Flexible Nanogenerator and Nano-Pressure Sensor Based on Nanofiber Web of PVDF and its Copolymers - Soft Fibrillar Materials: Fabrication and Applications - page 244

direction during electrospinning. The second case is that the preferential dipole
orientation can take place during electrospinning. Considering Figure 8.14a, the
second case is thought to give a greater ratio of A ν as ,CH 2 / A ν s ,CH 2 or A ν as ,CF 2 / A ν s ,CF 2
after electrospinning than in the case of random dipoles around a chain axis,
where A ν as is the absorption intensity due to the CH 2 or CF 2 asymmetric stretching
vibrationmode and A ν s is the absorption intensity due to the CH 2 or CF 2 symmetric
stretching vibration. Therefore, it is possible to distinguish these two extreme cases
by means of perpendicular polarized IR spectroscopy if we can prepare nanofibers
with a random dipole orientation even after electrospinning. Unfortunately, that is
not the case in a real situation. However, since it is well known that heating above
the T c is usually used to depolarize the poled P(VDF-TrFE) film [25], we selected
the method to heat the electrospun nanofiber at a temperature of 130 ◦ C, which
is well below the melting temperature ( T m ∼

147 ◦ C) but above the T c ∼ 115 ◦ C, to

obtain the nanofiber webs with a random dipole orientation.
Figure 8.15 shows FTIR spectra of the as-electrospun and heat-treated (130 ◦ C
for 15min) P(VDF-TrFE) nanofiber webs in different frequency ranges. In order to

As-electrospun
Heat treated (15 min at 130

0.5

° C)

B 2

B 1

A 1 and B 2 are sensitive
to dipolar orientation

0.4

0.3

B 2

A 1

A 1

0.2

0.1

0.0

1400 1200 1000 800

Wavenumber (cm − 1 ) (a)

0.3

0.016
0.014
0.012
0.010

As-electrospun
Heat treated (15 min at 130 ° C)

As-electrospun
Heat treated (15 min at 130 ° C)

B 2

A 1

ν s (CH 2 )

B 2 ν as (CH 2 )

0.2

A 1

0.008
0.006
0.004
0.002
0.000
3060

0.1

0.0

3040 3020 3000 2980

Wavenumber (cm − 1 )

2960 2940 2920

900 850 800

(b)

Wavenumber (cm − 1 ) (c)

Figure 8.15 Perpendicularly polarized FTIR spectra of P(VDF-TrFE) nanofiber web
(as-electrospun and heat treated at 130 ◦ C for 15min) in different frequency regions;
(a) 1450-700 cm − 1 ; (b) 940-775 cm − 1 ; and (c) 3060-2920 cm − 1 .