EPAPS Supplementary Material
Highly piezoelectric biocompatible and soft composite fibers
J. Morvan2, E. Buyuktanir1,3, J.L. West1.4, A. Jákli1,2
1
2
Liquid Crystal Institute, Kent State University, Kent, OH 44242, USA
Chemical Physics Interdisciplinary Program, Kent State University, Kent, OH 44242, USA
Department of Chemistry, Stark State College, North Canton, OH 44720, USA
4
Department of Chemistry, Kent State University, Kent, OH 44242, USA
Polylactic acid (PLA) (Mw = 186,000, Mw/Mn = 1.76, L/D~24/1) was obtained from Cargill
Dow LLC in Minnetonka, MN. Barium titanate, chloroform, and acetone were obtained from
Sigma-Aldrich (St. Louis, MO) and used as received. PLA was dissolved in a
chloroform/acetone solution (3:1 volume ratio) at 90 wt.% solvent and 10 wt.% of solute
concentration. BaTiO3 nanoparticles with sub-micrometer range size were added to the solution
to result in a 58/42 wt.% PLA/BaTiO3 ratio, adding up to 16wt.% of solid suspension. This was
mechanically stirred (Fisher Model 210T) for two days at room temperature. For the control 10
wt.% pure PLA solutions were prepared in chloroform/acetone and electrospun under same
conditions.
Figure S-1 shows SEM images of neat PLA and PLA/BaTiO3 fibers electrospun from their
solutions. Even though both neat PLA and PLA/BaTiO3 fibers were electrospun at E = 2 kV/cm
electric fields, the morphology and variation in diameters of the fibers are very different. The
neat PLA fibers Figure S-1(a-c) are 1.5 m thick and possess very uniform and smooth surface
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topology. On the other hand, Figure S-1 (d to f) shows that the BaTiO3 nanoparticles are spatially
confined and formed microdomains within the PLA matrix. The diameter of the PLA/BaTiO3
fiber varies between 0.2 and 3 m. The increase of the fiber size, and the appearance of the
porosity are already observed earlier.1,2,3,4 It is also reported that the use of multi-solvent
systems for PLA spinning is one of the important parameters to manipulate the surface
topography of the fibers.5
Figure S-1: SEM micrographs of (a)-(c) neat PLA fibers (electrospun at 20 kV, 10 cm, and 0.6
mL/h); (d)-(f) PLA/BaTiO3 fibers (electrospun at 20 kV, 10 cm, and 0.4 mL/h).
Figure S-2: shows the optical microscopic textures of a thin (d=20m) mat before (a) and
after (b) heating to 130oC for two hours; and in a thick (d=200m) mat containing 42wt% of BT
nanoparticles. One can see that the PLA polymer stays in fiber forms even after keeping the
sample at 130oC for two hours to unpole the BT particles. While the TG of PLA is around 70oC,
melting occurs above 160oC.
2
50m
(a)
( b)
( c)
Figure S-2: Polarizing optical microscopy images of a thin (d=20m) mat before (a) and after
(b) heating to 130oC for two hours; and in a thick (d=200m) mat containing 42wt% of BT
nanoparticles.
The electrospinning set-up consists of a KDS Model 100 programmable syringe pump and a
high-voltage power supply (Gamma High Voltage Research Inc., FL). The syringe was a Popper
Micro-mate 5 mL glass syringe with a 24-gauge metal needle. The fibers were collected on
indium-tin-oxide (ITO) coated glass substrates (Corning, 1.1 mm thick). The PLA/BaTiO3
(52/48 wt.%) suspension was drawn into the syringe and injected with a metal needle using a
syringe pump. The fibers were electrospun by applying 18 - 25 kV voltages at a rate of 0.4 mL/h
between the tip and the collector being 10-15 cm apart.
The movement of the top substrate under application of a DC voltage was measured by a
Leitz Mirau Interferometer as a function of the amplitude and sign of the external voltage applied
to the cell. A green filter was utilized to measure field-dependent changes of the interference
patterns found on the cell surface upon application of the field. The green filter gave a constant
periodicity as well as defining a single wavelength to the interference fringes. A movement of
the top substrate, d , would cause a change in the path length of the light beam reflecting off of
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the substrate. Consequently, a horizontal shift by x of the fringes occurs, as explained in Figure
S-2 (d). The vertical displacement can be related to the horizontal shift by the relation,
d  ( / 2)  (x / L)
(S.1)
where L is the distance between two maxima of the interference pattern. A typical microscope
image captured through the Mirau interferometer is shown in Figure S-2(a). The displacement of
the fiber mat measured at the center of the cell was obtained from video analysis of the
interference pattern shift for green light in comparison with Eq.(S.1). The results represent
mainly linear electromechanical (LEM) properties of the PLA/BaTiO3 fibers.
The spontaneous electric polarization of the ferroelectric BaTiO3 particles is approximately
P~0.15 C/m2 6, so the typical surface charge density of electrospun fibers is only in the range of
10-5nC/m2 , and contributes only to the quadratic electromechanical signal.7 Therefore, the role of
the surface charges was neglected interpreting the LEM results. Similarly, possible bound
charges that may result from non-homogeneous electric fields can be excluded, because they
would appear only in QEM signals.
4
Figure S-3. Experimental set-up for electro-mechanical characterization of fibers. The Mirau
interference patterns (a) are created between a reflection reference mirror in the center of the
objective lens (b) and the fiber cell top substrate (c). When the system is focused upon the
surface, localized interference fringes will appear due to the reflected light. The cell
compression or expansion due to applied field causes a shift in the interference pattern, which is
recorded and analyzed. Molecular structure of PLA is shown on the right (e).
For the AC field measurements a light weight (m = 2.5 g mass) accelerometer (BK 4375 from
Bruel&Kjaer) was attached to the cover plate of the fiber mat by a double-stick tape provided
by Bruel&Kjaer. With a EG&G 7265 Lock in Amplifier, we measured the amplitude and phase
of the electric current I = dQ/dt, where Q is the electric charge induced on the accelerometer by
an acceleration a  d 2 with a conversion coefficient of c=Q/a=0.318pC/(m/s2). As the
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applied voltage was set to sinusoidal waveform, we measured the 1st harmonic of the signal
(linear effect), I  Q   , where  is the angular frequency of the applied voltage, which means
that d  I /  3c  . To avoid any error arising from the electrical signal picked up from the
wires, we used a modified electric current I* = I-Inc, where Inc is the current measured when the
accelerometer was fixed to the vibration isolating table next to the cell. We found that below
150-200 Hz Inc was not negligible (~15%), and the noise was relatively large. For this reason
here we show results obtained only above 200 Hz, where the error is less than 5%.
The film thickness was measured by placing the cell perpendicular to the microscope stage
and viewing the cell using 5X magnification. This allowed the distance between the inner
surfaces of the substrates to be viewed at different edges of the cell. Measurements were taken at
each of the four corners of the cell in order to obtain an average of the cell thickness as well as
local thickness corresponding to the four different spots.
The Young modulus was measured using the Mirau interferometer mounted on the
microscope. Analysis was done by taking a circular ring of known mass and dropping the ring
onto the top substrate of the cell. This causes a change in the cell thickness due to the weight of
the ring compressing the mat. The horizontal displacement of the fringes was measured the same
way as for the DC displacement measurements using the top substrate. The Young modulus can
be calculated by Y  ( F / A)  ( S / s ) given that A  4cm 2 and S  200 m . With the mass of the
ring being m  1.4 g , the force applied to the mat is F  0.014N . The displacement of the
fringes was found to be s  440nm , giving a Young modulus of Y  1.6 104 N / m 2 .
The dielectric studies have been carried out by using a Schlumberger 1260 impedance/gainphase analyzer in the frequency range 200 Hz – 0.1 MHz with the maximum applied measuring
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voltage of 0.1 V (RMS). For the impedance measurements a 4 wired configuration has been used
in order to eliminate the distortive contribution of the connecting wires. To avoid high frequency
distortions, the instrument has been calibrated with a 1k resistor. The dielectric properties of
the composite mats were measured at room temperature. We found that the dielectric constant
was about 1.1±0.03 in the entire frequency range studied. To find the permittivity of a composite
from the dielectric constant of the constituent components, we use the Lichtenecker logarithmic
m
mixing rule8: log    i log  i , where i are the volume ratios of the individual components
i 1
(0.01, 0.04 and 0.95 for the BaTiO3, PLA and air, respectively), and i are the static dielectric
constants of the constituent materials (3000-5000 for the BaTiO3 nanoparticles9-13 , ~3 for the
PLA4, and ~1 for the Air). With these numbers the logarithmic mixing rule provides =1.14 in
good agreement with the measurements.
Neat electrospun PLA fibers showed only compressions and no expansion within the
experimental error of about 5 nm. Along with pure PLA fibers, only compression was seen for
casted films of PLA and BaTiO3. Linear electromechanical responses (LEM) are sensitive to the
sign of the external field, i.e. an external periodic field would result in mechanical vibrations at
the same frequency. The absence of any measurable LEM response is somewhat surprising,
because optically active PLA materials have been known to present shear piezoelectricity (which
is a LEM effect) with the piezoelectric constant of 10 pC/N.4 However, shear piezoelectricity
would not cause any thickness changes, and shear piezoelectricity occurs only in highly
oriented crystalline structure, which can only be obtained by drawing treatment.12-18 Thermal
behavior of pure electrospun PLA fiber mats has been studied in some of our previous work.19
Differential Scanning Calorimetry (DSC) results showed that the pure PLA mats exhibit a cold
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crystallization peak and very low crystallinity (6.6 %). The WAXD analysis also confirmed that
electrospun PLA fibers possess an amorphous structure.19 The reason for the low crystallinity of
PLA fibers is the result of rapid solidification of the fluid jets at high solvent evaporation rates
preventing the formation of ordered domains.20Therefore we understand the lack of LEM in pure
PLA fibers.
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