(54c) Cellulose Nanofiber and Polypyrrole Based Conducting Composite Films with Improved Conductivity, Strength, Water Resistance and Electromagnetic Shielding Properties

Polypyrrole (PPy) is
interesting conducting polymer due to its high conductivity, good environmental
stability, ease of synthesis and non-toxic nature. It has applicability in several
areas such as antistatic materials, electromagnetic shielding, biomedical
devices, various biosensing and electronic devices as a substitute for metallic
conductors or semiconductors. However, PPy is
difficult to process unlike the conventional thermoplastic and thermoset
polymers using methods such as melt processing and solution casting. It is
insoluble in common solvents and decomposes before reaching its melting
temperature. Several attempts were carried out in order to improve the
processability of PPy using various polymer matrices
such as rubber, polyamides, polyolefins and polyesters. Recently there has been
increasing attention towards utilization of biobased
materials such as cellulose nanofibers (CNF) as substrate for the preparation
of flexible and conducting composite films. This approach is more sustainable,
renewable and reduces the dependence on the petroleum based non-renewable
resources. Most methods in literature for preparation CNF based conducting
composite films have used in situ polymerization of pyrrole onto nanofibers
followed by film preparation (coded here as ISF). ISF approach produces
flexible conducting nanopaper with good conductivity
but has poor mechanical strength due to loss of hydrogen bonding interaction
between the nanofibers and high porosity of the film. Moreover, these films are
not water resistant which further limits their utility in various
electrochemical applications.

In this research we have developed a
novel and simple approach for improving the electrical and physical properties
of the CNF and PPy based composite films. Two
approaches were employed, in first approach in situ polymerization of pyrrole
was carried out on CNF film to obtain PPy/CNF
composite films while in second approach polyvinyl alcohol (PVA) coated CNF
film was used to obtain PPy/PVA-CNF composite films.
PVA coated CNF film was prior heat treated at 150^oC for 90 min to
increase its water resistance. The SEM images of the resultant composite films
revealed distinct difference in the surface morphology of these films (Figure
1). Highly porous and rough surface was observed for the ISF composite films,
while in situ polymerized films had more uniform PPy
coating compared to ISF. This has resulted in significant improvement in
conductivity of PPy/CNF and PPy/PVA-CNF
composite films (Table1). Higher conductivity in case of PPy/PVA-CNF
than PPy/CNF was due to smoother and more uniform PPy coating resulted from the partially crosslinked PVA
layer on CNF film and absorption of more monomer and oligomers inside the
swollen PVA layer during in situ polymerization. Besides this, weaker
interaction between partially crosslinked PVA and PPy
would retain the good conducting state for PPy
compared to CNF-PPy interphase. PPy/PVA-CNF
and PPy/CNF also has higher mechanical strength
compared to ISF composite films (Table 1). Tensile strength improvement for PPy/PVA-CNF and PPy/CNF compared
to ISF films is due to reduced porosity and improved hydrogen bonding
interaction between the nanofibers in these films. Moreover, the CNF/PPy and CNF/PVA-PPy films
exhibited water stability due to hydrophobic and compact PPy
coating while high porosity and poor interaction between PPy
coated nanofibers in ISF films drastically reduced their water stability.
Further, films were also characterized for their EMI shielding effectiveness
which showed similar trend as conductivity with CNF/PVA-PPy
films showing maximum shielding effectiveness (~15 dB). For a given thickness
this is promising EMI shielding behavior compared to reported values in
literature. These conducting composite films with improved performance have
potential applications in the electronic devices, antistatic and anticorrosive
coatings, intelligent clothes, sensors, flexible electrodes, and tissue
engineering scaffolds.