Polymer solution processability in
extensional-flow-dominated operations
is strongly influenced by polymer conformation and solution phase
behavior. Cosolvent addition can be used to tailor polymer conformation
and solution phase behavior to yield formulations that are amenable
to processes such as spraying and atomization, coating, and fiber
spinning. The addition of N,N-dimethylformamide
(DMF) to aqueous poly(N-isopropylacrylamide) (PNIPAM)
solutions induces unique phase behavior and microstructure formation,
yet the effects on solution processability have remained unexplored.
In this work, the effect of DMF cosolvent content on the rheology
(both shear and extensional) and microstructure of PNIPAM solutions
is investigated. While all examined PNIPAM solutions exhibit nearly
Newtonian steady shear behavior regardless of the DMF content, the
same solutions exhibit varying degrees of extensibility. Surprisingly,
the extensional relaxation time (λE) increases by
more than 20-fold with increasing DMF content in the water-rich regime.
In the DMF-rich regime, however, the solution extensibility dramatically
decreases. Interestingly, this unique variation in extensional flow
behavior does not scale as expected based on changes in the measured
intrinsic viscosity and radius of gyration. Instead, a mechanism is
proposed that relates the extensional flow behavior to the solution
microstructure, which is found to vary with DMF content in light scattering
measurements. In the water-rich regime, DMF molecules are proposed
to bridge PNIPAM chains via hydrogen bonding and hydrophobic interactions,
resulting in physically cross-linked aggregates. In extensional flows,
these aggregates behave like a polymer with higher apparent molecular
weight, increasing λE. In the DMF-rich regime, nonbridging
DMF molecules increasingly solvate individual PNIPAM chains; consequently,
more individual chains are stretched in extensional flows, leading
to a reduction in λE. These findings demonstrate
that interactions between components in these ternary systems have
unexpected but significant implications for solution extensional flow
behavior. Additionally, in the case of PNIPAM/DMF/water, the processability
of polymer-containing formulations can be modulated for spraying or
fiber spinning applications just by varying the cosolvent (DMF) content.
