Rubber, viscoelastic behavior

J.L. Leblanc, Investigating the non-linear viscoelastic behavior of filled mbber compounds through Fourier transform rheometry. Rubber Chem. TechnoL, 78, 54—75, 2005. 

The chemical structure of the epoxy matrix constituent as well as processing are reported to strongly influence 11 -I3> the thermoset network and hence the properties and durability of the crosslinked polymer 11 ,4-16). The cure of a reactive prepolymer involves the transformation of low-molecular-weight reactive substances from liquid to rubber and solid states as a result of the formation of a polymeric network by chemical reaction of some groups in the system. Gelation and vitrification are the two macroscopic phenomena encountered during this process which strongly alter the viscoelastic behavior of the material. 

The viscoelastic properties of the crystalline zones are significantly different from those of the amorphous phase, and consequently semicrystalline polymers may be considered to be made up of two phases each with its own viscoelastic properties. The best known model to study the viscoelastic behavior of polymers was developed for copolymers as ABS (acrylonitrile-butadiene-styrene triblock copolymer). In this system, spheres of rubber are immersed in a glassy matrix. Two cases can be considered. If the stress is uniform in a polyphase, the contribution of the phases to the complex tensile compliance should be additive. However, if the strain is uniform, then the contribution of the polyphases to the complex modulus is additive. The... 

The mechanical behavior of the hydrogels can be described by the theories of rubber elasticity and viscoelasticity, which are based on time-independent and time-dependent recovery of the chain orientation and structure, respectively. Mechanical properties due to rubber elastic behavior of hydrogels can be determined by tensile measurements, while the viscoelastic behavior can be determined through dynamic mechanical analysis. 

For a crossllnked rubber sample, one simple parameter which can be used to roughly characterize the material is the crosslink density (v) or the average molecular weight between crosslinks (Mg a 1/v). It should be clear that this single parameter cannot completely represent a network in general. Nevertheless, it is well known that the viscoelastic behavior of a polymer network will vary with crosslink density as schematically depicted in Figure 1 for the creep behavior of a polymer at two crosslink densities < Vq. Here the kinetic theory of rubber elasticity... 

As we have seen above, the transition that separates the glassy state from the viscous state is known as the glass-rubber transition. This transition attains the properties of a second-order transition at very slow rates of heating or cooling. In order to clearly locate the region of this transition and to provide a broader picture of the temperature dependence of polymer properties the principal regions of viscoelastic behavior of polymers will be briefly discussed. 

A fundamental difficulty in the study of the linear viscoelastic behavior of filled rubbers is the secondary aggregation of filler particles, which greatly influences the behavior at small strains, where the response is linear. The effect of this aggregation is overcome at large strains, but now non-linearity and a number of other complications become problems. 

Figure 15.5 shows a plot of the friction factor versus the Reynolds number as defined in Eq. 15.10. Because the Reynolds number has been defined by Eq. 15.10, the laminar-flow data must fall on the line shown. For flow at Reynolds numbers greater than 2000, two possible kinds of behavior are known. All slurries and many polymer solutions are represented by the solid curve in Fig 15.5. These do not seem to significantly suppress the turbulent behavior of the fluid. However, some polymer solutions and polymer melts, particularly those which show distinct viscoelastic behavior (such as rubber cement) obey the curves shown dotted at the right in Fig. 15.5. Visual observation [7] indicates that for these fluids the turbulence in the fluid is much less than it would be for a newtonian fluid at the same Reynolds number.

Numata and Kinjo (52) have shown rubber-modified isocyanurate-oxazolidone resins may be effectively modified with carboxyl-reactive nitrile liquids. The viscoelastic behavior of models using a polyglycidyl ether of phenol-formaldehyde novolac resin and di-phenylmethane-4,4 -diisocyanate is discussed. Such resins have suggested utility in thin films as electrical varnishes. 

Curemeters, oscillating disc, and reciprocating paddle types are forms of plastimeter that measure plasticity before the onset of cure, as well during the cure. The viscoelastic behavior of uncured rubber tested by dynamic measurement is characterized by in-phase and out-of-phase moduli and loss tangent. 

CHAPTER I 5 The Viscoelastic Behavior of Rubber and Dynamics of Blends GID... 

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