Systems which deviate from the hardness additivity law

1 Systems which deviate from the hardness additivity law 

In the preceding section it was demonstrated that multiphase systems comprising a phase with Tg below room temperature are distinguished by peculiarities in their deformation mechanism. While in the same subsection it was attempted to account for this peculiarity qualitatively, in this section it will be demonstrated that by means of eq. (3.4), which expresses the relationship between Tg and the microhardness, it is possible to describe the behaviour of the system in a quantitative manner. 

According to the additivity law, eq. (1.5), one can calculate the microhardness H of any multicomponent and/or multiphase system provided the microhardness of each component and/or phase //, and its mass fraction u), are known. This relationship is of great value because it offers the opportunity to characterize micromechan-ically components of a system which are not accessible to direct measurement. 

In the preceding chapters it has been demonstrated that many semicrystalline polymers, copolymers and blends obey the additivity law. Exceptions, such as blends of HDPE with PP are explained by a peculiarity in the morphological structure of the crystallites formed (mostly related to the surface energy) (see Sections 4.3 and 5.1.2). 

The same approach applied to thermoplastic elastomers of the PEE-type fails to explain the large discrepancy (up to 100 MPa when the measured H values are in the range 20-40 MPa) between the experimental values and those calculated according to eq. (3.4) (Fakirov el al, 1998 Apostolov et al, 1998). For this reason one has to look for other factors which may be responsible for such a discrepancy. Before disclosing these let us recall some of the characteristic features in the structure and morphology of thermoplastic elastomers of PEE-type which are closely related with the problem discussed. 

---