The determination of network chain density and the chemical stress relaxation of crosslinked polymers

Both initial network chain densities n M (0) and n s (0) of dicumyl peroxide‐cured natural rubbers were determined from the tensile stress and swelling method, respectively. The difference between n M (0) and n S (0) was usually constant, independent of the magnitude of network chain density. That is, it was found that the number of entanglement network chains in the crosslinked natural rubber was usually constant, independent of network chain density. The entanglement network chain density n II (0) was 0.7 × 10 −4 mole/cc. This led to the supposition that the molecular weight between entanglement points ( M e ) would be about 9000. Although this value is far from exact, it does not differ too greatly from the value found for noncrosslinked natural rubber. Next, in order to calculate the number of main‐chain scission of crosslinked polymers from their chemical stress relaxation, we proposed our modification of Tobolsky's equation. Using our equation, it was found that the scission of dicumyl peroxide‐cured natural rubber occurred in the main chain only. Furthermore, this value agreed with the one obtained from the oxidation of toluene solution of noncrosslinked rubber under the same conditions.