Linear Viscoelastic Properties of Putative Cyclic Polymers Synthesized by Reversible Radical Recombination Polymerization (R3P)

Linear viscoelastic properties in both melt and solution states are reported for a series of poly(3,6-dioxa-1,8-octanedithiol) (polyDODT) made by reversible radical recombination polymerization (R3P) under conditions designed to produce linear (LDODT), cyclic (RDODT), and linear-cyclic mixtures (LRDODT). PolyDODT is amorphous (T-g < -50 degrees C) and highly flexible (entanglement molecular weight M-e,M-lin asymptotic to 1850 g/mol for LDODT). PolyDODT's low T-g and low M-e,M-lin enable characterization over a wide dynamic range and a wide range of dimensionless weight-average molecular weight Z(w) = M-w/M-e,M-lin. Measurements at temperatures from -57 to 100 degrees C provide up to 18 decades of reduced frequency, which is necessary to characterize RDODT melts with Z(w) from 23 to 300. The two highest-molecular-weight polymers in the present RDODT series have such high M-w (406k and 556k g/mol) that mass spectrometry, NMR spectroscopy, and even chemical assays for chain ends are unable to rule out up to 2 mol % of linear contaminant. By studying the samples in solution (using dilution to reduce Z(w)), we could compare their dynamics with those of previously established high-purity polystyrene (PS) rings (limited to Z(w) <= 13.6). RDODT solutions with Z(w) < 15 (concentrations < 5 wt % for RDODT-406k and 556k) have dynamic moduli G* that accord with LCCC-purified PS rings in terms of the frequency dependence (including the absence of a plateau), the progression of shapes of G* as a function of Z(w), and the linear scaling of their zero-shear viscosity eta(0) with M-w. The shape of G* as a function of Z(w) for solutions of RDODT-406k and -556k also accords with lower M-w RDODT melts (which have <= 1.3 mol % of linear contaminant). Thus, the measurement of the linear viscoelastic properties of appropriate concentrations of high M-w (> 200k g/mol) putative cyclic polymers, in which linear chains evade spectroscopic detection, may provide an alternative means (though not fully proven) of validation of sample purity. When Z(w) > 15 (including all seven RDODT melts and eight of their solutions), G* has a rubbery plateau. This suggests that the onset of entanglement-like behavior in rings requires 4-5-fold greater Z(w) than is required for linear chains. Further, the plateau moduli of RDODT samples are indistinguishable from G(N)(o) of the corresponding LDODT (melt or matched-concentration solutions). In entangled linear polymers, the observation that G(N)(o) is independent of Z(w) follows from limitations on lateral fluctuations due to neighboring chains becoming independent of position along a given chain. The present results for RDODT suggest that this holds for sufficiently long endless chains, too. While the RDODTs have the same G(N)(o) as entangled LDODTs, when Z(w) > 60, the terminal relaxation, if reached at all, of RDODT extends to orders of magnitude lower frequency than an entangled linear polymer of the same Z(w). Consequently, the viscosity of RDODT with Z(w) > 60 increases with Z(w) much more strongly than the 3.4 power observed for entangled linear polymers. Finally, these novel polymers, with a disulfide-linked backbone and broad relaxation time distribution, may prove important in relation to biodegradable elastomers and materials with exceptional low-frequency dissipation, extending at least 12 decades below the onset of the rubbery plateau.

Automated summary

The linear viscoelastic properties of poly(3,6-dioxa-1,8-octanedithiol) (polyDODT) in both melt and solution states were studied. The samples in solution can be used to validate the sample purity.