Stress relaxation molecular interpretation

Studies have been made of the stresses produced in several non-steady flow histories. These include the buildup to steady state of a and pu — p22 at the onset of steady shearing flow (355-35 ) relaxation of stresses from their steady state values when the flow is suddenly stopped (356-360) stress relaxation after suddenly imposed large deformations (361) recoil behavior when the shear stress is suddenly removed after a steady state in the non-linear region has been reached (362) and parallel or transverse oscillations superimposed on steady shearing flow (363-367). Experimental problems caused by the inertia and compliance of the experimental apparatus are much more severe than in steady state measurements (368,369). Quantitative interpretations must therefore still be somewhat tentative. Nevertheless, the pattern of behavior emerging is suggestive with respect to possible molecular flow mechanisms. 

We now briefly discuss the molecular interpretation of the behavior exhibited by polymers that are held at constant temperature and studied as a function of time in a stress relaxation experiment covering the entire time scale (possibly 14 decades, or more). 

The models discussed here, which are phenomenological and have no direct relation with chemical composition or molecular structure, in principle enable the response to a complicated loading pattern to be deduced from a single creep (or stress-relaxation) plot extending over a long time interval. Interpretation depends on the assumption in linear viscoelasticity that the total deformation can be considered as the sum of independent elastic (Hookean) and viscous (Newtonian) components. In essence, the simple behaviour is modelled by a set of either integral or differential equations, which are then applicable in other situations. 

Those interested in the theory of viscoelasticity and in the relationship of materials properties to their molecular structure tend to concentrate more on stress-relaxation than creep measurements. One reason may be that stress-relaxation figures are generally more easily interpreted in terms of viscoelastic theory than are creep data. Stress-relaxation data also provide practical information such as determining the stress needed to hold a metal insert in a plastic product, evaluating the additives needed such as antioxidants, choosing cantiliver-type beams, and so on. 

The Eyring analysis does not explicity take chain structures into account, so its molecular picture is not obviously applicable to polymer systems. It also does not appear to predict normal stress differences in shear flow. Consequently, the mechanism of shear-rate dependence and the physical interpretation of the characteristic time t0 are unclear, as are their relationships to molecular structure and to cooperative configurational relaxation as reflected by the linear viscoelastic behavior. At the present time it is uncertain whether the agreement with experiment is simply fortuitous, or whether it signifies some kind of underlying unity in the shear rate dependence of concentrated systems of identical particles, regardless of their structure and the mechanism of interaction. 

The relaxation process may lead either to a decrease in residual stress or an increase in elastic properties, or both, with incubation time. However, independent of the actual interpretation, these results support the hypothesis that the chains in the films, which were initially out of equilibrium, tend to equilibrate during annealing. As shown above, this relaxation process was documented by dewetting. Interestingly, relaxations were much faster than according to bulk reptation at 125°C for the high molecular weight PS used here (Trep 2 x 10 s) [43, 104, 163, 164],... 

Many problems concerning the morphology of PLCs and its effect on properties still remain unsolved. A more reasonable interpretation of the origin of hierarchical structures and molecular orientation in PLC products, the mechanisms of interfibrillar stress transfer and the relaxation process of the rigid rod molecules all need more investigation. 

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