IV. Results Sample preparation was improved. Sonicating the CNF between each step

dispersed agglomerate clumps. Sonicating the composite in solution reduced the size and number of bubbles and cracks in the thin films. Casting the thin film at a

0.010 inch thickness allowed the dichloromethane solvent to evaporate from the

body of the film before the surface of the film trapped bubbles of solvent vapor.

The electrical properties of the thin films have begun to be tested. Preliminary analysis of the volume resistivity of the thin films are plotted in chart 1 and chart 2.

Error bars on each chart show a 95% confidence interval. In chart 1, pure PEI

shows the highest volume resistivity of !8E+17 "-m. A 2% untreated CNF

concentration shows the lowest volume resistivity of !1E+12 "-m. The sharp drop

in resistivity between a 0.5% and 0.75% concentration indicate an approach towards the percolation threshold.

In chart 2, a 3% CNF (treated for 3 hours) concentration shows the highest

volume resisitivity of !3E+14 "-m. Higher resistivity correlates to shorter

nanofibers, a result of sonication treatment. A 3% untreated CNF concentration shows the lowest volume resisitivity of !3E+12 "-m. Lower resistivity correlates to

longer nanofibers and greater fiber agglomeration compared to treated CNF.

The SEM scans of the extruded pieces show that the extrusion process mixes

the CNF evenly throughout the PEI. CNF treated with suspended agitation (see figure 5, right) appear shorter than untreated CNF (see figure 5, left). Verifying the

distribution using multiple fracture sites and an overlaid grid is needed for a larger

confidence in the distribution effects.

V. Conclusion

Thin film sample preparation was refined over the course of the experiment. Solvent ratios, dispersion techniques, and film thicknesses were explored in order

to avoid bubbles, cracks, and other deformities in a sample. A 0.010 inch (2.54

mm) film thickness allows the greatest uniformity.

The electrical conductivity of the nanocomposite increases as the CNF concentration increases. For untreated CNF, the percolation threshold appears at

concentrations greater than 0.50% CNF. This is indicated by a sharp decrease in

resistivity. Empirical data shows a minimum resistivity at 2.0% CNF, but finding a

definite optimal concentration requires further experimentation.

Sonicating suspended CNF correlates to an increase in volume resistivity. SEM

scans of treated and untreated CNF in the same concentrations suggest that

treated CNF is better dispersed throughout the matrix than untreated CNF. Studies

on the effect of sonication on CNF length would require TEM scans of bare fibers

undamaged by tensile fracture of the composite.

Acknowledgements

Faculty Advisor: Dr. Katie Zhong Project Supervisor: Dr. Gang Sui

Laboratory Assistant: Loren Baker

Financial support for this work was provided by the National Science Foundation’s Research Experience for

Undergraduates Site Program in the Division of Materials Research under grant number DMR 0755055, Characterization of Advanced Materials.

Chart 1: Volume Resistivity of

Untreated CNF + PEI Thin

Chart 2: Volume Resistivity of 3%

Treated CNF + PEI Thin Films

Rectangular Panel

Six composite mixes were prepared in different concentrations and methods in

order to explore the effects of such differences on the properties of the panel. The

components were mixed manually and by the extrusion process.

The first method mixed 0.5%, 1.0%, and 3.0% concentrations of untreated CNF

in PEI as dry powders. The second method mixed 1.0% concentrations of CNF treated for 60 minutes or 180 minutes in PEI as dry powders. The third method

mixed high concentration films with additional powdered PEI to a final weight. The

films were 10% concentrations of untreated CNF or CNF treated for 60 minutes in

PEI in solution. Additional powdered PEI was added for a final 1.0% CNF

concentration.

These samples were mixed and extruded by a polymer extruder, then formed in

a mold using a hot press, shown in figure 2, at 650°F into a uniform panel, shown in figure 3.

SEM Study A sample of each thin film and panel extrusion was fixed to a glass slide with

conductive tape and was sputtered with gold, shown in figure 4. The natural

surface of the polymer prevented a clear image of the embedded CNF, so the thin film samples were torn and the extrusion pieces were fractured in tension. These

fracture sites exposed the CNF, seen as bright strands in figure 5.

Preparation and Properties of Carbon-

Nanofiber / Polyethylamine Nanocomposites Graham H. Bratzel

!"#$%#&'()

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!"!!# !"!!!! $"!!!!

!"%!# !"!&%! '"()%!

!"*%# !"!''% '"(**%

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Table 1: Thin Film Concentrations

Figure 2: Resistivity Test Fixture

I. Abstract The polymer Polyethylamine (PEI) was mixed with carbon nanofibers (CNF) to

create a polymer-carbon nanocomposite with near-isotropic and enchanced properties. The volume resistivities of thin films of the nanocomposite were

determined for various CNF concentrations and compared to the known properties

of pure PEI. The effects of CNF sonication on volume resistivity was determined

for a 3% CNF concentration. SEM scans explored nanofiber size and distribution.

II. Background

Nanofiber Reinforced Polymer Composite

As composite materials are being used more in public aerospace applications,

e.g. the Boeing 787 Dreamliner (50% composite), isotropic and generally enhanced

mechanical, electrical, and thermal properties are all being investigated for stronger, lighter, and safer materials.

As a particle’s dimension is reduced to nanoscopic levels, the macroscopic

properties of the material give way to new properties. Polymers reinforced by

nanoparticles exhibit highly isotropic behavior, and at discrete concentrations as low as a few percent, the properties of the composite reflect the properties of the

nanoparticle filler more than that of the polymer matrix. This has been seen in

electrical properties using a conductive filler like carbon. Carbon nanofibers (CNF)

have a high aspect ratio (surface area/diameter ratio) such that electrons are

naturally on the surface, improving electrical conductivity. At concentrations of a few percent in a polymer matrix, electrons can jump among the CNF through the

dielectric matrix, allowing current flow. Volume conductivity improves as the CNF

concentration approaches the percolation threshold.

III. Method Polyethylamine (PEI) was chosen for the polymer matrix because it dissolves

easily and is flame retardant. Thin film samples were prepared to test the electrical

conductivity of the composite. Rectangular panels were prepared to develop nano-

pore foam in following stages of research. Samples were studied using an SEM to observe nanofiber distribution. In each case, sample preparation methods were

examined and refined.

Van der Waals and other adhesion forces cause the CNF to agglomerate into

clumps. These clumps were dispersed in order to improve distribution throughout the PEI matrix. CNF was suspended in acetone and was agitated by a horn-type

sonicator for 1 hour or 3 hours.

Thin Film

The thin film samples were prepared by dissolving the PEI and carbon nanofibers (CNF) in a common solution of dichloromethane (CH2Cl2) and pouring

the mix onto glass plates. Each pour was cast at a 0.010 inch (0.254 mm)

thickness. Seven concentrations were prepared, detailed in Table 1. Five films

were collected for each concentration. Of the five collected films, the film that was

smoothest, most uniform, and had the least bubbles was tested for surface and volume resistivity with an alternating voltage of 50V and a variable nano-ampere

current using a resistivity test fixture, shown in figure 1. The permittivity and other

electrical properties of the films were also tested and will be analyzed in further

stages of research.

Volume resistivity measures the resistance of a material through a known

thickness and across a known area, or , where ! is the resistivity (in ohm

meters, "-m); R is the electrical resistance of a uniform sample of the material (in

ohms, "); A is the cross-sectional area of the sample or the electrodes, whichever

is smaller (in square meters, m#); and l is the thickness of the sample (in meters, m). Conductivity is the inverse of resistivity and has the units "-1-m-1 or Siemens

per meter, S-m-1.

! =RA

l

Figure 5: SEM Scans of 1% untreated CNF (left) and 1% 3hr treated

CNF (right) at Extruded Piece Fracture Site

Figure 4: Fractured Thin Film (left) and Extruded Pieces (right) for SEM

Figure 3: Steel Mold and

Composite Panel

Figure 2: Hot Press for Panel Molding