Stress Relaxation Processability

The stress—relaxation process is governed by a number of different molecular motions. To resolve them, the thermally stimulated creep (TSCr) method was developed, which consists of the following steps. (/) The specimen is subjected to a given stress at a temperature T for a time /, both chosen to allow complete orientation of the mobile units that one wishes to consider. (2) The temperature is then lowered to Tq T, where any molecular motion is completely hindered then the stress is removed. (3) The specimen is subsequendy heated at a controlled rate. The mobile units reorient according to the available relaxation modes. The strain, its time derivative, and the temperature are recorded versus time. By mnning a series of experiments at different orientation temperatures and plotting the time derivative of the strain rate observed on heating versus the temperature, various relaxational processes are revealed as peaks (243). 

This stress relaxation process is important, and we shall revisit it after developing the second model of viscoelasticity. 

The use of a parallel plate plastimeter to determine both softness and recovery is a simple way of obtaining a measure of both the viscous and elastic components on deformation behaviour, albeit under conditions somewhat removed from those met during processing. An alternative approach is to measure the stress relaxation in a test piece and this was the basis of the Stress Relaxation Processibility Tester developed at RAPRA. 

Although the supramolecular polymers based on bifunctional ureidopyrimidinone derivatives in many ways behave like conventional polymers, the strong temperature dependence of their mechanical properties really sets them apart from macromolecular polymers. At room temperature, the supramolecular polymers show polymer-like viscoelastic behavior in bulk and solution, whereas at elevated temperatures liquid-like properties are observed. These changes are due to a 3-fold effect of temperature on the reversible polymer chain. Because of the temperature dependence of the Ka value of UPy association, the average DP of the chains is drastically reduced at elevated temperatures. Simultaneously, faster dynamics of the scission—recombination process leads to faster stress relaxation in an entangled system. These two effects occur in addition to the temperature-dependent stress relaxation processes that are also operative in melts... 

An hysteresis phenomenon is observed when the plateau pressure determined for an absorption isotherm is higher than the plateau pressure measured at the same temperature for the desorption process. Hysteresis is caused by the large stresses associated with the metal to hydride transformation which give rise to internal defects such as dislocations and stacking faults. Hysteresis decreases with increasing temperature as thermally activated stress relaxation processes set in. It is in general important to eliminate or at least minimize hysteresis for most applications. 

X increases with an increase in chain length. In the case of the Maxwell model, the relaxation of stress is complete, while with real materials, residual stresses that are unrelaxed may appear. It is also customary to express the stress relaxation process, by describing the time-history of the relaxation modulus G. 

The residual internal stress is a result of combined action of stress and stress relaxation process. Therefore to give a sufficient relaxation time for the coating, we often dry a coated film in a mild operating conditions, e.g., dry at lower temperature and velocity of air. 

The curves in Fig. 6.1a and b express the exponential decay of the strain rate during the transient stage, which may be expressed as in Eq. (6.22), which takes into account the initial strain rate, ci, and a characteristic relaxation time, t. The stress relaxation process in transient creep may be expressed by incorporating activation energy, AH, as given by Eq. (6.28) ... 

The stress-relaxation process is governed by a number of different molecular motions. To resolve them, the thermally stimulated creep (TSCr) method was developed, which consists of the following steps ... 

Fig. 44. Schematic of the stress relaxation process after a large step in deformation, (a) Equilibrium conformation of the tube prior to deformation, (b) Immediately after deformation, the primitive chain has been affinely deformed, (c) After the time Xr, the primitive chain retracts along the tube and recovers its equilibrium contour length (t=XR). (d) After the time Xd the primitive chain leaves the deformed tube by reptation (t=Xa). After Doi and Edwards (56), with permission.

Crystalline polymers exhibit more mechanical relaxations than amorphous polymers. It is not an overstatement to remark that the greater number of mechanical relaxations in crystalline polymers is the cause of the substantial difference in properties between crystalline and amorphous polymers (4.N.4). For example in linear (LPE) and branched (BPE) polyethylene at temperatures above — 200 "C, there is a sequence of relaxations (see Fig. 4.12). In branched PE the processes are y-relaxation at — 120 C, -relaxation at — lO C, and a-relaxation at 70 C. The presence of a relaxation is detected most easily by the peak in A this is one reason why this parameter is of value. The relaxation observed in creep in linear PE at room temperature and above (shown in Fig. 4.4) is the a-process. The torsion pendulum is a useful tool for yielding quickly a description of temperature regions where creep or stress-relaxation processes are to be expected. In addition, the relaxation temperatures often mark transitions in ductility the polymer becomes increasingly brittle as it is eooled. 

Compressive stress distributions raise the strength over the base glass by as much as a factor of seven, but near surface stress relaxation processes lead to some lowering of this enhanced value (to nearly three times of the base glass). 

The only stress relaxation tester commercially available is the Rapra/Monsanto SRPT (Stress Relaxation Processability Tester), although others have been described in the Uterature. The rationale behind this development is that viscosity alone is not able to predict rubber processability some elasticity characteristics must be taken into consideration. Further, since most processing problems are related to memory or relaxation effects, it makes sense to consider using stress relaxation. However, once again, this technique has found most application with compounded stock. It is claimed that the technique is sensitive to small variations in molecular weight distribution. Experience so far indicates that it may not be sensitive enough to differentiate levels of processability within grades. 

If, on the other hand, cross-links are forming and dissociating while the measurements are in progress, phenomena similar to chemical stress relaxation (Section D of Chapter 14) may be encountered. The stress may drop nearly to zero in a stress relaxation process, whereas the shear modulus as measured in a moderately rapid... 

For small values of single-stress relaxation processes are observed. However, for larger than unity, clear departures occur. These deviations can easily be represented in a Cole-Cole plot, where the loss modrdus G"((Si) is plotted as a function of the storage modulus G ((n). A pure exponential relaxation process appears in such a diagram as a semicircle passing through the origin ... 

In paper [43] acceleration of the stress relaxation process was found at loading of epoxy polymers under the conditions similar to those described above (Figure 6.8, curves 2-4). The authors [43] explained the observed effect by the partial rupture of chemical bonds. In order to check this conclusion in paper [39] repeated tests on compression of samples, loaded up to the cold flow plateau and then annealed at T < T, were carried out. It has been established that in the diagram o-e tooth of yield is restored. This can occur at the expense of the restoration of unstable clusters, since the restoration of failed chemical bonds at T < is scarcely probable. In this connection it is also necessary to note that yield tooth suppression as a result of preliminary plastic deformation was observed earlier for linear amorphous polymers, for example, polycarbonate [44], for which the chemical bonds network is obviously absent. 

It is obvious, therefore, that in order to eliminate the occurrence of both the cracking and the detachment phenomena, we ought to enable the gel-reformation and stress relaxation processes to occur predominantly It is most reasonable to analyze these processes from the point of view of polymer science. Doing so we can distinguish between three principle processes which take part in the course of this relaxation, as displayed in Figure 2 (from (26) copyright ACS). 

Inoue, T., Uematsu, T., Yamashita, Y., Osaki, K. Significance of the longest Rouse relaxation time in the stress relaxation process at large deformation of entangled polymer solutions. Macromol (2002) 35, pp. 4718-4724... 

Three reproducible transition processes were consistently observed. Near the glass transition temperature, subtle softening or smoothing occurred on the irregularly-shaped PS particles. Twitching motions were often noted for samples in this temperature range, presumably due to stress relaxation processes just above Tg. 

Elastic creep recovery can be observed after short term loading (10 sec) as illustrated in Figure 7. Since this recovery is primarily induced by the filler, one can observe distinct differences in the properties of the RAC s as the filler content is increased. Addition of 10% powdered rubber to an AC10 produced a mixture with a creep recovery better than that of an AC30 from the same source. The stress relaxation process is rather slow creep recovery after medium term loading (100... 

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