Crosslinking induced volume expansion in the glass state

We have studied a very unusual phenomenon that occurs in the glassy state, that the isothermal density and modulus of a thermosetting polymeric material pass through a maximum in the glassy state with increasing chemical conversion. The maximum occurs approximately after gelation, and the specific volume of the material after gelation increases via further crosslinking. We show that this anomalous behavior is not a kinetic phenomenon, though the glass transition is a function of the experimental time-scale. In addition, we show that the anomalous behavior strongly depends on the overall crosslinking densities of the polymeric material, and tends to intensify itself as the overall crosslinking density increases. We interpret that the nature of this anomaly is related to the configuration entropy criterion required for forming a glass, as it can be explained by the packing efficiency falling off of the highly crosslinked networks.