**Comparison of Friction Parametrization from Dynamics and Material
Properties for a Coarse-Grained Polymer Melt**

LC Johnson and F Phelan, JOURNAL OF PHYSICAL CHEMISTRY B, 127, 7054-7069 (2023).

DOI: 10.1021/acs.jpcb.3c03273

Inthis work, we extend an approach to coarse-grained (CG) modelingfor polymer melts in which the conservative potential is parametrizedusing the iterative Boltzmann inversion (IBI) method and the accelerateddynamics inherent to IBI are corrected using the dissipative Langevinthermostat with a single tunable friction parameter (J. Chem.Phys. 2021, 154, 084114). Diffusivemeasures from picoseconds to nanoseconds are used to determine theLangevin friction factor to apply to the CG model to recover all-atom(AA) dynamics; the resulting friction factors are then compared forconsistency. Here, we additionally parametrize the CG dynamics usinga material property, the zero-shear viscosity, which we measure usingthe Green-Kubo (GK) method. Two materials are studied, squalaneas a function of temperature and the same polystyrene oligomers previouslystudied as a function of chain length. For squalane, the frictionderived from the long-time diffusive measures and the viscosity allstrongly increase with decreasing temperature, showing an Arrhenius-likedependence, and remain consistent with each other over the entiretemperature range. In contrast, the friction required for the picoseconddiffusive measurement, the Debye-Waller factor, is somewhatlower than the friction from long-time measures and relatively insensitiveto temperature. A time-dependent friction would be required to exactlyreproduce the AA measurements during the caging transition connectingthese two extremes over the entire timespan at this level of coarse-graining.For the polystyrene oligomers for which we previously characterizedthe diffusive friction, the viscosity- parametrized frictions are consistentwith the diffusive measures for the smallest chain length. However,for the longer chains, we find different trends based on measurementmethod with friction derived from rotational diffusion remaining nearlyconstant, friction derived from translational diffusion showing amodestly increasing trend, and viscosity-derived friction showinga modest decreasing trend. This seems to indicate that there is somesensitivity of the friction measurement method for systems with increasedrelaxation times and that in particular, the unsteady dynamics ofthe individual parametrization schemes plays a role in this. Increaseddifficulty in applying the GK method with increasing relaxation timeof the longer chain systems is also discussed. Overall, we find thatwhen the material is in a high-temperature melt state and the viscositymeasurement is reliable, the friction parametrization from the diffusivefriction measures is consistent and the lower cost diffusive parametrizationis a reliable means for modeling viscosity. Our data give insightinto the time-dependent friction one might compute using a non- Markovianapproach to enable the recovery of AA dynamics over a wider rangeof time scales than can be computed using a single friction.

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