Plasma Polymerization

331 Pages·1979·5.12 MB·English

by  Mitchel Shen and Alexis T. Bell (Eds.)

Plasma Polymerization
Mitchel Shen and
Alexis T. Bell, EDITORS
University of California, Berkeley
Based on a symposium
sponsored by the
ACS Division of Polymer Chemistry
at the 176th Meeting of the
American Chemical Society,
Miami Beach, Florida,
September 15-16, 1978.
108
ACS SYMPOSIUM SERIES
AMERICAN CHEMICAL SOCIETY
WASHINGTON, D. C. 1979
In Plasma Polymerization; Shen, M., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
Library of Congress CIP Data
Symposium on Plasma Polymerization, Miami Beach,
Fla., 1978.
Plasma polymerization.
(ACS symposium series; 108 ISSN 0097-6156)
Includes bibliographies and index.
1. Polymers and polymerization—Congresses. 2.
Plasma chemistry—Congresses.
I. Shen, Mitchel C., 1938- . II. Bell, Alexis T.,
1942- . III. American Chemical Society. Division
of Polymer Chemistry. IV. Title. V. Series: American
Chemical Society. ACS symposium series; 108.
QD380.S938 1978 547'.84 79-15252
ISBN 0-8412-0510-8 ACSMC8 108 1-344 1979
Copyright © 1979
American Chemical Society
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In Plasma Polymerization; Shen, M., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
ACS Symposium Series
M. Joa
Advisory Board
Kenneth B. Bischoff James P. Lodge
Donald G. Crosby John L. Margrave
Robert E. Feeney Leon Petrakis
Jeremiah P. Freeman F. Sherwood Rowland
E. Desmond Goddard Alan C. Sartorelli
Jack Halpern Raymond B. Seymour
Robert A. Hofstader Aaron Wold
James D. Idol, Jr. Gunter Zweig
In Plasma Polymerization; Shen, M., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
FOREWORD
The ACS SYMPOSIUM SERIES founded i 1974 t provid
a medium for publishin
format of the Series parallels that of the continuing ADVANCES
IN CHEMISTRY SERIES except that in order to save time the
papers are not typeset but are reproduced as they are sub
mitted by the authors in camera-ready form. Papers are re
viewed under the supervision of the Editors with the assistance
of the Series Advisory Board and are selected to maintain the
integrity of the symposia; however, verbatim reproductions of
previously published papers are not accepted. Both reviews
and reports of research are acceptable since symposia may
embrace both types of presentation.
In Plasma Polymerization; Shen, M., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
PREFACE
Although the formation of high-molecular-weight materials through the
^ ^ use of ionized gaseous plasma has been known for many years, the
detailed mechanism of the process is still not fully understood. It is
obviously quite different from conventional polymerization since for
plasma polymerization monomers need not contain particular functional
groups, and the polymer
relation to the starting monomers
polymerized, but the resulting polymer has a chemical formula closer
to C H . Plasma polymerization has been further classified into "plasma-
2 3
state polymerization" and "plasma-induced polymerization." More re
cently, a hybrid type of polymerization using a plasma to initiate
conventional polymerizations has been discovered. These diverse and
interesting phenomena, as well as the properties and characteristics of
the materials produced therefrom, are fully discussed in this Symposium.
The twenty chapters included in this volume can be conveniently
divided into the following groups: review; plasma polymerization of
hydrocarbons; plasma polymerization of fluorocarbons; plasma polymeri
zation of organometallic systems; plasma-initiated polymerization; and
applications of plasma polymerization. Though the emphasis of this
Symposium is on the fundamental aspects of plasma polymerization, we
should not lose sight of the fact that it is the potential applications of
this technique that has stimulated the efforts in basic research. Potential
applications for plasma-polymerized films include membranes for reverse
osmosis, protective coatings for optical components, and insulating layers
for semiconductors.
We should like to take this opportunity to thank the contributors to
this volume for their efforts, and the expert referees whose anonymity
did not interfere with their conscientious and objective judgment in
upholding the standards of scholarship.
MITCHEL SHEN
ALEXIS T. BELL
University of California
Berkeley, California
April 9, 1979
vii
In Plasma Polymerization; Shen, M., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
1
A Review of Recent Advances in Plasma Polymerization
MITCHEL SHEN and ALEXIS T. BELL
Department of Chemical Engineering, University of California, Berkeley, CA 94720
A plasma is a partially ionized gas composed of
ions, electrons an
matter that can be
as flames, electrical discharges, electron beams,
lasers or nuclear fusion. The technique of most in­
terest to plasma polymerization is the glow discharge,
in which free electrons gain energy from an imposed
electrical field, and subsequently loses i t through
collisions with neutral molecules i n the gas. The
transfer of energy to gas molecules leads to the forma­
tion of a host of chemically reactive species, some of
which become precursors to the plasma polymerization
reaction.
The plasma created by a glow discharge possesses
average electron energies-in the..range of 1-10 ev and
electron densitites of 10 - 10 /c.c. In addition,
the electron temperature (T ) of the plasma is not
equal to the gas temperaturl (T ) but has a Τ /Τ
ratio of 10 - 100. I t is therefore possible fof g a S
plasma polymerization to proceed at near ambient tem­
peratures i n the presence of electrons to rupture co-
valent bonds i n the gas molecules. Thus plasmas pro­
duced by the glow discharge are called non-equilibrium
plasmas, i n contradistinction to equilibrium plasmas
created by arcs or plasma jets where Τ = Τ . The
very high temperatures i n these plasma! (i8aShousands
of degrees Kelvin) render them unsuitable for plasma
polymerizations, since polymers produced under these
conditions will be rapidly degraded.
Since de Wilde (1) and Thenard (2) first reported
the formation of solid products i n a plasma of
organic vapor more than a centrury ago, many workers
in the field of plasma organic chemistry have observed
the presence of high molecular weight materials as re­
action by-products. These materials adhered tightly
0-8412-0510-8/79/47-108-001$08.25/0
© 1979 American Chemical Society
In Plasma Polymerization; Shen, M., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
2 PLASMA POLYMERIZATION
to the walls of reaction vessels, and were insoluble
in most solvents. They were considered a nuisance un­
t i l Goodman (i) demonstrated that a 1 mm thick plasma
polymerized styrene film deposited on titanium foil
made a satisfactory dielectric for a nuclear battery.
Since that time plasma-derived polymers have been sug­
gested for numerous applications because of their
unique physical properties, ease of preparation, and
propensity for the formation of thin pinhole-free films.
A wide variety of organic and organometllic com­
pounds can be polymerized to form a thin film on a
substrate placed in a glow discharge. Although the
rate at which a monomer will polymerize depends on a
large number of parameters, one of the most unique fea­
tures of plasma polymerizatio i tha d
not possess such reactiv
bonds to be polymerizable. For instance, ethane
and benzene have been readily polymerized in the
plasma. Furthermore, polymers have also been prepared
from inorganic starting materials. Hollahan and
McKeever (4) reported that polymers were formed in an
electrodeless discharge sustained i n mixtures of CO,
H / and Ν · The structure of those materials was
2 2
found to resemble that of proteins.
The purpose of this paper i s to discuss some of
the recent advances in our understanding of the kin­
etics and mechanism of plasma polymerization, the
structure and properties of plasma polymers and some of
their potential applications. I t i s not intended to
be exhaustive, as earlier reviews (5-10) are already
available. Interested readers are referred to the
literature cited for further details.
Kinetics
The rate of plasma polymerization depends on the
nature of the monomer gas. In addition, such para­
meters as flow rate, pressure, power, frequency,
electrode gap and reactor configuration also strongly
influence the polymerization rate for a given monomer.
Generally at low flow rates there i s an abundance of
reactive species so the polymerization rate i s limited
only by the availability of monomer supply. At high
flow rates, however, there i s an overabundance of
monomer concentration and the polymerization rate now
depends on the residence time. At intermediate flow
rates these two competing processes result i n a
maximum. This behavior i s illustrated i n Figure 1 for
ethane, ethylene, and acetylene (11). These data also
demonstrate the effect of increased unsaturation in
In Plasma Polymerization; Shen, M., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
1. SHEN AND BELL Review of Recent Advances 3
Figure 1. Rates of plasma polymerization of acetylene, ethylene, and ethane as
a function of monomer flow rate
In Plasma Polymerization; Shen, M., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
4 PLASMA POLYMERIZATION
monomers on the polymerization rate. At comparable
reaction conditions, acetylene with a triple bond
polymerizes an order of magnitude faster than ethylene
which has a double bond; while ethylene in turn is
much more reactive than the saturated ethane.
The polymerization rates for three olefinic hydro
carbons (11) are shown in Figure 2. Isobutylene is
seen to polymerize more slowly than propylene, while
ethylene polymerizes faster than either isobutylene or
propylene by an order of magnitude. These data indicate
that in a given homologous series of hydrocarbons,
higher polymerization rates are favored by lower
molecular weights. Figure 3 compares three monomers
containing four carbon atoms. Here butadiene with two
double bonds polymerize
either cis-2-butene
bond (11).
Yasuda (12) surveyed 28 monomers and found that
monomers containing aromatic groups, nitrogen (e.g.,
-NH, -NH , -CN), silicon and olefinic double bonds are
more polymerizable while those containing oxygen (e.g.
-C=0,-0-,-OH), chlorine, aliphatic hydrocarbon and cyc
lic hydrocarbons tend to decompose. Brown (13.) report
ed in his studies of a series of vinyl halides that the
dihaloethylenes polymerize more rapidly than the corres
ponding monohalides and that chlorides and bromides
polymerize more rapidly than the fluorides. Kobayashi,
et. a l . (14) found that the additons of certain halo-
genated compounds to hydrocarbon monomer streams often
dramatically increases the polymerization rate. Thus,
these halogenated compounds may be considered to act
as gas phase catalysts for the plasma polymerization
of hydrocarbons.
The rate of plasma polymerization generally in
creases with increasing power, until at high power den
sities when i t becomes nearly independent of power.
This is illustrated in Figure 4 for tetrafluoroethyl-
ene (14). The interelectrode gap also exerts a similar
effect. Figure 5 shows that as the gap is narrowed
(higher electron density) the rate increases. For small
electrode gaps there is significant powder formation,
which is typical of the product formed at high polymer
ization rates (15). The effect of increased pressure
is to decrease.the rate of polymerization (15,16) as
shown in Figure 6 for styrene (16). The kinetics of
plasma polymerization is often affected by the reactor
configuration (9,17,18). Kobayashi, et. a l . (17)
showed that under otherwise identical conditions, the
polymerization rates of ethylene are not the same for
those using tubular type and bell-jar type reactors.
In Plasma Polymerization; Shen, M., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1979.
SHEN AND BELL Review of Recent Advances
τ 1 1 1 1 1 1 1 r
Flow Rate (STPcmVmin)
Figure 2. Rates of plasma polymerization of ethylene, propylene, and isobutyl­
ene as a function of monomer flow rate (11)
Figure 3. Rates of plasma polymerization of butadiene, cis-2-isobutylene, and
isobutylene as a function of monomer flow rate (11)
In Plasma Polymerization; Shen, M., et al.;
ACS Symposium Series; American Chemical Society: Washington, DC, 1979.