Stiffness of polymer

Many of the most floppy polymers have half-melted in this way at room temperature. The temperature at which this happens is called the glass temperature, Tq, for the polymer. Some polymers, which have no cross-links, melt completely at temperatures above T, becoming viscous liquids. Others, containing cross-links, become leathery (like PVC) or rubbery (as polystyrene butadiene does). Some typical values for Tg are polymethylmethacrylate (PMMA, or perspex), 100°C polystyrene (PS), 90°C polyethylene (low-density form), -20°C natural rubber, -40°C. To summarise, above Tc. the polymer is leathery, rubbery or molten below, it is a true solid with a modulus of at least 2GNm . This behaviour is shown in Fig. 6.2 which also shows how the stiffness of polymers increases as the covalent cross-link density increases, towards the value for diamond (which is simply a polymer with 100% of its bonds cross-linked. Fig. 4.7). Stiff polymers, then, are possible the stiffest now available have moduli comparable with that of aluminium. 

Reiirfbrcement. The strength and stiffness of polymers are improved by adding fibres of glass, carbon, etc. 

DMA can be applied to a wide range of materials using the different sample fixture configurations and deformation modes (Table 23.1) [10,11]. This procedure can be used to evaluate by comparison to known materials (a) degree of phase separation in multicomponent systems (b) amount type, and dispersion of filler (c) degree of polymer crystallinity, (d) effects of certain pretreatment and (e) stiffness of polymer composites [8,11]. 

All these tests are in common use to measure the tensile stiffness of polymers. For example, tests at constant extension rate are often carried out on an Instron tensile testing machine. Tensile creep is used in many cases while stress relaxation is not so common. Dynamic testing is commonly performed using the Rheovibron or other commercial equipment32 or home made equipment33,... 

The Strength and Stiffness of Polymers, edited by Anagnostis E. Zachariades and Roger S. Porter... 

The coils are mutually entangled, this fact is largely responsible for the special behaviour of polymers Between chain segments relatively weak interaction forces are present chain segments can move with respect to each other under the influence of relatively small external stresses. Consequently, the stiffness of polymers is rather low. 

Zachariades AE and Porter RS (Eds.), "The Strength and Stiffness of Polymers", Marcel Dekker, New York, 1983. Ziabicki AE and Kawai H (Eds), "High-speed Fibre Spinning", Wiley, New York, 1985. 

Among the applications discussed in this chapter, the most prominent in recent years is CNT-reinforced polymer nanocomposites. The use of CNTs in polymers can provide superior mechanical properties (60). For instance, the addition of 1% CNTs might increase the stiffness of polymers by 10% and increase their resistance to fracture however, improvements in the properties of CNT-reinforced polymers largely depend on the dispersion of CNTs within the polymer matrix and the polymer-CNT interfacial properties. The following section highlights several studies regarding the processing of PLA-CNT nanocomposites. 

Zachariades, A.E. Porter, R.S. Eds. "Strength and Stiffness of Polymers" Plastic Engineering Series, 1983 Vol. H. Marcel Dekker. 

It follows that the modulus, or stiffness of polymers cannot always be computed directly from the initial slope of the stress-strain graph, because of its previously mentioned non-linearity. It can be derived instead in various other ways. The initial tangent modulus is obtained using the slope of the tangent to the stress-strain curves at the origin, and similarly we... 

Figure 14.34 Temperature dependence of the elastic stiffnesses of polymer A. (Adapted from [13] by permission of The American Physical Society.)...

---