(102h) Micro/Nanofiber Assemblies with Controlled Fiber Morphology

                                          Micro/nanofiber
Assemblies with Controlled Fiber Morphology

     Ji Wang¹,
Amrinder S. Nain^2,3

                               
                           ¹ Engineering Science and Mechanics, ²
Mechanical Engineering, ³ Biomedical Engineering, Virginia Tech,
Blacksburg, VA

Introduction

Polymeric micro/nanofibers are
increasingly gaining importance due to their versatile application in diverse
fields of tissue engineering, smart textiles, sensors and actuators. However,
aligned deposition and continuous production of long fibers with sub-micron and
nanoscale diameters with desired morphologies has been challenging. Recently,
techniques of electrospinning, template synthesis and sequential micro dry
spinning have been used to deposit a wide variety of micro/nano scale polymeric
fibers in aligned configurations ^[1-6]. Among all these methods,
electrospinning is the most popular process, which allows for the production of
polymer/ceramic fibers ranging from tens of nanometers to a few microns in
diameter ^[7-11]. Recent research has been devoted to control electrospun
fiber morphology and surface porous /wrinkled fibers have been demonstrated  ^[12-13].
In addition, co-axial electrospinning has been well established as an easy and
cost effective method to fabricate tubular structures ^[14]. Using
specialized techniques, electrospinning methods are able to achieve control on
fiber alignment in single layer. However, fiber spacing and three-dimensional
stacking of fibers in precise locations has not been demonstrated using
electrospinning methods. Our recently reported non-electrospinning fiber
manufacturing strategy based on Spinneret based Tunable Engineering Parameter
(STEP) technique is able to deposit fibers and fiber arrays in aligned
configurations with control on spacing in single and multiple layers. This
allows fabrication for the very first time of fiber hierarchical structures
with each layer containing fibers of customized diameters (sub 100nm-micron),
spacing (sub 100 nm-microns), orientation ( 0 deg. -90 deg.) and variety of
polymeric materials.   In this study, we extend this capability to
include control on surface morphology of fibers through manipulation of solvent
and ambient atmosphere humidity.

Experimental

All chemicals were used as received without further
purification.

Polystyrene(PS)
Two molecular weights PS (650K and  2000K g.mol^-1, Polymer
scientific, USA) were dissolved in xylene and DMF mixture  with varying wt%
concentrations (from 1% to 18%) for one week
prior to experiments.

Poly(methyl methacrylate). PMMA(540K g.mol^-1, Polymer scientific, USA) dissolved
in chlorobenzene at 14%wt was used in this study.

Polyurethane  (PU5719, Thermal
Plastic, Lubrizol Advanced Material, USA)was dissolved in DMF at 30%wt  and
heated to  80 °C stirring for 24 hours to prepare homogenous
solution.

Polyacrylonitrile(PAN,150K g.mol^-1,SIGMA ALDRIC, USA) was dissolved in DMF at
11%wt overnight to prepare homogeneous solutions

STEP spinning.
All experiments were performed at 20°C.  As shown in Fig 1(a), the viscous
polymer solutions were pumped through the micropipette spinneret (ID=100mm) and formed a solution droplet at the nozzle. The substrate
(Thermanox, USA) was mounted to a DC motor, which in turn was mounted onto a
motorized three degree of freedom micro-positioning stage (VP-25XA, Newport
Inc., USA). After the substrates contacted the solution droplet, fibers were
collected on the substrate in aligned configurations. By adjusting the linear
movement speed of the substrate and the angle between micropipette and the
rotating axis, fiber spacing and orientation was tightly controlled.

Fiber Fusion: PMMA
fiber network were treated with chlorobenzene vapor at 20°C in an enclosed
chamber for 2 minutes.

Carbonization:
PAN fibers were calcinated at 900°C in nitrogen atmosphere for 3 hours to
produce carbon fibers.

Results and Discussion

As a non-electrospinning technique, STEP relies on the
rotating substrate to transform the polymer solution filament into nano/micro
scale fibers. Since no voltage was used in this technique, the classic jet
bending and spraying in electrospinning were avoided in STEP, and as a result,
the STEP fiber diameter is narrowly distributed (within 20%) and STEP fibers
maintain aligned configurations(angles between paralleled fibers are less than
1°) . In Fig 1(b), arrays of uniform diameter PS fibers were deposited with
equal inter-fiber space; in Fig 1(c), two PMMA fiber arrays were deposited
orthogonally, forming a crisscross fiber network, which was then treated by
solvent vapor using a custom in-house solvent evaporator, creating an
inter-connected structure composing of ?unit cells?.

Using the STEP technique, we have successfully
fabricated aligned nanofibers from a variety of polymers other than PS and
PMMA; typical examples include polyurethane(PU), polyacrylonitrile(PAN),
poly(ethylene oxide)(PEO), poly(lactic-co-glycolic
acid)(PLGA) and fibrinogen. Using these uniform diameter fibers as building
blocks, hierarchical fiber assemblies were achieved. Fig 2(a) shows a suspended
PS fiber assembly composing of 4 mm and 400 nm
diameter fibers. Fig 2(b) is a hierarchical assembly of PU fibers (300nm in
diameter) and PS fiber ( 4mm in diameter). Fig 2(c)
demonstrates a 10×10mm² PS fiber assembly, composing of 4 different
diameters (d₁=400nm, d₂=800nm,d₃=80nm, d₄=160nm).
 In addition, we demonstrated that inorganic nanofibers could be fabricated by
calcination of polymer precursors. In Fig 2 (d), crisscross carbon nanofibers were
obtained through calcination of PAN fibers at 900° C in nitrogen atmosphere for
3 hours.

By changing the solvent and adjusting the humidity of
atmosphere, we demonstrate that both the surface and inner morphology of STEP
fibers can be altered. As shown in Fig 3(a)-(c), at 60% relative humidity (RH),
by gradually adding DMF in xylene (from 0% wt to 50%wt and 75%wt), STEP fiber
surfaces evolve from smooth to porous and finally wrinkled. The mechanism for
surface attributes formation is solvent induced phase separation?the solvent
rich phase turns into pores while the solvent poor phase turns into the matrix.
As DMF concentration becomes dominating in the mixed solvent, the solvent
becomes more hydrophilic and the polymer-solvent interaction is weakened,
leading to larger dispersed phases. As a result, surface features evolve from
discontinuous pores (Fig 3(b))to parallel channels(Fig 3(c)). Fig 3(d)-(f)
demonstrate the corresponding inner fiber structures, which depend on the
relative rates of solvent evaporation to phase separation rate. When solvent
evaporation rate dominates over the phase separation rate (using solvent of 0%
wt DMF), a uniformly solidified cross section is obtained (Fig3 d). When the
solvent evaporation rate is faster( using solvent of 50%wt DMF), a thin skin
consisting of almost pure polymer is formed first, which subsequently develops
towards the core of the fiber, leading to the formation of a hollow fiber (Fig 3
e). To the best of our knowledge, this is first time tubular structures were
obtained through control of evaporation process. In electrospinning, the
tubular structures usually collapse into flat ribbons because of the lateral
pressure caused by cumulated electrical charges^[15]. In STEP,
tubular structures were well preserved due to abscence of electrical field. The
formation of a bi-continuous core (Fig 3 f) is attributed to the phase
separation process when the evaporation rate is slowed (using solvent of 75% wt
DMF). We go further to demonstrate that both the pore size and tube wall
thickness are function of solution concentrations. By changing polymer
concentration in the solvent, fibers with different diameters, surface
morphologies and inner structures were obtained.  Finally, fibers of different
morphologies were incorporated into 2D/3D assemblies as shown in Fig 3(g) to
(i).

Conclusion:

STEP turns out to be a very versatile
method, which allows controlled fiber deposition of various materials.  The
unique advantage of STEP in spacing and orientation control realizes the 3D nano/micro
fiber assembly. Fiber morphology is subjected to both processing parameters (
humidity) and material parameters( concentration and solvent). Compared with
electrospinning, STEP offers an alternative to achieve nano/micro fibers with
desired position and attributes.

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