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Research Papers

Enhanced Coarse-Graining of Thermoplastic Polyurethane Elastomer for Multiscale Modeling

[+] Author and Article Information
Md Salah Uddin

Department of Mechanical
and Energy Engineering,
University of North Texas,
3940 North Elm Street,
Denton, TX 76203-5017
e-mail: MdSalahUddin@my.unt.edu

Jaehyung Ju

Mem. ASME
Department of Mechanical
and Energy Engineering,
University of North Texas,
3940 North Elm Street,
Denton, TX 76203-5017
e-mails: jaehyung.ju@unt.edu;
jaehyung.ju@gmail.com

1Corresponding author.

Contributed by the Materials Division of ASME for publication in the JOURNAL OF ENGINEERING MATERIALS AND TECHNOLOGY. Manuscript received January 28, 2016; final manuscript received July 8, 2016; published online September 2, 2016. Assoc. Editor: Harley Johnson.

J. Eng. Mater. Technol 139(1), 011001 (Sep 02, 2016) (11 pages) Paper No: MATS-16-1041; doi: 10.1115/1.4034328 History: Received January 28, 2016; Revised July 08, 2016

The objective of this work is to develop a multiscale modeling tool of copolymers with long chains. We propose an enhanced coarse-graining method of thermoplastic polyurethane (TPU) with three beads. The proposed coarse-graining provides an accurate molecular modeling tool to keep the molecular interaction together with computational efficiency. The coarse-grained model with three beads is further improved with pressure-correction of the force-field. The improved coarse-grained model holds similar properties of a bulk model of TPU—varying density with temperature, a close density value of TPU at 1 atm, and the phase separation. Equating potential energy densities of the coarse-grained model to the strain energy functions of the continuum model at volumetric and isochoric deformation modes, bulk and shear moduli of TPU are directly obtained and used to estimate Young's modulus and Poisson's ratio. The molecular simulation with the coarse-grained model of TPU demonstrates its much greater bulk modulus than the shear modulus, which is typically observed in elastomers. Modifying the coarse-grained model of TPU with hard and soft segments, we successfully demonstrated the material design of bulk modulus and Poisson's ratio by varying hard and soft segments at the molecular level. The proposed coarse-graining tool will pave a new way to explore the multiscale modeling of copolymers with long chains and can be directly applied to the multiscale modeling of other thermoplastic elastomers (TPE).

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References

Figures

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Fig. 1

Construction of a coarse-grained model of TPU with three beads—A, B, and C

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Fig. 2

Probability distribution functions for the bond-stretching of the beads,  p(l) obtained from the full-atomic model, the IBM-derived effective coarse-grained bond potential, and the corresponding fitting of the potential

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Fig. 3

Probability distribution functions for the angle-bending of the beads, p(θ) obtained from the full-atomic model, the IBM-derived effective coarse-grained angle potential, and the corresponding fitting of the potential

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Fig. 4

g(r), radial distribution function (RDF) of the beads obtained from the full-atomic model, the IBM-derived effective coarse-grained pair potentials, and the corresponding fitting of the potential

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Fig. 5

RDFs obtained from a full-atomic model, RDFs obtained from a coarse-grained model without pressure-correction (forced to be squeezed to the desired density using NVT), RDFs obtained from a coarse-grained model with pressure-corrected force-fields, IBM-derived initial force-fields, and pressure-corrected force-fields

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Fig. 6

Construction of a coarse-grained model of TPU with/without pressure-correction of force-field

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Fig. 7

Comparison of density versus temperature graphs (during cooling) between pressure-corrected and without pressure-corrected force-field

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Fig. 8

Equilibrium structures cooled at 298 K and 1 atm with varying wt.% of hard segments: (a)  34.9%, (b)  41.5%, (c)  48.6%, (d)  54.1%, (e)  61.1%, and (f)  62.3%

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Fig. 9

Potential energy densities of the coarse-grained model of TPU for volumetric (a) and isochoric (b) deformation modes

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Fig. 10

Variation of the bulk modulus (a) and Poisson's ratio (b) concerning density due to different wt.% of hard segments

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