Microfibrillar substructure

Contrary to widespread opinion, the value of Ea is not a constant quantity. As was proved previously [52], the value of E is variable, since it depends on the ordering of macromolecules in the amorphous material of the fiber. At the same time, one can suppose that this ordering will be affected by the specificity of the fine structure of the fiber, and particularly by the type of substructure of the fiber. The relationship determining the modulus Ea appropriate for a definite type of fiber substructure can be derived from Eq. (11) when appropriate values of A are assumed. In the case of the microfibrillar substructure, i.e., for A < I, typical of PET fibers stretched, but not subjected to annealing, this equation has the form [52] ... 

Table 8 Fine Structure Parameters and Axial Elastic Moduli of PET Fibers of Microfibrillar Substructure...

For highly oriented films with microfibrillar substructure similar considerations on the origin of the voids hold. 

Another type of fibril substructure in PET fibers, besides the microfibrillar type already discussed, is the lamellar substructure, also referred to as the lateral substructure. The basic structural unit of this kind of substructure is the crystalline lamella. Formation of crystalline lamellae is a result of lateral adjustment of crystalline blocks occurring in neighboring microfibrils on the same level. Particular lamellae are placed laterally in relation to the axis of the fibrils, which explains the name—lateral substructure. The principle of the lamellar substructure is shown in Fig. 2. 

For details see Section III. A., Mechanical Properties.) For values of this parameter less than 1, we can conclude that a substructure of the microfibrillar type occurs, whereas for the values equal or close to 1 a substructure of the lamellar-type occurs. 

The parameter A determined by the above relationship can be called a substructure parameter. Its numerical value determines the type of substructure of PET fibers. For A < I we have to do with the substructure of microfibrillar type, while for A = 1 with the substructure of lamellar type. 

In the studies carried out by one of the authors [52], the values of Ea and E were determined for PET fibers of the microfibrillar and of the lamellar substructure. The results have been presented in Tables 8 and 9. The results obtained show that for both types of substructure the resistance to deformation, that is, the value of E, depends on the degree of molecular orientation of the amorphous material of the fiber fa) and the density of this amorphous phase of the fiber da)- However, this dependence assumes a different form for the microfibrillar and for the lamellar substructure. In the first case, it has the form ... 

---