Jump-like creep

Jump-Like Creep on the Submicro-, Micro-, and Meso-scale Levels and... 

It may be presumed, a priori, that the above-mentioned contradiction can be resolved and jump-like evolution of polymer deformation may be revealed in the case of (a) sharp improvement of the sensitivity and resolution of the method for creep rate measurements, and (b) coherent occurrence of many elementary microshear events (their cooperative or simultaneous manifestation) leading to observable jump-like creep. 

Figure 73 illustrates the jump-like creep process by the interferograms obtained for amorphous polymers and the scheme of stepped deformation. Two parameters were introduced to characterize the stepwise creep (1) deformation increment L corresponding to the period of creep rate variation (the height of the step in the scheme) and (2) the ratio h of the maximum ( max) to minimum (emin) creep rates within the same step L, i.e., jump sharpness. In some cases, the mean value... 

It was revealed that both parameters of jump-like creep typically changed with development of the deformation process. The deformation steps L of submicro-, micro-, and meso-scale sizes could be observed in polymers and composites depending on material composition and the deformation stage. It was found that the... 

Fig. 73 Typical interferograms (a) and a scheme of the stepwise (jump-like) creep (b) in amorphous polymers [11,321]. One oscillation (beat) corresponds to an increase of deformation on compression by 0.3 jam (usually 0.005%), and the creep rate e is determined by the beat frequency 1/x...

The peculiarities of jump-like creep at different stages of deformation may be illustrated, for instance, for PDMS networks and PDMS-silica nanocomposites with 40 wt% Si02 [319] studied at room temperature (above Tg). These nanocomposites were synthesized in situ as described elsewhere [266]. The silica domains formed had a diameter of about 10 nm. 

This phenomenon was discussed in terms of the possible direct correlations between stepwise creep and fiber morphology, namely, of micro-shear displacements of various fibrillar elements in a stick-slip mode. It was shown that the fibrillar units were weakly connected and loosely packed in these fibers interfibrillar regions contained pores and a small number of tie molecules. The length of microfibrils has been estimated to be microns, whereas the length of macrofibrils reached lOOpm and more. These sizes correlate satisfactorily with the observed deformation steps. Of course, this approach (slippage of fibrils upon creep) did not exclude the participation of the intracrystalline slip events and the process of scission of overstressed interfibrillar tie molecules in jump-like creep. Submicro- and microcrack formation could also contribute, to some extent, to creep heterogeneity and the total deformation of fibers [314]. 

Jump-like creep was also studied for UHMWPE films with intentionally varied structural organization of interfaces between fibrils at the expense of their different preparation (gel-casting, crystallization from the melt), various draw ratios X (from 7 to 119), and the special cross-linking of fibrils [315,317]. The fibrils were weakly connected and loosely packed, weakly tied but closely packed, or cross-linked by long molecular segments or connected by short crosslinks, in these samples. As a result, absolutely different jump-like creep rate vs deformation curves and jump sharpness parameter h vs strain dependencies were obtained for these model samples with different interfacial structures. Short interfibrillar crosslinks provided the largest effect on creep behavior. Creep occurred basically through shear of fibrillar structural units relative to one another in an acceleration-deceleration way deceleration was due to slip resistance by some stoppers. ... 

Of special interest are the results of studying jump-like creep on the micro-scale level when the controlled structural heterogeneities of the same sizes are present in polymers, namely, for epoxy networks with a globular sUiicture [311], for epoxy composites containing diabase microparticles [310], for POM plastics with different spherulite sizes [320], and for Pl-graphite composites [320], These materials can be considered as models for checking up the micro-plasticity vs sUiicture correlations. It was possible to compare directly the sizes of heterogeneities (solid microparticles or densely packed micro-domains of polymers) with the creep micro-jumps, and to draw the conclusions about their interrelationship. 

In [320], jump-like creep was studied for two POM semi-crystaUine polymers. Analysis of their polarizing microscope images showed that homoPOM contained predominantly spherulites 1-5 pm in diameter, whereas poly(oxymethylene-co-oxyethylene) (95POM/5POE) sample contained both the same small spherulites and larger ones, up to 25 pm in diameter. It might be supposed that the size of these more dense structural units will be reflected in the character of variation of the creep rate and the values of deformation jumps L. Then the loosely packed inter-spherulite boundaries could be considered presumably as the most probable points for local shear displacements (micro-plasticity). 

Similar structure vs jump-like creep correlations were revealed for neat epoxy network [310,311] and for the composite based thereon and filled with diabase particles of 5-10 pm size (weight ratio of both components was 1 1) [310]. Electton... 

Figure shows the example of the computerized measurement with construction of creep rate vs compressive strain plot obtained for polycarbonate (PC) near the yield point (deformation y = 6.5%) [12]. The data are presented on the different scales. Each point corresponds to the deformation increment of 300 nm (0.005%). Figure displays continuous changing of a creep rate even within the narrow deformation ranges, with minimum at y. It should also be stressed that the variance of the creep rate for neighboring experimental points is not scattering but reflects the jump-like development of deformation (see Sect. 5). 

Reliable detection and detailed studying of the jump-like character of polymer creep on the submicro-, micro-, and meso-scale levels, and the successful looking... 

Solids of different classes, including polymers, are characterized typically with a complex non-uniform structure on various morphological levels and the presence of different local defects. The theoretical approaches describe the deformation of solid polymers via local defects in the form of dislocations (or dislocation analogies ) and disclinations, or in terms of dislocation-disclination models even for non-crystalline polymers [271-275, 292]. In principle, this presumes the localized character and jump-like evolution of polymer deformation at various levels. Meantime, the structural heterogeneity and localized microdeformation processes revealed in solids by microscopic or diffraction methods, could not be discerned typically in the mechanical (stress-strain or creep) curves obtained by the traditional techniques. This supports the idea of deformation as a monotonic process with a smoothly varying rate. Creep process has been investigated in the numerous studies in terms of average rates (steady-state creep). For polymers, as the exclusion. 

PDMS-silica nanocomposite within the narrow strain intervals but at different deformation values. Each experimental point in these curves refers to a deformation increment of 0.3 p.m. One can see the jump-like development of creep and, simultaneously, very different plots in Fig. 76a-d depending on sample composition and deformation stage. 

The constant magnetic field affected not only the mean creep rate but also the jump-like character of deformation on the micro-scale level. It was shown... 

The creep of UPRs and the UPR composites filled with marble powder and powdery PVC was studied [235]. The authors proposed a jump-like character for the creep rate of materials. It was also assumed that the creep on the micron level reflected the structural inhomogeneity of UPRs and their composites. An optimized cure cycle with reduced process-induced residual stresses and the optimum temperature profile for the manufacturing of UPR composites were searched using the numerical simulation [236]. 

Polymers are a little more complicated. The drop in modulus (like the increase in creep rate) is caused by the increased ease with which molecules can slip past each other. In metals, which have a crystal structure, this reflects the increasing number of vacancies and the increased rate at which atoms jump into them. In polymers, which are amorphous, it reflects the increase in free volume which gives an increase in the rate of reptation. Then the shift factor is given, not by eqn. (23.11) but by... 

It has been recognized that the behavior of atomic friction, such as stick-slip, creep, and velocity dependence, can be understood in terms of the energy structure of multistable states and noise activated motion. Noises like thermal activities may cause the atom to jump even before AUq becomes zero, but the time when the atom is activated depends on sliding velocity in such a way that for a given energy barrier, AI/q the probability of activation increases with decreasing velocity. It has been demonstrated [14] that the mechanism of noise activation leads to "the velocity... 

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