Page 242 - Carrahers_Polymer_Chemistry,_Eighth_Edition
P. 242

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







                                                                                              9/14/2010   3:39:39 PM
         K10478.indb   205                                                                    9/14/2010   3:39:39 PM
         K10478.indb   205
   237   238   239   240   241   242   243   244   245   246   247