Structural Relaxation Time of a Polymer Glass during Deformation.

In order to determine the structural relaxation time of a polymer glass during deformation, a strain rate switching experiment is performed in the steady-state plastic flow regime. A lightly cross-linked poly(methylmethacrylate) glass was utilized and, simultaneously, the segmental motion in the glass was quantified using an optical probe reorientation method. After the strain rate switch, a nonmonotonic stress response is observed, consistent with previous work. The correlation time for segmental motion, in contrast, monotonically evolves toward a new steady state, providing an unambiguous measurement of the structural relaxation time during deformation, which is found to be approximately equal to the segmental correlation time. The Chen-Schweizer model qualitatively predicts the changes in the segmental correlation time and the observed nonmonotonic stress response. In addition, our experiments are reasonably consistent with the material time assumption used in polymer deformation modeling; in this approach, the response of a polymer glass to a large deformation is described by combining a linear-response model with a time-dependent segmental correlation time.

Temperature and pressure dependence of the alpha relaxation in ortho-terphenyl.

Molecular dynamics (MD) simulations of ortho-terphenyl using an all-atom model with the optimized potentials for liquid simulations (OPLS) force field were performed both in the high temperature Arrhenian region and at lower temperatures that include the onset of the super-Arrhenian region. From the MD simulations, the internal energy of both the equilibrium liquid and crystal was determined from 300 K to 600 K and at pressures from 0.1 MPa to 1 GPa. The translational and rotational diffusivities were also determined at these temperatures and pressures for the equilibrium liquid. It is shown that within a small offset, the excess internal energy Ux from the MD simulations is consistent with the experimentally determined excess internal energy reported earlier [Caruthers and Medvedev, Phys. Rev. Mater. 2, 055604, (2018)]. The MD mobility data {including extremely long-time 1 atm simulations from the study by Eastwood et al. [J. Phys. Chem. B 117, 12898, (2013)]} were combined with experimental data to form a unified dataset, where it was shown that in both the high temperature Arrhenian region and the lower temperature super-Arrhenian region, the mobility is a linear function of 1/Ux(T,p), albeit with different proportionality constants. The transition between the Arrhenian and super-Arrhenian regions is relatively sharp at a critical internal energy Ux α. The 1/Ux(T,p) model is able to describe the mobility data over nearly 16 orders-of-magnitude. Other excess thermodynamic properties such as excess enthalpy and excess entropy (i.e., the Adam-Gibbs model) are unable to unify the pressure dependence of the mobility.

Non-heme manganese catalysts for on-demand production of chlorine dioxide in water and under mild conditions.

Two non-heme manganese complexes are used in the catalytic formation of chlorine dioxide from chlorite under ambient temperature at pH 5.00. The catalysts afford up to 1000 turnovers per hour and remain highly active in subsequent additions of chlorite. Kinetic and spectroscopic studies revealed a Mn(III)(OH) species as the dominant form under catalytic conditions. A Mn(III)(µ-O)Mn(IV) dinuclear species was observed by EPR spectroscopy, supporting the involvement of a putative Mn(IV)(O) species. First-order kinetic dependence on the manganese catalyst precludes the dinuclear species as the active form of the catalyst. Quantitative kinetic modeling enabled the deduction of a mechanism that accounts for all experimental observations. The chlorine dioxide producing cycle involves formation of a putative Mn(IV)(O), which undergoes PCET (proton coupled electron-transfer) reaction with chlorite to afford chlorine dioxide. The ClO2 product can be efficiently removed from the aqueous reaction mixture via purging with an inert gas, allowing for the preparation of pure chlorine dioxide for on-site use and further production of chlorine dioxide.

Microemulsion-based synthesis and electrochemical evaluation of different nanostructures of LiCoO2 prepared through sacrificial nanowire templates.

For the first time, we demonstrate the use of a microemulsion reaction to synthesize different nanostructures of LiCoO2 cathode material. By varying the annealing temperature and time, porous nanowires and nanoparticles of LiCoO2 are obtained. The electrochemical performances of these different nanostructures obtained under the respective annealing conditions are evaluated. It is shown that nanoparticles formed under the annealing condition of 700 °C, 1.5 h perform the best, delivering an initial capacity of around 135 mA h g(-1), which is close to the theoretical capacity of LiCoO2, 140 mA h g(-1). They also exhibit a capacity retention of around 93% by 100 cycles at 0.1 C. Comparisons are made between our LiCoO2 material obtained under different annealing conditions and those in the literature.

Effects of pendant ligand binding affinity on chain transfer for 1-hexene polymerization catalyzed by single-site zirconium amine bis-phenolate complexes.

The kinetics of 1-hexene polymerization using a family of five zirconium amine bis-phenolate catalysts, Zr[tBu-ON(X)O]Bn2 (where X = THF (1), pyridine (2), NMe2 (3), furan (4), and SMe (5)), has been investigated to uncover the mechanistic effect of varying the pendant ligand X. A model-based approach using a diverse set of data including monomer consumption, evolution of molecular weight, and end-group analysis was employed to determine each of the reaction specific rate constants involved in a given polymerization process. The mechanism of polymerization for 1-5 was similar and the necessary elementary reaction steps included initiation, normal propagation, misinsertion, recovery from misinsertion, and chain transfer. The latter reaction, chain transfer, featured monomer independent ß-H elimination in 1-3 and monomer dependent ß-H transfer in 4 and 5. Of all the rate constants, those for chain transfer showed the most variation, spanning 2 orders of magnitude (ca. (0.1-10) × 10(-3) s(-1) for vinylidene and (0.5-87) × 10(-4) s(-1) for vinylene). A quantitative structure-activity relationship was uncovered between the logarithm of the chain transfer rate constants and the Zr-X bond distance for catalysts 1-3. However, this trend is broken once the Zr-X bond distance elongates further, as is the case for catalysts 4 and 5, which operate primarily through a different mechanistic pathway. These findings underscore the importance of comprehensive kinetic modeling using a diverse set of multiresponse data, enabling the determination of robust kinetic constants and reaction mechanisms of catalytic olefin polymerization as part of the development of structure-activity relationships.

Mechanistic detail revealed via comprehensive kinetic modeling of [rac-C(2)H4(1-indenyl)2ZrMe2]-catalyzed 1-hexene polymerization.

Thorough kinetic characterization of single-site olefin polymerization catalysis requires comprehensive, quantitative kinetic modeling of a rich multiresponse data set that includes monomer consumption, molecular weight distributions (MWDs), end group analysis, etc. at various conditions. Herein we report the results obtained via a comprehensive, quantitative kinetic modeling of all chemical species in the batch polymerization of 1-hexene by rac-C(2)H(4)(1-Ind)(2)ZrMe(2) activated with B(C(6)F(5))(3). While extensive studies have been published on this catalyst system, the previously acknowledged kinetic mechanism is unable to predict the MWD. We now show it is possible to predict the entire multiresponse data set (including the MWDs) using a kinetic model featuring a catalytic event that renders 43% of the catalyst inactive for the duration of the polymerization. This finding has significant implications regarding the behavior of the catalyst and the polymer produced and is potentially relevant to other single-site polymerization catalysts, where it would have been undetected as a result of incomplete kinetic modeling. In addition, comprehensive kinetic modeling of multiresponse data yields robust values of rate constants (uncertainties of less than 16% for this catalyst) for future use in developing predictive structure-activity relationships.

Theory of nonlinear creep in polymer glasses.

A statistical segment-scale constitutive equation theory for the nonlinear mechanics and relaxation of polymer glasses recently proposed by two of us is applied to study nonlinear creep and recovery. The key physics resides in a deformation-dependent elastic modulus and alpha relaxation time, which are determined by a segment-displacement-dependent dynamical free energy that quantifies the transient localization and activated hopping processes. For simple creep and recovery, the amplitudes of the instantaneous up- and down strain jumps are equal and exhibit upward deviations from a linear dependence on applied stress due to modulus softening. Nonexponential relaxation indicative of a distribution of alpha relaxation times is incorporated and shown to be crucial in determining the so-called delayed elastic deformation at intermediate times. The amount of delayed recovered strain appears to saturate at long times at a value equal to the total delayed elastic deformation during creep. Calculations of the time-dependent creep compliance covering the linear and nonlinear regimes are presented. Horizontal shifts can collapse the compliance curves at different stress levels onto a master plot as seen experimentally, and the extracted shift factor quantitatively agrees with the a priori computed normalized alpha relaxation time. Calculations for two-step creep at small stress are in reasonable agreement with experiments on poly(methylmethacrylate) glass, although systematic deviations occur at very high applied stresses.