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Free Radical Chain Polymerization 205
The electron density of PE and PTFE are also different. The electronegativity value for C is 2.5,
F = 4.0, and for H = 2.1. Thus, the electron density on the fluorine surface of PTFE is greater than
that for PE.
For high molecular weight linear PE, the repeat unit length is about 0.254 nm forming crystalline
portions with a characteristic thickness of about 10 nm. The chain length for tough solids from PE
is about 4.5 times the crystalline thickness. Thus, tough solids occur at molecular weights greater
than 5,000 g/mol or chain lengths greater than about 45 nm. In comparison, the repeat unit length
for PTFE is about 0.259 nm. The crystalline thicknesses for PTFE are about 100–200 nm or much
thicker than for PE. Chain lengths for tough solids are about 4.5 times the crystalline thickness.
Thus, much greater chain sizes, about 200,000–400,000 Da, are required to produce tough solids.
The greater size of the crystalline portions also probably contributes to its higher T and greater
m
difficulty in processing. The crystal thickness of PTFE is about 10–20 times the crystal thickness
found for most other semicrystalline polymers such as PE.
At low molecular weights, PTFE is waxy and brittle. To achieve good mechanical properties
ultrahigh molecular weights on the order of 10 million dalton is usually needed. These long chains
disrupt crystal formation because they are longer than a single crystal. But the long chain lengths
connect the crystals together adding to their strength. But these long chains result in extremely high
viscosities so that ultrahigh molecular weight PTFE does not flow when melted and is thus, not melt
processable. Form restrictive and costly methods are used to produce products from PTFE.
While vinyl fluoride was prepared in about 1900, it was believed resistant to typical “vinyl” poly-
merization. German scientists prepared vinyl fl uoride through reaction of acetylene with hydrogen
fluoride in the presence of catalysts in 1933 (Equation 6.51).
H F + HC CH H C (6.51)
2
F
It was not until 1958 that DuPont scientists announced the polymerization of vinyl fl uoride form-
ing poly(vinyl fluoride) (PVF); Equation 6.52. Polymerization is accomplished using peroxide cata-
lysts in water solutions under high pressure.
R
H 2 C (6.52)
F
R F
In comparison to PTFE, PVF is easily processable using a variety of techniques used for most
thermoplastic materials. It offers good flame retardancy, presumably due to the formation of HF that
assists in the control of the fire. Thermally induced formation of HF is also a negative factor because
of its toxicity. As in the case of PVC, elimination of the hydrogen halide (HF) promotes formation
of aromatic polycyclic products that themselves are toxic.
The difference in electronegativity between the adjacent carbons because of the differing elec-
tronegativities of H and F results in the C–F bond being particularly polar, resulting in it being
susceptible to attack by strong acids. The alternating bond polarities on the PVF chain gives a tight
structure, resulting in PVF films having a low permeability. This tight structure also results in good
resistance, resistance to cracking, and resistance to fading.
Friction and wear are important related characteristics. If a material has a high friction then it will
generally have a shorter wear time because water or other friction event chemicals pass over the mate-
rial with the higher friction causing greater wear. The friction eventually “wears” away polymer chains
layer-by-layer. The engineering laws of sliding friction are simple. According to Amontons’ laws, the
friction F between a body (rain drop, wind, or board rubbing against the material) and a plane surface
(the polymeric material) is proportional to the load L and independent to the area of contact A. The
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