Question

Heating natural rubber with 0.100  mol of peroxide/l000  cm3 gives a network that swells to 4.35 times its original volume in toluene (at 25°C). Each peroxide molecule yields one cross-link. Heating the same rubber with 28.0  g of sulfur/1000  cm3 (instead of the peroxide) gives a network that swells to 3.45 times its original volume (at 25°C). If all the sulfur atoms react to form bridges with variable numbers of atoms between chains, how many sulfur atoms are there in an average    cross-link? The atomic weight of sulfur is 32.0. Toluene: density  =  0.867  g/cm3, FW = 92.14  g/mol.

Answer 1

The Crosslink density pf an elastomer can be determined from swelling measurements.An crosslinked elastomer above its gel point absorbs solvents and swells, but do not dissolve. Swelling continues until the retractive forces in the elastomer balance the forces.

Flory Rehner Euation is used.

"Flary -Rehner equation N' = -1/2V x ln(1 -V_r) + V_r + x V_r^2/ Vr^''3 -vr/2 where n"" is the no. of moles of cross links per unit volume, VS rightarrow molar Volume of the swelling solvent, V_x is the volume fraction of suffer in the swoller gell, and x is the polymer -solvent interaction parameter"