Creep Polymers, typical behavior

Fig. 3. Typical creep behavior for rubber-modified styrene polymers.

Galgali and his colleagues [46] have also shown that the typical rheological response in nanocomposites arises from frictional interactions between the silicate layers and not from the immobilization of confined polymer chains between the silicate layers. They have also shown a dramatic decrease in the creep compliance for the PP-based nanocomposite with 9 wt% MMT. They showed a dramatic three orders of magnitude drop in the zero shear viscosity beyond the apparent yield stress, suggesting that the solid-like behavior in the quiescent state is a result of the percolated structure of the layered silicate. 

Creep behavior is similar to viscous flow. The behavior in Equation 14.17 shows that compliance and strain are linearly related and inversely related to stress. This linear behavior is typical for most amorphous polymers for small strains over short periods of time. Further, the overall effect of a number of such imposed stresses is additive. Non-creep-related recovery... 

The peeling off of a hook fitted with a pressure sensitive adhesive and attached to a ceramic or glass surface can be regarded as a typical example for the creep behavior of an adhesive layer. In particular, thermoplastic adhesives that, to a great extent, also include pressure-sensitive adhesives (Section 5.6) tend to creep under high strain. A reason for this behavior is the time-related failure of individual bonds between the polymer molecules due to the strain imposed from outside. The application of adhesives with a higher crosslink ratio can reduce the adhesive layers tendency to creep. 

Viscoelastic Effects. Time plays a very Important role in the properties of polymers. For metals, creep is generally significant only at relatively high temperatures, typically greater than half their absolute melting temperature. However, many polymers demonstrate time dependent behavior not only at room temperature but even at cryogenic temperatures (38). 

In general, creep behavior of ceramics is similar to that of metals. However, in ceramics it usually occurs at higher temperatures, typically >0.5 Tni. In comparison, creep is a consideration in aluminum alloys at 100°C and in polymers at room temperature. Creep is particularly important in ice, which creeps extensively at low temperatures. The creep of ice is responsible for the movement of glaciers and the spreading of the Antarctic ice cap. 

Equation 30 shows that the yield stress is both rate and temperature dependent, hence it captures some important features of yield in polymers. For example, Figure 10 (44) shows a plot of ax/T (or aJT in the notation of Reference (44)) as a function of log strain rate and, as predicted by equation 30, a linear relationship is seen at each temperature. It is worth noting that the Eyring equation (typically in the form of two activated processes acting in parallel) has been successfiilly applied not only to the yield behavior of polsrmers but also to the creep rupture behavior of isotropic and oriented polymers (45- 8). 

Polymer properties exhibit time-dependent behavior, which is dependent on the test conditions and polymer type. Figure 1.7 shows a typical viscoelastic response of a polymer to changes in testing rate or temperature. Increases in testing rate or decreases in temperature cause the material to appear more rigid, while an increase in temperature or decrease in rate will cause the material to appear softer. This time-dependent behavior can also result in long-term effects such as stress relaxation or creep. These two time-dependent behaviors are shown in Fig. 1.8. Under a fixed displacement, the stress on the material will decrease over time, and this is called stress relaxation. This behavior can be modeled nsing a... 

Figure 4.7 Typical creep compliance curve for a linear, entangied, monodisperse polymer, using logarithmic scales for both axes.The distinct characteristicsofthe creep behavior in severai ranges of time are apparent in this representation. We see the giassy, transition, plateau and terminal zones.

It is important to understand the creep behavior shown in Figure 16.4 is not a simple superposition of linear elastic and viscous responses. Figure 16.5 shows the typical strain-time curves of ideal elastic material, ideal viscous material, and viscoelastic polymer fibers under constant stress. The ideal elastic material deforms instantaneously as the stress is applied and the stain remains constant with time. The removal of the stress causes the ideal elastic material to return to its original dimension. For the ideal viscous material, the strain increases linearly with time as long as the stress is applied. The removal of the stress does not return the ideal viscous material to the original dimension. This is because the eneigy introduced by the woik of the external stress is dissipated in the flow, leading to a permanent deformation. Both the ideal elastic and viscous responses contribute to the creep-recovery curve of the viscoelastic polymer fibers. However, the creep-recovery curve of viscoelastic polymer fibers is not a simple superposition of these two ideal behaviors. In addition to the ideal responses, the creep-recovery curve of the polymer fibers also includes retarded elastic response, in which... 

---