Investigation of the Dielectric Properties of
Layered PVDF/CNF Nanocomposite Materials
Gavin Mitchell, Lili Sun, and Dr. Katie Zhong, WSU School of Mechanical and
Materials Engineering. Pullman, WA 99164.
Polymer nanocomposite are increasingly becoming attractive to
scientists due to their ease of fabrication and good combination
of thermal, mechanical, and electrical properties. In this study,
Poly (vinylidene fluoride) (PVDF) was used as the polymer matrix
due to its naturally high dielectric constant (ε’ = 12 at 1 kHz and
25 ˚C) and ease of processing. Carbon nanofibers (CNFs) were
chosen as the nanofiller in this study because of their excellent
electrical properties and low cost. A sandwich-structure sample
with composite-pure polymer-composite structure was prepared
to achieve a high dielectric constant while simultaneously
minimizing the increase in conductivity of the material by
preventing the formation of conductive networks.
Conductive Networks
Fig. 1: Formation of
conductive networks in
composite material
occurs when the nanosized fillers touch and
allow a continuous path
by which electrons may
flow. The black squiggles
represent CNFs.
Fig. 2: Inability of
conductive networks to
form with layered
composite-pure polymercomposite structure.
Cup-stacked carbon nanofibers (Pyrograf® PR-24 HHT) were
purchased from Applied Science, Inc. PVDF was purchased from
Aldrich Chemistry. Dimethylformamide (DMF) was purchased
from J. T. Baker. All materials were used as received.
PVDF/CNF composite films with CNF concentrations of 0, 1, 3,
and 5 wt% were prepared using the solution-cast method with
DMF and acetone used as the solvents. A Branson® 1510
ultrasonic bath was used to disperse the CNFs. Films were
allowed to dry in a 70 °C oven for at least 20 minutes. The films
were then stacked in layers such that a composite-purecomposite structured plate could be formed by using a Carver
hot press at 200 °C and 3.45 MPa.
The dielectric properties and conductivity of the samples was
tested by a Novocontrol Technologies Alpha-N high-resolution
Dielectric Analyzer made in Germany. Microscopy of the samples
was performed using an FEI QUANTA 200F model scanning
electron microscope (SEM).
Fig. 6: Dielectric
constant versus
electrical field
frequency. The
highest dielectric
constant is obtained
using 3 wt% CNFs in
the composite films.
Fig. 3
(left):
SEM
image of
a pure
PVDF
sample.
Fig. 4 (below): SEM image of a PVDF/5 wt% CNF sample.
Fig. 5 (below):
SEM image of
sandwich
structure
along dividing
line between
pure PVDF
(upper left)
and PVDF/
5 wt% CNF
composite
layer (lower
right).
Fig. 7: AC
conductivity as a
function of
electrical field
frequency.
1. The sandwich structure possesses significantly increased
dielectric constant, with the greatest increase in dielectric
constant occurring when the composite layers contain 3
wt% of CNFs.
2. The conductivity of the samples remained almost as low
as that of the pure polymer and significantly less than that
of a uniform composite structure with CNFs extending
throughout the entire matrix.
3. Further research will be directed into finding the optimal
layered structure that combines the highest dielectric
constant with the least conductivity.
Dr. Zhong for use of her lab, Ms. Lili Sun for her supervision,
instruction, and help in general, and Ms. Lili Newman for help
making the films.
This work was supported by the National Science Foundation’s
Research Experience for Undergraduates program under grant
number DMR-0755055, NSF GOALI-0758251, and a Boeing grant.