Behavior Above Tg

The viscoelastic response of equilibrium rubber networks can be obtained by measuring the shear and tensile moduli or compliances as a function of time, or the corresponding dynamic moduli and compliances as a function of frequency. As discussed in Section II, the measurements of any viscoelastic function can be converted to another viscoelastic function. 

The Epons 828,1001,1002,1004, and 1007 fully cured with stoichiometeric amounts of DDS are examples of well-characterized networks. Therefore, mechanical measurements on them offer insight into the viscoelastic properties of rubber networks. The shear creep compliance J t) of these Epons were measured above their glass temperatures [11, 12, 14]. From the statistical theory of rubber elasticity [1-5, 29-33] the equilibrium modulus Ge is proportional to the product Tp, where p is the density at temperature T, and hence the equilibrium compliance is proportional to (Tpy Thus J t) is expected to be proportional to and J(t)Tp is the quantity which should be compared at different temperatures. Actually the reduced creep compliance 

FIGURE 6 Reduced shear creep compliance curves cmVdyne, determined on Epon 

1007/DDS at seven temperatures, as indicated, presented logarithmically as a function of logarithmic time t. 

Traditionally it is assumed that the temperature dependences of the retardation times of all viscoelastic modes or mechanisms of polymers are proportional to one and the same monomeric friction coefficient Co [1,5,7,34], For rubber networks, the viscoelastic modes include those with shorter retardation times responsible for volume change and the glass temperature, and the longer retardation times of polymer strands between crosslinks contributing to rubbery deformation and Thus the retardation times A(T) of all the viscoelastic modes contributing to Jp(t) at any temperature T are related to that at a chosen reference temperature To by the same multiplicative factor given by 

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The physical interactions caused by "entanglements" between polymer chains gradually begin to play a more important role in determining the elastic behavior above Tg as the... 

The effects of annealing at 85 C (Figure 5) seems much more complicated. The peak at the glass transition temperature decreased rapidly and the broad peak dominated behavior above Tg. 

The Burger model represents the behavior above Tg in the elastomeric state, while the single dash-pot simulates melt flow at higher temperatures. Around the transition between the glassy and elastomeric states, the modulus of elasticity drops 10 to 10 fold, and the material becomes flexible and... 

Polymers can exhibit both viscous and solid mechanical behavior this phenomenon is called viscoelasticity. For a given polymer, the degree of viscous behavior depends on temperature. Below Tg, polymers will behave more or less as elastic solids with very little viscous behavior. Above Tg,... 

Recently, Souza and co-workers have described a special class of hybrid polyelectrolyte in which ion mobility presents an Arrhenius-type behavior above Tg, suggesting a segmental motion decoupled polymer system. Besides, the ion transport mechanism seems to be governed by thermally active ion hopping with the counter-ion fixed in the hybrid matrix. Based on this concept, many possibilities in solid state chenustry and physics arising within the several areas of elecfroactive and optically active... 

The above test provides a basis for evaluating a seal material s capability at the desired operating temperature. However, in realistic stack conditions, a seal material is under a shear stress. A double tube arrangement can be used to study the seal behavior. A disc can be sealed on both sides, and both tube enclosures can be pressurized to the same level. Such condition will eliminate the flexing of the membrane causing the seal to delaminate at a fairly low pressure when tested above Tg. In fact, a repeat test of the above seal with a double-tube arrangement showed that the seal could withstand 20 psi pressure before a small leak developed. 

The rate of all these processes, of course, depends strongly on the temperature in the vicinity of Tg the polymer chains are still relatively inflexible. Thus deformation requires considerable forces, and recovery occurs very slowly. Well above Tg the melt deforms more easily, but the tendency to flow as a result of increased macro-Brownian motion is still outweighed by the elastic recovery. The temperature range for pronounced elastic behavior of the polymer melt depends... 

Extensive measurements show that self-diffusivities in the relaxed glassy state are time independent and closely exhibit Arrhenius behavior (i.e., In Z), vs. 1/T plots appear as essentially straight lines) [8-11]. The diffusion therefore is thermally activated (in contrast to self-diffusion in the liquid above Tg as described in Section 10.1). 

An important difference between the PS-gas systems (Kwag et al., 1999) and the PDMS-C02 system (Gerhardt et al., 1997) is that the viscosity measurements of the PS-gas systems are conducted at temperatures within 75 °C of T of PS, whereas the PDMS-C02 measurements were performed nearly 200 °C above Tg of PDMS. The difference between these two thermal regimes leads to several differences in the observed rheological behavior. The viscosity reductions relative to the pure polymer are much greater for PS-gas systems than for PDMS-C02 systems at similar dissolved gas compositions, and the dependence of ac on temperature is much more pronounced for the PS-gas systems. These trends are consistent with the observations of Gerhardt et al. (1997, 1998) that the effect of dissolved gas on polymer melt viscosity occurs primarily through a free-volume mechanism. 

Tazuke, Ikeda, and their co-workers [53,54] were the first to use phenylene-diacrylate (PDA) as a mesogen and chromophore to explore the possibility of forming two-dimensionally reinforced polymer materials by photochemical cross-linking of a thermotropic LC polymer. Polyesters, 23, of para-phenylene-diacrylic acid and several diols were synthesized and found to exhibit LC behavior upon annealing above Tg. Polymer 23a exhibited [53] the texture of a N meso-... 

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