Vitrification as Compared to Gelation in Amorphous Polymers
November 19, 2014 - 10:00am
Gelation and vitrification are the most common mechanisms for a liquid-to-solid transition in amorphous materials. For both, a heterogeneous, percolating internal structure grows and reduces the mobility of internal constituents. Macroscopic rheological properties are strongly affected but appear to be very similar for gelation and vitrification. Here we propose a novel rheological test to distinguish between gelation and vitrification. The test is based on Boltzmann’s equation of linear viscoelasticity and focuses on the distribution of relaxation modes in samples near the liquid-to-solid transition. Short relaxation modes dominate gelation since the majority of the internal constituents is still unconnected or barely connected while the percolating structure is barely there and too weak to significantly support a macroscopic stress. The relaxation time spectrum of gelation is a decaying function, large for fast modes and small for the slower modes. 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, H(tau)~tau^n, adopts the same format for both phenomena near the transition, except that the relaxation exponent, n, is negative for gelation and positive for vitrification (see Macromolecules 46:2425-32, 2013). 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.