Polymorphism of Syndiotactic Polystyrene Crystals from Multiscale Simulations

Abstract

Syndiotactic polystyrene (sPS) exhibits complex polymorphic behavior upon crystallization. Computational modeling of polymer crystallization has remained a challenging task because the relevant processes are slow on the molecular time scale. A detailed characterization of sPS-crystal polymorphism by means of coarse-grained (CG) and atomistic (AA) modeling is reported herein. The CG model, parametrized in the melt, shows remarkable transferability properties in the crystalline phase. Not only is the transition temperature in good agreement with atomistic simulations, it stabilizes the main α and β polymorphs, observed experimentally. The propensities of polymorphs at the CG and AA levels are compared in detail, as well as finite-size and box-geometry effects. All in all, CG modeling stands as an efficient approach to characterize polymer-crystal poymorphism at large scale.in this work. The unit cells are projected onto the a − b plane, and the backbones of the chains are along the c axis. Experimentally, two types of crystalline phases have been identified: the zig-zag-chain forming α [12][13][14][15] and β [16,17] appear upon thermal annealing of a melt, whereas the other three helix-forming crystalline phases, γ , [18,19] δ, [20][21][22] and ε [23][24][25] are obtained by solution processing. The α and β forms of sPS are further classified into the limiting disordered forms (α and β ) and limiting ordered forms (α and β ). [12,14,16,26] In particular, melt crystallization procedures generally produce the limiting ordered α and limiting disordered β models. [26] The limiting disordered α model is obtained by annealing the amorphous sample, [26] whereas the limiting ordered β model is obtained by crystallization from solution, when the solvent is rapidly removed at higher temperatures above 150°C. [16] Two of the helical crystalline phases (δ and ε) can only be obtained by guest removal from co-crystalline phases. The δ e form is transformed into the solvent-free γ form by annealing above 130°C. [18,20,26] These findings illustrate that the experimentally observed structures depend on the experimental processing, making conclusions about their thermodynamic equilibrium difficult.Several molecular simulation studies have been performed to better understand the nanoporous cavity structures formed by crystalline sPS. Tamai and co-workers [27] studied the size, shape, and connectivity of the cavities in the crystal α, β, and δ forms. Some other properties were also studied, such as diffusion of gases, [28,29] reorientational motion of guest solvents, [30,31] and sorption of small molecules. [32][33][34] While these studies helped understand the behavior of specific forms of sPS crystals, they did not address the crystallization process or the relative stability of the polymorphs. By studying sPS on the nanosecond timescale, they could not observe self assembly and spontaneous polymorph interconversion, due to their metastability. Yamamoto observed the onset of isotactic polypropylene crystallization, but leading to a ...

A challenging topic of computer simulations: Polymorphism in polymers

“…Therefore, the all‐atom model is the only proper choice for simulations of polymorphism. Some united‐atom or coarse‐grained models may give correct crystalline structure in some particular cases, but they cannot be used to study the mechanical properties.…”

“…We estimate that the all‐atom model needs a simulation time of 40‐100 μs to obtain the α‐form of the iPP. Liu et al studied the α‐ and β‐forms of syndiotactic polystyrene (sPS) crystals from multiscale simulations by using the all‐atom and a particular coarse‐grained model …”

“…Both the REMD and AMD are general accelerating methods for all simulations, and usually are used to enhance the sampling for free energy calculation. In the simulations of polymorphism of sPS crystals mentioned above, the REMD was used to accelerate the simulations.…”

“…However, most of the reported simulations studying polymer crystallization are based on models ignoring some molecular details to boost simulations, resulting in faster crystallization but incorrect crystalline structure. So far, the reported simulations of polymorphism in polymers are limited as few studies of PE using all‐atom models, of iPP using fine united‐atom or all‐atom models, and of molecular modelings of specific polymers with experimental data …”

Guest Occupation Effects on the Transition of Crystalline Syndiotactic Polystyrene: Selection Criteria for Fundamental Structures γ ‐I and γ ‐II

“…Syndiotactic polystyrene (s‐PS) has attracted increasing attention from the standpoint of both fundamental science and industrial applications because of its complex polymorphic behavior and the variety of physical properties associated with its structural transitions, as reviewed by some authors . To understand these polymorphic behavior, much efforts have also been made by simulation studies . The main‐chain conformation of the s‐PS crystals is classified into two types: trans planar (α and β forms) and s(2/1)2 helical (γ, δ, and ε forms).…”