Giuseppe Ajroldi

New fluorinated thermoplastic elastomers

New 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 100°C), thanks to their multiphase morphology consisting of a continuous fluorinated phase with a very low Tg (−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.

Fluoroelastomers-dependence of relaxation phenomena on composition

Abstract The dynamic-mechanical properties of some fluoroelastomers were determined as a function of composition at low frequency (≈ 1 Hz), by means of a free oscillation torsion pendulum, between −180°C and the softening point. Vinylidenefluoride (VDF)-hexafluoropropene (HFP) copolymers of molar composition 0–39% HFP and terpolymers of VDF and HFP with up to 30mol% tetrafluoroethylene (TFE) and a constant VDF to HFP molar ratio of 3.4 were considered. Two relaxation processes typical of the amorphous phase were found. The first, located at about −87°C, is related to local motions and the transition temperature was found to be independent of composition for copolymers, while it depends on TFE molar content for terpolymers. The second is related to the glass transition and the transition temperature depends on the composition. However, for semicrystalline copolymers the double glass transition phenomenon was observed. When crystallinity goes to zero at about 20 mol% HFP, only one transition is observed. It was also found that ordered structures can take place for terpolymers when TFE molar concentration exceeds 20%. The crystal disorder transition of pure PVDF (75°C) is observed also for low HFP concentrations but the transition temperature is strongly reduced. Analogies between the VDF-HFP and E-P systems are also discussed.

Synthesis methods of fluorinated polyurethanes. 1. Effects on thermal and dynamic-mechanical behaviours

Abstract Fluorinated polyurethanes (FPUs) were synthesised from 4,4′-methylenebis(phenylisocyanate) (MDI), 1,4-butandiol (BDO), as a chain-extender, and soft perfluoropolyetheric block (ZDOLTX), by a two-step polymerisation technique, following specific procedures to obtain chains with a regular-segmented structure or polymers characterised by having high hard (MDI–BDO) segment polydispersity. Calorimetric and dynamic mechanical data, reported in this first part of our work, show that, independently of the synthesis method, the polymers are always characterised by a heterogeneous structure. This is true not only at molecular level but also at a macroscopic scale, in which the continuous soft phase contains different populations of crystalline hard domains. However, the crystallinity degree decreases on increasing the weight fraction of the soft phase. The so-called ‘regular synthesis’ seems to favour the MDI–BDO–MDI sequence, which is the largest constitutive part of the hard phase, but the macrostructure is less regular than that present in the other FPUs.

Molecular weight dependence of some rheological and thermal properties of perfluoropolyethers

The glass transition temperatures and the viscosities (over a very large temperature range) have been measured as a function of molecular weight, for two different series of perfluoropolyethers. The results have been interpreted according to well-known free-volume models. The Williams-Landel and Ferry equation describes the viscosity data with values of fg (the fractional free volume at Tg) close to 0.04 and with values of αf (the free-volume expansion coefficient) close to 7 × 10−4°C−1. The critical molecular weight, Mc, is approximately 5000–6000 for perfluoropolyethers from hexafluoropropene and 7800 for perfluoropolyethers from tetrafluoroethene.

Synthesis methods of fluorinated polyurethanes. 2. Effects on morphology and microstructure

Abstract A series of fluorinated polyurethanes, characterized by a segmented structure containing hard segments based on 4,4′-methylenebis(phenylisocyanate) (MDI) and 1,4-butandiol (BDO), were analyzed by small- and wide-angle X-ray scattering and transmission electron microscopy (TEM) in order to study the influence of the synthesis and of the soft and hard segments length on the structure. Theoretical models of lamellar morphology were used to fit calculated small-angle scattering patterns to the experimental ones. TEM and small-angle X-ray scattering analysis shows that these polymers are characterized by a heterogeneous structure, not only at the molecular level, but also at a macroscopic scale.

Some physical chemical properties of α–ω dihydroperfluoropolyethers

Abstract This paper describes the physico-chemical characterization and some properties of several α–ω dihydroperfluoropolyethers. Characterization has been carried out on the following properties: vapour pressure, density and viscosity as a function of temperature, glass transition temperature, refractive index and dielectric constant. The boiling point T b and the vaporization enthalpy Δ H v (calculated from vapour pressure data by means of the Clausius–Clapeyron equation) increase with the number of the backbone atoms N ; both T b and Δ H v are higher than the corresponding values for compounds with the non-polar chain end CF 3 and the difference decreases, but does not disappear, when the number of chain atoms is as large as 10–15. The cohesive energy density and the solubility parameter δ are also found to be larger. It is shown that the increase with respect to CF 3 terminated homologues has to be attributed, as expected, to the polar component due to the CF 2 H end group. The p – V – T properties can be predicted through a modified Peng–Robinson equation, where the critical temperature and pressure are obtained by group contributions. The comparison with the experimental data shows that the best group contribution data are those reported by Joback. The density increases and the expansion coefficient decreases with increasing chain length, as expected; at the same chain length the density is lower for materials with a CF 2 H end group; the difference decreases with increasing N . The viscosity also increases with molecular weight; its dependence on temperature is described by the well-known Arrhenius equation. The activation energy is a function of Δ H v , as predicted by the hole theory of Eyring for simple liquids. The glass transition temperatures are very low and depend both on the chain length and on the structure of the compound. While the refractive index is a weak function of chain length and structure, the dielectric constant is a strong function of both variables; a relationship between the dielectric constant and the polar component of the solubility parameter is suggested. Finally the group contributions of the CF 2 H group to the molar volume, vaporization energy, molar refractivity and molar polarizability have been determined, so that these data can be used to predict properties of new molecules.

The viscosity–molecular weight relationships for diolic perfluoropolyethers

Abstract It is shown that for diolic perfluoropolyethers having short chain lengths the “anomalous” (non monotonic) relationship between isothermal melt viscosity and molecular weight, reported in recent papers, is not anomalous at all but fully explainable when one takes into account the fact that the glass transition of the samples Tg is itself a function of the molecular weight. For low molecular weight polymers the viscosities have to be compared not at the same T but at the “normalised” temperature T−Tg.

The glass transition temperature of polymer-diluent systems

Abstract The theoretical treatment proposed by Couchman to describe the compositional dependence of T g for polymer-diluent systems has been applied, in its several forms, to selected pairs for which heat capacity data are found in the literature. The most accurate equation gives the least satisfactory description of the experimental T g data. This result seems to be accounted for by the assumption, first suggested by Couchman, that in some cases the entropy of mixing is not continuous at T g .

Glass transition of binary systems: peculiarities at low temperature

Abstract The present paper refers to the well known fact that the glass transition-composition curves of miscible binary systems (e.g. polymer-diluent and polymer-polymer mixtures) often exhibit a singular point when the Tg values of the two components are far apart. Two different equations apparently apply above and below the temperature of this singularity. Starting from Couchmans entropic treatment of the glass transition of mixtures, and from Angells suggestion for the calculation of T0 (‘ideal glass transition temperature’ of the component with higher Tg), it is shown that below T0 an equation should be applicable to binary mixtures which differs considerably from the equations normally used in the literature. The new equation gives a satisfactory description of selected experimental data in the proximity of the component with lower Tg.

Structure–Property Relationships in Perfluoropolyethers: A Family of Polymeric Oils

Among the different classes of fluorinated organic molecules the perfluoropolyethers (PFPEs), composed entirely of carbon, fluorine and oxygen, represent a well known family that is of interest to the researcher and the user for the novelty and uniqueness of their chemical and physical properties. PFPEs have therefore become, in the past two decades, very important technically and commercially for a variety of applications, the most diffuse of which is related to their use as high quality lubricants.