Page 241 - Carrahers_Polymer_Chemistry,_Eighth_Edition

204 Carraher’s Polymer Chemistry

•
R = k [M][M ] = k [M] (N/2) (6.49)
p p p
13
o
The rate of production of free radicals at 50 C is about 10 radicals/mL per 1 second. Thus, since
5
18
there are 10 micelles for every free radical produced in one second, inoculation of any of the 10
micelles/mL is infrequent. Hence, since propagation is a very fast reaction, long chains are produced
before termination by coupling, which takes place as the result of the entrance of a new oligo radical
in the active micelle. The DP is also proportional to the number of active micelles (N/2).
R k N
(/2)
DP = p = p •− (6.50)
R k [SO ]
i i 4

6.5 FLUORINE-CONTAINING POLYMERS

Polytetrafluoroethylene (PTFE), better known by its trade name of Teflon, was accidentally discov-

ered by Roy J. Plunkett, a DuPont chemist who had just received his PhD from Ohio State 2 years
before. He was part of a group searching for nontoxic refrigerant gases. On April 6, 1938, he and
his assistant, Jack Rebok, had filled a tank with tetrafluoroethylene. After some time, they opened

the value but no gas came out. The tank weight indicated that there was no weight loss—so what

happened to the tetrafluoroethylene. Using a hacksaw, they cut the cylinder in half and found a waxy
white powder. He correctly surmised that the tetrafluoroethylene had polymerized. The waxy white

powder had some interesting properties. It was quite inert toward strong acids, bases, and heat and
was not soluble in any attempted liquid. It appeared to be quite “slippery.”
Little was done with this new material until the military, working on the atomic bomb, needed

a special material for gaskets that would resist the corrosive gas uranium hexafluoride that was one
of the materials being used to make the atomic bomb. General Leslie Groves, responsible for the
U.S. Army’s part in the atomic bomb project, had learned of DuPont’s new inert polymer and had
DuPont manufacture it for them.
Teflon was introduced to the public in 1960 when the fi rst Tefl on-coated muffi n pans and frying

pans were sold. Like many new materials, problems were encountered. Bonding to the surfaces was
uncertain at best. Eventually, the bonding problem was solved. Teflon is now used for many other

applications, including acting as a biomedical material in artificial corneas, substitute bones for nose,

skull, hip, nose, and knees; ear parts, heart valves, tendons, sutures, dentures, and artifi cial tracheas.
It has also been used in the nose cones and heat shield for space vehicles and for their fuel tanks.
Over a half million vascular graft replacements are performed every year. Most of these grafts
are made of poly(ethyleneterephthalate) (PET) and PTFE. These relatively large-diameter grafts

work when blood flow is rapid, but they generally fail for smaller vessels.
Polytetrafluoroethylene is produced by the free radical polymerization process. While it has

outstanding thermal and corrosive resistance, it is a marginal engineering material because it is not
easily machinable. It has low tensile strength, resistance to wear, and it has low-creep resistance.
Molding powders are processed by press and sinter methods used in powder metallurgy. It can also
be extruded using ram extruder techniques.
PTFE is a crystalline polymer with melting typically occurring above 327°C. Because it is highly
crystalline, it does not generally exhibit a noticeable T .
g
The C–F bond is one of the strongest single bonds known with a bond energy of 485 kJ/mol.
While it is structurally similar to linear polyethylene (PE), it has marked differences. Because of
the small size of hydrogen, PE exists as a crank-shaft backbone structure. Fluorine is a little larger
(atomic radius of F = 71 pm and for H = 37 pm) than hydrogen, causing the teflon backbone to be

helical and forming a complete twist every 13 carbon atoms. The size of the fl uorine is suffi cient to
form a smooth “protective” sheath around the carbon backbone. The concentration of F end groups is
low in ultrahigh molecular weight PTFE, contributing to its tendency to form crystals.

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