Diisocyanate Dynamic mechanical properties

The dynamic mechanical properties of three epoxy resins cured with diphenyl-methane diisocyanate (MDI) are shown in Fig. 15. Since these resins consist of many bulky cyclic structures, Tg (a-dispersion) is above 200 °C and the transition region is wide. In the DEN 431 —L-MDI resin, Tg is higher than 300 °C. The resin is expected to be highly heat-resistant51). 

Hard- and Soft-Segment Polymers. The structure of the diisocyanate component has a significant influence on dynamic mechanical properties of both hard- and soft-segment polymers. Both 2,4- and 2,6-TDI/ BD a relaxations occur at about the same temperature and have a comparable activation energy. However, the symmetrical structure of... 

Above the -relaxation process, the 2,4-TDI/PTMO polymer displayed a short rubbery plateau at a storage modulus of about 5 MPa while 2,6-TDI/PTMO was capable of crystallization, as evidenced by the ac-loss process. This difference in dynamic mechanical properties demonstrates the effect of a symmetric diisocyanate structure upon soft-segment properties. As previously discussed, single urethane links can sometimes be incorporated into the soft-segment phase. The introduction of only one of these diisocyanate molecules between two long PTMO chains inhibits crystallization if the diisocyanate is asymmetric. In the case of a symmetric diisocyanate, soft-segment crystallization above Tg can readily occur. The crystals formed were found to melt about 30°C below the reported melting point for PTMO homopolymer, 37°-43°C (19), possibly because of disruption of the crystal structure by the bulky diisocyanate units. 

Figure 1. Dynamic mechanical properties of linear polyurethane encapsulants at 11 Hz. Formulation nomenclature the first number refers to the weight ratio of polyether (ET) in the soft segment and the remainder is assumed to be the polyester the second number designates the overall weight percent of the diisocyanate.

A systematic DMA study was carried out by Marcos et al on a series of aromatic and aliphatic PUs chain extended with diamine functionalised poly(oxypropylene)-tipped poly(oxyethylene) of various molar masses [224]. Four diisocyanates, MDI, 2,4-toluene (TDI), hexamethylene (HMDI) and isophorone (IPDI) were employed. The symmetry/asymmetry of the diisocyanate strongly influenced the dynamic mechanical properties. The symmetric diisocyanates (MDI, HMDI) had more phase... 

BD. They reported an improvement in shape memory properties as a result of introduction aromatic structure into the main chain. Yang and co-workers [122] compared the mechanical, dynamic mechanical and shape memory properties of PU block coPolymers with planar shape hard segment (1,6-diphenyl diisocyanate (PDI)) and bent shape hard segment (MDI). The PDl-based PU showed superior properties compared with MDI-based PU (Table 2.11) as a result of better interaction among hard segments due to the planar shape of PDI. 

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