Page 255 - Carrahers_Polymer_Chemistry,_Eighth_Edition

218 Carraher’s Polymer Chemistry

6.12 PLASMA POLYMERIZATIONS

Organic and inorganic molecules can be placed in the vapor state either through heating, low pres-
sure, simply spraying, or some combination of these. These molecules are then subjected to some
ionizing energy that forms active species that react with one another, eventually depositing them-
selves on a surface. Often the products are polymeric with complex structures. The term plasma
polymerization is generally used to describe the process resulting in surface film formation while

the term deposition is generally used to describe the deposition of powdery particles formed in the
gas phase. Others describe plasma polymerization as that polymerization that occurs at high rates
in the gas phase, resulting in powder formation and deposition as any sorption occurring on the

surface. In truth, it is difficult to separate the two reaction sequences because active molecules can
react both in the gaseous phase upon collision and on the surface.
Plasma environments are often created using plasma jets, ion beams, glow discharges, corona
discharges, laser induced plasmas, and electron beams. Low-temperature plasmas can also be cre-
ated using radio frequency, audio frequency, microwave, or direct-current energy sources. In general
terms, the molecules enter the reactor as neutral species. They become reactive species as electronic
energy is transferred to them. The reactive species can be ions, free radicals, or excited molecules.
Reaction can occur in the gaseous phase and/or at the solid surface. Commercially, reactors often
consist of a low pressure glow discharge of reactive species. Because a small amount of electromag-
netic radiation is emitted in the visible region, the term glow discharge was derived.
This approach allows the deposition of thin films at low temperatures. By comparison, polymer

deposition generally requires very high temperatures. For instance, the chemical vapor deposition
o
of silicon nitride requires a temperature of about 900 C whereas the plasma chemical deposition
o
requires a temperature of only 350 C.
A number of typical polymer-forming monomers have been polymerized using plasma polymer-

ization, including tetrafluoroethylene, styrene, acrylic acid, methyl methacrylate, isoprene, and eth-
ylene. Polymerization of many nontypical monomers has also occurred, including toluene, benzene,
and simple hydrocarbons.

Plasma films are usually highly cross-linked, resistant to higher temperatures, resistance to abra-
sion and chemical attack, and adhesion to the surface is high. Adhesion to the surface is generally
high both because the growing polymer complex can fit the surface contour and thus “lock-itself

in” (physical adhesion) and because in many instances, the species are active enough to chemically
react with the surface molecules to chemically bond to the surface. The surface can be prepared so
that the chemical reaction is enhanced.
Plasma surface treatment of many polymers, including fabrics, plastics, and composites, often

occurs. The production of ultrathin films via plasma deposition is important in microelectronics,
biomaterials, corrosion protection, permeation control, and for adhesion control. Plasma coatings
are often on the order of 1–100 nm thick.

6.13 SUMMARY

1. For classical free radical polymerizations the rate of propagation is proportional to the con-
centration of monomer and the square root of the initiator concentration. Termination usually
occurs through a coupling of two live radical chains but can occur through disproportion-
ation. The rate of termination for coupling is directly proportional to initiator concentration.
The DP is directly proportional to monomer concentration and inversely proportional to the
square root of the initiator concentration.
2. The first chains produced are high molecular weight products. Within the polymerizing

system, the most abundant species are the monomer and polymers chains.
3. Increasing the temperature increases the concentration of free radicals, thus decreasing the
chain length. Increasing the temperature increases the rate of polymer formation.

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