New fluoro-modified thermoplastic polyurethanes containing perfluoropolyether (PFPE) blocks were synthesized by the reaction of a fluorinated macrodiol with a hydrogenated prepolymer based on poly(tetramethylene glycol) and 4,4Ј-methylene-bis-phenylisocyanate, followed by subsequent chain extension with 1,4-n-butanediol. This multistep bulk process opened the way for a new family of polymeric materials whose tensile properties appear to be excellent and unchanged in comparison with the corresponding unmodified hydrogenated polymers. Dynamic mechanical analysis and differential scanning calorimetry revealed peculiar characteristics. These polymers showed an unusual multiphase structure in which not only the hard and the hydrogenated soft segments were self-organized, but also a second soft phase, constituted by the PFPE segments, was present. Moreover, an easier hard-phase segregation and self-organization were observed, as was evidenced by the higher melting temperatures of the semicrystalline phase. This unique characteristic combined with a selective enrichment of PFPE segments to the surface, as indicated by the unusually low coefficient of friction data and superior chemical resistance.

SYNOPSISNew fluorinated thermoplastic elastomers (FTE) with perfluoropolyether (PFPE) blocks have been synthesized by reacting a fluorinated macrodiol with aromatic diisocyanates in the presence of a solvent, followed by subsequent chain extension with low molecular weight aliphatic or aromatic diols. Tensile properties measurements and dynamical-mechanical analysis (DMA) have been carried out and the relationship between chemical structure and final properties has been determined. These new thermoplastic fluorinated polyurethanes show an elastomeric behavior over a wide temperature range (between -75 and lOO"C), thanks to their multiphase morphology consisting of a continuous fluorinated phase with a very low T, (-120°C) and a dispersed high melting hydrogenated hard phase, as verified by a calorimetric and dynamic-mechanical analysis. At the same time, some of the outstanding properties of fluorinated oligomers, such as chemical inertness and low surface tension, are retained in the final polymers. Thanks to these characteristics this new class of polymeric materials provides new opportunities for the application of thermoprocessable elastomers in advanced technological fields. 0 the soft segments or their crystallization tendency; this makes conventional polyurethanes unsuitable for temperatures lower than -40°C.PUTEs are block copolymers in which the soft segments are based on polyester or polyether chains; the polyester types offer some advantages in terms of mechanical properties, high-temperature performance, and thermal stability, while the polyetherbased materials have better resistance to hydrolysis and exhibit low-temperature elastomeric behavior (when relatively high molecular weight polyether diols areOn the basis of these considerations it seems attractive to develop new PUTEs offering the basic advantages of thermoplastic elastomers and exhibiting a lower Tg of the elastomeric block, adequate tensile properties, improved chemical resistance, and a lower surface tension. In principle, these improvements should be achievable by introducing fluorinated blocks into the polymer chains. As a matter of fact, thermoplastic fluoropolymers are commercially succesful thanks to their unique balance of properties, such as low surface energy, low coefficient of friction, nonflammability, low dielectric constant, and high solvent and chemical resistance.'l 311

The cohesive energy density (CED) of linear hydrogenated and perfluorinated paraffins and ethers has been investigated with regards to its dependence on chain length and oxygen content. For high molecular weight compounds (i.e., polymers) the CED has been obtained by extrapolation or by the group contribution method. The oxygen contribution to cohesive energy is found to be very different in the hydrogenated ethers with respect to the perfluorinated ones, and, correspondingly, the dependence of CED on the oxygen to carbon ratio (O/C) is the opposite for the two series. A similar trend, previously described for the glass transition temperature (Tg) is therefore roughly interpretable as related mostly to CED, although intramolecular flexibility parameters also play a role.

No abstract

Three copolymeric perfluoroethers with the structure CF3[(OCF2CF2)p(OCF2)q] OCF3, having different p/q ratios, have been fractionated. The fractions obtained have been characterized by Gel Permeation Chromatography and 19F‐NMR. The viscosity η the specific volume v and the glass transition temperature, Tg have been measured by standard techniques for all the above samples as well as for some other perfluorinated polyethers.The temperature dependence of viscosity of the unfractionated samples is described by the W.L.F. equation. The values of fg (fractional free‐volume at Tg) and of af (free‐volume expansion coefficient) are independent of composition, for p/q ratios from 0.53 to 1.15. The critical molecular weight, Mc, is of the order of 8–9,000. From the molecular weight dependence of specific volume, the contribution to the molar volume of the in‐chain CF2 group and the excess molar free volume of the chain ends have been determined. The limiting value of Tg for an infinite molecular weight polymer was found to depend linearly on the compositional ratio O/C and the extrapolated values for polytetrafluoroethylene and for the homopolymer (CF2O)n were found to be respectively 200 K and 120 K.