Appl Phys A (2012) 107:589–596
DOI 10.1007/s00339-012-6834-5
INVITED PAPER
Chemical and physical surface modification of PTFE
films—an approach to produce stable electrets
D. Rychkov · M. Yablokov · A. Rychkov
Received: 12 October 2011 / Accepted: 10 February 2012 / Published online: 25 February 2012
© Springer-Verlag 2012
Abstract The thermal stability of positive charge has been
investigated in chemically and physically treated polyte-
trafluoroethylene (PTFE) films. It has been found that vir-
gin films, oriented by the manufacturer, display an increase
in thermal stability of positive charge with an increase of
the initial value of surface potential. Such an anomalous
behavior is explained by the influence of a negative tri-
bocharge, trapped some small distance below the surface. In
PTFE samples treated with orthophosphoric acid and with
tetraethoxysilane, a considerable improvement of positive
charge stability has been achieved, but no influence of the
initial value of surface potential has been observed. How-
ever, this influence should be kept in mind when comparing
charge stability in virgin and modified samples. In nonori-
ented PTFE films, no influence of the initial value of sur-
face potential on charge stability has been observed. This
could be due to the fact that these films did not possess a
noticeable negative tribocharge. After the treatment in glow-
discharge defluorination, oxidation and appearance of polar
D. Rychkov ()
Applied Condensed-Matter Physics, Department of Physics and
Astronomy, Faculty of Science, University of Potsdam,
Karl-Liebknecht-Strasse 24-25, 14476 Potsdam-Golm, Germany
e-mail: rychkov@uni-potsdam.de
Fax: +49-331-9775494
M. Yablokov
Laboratory of Thermostable Plastics, Institute of Synthetic
Polymeric Materials, Russian Academy of Sciences,
Profsoyuznaya st. 70, 117393 Moscow, Russia
e-mail: yabl1@yandex.ru
A. Rychkov
Department of Technical Science, Herzen State Pedagogical
University of Russia, Moika emb. 48, 191186 St-Petersburg,
Russia
e-mail: richkov_andrey@mail.ru
groups have been detected on the surface. These changes
in chemical composition of a PTFE surface resulted in a
noticeable improvement in thermal stability of positively
charged electrets. This improvement is attributed to the for-
mation of deeper traps on the modified surface.
1 Introduction
It is known [1] that nonpolar polymers are the most suitable
material for the production of space-charge electrets. A de-
cisive role plays their relatively low conductivity, that allows
for the long storage of the charges injected into the polymer
and captured at energetically deep traps. The lifetime τ
t
of
the charge captured at a deep trap with an activation energy
E
t
and frequency factor ω, equals:
τ
t
= ω
−1
exp
E
t
kT
(1)
and, depending on E
t
, ω and the temperature, this time can
be very long. This, however, does not mean that a character-
istic time of charge decay τ in space-charge electrets would
have the same high value. This is only possible when the
conductivity γ in the bulk of the polymer film (γ = enμ,
where e is elementary charge, n and μ are the concentration
and mobility of the charge carriers, respectively) is very low.
In fact so low, that the Maxwell relaxation time
τ
M
=
0
γ
τ
t
(2)
In this case, the charges captured at the traps cannot be neu-
tralized by means of the intrinsic conductivity of the poly-
mer.
In nonpolar polymers, condition (2) is fulfilled due to
the fact that the concentration n of charge carriers is small