Chain stress

Environmental stresses through technical installations are usually transmitted through the carrier media of air and water which are both part of the environment. The noxious substances are introduced in gaseous or dusty form into the air or, as a solution, into the water. Tlirough the air we breathe and the water we drink, these noxious substances bring about a direct endangerment or, via the food chain, stress on man and animals. 

Mechanical stress causes changes in the physical properties and chemical reactivity of polymers [397]. Macroscopic extension of a polymer film causes anisotropic orientation and extension of polymer chains. Stress can cause chain breaks and introduce radicals, which can initiate degradative processes such as oxidation or microcracking. 

In the following section it will be discussed in more detail to what extent the orientation distribution of chain segments, the ensuing distribution of axial chain stresses, and the gradual accumulation of defects through successive breakage of chains can be held responsible for the discrepancy between actual and theoretical strength. 

Fig. 5.7. Schematic representation of a stress-induced defect. The point of maximum chain stress travels into the crystal as the leading chain group is pulled out of the crystal (after l22l).

The chain stress is derived from Eq. (5.24) replacing the chain elasticity constant K by the elastic constant of a series of elastic elements... 

The above equation shows quantitatively that large chain stresses will only be obtained if the interaction force constants Wj are of about the same order they are in a crystal and if the elastic moduli of the chain sections are large throughout. One weak section will greatly enhance average displacement and reduce the stress. It should also be mentioned that it is not the chain length L and the absolute number n of interaction sites which raise the chain stress but the intensity of the interaction forces per unit chain length. 

Due to the different elastic response the free chain would have to act against the lattice potential until chain stress and lattice interaction were in equilibrium at any point of the chain. This interaction can be described by identically the same mathematical formalism developed above. Since the chain ends are assumed to be free they cannot bear any axial stresses. This condition will exactly be met if one superimposes the hypothetical stress o (Eq. 5.35) by a compressive stress according to Eq. (5.33) where... 

Quite obviously the role of chain loading and scission within the three strength determining mechanisms will be quite different. It has been stated repeatedly throughout this book that the load carrying capabilities of chain molecules are utilized most effectively if chain orientation and intermolecular attraction permit the gradual accumulation of large axial chain stresses and oppose slip and... 

To date two methods have been employed for the determination of the distribution N(i//) of (large) axial chain stresses ... 

Fig. 8.5. Average orientation (cos 0)of highly stressed PETP segments as a function of stress as abscissa the frequency shifts of the 974 cm band and the corresponding axial chain stresses (using a stress sensitivity factor a = 3.6 cm per GN/m ) have been employed (after [5]).

---