The thermobalance used to measure crosslink density by solvent swell

Abstract

A micro-technique for rapid measurement of solvent swell of crosslinked polymers is described. Crosslink density by solvent swell provides a useful physical parameter (it is equivalent to Young's modulus). But even more important, it is often the only means of monitoring extent of cure or degradation of thermosets and elastomers. An additional benefit of this method is the measurement of the soluble fraction, i.e. that portion of the polymer not bound to the network. The uniqueness of this method eliminates evaporation from the sample so that small, thin samples, which equilibrate rapidly, can be used. Poly(dimethylsiloxane) elastomers, ranging in crosslink density from 7 to 35 × 10 −5 moles/cc and in sol fraction from 1 to 4%, were studied in hexane and toluene. A Perkin-Elmer TGS-2 thermogravimetric system was used. The critical measurement was the weight of polymer swollen with solvent, which was performed in a solvent-saturated environment. Repeated measurements on several samples demonstrated the method to be precise; comparison with a conventional technique showed it to be highly accurate. A direct correlation was established between swelling data and Young's modulus.