(33a) Rheology of Cyclic Macromolecules: Linear Dynamics and Extensional Rheology

Macrocyclic polymers represent a sort of last frontier in the polymer physics community’s efforts to understand dynamics of chain-like macromolecules. In this work we present results from an investigation of the linear viscoelastic responses and extensional rheology of cyclic poly(3,6-dioxa-1,8-octanedithiol) (polyDODT) and cyclic polyisobutylene disulfide (PIB-disulfide). The linear dynamics results focus on the polyDODT materials both in the melt and solution states with complementary results being provided for the PIB disulfide polymers. The polyDODT rings themselves had molecular weights from M_w=43.3 to 556 kg/mol, spanning an entanglement number Z_w from 23.4 to over 300. By diluting the rings with diethyl phthalate (DEP) the range of Z_w covered was extended to Z_w=0.11, i.e., unentangled molecules. The PIB-disulfide materials had molecular weights corresponding to Z_w=10.7 and Z_w=5.5, i.e., entangled for a linear chain, but just beginning to entangle in the case of the ring molecules.[1] Both classes of macrocyclic materials were synthesized by a novel synthesis route: Reversible Radical Recombination Polymerization (R3P)[2].

The dynamic frequency sweep measurements were performed over reasonable temperature ranges that permit measurements from the terminal region to the glassy region. The frequency range covered when using an Anton Paar MCR 501 stress-controlled rotary rheometer was from 0.002rad/s to 100 rad/s. When constructing the master curves using time-temperature superposition, the data at adjacent temperatures had at least one decade of overlapping frequencies. The linear shear rheology for the cyclic polyDODT has been reported previously[1]. For both the cyclic polyDODT and PIB-disulfide materials we show that in the low Z_w regime, there is overlap with literature results for highly purified ring molecules consistent with the lack of entanglement plateau in the dynamics. As Z_w increases, the entanglement plateau appears and has the same value as that of the corresponding linear polymer.

For the elongational viscosity studies, experiments were performed using an extensional viscosity fixture (EVF) from TA Instruments coupled to an ARES (TA Instruments, DE). The samples used were a polyDODT ring polymer with molecular weight of 107 kg/mol (Z_w=57.7) and the PIB-disulfide samples were the same as those described above. The extensional rates used in the experiments were between and , the maximum Hencky strain () limit of the EVF is 4. The temperature of the measurements for the cyclic polyDODT having M_w=107k was 30 °C, and the tests were performed in an air environment. The temperature of the measurements for the PIB-disulfide and a linear counterpart was 0 °C and liquid nitrogen was used as the cooling source.

Results

The salient results to be discussed at the meeting are the following:

The dynamics of the ring molecules show, consistent with prior work on low Z_w polystyrene materials [3,4,5] for Z_w less than approximately 10, that the relaxation is of a power law nature and shows no plateau modulus [1].

The viscosity of the rings with Z_w less than approximately 15 for the polyDODT follows a “Rouse-like” behavior in that the viscosity varies approximately as the first power of entanglement number Z_w [1]. The PIB-disulfide behavior suggests that the value at which the viscosity of the rings begins to deviate from the Rouse-like behavior may be smaller than for polystyrene and for the polyDODT, e., it falls between Z_w=5.5 and Z_w=10.7.

Above Z_w=15 for the polyDODT rings a plateau modulus appears that is similar in value to that of the linear polyDODT materials [1].

Above Z_w=15, the polyDODT ring viscosities vary very strongly with Z_w with the results suggesting a power-law like relationship of [1].

In contrast to literature reports[5] for a cyclic polystyrene with Z_w≈10, e., unentangled ring, neither the well entangled cyclic PolyDODT nor the unentangled cyclic PIB-disulfide shows a pronounced strain hardening behavior when compared with the linear polymer behavior. The Weissenberg number dependence of the steady-state extensional viscosity is similar for the R3P synthesized rings and the polystyrene rings.

Possible explanations for the discrepancies and similarities of the extensional rheology of the R3P synthesized cyclic polymers (PolyDODT and PIB-disulfide) and the cyclic polystyrene are discussed.

Acknowledgements

The authors greatly appreciate the U.S. Department of Energy, Office of Science, Basic Energy Sciences for its support under Awards DESC0018657 (DC and GBM), DESC0018891 (K.M., J.E.P. and G.K.) and DE-SC0018655 (H.K. and J.A.K). The authors at OSU also gratefully acknowledge funding from USDA-NIFA under Hatch project number OHO01417. GBM, DC and ZQ thank the John R. Bradford Endowment at Texas Tech University for partial support and GBM is thankful to the Department of Chemical and Biomolecular Engineering at NC State University for partial support.

References

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