Modulus changes chemical structure

A careful inspection of Tables 4 and 5 shows that changes, even limited, in the details of SAPA chemical structure may affect noticeably their fracture behavior. Although it has been carried out [1], a systematic study of their effects falls out of the scope of this article, because too many parameters are concerned. They include yield stress, p relaxation characteristics, tensile modulus, entanglement density, Mw / ratio, and also, in the high temperature range, the gap (Ta-T) between Ta and the test temperature T. Instead, emphasis will just be put here on two features whose explanation is quite simple. 

There are several ways of changing the properties of segmented polyurethane copolymers. In this study, the chemical structure of the hard segments was changed to adjust the modulus of the adhesive. Further fine tuning is possible by varying the concentration of the different blocks. For example, the 87-series adhesives have a lower modulus than the 88-series adhesives because their hard segment concentration is lower. Studies of concentration effects (16) have shown that much wider variation of modulus is possible tTian was achieved here. 

Eceiza et al investigated a series of thermoplastic PUs synthesized in bulk by two-step polymerization, obtained with the isocyanate-chain extender couple MDI-BG and various SS macrodiols. The chemical structure, the SS molecular weight and the HS content were varied. The effects on the thermal and mechanical properties were investigated [77]. The changes in the macrodiols in the case of PUs based on MDl-BG, resulted in a modulus curve that showed a plateau indicating the existence of physical crosslinks because of the increase in the size and inter-connectivity... 

The way in which the chemical structure of the various chain extenders of Table 3.16 influences the thermal stability of polyurethane elastomers based on the molecule Capa 225/CHDI/chain extender in the molar ratio 1 2 1, respectively, is given in the following figures and tables. For example, Fig. 3.7 shows the dynamic mechanical thermal properties of a series of the polyurethane elastomers in which the variable is the chain extender. The temperature at which the value of the storage modulus (log E ) changes significantly is considered to indicate the limit of thermal stability of the polyurethane elastomers. 

A concept that is of value in considering the relationship of viscoelastic behaviour to physical and chemical structure is that of relaxation strength . In a stress relaxation experiment, the modulus relaxes from a value G at very short times to Gr, at very long times (Figure 5.20(b)). Similarly in a dynamic mechanical experiment, the modulus changes from Gr, at low frequencies to G at very high frequencies. G is the unrelaxed modulus and Gr is the relaxed modulus (Figure 5.20(c)). 

Although several high modulus materials have been identified in this chapter as well as current commercial materials, a more wide spread acceptance of these materials will occur if their compressive properties are vastly improved. We have discussed several synthetic approaches to improve the compressive properties by structural changes of PBZX systems with small increments of improvement. From what has currently been done, crosslinking seems to have shown the greatest improvement. An in-depth study on a rigid-rod polymer system that forms three dimensional crosslinks by an addition mechanism without the evolution of volatile byproducts is required. For the most part, innovative approaches, both chemical and physical, are required to attack and solve this deficiency. 

---