Properties in the Rubbery State

Above Tg, the network chains have sufficient thermal energy to overtake the potential barriers linked to Van der Waals interactions. They imdergo fast conformational changes through cooperative segmental motions, but cross-linking prevents any liquid flow. We are thus in the presence of a peculiar state of matter, which displays at the same time liquid and solid (elastic) properties the rubbery state. 

Most of the physical properties of networks in the rubbery state can be linked to two groups of quantities that characterize respectively, the equilibrium entropic elasticity and the relaxation kinetics (linked to the segmental mobility). 

The equilibrium (relaxed) elastic properties of polymers in the rubbery state display two very important features  

The elastic modulus is an increasing function of temperature. In the ideal case, it is proportional to T. 

Elasticity is nonlinear the secant modulus (the stress/strain ratio) decreases rapidly as strain increases. It ean be reversible at very high strains, but this property can be rarely checked for in thermosets owing to their brittleness at T Tg. 

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The reaction of curing the epoxy-amine system occurring in the diffusion-controlled mode has little or no effect on the topological structure of the polymer 74> and on its properties in the rubbery state. However, the diffusion control has an effect on the properties of glassy polymers 76 78). 

The equilibrium properties in the rubbery state are almost exclusively governed by the macromolecular scale structure crosslinking suppresses liquid flow, decreases the number of available network complexions, and the gap between equilibrium (unstretched) and fully stretched network states. With regard to time-dependent properties (viscoelasticity, time-... 

Studies of epoxy-amine polymers, considered in the present article, offer a rather clear picture of structure-properties relationships for many properties of network polymers depending on their chemical composition in the glassy state near Tg and in the rubbery state. Mechanical properties in the rubbery state at a given chemical composition depend on network topology. 

Analytical methods for the determination of the number of chain scissions S per mass unit are scarce. When elastic properties in the rubbery state are measurable, one can use the theory of rubber elasticity (Hory, 1953), according to which ... 

Figure 1.1 describes the general regions of viscoelastic behavior for amorphous polymers where mechanochemistry may be conducted. The tensile and shear moduli for crystalline and amorphous polymers in the solid state are generally in the range of 10 dyn/cm. For the rubbery state, the value is about 10 dyn/cm, and it varies with the density of entanglements and chemical cross-links. The modulus can be calculated from the theory of rubber elasticity, and the short-time viscoelastic properties in the rubbery state are not unlike those of a common rubber band. 

In the preparation and processing of ionomers, plasticizers may be added to reduce viscosity at elevated temperatures and to permit easier processing. These plasticizers have an effect, as well, on the mechanical properties, both in the rubbery state and in the glassy state these effects depend on the composition of the ionomer, the polar or nonpolar nature of the plasticizer and on the concentration. Many studies have been carried out on plasticized ionomers and on the influence of plasticizer on viscoelastic and relaxation behavior and a review of this subject has been given 119]. However, there is still relatively little information on effects of plasticizer type and concentration on specific mechanical properties of ionomers in the glassy state or solid state. 

Amorphous polymers convert reversibly between the rubbery and glassy states as their temperature rises or falls. Below their glass transition temperature, amorphous polymers exist in a glassy state. Above their glass transition temperature they are rubbery. We can demonstrate this easily with a racquet ball, which is made of an amorphous polymer. At room temperature, as we all know, the ball bounces at this temperature it is in the rubbery state. If we immerse the ball in liquid nitrogen it becomes brittle and will shatter when we drop it, i.e., it has become a glass, If we were to allow the frozen ball to warm up to room temperature, it would become rubbery once more. We can freeze and thaw the same ball repeatedly with no loss of its properties at room temperature. 

Finally, we turn from solutions to the bulk state of amorphous polymers, specifically the thermoelastic properties of the rubbery state. The contrasting behavior of rubber, as compared with other solids, such as the temperature decrease upon adiabatic extension, the contraction upon heating under load, and the positive temperature coefficient of stress under constant elongation, had been observed in the nineteenth century by Gough and Joule. The latter was able to interpret these experiments in terms of the second law of thermodynamics, which revealed the connection between the different phenomena observed. One could conclude the primary effect to be a reduction of entropy... 

The latter result shows that to interpret the mechanical properties of networks we do not need to take into account the spatial inhomogeneities of crosslink distribution in a sample, at least in the rubbery state. The analysis of epoxy networks performed under the framework of a tree-like model and experiments 7,10-26) brought the... 

Several polymer properties are important in determining the ability to sorb vapors. The glass transition temperature, Tg, is the temperature at which a polymer changes from glassy to rubbery, as described in Chapter 4. Above Tg, (in the rubbery state), permeability is governed entirely by diffusional forces and sorption proceeds rapidly and reversibly. The sorption process is very much like absorption into a liquid and, as discussed later in the context of sorption mod-... 

List three examples of load-bearing engineering components where the properties of polymers in the rubbery state are erqrloited. Add brief notes on each to indicate what you believe were the primary reasons for the decision to employ a rubbery polymer. 

Techniques measuring the (thermo)mechanical properties as cure proceeds are very appropriate for the assessment of vitrification. One of the most important changes upon vitrification is the increase in modulus by two or three orders of magnitude (from 10 Pa in the rubbery state to 10 Pa in the glassy state), together with a change in cure shrinkage. 

Dufresne et al. studied stress vs strain curves (nominal data) for the chitin whiskers/unvulcanized NR evaporated composites, shown in Figure 14.12."" The polymeric matrix is in the rubbery state and its elasticity from entropic origin is ascribed to the presence of numerous entanglements due to high molecular weight chains. They further observed that the incorporation of anhydride and isocyanate modified chitin whiskers into NR lead to composite materials with improved mechanical properties. The study of the morphology of these nanocomposites leads to the conclusion that the various chemical treatments improve the adhesion between the filler and the matrix (Figure 14.13). However in some cases there is loss of performance, which could be due to the partial or total destruction of the three-dimensional network of chitin whiskers assumed to be present in the unmodified composites. 

As we have seen, most non-crystalline polymers can obtain a rubbery state, as shown in Fig. 1.4. At low temperatures, the polymer is glassy, with a high modulus (10 NM ). As the temperature is raised the polymer passes through the glass transition region (Tg), where it becomes visco elastic, with modulus very rate and temperature dependent. Beyond Tg, the polymer becomes rubbery, but not all polymers in the rubbery state show useful elastomeric properties most thermoplastics will flow irreversibly on loading - i.e. they are more visco than elastic. 

Glass transition temperature (Tg), coefficients of thermal expansion (CTE) at temperatures below and Ugher Tg, and the modulus of elasticity in the rubbery state (Eoo) were investigated. In the case of aligned polymers the thermal and mechanical properties were measured in the direction perpendicular and parallel to the magnetic field applied to the samples. 

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