivery carrier, etc. [8,9]; (2) electronic applications —

electronic packaging, sensors, wearable electronics, actuators, fuel cells, etc. [10,11]; (3) industrial

applications — filtration, structural toughening/reinforcement, chemical/bio-protection, etc.

238 Nanotubes and Nanofibers

0.1

100

1 10

Fiber diameter (micron meter)

SBS(TR2000)

PB(RB830)

Polystyrene

PB/VGCF(1phr)

Dia. = B

FIGURE 8.5 The relationship between fiber diameter and the Berry number.

[12,13]. It is of interest to note that these applications invariably made use of the large surface area

created by the fineness of the fibers. A few examples will be used herein to illustrate the potential

applications of nanofibers.

8.5.1 NANOFIBERS FOR TISSUE ENGINEERING SCAFFOLDS

It is well recognized, as articulated by Vacanti and Mikos [14], that the key challenges in tissue engi-

neering are the synthesis of new cell adhesion-specific materials and the development of fabrication

methods to produce reproducible three-dimensional synthetic or natural biodegradable polymer scaf-

folds with tailored properties. These properties include porosity, pore size distribution and connec-

tivity, mechanical properties for load-bearing applications, and rate of degradation. Of particular

interest in tissue engineering is the surface adhesion of the cells to the scaffold. Considering the

importance of surfaces for cell adhesion and migration, experiments were carried out in our labora-

tory using osteoblasts isolated from neonatal rat calvarias and grown to confluence in Ham’s F-12

medium (GIBCO), supplemented with 12% sigma fetal bovine on PLAGA-sintered spheres, 3-D-

braided 20 µm filament bundles, and nanofibrils. Four matrices were fabricated for the cell culture

experiments. These matrices include (1) 150–300 µm PLAGA-sintered spheres, (2) unidirectional

bundles of 20 µm filaments, (3) 3-D-braided structure consisting of 20 bundles of 20 µm filaments,

and (4) nonwoven consisting of nanofibrils. The cell proliferation, as shown in

Figure 8.7, is

expressed in terms of the amount of [

H]-thymidine uptake as a function of time. It can be seen that

there is a consistent increase in cell population with time for all the matrices. However, the nanofi-

brous structures demonstrated the most proliferate cell growth, whereas the cell growth in the struc-

tures consisting large diameter fibers and spheres were the least proliferate. Taking advantage of this

cell-friendly characteristic of nanofiber scaffolds, nanofibers are being evaluated for the regeneration

of skin, vascular grafts, ligament, cartilage, bone, and many other tissues and organs.

8.5.2 NANOFIBERS FOR CHEMICAL/BIO PROTECTIVE MEMBRANES

Nanofiber assemblies are excellent structure barrier systems for dust filtration [12] and for the pre-

vention of the penetration of chemical and biological agents [13]. Considering the various barrier

concepts shown in

Figure 8.8, it is concluded that neither the impermeable nor the permeable

Nanofiber Technology 239

High voltage

power supply

Polymer

solution

Rotary drum

Self-assembled

yarn

FIGURE 8.6 Schematic illustration of the formation of self-assembled yarns by the electrospinning process.

barrier systems are suitable because of the unacceptable level of heat generation in the imperme-

able barriers and the porous nature of the permeable systems. The semi-permeable barrier provides

incremental improvement but the addition of the sorptive layer may serve as a reservoir for bacte-

ria growth as well as adding bulk to the garment. Accordingly, the current design favors a system

that is carbon-free with selective permeable capability and which has the multiple functions of

repelling CB agents while allowing the body moisture vapor to escape. As an example, we illustrate

the feasibility of using the electrospinning process to construct a selective permeable membrane

using the polymers formulated and provided by Utility Development Cooperation (UDC).

An example of an solvent-based electrospun polymer is shown in

Figure 8.9 showing a three-

dimensional interconnected fiber network. The morphology of the polymer was found to be con-

trollable by varying polymer concentration.

The electrospun membranes were tested at UDC for moisture vapor transmission by using the

Thwing Albert permeation cups. The testing was performed at 110°F to simulate hot weather condi-

tions. The moisture vapor transmission results of dip-coated nylon fabric with the same polymer as the

electrospun polymer formulation were compared to uncoated nylon fabric. These dip-coated fabrics

were about 6 mil thick compared to electrospun membrane at 10 mil thick. As shown in

Table 8.2,

moisture vapor transmission through electrospun membrane was equivalent to uncoated nylon fabric

and there was no detectable moisture vapor transmission through the dip-coated nylon fabric. The test

duration was 5 h. Considering that the water vapor permeability of the state-of-the-art protective

membranes are of the order of 0.1 to 0.25 g/cm

, these results indicated that the electrospun mem-

branes meet the requirements for wearing comfort over an extended period. This favorable compari-

son with the performance of the state-of-the-art membranes appears to be consistent with the findings

240 Nanotubes and Nanofibers

0.001

Thymidine uptake (µCi)

1 3 7

Time (days)

10 11

3-D braid

Microsphere

Nano nonwoven

Control

Cell proliferation

0.002

0.003

0.004

0.005

0.006

0.007

FIGURE 8.7 Directed cell growth (top) and cell proliferation (bottom) behavior in PLA nanofiber scaffold.

Nanofiber Technology 241

Air

Liquids

Permeable

Semipermeable

Selectively permeable

Impermeable

Sorptive material

Liquids

Vapor

Moisture

vapor

Skin

This is Natick's

current R&D focus

Selectivel

semi-

ermeable barrier construction.

Source

: Wilusz, 1998.

Skin

Skin Skin

Moisture

vapor

Moisture

vapor

Moisture

vapor

Vapor

Aerosols

FIGURE 8.8 Concepts of barrier systems for chemical/biological agent protection.

of Gibson et al. [15], who showed that the water-transport properties of electrospun nonwovens are

superior to that of the state-of-the-art commercial membranes.

Chemical agent testing was performed at Geomet Technology using HD chemical agents for

these tests in accordance with MIL-STD-282 static diffusion test. As indicated in the report from

Geomet, the UDC-formulated product has more than 24 h of HD chemical agent resistance exceed-

ing the performance requirement for state-of-the-art chemical/bio protective membranes.

8.5.3 NANOCOMPOSITE FIBERS FOR STRUCTURAL APPLICATIONS

Carbon nanotubes (CNTs) [16] are seamless graphene tubule structures with nanometer-size diame-

ters and high aspect ratios. This new class of one-dimensional material is shown to have exceptional

mechanical, thermal, and novel electronic properties. The elastic moduli of the CNTs are in the range

of 1 to 5 TPa [17–19] and fracture strains of 6 to 30%—both parameters about an order of magnitude

better than those of commercial carbon fibers, which typically have 0.1 to 0.5 TPa elastic moduli and

0.1 to 2% fracture strains [20]. The factor of 10 enhancement in strength implies that, for the same

performance, replacing or augmenting commercial carbon fibers with CNTs will lead to significant

reduction in the volume and weight of the structural composites currently used in space applications.

Studies carried out in our laboratory have demonstrated that composite nanofibers containing 1

to 10 wt% single-wall nanotube (SWNT) can be produced by electrospinning [21,22]. Fiber diam-

eters in the range of 40 to 300 nm have been obtained by varying the solution concentration and

electrospinning parameters. The alignment of SWNT in the nanofibers was confirmed by Raman

spectroscopy and TEM. Bulk mechanical properties of the green (as-spun) composite nanofibers

were characterized in the random mat form. With proper dispersion of SWNT and preparation of

the spinning dope, a twofold increase in strength and modulus is observed for the polyacrylonitrile

(PAN)/SWNT fiber assemblies containing 1 wt% SWNT. In addition, a continuous process for the

conversion of nanofibers into continuous yarn has also been successfully demonstrated. These con-

tinuous yarns provide an effective means to translate the properties of the SWNT to higher order

textile structures suitable for advanced composites and numerous other applications.

242 Nanotubes and Nanofibers

FIGURE 8.9 Electrospun UDC843A polymer showing an interconnected fiber network.

TABLE 8.2

Water Vapor Loss of Various Membranes (g/cm

)

Uncoated Nylon Electrospun Membrane Dip Coated Nylon Cloth

on Nylon Cloth

0.402 0.348 0.055

0.323 0.105 0.012

0.478 0.235 0.035

Figure 8.10 shows the stress–strain properties of electrospun pristine PAN and 1 wt%

SWNT/PAN nanofibers. As can be seen in the random fiber mats, a more than twofold increase in

tensile strength and elastic modulus was obtained with the addition of 1 wt% SWNT. For the

aligned 1 wt% SWNT/PAN, a 33% increase in tensile strength and almost double strain were

obtained as compared with the random composite nanofibers. However, the increase in elastic mod-

ulus is not significant. A large increase in the area under the stress–strain curve of SWNT/PAN indi-

cates the effectiveness of SWNT in toughening and strengthening the fibers. Fibril as well as SWNT

alignments play an important role in maximizing properties translation from the nanoscopic level

to higher order structure. Even without any posttreatment, remarkable improvement in properties

was achieved in the electrospun composite nanofibrils. It is anticipated that posttreatment such as

mechanical drawing further aligns the SWNT in the fiber axis. Drawing of the fibers also induces

molecular orientation resulting in much higher properties. Furthermore, heat treatment enhances

interaction between the reinforcement and matrix leading to better load transferring across rein-

forcement–matrix interface.

The stress–strain behavior of the SWNT-loaded PAN suggested toughening and strengthening

of the fiber with the incorporation of 1 wt% SWNT.

8.6 SUMMARY AND CONCLUSIONS

The combination of high-specific surface area, flexibility, and superior directional strength makes

fiber a preferred material form for many applications ranging from clothing to reinforcements for

aerospace structures. The availability of fibers in nanoscale will greatly expand the performance limit

of materials and thus create new marketing opportunities leading to the birth of a new nanofiber-

based industry. Of the processing methods capable of fabricating nanofibers, electrospinning has

received the most intensive attention due to its simplicity and the potential for generating continuous

nanofiber assemblies. With the enormous increase in available surface area per unit mass, nanofibers

provide a remarkable capacity for the attachment or release of functional groups, absorbed mole-

cules, ions, catalytic moieties, and nanometer scale particles of many kinds. As a result, these

nanofiber assemblies provide favorable conditions for cell adhesion and cell migration for bioactive

nanofibers thus creating new opportunities for nanofiber-based scaffolds for tissue engineering and

for ultra-sensitive biosensors. Other areas expected to be impacted by nanofiber-based technology

Nanofiber Technology 243

0 0.05 0.1 0.15 0.2

Strain

Stress (MPa)

Random

PAN fibers

Random

1 wt%

SWNT/PAN

fibers

Aligned

1 wt%

SWNT/PAN

fibers

FIGURE 8.10 Tensile properties of electrospun pristine PAN and 1 wt% SWNT/PAN fibers in random and

aligned.

include drug delivery systems, wires, capacitors, transistors, and diodes for information technology,

systems for energy transport, conversion and storage, such as batteries and fuel cells, and structural

composites for aerospace structures. In order to capitalize on these new opportunities new process-

ing technology is needed, precise characterization tools for single nanofibers are required, and

nanoscale manipulation machines must be developed. These technologies must be supported by well-

verified engineering design tools and analytical methods. It is expected that nanofiber technology

will provide a critical link between nanoscale effect and macroscopic phenomena and become the

enabling pathway for the conversion of nanomaterials into practical large-scale applications.

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