Crosslinkers storage modulus curve

Figure 12.6 Schematic representation of the effects of plasticizers and crosslinkers on the storage modulus curve. Hollow and full symbols differentiate initial (polymer blend) and final (influence of plasticizer or crosslinker) situations.

Thermodynamic Analysis. As reported previously, the storage modulus G of PDMS networks with tetrafunctional crosslinks is independent of frequency between 10 3 and 1 Hz (21). This behaviour which is entirely different from that of vulcanized natural rubber or synthetic polyisoprene networks, was attributed to the lack of entanglements, both trapped and untrapped, in these PDMS networks. Figure 4 shows that G of a network with comb-like crosslinks is also frequency independent within an error of 0.5%. For comparison, two curves for PDMS having tetrafunctional crosslinks are also shown. The flat curves imply that slower relaxations are highly unlikely. Hence a thermodynamic analysis of the G data below 1 Hz can be made as they equal equilibrium moduli. 

Figure 7. Storage modulus vs. temperature curves for (A) linear amorphous polymer (B) crosslinked polymer (C) semicrystalline polymer (D) PTMA/MDI/BD-segmented copolymer (32% MDl by wt) (E) PTMA/MDI/BD-segmented copolymer...

Fig. 4. The five regions of viscoelastic behavior. All polymers exhibit these five regions, but crosslinking, crystallinity, and varying molecular weight alter the appearance of this generalized curve. The loss modulus Tg peak appears just after the storage modulus enters the glass transition region.

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