Consistent and Transferable Force Fields for Statistical Copolymer Systems at the Mesoscale

The statistical trajectory matching (STM) method was applied successfully to derive coarse grain (CG) models for bulk properties of homopolymers. The extension of the methodology for building CG models for statistical copolymer systems is much more challenging. We present here the strategy for developing CG models for styrene-butadiene-rubber, and we compare the quality of the resulting CG force fields on the structure and thermodynamics at different chemical compositions. The CG models are used through the use of a genuine mesoscopic method called the dissipative particle dynamics method and compared to high-resolution molecular dynamics simulations. We conclude that the STM method is able to produce coarse-grained potentials that are transferable in composition by using only a few reference systems. Additionally, this methodology can be applied on any copolymer system.

Role of many-body interactions in the structure of coarse-grained polymers

In developing coarse-grained (CG) polymer models it is important to reproduce both local and molecule-scale structure. We develop a procedure for fast calculation of the bond-orientation correlation and the internal squared distance 〈R^{2}(M)〉 through evaluation of the probability distribution functions that represent a CG model. Different CG models inherently contain or omit correlations between CG variables. Here, we construct CG models that contain specific correlations between CG variables. The importance of different correlations is tested on CG models of polyethylene, polytetrafluoroethylene, and poly-L-lactic acid. The chain stiffness and 〈R^{2}(M)〉 are calculated using both analytic evaluation and Monte Carlo sampling, and approximate model results are compared with exact results from all-atom simulations. For polymers with an exponential correlation decay, the bond-orientation correlation and 〈R^{2}(M)〉 indicate which CG variable correlations are most important to reproduce molecule-scale structure. Analysis of the bond-orientation correlation and internal-squared distance indicates that for poly-L-lactic acid the bond-orientation correlation requires qualitatively different additional terms in CG models and quantifies the error in neglecting this important behavior.

Tailoring the mechanical properties by molecular integration of flexible and stiff polymer networks

Designing a multiple-network structure at the molecular level to tailor the mechanical properties of polymeric materials is of great scientific and technological importance. Through the coarse-grained molecular dynamics simulation, we successfully construct an interpenetrating polymer network (IPN) composed of a flexible polymer network and a stiff polymer network. First, we find that there is an optimal chain stiffness for a single network (SN) to achieve the best stress-strain behavior. Then we turn to study the mechanical behaviors of IPNs. The result shows that the stress-strain behaviors of the IPNs appreciably exceed the sum of that of the corresponding single flexible and stiff network, which highlights the advantage of the IPN structure. By systematically varying the stiffness of the stiff polymer network of the IPNs, optimal stiffness also exists to achieve the best performance. We attribute this to a much larger contribution of the non-bonded interaction energy. Last, the effect of the component concentration ratio is probed. With the increase of the concentration of the flexible network, the stress-strain behavior of the IPNs is gradually enhanced, while an optimized concentration (around 60% molar ration) of the stiff network occurs, which could result from the dominant role of the enthalpy rather than the entropy. In general, our work is expected to provide some guidelines to better tailor the mechanical properties of the IPNs made of a flexible network and a stiff network, by manipulating the stiffness of the stiff polymer network and the component concentration ratio.

Molecular dynamics simulation of the viscoelasticity of polymer nanocomposites under oscillatory shear: effect of interfacial chemical coupling

In this work by adopting coarse-grained molecular dynamics simulation, we focus our attention on investigating the effect of the chemical coupling between polymer and nanoparticles (NPs) on the viscoelastic properties of polymer nanocomposites (PNCs). Firstly we examine the effect of the interfacial chemical coupling on the non-linear behavior, such as the change of the storage moduli, the loss moduli and the loss factor as a function of the strain amplitude. Besides the reinforcing effect contributed by the interfacial chemical interaction, a much smaller loss factor is also observed attributed to less molecular friction and dissipation. Meanwhile, the effects of temperature, frequency, and the interfacial physical interaction between NPs and polymers on the viscoelastic properties are also probed. To uncover the structural and dynamic effect of the interfacial chemical coupling, we calculate the radial distribution function of polymer chains around NPs, the content of the polymer beads in the first layer of the interfacial region under quiescent and dynamic conditions, the incoherent intermediate dynamic structure factor of the polymer beads, which are chemically or physically tethered to the NPs, and all the polymer beads of the system, the quantitative comparison of the mean relaxation time for different interfacial chemical coupling, and the mean-square displacement of the polymer chains. Lastly we analyze the change of the interfacial energy such as the physical and chemical energies during oscillatory shear. Through these analyses, we conclude that with the increase of the interfacial chemical coupling, the change extent of the interfacial physical interaction versus the periodic strain decreases, attributed to a much smaller adsorption-desorption reversible process. This can rationalize the much weaker non-linear behavior or the "Payne effect". Based on these results, we anticipate that a better molecular-level understanding is provided on the effect of the interfacial coupling on the viscoelastic properties of PNCs.

Simulational insights into the mechanical response of prestretched double network filled elastomers

This paper deals with molecular-dynamics simulations of the mechanical properties of prestretched double network filled elastomers. To this end, we firstly validated the accuracy of this method, and affirmed that the produced stress-strain characteristics were qualitatively consistent with Lesser's experimental results on the prestretched tri-block copolymers with a competitive double network. Secondly, we investigated the effect of the crosslinking network ratio on the mechanical properties of the prestretched double network homopolymers under uniaxial tension. We found that the prestretched double network contributes greatly to the enhanced tensile stress and ultimate strength at fracture, as well as to the lower permanent set (the residual strain) and dynamic hysteresis loss, both parallel and perpendicular to the prestretching direction. Notably, though, an anisotropic behavior was observed: in the parallel direction, both the first and the second crosslinked networks bore the external force; whereas in the perpendicular direction, only the second crosslinked network was relevantly effective. Finally, the polymer nanocomposites with a prestretched double network exhibited tensile mechanical properties similar to those of the studied homopolymers with prestretched double networks. Summing up the results, it can be concluded that the incorporation of prestretched double networks with a specified crosslinking network ratio seems to be a promising method for manipulating the mechanical properties of elastomers and their nanocomposites, as well as for introducing anisotropy in their mechanical response.

Designing polymer nanocomposites with a semi-interpenetrating or interpenetrating network structure: toward enhanced mechanical properties

Semi-interpenetrating and interpenetrating network structures for the uniform dispersion of NPs and the reinforced mechanical properties of polymer nanocomposites.

Transferability of a coarse-grained atactic polystyrene model: the non-bonded potential effect

In this paper, we construct an efficient and simple coarse grained (CG) model for atactic polystyrene (PS) by using a 1 : 1 mapping scheme at 463 K and 1 atm pressure and derive the corresponding bonded and non-bonded potentials in the CG force field (FF) via a direct Boltzmann inversion approach and a combined structure-based and thermodynamic quantities-based CG method, respectively.