Glassy polymers creep

Peschanskaya, N. N., Bershtein, V. A., Stepanov, V. A. (1978). The Connection of Glassy Polymers Creep Activation Energy with Cohesion Energy. Fizika Tverdogo Tela, 20(11), 3371-3374. 

Annealing can reduce the creep of crystalline polymers in the same manner as for glassy polymers (89,94,102). For example, the properties of a quenched specimen of low-density polyethylene will still be changing a month after it is made. The creep decreases with time, while the density and modulus increase with time of aging at room temperature. However, for crystalline polymers such as polyethylene and polypropylene, both the annealing temperature and the test temperatures are generally between... 

Creep rates of three glassy polymers are much greater during electron irradiation than before or after. Radiation heating is eliminated as a possible cause. Essentially the same concentration of unpaired electrons and ratio of cross-linking to scission were found in polystyrene samples in the presence or absence of stress. The effects of radiation intensity, stress, and temperature on creep during irradiation are examined. The accelerated creep under stress is directly related to a radiation-induced expansion in the absence of stress. This radiation expansion is decreased by increase in temperature or plasticizer content and decrease in sample thickness. It is concluded that gas accumulation within the sample during irradiation causes both the expansion under no stress and the acceleration of creep under stress. 

The results of the delayed stress on radiation studies presented above (Figure 7) are also consistent with the mechanism of gas buildup within the polymer specimens as the cause of the accelerated creep. An additional interesting conclusion is that applied stress should increase the rate at which gases diffuse out of a polymer specimen. This is not unreasonable in view of the fact that this conclusion is reached for stress application during irradiation, when expansion of the polymer matrix by the internally generated gas would be expected to facilitate gas diffusion. (Actually, one would expect increased gas diffusion in stressed glassy polymers, even in the absence of radiation, owing to the low Poisson ratio in such materials.)... 

Parallel to the stronger temperature dependence of E for semi-crystalline polymers is the stronger dependence on time they show a higher tendency to creep than amorphous, glassy polymers, (Figure 4.20), at least at temperatures above or not too far below Tg. 

The second equation appears to be applicable to a number of glassy polymers, and also to other materials the exponent m is always about 3, so that creep can be described by two parameters, Do and to, while the immediate elastic deformation is also taken into account (Do). As a matter of fact, Do and to are temperature dependent. When the experimentally found creep curves are shifted along the horizontal axis and (slightly) along the vertical axis, they can be made to coincide... 

The analogy in the creep behaviour of various (glassy) polymers and other substances is illustrated in Figure 7.5 on each material measurements have been carried out over a broad range of temperatures, and all results coincide, after shifting, into a master curve with the equation ... 

In 3.3 the effect of cooling rate on the free volume in the glassy state has been discussed. Rapidly cooled glassy polymers have a greater free volume, but they show volume retardation. This volume change, though very small, has a considerable effect on the creep behaviour all relaxation times for creep are shifted towards higher values. This phenomenon has been studied extensively by Struik (thesis Delft 1977). 

Anisotropy of creep behaviour in oriented glasses seems to be less well developed than in partially crystalline materials, but the anisotropy increases with time and presumably temperature. The low level of anisotropy in the time/temperature region investigated may well be a consequence of the polymer being well below its glass transition temperature. Qearly there is scope for systematic investigation of the contributions of the various relaxation phenomena to anisotropy in oriented glassy polymers. 

Plots of the product permeability versus time on a log-log coordinate system are often linear over relatively long time periods, as shown in Fig. 20.5-2. Similar behavior is observed in asymmetric reverse osmosis membranes. The log-log plotting approach provides a simple and reasonably satisfactory means of predicting the performance change of fibers under long-term operation by exuapolation of short-term data. Mechanical creep and volume recovery in glassy polymers aftw an initi perturbation also ate known to be reasonably represented on such log-log plots. ... 

It is convenient to define a retardation time t in the middle of the viscoelastic region to characterize the time-scale for creep. The distinction between a rubber and a glassy plastic is then seen to be somewhat artificial, because it depends only on the value of r at room temperature. Compared with typical experimental response times, which can rarely be less than 1 s, r for a rubber is very small at room temperature, whereas the opposite is true for a glassy polymer. As the temperature is raised the frequency of molecular rearrangements increases, so... 

Impact of a Static Magnetic Field on Creep in Diamagnetic Glassy Polymers. 205... 

Below we consider the results of our systematic research of deformation kinetics for glassy polymers over the wide ranges of temperatures and deformations, using the laser-interferometric technique under consideration [11,278,280-287], This research allowed us (1) to study the dependencies of kinetic parameters of creep on these factors, (2) to reveal the regular relations between the activation parameters of polymer creep, (3) to demonstrate their intimate connection with the parameters of relaxation transitions, and (4) to confirm directly the intermolecular physical nature of potential barriers of polymer plasticity. ... 

Figure 67 shows the creep rate value and creep kinetic parameters vs total deformation plots obtained for poly(methyl methacrylate) (PMMA) and PVC. For these and other glassy polymers studied, the values of Go, and o determined from the various points of the creep curves were found to depend on the strain value in a similar way. At constant stress and temperature, these parameters reach their maximum values at deformation y thereafter they decrease, and their changes become insignificant at > 10-15%. Over the range = 20-40%, the value of activation volume a remains constant, while the activation energy Go decreases slightly. 

Thus, creep of glassy polymers cannot be described by (20) with invariable values of the coefficients Go, a, and o, but the similarity of the a e), Qo e) and lg o( ) curves is observed that presumes their interrelationship and the integral nature of the potential barrier Go. 

Table 8 Activation parameters of creep at 20° C and their relationship to the cohesion energy in glassy polymers [11]...

These data indicate some similarity between the processes of creep in glassy polymers and flow. The mechanism of the deformation of solid polymers must be more complicated [271-275] however, overcoming of IMI barriers is the main point in the different models of deformation processes. 

Table 9 Comparison of activation energies and activation volumes for sub-Tg relaxations (Qb> Qy> "V) creep within the same temperature regions (Qo,m) in glassy polymers...

In [305,306,311,327], the effects of a static magnetic field (2 or 4 kOe) on creep processes at room temperature in linear and cross-linked glassy polymers (basically within or close to the p-relaxation region) were revealed and studied on the microscopic level. This became possible owing to using the LICRM setup and decrease in the relaxation times of dynamic processes in solid polymers under the action of mechanical forces. 

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