Composite networks stress-strain properties

After introduction of cross-links in the strained state, the composite network retracts, upon release, to a stress-free state-of-ease (J9 ) The amount of retraction is determined by the degree of strain during cross-linking and by the ratio >i/v2. The elastic properties relative to the state-of-ease are isotropic for a Gaussian composite network ( 8, 1 9,20). 

In order to check whether the mechanical properties of the PDMS network are really improved by the CAB network introduction in a final IPN material, the stress-strain curves of PDMS/CAB IPNs with different compositions were recorded (Fig. 2). The PDMS and CAB single networks break when less than 10 and 5% strains are applied respectively. On the other hand, the elongation at break is over 150% for the PDMS/CAB (50/50) IPN. Thus, the poor mechanical properties of the PDMS... 

The stress-strain behavior of thermosets (glassy polymers crosslinked beyond the gel point) is not as well-understood as that of elastomers. Much data were analyzed, in preparing the previous edition of this book, for properties such as the density, coefficient of thermal expansion, and elastic moduli of thermosets [20,21,153-162]. However, most trends which may exist in these data were obscured by the manner in which the effects of crosslinking and of compositional variation were superimposed during network formation in different studies, by... 

Metal alkoxides of silicon, titanium, zirconium, and aluminum have been used to prepare optically transparent composite films with the etheric phosphazene polymer poly[bis(2-(2-methoxyethoxy)ethoxy)-phosphazene] (MEEP). Dynamic mechanical and stress-strain techniques are used to evaluate and compare the mechanical properties of the different composites. Despite strong interactions with the inorganic oxide network, the glass transition temperature (Tg) of MEEP does not shift to higher temperature upon addition of the inorganics, but the polymer is reinforced by the oxides. The addition of potassium triflate to the composites resulted in materials with both excellent mechanical properties and increased conductivity. 

Dufresne et al. studied stress vs strain curves (nominal data) for the chitin whiskers/unvulcanized NR evaporated composites, shown in Figure 14.12."" The polymeric matrix is in the rubbery state and its elasticity from entropic origin is ascribed to the presence of numerous entanglements due to high molecular weight chains. They further observed that the incorporation of anhydride and isocyanate modified chitin whiskers into NR lead to composite materials with improved mechanical properties. The study of the morphology of these nanocomposites leads to the conclusion that the various chemical treatments improve the adhesion between the filler and the matrix (Figure 14.13). However in some cases there is loss of performance, which could be due to the partial or total destruction of the three-dimensional network of chitin whiskers assumed to be present in the unmodified composites. 

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