Elastomers relaxation processes

It has been reported (4-6) that elastomers undergo very longterm relaxation processes in stress relaxation and creep experiments. The long time behavior of shear modulus can be represented by (18)... 

An important feature of filled elastomers is the stress softening whereby an elastomer exhibits lower tensile properties at extensions less than those previously applied. As a result of this effect, a hysteresis loop on the stress-strain curve is observed. This effect is irreversible it is not connected with relaxation processes but the internal structure changes during stress softening. The reinforcement results from the polymer-filler interaction which include both physical and chemical bonds. Thus, deforma-tional properties and strength of filled rubbers are closely connected with the polymer-particle interactions and the ability of these bonds to become reformed under stress. 

We will discuss some preliminary results, which have been performed recently l01). In Fig. 39a the results for polymer No. 2d of Table 10 are shown, which were obtained by torsional vibration experiments. At low temperatures the step in the G (T) curve and the maximum in the G"(T) curve indicate a p-relaxation process at about 120-130 K. Accordingly the glass transition is detected at about 260 K. At 277 K the nematic elastomer becomes isotropic. This phase transformation can be seen only by a very small step in G and G" in the tail of glass transition region, which is shown in more detail in Fig. 39 b. From these measurements we can conclude, that the visco-elastic properties are largely dominated by the properties of the polymer backbone the change of the mesogenic side chains from isotropic to liquid crystalline acts only as a small disturbance and in principle the visco-elastic behavior of the elastomer... 

Solid-state NMR magnetisation relaxation experiments provide a good method for the analysis of network structures. In the past two decades considerable progress has been made in the field of elastomer characterisation using transverse or spin-spin (T2) relaxation data [36-42]. The principle of the use of such relaxation experiments is based on the high sensitivity of the relaxation process to chain dynamics involving large spatial-scale chain motion in elastomers at temperatures well above the Tg and in swollen networks. Since chain motion is closely coupled to elastomer structure, chemical information can also be obtained in this way. 

As is usual for PU elastomers, all the materials showed substantial deviations from hyperelastic response, indicating the presence of relaxation processes. The... 

Mixed Echo Decay. The mixed echo amplitude decay can be expressed in terms of the autocorrelation function of the dipolar Hamiltonian (64). Transverse relaxation processes described by a single correlation time are rarely found in amorphous polymers including elastomers. If a normalized distribution of correlation times (tc) is introduced, than the mixed echo decay is given by (64)... 

Segmental Motions of Elastomers. NOESY experiment under MAS on protons was used to study molecular motions in technical relevant materials such as rubbers (109,110). For the evaluation of these parameters, it is necessary to imderstand the cross-relaxation process in the presence of anisotropic motions and under sample spinning. Such a treatment is provided in Reference 110 and the cross-relaxation rates were found to weakly depend on fast motions in the Larmor frequency range and strongly on slow motions of the order of the spinning... 

In many studies of polymerizing systems, low frequency conductivity-related processes are large and may obscure the dipole relaxation process. Some evidence of these processes is seen in Figures 11.2-11.4 at low reaction times and low frequencies, but in other systems they may dominate the overall behaviour, e.g. as in the case of phase-separating elastomer-epoxy resins described by Pethrick and coworkers [80, 81] and by Maistros et al. [82]. In such cases the apparently dominating conductivity processes may be represented alternatively by the modulus representation [73] M = [l/e], which gives... 

The thermal transitions and the relaxation processes observed in multiblock terpolymers allow to evaluate their phase morphology. At room temperature, these polymers are composed of three phases hard, soft, and strongly expanded interphase. The two latter phases are amorphous and form a matrix (continuous phase), whereas the hard (crystalline) phase is the dispersed phase. The thermal transition and relaxation processes occurring in the interphase of the multiblock copolymers are not detected by the DSC and DMTA methods. The incorporation of the third short block into the copolymer chain causes an increase in the volume of the interphase. This facilitates the establishment of the processes occurring in this phase at various temperatures. Moreover, it enables the evaluation of the influence of the dimension and composition of this phase on the polymer properties. (About the number of phases in poly(ether ester) thermoplastic elastomers, see also Chapter 6.)... 

In addition, the findings of the work (75, 78) indicate that even for elastomers the effect of the filler on relaxation processes may be strong. 

The range of motions available to a polymer spans the high-frequency secondary relaxations, involving motion of pendant groups, to the slow so-called chain modes, which reflect motion over large (>10 nm) distances. The slowest relaxation process is the terminal mode, corresponding to motion of the entire molecule. These dynamics can be illustrated with an example, poly(vinylethylene) (PVE), an elastomer also known as 1, 2-polybutadiene. 

The progress achieved is closely linked to the development of both powerful detectors and brilliant X-ray sources (synchrotron radiation, rotating anode). Such point-focus equipment has replaced older slit-focus equipment (Kratky camera, Rigaku-Denki camera) in many laboratories, and the next step of instrumental progress is already discernible. With the X-ray free electron laser (XFEL) it will become possible to study very fast processes like the structure relaxation of elastomers after the removal of mechanical load. 

One type of block polymer is known as thermoplastic elastomers. They consist of a number of rubber blocks tied together by hard crystalline or glassy blocks. These materials can be processed in injection molding and extrusion equipment since the crystalline blocks melt or the glassy ones soften at high temperatures. However, at lower temperatures, such as at room temperature, the hard blocks behave very much as cross-links to reduce creep and stress relaxation. Thermoplastic elastomers have creep behavior between that of very lightly cross-linked rubbers and highly cross-... 

Before briefly discussing each type it is necessary to consider the performance of thermoplastic elastomers, and the problem of defining service temperature limits for them. The structural features that convey the ability to be processed as a thermoplastic are also a limiting factor in their use. Since it is the pseudocrosslinks that allow these materials to develop elastomeric behaviour, any factor which interferes with the integrity of the pseudocrosslinks will weaken the material, and allow excessive creep or stress relaxation to occur under the sustained application of stress and strain. Temperature is obviously one such factor. 

The large scale molecular motions which take place in the rubber plateau and terminal zones of an uncross-linked linear polymer give rise to stress relaxation and thereby energy dissipation. For narrow molecular weight distribution elastomers non-catastrophic rupture of the material is caused by the disentanglement processes which occur in the terminal zone, e.g., by the reptation process. In practical terms it means that the green strength of the elastomer is poor. 

The RIM elastomer A, exposed to the stress relaxation test after one hour at 100°C, was able to retain only 37.2% of its physical crosslinks. The dissipation of crystalline aggregates by the DSC measurements occurred at a temperature approximately 50 higher than determined by the stress relaxation method. Seemingly, the stress which was applied in the process of the stress relaxation method was an additional factor enhancing decrystalization of the physical crosslinks. 

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