To explore the relationship between microscopic structure and viscoelastic properties of polyurea, a coarsegrained (CG) model is developed by a structure matching method and validated against experiments conducted on a controlled, benchmark material. Using the Green-Kubo method, the relaxation function is computed from the autocorrelation of the stress tensor, sampled over equilibrium MD simulations, and mapped to a real time scale established by matching self-diffusion rates of atomistic and CG models. Master curves computed from the predicted stress relaxation function are then compared with dynamic mechanical analysis experiments mapped to a wide frequency range by time-temperature superposition, as well as measurements of ultrasonic shear wave propagation. Computational simulations from monodisperse and polydisperse configurations, representative of the benchmark polyurea, show excellent agreement with the experimental measurements over a multidecade range of loading frequency. V C 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016, 54, 797-810 KEYWORDS: coarse-grained molecular dynamics; mechanical properties; polyurea INTRODUCTION Knowledge of the connections between chemistry, structure, and properties is needed to develop improved polymers with a materials-by-design approach. Computational models offer promise in identifying these relationships, but unfortunately they typically lack predictive capability beyond a small range of properties. Molecular dynamics (MD) simulations can provide tremendous insight into how the fine details of chemistry, chain architecture, and microstructure affect many physical properties; however, they face well-known limitations in both time and length scales. The goal of this work is to develop coarse-grained (CG) models that enable molecular simulations to reach more representative time and length scales to investigate the viscoelastic properties of polyurea, a thermorheologically complex block copolymer, for which theoretical rheological models are difficult to apply.

Polyurea is a block copolymer that has been widely used in the coating industry as an abrasion-resistant and energy-dissipative material. Its mechanical properties can be tuned by choosing different variations of diamines and diisocyanates as well as by adding various nano and micro-inclusions to create polyurea-based composites. Our aim here is to provide the necessary experimentally-based viscoelastic constitutive relations for polyurea and its composites in a format convenient to support computational studies. The polyurea used in this research is synthesized by the reaction of Versalink P-1000 (Air Products) and Isonate 143L (Dow Chemicals). Samples of pure polyurea and polyurea composites are fabricated and then characterized using dynamic mechanical analysis (DMA). Based on the DMA data, master curves of storage and loss moduli are developed using time-temperature superposition. The quality of the master curves is carefully assessed by comparing with the ultrasonic wave measurements and by Kramers-Kronig relations. Based on the master curves, continuous relaxation spectra are calculated, then the time-domain relaxation moduli are approximated from the relaxation spectra. Prony series of desired number of terms for the frequency ranges of interest are extracted from the relaxation modulus. This method for developing cost efficient Prony series has been proven to be effective and efficient for numerous DMA test results of many polyurea/polyurea-based material systems, including pure polyurea with various stoichiometric ratios, polyurea with milled glass inclusions, polyurea with hybrid-nano particles and polyurea with phenolic microbubbles. The resulting viscoelastic models are customized for the frequency ranges of interest, reference temperature and desired number of Prony terms, achieving both computational accuracy and low cost. The method is not limited to polyurea-based systems. It can be applied to other similar polymers systems.

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