Tensile properties modulus, equilibrium

The equilibrium shear modulus of two similar polyurethane elastomers is shown to depend on both the concentration of elastically active chains, vc, and topological interactions between such chains (trapped entanglements). The elastomers were carefully prepared in different ways from the same amounts of toluene-2,4-diisocyanate, a polypropylene oxide) (PPO) triol, a dihydroxy-terminated PPO, and a monohydroxy PPO in small amount. Provided the network junctions do not fluctuate significantly, the modulus of both elastomers can be expressed as c( 1 + ve/vc)RT, the average value of vth>c being 0.61. The quantity vc equals TeG ax/RT, where TeG ax is the contribution of the topological interactions to the modulus. Both vc and Te were calculated from the sol fraction and the initial formulation. Discussed briefly is the dependence of the ultimate tensile properties on extension rate. 

Figure 22 is a plot of the initial tensile modulus of the epoxy matrix after equilibrium moisture exposure and dehydration. At both 20 °C and 70 °C, the effect of moisture absorption on the matrix is reversible as evidenced by the reattainment of dry properties. The exposure at 125 °C is not completely reversible as shown by the data. 

Because of the time dependence of polymer properties, it is important that the time scale used in our standard tests corresponds exactly with that of the moment loading. That is, if the moment is to be a sustained load, then the tensile test should also be at very long loading, for example, in the equilibrium modulus region. 

UHMW PE fibers grown by the surface growth technique above the thermodynamic equilibrium tenperature obtained a tensile modulus of 133 GPa and a breaking stress as high as 5.04 GPa at a breaking strain of 3.1 %, showing good physical properties. 

Hydrogen bonding makes a major contribution to the intermolecular interactions in PBU and the presence of H-bonds contributes to mechanical properties. As a result of the swelling, the concentration of elastically active chains in a labile network of elastomer is reduced. The theoretical evaluation of change in H-bond concentration in PBU on plasticization by a proton acceptor (DOS) shows that decrease in concentration of H-bonds occurs even with small amounts of plasticizer (< )i = 0.05 - 0.2). The density of a labile network determined by H-bonds decreases sharply and modulus and tensile strength of polymer decrease as well. Figures 10.71 and 10.72 show that the character of influence of a plasticizer on elastomer is similar for two very diiferent strain rates. This allows to carry out evaliration of the physical network density of elastomer under the non-equilibrium conditions of deformation using Eq. [10.33] as previously reported. ... 

The indicated equilibrium compliance of 10 7 1 cm /djme corresponds to an equilibrium shear modulus of 12 x 10 dynes/cm, close to but considerably higher than that found. This may reflect an unexpected difficulty in preparing reproducible samples or it may reflect a different sample histoiy. The tensile specimens were tested shortly after preparation while the dynamic mechanical properties were measured some l8 months after preparation. 

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