Linear and Nonlinear Shear Rheology of ‘Pure’ Ring Polymers Free from Linear Contaminants Using a Low-Ceiling Temperature Chemistry

Ring polymers have fascinated polymer chemists and physicists for decades, yet achieving a complete understanding of the dynamics of pure ring polymers free from linear contaminants has remained challenging. Despite recent advances in polymer purification methods, nearly all prior work on ring polymers are known to contain trace amounts of contaminant linear polymers, which have been shown to greatly affect the rheological response of these samples. In this work, we study the linear and non-linear rheology of ‘pure’ ring polymers that are effectively free of linear contaminants. In particular, we focus on cyclic poly(phthalaldehyde) (cPPA), which is a low-ceiling temperature polymer whose metastable chemistry results in kinetically-trapped cyclic polymers at room temperature. Above the ceiling temperature (-40°C), all linear chains rapidly degrade into monomer via unzipping from the free ends, thereby resulting in a system comprised of highly pure and stable ring polymers. Facile synthesis of cPPA is performed at the multi-gram scale, yielding a large quantity of ring polymer samples. Due to the self-immolating chemistry of uncapped linear chains at room temperature, no additional purification after synthesis is needed to separate linear and circular chains. Here, we report the linear and nonlinear viscoelastic properties of highly concentrated cPPA samples (>70 wt%) of various molecular weights (20-300 kDa). The transient nonlinear start-up shear responses show stronger shear-thinning than a previously-reported polystyrene ring melt, but exhibit drastically reduced scaling behavior in the overshoot viscosity and strain with respect to shear rate. We attribute these differences to the large molecular weights of our samples (Mw > 200 kDa) and the highly pure nature of these polymers with respect to ring/linear purity. In addition to the rheological results, we further characterize the thermal stability of cPPA using thermal gravimetric analysis (TGA) scans and isothermal holds. We also show rapid acid-triggered depolymerization of cPPA, with complete depolymerization occurring in less than 10 seconds with 10 mM trifluoroacetic acid using NMR spectroscopy. Overall, these results give experimental insight into the dynamics of ring polymer systems with unprecedented purity.