Viscoelastic state

As already stated, mastication reactions are not limited to elastomers but can be extended to all polymers in the viscoelastic state. It is thus interesting to note that before the fundamental study of Watson and coworkers on cold rubber... 

Spin-spin relaxation times (T2) in polymer systems range from about 10-5 s for the rigid lattice (glassy polymers) to a value greater than 10-3 s for the rubbery or viscoelastic state. In the temperature region below the glass transition, T2 is temperature independent and not sensitive to the motional processes, because of the static dipolar interactions. The temperature dependence of T2 above Tg and its sensitivity to low-frequency motions, which are strongly affected by the network formation, make spin-spin relaxation studies suitable for polymer network studies. 

A pseudo solid-like behavior of the T2 relaxation is also observed in i) high Mn fractionated linear polydimethylsiloxanes (PDMS), ii) crosslinked PDMS networks, with a single FID and the line shape follows the Weibull function (p = 1.5)88> and iii) in uncrosslinked c/.s-polyisoprenes with Mn > 30000, when the presence of entanglements produces a transient network structure. Irradiation crosslinking of polyisoprenes having smaller Mn leads to a similar effect91 . The non-Lorentzian free-induction decay can be a consequence of a) anisotropic molecular motion or b) residual dipolar interactions in the viscoelastic state. 

Most of us think of matter as being segregated into one of three states gas, liquid, or solid. Gases flow easily and expand to fill their container. Liquids also flow, but they take the shape of their container and have a fixed volume. Solids have a fixed shape and do not flow. As a general rule, polymers do not fall neatly into any of these categories. Most uncrosslinked (or lightly crosslinked) polymers do not behave exactly either as liquids or solids. Their behavior has characteristics of both liquid and solid states. We call this state the viscoelastic state, the same term we applied to certain solutions in Chapter 6. 

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