Viscoelastic state rubbery

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. 

Plasticizers are particularly used for polymers that are in a glassy state at room temperature. These rigid polymers become flexible by strong interactions between plasticizer molecules and chain units, which lower their brittle-tough transition or brittleness temperature (Tb) (the temperature at which a sample breaks when struck) and their Tg value, and extend the temperature range for their rubbery or viscoelastic state behavior (see Figure 1.19). 

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 co-workers on cold rubber mastication, Reid [110] had already found that when vinyl polymers and monomers are subjected to mechanical deformation, both degradation and polymerization occur simultaneously. If, however, the polymer is too soluble in the monomer, this method cannot be applied since the rubbery state, necessary to have effective shear, cannot be achieved. 

An amorphous polymer is in a glassy (vitreous) stale below its glass liansitioo temperature and in a rubbery (viscoelastic) state above it In non-croas-linkcd polymers the viscoelastic behavior is attributed to chain cnianglemenis. 

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. 

In the molten state polymers are viscoelastic that is they exhibit properties that are a combination of viscous and elastic components. The viscoelastic properties of molten polymers are non-Newtonian, i.e., their measured properties change as a function of the rate at which they are probed. (We discussed the non-Newtonian behavior of molten polymers in Chapter 6.) Thus, if we wait long enough, a lump of molten polyethylene will spread out under its own weight, i.e., it behaves as a viscous liquid under conditions of slow flow. However, if we take the same lump of molten polymer and throw it against a solid surface it will bounce, i.e., it behaves as an elastic solid under conditions of high speed deformation. As a molten polymer cools, the thermal agitation of its molecules decreases, which reduces its free volume. The net result is an increase in its viscosity, while the elastic component of its behavior becomes more prominent. At some temperature it ceases to behave primarily as a viscous liquid and takes on the properties of a rubbery amorphous solid. There is no well defined demarcation between a polymer in its molten and rubbery amorphous states. 

Since the viscoelastic properties of the solid undergo a significant change as the solid undergoes a transition from the amorphous to rubbery states (due to elevation of temperature at constant moisture content or to an increase in moisture content at constant temperature), one also expects marked changes in the processing properties of these solids as this transition occurs. Some properties that are likely to be affected include tablet compaction [76], gelatin capsule... 

Filler-filler interaction (Payne effect) - The introduction of reinforcing fillers into rubbery matrices strongly modifies the viscoelastic behavior of the materials. In dynamic mechanical measurements, with increasing strain amplitude, reinforced samples display a decrease of the storage shear modulus G. This phenomenon is commonly known as the Payne effect and is due to progressive destruction of the filler-filler interaction [46, 47]. The AG values calculated from the difference in the G values measured at 0.56% strain and at 100% strain in the unvulcanized state are used to quantify the Payne effect. 

The possibility for the existence of mesophase in a rubbery state 36,46), typical only for macromolecular compounds with their natural ability to display big reversible deformations, reveals interesting prospects from the viewpoint of creation of new types of liquid-crystalline materials in the form of elastic films, as well as for development of the theory of viscoelastic behaviour of such unusual elastomers. 

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-... 

The only "normal" phase state for polymers, known from the physics of small molecules, is the liquid state, though even here polymers show special properties, like viscoelasticity. The typical states of polymers are the glassy, the rubbery and the semi-crystalline state, all of which are thermodynamically metastable. 

We have approached the subject in such a way that the book will meet the requirements of the beginner in the study of viscoelastic properties of polymers as well as those of the experienced worker in other type of materials. With this in mind. Chapters 1 and 2 are introductory and discuss aspects related to chemical diversity, topology, molecular heterodispersity, and states of aggregation of polymers (glassy, crystalline, and rubbery states) to familiarize those who are not acquainted with polymers with molecular parameters that condition the marked viscoelastic behavior of these materials. Chapters 1 and 2 also discuss melting processes and glass transition, and factors affecting them. 

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