The Solidification Rheology of Amorphous Polymers − Vitrification as Compared to Gelation

Abstract

The liquid‐to‐solid transition of polymeric materials results from the growth of a heterogeneous, percolating internal structure. The increased connectivity reduces the mobility of internal constituents and dominates the macroscopic rheological properties, which often appear to be very similar for gelation and vitrification. An example near the liquid‐to‐solid transition is the diverging shear viscosity, which is accompanied by high elasticity and a diverging relaxation time (critical slow down). Gelation and vitrification are hard to distinguish in this way. A distinctive difference, however, was found in the distribution of relaxation modes. Short relaxation modes dominate gelation since most internal constituents maintain their high mobility while, in comparison, the percolating structure is too weak to significantly contribute to the macroscopic behavior. The opposite is found for vitrification, which originates from large, cooperatively‐moving regions which finally connect into a percolating structure at the glass transition. As a consequence, the long relaxation modes dominate the approach of the glass transition. Surprisingly, the relaxation time spectrum adopts the same format for both phenomena near the transition, except that the relaxation exponent, n, is negative for gelation and positive for vitrification. Mathematically, one is the inverse of the other. The spectrum is cut off by the diverging longest relaxation time. Examples will be shown for these phenomena.