The rubber state

The rubber state is unique to linear and Hghtly cross-linked polymeric materials. It is characterised by the ability to undergo very large reversible deformations, to recover its original shape, to absorb energy and to damp vibrations. 

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Deformation of a polymer in the rubber state—of importance in vacuum forming, pressure forming and warm forging techniques. 

While in the temperature range called the rubbery plateau, the soft polymer responds instantaneously and reversibly to applied stress and tends to be Hookean. In the rubber state, the polymer approaches Hooke s law for... 

As already mentioned earher (see Section 7.1.1), because of its high glass-transition temperature, that is, T = 208°C, PPO has to be melt-processed at elevated temperatures. As a result, degradation of the polymer may occur at such temperatures (particularly through oxidation reactions at the methyl substituents), furthermore, upon cooling from the liquid to the rubber state, two unwanted events can take place (i) the polymer crystallizes and (ii) the molecular motions are frozen and the rubbery polymer turns to a glass. As a consequence, the material becomes brittle and cannot be used for practical applications. Fortunately, PPO exhibits unusual and remarkable blending properties [36]. 

POLYMER SCIENCE ASPECTS POLYMERS IN THE RUBBER STATE... 

The respective kinetics correlates with and can be predicted from the glass transition temperature of the carrier materials. Plasticization by water absorption under conditions of high humidity may cause reduction of the glass transition temperature below room temperature. Then, the structural change in the wall material leads to collapse of the food powder, resulting in flavor release from the rubber state of the carrier matrices (Ubbink and Schoonman, 2003). 

The viscoelastic state is also known as the rubber state. A piece of rubber under external force can be stretched. When the external force stops, the rubber recovers to its original position. Usually long-chain polymers can be induced to exhibit typical rubberlike behavior, for example, chains such as polyesters, polyamides, elastic sulfur (sulfur cooled from the liquid), and cellulose derivatives. 

The consideration of fiee space changing with temperature brings us to some of the earliest studies of the effect of time on both the transition temperature and the polymer volume. A French scientist, Kovacs, carried these out. He heated a sample of polymer to the rubber state and then cooled it to the glassy state, all the while making precise measurements of the volume (Figure 4.13). 

To demonstrate the entropic origin of the elastic response of the rubber state, we consider the thermodynamics of this state, in which changes in the internal energy E are given by... 

The second term on the right-hand side is usually negligible because the change in volume with extension of the solid is very small, (dQ/dL)pj 0. This leaves two important contributions to the force, one being energy-related, the other entropy-related. It turns out that in the rubber state... 

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