Elastomer relaxation, physical

Another vivid example of the exceptional role of network topology is the unexpectedly high deformation abUity of hypercrosslinked polystyrenes under loading, which is usuaUy characteristic of conventional slightly cross-linked networks or linear polymers in the rubber elasticity state. Hypercrosslinked polymers, however, differ from the latter in that they retain their mobUity even at very low temperatures. In fact, hypercrosslinked materials do not exhibit typical features of polymeric glasses, nor are they typical elastomers. Their physical state thus cannot be described in terms of generaUy accepted notions. More likely, the hypercrosslinked networks demonstrate distinctly different, unique deformation and relaxation properties. 

In addition to the physical stress relaxation effect, chemical changes to the elastomer (e.g., by oxidation) can also increase relaxation rate. We then have... 

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. 

These results clearly indicate that the multi-frequency dynamic analysis method allows us to estimate the contribution of different relaxation mechanisms during curing of elastomers, and the changes in chemical and physical networks densities can be studied separately. 

HDI Heat buildup An abbreviation for hexamethylene diisocyanate. The temperature rise within an elastomer due to hysteresis. In many end-use applications, an elastomer can be subjected to repeated cycles of deformation-relaxation. As this occurs, friction between the elastomer molecules generates heat. As elastomers have relatively poor thermal conductivity, the heat generated builds up over time, progressively increasing the internal temperature of the elastomer. If the temperature increases above 70°C, the elastomer physical properties can begin to reduce. Design of the elastomer part can play an important role in minimizing the effects of heat buildup. 

Solid state NMR offers several advantages for the investigation of filled rubbers since molecular properties of elastomer chains can be measured selectively by NMR e>q)eriments. The method is very sensitive to the molecular scale heterogeneity in a sample. The network structure which is composed of chemical, physical and topological junctions can also be andyzed by NMR relaxation experiments [11,12,14,15],... 

Effects of Temperature on Physical Crosslinks Induced by Crystallites. All five RIM elastomers, containing different amounts of rigid segments, and four RIM elastomers A, modified with a plasticizer, were evaluated for crosslink densities by stress relaxation method at a low extension of 50% and at three temperatures of 25°C, 50°C, and 100 C. Crosslink densities, c, were calculated using Equation 6 ... 

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. 

Figure 5-16. Physical stress relaxation at 25 °C in a crosslinked network made by a zinc oxide cure of a halobutyl rubber. The elastomer has been soaked in oil to speed the relaxation. The line is the fit to the data using the Chasset-Thirion equation with E =0.018 MPa, t= 1.04 s and m = 0.31. The nature of the polymer and the cure makes for a chemically stable structure otherwise, the plot would begin to curve downward at long time.

Introduction of small amounts of ionic groups in hydrocarbon polymers exerts a profound effect on their mechanical properties. These ionic groups neutralized with suitable metal ions act as physical cross-hnks within the polymer matrix. The ionic associations can be thermally relaxed to permit sufficient melt flow at the processing temperature. The ion containing polymer thus behaves as a thermoplastic elastomer having the unique ability to act as cross-linked elastomer at ambient temperature, and to melt and flow at elevated temperatures like thermoplastics [50-53]. 

Of primary importance in moist environments is the plasticization, or softening, of the adhesive, a process that depresses Jg and lowers the modulus and strength of the elastomer [89-91]. Plasticization of the adhesive may also allow disengagement from a microrough adherend surface to reduce physical bonding and thus reduce joint strength and durability [37]. On the other hand, it may allow stress relaxation or crack blunting and improve durability [92]. 

In a stress-relaxation experiment, the sample under study is deformed by a rapidly applied stress. As the stress is normally observed to reach a maximum as soon as the material deforms and then decreases thereafter, it is necessary to alter this continually in order to maintain a constant deformation or measure the stress that would be required to accomplish this operation. The apparatus used varies in complexity with the physical nature of the sample, being simplest for an elastomer and becoming more sophisticated when the polymer is more rigid. 

Schneider, K Schone, A. (2008). Online-structure characterisation of polymers during deformation and relaxation by Synchrotron-SAXS and WAXS, In Reinforced Elastomers Fracture Mechanics, Statistical Physics and Numerical Simulations Kaliske, M. Heinrich, G. Verron, E. (Eds.) EUROMECH Colloquium 502, Dresden, 2008 pp. 79-81... 

Thirion relaxation, which usually is reversible, involving complex adjustments of the chains in a network in response to a stress, and possibly involving chain motion of physical entanglements. The long-term relaxation of thermally stable elastomers illustrates this phenomenon (Nielsen, 1962, Chapter 3 Thirion and Chasset, 1962). 

The book covers aspects from the morphology to mechanical aspects focused on the elasticity and inelasticity of amorphous to crystalline polyurethane elastomers, in relation to their sensitivity to chemical and physical structure. In such polymers, resilience of the material is an important attribute. In many applications they are in commercial competition with other, relatively soft, elastomeric materials. The choice of material for any given application then hinges on a spectrum of key properties offered by relatively soft polymers—stiffness and strain recovery characterizing their elasticity, but also inelastic effects such as hysteresis and stress relaxation. In these respects the mechanical properties of polyurethane elastomers are similar to those of other elastomers. 

Characterization of Poiymer Networks by Transverse Relaxation. The NMR transverse magnetisation relaxation experiments were extensively used for quantitative analysis of the density of chemical cross-links, temporary and trapped chain entanglements and physical network junctions which are formed in fllled rubbers, semicrystalline and ionic containing elastomers, and for determination of the molecular-scale heterogeneity of polymer networks ((83), and reference therein). 

Since the basic notions of chain motion in the bulk state are required to understand much of physical polymer science, a brief introduction is given here. Applications include chain crystallization (to be considered beginning in Chapter 6), the onset of motions in the glass transition region (Chapter 8), and the extension and relaxation of elastomers (Chapters 9 and 10). 

Amorphous polymers when heated above Tg pass from the hard to the soft state. During this process, relaxation of any internal stress occurs. At the Tg many physical properties change abruptly, including Young s and shear moduli, specific heat, coefficient of expansion and dielectric constant. For hard polymeric materials this temperature corresponds to the highest working temperature, for elastomers, it represents the lowest working temperature. 

Figure 5.36. Diagram of the morphology of relaxed S-B-S thermoplastic elastomers revealing the physical cross-linking of the system by the glassy nodules of polystyrene.

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