Françoise Méchin

Relationships between synthesis and mechanical properties of new polyurea materials

New segmented polyureas were prepared from 4,4′-diisocyanato dicyclohexylmethane and amino terminated polyoxypropylene. Different cycloaliphatic and aromatic diamines were used as chain extenders as well as to synthesize the corresponding model hard segments (HSs). The competition between polycondensation and HS crystallization requires a sufficiently fast reaction (e.g., cycloaliphatic diamines associated with a catalyst, if necessary in solution), although low reactivity monomers would be necessary to avoid demanding processing techniques such as reaction injection molding; otherwise, materials with isolated (i.e., not chemically linked) HS are obtained and they display poor mechanical properties. In contrast, when an appropriate synthesis procedure is used, elastomers with high molar masses and narrow molar mass distributions can be obtained. These characteristics, associated with the high melting temperature of the hard domains, result in very good mechanical behavior, up to high enough temperatures (∼ 180–190°C). The presence of low molar masses can be responsible for a rather low but continuous energy dissipation between the relaxations of the soft and hard domains (and particularly at room temperature), but can be well limited by a short thermal treatment at high temperature.

Characterization, structural transitions and properties of a tightly crosslinked polythiourethane network for optical applications

Abstract A new polythiourethane thermosetting system based on a diisocyanate and a trithiol was studied. After characterization of the reactive species, two critical temperatures, namely Tg∞ (maximum glass transition temperature of the thermosetting system) and gelTg (Tg of the material at the gel point), were determined. The conversion at gel point was also determined and compared to the theoretical prediction. Two different characteristics of the evolution of a reactive system, Tg and conversion x as a function of time, were related. This is of particular interest for understanding the curing process, especially when non-isothermal cure schedules are used. Viscoelastic and plastic properties were investigated, with the aim of establishing connections between the chemical structure and properties through detailed analysis of the polymer chain motions (β and α relaxations). Measurements of the storage modulus at the rubbery plateau and of the critical strain intensity factor KIc complete the study.

New segmented polyurethane ureas based on 4,4′‐dicyclohexylmethane diisocyanate and on various soft segments

Linear segmented polyurethane ureas were prepared from 4,4′-dicyclohexylmethane diisocyanate (H12MDI), 4,4′-diamino-3,3′-dicyclohexyl methane (3DCM), and various hydrophilic and hydrophobic soft segments. Kinetic studies of the synthesis of the diisocyanate-terminated prepolymers revealed that the use of too little reactive polyols (that is, polyoxypropylene that bears secondary hydroxyls) could be rather tricky; the noncatalyzed reaction is very slow, but the use of a catalyst soon triggers the formation of side products, and the processing window consequently becomes quite short. Microcalorimetric and dynamic mechanical measurements showed that all the materials were highly phase-segregated elastomers and displayed good mechanical properties up to high temperature (typically 180°C), provided that they had been postcured properly; in this respect, the dramatic effects of isolated (nonchemically linked) hard segments, as well as of too low postcuring temperatures, were demonstrated. Polyurethane ureas compare well with polyureas, and their synthesis can be a good way to cope with the lack of well-adapted commercial diamino-terminated prepolymers.

Preparation of polyurethane microspheres dispersed in an epoxy prepolymer

HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés. Preparation of polyurethane microspheres dispersed in an epoxy prepolymer Abderrahim Djahieche, Françoise Méchin, Jean Pierre Pascault

Novel nucleophilic/basic and acidic organocatalysts for reaction between poorly reactive diisocyanate and diols

Abstract Original basic or acidic organic compounds derived from guanidine or phenyl phosphonic acid were specifically designed and tested as new catalysts for the bulk synthesis of polyurethane prepolymers from a precursor system with particularly low reactivity (secondary alcohol + aliphatic isocyanate at low temperature). Both families showed interesting catalytic activities at 60–80 °C, but must nevertheless be used in much higher amounts (1 mol%, i.e. between 0.15 and 0.50 wt%) than traditional metal-based catalysts. The efficiency of guanidine derivatives seems to be related to their nucleophilicity, whereas that of phosphonic acid derivatives depends on their acidity. However, the solubility of the considered species in the reactive medium also plays a major role. The water/alcohol selectivity of the catalysts, especially at room temperature, was then examined as an additional criterion. Guanidines are not selective and favor the reaction of isocyanate groups with water as much as that with alcohols. Phenyl phosphonic acid derivatives are more selective, and particularly pentafluorophenyl phosphonic acid displays a remarkable catalytic activity together with an acceptable selectivity and could represent an interesting and safer alternative to toxic tin and mercury derivatives for many industrial polyurethanes.