Polyurethane network formation

Prochazka, F., Nicolai, T., and Durand, D. (1996) Dynamic viscoelastic characterization of a polyurethane network formation. Macromoiecules, 29, 2260 2264. 

Polyurethane Networks. Again, for Tqjj. 1 the sol fraction fits well the theoretical curves, if the measured values of the average functionality fn of the polyoxypropylene (POP) triol, the final conversion of isocyanate groups, and the formation °f... 

One alternative is to select precursors which form a gas as a reaction product in situ during the network formation of thermosets. However this approach is restricted to a very limited number of precursors reacting via a polycondensation mechanism to split off a gas. For example, flexible polyurethane foams are commercially produced using CO2 that is liberated as a reaction product of the isocyanate monomer with water [5]. Very recently, Macosko and coworkers studied the macroscopic cell opening mechanism in polyurethane foams and unraveled a microphase separation occurring in the cell walls. This leads to nanosized domains, which are considered as hard segments and responsible for a rise in modulus after the cell opening [6]. 

It is shown that model, end-linked networks cannot be perfect networks. Simply from the mechanism of formation, post-gel intramolecular reaction must occur and some of this leads to the formation of inelastic loops. Data on the small-strain, shear moduli of trifunctional and tetrafunctional polyurethane networks from polyols of various molar masses, and the extents of reaction at gelation occurring during their formation are considered in more detail than hitherto. The networks, prepared in bulk and at various dilutions in solvent, show extents of reaction at gelation which indicate pre-gel intramolecular reaction and small-strain moduli which are lower than those expected for perfect network structures. From the systematic variations of moduli and gel points with dilution of preparation, it is deduced that the networks follow affine behaviour at small strains and that even in the limit of no pre-gel intramolecular reaction, the occurrence of post-gel intramolecular reaction means that network defects still occur. In addition, from the variation of defects with polyol molar mass it is demonstrated that defects will still persist in the limit of infinite molar mass. In this limit, theoretical arguments are used to define the minimal significant structures which must be considered for the definition of the properties and structures of real networks. 

In discussing the formation of interpenetrating networks, we may reasonably assume as a first approximation that the synthesis of each component occurs irrespective of the others, and that the kinetics of the process is determined by the concentration of one or another component. However, this is a rather rough approximation, which is more or less valid at low concentrations of the polyurethane component. As was shown before,51 when the content of polyurethane exceeds 50%, its network begins to work as a cage, preventing polyester formation because the primary polyurethane network hampers diffusion of the ester. This means that in such systems, mutual interference of the components occurs even in the absence of chemical interactions between them. 

In this case of three-monomer polyurethane synthesis, there is no thermodynamic driving force for phase separation. The formation of clusters is fully controlled by the initial composition of the system, the reactivity of functional groups, and the network formation history (one or two stages, macrodiol or triol reacted with diisocyanate first, etc.). 

In the case of Fig. 7.6a the cluster formation and the size distribution can be influenced not only by chemical reactions but also by partial miscibility of the substructures during reaction. Polyurethane networks prepared from polyolefin instead of polyester or polyether as macrodiol, can serve as an example. In this particular case an agglomeration of hard domains takes place in the pregel stage, produced by a thermodynamic driving force. 

Zammarano M, Kramer RH, Harris R, Ohlemiller TJ, Shields JR, Rahatekar SS, Lacerda S, Gilman JW. Flammability reduction of flexible polyurethane foams via carbon nanofiber network formation. Polym. Adv. Technol. 2008 19 588-595. 

The peculiarities of the process described above are explained by the fact that branchings may create nuclei of insoluble fractions [43]. This assumption is confirmed by the data given in Fig. 8 where the viscosity increase is compared for network formation of polyurethanes from bi- and multi-functional diamine. The fact that the shape of the rj (t) dependences is similar and the exponents a = 4.6 indicate that equivalent physical effects are operative. 

In addition, PET can be used to form semi-IPNs with other naturally functionalized triglyceride oils, such as vemonia oU (31). The procedures for PET/vemonia semi-IPNs are essentially the same as those of PET/castor ones, but with important differences. For PET/castor mixtures, the diisocyanate cross-hnker was added at 240°C and the mixture was poured into the molds rapidly before the castor gel point had been reached. In this case, PET/castor polyurethane semi-IPNs were formed, in which crystallization and gelation occurred simultaneously resulting in a single, broad glass transition temperature. For PET/vernonia, the sebacic acid was added at 280°C, which reduced the temperature to about 250°C, where the mixture was held for another 5 min, then poured into a preheated mold and allowed to cool, during which time the PET crystalhzes. In this case, PET/vernonia polyester network was formed, and the PET crystallized prior to network formation because the latter... 

ONEl-SHOT PROCEDURE. All reactants were mixed together at room temperature and catalysts, and initiator were added to promote polyurethane and PMMA network formation. Prior to gelation, the solution was quickly degassed and poured into a mould. After the gelation, the casting was cured at 60 C for 48 hours. 

Many thermoset polymers of major commercial importance are synthesized by step-growth polymerization, as the case of unsaturated polyester, polyurethanes, melamines, phenolic and urea formaldehyde resins, epoxy resins, silicons, etc. In these systems, the crosslinking process, which leads to a polymer network formation, is usually referred to as curing. 

Zhang, R. Dowden, A. Deng, H. Baxendale M. Peijs T. (2009) Conductive network formation in the melt of carbon nanotube/thermoplastic polyurethane composite. Compos. Set. Technol. Vol.69, No.lO, pp.1499-1504... 

Comparing the results of kinetic studies with these conclusions, we see that the surfactant availabihty in the reaction system does not affect the formal kinetics of polyurethane formation precisely at the transition state of the siufactant adsorption layer. The acceleration of the reaction is achieved when IiEP-2 is introduced into the system at concentrations other than 0.03% and 0.15%. It is shown that there is an important relation between the effect of surfactant on the formation kinetics of polyurethane networks and the structure of surfactant layers in the ohgoglycol in the formation of these polymers. 

Thermodynamic functions of surfactant surface layers in oligomer mixtures. For task-oriented investigation of the influence of surfactant on the formation of polyurethane networks, information concerning the structure of the surfactant surface layers is required for various surfactant contents as well as identification of the concentration interval in which the surfactant is fully soluble in the oligomer mixtures. For this reason, surface tension isotherms of smfactant solutions were studied and thermodynamic analysis was imdertaken. 

Swelling of polyurethane networks. Proceeding from the general conception of the role of surfactants in the formation of the secondary structure of polyurethanes, studies of the influence of KEP-2 on the distribution of hydrogen bonds in polyurethane networks were required. The literature on the thermodynamic properties of polymer solutions and gels was searched and, in particular, the work of the authors of [117] is of direct interest in this context. This work... 

Physical and mechanical characteristics of pol5nirethane nets based on PPG and ODA mixtures at 343 1K were studied—that is, the properties of polymers formed in the conditions of macrolevel heterogeneity were investigated. As has been shown [167], the separation of the system into phases occurs much earlier than the emergence of microheterogeneous structures connected with network formations when the formed polyurethanes are incompatible. This process is accompanied by stick-slip increase of surface tension in the system... 

Thus, one can identify at least three factors that increase the longterm strength of Sprut-5M adhesive at the stage of formation of the polyurethane network but which have no further effect after the adhesive cures completely. Actually, in this state of the adhesive there is no indication of the structure that is observed with the orientation of the molecular chains there is an abrupt reduction of the opportunity for macroradicals to recombine and for the submicrocracks to be filled. At this stage of curing, despite some decrease of the long-term strength under a load of 50% of the fracture load, the Sprut-5M adhesive has much better resistance to the effect of constant loads than does the polyester adhesive (see Fig. 3.2, curve 1 (after 102 days) and curve 4). 

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