Viscoelastic state glassy

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. 

When a liquid supercools (i.e., does not crystallize when its temperature drops below the thermodynamic melting point), the liquidlike structure is frozen due to the high viscosity of the system. The supercooled liquid is in a so-called viscoelastic state. If the crystallization can be further avoided as the ten ierature continues to drop, a glass transition will happen at a certain temperature, where the frozen liquid turns into a brittle, rigid state known as a glassy state. A well-accepted definition for glass transition is that the relaxation time t of the system is 2 X10 s or the viscosity / isio Pas (an arbitrary standard, of course). 

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). 

Figure 11-12. Schematic representation of the compliance C as a function of time, tr is the orientation time. I, Glassy state II, viscoelastic state III, entropy-elastic state IV, viscous flow.

The glass transition phenomenon has been the object of many molecular theories, that of Ferry [29], further developed by Bueche [30], on the free volume concept being widely accepted in the polymer field. The volume occupied by a chain of amorphous polymer consists partly of free space, i.e., the volume excluded by the movements of segments about their equilibrium position. As shown in Fig. 1.8, the temperature coefficient for the polymer volume at constant pressure dV dT)p is higher for the viscoelastic state (curve b) than for the glassy state (curve a), and it changes abruptly at Tg this is the mathematical definition of the glass transition temperature. It can also be defined as the... 

Fig. 1.8. Variation of the total free and filled volume with the temperature for a polymer, (a) Glassy state, (b) Viscoelastic state.

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. 

The interphase (intermediate phase) is a result of an imperfect separation of the soft and hard phases it can be in the high-elastic, viscoelastic, or glassy state, and these physical states determine the material flexibility. 

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. 

Mechanical properties of plastics are invariably time-dependent. Rheology of plastics involves plastics in all possible states from the molten state to the glassy or crystalline state (Chapter 6). The rheology of solid plastics within a range of small strains, within the range of linear viscoelasticity, has shown that mechanical behavior has often been successfully related to molecular structure. Studies in this area can have two objectives (1) mechanical characterization of... 

Results of the next section indicate that the three-dimensional lattice acts much like an Einstein solid with a single phonon frequency vE. Viscoelastic experiments (5) on rosin in the glassy state are consistent with the sharp distribution of relaxation times thus predicted. In the dielectric case where each oscillator contains a dipole,... 

In 97- it was also shown on the basis of dilatometric data that the free-volume of PMMA in the mixture with polyvinylacetate increases with the increase in FVA concentration. In 98) a large difference was reported in the viscoelastic behavior of block copolymer from that predicted by WLF theory. This theory is believed to be useful only near the Te of each component, not in the broad temperature interval including the transition from glassy to rubberlike state. This anomaly is thought to be connected with certain motions in the interphase regions, which should be looked upon as independent components of the mixture. 

These new variables are necessary to take into account viscoelastic effects linked to molecular motions. These effects are non-negligible in the glassy domain between boundaries a and (3 in the map of Fig. 11.2, and they are very important in the glass transition region (around boundary a). Here, we need relationships that express the effects of s, d (the stress rate may be used instead of the strain rate), and T on the previously defined elastic properties. Also numerical boundary values of elastic properties are required, characterizing unrelaxed and relaxed states (see Chapter 10). 

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