Prof. Nicolas Alvarez

Drexel University, USA

The Effect of Chemistry on Rheology in Supramolecular Polymers

Abstract

Supramolecular polymers possess versatile mechanical properties and a unique ability
to respond to external stimuli. Understanding the rich dynamics of associative polymers
is essential for tailoring user-defined properties in polymeric materials. In this talk we will
examine the effect of varying associating chemistry, e.g. ionomers, hydrogen bonds, and
hydrophilic-hydrophobic interactions, on linear viscoelasticity and nonlinear extensional
rheology. The investigation of nonlinear extensional rheology is particularly of interest as
such materials are questionably more solid than fluid on certain timescales. In general,
associating polymers have two adjustable parameters: the interaction strength and the
number of associating groups per chain. We will show that independent of chemistry there
are three effects on linear viscoelasticity due to an increase in interaction strength and/or
the number of associations per chain: (i) the longest relaxation time increases, (ii) the
plateau modulus increases, and (iii) the power law scaling in the terminal regime deviates
toward lower magnitudes for nonuniform distributions of association groups along the
backbone. Increasing both interaction strength and number of interactions per chain has
a profound effect on nonlinear extensional rheology. In unentangled systems, too strong of
an interaction causes the material to behave like a brittle solid, i.e. fractures at low strain,
when the experimental time is shorter than the relaxation time of the association groups,
s. When the experimental time is longer than s, the material is deformable and behaves
similar to an non-associating entangled linear polymer melt. Entangled associating polymer
systems behave quite differently and exhibit very strong strain hardening dynamics;
achieving stresses orders of magnitude larger than their plateau modulus. Throughout
the discussion, effort will be made to make comparisons between associating and nonassociating
linear polymer melt rheology in order to contextualize our understanding of
supramolecular polymer dynamics.



Date: 31/3/2017
Time:12:00 (coffee & cookies will be served at 11:45)
Place:FORTH Seminar Room 2