First Measurement of Bond Dissociation During Shear in a Transient Network

Intellectual Merit
Olsen and Holten-Andersen have developed a new tool to simultaneously measure rheology and fluorescence from hydrogel materials.  They have constructed transient polymer networks where bonds fluoresce only in the dissociated state and built a rheo-fluorescence apparatus to quantify this effect in shear. When the gels are studied with steady shear experiments, the fraction of free bonds increases steadily with increasing Deborah number.  However, the fraction of dissociated bonds remains quite low, less than 0.1%.  In contrast, many transient network theories predict that the fraction of dissociated bonds should be substantially larger than 10%, more than two orders of magnitude different.  Therefore, this new measurement provides a powerful means to differentiate between transient network theories, moving forward the state of fundamental knowledge in associative polymer design.

Graph showing the fraction of dissocated bonds as a function of the Deborah number in steady shear.  The number of dissociated bonds increases with shear as expected, but the absolute number is much lower than predicted by most transient network theories.


Broader Impact
During the spring of 2018, Olsen co-organized the American Physical Society Division of Polymer Physics short course entitled the Gels, Elastomers, and Networks Experience (GENE) with Prof. Costantino Creton from ESPCI. This course extended for two days.  The first day covered an advanced introductory treatment of polymer gels and networks, including chemistry, mechanics, fracture, and transient network physics.  The second day covered advanced topics including responsive networks, network topology, theory, and control over network toughness. The class was co-taught by 11 expert instructors from across the united states (including Olsen and Creton) and attended by more than 75 students representing a cross-section of graduate students, postdocs, young faculty, and industrial guests.