H. Henning Winter, University of Massachusetts (Amherst)
The materials in this experimental study are layered silicates (stacks of nm-thick organo-clay sheets), such as organically modified montmorillonite DK1, as the reinforcing component and an end-functionalized polybutadiene as matrix polymer. As the clay exfoliation proceeds and the clay surface accessible to the polymer increases, polymer molecules and clay connect into a sample-spanning network, a physical gel, with increasing modulus and decreasing relaxation times. Thus, small amplitude oscillatory shear (SAOS) is a sensitive probe for the evolving structure and is chosen here for exploring the exfoliation dynamics. The growth of the storage modulus is attributed to the exfoliation, both being similar in their temperature dependence With SAOS, a maximum rate of exfoliation was found at intermediate temperatures. A sharp decrease in rate is observed above 80ºC and below 40ºC.
SAXS enabled direct observation of increasing spacing between clay sheets. The SAXS analysis shows a very rapid increase in the spacing of the clay sheets with a maximum early spacing at around 80°C. Also of interest is the rate with which the galleries expand. There appear to be two clear expansions; a rapid early expansion followed by a considerably slower expansion. However, the data suggests that below a certain temperature the second expansion does not occur to any significant degree. This is evident in the 40°C experiment. The SAXS peaks did not show significant broadening, leading to the conclusion that all the clay aggregates are expanding at approximately the same rate.
Results from SAXS agreed with results from SAOS in that the clay sheet spacing rapidly increased. The results were analyzed with regard to several previously proposed mechanisms of exfoliation and some inconsistencies were found. Alternative exfoliation mechanisms are therefore hypothesized to fully explain the SAOS and SAXS data. A newly proposed mechanism is consistent with a diverse set of experimental observations, not just the unexpected temperature dependence. At this point the hypothesized mechanism is strictly qualitative and much work still needs to be done. Work is actively being pursued on further understanding this system in a quantitative way. Preliminary optical microscopy experiments are consistent with the hypothesized mechanism.
Fig. 4 Comparison of SAOS data in the approach of the gel point and beyond, left side and in the approach of the glass transition, right side.
A related study, but also an exploration into a new direction for the PI, focused on the relaxation dynamics of amorphous materials in the approach of a liquid-to-solid transition from the liquid side (LSTLS). Linear viscoelastic experiments on two representative materials focus on the distribution of relaxation modes and the increasing elasticity in the LSTLS approach. A concentrated colloidal suspension to represent the glass transition and a crosslinking polymer to represent gelation. For both, the relaxation time spectrum H broadens significantly near LSTLS and was found to share the same powerlaw format, for . A distinctive difference comes from the powerlaw exponent, n, which is positive for the glass transition, , and negative for gelation, -1<n<0. The cut-off of the spectrum , H= 0 for , depends on the longest relaxation time, , which belongs to the diverging, largest cluster in the glass or gel. The front factor, H0, determines the basic stress level in a relaxation process. Several examples are shown for each class of materials in order to test the proposed transition behavior for glasses (colloidal and molecular) on the one hand and chemical/physical gels on the other. Among several results, this experimental study provides a decisive criterion that distinguishes the glass transition from gelation. Reference: Winter HH (2013) The Glass Transition as Rheological Inverse of Gelation. Macromolecules 46:2425