Filler networking in elastomers

Flocculation of fillers during heat treatment

For a deeper understanding of filler networking in elastomers it is useful to monitor structural relaxation phenomena during heat treatment (annealing) of the uncrosslinked composites. This can be achieved by investigations of the time development of the small-strain storage modulus G′0 that provides information about the flocculation dynamics [138,221–224]. Figure 10.1(a) shows the time development of the small-strain storage modulus G′0 at 0.28% strain and 1 Hz of three elastomer composites containing 50 phr carbon black of different grades. The sample with the smallest primary aggregate size (N115) exhibits the most pronounced increase of the storage modulus with annealing time, which levels out after about 10 minutes in this example. The extent of modulus gain reduces with increasing primary aggregate size and the N550 sample shows almost no effect. With increasing dynamic strain amplitude, as depicted in Fig. 10.1(b), the storage modulus decreases by about one order of magnitude (the Payne effect). Thus, it appears that during heat treatment a weakly bonded superstructure develops in the systems which stiffens the polymer matrix, indicating that the increase of the modulus results from flocculation of primary aggregates to form secondary aggregates (clusters) and finally a filler network. The dependence of the effect on the primary aggregate size is in accordance with the picture of a kinetic aggregation process.

Figure 10.2(a) shows the time development of G′0 of S-SBR melts of variable molar mass filled with 50 phr carbon black (N234), when a step-like increase of the temperature from room temperature to 160 °C is applied. Figure 10.2(b) shows a strain sweep of the same systems after 60 minutes annealing time.